STEADY-STATE HEAT REJECTION RATES FOR A COAXIAL BOREHOLE HEAT EXCHANGER DURING PASSIVE AND ACTIVE COOLING DETERMINED WITH THE NOVEL STEP THERMAL RESPONSE TEST METHOD

Abstract

At three locations in Zagreb, classical and extended thermal response test (TRT) was conducted on installed coaxial heat exchangers. With classic TR test, thermogeological properties of the ground and thermal resistance of the borehole were determined at each location. It is seen that thermal conductivity of the ground varies, due to difference in geological profile of the sites. In addition, experimental research of steady-state thermal response step test (SSTRST) was carried out to determine heat rejection rates for passive and active cooling in steady state regime. Results showed that heat rejection rate is only between 8-11 W/m, which indicates that coaxial system is not suitable for passive cooling demands. Furthermore, the heat pump in passive cooling mode uses additional plate heat exchanger where there is additional temperature drop of working fluid by approximately 1,5 °C. Therefore, steady-state rejection rate for passive cooling is even lower for a real case project. Coaxial heat exchanger should be always designed for an active cooling regime with an operation of a heat pump compressor in a classical vapour compression refrigeration cycle.

Keywords

• shallow geothermal energy
• thermogeology
• borehole heat exchanger
• ground source heat pump