The principle of solar thermal captors is simple:
- A
plain
surface, the captor itself, to recover solar heat;
- A circulation system to carry that heat to a storage place or to its end use;
- A system of control, to maintain the desired temperature.
Captors are of 3 types, according to the temperature required:
Non-vitreous captors: they are long black tubes (the colour which best absorbs heat) made of plastic or metal, in which water circulates. They are not insulated, which means that the temperature increase obtained is low: +20°C compared to the temperature of the air. These captors are very suitable for heating swimming pools.
Plain
captors: they consist of an insulated box with a glass or plastic window fixed on top. Inside, a black metallic sheet absorbs the heat of the sun, which is retained in the box. This heat is transmitted to air, water, or another thermal carrying fluid that does not freeze in winter. The thermal carrying fluid circulates freely, or in pipes, to the point where the heat is used. The temperature increase compared with the ambient air temperature can reach +70°C. It is ideal for producing household hot water or for heating all types of buildings.
Vacuum captors: they take the form of a panel on which a series of transparent glass tubes are aligned. Inside these tubes, there is a vacuum, which is one of the best thermal insulations that exist. In each tube, an absorber captures the solar heat that is transmitted to a thermal carrying fluid by means of a heat exchanger system. In the same way as for the other types of captor, the thermal carrying fluid then circulates to the point where the heat is used. In this system, heat losses are very low. The temperature can rise to 100/140°C. Such captors are suitable for industrial applications where high temperatures are required.
The circulation system is driven by a circulation pump or a thermo-siphon. The latter device depends for its operation on the fact that hot water is lighter than cold. In a circuit of vertical pipes, the hot water rises by itself and the cold water goes downwards. A thermo-siphon can only work if the point where the hot water is used is located above the captor. A system of temperature probes and a thermostat controls the starting and stopping of the circulation pump.
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