Thermal Power from a Notional 6km Deep Borehole Heat Exchanger in Glasgow


Isa KOLO, Christopher S BROWN, Gioia FALCONE

Key Words:

deep borehole heat exchanger, closed-loop geothermal system, single well, finite element method, Glasgow


Stanford Geothermal Workshop







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The UK was the first major economy to legislate for net zero carbon emissions by 2050. In its Climate Plan, Glasgow – the third largest city in the UK – set out its strategy for net zero carbon emissions, healthy biodiversity, and climate resilience by 2030. The city also hosted world leaders in 2021 to deliberate on climate change where the Glasgow Pact was adopted at the United Nations Climate Change Conference. To mitigate greenhouse gas emissions, alternative energy sources must be explored. Unlike intermittent sources (wind, solar), geothermal energy represents a viable candidate for year-round base-load provision. While conventional geothermal electrical power generation systems are increasingly contributing to green power production worldwide, the geological risk inherent in such geothermal developments can be high. An alternative lower risk approach to exploiting geothermal energy is to employ a deep borehole heat exchanger (DBHE), where a supply of natural fluid from a geothermal reservoir is not required. A single well with concentric pipes is constructed so that a heat transfer fluid can flow down the annular section and return extracted heat to the surface via a central pipe. There is no hydraulic interaction with the reservoir and only a single well is required. In principle, the concept can be applied in any geological setting. While it is mainly used for space heating applications, some studies have suggested the possibility of generating electricity in combination with binary power stations. In this work, Central Glasgow is taken as a case study; a notional 6 km deep borehole heat exchanger is modeled to determine the thermal power that could be extracted. At the assumed depth, the deep borehole heat exchanger is likely to penetrate crystalline basement with a bottom-hole temperature of 225 °C. Results indicate that, with a circulation mass flow rate of 8.33 kg/s, 800 kW of heat could be extracted for 6 months from the DBHE without the fluid inlet temperature going below 36 °C using a power-controlled simulation. With a constant inlet temperature of 10 °C, up to 1096 kW of heat could be extracted using the same mass flow rate within the same period. The outlet fluid temperature from the DBHE goes into the inlet of the binary power plant which typically requires a minimum temperature of 100°C. To maintain a DBHE outlet temperature of 100°C, only a thermal power of 150 kW can be supplied based on the assumptions made in this work. The effects of mass flow rate, varying heat load and varying rock thermal conductivity relevant to the Glasgow area are investigated. Under present economic conditions, it seems unlikely that the significant capital cost of inner-city deep drilling would offer a viable economic return.

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