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

Croatian. Plitka, potpovršinska, temperatura (na dubinama plićim od 50 m) nije konstatna, niti u prostoru, niti vremenu. Takve promjene posljedica su utjecaja toplinskih „pulseva” različitoga podrijetla poput Sunčeva, geotermalnoga ili ljudskoga. Točna procjena temperature ključni je čimbenik kod planiranja energetskih sustava temeljenih na plitkoj geotermalnoj energiji. U takvim projektima, temeljenim na izmjenjivačima topline u plitkim bušotinama, potpovršinska je temperatura promjenjivija, što utječe na iznos pridobivanja topline, tj. utiskivanja fluida. Praćenje takvih promjena važno je stoga kod svih projekata toplinskih izmjenjivača vezanih uz plitka geotermalna ležišta. U radu je prikazan praktičan oblik toga, ali i glavni problemi koje je moguće susresti tijekom instaliranja, testiranja ili uporabe potrebne geotermalne opreme. Dan je primjer polja u kojemu je smješteno osam koaksijalnih izmjenjivača topline, 30 metara dugačkih te povezanih s prototipom uređaja dvostruke toplinske crpke (zračne i dubinske).

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.