Solar thermal collectors and applications

Solar collector applications

Solar collectors have been used in a variety of applications. These are described in this section. In Table 10 the most important technologies in use are listed together with the type of collector that can be used in each case.

5.1. Solar water heating systems

The main part of a SWH is the solar collector array that absorbs solar radiation and converts it into heat. This heat is then absorbed by a heat transfer fluid (water, non-freezing liquid, or air) that passes through the collector. This heat can then be stored or used directly. Portions of the solar energy system are exposed to the weather conditions, so they must be protected from freezing and from overheating caused by high insolation levels during periods of low energy demand.

In solar water heating systems, potable water can either be heated directly in the collector (direct systems) or

indirectly by a heat transfer fluid that is heated in the collector, passes through a heat exchanger to transfer its heat to the domestic or service water (indirect systems). The heat transfer fluid is transported either naturally (passive systems) or by forced circulation (active systems). Natural circulation occurs by natural convection (thermosyphon­ing), whereas for the forced circulation systems pumps or fans are used. Except for thermosyphon and integrated collector storage (ICS) systems, which need no control, solar domestic and service hot water systems are controlled using differential thermostats.

Five types of solar energy systems can be used to heat domestic and service hot water: thermosyphon, ICS, direct circulation, indirect, and air. The first two are called passive systems as no pump is employed, whereas the others are called active systems because a pump or fan is employed in order to circulate the fluid. For freeze protection, recircula­tion and drain-down are used for direct solar water heating systems and drain-back is used for indirect water heating systems.

All these systems offer significant economic benefits with payback times, depending on the type of fuel they replace, between 4 years (electricity) and 7 years (diesel oil). Of course, these payback times depend on the economic indices, like the inflation rates and fuel price applied in a country. A wide range of collectors have been used for solar water heating systems. A review of the systems manufac­tured in the last 20 years is given in Ref. [128].