Optimisation of industrial size cold production from a ground source heat pump plant using borehole heat exchangers

Abstract

A ground source heat pump installation was designed and built for a factory near Limburg, Germany. The basic design of 50 borehole heat exchangers (BHE) was sized for covering the heating demand and the basic cooling demand. Due to the intended production methods in the factory, a higher cooling load was required than anticipated. Numerical simulations were used to devise an optimum operational strategy for covering a maximum of the cooling load. The original, classical concept used the ground mainly as a seasonal storage, extracting heat in winter for the heating of the building and injecting heat from cooling in summertime into the BHE. With the increased cooling load, this was no longer sufficient. Re-cooling of the underground also during summer became necessary. This re-cooling can use the colder outdoor air at night to extract heat from the ground and dissipate it into the ambient air. Because also this operational mode requires energy for pumps and fans, an energetic optimisation is crucial. This optimisation was achieved through extensive numerical modelling, taking into account the temperatures in air and ground in summer nights, the heat transfer to the BHE and further to the air, and the energy input into the system. The simulation had to be done with maximum hourly temperature values in order to correctly simulate the temperature development. The result of the simulation was an operational strategy for the system, but also the concrete control parameters for the direct control of the valves, pumps and fans. The optimisation resulted in an increase of the available energy for cooling from about 800 MWh in the original design to more than 2000 MWh in the final case, without enlarging the BHE field.