Heat pumps are a great energy saving, cost effective, water heating method that is becoming increasingly popular in South Africa. Water heating in a domestic setting accounts for a large portion of electricity usage. With escalating costs of power, a more energy efficient source has become vital.
How a heat pump works
Water heat pumps work in a similar principle to air conditioners and chillers. The refrigerant gas is compressed, condensed to a liquid, expanded back to a gas and then back to the compressor. In the process of condensing the compressed refrigerant to a liquid, heat is rejected which is absorbed by the water in the condenser heat exchanger.
The liquid is then expanded before it enters the evaporator (heat exchanger). Ambient air is then drawn through the evaporator removing the cold air at approximately 8oC and ‘dumping’ it to atmosphere.
Putting it to work
A heat pump replaces the electric elements of a geyser and uses a third of the electricity. The heat pump heats small quantities of water at a time and recirculates it with the aid of a pump to and from a tank (the geyser). Heating the water gradually until it reaches the desired temperature. The water from the mains enters near the bottom of the tank and the hot water exits at the top. An expansion relief safety valve is also fitted on the tank. The temperature in the tank that can be obtained from a heat pump is approximately 55oC, but if higher temperatures are required, electric elements can be installed in the tank to raise it.
Making use of the Reject air
If, for example, the hot water is only required by a factory for showers for the staff at the end of the day, the heat pumps can be set to heat the water from midday.
By doing this the cold reject air from the evaporator fan can be used to cool the factory. It must be understood that this is not an air conditioning system, but just an aid to a system or simply to help with the hot afternoon conditions in the factory. The temperature cannot be controlled, because when the desired water temperature is reached the heat pump will switch off. When designing such a cooling system, a booster fan must be installed to cater for all the resistance in the ducting system as the evaporator fan is not capable of generating pressure. Another consideration is an air bypass to prevent cold air being blown into the factory when not required.