Heat pumps capture the energy potential that is produced in nature – in the air around us or the ground beneath our feet – and use it for heating and hot water production.

The limitations of traditional electric heating

A normal electric heating element in a water tank may have a capacity of, say, 2 kilowatts (kW) and over an hour will only be able to provide 2 kilowatt hours (kWh) of energy into the water by joule heating. This is effectively a 1:1 ratio, since the electric energy is the only source producing hot water, making it an energy intensive process when compared to the heat pump heating process.

The heat pump heating process

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In a heat pump, there are two sources of energy for hot water production.

The heat present in the air produces most of the hot water. This ‘air energy’ is renewable and is a consequence of natural heating from the sun. This heat is then transferred – or ‘pumped’– into the water by the clever use of a fan and a refrigeration compressor. This is where the use of the second energy source, electricity, comes in.

However, the beauty of this technology is that the amount of electrical energy required to move the refrigerant and create the evaporating and condensing cycles, which ‘pump’ this heat into the water, is much less than the amount of heat that can be transferred.

When compared with the joule heating of an electrical element, the ratio of electrical energy used to the hot water produced is higher than 1:1. Depending on air and tank water temperatures and other conditions, the ratio for heat pumps can exceed 1:5.

The case for heat pumps as renewable

The fan and compressor of the Stiebel Eltron WWK300A, for example, may use 1 kWh of energy over a period of operation. If the air and tank water temperatures measured are 25° Celsius, you may expect 4 kWh of energy to go into your water supply tank (1:4 ratio). Seventy-five per cent of the energy used for heating is renewable in this case, regardless of how the electrical energy is created.

The higher the air temperature, the higher the ratio of energy use to energy output, and consequently, the more efficient the process is. This measure of efficiency (heat output to electrical input) is defined as the Coefficient of Performance (COP). The idea is to always have as high a COP as possible.

Heat pumps in cold climates

Even at cold temperatures, heat pumps are much more efficient than an electrical element. In Europe, heat pump technology has been used for decades to provide comfort heating and domestic hot water.

Stiebel Eltron has systems that can work down to –20° Celsius air temperature and still provide 40 per cent more heating than pure electric heating, with a ratio of 1:1.4, equivalent to a COP of 1.4.

A recent test conducted by the Consumer New Zealand group tested the Stiebel Eltron WWK300A and four heat pump models of other manufacturers. The criteria used were the provision of a hot water load for a family house for three different air temperature conditions of 15° Celsius, 7° Celsius and 2° Celsius.

This testing revealed that the Stiebel Eltron system, at 2° Celsius continuous air temperature for a full heating cycle was able to provide

80 per cent more energy into the water than a conventional electric heating element, or a COP of 1.8. At air temperatures of 15° Celsius, the system was able to provide 160 per cent more energy, or COP of 2.6.

The WWK300A achieves 30 Renewable Energy Certificates, meets all state laws for installation and meets state energy efficiency schemes.