18 May 2017
A comprehensive energy analysis was performed on a strongly nonequilibrium unsteady thermodynamic system comprising ground/soil, borehole heat exchanger (BHE) and ground source heat pump (GSHP) in vertical geothermal systems (VGS). The analysis was based on development of an unsteady theory for VGS, taking into account both spatial and temporal variability of ground temperature and the energy characteristics of the heat pump.
The analysis showed that all energy characteristics of VGS can be determined by two similarity parameters: asb,the thermal diffusivity ratio of soil and brine, and ksb, the thermal conductivity ratio of soil and brine. The mathematical model developed was used to study quantitative and qualitative characteristics of extractable energy. In particular, the correlation between the amount and quality of the energy produced by VGS was analysed in detail.
The fundamental energy characteristic of an energy well for a given VGS was developed. It was shown that this characteristic completely defines the quantitative and qualitative energy characteristics in time for any given VGS and depends on three parameters.
These results allowed geometric and energy parameters of the energy well to be linked to the coefficient of performance (COP) of GSHP. It was demonstrated that the energy efficiency of each individual GSHP system requires its own analysis and that the suggested approach can be used for optimisation of GSHP systems. This enables innovative strategies for future VGS design.