Heat Pumps and how they work

To understand how a heat pump works we must first understand how a simple refrigerator works. The refrigeration circuit contains a refrigerant that acts as a heat transfer media which is  pumped around the circuit. The refrigeration circuit consist of a few basic components and in the following order.

Stage 1 – The compressor does work by compressing a cool refrigerant vapour into a hot refrigerant vapour.

Stage 2 – The condenser at the back of the refrigerator reject the heat and convert the hot refrigerant vapour into a cool refrigerant liquid. Heat is rejected outside of the refrigerator.

Stage 3 – The expansion valve also known as a metering device controls liquid refrigerant flow from the condenser into the evaporator.

Stage 4 – The evaporator inside the refrigerator absorbs heat from the food converting the liquid refrigerant back into a refrigerant vapour, which brings us back to stage 1 for the whole cycle to repeat itself.

A heat pump works in the same way but in reverse. A heat pump cools the outside of the refrigerator and rejects the heat into the refrigerator. The purpose of a heat pump is to transfer heat from a low temperature heat source (outdoors) to a higher temperature heat sink (indoors) for heating. It does this by way of a specially designed four-way reversing valve that reverses the function of the evaporator and condenser. In this way, we can heat or cool the inside of the refrigerator or our living space.

heat pump diagram


Are Heat pumps useful?

Yes, now that we know how a heat pump works we can use this knowledge to cool heat or heat our living spaces. In summer, we can cool the space and dump the unwanted heat to the outside. In winter, we can cool the outside, dump the heat into our living space, and give us very cheap heat.

How cheap are heat pumps to run?

The manufactures use the Coefficient of Performance (COP) to give us some idea how efficient a heat pump is so for example if a heat pump claims to uses 3.48kW of power to run your compressor and fans and delivers 12kW of heat its COP is 3.45 w/w i.e.  (12 / 3.48 = 3.45). Therefore, for every 1kW of electrical energy you use you receive 3.48kW of heat. This sound good, you are receiving 2.48kW of free heat for the cost of 1kW of electricity. These values are given to us buy manufactures that are competing in a very competitive industry and are tested in controlled conditions. The reality is the heat pump is used all year round for heating and cooling in an environment that is constantly changing from winter temperatures of -5oC to summer temperatures of +25oC and therefore the heat pump is not as efficient as theoretically calculated.

A more realistic approach is to use the Seasonal Energy Efficiency Ratio (SEER). This uses the actual energy (kW) used to delivered heating and cooling energy (kW) throughout the year. Therefore, in reality the SEER is lower than the COP . in our example the SEER is 3.0w/w.

How much does it cost to run a Heat Pump?

With the advent of Inverter compressor technology and when the correct selection of a heat pump has been made massive saving can be made in heating and cooling your space. The inverter technology allows the heat pump to vary the heat/cooling output from 100% to as low as 25% according to the demands hence vary the power input to the heat pump and maximises efficiency.

Therefore, to calculate the actual energy cost to run a heat pump should be done by someone that that has clear understanding on how a heat pump is selected and operates. So again, in our example an inverter heat pump could cost from €0.12 per hour to €0.50 per hour.

BSE Air conditioning has over 25 years of experience on the selection, installation and operation of heat pumps.

For more information on heat pumps, energy efficiency and running cost contact info@bse.ie for professional advice.

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