Solar Energy - Chapter 1

Introduction

PURPOSE OF THIS PUBLICATION

This publication is aimed at people who are interested in finding out whether they can use solar energy for one or more applications in their daily lives, or in the lives of those with whom they work. This means that the authors have chosen to limit themselves to small-scale applications of solar energy. In other words, the reader will find nothing about large scale thermal power stations, solar heat for industrial processes, solar air-conditioning, etc.

Instead, thermal applications such as water heating, drying and cooking will be discussed, as well as electricity generation using small photovoltaic systems.

HISTORY

Solar energy has been used since time immemorial to dry agricultural products, to provide space heat in cold seasons or to create ventilation in homes, applications which are still used in many developing countries. More than two thousand years ago, Heron of Alexandria constructed a simple water pump driven by solar energy and in 214 B.C. Archimedes of Syracuse used concentrating solar mirrors to set fire m Roman ships.

The daily work of those complex and elegant solar collectors, the leaves of plants and trees, directly or indirectly provides our food, creates the cooking fuel for millions of households throughout the world, and has created all our fossil fuel reserves in the past.

This does not imply that there is nothing new in applying solar energy (solar photovoltaic cells are only a few decades old), but some historical insight helps to put things in perspective. People have been using and are still using solar energy technologies without even knowing the term, simply because it is useful and practical to them.

CHARACTERISTICS

At the present moment two methods exist by which sunlight can be converted into directly usable energy: conversion to warmth (thermal energy) and conversion to electricity (photovoltaic energy). In the first method, for example, sunlight is absorbed by a blackened surface, which then warms up. If air or water is passed alongside or through this warmed surface, it too will be warmed. In this way the warmth can be transported to wherever it is needed. For storage, an insulated chamber is usually employed, From which, for example, hot water can be drawn. This, in brief, is a principle of thermal conversion.
In photovoltaic conversion, sunlight falling onto a 'solar cell' induces an electrical tension; a number of cells combined in a panel are capable of generating enough current to drive an electric pump or to charge a battery.

LIMITATIONS

Whenever one is convinced that new solar technologies should be used by rural people, one should start by appreciating their own experience, looking at how they use their own resources and then find out together whether the new technology could be of any use to them and how it could be introduced. An important part of this process is a discussion of not only the advantages but also the limitations of the new technology.

The source of energy, solar radiation, is free, but the equipment needed to persuade the solar rays to do useful work can sometimes be expensive, usually requires maintenance and needs certain understanding of how things work.

It is a pity that there are so many examples in developing countries of solar energy equipment which has been 'dumped' into villages, without even asking if it could be of any use to the inhabitants. The ability of rural people to recognize immediately the benefits of a new technology, and to rapidly absorb it, is often underestimated. The first step For the introduction of any new technology should be the needs of the people for which it is aimed, and usually they know their needs much better than we do.

FLEXIBILITY

One of the beautiful characteristics of a solar equipment is that it can be made in varying degrees of perfection and in a wide range of sizes and costs. This implies that it can be of use for a wide social range as well, from the farmer who dries his grains, to a Minister of Agriculture who uses a solar water heater for his shower.

Let us make the example of solar drying, one of the oldest solar applications of mankind. The simplest solar dryer, at zero cost, is a black asphalt road on which people spread their grains to increase the natural (solar) drying process. The bamboo racks on which Thai fishermen put their fish are a little more sophisticated, but still represent a very cheap type of solar dryer. The solar timber kilns, which have been tested in many Asian countries, require much more care in design, can be quite expensive (although locally made) and are not meant for individual small-scale use. In Chapter 2 the reader will find more on the subject of solar dryers.

Solar water heating shows the same wide range of sophistication. On the one hand there is a blackened water tank which was used in Japan to heat bath water to fill the family bath at the end of the day. Or a cheap plastic tube filled with water which will heat up rapidly during a sunny day. Then there is the solar collector, such as discussed in Chapter 3, which can be made of locally available materials, and provides sufficient amounts of hot water for a small dispensary to save them collecting (or paying for) a large amount of firewood.

Solar cooking is one of the most debated applications, which is partly explained by the fact that making is one of the most important daily activities of every households in the world. Anyone who has cooked meals by himself knows that cooking energy by preference should be easy to handle, the power it produces should be easy to control, and the power should be available when the cook wants it. This is why gas cookers are so easily introduced, once people can afford it. Solar cookers have difficulty complying with these demands, which is why many rural households are not actually using them. In particular, the parabolic solar cookers, which are such nice toys for researchers and policy makers, have hardly been applied. The solar hotboxes are much better suited for their purpose, although Chapter 5 clearly warns that the users should be told that it is an additional cooking device.

Another example, at the other end of the technology scale, is the photovoltaic (PV) cell. They are applied in cheap solar calculators, in use throughout the world, but also the use of PV panels for street lighting, home lighting and for powering refrigerators in rural hospitals is increasing rapidly as discussed in Chapter 4.
A solar-powered telephone can be immensely useful for farmers wanting information about market prices or to arrange transport. These developments take place in spite of the fact that PV systems are (still) expensive, have to be imported, require care in handling, etc. In other words, people see a benefit in using them, and are therefore prepared to pay for it.

MORE INFORMATION

For those who require more detailed information references are presented and, if necessary, additional information can be provided by the TOOL Foundation in Amsterdam.

1.1 REFERENCES

• GATE, Solar energy. Status report, Vieweg, Braunschweig, FR Germany, 1986.