Saskatchewan's Green Directory Helping Saskatchewan choose a better environment

Geothermal: some background information

The word geothermal has come to be used for two different things.

True geothermal energy is energy extracted from hot rocks at depth; the means to do this is not within reach of the typical Saskatchewan householder.

In common terms, however, it has also (not strictly accurately) come to mean a heating system based on ground-source heat pumps (GSHPs). In such a system most of the heat is pumped from a lower to a higher temperature (from the ground to the house), but the laws of thermodynamics require that in order to do this a certain minimum electrical input is required.

This technology has become publicly associated with energy efficiency. There are indeed occasions when it is a good environmental choice. However, this is by no means always true, and each situtation needs to be assessed on its own terms. The suitability of a GSHP for any particular application is dependent on a number of factors:

  • What are the householders' criteria of choice? (environmental? long-term financial? short-term financial? or more likely some combination).

  • What other heating options are available locally?

  • What is the source of electricity?

  • How will heat be delivered? (underfloor water? radiators? blown air?)

  • Will summer cooling be required?

  • Is the system for a single dwelling or for something larger?

  • Is a heat pump / solar hybrid system an option?

  • How much land is available under which to bury pipes?

In making a green choice, a vital concern in choosing one method of heating over another is the relative level of carbon dioxide emissions: carbon dioxide (CO2) is the principal greenhouse gas contributing to climate change. Customers choosing a heating system on environmental grounds should check the numbers, or hire someone competent to do so for them.

So let's first look at the alternatives:

  • If you heat your house with a 94% efficient gas furnace or boiler, every kilowatt-hour of useful heat will result in emissions of about 0.2 kg of CO2 - we write this as 0.2kgCO2/kWh

  • If you use oil for heating, with a similar efficiency, you emit about 0.3kgCO2/kWh

  • If instead you use propane, the emissions are about 0.24kgCO2/kWh

  • If you heat using electricity from the SaskPower grid, our calculations indicate that the emissions are currently about 0.68kgCO2/kWh. (This figure changes from year to year, depending mostly on the amount of hydroelectric generation, but SaskPower's current generation plans suggest it will decrease only slightly in the next few years. Our figure is based on SaskPower's own published data in the latest annual report.) The main reasons for this high figure are that (i) nearly 60% of Saskatchewan's electricity supply is from coal, a very carbon-intensive fuel, and (ii) electricity cannot realistically be generated at efficiencies over about 35% in coal-fired power stations.

So how does a heat pump compare? If you use a GSHP, some of your heat will come from electricity and some will be pumped out of the ground. How much of each depends on the effectiveness of the heat pump: this is measured by something called the heating coefficient of performance (or COPh). A heat pump with a COPh of 2 gives 2 units of heat per 1 unit of electricity put in. A heat pump with a COPh of 5 gives 5 units of heat per 1 unit of electricity put in. To obtain the rate of CO2 emissions, you need to divide the figure for electricity (0.68kgCO2/kWh - see above) by the COPh.

The COP of a heat pump depends on its design, and on the working fluid used inside it to transfer heat, but it also depends on the climate, and in particular on the ground temperature adjacent to the underground pipes. A colder ground temperature results in a lower COPh. This means in practice that the COPh quoted by the manufacturers, determined by a standard test procedure, will generally overestimate the energy-efficiency of a heat pump performance in any given Saskatchewan location. This is borne out by some of the industry's own literature. In a 2003 report by Caneta Research Inc for the Canadian Geoexchange Coalition (some of whose other assumptions are very generous towards the case for GSHP use), a GSHP rated at COPh=3.5 is found by computer modelling to have a seasonal average COPh of 2.61 in Regina (which would give 0.26kgCO2/kWh, worse than natural gas or propane).

To outperform a comparable gas system, a GSHP would need to have an average seasonal COPh better than 3.4: it is not clear at this point whether any units currently on the market can achieve this under Saskatchewan conditions. Two papers given at the Canadian Geoexchange Coalition's conference in 2007:Dec showed the use of the technology (even with some favourable assumptions) to be marginal at best when compared with natural gas.

However, there is every reason to believe that a competently-installed GSHP can outperform an oil furnace or boiler on CO2 emissions. And it will of course always perform substantially better than direct electrical resistance heating.

This distinction is now recognised in the provincial subsidy regime. Subsidies are available for replacement of oil or electrical furnaces with GSHPs. But if you are replacing a natural gas furnace or boiler you also need to install on-site electrical generation using renewables.

There is another problem, which has not as yet been adequately researched. A GSHP pulls heat out of the ground, thereby reducing its temperature. As the ground temperature reduces, the COPh of the unit also reduces. Thus, over a number of years, many installations may be expected to require steadily increasing inputs of electricity if the land area available for underground pipework is small. Summer use as an air-conditioner only partially compensates for this effect (by putting heat back into the ground), as the winter heating load in Saskatchewan is so much larger than the summer cooling load.

There are a few ways in which GSHP carbon emissions can be reduced:

  • use a well-designed hydronic (underfloor) system instead of blown air. This way, you can reduce the temperature at which you feel comfortable by perhaps 3 degrees (say a reduction from 23°C to 20°C), and thereby reduce your heating requirement by maybe as much as 15%. GSHPs work well with this delivery system —but so do efficient gas boilers.

  • operate a hybrid solar-GSHP system: not only is some of the heating load transferred to a renewable source, but also seasonal COP values may be improved by charging the ground with solar-heated water.

  • obtain your electricity from primarily renewable sources - there is now a government grant available for this option.

  • if a substantial level of air conditioning is a basic requirement in your building, summer use of a GSHP for this purpose will result in carbon savings when compared with a conventional (air source heat pump) unit.