Possible Seasonal Injection of Surplus Hot Water from the Hengill Area Into A Low Temperature System Within Iceland´s Capital Region

Abstract

The district heating system in Reykjavík and its neighbouring communities is operated by Veitur Utilities, a subsidiary of Reykjavík Energy. The system receives water from two different sources; on one hand it receives geothermal water from low temperature fields in the city´s vicinity and on the other hand it receives heated groundwater from two co-generation high temperature geothermal power plants in the Hengill Area. These two water types have a different chemical composition and cannot be mixed within the system due to precipitation of magnesium silicates. To avoid this, the two waters are kept separated within the distribution system. The space heating demand in the capital region is growing fast, especially within areas that mainly use water from the low temperature systems. The production capacity of the low temperature systems is, however, limited. The heat production of the power plants in the Hengill Area, on the other hand, can and will be increased. They operate on base load in electricity generation and consequently they produce excess hot water during the summer months when the space heating demand is lower. This discrepancy between demand and supply requires new solutions. One solution could be to inject the surplus hot water produced in the Hengill Area during the summer into one of the low temperature systems. The two water types would then be mixed within the reactive basaltic bedrock instead of within the distribution system, which presumably would solve chemical issues related to the mixing. This solution would reduce energy waste, provide pressure support for the low temperature systems and possibly allow heat storage until the following winter when demand rises again. Numerical simulations need to be carried out to test the effect of such an injection. A simplified flow model of the Reykir/Reykjahlíð low temperature system has been developed as part of the HEATSTORE project. The model was calibrated against formation temperature profiles and production history – i.e. pressure draw-down and temperature of produced fluid. This paper describes the model development and presents preliminary results from different injection scenarios. The model will estimate the feasibility of storing the surplus water from the high temperature systems in low temperature areas. The simulations will analyze the system’s response to seasonal injection of water and the effect of injecting water with a different chemical composition than the formation water. The aim is to see whether this method could be one of the solutions to the capital region's future heating demand.