Classic and extended step thermal response test were conducted on three diff erent locations in Zagreb. Measurements with the classical thermal response test were used to determine thermogeological properties of the ground and thermal resistance of the borehole for each location. Diff erent values of thermal conductivity are the result of diff erences in the geological profi le and depth of the sites. In addition, experimental research of the steady-state thermal response step test (SSTRST) was carried out to determine heat rejection rates for passive and active cooling in a steady state regime. Results showed that the heat rejection rate is only between 8-11 W/m, which indicates that the coaxial system is not suitable for passive cooling demands. Furthermore, the heat pump in passive cooling mode uses an additional plate heat exchanger which causes heat losses and additional temperature rise of the working fl uid by approximately 1.5 °C. Therefore, the steady-state rejection rate for passive cooling is even lower for a real case project. The coaxial heat exchanger should always be designed for an active cooling regime with an operation of a heat pump compressor in a classical vapour compression refrigeration cycle.
The aim of our research is to use abandoned deep-hydrocarbon reservoirs and dry wells in the Croatian part of the Pannonian Basin as a geothermal energy source. Croatia has been exploring and exploiting hydrocarbon reserves in the Pannonian Basin from the mid-20th century. Therefore, many oil and gas wells are reaching the end of their production phase and many are already abandoned. These wells could be considered for geothermal energy production through the coaxial heat exchanger principle, which is usually used in shallow geothermal energy extraction. Using the abandoned deep well Pčelić located in the Drava subbasin as a case study, we have derived the available energy and fluid temperature changes during 20 years of operation for two cases: one with a constant base heat load throughout the year and the second as a variable heat load depending on outside air temperatures. We determined that the maximum potential heat extraction in a variable system is 1750 MWh per year, with 1.5 MW of peak heating power in winter, depending on the sink temperature, climate, and consumer input data. The maximum theoretical constant heat extraction for possible industrial direct heating could be 400 kW during the entire period of 20 years, with fluid temperature reaching steady state at a favorable 50°C. To define steady-state ratio between extracted heat and consumed energy at the circulating pump, we evaluated seasonal performance factor (SPF) analysis similar to heat pump systems. Lower values of SPF linked to a higher flow rate implies higher energy extraction. Our results show that when using a lower flow, steady-state SPF ratio is as high as 280, and for a higher flow, steady-state SPF ratio drops to only 25.
The utilization of shallow geothermal energy is well known in several European countries. Even though large parts of the Republic of Croatia show signifi cant potential for its use, the installation of ground source heat pump systems (heat pump and heat exchangers) is slowly progressing. Therefore, a short overview of research published thus far concerning the utilization and assessments of shallow geothermal potential for Croatia was done. In Croatia, there is no agency or government department in charge of collecting and publishing data concerning installations of heat exchangers. Therefore, a study was done to collect the available data on installed closed-loop heat exchangers either from scientifi c research or from personal contacts with drilling companies. Based on the collected data, a map was produced that shows general locations of installed heat exchangers. From the obtained data of installed heat exchanger length, a fi rst assessment of utilizing shallow geothermal potential in Croatia was given.
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