Direct mixing of geothermal water and condensate into heated groundwater at Nesjavellir power station

Abstract

The focus of this study is on the Nesjavellir geothermal power station in Iceland, which plays a vital role in supplying hot water for space heating in the capital area. The power station operates as a co-generative single-flash geothermal plant, generating electricity using high-temperature geothermal steam and utilizing geothermal water to heat groundwater for hot water production. Opportunities exist at the Nesjavellir power station to enhance the utilization of underutilized flow streams, such as geothermal water and condensate. The objective of this thesis is to investigate the potential of incorporating a small amount of geothermal water and condensate into heated groundwater at Nesjavellir in order to improve resource utilization without increasing scaling risks. The study involves analyzing the scaling tendencies of different mix combinations. Laboratory experiments were conducted to evaluate the scaling potential of various mixing ratios. PHREEQC modeling and simulation techniques were employed to determine the optimal mixing ratio that minimizes scaling while maximizing energy utilization. The research findings suggest that mixes containing condensate water potentially decrease the rate of magnesium silicate scaling in laboratory experiments. This indicates that using condensate water as a component in the mixture can be beneficial in mitigating the formation of magnesium silicate scale. Additionally, the PHREEQC modeling results provide further insight into the behavior of magnesium silicate minerals, specifically Antigorite, Chrysotile, Talc, and Enstatite. The modeling suggests that the supersaturation of these minerals in the presence of mixes containing condensate water would be either lower or comparable to that observed in 100% heated groundwater. By identifying the optimal mixing ratio of geothermal water, condensate, and heated groundwater, it becomes possible to maximize the utilization of geothermal resources without significant modifications to the current production process