By the middle of February, Sustainable Energy Ireland had received 3,500 applications for grants from people wishing to install air- or ground-source heat pumps in their homes under the Greener Homes Scheme which was launched at the end of March last year. The grants available range from €4,000 for an air-source heat pump and €4,300 for a ground source pump using horizontal pipes, to €6,500 for one with vertically bored pipes. Consequently, if all these applications are approved and the installations go ahead, the public purse will pay out at least €15 million.
What will the public get for that subsidy? The Greener Homes Scheme is designed to encourage people to heat their homes with renewable energy but, since heat pumps are powered by electricity and only 5% of Irish electricity comes from renewable sources, should their installation really be grant-aided at all?
I began to ask myself that question when I came across a document on the website of a small German electricity company which concluded: “A ground-source heat pump can only be considered acceptable as an alternative to conventional heating systems from an environmental point of view if it has a coefficient of performance of over 4, gets its electricity from a CO2 free source and uses a climate-friendly refrigerant. As most current heat pumps do not meet these criteria, we advise against their use.”
I asked a friend to do a quick translation of the whole document because I knew a little about the company involved, Schonau Electricity Works, (EWS). EWS was set up ten years ago to take over the electricity grid in Schonau, a town with around 2,500 inhabitants in the Black Forest in Southern Germany, not far from the Swiss border. The driving force behind it is Ursula Sladek, the wife of a local GP, Michael, who, like 200 other townspeople, is also involved in the company. Since EWS does not own any generating capacity, its emphasis has consistently been on getting its customers to use less electricity rather than more!
“Heat pumps are subsidised by being given special electricity price tariffs” the article, which was written by Mrs. Sladek, says. “If, as one must suppose, power prices rise massively in the future, the economic viability of heat pumps will decline rapidly. What is more, heat pumps users are making themselves dependent on their power suppliers for the long-term”.
Let's look at these criticisms in more detail. A heat pump's coefficient of performance (COP) measures how much more heat it gives out than would be produced by using the electricity directly in, say, a conventional electric fire. If the COP is 3, for example, the pump delivers 3kWh of heat for every 1kWh of electricity it consumes. That seems straightforward, but, unfortunately, a pump's COP is not fixed. It varies according to the difference between the temperature at which the pump takes in heat from the outside world and the temperature at which it puts the heat out into the building. This difference is known as the 'thermodynamic lift'. “For any given lift, the actual incoming and outgoing temperatures don't make a lot of difference” says Paul Sikora of Dunstar, who supplied Table 1.
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The number of kWh of heat a heat pump can put outfor each kWw of electricity it uses is largely determined by the difference between the temperature of its heat source and that of its output. In this chart, the temperature of the heat source is describee as the evaporating temperature, and the temperature of the output is called the condensing temperature. Thus, if the fluid in the underground coil of a ground source heat pump enters the pump at 10 deg C and the pump is putting hotter water into an underfloor heating system at 35 deg C, the chart shows that it should be giving 7.99 times more heat out than came from the electricity alone.
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