Indirect Water Heating Systems

A schematic diagram of indirect water heating systems is shown in Figure 5.11. In this system, a heat transfer fluid is circulated through the closed collector loop to a heat exchanger, where its heat is transferred to the potable water. The most commonly used heat transfer fluids are water-ethylene glycol solutions, although other heat transfer fluids such as silicone oils and refrigerants can be used. When fluids that are non-potable or toxic are used, double-wall heat exchangers should be employed; this can be two heat exchangers in series. The heat exchanger can be located inside the storage tank, around the storage tank (tank mantle), or external to the storage tank (see Section 5.3). It should be noted that the collector loop is closed; therefore, an expansion tank and a pressure relief valve are required. Additional over-temperature protection may be needed to prevent the collector heat-transfer fluid from decomposing or becoming corrosive.

Systems of this type using water-ethylene glycol solutions are preferred in areas subject to extended freezing temperatures, because they offer good freeze protection. These systems are more expensive to construct and operate, since the solution should be checked every year and changed every few years, depending on the solution quality and system temperatures achieved.

Typical collector configurations include the internal heat exchanger shown in Figure 5.11, an external heat exchanger shown in Figure 5.12a, and a mantle heat exchanger shown in Figure 5.12b. A general rule to follow is that the storage

Array of solar collectors

Outdoor equipment

Roof slab

Indoor equipment Relief valve

Outdoor equipment

Roof slab

Indoor equipment Relief valve

Gravity Indirect System

Cold water IN

FIGuRE 5.11 Indirect water heating system.

Cold water IN

FIGuRE 5.11 Indirect water heating system.

From solar collector

External ^

To solar collector

Hot water OUT Relief valve -►

Hot water OUT Relief valve -►

From solar collector

External ^

To solar collector

Indirect Hot Water System

Hot water OUT Relief valve -►

From solar collector

Tank mantle

To solar collector

Hot water OUT Relief valve -►

From solar collector

Tank mantle

To solar collector

Mantle Tank For Solar Heating System

FIGuRE 5.12 External and mantle heat exchangers. (a) External heat exchanger. (b) Mantle heat exchanger.

tank should be between 35 and 70 L/m2 of collector aperture area, while the most widely used size is 50 L/m2. More details on internal heat exchangers are given in Section 5.3.2.

For freeze protection, a variation of the indirect water-heating system, called the drain-back system, is used. Drain-back systems are generally indirect water heating systems that circulate water through the closed collector loop to a heat exchanger, where its heat is transferred to potable water. Circulation continues as long as usable energy is available. When the circulation pump stops, the collector fluid drains by gravity to a drain-back tank. If the system is pressurized, the tank also serves as an expansion tank when the system is operating; in this case, it must be protected with temperature and pressure relief valves. In the case of an unpressurized system (Figure 5.13), the tank is open and vented to

268 Solar Water Heating Systems Solar collector array m

Roof slab

Vent-

Fill line1

Drain-back-tank

To drain

FiGURE 5.13 Drain-back system.

Outdoor equipment

Sight glass Indoor equipment ff Relief valve Hot water OUT

Fill line1

Drain-back-tank

Cold water IN

the atmosphere. The second pipe directed from the collectors to the top of the drain-back tank is to allow air to fill the collectors during drain-back.

Because the collector loop is isolated from the potable water, no valves are needed to actuate draining and scaling is not a problem; however, the collector array and exterior piping must be adequately sloped to drain completely. Freeze protection is inherent to the drain-back system because the collectors and the piping above the roof are empty whenever the pump is not running. A disadvantage of this system is that a pump with high static lift capability is required in order to fill the collector when the system starts up.

In drain-back systems, there is a possibility that the collectors will be drained during periods of insolation; it is therefore important to select collectors that can withstand prolonged periods of stagnation conditions. Such a case can happen when there is no load and the storage tank reaches a temperature that would not allow the differential thermostat to switch on the solar pump.

An alternative design to the one shown in Figure 5.13, which is suitable for small systems, is to drain the water directly in the storage tank. In this case, the system is open (without a heat exchanger) and there is no need the have a separate drain-back tank; however, the system suffers from the disadvantages of the direct systems outlined in Section 5.2.1.

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