DEVELOPING WORK ON GROUND HEAT EXCHANGERS P.Platell

Ground couple heat pump performance is to a great extent depending on the GHEX (Ground Heat Exchangers), the ground loop. The conventional GHEX for vertical bore holes looks as a U when it is installed in the ground and therefore called a U-loop. In order to improve thermal performance expressed in terms of W per installed boreholes meter sometime double U-loops are installed in the borehole. However, a conventional U-loop doesn’t utilize the investment that the borehole constitutes in the very best way. It has to operate at high mass flow, i.e. turbulent flow, in order to offer high heat transfer performance from the fluid in the pipe to the ground. High flows means high pump losses and hence electric consumption. If the GHEX could operate with lower flows, preferably in the laminar flow regime the pump losses could be reduced considerably. Further more, in the conventional U-loop there is a heat transfer between the flow upward and the flow downwards which reduce heat transfer from the fluid in the U-loop to the ground. There are new ideas coming up regarding more efficient GHEX. The most promising type of GHEX seams to be a type that is often called coaxial pipe. This paper describes a recent developing work on such type of coaxial GEHX, which is also characterized by having one of the legs thermal insulated. 2. NEW GROUND HEAT EXCHANGERS, TIL-PIPE (THERMAL INSULATED LEG) As mentioned above in the background conventional U-loops suffer from some thermal deficiencies. For instance