Direct Circulation Systems

A schematic diagram of a direct circulation system is shown in Figure 5.9. In this system, 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. Because a pump is used to circulate the water, the collectors can be mounted either above or below 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. An important feature of this configuration is the spring-loaded

Array of solar collectors

Outdoor equipment

Roof slab

Indoor equipment Relief valve

Outdoor equipment

Roof slab

Indoor equipment Relief valve

Direct Circulation System

Cold water IN txM-

FIGURE 5.9 Direct circulation system.

FIGURE 5.9 Direct circulation system.

Cold water IN txM-

check valve, which is used to prevent reverse thermosiphon circulation energy losses when the pump is not running.

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 (calcium) deposits may clog or corrode the collectors.

Direct circulation systems can be used in areas where freezing is infrequent. For extreme weather conditions, freeze protection is usually provided by recirculating warm water from the storage tank. This loses some heat but protects the system. A special thermostat that operates the pump when temperature drops below a certain value is used in this case. Such recirculation freeze protection should be used only for locations where freezing occurs rarely (a few times a year), since stored heat is dumped in the process. A disadvantage of this system occurs in cases when there is power failure, in which case the pump will not work and the system could freeze. In such a case, a dump valve can be installed at the bottom of the collectors to provide additional protection.

For freeze protection, a variation of the direct circulation system, called the drain-down system, is used (shown schematically in Figure 5.10). In this case, potable water is also 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 the collector array and exterior piping from the make-up water supply with the normally closed (NC) valve and draining it using the two normally open (NO) valves, shown in Figure 5.10. 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 (see Section 5.4.2).

Array of solar collectors

Outdoor equipment

Roof slab

Indoor equipment

Outdoor equipment

Roof slab

Indoor equipment

Direct Circulation System

Cold water IN

FiGuRE 5.10 Drain-down system.

Cold water IN

FiGuRE 5.10 Drain-down system.

The check valve shown on the top of the collectors in Figure 5.10 is used to allow air to fill the collectors and piping during draining and to escape during fill-up. The same comments about pressure and scale deposits apply here as for the direct circulation systems.

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