Geothermal Energy Research
Geothermal Energy in Nova Scota
Geothermal energy is energy within the subsurface that can be recovered and exploited. Geothermal has a range of uses, and how it is utilized can depend on the geologic setting where it is generated. There are two main types of geothermal energy uses; power generation (high-enthalpy) and direct use (low-enthalpy).
High-enthalpy
High-enthalpy geothermal energy can be generated where the local geothermal gradient is steep, meaning higher temperatures at shallower depth. According to Chandrasekharam & Bundschuh (2008) a reservoir is considered to be a high-enthalpy resource if the reservoir temperature exceeds 150°C. They are restricted to plate boundaries and areas with active volcanism. Due to the pressure at depth, water can still remain in liquid form at high temperatures. When it is raised to the surface it expands to a gas, so it is possible to use the expanded fluids to spin turbines in order to generate electricity (Kranz, 2007). This is used in regions of Iceland. In Nova Scotia the geothermal gradient is much deeper, and so there is not much potential for high-enthalpy geothermal projects.
Low-enthalpy
Low-enthalpy geothermal can be utilized at average (~25°C/km), and even below average geothermal gradients. According to Chandrasekharam & Bundschuh (2008) a reservoir is considered to be a low-enthalpy resource if the reservoir temperature is below 150°C. These reservoirs are not restricted to any specific geologic or tectonic setting. Rather than tectonic restrictions, low-enthalpy reservoirs are assessed The primary system consists of using geothermal (or ground-sourced) heat pumps that pipe water in a closed-loop system into the subsurface, as deep as a few hundred meters. Energy is exchanged between the water in the pipes and the subsurface, and is then pumped directly into a building for space heating and cooling, and bathing. Heat pumps are most commonly used for personal dwellings, but can be used at larger scale buildings such as offices, swimming pool complexes, and apartments (O’Connell, Cassidy, 2003). It is possible to generate electricity with geothermal fluid temperatures in a reservoir as low as 74°C, however due to low feasibility these reservoirs are seldom targets for generation plants.
In a seasonal climate such as Nova Scotia, a geothermal heat pump can utilizes the energy stored in the shallow subsurface in the winter to warm water moving through subsurface pipes, where it is then transferred into a house’s duct system. In the summer it essentially works in reverse. As the shallow subsurface temperature is roughly 10-20°C (depending on specific site and season) (Franz, 2007) it is favourable for both heating in the winter and cooling in the summer.
Local Geothermal Gradient
As mentioned earlier, the reason large-scale geothermal energy generation is not utilized in Nova Scotia is to do with the low geothermal gradient. Being on a passive margin Nova Scotia naturally does not lie over any “hot-spot” or otherwise anomalously high-temperature crust, whereas regions of Iceland and Indonesia do.
Sedimentary Geothermal Systems
As shown below by Drury (Figure 3) the average regional sedimentary basin geothermal gradient in Atlantic Canada is ~17°C/km with a surface temperature of 8°C. For example, the poorly permeable basaltic bedrock in Arskogsstrond, Iceland has a gradient over 200°C/km (Kranz, 2007). According to an assessment of the geothermal resources of Atlantic Canada (Drury, 1984) Atlantic Canada as a whole has, at best, a marginal sedimentary basin geothermal resource. The energy in sedimentary geothermal systems is sourced from water moving in aquifers. For this reason, adequate supply of water is also necessary. Porosity and permeability can be as high as 35-40% and 650-700mD, but deeper than a few hundred meters little is known of the hydrological state of the basins (Drury, 1984).
Hot Dry Rock Geothermal Systems
Limited research has been undertaken in Nova Scotia on the potential for geothermal energy generation in hot dry rocks (HDR), which is to say igneous rocks with radiogenic heat. The heat flow vs. heat production of HDRs in Atlantic Canada suggests an average gradient of 16-18°C/km. Samples from the Wedgeport pluton near Yarmouth have suggested high heat generation, but further investigation is required. According to Drury, 300Ma-400Ma granite batholiths (e.g. the South Mountain Batholith at 371Ma) do not normally have the HDR potential of young intrusives, due to the extended time of radiogenic decay and therefore decrease in heat production (cite somewhere?).
Nova Scotia Geothermal Projects
Two areas in Nova Scotia where there is geothermal potential are Springhill and Stellarton. In the case of Springhill, geothermal energy use is on-going. The local township utilizes old mined coal seams as sources for geothermal space heating and cooling. Large-scale mining of the coal began in 1884, but stopped after the Springhill Mine Disasters in 1956 and 1958.
The mine openings were flooded by groundwater, and warmed below surface, which offers a geothermal heat source. Since the early 1990’s ~25 geothermal wells have been drilled, of which 17 are currently used by businesses/facilities. Six wells maintained and operated by Town of Springhill, the remaining 11 wells owned and maintained by private operators for heating, cooling.
NSCC Springhill currently utilizes some of the geothermal technology to instruct students in their Refrigeration and Air Conditioning Program, sharing the technical data associated with well pump tests and analysis of mine water. Some areas of the campus utilize the Springhill mine for heating. A “Geothermal Industrial Park” development is currently undergoing concept design.
The review of existing data, especially on geology (structural geology, stress field, hydraulic transport properties, (thermal) rock properties, petrography and mineralogy), temperature, heat flow, and geochemistry form the base to identify the potential geothermal reservoir and to estimate the size and the heat content of it.
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