Modelling of ground source heat exchanger performance

The ground presents a practical heat source for small scale heat pump applications, such as single family dwellings. Where little space is available, vertical ground source heat exchangers (GSHX) extending to a depth of 15 - 100 metres can be used. The number of pipes, and their length, is of great importance to the temperature at which the heat is extracted, and consequently to the heat pump COP. In the design phase of a GSHX system, computer programs like EED (Earth Energy Designer) can be used to calculate the number and length of GSHX pipes needed. To simulate actual (dynamic) behaviour of the GSHX system, more powerful computer codes are needed. The "Bodemtix" program, developed by TNO - MEP from earlier simulation programs [1,2] is especially suited to perform calculations on dynamic behaviour of GSHX applications. The program can be used to calculate - based on an hourly demand pattern - the temperature of the medium circulating inside the heat exchanger, for periods extending up to 25 years. Another additional feature of Bodemtix is, that the program is suited to simulate freezing of the ground surrounding the pipes - which may occur during peak heat demands. A proper simulation at peak heat demands is essential for monovalent systems (heat pumps without additional heating). Furthermore, the program can also make simulations with ground water flow - which is important in the Dutch situation with sandy, wet soils. The Bodemtix program has been validated on the basis of existing and new measurements in the field. The results are quite satisfactory, although in the trajectory of freezing of the ground the differences between measurements and simulation are relatively high. A comparison to other available programs for GSHX calculation shows that Bodemtix results keep the middle between the results of these other programs.