Small scale applications in
ISBN 90 70857 19 7
(c) Stichting TOOL, Amsterdam 1990. All rights reserved.
Table of contents
4. Cooking with solar energy
4.2 Social aspects
2. Solar dryers
4.3 The parabolic solar cooker
4.4 The cooking box
2.2 Principles of drying with the sun's warmth
2.3 The principle of the flat-plate collector with
2.4 Different designs and constructions of solar
5. Electricity from the sun
2.5 Practical tips
5.2 The case for PV
5.3 Solar cells
3. Warming water with solar energy
5.4 Balance of system
5.5 PV-system characteristics
General recommendations for PV-system
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.
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.
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.
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.
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.
For those who require more detailed information references are presented and, if necessary,
additional information can be provided by the TOOL Foundation in Amsterdam.
• GATE, Solar energy. Status report, Vieweg, Braunschweig, FR Germany, 1986.
2.2 Principles of drying with the sun's warmth
2.3 The principle of the flat-plate collector with cover
2.4 Different designs and constructions of solar driers
2.5 Practical tips
In contrast to water heating and the generation of electricity, crop drying utilizes the sun's energy
Using solar energy to dry crops is nothing new in the tropics. Many edible, and even cash crops
such as cocoa and coffee beans, have for decades been dried on racks placed in the sun.
2.2 PRINCIPLES OF DRYING WITH THE SUN'S WARMTH
Imagine a closed heated space in which a damp agricultural crop has been stored.
Two things happen:
• the crops is warmed by the heat from the stove of fire
• air around the heat source is heated up - whereby it can take up a great deal of moisture -
and, rising, is continually replaced.
As the crop is warmed up, including the air between the plant fibres, the water it contains quickly
evaporates. Pretty soon the air within and surrounding the crop is saturated with water vapour.
Fortunately the air moving alongside, warm and unsaturated, can take up this moisture and
transport it away. A small fan will of course help this process, but it is not strictly necessary.
At a certain moment the air in the room has taken up so much moisture from the crop that the
windows suddenly mist up (though this will depend on the outside temperature); the air against
the cold windows has been cooled to below the 'dew point'.
In this way the water in the crop is transferred to the window panes, where it can be wiped off, or
allowed to fall into a gutter which leads outside the room.
If in this account 'heat source' is replaced by 'sun', a solar drier has effectively been described.
The 'cold window' (which works as a condensor) is sometimes encountered in indirect drying,
where the warming of the air and the drying of the crop are separated, if the product has been
stacked too high or too close together. See also paragraph 3.2.
Solar drying is a technique particularly suited to the warmer parts of the world, since:
• there is abundant sunlight.
• the air temperature is high and relatively constant over the whole year.
Figure 1. Annual mean global irradiance on a horizontal plane at the surface of the earth W/m averaged over 24
hours (Source: Budyko, 1958)
A high and stable air temperature is actually just as important as the sunshine itself, since it
limits loss of generated warmth. It allows a simple solar drier to maintain the temperature of the
drying crop during the day around 40°C.
Drying edible crops
The temperature within the solar drier is higher than that outside it. Consequently water on and
in the product evaporates. The air takes up more and more of this moisture until a certain
equilibrium is reached. Ventilation ensures that this saturated air is replaced with less saturated
air, and so the product eventually dries out.
Drying is intended to evaporate and dispel the free water in a product, to make it unavailable to
micro organisms. This water can also be bound, by adding salts (pickling) or sugar (preserving).
Both techniques can also be used after drying.
Dried products attract moisture from the air, just as salt does. This moisture remains much freer -
to micro organisms - than the moisture which was removed from the product; so even in
conditions of relatively low humidity the product will rot.
This means that dried products must be given airtight packing unless the humidity is otherwise
controlled, for example in a silo.
The level of dessication, i.e. the unavailability of free water, at which decay is stopped varies
from product to product.
Table 1. Specifications for drying of agricultural products.
(Source: Herbert et al., 1984)
(water content %)
The warmth in the drier actually encourages rotting in products that are not yet completely dried.
For this reason the speed at which the drying takes place is important. The fastest drying is
brought about by strong ventilation with dry air.
Under such circumstances the difference between the internal and external temperature is less
important than simply getting rid of the moisture as fast as possible. At a later stage the
evaporation is less abundant, and much more temperature dependent. If the ventilation is now
limited, the air in the drier will be warmed up, and the drying process improved further.
These considerations apart, the quality of the original product (its freshness and cleanliness) and
of the drying air both exert a critical influence on the quality of the end product.
Forced drying using warm air circulation
Good ventilation is of crucial importance. It determines on the one hand the exchange of warmth
from the absorbent surface to the air next to it and on the other hand the evaporation of the water
on and in the product. A stronger ventilation leads to a lower average temperature but also to a
more efficient overall transfer of warmth. This leads to a reduction in the relative humidity and
Electric fans strongly increase the transfer of warmth to the drying air. This is especially true if
the product is stacked close together, impeding the air circulation. It is important, therefore, to
rack and shelve the products in such a way that the air circulation is impeded as little as possible.
Forced air circulation is only worthwhile if sufficient solar energy can be taken in by the drier;
this supposes a large enough (with regard to the mass to be dried) and efficient enough absorbent
surface (for example, porous materials), and special glass for covering.
If these factors are not taken into account, the temperature within the drier will not be much
higher than that outside it - which of course does not promote efficient drying, and certainly not
at the last drying stage. Forced air circulation becomes economic in larger installations drying
50-100 kg per day or more. In non-forced air circulation, or natural ventilation a site is chosen
which makes best use of prevailing winds, the air inlet and outlet being oriented accordingly, or a
chimney is added to improve the draught.
2.3 THE PRINCIPLE OF THE FLAT-PLATE COLLECTOR WITH
The principle underlying the solar collector is that 'visible light' falling onto a dark object is
converted into tangible warmth. The colour of the object does not in fact need to be black; it is
rather the absorptive qualities of the material which determine the effect. A painted plate can be
warmed, but so can a suitable fibrous material such as charred rice chaff.
The cover is of secondary importance, but still has a decisive influence on the total working
efficiency; it prevents the created warmth from being blown away and also limits the warmed-up
objects' heat loss through reradiation. Moreover it allows a controlled airstream over the warmed
objects, which would not otherwise be possible.
To exploit the warmth in the heated objects or surface a medium (water, air) is directed alongside
which takes up the warmth and takes it to wherever it is needed. When air is used, it can pass
under the collector, above it, or through canals embedded within it. It can be a 'forced' or a
'natural' current. The various possibilities are examined in paragraph 3.2.
In drying, the relative and absolute humidity are of great importance. Air can take up moisture,
but only up to a limit. This limit is the absolute (= maximum) humidity, and is temperature
In practice, however, the air is very rarely fully saturated with moisture. The degree of saturation
at a given temperature is called the relative humidity and is expressed as a percentage of the
absolute humidity at that temperature.
If air is passed over a moist substance it will take up moisture until it is virtually fully saturated,
that is to say until absolute humidity has been reached.
However, the capacity of the air for taking up this moisture is dependent on its temperature. The
higher the temperature, the higher the absolute humidity, and the larger the uptake of moisture.
If air is warmed the amount of moisture in it remains the same, but the relative humidity falls;
and the air is therefore enabled to take up more moisture from its surroundings.
If fully-saturated air is warmed and then passed over the objects to be dried, the rise in absolute
humidity (and the fall in relative humidity) allows still more water to be taken up.
Figure 2. Simple solar dryer
Basic technical details of the drier
Every solar drier is constructed using the same basic units, namely:
a. A transparent cover which admits sunlight and limits heat loss (glass or plastic)
b. An absorbent surface, made dark in colour, which takes up sunlight and converts it to
warmth, then giving this warmth to the air within; this can also be the product that needs
c. An insulating layer underneath
d. An air intake and an outlet, by which means the damper air can be replaced with fresh
These four elements can be modified if necessary, and/or other elements added, for example a
fan or a chimney.