In the last decade, due to climate change concerns and new environmental regulations in the EU, there was a tremendous rise in installed heat pump systems in new homes and buildings. The majority of ...these installed units are related to air-source heat pumps, as they offer a good trade-off between capital and operating expenses. However, when analysing heating and cooling heat pump systems from the primary energy consumption and ecological aspects, groundwater and shallow geothermal heat pump systems offer superior efficiency, compared to all market-available thermo-technical systems today. In the last decade, ground-source systems have seen some technological improvement by employing new borehole heat exchanger designs, such as piping with internal fins and a wider diameter (so called Turbocollector) to enhance the heat transfer between fluid and rock, as well as to reduce the pressure drop in the system. Furthermore, the process of drilling deeper offers higher ground temperatures and consequently higher seasonal performance factors in the heating cycle, due to the effect of the geothermal gradient. Nevertheless, although deeper boreholes provide better heat extraction rates per meter during the heat pump heating cycle, at the same time, it reduces heat rejection rates during the heat pump cooling cycle. The objective of this paper is to analyse and evaluate benefits and downsides of a new approach in the heat pump system design with deeper borehole heat exchangers of up to 300 m, comparing it to the traditional design of double-loop exchangers with 100 m depth. The geothermal borehole grid design simulation model, along with heat extraction and rejection, is performed on a yearly basis. The results are showing that the benefits of shallow geothermal boreholes, from the hydraulic and thermodynamic point of view, still dominate over deeper solutions.
The Pannonian Basin, partly located in Croatia, is well known for its higher-than-average geothermal gradient with good potential for geothermal energy exploitation. Most of the currently known ...geothermal potential locations in the Croatian part of the Pannonian Basin (CPB) were discovered during the oil and gas exploration and exploitation from the mid-twentieth century onward. Unfortunately, recent geothermal energy utilisation in Croatia, which began in the late 1980s, developed very slowly and sporadically, even though the utilisation of it has been known since the Roman times. Most of the discovered geothermal sources are used for balneology in numerous thermal spas. In the last decade, low-temperature geothermal resources have also been used in agriculture, namely in greenhouses. However, with the change of legal framework in 2018, the market showed an increase in the number of issued geothermal exploration blocks. With Croatia’s first geothermal ORC power plant Velika 1, commissioned in 2019, the interest in developing geothermal projects is seen in 13 exploration and six production licenses issued in the last three years, focusing on deep geothermal potential. The planned use of these granted licenses varies from electricity production to agricultural use. Aside from classic geothermal brine production, there is also a good potential of geothermal brine exploitation from bottom aquifers in depleted oil and gas fields. Many hydrocarbon reservoirs in Croatia consist of oil and gas in the upper part of the reservoir and aquifer in the bottom part. During initial depletion-drive exploitation, pressure in the reservoir declines, causing brine from the aquifer to slowly invade the oil zone. While the reservoir is in its final stages of production, some waterflooded peripheral wells could be turned into geothermal ones, even if oil is still produced or after the field is abandoned. So far, several locations with relatively high temperatures of the bottom aquifer have been identified as a good potential for deep geothermal energy exploration and exploitation. This work gives an overview of the current state of geothermal energy utilisation in Croatia and future prospects.
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The paper analyzes the technical solutions, economic feasibility and institutional aspects of geothermal energy use in a district heating system by retrofitting wells on a mature oil field based on ...the case study. The objective is to describe institutional procedures for brine production from the mature oil field. The research methodology includes the well data and the condition of the existing oil infrastructure analysis, to evaluate the brine production possibility. The methodology also analyzes existing hydrocarbon contracts. Institutional steps to obtain a geothermal concession on the brownfields are proposed. The hypothesis assumes that, with the legislation change, a vertical distribution of concessions for simultaneous production of oil and brine is achievable. The conclusion brings the novelty of this paper as the provision of guidelines for the development of the projects and the start of the production of brine from a mature oil field for the investors and the legal authorities.
When considering implementation of shallow geothermal energy as a renewable source for heating and cooling of buildings, special care should be taken in the hydraulic design of the borehole heat ...exchanger system. Laminar flow can occur in pipes due to the usage of glycol mixtures at low temperature or inadequate flow rates. This can lead to lower heat extraction and rejection rates of the exchanger because of higher thermal resistance. Furthermore, by increasing the flow rate to achieve turbulent flow and satisfactory heat transfer rate can lead to an increase in the pressure drop of the system and oversizing of the circulation pump which leads to impairment of the seasonal coefficient of performance at the heat pump. The most frequently used borehole heat exchanger system in Europe is a double-loop pipe system with a smooth inner wall. Lately, development is focused on the implementation of a different configuration as well as with ribbed inner walls which ensures turbulent flow in the system, even at lower flow rates. At a location in Zagreb, standard and extended thermal response tests were conducted on three different heat exchanger configurations in the same geological environment. With a standard TRT test, thermogeological properties of the ground and thermal resistance of the borehole were determined for each smooth or turbulator pipe configuration. On the other hand, extended Steady-State Thermal Response Step Test (TRST) incorporates a series of power steps to determine borehole extraction rates at the defined steady-state heat transfer conditions of 0/−3 °C. When comparing most common exchanger, 2U-loop D32 smooth pipe, with novel 1U-loop D45 ribbed pipe, an increase in heat extraction of 6.5% can be observed. Also, when the same comparison is made with novel 2U-loop D32 ribbed pipe, an increase of 18.7% is achieved. Overall results show that heat exchangers with ribbed inner pipe wall have advantages over classic double-loop smooth pipe designs, in terms of greater steady-state heat extraction rate and more favorable hydraulic conditions.
Classic and extended step thermal response test were conducted on three different 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. Different values of thermal conductivity are the result of differences in the geological profile 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 fluid 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.
Kindergarten Grdelin in the city of Buzet, Istria, Croatia, was chosen to undergo a deep retrofit of the current thermotechnical system, as a part of the HORIZON 2020 HAPPEN project1. The existing ...shallow spiral heat exchanger field is insufficient to cover heating loads of the building. Therefore, additional BHEs were drilled and completed to determine optimal borehole heat exchanger type within the same geological environment. Four BHEs, either single U (1U) or double U (2U), with different geometrical setting and depth were tested: BHE-1 (50 m, 2U DN32 ribbed), BHE-2 (75 m, 2U DN40 ribbed), BHE-3 (100 m, 2U DN32 smooth) and BHE-4 (150 m, 1U DN45 ribbed). A thermal response test (TRT) was performed to obtain the ground thermal properties. Furthermore, synthetic TRT curves were calculated to describe temperature response in the case of different heat pulses. This was done to determine heat extraction rates and the capacity of each BHE type, according to EN14511 norm. It was established that the BHE-4 is the optimal design for heating and cooling purposes on the selected site due to positive impact of the geothermal gradient, higher initial borehole temperature and a positive effect of the ribbed inner wall.
The utilization of shallow geothermal energy is well known in several European countries. Even though large parts of the Republic of Croatia show significant 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 scientific 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 first assessment of utilizing shallow geothermal potential in Croatia was given.
Wider Zagreb city area, capital city of Republic of Croatia, has significant potential for major implementation of geothermal heat pump systems in not just the residential sector, but in commercial ...sector as well. Geothermal heat pumps often cumulatively describes different earth energy heat exchanger installations, from groundwater up to shallow ground horizontal and borehole heat exchangers installations. Zagreb area has especially favourable both hydrogeological and thermogeological parameters, which allows implementation of different designs for residential and commercial heating and cooling. Analysis of comparative energy and economy advantages of heating and cooling with heat pump systems (air, ground or groundwater) was made for capital city of Zagreb. Since heat pump systems in residential area is always higher-class investment, there is often a need for whole year heating since projects increasingly incorporate outside swimming pool. Up till now, many authors investigated comparison of winter heating energy cost for different heat pump systems. However, outdoor swimming pool sometimes consumes as much heating energy during spring-summer-autumn season, as do house in autumn-winter-spring season. Since air temperatures during spring-summer season are much more favourable then sub-cooled ground or groundwater temperatures, air-source heat pumps when used in whole year round for heating, could offset economically unfavourable coefficient of performance (COP) during coldest days of winter. On a real residential home, currently in construction phase, techno-economic analysis was performed comparing seasonal performance factor (SPF) for different heat pump systems during whole year round heating.
Carbon capture and storage (CCS) technology is a beneficial greenhouse gas mitigating strategy carried out in the last 20 years. Depleted gas reservoirs are promising candidates for the storage of ...carbon dioxide (CO2). Therefore, a depleted gas reservoir in the Upper Miocene sandstone located in Northern Croatia was taken as an example. The purpose of this study was to compare CO2 storage capacity obtained with two analytical equations to total storage capacity obtained through the simulator, in order to validate the equations. The first equation takes the average reservoir pressure and available production data into account, while the other one is more general and includes produced volume, CO2 density and formation volume factor of the original fluid. The tools used for these calculations were Schlumberger PVTi software, in which the equation of state was obtained, and ECLIPSE (E300 Module) which is a reservoir engineering simulator used for reservoir behaviour prediction. The results conformed analytical solutions, indicating that, depending on the depth, the mass of the CO2 that can be injected is twice as big as the mass of CH4 produced. The results of analytical solutions, 16.7 × 106 m3 and 14.6 × 106 m3, are in accordance with the results obtained by the simulation of CO2 injection in depleted reservoirs - 16.2 × 106 m3. Based on this, a conclusion is derived that these analytical solutions can be used as a first approximation of injection in a depleted gas reservoir.
Based on the statistical analysis of the time series of data, the influence of the change of Sava River temperature and geothermal anomalies on the changes of the groundwater temperature of the ...Zagreb and Samobor-Zaprešić aquifers is described. In the analysis, data were used from daily measurements of the Sava River temperature and from the quarterly measurements of the groundwater temperature. Statistical methods of correlation, cross-correlation and linear regression were applied and the maximum, mean and minimum groundwater temperatures were analysed. Obtained results are presented in the form of statistical parameters, diagrams and maps of isotherms. This data is indispensable for development of shallow geothermal applications related to open loop groundwater heat pump systems. Since efficiency of the heat pump is directly dependent source temperature, presented analyses is necessary for prefeasibility study of geothermal projects and comparison between different designs. Furthermore, currently operating projects in the Zagreb and Samobor-Zaprešić are systematically elaborated, giving first approximation of energy consummation from this renewable energy resource.
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