Life cycle assessment of a geothermal combined heat and power plant based on high temperature utilization

Abstract

This study applies life cycle assessment (LCA) to examine environmental impacts of generating 1 kW h of energy in a geothermal combined heat and power (CHP) plant based on high temperature geothermal utilization. The Hellisheidi geothermal CHP plant located in SW Iceland, producing 303 MWe and 133–267 MWth in a double flash cycle, is used as a case study for the LCA. The CML-IA baseline and Cumulative Energy Demand (CED) methods are used to perform the life cycle impact assessment (LCIA), providing results for common environmental impact categories investigated in most LCA studies. The impacts associated with joint production processes for electricity and heat are allocated to the two products by energy allocation. The result show that the construction phase of the plant, mainly drilling and casing of geothermal wells along with construction of collection system for geothermal fluid, is largely responsible for most of the impact category results. However, the global warming potential (GWP100), acidification (AP) and the renewable cumulative energy demand from wind, solar and geothermal energy (CEDR,w,s,g) are mainly affected by the operational phase of the plant, due to direct emissions of gases (mainly CO2 and H2S) and the extraction of geothermal fluid from ground. To explore the effects of currently installed mitigation methods and operational improvements at the Hellisheidi plant, two operation scenarios are investigated within the study; the first based on a previously published dataset for operating conditions at year 2012 and the second, an updated dataset based on inclusion of implemented mitigation methods until the operating year 2017. Due to carbon capture and storage (CCS) by reinjection of CO2 using the CarbFix method developed at Hellisheidi, the GWP100 reduced from 15.9 g CO2eq/kWh to 11.4 g CO2eq/kWh for electricity and 15.8 g CO2eq/kWh to 11.2 CO2eq/kWh for heat over the 30-year operational time under investigation. Similarly, the SulFix method used for reinjection of H2S at Hellisheidi resulted in decreased AP from 9.7 g SO2eq/kWh to 3.6 and 3.5 g SO2eq/kWh and human toxicity (HTP) from 5.8 and 5.5 g 1,4-DB eq/kWh to 5.1 and 4.8 1,4-DB eq/kWh for electricity and heat respectively. The overall CED resulted in 5.2 kW h (18.7 MJ) of energy demand to produce 1 kW h of either electricity or heat, dominated by the use of geothermal energy as mentioned earlier. Non-renewable energy demand (CEDNR) decreased from 6.8 × 10−3 and 5.9 × 10−3 kWh (0.024 and 0.021 MJ) to 5.8 × 10−3 and 5.0 × 10−3 kWh (0.021 and 0.018 MJ), for electricity and heat respectively, by using electrical drills instead of diesel fueled drills for additional wells during the operational time of the power plant. In conclusion, these results indicate a very high environmental performance of the Hellisheidi plant compared to other energy conversion technologies and emphasizes the potential of geothermal energy as a clean energy source for producing electricity and heat.