Operation performance of a ground source heat pump system in the mediterranean climate zone. First results

Unlike fossil fuels, renewable energy sources emit little to zero greenhouse gases and in most cases are cheaper than coal, oil or gas (UN, n.d.). These clean and sustainable alternative energy solutions include solar energy, hydropower, wind energy, geothermal energy, biomass energy, and so on. Such forms of energy can be used in the collective effort toward giving solutions to major issues arising from the energy crisis Europe is now facing.

Due to this century’s energy crisis, the European Union (EU) has developed energy strategies in order to become more resource-efficient and to ensure that greenhouse gas emissions are generated at a minimum. It has also invested in new technologies in the energy sector and developed legislation that can lead to higher energy efficiency of buildings (Massarotti et al., 2021). Additionally, relevant technologies developed for the exploitation of this free type of energy are getting more important, as heating and cooling represents 50% of EU’s final energy consumption (ReGeoCitie Project, 2015) (2015 figure). Currently, in EU the residential sector is responsible for more than one quarter of the total primary cooling energy demand (Founda et al., 2019) and buildings are responsible for approximately 40% of total EU energy consumption, in spite of the fact that EU Commission has established a set of standards for the energy performance of buildings, called EPB standards (Energy EC Europa EU, n.d.). This still leaves a large untapped research potential with regard to using renewables and improving energy efficiency of systems. One manifestation of the latter is the installation of Geothermal energy systems (mainly Ground-Source Heat Pumps—GSHPs) for space heating and cooling in residential and commercial buildings, which has become increasingly popular in the last decades as compared to the more common air-to-water or air-source heat pumps (AWHP or ASHP) (Christou et al., 2024). Geothermal energy systems give an opportunity to achieve significant energy, environmental and at the same time economic benefits, if well applied (Atam & Helsen, 2016). In comparison with ASHP systems, for each kWh electric, GSHPs can produce (or remove) 50% extra thermal energy (Zhang et al., 2018). In particular, a vertical GSHP system for heating and cooling in typical conditions of the European Mediterranean coast consumed in average up to 43 ± 17% less primary energy than the alternative ASHP system (Urchueguía et al., 2008). Even if ASHP systems can be slightly more financially attractive for a short period, GSHP systems can offer significant savings for a longer design life (Lu et al., 2017). Studying the thermal performance of a hybrid solar GSHP system, the efficiency of the ground heat storage (in a water tank) mode can reach 47.9%, depending on the collector area and the number of GHEs (Yang et al., 2018). Moreover, a ground water GSHP was shown to lead to a primary energy consumption reduction of 42.9% compared to an original mainstream system (Zhu et al., 2015), while a GSHP system was shown to lead to CO2 emissions reduction of 36.7% and 28.9%, compared to an ASHP system and a natural gas system respectively (Kapıcıoğlu & Esen, 2022).

To this end, as other EU countries, Cyprus has adopted the EU Directives 31/2010/EC and 27/2012/EC, and in order to support, among other goals, the reduction of energy consumption and the increase of the energy efficiency in the building sector (Cyprus Bar Association, 2006), three financial subsidy schemes were launched in December 2014, March 2015 and March 2018 respectively. Moreover, Cyprus had adopted the EU Directive 2009/28/EC regarding geothermal energy development and, as a result, the Government was obliged to establish certification schemes for GSHPs installers. The GSHP system is one of the most well-known types of geothermal systems and renewable energy technologies for heating and cooling buildings due to its high efficiency and its environmental friendliness. GSHPs make use of GHEs, which are designed to meet the energy demand both in summer and winter (Ma et al., 2019).

In the case of any space air conditioning geothermal installation, the purpose is to cover the needs of the summer and winter air conditioning of a building according to its use. In recent years, applications for the utilization of shallow geothermal energy have been successfully developed in Cyprus. In particular, previous studies have performed measurements of ground temperatures in Cyprus that can be used for ground thermal applications (Pouloupatis et al., 2011), as well as measurements and analysis of thermal properties of rocks that can be used for the compilation of geothermal maps of Cyprus (Stylianou et al., 2016). Also, a methodology for estimating the ground heat absorption rate of GHEs in Cyprus (Stylianou et al., 2017), a parametric analysis of the factors affecting the efficiency of GHEs in Cyprus (Pouloupatis et al., 2017) and the effect of groundwater in modeling GHEs in Cyprus (Stylianou et al., 2019), have been addressed. The design of a geothermal system and the methodology used for estimating the ground heat absorption rate of GHEs (Stylianou et al., 2017), both play a vital role for low energy consumption, whereby reducing the amount of energy required and, hence, maximizing the efficiency (Michopoulos, 2020). Note that, for maximum efficiency, an equally important role is played by the ground thermal properties (Stylianou et al., 2016), which is an issue beyond the scope of this study though. Once the system geometry is established, the working temperature profiles must be set. If the velocity of the heat carrier fluid stays constant, then the hydraulic and thermal problems can be uncoupled. This allows performing the calculation of the energy balance and solve problems of temperature distribution.

Naranjo-Mendoza et al. (2019) used subsurface as an Earth Energy Bank and calculated the whole system’s performance assessed through calculating the energy fluxes involved in it. The obtained results showed that after 19 months of operation the system performed well with a COPsys of 2.51 and covered the space heating requirements of the building in winter. Liu et al. (2023) went into more detail as it was required to keep all winter time the temperature of a greenhouse constant at 25°C. So, the general energy balance of the geothermal system was calculated as the net heat input rate equal to network output rate, if it is assumed that there are no significant changes in kinetic or potential energy, and no heat or work transfers (Ozgener & Hepbasli, 2005).

Cooling or heating performance of a GSHP system is represented by the Coefficient of Performance (COPsys), as opposed to the COP of the HP and the seasonal COP (SCOP) (Man et al., 2012). It should be emphasized that HP performance depends on the temperature of the fluids exchanging heat at the evaporator and the condenser stages. Therefore, different climatic conditions like the Mediterranean climate zone and different demand scenarios over a season can lead to large variations in the overall efficiency of the machine.

Concerning existing studies of GSHP systems that have been conducted in the last decade, just a few of them concentrate on the practical operation performance of the GSHP systems, as most of them are focused on theoretical and simulation models.

Hwang et al. (2009) presented the actual cooling performance of a GSHP system installed in Korea, based on specific dates where temperatures were the highest of the year. With the help of these computational measurements the COPsys was found to be 5.9 at 65% of partial load conditions. Man et al. (2012) presented comprehensive data concerning the GSHP experimental rig installed in the Hebei University of Engineering, China. The GSHP system, used for both cooling and heating, was analyzed in intermittent and continuous mode, based on experimental data. It was found that the performance of the system was affected by its operation conditions and modes. In more detail, the system operated in intermittent mode possesses higher COP compared to that operated in continuous mode by 11.57% after 40 h of operation for cooling provision, and by 9.47% after 100 h of operation for heating provision. The advantage of intermittent mode over continuous mode increases as operation time increased.

Luo et al. (2015) examined the thermal performance of a GSHP system installed in an office building in Nuremberg, Germany. A 4-year continuous monitoring of the heating and cooling performance led to a COP of 3.9 for a typical winter day and a COP of 8.0 for a typical summer day (when the HP is shut and only water circulating pumps are in operation).

In the geographic area of the Mediterranean climate zone, Emmi et al. (2017) investigated the energy behavior of two case-studies in the environment of Venice and Florence, where GSHP systems were compared to ASHP systems and to common systems consisting of condensing gas boiler and air-to-water chiller. The obtained results showed that the GSHP was the best solution in terms of primary energy savings. In more detail, the results of the simulations were calculated in terms of the seasonal energy efficiency where the seasonal coefficient of performance (SCOP) and seasonal energy efficiency ratio (SEER) of the HP were assumed as the ratio between the annual energy need for heating and cooling and the annual electrical energy demand for the two operation modes. The mean SCOP of the GSHP system was found to be 3.5, compared to the AWHP system, which was 2.5, and the condensing boiler coupled with an air-to-water chiller system, which was 0.95, for the Mediterranean climate of Venice and Florence. Simulations were executed for a 10-year period. The issue of heat dissipated to the ground, especially in the case of highly insulated buildings, was also addressed as this could lead to long-term, ground temperature shifts, resulting in a reduction of the COP in cooling mode.

Esen et al. (2006) analyzed the energy performance of a low enthalpy GSHP system. The study case was located in the hot climate of Turkey were the annual COPsys and the HP COP reached 2.82 and 3.42. Another study in Turkey by Pulat et al. (2009) tested the performance of a GSHP system with horizontal GHEs. As the system was tested in heavy winter, it displayed a COPsys of 2.58 and a considerably higher SCOP of 4.18 for the HP unit. As for different installation configurations of GHE, Esen et al. (2017) showed that the COPsys and SCOP were 2.88 and 3.55 respectively for horizontal slinky-type GHEs, while the COPsys and SCOP reached 2.34 and 2.91 respectively for vertical slinky-type GHEs.

García-Cespedes et al. (2020) focus on methods to characterize the efficiency losses in existing GSHP installations under partial-load conditions. The closed-loop system consisted of 10 vertical BHEs with an average drilling depth of 140 m. The efficiency losses of the GSHP were identified and quantified by analyzing key parameters measured and recorded by its monitoring system. The methodology was based on the EN-14825 standard and the literature. However, in order to obtain significant results, minute-resolved data acquired over time periods with almost constant thermal load (quasi-steady state conditions) proved to be necessary. The analysis yielded a value around 9% for the maximum partialization losses due to cycling operation. On the other hand, overall heating and cooling capacity reductions in the GSHP were observed. An overall 20% reduction was determined for the cool season (heating) and a 24% reduction for the warm season (cooling). The obtained SCOP for summer and winter were 3.14 and 3.09, respectively. These values are below what is expected in such heating/cooling facilities, where SCOP can easily exceed a value of 3.5 (Kim et al., 2012; Pospísil et al., 2018).

Lim and Lee (2021) experimented on the performance of standing column wells (SCW) used in a geothermal project and proposed the cross-mixing balancing well heat exchanger method. Measurements showed that using this technique in operation from a regular SCW-type heat exchange system to a well-intersected heat exchange system, not only improved the COPsys, but also ensured a stable supply of geothermal sources. The COPsys had an improvement of 23% and 12% during the cooling and heating operations, respectively, yielding values of 3.76 and 3.27 respectively.

Michopoulos et al. (2016) presented a theoretical feasibility analysis for the installation of GSHP systems in Cyprus, referring to two different types of buildings, a single and a multifamily one. The energy demand of the buildings for heating and cooling in the climate conditions of Cyprus was calculated. The authors performed a comparison of conventional heating and cooling systems (ASHP) that are widely used in Cyprus and a GSHP system consisting of a vertical GHE and water-to-water HPs. Significant reductions in primary energy use for both types of buildings were observed upon installation of the GSHP system. From an economic point of view, the geothermal system compares favorably with the conventional systems in many cases, having SCOP for the heating and the cooling mode, range between 4.0 and 4.5 and 5.0 and 5.5 for both building types.

Mahmoud et al. (2024) studied the advantages/disadvantages between the different types of GSHPs with highlights on the GSHP’s performance in coloration with its geometry and the construction cost. Published results showed that the COP is mostly affected by the GHEs’ depth and borehole spacing.

For comparison reasons, regarding ASHP systems, such a system installed in Woodbridge, Ontario, Canada performed well in the cooling mode with a SCOP average at 5.2, but in heating mode, the SCOP turned out to be only 3.4 (Safa et al., 2015).

Finally, in Table 1 a summary of real case studies regarding GSHP systems installed in various countries is given, with the respective COP, SCOP or COPsys.

The purpose of the current study is to present the hybrid configuration and the practical operational results of the GSHP system used for space air conditioning at the building where the library of Cyprus University of Technology (UMLL) is housed. In particular the goal is to estimate the efficiency of the system in relation to the specific area conditions, and to make the comparison with the theoretical efficiency given by the manufacturer, thereby assessing the success and usefulness of the system’s employment. Section "Methods and Materials" presents the Methodology, containing the case study presented, the GSHP system operation and the associated Building Management system. In Section "Results" the results and their discussion are presented both for heating and cooling mode of the system operation. Finally, we conclude with Section "Discussion".

The study presented here is a case study of a GSHP system installed in a relatively large building in the city of Limassol at the coastline of the Mediterranean island of Cyprus. In the sequel, the case study itself, the GSHP system operation and the associated Building Management system are all addressed in detail.

The data used for the preparation of graphs in the sequel, were collected through the BMS platform tools. Then using MS Excel, the efficiency of each part F (recall Table 3) was calculated separately. In order to confirm the calculated results, measurements were also manually taken directly from the Plant room.

As mentioned above, the UMLL GSHP system operates with two chillers, which function in alternating days. Here chiller 2 is studied.

With regard to the GHE type, the analysis presented above has shown higher heat exchange values, especially in summer, inside the open well, compared to the boreholes and the helicoidal coils.

The chillers’ operation was also examined as it has a great impact on operational costs by being the largest electricity consumer within the System. The chillers’ operation was monitored through the SCOP and by analyzing the results one comes at the conclusion that chillers in a GSHP system working in Mediterranean climate zone are more efficient in cooling mode than it heating mode for the tested case; this is probably due to the specific choice of April.

The obtained SCOP values (4.7 for heating, 5.0 for cooling) compare well with all reported values for similar real case studies (recall Table 1). Actually, the obtained SCOP values for the current case study are higher than most of the reported cases, whether these are concerned with boreholes in Germany (Luo et al., 2015), Italy (Emmi et al., 2017), Turkey (Esen et al., 2017), Spain (García-Céspedes et al., 2020), or other GHE types in Turkey (Esen et al., 2017; Pulat et al., 2009) or Korea (Lim & Lee, 2021). This can be attributed to the Mediterranean weather conditions of the coastline city of Limassol, a climate well suited for geothermal installations. In addition, the experimental results obtained here verify the theoretical values as suggested by Michopoulos et al. (Michopoulos et al., 2016) for close weather conditions and geographical location. Of note is that previous numerical studies, validated using borehole data in various locations in Cyprus led to a positive conclusion about the suitability of geothermal systems in Cyprus, as they can lead to efficient use of GSHPs, as performance is not affected by the ambient conditions (Florides et al., 2012, 2013; Pouloupatis et al., 2017). The results obtained here indeed verify this conclusion.

Measurement errors can have significant effects on the accuracy and reliability of experimental or observational results. Inaccurate measurements lead to inaccurate results by introducing deviations from the true values of the quantities being measured. These can lead to inconsistencies or difficulties in the reproducibility of results. Measurement errors therefore can undermine the validity of conclusions drawn from the data. Researchers might make incorrect assumptions or interpretations about the phenomenon under study due to the presence of measurement errors. To avoid these errors measurement instruments should regularly be calibrated to ensure their accuracy and reliability, implement quality control measures during experiments or observations to identify and correct errors and through replication to conduct multiple measurements or experiments to assess the consistency of results and reduce the impact of random errors. It is important to recognize that complete elimination of measurement errors is often not feasible, but a systematic approach to minimizing and managing these errors can improve the overall quality and reliability of scientific research and experimentation. In order to eliminate errors in the case study we would recommend temperature readings to be recorded with an increased accuracy (0.1⁰C instead of 0.5⁰C) and monitoring the system and reporting results for much longer periods (whole year/s).

The overall aim of this paper was to present the practical operational results of a GSHP system used for space air-conditioning installed in the Mediterranean climate zone. Another goal was to compare different types of GHE, as the system consists of five different types of GHEs U-tube configuration, one GHE of helicoidal configuration and a complex of open wells. The evaluation of the collected data was done both for heating and cooling operation mode.

Regarding heat exchange value, in cooling mode, the open well had values between 60 and 80 kW, which are higher than the heat exchange achieved by the sum of the boreholes (between 40 and 60 kW) or the helicoidal coils in the well (between 10 and 15 kW). Regarding the chillers’ performance, analysis of the SCOP lead to the finding that the chillers in a GSHP system working in cooling mode were more efficient than working than in heating mode (SCOP values 5.0 for cooling versus 4.7 for heating). In both modes one could conclude that the electric power consumed by the chillers is approximately five times lower than the power placed in the building by the Geothermal System. The estimated efficiencies indeed match the theoretical manufacturer’s efficiencies.

It is crucial to recognize that the ranges for COP can fluctuate due to variables such as the specific ground temperature and the geological characteristics of the region, the level of installation quality, the design temperatures of the system, the type of ground loop implementation, as well as the indoor and outdoor air temperature and humidity. Still, the good results in terms of energy performance confirm that the GSHP systems located in Mediterranean climate zone are high efficiency solutions in terms of primary energy savings.

The next step in the GSHP under consideration performance assessment is to re-test the system with present day data, where the Library was in full operation after the Covid19 pandemic years. Before doing that the whole BMS system should be re-calibrated in line with the mechanical system. This points to the limitations of the current study, the most important of which is the lack of data related to the additional energy consumed by the handling system, i.e., ground heat exchanger circulating pumps, pumps and fans related to the handling system (to be used for COPsys). However, as a very first verification of the high SCOP obtained for the year 2019, come some preliminary results for the year 2023 (the first post-Covid19 year with full operation of the UMLL), with calculated respective SCOP values of 5.2 and 5.1 for summer and winter. Moreover, the present-day data (with the library in full operation) will also enable a complete analysis of the performance of all ingredients of the mechanical system, as well as a cost analysis.