Heat pump Systems

Active solar energy systems can also be combined with heat pumps for domestic water heating or space heating. In residential heating, the solar energy 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.

A heat pump is a device that pumps heat from a low-temperature source to a higher-temperature sink. Heat pumps are usually vapor compression refrigeration machines, where the evaporator can take heat into the system at low temperatures and the condenser can reject heat from the system at high temperatures. In the heating mode, a heat pump delivers thermal energy from the condenser for space heating and can be combined with solar heating. In the cooling mode, the evaporator extracts heat from the air to be conditioned and rejects heat from the condenser to the atmosphere, with solar energy not contributing to the energy for cooling. The performance characteristics of an integral type of solar-assisted heat pump are given by Huang and Chyng (2001).

Return air

Return air

Return Air Diagram
FIGURE 6.16 schematic diagram of a domestic water-to-air heat pump system (series arrangement).

Heat pumps use mechanical energy to transfer thermal energy from a source at a lower temperature to a sink at a higher temperature. Electrically driven heat pump heating systems have two advantages compared to electric resistance heating or expensive fuels. The first one, as was seen in Chapter 5, Section 5.2.4, is that the heat pump's ratio of heating performance to electrical energy (COP) is high enough to yield 9-15 MJ of heat for each kWh of energy supplied to the compressor, which saves on the purchase of energy. The second is the usefulness for air conditioning in the summer. Water-to-air heat pumps, which use solar-heated water from the storage tank as the evaporator energy source, are an alternative auxiliary heat source. Use of water involves freezing problems, which need to be taken into consideration.

Heat pumps have been used in combination with solar systems in residential and commercial applications. The additional complexity imposed by such a system and extra cost are offset by the high coefficient of performance and the lower operating temperature of the collector subsystem. A schematic of a common residential type heat pump system is shown in Figure 6.16. During favorable weather conditions, it is possible with this arrangement to have solar energy delivered directly to the forced air system while the heat pump is kept off.

The arrangement shown in Figure 6.16 is a series configuration, where the heat pump evaporator is supplied with energy from the solar energy system, called a water-to-air heat pump. As can be seen, energy from the collector system is supplied directly to the building when the temperature of the water in the storage temperature is high. When the storage tank temperature cannot satisfy the load, the heat pump is operated; thus it benefits from the relatively high temperature of the solar energy system, which is higher than the ambient and thus increases the heat pump's COP. A parallel arrangement is also possible, where the heat pump serves as an independent auxiliary energy source for the solar energy system, as shown in Figure 6.17. In this case, a water-to-water heat pump is used.

Heat Pump Install Parallel

Pump Pump Pump

FIGURE 6.17 schematic diagram of a domestic water-to-water heat pump system (parallel arrangement).

Pump Pump Pump

FIGURE 6.17 schematic diagram of a domestic water-to-water heat pump system (parallel arrangement).

The series configuration is usually preferred because it allows all the solar collected power to be used, leaving the tank at a low temperature, which allows the solar energy system to work more effectively the next day. Additionally, the heat pump performance is higher with high evaporator temperatures. An added advantage of this system is that the solar energy system is conventional, using liquid collectors and a water storage tank. A dual-source heat pump can also be used, in which another form of renewable energy, such as a pellets boiler, can be used when the storage tank is completely depleted. In such a case, a control system selects the heat source to use for the best heat pump COP, i.e., it selects the higher of the two heat sources. Another possible design is to use an air solar heating system and an air-to-air heat pump.

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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Responses

  • Ed Johnson
    I am looking for any information on assisting an air-to-air heat pump, especially in winter for Residential use. Has anyone actually DONE IT. I will with in the year, but I would like to not reinvent the wheel if someone else has already got ideas. I plan to house it the outside unit in a small room size 'house' to have solar gain in the winter and shade and cool breeze in the summer.
    8 years ago

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