Heat pumps

Why all the fuss about heat pumps? 

I’m sure heat pumps are a mystery to most people, so I figured a brief commentary on them might be helpful.

So why have heat pumps have been in the news lately? The technology is not new, but has been developed a lot in recent times, making heat pumps a competitive alternative to other forms of heating.  You probably own a heat pump without realising it – a refrigerator is actually a type of heat pump. The key to the technology is a fluid (refrigerant) that is pumped by a compressor around a circuit that includes a condenser (that gets hot) and an evaporator (that gets cold) – see diagram.

When used for heating, heat pumps save energy by extracting heat from an outside source, and delivering it for use within the building. They can be used for any normal heating need. This apparently magical technology is not new: in the 1950’s several heat pumps were installed in a bid to save energy and fuel costs. One of the most famous of these was used to heat the Royal Festival Hall in London by extracting heat from the River Thames.

So what is the reason for the big revival in interest?  The key here is that the efficiency of the overall process is very good because substantially more heat can be ‘pumped’ than the device uses in electrical energy (for the compressor and fans).  For example a heat pump using 1 kW of electrical power can transfer, say, 2.5 kW of heat energy from outside, resulting in 3.5 kW  (2.5 + 1 = 3.5 kW) of heat energy delivered to the house. (NB: kW = kilo Watts, ie 1000 Watts)

In making the comparison between various alternatives the ‘Coefficient of performance’ or COP is used.  The COP is found by dividing the useful heat output by the energy input. e.g. a heat pump that produces 3.5 kW of heat for 1 kW of input power has a COP of 3.5. By comparison a simple electric heater has a COP of 1 and a modern condensing central heating boiler with an efficiency of 90%  has a COP of 0.9.

However, in making the comparison between a heat pump and a gas fired boiler the higher basic cost of electricity has to be taken into account as well.  Let’s say for example that the electricity cost is 2.5 times higher than gas and the heat pump COP averages 3.0, then the ‘effective COP’ compared to gas energy cost is 3.0 / 2.5 = 1.2 compared to the 0.9 for a modern gas boiler; ie a benefit in energy cost of 1.2 / 0.9 = 133%  (benefit = 33%). The corresponding benefit in terms of CO2 emissions is about 50%.

So you can see the benefits are not huge (although significant), but do become much greater if the comparison is to an oil or LPG fuelled system. However, if we include the expected Renewable Heat Incentive tariffs (of 7 or 7.5 pence / kWh for ground or air source heat pumps), then the economic case for considering heat pumps becomes strong.

Some of the key factors to be considered in making effective use of heat pumps in homes.

I gave as an example a heat pump using 1 kW of electrical power that can transfer (per hour) 2.5 kWh of heat energy from outside, resulting in 3.5 kWh  (2.5 + 1 = 3.5 kWh) of heat energy delivered to the house.  In the trade they use the term ‘Coefficient of performance’ or COP, which is found by dividing the heat output by the energy input. e.g. a heat pump that produces 3.5 kWh of heat for 1 kWh of input energy has a COP of 3.5. By comparison a simple electric heater has a COP of 1 and a modern condensing central heating boiler with an efficiency of 90%  has a COP of 0.9.

However, in making the comparison between a heat pump and a gas fired boiler the higher basic cost of electricity has to be taken into account as well.  If the electricity cost is typically 2.5 times higher than gas and the heat pump COP averages 3.0, then the ‘effective COP’ compared to gas energy cost is 3.0 / 2.5 = 1.2 compared to the 0.9 for a modern gas boiler; ie a benefit in energy cost of about 33%.  The corresponding benefit in terms of CO2 emissions is greater, at about 50%.

So you can see the benefits are not huge (although significant), but do become much greater if the comparison is to an oil or LPG fuelled system, and if the expected Renewable Heat Incentive tariffs are included then the economic case for considering heat pumps becomes strong.

In practice it is also vital to understand that the hotter the water produced by the heat pump, the lower the energy efficiency, so running at a lower temperature will be more efficient. The following figures for a typical ground source heat pump system illustrate this:

Water heated to 55°C, COP = 2.4

Water heated to 45°C, COP = 3.2

Water heated to 35°C, COP = 4

This can lead to big variations in efficiency claims, and means that the water flowing to the radiators needs to be at a lower temperature than the 60 to 80°C used for conventional heating systems.

So – can the heat pump heat the whole house?  Yes, it can in newly built or well-insulated buildings. But in older buildings with poor insulation there is likely to be a problem in cold weather, with the temperature of the radiators having to exceed the efficient working temperature levels for the heat pump. Even in very well insulated houses, it is not uncommon to retain a fire or stove for extra comfort in cold weather. Generally, air source systems require more back-up heat then ground source systems.

John Willis

 

 

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