Horizontal Ground Source Heat Pump Research Articles

Technical Performance Comparison of Horizontal and Vertical Ground-Source Heat Pump Systems

The configurations of ground heat exchangers (GHEs) play a significant role in the efficiency and sustainability of ground-source heat pump (GSHP) systems. However, there is a knowledge gap in understanding the performance differences between the horizontal and vertical GSHP systems in the same project under various heating and cooling demands. In this study, a technical performance comparison between GSHP systems coupled with horizontal ground loops and vertical boreholes under three scenarios of heating-to-cooling ratios (6 : 1, 2.4 : 1, and 1 : 1) was conducted. The simulations were based on a coupled thermal–hydraulic model for unsaturated soils that takes into account realistic ground surface boundary, GHE boundary, and the dynamics of heat pump efficiency. The GHEs were designed based on an experimental site located on the campus of a UK university. Results showed significant differences in the development of fluid temperatures and coefficient of performance (COP) of heat pumps between the horizontal and vertical GSHP systems due to the differences in the soil profiles and temperature boundaries. Both the fluid temperatures and heat pump COPs in the horizontal GSHP system reached a steady annual cycle after 2 years regardless of the heating-to-cooling ratios. For the vertical system, a general downward trend in the fluid temperatures and the COP of the heat pump in the heating mode can be found when a heating-to-cooling ratio was 6 : 1 or 2.4 : 1, while an overall upward trend in the fluid temperatures and the COP of the heat pump in the heating mode can be noted in the case of 1 : 1 heating-to-cooling ratio. Additionally, the heat pump operating in the cooling mode was off most of the time when a heating-to-cooling ratio was 6 : 1 or 2.4 : 1, while a declining trend in the COP of the heat pump in the cooling mode was exhibited in the case of a heating-to-cooling ratio of 1 : 1. The technical comparison reveals that the heating-to-cooling ratios would significantly affect the efficiency and sustainability of both GSHP systems.

A numerical study on performance efficiency of a low-temperature horizontal ground-source heat pump system

Exploitation of shallow ground and its low-grade heat potential is fundamental to designing 5th generation district heating and cooling (DHC) networks. Horizontal ground-source heat pump (HGSHP) systems are a common way to utilize shallow geothermal energy. Realistic estimation and prediction of performance of a HGSHP system and shallow ground thermal behaviour should consider the whole system including building heating and cooling load, heat pump and ground heat exchanger, and the ground. This should be accompanied by realistic atmospheric and ground conditions. In this paper, a three-dimensional coupled thermal–hydraulic model with realistic boundary conditions adopting a whole system approach is presented. Dynamic heat pump coefficient of performance (COP) that depends on seasonal variation of heating/cooling demand and ground conditions are also considered. Model validations are conducted against experimental and analytical results in literatures. The model is applied for evaluating a HGSHP system to support development of a 5th generation DHC network on a potential site in the UK. Several influencing factors, such as ground moisture transfer, building thermal load mode, buried depth of ground loops, and initial ground temperature profile are studied to assess performance efficiency of the HGSHP system and evolution of ground thermal behaviour in response to heat extraction or rejection into the ground. The results show that 5% of the monthly total heat demand of the site could be met by the designed HGSHP system, consisting of 200 U-shaped ground loops buried at the depth of 3 m and pure water as the heat carrier. Overlooking the ground moisture transfer or hyperbolizing the ground saturation would overestimate the load-carrying capacity of the HGSHP system. The HGSHP system is more efficient with a higher heat pump COP under the heating and cooling mode than under the heating-only mode. Predicted performance of the HGSHP system improves with buried depth of the ground loops. The results also show that a 1 ℃ increase in the undisturbed ground temperature could suffice up to 8% of the monthly total heat demand of the site.

Solar harvest: Enhancing carbon sequestration and energy efficiency in solar greenhouses with PVT and GSHP systems

It is universally acknowledged that climate change brings widespread attention to solar greenhouse plant carbon sequestration. Suitable technologies in solar greenhouses were, are, and will be play a leading role in this vital transition. The primary aim of this research is to examine the energy efficiency and carbon sequestration potential of a solar-assisted ground-source heat pump (SAGSHP) heating system. This hybrid system, which integrates a horizontal ground-source heat pump (GSHP) system with PVT and heat storage, can efficiently fulfill the heating demands of a greenhouse and function as a positive energy building. Four plants include cucumber, tomato, cowpea, and lettuce were selected to compare the carbon absorption effects. Results show that the hybrid system outperforms conventional systems, with a coefficient of performance (COP) of 6.71 during peak hours and PVT efficiency over 57.88%, which effectively meet the heat load of the greenhouse and keep the indoor heat comfortable. In addition, for the carbon sequestration potential of four plants, tomato exhibited the highest photosynthetic carbon sequestration of 3522 kgCO2·m−2. Cowpea showed the strongest daily carbon sequestration capacity at 26.86 gCO2m−2d−1 and better economic income. Through the application of this enhanced solar greenhouse, people can enhance the utilization of solar energy, establish flexible interaction between energy and information flow, and make a promising option for sustainable building design.

Improving computational efficiency of numerical modelling of horizontal ground source heat pump systems for accommodating complex and realistic atmospheric processes

Modelling horizontal ground loops for a horizontal ground source heat pump (HGSHP) system is complex and computationally expensive. The computation precision is highly reliant on the prescription of an undisturbed ground temperature in the unsaturated ground as well as realistic and accurate atmospheric processes at the ground surface boundary. Conventionally, modelling of such a system would include direct application of the atmospheric processes at the soil-atmosphere boundary and solve it in a single-stage approach. However, low efficiency is found for large spatial domain and long-term transient problems as the boundary processes need to be solved and expressed in terms of primary model variables at each simulation time-step. This paper proposes an equivalent two-stage modelling approach, for the first time, based on an advanced coupled thermal-hydraulic (TH) model to improve computation efficiency while maintaining adequate accuracy. In this approach, firstly, the model is solved for an intact ground that is imposed by complex atmospheric processes, e.g., rainfall, solar radiation, humidity, evaporation, etc. at the soil-atmosphere boundary, and the spatial and temporal variations of the primary model variables are recorded. Afterwards, the recorded data are incorporated in the simulator, as model inputs, for the same ground including a HGSHP system. Predicted results from both 2D and 3D simulations show that the ground temperatures calculated by the proposed two-stage approach are in good agreement with that of the traditional single-stage approach. However, the two-stage approach is computationally robust. For the presented 2D and 3D simulations, it required only 32% and 37% of the time of the single-stage approach, respectively, while maintaining great accuracy. This demonstrates the utility of the proposed two-stage approach for modelling complex scenarios of realistic HGSHP systems installed in a large spatial domain and for long-term operation.

Techno ‐economic analysis of Al 2 O 3 / CuO nanofluid applied in various horizontal ground heat exchangers

A ground source heat pump (GSHP) is widely regarded as a promising technology for building heating, ventilation, and air conditioning because of its higher energy efficiency. Nanofluids (NFs) have also received extensive attention due to their excellent heat transfer characteristics. In this paper, CuO/Al2O3 particles mixed with propylene glycol/water (PG/W, volume ratio: 60/40) are applied to GSHPs as a heat transfer medium. The combination of different pipe types and different concentrations of NFs is studied by comparing the coefficient of performances (COPs) and outlet temperatures. The results show that the 2% CuO NF combined with the spiral type offers the best performance with a 28% rise in the outlet temperature in the pipe and an average COP value of 4.6. When the concentration of Al2O3 NF is 2%, the combined effect of the meander type is the best. The optimal combination of CuO (2%) and spiral type is used for economic analysis by comparing with the performance of natural gas boilers, air source pumps, and GSHPs under similar conditions. The levelized cost of heat analysis results indicate that the horizontal GSHP system coupled with NF can maximize heat exchange efficiency and mitigate environmental effects, but the cost is still high as ¥ 1355/MWh ($212.46/MWh).

Thermal and economic performance of horizontal ground source heat pump systems with different flowrate control methods

The horizontal ground heat exchanger (HGHE) integrated with the heat pump unit can be defined as the horizontal GSHP system. Most horizontal GSHP systems are always operating at a rated flowrate in the secondary loop, resulting in the electricity waste of circulation pump when the system operates in partial load. This study aimed to compare the thermal and economic performance of horizontal GSHP systems with different flowrate control approaches. Two novel variable flowrate regulation methods, namely the segmented and time-varying flowrate control (SFC and TFC) method, together with the traditional constant flowrate control (CFC) method, were employed in the secondary loop of system. 3D numerical model of the HGHE considering the isotropic porous soils and varying ambient environmental conditions was built and coupled with the model of heat pump to analyze the heating and economic efficiency of the horizontal GSHP system. Considering that the buried depth of HGHE, H, may vary in different conditions, the impact of flowrate control methods in various H was analyzed. The results reveal that compared with the CFC and SFC method, the variation range of the fluid temperature difference between the inlet and outlet of the HGHE, ΔTf, and system COP, COPsys, are smallest and the level of ΔTf and COPsys are greatest for the TFC method. The system operation costs and electricity charges proportion of water pump can decrease by up to 9.80% and 47.26% as the control method varies from CFC to TFC, but the differences between the variable flow control technologies are not significant. The discrepancy of COPsys and operation costs between the constant and variable flowrate control methods decrease by 39.13% and 48.72% with the increases of H. This study highlights the significance of using variable flowrate regulation methods in the secondary loop of the horizontal GSHP system to improve thermal and economic performance.

A study on heating and cooling requirements for green buildings and refugee settlements

In this study, geothermal-based three different systems are examined comparatively and a decision-making method is used to find the best option to use in refugee settlements. Earth air heat exchanger (EAHE), vertical ground source heat pump (VGSHP), and horizontal ground source heat pump (HGSHP) are evaluated from the viewpoint of environmental impact, performance, economy, and health. This study is the first study related to the evaluation of EAHE and GSHP systems by using the analytical hierarchy process (AHP). It is concluded that EAHE is the best option for refugee settlements; also EAHE is a healthier option for the COVID-19 pandemic.

Comparison of long-term performance and initial cost of a horizontal ground source heat pump with an air source heat pump in hot regions

Ground source heat pumps (GSHPs) have received much attention due to their high coefficient of performance (COP). In these systems, the ground is used as a sink/source for heat pumps and the heat transfers between the ground and the heat pump is performed by the ground heat exchanger (GHE). In this study, the long-term performance and initial cost of these systems have been compared with conventional air-source systems in a hot region. To evaluate the performance of the GSHP, using fluent software, numerical modeling of a linear horizontal GHE was carried out in Bandar-Abbas. Also, the effect of various system operating parameters, i.e., length, distance, buried depth, and diameter of GHE pipes have been studied. According to results, the five-year COP and exergy efficiency of GSHP systems is 19.9% to 24.30% and 5.95% to 6.55%, respectively, more than that of the air-source system. However, 1.2 to 2.5 million Tomans is needed as the installation cost for each kW of maximum building load per year. Also, it is demonstrated that by reducing the length of the GHE, the initial cost is reduced, and the performance increases, the pipe spacing is the most important factor in the required ground surface, the depth has the most impact on the initial cost and the performance, and the pipe diameter does not affect the system performance.

Performance analysis of horizontal ground source heat pump for building cooling in arid Saharan climate: thermal-economic modeling and optimization on TRNSYS

This paper presents a feasibility and performance study of ground source heat pump (GSHP) coupled with horizontal ground heat exchanger (HGHX) used for cooling residential unit equipped with radiant floor system (RFs) under the meteorological conditions of Saharan environment in Ouargla, city located in Southeast Algeria. A dynamic simulation system is developed using TRNSYS software for modeling the performance of the GSHP system. To verify the reliability of GSHP including HGHX system programs, the modeling procedure was validated against experimental data from a horizontal ground source heat pump system (HGSHPs) installed at the Research and Technology Center of Energy (CRTEn), Tunisia, and a good agreement was obtained. Then, to obtain an acceptable balance between system efficiency and total cost of HGSHPs an economic analysis was carried out to determine the optimum design parameters of the HGHX. The simulation results obtained from this study indicated that the HGSHPs could effectively solve cooling problem and reduce traditional energy consumption in the Saharan areas; it is possible to lower the mean indoor air temperature below 27 °C and raise the average relative humidity to reach 73.97%. By concentrating principally on the thermal-economic optimization, the optimized COP of the GSHP that combines the reliability and economy of cooling in long term was found to be 3.89.

The application of thermoelectric and ejector in a CO2 direct-expansion ground source heat pump; energy and exergy analysis

The thermodynamic analysis of a horizontal CO2 direct-expansion ground source heat pump in transcritical cycle was performed to investigate the influence of using thermoelectric sub-cooler and the ejector on the performance parameters of the cycle. A comparative analysis was conducted between the Base cycle and the cycles with ejector and thermoelectric sub-cooler considering the pressure drops in the gas cooler and the ground heat exchanger. The effects of compressor inlet temperature, soil temperature, thermoelectric sub-cooler length, thermoelectric sub-cooler cooling water mass flow rate, and ground heat exchanger outlet temperature on different performance parameters of the cycles were studied. The results demonstrated that the application of ejector exhibits a more stable operation of the cycle and increasing its COP by 16.5% as compared to the Base cycle considering the same ground heat exchanger length. Also, the length of ground heat exchanger is considerably decreased by using the ejector considering the same COP for the Base and EJR cycles. Besides, thermoelectric sub-cooler decreases the irreversibilities of the cycle by generation more power, thereby COP is boosted by 16% as compared to the Base cycle. Furthermore, the major irreversibility in the cycle with thermoelectric and ejector is occurred in the compressor and followed by ejector, ground heat exchanger, gas cooler, thermoelectric sub-cooler, and the expansion valve, respectively.

Comparative study on thermal performance of horizontal ground source heat pump systems with Dirichlet and Robin boundary conditions on ground surface

The heat transfer of horizontal ground heat exchanger (HGHE) is susceptible to the ground surface boundary condition (BC) due to the relatively shallow buried depth. This study aimed to compare the thermal performance of HGHEs with different ground surface BC by developing a 3D numerical model. Three commonly used HGHEs, namely spiral, slinky, and linear-type HGHE, and three ground surface BCs, namely constant temperature, time-varying temperature, and energy balance BC, were considered in the numerical models to compare the short-term performance of HGHEs with various BCs. The ground source heat pump (GSHP) system model, coupling the heat transfer model of HGHE and the model of heat pump, was further proposed to analyze the long-term heating efficiency of horizontal GSHP systems applied in a residential building with different BCs. Concerning that the influence of ground surface BC may vary with the buried depth of HGHE, a series of sensitivity analyses were lastly performed. The results reveal that there is no affirmatory relationship between the ground surface temperature and time, meaning that the use of sinusoidal or other forms of functions cannot accurately evaluate the ground surface temperature. The predicted fluid temperature and coefficient of performance (COP) of the GSHP system are underestimated by forcing the surface temperature to follow the constant or time-varying temperature, and those underestimations decline with the increase of the buried depth. Neglecting the thermal interaction of HGHE and ground surface results in small variation ranges in fluid temperature and COP. Compared to the spiral and slinky-type GSHP system, the heat exchange performance of linear-type GSHP system are more sensitive to the ground surface BC. This paper underscored the importance of using proper BC in modelling HGHE to accurately evaluate the thermal performance of HGHE.

PERFORMANCE ANALYSIS OF COOLING SYSTEM WITH HORIZONTAL TYPE GROUND SOURCE HEAT PUMP FOR DIYARBAKIR CONDITIONS

In this study, the performance of the horizontal type ground source heat pump cooling system in the summer was analyzed for cooling the testing area with a size of 9x8x2.8m in Diyarbakir province with a warm climate. Energy and exergy analysis was applied to the system and system units by examining the results obtained in the summer in the evaluated system. The monthly and seasonal mean values of the data in the summer (01.06.2018 / 30.08.2018 ) were taken for 11 node points determined in the installed system. In the energy analysis performed with the results obtained, it was calculated that the energy loss in the system was 2.499kW and the energy efficiency 76.86%. For the system units, the unit operating with the lowest energy efficiency was determined as the compressor. According to exergy analysis applied to the system, it was calculated that the exergy loss of the system was 2.150 kW and the second law efficiency was 20.70%. The compressor was also determined as the unit with the highest exergy loss in the system. Compressor should be prioritized for improvement studies to be planned according to both energy and exergy analysis results. The COP value of the system was found to be 4.05, and it was determined that the system, the use of which was examined in the summer, could be used for Diyarbakir province.

The thermal behavior of a novel wall radiator panel coupled with horizontal ground source heat pump heating system: improve indoor environment to reduce the airborne transmission of infectious diseases

Motivated by the rapid spread of the novel pandemic disease (COVID-19) that swept the most countries in the world, a new radiation heating system consists of wall radiator panel system connected to a reversible geothermal heat pump (GHP) coupled with horizontal ground heat exchanger (HGHX) was proposed as fast and permanent solution to the risks of the dispersion of airborne infectious diseases in air-conditioned enclosed spaces. An experimental system was installed and tested in the laboratory of thermal process of Research and Technology Center of Energy (CRTEn), Tunisia, in order to achieve the two main goals of this work: developing a new radiation heating system with quick and inexpensive implementation while ensuring high efficiency and environment-friendly performance for the entire system. The results obtained show that it is feasible to use the novel RPHs as heat rejecter of the horizontal ground source heat pump system (HGSHPs) for heating buildings with limited surface land areas epically those located in the Mediterranean regions such as Tunisia, the average performance coefficients of the geothermal heat pump COPhp and the overall system COPsys are found to be 6.3 and 3, respectively. The thermal comfort analysis indicates that there is only a small vertical temperature fluctuation in the test room that would not produce any negative effect on thermal comfort.

Thermal performance of the ground in geothermal pavements

Shallow geothermal energy utilises the ground at relatively shallow depths as a heat source or sink to efficiently heat and cool buildings. Geothermal pavement systems represent a novel concept where horizontal ground source heat pump systems (GSHP) are implemented in pavements instead of purpose-built trenches, thus reducing their capital costs. This paper presents a geothermal pavement system segment (20m × 10m) constructed and monitored in the city of Adelaide, Australia, as well as thermal response testing (TRT) results. Pipes have been installed in the pavement at 0.5 m depth, and several thermistors have been placed on the pipes and in the ground. A TRT has been performed with 6kW heating load to achieve an understanding of the thermal response of the system as well as to estimate the effective thermal conductivity of the ground. The results show that the conventional semi-log method may be applicable to determine the thermal conductivity for geothermal pavements. The geothermal heat exchanger at shallow depth is considerably under the influence of the ambient temperature; however, it is still acceptable for exchanging the heat within the ground. It is also concluded that the impact radius of heat exchanger in geothermal pavement during the TRT is around 0.5m in the vertical and horizontal directions for this case study.

Optimization of spatially distributed soil conductivities for horizontal ground heat exchangers

Horizontal ground source heat pumps have the potential to significantly reduce the operating energy utilized for space heating and cooling thus reducing greenhouse gases and pollution. However, the amount of land required for the installation of these systems is often a drawback in widespread implementation. It follows, therefore, that any considerable increase in ground heat exchanger performance would reduce the necessary footprint thereby making these systems more attractive. This paper presents an algorithm to determine the optimal spatial distribution of heterogeneous soil conductivities to increase the performance of ground heat exchangers. The algorithm utilizes a gradient-based optimization approach. Using this algorithm, the heat extraction rate for straight pipe and slinky configurations was significantly increased for winter conditions.

Meeting the demand: geothermal heat supply rates for an urban quarter in Germany

Thermal energy for space heating and for domestic hot water use represents about a third of the overall energy demand in Germany. An alternative to non-renewable energy-based heat supply is the implementation of closed and open shallow geothermal systems, such as horizontal ground source heat pump systems, vertical ground source heat pump (vGSHP) systems and groundwater heat pump systems. Based on existing regulations and local hydrogeological conditions, the optimal site-specific system for heat supply has to be identified. In the presented technical feasibility study, various analytical solutions are tested for an urban quarter before and after building refurbishment. Geothermal heat supply rates are evaluated by providing information on the optimal system and the specific shortcomings. Our results show that standard vGSHP systems are even applicable in older and non-refurbished residential areas with a high heat demand using a borehole heat exchanger with a length of 100 m or in conjunction with multiple boreholes. After refurbishment, all studied shallow geothermal systems are able to cover the lowered heat demand. The presented analysis also demonstrates that ideally, various technological variants of geothermal systems should be evaluated for finding the optimal solution for existing, refurbished and newly developed residential areas.

Influence of ground surface boundary conditions on horizontal ground source heat pump systems

In this paper, the effects of ground surface boundary conditions, especially diurnal shading, on performance of horizontal ground source heat pump (GSHP) systems were studied. Firstly, a GSHP system model, which consists of the heat transfer of horizontal ground heat exchangers (GHEs) and the integration of the heat pump, and the ground surface boundary model are introduced. The diurnal shading model for canyon streets is also introduced. Then, effects of ground surface boundary conditions and shading on horizontal GSHP systems are studied. Results show that different assumptions on ground surface boundary conditions have significant impact on outcomes and should carefully be considered during analysing. The outlet fluid temperature and fluctuation in North-South (N-S) streets are higher than those of East-West (E-W) streets. The increase in street aspect ratios and latitudes could decrease sunshine exposure time of the streets. Daily variations of solar radiation and shading influence the outlet temperatures of horizontal GHEs up to buried depth of 2.5 m. Therefore, the effects of solar radiation and shading should be considered when studying shallow horizontal GHEs.

Dynamic thermal performance of horizontal ground source heat pumps – The impact of coupled heat and moisture transfer

A ground heat exchanger is a key component of a ground source heat pump system, and heat and moisture transfer occurs simultaneously in soil with a horizontal ground heat exchanger in operation. A new method has been developed to generate moisture and temperature profiles in soil with spatially and temporally varying properties. The profiles are used as initial data for accurate solution of the equations for transient heat and moisture transfer in soil containing a buried horizontal ground heat exchanger. The impacts of initial conditions of soil and coupled heat and moisture transfer are assessed on the thermal performance of a horizontal ground heat exchanger for a ground source heat pump for different installation depths and soil textures. Seasonal heat transfer through a horizontal heat exchanger increases with installation depth and a heat exchanger installed at 2 m deep can transfer 19% more heat than that at 1 m deep. Heat transfer in sandy soil is 17% higher than in loamy sand soil which is 14.5% higher than in clay loam soil. The maximum differences between models with and without moisture transfer for the prediction of heat transfer through a heat exchanger are 24%, 17% and 18% in clay sand, loamy sand and sandy soils, respectively. In conclusion, it is necessary to use a coupled heat and moisture transfer model in order to predict accurately the seasonal thermal performance of a ground heat exchanger in shallow ground.

寒冷地における水平採熱型地中熱ヒートポンプ暖房システムの検討 （その4）設計用年間採熱量

寒冷地における水平採熱型地中熱ヒートポンプ暖房システムの検討 （その4）設計用年間採熱量

Can brownfield land be reused for ground source heating to alleviate fuel poverty?

Abstract Brownfield land is a legacy of industrial retraction in many towns and cities worldwide, where land remains vacant long after it has gone into disuse, and is often a barrier to redevelopment. Using this land for renewable energy generation is one option that can support development of a low carbon economy and also stimulate regeneration. Fuel poverty is an increasingly pertinent social issue due to rising energy costs. This is particularly true for space heating, accounting for nearly half of all the energy consumed in North European climates. Addressing fuel poverty has become a key consideration in Scotland's internationally leading renewables policy. This article considers how deployment of renewables on brownfield land can be targeted towards addressing heat poverty in social housing. Using Glasgow as a case study, the quantity of available derelict land is calculated, then the spatial association of social housing and urban brownfield land is demonstrated. Technology options for meeting household heat requirements from brownfield land are presented, including scenarios using vertical or horizontal ground source heat pumps. The results suggest that the available urban land could easily supply the needs of all households in fuel poverty, if this scale of investment and non-market intervention was justified.