case study of solar stoves

8/13/2019 Case Study of Solar Stoves

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Case Study of Solar Stoves

Made and Used in the Peruvian Andes

For the Peru Childrens Trust.

A. Introduction.

ThePeru Childrens Trustis a small Christian charity run by a couple called Manuel andGay Reynaga. They work in the high Andes of Peru in the city and shantytowns ofHuancayo. The main work of the charity is to support 100 of the more disadvantagedchildren through sponsorship, which pays towards schooling costs. They also support thefamilies of these children with medical and pastoral care and have initiated a number ofsmall business development schemes from carrot farming to brick manufacturing.

In 1997 Manuel and Gay came to our Youth Group (EnigmaSt Marys Maidenhead)totalk about the Peru Childrens Trust. Among other things they mentioned the fact thatmuch time and money is spent on collecting firewood and buying gas to cook with. Theyalso mentioned that the sun is very hot during the day and it should be possible to use thesuns energy to cook with.

Being an engineer, this got me thinking, so I did some research and found a number ofuseful resources. The most inspiring book was Cooking with the Sun: How to Buildand Use Solar Cookersby Beth and Dan Halacy.

So during the summer of 1999 my wife and I set about making ourfirst andsecond

prototypes.The first was a bit wobbly and would only boil water if there were no wind.The second was much more efficient but a bit time consuming to make. But the joy andglee of boiling your first potatoes, or frying your first sausages is immense!

Ourthird model was more stable, simpler to make and more efficient. It is this model thatwe decided would be best for Peru, so this case study is mainly concerned with themanufacture and use of this solar stove.

In September 1999 a group of seven people from St Mary's went out to Peru to visit theReynagas and to help with their work in a variety of ways. The seven included David &Gill Coe, Jim & Ali Peck, Quinton & Jody Stowell and Vicky Warren. We spent three

days in Lima, did a ten day tour around Peru and spent the remainder of our four weektrip with the Reynagas. Apart from visiting many of the sponsored families and passingon gifts from English sponsors, we also organised two 'Fun Days' for the 100 sponsoredchildren. This involved sketches (The Prodigal Son), art and crafts, active games, lots offood, and for some of the older children - building stoves. It was an exhausting few daysbut very rewarding.

A. Why use a Solar Stove?
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The heat energy produced by the sun is immense. In equatorial regions the solar radiationcan exceed 1000 Watts/m2. That is equivalent to half the power of an electric kettlewhenever there is good sunlight. It only takes 1015 minutes to boil water on a solarstove. And its free, as long as you have clear blue skies! (Not much use for us British

then!) The material costs are about $10 per stove which with some sponsorship is feasible

to raise, and they are fun to make.

In Peru, like many equatorial countries the main method of cooking is with wood fires orbutane gas. Among the poorer families firewood is usually the main option as gas isrelatively expensive. So a free source of energy for cooking is an exciting possibility.

B. Firewood and Health issues.

The Manual for solar box cookers published by Technology forLife, Finland quotes: -

"About 2000 million people, over one-third of the population of the world, are daily

dependent on firewood as the source of their cooking and heating energy. They live in thetropics, in the most favourable areas for the use of solar energy. Every year the cutting offirewood results in the loss of 20,000 - 25,000 km2of tropical forests (UNEP).

The use of solar cookers also brings with in important health benefits. Diseases spreadthrough contaminated water cause 80% of the illnesses in the world (WHO). Heatingwater to the pasteurisation temperature of about 60 0C destroys disease organisms. Thistemperature is easily achieved with solar cookers. Acute respiratory infections (ARI) arethe cause of death for millions of children in the world each year. The large majority ofthese casualties occur in the developing countries as a result of polluted indoor air due tocooking over open fires in houses without chimneys or ventilation. This problem could be

greatly reduced by using solar cookers, which are, of course, completely smokeless."

C. How does a solar stove work?

There are two basic methods of collecting enough heat from the sun to cook. These arecommonly described as the Solar Box Cooker and the Solar Stove.

Solar Box Cooker.

The basic principle is tocollect the heat by letting

the sun light pass througha clear glass plate into awell-insulated enclosure.The light trapped in thebox and turns into heatwhen it is absorbed by theblack cooking pot. Thesecret of a good Solar Box


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The main enclosure is cut from 4 mm thick Triple A plywood. An 8ft x 4ft sheet willmake 21 stoves and only costs $5 in Peru. A few other bits of wood are required to makethe framework within the stove (sizes detailed below).

As the focal point gets very hot it is necessary to make the grill and grill support from

metal. We used mild steel and painted it silver, but any metal will do.

Many thanks to Pat Keogh for his kind donation that covered the cost of all the materialsrequired making 10 stoves.

F. Parts List and Technical Drawings.General assembly drawing

To view and print all the drawings togetherclickhere

To download an Assembly Drawing as an AutoCAD file 49KBclick here

To download an Assembly drawing as a DXF (Drawing Exchange File) 94KBclickhere

To download a CorelDraw 8 Assembly drawing 50KBclick here

To download an Assembly drawing as a PostScript file 52KBclick here

To download an Assembly drawing as a Windows Metafile 34KBclick here

Description Size Material Quantity

Base 710 x 570 x 4 Plywood 1

Side 710 x B x 4 Plywood 2

End 560 x A x 4 Plywood 2

Frameside 25 x 25 x 700 Wood 2

Frameend 25 x 25 x 510 Wood 2

Framecorner 25 x 25 x 130 Wood 4

Rib drawings 560 x B Cardboard 2

Rib 560 x C Cardboard 2

Rib 560 x D Cardboard 2

Rib 560 x E Cardboard 2

Rib 560 x F Cardboard 1

Cross Rib 700 x 60 Cardboard 2

Cross Rib 700 x 30 Cardboard 2
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Grill 350 x 12 x 2.5 Mild steel bar 2

Grill support 16 x 430 Mild steel tube 1

Pin 5 x 105 Mild steel bar 1

Base supportlong 75 x 25 x 580

Wood 1

Base supportshort 75 x 25 x 100 Wood 1

Reflector 620 x 400 x 0.3 Anocoilaluminium 2

G. Tools Required

The Peru Childrens Trust has run a number of courses for the children in workshoptraining so we were blessed with a good variety of tools and a couple of youngsters who

could weld!

Useful tools include: Drill, jigsaw, hacksaw, vice, wood saw, sharp Stanley knife, tapemeasure, square, metal file and hammer.

Optional tools include: Grinder and welder.

H. Building the solar stove.

We split the work up into three sections. Manuel Reynaga was in charge of themetalwork, David Coe organised the lads (and one girl) in cutting the cardboard ribs, and

I was in charge of the woodwork. We rotated the work, so they all had a go at eachdiscipline. I was amazed how keen they all were to get things done. Once they saw thewater boiling on our prototype they all wanted one!

Woodwork.

We made up some templates(thanks Ali Peck) for thesides and base, and cut themout using a 'jigsaw' powertool. The edges were filed

flat. We then cut the frameparts to length and nailed thewhole box together aroundthe frame. SeedrawingWOODfordimensions. Once theenclosure was assembled wemade up the
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woodenSUPPORTat the base. This had the main purpose of holding the 16mm bar andgrill in place. The support was nailed to the underside of the wooden box. To help alignthe stove with the sun we made a sundial by fixing a nail through the support at the edgeparallel to the 16mm bar. By adjusting the stove to eliminate any 'shadow' of the nail it iseasy to find the sun's location. See drawingGAfor details.

Cardboard.

Finding enough cardboard for 10 stoves was surprisingly difficult, but the 'Lord provided'just enough. The reflector profile is spherical with a radius of 0.7 m (equal to the squaresize of the enclosure). This gives an almost parabolic shape with a focal point at 0.35 mfrom the reflector. We used the 'wine box' principle of interlocking cardboard to createthe required shape for the reflector to fit into. Ideally the box should be square (0.7 m x0.7 m) but as our aluminium was not big enough we had to make it only 0.56 m wide by0.7 m long. I designed the rib shapes on the computer, but it only takes a bit ofPythagoras to work out each radius. See drawingRIBSandRIBSXfor details. Each rib is

0.07 m apart and 11 ribs are required to make the shape (including the two wooden ends).Each rib has a different radius depending on its location. I have designedaGENERICprofile that can be scaled up to any size of stove. The larger the reflectorarea, the more effective the stove, so I have also designed a unit 1 m x 1mcalledSolar1000 that will give 2.5 time the heat of our Peruvian stove.

Metalwork

The hacksaw, drill and vice came in very handy. The three main items aretheBAR,GRILLandPIN.The bar pushes into the wooden base and is located in placewith the pin. The grill is fixed to the bar with an M5 x 20 screw with a wingnut and

spring washer. This allows the grill to be secure but also adjustable as the sun changesheight. The bar and grill were painted silver to reflect more light. See drawingGAfordetails.

Reflector

Once all the parts have beenmade they can be assembledinto the box. The cardboardribs should just drop in andsit flush with the base. Ifthere are gaps under someof the ribs then it isprobably because the slotsin the ribs are not deepenough. Fitting the reflectoris the final thing to do. Wemarked the reflector on 15degree angles and then cut
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through with a very sharp knife leaving an area in the middle uncut. Take care of yourfingers, we only had one accident! SeeREFLECTORdrawing for details. We then placedthe reflector on top of the box and ribs and cut the edges to length. The reflector was thentaped down around the edges, which produced an accurate focal point. We used shinyaluminium foil to tape the reflector in place. We found that the best method was to tape

alternate edges first. This enabled us to push the other segments firmly on top of the firstsegments. It is worth spending time in getting each segment just right as this affects thefocusing.

I. Maintenance and caring for your solar stove.

When not in use, keep the stove in the dry. To store the unit in winter, remove the grilland grill support and turn upside down.

A dirty reflector will slow down the cooking times. Clean the reflector with a dry cloth or'alcohol' if available.

J. Cooking tips.

Black pots work a lot betterthan silver pots. The potneeds to absorb as muchlight as possible and silvertends to reflect the light.Dull or 'matt' finishesabsorb more light than'shiny' surfaces.

Pots with close fitting lidskeep the heat in and help thecooking process. Placing the
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stove in a sheltered area stops the wind from cooling the outside of the pot.

The "Easy Lid" Cooker Designed by Chao Tan andTom Sponheim

Although designs for cardboard cookers have gotten simpler, fitting a lid can still bedifficult and time consuming. In this version, a lid is formed automatically from theouter box.

Making the Base

1. Take a large box and cut it in half as shown in Figure 1. Set one half aside to beused for the lid. The other half becomes the base.

2. Fold an extra cardboard piece so that it forms a liner around the inside of thebase (see Figure 2).
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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8. Fold these flaps down to fit down around the top of the liner and tuck them into

the space between the base and the liner (see Figure 6).9. Fold the tabs over and tuck them under the flaps of the inner box so that they

obstruct the holes in the four corners (see Figure 6).10.Now glue these pieces together in their present configuration.11.As the glue is drying, line the inside of the inner box with aluminum foil.

Finishing the Lid

1. Measure the width of the walls of the base and use these measurements tocalculate where to make the cuts that form the reflector in Figure 7. Only cut onthree sides. The reflector is folded up using the fourth side as a hinge.

2. Glue plastic or glass in place on the underside of the lid. If you are using glass,sandwich the glass using extra strips of cardboard. Allow to dry.


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3. Bend the ends of the wire as shown in Figure 7 and insert these into the

corrugations on the lid and on the reflector to prop open the latter.4. Paint the sheet metal (or cardboard) piece black and place it into the inside of

the oven.

Improving Efficiency

1. Glue thin strips of cardboard underneath the sheet metal (or cardboard) piece toelevate it off of the bottom of the oven slightly.

2. Cut off the reflector and replace it with one that is as large as (or larger than)the entire lid. This reflects light into the oven more reliably.

3. Turn the oven over and open the bottom flaps. Place one foiled cardboard panelinto each airspace to divide each into two spaces. The foiled side should facethe center of the oven.

Solar Box Cooker

A great solar oven you can build quickly from two cardboard boxes


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Experiments in Seattle and

Arizona have proven that

solar box cookers can be

built more simply than

even the simple method

we have been using.

These discoveries have

paved the way for a

simpler construction

method that allows a

cooker to be built in a few hours for very little

money.

The following developments make this design possible:

Insulation material is not essential in the walls -- a foiled airspace is all that is

necessary. Aluminum foil can be reduced to just one layer (though a layer on the inside of each

box makes a hotter oven).

The airspace between the walls can be very small.

Almost any size oven will cook. In general, larger ovens get hotter and can cook more

food, but the limiting factor is still the ratio between the mass of the food and the

size of the oven. In general it is best to make an oven as large as is conveniently

possible so that it will perform adequately even under marginal conditions.

Our experience shows that a double layer of plastic film (such as Reynolds Oven

Cooking Bags) works at least as well as a single sheet of glass.

Shallower ovens cook better since they have less wall area through which to lose

heat. It's best for the inside of the oven to be just slightly taller than the biggest pot

you plan to use.

A New Simp ler DesignTaking these factors into account, we are able to take our best shot at describing theminimum solar box cooker -- one that can be built by anyone with access to cardboard,foil, glue, and plastic or glass.


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What You Will Need

Two cardboard boxes. We would suggest that you use an inner box that is at least 15"

x 15" (38cm x 38cm), but bigger is better. The outer box should be larger all around,

but it doesn't matter how much bigger, as long as there is a half inch (1.5cm) or more

of an airspace between the two boxes. Also note that the distance between the twoboxes does not have to be equal all the way around. Also, keep in mind that it is very

easy to adjust the size of a cardboard box by cutting and gluing it.

One sheet of cardboard to make the lid. This piece must be approximately 2" - 3" (4 -

8cm) larger all the way around than the top of the finished cooker.

One small roll of aluminum foil.

One can of flat-black spray paint (says on can "non-toxic when dry") or one small jar

of black tempera paint. Some people have reported making their own paint out of

soot mixed with wheat paste.

At least 8 ounces of white glue or wheat paste.

One Reynolds Oven Cooking Bag. These are available in almost all supermarkets in

the U.S. and they can be mail-ordered from Solar Cookers International. They are

rated for 400 F (204.4 C) so they are perfect for solar cooking. They are not UV-

resistant; thus they will become more brittle and opaque over time and may need to

be replaced periodically. A sheet of glass can also be used, but this is more expensive

and fragile, and doesn't offer that

much better cooking except on windy

days.

Buildin g the BaseFold the top flaps closed on theouter box and set the inner box on

top and trace a line around it ontothe top of the outer box, Removethe inner box and cut along thisline to form a hole in the top of theouter box(Figure 1).

Decidehow deep you want your oven to be (about 1" or 2.5cm biggerthan your largest pot and at least 1" shorter than the outer box)and slit the corners of the inner box with a knife down to that

height. Fold each side down forming extended flaps (Figure 2).Folding is smoother if you first draw a firm line from the end ofone cut to the other where the folds are to go.

Glue aluminum foil to the inside of both boxes and also to theinside of the remaining top flaps of the outer box. Don't waste your time being neat onthe outer box, since it will never be seen, nor will it experience any wear. The innerbox will be visible even after assembly, so if it matters to you, you might want to take


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more time here. Glue the top flaps closed onthe outer box.

Place some wads of crumpled newspaper intothe outer box so that when you set the inner

box down inside the hole in the outer box, theflaps on the inner box just touch the top of theouter box (Figure 3). Glue these flaps onto thetop of the outer box. Trim the excess flaplength to be even with the perimeter of the

outer box.

Finally, to make the drip pan, cut a piece of cardboard, the same size as the bottom ofthe interior of the oven and apply foil to one side. Paint this foiled side black and allowit to dry. Put this in the oven (black side up) and place your pots on it when cooking.The base is now finished.

Build ing the Remo vable LidTake the large sheet ofcardboard and lay it on top ofthe base. Trace its outline andthen cut and fold down theedges to form a lip of about 3"(7.5cm). Fold the corner flaps

around and glue to the side lidflaps. (Figure 4). Orient thecorrugations so that they go from left to right as you face the oven so that later the propmay be inserted into the corrugations (Figure 6). One trick you can use to make the lidfit well is to lay the pencil or pen against the side of the box when marking (Figure 5).Don't glue this lid to the box; you'll need to remove it to move pots in and out of the

oven.

To make the reflector flap, draw a line on the lid,forming a rectangle the same size as the ovenopening. Cut around three sides and fold the

resulting flap up forming the reflector (Figure 6).Foil this flap on the inside.

To make a prop bend a 12" (30cm) piece ofhanger wire as indicated in Figure 6. This canthen be inserted into the corrugations as shown.

Next, turn the lid upside-down and glue the oven


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bag (or other glazing material) in place. We have had great success using the turkeysize oven bag (19" x 23 1/2", 47.5cm x 58.5cm) applied as is, i.e., without opening itup. This makes a double layer of plastic. The two layers tend to separate from eachother to form an airspace as the oven cooks. When using this method, it is important toalso glue the bag closed on its open end. This stops water vapor from entering the bag

and condensing. Alternately you can cut any size oven bag open to form a flat sheetlarge enough to coverthe oven opening.

Improvin g Eff ic iencyThe oven you havebuilt should cook fineduring most of thesolar season. If youwould like to improve

the efficiency to beable to cook on moremarginal days, youcan modify your ovenin any or all of thefollowing ways:

Make pieces of foiled cardboard the same size as the oven sides and place these in

the wall spaces.

Make a new reflector the size of the entire lid (see photo).

Make the drip pan using sheet metal, such as aluminum flashing. Paint this black and

elevate this off the bottom of the oven slightly with small cardboard strips.

Thermodynamic Review of Solar Box Cookers

(Excerpted from the thesis of Petri Konttinen for the Helsinki University ofTechnology. Submitted September 25th, 1995)

3.1 Introduction

A typical solar box cooker consists of two boxes with insulation between them, ablack absorber plate at the bottom of the inner box, a transparent top cover and areflective lid as an energy booster (Fig. 3.1.). There are hundreds of different designsof solar box cookers in use. These vary in size, material, insulation and componentsused (Grupp, 1991).


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Generalisations about what works and what does not work, what is important andwhat is not important under certain conditions cannot be easily drawn. It is difficultfor the non-professional people to understand just by common sense. The reason forthis is that there are many variables among the choices: the box, insulation,transparent cover and reflective materials; as well as foods, latitude, month, hour and

time of cooking (Pejack, 1992).

Fig. 3.1. Cross-section of a solar box cooker with a cooking vessel.

This study is not meant to cover all the possible thermodynamical factors of solar boxcookers (because of their large number and complexity). The purpose is to give thereader some guide-lines to successful design of an operating solar box cooker. Thelaws of thermodynamics always determine the function of a solar box cooker,irrespective of its design and materials used (for basic information concerning theutilisation of solar energy in thermal processes, see Duffie and Beckman, 1991).

Instead of covering everything, the most important factors that affect the efficiency of

a solar box cooker (excluding operating conditions) will be examined separately.Those included in this study are: heat gain into a solar box cooker, heat loss from thebox, heat transfer from the solar box cooker to the cooking vessel, structural materialsof the box, materials and design of the reflective lid and transparent top cover, and thevolume of the cooking chamber. All of these will be looked at more closely (for moredetailed analysis on the thermodynamics of solar box cookers see: Pejack, 1990, 1991,


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1992, Thulasi Das et al. 1994, Grupp et al. 1991, Channiwala, 1989, Allfs, 1992, toname a few).

3.2 Heat gain into a solar box cooker

From a thermodynamical point of view the function of a solar box cooker is to trapand contain the heat of the sun inside it and to transfer the heat to the cooking vesselas efficiently as possible. The heat retained inside the insulated box with a transparentcover is based on the greenhouse effect, which is illustrated in Fig. 3.2.

Fig. 3.2. The greenhouse effect. Short-wave sunlight is absorbed into the black

materials inside the solar box cooker and converted into longer wavelength heat

Fig. 3.2. illustrates the idea of the greenhouse effect inside a solar box cooker. Thelight energy (which is short-wave energy) that enters the cooker through thetransparent top cover is absorbed by the black pots and the black bottom metal plate.The short-wave light energy is then converted into longer wavelength heat energy and

radiated from the interior materials. Most of this radient heat energy is trapped insidethe cooker and can (mostly) not radiate back out because of its longer wavelength.Although the transparent cover traps most of the radient heat, some does escapedirectly through the lid (Aalfs, 1992).


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3.3 Heat loss from solar box cookers

Heat loss from a solar box cooker consists of conduction, convection and radiation.Heat is lost by conduction, when it travels through the molecules of aluminium foil,glass, cardboard, air, bottom metal plate, and insulation, to the air outside of the box.

Hot air has a tendency to move upwards due to its lower density. If there are anycracks around the top lid, or side door, or construction imperfections, the hot airtravels (convects) out of the box and cooler air from outside enters. This lowers thetemperature inside the cooker.

The third heat loss mechanism is radiation. Any hot object give off heat waves, orradiates, to its surroundings (which are at a lower temperature). These heat waves areradiated through air or space. Most of the radiant heat given off by the warm potsinside the cooker is reflected back from the foil, bottom metal plate and the glass. The

transparent top cover (usually glass or plastic) traps most of the long-wave radientheat, but some does escape directly through the glazing (Aalfs, 1992).

The main heat loss mechanisms are conduction from the walls and floor, convectionfrom the cover and re-radiation out of the cover (Pejack, 1990). These will beexamined separately.

3.3.1 Heat loss from walls and floor

Heat loss from the walls and floor (The floor of the cooker is treated the same as the

walls) consists of conduction, convection and radiation.

Conduction from the walls can be reduced by increasing the thermal resistance of thewalls. Thermal resistance can vary significantly, depending on the construction andinsulation materials used. At the operating temperatures of solar box cookers thermalresistance can be defined as

q''= T1 - T2 /R (1)

where q is the heat flux (W/m2) and R is the thermal resistance in units of (m2

W/C). T1 and T2 are the temperatures of the opposing walls of the cooker (inabsolute Kelvin).

An empirical equation (Incropera and Dewitt, 1985, eq. 9.38) predicts a thermalresistance of 0.39 units for the 5.0 cm thick wall (with only air in between), when T1= 368 K (95 C) and T2 = 298K (25 C). Doubling of the wall to 10.0 cm results inonly about a 6 % increase in resistance R.


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Inserting a thin parallel plane of material, or a baffle, causes impediment to theconvection currents (and also diminishes radiation) between the walls. An empiricalequation (Incropera and Dewitt, 1985, eq. 9.40) predicts a resistance of 0.66 unitswhen using one parallel plane. Theoretically, inserting many planes would cause theoverall resistance to approach 2.0 units, but planes touching each other would "short"

the resistance by conduction.

The radiation componentof the heat flux across the cavity between the walls is givenby the equation

qr = (T14 - T24) (2)

where is the emissivity of the wall surfaces and is Bolzmann's constant of 5.67 x 10-8W/m2K4. Emissivity is a property of the wall material and it varies from less than 0.1for polished metals to 0.8-0.9 for wood, paper, etc.

The total wall resistance, by combining in parallel, is

R = RradRc /(Rrad + Rc) (3)

where Rrad is the radiative resistance and Rc is the convective resistance.

The relative radiation heat loss (using emissivity of 0.9 for paper, and the wall spaceof 5 cm) is three times more than the heat loss by convection, therefore it is importantto reduce radiation through the walls. This can be done by lowering the emissivity of

the walls; for example, covering the wall(s) with common household aluminium foil (= 0.05). Another way is to insert parallel lines (radiation shields) between the walls. Acertain level of thermal resistance is needed for a sufficient cooker operation. Pejack(Pejack 1990) has empirically measured that cooking at or above 100 C wouldrequire a wall resistance of about 1 unit or more for a cardboard box cooker. (see Fig.3.3).


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Fig. 3.3. Box temperature as a function of wall resistance for several values of input

solar flux. (Pejack, 1990)

In his test procedure Pejack used corrugated cardboard as a box material. Thethickness of cardboard was 4.3 mm, 1 corrugations per cm, and a mass density of0.87 kg/m3. The internal dimensions of the cooker were 30x40x60cm.

Fig. 3.4 shows box wall experimental values of resistance with different insulationmaterials. As we can see, there are many ways to reach the level of 1 unit.


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Fig. 3.4. Experimental values of resistances of the test walls. Circles and triangles

represent 5 and 10 cm wall spaces. (Pejack, 1990)

In practise an adequate thermal resistance can be achieved either by inserting foiled

baffles between the walls or by foiling the walls and adding some filling material inbetween. Using all of these together brings the maximum resistance.

3.3.2 Materials used to prevent heat loss, experiences from Namibia


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Cardboard sheets, aluminium foil and newspaper (or other similar filling) can be fairlyeasily found in developing countries. These materials are relatively low-cost and easyto use.

In Namibia we used very strong and robust reinforced aluminium insulation foil,

newspaper filling and cardboard walls. The foil is very waterproof, which is a bigadvantage considering the lifetime of the cooker. The price of the foil was 295Namibian dollars (about 70 US$) per 50 m2. It is more expensive than a normalkitchen foil, but it also lasts ten times longer.

3.3.3 Heat loss from cover

Heat loss from the cover occurs by convection from the cover, re-radiation out of thecover and through channels such as sealing, edges and corners. Estimation of top heatlosses is a complex problem due to the tray-shaped absorber plate and the presence of

combined convective and radiative modes of heat transfer.

Channiwala and Doshi (Channiwala and Doshi, 1989) have experimentally evaluatedthe top heat loss coefficient. They measured the cooker's temperatures with 12thermocouples, installed at various places in the cooker. Their measurements weremade in three stages:

1. A single glass cover with the absorber plate temperature range of 50 C to 180C

2. A single glass cover at different wind speeds, varying from 0 to 3.33 m/s.

3. The number of glass covers is changed to two, three and four respectively and eachone is tested at different wind speeds as in stage 2.

Their total results (for one and two glass covers, respectively) are shown in Figs. 3.5and 3.6, where Nc = number of glass covers, Ta = Ambient temperature, Tpm = Meantemperature of the absorber plate; Ut = top heat loss coefficient, and V = windvelocity. The correlation curve is obtained from theoretical calculations (Channiwalaand Doshi, 1989, pg. 494). As it would require several pages to explain it, it is notincluded in this study.

It can be clearly seen from figures 3.5 and 3.6 that as the absorber plate temperatureincreases, the top heat loss coefficient increases due to higher losses at highertemperatures. Wind velocity increases the convective heat losses significantly andthus reduces the cooker's temperature. Therefore it is important to place the cooker ina sunny spot out of the wind in the cooking area.


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An increase in the number of glass layers affects the reduction of the heat losscoefficients due to the increase in thermal resistance offered by successive air layers.However, increasing the number of glass covers from one to two only has the effect ofabout a 20 % decrease in the heat loss coefficient. Therefore in good solar conditions,e.g., in Namibia, it is not necessarily worth the trouble and cost to use a double-glass.

(Of course this depends on other factors also). For windy and/or moderate sunshineconditions two glasses might prove necessary.

Fig. 3.5. Top heat loss coefficients (Nc = 1). Channiwala and Doshi (1989)


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They computed the cooking times for plates with a thickness of between 0.1 and 10.0mm. (Fig. 3.7)

Fig. 3.7 Effect of absorber plate thickness on cooking time.Circles 15th April,

triangles 10th December (Thulasi Das et al., 1994)

The cooking time was minimum for plates of 0.5 and 1.0 mm in thickness. However,to ensure a good thermal contact between the vessel base and the plate, the plate andthe vessel base have to be smooth, even and rigid. Hence, the use of a 1.0 mm thick

plate is advisable. The plate should be painted dull black.

Thulasi Das et al.computed the meaning of other parameters, such as emissivity ofthe vessel and the contact resistance between the vessel base and the plate. These areshown in Table 3.1.
http://solarcooking.org/research/fi/pics/37.gif


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S. No. Upb e in (cm) Cooking time (min)

(W/m2 K)

1 100 1.0 10.0 60

2 100 1.0 7.5 62

3 100 1.0 5.0 66

4 100 0.5 7.5 66

5 50 0.2 7.5 72

6 20 1.0 7.5 84

Table 3.1. Effect of contact resistance, emissivity and insulation on cooking time

(Thulasi Das et al. 1994)

Thulasi Das et al.considered an insulation thickness of 7.5 cm adequate. Theemissivity e (of the cooking vessel) between 0.5 and 1.0 (compare S. No. 2 and 4) didnot make much difference. This was because the radiative transfer to the vessel wasnot very significant. Hence, weathered (oxidized) stainless steel (e = 0.5) oraluminium (e = 0.4) vessels can be used without the black paint. On the other hand,users might consider a black exterior aesthetic, and when they paint the base trayblack they could use the left-over to paint the pots. Paint also protects the outersurface of the pot.

With the increase in the contact resistance (1/ Upb) the cooking time increasedconsiderably. Therefore the plate and the vessel base should be as smooth, even andrigid as possible to ensure a good thermal contact as mentioned earlier. A thinglycerol film between the vessel base and the plate can enhance the coefficient (Upb)by threefold and thus improve the thermal contact.

Grupp et al.(Grupp et al1991) have computed that elevating the absorber plateslightly has the advantage of higher air temperature inside the cooker. Its drawback isa larger heat transfer between the absorber and the air in the box, which means higherheat losses. Elevating the absorber plate does not give any significant advantage,therefore it is not necessary.

3.5 Structural materials used for a solar box cooker

3.5.1 Introduction

Structural materials used for solar box cookers in industrial countries can be anythingtechnologically appropriate. They (and thus the price of the cooker) depend only onthe customers' wishes. This is not the situation with most of the people in thedeveloping countries. Materials used for their cookers (and why not for ours, too)should be easily available, inexpensive, easy to repair and replace. The more thematerials can be manufactured locally, the better. The possible materials for the


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structure include cardboard, wood, plywood, masonite, bamboo, metal, cement,bricks, stone, glass, fiberglass, woven reeds, rattan, plastic, papier mache, clay,rammed earth, tree bark, hardboard, reed, adobe bricks, cloth stiffened with glue, etc.(Aalfs, 1992). The list could be continued endlessly. Finally the cost, availability,personal wishes and humidity of the climate determine which ones are applicable in

each case.

A common and widely available totally free or low-cost cost material is corrugatedcardboard. It has been criticised for poor durability and not being waterproof.However, if the box is designed and constructed properly, it is possible to make acardboard cooker, which lasts for years with everyday use.

I saw cardboard solar box cookers in Namibia that have been used continually since1992. Most of those cookers are still in very good operating condition. Only one ofthem has become a little bit soaked, but it stayed out night and day for eight months

last year.

3.5.2 Moisture resistance: using a vapour barrier

When using cardboard (or any other material that will be easily soaked) it is crucial tomake a good vapour barrier inside the box. Water that vapourises from food whilecooking will soak the materials of the cooker if it is not prevented from entering to thestructure. For example, a strong, plastic-coated aluminium foil can be used to seal theinside of the inner box so, that moisture can not penetrate through the foil to thecardboard. This has to be done very carefully, as hot steam or water vapour can

penetrate the smallest holes.

3.5.3 Practical experiences of materials in Namibia

I made one cooker from corrugated plastic (polypropylene profile or fluteboard, idea:Magney 1992) in Finland and brought it to Namibia. In comparison to a cardboardcooker of a similar design (Figs. 3.8-3.9) it proved very effective (and being abeautiful white, also attractive to the people). In Namibian autumn conditions themaximum cooker air temperature inside the polypropylene cooker was 20 C higherthan the maximum air temperature inside the cardboard cooker.

Due to primitive test conditions and lack of equipment I could only measure watertemperature inside the pot of the cardboard-cooker. I assumed that the watertemperature inside the pot of the polypropylene-cooker was similar, due to strongsteam formulation from both pots at the same time. The only difference might be aslightly faster increase in temperature, which has no real importance under thoseconditions. Anyway, it took exactly two hours for water to reach the boiling


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temperature in the morning, and it continued boiling till the end of the test in lateafternoon. Clouds appearing in the sky (which can be seen as three sudden drops inthe radiation level in Fig 3.9), had a delayed lowering effect on the air temperature,but the water temperature remained constant from the beginning of boiling. The windwas from low to moderate during the whole testing time.

Solar radiation was very good on the test day morning. Perpendicular radiation fluxwas more than 1000 W/m2 from the very beginning of the test, therefore one couldbegin solar cooking when the sun came up. This would speed up solar cooking if thecook is in a hurry. At the other end of the day, perpendicular radiation remained highuntil the end of the test at four o'clock.. My experiments between February and Mayin Namibia showed that solar cooking can be continued up until 5-7 o'clock,depending on the weather and month. By using an adjustable reflective lid, horizontalradiation can be transferred almost totally perpendicularly to the cooker and thusextend the available cooking time.

Fig. 3.8. Solar box cooker test, temperatures


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Ongwediva, Namibia, April 4th, 1995

Fig. 3.9. Solar box cooker test, solar radiation

Ongwediva, Namibia, April 4th, 1995

Figs. 3.8-3.9 shows that irrespective of the fact that the polypropylene cooker reachedconsiderably higher temperatures, the cardboard cooker is perfectly sufficient forgood solar radiation areas, such as Namibia. The sudden drops in temperatures andradiation are caused by clouds. If the weather would have continued cloudless, thecurves would have followed (almost) linear lines till 14.30, when the cloudsdisappeared.

Test equipment: Two solar box cookers (Design: Appendix two, polypropylene modelmade by myself, cardboard model made by women at the building course). Two 5-litre pots with glass lids with one litre of water at ambient temperature inside each(manufacturer: Hackman Oy Ab, Finland). Two oven thermometers, (manufacturer:Suomen Lmpmittari Oy, Finland) and a Fluke 51 thermometer with j-type bi-metalprobe.


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3.6 Materials and design of the transparent top cover and reflective lid

The material of the transparent top cover (or window) can be glass, plastic or someother suitable heat resistant material. Glass is widely available in developing countriesand therefore by far the most common material used. Glass has some disadvantages; itis heavy, awkward to carry and it breaks easily. Despite these facts glass is difficult tosurpass, because it is a relatively low-cost material, and easy to get and replace. Oneother important aspect is that when glass gets hot it does not become distorted, asmany plastics do. If many cookers are planned, for example, to be transported byaeroplane to refugee camps, a heat resistant plastic window is a good choice becauseit is light and durable compared to glass.

Several substitute materials for glass are under development. For example, SolarCooking International (SCI) is using a plastic film on some of their cookers. It is aspecial heat resistant polyester that is also somewhat resistant to the effects of ultra-violet light. It is produced experimentally by the 3-M Company. (Source: electronicmail from Kevin Coyle; SCI Resources Co-ordinator, 22.6.1995)

Whichever window material and number used, it is very important to seal themproperly. Air leaks between the window and the cooker frame reduces the temperaturedrastically. A suitable sealing material can be silicone sealing strip with silicone glue(see Appendix two). Another possible sealant, if silicone is not available or it is tooexpensive to use, could be strips cut from common plastic foam, or even a narrowstrip of cardboard (in a case of emergency).

The reflective lid can be just a thin plane, which is coated with aluminium foil. Thearea of the lid should be equal to, or bigger than, the area of the window. Boosters canbe used to maximise solar radiation transfer to the box. One way to do this is to makean extra reflector, which is attached vertically to the lid. The extra reflector can be alittle bit bent (convex) to concentrate solar radiation inside the box.

If the food is to be kept warm after cooking, it is advisable to design the lid to help inthis task (see Appendix four). A thin lid does not prove effective enough to preventthe heat from escaping through it and the glass, after the solar radiation to the box hasceased. A thicker lid with some insulation in between (and good sealing to the glass)helps to keep food warm even several hours after cooking, when closed carefully.Heat retention can be rendered even more effective by putting a blanket on top of theclosed lid.

The reflective material should have high specular reflectance, high durability, and, ofcourse, low cost (Funk and Wilcke 1992).


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Material Durability Cost

Specular reflectance

Mirrors Breakable Very high 0.88

Aluminium foil Tears Moderate 0.86

Aluminium sheet Good High 0.85Aluminised polyester Tears Moderate 0.75-0.85

Metal from fuel tins Rusts Moderate not available

Table 3.2. Potential reflector materials and normal specular reflectance

(Funk and Wilcke, 1992)

None of the available materials can fulfil all the necessary expectations. It should alsobe easily available locally in developing countries.

In Namibia, the same reinforced aluminium insulation foil, which was used for foilingthe inner walls of the box, was also used as reflective material for the lid. It proved tobe much more tear resistant than normal aluminium foil and thus is to berecommended. Its specular reflectance is not available. It looked slightly dullcompared to the kitchen foil, but it worked adequately well.

3.7 Size of cooker and volume of cooking chamber

The size of the cooker is an important factor not only for the amount of food it cooks,

but also it dictates the cooking time. As a rule, the bigger cooker (and thus the biggersurface to receive solar radiation) you have, the more and faster you can cook.However, the design of the cooker is a much more important factor than it wouldseem at first sight.

Malhotra et al. have measured the effect of reducing the cooking chamber volume(Malhotra et al. 1983). They changed the inside of the box from a square shape bytilting the inner angle to four different shapes and got a considerable improvement inits performance (see Fig. 3.10)


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Fig. 3.10. Effect of cooking chamber volume on the air temperature inside.

Malhotra et al. (1983)

They calculated that the optimisation factor (Fo) of the optimised cooker is:

Fo = Volume of chamber/Area of window 14 cm (4)

provided the concentration ratio of the reflecting assembly is 3.6.
http://solarcooking.org/research/fi/pics/310.gif


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In any case, Fig. 3.10 can be seen more as a guide-line to the tilting of the inner sideof the box. Tilting it too much is not advisable because the base tray surface area (=main heat conduction surface area) also decreases.

3.8 Estimate of efficiency of solar box cookers

Estimating the power efficiency of a solar box cooker in theory is a complicated taskdue to multiple heat gain, loss and transfer mechanisms (for calculating energybalance alone, see Thulasi Das et al. 1994). I do not even try to include thesecalculations here, because deriving even a rough estimate would require severalpages. An estimation of thermal efficiency found from literature is between 20 % and50 % (Kuhnke et al., 1990). Another estimation says the power efficiency may beabout 20 % (Currin, 1994). Still, due to sunshine being an energy source totally free ofcost, the cost - efficiency ratio is more important than the power efficiency alone.

The users I have met in Namibia are more concerned about the temperature reachedand the heat transferred to the food, or the cooking potential of a cooker.

My experiences from Namibia are, that one solar box cooker (TFL - type, seeAppendix two) cooks enough food for at least 11 persons for one meal. The timeneeded was about two hours at midday, and the perpendicular radiation was around1000 W/m2.

The results presented in this thermodynamical review are mostly based onmeasurements and experiments. One has to take into account his or her needs (amount

of food needed to be cooked, money willingly paid for a cooker, etc.), when designingor choosing a proper solar box cooker for any climatic condition.

3.9 Conclusions (and sources of error)

This chapter deals with the most common factors of optimising a solar box cooker. Itis based mostly on literature (as shown in previous chapters), as well as my ownexperiences from Namibia.

Solar box cookers reported in researches are different in size, design and construction.

Research conditions and the equipment used are not equal either. Therefore the resultsshown in this chapter should be read critically and they should be seen more as guide-lines to the quite complex matter; design of a low cost, reliable and robust solar boxcooker.

The main functions of a solar box cooker are to heat up (and contain heat) properlyand to transfer this heat to the cooking vessel effectively. By far the main heat transfer


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method is by conduction from the black base tray to the bottom of the cooking pot (ifthere is enough space in the cooker to allow the light to reflect to the bottom).

Materials for the box can be cardboard, hardboard, wood, etc. Polypropylene plasticproved to function very well (see Figs. 3.8-3.9) and also to be attractive to the users.

Cardboard must be coated with foil, painted or otherwise treated properly to make itwaterproof (at least to some extent).

Heat loss from the cooker walls can be reduced by using extra aluminium foiledbaffles between the walls of the cooker. A wall and floor thickness of 5 cm isadequate with proper insulation. The walls should be foiled in order to be waterproofand reflective (to the base tray). Insulation material can be rolled newspaper, feathers,rice hulls, dry loose material, etc. Heat loss from the transparent top cover can beminimised by using several cover layers, sealing them properly and placing the cookerin a windless place. In good solar conditions even one glass cover is good enough.

The absorber plate should be 1-2 mm thick for maximum heat transfer efficiency. Itshould be dull black, smooth, even and rigid. There should be a good thermal contactbetween the absorber plate and the cooking vessel to ensure maximum heat transfer. Athin film of glycerol can be used to enhance this.

The cooking vessel should have a tightly closing lid to prevent steam from escaping.The material of the vessel can be weathered (oxidised) stainless steel or aluminium,even without black paint. However, the paint protects the surfaces and is aesthetic.The bottom of the cooking vessel should also be smooth, even and rigid.

The material of the transparent top cover can be glass or heat resistant plastic. Glass ismore widely available, but it is heavy and it breaks more easily than plastic. The covershould be sealed properly to prevent air leaks between it and the box frame. Thereflective lid can be coated with reinforced aluminium foil or some other availablereflective material. Boosters can be used to maximise reflection. The lid should be afew cms thick with some insulation in between, if it is supposed to keep the foodwarm after cooking.

The inner walls of the cooker can be tilted to enhance the operating temperature and

to maximise the solar radiation reflection to the base tray.

The Solar Altitude is the angle of the sun above the horizon.

Calculations of Solar Angles from:

http://holodeck.st.usm.edu/vrcomputing/vrc_t/tutorials/solar/sa_intro.shtml


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22-Mar 1-Jan

Equinoxes Summer Sol

Declination 22.48657371 23.2406Hour Angle, Noon 0 0Hour Angle, 2 PM 30 30

Lattitude in Oakland, CA 37.8 37.8

Sun Angle, Noon 84 84Sun Angle, 2 PM 9 9

sin-1[[cos L) * cos ) * cos hs)] + [sin L) * sin )]]1 radians = 57.2957795 degrees

Step 1. Determine declination d

Step 2. Determine Hour Angle hs

Step 3. Determine Solar altitude,a:

Step 4. Determine Solar azimuth, as

Get latitude and longitute: http://www.gorissen.info/Pierre/maps/googleMapLocationv3.php

Latitude, Lake Merritt 37.805444

Longitude, Lake Merritt -122.257919

Get Solar Position: http://solardat.uoregon.edu/SolarPositionCalculator.html

Solar position calculatorresults at Lake Merritt

Lat: 37.80 Long: -122.00

March 21 and September 21 (Equinoxes)

Time: 12:00 PM Time: 2:00 PM

Declination0.3

Declination0.3

Solar zenith angle 38 Solar zenith angle 46

Solar altitude 52 Solar altitude 44

June 21 (Summer Solstice)

Time: 12:00 PM Time: 2:00 PM

Declination0.3

Declination0.3
http://holodeck.st.usm.edu/vrcomputing/vrc_t/tutorials/solar/declination.shtmlhttp://holodeck.st.usm.edu/vrcomputing/vrc_t/tutorials/solar/hourangle.shtmlhttp://holodeck.st.usm.edu/vrcomputing/vrc_t/tutorials/solar/hourangle.shtmlhttp://holodeck.st.usm.edu/vrcomputing/vrc_t/tutorials/solar/altitude_angle.shtmlhttp://holodeck.st.usm.edu/vrcomputing/vrc_t/tutorials/solar/altitude_angle.shtmlhttp://holodeck.st.usm.edu/vrcomputing/vrc_t/tutorials/solar/azimuth.shtmlhttp://holodeck.st.usm.edu/vrcomputing/vrc_t/tutorials/solar/azimuth.shtmlhttp://holodeck.st.usm.edu/vrcomputing/vrc_t/tutorials/solar/azimuth.shtmlhttp://holodeck.st.usm.edu/vrcomputing/vrc_t/tutorials/solar/altitude_angle.shtmlhttp://holodeck.st.usm.edu/vrcomputing/vrc_t/tutorials/solar/hourangle.shtmlhttp://holodeck.st.usm.edu/vrcomputing/vrc_t/tutorials/solar/declination.shtml


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Solar zenith angle 15 Solar zenith angle 28

Solar altitude 75 Solar altitude 62

Zenith angle is the angle the sun is off of vertical

Altitude is the angle the sun is off of horizontal

http://asd-www.larc.nasa.gov/SCOOL/definition.html

Principles of Solar Box Cooker Design

By Mark Aalfs, Solar Cookers Internationale-mail:[email protected]

The purpose of this paper is to summarize the basic principles thatare used in the design of solar box cookers.

People use solar cookers primarily to cook food and pasteurizewater, although additional uses are continually being developed.Numerous factors including access to materials, availability oftraditional cooking fuels, climate, food preferences, culturalfactors, and technical capabilities, affect people's approach to solarcooking.

With an understanding of basic principles of solar energy andaccess to simple materials such as cardboard, aluminum foil, andglass, one can build an effective solar cooking device. This paperoutlines the basic principles of solar box cooker design and

identifies a broad range of potentially useful construction materials.

These principles are presented in general terms so that they are applicable to a wide variety ofdesign problems. Whether the need is to cook food, pasteurize water, or dry fish or grain; thebasic principles of solar, heat transfer, and materials apply. We look forward to the application ofa wide variety of materials and techniques as people make direct use of the sun's energy.

The following are the general concepts relevant to the design or modification of a solar box

cooker:

Heat Principles

Materials Requirements

Design and Proportion

Solar Box Cooker Operation

Cultural Factors

Back to the top
mailto:[email protected]:[email protected]://solarcooking.org/sbcdes.htm#heat_principleshttp://solarcooking.org/sbcdes.htm#heat_principleshttp://solarcooking.org/sbcdes2.htm#materialshttp://solarcooking.org/sbcdes2.htm#materialshttp://solarcooking.org/sbcdes2.htm#design_tophttp://solarcooking.org/sbcdes2.htm#design_tophttp://solarcooking.org/sbcdes2.htm#operationhttp://solarcooking.org/sbcdes2.htm#operationhttp://solarcooking.org/sbcdes2.htm#culturalhttp://solarcooking.org/sbcdes2.htm#culturalhttp://solarcooking.org/sbcdes.htm#principles_tophttp://solarcooking.org/sbcdes.htm#principles_tophttp://solarcooking.org/sbcdes.htm#principles_tophttp://solarcooking.org/sbcdes2.htm#culturalhttp://solarcooking.org/sbcdes2.htm#operationhttp://solarcooking.org/sbcdes2.htm#design_tophttp://solarcooking.org/sbcdes2.htm#materialshttp://solarcooking.org/sbcdes.htm#heat_principlesmailto:[email protected]


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HEAT PRINCIPLES

The basic purpose of a solar box cooker is to heat things up - cook food, purify water, andsterilize instruments - to mention a few.

A solar box cooks because the interior of the box is heated by the energy of the sun. Sunlight,both direct and reflected, enters the solar box through the glass or plastic top. It turns to heatenergy when it is absorbed by the dark absorber plate and cooking pots. This heat input causesthe temperature inside of the solar box cooker to rise until the heat loss of the cooker is equal tothe solar heat gain. Temperatures sufficient for cooking food and pasteurizing water are easilyachieved.

Given two boxes that have the same heat retention capabilities, the one that has more gain, fromstronger sunlight or additional sunlight via a reflector, will be hotter inside.

Given two boxes that have equal heat gain, the one that has more heat retention capabilities -better insulated walls, bottom, and top - will reach a higher interior temperature.

The following heating principles will be considered first:

Heat gain

Heat loss

Heat storage

Heat Principles:Heat gain,Heat loss,Heat storage |Materials Requirements

Design and Proportion |Solar Box Cooker Operation |Cultural Factors |To top

A. Heat gain

Greenhouse effect: This effect results in the heating of enclosed spaces into which the sunshines through a transparent material such as glass or plastic. Visible light easily passes throughthe glass and is absorbed and reflected by materials within the enclosed space.

The light energy that is absorbed by dark pots and the

dark absorber plate underneath the pots is converted

into longer wavelength heat energy and radiates from

the interior materials. Most of this radiant energy,because it is of a longer wavelength, cannot pass back

out through the glass and is therefore trapped within

the enclosed space.

The reflected light is either absorbed by othermaterials within the space or, because it doesn't
http://solarcooking.org/sbcdes.htm#heat_gainhttp://solarcooking.org/sbcdes.htm#heat_losshttp://solarcooking.org/sbcdes.htm#heat_storagehttp://solarcooking.org/sbcdes.htm#heat_principleshttp://solarcooking.org/sbcdes.htm#heat_gainhttp://solarcooking.org/sbcdes.htm#heat_losshttp://solarcooking.org/sbcdes.htm#heat_storagehttp://solarcooking.org/sbcdes2.htm#materialshttp://solarcooking.org/sbcdes2.htm#design_tophttp://solarcooking.org/sbcdes2.htm#operationhttp://solarcooking.org/sbcdes2.htm#culturalhttp://solarcooking.org/sbcdes.htm#principles_tophttp://solarcooking.org/sbcdes.htm#principles_tophttp://solarcooking.org/sbcdes.htm#principles_tophttp://solarcooking.org/sbcdes2.htm#culturalhttp://solarcooking.org/sbcdes2.htm#operationhttp://solarcooking.org/sbcdes2.htm#design_tophttp://solarcooking.org/sbcdes2.htm#materialshttp://solarcooking.org/sbcdes.htm#heat_storagehttp://solarcooking.org/sbcdes.htm#heat_losshttp://solarcooking.org/sbcdes.htm#heat_gainhttp://solarcooking.org/sbcdes.htm#heat_principleshttp://solarcooking.org/sbcdes.htm#heat_storagehttp://solarcooking.org/sbcdes.htm#heat_losshttp://solarcooking.org/sbcdes.htm#heat_gain


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change wavelength, passes back out through theglass.

Critical to solar cooker performance, the heat thatis collected by the dark metal absorber plate and

pots is conducted through those materials to heatand cook the food.

Glass orientation: The more directly the glass faces

the sun, the greater the solar heat gain. Although the

glass is the same size on box 1 and box 2, more sun

shines through the glass on box 2 because it faces the

sun more directly. Note that box 2 also has more wall

area through which to lose heat.

Reflectors, additional gain: Single or multiple

reflectors bounce additional sunlight through the

glass and into the solar box. This additional input of

solar energy results in higher cooker temperatures.

Heat Principles:Heat gain,Heat loss,Heat storage |Materials Requirements

Design and Proportion |Solar Box Cooker Operation |Cultural Factors |To top

B. Heatloss

The Second Law of Thermodynamics states that heat always travels from hot to cold. Heatwithin a solar box cooker is lost in three fundamental ways: Conduction, Radiation, andConvection

Conduction:

The handle of a metal pan on a stove or fire becomes hot through the transfer of heat from thefire through the materials of the pan, to the materials of the handle. In the same way, heat within

a solar box is lost when it travels through the molecules of tin foil, glass, cardboard, air, andinsulation, to the air outside of the box.
http://solarcooking.org/sbcdes.htm#heat_principleshttp://solarcooking.org/sbcdes.htm#heat_gainhttp://solarcooking.org/sbcdes.htm#heat_losshttp://solarcooking.org/sbcdes.htm#heat_storagehttp://solarcooking.org/sbcdes2.htm#materialshttp://solarcooking.org/sbcdes2.htm#design_tophttp://solarcooking.org/sbcdes2.htm#operationhttp://solarcooking.org/sbcdes2.htm#culturalhttp://solarcooking.org/sbcdes.htm#principles_tophttp://solarcooking.org/sbcdes.htm#principles_tophttp://solarcooking.org/sbcdes.htm#principles_tophttp://solarcooking.org/sbcdes2.htm#culturalhttp://solarcooking.org/sbcdes2.htm#operationhttp://solarcooking.org/sbcdes2.htm#design_tophttp://solarcooking.org/sbcdes2.htm#materialshttp://solarcooking.org/sbcdes.htm#heat_storagehttp://solarcooking.org/sbcdes.htm#heat_losshttp://solarcooking.org/sbcdes.htm#heat_gainhttp://solarcooking.org/sbcdes.htm#heat_principles


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The solar heated absorber plate

conducts heat to the bottoms of the

pots. To prevent loss of this heat via

conduction through the bottom of the

cooker, the absorber plate is raised

from the bottom using small

insulating spacers as in figure 6.

Radiation:Things that are warmor hot -- fires, stoves, or pots andfood within a solar box cooker --give off heat waves, or radiate heatto their surroundings. These heatwaves are radiated from warmobjects through air or space. Most

of the radiant heat given off by thewarm pots within a solar box isreflected from the foil and glassback to the pots and bottom tray.Although the transparent glazingsdo trap most of the radiant heat,some does escape directly throughthe glazing. Glass traps radiant heatbetter than most plastics.

Convection:Molecules of air

move in and out of the box throughcracks. They convect. Heated airmolecules within a solar boxescape, primarily through thecracks around the top lid, a side"oven door" opening, orconstruction imperfections. Coolerair from outside the box also entersthrough these openings.

C. Heat storage:

As the density and weight of the materials within the insulatedshell of a solar box cooker increase, the capacity of the box tohold heat increases. The interior of a box including heavymaterials such as rocks, bricks, heavy pans, water, or heavyfoods will take longer to heat up because of this additional heatstorage capacity. The incoming energy is stored as heat in these


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heavy materials, slowing down the heating of the air in the box.

These dense materials, charged with heat, will radiate that heat within the box, keeping it warmfor a longer period at the day's end.

case study of solar stoves

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