ELSEVIER Desalination 116 (1998) 45 -56

DESALINATION

Solar distillation: a promising alternative for water provision with free energy, simple technology and a clean environment

Hassan E.S. Fath Mechanical Department, Faculty of Engineering, Alexandria University, Alexandria, Egypt

Tel. (203) 546-9378; Fax (203) 587-8294, 597-1853

Received 27 December 1997 ; accepted 13 May 1998

A b s t r a c t

Solar distillation presents a promising alternative for saline water desalination that can partially support humanity' s needs for flesh water with flee energy, simple technology and a clean environment. The development of solar distillation systems has demonstrated their suitability for the desalination process when the weather conditions are favorable and the demand is not too large, i.e., less than 200 m3/d. The problem of low daily productivity of the solar stills triggered scientists to investigate various means of improving still productivity and thermal efficiency in order to reduce water production cost. This paper presents an overall review and technical assessment of the various and up-to-date developments in single and multi-effect solar stills. The development in still configurations, the problems encountered with units during the course of operation as well as the impact on the environment are addressed.

Keywords: Solar energy; Desalination

1. Introduction

Where the demand for fresh water exceeds the amount that fresh water sources can meet, desalination of lower quality water provides a reasonable new fresh water source. Desalination (desalting) of brackish water and seawater to provide the needed drinking water fulfills a basic social need and, in general, it does this without any serious impact on the environment. As a result, there has been a dramatic worldwide increase in the number and capacity of desalination processes

and plants. A diversity of desalination technol- ogies are being used to separate fresh water from saline water; including multi stage flash (MSF), multiple effect (ME), vapor compression (VC), reverse osmosis (RO), ion exchange, electro- dialysis, phase change and solvent extraction. These technologies are expensive, however, for the production of small amount of fresh water. On the other hand, the use of conventional energy sources (hydrocarbon fuels) to drive these technologies has a negative impact on the environment.

0011-9164/98/$ - see front matter © 1998 Elsevier Science B.V. All fights reserved PII S 0 0 1 1 - 9 1 6 4 ( 9 8 ) 0 0 0 5 6 - 3

46 H.E.S. Fath / Desalination 116 (1998) 45-56

Solar distillation, on the other hand, provides a promising alternative desalting process that can partially support humanity's needs for fresh water with free energy, simple technology and a clean environment. The development of solar distillation has demonstrated its suitability for saline water desalination when the weather conditions are favorable and the demand is not too large, i.e., less than 200 m3/d. The problem of low daily product- ivity of the solar stills triggered scientists to investigate various means of improving still productivity and thermal efficiency for minimum water production cost. These means include various passive and active methods for single- effect stills. Several workers have also tried to condense the produced water vapor externally (in additional condensing surfaces). On the other hand, the wasted latent heat of condensation was also recovered so as to increase the production of the distillate water and improve system efficiency. This was carried out in two or more stages, generally referred to as a multi-effect solar distil- lation system.

This paper presents an overall review and technical assessment of the various and up-to-date developments in single- and multi-effect solar stills, The developed still configurations, the various problems encountered with the units during the course of operation as well as the impact of solar distillation technology on the environment are addressed. The developments in

single-effect stills, the various problems encoun- tered with units during the course of operation and a typical economic break-down will be presented and discussed in Section 2, while the development in the multiple-effect stills will be presented in Section 3. The characteristics of solar distillation concentrates disposal and concerns on the environ- ment will be addressed in Section 4.

2. Developments in single effect stills

The thermal efficiency and the daily production per m 2 of the single effect solar still (Fig. 1) can be increased by various passive methods such as lowering depth of water in the basin, injecting black dye in the water mass, and reducing side/bottom heat losses. It could also be improved through active methods of integrating the still with either a solar heater or solar concentrator. The classification of these develop- ment techniques are shown in Fig. 2. These techniques are addressed and summarized below.

2.1. Modifications using passive methods

Different techniques and configuration modifi- cations and developments have been introduced in literature to passively improve the single-effect stills. These techniques, configuration modifica- tions and developments are enormous and will, therefore, be highlighted and briefly discussed:

Gloss cover

..... • ° ° : ,.o::o

B¢ine drain

Fig.1. Single-effect basin still.

H.E.S. Fath / Desalination 116 (1998) 45-56 47

Single Effect Solar Stills

Passive Stills Active Stills

Basin Wick Integrated Wifll Waste Heat Solar Collee[ing System Rccovc~

Diffusion Ofl~exs ~. ,I, Solar Solar H~ate~ Concet*tTato~s

Fig. 2. Classifications of developments for single-effect solar stills.

2.1.1. Basin stills

1. Single-slope vs double-slope basin stills: Comparison of the two configurations of basin type stills, single slope and double slope, shows that on the basis of motion of the sun, in different seasons and locations, the maximum radiation may be higher in double-slope stills and the perfor- mance may be better. On the other hand, single slope has less convection and radiation losses, and the shaded region may be utilized for additional condensation as will be seen below. On the basis of yearly performance data for Delhi climatic conditions, Tiwari and Yadav [1] concluded that a single slope still gives better performance than a double slope for cold climatic conditions. For summer climatic conditions the double slope gives better performance.

2. Still with cover cooling: Evaporation rate can be increased if the difference in temperature between the basin (heat sources) and the glass cover (heat sink) increases. This can be achieved by either increasing the basin temperature or decreasing the cover temperature or both. Two cooling arrangements have been suggested, both using a double glass cover. These two methods are shown in Fig. 3, and are termed feed back flow and counter flow. Results have shown that cover cooling produces an increase in the productivity of the still, with the improvement when using the feed back flow being greater than when using the counter flow [2], since part of the sensible heat gained by cooling the glass cover is utilized in the feed.

6 ~ o ~ ~. . ~ , ~ ~ t '~s

off,,o~ ~ t e r tlow ~6~ ¢ .

Fig. 3. Cooling of glass cover by (a) feed back flow, and (b) counter flow.

3. Still withtreated cover surface: Baladori and Eldin [3] showed that the use of sodium recta- silicate or hydrofluoric acid to treat the inner surface of the glass cover increases its wettability and reduces the minimum permissible cover slope 1.5 o from the horizontal plan, which increases the yield of the still. Treated cover surface adds, how- ever, the initial cost of the still.

4. Still with additional condenser: Fath and Elsherbiny [4] added a passive condenser in the shaded region of a single-slopped still, shown in Fig. 4. The authors indicated that purging the vapor from the still to the additional condenser is a function of the volume ratio of the additional condenser, and the still efficiency increased by 45%. Natural circulation increases the still productivity by up to 75% depending on the circulation flow resistance.

5. Still with black dye/muddy water: Lawrence et al. [5] indicated that: (a) there is a significant effect of dye on still performance particularly for large water depth, and (b) black dye gives a better performance than violet and red dyes.

48 H.E.S. Fath / Desalination 116 (1998) 45-56

Condenser

I

Fig. 4. Single-slopped still with passive condenser [4],

Muddy water is opaque, and so the incident solar radiation gets absorbed mostly around the top layer. The results of Onyegegbu [8] for distillation of muddy pond water indicated (on the daily basics) that the muddy and clear water samples yielded the same distillate output. Not much information is available on muddy and polluted water, which in some regions of the world may be the only source of feed water.

2.1.2. Wick stills

It has already been established that a reduction in the depth of brine in the still improves the productivity, mainly due to the higher basin temperature. The advantage of the wick is to keep the brine as shallow as possible (with low heat capacity) while avoiding dry spots.

1. Single-wick still: The results of a still of this type using a plastic cover located at Valparasio, Chile, showed a production rate of 3.8 to 4.4 l/m2/d with an operational efficiency of about 40-46%. An improved design for the wick-type collector- evaporator still was created by Moustafa et al. [6]. The results of this design indicate an improvement in productivity and operational efficiency.

2. Multi-wick stills: Tiwari et al. [7] proposed a double-condensing, multi-wick still. Excess vapor can then be condensed on the additional surface and reduce the heat load on the glass

cover, reduces class cover temperature, which in turn enhances evaporation rate. The experimental results showed a 20% increase in the still productivity over the simple multi-wick still. Fig. 5 shows a cross-sectional view and plant layout of a double-slope multi-wick solar distillation unit with a capacity of 85 l/d [8] that was installed in Delhi, India, in 1981.

3. Wick vs basin stills: The productivity of the multi-wick stills is always higher than conven- tional basin stills due to the negligible heat capacity of the water mass in the multi-wick stills. Tiwari and Yadav [8] indicated that the multi-wick distillation plant will be more economical for a medium-scale installation. For larger scale supply of distilled water, the basin type is preferred because of its simplicity and low cost.

4. Combined wick-basin stills: Minasian and Al-Karaghoul [9] connected a conventional basin type still (installed in a shadow and having an opaque cover) with a wick-type solar still so that the hot waste brine water leaving the wick-type feeds directly into the basin-type, with the basin still cover cooled. The combined stills showed higher efficiency than the two stills separately, and the yearly amount of distilled water was 85% more than the basin type and 43% more than the wick type.

5. Economical breakdown: An economic analysis of the above multi-wick solar distillation

H.E.S. Fath / Desalination 116 (1998) 45-56 49

Solar r ad i a l i on ~ . ~ Soicar r a d i a t i o n

AI / U FRP Body distilled ~ater

i- Drainage for excess water Jute troth . . . . . Black polythe ne

I. SllU. ~Conneoting Dipes Om • flld ~ .Conn i c f l ng O i p l i I0 ¢o~.te©t

I x C l | S wOl I l l

From mains

l x C e $ $ ",~Gler I - - ) Pump

Fig. 5. Double-slope multi-wick solar still [8]. Top: cross sectional view. Bottom: distillation plant layout.

plant has been presented by Tiwari and Yadav [8], taking into account the various factors, viz. the lifetime of the system, salvage values of the system, interest rate and maintenance cost. The cost breakdown of the plant is as given in Table 1.

6. Operation difficulties: Various problems were encountered during the course of operation of the system (4 years) as reported by Tiwari and Yadav [8]. These problems were:

• The distribution of saline water in the reservoir of the stills was not uniform. This may arise due to (a) the diameter of the connecting pipes between the stills was small, leading to the formation of air bubbles preventing the uniform flow of saline water; and (b) the stands of the stills were inclined, hence the solar stills could not be placed at the same level.

• Corrosion took place at the joint of galvanized

50 H.E.S. Fath / Desalination 116 (1998) 45-56

Table I Cost breakdown of the multi-wick solar distillation plant [15]

Still components Cost in Indian rupees

Steel and all structure 4,680 One 1000 1 and three 50 1 tanks 660 24 pcs. of glass cover (4 mm thick) 1,440 Jute cloth 840 Black polythene 360 Foam and solution 720 GI pipe (1", 3/4", 1/2") 1,000 PVC pipes and brass nipples 218 Two ball cup and GI fitting 386 Pump 600 Labor 3,000 Total cost 13,904

iron sheet, and MS stands in the presence of saline water, leading to leakage in the system.

• Due to non-uniform distribution of saline water in the reservoir, the jute cloth in the stills dried out.

• The black polythene sheet and the foam of the walls of the stills were damaged by birds; this required a frequent change of the materials.

• Since the bottom of the still was made of galvanized iron, conduction losses occurred from the base which reduced the output significantly.

• There was a possibility of mixing distilled water with saline water in the tray due to overflow in the reservoir because: (a) the slop of the tray kept excess saline water just below the drainage of distilled water, and (b) the vertical walls were too small, hence there was a small gap between the drainage and outlet for excess water.

• The purity of distilled water decreased due to the Al sheet used for drainage.

• There was a leakage of distilled water at the lower edge of the glass cover through the

drainage due to improper sealing between the glass cover and the drainage.

• There was a significant effect on output due to the high temperature of excess water.

• A large amount of black dye was wasted when coloring the jute cloth. In order to solve the these problems, a new

design of a fiber-reinforced plastic stills plant of 1001/d was proposed to increase the plant life time.

2.1.3. Diffusion stills

Basic diffusion stills and comparison with basin stills: Fath and Elsherbiny [10] showed that daily yield of diffusion still varies between 0.5 and 5.0 kg/m 2 under the climatic conditions of Egypt. Based on a similar design, operational and environmental parameters , Elsayed [2] numerically compared a single-effect diffusion still with a basin-type still and showed that the use of the diffusion-type still leads to an improvement in both production rate and operational efficiency.

2.1.4. Stills integrated with greenhouse

Fath [11] installed a solar still on the top of greenhouse roof with waste heat and a mass recovery system (WHMRS). The greenhouse still integrated system could ventilate and reduce the heat load on the greenhouse (particularly in hot climates), as well as supply the greenhouse with the fresh water requirements. The system is claimed to be self- sufficient of energy and irrigating water.

2.1.5. Other still configurations

Other still configurations have been presented in the literature, including a vertical microporous evaporator still and a cascade solar still [2].

2.2. Externally heated (active) solar stills

To enhance the evaporation rate and obtain higher still productivity, the basin cover tempera-

1t. E.S. Fath / Desalination 116 (1998) 45-56 51

ture difference must be increased. The basin operating temperature can be increased through additional (external) heating. A high operating temperature range can be achieved by integrating the still with: (1) a solar heater, (2) a solar concentrator, or (3) a waste heat recovery system. The solar still integrated with a heater or concen- trator panel is generally referred to as an active solar distillation system. Circulation inside the heater or the concentrator could either be through natural circulation (Thermosyphon) or through a circulating device (pump). In a concentrator heating system, possibilities of bubbles formation and two-phase flow that can influence the circulation should be taken into consideration.

The heat derived from the (external) collector could either be directly supplied to the still and increased rate of evaporation or could be indirectly supplied to the still through a heat exchanger. The overall system efficiency will be reduced for the

indirect system due to the chances for lower temperature heat supply to the still and due to more energy losses. However, such an indirect still collector system will protect the collector from corrosion and scale deposits caused by saline water. Other configurations include the use of dye and cover cooling, and other designs of solar heaters.

2.2.1. Stills coupled with a solar collector

Fig. 6a shows the integrated system proposed by Rai and Tiwari [12]. The water is circulated between the still and collector with the help of a small pump. The heat derived from the collector is directly supplied to the still and increases the rate of evaporation of the still. The daily production is 24% higher than that of an uncoupled one. Fig. 6b shows the concept of a naturally circulated system. The still should be placed high enough to create the sufficient pressure for the Thermosyphon flow.

(a)

P u m p

lbl 9 . ~ a r stit t

~ OII~c10¢ p o n e |

- - 8 locke ned s u r f o c e

• I n s u l o t i o n

Fig. 6. Directly heated still coupled with flat plate collector. a: forced circulation [24]. b: natural circulation.

52 H.E.S. Fath / Desalination 116 (1998) 45-56

Recently Sinha et al. [ 13] evaluated a collector assisted solar distillation system as an investment alternative to a solar hot water system. A techno- economic analysis was performed for both system s in the same economic environment and considering the same capacity. It was concluded that the cost of energy from the distillation system is much less than the cost of energy obtained from the water heater, and the annual operation cost of the solar water heater is higher than that of the solar still because of the higher initial investment in the former.

condensation. The re-utilization of latent heat in two or more stages is generally known as a multi- effect distillation system. The additional pro- duction resulting from the multi-effect stills as compared with that from the single effect should be justified, however, with the additional cost incurred in the construction of the more complicated multi-effect still. The classification of multi-effect solar stills is very much the same as of single-effect stills. Only the main developments in the multi-stage stills will be presented.

2.2.2. Stills coupled with a solar concentrator

The performance of a solar still coupled with an external heating system can be improved by replacing the collector with a concentrator. In the concentrator the heat loss is reduced compared to the collector due to the smaller concentrator surface area. In addition, the saline water tempera- ture in the still is higher which causes the improve- ment in the still performance. Kumar and Sinha [14] concluded in their study that the yield of the concentrator-assisted solar still is much higher than any other passive/active solar distillation system.

3.2. Double-effect basin stills

3.2.1. Double-basins stills (Fig. 7)

Water of the second basin can either flow over the glass cover (Fig. 7a), or be stationary (Fig. 7b).

Solar r ~ d ~ t ~ {a)

",,_

2.2.3. The use o f waste heat in solar stills

One way to increase production is to utilize waste heat (from gasoline or diesel engines) for still water heating so as to keep it operating (at a higher temperature) particularly during sunless periods. The external heat source will increase tile saline water heat capacity and therefore enhance evaporation (see Sayigh [15] and Fath [16]).

3. Developments in multi-effect stills

3.1. Why multi effect?

The thermal efficiency of a solar distillation unit in terms of daily production per m 2 can be increased by the utilization of the latent heat of

Single bosln Double - bosln

. ~ (b)

" / / / / / / / / / / / / / / Ambiem oir

Fig. 7. Double-basin solar stills. (a) Schematic of single- and double-basin stills. (b) Stationary double-basin still with flowing water over upper basis.

H.E.S. Fath / Desalination 116 (1998) 45-56 53

Sodha et al. [17] showed that double-basin type produced about a 5 6% higher yield than the single- effect still.

For a higher daily yield, the temperature difference between the basin and the glass cover should always be kept large. This can be achieved by connecting the basin of the still to an external heating source (solar collector, solar concentrator or waste heat system) in addition to arranging water flow over the glass cover. Tiwari [18] incorporated the effect of water flow over the glass cover and flow of hot water in the lower basin through a flat plate collector. The author concluded that: (a) there is no significant effect of the flow rate of flowing water above the upper glass cover (may be due to the low available energy at the upper glass cover), (b) the collector must be disconnected from the still during off- sunshine hours to avoid heat losses through the collector, and (c) the system gave about a 50% higher yield than that of an ordinary double-basin still.

3.2.2 Purging vapor to a second effect

Fath [ 16] proposed a second effect connected to a single-slopped still through a shutter fashion reflector and in its shaded zone. The second effect acts as an additional heat and mass sink. Vapor purged from the first effect to the second effect relieves the first effect pressure and utilizes the latent heat to generate additional 30 % water in the second effect. The author showed that the product- ivity increases to as high as 10.7kg/m 2 for the proposed design under the climatic conditions of Saudi Arabia (of 1000W/m 2 midday solar intensity and 30-40 °C ambient temperature). The proposed unit is simple, passive and adds no design, operation, or maintenance complexities over the conventional single-effect basin stills.

3.3. Multi-effect multi-wick stills

In the multi-effect multi-wick type solar stills (Fig. 8), the availability of latent heat of vapor- ization can be maximized and equalized for least

Sclzr rcdl~.tion $olot ' rod ic t ion

l 1 1 1 1 1 Woter f low ~ Pipe w i th

I ,oo° .U L--Droinoge ~or Dfclno~e for

excess waLet d~st l l led =oter . . . . Block jute cloth

Block polythene

Fig. 8. Typical multi-effect multi-wick solar still [19].

54 H.E.S. Fath / Desalination 116 (1998) 45-56

4 5

w

6 M- j , I=10 "2 k9/$ (O) • Mwj • Iz~G -~ k~;/s

Mwj. lllO -3 k,/.~ / / M.w i " IxlO *I' kQ/s

S 2 4

Number of effect

45 A Mwj. I~I0-2 kglS Mw]. I z h,~-SXgls ( b l 13 Mw,i • IxlO -4 k915 • Mwj • IzlO -3 kgls . , ~ • Mwj • IxlO " l kgl$ / 0 MwJ'l ' l 'O-'5 k~/s f J

,

l I ' '

2 4

Number of effect

Fig. 9. Effect of water flow rate and number of effects on multi-effect multi-wick type solar still [20].

water depth in each effect including the lower basin (multi-wick stills). The multi-effect multi- wick solar stills with the first effect of least water depth was designed by Sodha et al. [19] to achieve the above conditions. When the water flow rate increases significantly over the glass cover, the flowing water does not have sufficient time to evaporate, and thus the performance is not as good as with a low flow rate. Different flow rates have been studied by Singh and Tiwari [20] as shown in Fig. 9, where the still production can be highly increased.

3.4. Other distillation systems

Other solar distillation systems have been presented in the literature including the stacked tray stills, multi-effect humidification-dehumidi- fication, multi-effect diffusion stills, and water recovery from air.

4. Environmental considerations

The major waste stream produced by the solar distillation process--the concentrate-- is not char-

acterized by intentionally added little process chemicals. Rather, the concentrate reflects the raw water characteristics (almost the same composition at a more concentrated level). Solar distillation processes do not, therefore, produce more pollutant material or mass; they redistribute (concentrate) that which is present in the raw water. The other wastes produced, such as cleaning wastes, may be mixed with the concen- trate and discharged together. The waste contains the scaling and fouling materials that are cleaned from the system in addition to metals due to corrosion/etching.

4.1. Concentrate disposal

Similar to other desalting processes, there are several means of disposal of concentrates that are practiced worldwide. These include [21]: surface water discharge, disposal to front end of sewage, treatment plants, deep-well disposal, land appli- cations, evaporation ponds, brine concentrators, the discharge to the effluent end of a sewage treatment plant, and spray-drying to solids or crystallizer (zero discharge option).

H.E.S. Fath / Desalination 116 (1998) 45-56 55

Evaporation ponds are most appropriate for a relatively warm, dry climate, with high evapo- ration rates and low land cost. In the zero discharge case, the concentrate is taken to dry solids as a result of complete drying or further treatment. The environmental concern is with the disposal of the solid waste from the land-fill site and eventually into nearby surface and under- ground waters. Salts recovery may be an option to utilize and get rid of the large mass of concen- trated salt components.

4.2 Environmental concerns and mitigating methods

The environmental concerns associated with the disposal of concentrates center on the contami- nation of surface and ground waters, soil, and air by the salinity level of the concentrate. Table 2 lists the particular environmental concerns for each of the disposal options as well as a possible mitigation method.

Table 2 Environmental concerns for disposal options and a possible mitigation method [21]

Disposal option Environmental concern

Surface water

Sewer system Land application

Deep-well injection

Evaporation pond

Zero discharge

Contamination of receiving water Eventual contamination Contamination of underlying groundwater and soil Contamination of overlying drinking water aquifers due to well leakage Contamination of underlying higher quality aquifers due to pond fill leakage Contamination of underlying higher quality aquifers due to land-fill leakage

The mitigation methods involve the following technical and regulatory approaches:

• Additional processing: treatment or blending to remove or dilute the chemical of concern (if any).

• Changing of materials: use non-toxic additives, non-corrosive materials to reduce or eliminate the problem.

• Reducing effluent impact on receiving water: use of diffusers to afford an immediate dilution factor.

• Better chemical control of chlorine to reduce residual levels or other process additives to limit their level.

• Continuous blending of cleaning wastes and concentrate or separate disposal of cleaning wastes and concentrates.

The awareness of the environmental consider- ation and the industry-regulation interface is relatively new and is in an early stage of development. There is a need to identify, understand, and address environmental concerns that are increasingly being raised, and eventually the legislative and regulatory process. The trend has been and will likely continue to be of both increasing concern for and increasing regulation of environmental impact. For negative environmental impact, the water-producing industry needs to recognize this trend and determine ways to mitigate the problems.

5. Conclusions

1. Solar distillation presents a promising alter- native for saline water desalination that can partially support humanity's needs for fresh water with free energy, simple technology and a clean environment. Producing fresh water by solar distillation can support community living activi- ties, particularly in rural areas.

2. The development of solar distillation systems has demonstrated their suitability for the

56 H.E.S. Fath / Desalination 116 (1998) 45-56

desalination process when weather conditions are suitable and demand is not too great, i.e., less than 200m3/d.

3. Although many researchers were very much concerned with increasing the stills' efficiency and productivity, not much cover the economical considerations of these development so as to assess the ultimate cost o f product water.

4. Based on the discussion presented in this paper, a combination of the following design and operational parameters should be considered in future developments in solar distillation systems: • higher basin temperature (lower water level,

use of wick, adding black dyes, additional external heating-collector, concentrator, waste heat recovery, etc.)

• lower cover temperature (cover cooling, multi- effect, overnight with basin energy storage, additional condenser, .etc.)

• large evaporation and condensation surface areas

• re-utilization of the latent heat of condensation (multi effect)

• minimize heat losses (good side and bottom insulation)

• utilization of the shaded area (additional condenser, combined stills, .etc.). 5. Solar distillation technology problems such

as corrosion, scale deposits and concentrate (brine) disposal can be handled in similar way as other conventional desalination technologies (corrosion- resistant materials, continuous blow-down, adding corrosion inhibitors and anti-scalant, frequent cleaning, etc.).

A pilot plant should therefore be installed to assess all these aspects vs the economical penalties. Continuous research will ultimately lead to a water production cost that can compete with other technologies, in addition to the basic advantages of solar distillation. Larger distillation units will help the overhead costs of common and auxiliary systems and components to have a smaller effect on the water production cost.

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