A preliminary investigation into the geothermal application of long heat pipes

It is thought that a two phase thermosyphon may be used to remove geothermal energy from a geothermal aquifer to supply energy at a rate of the order of 10 kW to a small scale user, such as a domestic dwelling. In this project a two phase thermosyphon, approximately 8m long, was constructed and tested in order to examine design procedures and to find optimum operating conditions. The thermosyphon consisted of a 0.6m long evaporator section of 35mm pipe, a 6.20m long, vacuum jacketed, adiabatic section of 35mm pipe and a 1.06m long condenser section. Heat was supplied to the evaporator through a steam jacket using saturated steam at 140°C and cooling water was supplied to the condenser. Two working fluids, water and hexane, were tested and a 60 mesh stainless steel wick was also used in the evaporator. When using water the maximum performance of 8740 ± 700 W was obtained at a cooling water flowrate of 34.0 ml/s, without using a wick. This heated the cooling water to a temperature of 81°C. When using hexane the maximum performance obtained was 2150 ± 300 W at a cooling water flowrate of 7 ml/s, without using a wick. These results compare well with the predicted values found using a calculation procedure presented by Engineering Services Data Unit, Item No. 81038. The use of accurate operating conditions when predicting the thermosyphon performance was found to be essential. The use of a simple wick in the evaporator was found to hinder heat transfer. The limiting factor is thought to be the occurrence of film boiling in the evaporator. This was not predicted by ESDU 81038. The prediction of limits to heat transfer was not accurate and more work is needed in this area. The overall results are encouraging and show that it is possible to transfer heat at a rate of the order of 10 kW, using a temperature drop of around 50°C as is required in a domestic geothermal application