Solar Energy - Chapter 3

Warming water with solar energy

CONTENTS

3.1 Introduction

3.2 Technology

3.3 Applications

3.4 References

3.1 INTRODUCTION

In this chapter we will examine the heating of water by means of solar energy. This has several applications; we consider the distillation of dirty water, the heating of clean water and the sterilization of water and of medical instruments.
Each of these applications makes specific demands on the design of a solar-energy installation. Nevertheless the basic principle of solar heating remains the same in all cases. The general introduction contained a summary of the principle; in the first section the technical details will be examined. Where possible, for each application we specify whether the installations are commercially available or whether they are suitable for do-it- yourself construction.

3.2 TECHNOLOGY

Collector

A surface faces the sun's rays and absorbs them, converting the radiation into warmth. The temperature of this surface, the so-called absorber, therefore rises. Every object placed in the sun exhibits this effect to a greater or lesser degree. A black surface shows the greatest rise in temperature; it absorbs about 90% of the sun's incident radiation and reflects very little.
The warmed absorber would however give its warmth to the surroundings again, if we take no further action. A heat-insulating layer can be brought against the underside of the absorber. The upper surface can be covered with a transparent screen of glass or ultraviolet-resistant plastic. In this way a sheet of air above the absorber is formed which also acts as an insulator. For the absorber's warmth to be employed it has to be conveyed away; if water channels are built into the absorber a heat exchanger has been made; if water is passed through, it takes the heat from the absorber.
These components, the absorber, the insulation and the screen are finally built into a closed container provided with connections for the supply and removal of the water. The whole thing is called a collector.

Figure 5. Solar collector schematic (Source: Deuss, 1987)

Besides this type there is a 'concentrating collector', in which the sun's rays are concentrated before falling onto the absorber. In principle, this type of collector can generate much higher temperatures. However, it has several disadvantages compared with the flat plate collector.
In the first place this type operates only under 'direct' illumination. Only those rays coming directly from the sun are effective; much 'diffuse' radiation, scattered by cloud, mist or dust, has little effect. The proportions of direct and diffuse radiation depend on the climate, the season and frequently also the time of day. The flat plate collector makes use of both direct and diffuse radiation. The use of the concentrating collector is therefore much more exacting.
In the second place the concentrating collector must be moved continuously to face the sun. There are mechanisms which make this possible but they are expensive, delicate, difficult to obtain and anything but maintenance-free.
Thirdly, a concentrating collector can easily inflict burns.

For these reasons we shall pay no more attention here to the concentrating collector.

Storage

The second important component in a solar energy water warming installation is the storage. The purpose of this is to bridge the intervals between the collector's supply and the user's demand for warm water. If the warmed water is held in an insulated tank, then in principle it is made available in the evening and the following morning. In the following applications we shall encounter heat-storage in various forms.

3.3 APPLICATIONS

Water distillation

The simplest application of a thermal solar energy installation is in the distillation of water. The solar distiller purifies water by first evaporating and then condensing it.
Distilled water contains no salts, minerals or organic impurities. It is not, however, aseptic, as is sterilized water; of which more later. Distilled water can be used for: drinking water, applications in hospitals, battery water, and so on.
Such an installation is suited to areas where water is ample but polluted, salty or brackish; naturally, there must also be abundant sun. Finally, glass or UV-resistant transparent foil - the most important materials in the construction - must be available and affordable.
A reasonably functional solar distiller is able to produce an average of four litres of distilled water per day per square meter of working surface.

The operation of the distiller will be described with reference to Fig. 6.

Figure 6. Solar still: simplified cross-sectional diagram

The radiation (A) falls through the glass or plastic screen (D) onto the absorber. In this case the absorber is a tray or basin filled with dirty water (B). Just as in the flat plate collector's absorber, this absorber works best if the basin is black. This is especially important if the water is clear; turbid water absorbs well enough on its own. The absorbed radiation, then, warms the basin and, gradually, the water. To reduce heat loss to a minimum it is vital to insulate the sides and bottom of the basin; if the basin rests on a dry surface this actually forms a reasonable insulation.
The water warms and then evaporates, leaving the impurities behind. This vapour (C) condenses on the underside of the screen (D) when this has a temperature appreciably lower than that of the water and the water vapour. This will certainly be the case if wind is cooling the screen, or the outside temperature falls as night falls. The condensate runs along the sloping screen and into a collecting gutter. To prevent the condensate from falling back into the water, the screen must be filled by at least 10 degrees from the horizontal. The whole distiller must be made as airtight as possible, to prevent loss of vapour. To achieve the best results the dirty water must be daily replaced by more water. Setting the whole distiller at a slight angle makes this straightforward.

It will be clear from this description that this distiller lends itself well to independent construction. More information on the various forms of construction are available from the WOT (1).

Solar boiler

Slightly more complex than the distiller is the solar boiler. This consists of one or more flat plate collectors and a insulated storage tank, and is designed for use as a water heater for hospitals, laundries, kitchens, showers, and so on.
A solar boiler with a collector surface of 3 to 4 m² and a storage capacity of 200 litres can provide 300 to 400 litres per day of water between 40°C and 60°C in temperature.
The yield is naturally dependent on the amount of sun and on a judicious use of the installation.
The conditions for the useful application of such a solar boiler are: one, a spot in direct sunlight as close as possible to the point of water use, and two, the straightforward supply of (unheated) water. If a water mains system is available, this of course offers the best solution.
Finally, the collector surface area and tank volume demand for warm water. The yield already mentioned, 300 - 400 litres per day at 40 - 60°C for a collector of 4 m and a 2001 tank, can be used as a guideline. If more water is drawn off, its average temperature will fall.

Figure 7. Solar collector of 1.6 m²

The operation of the solar boiler will be explained with reference to Fig. 7. The collector (A) is constructed according to the principles already described in paragraph 3.2. The collector is fixed at an angle to the horizontal, which approximates to the degree of latitude of the location itself.
The panel faces south if the location is in the northern hemisphere, north if in the southern. The storage tank (B) is so arranged that its lowest point is about 40 cm above the highest point of the collector. The warm water outlet (C) connects the higher outlet of the connector to the top of the warm water tank; a cold water supply pipe runs from the bottom of the storage tank to the lower part of the collector. The cold mains water supply also runs into the bottom of the tank, and the warm water draw-off pipe is fixed to the top of the tank. If the tank is now filled with cold water it will pass via (D) into the (lower) collector. If the sun shines the water will warm up and will pass via (C) to the top of the tank. This happens on its own, because warm water moves upwards. The sun therefore generates a circulation of water between the collector and the tank, called 'natural circulation' or 'thermosyphon'. No pump is needed. The water in the tank gradually warms up until it is drawn off and cold water comes in to replace it. If the storage tank is well insulated, the water remains warm into the evening and even the early morning. The nice thing about this self-starting system is that it is practically maintenance-free.

The construction of a solar boiler calls for more skilled work than is needed to make a solar distiller. Still, a solar boiler is suited to local production. An excellent handbook, in English, containing many practical tips and illustrations has been compiled by Bart Deuss. Step by step he describes the construction of the so called zig zag collector. He makes use of a special toolset developed by the Dutch firm Zonnevang (2), shown in Fig. 8. If, after studying the abovementioned handbook, it is decided against home construction of the collector, the same firm sells ready-made examples made especially for the tropics. Connecting a prepared oil drum to this form a storage tank is then comparatively easy.
It is also possible to make the principle tools (like a pipe bender) by your- self. More information on this can be obtained via the WOT (1).

Figure 8. Solar collector manufacturing tool box (Source: Zonne-energie Nederland BV)

Solar disinfector and sterilizer

It is possible to reach higher temperatures using solar energy than were quoted for the solar boiler. For most applications of the solar boiler these higher temperatures are not needed, however, if water has to be disinfected, or medical instruments sterilized, higher temperatures are required, and can be achieved using solar energy. The apparatus is considerably more complex and is not well suited to local construction.

Figure 9. Autoclave plant with portable sterilizer (Source: SUNICE)

Here we would like to refer to a water disinfection unit developed by EEG-International (3). The unit has a maximum capacity of eleven litres per hour of disinfected water and can reach a temperature of 95°C. It also makes use of natural circulation between collector and tank; when the tank water reaches the desired temperature, a thermostatically-controlled tap opens and the contents of the tank pass into a separate reservoir.

The Danish firm SUNICE (4) manufactures an 'autoclave plant'. This generates steam at a temperature of about 130°C. A portable autoclave can be connected to it, in which medical instruments can be sterilized.
According to the manufacturer, the contents of the autoclave are sterilized within 25 minutes - including deactivation of the most resistant bacteria, such as Clostridium tetani, and the virus for Hepatitus B. Fig. 9. shows a photograph of the steam-producing unit and the free-standing sterilizer.
The firm also makes a vaccine cooler also powered by solar collectors, as opposed to the vaccine cooler powered by photovoltaic cells which shall be described in another chapter.

3.4 REFERENCES

• Bachman A., Solar water heaters in Nepal. Installation manual, SATA, Kathmandu, Nepal, 1979.
• Deuss B., Zig zag collector. Manual on the construction of u solar water beater, TOOL/BACIBO, Amsterdam, The Netherlands, 1987.
• Streib J., Hot water from the sun. How to construct your own solar panel, FAKT, Stuttgart, FR Germany, 1989.