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2.3 How a heat pump works In broad terms, air-to-air heat pumps work by using electrical energy to extract energy from the outside air and transfer it inside as heat, which makes it an efficient way of heating. Reversing the process enables indoor heat to be removed to provide cooling. A more detailed illustration of how a heat pump operates in heating mode can be seen in Figure 2.9. There are five main processes in the cycle: 1. In the evaporator (outdoor unit), low-pressure, low-temperature liquid refrigerant absorbs heat from its surroundings and evaporates, converting to a gas state and absorbing energy as it does so (latent heat of evaporation). 2. It passes through the compressor where the low temperature gas is reduced in volume, resulting in a rise in both temperature and pressure. 3. As a heated and high-pressure gas, it passes through the condenser (indoor unit) where the gas condenses (latent heat of condensation) with a release of heat into the air surrounding the coil. A fan moves the warmed air away from the coil to distribute it throughout the indoor space. 4. Still under pressure, the cooled refrigerant, now in liquid state, passes through the metering device, where rapid expansion results in a reduction in pressure. 5. In the low-pressure, low-temperature state, the refrigerant flows back into the evaporator, and the cycle is repeated. 2.4 Fixed-speed and inverter compressors As noted in Section 2.2, the compressor component of a heat pump can be either: • a fixed- or single-speed compressor; or • a variable-speed compressor – also known as an inverter. Figure 2.10 Fixed-speed compressor operation A fixed-speed compressor operates at maximum refrigerant flow and capacity at start-up and during operation. It must continually stop and start to maintain the desired room temperature, switching off when the set point temperature is reached and switching on again when the temperature drops. This can produce large fluctuations of temperature and reduce the system’s energy efficiency as it switches on and off (Figure 2.10). An inverter or variable-speed compressor has a number of advantages over fixed-speed units. They have a ‘soft start’ and run at variable speed, decreasing as the temperature approaches set point and increasing as the temperature begins to fall. The varying speed delivers heating or cooling as required and maintains a more constant temperature that has smaller fluctuations (Figure 2.11). This results in improved efficiency, reduced vibration and a quieter operation when compared to fixed-speed compressors. Most heat pumps currently sold in New Zealand are inverter systems. set temperature Figure 2.11 Inverter operation set temperature Time temperature range fluctuation Time 12 temperature range fluctuation Temperature (°C) Temperature (°C)PDF Image | Heat pump installation Good Practice Guide
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