Solar thermal collectors and applications

Direct circulation systems (active)

In direct circulation systems, shown schematically in Fig. 29, a pump is used to circulate potable water from storage to the collectors when there is enough available solar energy to increase its temperature and then return the heated water to the storage tank until it is needed. As a pump circulates the water, the collectors can be mounted either above or below the storage tank. The optimum flow rate for such units is about 0.015 l/m2 of collector area. Direct circulation systems can be used in areas where freezing is not frequent. For extreme weather conditions, freeze protection is usually provided by recirculating warm water from the storage tank. Direct circulation systems often use a single storage tank equipped with an auxiliary water heater, but two-tank storage systems can also be used.

Direct circulation systems can be used with water supplied from a cold water storage tank or connected directly to city water mains. Pressure-reducing valves and pressure relief valves are required however when the city water pressure is greater than the working pressure of the collectors. Direct water heating systems should not be used in areas where the water is extremely hard or acidic because scale deposits may clog or corrode the collectors.

A variation of the direct circulation system is the drain - down systems shown in Fig. 30. In this case also potable water is pumped from storage to the collector array where it is heated. When a freezing condition or a power failure occurs, the system drains automatically by isolating

SHAPE * MERGEFORMAT

the collector array and exterior piping from the make-up water supply and draining it using the two normally open (NO) valves, shown in Fig. 30. It should be noted that the solar collectors and associated piping must be carefully sloped to drain the collector’s exterior piping when circulation stops. The same comments about pressure and scale deposits apply here as for the direct circulation systems.