Heat Distribution for Liquid Types of Solar Systems

The temperature requirements of a hydronic heating system depend on the amount of heat exchanger surface. Most baseboard heaters have comparatively small surface areas, so they require higher temperatures, typically about 180F. If larger heat transfer areas are available, as in older or modified hot-water systems, temperatures of 120F maybe sufficient. Temperatures of 100F to 120F are adequate for the system that uses entire floors, walls, and ceilings as radiator surfaces.

During the winter, typical liquid types of solar systems are seldom operated at delivery temperatures above 150F. Clearly the use of

Figure 15-11.-Minimum heating system showing relationship of collector, storage, and room unit heater.

Figure 15-12.-Space heating system with a closed-loop collector.

solar-heated water in standard baseboard heaters is impractical. Only modified baseboard heaters of adequate size or radiant panels are suitable for use in hydronic systems that use solar-heated water. One economical means of auxiliary heat supply and heat distribution for liquid types of solar systems involves the use of a warm-air system. A typical system is shown in figure 15-15. In this system, the warm-air furnace is located downstream from a liquid-to-air heat exchanger supplied with solar-heated water. The furnace can then serve to boost air temperature when insufficient heat is available from the solar-heated water, or it can meet the full heat load when no heat is available in solar storage. Auxiliary heat can be supplied by a gas, oil, or electric furnace, or by the condenser of an air-to-air heat pump. Another method of heat distribution is to use a water-to-air heat pump that draws heat from the solar storage tank and pumps it to a condenser coil placed in a central air duct. The advantage of this system is that it can effectively use heat from solar storage at temperatures down to 45F; thus more of the stored heat is available. Also, average storage temperatures are lower, resulting in significantly increased collector efficiency. Some manufacturers are combining solar systems with heat pumps to reduce auxiliary energy costs. When a heat pump and solar system are combined

Figure 15-13.-Space heating and domestic hot-water systems.

in this manner, the system is usually called a solar-assisted or solar-augmented heat pump (SAHP) system.

Solar-assisted heat pump systems are configured in many different ways. For example, the solar collectors can be either water or air types; the heat storage medium can be water or a solid material, such as rock or brick; and the heat pump can be of either the air-to-air design or the water-to-air design. But heat pumps have a characteristic that can limit their effectiveness: the efficiency and capacity of a heat pump decrease as the temperature of the heat source (usually outdoor air) decreases. This deficiency can be overcome, however, by using solar collectors to gather the energy of the sun to keep the heat source in the temperature range required for efficient heat pump operation.

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