Geothermal Heat Pump System Research Articles

Heat Transfer Performance Factors in a Vertical Ground Heat Exchanger for a Geothermal Heat Pump System

Ground heat pump systems (GHPSs) are esteemed for their high efficiency within renewable energy technologies, providing effective solutions for heating and cooling requirements. These GHPSs operate by utilizing the relatively constant temperature of the Earth’s subsurface as a thermal source or sink. This feature allows them to perform greater energy transfer than traditional heating and cooling systems (i.e., heating, ventilation, and air conditioning (HVAC)). The GHPSs represent a sustainable and cost-effective temperature-regulating solution in diverse applications. The ground heat exchanger (GHE) technology is well known, with extensive research and development conducted in recent decades significantly advancing its applications. Improving GHE performance factors is vital for enhancing heat transfer efficiency and overall GHPS performance. Therefore, this paper provides a comprehensive review of research on various factors affecting GHE performance, such as soil thermal properties, backfill material properties, borehole depth, spacing, U-tube pipe properties, and heat carrier fluid type and velocity. It also discusses their impact on heat transfer efficiency and proposes optimal solutions for improving GHE performance.

Enhancing nocturnal microclimate in Tunisian greenhouses: Experimental study on integrating an innovative conic helicoidal heat exchanger with a geothermal heat pump system

Ensuring an optimal nocturnal microclimate within greenhouses is essential for successful crop cultivation. In regions like Tunisia, characterized by arid and semi-arid climates, managing temperature and humidity during the night, particularly in cold winter months, is challenging and often requires external heating sources. This study aims to investigate the use of geothermal energy as a sustainable solution for greenhouse heating, focusing on an innovative helicoidal geothermal system to maintain ideal nocturnal conditions.The primary objective of this research is to evaluate the effectiveness of the helicoidal geothermal system in achieving optimal temperature and humidity levels for plant growth during the night. Conducted at the Research and Technology Centre of Energy in Tunisia, the experimental investigation involved a new ground heat exchanger coupled with a heat pump system.The experimental setup consisted of two greenhouses: one serving as a control and the other equipped with two Conic Helicoidal Ground Heat Exchangers (CHGHE) and a Ground-Source Heat Pump (GSHP) system, which includes both underground and suspended heat exchangers. The results demonstrated that the geothermal system, operating at an optimal flow rate of 0.6 kg/s, successfully maintained the nocturnal comfort temperature required for plant growth. The coefficients of performance for the geothermal heat pump (COPhp) and the overall system (COPsys) were calculated to be 3.12 and 2.7, respectively.These findings underscore the potential of near-surface geothermal energy, utilizing a geothermal heat pump, to efficiently heat greenhouses under the climatic conditions of Tunisia. This approach not only supports sustainable agricultural practices but also offers significant energy efficiency and cost-saving benefits.

Optimization of Geothermal Heat Pump Systems for Sustainable Urban Development in Southeast Asia

This study examines the optimization of ground source heat pump (GSHP) systems and energy piles for sustainable urban development, focusing on Southeast Asia. GSHPs, which utilize geothermal energy for indoor HVAC needs, offer a sustainable alternative to traditional systems by utilizing consistent subsurface temperatures for heating and cooling. The study highlights the importance of understanding thermal movement within the soil, especially in soft marine clays prevalent in Southeast Asia, to improve GSHP system efficiency. Using a one-dimensional finite difference model, the study examines the effects of soil thermal conductivity and density on system performance. The results show that GSHP systems, especially when integrated with energy piles, significantly reduce electricity consumption and greenhouse gas emissions, underscoring their potential to mitigate the urban heat island effect in densely populated areas. Despite challenges posed by the region’s hot and humid climate, which could affect long-term effectiveness, the study highlights the need for further study, including field experiments and advanced modeling techniques, to optimize GSHP configurations and fully exploit geothermal energy in urban environments. The study’s insights into soil thermal dynamics and system design optimization contribute to advancing sustainable urban infrastructure development.

Scenarios for replacement of electric resistive space heating by a geothermal heat pump - Environmental amortization

Geothermal energy has become of increasing scientific and industrial interest; hence, many works discuss and study its principles, uses, and technical/economic viability. However, much less research focuses on the environmental impact of geothermal energy projects.In this research, we perform a life cycle assessment of five different geothermal heat pump systems (GHPS) installed across five different facilities: three single-family detached houses (heating capacity of GHPS: 10 kW, 15 kW, and 45 kWh, respectively), a multipurpose building (heating capacity of GHPS: 110 kW), and a residential area of semi-detached houses (heating capacity of GHPS: 210 kW).The overall results demonstrate that the higher the heating capacity of a GHPS, the greater the environmental impact. Such results allow us to establish correlations between GHPS heating capacity and environmental impact across the 11 environmental impact categories.Amortization periods were calculated with respect to the annual demand for heating and cooling of each facility. In the 10 kW and 15 kW GHPS, the amortization period is shorter than 11 years, followed by an eight-year period for the 45 kW GHPS. Similarly, the amortization period for the 210 kW GHPS decreases up to five years. Conversely, the 110 kW GHPS has an 18-year amortization period.

Effect of Different Control Strategies on the Heat Transfer Mechanism of Helical Energy Piles

In this paper, numerical simulations of a special energy pile, which constitutes a spiral-injected pipe and one straight discharge pile for Geothermal Heat Pump Systems (SGHEs-P(parallel)), were conducted by Fluent software. The effects of the spiral pitches on the heat transfer rate based on the G-function method and peripheral soil temperature of the pile were investigated under continuous and intermittent operation strategies. The impact of spiral tube sizing on the surface heat transfer coefficients was studied. The results indicated that SGHEs-P may be preferred for office buildings under intermittent operation conditions. For a short period, the temperature profiles and heat transfer efficiency of SGHEs-P were mainly influenced by the fluid type, length of the spiral tube, and spiral pitch. The smaller the spiral pitch, the more uniform the temperature distribution, and the better the heat transfer effect, but the heat transfer per unit depth of pile decreased. The average temperature variation curve of the soil around the energy pile with different spiral pitches was simulated and obtained over time. Meanwhile, the impact of spiral radius, spiral pitch, and spiral tube radius on the convective heat transfer coefficient was also presented. Through data fitting, the formulas for the correction coefficients of spiral radius, spiral pitch, and spiral tube radius on convective heat transfer coefficient were obtained, respectively.

Effects of varying heat transfer rates for borehole heat exchangers in layered subsurface with groundwater flow

The existence of groundwater flow adds a convective heat component to shallow geothermal heat pump systems, significantly improving heat transfer efficiency. Numerous analytical and numerical methods have been proposed to assess the impact of groundwater flow on ground heat exchangers. However, numerical simulations often require significant resources and time, while conventional analytical solutions overlook the time and depth-varying heat transfer rates, leading to inaccurate temperature predictions. This study proposes a computationally efficient semi-analytical solution by extending the line source solution and coupling it with a thermal resistance model. This approach addresses variations in heat transfer rate over time and depth, overcoming the limitations of conventional analytical solutions. Compared with a three-dimensional numerical model constructed in COMSOL Multiphysics, the average absolute percentage error (MAPE) of borehole wall temperatures for a U-shaped borehole heat exchanger (BHE) is approximately 1 %, across a wide range of water flow velocities and thermal conductivity ratios between two contacting geological layers. A significant advantage of the proposed solution is its computational efficiency. The proposed solution can complete long-term simulations with over 9,000 time steps in about 10 min, as opposed to numerical models that typically require several hours. The excellent computational efficiency and substantial accuracy make the proposed solution a simple and effective tool for the design and optimization of BHEs in engineering applications.

A variable water flow rate control method of hybrid geothermal heat pump systems

In the long-term operation of a geothermal heat pump system, when the cooling load is high, the temperature rise in the ground due to heat accumulation occurs. Changes in ground temperature hinder recovery of the temperature on the heat source side of the heat pump, and the performance of the geothermal system deteriorates because the heat exchange efficiency of the circulating water decreases. This problem can be solved by operating a hybrid geothermal system composed of an additional heat source system to the geothermal system. Therefore, in this paper, a method for operating a hybrid geothermal heat pump system was proposed to solve the degradation of system performance due to the temperature rise in the ground due to heat accumulation. The operating conditions for starting the operation of the hybrid geothermal heat pump system were derived, and the coefficient of performance prediction model was proposed for flow rate control considering the energy of the pump and the heat pump system. In addition, the proposed control method was evaluated for energy saving and performance improvement through simulation, and was also evaluated in terms of economic feasibility.

Numerical investigation of the deep borehole heat exchanger in medium-depth geothermal heat pump system for building space heating

The deep borehole heat exchanger (DBHE) is a vital part in medium-depth geothermal heat pump system (MD-GHPs). In comparison to other forms of heat exchangers, the DBHE has the following outstanding advantages: the small footprints, the little damage to rock as well as the high efficiency. However, numerous DBHE models were fixed variable values such as flow rate or inlet fluid temperature and there were few studies combining the user heating load variation with the temperature fluctuation on the DBHE side. In order to fill this gap, a DBHE heat transfer model with dynamic building heating load constraint is developed. In this heat transfer model, the load boundary condition is the hourly flow rate and the difference in inlet and outlet water temperature, and the outlet water temperature is obtained through iteration. In addition, this model is validated by a building space heating experiment data in Harbin and guaranteed its correctness. The average heat exchange per meter at the coldest heating moment also ensures the viability of MD-GHPs, which is 141.5 W/m. A series of typical coordinates are used to analyze the effect of DBHE on the surrounding rock, and the concept of DBHE recommended insulation length are proposed. One single DBHE with 2000 m burial depth possesses a temperature effect radius of 13.0 m for one heating period. Besides, some critical conclusions are as follows: For the structural parameter, the recommended insulation length is 178 m. For the operating parameter, there is a minimum value for the total power consumption and 18 m3·h−1 is the recommended optimal flow rate in this situation. For the geological parameter, raising the geothermal gradient can dramatically improve inlet fluid temperature (IFT) and outlet fluid temperature (OFT), and enhance the ability of DBHE to carry the building heating load. When maintaining the same IFT at the coldest moment, the geothermal gradient of 0.04℃·m−1 is 1.33 times more capable of carrying loads than the 0.03℃·m−1. This paper can provide basis for a series of DBHE thermal performance in practical space heating engineering applications of MD-GHPs.

What requirements for geothermal heat pump systems for heating & cooling

In termini più sintetici rispetto a quanto già fornito in precedenti numeri di questa rubrica relativamente agli impianti geotermici a pompa di calore per la climatizzazione, si ripropongono nel seguito alcuni presupposti ed alcuni obiettivi aggiornati che, se condivisi, potrebbero essere un utile riferimento per l’aggiornamento degli strumenti normativi vigenti e per l’impostazione di quelli in divenire... [Article in Italian]

Accurate design of BHE fields for geothermal heat pump systems: The ASHRAE-Tp8 method compared to non aggregated schemes applied to different European test cases

The European targets addressed to greenhouse gas emissions reduction require a series of actions, including the role to be played by heat pumps and ground-coupled (GCHP) high performance ones. The correct design of the Borehole Heat Exchangers (BHE) field is the core of any GCHP project. The building heating and cooling demand is one of the inputs that shape the problem, to be associated with the use of precalculated temperature response factors describing the ground thermal response to different BHE field arrangements. Commercial codes (e.g. EED, GLHEPRO) can address such a complex computing task by referring to the monthly step thermal loads. In this paper, the monthly approach is compared with the three thermal pulses approach (modified ASHRAE-Tp8 method) by investigating a series of typical building structures in Europe (Tabula webtool database). Hourly heat load profiles for different European countries (Italy, France, Sweden, Germany, Spain) are calculated by EnergyPlus simulations. The present analysis, based on a representative set of building heat demands in Europe, demonstrates that the simple three-pulses approach with the Tp8 formulas is able to provide the correct BHE field overall length with 5% accuracy (average error 2.22%) with respect to the reference monthly calculations performed with commercial codes.

Effect of a geothermal heat pump system on cooling residential buildings in a hot, dry climate

ABSTRACT In the last century, electricity demand has doubled due to urban expansion, which has contributed to the formation of more urban heat islands (UHI) and the appearance of environmental hazards such as the global climate change phenomenon. Since residential buildings are considered the main electricity consumer sector in Egypt, they consume up to 42% of total energy consumption, which contributes to increasing temperatures and constitutes UHI in cities. In this context, the research aims to examine the effectiveness of using the closed vertical loop geothermal system (GSHPCV) for cooling residential buildings in a hot, dry climate, such as Cairo, Egypt. In addition to determining its effect on carbon emissions and the amount of energy consumed for cooling, this was done using TRNSYS-17 software as a simulation tool to compare the traditional building in a hot, dry climate before and after adding GSHPCV in Cairo, Egypt. The simulation revealed that using GSHPCV in hot, dry climate zones has a significant impact on residential buildings. It reduced the electricity consumption up to 19.7% of the electricity consumed for cooling, and it reduced temperatures indoors for buildings up to 15.8% on the ground floor, up to 11.3% on the first floor, and up to 3.5% on the roof floor. In addition, it reduced up to 19.7% of the carbon emissions and reached human thermal comfort on the ground floor for the study case at peak times during the summer period. Which provide a solution to reduce UHI and environmental hazards such as climate change.

Numerical investigation of a district scale groundwater heat pump system: A case study from Colchester, UK

Nearly 40% of Europe’s total energy consumption is dedicated to buildings and heating/cooling make a significant part of this consumption. Groundwater heat pumps (GWHP) are highly efficient, and low-carbon technology that can supply heating/cooling to buildings on small or large scales. Thus, they contribute to achieving European targets of net-zero greenhouse gas emissions by 2050. In the literature, studies on the utilisation of GWHP at a district scale, particularly in chalk aquifers, are relatively rare. The implementation of district-scale geothermal heat pump (GWHP) systems poses several challenges, including dealing with the scale and complexity of the systems, addressing geological variability, managing high initial investments, balancing energy demand and supply, ensuring proper maintenance and monitoring, and mitigating potential environmental impacts. These challenges require careful consideration and strategic planning to ensure the successful deployment and sustainable operation of these systems., This study numerically investigates a district-scale GWHP system and analyses the thermal plume development created due to the heating operation, offering insights into system performance. A good match was found between field results and simulation results for water level increase and drawdown. However, there is a difference of approximately 11% in system efficiency between field tests and simulations due to the lower abstraction temperature detected in the simulation. The simulation results show that cooler water injection into the fractured chalk aquifer creates a thermal plume radially spanning out to 50 m. The thermal plume has no effect on the abstraction temperature and system performance. This result can be attributed to the large distance between injection and abstraction wells and the low hydraulic gradient.

Experimental energy performance in electrified renewable energy-sharing community

In this study, the empirical results of renewable energy self-sufficiency for communities consisting of residential and nonresidential buildings were analyzed. These buildings were combined with a grid-connected electrified community. The community was primarily powered by building-integrated photovoltaic (BIPV) systems, and cooling and heating water were supplied by geothermal and hydrothermal source heat pump systems. The community building comprised 56 detached houses and two office buildings. The community’s primary electricity generation was provided by a 500 kWp BIPV, approximately 7.1 kWp installed per household. This community had facilities that provided cooling, heating, and domestic hot water from heat supply stations. Thermal energy for cooling and heating was mainly generated by geothermal and sewage water-source heat pumps, stored in a daily heat storage tank, and supplied to households. Based on the analysis using ISO 13790-based software, the cooling and heating requirements were 51 and 42 kWh/m2, respectively. The cooling and heating requirements were 49 and 44 kWh/m2, and a difference of approximately 5% between the simulation and experiment results was observed. Consequently, the annual zero-net energy balance was 138.2% when the plug load was excluded but 46.8% when the plug and electric-vehicle electricity loads were included.

How to improve the energy performance of mid-deep geothermal heat pump systems: Optimization of heat pump, system configuration and control strategy

Mid-deep borehole heat exchangers (MDBHEs) extract heat from geothermal energy with depth of 2–3 km, and provide high-temperature heat source for heat pump heating system. To improve the energy performance, this paper conducted optimization analysis on heat pump, system configuration and control strategy. Firstly, the energy efficiency and regulation characteristic of magnetic bearing variable-speed centrifugal heat pump (MBC-HP) was examined through field tests and simulation analysis. Results showed that MBC-HP performed efficiently with wide-range variation of heating load and compression ratio. And average COP of MBC-HP reached 6.21 during heating season. Besides, as evaporator-side water temperature difference (dTe) reached about 10 °C, two-stage MBC-HPs connected in series was raised to fit the large dTe, and the average COP of heat pump was improved to 6.97. Furthermore, the evaporator-side water flow rate (Ge) affected the energy efficiency of heat pump system and heat transfer performance of MDBHEs significantly. Thus the water pumps were recommended to be connected in parallel and then connected with heat pumps. Then the numbers and frequency of water pumps should be regulated to maintain dTe about 9.0 °C. Compared with control strategy of constant Ge, the COPs of heat pump system increased to 5.28, showing 18.7 % improvement.

Effect of soil moisture content on thermal performance of ground source heat exchangers: An electromagnetism topology-based analysis

Ground source heat pump (GSHP) technology has gained significant attention as a viable solution for the heating, ventilation, and air conditioning (HVAC) of buildings. One of the factors that can impact the performance of GSHP systems is the soil moisture content. In this study, an innovative approach, referred to as soil impedance topology, is introduced as an alternative to traditional methods for measuring soil moisture content in geothermal heat pump systems. This method is inspired by pipeline grid utilizes impedance measurement, which provides a direct measurement of soil moisture content without the need for sensors. The study also proposes a changing rule of impedance with varying moisture content, where capacitance increases and resistance decreases when the soil moisture content is less than 20% but stabilizes beyond a water content of 20%. The accuracy of the topological mechanism prediction method is high, as demonstrated through the verification of the Smith circle and measured data. However, it is noted that the performance may be insufficient in the high frequency band and high soil moisture content. The use of impedance topology provides a more accurate and reliable method for measuring soil moisture content as well as for modeling and predicting heat transfer rates and system performance. This technology has strong potential to improve efficiency, reduce energy costs, and enhance the environmental sustainability of geothermal heat pump systems.

The novel designs for increasing heat transfer in ground heat exchangers to improve geothermal heat pump efficiency

Heat transfer between ground heat exchangers (GHE) and the soil is highly important for the overall efficiency of geothermal heat pump systems. Therefore, the present study investigates how adding materials with high thermal conductivity influences the efficiency and outlet temperature of horizontal heat exchangers. To this end, novel methods were developed and evaluated. Initially, metal sheets were used under pipes as fins or blades to enhance the heat transfer rate (HTR) between GHE and the ground and involve a larger portion of the soil. Due to soil moisture effects on thermal conductivity, the boxes used in this study were filled using two distinct methods. Given its higher moisture content, the soil (backfill material) within the box demonstrated a higher thermal conductivity than the soil surrounding the box. In the first method, the box was connected to the ground so that water could enter it when irrigating the green space, thereby increasing the soil's moisture content and the thermal conductivity of the backfill material. In the second method, the boxes were equipped with aluminum and steel blades to enhance heat transfer through pipes. The simulation was performed using the 1D-3D method and COMSOL software was used. After confirming the efficiency of the proposed methods, their dimensions and thermal properties were altered to produce optimal modes. The temperature of the outlet fluid was reduced by 3.8% in cooling mode using only one aluminum sheet, and the overall efficiency of the GHE increased by 37%. Moreover, the reduction in the outlet temperature of the GHE when the blade was connected to the box filled with the proposed backfill materials was reduced by 10%, which is a very significant result.

Investigation of system optimization and control logic on a solar geothermal hybrid heat pump system based on integral effect test data

The solar geothermal hybrid heat pump systems hold great promise for sustainable heating and cooling applications. However, the practical challenges and limitations have resulted in a lack of extensive field trials due to the time and financial constraints associated with conducting field trials. As a result, previous studies have primarily relied on dynamic simulation tools to estimate the performance of prototype systems. The annual performance and efficiency of these hybrid systems are heavily influenced by the control logic of their individual components such as PVT and GSHP unit and the overall operation of the system. Consequently, there is a pressing need to conduct a comprehensive study on the sensitivity of system performance to variations in the operation control, particularly when employing advanced logic, such as fuzzy logic control. When it comes to real-world implementation, there are concerns about the feasibility of integrating additional components like PVT and GSHP into the system. Advanced control strategies like fuzzy logic may also raise questions about the initial capital costs and whether the benefits outweigh the investments. Thus, the system optimization study needs regarding to energy consumption and economic feasibility analysis based on precisely predictable simulation model and practical initial capital cost. In the present study, a solar geothermal hybrid heat pump system has been modeled with simulation and designed IET (Integral Effect Test) facility in an industrial building based on component SET (Separate Effect Test) data, conducted a comparison study for validation of the simulation model based on real field heating and cooling weekly IET data. To optimize the system's performance, annual Operation and Maintenance (O&M) costs sensitivity study was conducted for four different Cases, considering the effects of PVT, GSHP components, and fuzzy logic control. Furthermore, an Net Present Value (NPV) method economic analysis was conducted to evaluate the economic feasibility analysis, considering the initial capital cost, inflation rate, and annual O&M costs savings. The results demonstrate that the simulation model can precisely estimate annual energy consumption within 7.4 % compared to heating and cooling IET operation data. The sensitivity study revealed that Case 4, incorporating fuzzy logic control, exhibited the lowest energy consumption of 9450 kWh/year and the highest annual O&M savings of $1869 among the four Cases studied. However, the advanced system control logic increased the initial capital cost by 34 % ($12,490) compared to Case 1's single-stage GSHP. The NPV economic feasibility analysis, considering the initial capital cost, inflation rate, and annual O&M savings in comparison to Case 1, indicated that Case 4 with fuzzy logic control system showed the shortest payback period of 8.5 years.

Geothermal heat pump systems for underground geoexchange: a ripe opportunity for the present, a prospect to be developed for the future

[Article in Italian]

Drilling Deeper in Shallow Geoexchange Heat Pump Systems—Thermogeological, Energy and Hydraulic Benefits and Restraints

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.

Exergoeconomic and Environmental Evaluation of a Ground Source Heat Pump System for Reducing the Fossil Fuel Dependence: A Case Study in Rome

By 2050, the European Commission aims to achieve a 90% reduction in greenhouse gas emissions within the construction sector due to new targets set for greenhouse gases by the Commission. One of the most effective solutions for mitigating the environmental impact of buildings is to integrate renewable-energy systems such as air-to-water heat pumps or geothermal heat pumps. Several works in the literature investigated the advantages of heat pumps, particularly ground source heat pumps, for specific contexts. Furthermore, the evaluation can encompass not only energy considerations but also exergetic aspects, and this paper makes a significant contribution to the latter. The study presented here applies exergy analysis to a geothermal heat pump system that is interconnected with photo-voltaic and solar collector systems in a building located in Rome. Feasibility and environmental assessments were also conducted. It is evident that the exergy efficiency obtained is much lower than the energy efficiency. The heat pump demonstrates higher exergy efficiency by producing high-temperature thermal power compared to a constant dead state at relatively low temperatures. Following the heat pump, the adiabatic mixer exhibits the second highest exergy efficiency, trailed by two heat exchangers. Compared to a conventional plant case, the money saved is EUR 16,772 per year, translating to a Payback Period of 23 years. Furthermore, the average annual reduction in fossil fuel emissions is estimated at 26.2 metric tons of CO2.