By Brian Brunskill, P. Geo. And Laurence ViGrass, P. EnG. , P. Geo.
Deep Geothermal’s PotentialEngineering
Geothermal energy comes in various forms. Geological hot spots like those in Iceland and in northern California produce steam that is used to generate electricity and hot water that is used for space ...
Geothermal energy comes in various forms. Geological hot spots like those in Iceland and in northern California produce steam that is used to generate electricity and hot water that is used for space heating. In Canada and many other places, GeoExchange equipment is being installed to provide both heating and cooling for buildings, using near-surface, ground source heat pumps.
This article, however, is concerned with “deep geothermal energy,” which refers to the recovery of heat from hot water in rocks at depth.
The earth’s crust becomes progressively warmer with increased depth due to heat generated from deep inside the earth. Generally, in southern Saskatchewan this temperature gradient increases about 30 Celsius per kilometre of depth. The rocks underlying southern Saskatchewan consist of sedimentary layers, and some of these layers host prolific aquifers which contain hot, geothermally-heated water. The energy potential is enormous, and it is literally “beneath our feet.”
Many other areas in Canada also host deep aquifers that contain hot water. There may be potential to develop this resource in parts of Alberta, B. C., southern Ontario and the Maritimes.
Geothermal energy is a sustainable resource that can provide a very long term, reliable, nearly emissions-free form of energy. It represents a stable, long-term revenue stream. Geothermal energy is clean, quiet, and is practically inexhaustible. It can meet some of our heating needs for many generations with very little impact on the environment.
How Does It Work?
The Deadwood Formation is present throughout much of southern Saskatchewan, and is one layer of rocks that contains a prolific aquifer. Near Regina these rocks are about 2,200 metres deep with a temperature of about 60C.
Two wells must be completed in the source aquifer to create the geothermal-water loop. A source or production well brings geothermal water to the surface, where it passes through a heat exchanger, transferring useful heat to a fresh water circuit. The fresh water circuit carries the heat to the buildings or other heating load. The cooled geothermal water from the exchanger is pumped back to the aquifer via the disposal well; the geothermal water must be disposed underground because of its high mineral content. Putting the water back into the same aquifer also helps to maintain the pressure in the aquifer.
A single, centralized geothermal utility could serve as a district heating system, providing heating and domestic hot-water to industry, commercial space, residential subdivisions, com munity centres, swimming pools, etc. Heat pumps can be incorporated in the system to extract additional heat from the geothermal source.
Deep geothermal energy is used in many parts of the world and all the equipment required for these systems is available. In France, by the end of 2008 a total of 61 plants were operating, providing heating and domestic hot water for about 200,000 residences.
Deadwood aquifer near Regina
Development of a deep geothermal energy source requires significant up-front capital investment. However, the resulting energy supply is inexpensive and predictable.
Depending upon what mechanical design were to be used, the Deadwood aquifer near Regina could provide up to 19.5 million BTU/ hr (20.5 GJ/ hr or 5700 kW thermal) of energy. This energy would be available for district heating from a single geothermal loop. Assuming no distribution losses, and using the average rate requirement of 16 BTU/ hr/ ft2 of energy for LEED Silver designation for new commercial construction, the geothermal loop could provide heating for up to 1.2 million square feet of space (111,500 m2) –an area roughly equivalent to nearly 14 CFL football fields! These values do not inc lude any potential benefit from using heat pumps in the system.
Purchasing electricity for pumping is the largest single operating expense. Near Regina a total of about 400 HP would be necessary to operate the loop, having a load of about 300 kW. The ratio of the heat energy produced to the electrical energy used is as high as 19:1.
Most buildings in Saskatchewan are heated by burning natural gas, with the combustion process releasing carbon dioxide (CO2) to the atmosphere. Use of geothermal energy produces no direct CO2 except for that produced by coal-generating plants to create electricity to operate the pumps. The hourly net production of 19.5 million BTU using geothermal rather than natural gas would avoid the emission of about 28 tonnes of CO2 per day.
In its efforts to reduce greenhouse gas emissions and develop new energy sources, Canada should be looking seriously at developing deep geothermal sources.
Brian Brunskill, P. Geo. is a consulting geologist in the energy industry in Saskatchewan, e-mail firstname.lastname@example.org.Lawrence Vigrass is professor emeritus of geology at the University of Regina.
Benefits of Deep Geothermal Energy
• Long term. The productive life of a single project will be for 35-50 years, or more;
• Sustainable. Geothermal systems are not subject to seasonal variations or weather conditions. Energy production is continuous and predictable 24/7/365;
• Adjustable. Differences in daily and seasonal energy use could be accommodated by adjusting the pumping rate of the source well;
• Reliable. The temperature of the water is essentially constant over the life of the project due to the massive geological heat-reservoir. The heat is always available without any energy storage requirements;
• Stable. The cost of geothermal energy would be relatively constant;
• Secure. Geothermal energy is always available;
• Environmentally friendly. Deep geothermal energy does not involve combustion so the direct production of CO2, SOx or NOx is avoided;
• Adaptable. Experience in other countries has shown that district heating can be adapted to many urban environments. In the future, builders and developers could come to see access to geothermal resources as being as important as access to electricity and water.