The primary interest in heat pumps in South Africa revolves around its ability to dump heat which has been extracted somewhere else and, to be more specific, the ability of heat pumps to heat water in this way.

At this stage it has to be noted that the heat which has been extracted at the source is exponentially higher the moment it reaches the place of delivery, the “heat sink”. This is made possible by the heart of the heat pump system, the reverse of the refrigeration cycle, which will be explained with reference to the following diagram:

The system consists of a panel of thin pipes, the evaporator (like at the back of a refrigerator), a compressor, a coil of pipes (heat exchanger), an expansion valve, pipes connecting everything and a fan. The system is filled with a liquid known as a refrigerant which has very specific features: It heats up to boiling point when compressed and when expanded, it turns into liquid, cooling down sharply at the same time.

The fan draws in hot air through the evaporator, where the liquid inside the pipes is at a very low temperature. The liquid absorbs heat rapidly (due to the huge difference in temperature between the liquid in the pipes and the ambient temperature) and turns into warm gas, which is then compressed by the compressor into a high temperature gas. This high temperature gas is then circulated through the coil (heat exchanger, “heat tank”) inside the geyser (water storage tank), where the heat is released to the surrounding water inside the tank, gradually heating up the water. As the gas cools off (releasing its heat to the surrounding water inside the tank), it turns into a warm liquid. From there the cooled-off refrigerant (still a liquid) reaches the expansion valve, where it expands, sharply dropping in temperature at the same time and enters the evaporator as a cold liquid, but turns into a gas as it absorbs heat in the evaporator, and the process is repeated.

The cycle continues uninterrupted until a pre-set temperature, controlled by a thermostat, is reached. The power is then cut off, the fan and compressor stops and the cycle is only resumed once the temperature drops beneath the pre-set level (like when someone extracts water to take a shower).

There are two types of heat pumps manufactured for heating water:

1. The Integral heat pump geyser, where the heat pump is integrated with the storage cylinder.
2. The separate heat pump geyser or split heat pump geyser, where the heat pump is separate from the storage cylinder and joined by a pipe taking the hot refrigerant to the heat exchanger in the storage tank and another pipe returning the cooled refrigerant through the expansion valve to the evaporator on the outside.

In South Africa both types are marketed, but only two distributors sell the integrated type; the split type is widely marketed and easy to come by. In its case the heat pump in its entirety is located on the outside of the building, either at the northern, sunny side of the building or on top of the roof (like in the case of mass volumes of hot water needed in big buildings).

With the exception of one system, all of the systems are air-source based, meaning that the heat is extracted from the ambient air as pulled in by the fan. In a Spanish system recently introduced to the local market by Emergy, the refrigerant is actually circulated in a solar panel which is placed strategically on the roof. In this way the absorption of heat is considerably higher than in the case of air-source systems.