engleski

IMPORTANCE OF GEOTHERMAL GRADIENT IN MODELING OF GROUND SOURCE HEAT PUMP SYSTEMS

Ground surface is exposed to the solar radiation effect to the certain depth. The intensity of solar radiation is different because of geographic location, morphology and plant diversity. Ground temperature is generally a function of solar heat transferred by radiation, convection and conduction. Undisturbed ground temperature could be considered to occur at depth where annual ground temperature amplitude becomes as low as 0, 1°C. It represents temperature at the depth where equilibrium between surface solar radiation from and geothermal heat flow from Earth’s crust exists. Most of today’s software for borehole heat extraction analysis considers only undisturbed ground temperature as input value, while geothermal gradient and surface variations are usually neglected. This method usually has good accuracy but in areas with high geothermal gradient it can lead up to oversized system in some extent. Northern part of the Republic of Croatia has geothermal gradients with values of 5 - 7 °C/100m, which is much higher than a Europe average value. Experimentally, effect of geothermal gradient can be determined by circulating the heat carrier fluid without heat extraction or injection (prior to Thermal Response Test). The circulating fluid assumes steady temperature (or mean borehole temperature) after a short period. This paper will discuss difference in results of borehole grid array design in commercial building heating/cooling process. Eskilson/Lund model will be applied in simulation of geothermal heat pump system. Model will include two separate ground temperature inputs: one with entering solely undisturbed ground temperature at the site, and the other by entering effective ground temperature with applied geothermal gradient and surface temperature variations.

geothermal gradient; heat pump; borehole heat exchanger; thermogeology

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