This chapter characterizes the four basic types of solar cookers, describes the physical fundamentals of solar cooking, and scrutinizes solar cooker projects in India, Kenya, Mali, Pakistan and the Sudan. Subsequently, it examines the present state of the art and dissemination. The emphasis is on technical aspects.
2.1.1 Box-type solar cookers
2.1.2 Reflector cookers and
concentrators
2.1.3 Heat-accumulating solar cookers
2.1.4 Steam cookers
Solar cookers and ovens, absorb solar energy and convert it to heat, which is then used for cooking or baking various kinds of food.
Differentiation is made between four basic types of solar cookers:
- box-type solar cookers
- concentrating-type or reflector
cookers
- heat-accumulating solar cookers
- solar steam cookers.
There are also some hybrid types, e.g. heat-accumulating solar
cooking boxes with reflectors. Various types of solar cookers are described in
the appendices.
2.1.1
Box-type solar cookers
Box-type solar cookers - or more precisely: solar cooker-cumovens (or solar ranges) - consist of a well-insulated box with a black interior, into which black pots containing food are placed. The cover of the box usually comprises a two-pane "window" that lets solar radiation enter the box but keeps the heat from escaping. This in addition to a lid with a mirror on the inside that can be adjusted to intensify the incident radiation when it is open and improve the box's insulation when it is closed.
Figure 1: Box-type solar cooker
The main advantages of box-type solar cookers are:
- They make use of both direct and diffuse solar radiation
-
Several vessels can be heated at once
- They can double as an oven (not for
crispy baked goods)
- They are light and portable
- They are easy to
handle and operate
- They needn't track the sun
- The moderate
temperatures make stirring unnecessary
- The food can be kept warm until
evening
- The boxes are easy to make and repair using locally or regionally
available materials
- They are relatively inexpensive (compared to other
types of solar cookers).
There are some disadvantages too, of course:
- Cooking must be limited to the daylight hours
- The
moderate temperatures make for long cooking times
- The food is not
accessible for stirring, turning, etc.
- The glass cover causes considerable
heat losses
- Such cookers cannot be used for frying or grilling.
Thanks to their simple construction, relatively low cost, uncomplicated handling and easy operation, solar cooking boxes are the most widely used type of solar cooker. Their practical significance warrants closer examination.
There are all sorts of box-type solar cookers: mass-produced, hand-crafted, do-it-yourself types ..., with shapes resembling a suitcase or a wide, low box, and stationary types made of clay, with a horizontal lid for tropical and subtropical areas or an inclined lid for more temperate regions. Standard models with aperture areas of about 0.25 m² are the rule for a family of five, and larger versions measuring 1 m² and more are available for schools, clinics, etc.
Many technically mature solar cooking boxes intended for use in the tropics (high solar altitude) are of shallow design. The inner box, usually made of blackened sheet metal, must be watertight - or better: vaporproof - with respect to the insulation. The bottom must also be painted (or otherwise coated) flat black to achieve good absorption; the sides of some types (Indian, for instance) are also black, while other designs prefer reflecting walls (ULOG cookers).
Since the heat absorbed by the inner box needs to be conducted to the area beneath the cooking pots, the best choice of material is aluminum, because it is a very good heat conductor, additionally, aluminum is good for reasons of corrosion prevention, i.e. iron sheet boxes, even galvanized ones, could not stand up indefinitely to the hot, humid conditions that are created inside during the cooking process. Sheet copper is prohibitively expensive. Oehler uses old aluminum offset-printing plates as an inexpensive source of material for the inner boxes of his ULOG solar cooker/ovens /130/.
No metal parts should extend to the outside around the top rim of the inner box: thermal bridges must be avoided. The insulation may consist of spun glass, rock wool or some natural material like residue from the processing of peanuts, coconuts, rice, corn, etc. Whatever kind of material is used, it must be kept dry.
The cover must consist of two panes of glass with a layer of air between them. The pane-to-pane clearance usually amounts to 10...20 mm. Recent experiments have shown that a honeycomb structure of transparent material that divides the interspace into small vertical compartments can substantially reduce the cooker's heat losses, thus increasing its efficiency accordingly /145/. The inside cover pane is exposed to substantial amounts of thermal stress, for which reason tempered (safety) glass is frequently used; otherwise, both panes may consist of normal window glass with a thickness of about 3 mm.
Set in a (wooden) frame, the two panes of glass yield a tillable cover. The glass must be held flexibly and free of stress in the frame; the joint must be heat-resistant up to 150ºC (e.g. silicone). The double-pane cover must fit tightly around the top of the box in order to trap as much heat as possible, thus maximizing the cooker's efficiency and minimizing the required cooking time:
The outer cover, or lid, of the solar cooking box always serves as a reflector to amplify the incident radiation. The reflecting surface may consist of an ordinary glass mirror (heavy, expensive, fragile, but easily obtainable anywhere), plastic sheet with a reflecting coating (Mylar, Tedlar, etc.; cheap, but not very durable and hard to find), or a metal mirror (unbreakable, but dulls easily). In an emergency, even foil from empty cigarette packs will do the job.
A simple catch device keeps the lid in the proper reflection position (depending on the altitude of the sun), and a latch is used to close it like a suitcase. Both the positioning mechanism and the latch must be fail-safe to the extent that they cannot get between the lid and the cover and break the glass. The closed lid - with or without a layer of insulation behind the mirror - keeps the food warm. The hinges for the reflecting lid must be strong enough and carefully mounted to ensure that they stand up to everyday use: the hinges are a frequent weakpoint. Instead of metal hinges, strips of leather or textile can be used (cf. Appendix 2, item no.13).
The outer box of the solar cooker may be made of wood, glass-reinforced plastic (GRP) or metal. GRP is light, inexpensive and fairly weather-resistant, but not necessarily stable enough for continuous use. Wood is more stable, but also heavier and less weather-resistant. A metal case "aluminum" with wooden bracing offers the best finish (the cooker's image and appeal are important acceptance factors) and is adequately stable with regard to mechanical impact and the effects of weather. An aluminum-clad wooden box is the most stable of all, but it is expensive and time-consuming to make, in addition to being heavy.
All box-type solar cookers have one or more handles on the sides or front; some have castor wheels. Still others are of the stationary type, i.e. home-built clay structures that are extremely inexpensive (Dhauladhar solar cooker), but which have the disadvantage of not being able to track the sun during the morning and afternoon hours. In addition, special precautionary measures must be taken to get the cooker through the rainy season unscathed.
The basic shape of all solar cooking boxes is rectangular (with the exception of the Franco-Indian one-pot rice cooker, which is round). A quadratic design minimizes heat losses through the joints and side walls, while a rectangular design has other advantages: the hinges are exposed to less wear & tear and the long reflecting lid makes solar positioning less critical (cf. ATRC solar cooker in Appendix 1).
Suitable cooking pots are important prerequisites for successful solar cooking. Shallow flat-black aluminum vessels with flat bottoms that make good thermal contact with the metal floor of the inner box are most suitable. The pots used in India are no more than 7 cm high and have flat, tightly closing lids. According to Indian experience, the bottom pane of glass in the cover should be about 2 cm above the lid of the pot. A smaller clearance would tend to aggravate losses by heat conduction, and a larger clearance would promote convection with largely the same results.
According to Parikh /136/, there are three ways to effect heat transport to the food. These are:
1. absorption by the sheet-metal bottom, heat conduction into the area below the pot, heating of food from below
2. absorption of solar radiation by the lid and other exposed parts of the pot
3. heating of the food by convection of the hot air surrounding the pot.
Oehler reports that a grid supporting the pot 12 cm above the box bottom, increases convective heat transfer considerably and improves the efficiency of the cooking box. If the cooking vessel contains only a shallow layer of food, heating occurs quite rapidly, and the food requires less cooking time. The optimal pot filling height is 23 cm, or roughly 1 inch.
The capacity of a normal box-type solar cooker with a 0.25 mº area of incidence (aperture) amounts to 2 kg solids, i.e. about 4 kg ready-to-eat food, or enough to feed a family of five.
The inside of a solar cooking box can reach a peak temperature of over 150ºC on a sunny day in the tropics; that amounts to a thermal head of 120 K, referred to the ambient temperature. Since the water content of food does not heat up beyond 100ºC, a loaded solar cooker will always show an accordingly lower inside temperature.
Figure 2 shows a typical time history of temperature in a food-containing solar cooking box /138/.
FIGURE
Figure 2: Global radiation and air temperature in a food-containing solar cooking box vs. time
The temperature inside of the solar cooker drops off sharply when the vessels are placed inside it. Also conspicuous is the fact that the temperature remains well below 100ºC for the greater part of the cooking time. The boiling temperature of 100ºC is not necessary for most vegetables and cereals.
The average achievable cooking times in box-type solar cookers amount to somewhere between 1 and 3 hours for good insolation and a reasonable fill volume. Thin-walled aluminum vessels yield much shorter cooking times than, say, stainless steel pots. The time taken for cooking is also influenced by the following factors, of course:
- The cooking time is shortened by strong insolation and
viceversa
- High ambient temperatures shorten the cooking time, and
viceversa
- Small volumes (shallow fill) in the pot make for shorter cooking
times, and vice versa.
Parikh drew up a cooking factor scheme that combines various factors with an influence on the qualitative and quantitative performance of box-type solar cookers and provides information on the various cooking options and times /75/. The empirical values refer to the western part of Central India (cf. table 1). The basic criteria in Parikh's scheme are - the weather (season) - time of day (when cooking begins) - kind of food - fill height of food in the pot.
Each of the four main criteria can assume any of three different
values, as presented by the multipliers 1, 2 and 3. Once the multipliers have
been defined for all four basic factors, the results are multiplied with each
other to arrive at a total value situated somewhere between 1 and 81. The
results break down into categories that correspond to different cooking times
(cf. table
1). Example: vegetables (multiplier 1), 4 cm high in the cooking
pot (multiplier 23), placed in the cooking box at about 1:30 p.m. (multiplier
3), requiring a cooking time of 3.5...4 hours in
Central India in May
(multiplier 1), because the product of multipliers is 2xlx3xl = 6, meaning 3.5 -
4 hours as per table 1.
Table 1: R. Parikh's cooking-factor /cooking-time multiplication table /75/
Sl. No. |
Factors |
1 |
2 |
3 |
| |
Summer |
Moderate |
Winter |
1. |
Weather |
April, May, June |
October, November, March |
December, January, February |
| |
Noon |
Morning |
Afternoon |
2. |
Time of placing food in the cooker |
11:00 a.m. to3:00 p.m |
Up to 3 hours after sunrise6:00 a.m. to 10:00 a.m. |
3:00 to 4:00 p.m |
3. |
Kind of food (from cooking point of view) |
Soft (vegetables) |
Medium (rice) |
Hard (dal, meat, etc.) |
4. |
Thickness of the food material |
2 cm (¾ inch) |
4 cm (1 ½ inch) |
6 cm (2 ¼ inch) |
Sl. No. |
Product of multiplication factor |
Time taken for cooking | |
| |
Hours |
Minutes |
1. |
1 |
1 |
00 |
2. |
2 |
1 |
30 |
3. |
3-4 |
2 |
00 |
4. |
6-8-9 |
3 |
00 |
5. |
12-16-18-24 |
4 |
00 |
6. |
27-36-54-81 |
Food cannot be cooked |
2.1.2 Reflector cookers and concentrators
The most elementary kind of reflector cooker is one that consists of (more or less) parabolic reflectors and a holder for the cooking pot situated at the cooker's focal spot. If the cooker is properly aligned with the sun, the solar energy bounces off of the reflectors such that it all meets at the focal spot, thus heating the pot. The reflector can be a rigid axial paraboloid, made for example from sheet metal or from a reflecting foil by application of high or low pressure, possibly with some manner of sectionalization provided for taking it apart, folding it together or otherwise rendering it portable. The reflecting surface is usually made of treated aluminum or a mirror-finish metal or plastic sheet, but it may also consist of numerous little flat mirrors cemented onto the inside of the paraboloid. Depending on the desired focal length, the reflector may have the shape of a deep bowl that completely "swallows" the pot (short focal length, pot shielded from the wind) or that of a shallow plate with the cooking pot mounted in the focal point a certain distance above or in front of it.
FIGURE
A somewhat more detailed introduction to the mechanics of reflector cookers is offered in /5/. Some special designs are touched upon below.
- The Fresnel cooker has a parabolic reflector consisting of several concentric rings arranged in a single plane. This approach facilitates manufacturing and gives the device a lighter, more slender appearance, e.g. the VITA cooker;
- Linear paraboloids, e.g. the Sobako, have a focal line instead of a focal point. Several cooking vessels can be strung out along the focal line;
- Fixed-focus solar cookers are designed such that, as the day progresses, the reflector can be rotated about its own polar axis (parallel to the earth's axis) while the focal point does not change its location at all. Consequently, the . cooking pot can be rigidly mounted for added stability and even provided with thermal insulation. The cook can stand in the shade and, with some effort, the "hearth" can even be installed inside the house /95;19/
- Eccentric axis reflector cookers have reflectors that represent only the lower part of a parabolic dish. Thus, all intercepted solar radiation is reflected from downside to the bottom of the pot, which improves thermal efficiency. The reflector structure is usually made form fiberglass reinforced cement or from wood, in the latter case foldable for easier transport and handling. Nearly all Solar cookers in service in China are of the eccentric axis reflector type.
All reflector cookers exploit only direct insolation and must track the sun at all times. The tracking requirement makes them somewhat complicated to handle, depending on the nature and stability of the stand and adjusting mechanism.
The advantages of reflector cookers include:
- the ability to achieve high temperatures
- and accordingly
short cooking times
- relatively inexpensive versions are possible
- some
of them can also be used for baking.
FIGURE
FIGURE
FIGURE
The aforementioned merits stand in contrast to the following drawbacks, some of which are quite serious:
- Depending on its focal length, the cooker must be realigned
with the sun every 15 minutes or so
- Only direct insolation is exploited,
i.e. diffuse radiation goes unused
- Even scattered clouds can cause high
heat losses
- The handling and operation of such cookers is not easy; it
requires practice, a good grasp of the working principle, and constant close
attention to the job at hand
- The reflected radiation is blinding, and
there is danger of injury by burning when manipulating the pot in the cooker's
focal spot
- Cooking is restricted to the daylight hours
- The cook must
stand out in the hot sun (single exception: fixed-focus cookers)
- The
reflector is somewhat fragile and its mirror finish normally requires the use of
nonlocal materials
- The efficiency is heavily dependent on the momentary
wind conditions
- Any food cooked around noon or in the afternoon gets cold
by evening.
Particularly the cooker's complicated handling, in combination
with the fact that the cook has to stand out in the sun, is a major impediment
with regard to the acceptance of reflector cookers. But in China, where the food
demands high cooking power and temperature, eccentric axis reflector cookers
have been disseminated and accepted in a large number.
Box-type solar cookers, too, can be fitted with reflectors to
amplify the incident radiation (booster principle). In most cases, the
reflectors are arranged in the form of quadratic or octagonal reflecting funnels
that fit onto the top of the cooking box such as to enlarge the effective
aperture by a factor of 2...3, resulting in a corresponding increase in energy
capture without the need to concentrate the radiation at a focal point. There
are booster cooking boxes with and without heat stores.
2.1.3 Heat-accumulatina solar
cookers
Heat-accumulating solar cookers gather the heat of the sun all day long and store it for use sometime after sundown - or even the next morning. They eliminate one of the main drawbacks of all other solar cookers, which are only useful during sunshine hours. Heat storage, by contrast, extends the cooking option to the time of day when most cooking is done in Third World cultures: in the morning and in the evening.
The simplest type of heat-accumulating solar cooker is an ordinary box-type solar cooker containing a few bricks. That alone is enough to shift the cooking time from the late afternoon towards the early evening. However, the method is not very efficient and really only useful for keeping the food warm somewhat longer.
Strictly speaking, turning a solar cooker into a heat accumulator requires some very special design considerations. Such devices contain substantial amounts of heat-retaining material and are usually designed to withstand very high temperatures, since the mass required to store a certain amount of thermal energy is inversely proportional to the temperature. The weight of the heat store, the heat-transfer mechanism and the (highly efficient or concentrating-type) high-temperature collector make the cooker quite heavy and voluminous, complicated to build, and usually quite expensive.
The more familiar designs use heat stores consisting of iron, magnesite, water or a high-boiling fluid (thermal oil). The Bomin Solar Hot Plate Cooker uses a funnel-shaped reflector to amplify the incident radiation by a factor of roughly 3 in order to heat a set of iron plates to very high temperatures in a sort of cooking box. The iron plates are then removed and transferred to a separate cooking area, where they impart their heat to cooking pots. The heat-accumulating solar cooker designed by Pohlmann/Stoy uses high-efficiency evacuated collector pipes to trap solar energy. The heat is transferred by heat pipe to the solid heat store, which then delivers controlled amounts of heat to a hot plate.
The ISE heat-accumulating solar cooker uses a highly efficient oil-filled flat plate collector that operates on the thermosiphon circulation principle to carry heat to an elevated hot-oil storage tank. Gravity circulation transfers the heat to the oil tank without need of a pump. The cooking pot stands in a matching depression in the top of the oil tank. The heataccumulating steam cooker devised by Mills and Qiu (solar cooking stove) stores heat in pressurized water at more than 100ºC. Heat extraction via the cooking pot lowers the system's internal pressure, thus causing the water to boil and carry more heat to the pot according to the heat-pipe principle.
The possibility of storing latent heat in solar cookers has been investigated in detail /99/. Latent-heat stores can collect heat without experiencing an increase in temperature. Instead, they undergo a physical change of state. In principle, this makes it possible to store more heat at a lower temperature and/or in less storage mass than in the case of normal (sensible) heat storage. The lower temperature reduces heat losses, a fact which is of advantage for collecting heat as well as for storing it.
A great number of latent-store substances and mixtures were investigated in /99/. The main preconditions were that the conversion (i.e. latent-storage) temperature had to be situated within a range regarded as acceptable for solar cooking (150...300ºC), and the material had to be relatively nontoxic, inexpensive, stable and available. Of all the tested metals, alloys, organic compounds and inorganic saline mixtures, the mixture NaNO2/NaNO3 emerged as the most favorable (in relation to the others), but there is still a long way to go before a really serviceable latent-heat-storage solar cooker can hit the market. Among the problems still to be remedied:
- corrosion due to the mixture's chemical aggressiveness
-
the design of the heat box and the fit to a suitable type of focussing
reflector
- the elimination of phase-separation problems in long-term
operation - sundry technical problems and
- ascertainment of the system's
acceptance potential.
To the knowledge of the authors, there is presently no work being done anywhere on a solar cooker featuring latent-heat storage.
All heat-accumulating types of solar cookers have one thing in common: they are so expensive as to be unaffordable for the average family in a developing country. Consequently, it is precisely the rural population that loses out. With that in mind, two cooker designers sized their units large enough to serve at the institutional level, i.e. in schools, hospitals, etc.
Some of the advantages of heat-accumulating solar cookers are:
- Cooking can also be done in the evening, i.e. after
sunset
- The cook needn't stand out in the sun ,
- The food is easily
accessible,
- Some units can be used for baking, too
- The achievable
temperatures are higher than those produced by cooking boxes
- Some
collectors require little or no solar tracking.
Their drawbacks:
- Their extremely high cost (up to US$ 2000 or more) could only
be lowered by mass production
- Various "exotic" materials and components are
used
- The storage units are heavy and immobile
- It is hard to get a
good, uniform transfer of heat from the heat store to the cooking pot; one such
cooker requires the use of pots with surface-ground bottoms (electric cooking
pots).
FIGURE
2.1.4 Steam cookers
Solar steam cookers use an efficient (flat plate) collector to generate steam, which then rises to an elevated cooking box, where it heats the bottom and sides of the cooking pot and, hence, the food.
Fixed-focus concentrators, heat-accumulating solar cookers and other designs in which the collector is separate from the stove offer the potential advantage of being able to cook in the shade or even indoors. The cooking temperatures are moderate, and the efficiency is very low (about 15%), with accordingly long cooking times; such cookers are quite elaborate and expensive in relation to their performance.
In addition to the advantage of being able to cook in the shade or indoors (thanks to separation of the collector from the stove), steam cookers also afford easy access to the food, the flat plate collector makes use of diffuse radiation, and the unit rarely or never has to be aligned with the sun.
Despite their relatively elaborate design, solar steam cookers have relatively modest cooking capacities, since the transfer of heat from the steam to the cooking pot is rather inefficient, and the cooking temperature is almost always limited to about 100ºC.
FIGURE
2.2.1 Solar radiation energy
2.2.2 Transfer mechanisms
2.2.3 Loss mechanisms
2.2.4 Efficiency
2.2.5 Cooking and baking:
temperatures and performance
2.2.6 Thermal output
Solar cookers/stoves absorb solar energy and convert it to heat
for cooking or baking.
2.2.1
Solar radiation energy
Solar radiation is electromagnetic radiation in the 0.28...3.0
µm wavelength range. The solar spectrum includes a small share of
ultraviolet radiation (0.28...0.38 µm), the visible light range
(0.38...0.78 µm) and infrared rays (0.78...3.0 µm), with the
latter accounting for nearly half of the solar spectrum.
In the earth's atmosphere, solar radiation is received directly (direct radiation, S) and by diffusion in air, dust, water, etc., contained in the atmosphere (diffuse radiation, H). The sum of the two is referred to as global radiation, G:
G = S + H (1)
On a clear day at high noon, the global irradiance can amount to as much as 1000 W/m² on a horizontal surface. Under very favorable conditions, even higher levels can occur.
The amount of incident energy per unit area and day depends on a number of factors, e.g.:
- latitude
- local climate
- season of the year
-
inclination of the collecting surface in the direction of the sun.
The average annual global radiation impinging on a horizontal surface amounts to approx. 1000 kWh/(m²a) in Central Europe, Central Asia, and Canada approx. 1700 kWh/(m²a) in the Mediterranian and most equatorial regions approx. 2200 kWh/(m²a) African, Oriental, and Australian desert areas (see fig below).
In general, seasonal differences in irradiation are considerable and must be taken into account for all solar energy applications.
Tilting the collecting surface some 30...50º to the South
in the Northern Hemisphere or to the North in the Southern Hemisphere yields
somewhat better wintertime results for the region in question, but also some
losses in summer. In the tropics, a nearly horizontal receiving surface is
generally most advantageous because of the sun's high altitude.
2.2.2 Transfer mechanisms
It takes several successive steps to transmit and convert solar energy from its reception to the point of actually heating food. Depending on the type of solar cooker being used, those steps differ from case to case:
Box-type solar cookers make use of both direct and diffuse radiation, with part of it being reflected off of the mirror lid and the remainder entering the cooking space directly through the transparent cover. Since practically everything in the box is black - the pot, its lid, the walls and floor of the box itself - most of the incoming radiation is absorbed. The black surfaces heat up and begin emitting longwave thermal radiation that is unable to escape through the glass cover. In a quasi-static state of radiation equilibrium, the pot also heats up and cooks the food by thermal conduction, with the aid of radiant heat from the lid. Additionally, steady circulation of the air in the box supplies more heat to the pot by convection, including from below, if the pot is not standing directly on the floor of the box. Transversal heat conduction through the floor of the box to the bottom of the pot (if the pot is standing directly on the floor of the box) has a similar though somewhat less pronounced effect.
Reflector cookers exploit only the insulation that impinges directly onto the reflecting mirrors, which redirect it toward the cooking pot. The dark-colored bottom and sides of the pot absorb the radiation and transfer the heat to the food. The concentrated radiation has a higher power density and is therefore able to generate higher temperatures and temperature gradients at and within the pot. While that does make for faster cooking, it also makes the food more susceptible to burning than would be the case in a solar cooking box.
Heat-accumulatinq solar cookers store heat in a solid or liquid medium. In a simple case, e.g. a cooking box equipped for heat storage, the process of heating up the storage mass can be exactly the same as for heating food: insulation, absorption, thermal radiation, convection, heat conduction. The food (in a pot in or on the heat store) gets most of its heat by conduction and radiation from the heat store. The more advanced types of heat-accumulating solar cookers have very elaborate heat transfer mechanisms. They collect solar energy not directly in the cooking containment, but in highly efficient tube/flat-plate collectors. The heat is transferred at a relatively high temperature to the heat store by heat pipes or a thermal oil loop; in the former case a solid store is used, and in the latter case the heat is stored in a volume of liquid serving as an integral part of the collecting cycle.
The transfer of heat from the solid store to the hot cooking plate may occur directly by conduction or via an intermediate controllable heat pipe. In either case, the hot plate imparts heat to the pot by heat conduction (assuming good contact between the two).
The liquid heat store may include a depression that serves as
the cooking containment, in which case both the storage medium and the air
between the containment wall and the cooking pot give rise to lively convection.
Together with thermal radiation from the walls of the containment and heat
conduction into the pot from below, this arrangement provides for efficient
heating of food.
2.2.3 Loss
mechanisms
Not all of the energy irradiated on a solar cooker can actually contribute directly to the cooking process by heating the food and/or boiling the water. The losses occurring along the way are due mainly to:
- optical radiation (l< 3 ym) through partial reflection from
and absorption by the cover (glass) in combination with incomplete absorption
inside the cooking box/pot, with the remainder being reflected;
- thermal
radiation (long-wave, l > 3 ym) given off by all warm-to-hot parts of the
solar cooker. Box-type solar cookers exploit the thermal radiation given off by
the inside of the box and the cooking pot by trapping it under a glass cover
that does not let the thermal radiation pass through, but instead absorbs it,
heats up and then radiates heat itself;
- heat conduction, despite careful
thermal insulation: through the insulation in the cooking box, through its
transparent cover (two panes of glass) and, in the more elaborate versions,
through the collector, the heat store and the pipes;
- convection by air,
either circulating within the cooker due to differences in temperature at
different points of the cooker, or due to the cooling effect of wind on the
outside (and, to a certain extent, the inside) of the cooker.
With due consideration of the aforementioned transfer and loss mechanisms, the effective thermal power, Q'eff (heat gain), inside a solar cooking box can be described by way of the following 4 terms:
i) optical gain through radiation, transmission and absorption (including reflection losses, since y = 1 and y = 1):
Q'opt = G'*A*T*a (2)
Q'opt |
optical power gain |
G' |
irradiance |
A |
aperture of solar cooker |
t |
transmittance through the cover |
a |
mean absorptance inside the cooker |
ii) heat losses through the transparent cover, summarized by a coefficient of heat transmission for the aperture, UA, including heat conduction, radiation and convection, and which, to be precise, is a function of temperature:
QL,A = UA (T) . A . DT ( 3)
Q'L,A |
thermal power loss through the aperture |
UA |
heat transfer coefficient of cover (aperture) |
T |
temperature |
DT |
temperature head between the inside and outside of the cooking box |
iii) heat losses through the bottom and sides; like for ii), but with a better (read: lower) heat transfer coefficient, UW:
QL,W = UW . (A+2bh+2dh) . DT (4)
Q'L,W |
thermal power loss through floor and walls |
UW |
heat transfer coefficient of the floor and walls |
b |
width |
h |
height |
d |
depth |
(The dimensions of the bottom, b and d, are taken as approximately equal to those of the aperture, A.)
iv) convection losses through the joints of the cooking box. These depend on wind-induced differential pressures, the length of individual joints, li (particularly around the tillable cover), the quality of the joint (coefficient of air permeability, a) and - instead of temperature - the enthalpy of the air: any moist warm air escaping from the box increases the total loss by the amount of its heat of vaporization:
Q'L,J = V' . r . DH' = a . r * DH Sum(li) . Dpi(2/3) (5)
Q'L,J |
thermal power loss through joints |
V' |
volumetric flow of air through the joints |
r |
density of air |
DH' |
difference in the specific enthalpy of the air inside and outside of the box |
a |
coefficient of air permeability of joint |
li |
length of a joint |
Dpi pressure drop across a joint |
(From window engineering, we know that the volumetric flow of air is roughly proportional to Dp2/3; accordingly the unit of a is defined as m³/(h m Pa2/3).)
In sum, then, the effective thermal power Q'eff of the solar cooking box, from eguations (2) through (5), figures to:
Q'eff = G' . A = T . a - UA . A . DT - UW (A+2bh+2dh) DT - a . r * DH' . S li * Dpi2/3 (6)
This equation is similar to the one used for calculating the effective power of a flat plate collector, the difference being that the collector's comprehensive effective U-value has been replaced by two different heat transfer coefficients: UA for the aperture and Uw for the walls and floor (this in order to do more justice to the given geometry = box), and that the losses have been supplemented by joint convection (5) - a very important term for cooking boxes.
Reflector cookers usually show especially high radiant and
convective heat losses, since the cooking pot is exposed to the effects of wind
and radiation imbalance with respect to the surroundings. Add to that the
optical losses attributable to less-than-ideal reflection by the mirror (partial
absorption, diffuse reflection and geometrical defects in the mirror) and
imperfect absorption by the cooking pot.
2.2.4 Efficiency
As far as solar cookers are concerned, the term efficiency is not dealt with uniformly in professional circles. Different experts take different degrees of liberty in defining the term, and the results are accordingly open to debate. A recent proposal of definition and experimental evaluation of solar cooking box efficiency are described in /180/.
Defining the efficiency, h, as the ratio between effective thermal power and the incident radiant power (global irradiance x area of aperture)
h = Q'eff / A . G' (7)
the efficiency equation emerges from equation (6) as follows:
As in the conventional equation for flat plate collectors, this
equation covers the optical efficiency, an approximately linear loss term as a
function of the reduced differential temperature DT/G' with two coefficients of heat transmission,
UA and UW, both of which, in second approximation, may
also be regarded as temperature-dependent, supplemented by an enthalpy-dependent
loss
term accounting for joint convection.
A mean empirical efficiency can also be defined for non-accumulating solar cookers with little heat capacity: the ratio of useful heat yield divided by the global radiation passing through the aperture in the same time interval, with constant irradiance, applied to the process of heating l l of water from 20ºC to 100ºC. This efficiency term averages out the various operating states of the heating process, during which the instantaneous efficiency is subject to change: for cooking boxes, as for flat plate collectors, the instantaneous efficiency decreases with increasing temperature and vice versa.
m |
mass (here: 1 kg water) |
c |
specific heat capacity (l.l9 kJ/kg K) |
DT' |
temperature interval of the heating process |
G is the global radiation, i.e. the integral of irradiance, G', passing through the aperture plane during the time it takes to heat the water from 20ºC to 100ºC:
(10)
Capacitive effects (e.g. preheating of the solar heater prior to testing, thermal mass of the cooking pot, ...) can influence the results, as can wind and (for some types of cookers) the angle of radiation incidence relative to the aperture, in addition to variations in irradiance. The strong point of this definition of efficiency lies in the simplicity of its experimental realization.
No such efficiency values can be stated for heat-accumulating
solar cookers, at best for the heat-collecting components. Those, however, would
be pure collector-efficiency values that would leave unaccounted for the heat
storage losses and those related to the heat transfer from the store to the
food.
2.2.5 Cooking and
baking: temperatures and performance
Cooking is the process of treating food with heat in order to enhance its taste and nutritional value and/or neutralize certain ingredients /71/.
During the cooking process, the cell structure is loosened and, in part, broken down, thus allowing more efficient use of the nutritive substances. At the same time, certain aromatics and flavoring substances can form, thus stimulating a person's appetite and causing increased secretion of digestive juices. With a view to keeping the loss of heat-sensitive vitamins and amino acids within limits, the cooking temperature and duration should not exceed what is necessary /71/. The extent to which overcooking and/or extended warming of food, specially in aluminum vessels, may lead to the formation of toxic substances in some dishes should be investigated separately /113/.
Some types of solar cookers are good for certain kinds of cooking processes, and others are good for others. Reflector cookers, for example, reach such high energy flow densities and correspondingly high temperatures (over 100ºC) - that those of appropriate design can be used for frying, roasting and barbecueing - or even baking crispy bread and cake. The problem with such high temperatures, of course, is that the food might burn. The same applies by analogy to heat-accumulating solar cookers.
Box-type solar cookers, by contrast, have a concentration factor of somewhere between 1 and 2 (with a reflecting lid) and a correspondingly low energy flow density. Consequently, they do not generate such high temperatures, i.e. most of the cooking is done in the sub-100ºC temperature range.
For most kinds of food, though, that much heat is thoroughly
adequate: protein (e.g. in meat) begins to undergo denaturation at about
45ºC; the accelerated swelling of rice, lentils, etc. sets in at roughly
80ºC, and the fibrocellular structure of both meat and vegetables is
usually destroyed at temperatures of less than 100ºC.
The main
reason why cooking is preferentially and effectively accomplished at the
temperature of boiling water (at atmospheric pressure) or at higher temperatures
(e.g. in pressure cookers) is that the high temperature gradient, i.e. a large
difference between the temperature inside of the food and the temperature of its
surroundings, helps the heat penetrate into the food more quickly; lower
temperatures would usually suffice to induce swelling and disintegration of the
fibrocellular structure - but "low-temperature cooking" would naturally take
longer, too.
The same applies to baking. Many kinds of dough bake well at less than 100ºC. Here, too, a relatively low baking temperature, correspondingly small differential temperature and accordingly poor transfer of heat into the bread or cake would make the baking process take much longer. In addition, the substantially higher temperatures needed to give bread a crispy crust can only be provided by concentrating-type solar ovens and some, but not all, heat-accumulating solar cookers.
The thermal power needed for cooking serves two purposes: - for
heating the food to cooking temperature: This requires a relatively high thermal
power input (by comparison, an electric range would be set at 1500...2000 W).
Reflector cookers/concentrators and some types of heat-accumulating solar
cookers (fully charged) are able to develop such high power levels, while
box-type solar cookers cannot. The latter therefore take longer to complete the
hetup process. The time expenditure can be shortened by placing some sort of
heat-storing element (dark-painted rocks, bricks or the like) in the solar
cooking box. - for keeping the food warm or maintaining the cooking temperature:
The heat requirement of an ideally insulated - unit can approach zero for this
function, which corresponds to that of the old fireless cooker (haybox); by
comparison, an uninsulated pot on an electric range would need about 900 W
without a lid and only about 300 W with a lid (presupposing good heat transfer
to the bottom of the pot). Putting the same pot in a fully insulated cooking
containment (solar cooking box, fireless cooker) further reduces the power
requirement, which is primarily a function of the box's heat losses. As long as
adequate thermal power is available, all watery dishes eventually stabilize at a
temperature of roughly 100ºC; the excess power is dissipated in the form of
heat of evaporation as the water in the food gradually evaporates.
2.2.6 Thermal output
The thermal output of a solar cooker is determined by the solar irradiance level (max. 1000 W/m²), the cooker's effective collecting area (usually between 0.25 m² and 2 m²), and its thermal efficiency (usually between 20% and 50%). Table 2 compares some typical area, efficiency and cooking-power values for a box-type solar cooker and a concentrator.
Table 2: Standard values for area, efficiency and power output of reflector cookers and cooking boxes
|
Area (m² ) |
Normal efficiency |
Output W at 850 W/m² |
Time needed to cook 1 liter of water |
Reflector cooker |
1.25 |
30 % |
320 |
17 min |
Cooking box |
0.25 |
40 % |
85 |
64 min |
As a rule, reflector cookers have a much larger collecting area than do cooking boxes. Consequently, they are able to generate a much higher power output, meaning that they can boil more water, cook more food, or process comparable amounts in less time. On the other hand, their thermal efficiency is lower, because the cooking pot is completely exposed to the cooling effects of the surrounding atmosphere.
In many tropical and subtropical countries, one can count on clear skies and normal daily insolation patterns for most of the year. At about midday, when the global radiation reaches a good 1000 W/m², the table-2 power levels (~ 50...350 W, depending on the type and size of the cooker) may be regarded as quite realistic. The solar irradiance is naturally lower during the morning and afternoon hours and cannot be fully compensated for by solar tracking. (Note in that connection that box-type solar cookers also can be kept more or less in line with the sun.)
By way of comparison: burning 1 kg of dry wood in one hour yields approximately 5000 W times the thermal efficiency of the cooking facility (15% for a three-stone hearth and 25-30% for an improved cookstove). The thermal power actually reaching the cooking pot therefore amounts to between 750 and 1500 W.
Figure 10 shows the typical time history of global irradiance from sunrise to sunset, with the lower curve indicating the diffuse radiation. Reflector cookers cannot exploit the latter, since they are only able to focus the direct radiation, but solar cooking boxes and cookers with non-focussing collectors can.
Solar irradiance drops off sharply under cloud and during the rainy season. The lack of direct radiation leaves reflector cookers without the slightest chance, and cooking boxes can do little more than keep prepared food warm. Which brings us to the intrinsic weak point of solar cooking - no matter what kind of device is used: on cloudy and rainy days - adding up to between 2 and 4 months per year in most Third World countries cooking has to be done according to conventional methods, e.g. Over a wood/dung fire or on a gas/kerosene-fueled cooker.
FIGURE
2.3.1 India's national solar cooker program
2.3.2
Indo-German Dhauladhar project
2.3.3 ATDO project in Pakistan
2.3.4
Orangi project in Pakistan
2.3.5 SERVE solar cooker project in Pakistan
2.3.6 Solar cooker project in North Horr, Kenya
2.3.7 GTZ project Sobako
1 in Kenya
2.3.8 GTZ solar cooker project in Mali
2.3.9 Sudanese special
energy program
2.3.10 The Chinese solar cooker development and dissemination
project
2.3.11 Lessons to be drawn
In some countries, projects promoting the use of solar cookers
are being sponsored by the government or public or private institutions. Those
being implemented in India, Kenya, Mali,
Pakistan and the Sudan are
exemplified below.
2.3.1
lndia's National Solar Cooker Program
The solar-cooker dissemination project is part of India's National Program for the Development and Use of Renewable Energy Sources. The star of the project is the Indian Box-type Solar Cooker. Also in use are the Suryamuklu Box-type Cooker and a solar cooking box developed by the Agricultural Tools Research Center (ATRC-cooker). Together with the activities aimed directly at the promotion of solar cooking, the program also addresses the development of improved adobe stoves.
The dissemination of solar cookers is presently being undertaken in eight states: Delhi, Haryana, Uttar Pradesh, Rajasthan, Gujarat, Bihar, Madhya Pradesh and Andhra Pradesh. All told, some 70000 box-tpye solar cookers were sold during 1985 and 1986, including over 21000 in Gujarat. The cookers are made by local craftsmen who pledge to observe certain official stipulations regarding materials and workmanship. It costs somewhere between US$ 60 and 70 to build the average solar cooker. The subsidized selling price is US$ 30...35 (US$ 15 in low-income regions). The following information applies mainly to the two states Gujarat and Delhi.
Target group: The solar cooker dissemination program is presently targeted on families with medium and above-average incomes. In urban areas this means successful shopkeepers, craftsmen and senior civil servants. In rural areas, those involved in the project are also salaried employees, prosperous landowners and cattle farmers. Members of this group earn between US$ 15 and US$ 200 per month. In addition, the Indian army has ordered 300 solar cookers. The lower social strata cannot afford to buy such cookers.
The target groups in Gujarat and Delhi use mostly wood, propane, kerosene, charcoal and agricultural residue as fuel for cooking. Due to limited resources, commercial energy carriers like propane and kerosene are routinely rationed. Consequently, most women use whatever combination of fuels they can get. Between 10% and 20% of the monthly family income is spent on fuel.
Dissemination program: The gradual diffusion of solar cookers is being promoted with the aid of a large-scale publicity program. At the supraregional level, the advantages of solar cooking are expounded on television, on the radio, in newspapers and in periodicals. Additionally, numerous craftsmen, salespeople, institutions and development organizations are participating in the solar cooker dissemination drive. At the village level, solar cooking demonstrations are held at village meetings and markets and in schools, hospitals and other public-access buildings. At dancing competitions, for example, the winning couple gets a solar cooker. In urban areas, solar cookers are demonstrated and sold in shopping centers and at seminars and exhibitions.
Also, all kinds of compainies recently have begun to participate in the dissemination program by offering their employees a US$ 10-premium toward the purchase of a solar cooker. Other firms pass solar cookers out to interested parties in return for a small monthly payment. Some coal miners are being given free solar cookers instead of free coal. Each solar cooker comes with a pamphlet explaining its proper function and operation. Basically, the dissemination program embraces two approaches:
- convincing the target group through personal experience
(seeing, tasting, feeling)
- enlisting the aid of influential
(opinion-forming) individuals and groups to help motivate women.
According to the first approach, women are expected to attend cooking demonstrations to "experience" how solar cookers work by doing some experimental cooking themselves. In Gujarat, solar cookers are loaned out to families interested in finding out what solar cooking is all about.
The other approach assumes that women will be quicker to accept
solar cookers, if they have the opportunity to see them being used by
influential, prominent or highly educated individuals or groups. In other words,
if it becomes known that members of the upper social strata have taken to using
solar cookers, other - lower - social strata should follow suit. The presumption
here, of course, is that low-income families will presumably try to "keep up
with the Jones's". The initiators therefore push the program with posters in
which solar cookers are depicted as a symbolic part of a better way of life. The
aim is to give solar cookers such a good image that the potential buyer views
them in the same light as any other luxury article like a radio, television or
refrigerator.
In contrast to other government agencies, the Gujarat Energy
Development Agency (GEDA) does not measure the success of the dissemination
project against the number of solar cookers sold, but against the number of
those actually in use. That is, the promotion campaign is not supposed to merely
help sell the cookers, but should also and simultaneously appeal to people's
curiosity - get them to ask questions and try it themselves. There is at present
no regular backstopping being provided to families who already have a solar
cooker, though such a program is planned for the near future in Gujarat.
Systematic studies of the basic socioeconomic situation of the target group are
also planned.
The heavily subsidized, state-controlled sale of more than 70000
solar cooking boxes all over the country makes India the only country (possibly
outside of China) in which solar cookers have achieved any appreciable degree of
diffusion. This is probably attributable to a combination of governmental
efforts, particularly at the state level, and the technical character of the
cookers themselves. At any rate, just how durable that success will be remains
to be seen. Box-type solar cookers appear to be the only approach to solar
cooking that has gained nominal - if conditional - acceptance. But the
relatively large number of units sold should not be allowed to obscure the fact
that those with a solar cooker constitute only a tiny minority of India's 764
million inhabitants. Despite the genuine merits of box-type solar cookers, and
despite the large-scale publicity campaigns in their favor, no massive
dissemination of such cookers has been achieved to date.
2.3.2 Indo-German Dhauladhar
Project
Measures taken to promote the testing and introduction of solar cookers in the northern state of Himachal Pradesh are part of an overall scheme aimed at achieving ecological stability in the erosion-imperiled project area. The Indo-German Dhauladhar Project (IGDP) includes erosion-prevention and afforestation programs, activities designed to improve crop yields and animal production, and the introduction of energy-saving adobe stoves (Dhauladhar.Chulha). Work on the development of solar cooking equipment began in 1985. By 1986 some 20 solar cookers were built and dispensed. Existing plans called for a doubling of the number of solar cookers in use by year's-end 1986. The "Dhauladhar Solar Cooker" is a stationary, partly home-built, box-type solar cooker made of adobe; the requisite material costs US$ 4...10, depending on the desired model.
Target group: The target group in this case is the general populace in the project area, about 12% and 23% of which belong to the highest and lowest castes, respectively. Some 90% of the project-area inhabitants are farmers. Nearly 40% of the families do sharecropping on at least part of their land, and 39% avail themselves of seasonal labor. More than 80% own cattle (cows, sheep and goats). Nearly all have some form of supplementary income from a family member in military or civil service.
Wood is relied on for most cooking and heating; about 10% of the families buy their firewood, while the other 90% spend two or three hours daily gathering it. Kerosene is used in about every other household to cover a small fraction of the total fuel requirement. If all cooking and heating were to be done with kerosene, consumption would run at about 13 1 per month and family, at a cost of roughly US$ 13. Agricultural waste products like cow manure, rice straw or pine cones play a lesser role as household energy sources, being used mainly during the rainy season due to a shortage of dry wood. All women in the project area do their cooking on a traditional indoor Chulha made of adobe clay. A second means of cooking mainly for tea - is sometimes available in the form of a kerosene stove.
Dissemination Program: Once some initial positive experience had been gathered in the course of the technical shakedown phase, solar cookers were installed in the homes of potential promotors, i.e. field workers, teachers, women's group chairwomen, etc.. The local craftsmen are familiarized with the production techniques by way of on-the-job training during construction of the stove. From October 1986 to September 1987, 133 solar cookers of the stationary Dhauladhar have been installed.
Meanwhile it turned out that the cooker is only accepted to a limited degree. The limited acceptance is considered to be due to climatic constraints and the firewood situation: It is assumed that the solar cooker is principally accepted in those areas where only commercial fuel is available. For this reason, it is intended to disseminate further solar cookers mainly in those villages which have no free access to firewood.
In 1988, a 6-page information/advertisement paper has been
prepared for decision makers and/or for potential users of the Dhauladhar s.c.,
more printed information is currently being prepared.
2.3.3 ATDO Project in Pakistan
The Appropriate Technology Development Organization (ATDO), organized within the Ministry of Science and Technology in Karachi, began building solar cookers in early 1986. The ATDO cooker is described in the Appendix. The project objective is to test and disseminate solar cookers in the western regions of the country, where wood is scarce. The organization is also involved in the development of wood-saving clay stoves. The solar-cooker program; which represents only part of ATDO's numerous activities, is presently in its shakedown phase. Since work began, between 15 and 20 box-type solar cookers have been produced at a cost of US$ 24 a piece and sold for US$ 30 apiece. The project is being funded by the competent ministry. Target group: Up to now most project activities have been concentrating on the Greater Karachi Area. The target group comprises independent tradespeople, middle-income salaried employees and others who can afford to buy a solar cooker at unsubsidized prices. The average monthly income of the target group "amounts to US$ 85...90, of which roughly 8% is spent on fuel. The project activities were recently extended to the rural areas of Sind Province, where the main thrust is being directed at the wood-starved Thar Desert, where most of the people work on the farming estates of major landowners. A few manage to produce some extra food that can be sold to supplement the average farm worker's monthly income of US$ 13...17. Due to the scarcity of wood, about 30% of the housewives fuel their fires with dry cow pats.
Dissemination program: ATDO's involvement in the production of solar cookers began in early 1986. Accordingly, the project is still in its initial phase, i.e. testing is still more important than mass dissemination. The introduction of solar cookers in rural Sind is being closely coordinated with local production centers, national development organizations and field workers from the United Nations Educational, Scientific and Cultural Organization (UNESCO). Some 500 villages have been selected to participate in field testing of the solar cookers. In contrast to the urban dissemination concept, the rural women are supposed to be advised and informed less individually, i.e. more within the framework of communal events. In that connection, the organization relies heavily on the female staff of the women's education centers (WEC's) for providing information and advice. The solar cookers are demonstrated at the WEC's. In addition, the women are given pamphlets containing further information. Any woman participating in such events is expected to become a multiplier within her own household environs. With deference to anticipated acceptance problems, ATDO is giving its activities a two-prong thrust, namely solar cookers and fuel-saving adobe stoves, the handling characteristics and function of which are more in line with the women's traditional cooking habits.
Gradual expansion of the project activities into regions affected by a distinct shortage of fuels would certainly be worthwhile, since the broader base would enable a more comprehensive assessment of the real acceptance potential for solar cookers among the general populace. Incorporating such activities into existing organizational structures (women's education centers, etc.) and cooperation with the local staff of various institutions/agencies will not only yield positive results, but also force alteration of the previous dissemination strategy, which addressed only individual households. In future, solar cookers will be presented and demonstrated at joint-effort gatherings and seminars for women.
2.3.4 Orangi Project in Pakistan
The Orangi Pilot Project is sponsored by a nongovernmental organization operating in Orangi Town, a Karachi Suburb. Alongside of numerous self-help activities and income-generating projects, the use of solar cooking boxes was added to the scheme in 1986. By the end of that year, about 30 solar cookers had been built and demonstrated to 350 families. The cooker in question is a rectangular box-type solar cooker with a reflecting lid. Due to lack of interest on the part of the local population - not a single family was willing to test-use a solar cooker for a prolonged period of time - the solarcooker activities are being discontinued. The project staff attributes the lack of acceptance not so much to financial aspects,.since part of the local populace has regular income as to the technical inadequacy of the cooker itself and to the women's lack of willingness to alter their traditional cooking habits. As a result, the organization is now attempting to introduce wood-saving adobe and metal stoves. A major share of the solar cookers built to date were sent up north to the mountainous Gilgit region for field testing; to the author's knowledge, no evaluation has been conducted yet.
2.3.5 SERVE Solar Cooker Project in Peshawar, Pakistan
SERVE (Serving Emergency Relief and Vocational Enterprises), a
voluntary agency based in England, initiated a pilot project to test the
acceptability of solar cookers among Afghan refugees in
Northern Pakistan in
1984. In 1985, a solar cooker dissemination project was started, which is still
going on /184/.
Target group: Since the invasion of Afghanistan in 1979, over three-million Afghans have fled to Pakistan. In and around the refugee camps, there is a severe shortage of firewood and other natural fuels, and the ongoing fuelwood need and consumption of the Afghans has resulted in threatening the ecology of the region and gives rise to serious tensions between Afghan and Pakistani people.
A 1983 survey in the Northwest Frontier Province of Pakistan /184/ showed that 88% of the refugees burnt wood (approx. 0.85 kg per person and day), and 83% of them used kerosene (appr. 1 1 per day and family). Three man-hours were spent per day and family for fuel collection. The refugees have seen cooking fuel as one of their greatest needs.
Most of the refugee women are illiterate, which is important with regard to their capability and willingness to adapt their cooking and eating habits to the use of solar cookers.
Pilot Project: In 1984, a square box fiberglass cooker with mirror lid was introduced. 2240 items of this type have been produced and distributed. Over 80% of the families in the project were reported to use the ovens on a regular basis /184/. Altogether 16 models of different solar cookers were tested. Parabolic concentrators were judged not to be suitable for the needs of Afghan refugees.
Model Change, Production, and Marketing: The oven went through a number of design changes between the completion of the pilot project and the actual beginning of production in mid 1985. The square box shape was replaced by a rectangular shape. The interior oven liner and pots were painted with a high-temperature, non-glossy black paint. Handles and wheels made the oven easier to move and adjust. Each unit contained four 1 1/2 1 pots, a pair of gloves, and illustrated cooking instructions.
Sales teams were formed to market the ovens in the refugee camps to display the cookers and prepared demonstration meals. Instructors taught refugee women how to use the cookers. The manufacturing price was US$ 56.00, the subsidized sale price US$ 18.00. Over 1780 cookers were sold from June 1985 to June 1987 /184/. Over 70% of the families are reported to use their solar oven consistently.
The rectangular 1985 solar oven model still had some drawbacks, and the following improvements have been suggested /185/:
- inclination of glass cover 20º30º
- more distance
between the cover glass panes
- clear glass with low or no iron content to
minimize absorption
- more thermal insulation, 5...8cm thick
- more heigt
in the inner box to obtain space for grid and convection distance under the
pot
- Mylar foil reflector instead of glass mirror
- substitution of
offset print aluminum sheet instead of iron sheet material for the inner
box
- light wooden support instead of 4 metal wheels
- to replace the
galvanized sheet metal casing by a fiberglass box with low weight and high life
expectancy
- other measures in order to decrease weight and make the oven
handy and transportable by one person
- to split the lower glass pane into
two parts to relieve thermal stress and reduce breaking rate.
The 1987 Model: Most of these suggestions have been realized early in 1987, which led to a new SERVE Solar Oven model with less weight, more aperture, better performance, and higher reliability and lifetime which has a 20 K higher stagnation temperature. The manufacturing cost is about US$ 63.00, the sales price still US$ 18.00. The subsidy of $ 45.00 per oven is proviced by donors and international organizations like UNHCR, World Vision International, etc.
It was planned to extend the production of this new type, thus manufacturing 3700 ovens per year.
Conclusion: SERVE has produced and disseminated a solar cooker
that performs satisfactorily and has been accepted by the refugees. This project
is said to provide a model for introducing solar cooking to other segments of
Pakistan society and in other parts of the world where similar needs and
conditions exist /184/.
2.3.6 Solar cooker project in
North Horr, Kenya
In cooperation with the Catholic mission in North Horr, the local production of a simple box-type solar cooker (ULOG cooker, tropical model 85, see appendix 1) began in 1985. The local carpenter and three helpers were shown how to build the cookers. The solar cookers, which are being built at the rate of roughly one per day, are sold at the prime-cost price of about US$ 25. Most of the > 40 units produced by end of 1986 had been sold, some of them were in service at the mission, and others were being used for demonstration purposes. All told, some 80 solar cooking boxes have been built since 1985 and sold to various missions and development organizations in northern and central Kenya, on the east coast and to the southeast of Nairobi. Most of the funding for the project is provided by the mission.
Target group: Basically, the target group consists of the approximately 3000 inhabitants of North Horr, some of whom are now-sedentary nomads (Gabra). The settlement is located on the fringe of Chalbi Desert, having appeared there long ago by reason of sweetwater springs. There are bushes in and around North Horr, and even trees grow along the riverbeds. The local inhabitants gather wood for their own use or for selling (10 branches for about US$ 10). The main sources of income are animal husbandry, handicrafts and odd jobs.
Dissemination program: The following activities are designed to inform and motivate the inhabitants of North Horr in order to promote the introduction and dissemination of solar cookers: - exemplary use of the solar cooking boxes already in service at the settlement - demonstrations at a central location - solar-cooker demonstrations in various women's groups concerned with the production of palm-frond wickerwork. Those women are expected to convince other women of the advantages of solar cooking. Also, individual household consultancy sessions were held for various groups of users. Local craftsmen are being taught how to build solar cooking boxes. Moreover, a three-day course in cooking-box construction was held for fifteen carpenter's apprentices at Marsabit Technical College.
The solar radiation conditions of the region (about 6.2 kWh/(m²d) all year round) are favourable for solar cooking during the whole year (see radiation table of Lodwar, table 3). Nevertheless, users complain about too many cloudy days which aggravate a regular use of the solar cooker.
Some of the cookers had been given to several development organizations for field testing. However, no definite findings about the acceptance and use of the cookers, which had been distributed since 1985, could be established.
Big solar cooking box: In 1986, a big stationary box-type solar
cooker was installed at the mission of North Horr. The 3.2 x 1.5 x 0.5 m box is
made of bricks, the two 50 cm diem. flat cooking vessels are suspended at the
level of the horizontal double glass cover, partially immerged into the box,
where they are heated by convection and radiation from the absorbing walls of
the inner box.
This stationary box-type solar cooker is used to cook for 70
school children if weather conditions are good.
Fixed-focus hybrid solar cookers: From 1986 to 1988, three
hybrid solar and wood fire cookers with 1, 7.6, and 22.4 m² apertures for
5, 54, and 500 1 of food were constructed at a mission and two schools in Kenya.
(For the fixed-focus principle see fig. 6; for details of these cookers see
appendix 1) .
2.3.7 GTZ
project Sobako in Kenya
The project objective was to field test the Sobako 1 solar cooker
- a linear-parabolic reflector cooker - with regard to: - its
appropriateness as a cooking appliance in an average household .
- the
anticipated savings on fuel - improvement via design modification.
In addition, field trials were supposed to yield information on which to base suggestions for the future mass production of identical or similar cookers.
To that end, a solar cooker built in the Federal Republic of Germany was sent to Kenya. Two other cookers were produced by the Department of Mechanical Engineering, University of Nairobi, and by Nairobi-based Kenya Industrial Estate. The performance of all three solar cookers was investigated over a period of twelve months in various regions of Kenya. The target group to be familiarized with solar cooking comprised various low-income groups, primarily in rural areas.
It turned out that Sobako 1 and also its simplified successor model Soba 1 were complicated, high-cost, low performance solar cooking devices which were extremely unadapted to the user's needs (see description in appendix 1).
At the time of project implementation in 1977/78, it cost well over US$ 420 to build either a Sobako 1 or a Soba 1. Under assumed favorable conditions, the estimated cost price for economic lot sizes was expected to be about US$ 25. The calculated payback period based on fuel savings was 3.5 years; allowing for the money saved by home-baked bread shortened the calculated payback period to 1.8 years.
A six-month social-acceptance study was supposed to yield
additional data. In that connection, eight additional solar cookers were to be
built and demonstrated all over the country in cooperation with various
governmental and non-governmental organizations. In addition, plans for
producing and disseminating the cooker in large numbers were drawn up, and
pertinent recommendations on a subsidy program were formulated for the
government. Eventually, though, the severe shortcomings of the design principle
led to discontinuation of all relevant activities.
No methodical
evaluation of acceptance was made that would satisfy even minimum criteria.
2.3.8 GTZ solar cooker
project in Mali
Upon completion of a 1978 survey of existing solar cooking and baking appliances, the Munich-based Arbeitsgemeinschaft für Entwicklungsplanung (~ working group on development planning) received from GTZ a follow-up commission concerning the development and testing of solar cookers in Mali. To begin with, ten reflector cookers were built and tested in 1979/80 in cooperation with the Mali project partner. The cookers were field tested in two hospitals and four "Centres d'Animation Rurale" (CAR) from May through September 1980. The empirical data was evaluated in the form of a study entitled "Solare Koch- und Backgerate - Prototypenbau und Erprobung" (solar cooking and baking devices - prototype construction and testing) /6/, and the following recommendations concerning the further development of solar cookers were made: - appropriate long-term testing of solar cookers - local production - program for the broad-scale dissemination of solar cooking and baking devices. Since the efforts aimed at introducing solar cookers were only moderately successful, the above recommendations were never implemented.
Target group: The "Comite Interservice pour les Energies Renouvelables" had eight solar cookers tested in four CARs (in Narena, Quelessebougou, Yangasso and Sorobasso - located 80 and 90 km to the south and southwest, and 400 to 450 km to the east of Bamako, respectively). A pair of cookers was given to two hospitals in San and Dioro to determine how well they would serve in heating water and sterilizing medical instruments. A CAR is a rural training center in which young farmers can learn to read and write, take courses in modern agricultural production, and receive paramilitary training. Between 40 and 60 people live in the average CAR. Two women cook the meals for the trainees and their families. The two staple dishes eaten alternately for breakfast and supper are Tot and Couscous.
The women fuel their cooking fires primarily with wood, which
the men gather between once and three times a week. Gathering wood takes between
2.5 and 8 hours, depending on the region. Only dead wood is used; indeed, the
use of freshly felled wood is prohibited. The CAR women make tea on a portable
charcoal stove, but cook the meals over an open fire (three-stone hearth). Solar
cookers were introduced in the CARs and hospitals by way of demonstration and
explanation of their function. The personnel received operating instructions and
solar-cooker-reporting sheets, but the users were given no trial-phase
backstopping assistance.
2.3.9 Sudanese Special Energy
Program
The use of solar energy for cooking is part of the Sudan's Special Energy Program, which took shape in the early 1980s and was promoted by the Kreditanstalt für Wiederaufbau (KfW, reconstruction loan corporation) and the Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ) GmbH. Some of the program's many planned and in part already implemented elements are afforestation, carbonization of cotton stalks, charcoal processing, energy-saving stoves, photovoltaic refrigerators in hospitals, solar-assisted educational television, windmills, and river pumps. The development and testing of a box-type solar cooker with reflecting lid, 80 of which were built, was more or less a spin-off from the mainstream project. Other types of solar cookers that have been tested in the Sudan, e.g. the axially parabolic reflector cooker Falco S/C and the Convective Solar Cooker, the latter being a rather large apparatus with convective solar collectors, have failed to gain acceptance or multiplication; no evaluations are known to have been conducted. According to a World Bank report, heating water is about the only use for solar energy that could find a market in the Sudan.
Dissemination program: As of this writing, prototypes have been distributed in 15 towns and given to interested institutions. Systematic testing is planned. The only nominal-scale test run as yet under controlled conditions was in Khartoum: relatives of staff members at the National Renewable Energy Research Institute (RERI) obtained the cookers at a special price of about US$ 20 - compared to the going market price of US$ 40...80 - and used them for several months. Thirteen families participated in the test, and eleven families responded to a subsequent survey.
Target group: Even though the test families enjoyed incomes amounting to more than twice that of the average family, the heavily subsidized price was still too high - considering the cooker's meager performance. The potential users of the Sudanese solar cooker (if any) will most likely be limited to well-to-do urban households wishing to save charcoal.
Evaluation aspects: The makers of solar cooking boxes bemoan the
lack of both a market analysis and a dissemination program with promotors,
consumer surveys, media advertising and market strategy. One private producer
has 14 cooking boxes waiting in vain for buyers. Observers report that the
cooking boxes are of varying design and quality and that the product development
phase is far from over; in other words, dissemination via the free market would
still be premature, and the Ministry of Energy still does not have access to
adequate dissemination channels. The test families tend to regard solar cookers
as a supplementary means of cooking, mainly because the cooking times are still
much too long. Other aspects are dealt with in other sections, e.g. chapter 3.
At any rate, the solar cooker is not yet ripe for general diffusion; considering
the given acceptance situation, it remains to be seen (and should be
investigated) whether or not that ever will be the case.
2.3.10 The Chinese Solar Cooker
Development and Dissemination Project
In September 1983, at the Conference of National Comparison and Exchange of Solar Cookers, 48 institutions presented 102 models of solar cookers, of which 80 had been tested. 41 of these had an efficiency above 60%, and 41 also had a thermal power above 1000 W. For most of them, the operating height was lower than 1 25 m. The specific cost ranged from 30 to 140 yuan/kW (US$ 8.00 - 38.00 per kW). Finally, the conference selected 24 models for dissemination because of their thermal efficiency, easy use, low cost, and adaptedness to the needs of a Chinese family of five /187/.
Tongging (Yongjing) County, in Gansu Province, started to popularize solar cookers already from 1978, and by end of 1983, there were about 20,000 solar cookers in use, 40% of the total number in the whole country.
As of this writing, more than 100.000 solar cookers have been in service in China. Almost all of them are of the eccentric axis concentrator type (see below).
Cooking needs and model selection: The preparation of chinese food comprises boiling' roasting, frying in boiling oil, etc. For these processes, high thermal power and high temperatures are needed. Solar cooking boxes, e.g. of the Indian type, cannot be used for many dishes/components of chinese food. For this reason, the solar cookers in use have mainly changed from the box type to the concentrating reflector type. The high thermal power is obtained by the choice of a large aperture (1...2.5 m²), which results in 400...1200 W of thermal power. Thus, for some dishes, cooking on the solar cooker takes only half of the time that would be necessary on a wood fire. Maximum temperatures measured in the focal spot are - depending on the paraboloid precision and the concentration factor 400...1000ºC /187/.
Fuel prices and savings: Investigations in Taxang village have
shown that a family of 5 persons consumes about 5 kg of firewood per day, which
is 1825 kg per year. In a sunny region of China
(2000 h/year of sunshine), a
solar cooker can save 500...1000 kg of firewood per year. At the price of 0.20
yuan/kg (US$ 0.05/kg), this results in the saving of 100...200 yuan/year (US$
30...60/year). "Practice has proven that in areas with better solar radiation
and a serious lack of firewood, the solar cooker is feasible" /187/.
Solar Cooker Technique: Development of solar cookers in China started from the round parabolic dish. But with this design, it is not possible to focus all radiation to the bottom of the pot, which results in low thermal efficiency, specially when the sun's altitude is low.
Using only a section of the lower part of the paraboloid allows the concentration of all radiation under the pot. This socalled eccentric axis concentrator has been developed to a large extent both in theory and practice in the Energy Research Institute of the Henan Academy of Sciences at Zhengahou. The eccentric axis concentrator type has become the most common type of solar cooker in China /186-188/.
The eccentric parabolic reflector was first made from conventional cement (160 kg), then from a thin layer of fiberglass reinforced cement (70 kg). If the glass fibers are cut short, they can be sprayed with the cement; this is suitable for industrial production.
Other cookers of this type have a wooden or metallic reflector shell. It can be folded once or twice, resulting in a handy transportable box. This so-called eccentric axis box-style solar cooker is still a pure reflector model and has nothing in common with other box-type models such as the Indian solar cooking box etc.
The reflecting surface was first realized by small pieces of
mirror glass, then by a reflecting aluminum foil. But there have been problems
concerning the quality and the stability of the reflecting foil surface.
2.3.11 Lessons to be drawn
No systematic evaluation of the above solar cooker projects was conducted prior to, during or after their implementation; the only relevant publications deal with India's national solar cooker program /116; 139/. Consequently, the lessons to be drawn from successful and unsuccessful projects must be of an extensively intuitive nature. The most immediate needs are:
- exact analyses of dietary, eating and cooking habits of the
target groups, and which-forms of energy they use
- more consideration of the
target groups' buying power and "emergency strategies" for securing energy
-
organization and implementation of training measures for local craftsmen
-
regular quality controls at the place of manufacture in order to enable
detection of hidden defects, e.g. forgotten insulation - methodical field
testing
- identification and exploitation of existing infrastructures and
useful organizations (artisan groups, self-help groups, women's groups, etc.)
for testing and disseminating solar cookers
- implementation of user
sensitization measures designed to get across the importance of energy
conservation
- including women in the project activities and realizing their
importance as multipliers and promotors
- incorporating the project
activities into existing women's programs (courses in dietetics, health &
hygiene, handicrafts, etc.)
- familiarizing women with the utilization of
such equipment
- volunteer participation in field-testing and/or
dissemination programs- provision of regular backstopping for all concerned,
from the users to the makers, including any necessary follow-up studies
-
making sure that the target group is the user/beneficiary and vise versa.
It
would also be very good to promote back-up research. Most results of evaluation
still rest more on anecdotes than on
analyses.
Comparing prior surveys on solar cookers, e.g. /58/, with this study, one notices that the former make little mention of box type solar cookers, while the present report is practically dominated by solar cooking boxes. Indeed, there seems to be a trend taking hold in the entire solar-cooking scence. While tens of thousands of box-type solar cookers are being produced - and sold - in India, other once dominant types of solar cookers appear to be losing favour. Consider for example the Fresnel reflector cooker by VITA, the reflector cooker from Mali, the Sobako, and the BRACE steam cooker. But in China, the majority of solar cookers in use are reflector type ones.
Initial successes in India and China are still infinitesimal in relation to the total population. The fact that "practically nobody in the world" is still cooking with solar energy, even though the advantages are quite obvious (and few are aware of the drawbacks), continues to spur the imaginations of engineers and solar experts, whose research data, inventions and new products - like the VIAX cooker and all four heat-accumulating solar cookers - must stand up to extremely stringent economic, technical and socio-cultural conditions of acceptance in the Third World.
The latest developments document the fact that inventors and engineers still have ample latitude for new ideas. They also confirm the observation that appropriate technology cannot be developed independently of the target group.
Solar cooking boxes have by no means reached the end of their technical evolutionary process, either. While the Telkes type (cooking box with a round floor, freely suspended pot holder and a funnel-shaped array of reflecting surfaces) was just plain too expensive, simple cooking boxes can now be made much more efficient at little or no extra expense. Using transparent insulation between the two panes of glass, for example, lets the sunlight enter practically without hindrance, but prevents the resultant heat from escaping through the cover. One such configuration is the transparent plastic honeycomb structure described in /145/.
New kinds of transparent insulation are being developed and tested /143/; they are expected to revolutionize the entire field of solar thermal technology and give new impetus to the spread of solar cooking, particularly in box-type solar cookers.
The usefulness of a solar cooking box can also be increased by including some means of internal heat storage, e.g. rocks. While that would mean that the cooker takes longer to heat up in the morning, it would also increase its primary cooking performance and help keep food warm until long after sunset. Such a simple and for all practical purposes free means of expanding the function of solar cooking boxes surely must have been tried out by various Third World users by now, but the authors have received no relevant reports. One possible drawback, namely the fact that adding a heat store to a solar cooking box would make it considerably heavier, still needs to be clarified.
India and China are the only countries in which efforts aimed at disseminating solar cookers have been at least partially successful. In India, where some 100,000 solar cookers have been sold to date (still minuscule in comparison with the total population), the "success" has been attributable to a massive dissemination campaign (cf. chp. 2.3.1).
In China, more than 100,000 solar cookers are already distributed and sold. Most of them are reflector cookers that develop the high temperatures needed to accommodate Chinese cooking habits. The authors do not know how many of these cookers are really in use at this time.
Whether or not solar cookers finally do gain widespread acceptance in some parts of the world remains to be seen. But one thing is for sure: wood and other natural fuels are becoming increasingly scarce in numerous developing countries with ample solar radiation; and fossil fuels can also be expected to become much more expensive in the not too distant future. Soon' most families in many areas will be finding no more natural fuels and will not be able to afford commercial fuels. Such families may find that solar cooking constitutes their only long-term alternative. The extent to which such arguments are being accepted by the target groups of solar cooker projects is still an open question - one that is examined more closely in the following chapter.