There are some disadvantages too:
Cooking must be limited to the daylight hours.
The moderate temperatures make for long cooking times.
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. There are all sorts of box-type solar cookers: mass-produced, hand-crafted, do-it-yourself types etc. 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 on the market.

GUIDELINES FOR CONSTRUCTION
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 aluminium, because it is a very good heat conductor. Additionally, aluminium 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.
No metal parts should placed to the outside around the top rim of the inner box: thermal bridges must be avoided. The insulation may consist of 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 could consist of one or 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 inner space into small vertical compartments can substantially reduce the cooker’s heat losses, thus increasing its efficiency accordingly. 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.

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). In an emergency, even foil from empty cigarette packs will do the job.
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 aluminium with wooden bracing offers the best finish and is adequately stable with regard to mechanical impact and the effects of weather. An aluminium-clad wooden box is the most stable of all, but it is expensive and time-consuming to make, in addition to being heavy.
The capacity of a normal box-type solar cooker with a 0.25 m2 area of incidence (aperture) amounts 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 150 deg.C on a sunny day in the tropics; that amounts to a thermal head of 120 deg.C, referred to the ambient temperature. Since the water content of food does not heat up beyond 100 deg. C, a loaded solar cooker will always show an accordingly lower inside temperature. The temperature inside of the solar cooker drops off sharply when the vessels are placed inside it. Also important is the fact that the temperature remains well below 100 deg.C for the greater part of the cooking time. Nevertheless the boiling temperature of 100 deg. C 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 aluminium vessels yield much shorter cooking times than stainless steel pots.

The time taken for cooking is also influenced by the following factors:
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.

REFLECTOR COOKERS
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. The reflecting surface is usually made of treated aluminium 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.
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.

The above mentioned 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.
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 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.

THERMAL OUTPUT
The thermal output of a solar cooker is determined by the insolation level, the cooker’s effective collecting area (usually between 0.25 m2 and 2 m2), and its thermal efficiency (usually between 20% and 50%). Table below compares some typical area, efficiency and cooking-power values for a box-type solar cooker and a reflector.

Standard values for area, efficiency and power output of reflector cookers and cooking boxes

	Area in m2	Normal efficiency	Output in W at insolation of  850 W/m2	Time needed to cook 1 litre 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 up to 1000 W/m2 , the thermal output levels (50 to 350 W, depending on the type and size of the cooker) may be regarded as quite realistic. The insolation is naturally lower during the morning and afternoon hours and cannot be fully compensated for by solar tracking.
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 used in developing countries). The thermal power actually reaching the cooking pot therefore amounts to between 750 and 1500 W.
Insolation 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. The weak point of solar cooking is that no matter what kind of device is used: on cloudy and rainy days (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-fuelled cooker.

SOLAR RADIATION
The first and foremost prerequisite for success in a solar cooker application is adequate insolation, with only infrequent interruptions during the day and/or the year. The duration and intensity of solar radiation must suffice to allow the use of a solar cooker for prolonged, worthwhile regular periods. While cooking with solar energy is possible in Central Europe on a sunny summer day, a minimum irradiation of 1500 kWh/m2 per year (corresponding to a mean daily insolation of 4 kWh/m2 per day) should be available for any solar cooker. But these annual data can sometimes be misleading. The essential condition for solar cooking is a reliable “summer weather”, i.e. essentially predictable sequences of regular cloudless days.
Supply of solar energy varies substantially from country to country, even within the Third World’s tropical belt. Thus, local data must be referred to - and they are not always available. Some examples: In India solar radiation in most regions is good to very good for purposes of solar energy exploitation. The yearly averages of daily annual global radiation range from 5 to 7 kWh/m2 per day, depending on the region. In most places, the insolation reaches its minimum during the monsoon season and is nearly as weak again during the months of December and January.
In Kenya’s climate and insolation potential are favourable to the use of solar cookers. Kenya is close to the equator and therefore has a purely tropical climate. In Nairobi, the daily irradiation alternates between 3.5 kWh/m2 per day in July and 6.5 kWh/m2 per day  in February, but it remains practically uniform (6.0...6.5 kWh/m2 per day) in other regions of Kenya like Lodwar. Solar irradiation in Nairobi is adequate for cooking with solar energy nine months a year (excluding June through August). On the other hand, conventional cooking facilities must be relied on for cloudy or hazy days. In the Lodwar area, though, solar cookers can be used year-round.

SOLAR COOKERS FOR DEVELOPING COUNTRIES
The purpose of solar cookers, of course, is to save energy in the face of a double energy crisis: the poor people’s energy crisis is the increasing scarcity of firewood, and the nation’s energy crisis is the growing pressure on its balance of payments. Solar cooker should be judged with that in mind.
Compared to other nations, developing countries consume very little energy. For example, India’s 1982 per capita energy consumption rate, at 7325 GJ, was one of the world’s lowest. But the country’s energy consumption rate is increasing nearly twice as fast as its gross national product. The same is true for the  most developing countries.
The poor majority of the people in developing countries cover most of their energy requirement in a non-commercial way, using traditional, locally available sources of energy and their own physical labour. They simply cannot afford to buy any appreciable amounts of commercial energy.
The logical consequence is a relative shortage of fuel for use by the poor, whose living conditions deteriorate even more as a result. Solar cookers could at least try to compensate.
If the “poor” majority of the Third World’s people is the target group, then solar cookers must be first and foremost to the benefit of the rural population.

COOKING-ENERGY QUANTITIES
The daily fuel requirement varies according to the kind of food being cooked and the number of warm meals. In the typical developing country, each native burns one ton of firewood each year.
In India, the average family needs somewhere between 3 and 7 kg of wood per day; in the cooler regions, the daily firewood demand varies between just under 20 kg in the winter and 14 kg in the summer. In the southern part of Mali, the average 15-member (!) family burns about 15 kg of wood each day. A survey conducted in an Afghan refugee camp in Pakistan showed a daily firewood demand of up to 10 kg per family and day. More than half of the wood used in the average household goes for baking, and the remainder is used for cooking. Additional wood is needed for heating in the wintertime, of course.
Despite the fact that above examples indicate that the required amounts of cooking energy are extremely variable much cooking energy can be saved by using solar cookers.
The prime function of solar cookers is to help reduce firewood consumption, since most cooking fires are still fuelled with firewood. The trouble is, firewood is usually quite inexpensive in comparison with kerosene, bottled gas or electricity (based on relative energy content).Increasing, uncontrolled felling of wood for people’s own use and for selling are a main cause of deforestation, desertification, erosion, receding groundwater levels and it has long-term adverse effects on the ecological balance. Pakistan’s meager forest heritage and rampant deforestation in Kenya show that such fears are well-founded. If denudation of the Sudan’s forests continues at the present rate, they will be gone by the year 2005.
For most solar cookers, little data is available on the actual cost of production. Since most of those solar cookers are prototypes that do not yet display the technical maturity needed for series production, pertinent information is of low indicative value. Due to the chronic shortage of foreign currency in the Third World, preference should be given to cookers that can be made locally using indigenous materials.
The problem is that practically any amount of money paid for solar cooker, however small, would still be too expensive for most rural households as long as firewood can be gathered for free and the farmers earn very little money.
On the whole, solar cookers could, at best contribute little toward a national energy policy. But they could make a very substantial contribution toward improving the living conditions of the poor and helping them overcome their own energy crisis.

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