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. However, its suitability 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). How low are typical local winter temperatures? Is the system for a single dwelling or for something larger? How will heat be delivered? (underfloor water? radiators? blown air?) Will summer cooling be required? Is a heat pump / solar hybrid system an option? What other heating options are available locally? What is the source of electricity?
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.
If you heat your house with a 94% efficient gas furnace, every kilowatt-hour of useful heat will result in emissions of about 0.19 kg of CO2. If instead you use electricity from the SaskPower grid, our calculations indicate that the emissions are currently about 0.69 kg/kWh, or over 3.6 times as much. (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 data. Others, using other data sources, have reached a somewhat higher figure.) The main reasons for this difference are that (i) much 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.
If instead you use a heat pump, 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. Hence, to keep CO2 emissions down to what they would be with an efficient gas furnace, a SaskPower customer needs a heat pump with a seasonal average COPh of 3.6 at present. [If, however, the only realistic alternative is direct electrical heating, a GSHP is a significant improvement.]
The COP of a heat pump depends on its design, but it also depends on the climate, and in particular on ground temperature. While ground temperature doesn’t vary as much as air temperature, it is still affected by cold winter temperatures, resulting in lower COPhs—so the seasonal average COPh of a heat pump will be lower for a Saskatchewan installation than the standard quoted COPh for that unit. The result is that for a typical Saskatchewan installation we need a quoted figure not of 3.6 but of about 3.9 for a well-installed unit. Several GSHPs currently on the market achieve this figure. However, there is another problem. 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, especially if the land area available 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.
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.