Solar thermal collectors and applications

Solar space heating and cooling

The components and subsystems discussed in Section 5.1 may be combined to create a wide variety of building solar heating and cooling systems. There are again two principal categories of such systems, passive and active.

The term passive system is applied to buildings that include as integral part of the building elements, that admit, absorb, store and release solar energy and thus reduce the needs for auxiliary energy for comfort heating. As no solar collectors are employed in the passive systems in this paper, only active systems are considered.

Systems for space heating are very similar to those for water heating, described in Section 5.1, and as the same considerations for combination with an auxiliary source, boiling and freezing, controls, etc., apply to both these may not be repeated again. The most common heat transfer fluids are water, water and antifreeze mixtures and air. The load is the building to be heated. Although it is technically possible to construct a solar heating or cooling system which can satisfy 100% the design load, such a system would be non­viable since it would be oversized for most of the time. The size of the solar system may be determined by a life-cycle cost analysis described in Section 4.7.

Active solar space systems use collectors to heat a fluid, storage units to store solar energy until needed, and distri­bution equipment to provide the solar energy to the heated spaces in a controlled manner. A complete system includes additionally pumps or fans for transferring the energy to storage or to the load which require a continuous availability of non-renewable energy, generally in the form of electricity.

The load can be space cooling, heating, or a combination of these two with hot water supply. In combination with conventional heating equipment solar heating provides the same levels of comfort, temperature stability, and reliability as conventional systems.

Active solar energy systems can also be combined with heat pumps for water heating and/or space heating. In residential heating the solar system can be used in parallel
with a heat pump, which supplies auxiliary energy when the sun is not available. Additionally, for domestic water systems requiring high water temperatures, a heat pump can be placed in series with the solar storage tank.

During daytime the solar system absorbs solar radiation with collectors and conveys it to storage using a suitable fluid. As the building requires heat it is obtained from storage. Control of the solar system is exercised by differential temperature controllers, i. e. the controller compares the temperature of the collectors and storage and whenever the temperature difference is more than a certain value (7-10 °C), the solar pump is switched ON. In locations where freezing conditions are possible to occur, a low-temperature sensor is installed on the collector which controls the solar pump when a pre-set temperature is reached. This process wastes some stored heat, but it prevents costly damages to the solar collectors.

Solar cooling of buildings is an attractive idea as the cooling loads and availability of solar radiation are in phase. Additionally, the combination of solar cooling and heating greatly improves the use factors of collectors compared to heating alone. Solar air conditioning can be accomplished by three types of systems: absorption cycles, adsorption (desiccant) cycles and solar mechanical processes. Some of these cycles are also used in solar refrigeration systems and are described in Section 5.3. The rest of this section deals with solar heating and service hot water production. It should be noted that the same solar collectors are used for both space heating and cooling systems when both are present.

A review of the various solar heating and cooling systems is presented in Ref. [139]. A review of solar and low energy cooling technologies is presented in Ref. [140].