Heat pumps form a critical element in the energy policies of many countries, but in the UK their use is still at a relatively low level. As EC&M reports, this is about to change.
n a climate where heating plays a more important role than cooling in maintaining comfortable conditions for most of the year, you might expect that the UK would be leading the way in the use of heat pump technology. Surprisingly, the reverse is the case, with our use of heat pumps for heating tailing way behind most of northern Europe.

Part of the reason for this is historical. In a report for the Government's Energy Efficiency Best Practice programme, the Energy Technology Support Unit (ETSU) recognises one of the factors holding back the adoption of electric heating using heat pumps. "The use of electricity is hindered by the UK preference for gas-fired wet central heating systems, with strong marketing of the condensing boiler as the current technology for improving energy efficiency," says the report.

Ironically, there are a lot of heat pumps installed in the UK, but these tend to be split and multi-split air conditioning systems that were chosen for their comfort cooling. The prices of cooling-only and heat pump models are now so close that heat pumps are often installed for cooling-only applications because it is easier to obtain them.

"About 60% of the split systems installed in the UK are heat pumps but we have no way of knowing how many of them are used for heating," notes Sandra Gomez of the Building Services Research and Information Association (BSRIA). "Part of the problem is that many people may not even be aware of the heating capability," she adds.

So there are loads of buildings with the potential to reduce their heating bills at no capital cost at all, but they are not taking advantage of the opportunity.

According to David Birtles, managing director of Climate Equipment, those potential benefits are significant: "In situations where there is already a heating system in place, running costs can be greatly reduced by using heat pumps. And in a new-build or refurbishment situation, capital costs can be cut significantly because there is no need to buy and install separate heating plant.

"Also, there is the advantage of needing only one subcontractor to install both cooling and heating. For all of these reasons, we expect to see air conditioners in general, and heat pumps in particular, becoming more of a commodity item like other electrical appliances."

The types of heat pumps that are likely to become commodities are the smaller models, where there ought to be huge demand in the domestic sector – and where currently hardly any can be found. In comparison, Switzerland has around 25% of its homes heated by heat pumps and the figure is even higher for Sweden.

In the case of Switzerland, heat pump use has been encouraged by government incentives, as part of the country's policy of reducing oil consumption. With the introduction of the Climate Change Levy there is a possibility that Enhanced Capital Allowances (ECAs) will be introduced for heat pumps, but there is concern in government circles that this would encourage the use of air conditioning. Current proposals, therefore, are that ECAs will only be granted for heat pumps that are to be used mainly or solely for heating.

While this may encourage the introduction of heat pumps in certain areas, especially where there is no mains gas available, it ignores the fact that air conditioning is going to grow, whatever the Government wants, because people want to be comfortable all year round. In that case, it makes sense to encourage the use of heat pump air conditioners, as opposed to cooling only, in situations where air conditioning is to be installed.

According to the European Heat Pump Network, studies have shown that electric- or gas-driven heat pumps can reduce emissions of CO2 and other pollutants associated with space heating in Europe by between 30% and 50%. Assuming that heat pumps were used to provide 30% of the heating in residential and commercial buildings, the International Energy Agency calculates that some one billion tonnes of CO2 emissions can be saved per annum. A further 0·2 billion tonnes would be saved by the increased use of industrial heat pumps.

The energy saving benefits of heat pumps come from their ability to use a relatively small amount of high quality drive energy (eg electricity, gas or high temperature waste heat) to transfer heat from natural heat sources such as the air, ground or water.

Clearly, it is important that the amount of heat energy coming out of the system is greater than the energy put in to drive the system. Electrically-driven heat pumps for buildings typically supply 100 kWh of heat with just 20-40 kWh of electricity. Industrial heat pumps can achieve even higher performances, and supply the same amount of heat with only 3-10 kWh of electricity.

One of the problems in comparing the energy efficiency of heat pumps to other options is that there are several types of heat pump and the most appropriate type will depend on the application.

Most heat pumps in current use operate on the vapour compression cycle, where the main components are the compressor, the expansion valve and two heat exchangers, referred to as evaporator and condenser, connected to form a closed circuit. In the evaporator the temperature of the liquid working fluid is kept lower than the temperature of the heat source, causing heat to flow from the heat source to the liquid, and the working fluid evaporates. Vapour from the evaporator is compressed to a higher pressure and temperature. The hot vapour then enters the condenser, where it condenses and gives off useful heat. Finally, the high-pressure working fluid is expanded to the evaporator pressure and temperature in the expansion valve. The working fluid is returned to its original state and once again enters the evaporator.

The energy to drive this process is usually an electric motor but it can also be a combustion engine. Most of the heat pumps installed in the UK use air as the heat source, as in reverse cycle split system air conditioners, but there are a number of studies looking into using the ground as the heat source for heating-only applications – a technique that is used widely in Europe and the USA. In these cases, the ground is an attractive temperature source because ground temperature a few feet below the surface tends to remain fairly stable throughout the year.

The other common type of heat pump is the absorption heat pump, which is thermally driven and uses the ability of liquids or salts to absorb the vapour of the working fluid. The system consists of an absorber, a solution pump, a generator and an expansion valve. Low-pressure vapour from the evaporator is absorbed in the absorbent, a process that generates heat. The solution is pumped to high pressure and then enters the generator, where the working fluid is boiled off with an external heat supply at a high temperature. The working fluid (vapour) is condensed in the condenser, while the absorbent is returned to the absorber via the expansion valve.

Heat is extracted from the heat source in the evaporator. Useful heat is given off at medium temperature in the condenser and the absorber. In the generator high-temperature heat is supplied to run the process. A small amount of electricity may be needed to operate the solution pump.

Absorption heat pumps are based on well-established technology and are enjoying a resurgence of interest in line with the growth of combined heat and power (chp). Using absorption technology, the waste heat from the chp engine can be used to provide cooling and additional heating with an overall gain in energy efficiency.

In heat pump circles there is a great deal of enthusiasm for ground source heat pumps and absorption heat pumps and we can expect to see gradual growth of these technologies in the UK. The fact remains, however, that the largest installed base of heat pump technology is in the split and multi-split system air conditioning market, and it is likely to remain so for some time to come. While these may be less efficient than other methods, the fact is that they are being installed in buildings to provide cooling, so why not make use of the highly efficient heating that goes with them?

Deep impact

It is now common practice to measure energy efficiency in relation to the amount of carbon dioxide emissions associated with each technology. The list below indicates the various types of heating available in increasing order of CO2 emissions and, therefore, decreasing order of efficiency.
Lower CO2 impact
Gas engine-driven heat pump
Gas-fired absorption heat pump
Electric ground-source heat pump
Electric air-source heat pump
Gas-fired boiler
Oil-fired boiler
Direct electrical heating (ie electrical resistance heating)
Higher CO2 impact