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TRANSCRIPT
5101-144
Low-Cost Solar Array Project
Encapsulation Materials Status to December 1979 Edward F. Cuddihy
January 15, 1980
Prepared for
U.S. Department of Energy
Through an agreement with National Aeronautics and Space Administration
by Jet Propulsion Laboratory California Institute of Technology Pasadena. California
Prepared by the Jet Propulsion Laboratory. California Institute of Technology, for the Department of Energy through an agreement with the National Aeronautics and Space Administration.
The JPL Low-Cost Solar Array Project is sponsored by the Department of Energy (DOE) and forms part of the Solar Photovoltaic Conversion Program to initiate a major effort toward the development of low-cost solar arrays.
This report was prepared as an account of work sponsored by the United States Government. Neither. the United States nor the United States Department of Energy. nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information. apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights.
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SUMMARY
The first LSA report on encapsulation materials, entitled "Encapsulation Material Trends Relative to 1986 Cost Goals" (Reference 1), was published in April 1978. An essential feature of the first report was a discussion of the LSA criteria for selection of low-cost encapsulation material candidates.
This second report is an update of the April 1978 report. It describes the current status and level of development of the LSA encapsulation material candidates.
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CONTENTS
INTRODUCTION---------------------------------------------------- 1
GENERAL MODULE DESIGN------------------------------------------- 3
CANDIDATE MATERIALS--------------------------------------------- 6
Pot tan ts-------------------------------------------------- 6
Top Covers
Substrates
17
20
Electrical and Mechanical Isolator------------------------ 23
Back Covers----------------------------------------------- 25
Surfacing Materials and Modifications--------------------- 29
Primers and Adhesives------------------------------------- 33
APPENDIX A: WOOD----------------------------------------------- A-1
INTRODUCTION---------------------------------------------------- A-1
Reconstituted Wood Products------------------------------- A-1
Mechanical Properties------------------------------------- A-4
Cost------------------------------------------------------ A-5
APPENDIX B: SURFACE-SOILING MECHANISMS: THEORETICAL CONSIDERATIONS------------------------------------- B-1
INTRODUCTION---------------------------------------------------- B-1
SOILING BEHAVIOR------------------------------------------------ B-1
SOILING MECHANISMS---------------------------------------------- B-4
Cementation ----------------------------------------------- B-4
Organic Deposition---------------------------------------- B-6
Surface Tension------------------------------------------- B-6
Particle Energetics--------------------------------------- B-7
DISCUSSION------------------------------------------------------ B-12
SUMMARY--------------------------------------------------------- B-16
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Figures
Tables
1. Generalized Flat Module Design---------------------- 4
2. Wood Products Manufacturing Sequence---------------- A-2
3. Cementation Process--------------------------------- B-5
4. Weathering of Soda-Lime Glass----------------------- B-5
5. Effect of Particle-Particle Interactions on Sedimentation Volume----------------------------- B-7
6. Removal by Wind of Dust Particles from the Surface of a Clean, Oil-Free Glass Slide-------------------- B-9
7. Removal by Wind of Dust Particles from an Oily Glass Slide Surface--------------------------------- B-10
8. Removal by Wind of Dust Particles from a Glass Slide Surface after One Dew Cycle------------------- B-11
1. Status of Encapsulation Materials------------------- 4
2. Pottant Materials----------------------------------- 7
3. Ethylene Vinyl Acetate Compared With Polyvinyl Butyral --------------------------------------------- 10
4. Ethylene Vinyl Acetate - Processing Issues and Problems-------------------------------------------- 10
S. Current Formulation of Encapsulation-Grade Ethylene Vinyl Acetate------------------------------ 12
6. Current Formulation of Encapsulation-Grade Ethylene Propylene Rubber------------------------------------ 13
7. Non-Woven Glass Mats for Electrical and Mechanical Spacer Application---------------------------------- 24
8. Aluminum Foil/Polymer Laminates--------------------- 27
9. Polymer Films With Vapor-Deposited Aluminum--------- 28
10. Connnercial Surfacing Materials---------------------- 32
11. Status of Primers and Adhesives for Solar Cells and Encapsulation Materials------------------------- 34
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12. List of Interfacial Bonding Materials and Techniques-------------------------------------- 36
13. Summary of Problems Experienced with Photovoltaic Modules after more than One Year of Outdoor Exposure at the 12 JPL/Lewis Endurance Test Sites------------ B-2
14. Outdoor Soiling Experience of Photovoltaic Modules Fabricated with Different Exterior Surfaces-------------------------------------------- B-15
REFERENCES------------------------------------------------------ R-1
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INTRODUCTION
The Encapsulation Task of the Low-Cost Solar Array Project (LSA) has defined two task objectives in achieving 1986 technology readiness of encapsulation systems:
1. Materials and Processes
Define, develop, demonstrate and qualify encapsulation systems, materials, and processes that meet the LSA project life, cost and performance goals.
2. Life Prediction Method
Develop and validate a module life prediction method based on modeling life-limiting failure modes and on conducting and analyzing accelerated aging tests of candidate encapsulation systems.
This is a status report on the first task objective, to date.
The LSA project goal is to sponsor and stimulate activities that will reduce solar array prices to $0.70/Wpk (1980 dollars) by 1986. Of this price goal, the allocation for encapsulation materials* is approximately $0.14/Wpk or $14/m2 of completed module including an edge seal and gasket.
Surveys of encapsulation materials capable of meeting the LSA project goal have been carried out (References 2, 3) and were reported in April 1978 in the LSA Project report ''Encapsulation Material Trends Relative to 1986 Cost Goals" (Reference 3). This was followed by a shortened version entitled ''Low-Cost Encapsulation Materials for Terrestrial Solar Cell Modules" (Reference 4). The 1978 articles reported on a broad class of candidate materials by generic description, such as ethylene vinyl acetate, recognizing that many of the reported materials were not innnediately useful for encapsulation. Since April 1978 no new generic classes of materials have been identified, and the emphasis in the intervening period has been on the identification, development and evaluation of specific materials within the generic classes, and the evolution of encapsulation processes and of module designs with the low-cost materials.
In April 1978, encapsulation materials industrially used consisted essentially of two castable silicone elastomers (Sylgard 184 and RTV 615), a silicone gel, polyvinyl butyral (PVB) laminating film,
*The former cost allocation for encapsulation materials, reported in Reference 1, was $2.SO/m2 ($0.25/ft2) in 1975 dollars, or $3.50/m2 ($0.35/ft2) in 1980 dollars. The current cost allocation of $14/m2 is an aggregate allocation for all encapsulation materials including an edge seal and gasket.
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a hard silicone soil-resistant top coat, Tedlar and Mylar films, glass superstrate, and several substrate panels such as aluminum, NEMA-GIO epoxy board, and glass-reinforced polyester. Many of these materials are still being used and evaluated by module manufacturers; therefore the LSA program has chosen not to duplicate evaluation of them.
Since carrying out the surveys, about 15 to 20 additional materials have been identified and developed, or are under development, by the LSA Encapsulation Task. Of these, about 13 are now available for industrial encapsulation; only two or three identified in the 1978 surveys were then in a form inunediately useful for encapsulation of solar cell modules.
Industrial participation in evaluation of these materials is desired·and encouraged by the Encapsulation Task in order to identify industrial preferences, problems, and needs. Industrial evaluation aids in screening and ranking the list of candidate materials, as the list of encapsulation materials for continued development will be reduced. It is desirable that the shortened list of candidate materials be those favored by industry on the basis of practical experience.
An example is ethylene vinyl acetate (EVA) laminating film, which has been evaluated by a large part of the photovoltaic industry. A measure of its acceptance is that three of the Block IV manufacturers are using the film. This film material is currently in advanced development, primarily as a result of industrial reconnnendations of desired characteristics and features that should be incorporated into the film. Yet EVA is only one of the 15 to 20 encapsulation materials that have become or are becoming available since April 1978. This report identifies the available materials and their sources.
Since April 1978 activities toward the second encapsulation task objective, Life Prediction Methods, have been initiated, and are beginning to influence materials and module-design considerations.
Two examples are: first, identification of UV screening protection required of each candidate pottant, and therefore the level of UV screening property needed in protective outer covers, and second, efforts to limit UV degradation by the proper selection of UV stabilization additives incorporated into the bulk pottant. Additives can influence the types of chemical reactions that occur when materials are exposed to UV, the chemistry of which can proceed along different reaction paths. Some of these reactions are auto-catalytic, accelerating degradation, and others are catalytically non-active. The latter are the favoured reactions, which can be selected preferentially by the appropriate choices of UV stabilization additives. Inputs of this kind have the potential of payoffs in outdoor-life extension, generally at acceptable additional costs. The influence of life-prediction activities will increase substantially in the inunediate future.
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Recently a new contract activity (Reference 5) was initiated in the engineering optimization of a module design involving four design factors: structural integrity, optical transmission (maximized cell power output), electrical isolation (safety), and low operating temperature, at the lowest cost. Encapsulation materials, in terms of their properties and cost, will be treated as design variables. Engineering design data on the encapsulation materials will be documented and reported as part of this contract activity.
This report describes the current status and level of development of the viable material candidates, and includes a description of work remaining to be done to achieve usefulness of all of the encapsulation candidates. All material costs quoted in this report are in 1980 dollars.
GENERAL MODULE DESIGN
Since April 1978, it has been required of Task III contractors that module fabricability of all candidate materials be demonstrated, and that the modules pass the JPL temperature-cycle qualification tests (-40 to +900c). From these activities with low-cost materials, coupled with an awareness of the engineering and life requirements of a module, there emerged a module design connnonality in that all of the modules were being fabricated with combinations of six distinct functional layers. These are shown in Figure 1, which depicts a cross-sectional view of a module, with all six of the separate component layers separated according to their function, and in the relative positions they would occupy in a completed module. All six of these functional layers need not be employed in any given module, but all modules fabricated with the low-cost materials employ combinations of some or all of these six layers.
Table 1 lists the encapsulation materials, according to their functions (Figure 1), that have been identified or developed under Task III activities. A few of these materials are still under development and are not ready for industrial evaluation by module manufacturers. The majority are either connnercially available, or in such a stage of development that they can be subjected to industrial evaluation. Of the pottant materials, ethylene vinyl acetate has undergone an extensive evaluation by industry, and has been selected as the encapsulation pottant by three Block IV module manufacturers. Ethylene vinyl acetate is currently in an advanced stage of development.
In addition to materials, four encapsulation processes are being investigated:
I) Vacuum bag lamination
2) Liquid casting
3) Spraying
4) Direct extrusion
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MODULE SUNS I DE LAYER DESIGNATION FUNCTION
• LOW SOI LI NG SURFACE • EASY CLEANABILITY
1) MATERIAL • ABRASION RESISTANT 2) MODI Fl CATION • ANTI REFLECT I VE
TOP COVER • UV SCREENING • STRUCTURAL SUPERSTRATE
~ POTTANT • SOLAR CELL ENCAPSULATION
~ SPACER • ELECTRICAL ISOLATION • MECHANICAL SEPARATION
~ SUBSTRATE • STRUCTURAL SUPPORT
~ BACK COVER • BACKS I DE MECHANICAL PROTECT! ON
• BACKSIDE WEATHERING BARRI ER
Figure 1. Generalized Flat Module Design
1.
2.
3.
Table 1. Status of Encapsulation Materials
Surface materials and modification
Top covers
(with UV screening property)
a. Glass b. Tedlar XOO BG 30 UT
(Note 1) c. Korad 212 d. Silicone/Acrylic
Pottants
a. Ethylene Vinyl acetate b. Ethylene propylene rubber c. Aliphatic polyether urethane d. Polyvinyl chloride
plastisol e. Silicone elastomer,
534-044
4
Under development
Available Available (Du Pont)
Available (XCEL) (Note 2) Under development
Available (Springborn) (Note 3) Available (Springborn) (Note 4) Available (M. J. Quinn) (Note 5) Available (Springborn) (Note 5)
Available (GE) (Note 5)
f. Silicone resin, Ql-2577
g. Poly-n-butyl acrylate h. Silicone/acrylic polymer
4. Electrical and mechanical spacer
a. Non-woven glass mats
5. Substrate panels
a. Hardboards
b. Strandboards
c. Glass-reinforced concrete
d. Mild steel (incl. galvanized and enameled)
6. Back covers
a. Aluminum foils and polymer laminates
b. Tedlar, Mylar, Korad (polymer films)
c. Pigmented ethylene vinyl acetate
d. Others
Available (Dow Corning) (Note 5) Under development Under development
Available (Craneglas)
Available (Masonite, "SuperDorlux," Laurel 200, Ukiah Standard Hardboard) Under development (Potlatch Corp.) Under development (MBAssociates) Available
Available
Available
Available (Springborn)
Under development
Note 1: X = 1, 2, 3, 4, designating nominal film thickness in mils
Note 2: Needs additional development to achieve permanence of UV screening
Note 3: Suitable for industrial evaluation; advanced development in progress
Note 4: Suitable for industrial evaluation; advanced development not started
Note 5: Suitable for industrial evaluation; need for advanced development undetermined
The suitability of these processes for automation is also being investigated; however, the selection of a process is almost exclusively dependent on the processing properties of the pottant. This interrelationship may have a significant influence on the eventual selection of pottant materials.
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CANDIDATE MATERIALS
POTTANTS
In addition to being low in cost, these materials must be transparent, processable, commercially available, and pliable enough not to damage solar cells from mechanical toughness or thermal expansion mismatch. In many cases, the commercially available material is not physically or chemically suitable for encapsulation use, and therefore must also be amenable to low-cost modification. Additionally, the pottant material must either have inherent weatherability, i.e., retention of transparency and mechanical integrity under weather extremes, or the potential of long life provided by cost-effective protection that can be incorporated into the material or into the module design.
The candidate pottants are all transparent polymeric materials, and in an outdoor weathering environment polymeric materials can degrade from one or more of the following weathering actions:
1) UV photo-oxidation
2) UV photolysis
3) Thermal oxidation
4) Hydrolysis.
For temperature levels anticipated in operating modules, perhaps up to 105°c, three generic classes of transparent polymers are generally resistant to the four weathering actions: silicones, fluorocarbons, and acrylics. Another group of transparent polymers are resistant to thermal oxidation and hydrolysis, but not to degradation by UV photo-oxidation or UV photolysis. These are:
1) Ethylene vinyl acetate
2) Ethylene propylene rubber
3) Aliphatic polyether urethane
4) Polyvinyl chloride plastisol.
For these four transparent polymers, the top-cover material (Figure 1) must function as a UV screening layer in order to provide UV protection for these polymers. These four polymers, plus two silicones, one acrylic and one silicone/acrylic copolymer, comprise the list of candidate pottants. Six of these polymers have soft surfaces that are not easily cleaned and therefore necessitate coating with an easily cleanable surface material. The eight pottant materials are listed in Table 2.
Ethylene vinyl acetate (EVA) is a copolymer of ethylene and vinyl acetate typically sold in pellet form by Du Pont and United States Industrial Chemicals Incorporated (USI). The Du Pont trade name is Elvax;
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Table 2. Pottant Materials
COST Soil
Commercial or UV Screening Resist Source and Encapsulation Experiment Protection Layer
Material Identification Process Evaluation Projected Needed Needed
1) Ethylene vinyl acetate Springborn Laboratories Vacuum-bag lamination Yes Yes Enfield, CT
a) Clear 20-mil film A-9918 35¢/ft2 95¢/lb 0.48¢/ft2/mil
b) White-pigmented A-9930B 40¢/ft2 98¢/lb 14-mil film O. 50¢/ ft2/mil
2) Ethylene propylene Springborn Laboratories Vacuum-bag lamination To be negotiated $1.09/lb Yes Yes rubber Enfield, CT 0.50c/ft2/mil
a) Clear film A-8945-A
3) Aliphatic polyether H,J, Quinn Liquid cast Yes Yes urethane
2-part system:
a) Resin (3,86 parts) Q-626 $1.24/lb ?
b) Catalyst (1 part) Q-621 $1.49/lb ?
4) Polyvinyl chloride Springborn Laboratories Liquid cast To be negotiated 83¢/lb Yes Yes plastisol Enfield, CT 0,53¢/ft2/mil
A-10585-1
5) Silicone resin Dow Corning Spray $11.26/lb solids (Note 1) ? No No Midland, MI 5.85¢/ft2/mil
Ql-2577
6) Poly-n-butyl acrylate Under development Liquid cast To be determined $1.50/lb 0,93¢/ft2/mil
? Yes
7) Silicone/acrylic Under development Spray -To be determined $6.40/lb ? (Note 2) copolymer 3. 5¢/ft2/mil
8) Silicone elastomer GE Silicone Prod. Dept. Liquid cast $3.00/lb ? No Yes Waterford, NY (requires catalyst) 1.6¢/ ft2 /mil
(Experimental 534-044 photovoltaic pottant)
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Notes: (1) Sold as solvent solution at $8.45 per pound, consisting of 75% by weight solids and 25 wt,% solvent.
(2) Preliminary tests indicate good soil resistance and easy cleanability,
the US! trade name is Vynathane. The cost of EVA typically ranges between 55¢ and 65¢ per pound. Springborn Laboratories (Reference 6) screened all commercially available grades of EVA and reduced the list to four candidates on the basis of maximum transparency: Elvax 150, Elvax 250, Elvax 4320, and Elvax 4355. Because EVA is thermoplastic, processing into a module is best accomplished by vacuum-bag lamination with a film of EVA or by direct extrusion as a viscous melt. Based on extrudability and transparency, the optimal choice is Elvax 150, at a cost for high-volume purchases of about 60¢ per pound.
Elvax 150 softens to a viscous melt above 70°c, and therefore is not suitable as purchased for high-temperature service in a fabricated module. Springborn Laboratories (Reference 6) developed a cure system for Elvax 150 that results ih a tough, temperature-stable elastomer. In addition, Springborn Laboratories compounds Elvax 150 with an anti-oxidant and UV stabilizers, which improves its weather stability but does not affect its transparency. Preliminary tests at Springborn indicate that the compounded and cured EVA is chemically inert and that it provides corrosion protection to copper and mild steel. Although these results are encouraging, more investigation is needed, especially into long-term effects, if any.
In addition to clear EVA, Springborn makes a white-pigmented (with Zn02 and Ti02) EVA film that can be positioned on the back side of the solar cells in a module lay-up. The pigment provides a light-reflecting background for those module areas not covered by round solar cells, and increases module power output by means of internal reflection. This use of white pigmented EVA may be phased out when cell geometries are changed to achieve full-area cell packing, but white-pigmented EVA combined with a non-woven glass cloth is showing promise as a low-cost backside cover (Figure 1).
Elvax 150 film with two different compounding formulations has been available from Springborn. The first formulation was available in the fall of 1978, and several module manufacturers evaluated it by fabricating modules using their conunercial solar cells. The processing technique in all cases was vacuum-bag lamination. The manufacturers observed no basic fabrication problems with EVA, and reported on certain advantages of EVA when compared to polyvinyl butyral (PVB), the laminating film material in common use within the module industry. The reported advantages are listed in Table 3.
Further, the manufacturers suggested improvements for and raised questions about EVA, and expressed concerns about the associated vacuum-bag process. These are listed in Table 4. To date, the EVA questions relative to storage, handling, and repairability have not been answered, and the vacuum-bag process concerns have all but disappeared with improvements in fabrication techniques. Specific EVA improvements have been elimination of gassing additives and increased whiting in the pigmented EVA. Embossed EVA has been made experimentally, and activities are under way to incorporate a primer into the EVA, providing a "self-priming" property. A considerable and promising effort has been expended to reduce the time and temperature
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Table 3. Ethylene Vinyl Acetate Compared With Polyvinyl Butyral
EVA ADVANTAGES
• Cost
• Appearance
• Clarity
• Non-yellowing
• Eliminates cold storage
• Dimensional stability
• Processing advantages
1) Reduces time 2) Eliminates pressure autoclave
• Good flow properties and volumetric fill
Table 4. Ethylene Vinyl Acetate - Processing Issues and Problems
SUGGESTED IMPROVEMENTS
Incorporate primer
Increase whiting content
Reduce time/temp. for faster processing
Emboss for air removal and film winding
Avoid gassing additives
APPLICATION QUESTIONS
Maximum storage time/humidity?
Maximum handling temperature (blocking)?
Repairability?
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Table 4. Ethylene Vinyl Acetate - Processing Issues and Problems (Continued)
VACUUM-BAG PROCESS
CONCERNS
• Handling of large area pre-fabricated cell strings
• Air removal from large-area laminated modules
• Provisions for external connectors and leads
• Cell shifting
required to cure EVA, but has not yet succeeded. Nevertheless, this activity resulted in identification of a better anti-oxidant (Naugard-P) for Elvax 150, which is now used in the second and current formulation available from Springboro Laboratories for industrial evaluation.
The current formulations for both the clear and white-pigmented EVA films are listed in Table 5. Springboro compounds these ingredients into Elvax 150 pellets, then extrudes the compounded pellets at about 85°c to form a continuous film. The thickness of the clear film is nominally 20 mils; the white-pigmented film is nominally 14 mils thick. The curing system is selected to become active only above 120°c, so that film extruded at 85°c undergoes no curing reaction. The extruded film therefore retains the basic thermoplasticity of the Elvax 150, so that during vacuum-bag lamination the material will soften and flow above 70°c like any conventional laminating resin.
The curing process for Elvax 150 begins above 120°c, but is maximally effective above 140°c. The reco11DDended curing schedule for the formulations listed in Table 5 is 20 minutes at 150°c. The EVA curing system is inhibited by oxygen, so curing must proceed in an oxygen-free environment, which is ordinarily the condition in vacuum-bag lamination. It is important that the EVA be heated 20 minutes at 1S0°c in order to assure complete cure.
EVA films with the formulations listed in Table 5 are available from Springboro Laboratories for industrial evaluation. The cost of experimental quantities of the 20-mil clear film is 35¢/ft2; the 14-mil white-pigmented film costs 40¢/ft2.
Cost projections made by Springborn Laboratories indicate that as the annual production of encapsulation-grade EVA approaches 106 pounds, its cost should approach about 95¢/lb, or about 0.48¢/ft2/mil of thickness. This cost assumes that the pellets are purchased from Du Pont (60¢/lb), then compounded and extruded in a separate manufacturing operation. If encapsulation-grade EVA films were to be produced in a continuous one-company operation from monomer
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Table 5. Current Formulation of Encapsulation-Grade Ethylene Vinyl Acetate
Ingredient Function
Elvax 150 Base EVA
Lupersol 101 Curing agent
Naugard-P Antioxidant
Tinuvin 770 } UV stabilizers
Cyasorb UV-531
TiOz
} White pigments ZnOz
Ferro AM-105 UV stabilizer
*composition - parts per hundred of rubber.
Springborn formulation identification number
A-9918 Clear (phR)*
100
1.5
0.2
0.1
0.3
A-9930B Pigmented
(phR)*
100
1.5
2.0
5.0
0.5
to final product, its cost is estimated at about 76¢/lb, or about 0.38¢/ft2/mil of thickness. These projected costs can be compared with PVB, which currently sells for about l.46¢/ft2/mil of thickness.
The currently available encapsulation grade EVA has been favorably received by the industry. However, its status is still considered experimental, because of lack of sufficient data on long-term performance. Several developmental tasks remain to be completed:
1) Faster processing, primarily the cure schedule, which involves a reduction in both cure time and temperature. The minimum cure temperature will be dictated by the requirement that the curing system must not become active during film extrusion.
2) Minimization of the concentration of peroxide curing agent to the level just necessary for cure. Although not demonstrated, a long-term effect could result from the presence of excess peroxide in the EVA. The potential problem may be avoided by reducing peroxide concentration to the minimum needed for cure.
3) Optimization of the UV stabilization additives. The current additives were selected on the basis of literature
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citation and industrial experience with polymers similar to EVA. As remarked in the Introduction, UV stabilization additives that limit UV degradation to reaction products that are not auto-catalytic can be selected.
4) Embossed and dusted surfaces to permit rolling and unrolling EVA film for high-speed automation. The dusting material should be chemically inert and nonhygroscopic, as it is expected that the dusting material would be retained on the EVA and incorporated into the laminated module.
5) Self-priming; fabricating modules with the currently available EVA film requires a separate operation to prime the surfaces of other module components that will be in contact with the EVA.
Ethylene propylene rubber (EPR) is a copolymer of ethylene and propylene having cost and thermoplastic properties that are similar to EVA. EPR is processed by vacuum-bag lamination and is considered an alternative to EVA. Springborn screened the conunercial grades of EPR and selected Nordel 1320 (Du Pont) on the basis of transparency and extrudability. The cost of Nordel 1320 is about 65¢/lb for high-volume purchases. Springborn developed a curing system for Nordel 1320, and compounds this material with UV stabilization additives and an anti-oxidant reconnnended by Du Pont. The formulation for clear EPR is listed in Table 6. A white-pigmented EPR has not yet been developed.
A mini-module fabricated by vacuum-bag lamination with clear EPR film has successfully passed the JPL thermal cycle test (-40 to +90°c), and preliminary tests are indicating that the compounded and cured EPR is chemically inert and capable of providing corrosion
Table 6. Current Formulation of Encapsulation-Grade Ethylene Propylene Rubber*
Ingredient
Nordel 1320
Lupersol 231
Goodrite 3114
Cab-o-Sil MS-7
Tinuvin 770 }
Cyasorb UV-531
Function
Base EPR
Curing agent
Antioxidant
Crystallization inhibitor
UV stabilizers
*springborn formulation identification No. A-8945-A **composition - Parts per hundred rubber
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Clear (phR)**
100
1.0
0.2
3.0
0.1
0.3
protection to copper and mild steel. The cure schedule for EPR is 20 minutes at 150°c, and like EVA must proceed in an oxygen-free environment.
For annual production levels approaching 106 lb, the cost of encapsulation-grade EPR is projected at about $1.09/lb, or about 0.50¢/ft2/mil of thickness. Experimental quantities of encapsulation-grade clear EPR films can be purchased from Springborn Laboratories, with the price subject to negotiation.
As of this writing there has been no industrial evaluation of this film material, so there are no industrial opinions or guidelines for future developmental needs.
A potential advantage for compounded EPR may be an ability to encapsulate by continuous extrusion of the melt, but this has not yet been done. Attempts to do this with EVA have not been entirely successful, presumably due to a peculiar bubbling characteristic of EVA melt (this is not a problem for film extrusion or film lamination).
Aliphatic Polyether Urethane. This is a two-part liquid casting pottant available from H.J. Quinn in Malden MA. The resin, designated as Q-626, costs $1.24/lb, and the catalyst designated Q-621, costs $1.49/lb. The mix ratio is approximately 3.86 parts resin to 1 part catalyst, which yields a system cost of $1.29 per pound.
Both the resin and the curing agent are viscous fluids, about the consistency of heavy motor oil. The curing agent is chemically a diisocyanate, which is chemically very reactive with water, generating CO2. It is necessary that the surfaces of all module components that will come in contact with the mixed liquid system be dry in order to avoid gas generation and trapped bubbles. Attempts to use this material with wood substrates were disastrous, since wood is highly hygroscopic and extremely difficult to dry of all absorbed water. There has been partial success with glass superstrate and metal substrate modules, which are easier to dry. More work remains to be done with this material, particularly on the practicalities and cost impact of achieving the necessary desiccation.
After the resin and the curing agent are mixed, the system must be de-aerated in conventional liquid de-aeration equipment. The pot life at room temperature (20 to 30°c) is about 3 h; after casting the system develops full cure at room temperature in about 24 h. The cure may be accelerated by heating at 900c for 2 h.
Surveys* of other urethane candidates are still under way at Springborn Laboratories, but the chemical characteristics required of an encapsulation-grade urethane are not generally found in connnon connnercial products.
*At press, a new two-part liquid-casting aliphatic urethane resin, introduced recently into the market by Henkel Corp., Minneapolis MN, was identified.
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Poly-n-Butyl Acrylate. The desirability of an acrylic elastomer suitable for use as an encapsulation pottant was previously reported (Reference 1), but no acceptable connnercial materials exist. As a class of polymers, acrylics are highly desirable for module application because they are the lowest-costing transparent polymers that are generally resistant to degradation by weathering action. They offer the potential of long outdoor service life with a minimum of or no UV protection required for the top cover.
A requirement of encapsulation grade pottants is retention of elastomeric properties over the temperature range from -4o0 c to +9ooc. This requirement is met by poly-n-butyl acrylate, which has a glass-transition temperature of -54°c (Reference 7).
Poly-n-butyl acrylate is not commercially available in a form suitable for use as an encapsulation pottant, but then-butyl acrylate monomer is readily available at a bulk cost of 45¢/lb. In a developmental program undertaken at JPL, a 100%-pure poly-n-butyl acrylate liquid was developed that can be cast like any conventional liquid casting resin, and that subsequently cures to a tough, temperature-stable elastomer. A module fabricated with poly-n-butyl acrylate elastomer has successfully passed the JPL thermal cycle test, and another module covered with a hard acrylic soil-resistant layer has been outdoors for about one year with no visual evidence of change.
In general, the process for producing the liquid poly-n-butyl acrylate consists of first polymerizing a batch of n-butyl acrylate to achieve a high-molecular-weight elastomer, followed by dissolving the elastomer inn-butyl acrylate monomer to obtain a solution of acceptable viscosity. A polymerization initiator called Vazo is also added to the solution. After casting, the liquid is heated for 4 hat sooc in an oxygen-free environment, which results in polymerization of then-butyl acrylate monomer, and crosslinking of the resultant elastomer to a tough, temperature-stable material.
The detailed techniques of producing the poly-n-butyl acrylate have been transferred to Springboro Laboratories for further development and scale-up in order to produce a sufficient quantity for industrial evaluation.
A projected commercial cost for poly-n-butyl acrylate has not been established, but the cost of a related industrial acrylic product ~ay serve as a guide. Polymethyl methacrylate (PMMA) is familiar as Lucite (Du Pont) and Plexiglas (Rohm and Haas). The bulk cost of the monomer is 55¢/lb, and PMMA molding powder markets for about 68¢/lb, an increase ratio of about 1:1.2. As sheets sold in standard thicknesses, PMMA markets for about $1.07/lb to $1.92/lb, an increase ratio between 1:1.9 and 1:3.5. The bulk cost of n-butyl acrylate monomer is about 45¢/lb, and using an increase ratio of 1:1.2 as a lower limit and of 1:3.5 as an upper limit, the commercial cost of poly-n-butyl acrylate is estimated to be within the range of 54¢/lb to $1.57/lb. At the higher figure, this comes to an area cost of 0.97¢/ft2/mil of thickness.
15
Polyvinyl Chloride Plastisol. This is a liquid casting system developed by Springborn Laboratories for other cotIDllercial applications. It can be used for encapsulation in its current connnercial form, and modules fabricated with this material have successfully passed the JPL thermal-cycle test. Other modules with the plastisol have had up to ·one year of outdoor exposure without any visual evidence of change. Experimental quantities of this material can be purchased from Springborn Laboratories at a negotiated price. The material cures in air to a tough, temperature-stable elastomer after 20 minutes at 150°c. The cured material has a natural slightly yellow color that has not been observed to affect the power output from encapsulated solar cells. The projected cost of this material in volume is about 83¢/lb, or about 0.53¢/ft2/mil of thickness.
Silicone Elastomer. The Silicone Products Department of General Electric, Waterford NY, has developed an experimental silicone photovoltaic pottant at a current price of about $3.00/lb, or about 1.6¢/ft2 mil of thickness. The designation for this experimental material is 534-044.
The liquid silicone is a room-temperature curing material requiring a catalyst. After adding the catalyst, the pot life is very short, in the order of 15 to 25 min. In this time the liquid must be de-aerated and cast. Full cure occurs in about 4 hat room temperature. To the touch, the cured silicone elastomer is similar to that of Dow Corning's Sylgard 184 and GE's RTV/615. Springborn has successfully fabricated a small two-cell glass superstrate module with this silicone, which passed the JPL thermal cycle test. Springborn noted that adhesion of this silicone to glass was improved by using a Dow Corning primer, Z-6020.
Independently, GE's Silicone Products Department is investigating alternative or reduced use of the catalyst to increase pot life. At present, pot life can be increased to 30 to 40 min by reducing the amount of catalyst below current reconnnendations without changing the properties of the cured silicone.
In addition, GE is developing methods of fabricating minimodules with GE534-034 using liquid-application systems such as automatic mixing equipment and airless spraying. The automatic mixing equipment mixes catalyst and base resin in precise proportions with practically instantaneous mixing and without the necessity of de-aerating so that short-pot-life material can be easily used in an automated set-up. Likewise, airless spraying is capable of applying thin, uniform layers of catalyzed resin without the inclusion of air.
GE is doing this work with their own funding, aside from cells and miscellaneous encapsulation materials supplied by JPL.
There are no cost projections for high-volume usage of this material. Although silicone pottants will in general cost more than the other pottant candidates cited above, their use may become attractive under conditions where module flannnability is a critical concern.
16
Silicone/Acrylic Copolymer. Dow Corning has developed a silicone/acrylic block copolymer film-forming material to function as a UV screening cover for UV sensitive pottants (Reference 8). The chemistry of this material permits the incorporation of polymerizable UV screening agents for permanance of the UV screening property. As currently made, the polymer is dissolved in toluene and the solution is sprayed onto a release paper. After air drying for 30 min at 75°c, the film can be lifted off the release paper.
From preliminary tests, the cleanability of the film and its resistance to soiling is proving to be remarkable. In some cases settled particles can be blown off, and the surface can be cleaned readily by gentle wiping with a dry or slightly moist cloth.
In light of this, Dow investigated the potential of this material as a conformal coating pottant. A substrate module with adhesively attached solar cells was spray coated and dried at 75°c. The fabricated module successfully passed the JPL thermal cycle test, and is now outdoors on the Dow Corning weathering racks.
The projected cost of the film material is estimated at $6.40/lb, or 3.5¢/ft2/mil of thickness. Continued development and
quantity scale-up of this material will be carried out at Springborn Laboratories.
Silicone Resin. Dow Corning has additionally identified a connnercial silicone resin, Ql-2577, which can be spray coated (Reference 8). The material is functional as a conformal coating pottant without need for an additional overlay of a soil-resistant cover. Modules up to 11 x 16 in. have been spray coated with Ql-2577; all passed the JPL thermal cycle test. After spray coating the modules can be air-dried at room temperature for 24 h, or the air-drying can be accelerated to a few hours at 75°c.
Ti02 whiting pigment can be readily dispersed in Ql-2577 and can be spray coated as a thin layer on substrate panels to provide a light-reflecting white background on module areas uncovered by round solar cells. The pigmented Ql-2577 has also been used as a back cover for glass superstrate modules.
Ql-2577 is commercially available as a solution at a bulk cost of $8.45/lb. The solids content of the solution is 75% by weight, which correspond to a dry-solids cost of $11.26/lb, or 5.85¢/ft2/mil of thickness, assuming no spray loss.
TOP COVERS
A top cover is in direct contact with all the weathering elements--UV, humidity, dew, rain, oxygen, etc.--and therefore the materials selected must be weatherable. Only four classes of materials are known to be inherently weatherable:
17
1) Glass
2) Fluorocarbons
3) Silicones
4) Acrylics
In addition to weatherability, the top cover must also function as a UV screen, and possibly as an oxygen barrier, to protect underlying pottants that are sensitive to degradation by UV photo-oxidation or UV photolysis. The outer surface of the top cover should be easily cleanable and resistant to atmospheric soiling, abrasion-resistant, and anti-reflective to increase module light transmission. If some or all of these outer-surface characteristics are absent in the top-cover material, additional surfacing materials, to be described in a later section, may be applied. The outer surface may also be chemically modified to achieve a desirable surface characteristic.
To date, only four transparent and UV-screening top covers have been identified or developed.
Glass, such as ordinary soda-lime window glass, tempered glass, low-iron glass, etc., is also a structural material that can be used for glass-superstrate modules without the need of additional mechanical support from a substrate panel. Modules with thin glass top covers would need an additional substrate panel for mechanical strength.
The UV absorption characteristics of glass, such as magnitude of absorption and the UV cut-off point can be different for different glasses, and therefore the selection of a top-cover glass may be keyed to the UV protection requirements of the underlying pottant. Unlike polymer-film top covers, however, glass is impermeable to oxygen. The degradation of underlying hydrocarbon (i.e., EVA) pottants requires both UV and oxygen for UV photo-oxidation. Hence the oxygen-barrier property of glass, rather than UV absorption, may be the feature that prevents pottant degradation, especially if the back-cover and edge seal is also an oxygen barrier, such as aluminum foil.
The lowest-costing glass is ordinary soda-lime window glass, estimated by Battelle (Reference 9) to be on the order of 24¢/ft2 for 1/8-in. thickness, when purchased wholesale in large volume. At that thickness, all other glasses cost more. (The cost quote in Reference 9 is expressed in 1975 dollars as 17¢/ft2).
Tedlar. Du Pont markets a Tedlar fluorocarbon UV screening film designated Tedlar XOO-BG-30-UT where X represents the thickness of the film in mils (1, 2i 3, and 4). The cost of this film material is in the order of 5¢/ft /mil of thickness. The UV screening agent (proprietary) is chemically incorporated in the film to ensure permanence of the film's UV screening property. The permissible
18
combinations of Tedlar top cover and underlying pottants will probably be dictated by the film's UV absorption characteristics and by the UV protection requirements of the underlying pottant. A potential disadvantage of the Tedlar film is that the concentration of UV screening agent is fixed by the manufacturer; to increase UV absorption if necessary, thicker films would be needed at a higher cost. Another potential disadvantage is that the manufacturer will not warrant the film for longer than 10 years outdoors. An advantage of Tedlar film is that the surface is easily cleaned and is generally resistant to retention of atmospheric soiling.
Korad. XCEL Corporation of Newark NJ markets an acrylic UV screening film under the trade name Korad 212. The current cost of the film for low-volume purchases is in the order of 3¢/ft2/mil of thickness; the minimum thickness now is 3 mils. XCEL expects to market films as thin as 1 mil, with a projected high-volume purchase price no lower than 1.5¢/ft2/mil of thickness, but probably closer to 2¢/ft2/mil.
XCEL purchases Korad pellets from Rohm and Haas, and before extruding the film, compounds a low-molecular-weight UV screening agent with the pellets, which then is distributed physically throughout the film material. The big problem is that the incorporated UV screening agent becomes depleted within two years; the film then loses its UV screening properties.
Achieving permanent UV screening has become a major developmental activity involving JPL, Springborn Laboratories, University of Massachusetts (Amherst MA), and the XCEL Corporation. Several technical approaches are under investigation, but the most promising involves the use of polymerizable UV screening monomers developed by Professor Otto Vogl's group at the University of Massachusetts. One plan is to produce a high-molecular-weight random copolymer of methyl methacrylate, n-butyl acrylate, and the UV screening monomers such that the resultant copolymer material has mechanical and film extrudability properties closely matched to those of Korad. The copolymer material and the Korad pellets will then be blended and extruded into a film. Because the UV screening agent is chemically bound in a high-molecular-weight polymer, it should be retained permanently in the film.
Professor Vogl's group is capable of producing UV screening monomers having distinctly different UV absorption characteristics. It is foreseen that the selection of, and the film concentration of, a particular UV screening monomer should permit latitude in matching the UV absorption characteristics of the film to the UV protection requirements of the underlying pottant.
Silicone/Acrylic Copolymer Film. Dow Corning has successfully developed a process to produce a UV screening film that is a block copolymer of silicone and acrylics containing a chemically incorporated UV screening agent (Reference 8). The polymerizable UV screening monomer currently used is Permasorb-MA, available in limited
19
quantities from National Starch and Chemical Corporation in Bridgewater NJ. The acrylic block is a random copolymer of methyl methacrylate (MMA) and n-butyl acrylate (BA), and the silicone block is an acrylic terminated polydimethyl siloxane. The concentration of the acrylics and silicones, as well as the amount of Permasorb-MA, can be varied. From an experimental study relating the relative composition of acrylics and silicones with properties such as mechanical and thermomechanical behavior, the best film composition to date consists of 20% by weight silicones and 80% by weight acrylics, with the latter in turn consisting of 50% MMA and 50% BA by weight.
The UV protection afforded by this film, its inherent weatherability and the permananence of its UV screening property are now being tested. As a UV-degradable control, Dow Corning uses cellulose acetate (CA). After 904 h in the Dow Corning Atlas Weatherometer (the accumulated hours to date) the CA control is virtually destroyed, whereas CA coated with 3 mils of the film containing 1% by weight Permasorb-MA is visually unchanged. The screening film itself and its level of UV absorption are also unchanged.
The Dow Corning film is exhibiting good resistance to dirt and soil retention, an~ is also easily cleaned by wiping with a dry or slightly damp cloth. The wiping action does not visually mar or abrade the surface of the film.
The chemical technology for producing this film has been transferred to Springborn Laboratories for further development and scale-up to produce sufficient quantities for module fabrication and industrial evaluation. Springboro Laboratories will also investigate the use of UV screening monomers from the University of Massachusetts in this silicone/acrylic film. The projected cost for this UV screening film material is estimated to be no higher than 3.5¢/ft2/ mil.
SUBSTRATES
Three low-cost structural materials have been identified:
1) Mild steel
2) Wood
3) Glass-reinforced concrete
Mild Steel is the cheapest metallic material, on the order of 24¢/ftZ for a 28-mil-thick sheet. An advantage of mild steel is that it can be shaped easily to a flat panel with integral stiffening ribs on the backside. The stiffening ribs would reduce the panel thickness as compared with a flat panel without ribs carrying the same area load. Optimization of a ribbed substrate design in terms of minimizing the material cost/strength ratio has not been done.
20
A disadvantage to mild steel is its corrosion sensitivity. There is evidence (References 10, 11, 12, 13, 14) that metallic corrosion is arrested when the metal is wholly enclosed within a polymeric conformal coating chemically bonded to the metallic surface. The chemical bonding between the metal surface and the polymer coating is accomplished by use of chemical coupling agents; e.g., silanes. Reference 10 points out that coated metals have remained uncorroded after 19 years of direct exposure to salt water.
This concept is being experimentally tested at Springborn Laboratories. Pieces of mild steel and galvanized (zinc coated) steel have been wholly enclosed in clear ethylene vinyl acetate (EVA) that is chemically coupled to the metallic surfaces with silane coupling agents recommended by Dow Corning (Reference 15). After several thousands of hours of exposure to corrosive environments such as salt spray, there is no visual evidence of metallic corrosion. On the other hand, unprotected metallic controls rapidly corroded.
Additionally, metallic strips were partially coated, one-half of the strip wholly enclosed and the remaining half left as is. The uncoated half rapidly corroded, and there is visual evidence that surface corrosion is proceeding slowly past the edge of the EVA coating.
The tentative conclusion is that mild steel substrates, no matter how shaped, would have to be wholly enclosed, top-side, back-side, and edges, within a continuous and unbroken corrosion-protection coating.
Springborn Laboratories and Solar Power Corporation are fabricating 11 x 16 in. modules with flat sheets of mild steel as the substrate, and EVA pottant. The white-pigmented EVA is being used as the conformal coating to enclose the mild steel wholly, which is achieved during vacuum-bag lamination. Modules of this design will be mounted outdoors in marine, desert, and urban environments.
Alternatives to mild steel are galvanized (zinc coated) and enameled steel. In general, galvanized steel will cost about 20% more than mild steel. Steel sheet, which can be enameled, costs about 15% more than mild steel, and there are additional costs for the enamel and the enameling process. Detailed cost analysis of enameled steel products have not been done.
An ion-plating technique (Reference 16) for achieving corrosion resistance will also be investigated.
Wood is the cheapest structural material that has been identified. Structural wood products are divided into two classifications: prime lumber and reconstituted wood products. Lumber consists of the familiar 2 x 4s, 4 x 4s, planks and boards that are cut directly out of logs. These are not substrate candidates, not only because of high cost but also because large-area panels cannot be directly cut out of a log. Large-area wood panels useful as module
21
substrates are reconstituted wood products, such as particle boards, plywood, fiberboards, and so on. An expanded description of reconstituted wood products is detailed in Appendix I.
Of all of the varieties of reconstituted wood panels, only two kinds are considered viable candidates: strandboards and hardboards. The latter are fiberboards with densities greater than 50 lb/ft3. Both of these wood products are moldable, and can be molded as flat panels with integral reinforcing ribs. Optimization of a rib design has not been done.
Hardboard panels are readily available; Masonite Corporation, particularly, markets several 1/8-in.-thick panels with modulus values on the order of 800,000 to 1,000,000 psi. The price of these panels are in the order of 12¢/ft2. The specific hardboard being experimentally evaluated by Task III as a substrate panel is called Super-Dorlux.
Strandboard panels are being developed by Potlatch Corporation, which will begin commercial production in the fall of 1980. Strandboard panels having modulus values around 800,000 psi are currently being manufactured for evaluation at pilot-plant production levels. The projected price of strandboard panels is on the order of 13 to 14¢/ft2 for 1/4-in. thickness, and about 16¢/ft2 for 3/8-in. thickness.
In an outdoor weathering environment, wood products generally deteriorate from one or more of the following actions:
1) UV photo-oxidation of the lignin
2) Structural breakdown from extremes of hygroscopic expansion and contraction
3) Water rot
Water rot, also called wood rot, and also erroneously referred to as dry rot, is a bacterial attack on wood that begins when the absorbed water content of the wood exceeds 20 to 30% by weight. This level of water absorption only occurs from direct immersion or sustained exposure of wood to liquid water, such as, for example, sunken posts in wet soil or in ground areas with high water tables. When exposed only to atmospheric water vapor, the maximum water absorption at 100% RH is in the order of 9 to 16% by weight. Reconstituted wood-panel substrates mounted above ground will not undergo water rot. (Dry rot is the spread of water rot to drier sections of the wood, but the bacterial attack is initiated in the wet sections of the wood.)
For long service life outdoors as a substrate panel, the wood product must be protected from UV and hygroscopic expansion extremes. One ap~roach is to enclose the wood panel wholly within a polymeric conformal coating -- e.g., white-pigmented EVA. First, the wood panel is isolated from exposure to UV, and second, the hygroscopic response
22
rate of the wood panel is highly attenuated with respect to exterior fluctuations in relative humidity. Given sufficient thickness of the polymeric conformal coating, the hygroscopic response fluctuations of a wood panel can be damped to the point where the wood's absorbed water content is in virtual equilibrium with the annual average relative humidity of its locale. In effect, hygroscopic expansion and contraction of the wood panel is arrested. The optimum thickness of wood coating has not yet been determined.
It is interesting to note from Weather Bureau statistics that approximately 85% of the United States has an annual average relative humidity of about 55% to 65% RH. These statistics suggest that wood panels should probably be equilibrated to near 60% RH before being fabricated into modules.
An additional problem with wood panels is hygroscopic warp, primarily a result of localized variations in absorbed water content, or differences in absorbed water content on opposing surfaces. The level of absorbed water content is also a function of temperature. In a wholly enclosed environment, temperature differences across the thickness direction of the wood could result in an unbalanced distribution of the absorbed water content across the thickness direction, resulting in the potential of warp. This consideration will be assessed experimentally.
Springborn Laboratories and Solar Power Corporation are fabricating 11 x 16 in. EVA modules with Super-Dorlux hardboard substrate panels. The hardboard panels are being wholly enclosed within a continuous conformal coating of white-pigmented EVA. Modules of this design will be exposed outdoors in marine, desert, and urban environments.
Glass-Reinforced-Concrete substrate panels are being developed by MBAssociates, San Ramon, California (Reference 17). The panels are 1/4 in. thick, and have integral reinforcing ribs on the backside of the panel. The projected cost is 62¢/ft2, but this high cost is offset by the fact that its inherent mechanical rigidity reduces the cost of rack materials required for outdoor mounting. Total-costanalyses recently carried out indicate that glass-reinforced concrete is only cost-effective if it is part of the solar array field structure in addition to serving as a module substrate.
MBAssociates have manufactured a 4-x-8-ft demonstration module with this substrate material, using EVA as the encapsulation pottant and Korad 212 as the top cover. The demonstration module will be mounted on 6-x-6-in. pressure-treated wood posts, simulating an array field structure.
ELECTRICAL AND MECHANICAL ISOLATOR
Substrate modules employing metallic substrates, or glass superstrate modules employing metallic foils as back covers, must be fabricated in such a way that electrical contact with the solar cells
23
and their circuitry is avoided. In low-cost module designs this has been accomplished by positioning a non-conductive spacer between the solar cells and the metallic surface. A great variety of candidate spacer materials were investigated (Reference 6), and the best materials identified to date, in terms of handling, fabrication, and cost, are non-woven glass mats manufactured by the Crane Company, Dalton MA. The materials are sold under the trade name "Craneglas," and are distributed by Electrolock, Inc., Chagrin Falls, OH. The designation and cost of these materials are listed in Table 7. The particular mat being used is the Type 200, 5 mils thick, at a cost of 0. 78¢/ ft 2.
Table 7. Non-Woven Glass Mats* for Electrical and Mechanical Spacer Application
230
210
200
3
1.32
0.66
*craneglas, distributed by Electrolock, Inc. Chagrin Falls OH
5
1. 76
0.78
Thickness in Mils
7
Cost, ¢/ft2
2.2
1.56
0.97
9
2.8
1.36
12
3.7
1.81
The level of DC voltage required for electrical breakdown of EVA encapsulated modules with these spacer materials is under investigation. The first module tested was constructed with the following materials:
1) Soda-lime window glass
2) 20-mil clear EVA film
3) Cell string
4) 5-mil non-woven glass mat
5) 14-mil white pigmented EVA
6) 1-mil aluminum foil
Under lamination pressure, the minimum separation between cells and the aluminum foil would be limited to 5 mils, the thickness of the
24
non-woven glass mat. The DC electrical breakdown voltage of this module was measured at 5.8 kV. Although encouraging, it should be recognized that this a single result, and more work has to be done.
The non-woven glass mats also facilitate air removal from the module assembly when vacuum is applied inside the laminating fixture. For this reason, the non-woven glass mats are also used in modules fabricated with wood substrates. In this case the spacer also serves to provide a minimum separation between the wood substrate and the solar cells. There is an unproven hypothesis that it is desirable to maintain a minimum separation between the cells and the wood substrate because of the differences in thermal expansion coefficients of these dissimilar materials.
The non-woven glass mats are foreseen to provide another useful function.on the backside of wood substrate modules. As mentioned earlier, the hygroscopic response of wood panels can be arrested given sufficient thickness of the conformal polymer coating. Wood substrate modules fabricated by vacuum-bag lamination include in the lay-up a 14-mil white-pigmented EVA film on the backside of the wood panel. Under lamination pressure, this film thins to an uncontrollable thickness. By including a non-woven mat on the backside, the thickness of the backside EVA coating is always assured to be at least the thickness of the mat. Currently the 5-mil-thick mat is being used; eventual experimentation will identify the optimum required thickness, as well as how thin the pigmented EVA film may be (for cost savings).
It is speculated that the non-woven glass mats can be positioned above the top surface of solar cells in the light path, without affecting light transmission. In this position, in substrate modules, the spacer would provide a fixed separation between the topside solar cell metallization and the outer surface of the module, for topside electrical isolation.
BACK COVERS
Back covers evolve from the specific requirements needed on the backside of a low-cost module. Currently there are three backside materials recognized for low-cost modules: wood and mild steel for the substrate designs, and the laminating pottant for glass superstrate designs. Wood and mild steel require back covers for reasons stated earlier; as UV isolation and humidity fluctuation barriers for wood, and corrosion protection for mild steel. Laminating pottants, which are polymeric materials, may need prot~ction from humidity or from back-scattered UV, or may need durable back covers for protection during storage, shipment, and mechanical action such as blowing sand. The need of hermetic metal-foil back cover in the glass superstrate design may be determined by the moisture sensitivity of low-cost cell metallization.
For manufacturing simplicity and materials compatibility, the back covers for wood and mild steel are envisioned to be the pottant. This approach has already been taken with EVA, and will be taken with
25
EPR when a white-pigmented version is developed. Both of these pottant materials are processed readily by vacuum-bag lamination, and fabrication techniques have been worked out to achieve total enclosure of the wood and mild-steel panels during lamination.
This is accomplished by cutting the non-woven glass mats and white-pigmented EVA films larger in both length and width than the panel. The wood panel is machine sanded at the edges to a taper, an operation not required for the thinner mild-steel panel. During lamination, the overhanging edges of the white EVA and non-woven glass mats are pinched together to conform to the wood taper or the thin edge of the mild steel, and subsequently are fused together at and beyond the edge of the panel to form a continuous seal around the edges. Excess material can be trinnned.
For liquid casting pottants, back-cover fabrication techniques for wood and mild steel have not yet been worked out.
Back-cover films for glass superstrate modules can include Mylar, Tedlar, Korad, or metal foils. Alternatively, the back cover could be a pigmented layer of the laminating pottant. The pigment filler increases the mechanical toughness of the pottant, and if properly selected, also enhances UV protection. An example of an experimental module having, as a back cover, a pigmented layer of the laminating pottant is
1) Soda-lime window glass
2) 20-mil clear EVA film
3) Cell string
4) 5-mil non-woven glass mat
5) 14-mil white-pigmented EVA film
The back surface is mechanically tough and durable, and the glass mat isolates the solar cells electrically from the back surface. From the front, the white pigmentation provides a light-ref~ecting background that increases module power output.
Of all of the metal foil candidates, aluminum is the most appealing for reasons of cost, availability, and weatherability. Other metal foils such as stainless steel will, however, be investigated. The minimum thickness of aluminum foil that can be warranted pin-hole free is 1 mil, at a high-volume cost of 1.9¢/ft2. The use of aluminum foil requires an electrically non-conductive layer between itself and the solar cells, such as non-woven glass mats. Alternatively, the aluminum foil could be coated or laminated with a nonconductive material, such as Mylar, obviating additional spacer material.
Laminates of aluminum foil and polymer films are commercially available, as are polymer films that have been coated with vapor-
26
deposited aluminum. Connnercial products and their suppliers are set forth in Tables 8 and 9. The prices quoted in these Tables are for routine low-volume sales. Cost projections for high-volume sales have not been made, but as a beginning guideline, the cost of 92-gauge Mylar (0.00092 in.) for high-volume sales is l.4¢/ft2 (Mylar is a polyethylene terephthalate PET product), and as noted earlier, I-mil aluminum foil is l.9¢/ft2. The combined cost is therefore 3.3¢/ft2, which can be proportioned appropriately for comparison with aluminum/PET materials described in the Tables. The combined cost of the aluminum foil and the 5-mil non-woven glass mat is the lowest identified to date, at 2.7¢/ft2.
Table 8. Aluminum Foil/Polymer Laminates
Price Supplier ¢/ft2
1) Acme Backing
Polyester/Al/polyester 0.5 mil PET/I mil AL/0.5 mil PET
2) 3M Company
No. 431 Al foil tape w/acrylic adhesive No. 425 Al foil tape w/acrylic adhesive
3) Spaulding
0.007 Tedlar (PVF)/0.001 Al regular adhesive
0.001 Al/0.001 Mylar regular adhesive
0.001 Al/0.001 fiber regular adhesive
4) Surgicot
SPF-2273 0.066 Nylon/ream polyethylene 0.00035 Al foil/ream polyethylene 0.002 Polyethylene film
NOTES: PET= Polyethylene terephthalate (polyester) PVF = Polyvinyl fluoride
27
10
35 35
16 40
19 44
22 47
8
Table 9. Polymer Films With Vapor-Deposited Alwninum
Price Supplier ¢/ft2
1) Coating Products
Metallized Mylar 0.05 mil. 2 I mil. 3 2 mil. 6
1/2 mil. Mylar/~hite polyvinyl chloride (PVC) 4 mil. PVC 9 8 mil. PVC 11 12 mil. PVC 14
Mylar/60 lb. paper 1 mil. Mylar/paper 6 2 mil. Mylar/paper 10 3 mil. Mylar/paper 13
0.5 mil. Mylar/Al dep/adh. 26 5 mil. Mylar
0.5 mil. Mylar/Al dep/adh. 22 1 mil. Mylar
2) Hilcor Plastics
Metallized Mylar 2 mil.
3) King Seeley
Metallized Mylar 0.5 mil. 1.0 mil. 2.0 mil.
4) IC!
Melinex polyester 0.5 mil. Clear Metallized
5) 3M Company
No. 852 metallized polyester (adhesive)
28
8
2 3 6
7 10
68
Table 9. Polymer Films With Vapor-Deposited Aluminum (Continued)
Price Supplier ¢/ft2
6) Milprint
Metallized polypropylene 0.9 mil.
Metallized oriented nylon 0. 9 mil.
Metallized high-density polyethylene
7) Mobil Chemical Company
Oriented polypropylene
Metallized on side with a heat sealing resin on the other side
Metallized Bicor 0. 7 mil. O. 9 mil.
8) National Metallizing
A. Saxon Corp., Metallized polyester
laminated to polyethylene
9) St. Regis
Metallized polyester 0.5 mil.
10) Vacumet
Metallized polyester 1 mil. 2 mil.
SURFACING MATERIALS AND MODIFICATIONS
The top surface of a module is where all the action begins.
2
2
1
1 1
2
1
3 6
Incident light either enters into the module, or it does not, or is partially blocked by surface accumulations of atmospheric soiling, or is partially reduced by back-scattering losses from an abraded or roughened surface. The top surface should be resistant to permanent retention of atmospheric soiling, easily and readily cleanable,
29
resistant to abrasion and roughening, and if possible, anti-reflective to increase the transmission of incident light. One or more of these surface requirements may be inherent in the top-covering materials, but if not, additional surfacing materials or surface modifications may be needed. This area is under intense investigation.
Identification of module soiling as a problem has resulted from field observations. However, the LSA project is not aware of any incidences of surface abrasion that have naturally occurred to modules in the field. Efforts to develop or to qualify commercial abrasion-resistant surfacing materials for modules should be scaled to the seriousness of material abrasion occurrences. LSA is not aware of any scientific or organized study of the reality and characteristics of "natural abrasion"--for example, how high above ground level does abrasion occur on a stationary object during a sandstorm?
An understanding of the properties and behavior of natural abrasion would permit materials developers to focus more easily on surface requirements and simulation tests. Equally, designers of outdoor array structu~es may be able to achieve elimination or minimization of natural abrasion. If it happens that natural abrasion is in reality not a problem, attention could then be more constructively focused on non-abrading cleaning strategies.
The LSA Engineering Task has sand-blasted the surface of an annealed glass-superstrate module and observed only a 10% decrease in module power output. Abraded surfaces, whether natural or as a result of cleaning, may have more significance relative to enhanced soil retention.
Considerations of soiling resistance and ready cleanability are the farthest advanced, although there is still a long way to go. An expanded discussion of soiling mechanisms, both known and postulated, is presented in Appendix B. Based on these mechanisms, it can be inferred that the requirements for low soiling and easily cleanable surfaces appear as a minimum to be:
1) Hard
2) Smooth
3) Hydrophobic
4) Low surface energy
5) Chemically clean of sticky materials
6) Chemically clean of water soluble salts
and evolving requirements for low-soiling environments appear to be:
30
1) Low to zero airborne organic vapors
2) Frequent rains, or generally dry (low dew, low RH)
3) Few dew cycles or occurrences of high RH between heavy rain periods.
The natural cleaning forces are seen to be as a minimum:
1) Rain, including dew that may be heavy enough to run off the module surface in a liquid phase.
2) Snow
3) Wind
Given the validity of the six requirements for low soiling surfaces, only two of the four inherently weatherable and transparent materials came close to meeting these requirements: glass and fluorocarbons. The soiling and cleaning qualities of glass are familiar; these same qualities are observed for hard fluorocarbon films such as Tedlar. Hard acrylics such as Plexiglas (Rohm and Haas), Lucite (Du Pont) and Korad are also observed to have qualities of low soil retention and easy cleanability despite being less hydrophobic, but acrylic materials are more sensitive to abrasion, which changes their surface characteristics. Acrylic materials such as Korad may need additional surfacing materials, more for abrasion resistance than for soiling behavior. Dow Corning's silicone/acrylic copolymer film exhibits easy cleanability, but is extremely sensitive to abrasion, and like Korad, may need an additional surfacing material.
From considerations of soiling and abrasi~n resistance, glass superstrate modules are excellent candidates. For substrate modules having polymer films as top covers, fluorocarbon films, or a surfacing material consisting of a dispersion of colloidal glass or silica in a hard but not brittle fluorocarbon binder, appear as ~iable candidates. The latter mixture should be appliable as a surface layer in the order of 5 to 10 µm thick in order to achieve a cost-effective utilization of the expensive fluorocarbon. Such a surfacing material is not available.
There are three classes of surfacing materials available cotmnercially that can be applied as thin coats from solutions or liquid resins, or as thin film layers. These are listed in Table 10.
The dispersions of colloidal silica in a hard silicone resin are the commercially available abrasion-resistant coatings. The Dow Corning ARC coating on Korad is currently being exposed outdoors at the JPL test site. Although results are encouraging, it would be premature to make a judgment at this time.
31
Table 10. Connnercial Surfacing Materials
1) Fluorocarbons
Tedlar (Du Pont)
Kynar (Pennwalt)
Halar (3M)
Teflon (Du Pont)
Abcite (Du Pont)
2) Glass Resins
Glass Resin 650 (Owens-Illinois)
3) Silicon Resins with Colloidal Silica
ARC (Dow Corning)
SAR (Du Pont)
SHC-1000 (General Electric)
Thin layers of the fluorocarbon materials have not yet been investigated.
Another investigation involves a chemical modification of the surface of the Korad film. It has been observed that certain grades of glasses are more resistant to soiling retention, and to surface etching by chemical action with atmospheric components. These glasses contain aluminum ions rather than the sodium and other alkali ions found in soda-lime window glass. It is speculated that the improvement in surface properties is attributable to aluminum, which, unlike the monovalent alkalis, is non-hygroscopic (Reference 18), forms stable salts resistant to chemical reactions with water, and is trivalent, thus ionically crosslinking the surface to achieve hardness and toughness. The acrylics in Korad are organic esters that can be saponified to introduce ions such as aluminum. Acrylic reverse-osmosis membranes have been ionically crosslinked by aluminum (Reference 19), and the membranes, which are colorless and transparent, achieve mechanical toughness and durability and resistance to swelling when innnersed in water. The surface modification of Korad with aluminum is under investigation. The concept of aluminum-ion surface modification could be extended to the low-cost soda-lime window glass in order to achieve a tougher, more weather-stable surface. The concept will be investigated using ion plating (Reference 16).
32
With respect to generating anti-reflective (AR) surfaces, work is going on at Motorola in Phoenix (LSA Contract No. 995387) to achieve AR surfaces on glasses by chemical etching techniques. Transmission of 98% has been demonstrated, and automated mass production procedures are being developed. Another approach is through the use of ion plating (Reference 16). This process for depositing thin surface films is capable of regulating the morphology of the deposited phase, which has an influence on the level of light reflection. Thus there is the potential of depositing hard surface materials having soiling and abrasion resistance, coupled with the preferred morphology for AR properties. The process can deposit on glass and non-glass surfaces. This work is just beginning.
PRIMERS AND ADHESIVES
Encapsulation modules must hold together for 20 years, reliably resisting partial delamination and separation of any of the component materials. Delamination of encapsulation materials from solar cells or interconnects can create voids for accumulation of liquid water and potential corrosive failure. Delamination of silicone elastomers from unprimed surfaces was a connnon occurrence with Block I modules, but the incidences of silicone delamination with Block II and Block III modules decreased when adhesion promoters recommended by the silicone manufacturers were employed. An investigation of silicone delamination from unprimed surfaces successfully identified the mechanism, which is reported in References 20 and 21.
It would be desirable to have all of the interfaces between encapsulation materials and between encapsulation materials and solar cells held together by environmentally stable primary chemical bonds (Reference 22). Some materials bond to each other chemically during the module fabrication process, such as EVA and Korad, but the majority of interfaces will probably require the use of a chemical coupling primer or adhesive. A recent LSA report (Reference 15) describes the fundamentals of chemical bonding technology, and reports on initial reconnnendation of chemical coupling primers for ethylene vinyl acetate modules. There is also accumulating evidence that proper selection of chemical bonding agents or adhesion promoters can also prevent metallic corrosion (References 10, 11, 12, 13, 14), opening the possibility of employing mild-steel substrates, and copper as a solar cell metallization material.
The identification and qualification of chemical bonding agents for all of the practical interfacial combinations of encapsulation materials requires a substantial increase in activity. A perspective of the task is presented compactly in Table 11.
This table lists vertically and horizontally all of the encapsulation materials under investigation by Task III, and includes solar cells in the list. All combinations of materials and solar cells are generated by this matrix format; combinations that are not seen as practical or possible are identified by an X in the common location defined by the crossing of the row and column of the respective materials (or solar cells).
33
Table 11. Status .of Primers and Adhesives .for Solar Cells and Encapsulation Materials
SOLAR CELLS -----------------~ . A) SOLAR CEUS SURFACING MATERIALS ------·---·----a--•---. .--.. -·----O----<
8) SURFACING MATERIALS X GLASS 1--~~------~ --~·-·------~-+e--,,-t-----1
KORAD 212 1,2, 3.7 C) TOP COVERS 1) GLASS
-----·----·-·-·----- ··-···-·-·-----+-"-"--------, 2) KORAD 212 X X
------------------·--------11--.....-.--+-+---t 3) TEDLAR XOO-BG-~UT X X X
TEDLAR XOO-BG-3~T
SILICONE/ACRYLIC COPOLYMER ---··----·--·--. --· .. -·- ······- --------------t--~ > TOPCOVERS
.C} SILICONE/ACRYLIC COPOLYMER T X X X ETHYLENE VINYL ACETATE (CLEAR)
.... D_)_P_o_n_A_N_T_s ____ 1,-ET-H-YLENE VINYL ACETATE(CLEAR -;-'"·;- ;,6- u T T I ETHYLENE PROPYLEN~
2) ETHYLENE PRO)'YLENE RUDER 5 X 5,6 T X j I POLY•n•BUTYL ACRYLATE
i------------3)-PO_L_Y:n-BUTYL ACRYLATE X ' t---~--~_c~-~ POLYVINYL CHLORIDE PLASTISOL
.C) POL VVINYL CHLORIDE PLASTISOL X .C, 5 i X X I X POLY URETHANE ' l
5) POLY URETHANE ---- X T_J_ ' _ _L_J-:<_l!'-!.-~~~ SILICONE RESIN (Ql-2577)
} POTTANTS
6) SILICONE RESIN (Ql-2577) u x u ! xi x I x · x : x i x l x ! x I SILICONE ELASTOMER (GE) _,
n. SILICONE ELASTOMER (GE) x .. ··=0-~~-~ : x_ I x , ~ _ -l~~~~e/~~~.~~P~L-;~-;
a, SILICONE/ACRYLIC cOPOL YMER T x T : x t-x--r ~ x : x . x t-;T x x I x , jCRANEGLAS } ELECT/MECH ISOLATOR
E) ELECT/MECH ISOLATOR (CRANEGLAS) x x Tf; r~ 1 ~1~==u- ~-~ -_ : ~- ~:~ LlJ HAR~!-RD · - ..
SUBSTRATE PANELS
~TRATE PANELS 1) HARDBO~D _______ ~~--.~~ ;_x_: ~+-x i~~6-t- _ ·-- : J . ~ i L'-~ ~-~ STRANDBOARD
----~~N_DBO~D _________ _,~x -~lx-f--x-Jxj_ ~ __ _LL_ J_+_Lx ;~~~'-- .. _ 1------ 3) MILDSTEEL ---------~-~~~ ·~- . ,--- -+-- __
1 __ ··t·X_, ~ ~"T GALVANIZED_ST!~
.__ _______ .. _)G_A_Lv_A_N_1z_E_D_S!EEL _____ ..'::.__ X j X tx~ "...8__,__L_ J-, xr~ Tx I X ~ GLASS·RE!Nf~_B>C<>NCR~- _ _ _ -- _ ---
_________ S) ___ G_LAS_S-_RE_IN_F~CED CONCRETE X : X / X : ~ti -~i' . 1
, i __ - t -~ ; ~-!_x. l. _ X ·. X 3YLAR
G) BACKCOVERS 1) MYLAR x ~l~J!.L~..._~..J.-i __ ~- j I x_L :_ ,~ J ~-4 ~ 1 ~_I _xJ _1_SoRAo 1--------·2-, .OW> x x x j x, x_: x I u' : ____ ·_ .- ~~·-;-~~-~-:~T~~-__J_~DLAR
3) rmw x x x ili x r . _ I I ___ t _ x ; --~ ! x-J-~x.h:!" x J _ j_ x GJ§UMINUM FOIL
.C) ALUMINUM FOIL X X r·~ X X s I s ' ! I X I ~ i X I ; tt-' J T ; i =-m· WHITE-PIGMENTED EVA ------- - t- l : - +-·- -+--+-:-! . -~
5) WHITE-PIGMENTED EVA X X I X X U X i X X . X X I X X I X !S,6 1 ; : ! ! i i OTHER P1GMENTED POTTANTS
1-------6-~-0-TH_E_R PI-G-MENTED POTTANTS x x x x- j x \ i--.~;fjx l j I j j i i -] i ; I x I
BACK COVERS
The common locations of all other interfacial combinations seen as practical or possible are identified by a letter, number, or blank (Table 11).
The letter U indicates that the interfacial combination requires no primers or adhesives, such as EVA and Korad, which bond chemically during module fabrication. The letter T indicates that a chemical bonding agent is being investigated for the particular combination, but is not qualified sufficiently to be recommended.
The numbers identify primers and adhesives listed in Table 12 that are recommended for the particular interfacial combination. Details concerning the preparation and use of the two primers numbered 4 and 5 in Table 12 are to be found in Reference 15.
A blank (no letter or number) means that no primer or adhesive has been identified or recommended for the combination.
Included in activities related to the identification and recommendation of primers and adhesives, is work being done at the Rockwell International Science Center (LSA Contract No. 954739) investigating the physical and chemical quality of the interfacial bond. This investigation includes an evaluation of nondestructive techniques that may be applicable to the analytical and diagnostic evaluation and assessment of an interface after module fabrication and during outdoor exposure.
35
Table 12. List of Interfacial Bonding Materials and Techniques
Materials
A) Adhesives
1) Xl-2561
2) Ql-2577
3) Q96-083
(Dow Corning)
(Dow Corning)
(Dow Corning)
B) Primers
4) Z-6020 (Dow Corning)
5) Z-6020/ Z-6030 Mix (Dow Corning)
6) SS-4179 (General Electric)
Techniques
A) Non-material adhesion
7) Electrostatic bonding (Spire)
Letter Codes
X
u
T
Blank Space
No interfacial combinations foreseen
Self-bonding; no primers or adhesives needed
Test in progress
Not Determined
36
APPENDIX A
WOOD
INTRODUCTION
Wood has been identified (References 1, 3) as a low-cost candidate material for the substrate panels of photovoltaic modules. Conunercially, wood products divide into two broad classifications: prime lumber and reconstituted products. Prime lumber consists of studs, planks, boards, etc. which are directly sawed out of logs; reconstituted wood products are produced by pressing wood pieces such as sawdust, flakes, slivers, fibers, etc., together with organic binders, and from gluing laminas of thin veneers (i.e., plywood).
Large-area panels that can be considered candidates for module substrates are reconstituted wood products.
In this appendix, the technology of reconstituted wood products as it relates to substrate application will be briefly described, and cost information will be provided. Figure 2 is an illustration of the wood-product production sequence.
As illustrated, the log, after its bark is removed, could optionally proceed along any of three different production paths, designated as A, B, and C (Figure 2). Path A leads to plywood, Path B leads to reconstituted wood products, and Path C, leads to prime-lumber production.
RECONSTITUTED WOOD PRODUCTS
A discussion of reconstituted wood products involves three topics: the properties of natural wood, the size and shape of the wood pieces, and organic binders.
Natural wood is mechanically anisotropic, having maximum tensile modulus and strength along its fiber axis, and minimum tensile modulus and strength in directions perpendicular to the fiber axis. Depending on the tree, the ratio of maximum to minimum modulus varies on the average from 3 to 5. The maximum modulus will typically not exceed 1.5 million psi, while the minimum will typically be no lower than 300,000 psi (Reference 23). The level of modulus in any direction is related to tree density and, generally, strength increases with increasing density. Dense trees are sought for prime lumber production. Low-density trees are useful for reconstituted wood products because, under pressure, the wood density and therefore the strength can be increased.
A-1
STEAM ENERGY
SAWDUST +
UREA FORMALDEHYDE BINDER
PARTICLE BOARD
BARK REMOVAL
PATH "C" PRIME I LUMBER
~QREMNANT f CORE
_____- SAWING ---~ l -------... PR I ME LUMBER, STUDS,
POSTS, BOARDS, ETC.
WOOD REMNANTS (IRREGULAR PIECES UNSUITABLE (j c:::::l ~ FOR PR I ME LUMBER)
0...-... __n-~ PRODUCTION OF ... ~ c,; RECONSTITUTED
SAWDUST
WOOD FLAKES +
PHENOL FORMALDEHYDE BINDER
WOOD PRODUCTS
MASONITE "HOT -WATER11
PROCESS
WOOD STRANDS (SLIVERS) +
PHENOL FORMALDEHYDE BINDER
STRAND BOARD Fl BER BOARD
......_ _____ ~ RECONSTITUTED WOOD ---------·--PANEL PRODUCTS
Figure 2. Wood Products Manufacturing Sequence
A-2
Wood pieces divide into 5 distinct types that dominate the reconstituted wood market. These are:
1) Sawdust
2) Flakes
3) Slivers
4) Fibers
5) Veneers
Sawdust, flakes, and slivers generate a class of wood products referred to as particle boards. Fibers produce a class of wood products referred to as fiberboards, and if the board density is greater than 50 lb/ft3, they are called hardboards. Veneers are used for plywood.
Sawdust is produced as a by-product of sawing prime lumber, or as a primary product by mechanically shredding large wood pieces to a fine powder.
Flakes and slivers are large wood particles produced by mechanical shredding of whole logs or large wood pieces. Flakes are irregularly shaped wood pieces, generally rectangular, averaging about 2 to 3 in. long and about 1/2 to 1 in. wide. The thickness along the long axis tapers from a few mils at one end to about 5 to 10 mils at the other end. The flakes are randomly shredded out of the wood stock, so that there is no orientation pattern of mechanical properties. Reconstituted wood products made with flakes, referred to as "flakeboards," are mechanically isotropic in the plane of the board, but may have different properties in the thickness direction.
Slivers are oriented flakes, and are narrower, being about 1/4 in. wide. The slivers are shredded to have the high-modulus axis of the wood aligned with the long axis of the sliver. Slivers are used to make boards that have mechanical-property orientation; the products are referred to as "strandboards" (Reference 23).
Fibers are obtained by heating wood pieces in hot water until they fall apart. The fibers are not soluble in water, and can be collected by filtration. The resultant fiber bundle looks like a bale of cotton; it can be placed in a press and compressed under heat to a solid board called fiberboard. If the density of the fiberboard exceeds about 50 lb/ft3, the resultant product is generally referred to as "hardboard." The mechanical anisotropy of the wood stock is not preserved by the hot-water fiber process, and fiberboards are mechanically isotropic in the plane of the board, but may have different properties in the thickness direction. Some manufacturers add organic binders to their fiberboard products.
A-3
Veneers are 1/8-in.-thick (nominal) sheets of wood that have been continuously sliced from rotating logs. The veneer sheets are mechanically anisotropic, preserving the mechanical anisotropy and level of mechanical properties of the source log. In order to make a mechanically balanced plywood, the veneers are stacked alternately and glued together under pressure. That is, using compass points as frames of reference, the high-modulus direction of the first veneer is aligned north-south, that of the second veneer east-west, and so on. The thickness of the plywood product is dictated by the number of veneer plies, and the modulus is dictated by the parent tree.
Organic binders used in reconstituted wood products are typically urea formaldehyde and phenol formaldehyde. Urea formaldehyde is susceptible to hydrolysis and reconstituted wood products produced with this binder are recommended for interior use only. Phenol formaldehyde is resistant to hydrolysis, and wood products prepared with this binder can be used outdoors as long as the binder is protected from UV. Phenol formaldehyde costs twice as much as urea formaldehyde and its processing is slower than that of urea formaldehyde, thus reducing production rate. Phenol formaldehyde products are more expensive than urea formaldehyde products because of both base cost and reduced production rate.
Urea formaldehyde wood products are not recommended for module panels by the wood industry. Not only is hydrolysis of concern to them, but more important, urea formaldehyde has a tendency to release formaldehyde vapors. This is coming under increasing scrutiny by the Environmental Protection Agency. Urea formaldehyde binders may be phased out of use in the foreseeable future.
MECHANICAL PROPERTIES
As unfilled materials, both urea formaldehyde and phenol formaldehyde have a Young's modulus in the order of 250,000 to 300,000 psi, which therefore becomes the lower limit of modulus for particle boards made with these binders. Sawdust is a nonreinforcing filler, which means that the modulus of sawdust particle boards is not increased to any extent above the base value of the unfilled binders. In other words, the modulus of sawdust particle boards is about 300,000 psi. Because of this property, exterior construction applications are limited. Generally, sawdust particle boards are used for interior applications as simulated wood for furniture, door panels, etc. With the evolutionary trend toward indoor applications for sawdust particle boards, almost all of them are made with urea formaldehyde binders. Sawdust particle boards are not recommended as module panels.
On the other hand, slivers and flakes are reinforcing fillers, and particle boards made with them can yield modulus values approaching 1.5 million psi; the upper limit is basically dictated by filler concentration, modulus of the source tree, and processing pressure. Typically, however, the modulus of commercial flakeboards
A-4
and strandboards is in the order of 800,000 psi, to be competitive with commercial plywood. Flakeboards and strandboards are recommended candidates for substrate panels.
The modulus of fiberboard products is related almost exclusively to processing pressure and temperature, which act to make the board denser. Fiberboards with densities greater than 50 lb/ft3 (hardboards) are the boards recommended for substrate panels. For example, Masonite markets two hardboard products, Super-Dorlux, with a modulus of about 800,000 psi, and Tempered Duolux, with a modulus slightly over 1 million psi.
Plywood, as we know it today, will be phased out gradually as a commercial product, or will be manufactured in limited quantities for specialty markets. The cost will be substantially higher than for other reconstituted wood products of equal strength. Plywood is not recommended as a substrate panel.
Tomorrow's plywood will probably be made from strandboard, and commercial development is already underway. The process consists of building up a (nominally) 1/8-in.-thick layer of strands all aligned north-south, followed by another (nominally) 1/8-in.-thick layer of strands all aligned east-west, and so on. A liquid binder is added and the product is pressed and cured to yield a substitute for veneer plywood. Some of its advantages are: no knotholes, 100% utilization of low-density and rapidly growing trees such as aspens, and elimination of wasted cores of valuable high-density trees.
Potlatch plans to produce commercially an alternating strandboard as a substitute for veneer plywood by late 1980.
The two primary candidates for photovoltaic module panels, fiberboards and strandboards, are capable of being manufactured with integral stiffening ribs.
COST
Cost data for reconstituted wood products fluctuates because these wood products are commodities subject to supply and demand of the open market. Another factor is that the industry considers cost as what they pay to produce and deliver the product to their shipping dock. Price is what the customer pays F.O.B. for the product, which can vary from 25% to 50% above cost, averaging about 33%. Depending on the volume of purchase and the contracted period over which purchases are to be made, the markup between cost and price may be negotiable.
The cost of reconstituted wood products is bracketed on the low side by the cost of natural veneer, directly sliced from rotating logs, and on the high side by the cost of fiberboards (hardboards). Fiberboards generally are the most costly reconstituted wood product because of the included cost of energy for the hot-water process.
A-5
The cost of natural veneer is currently about 3¢/ft2 for a nominal 1/8-in. thickness. Natural veneer is highly anisotropic, having a unidirectionally high modulus value which can be as high as 1.2 million psi, to a value as low as 300,000 psi in the perpendicular direction. The cost of natural veneer is published periodically in a wood-industry publication.
The cost of fiberboard is typically not published, but F.O.B. prices will be quoted by manufacturers. Currently, Masonite's lowest price quote for high-volume purchases is about 12¢ per ft2 for 1/8-in.-thick Super-Dorlux and Tempered Duolux. Super-Dorlux has a modulus of about 800,000 psi; Tempered Duolux has a modulus slightly higher than one million psi.
Potlatch forecasts a price in late 1980 of about 16¢/ft2 for 3/8-in. thickness, and 12 to I3¢/ft2 for 1/4-inch thickness for their alternating strandboard. These wood products are designed to have a modulus of about 800,000 psi. Although a 1/8-in.-thick strandboard will not be made, its price for comparison purposes can be estimated at about 9 to 10¢ per ft2.
A rule of thumb for estimating wood product costs and prices assumes that the cost of wood in each 1/8-in. increment of thickness is the currently published cost of natural veneer, about 3¢/ft2 at the time of this writing. Further, assume a cost-to-price markup of about 1/3 above cost. Additional cost not accounted for is assumed to be the fixed cost of manufacturing, which is treated as independent of the thickness of the wood product.
The cost of natural veneer may also serve as a useful gauge for monitoring not only trends in the cost of wood products, but also the tendency of an energy-consuming society to increase its use of wood as a fuel. The competition for wood could drive the price up substantially.
A-6
APPENDIX B
SURFACE SOILING MECHANISMS: THEORETICAL CONSIDERATIONS
INTRODUCTION
The performance of photovoltaic modules is adversely affected by surface soiling, and generally, the loss of performance increases with the quantity of soil retained on their surfaces. To minimize performance losses caused by soiling, photovoltaic modules not only should be deployed in low-soiling geographical areas, but also should have surfaces or surfacing materials with low affinity for soil retention, maximum susceptibility to natural cleaning by wind, rain, and snow, and should be readily cleanable by simple and inexpensive maintenance cleaning techniques. This appendix describes known and postulated mechanisms of soil retention on surfaces, and infers from these mechanisms that low soiling and easily cleanable surfaces should have low surface energy, and be hard, smooth, hydrophobic, and chemically clean of sticky materials and water-soluble salts.
SOILING BEHAVIOR
The action of soiling is considered to include accumulation, natural removal by wind, rain, and snow, and activation of mechanisms that result in surface soiling that resists natural removal, thus requiring maintenance cleaning methods.
Recently, Hoffman and Maag (Reference 24) reported on the soiling behavior of photovoltaic modules with hard, soft, and chemically different surfaces, and concluded that the rate of soil accumulation in the same geographical area is material-independent.
In addition, they reported on the results of an experiment wherein two separate strings of electrically interconnected solar cells encapsulated in silicone rubber were maintained at a positive 1500 VDC and at a negative 1500 VDC respectively, relative to electrical ground. Compared with a module whose encapsulated solar cell string was maintained at O VDC (electrical ground), the existence of a positive or negative 1500 VDC potential in the interior of the silicone rubber, and any induced electrostatic charge on the surface, did not appear to affect the rate or behavior of soil accumulation.
Their major observation was that rain functioned as a natural cleaning agent, but that the effectiveness of soil removal by rain was material-dependent.
Relative to this, the LSA project has deployed commercial photovoltaic modules in 12 locations, which are listed in the heading of Table 13. The modules in these 12 locations are periodically
B-1
to I
N
Table 13. Summary of Problems Experienced With Photovoltaic Modules After More Than 1 yr of Outdoor Exposure at the 12 JPL/Lewis
Endurance Test Sites
(j/~ ·~ ~ ~ ~ ~ <->' PROBLEMS ~ ... ,~ ~ ~ ~ <-, ~ ~ <if'§ ~ # e
~~ ~~~~~~~~~ ~ ~~#71;~ !4; ~~ ELECTRICAL DEGRADATION
FA I LED MODULES I 1 DEGRADED MODULES 2 I I I 2
PHYSICAL DEGRADATION CRACKED CELLS METALLI ZA Tl ON DISCOLORED/CORRODED • • • • DELAMINATION • FRAME/HARDWARE CORROSION • • • • • • CONNECTOR DETERIORATION • • • • • • WIRING DETERIORATION EMBEDDED DI RT • • • • • • • • • • STAND CORROSION • • • • • • • • • • MODERATE AMOUNT e SUBSTANTIAL AMOUNT
0 MODULE SURFACES AT ALL SITES INCLUDE SOFT SILICONES, HARD SI LI CONES, AND GLASS
inspected, and problems and failures are recorded by the inspectors and tabulated. The important observation to be noted from Table 13 is that modules deployed in the two areas characterized by heavy and frequent rainfall, the Canal Zone and Mines Peak in Colorado, showed no visual evidence of surface soiling. The other locations, characterized by less frequent and less intense rainfall, show soiled surfaces.
The inference is that rain is an effective cleaning agent when it is frequent and intense, apparently independent of the surface material. But when rainfall becomes less frequent and less intense, its natural cleaning action diminishes, and its effectiveness becomes material-dependent.
With respect to snow, observations have been made by the inspectors and by Sandia Laboratory (Reference 25) that the surfaces of photovoltaic modules are noticeably quite clean after a heavy snow pack has slid off the tilted modules. The presumption is that cleaning is accomplished by a combination of abrasive action and the presence of liquid water at the module-surface/snow-pack interface.
Wind is not considered to be an effective natural cleaning agent. Reference 24 reports on a spacecraft cleaning experience that observed that the Van der Waals attractive forces between surfaces and particles smaller than SOU are greater than the aerodynamic forces generated on a particle by wind. Small particles would cling to surfaces and resist removal by wind forces, even at air speeds approaching hurricane velocities. However, particles clinging to surfaces only by Van der Waals forces can be readily removed by water washing.
The inference from all the foregoing is that particles smaller than 50µ will cling to surfaces, resisting removal by wind, but not by frequent and intense rainfall. When rainfall becomes less frequent and less intense, particle removal becomes increasingly difficult, with the effectiveness of removal becoming surface-material dependent. The inference is that between periods of rainfall, mechanisms become activated on the surface that result in resistance of the soiling particles removal by rain.
The observation of material dependence suggests that certain chemical, physical, or mechanical properties of the surface are operative in the retention of soiling particles. This article describes soil retention mechanisms, both known and postulated, that may be operative in causing outdoor soil retention resistant to removal by rain, and therefore suggests directions for defining the chemical, physical, and mechanical requirements of low-soil-retention surfaces, and for establishing maintenance cleaning strategies.
The soiling mechanisms to be discussed include:
1) Cementation by water-soluble salts
B-3
2) Deposition of organic materials
3) Surface tension
4) Particle energetics
SOILING MECHANISMS
Cementation. Adherent particles clinging to the surfaces of video discs interfere with both record and playback signal quality. This problem was encountered in the development of flat-surface video discs. RCA (Reference 26) identified a specific mechanism causing the problem that involved soluble salts and relative humidity.
Atmospheric dust contains a distribution of inorganic and organic particulates. The inorganic particulates in turn contain a distribution of water soluble and non-soluble salts. At high humidities, water-soluble dust particles on the surface formed microscopic droplets of salt solutions that also retained any insoluble particles. As the relative humidity decreased, the drops of salt solution dried out, leaving the precipitated salt to function as a cement to anchor insoluble particles to the surface (Figure 3). RCA reported that a crosslinked surface layer of General Electric silicone product designated SF-1147 aided in reducing the affinity of the surface for particles by cementation mechanism.
The RCA study used house dust, which required a relative humidity of 80% or more to generate salt solutions. RCA did not report on the chemical composition of the house dust, but it can be pointed out that common table salt, sodium chloride (NaCl), will form a salt solution at and above 76% relative humidity (RH).
Other water-soluble salts have different relative humidities associated with the formulation of solutions. Additionally, not all salts are soluble in water, but may be hygroscopic (waterabsorbing). Thus, an accumulation of dust particles on a surface may be quite heterogeneous in the distribution of soluble and hygroscopic particles, and this heterogeneity on a hydrophilic surface may induce mechanical entrapment of particles. This will be discussed below.
McDonnell-Douglas recently published a report (Reference 27) on the soiling of mirror surfaces that also identified cementation as a retention mechanism.
The weathering of soda-lime glass is schematically illustrated in Figure 4, which is reproduced from Reference 28. Observe that one result of glass weathering is the formation and deposition of water-soluble sodium salts on the surface. Water-soluble salts for cementation can orginate not only as a component of atmospheric particles, but also in the surface material itself, either inherently or by weathering.
B-4
WATER SOLUBLE PARTICLE
SURFACE
SALT WATER INSOLUBLE SOLUTION PARTICLE
HIGH HUMIDITY
~--> DEW
DRY
PERIOD
BUILDUP OF LAYERS OF CEMENTED PARTICLES
Figure 3. Cementation Process
WATER INSOLUBLE PARTICLE
I ON EXCHANGE AND / NaOH
/
SALT FORMATION Naf03
WATER ADSORPTION~
H20 H+
Figure 4. Weathering of Soda-Lime Glass
B-5
SALT DEPOSITS
HYDRATION DEHYDRATION
Organic Deposition. An investigation into the mechanisms by which salt deposits built up on desalinization membranes revealed (Reference 29) that deposition sites were first coated with an ultra-thin layer of organic matter, presumably organic colloids present in the water. Cleaning of the salt deposits was facilitated by the use of surfactant and/or detergent solutions to remove the organic layer, but the efficiency of cleaning was time-dependent. This was because the salt buildup became so thick that the cleaning solutions could not efficiently reach the organic layer.
With respect to cementation, this same characteristic of time dependence on the efficiency of cleaning was mentioned in the McDonnell-Douglas report (Reference 27). Repetitive cycles of dew formation and evaporation and high RH presumably resulted in a gradual buildup of cemented layers (Figure 3), and the eventual inability of cleaning agents and rain to penetrate and dissolve the cement.
Surface Tension. Early photomicrographs of bacteria revealed them to be flat and saucer-shaped, contrary to expectation. In preparation for photography, fluid dispersions of bacteria were deposited on a slide and the fluid was allowed to evaporate away. Eventually an alternative preparative technique called freeze drying was employed, and bacteria were then observed in their expected shape.
An investigation of the evaporative technique revealed that the bacteria were being flattened by mechanical forces associated with surface tension. In the final stages of fluid drying, the fluid formed an enveloping film around the bacteria particle that terminated on the surface of the slide. This termination functioned to anchor the film around the particle. As fluid continued to evaporate away, the surface-tension forces were sufficient to flatten the bacteria particle against the surface of the slide.
Pressures inside a fluid drop can be estimated from the following equation:
p p 2Y = + -0 R
where p = pressure inside drop
Po = external pressure
R = radius of drop
and y = surface tension of fluid
If water (Y = 72.8 dynes/cm) is drying around a particle of O.lµro diameter, then the differential pressure 6P acting on the particle is estimated at 2.9 x 107 dynes/cm (420 psi). If, further, the particle is irregularly shaped with sharp points, then forces acting on these sharp points from surface tension could become enormous, and drive the particle into a soft surface.
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Photomicrographs taken of the soiled surfaces of soft silicone elastomers revealed that the surfaces acquired "craters" with the appearance typical of impact craters. As a speculation, the action of surface tension forces pushing down on particles could squeeze up the pliant elastomer around the particle perimeter and generate the crater appearance seen in SEM photographs.
Particle Energetics. The energetics of particles, and therefore particle-particle attraction, increases wihth decreasing particle size below 10µ. The attractive forces are considered to be London-Van der Waal forces. One experimental method that has been successfully used to measure the relative magnitude of particle-particle attraction forces as a function of particle size is sedimentation. For this experimental method, particles of a known and narrow size distribution are dispersed in an inert liquid, and the sedimentation volume ~mis measured after centrifuging. The sedimentation volume ~m decreases as particle-particle attraction increases (Reference 30).
Figure 5 is reproduced from Reference 30, and is a plot of experimentally measured values of ~mas a function of particle size for a variety of chemically different particles. The results show that particle-particle attractions that limit the sedimentation volume begin at a nominal particle size of 10µ.
60
55
50
45
40
35
0. 4 o. 6 1. 0 2.0 4. 0 6. 0 10. 0 20. 0 40. 0 60. 0 I 00 PARTICLE DIAMETER,µ
Figure 5. Effect of Particle-Particle Interactions on Sedimentation Volume
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It can be suggested that the natural attractive forces that develop between small particles might also develop between a small particle and a surface. During the development of the Mars landing probe for the Viking mission, it was required that the probe be decontaminated so as not to carry earth bacteria into the Mars environment. One carrier of bacteria is atmospheric dust, which would settle freely on the surfaces of the landing probe. Efforts to remove the settled dust with a revolving-brush vacuum cleaner were fruitless. Dust particles of a size greater than 50µ were readily removed, but particles of a size 10µ and less could not be removed at all. There was some removal of particles whose size was intermediate between 10 and 50µ.
The vacuum cleaner generates a wind velocity across the surface that produces a force on the dust particles. It was decided to measure ·experimentally the efficiency of particle removal as a function of wind velocity and particle size. The experimental surface selected was a glass slide. Details of the experimental technique are described in Reference 31. The dust, referred to as "facility dust", consisted of 90% silicon-based materials, with the remaining 10% being essentially equal parts of non-magnetic metals, cellulose, Teflon, and some unidentified colored materials.
The efficiency of removal of the facility dust from dry, oil-free glass slides as a function of particle size and air velocity is shown in Figure 6, which is reproduced from Reference 31. The measurements were made in a room with an average relative humidity of 40%. Note the dramatic resistance of the facility dust to removal by wind forces, with the resistance significantly increasing as the particle sizes decrease below SOµ. It is pointed out in Reference 31 that no particles of size 10µ and smaller were removed at air velocities below 25 m/sec ( ~ 55 mph). The important finding is that the magnitude of the attractive forces between particles and a surface can be greater than wind forces, and that the magnitude of the attractive forces increases with decreasing particle size. This latter observation parallels the behavior of particle-particle attraction, and suggests that the magnitude of particle-surface attraction may be regulated primarily by particle size.
Two additional experiments with dust removal by wind forces were carried out. The first was to allow facility dust to settle on a glass slide whose surface had been touched by human fingers, in order to generate a thin oily layer. The second consisted of placing a pre-cleaned glass slide with settled facility dust in a refrigerator, and then back out in the ambient air to allow the cold surface to fog with condensed moisture. The test results for both slides are shown in Figures 7 and 8, which are reproduced from Reference 31. The effect of an oily layer and one dew cycle are indeed spectacular. Interestingly, these data suggest (but do not explain) that the resistance to removal increases with increasing particle size, just the reverse of the behavior observed for the clean, oil-free glass slide (Figure 6).
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mph 0 100 200 300 400 500 100.--~----..~~---~~---~~---~--~-
0 L.a.J 80 > 0 ~ L.a.J 0:: V') L.a.J
60 _.J
u I-a::: <( ~ 10µ u..
40 0 L.a.J C!) <( I-z L&.J u
20 0:: L&.J ~
0 L----.......... -----'-------I......_ __ ......__ __ --J
0 50 100 150 200 250 AVERAGE A IR SPEED, mis
Figure 6. Removal by Wind of Dust Particles from the Surface of a Clean, Oil-Free Glass Slide
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mph
60 O 100 200 300 400 500
Cl LLJ > 0
PARTICLE SIZE ~ LLJ a::: V')
40 •5 - IOµ
LLJ •20 - 25µ _, u
~ o>40µ
<( a.. u. 0 LLJ 20 (!) <(
~ LLJ u a::: LLJ a..
0 0 50 100 150 200 250
AVERAGE Al RSPEED, mis
Figure 7. Removal by Wind of Dust Particles from an Oily Glass Slide Surface
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mph 0 100 200 300 400 500
0 60 LLJ > 0 :E
PARTICLE SIZE LLJ a:::: V')
• 5 -10µ LLJ 40 _J
u • 20 - 25µ ~ o >40µ a:::: <( a.. LL 0 LLJ 20 (.:) <( ~ z: LLJ u a:::: LLJ a..
0 0 50 100 150 200 250
AVERAGE A I RS PEED, mis
Figure 8. Removal by Wind of Dust Particles from a Glass Slide Surface after One Dew Cycle
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DISCUSSION
The considerations relative to particle energetics, and the experimental investigation of particle removal by wind forces strongly suggest that for small particles (<50µ), wind is nonfunctional as a natural cleaning agent.
The cementation mechanisms identified by RCA and McDonnell-Douglas, and the significant increase in resistance of particles to be removed by wind after just one dew cycle, strongly suggest for outdoor soiling that relative humidity and dew are involved in soil retention. For the cementation mechanism, frequent rains should dissolve the salts and reomve the trapp~d particulates. However, for low rain locations, the buildup of cemented particles should begin to resist water penetration to lower layers. The RCA article suggests that surfaces such as those generated with the General Electric's SF-1147 silicone product may have a reduced affinity for soiling caused by cementation. The surface can be characterized as hard and hydrophobic.
In the case of the surface tension mechanism of drying water, this could cause particulates to be pressed into surfaces permanently. Hard surfaces are suggested in order to counter this postulated mechanism of soil retention. Additionally, it would seem that both particles and the surface should be wetted by water in order to set up the surface tension mechanism. This would not be expected with organic colloids or organic-coated dust particles, and/or hydrophobic surfaces.
Considering that water is brought to a surface by dew or relative humidity, it can be postulated that hydrophilic properties of the surface may be operative in the soiling mechanism. These properties may be water permeation, water solubility, and water swell.
Subsurface deposition of particulates may be caused by permeation and/or solubility of salt solutions. During a dry cycle, the water dries out of the material and leaves behind subsurface precipitated salts, i.e., water spots. An opposing consideration is that subsurface water dissolves impurities and "processing additives" present in the bulk of the material, and draws these agents to the surface (i.e., by elution). Some of the agents may be sticky and tacky.
Certain polymeric materials are susceptible to being ''plasticized" by water, that is, water softens the material; this may also be accompanied by swelling. This could lead to two operative mechanisms.
The first relates to local droplets of water generated either by water absorption of salts at higher relative humidity, or by water that remains in the final stages of dew drying; both create surfacetension forces acting to push down on a non-soluble particle. The local water can also plasticize the underlying surface layer, that
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is, soften the material sufficiently to render it more readily imbeddable by a particle. In addition, if there is concomitant swelling, it would seem that surface material would have to expand, possibly upwards to create crater-shaped appearances.
The second relates to the boundaries between wet and dry locales. Differential swelling could lead to mechanical distortions at the boundaries, such as wrinkling, puckering, etc., which may act mechanically to entrap non-soluble particulates. In general, if the surface were covered with a distribution of diverse soluble and hygroscopic salts, it could happen that the total surface becomes heterogeneously swollen, creating a broad surface area condition for mechanical entrapment.
If these postulated mechanisms of soiling retention that resist removal by rain have merit, there could be a connection then between the rates and magnitudes of retained surface soiling with some or all of the following:
I) Concentration of soluble salts and insoluble particulates in the atmosphere, perhaps the ratio of salts to particulates;
2) Types of salts, and
3) Particle size distributions in various geographical locations.
A complication to the removal of soil by rain from hard, smooth, and hydrophobic surfaces involves the deposition of atmospheric organic matter. An organic coating may generate a sticky surface for particulates (including soluble salts), and therefore keep them from being removed by rain. Atmospheric organic matter (such as colloids or aerosols and vapors) can originate in factories, oil refineries, automobiles, and the evaporation of oils from plants, trees, etc. Rain is not seen as an effective cleaning agent for removing deposited organic layers, and therefore artificial cleaning methods must be employed. As a corollary, this hypothesis indicates that significant soiling problems are to be expected in locations having high concentrations of atmospheric organic matter.
The artificial cleaning methods must be capable of readily removing the organic materials, without in turn chemically or physically harming the surface. If a washing solution is used, the organic materials should have greater affinity for the solution than for the surface. Such a requirement suggests that the surface should have low surface energy.
An additional argument for low surface energy involves the attractive forces between surfaces and particles that cause clinging. It can be speculated that these forces, basically Van der Waals in nature, arise both from the level of surface energy and from the energetics of particles, which increases with decreasing particle size. Using surfaces with low surface energy may shift downwards the sizes of particles which tend to naturally cling to surfaces.
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The foregoing suggests that low-soil-retention surfaces can be characterized as being hard, smooth, hydrophobic, and with the lowest possible surface energy. In turn, low-soiling-retention environments can be characterized as those having little to no atmospheric organic vapors, and either extreme of very rainy or very dry climate (low relative humidity and incidence of fogging and dew).
Evolving support for these concepts is indicated in the experimental data of Tables 13 and 14. Table 13 is a summary of problems encountered with a wide variety of connnercial photovoltaic modules in 12 geographic locations. The surfaces of the modules range from soft to hard. The data were taken by inspectors during routine examination of the conditions of the modules, and were not intended to be scientific investigation. The point of interest is that the inspectors did not observe embedded dirt on the surfaces of modules situated in the Canal Zone and atop Mines Peak. Both geographical locations are characterized as very rainy, and low to zero in atmospheric organic matter.
Table 14 documents soiling data measured on connnercial photovoltaic modules (Reference 32). As tabulated, the surface hardness of the modules increases from left to right, and the atmospheric soiling environment, in terms of atmospheric organic matter, decreases from bottom to top. In terms of water, DSET is typical of a desert environment, low RH and rain; the Key West site is rainy. The modules were exposed without cleaning to these environments for 12 months, or less, as indicated by the number of months in parentheses. At the end of the exposure period, the power output from the modules was measured before, and then after a very vigorous cleaning of the surfaces. The power recovery of the cleaned modules, expressed as a percentage of the before-cleaning condition, are recorded for each module and environment as the positive numbers in Table 13.
Note that in general the recovery in power, which is essentially an index of the magnitude of surface soiling, decreases from left to right and from bottom to top. The implication is that retained soiling is reduced by increasing surface hardness in combination with exposure in environments reduced in atmospheric organics.
Defining soil retention mechanisms as those that result in resistance of soil particles to be removed by rain, particle-surface attraction would not appear to be a mechanism. Rather, particle-surface attraction keeps the small particles (<<50µ) clinging to the surface until rain-resistant mechanisms occur. In unpolluted geographical areas characterized by heavy and frequent rains, such as the Canal Zone and Mines Peak, little to no retained soil particles are observed on the surfaces, suggesting that the clinging forces from particle surface attractions cannot resist the removal action of the rain.
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t:::d I .....
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Table 14. Outdoor Soiling Experience of Photovoltaic Modules Fabricated with Different Exterior Surfaces
DSET (DRY)
KEY WEST (WET)
MIT
NYU
COLUMBIA U.
SOFT SI LI CONE ELA STOMERS RTV615
+9
+9
+ 13 (5)
+23 (6) +33 (12)
+21 (6) +29 (12)
SYLG. 184
+9
+9
+14
+29 (5) +38 (12)
+22 (6) +33 (12)
[)
INCREASING SURFACE HARDNESS
SI LI CONE HARD COAT
+4
0
+ 10 (5)
+22-26
-
GLASS
+1
+2
+6 (5)
+11
+ 12 (6)
0 DECREASING AIRBORNE ORGANICS
SUMMARY
In suunnary, the evolving required characteristics of lowsoiling surfaces appear to be:
1) Hard
2) Smooth
3) Hydrophobic
4) Low surface energy
5) Chemically clean of sticky materials
6) Chemically clean of water soluble salts
and the evolving requirements for low-soiling environments appear to be:
1) Low to zero airborne organic vapors
2) Frequent rains, or generally dry (low dew, low RH)
3) Few dew cycles or occurrences of high RH between heavy rain periods.
Evolving cleaning strategies and concepts that are being suggested include:
1) Water washing of surfaces while wet with dew (dew has an opportunity to soak down and loosen cemented layers).
2) Washing with detergent and/or surfactant solutions selected for organic deposits characteristic of the local environment.
3) Washing with solvent/water mixtures for both deposited organics and inorganic salt "cements".
4) Replenishment of low-soil surfacing chemicals during routine washing.
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REFERENCES
1. Cuddihy, E.F., "Encapsulation Material Trends Relative To 1986 Cost Goals," LSA Project Task Report 5101-61, Jet Propulsion Laboratory, Pasadena CA, April 13, 1978.
2. Willis, P., Baum, B., White, R., and Kucejko, R., Encapsulation Task 1st Annual Report, ERDA/JPL-954527, Springborn Laboratories, Inc., Enfield CT, July 1977.
3. Willis, P., Baum, B. and White, R., Encapsulation Task 2nd Annual Report, ERDA/JPL-954527, Springboro Laboratories, Inc., July 1978.
4. Cuddihy, E.F., Baum, B., and Willis, P., "Low-Cost Encapsulation Materials for Terrestial Solar Cell Modules," LSA Project Task Report 5101-78, Jet Propulsion Laboratory, Sept. 1978. Also Solar Energy, Vol. 22, p. 389 (1979).
5. Spectrolab, Inc., Sylmar CA, LSA Contract No. 955567.
6. Springborn Laboratories, Inc., Third Annual Report for the Jet Propulsion Laboratory's Low-Cost Solar Array Project (LSA), Contract No. 954527, July 1979.
7. Brendlay, W.H., Jr., "Fundamentals of Acrylic Polymers," presented at and published in Paint and Varnish Production, July 1973 issue.
8. Dow Corning Corporation, Final Report for the Jet Propulsion Laboratory's Low-Cost Solar Array Project (LSA), Contract No. 957995, in preparation.
9. Carmichael, D.C., et al, "Review of World Experiences and Pro~erties of Materials for Encapsulation of Terrestrial Photovoltaic Arrays," Final Report, Battelle Columbus Laboratories, July 21, 1976.
10. Longo, F.N., and Durman, G.J., "Corrosion Prevention with Thermal-Sprayed Zinc and Aluminum Coatings," Atmospheric Factors Affecting the Corrosion of Engineering Metals, ASTM STP 646, S.K. Coburn, Ed., American Society for Testing and Materials, 1978, pp. 97-114.
11.
12.
White, M.L., "Encapsulation of Integrated Circuits," Proceedings of the IEEE, Vol. 57, 1969, p. 1610.
Jaffe, D., "Encapsulation of Integrated Circuits Containing Beam Leaded Devices with a Silicone RTV Dispersion," IEEE Transactions on Parts, Hybrids, and Packaging, Vol. PHP 12, 1976, p. 182.
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23.
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25.
White, M.L., "Encapsulating Integrated Circuits," Bell Laboratories Record, March 1974, p. 80.
Shar, N.L., and Feinstein, L.G., "Performance of New Copper-Based Metallization Systems in an 85°c, 80% RH, Cl2 Contaminated Environment," Proceedings of 1977 Electronic Components Conference, pp. 84-95.
Plueddemann, E.P., "Chemical Bonding Technology for Terrestrial Solar Cell Modules," LSA Project Task Report 5101-132, Jet Propulsion Laboratory, September 1, 1979.
Illinois Tool Works, Endurex Division, Elgin IL, LSA Contract No. 955506.
MBAssociates, San Ramon CA, JPL Contract No. 954882.
Erlander, S.R., series of 12 articles on the intersections between water and salts and ions, published in J. Macromol. Sir.-Chem., A2, pp. 595 to 1542 (1968).
Habert, A.C., Burns, C .M., and Huang, R. Y .M., "Ionical ly Crosslinked Poly (Acrylic Acid) Membranes, II, Dry Technique," J. App. Polym. Sci., Vol. 24, p. 801 (1979).
Gupta, A., "Photodegradation of Polymeric Encapsulants of Solar Cell Modules," JPL Document 5101-77, Jet Propulsion Laboratory, August 10, 1978.
Gupta, A., "Effect of Photodegradation on Chemical Structure and Surface Characteristics of Silicone Pottants Used in Solar Cell Modules," JPL Document 5101-79, Jet Propulsion Laboratory, August 18, 1978.
Runge, M.L., and Dreyfuss, P., "Effect of Interfacial Chemical Bonding on the Strength of Adhesion of Glass-Polybutadiene Joints," J. Polym. Sci., Vol. 17, 1067 (1979).
Snodgrass, J.D., Saunders, R. J., and Syska, A.D., "Particleboarps of Aligned Wood Strands", Presented at and Published in the Proceedings of the 7th Washington State University Symposium on Particleboard, Pullman WA, March 1973.
Hoffman, A.R., and Maag, C.R., "Airborne Particulate Soiling of Terrestrial Photovoltaic Modules and Cover Materials," Proceedings of the Institute of Environmental Sciences, May 11-14, 1980, Philadelphia PA.
Freese, J.M., "Effects of Outdoor Exposure on the Solar Reflectance Properties of Silvered Glass Mirrors," Sandia Laboratory Report 78-1649, September 20, 1978.
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26. Ross, D.L., "Coatings for Video Discs," RCA Review, l2_, p. 136 0978).
27. Sheratte, M.B., "Cleaning Agents and Techniques for Concentrating Solar Collectors," Final Report, No. MDCG 8131, McDonnell-Douglas Astronautics Company - West, September 1979.
28. Adams, P.B., "Glass Containers for Ultrapure Solutions," Chapter 14 in Ultrapurity, Marcel Dekker, Inc. (1972).
29. Porter, M.C., "Membrane Filtration," Section 2.1 in Handbook of Separation, McGraw-Hill, New York, p. 2-3 (1979).
30. Moser, B.G. and Landel, R.F., "A Theory of Particle-Particle Interaction Describing the Mechanical Properties of Dental Amalgam," JPL SPS No. 37-40, Vol. IV, P• 84 (1966).
31. Schneider, H., "Mechanical Removal of Spacecraft Microbial Burden," Subtask I of Spacecraft Cleaning and Decontamination Techniques, Chapter 6 of Planetary Quarantine, Annual Review, Space Technology and Research, JPL TR-900-597, February 1973.
32. Hoffman, A.R., and Dumas, L., "Module Soiling Field Experience and Cleaning Procedures," JPL IOM 353/ARH:LD/048, August 28, 1978.
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