Ground temperature monitoring for a coaxial geothermal heat exchangers field: Practical aspects and main issues from the first year of measurements

Ground temperature at shallow depths (< 50 m) is not stable, nor in space, neither in time, and its behaviour is the result of the superimposition of effects of heat pulses of different origin: solar, geothermal and anthropic. The correct assessment of ground temperature is a crucial point when designing a shallow geothermal energy system. In geothermal closed loop projects using short borehole heat exchangers, the ground temperature has more variability and affects the rate of heat extraction/injection. Monitoring of the ground temperature can therefore be useful in ground source heat pump projects to correctly understand the behaviour of a shallow geothermal reservoir subjected to heat extraction/injection. This paper illustrates the practical aspects and main issues occurred in the installation, testing and working phases of a monitoring system realised to record ground temperature in a geothermal application. The case study is a field of eight coaxial borehole heat exchangers, 30 m long, connected to a novel prototype of dual source (air and ground) heat pump.