Heat pumps have been with us for quite some time now, but as the dangers of global heating and spiking electricity prices have become the order of the day, they have gained ground in popularity. They simply use less electricity to give higher energy output – they use about only a third of their traditional rivals, so why should they not have the edge?
Heat pumps have a wide array of applications both industrial and domestic. In the industry they are unequalled as far as heating of big buildings are concerned, as well as heating of mass volumes of water, ranging from hot water for big buildings to heating of huge Olympic size swimming pools. The same heat pump can be used for both heating a building and cooling it depending on the season, by simply inserting a particular type of valve at a strategic point in the cycle. Domestically they have finally been accepted as the most eco-friendly way of heating and cooling homes, as well as heating sanitary water as a geyser heat pump.
All heat pumps have one common feature: They have the ability to move heat from one place, raise the temperature of the heat and release the heat at this higher temperature elsewhere – using only about one third of the energy it releases elsewhere. This seems like breaking the laws of physics, but it is not. It all becomes possible due to the way in which a heat pump works. The secret boils down to the liquid which runs through the veins of a heat pump called a refrigerant. Although different refrigerants are available, they all have the same feature: They cool down sharply when de-compressed and superheats when compressed. This applies to a geyser heat pump – used for heating domestic sanitary water in a geyser (storage tank) – as well.
So how does a geyser heat pump work?
A geyser heat pump consists of two heat exchangers: One called is the “evaporator” (+ fan) and the other one the “condenser”, which is located inside the geyser; it also has two other components, viz. a compressor and an expansion valve. Finally, without the refrigerant flowing through the set of thin pipes, the heat pump will not work.
In the case of the geyser heat pump the one heat exchanger, the evaporator, is exposed to the ambient air and the other one, the condenser, inside the geyser. The compressor circulates the refrigerant through the system. When the system is switched on, the fan draws air from outside, heated by the sun, through the evaporator, which looks like the grid behind a fridge or the radiator of a motor car. The heat is quickly absorbed by the refrigerant inside the pipes of the evaporator. This is made possible only since the refrigerant in the evaporator is at a very low temperature (see reason later) – in some cases even as low as 32.3 degrees C. So, even if it’s freezing outside (0 degrees C), there will still be enough heat to be absorbed, since it will still be “hotter” than the fluid in the evaporator.
Absorbing so much heat in a short space of time the refrigerant changes from a fluid to a low pressure vapour and then enters the compressor. The compressor compresses it into a high pressure gas and the temperature shoots up to about 60 degrees C due to the compression. The compressed, superheated vapour enters the geyser and releases most of the heat in the heat exchanger inside the geyser, the condenser, to the surrounding water, which is at a lower temperature. It changes back to a warm fluid and enters the thermal expansion valve, where the abrupt decompression causes it to loose pressure and drop very sharply in temperature. This is the state of the refrigerant when it finally enters the evaporator where the cycle started – a very cold and low pressure fluid, again exposed to the heat of the surrounding environment.
A few final remarks:
1. The main reason why a geyser heat pump is becoming increasingly popular is the fact that it uses only a third of the energy a conventional geyser element would use to get the same output. Due to the incredible hikes in the price of electricity in South Africa this actually means that it takes less than two years (some suppliers actually say 18 months) the regain the capital investment installing a geyser heat pump.
2. A geyser heat pump actually comes in two configurations:
a.The first configuration: The heat pump and geyser separated as in the following picture:
b.The second configuration: With the heat pump and geyser completely integrated, as in the following image:
Suppliers of geyser heat pumps claim that their systems can operate when ambient temperatures are below 0 degrees Celsius. Theoretically this is true, but there’s a snag to it: The higher the ambient temperature, the more effective the unit and the lower the ambient temperature, the less effective. So, if you’re staying in a region with very low winter temperatures (like in Sutherland!), you will not be saving energy in winter expecting it to heat your water with very little heat available.