Economic Factors and Their Implications for the Development of a Commercially Viable Geothermal ProjectThursday, November 22, 2018 - 12:20 to 12:39 Theatre 2
Geothermal energy requires accessing warm to hot brines useful for electrical generation or direct-use. The development pathway can be an up-hill battle; both time consuming and costly. Although geothermal energy provides developers with significant long-term stability, low operating costs (OPEX), and high energy availability (above 90%); getting into production is the challenge. Currently, brine temperatures above 170°C are best suited for electrical generation with greater efficiencies the higher the brine temperatures. Systems generating electricity from dry steam (for example the Geysers in California) offer the greatest efficiencies. Electricity can also be generated from brines with temperatures between 120°C and 170°C. These systems generally use an Organic Rankin Cycle system. Wells must be pumped as they do not spontaneously flow like those at higher temperatures and the brine is used to heat a secondary fluid that run the generator. Several companies are working on turbines and heat-engines that produce electrical energy from waters as low as 70°C. It is project specific whether these low-temperature systems are efficient enough to be economical because they require large pumped water flows. Pumping requires significant energy called “parasite load”. Geothermal Power plants that use pumped wells typically expend 50% of their energy needs on parasite load. Pumping costs become an important consideration for the OPEX of these plants; ultimately determining if the plant can be economic. As an example, to generate 1 MWe from water with temperatures between 110°C and 120°C, requires between 60 and 70 kg/sec of mass flow.
Some authors have suggested that existing gas and oil wells can be used to produce geothermal power. Typical gas and oil wells have a diameter of 4 ½ or 5 ½ inches. Volumetrically, a one-foot section of pipe holds 3 to 4.5 liters of brine at these pipe diameters. Geothermal wells use larger diameter pipe (9 5/8 to 13 3/8) which can hold between 10.5 and 31 liters of fluid in a one-foot section. This volume difference means that the potential mass flux of larger diameter wells is significant and can result in the useful production of brines to power electrical generation. It is also why oil and gas wells are unlikely to be useful for electrical generation. Oil and gas wells might flow enough fluid for direct-use purposes, but the value of the energy may not be enough to offset the pumping costs.
In the context of the North West Territories (NWT), the most likely geothermal development area is within the northern parts of the Western Canada Sedimentary basin extending northward from Alberta. Currently, information is limited as to the potential brine flows that could be produced from purpose drilled geothermal wells in NWT. Although transmission is available in some area, building infrastructure is costly. For direct-use applications the load (consumers) must be relatively close to the source. But given the mean monthly temperatures in NWT are below 0°C for 7 months of the year, the heat value from even a low-temperature geothermal resource could be of significant value even for small communities.