Geotermisk fjärrvärme i Gunsta

Sammanfattning: Gunsta is situated about 10 km east of Uppsala. Today there are plans to establish 1000 new households in Gunsta before year 2030. The required energy for heating houses and water would be distributed via a district heating system, which also includes 200 existing residences. The annual required energy, according to this study, would be 16.2 GWh with a maximal peak effect of 6.9 MW. To meet this need, the study suggests a system with heat pumps to upgrade the 31˚C water from a 1.8 kilometre deep borehole. In order to achieve the desired water flow, hydraulic fracturing will be necessary since it is assumed that the rock would be relatively solid at that depth. Water at 10˚C will be pump down between an injection hole and a production one. During circulation water absorbs heat from the surrounding ground and increases temperature. The main source of energy is the difference between the initial and after circulation temperatures. The proposed system consists of a primary and a secondary circuit connected via a heating exchanger. The primary circuit includes the boreholes and necessary equipment for filtering and pumping the thermal water. The secondary circuit involves two paralleled series of heat pumps, a pellet boiler for peak production and a storage tank to even out the daily consumption variations. The system is then connected to the Gunsta ’s local heating system. Uncertainties regarding the extent of the drilling make it difficult to estimate total costs of the geothermal district heating plant in Gunsta. The investment costs for the plant were roughly estimated to 88 million SEK, with a production cost of 0.19 SEK/kWh and a payback time of 12 years.