Data Analysis of the Deep Borehole Heat Exchanger Plant Weissbad (SWITZERLAND)

Abstract

The operation of the 1213 m deep borehole heat exchanger (BHE) plant Weissbad has been measured in over two year lasting measurement period. The temperature and performance data were reported at approximately one week intervals. It became evident from these measurements that the BHE output temperatures were significantly lower than expected. Understanding the reason for this discrepancy was the goal of the present study. By numerical simulation an axis-symmetrical model was developed which accurately represented the subsurface casing installations and the operational history. Sensitivity studies showed that different operation cycles which might not have been registered by the measurement intervals do not significantly change the model results. The initial simulations result in a rather constant offset of 1.8?C to the measured data. The produced temperature shows a strong sensitivity to the existence of a loose contact zone between the cementation and the casing. The measurements could be fitted perfectly by the assumption of a 1 cm thick layer between these two units with 75% porosity extending over 1/3 of the total depth range. Another reason for the low temperatures is the steel tube which hydraulically separates but thermally connects the upcoming from the downgoing flow path.