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Introduction to Heat Pump Systems

Overview:

A heat pump channels heat from a region of lower temperature (e.g. external air, soil, water, etc.) to a region of higher temperature (e.g. the interior of a building). Although this sounds counter-intuitive, it is the same basic refrigeration cycle which we see every day in fridges, freezers and air-conditioners.

The concept of the heat pump was initially proposed by Lord Kelvin in 1852, and the first working examples began to see active service during the latter half of the 19th Century. Since then their use has grown in commercial, industrial and domestic applications. Now, as fossil fuel prices continue to rise and an ever-growing share of our electricity is coming from renewable sources, heat pumps are the low-cost, low-carbon and stress-free way to heat your building.

How do Heat Pumps Work?

A compressor, powered by electricity, circulates refrigerant fluid around a circuit consisting of an expansion valve, evaporator and condenser. This causes the expansion and contraction of the fluid which heats up and cools down as it condenses and evaporates, thus allowing heat to be extracted from one medium and transferred to another. This is why the coil at the back of a fridge gets warm while the inside is kept cold. Reverse this process, and this simple, every-day circuit can be used for heating!

Fuel and Efficiency:

Heat pumps do not use oil, gas, coal or any other fossil fuel. They are powered by electricity and their efficiency is measured by the coefficient of performance, or COP. The COP is the ratio of delivered thermal energy versus electricity consumed.

A COP of 1.0 means that 1 kilowatt of electricity provides 1 kilowatt of heat. This describes a traditional electric bar heater where Energy In = Energy Out. Oil and gas boilers have an efficiency of anywhere between 70 – 90%; this can be expressed as a COP of 0.7 to 0.9.

In the very worst conditions, a heat pump will have a minimum COP of 2.5. This means that 1kW of electricity gives 2.5 kW of heat. However in general use the COP can be as high as 4.0 or 5.0, and even higher again where heat-recovery mechanism is employed. If we take a conservative figure of 3.2 (as recommended by the British Standard Assessment Procedure), this means that 3.2kW of heat is provided by 1kW of electricity.

Sample Calculation:

To illustrate the concept we will take a standard three-bedroom house with a heat demand of 16kW:

16kW x 200 days x 12hours = 38,400 kWh to heat the building for one year.

To heat the building using oil, with a COP of 0.8 and which costs 8 cent, or €0.08, per kWh:

38,400 kWh / 0.8 = 48,000kWh

48,000kWh x €0.08 = € 3,840.00

To heat the building using a heat pump, with a COP of 3.2 and which costs 12 cent, or €0.12, per kWh:

38,400 kWh / 3.2 = 12,000kWh

12,000kWh x €0.12 = €1,440.00

Thus the annual running cost of a heat pump is roughly 40% that of oil or gas.

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