Heat Pumps

A heat pump is basically a device that uses the fact that heat is released when a gas is compressed and condenses into a liquid and conversely that heat is absorbed when the liquid evaporates. By pumping a refrigerant fluid around in a circuit in which it is compressed into a liquid at one end and then allowed to evaporate at the other, heat is absorbed in the evaporator, transported around the circuit by the fluid to the condenser where it is released.

This circuit is called a vapour compression cycle and is used in most fridges and air conditioners. The heat exchanger bolted onto on the back of your fridge will feel warm because it is releasing heat extracted from the inside of the fridge by the circulating refrigerant fluid.

A heat pump uses the same circuit, but the sides are reversed. Typically, it absorbs heat from the air outside your home and transfers that heat to a heat exchanger that warms water inside the house. Even though the outside air might not be that warm, it’s a virtually limitless source of heat because there is lots of it, compared to the amount of water being heated. Water can be heated to much higher temperatures than the outside air (typically about 60 degrees C) in this way. Heat pums are well suited to heating water using the outside air as a heat source in our South African climate, but start to struggle when the outside temperature drops below -5 C.

Heat pumps use relatively small amounts of energy compared to the amount of heating they provide because they don’t “make” heat; they just move it from one place to another. The they use electricity to drive the compressor that circulates and compresses the refrigerant fluid as well as fans that circulate the outside air, as well as a water pump to move the water through the system. Heat pumps can save up to 67% of the electricity used to heat water to the equivalent heat in a normal geyser.

A recent study comparing the energy efficiency of heat pumps to other methods of water heating is available here.

Heat pumps are used in large facilities such as hotels instead of geysers to heat the water for guests; because they use far less electricity to heat the large amounts of hot water needed each morning/evening.

In summary the benefits of heat pumps are:

They use far less electricity than a conventional geyser to heat water.
They are cheaper than most solar water heating systems and take up less space than equivalent solar heat collectors. They are also not normally dependant on the weather in the same way solar systems are.

Their main draw backs are:

They still require electricity for operation and thus may require alternative electrical power during power failures. Although the typical 2 hour load shedding break shouldn’t have a large affect provided the heated water was stored in a suitably insulated tank.
Because they use the outside air as their source of heat, they normally use quite substantial fans to circulate the air over the external heat exchanger. These fans can generate noise, similar to an air-conditioner.