22 May 2002
Vertical boreholes in dry bedrock are commonly used as heat source in Swedish heat pump installations. Currently used design depths result in minimum brine temperatures in the range -3 to -5 °C. This temperature can be raised by means of a deeper borehole or recharging from an alternative heat source (e.g. exhaust-air or a solar system). The paper discusses a combination of borehole/exhaust-air heat-recovery including an optional supply-air coil to achieve summer recharging/’free’ air-conditioning and high-efficiency heat recovery ventilation.
In situ measurements confirm calculated brine temperatures and improved performance. Furthermore, results underline the importance of minimizing power input to pumps and fans that are used in the distribution systems. With heat-recovery, the brine temperature never goes below zero and hence defrosting of the exhaust-air coil will not be necessary. As a consequence of the raised brine temperature, a smaller amount of anti-freeze suffices and this will both improve heat transfer and decrease electric input to the brine pump.