Solar Energy - Chapter 4

Cooking with solar energy

CONTENTS

4.1 Introduction

4.2 Social aspects

4.3 The parabolic solar cooker

4.4 The cooking box

4.5 Conclusion

4.6 References

4.1 INTRODUCTION

Cooking by means of solar energy started in the '70s in response to the growing shortage of firewood resulting from chronic deforestation. A solar cooker needed no wood - just sun! With this in mind, dozens of research institutes developed solar cookers.
There are several types. The most important is the solar cooking box, which will be described in more detail in the last section. To start with we will look at another type that enjoyed much attention in the past - the parabolic model.

However, first of all something will be said about problems that may be expected in the introduction of solar cookers; it has become clear that several projects in which solar cookers were introduced were failures because insufficient account was taken of the local population.

4.2 SOCIAL ASPECTS

The social aspects of solar cooker introduction schemes can cause important problems, as cooking with solar energy differs radically from other cooking methods. Because of this, solar energy offers only a supplement to other energy sources, and can never provide a 100' replacement. Besides this, buying a solar cooker will cost a lot of money.
In practice it is hard to explain that a technique with a limited application can still be economic. The applications are limited because:

• the sun does not always shine
• not all meals can be prepared in a solar cooker.

To begin with the sunshine: many people cook at dawn or dusk. A solar cooker cannot, therefore, be used. It is often difficult and sometimes impossible to change these cooking habits. Someone who walks an hour to the fields to spend all day there is not going to come home at midday to cook and eat. On sunless days the cooker is also useless.
Not all meals can be prepared in a solar cooker. The more important the meal is, the worse this problem becomes. The cooking box has two serious drawbacks: it cannot be opened during the cooking, to stir a meal or to add ingredients; and food cannot be fried or roasted in it. In the cooking box that will here be described the temperatures are not high enough.
There are models, which allow for frying and roasting, but they are complicated and less convenient. A cooker using a parabolic reflector is less of a problem but this can heat only one pan at a time.

The purchase price is a problem for the poorer groups especially; the cost recovery time varies widely according to local circumstances and in many instances can be as long as two or three years. The apparatus must therefore be made to a high quality, using accordingly expensive materials.
The image of the cooking box also plays a part. In areas where the 'modern, western life' is in demand, such a simple appliance may be rejected for vague reasons even when it is economically acceptable.

Finally a few remarks concerning its use. The cooker must obviously be so sited as to receive full sunlight. Children at play can easily damage a solar cooker so it also has to be out of their way; in towns, a flat roof is sometimes a suitable place.
The different appliances can be heavy or awkward to move; facilities for an eventual repair should be nearby.
Lastly, the users have to learn how to handle the cooker. The cooking technique is so unfamiliar, especially for the cooking box, that guidance during its introduction and afterwards is vital. If its users are at a loss to know how to use it, it quickly falls into disuse.

The problems outlined above are not intended to discourage the introduction of a solar cooker, but to illustrate that careful planning and preparation are a more than usually important aspects of the introduction of this technique.

4.3 THE PARABOLIC SOLAR COOKER

The parabolic or concentrating solar cooker reflects the sun's rays in such a way that these are converged onto a small area. In this area a dark metal cooking pot is fixed. Because of the small size of the area of convergence there is room for only one pot. It can be warmed up between 150 and 350°C; enough to fry. See Fig. 10.

Figure 10. Parabolic solar cooker

However, the design does suffer some serious disadvantages. The reflector has to be realigned as often as every ten minutes, because the sun's movement shifts the convergence point accordingly. This means continual attention during cooking. Another disadvantage is the higher temperature - carelessness can result in burns - and the disagreeable reflections of light.
Besides the problems of its use, the construction of such a parabolic reflector poses difficulties. The reflecting surface's curve must be extremely precise; this exactness is not always achieved. The apparatus is, moreover, often sensitive to wind.

All in all the parabolic solar cooker suffers considerable technical drawbacks. These are partly obviated by the solar cooking box described below.

4.4 THE COOKING BOX

The cooking box works on the principle of the retention of warmth. The box must be well insulated, so as to retain the energy that shines into it; for this reason the box cannot be opened during cooking. If this should happen, the loss of heat will considerably slow down the preparation of the meal. This limitation also means that meals that need to be prepared step by step are unsuited; so an effective menu plan, prepared beforehand, is viral.
A cooking box is easy enough to make on one's own; the following introduction looks at what comes into such construction.

The highest attainable temperature depends largely on the design. The more sophisticated it is, the higher the temperature. The simplest example of such a cooking box is shown in Fig. -11. It is a box, with double walls of wood or of metal, and a lid comprising two layers of glass. Insulating material fills the space between the double walls. This simple version can reach a temperature of 50 to 70 degrees above the outside temperature.

Figure 11. Simple cooking box

An excellent and long-lasting insulation material is rock wool, but in most developing countries this is either unavailable or very expensive indeed.

Other insulating materials include cotton, wood shavings, coconut fibre and straw.

These materials do not need to be packed densely in, or scattered loosely; firm pressure is enough. The material must be very dry and it must remain so; for this reason a metal inner box is preferable to a wooden one which is comparatively difficult to make watertight.
It is advisable to paint the bottom of the inner surface of the cooker matt black. The sides can be painted reflective (shining). As this is to obtain the best possible absorption of the sun's rays, a matt paint is most important, and a matt dark blue works better than a shiny black.
It is advisable to have outward-tapering walls rather than upright ones, as these prevent shadowing the interior; and high temperatures are more likely if the whole box is shallow.

The food to be cooked is placed, in a matt black pan, in the box. The pan may be made of thin metal; this often gives the best results, but such pans are not too strong. The pan should be covered with a lid, to prevent condensation forming on the glass and to speed the cooking. The cooking time in such a cooking box is considerably longer than normal.

The temperatures reached by the design shown in Fig. 11. would be too low for many cooking purposes. Filling the box with one or more reflectors can appreciably raise the maximum temperature. An example of a cooking box filled with a reflector is shown in Fig. 12. The reflector increases the amount of incident light, whereby higher temperatures are attained more quickly.

However, the use of reflectors also has its disadvantages:

• The box must be aimed at the sun more frequently
• The construction becomes a little more complex
• The reflector makes the cooker more expensive
• The cooker is less stable (gusts of wind)
• The more reflectors are used, the more these disadvantages will prevail

Figure 12. Example of a solar cooking box with reflector

The reflector can be made from various materials. Ordinary mirrors can be used, and are good reflectors, but they are expensive and fragile. Aluminium foil is an alternative but is hardly durable; plastic containing a layer of aluminium, sometimes sold under the name Mylar, is longer-lasting.
Lastly, a simple reflector can be made from a polished aluminium sheet. In most instances it provides an adequate degree of reflection.

4.5 CONCLUSION

Cooking with solar energy remains a fuel-saving technique, which can provide definite help in situations of fuel scarcity. Solar cookers and especially cooking boxes can be successfully locally made.
However, the introduction of a new way of cooking results in radical changes in the rhythm of life in a community. Many projects failed because of disregard for these social aspects, or because mistaken priorities meant that insufficient attention was paid to the problems of introduction.
In fact, intensive guidance during the introduction period is of crucial importance.

4.6 REFERENCES

• Hoda M.M., Solar Cookers, Appropriate Technology Development Association, Lucknow, India 1979.