Home-immediately access 800+ free online publications. Download CD3WD (680 Megabytes) and distribute it to the 3rd World. CD3WD is a 3rd World Development private-sector initiative, mastered by Software Developer Alex Weir and hosted by GNUveau_Networks (From globally distributed organizations, to supercomputers, to a small home server, if it's Linux, we know it.)ar.cn.de.en.es.fr.id.it.ph.po.ru.sw

CLOSE THIS BOOKSolar Cookers in the Third World (GTZ, 1990, 228 p.)
Appendix 1: Solar Cooking Devices
VIEW THE DOCUMENTPreliminary notes
VIEW THE DOCUMENTBox-type solar cookers
VIEW THE DOCUMENTReflector Cookers
VIEW THE DOCUMENTHeat-accumulatinq solar cookers
VIEW THE DOCUMENTConvective solar cookers
VIEW THE DOCUMENTComparative Survey

Solar Cookers in the Third World (GTZ, 1990, 228 p.)

Appendix 1: Solar Cooking Devices

Preliminary notes

Various solar cookers that were devised in developing and industrialized countries for use in developing countries are described below. The individual illustrated descriptions precede a tabular summary comparison.

Most of the data listed in the table were taken from questionnaires that were filled out by individuals and institutions involved in the development and dissemination of solar cookers. A total of 92 individuals and institutions in 31 different countries were written to. Of the 46 replies received, 21 yielded data that could be included in the comparative survey included at the end of this appendix. The various individual solar cooker descriptions contain more such data. It should be noted that practically all such data is more or less subjective, i.e. not free of personal opinion. Some devices are described individually but not listed in the table, which may be taken as an indication of the questionnaire not having been answered properly, so that all or most of the data needed for tabular comparison were missing.

Very little empirical data is available on longevity, so that the information provided is estimative, and the performance data are also influenced by subjective opinions, if not to say extenuation.
Even the prices cannot be taken at face value unless a certain minimum quantity of units produced is guaranteed. Frequently, rash conclusions concerning the low cost of future mass-produced devices are drawn on the basis of prototype production.

Questions regarding the cookers' stability and ease of cleaning almost invariably drew such positive answers that the results offered little in the sense of differentiation. Consequently, despite the practical importance of stability and cleanability, those two factors were omitted from the table. Some information on stability is offered in the individual texts.

The definition of efficiency is a critical aspect (cf. sections 2.2 and 3.9). Some respondents went so far as to openly refuse to answer that question. The efficiency of solar cooking boxes and reflector cookers can be described as the ratio between the available heat and the incident energy. To determine the efficiency, the available heat is measured by heating a certain amount of water from a certain initial temperature up to the boiling point. The incident energy is the measured global radiation striking the aperture plane of the device during that same period of time.

But for heat-accumulating solar cookers, only the partial efficiency of the energy-collecting components (collector and heat storage) are directly measurable and, hence, known.

Conspiciously unfounded and/or unrealistic data are shown with question marks and/for in parentheses. To the extent available, more realistic data were inserted.

For the second edition, which was updated and translated to English in 1988/89 some more types of solar cookers have been added both to the descriptive part and to the synoptic comparative survey table.

Box-type solar cookers

Designation:

ATDO Reflector Box Type Solar Cooker

Type:

Solar cooking box with reflecting lid

Designed/produced by:

Appropriate Technology Development


Organization (ATDO)


Government of Pakistan, Karachi

Promoted/ sponsored by:

All Pakistan Women's Association


67 b, Garden Rd.


Karachi 3, Pakistan


ATDO works with several different kinds of box-type solar cookers. They are all larger than a "normal" Indian solar cooking box but designed to hold about the same amount of food. Some (proto)types open at the side (in order to reduce heat losses when opened); the standard version has a tilting double-pane glass cover.

The activities are new; about 20 cookers have been produced since 1986.

Designation:

ATRC Solar Cooker

Type:

Solar cooking box with reflecting lid

Invented/developed by:

Agricultural Tools Research Centre


Suruchi Campus, P.O.Box 4


Bardoli - 394 601, India

Manufactured/marketed by:

Yantra Vidyalaya, P.O.Box 4


Bardoli - 394 601, India

The ATRC solar cooker is a solar cooking box with a reflecting lid. It differs from normal Indian box-type solar cookers mainly by reason of its rectangular shape and stable construction as a robust, all-aluminum-clad wooden box, which also makes it heavy.

While a quadratic shape is optimal for minimizing heat losses through joints and the sides, a rectangular shape has two other advantages:

- it reduces the lateral stress on the hinges when the lid is lifted or lowered unevenly (sometimes problematic for square bottomed fiber glass cooking boxes), and
- it reduces the amount of radiation lost by not keeping the cooking box accurately (azimuthally) aligned with the sun and/or by neglecting to track the sun at adequately frequent intervals.

At 5 years, the maker's service-life prognosis is quite modest; the device is certainly just as robust, if not more so, than other Indian-made cooking boxes. The cooker comes with a simple, revolving, three-legged stand that puts the unit at a normal working height and facilitates solar tracking.

An appreciable number of ATRC solar cookers have been sold at a quoted price of US$ 60 (converted); however, the major share of the sold units was government subsidized (50 %).

In addition to the solar cooker described here, ATRC also has do-it-yourself instructions for building a cheap, stationary solar cooker. Anchored firmly in the ground, it consists mainly of a double-pane glass cover on a black aluminum receiver surrounded by natural insulating materials (straw or the like and embedded in the ground. However, the type in question has made no appreciable progress toward dissemination.


Figure 19: ATRC Solar Cooker on a revolving stand

Designation:

Cocina Solar

Type:

Box-type solar cooker with reflecting lid

Developed by:

Grupo de Investigaciones Agrarias


Academia de Humanismo Cristiano


Casilla 6122 Correo 22


Santiago, Chile

This solar cooking box has an unusually small aperture (28 x 38 cm² + mirror of the same size). It is of simple construction and easy to build by following the well-written instructions.

The inside and outside boxes are made completely of wood. The (uninsulated) wooden lid has an aperture-size mirror mounted on the inside. The sides of the inner box reflect radiation, but the black bottom absorbs it. The cover is a single pane of glass.

No information is available on the cooker's thermal output. Its small aperture, its relatively small mirror (which requires constant, very accurate adjustment to reflect fully into the box) and the single-pane cover seem to indicate relatively poor performance. However, the device is promising and capable of improvement.


FIGURE

Designation:

Dhauladhar Solar Cooker

Type:

Stationary adobe cooking box Mrs. D. Contractor

Developed/sponsored by:

The Mirage


V.P.O. Andreta


Palampur Tehsil


Kangra District


H.P. 176-103 India


The Dhauladhar Solar Cooker is a table-level clay-brick solar cooking box, with or without an intensifying reflector.

Larger than most customary types of box-type solar cookers (outside dimensions: 170x75x75 cm; inside dimensions: 86x56x17 cm), this one is designed to handle accordingly large quantities of food (6 kg and more) for large families.

The double-pane glass cover is rigidly attached to the adobe structure (no tilting); the cooking space is accessible from the front, i.e. it corresponds roughly to a drawer. The inclined sheet metal walls of the inner box are made of tin on rice-rusk insulation. Thanks to its modular construction, the individual parts of the inner box can be extracted through the drawer opening without disturbing the glass-cover seal. In practice, this is sometimes necessary for cleaning, repainting, etc. The supporting masonry can be made of bricks or rocks and mortar, while the actual cooking box should consist of adobe clay, since the latter is a good insulator, thus contributing substantially to the box's good heat retention capacity. The cooker still works when individual clouds move past.

Adobe construction makes good weather protection indispensable. In fact making the "umbrella" is part of the building instructions. Nonetheless, the adobe is still subject to gradual erosion, so the user will have to keep it in good repair.

Dhauladhar solar cookers can be built by the users themselves, but they will need the help of an experienced craftsman, since some parts require accurate workmanship.

For better promotion of the cooker, information papers including photos, sketches, recepes, and self-help construction guidelines are being published.

The building materials cost about US$ 22.-, including two shallow pots, but this price does not cover the cost of labour and/or locally available materials. About 150 such cookers have been built since 1985 (see also chapter 2.3.2).

Designation:

Indian Box Type Solar Cooker

Type:

Solar cooking box with reflecting lid

Institutions:

Consortium on Rural Technology (CORT) E-350 Nirman Vihar Delhi 110 092, India; Gujarat Energy Development Agency (GEDA) B.N. Chambers, R.C. Dutt Rd. Vadodara 390 005, India and others

Marketed by:

GEDA (see above) and others

The Indian Box Type Solar Cooker is the subject of a national campaign for introducing solar cookers throughout India.

The design and characteristics of the solar cooker are specified by the government and must be adhered to by all manufacturers. The cooker has been type-tested by various institutes. Manufacturers willing to build the cooker according to the official specifications are admitted to the marketing network by the regional or state agency, e.g. GEDA in the state of Gujarat. CORT tan NGO) and the Department of Nonconventional Energy Sources (DNES), Federal Ministry of Energy, among other organizations, are involved at the national level.

The specifications call for fiber glass-reinforced plastic, aluminum-sheet or wood construction. The basic shape is that of a square suitcase, the aperture dimensions amounting to 50 x 50 cm, the lid being fitted with a glass mirror (no reflecting foil) and the inner glass cover pane consisting of tempered (safety) glass. A simple support holds the cover open for introducing and removing food; a second support is provided for adjusting the reflector to the momentary solar altitude. The bottom of the case is fitted with rollers, and a handle is provided for carrying it. Some versions will accept a padlock to protect the food.

Designation:

Kerr-Cole Solar Box Cooker

Type:

Box-type solar cooker made of corrugated paper

Designed/marketed by:

Kerr Enterprises, Inc. P.O.Box 27417, Tempe Arizona 85282 0410, USA

The Kerr-Cole Solar Box Cooker is a low-cost cardboard model made in appreciable numbers. It has about the same inside dimensions as an Indian Box Type Solar Cooker, but its cover consists of a single pane of glass. A maximum inside temperature of 130...140ºC is achievable.

The lid, also made of corrugated paper, has a reflecting foil slicked onto the inside. The optical quality of the reflector is clearly inferior to that of a glass mirror.

Unfortunately, no performance data or prior production figures were available at the time of this writing.

Designation:

MECTAT Solar Ovens

Type:

Solar Cooking Boxes of different types

Institution:

Middle East Centre for the Transfer of Appropriate Technology (MECTAT) P.O.Box 113, 5474 Beirut, Libanon

The Middle East Centre for the Transfer of Appropriate Technology (MECTAT) has developed 9 different types of box type solar cookers (solar ovens), made from different materials such as mild steel sheet, plywood, pressedwood, pasteboard, cardboard. The boxes have partly horizontal, partly inclined (0...40º) glazed covers, different numbers of booster mirrors (0...4), and the apertures vary from 0.08 to 0.26 m². There are low-cost models made from recycled materials at approx. US$ 5.00, as well as metal sheet boxes with stainless steel reflectors at the price of US$ 95.00 /181/.

As of this writing, MECTAT is carrying out research on heat storage in pressurized water vessels inside the solar oven in order to make cooking after sunset possible.


Figure 24: Sectional views of MECTAT’s four solar oven designs /181/

Designation:

Mina Solar Pressure Cooker

Type:

Cooking box with booster mirrors and integrated steam pot

Developed by:

A.M. Khalifa, M.M. Taha, M. Akyurt Mech. Eng. Dept., College of Engineering King Abdulaziz University P.O.Box 9027, Jeddah 21413, Saudi Arabia

The Mina Solar Pressure Cooker consists of an air-tight pressure cooking pot with attached heat-conducting metal absorber wings, integrated in an insulated box with a glass or plastic foil cover and a set of 8 booster mirrors in a "funnel" arrangement.

Incoming solar radiation is concentrated by a factor of approx. 3.5 by the booster mirrors, passes through the glass or plastic cover and hits the absorber wings and the pot. Energy is transferred by heat conduction through the wings to the pot walls. The pressure pot allows temperatures over 100ºC nearly without evaporation losses, thus enhancing heat conduction within the food and accelerating the cooling process (see ref. 192).

Theoretical studies have shown that the air and vapour tight vessel yields a considerable improvement of efficiency. Smaller cookers of this type perform better than large ones due to their smaller thermal mass. A bigger pot diameter relative to the overall absorber width leads to a higher effective heating power (see ref. 193). Studies are going on.

Adjusting the cooker to the sun makes inclination-of the cooking vessel necessary. This might be acceptable as long as the lid is really air and water tight. Stirring during the cooking process is impossible. Pot handling (taking, pooring out the food, cleaning the pot) is difficult because of the attached absorber wings (the wings must be welded to the pot walls to assure good'heat transfer, and cannot be taken away).


FIGURE

Figure 24a: Mina Solar Pressure Cooker (Saudi Arabia)

Designation:

Orangi Cooking Box

Type:

Box type solar cooker with reflecting lid

Institution:

Orangi Pilot Project, Orangitown, Karachi, Pakistan

An initiative aimed at promoting the construction and distribution of solar cooking boxes emerged in Orangitown (Karachi) in 1985 and 1986 in connection with the Orangi Pilot Project. The cookers in question are of the conventional rectangular type with a reflecting lid. Their main dimensions are 71 x 43 x 20 cm, and the empty unit weighs 20 kg. The somewhat modest maximum temperature of 114ºC is related to an equally modest efficiency rating of roughly 25 %. The technical details responsible for the meager performance could not be ascertained.

The production/selling price of US$ 30 would certainly not be too high, if the cooker's performance were commensurate. However, the maximum achievable temperature and performance data are taken as indications of excessively long cooking times, even for standard dishes, so that even small clouds would condemn to failure any attempt to use the box for cooking. In addition, the heavy weight of 20 kg makes it too heavy for a housewife to carry around.

After an initial lot of 30 such cookers was produced, a lack of acceptance led to discontinuation of solar-cooker activities within the Orangi Project. It may be safely assumed that the technical inadequacy of the Orangi cooker - in combination with other socio-eco-cultural factors - prevented it from finding acceptance, so that the project had to be scrapped.


FIGURE

Designation:

RERI-SEP Solar Cooking Box

Type:

Box type solar cooker with reflecting lid

Institutions:

Renewable Energy Research Institute (RERI) Energy Research Council (ERC) P.O.Box 4032, Khartoum Centre Special Energy Programme (GTZ) P.O.Box 8192, Khartoum, Sudan

The solar cooking box being promoted via the Special Energy Programme in the Sudan differs little from other solar cooking boxes in its basic design and construction.

Its basic features are the following:

- the box is rectangular and larger than, say, the Indian type
- it will hold more food
- it is nonetheless handy to use and easy to build
- it cannot be carried like a suitcase
- and is very good at cooking a national dish (foul, a dish based on beans)


FIGURE

Designation:

RIIC Solar Oven

Type:

Inclined box type solar cooker with 4 mirrors

Developed/tested by:

Rural Industries Innovations Centre (RIIC) Private Bag 11 Kanye, Botswana

The RIIC Solar Oven is a small box type solar cooker with an aperture measuring only 0.1 m² in area and an inclined cover with 4 roughly 30 x 30 m³ mirrors that help concentrate the incident solar radiation (so-called modified TELKES type).

The intensifying mirrors effectively increase the aperture size by a factor of 3. Test reports indicate that this arrangement makes a decisive contribution toward achieving higher temperatures.
Conscquently, it is reported that the comparatively small receiver can handle a large amount of food (6 kg). The maximum temperature is situated at about 175ºC. Nonetheless/ the box is reportedly not able to heat water efficiently; in fact, the efficiency amounts to a mere 10 %. A purchase price of US$ 40 is stated (1981).

About 30 of these models were built prior to 1981 and tested with various degrees of success - by rural families. In 1981, RIIC discontinued its research & development activities in the field of solar cookers, but, as of this writing, such activities were scheduled for renewal in 1986/87.

Designation:

SERVE Solar Oven

Type:

Inclined solar cooking box with reflecting lid

Dissemination by:

SERVE, P.O.Box 477 Peshawar, Pakistan

The SERVE Solar Oven is a rectangular solar cooking box with double glass cover, a reflecting lid, and approximately the double size of the Indian solar cooking box (width about 1 m).

In the 1987 model, several improvements have been realized, such as 20º inclination of the glass cover, Mylar foil reflector instead of glass mirror, fiber glass casing instead of sheet metal box, etc.
These measures reduced the weight and raised the efficiency of the oven.

As well, the lower glass of the cover was cut into two pieces, then sealed again with silicone to make the glass less susceptible to thermal tension. Thus, glass breakage was reduced from 15 % to less than 5 % /184/.

More than 4000 ovens of previous design types (square shape, horizontal cover, etc.) have been produced by 1987. For the improved 1987 type, a production of 3700 items p.a. has been planned.

Designation:

Suryamuklu Box Type Cooker

Type:

Box type solar cooker with reflecting lid

Produced/marketed by:

Centre of Energy Studies IIT Delhi

The Suryamuklu Cooker is a technically improved version of the Indian Box Type Solar Cooker and costs only about half as much to manufacture. It is substantially smaller, easier to use, and reportedly handles the same amount of food as an Indian Box Type Solar Cooker - even during periods of weaker insulation (or in less time). However, such information stands in contradiction to the box's very low maximum achievable temperature of 100ºC. No reliable performance data were available.

Production has either not yet started or only did so recently. No illustration/photo and no details on efficiency-enhancing measures were available as of this writing.

Designation:

22º Solar Cooker

Type:

Lightweight box type solar cooker

Developed/produced by:

I. de Klerk Harare Polytechnic Box 8074 Harare, Zimbabwe

The 22º Solar Cooker is a do-it-yourself cooking box made of glued cardboard. The name derives from the top's angle of inclination, which roughly corresponds to the geographic latitude of Harare
(18ºS).

According to the instructions, the user is free to size the box as desired. About 21 of these solar cookers have been built since 1982.


FIGURE

Designation:

ULOG-Tropical Solar Cooker, Model 85

Type:

Box type solar cooker with reflecting lid

Invented/developed by:

U. Oehler Morgartenring 18 CH-4054 Basel

Institution/marketed by:

U. Oehler

The ULOG-Tropical Solar Cooker, model 85, is a stable, robust, wooden solar cooking box designed for self construction or commercial fabrication by simple means.

The box is quadratic, covered with two panes of glass and a wooden lid with a reflecting foil on the inside to serve as an intensifier for cooking.

The sides of the inner box are slanted. The floor is painted flat black, and the walls are silvery reflecting. Used sheets of aluminum offset-printing foil are used for lining the inner box. In fact, the cooker design is based on the dimensions of those foils.

The stiff aluminum foil is bent such as to produce a thoroughly vapor proof box (important, because the insulation between the inner and outer boxes must be kept dry).

The wooden outer box is built for stability, not for light weight. By employing a few tricks of the trade, the assembly procedure was kept relatively simple without sacrificing any of the precision that is necessary to keep the cooker from losing heat through cracks and joints.

Well-illustrated and equally well-written assembly instructions are available in English, French and German (Spanish and Portuguese in preparation). The self-construction directions are included here as Appendix 2.

One drawback of the Model 85 Tropical Solar Cooker is its somewhat bulky shape due to stable construction. It has two handles but cannot be carried like a suitcase.

Its predecessor, the ULOG Tropical Solar Cooker, Model 80 (rectangular) could be carried like a suitcase, but it also had less space inside (for not more than 3 cooking vessels).

About 450 ULOG Tropical Solar Cookers (both models) have been built to date, 90 of these in Sudan, 60 in Senegal, 100 in Turkey, etc.

Designation:

ULOG European Solar Cooker

Type:

Box type solar cooker with inclined cover

Invented/developed by:

U. Oehler, Morgartenring 18 CH-4054 Basel

Institution/ marketed by:

U. Oehler

The ULOG European Solar Cooker is very similar to the ULOG Tropical Solar Cooker model 85 in design and construction, except that it has an inclined cover that allows the capture of enough insulation for cooking in moderate climates (where the sun has a lower altitude).

The ULOG cooking box has also been successfully tested in combination with a heat store consisting of 4 kg dark-colored stone. For the stones to accumulate enough heat for cooking, the device must be set up early in the morning. By the time the food is loaded into the box, the stored heat provides a relatively high starting power. It also serves well as a source of bottom heat for baking.

Some 1600 European Solar Cookers have been built (from kits and in home construction), most of them in Switzerland. A set of instructions can be obtained from U. Oehler.

Designation:

Four-mirror Cooking Box

Type:

Box type solar cooker with inclined aperture and four intensifying mirrors

Developed by:

Gabriel Rodriguez Jaque Seccion Fisica de la Construccion en IDIEM Universidad de Chile, Plaza Ercilla 847-2º Casilla 1420 Santiago, Chile

This solar cooking box measures 0.9 x 0.9 m² at the base and has a 0.49 m² aperture set at an angle of 30º. Four mirrors of equal size are hinged on around the top of the box to provide intensified radiation input.

Performance data: Depending on the season, location and momentary weather conditions, the device can collect up to 2.3 kWh per day, or enough to heat 18 1 of water from 15ºC (average spring temperature in Chile) to 95ºC.

The prototype for this cooker took first prize in a UNESCO-sponsored solar cooker competition in Chile /65/.

Reflector Cookers

Designation:

Advanced Reflector Cooker for Mali

Type:

Arbeitegemeinschaft fr Entwicklungsplanung (AE), Munich

Invented/developed by:

Deutsche Gesellschaft fr Technische

Project-executing agency:

Zusammenarbeit (GTZ) GmbH, P.O.Box 5180, D-6236 Eschborn

The Advanced Reflector Cooker was developed by AE Munchen /6/ in 1979/80 on the basis of the then latest knowledge concerning solar cookers. The design stage of development was preceded by a thorough global research effort involving detailed testing and evaluation of sundry solar cookers /5/. The advanced reflector cooker was supposed to emerge from knowledge gained on other devices and incorporate/optimize as many positive characteristics as possible.

The basic unit consisted of a short-focusing glass-reinforced polyester reflector shell in a swivel mount on a frame made of angle irons and/or square bar steel. The diameter of the reflector was 1.4 m, and the shell had cutouts on three sides, so that

1) the load-bearing frame and, hence, the entire reflector cooker would be much more compact and stable, and
2) the cook could use one of the cutouts to get close enough to the pot for comfortable handling and stirring.

Other improvements over prior-art reflector cookers included:

3) coincident pivotal point, center of gravity and focal point of the reflector shell, thus enabling easy adjustment without the danger of injury caused by the reflector swinging back;
4) the option of turning the shell away and out of the sun to prevent accidents and avoid unnecessary soiling of the reflective surface while manipulating the pot;
5) minimum convection losses, since the pot was protected from the wind by virtue of its position below the shell's rim;
6) the possibility of turning over the swivel-mounted reflector when the device was not in use in order to protect it from damage and dust accumulation;
7) an additional worktop (pot setdown area) provided at normal working height beside the shell;
8) the extreme importance attached to stability e.g. for stirring;
9) but with the cooker still remaining so light (10-kg reflector, 15-kg frame) and compact that it could be brought into position by one person, meaning that no supplementary adjusting axis, e.g. parallel to the earth's axis, was necessary;
10) the short focal length and the in-reflector position of the pot, yielding uniform heating from all sides at once and, hence, good efficiency (59 % at 770 W/m2);
11) the short focal length (0.26 m), permitting slight inaccuracies in the shape of the parabola without undue loss of efficiency.


FIGURE

The cooker's maximum capacity was 10 l of food.

The improved reflector cooker naturally had some of the same drawbacks with which all reflector cookers are afflicted, e.g. no utilization of diffuse radiation, necessity of standing in the sun for cooking, no chance of cooking under cloud cover, necessity of stirring due to (excessively) high temperatures, solar tracking, no attendance-free cooking, no warm food in the evening.

The aluminum-coated PE foil used for lining the inside of the reflector caused problems during prototype development as well as during the subsequent testing phase.

Alternative prototypes were made with wooden supports and papier mache reflector shells, but had to be rejected for lack of stability. Various experiments were conducted (some in the Federal
Republic of Germany and some in Mali) with smaller and larger diameters, other kinds of foil Lining, etc. Various design modifications were tried out in order to make the device as inexpensive as possible and more amenable to locally available materials.

The cost of producing a prototype in 1980 came to roughly US$ 100, or about as much as a large family in Mali would save on firewood in the course of 1 1/2 years by using a solar cooker.

About 16 such devices were built. Field testing of 10 units in Mali yielded negative results, due in part to organizing problems and in part to the inherent disadvantages of cooking with a solar reflector cooker in general.

In sum, it may be said that, thanks to the aforementioned improvements, the Advanced Reflector Cooker probably was despite its drawbacks - one of the best-conceived (from a technical standpoint) solar reflector cookers ever built (cf. Sun Basket and Falco S/C).

Designation:

Bottom-Heated Concentrator Box

Type:

Bottom-heated box-type solar cooker with spiral concentrator

Developed by:

A.M.A. Khalifa, M.M.A. Taha, M. Akyurt Mech. Eng. Dept., College of Engineering King Abdulaziz University P.O.Box 9027, Jeddah 21413, Saudi Arabia

The bottom-heated concentrator box is the result of the inventors' efforts to combine efficiency, ease of construction and operation, and simplicity of box type solar cookers with the suspension principle of the Telkes design and the concentrated energy flow and high power of concentrator s.c.s.

An insulated cooking box with glass bottom and side walls is suspended by a metal rod frame in the focal spot on top of a spiral metal sheet reflector of 1.13 m² aperture area. The pot is heated from the bottom and not - as in most of the other solar cooking boxes, from the lid. This makes heating more effective and cooking times shorter. High energy flux and good box insulation lead easily to temperatures about 180ºC. This enhances the possibilities of food processing: Grilling and roasting are possible.

A boiling test of 6 kg of water resulted in an efficiency of approx. 18.5%. The heating process does almost not depend on wind velocity as it does for most reflector cookers. Sun tracking of the concentrator does not affect the horizontal position of the cooking pot; food does not spill over.

Handling of the cooker is not easy, stirring the food during the cooking process is hardly possible, though it is necessary for many types of food when cooked in a concentrator (turn the reflector away, open the swinging box, open the pot, stir in the swinging device...). Research and development are going on (see ref. 191).


FIGURE

Designation:

External Concentrating Eccentric Axis Box Style Solar Cooker (EEB Cooker)

Type:

Collapsable eccentric reflector cooker

Institution:

Energy Research Institute Henan Academy of Sciences Huayuan Road Zhengzhou, China

The eccentric axis reflector is - geometrically speaking - a section of the lower part of a parabolic reflector dish. This allows all sun rays to be reflected from below to the bottom of the cooking pot, which results in improved cooking efficiency. The focal spot has an area of about 120 cm².

Several hundreds of small silvered glass mirrors are arranged in the parabolic reflector shell, which is divided in two or three parts and can be folded to become a handy transportable box.

The radiation collecting aperture is about 1 m², the effective power about 500 W on sunny days. Steaming, boiling, and frying is possible in the cooker. The working height (cooking pot level) is between 1 m and 1.25 m. It can provide a meal for 3-4 persons.

Other external concentrating eccentric axis solar cookers are made from fiberglass reinforced cement. Some have an aluminum foil reflecting surface. Nearly all of the solar cookers in use in China are of the external concentrating eccentric axis type (see figs. 13 and 32 c).

Designation:

Falco S/C

Type:

Parabolic reflector cooker

Developed/produced by:

B.B.K. J.D. Kaller Kleingsenget Haus 111, P.O.Box 11 D-8391 Neureichenau, W-Germany

The Falco S/C is a reflector cooker with a medium focal length. It is conspicuous by virtue of its simple construction.

The mirror is adjustably mounted in a simple frame equipped with a simple chain mechanism to keep it in line with the sun's altitude. The axis of rotation itself is stationary, running through the center point of the mirror and including a holder for the pot (or two) at the focal spot.

The metal mirror is an industrial, single-piece product with a diameter of 115 cm. The polished surface has a reflectance of about 80 %. A significant share of the incident radiation is reflected diffusely and therefore unavailable as cooking energy.

The complete unit, including mirror and frame, weighs less than 20 kg. It comes with two shallow flat-bottomed pots (3 1 and 4 1) that can be used simultaneously, i.e. while food is cooking in one pot, the other pot can be set on top of the first for slow cooking, warming up or keeping food warm.

Efficiency of between 20 % and 35 % for global radiation intensities above 800 W/m2 and wind velocities of mostly over 1 m/s were measured at the RERI Institute in Karthoum. The modest efficiency is probably due to diffuse reflection (= energy loss) by the parabolic reflector, in combination with the fact that the pots project beyond the spherical segment and are therefore exposed to the wind. In addition, the absorptance of the pots appears less than optimal.

In quantities of 100 or more, the Falco S/C costs US$ 240 apiece. To the extent possible, regionally dependent "special options" are allowed for at the time of production.

Designation:

Fixed-Focus Hybrid Solar Cooker

Type:

Hybrid wood and solar off-axis parabolic reflector cooker

Invented/ developed by:

W. Scheffler c/o German Water Team P.O.Box 19512 Nairobi, Kenya

Fixed-focus solar cookers are composed of one or more off-axis parabolic mirrors and a stand for one or more pots with a secondary mirror underneath which reflects the concentrated radiation coming from the parabolics, to the bottom of the cooking vessel(s).

Furthermore, these cookers are hybrid S.C.S. This means that, whenever there is not sufficient direct solar radiation, the secondary reflector under the cooking pot can be removed, and the pot can be heated by a conventional wood fire.

3 fixed-focus hybrid solar cookers have been established in Kenya till 1988, one family-sized device and two big ones for institutional cooking at schools. The latter are equiped with an additional tank for warm water generation.

The technical data can be taken from the following table:


FIGURE


FIGURE

Designation:

Sobako 1/SOBA 1

Type:

Linear parabolic reflector cooker

Invented/developed by:

N. Kuhnert, D-8172 Lenggries Institutions: GTZ, Nairobi University (1977/78)

Sobako 1 (German acronym for "Solar Baking and Cooking" Device) is a linear-parabolic reflector cooker, i.e. one that focuses direct solar radiation along a focal line.

The reflector comprises two plywood paraboloids with a reflecting chrome-nickel steel lining. The oven is arranged along the focal line; it is surrounded by glass to minimize thermal losses during cooking and baking.

The reflector is swivel-mounted on a wooden support and held in place by a counterweight. It has to be realigned with the sun by hand every 15...20 minutes.

The cooker's span width comes to 2.46 m, the width of the reflector/oven is 84 cm. The aperture figures to just over 2 m². The inside diameter of the oven is ll.5 cm. The cooking capacity is listed as adequate for preparing a rich European midday meal for four or for baking two 1-kg loaves of wholegrain bread at once.

When not in use, the cooker can be folded up to protect the internals and the reflector from the effects of wind, rain and dust. A cover is also provided.

Considering the unit's cooking/baking capacity (2 kg bread or food) its collecting area of roughly 2 m² and span width of nearly 2.5 m appear quite large, and its mechanical design is relatively elaborate (numerous different parts). Additionally, its projecting structure necessitates a broad, heavy, revolving foot.

At 170...180ºC, the reported maximum achievable temperature inside the oven is rather modest, considering how large the collecting area is. This corresponds to other reports, according to which the special steel reflectors are quick to dull, after which they are heated by the sun and display poor reflectance (estimated 50 %).

According to the reports (with the exception of the designer's), Sobako 1 was expensive, complicated, unfunctional and awkward to use. In other words, it was inappropriate.

Some of the points drawing criticism were that

- even two people had a hard time transporting the device,
- setting it up required so much technical knowledge, carefulness and physical strength that the users were hardpressed to accomplish the task,
- the oven had to be taken apart for cleaning,
- the insulating glasses surrounding the oven were inadequately mounted,
- the mirrors had a blinding glare,
- the cooking vessels intended for use in the oven were prone to rusting,
- the oven was 1 cm too small for standard-size pots,
- even hazy weather (no clouds!) made cooking impossible,
- the cooking capacity was too small for a "normal family" with 4 to 8 children,
- the device had to be made of high-quality waterproof plywood, with no possibility of using alternative materials,
- various parts of the cooker tended to swell up and become distorted due to the effects of heat and humidity,
- the oven was heated unevenly,
- the necessary tracking interval of 10 minutes required someone's constant attention,
- the device hardly ever had enough direct radiation to work efficiently in Nairobi's constantly smoggy atmosphere,
- it was incompatible with local eating and cooking habits, and, finally,
- it "disqualified itself by reason of its exorbitant cost".

Sobako 1 was succeeded by a simplified type called Soba 1. Each of the two prototypes built at the University of Nairobi cost well over US$ 420. Assuming otherwise favorable conditions and production in considerable quantities, a prime cost of US$ 252 was regarded as achievable. Payback periods of 3.5 years based on fuel savings and 1.8 years based on the added advantage of home-baked bread were calculated. Partial improvement of the reflecting liner yielded higher efficiency.

Plans were drawn up for series production and dissemination, and recommendations concerning a subsidization program were elaborated for submittal to the government. A successor type Soba 2 was never built. The high cost of construction, awkward handling (even of the simplified Soba device), the natives' habit of cooking their main meal in the evening, the cooker's dependence on direct radiation and the practically permanent haziness in and around Nairobi were presumably the main reasons why Sobako 1 and Soba 1 never got past the prototype testing stage.


FIGURE

Figure 34: Sobako 1 (at left) and Soba 1 (at right)

Designation:

Sun Basket

Type:

Reflector concentrator with semi automatic tracking feature

Invented/developed by:

Dr. M. v. Oppen, Steinwaldstr. 25, Hohenheim-Stechfeld, 7000 Stuttgart 70, Fed. Rep. of Germany

Manufactured/marketed by:

Mr. Kiram Chandwalker, Stiletto Engineers Shed No. F5/A, IDA, Kukatpally Hyderabad 500 037, India

Sponsored by:

Sonnenkorb Association Barckhausenstr. 47 D-2120 Luneburg, Fed. Rep. of Germany

Sun Basket is a large, relatively short-focussing parabolic reflector in which a pot is held in the focal point by an attachment on an iron rod. The parabolic reflector measures about 1.3 m in diameter and is made of glass-reinforced plastic (GRP, fiberglass) over a masoned mold. It is relatively expensive to make and accounts for most of the device's unit price. The reflective liner consists of vapor-deposited aluminum foil, which must be replaced periodically.

Earlier Sun Basket models had parabolic reflectors made of curved strips of bamboo - hence the name. But problems with stability and accurate focussing made it necessary to discontinue that very inexpensive type of construction.

A different Sun Basket model has an ingenious solar tracking mechanism that operates according to the water-clock principle in keeping the reflector almost exactly aligned with the sun. The pot hangs on a sort of jib over or in the bowl of the reflector. While this arrangement is quite elegant, it also has the drawback of having to hold the pot by hand for stirring its contents - and manipulating the "floating" pot in the cooker's focal point poses the danger of injury by burning. That model has also been discontinued.

The reflector is quite large and has no recess that would let the cook get closer to the pot (as the Advanced Mali Cooker has). Having to lean over the basket to stir food, for example, is not only tedious but also hazardous (blinding, burning). In addition, the cook must stand out in the sun.

About 20 such Sun Baskets have been built since 1985. Work is still being invested in making the reflector more reflective and less expensive.


FIGURE

Designation:

Sungril

Type:

Collapsable transportable aluminum reflector cooker

Manufacturer:

Interel S.A., Clarastr. 2, CH-4005 Basel, Switzerland

Sungril is an industrially manufactured, light-weight collapsable reflector cooker of 1 m diameter which can easily be transported by hand in a 50 x 36 x 12 cm briefcase box. The pot holder is fixed on a vertical stand in the 15 cm diem. focal spot. It does not move when the reflector is adjusted to track the sun's position. All elements are made from aluminum and free of corrosion.

The light-weight construction is easy in handling and transport, but not very stable when stirring of the food is required or the wind is blowing.

Sungril has been produced for 10 years and is patented in Switzerland and the USA.


Figure 35 a: Sungril

Designation:

Suryakund

Type:

Small, nonfocussing elliptic reflector cooker with glass cooking vessel

Contact:

CORT (cf. Indian Box Type Solar Cooker) and others

The Suryakund is the simplest known type of reflector cooker. It comprises four parts:

- an asymmetrical reflector bowl made of plastic, with an aperture size of approx. 0.2 m²,
- an asymmetrical support that will accommodate the reflector bowl in practically any position,
- cooking utensils: two black pots (1-liter and 2-liter capacity) that fit together (nest) and have tight-fitting lids, and
- a glass globe to reduce heat losses.

The food is put in one or both pots. The pot(s) is/are placed in the globe, which completely surrounds the pot(s) and retains heat by virtue of its greenhouse effect. Then, the ensemble is placed in the reflector's concentrated field of solar radiation. The solar rays penetrate the globe to uniformly heat the bottom part of the cylindrical pot.

Since the cooking vessel is not all that small in relation to the reflector bowl, pinpoint concentration is unnecessary. Consequently, the device does not require precision adjustment. With a view to minimizing heat losses during cooking, the vessels should not be opened, and the food should not be manipulated (by stirring, etc.).

Suryakund is one of the simplest solar cookers in the world. Made in India, it is sold in large numbers. Its drawbacks are its small area and small capacity, i.e. it doesn't readily accept more than one pot at a time, and it mostly makes use of direct solar radiation only.

Designation:

Table-type Reflector Cooker

Type:

Reflector cooker with its own worktop

Developed by:

Roger Bernard, Laboratoire d'Energie Solaire, Universite Claude Bernard, 43 Boulevard du 11 Novembre 1918, F-69622 Villeurbanne, Cedex, France

The Table-type Reflector Cooker is a typical reflector cooker with no means of heat accumulation, but which avoids some of the drawbacks of conventional reflector cookers.

The "hearth" is stably integrated into a worktop standing on four legs about 70 cm above the adjustable reflector. This extensively precludes problems concerning pot-mount stability and general handling.

The rectangular parabolic reflector consists of 35 flat mirrors. Its maximum thermal output in bright sunshine peaks below 400 W. That amount of energy generates a temperature well in excess of
200ºC at the bottom of an empty pot. For a more detailed description see refs /11/, /12/, /183/.

This cooker has a noteworthy adjusting mechanism: a small positioning mirror reflects onto a focusing plate used for checking the momentary quality of alignment. When the unit is not needed for cooking, it can be put aside with its reflector turned vertical or upside down.

Apart from the aforementioned merits, this cooker also has some of the drawbacks of conventional reflector cookers: the cook has to stand out in the sun, and cooking is impossible without adequate direct radiation, which excludes cloudy days and the morning/evening hours.


FIGURE

Designation:

Tube Solar Oven

Type:

Linear parabolic reflector cooker with cooking/baking tube

developed by:

K. Wippermann, Inst. f. Angewandte Physik, Universitat Karleruhe, Kaiserstr. 12, D-7500 Karlsruhe, FRG

The Tube Solar Oven is an adjustable linear parabolic 0.6 x 1.0 m² reflector with a 15 cm diem. glass baking/cooking tube containing a long 8 liter food vessel in the focal Line. At first glance, it has the appearance of a comfortable arm chair.

The main features of this s.c. are:

- The mirror is a one-dimensional parabolic reflector which is easy to manufacture.
- Solar tracking is simple and necessary only every 40 min because of the relatively wide acceptance angle of the tube (15º)
- The glass tube acts as a collector, collects direct, diffuse, and indirect concentrated radiation from all sides and therefore does not need any insulation.
- The parasitic (= unwanted) heat capacity is low.
- The energy flow to the food vessel is high enough to make brown cake and bread, but low enough to make stirring unnecessary.
- Normal cooking pots cannot be used; special cooking vessels are required.

Optimization has lead to a medium-cost low-capacity' high efficiency construction with 46 % efficiency in the water heating test. Till 1988, one prototype was constructed; research is going on in the directions of heat accumulation and steam generation for sterilization.

Designation:

Valparaiso Reflector Cooker

Type:

Parabolic reflector with mirror mosaic

Developed by:

Jaime Lopez C., Luis Seguel R., Juan Jerez I., Universidad de Diseno, Av. El Parque s/N. PYA. Ancha, Valparaiso, Chile

This cooker is characterized by extreme simplicity. A well conceived set of instructions makes it relatively easy to build and use. Building is possible at home (self-made) or by local craftsmen.

The parabolic shape of the shell is achieved on a stationary adobe mold with a reinforced-concrete core. The inside of the parabola is studded with numerous fragments of mirrors. The cooking pot rests on a wire grate situated at the cooker's focal point. The grate hangs on the two arms of a supporting fork such that the pot always maintains a horizontal position. The reflector measures 1.20 m in diameter, corresponding to an aperture area of roughly 2 m² and a depth of about 0.3 m. Depending on how carefully the parabola was molded and how accurately the mirror fragments were installed, the diameter of the focal spot amounts to about 10 cm.

This model took the second prize in Chile's 1987 solar cooker competition /65/. One of its main advantages is that it can be built at little expense (but with a lot of work). Unfortunately, in addition to having the standard disadvantages of conventional reflector cookers, it is also somewhat awkward to operate: to stir the food, for example, the cook has to bend over the reflector to reach the swaying pot at the device's focal spot.

Designation:

VIAX Solar Cooker SK 10

Type:

Reflector cooker

Invented/developed by:

D. Seifert, Siedlungsstr. 12 D-8265 Neuotting, FRG

The VIAX Solar Cooker is a customer-assembled metal reflector cooker with an especially sophisticated type of tracking mechanism (see figs.)

The reflector is vertically (2) and horizontally (3) adjustable. As it is turned by hand around its vertical axis in the course of a day (azimuthal adjustment), the proper horizontal adjustment with respect to the sun's elevation is achieved automatically by way of a fixed guide element (5), the constant length of which ensures that the reflector is aimed skyward when pointing to the south, gradually tilting downward when moved toward the east or west (depending on the time of day). Altogether, this corresponds to revolution around a virtual axis (hence the name) - the polar axis.
A seasonal adjusting mechanism (7 and 8) enables accommodation of seasonal variation in solar altitude. Tracking is necessary only every 25 min.

Appropriately, the VIAX mechanism is installed behind the reflector. The journal "Sonnenenergie" 10, 3, 1985 includes a detailed description. For checking and testing the food, the SK 10 parabolic mirror can easily be turned away without moving the cooling vessel in the pot holder. Consequently, there is no danger of dazzling, blinding, or burning. The rest and night position of the reflector is upside down for dust protection. Several VIAX cookers have been tested so far.


FIGURE

Heat-accumulatinq solar cookers

Designation:

Heat-accumulating Steam Cooker

Type:

Linear-parabolic reflector cooker with heat storage in steam and water

Developed by:

David R. Mills, School of Physics, University of Sidney, Australia Mao Yin Qiu, Gansu Natural Energy Research Institute, Lanzhow, Gansu Province, China

Compared to most other solar cookers, the solar cooking stove, or solar steam cooker, is a relatively sophisticated device that unites some of the characteristics of reflector cookers, steam cookers and heat-accumulating solar cookers.

The principle-of operation: Water is heated and subsequently evaporated in an evacuated glass absorber tube situated in line with the focal line of a linear parabolic reflector. The water is transported to a barrel-shaped, water + steam-filled heat store by natural (thermosiphon) circulation; as more water evaporates, the pressure increases steadily. The top of the cylindrical heat store serves as a hot plate that imparts the heat of the steam to the cooking pot/food. The process involved is similar to that of a heat pipe: steam condenses on the relatively cool surface of the pressure vessel and gives off latent heat (of condensation). That causes the pressure to decrease, consequently allowing more water to evaporate, and the process continues.

According to Mills and Mao Yin Qiu, the maximum achievable temperature is about 170ºC. The system is designed for a corresponding watervapour pressure. The device can store about 2 kWh per day of good sunshine, or enough to prepare about 50 sausages or 50 fried potatoes. At 4 kW, the starting power is relatively high. The linear parabolic reflector has to be adjusted to the seasonal solar altitude 10-20 times a year. Daytime solar tracking is unnecessary.

The reflector area is 2 x 0.9 m², and the storage volume is 23 l.

A solar cooking stove in combination with a linear reflector constitutes a totally new combination of ideas and action principles in solar cooking. However, it is comparatively expensive. One prototype has been built to date. A detailed description along with test data is contained in /110/.


Figure 41: Heat-accumulating Steam Cooker

Designation:

Hybrid fixed-focus cooker

Type:


Invented/developed by:

W. Scheffler, c/o Oehler, Norgartenring 18, CH-4054 Basel, Switzerland Ministry of Energy and Resources Nairobi, Kenia

Big combined solar and fuelwood cooking device for institutional use in schools, etc.

The cooker consists of 2 parabolic reflectors and a central unit with 2 glass windows of 0.6 m diameter. Reflected sunlight from the parabolics passes the windows and is reflected by two cured aluminum secondary reflectors to strike sides and bottom of the pot.

In case the sun is not shining, the secondary reflectors can be removed and replaced by a fuelsaving woodstove under the pot for conventional cooking.

Technical data:
2 parabolic reflectors of 3.8 m² each with 21 flat segments in an ellipitic frame of 2 m x 2.7 m.
Total aperture 5.5 m², focal length 2.0 m.
Focus diameter 0.6 m.
Pot diameter 0.5 m, volume 54 1.
Effective heating power 1.62.2 kW (at clear sunshine 900 m above sea level, depending on pot temperature), heat losses of the pot at 100ºC approx 0.5 kW.
Estimated thermal efficiency 35...45 %.

Prototype constructed in 1987; a second item with 2 pots of 100 l each is under construction.

Designation:

ISE Solar Cooker with Integral Oil Storage

Type:

Heat-accumulating solar cooker with flat collector and fluid heat store

Developed by:

Fraunhofer-Institut fr Solare Energiesysteme (ISE), Oltmannstr. 22, D-7800 Freiburg, FRG

Manufactured by:

Cambra GmbH, D-7260 Calw, FRG


The ISE Solar Cooker with Integral Oil Storage is a heat accumulating type with numerous virtues. First of all, it will cook food anytime day or night, has-medium-high temperatures and reasonable cooking times. Additionally, the cook needn't stand out in the hot sun; indeed, the "range" part can even be installed indoors. The device exploits both direct and diffuse radiation, requires no solar positioning, allows access to the pot during cooking, and is easy and simple to operate. On the other hand, those numerous advantages are obtained at the cost of technical complexity and a very high price: Units from short-series production cost between US$ 1250 and US$ 1500. Consequently, though an early version with a 1.9-m² aperture held 5 kg food, and the capacity of a later model is 10 kg, the cooker is still disproportionately expensive for its size.

A closed-loop heat cycle in which oil transfers thermal energy from a highly efficient flat plate collector to an elevated heat-storage vessel (oil storage), serves as the device's functional basis. The thermal oil serves both as the energy vehicle and the heat store. Circulation between the collector and the oil storage tank requires no pump, since it is based on natural circulation, i.e. the thermosiphon effect. The cooking pot is placed in a cavity in the top of the oil storage tank. Heat losses are minimized by an insulating cover that can be raised during cooking, thus allowing access to the food for stirring etc. without wasting too much heat.

For an ambient temperature of 25ºC, the cooker can achieve a maximum temperature of 160ºC. The 50-1 oil store will retain enough heat to cook two consecutive batches of food (3.5 to 4 kg each).

About 20 prototypes have been built to date and are being tested in various parts of the world. Ongoing developments continue to aim for larger units (for use at the institutional level) with more efficient collectors, local producibility and, above all else: lower prices.


FIGURE

Designation:

Solar Hot Plate Cooker

Type:

Heat-accumulating solar cooker with radiation-intensifying reflectors in split construction

Developed by:

Bomin Solar, Industriestr. 8, D-7850 Lorrach, FRG

The Solar Hot Plate Cooker is the only solar cooker ever to be strictly designed as a split device. It consists of two units: one for collecting the radiation and converting the energy into heat, and the other for storing the heat and doing the actual cooking. Collecting the radiation and cooking are therefore temporally and spatially separate functions, the advantage being that meals can be cooked indoors and whenever so desired. The collector essentially consists of a rather large box type solar cooker into which the solar radiation impinging on a reflecting funnel made of 8 flat mirrors is directed at 3.5 fold intensity; passing through 3 sheets of fluoropolymer foil, the incident energy heats up a set of selectively coated iron slabs. The whole device is mounted in a tracking mechanism that allows hourly repositioning with the aid of a crank.

When the slabs are hot enough (200...300ºC), a transport box is used to remove them from the collector box through an opening in the side and transfer them to the indoor heat-storing stove. The latter is a very good heat retention. For cooking, one merely folds it open and places the pot(s) on the hot slabs of iron. The device has a very high starting power. The contact between the hot slabs and the cooking pot can be varied to yield a sort of heat control for fast or slow cooking.

In addition to its main advantage Of making the cooking process temporally and spatially independent of energy collection, the Solar Hot Plate Cooker also has some interesting design details: the funnel of mirrors reflects the radiation at Brewster's angle, i.e. with almost no loss; two-dimensional solar tracking occurs once per hour via crank, with a sighting instrument provided for easy checking of the adjustment; the heatproof plastic cover costs less and yields higher efficiency than two panes of glass.

The drawbacks of a Solar Hot Plate Cooker are its elaborate design and somewhat complicated handling and operation. Controlling the thermal output of the cooker requires experience and a delicate touch. If food boils over or a pot is spilled, subsequent cleaning can be problematic. For this reason, an additional joint around the pot was provided to catch the food spilled over.

At a large-series price of US$ 400 (individual local production: US$ 800, the Solar Hot Plate Cooker is less expensive than other heat-accumulating cookers, but still too expensive to gain widespread acceptance in developing countries.

Designation:

Heat-pipe Storage Cooker

Type:

Heat-accumulating solar cooker with evacuated tube collectors

Invented by:

Stoy/Pohlmann

Developed/manufactured by:

AEG, Geschaftsbereich Hausgerate Muggenhofer Str. 135, D-8500 Nurnberg 80, FRG

This solar cooker consists of a heat-storage block under a hot plate and lid, with an inclined bank of 6 evacuated collector pipes in a reflective tray with a transparent protective cover, all arranged as shown in figure 45.

The evacuated tube collectors function according to the heatpipe principle (modified Philips collector tube); the heat collects at the head of the tube, from where it is directly transferred to the heat store without need of a special circulating loop.

The heat store is made of magnesite (the prototypes contained 50 kg aluminum). The combination hot-plate + cooking cavity arranged above the heat store takes its heat through a second short high-temperature heat pipe. Heat control is provided by a valve in the heat pipe.

The device has essentially the same advantages as those inherent to any heat-accumulating type of solar cooker: cooking is possible anytime day or night, and the cook needn't stand out in the sun. Additionally, it is smaller and lighter than the other two types of heat-accumulating solar cookers described above and can be pulled along on its two wheels (or skids if necessary). Though still in the development stage, its external design is already practically perfect.

The hot plate requires the use of flat-bottomed pots (almost unknown in developing countries); since the pot is only heated from the bottom, the hot plate must develop a higher temperature than would be necessary in a cooking cavity or box-type solar cooker, and the size of the hot plate leads to substantial heat losses around the pot. Under such conditions, the collecting/storage capacity is relatively modest: the heat store can deliver about 1 kWh for the useful temperature range of 140...220ºC.

To make use of at least part of the waste heat, the insulated cover has been replaced by a dome that covers cooking plate, cooking mould, and cooking pot.

According to the manufacturer, the device could be improved by installing evacuated collector pipes that function at higher temperatures and by intensifying the transfer of heat from the heat pipe to the heat store.

The two main problems are a) inefficient transfer of heat to the cooking vessel and b) sophisticatedness and price (approx. US$ 1750, or half that much if mass-produced), the latter factor having the effect of precluding use of the Heat-pipe Storage Cooker in developing countries.


FIGURE

Convective solar cookers

Designation:

Convective Solar Cooker (CSC)

Type:

Large-scale solar range with air convection

Invented/developed by:

Synopsis, Institut de Recherche Alternative, Domaine de Belbezet, Route d'Olmet, F-34700 Lodeve, France

The Synopsis Convective Solar Cooker has an aperture area of nearly 10 m². Its design capacity is 182 l, corresponding to a warm meal for roughly 200 people.

Heat transfer is effected mainly by air convection, with the air heating up in a flat plate collector, ascending to the elevated cooking pots and heating them from all sides Insulation onto the flat plate collector is intensified by a reflecting mirror.

The cook stands on a platform to tend 7 (I) large vessels (each with a capacity of 26 l) above the large-area collector. The platform is shaded by a porch roof.

In addition to cooking, the CSC range also heats 150 l of water to approx. 80ºC.

The device is a big and expensive construction, but is relatively easy to use (by a professional cook). The "classic'' disadvantages like standing out in the sun, handling pots in the unit's focal spot, etc. have been done away with. On the other hand, the size of the cooker and its stationary installation preclude optimized solar tracking for a particular time of day (morning or afternoon).
Though air is a poor heat conductor, the device is reported to be relatively efficient ([n] = 43 %).

The only large-scale CSC ever commissioned is in operation at a school in the Sudan, where it is used to cook for 250 people. It is intended to install more, improved CSCs in other places.

Since the device was designed for use by professional cooks at the institutional level, the question of acceptance by the users reads differently than in the case of family-size cookers. An institution can be expected to base its investment decisions on more long-range planning than an individual family would, and the cooking staff can be expected to have been both properly familiarized with the cooker and trained in its proper use. Under comparable circumstances, some types of solar cookers must be regarded as acceptable, while they would have no chance at all of being accepted by individual families.

Only one CSC has been built to date, and it is still undergoing field and acceptance testing. There were also CSCs of smaller size developed by Synopsis. The series production price 20 to 500 units per month of the "family model" has theoretically been calculated to US$ 152 to 582, for the 50 persons module US$ 223 to 698.


FIGURE

Designation:

Steam Immersion Heater Solar Cooker

Type:

Closed-loop steam s.c. with high efficiency collector

Developed by:

V. Heinzel, J. Holzinger Institut fr Reaktorentwicklung, Kerforschungszentrum Karlsruhe, Postfach 3640, D-7500 Karlsruhe, FRG

Solar collector tube, steam separator, riser and downcomer tubes, air well, siphon tubes, and immersion heater together make up a closed circuit, partly filled with water. Solar radiation striking the collector tubes generates steam which rises up to the immersion heater inside a cooking pot. Heat passes from the steam to the food, thus condensing the steam to water. A siphon tube evacuates the condensate via the downcomer back to the collector tube. An additional siphon breaker tube is installed to avoid water-air pulsations in the system.

As the immersion heater is attached to the circuit with flexible hose pipes, it can be used for indoor cooking in any type of cooking pot. Providing that the pot is located 50 cm higher than the collector, the circuit runs spontaneously without any auxiliary energy.

Allowing indoor cooking with different types and sizes of cooking vessels, the system is extremely flexible and comfortable. Because of the closed circuit, higher pressure and higher temperatures are possible than with atmospheric steam cookers, leading to better heat transfer and shorter cooking times.

This s.c. is a high-cost solution requiring high efficiency collectors (e.g. evacuated tubular collectors) and an accurately assembled immersion heater and siphon tube system.

Till 1988, one prototype was realized to demonstrate the physics of the self-acting siphonic heat transfer system. (See also ref. /l95/).


FIGURE

Comparative Survey


FIGURE

FIGURE

FIGURE

FIGURE

FIGURE

TO PREVIOUS SECTION OF BOOK TO NEXT SECTION OF BOOK