Monitoring and Forecasting the Thermal Response of an Existing Borehole Field

Authors: Marc DEROUET, Patricia MONZÓ, José ACUNA
Keywords: ground source heat pump, borehole heat exchanger, g-function, analytical solution, temperature prediction
Conference: World Geothermal Congress Session: Geothermal Heat Pumps
Year: 2015 Language: English
Abstract: Ground coupled heating and cooling systems have become very popular during the last decades in Sweden, with about 425000 small Ground Source Heat Pumps (GSHP) and 400 large Borehole Thermal Energy Storage (BTES) systems. The large installations have a total installed capacity of about 140 MW and deliver around 800 GWh of energy, out of which circa 80% are used for heating and about 20% for cooling. Normally, all installations are monitored to some extent. At most of them, temperatures and energy flows on the building side are followed up and even logged. Electricity consumption is also known, as well as energy used by secondary back-up systems. On the ground side only temperatures going in and out from manifolds are followed up in the best case. However, no information is recorded about how the thermal loads are distributed across the borehole field or along the depth. This paper is the very beginning of monitoring activities where several new and existing GSHP installations are going to be studied and forecasted during the next coming years in terms of their thermal response, the object being in this case an existing borehole field consisting of 26 boreholes located in Sweden that has been operating during 15 years. The boreholes are connected to 3 heat pumps that provide space heating to 150 apartments. The field is divided in two sub-groups: one consisting of 14 boreholes drilled in 1998 and connected to two of the heat pumps and a second group drilled in 2009 which is connected to the third heat pump. The layout of both fields is uneven (e.g. not following linear or rectangular pattern) and comprise vertical and inclined boreholes, which is normal in Sweden. The predicted lack of thermal interaction between the borehole groups allowed the independent study of each sub-borehole field. A method based on the finite line source theory was used to calculate the g-function of both borehole fields and measured thermal loads were subsequently used as inputs to predict secondary fluid temperatures.
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