Ground-coupled Heat Pump System Research Articles

The Ground-Coupled Heat Pump Technology in China

The ground-coupled heat pump (GCHP) system utilizes the ground as a heat source/sink of the heat pump, and has been identified as one of the best sustainable energy technologies for space heating and cooling in residential and commercial buildings. Applications of this relatively-new technology in China have grown dramatically in the past decades owing to the urgent demands of energy conservation and environment protection for sustainable development. Government decrees and incentives on the GCHP technology in China are presented in this paper. Research and development on the GCHP technology in China are summarized, including those in the heat transfer modelling of borehole heat exchangers as well as technical developments of energy piles, deep borehole heat exchangers and hybrid GCHP systems.

Thermal response factors for fast parameterized design and long-term performance simulation of vertical GCHP systems

Ground-coupled heat pump (GCHP) system is one of the important renewable energy systems. Vertical ground heat exchanger (GHE) is the most expensive part of GCHP systems. Fast and accurate computation of fluid and ground temperatures is desired for the long-term prediction of GCHP performance and the optimal sizing of borehole fields. In this paper, a thermal response factors, δ-function is proposed for computing fluid and ground temperatures of borehole fields. δ-function is a non-dimensional ground temperature response and an accurate analytical solution of finite line source model to a unit rectangular heat pulse. Numerical computation of δ-function is quite fast for its small integral range. The accuracy of δ-function is validated by the simulation results of g-function. The comparative computations are conducted for the hourly simulation of various long periods by combining the g- and δ-functions with both time and spectral domain methods. It is found that the combination of δ-function with fast Fourier transform (FFT) provides dramatically fast computing speed. Therefore, the computation approach of δ-function combined with FFT is the most suitable to quickly and accurately compute the fluid and ground temperatures in parameterized design and long-term simulation of vertical GHE and GCHP systems.

Prediction of ground thermal diffusivity from thermal response tests

Ground-coupled heat pump (GCHP) systems are being increasingly utilized in recent years. There are several important parameters in the design of GCHP systems. As ground thermal conductivity is one of the most crucial parameters, it should be measured by conducting in-situ thermal response tests (TRTs). This paper presents the experimental results and analysis of thermal response tests to estimate the ground thermal conductivity and ground thermal diffusivity for three types of ground heat exchangers (GHEs), including U, 2U, and W-type ground heat exchangers (GHEs). Three different types of GHEs were installed in a dredged soil deposit, and continuous TRTs were conducted for 48 h. This study suggests a method to predict ground thermal diffusivity using the infinite line source model from the TRTs. Furthermore, soil samples were collected from different ground layers, and ground thermal properties were measured using laboratory tests. The equivalent ground thermal conductivity and ground thermal diffusivity measured through laboratory tests were compared with the in-situ TRT results that were newly derived in this paper. In addition, ground thermal diffusivity and ground thermal conductivity values that were obtained through research were used as input parameters in numerical analysis. The in-situ TRT results were numerically modeled using the finite element method.

Performance assessment between a ground coupled and air source heat pump used for domestic hot water preparation

A ground-coupled heat pump system (GCHP) and an air source heat pump (ASHP) driven by photovoltaic panels are used to provide domestic hot preparation for a NZEB house. The experimental measurements are used to test both the heat pump models in the same conditions of water temperature and volume of domestic hot water. A comparative analysis of the two heat pumps for domestic hot water preparation is performed. In addition, using the software TRNSYS (Transient Systems Simulation), two numerical simulation models of thermal and electrical energy consumption in DHW mode are developed. Finally, the simulations obtained using TRNSYS software are analysed and compared to the experimental data.

Distributed Thermal Response Tests Using a Heating Cable and Fiber Optic Temperature Sensing

Thermal response tests are used to assess the subsurface thermal conductivity to design ground-coupled heat pump systems. Conventional tests are cumbersome and require a source of high power to heat water circulating in a pilot ground heat exchanger. An alternative test method using heating cable was verified in the field as an option to conduct this heat injection experiment with a low power source and a compact equipment. Two thermal response tests using heating cable sections and a continuous heating cable were performed in two experimental heat exchangers on different sites in Canada and France. The temperature evolution during the tests was monitored using submersible sensors and fiber optic distributed temperature sensing. Free convection that can occur in the pipe of the heat exchanger was evaluated using the Rayleigh number stability criterion. The finite and infinite line source equations were used to reproduce temperature variations along the heating cable sections and continuous heating cable, respectively. The thermal conductivity profile of each site was inferred and the uncertainly of the test was evaluated. A mean thermal conductivity 15% higher than that revealed with the conventional test was estimated with heating cable sections. The thermal conductivity evaluated using the continuous heating cable corresponds to the value estimated during the conventional test. The average uncertainly associated with the heating cable section test was 15.18%, while an uncertainty of 2.14% was estimated for the test with the continuous heating cable. According to the Rayleigh number stability criterion, significant free convection can occur during the heat injection period when heating cable sections are used. The continuous heating cable with a low power source is a promising method to perform thermal response tests and further tests could be carried out in deep boreholes to verify its applicability.

Techno-economic evaluation of multiple energy piles for a ground-coupled heat pump system

A technical and economic feasibility study of multiple energy piles (EPs) for a ground-coupled heat pump (GCHP) system is presented in this paper. The GCHP system energy performance and life-cycle cost (LCC) are evaluated, it is found that the system energy output (heating and cooling) could meet a domestic building comfortable environment requirement with the annual average COP of 3.63 and EER of 4.62. The LCC evaluation indicates that the system net present value (NPV) is approximately £26,095 at the market discount rate of 8.75% for a 20-year operating period. Moreover, the payback period of the GCHP system is approximately 4.31 years, which is sensitive to the main parameters including electricity price, capital investment and energy generation. Furthermore, the low discount rate and high energy generation are beneficial to the GCHP system with the high NPV and cash flows. The capital price of the system should be regulated to a lower level for the larger market potential.

Characterizing lithological effects on large scale borehole heat exchangers during cyclic heating of the subsurface

This paper presents the results of a study where subsurface temperatures were monitored adjacent to the u-tube of a central borehole of a large scale 144 borehole heat exchanger (BHE) at multiple depths. This was an effort to develop a method to characterize the variability of thermal properties of the various lithological units surrounding the BHEs of large scale ground coupled heat pump systems during the operation of the system. Saturation of the units and lithology was shown to affect the performance of the BHE resulting variable temperature fluctuations and heat flux at different depths within the borehole. Median heat flux tended to increase with decreasing temperature ranges measured by the sensors. The lowest median heat flux was in the location nearest to the surface in the least saturated zone. In the more saturated zone, the highest median heat flux were at locations where the lithological units were composed of 75 percent or greater dolomite while the lowest median heat flux where in lithological units composed of 30 percent or greater chert.

Simple equations to evaluate the mean fluid temperature of double-U-tube borehole heat exchangers

The design of ground-coupled heat pump systems requires the knowledge of the mean temperature Tfm of the working fluid in borehole heat exchangers. This quantity is usually approximated by the arithmetic mean of inlet and outlet temperature. The approximation yields an overestimation of the thermal resistance of the heat exchanger in thermal response tests, as well as errors in the estimation of the outlet fluid temperature in the dynamic simulation of ground-coupled heat pumps. Recently (Applied Energy 206, 2017, 1406–1415), Zanchini and Jahanbin determined, by finite element simulations, correlations that allow an immediate evaluation of Tfm in any working condition, with reference to double U-tube boreholes with length 100 m and shank spacing 85 mm. In this paper, the analysis presented there is extended to double U-tube boreholes with any length, between 50 and 200 m, and any shank spacing, between 65 and 105 mm. The results hold for every thermal conductivity of the sealing grout between 0.9 and 1.6 W/(m K), every volume flow rate between 12 and 24 L per minute, every diameter of the BHE and every working condition (heating, cooling, thermal response test), both in quasi-stationary and in transient regime.

A mathematical model and thermal performance analysis of single-well circulation (SWC) coupled ground source heat pump (GSHP) systems

Owing to the limitation of the available land in China, among various ground source heat pump (GSHP) system configurations, the single-well circulation (SWC) coupled GSHP systems are being intended to provide heating and cooling for building in recent years, especially utilized in this area with suitable hydrogeological and thermogeological conditions. This is due to the fact that the SWC system could not only substantially provide shallow geothermal energy for space heating or cooling in small-scale applications, but also reduce the number of boreholes needed for large-scale geothermal applications. In this work, a mathematical model has been established to analysis the groundwater seepage of SWC system, and analytical solution of steady drawdown was derived. Meanwhile, a numerical model was constructed to evaluate the thermal performance by using SWC coupled GSHP systems. Numerical experiments were performed to observe the evolution of outlet temperature, the distribution of subsurface temperature field, and the long-term development of outlet temperature. It was found that the thermal effective radius (TER) of SWC system is much larger than that of ground-coupled heat pump (GCHP) systems. Also, the temperature field in vertical section caused by the operation of SWC system is funnel-shaped. In addition, the outlet temperature fluctuates annually, and it rather starts a long-term decaying process, until reaching a quasi-steady state after about 8–10 years.

Investigation on the thermal behavior of energy piles and borehole heat exchangers: A case study

Constant-flux and operation thermal behavior of ground heat exchangers are meaningful for system design and control strategy of ground-coupled heat pump systems. Based on a ground-coupled heat pump project, four thermal performance tests and an operation test were conducted to compare the constant-flux and operation thermal behavior of spiral-tube energy piles with that of double U-tube borehole heat exchangers. The thermal performance of energy piles is more efficient than that of borehole heat exchangers. A large radial scale of heat source, a long heat exchange tube per unit depth of holes and a backfilled material with large thermal conductivity is helpful to improve the heat transfer efficiency of ground heat exchangers. The short-term thermal response of energy piles is more rapid than that of borehole heat exchangers. The short-term thermal resistance of energy piles is much less than that of borehole heat exchangers. In operation test heat transfer rate per unit length of pile and per unit length of pipe for energy piles is greater than those for borehole heat exchangers. The regular fluctuations in fluid temperature and heat transfer rate in operation should be determined by heating capacity of heat pumps and thermal loads of buildings.

An analytical method to simulate the dynamic performances of truncated cone helix ground heat exchangers

This paper proposes a dynamic analytical method to simulate the thermal performances of truncated cone helix ground heat exchangers (i.e., the so-called “energy baskets”). These ground-coupled devices are attractive solutions to reduce the initial cost of ground-coupled heat pump systems, as they require lower cost to be drilled and installed with respect to traditional boreholes. However, both design methodologies and performance assessment models are still not well developed, producing substantial uncertainties on final operative performances. This work presents a plain evaluation method based on the heat exchangers theory and the analytical solution of the truncated cone helix heat source in a semi-infinite medium. It can be advantageously used to simulate the thermal performance of truncated cone helix ground heat exchangers as a function of helix geometries and operative conditions evolution (e.g., inlet temperature, fluid flow rate, ground temperature…). Specifically, in this paper, we perform a sensitivity analysis of the thermal performances of a case study by varying the main geometrical parameters. Besides, we compare the heat transfer of the reference configuration with an equivalent cylindrical arrangment. The truncated coil configuration is more effective than cylindrical one as the cone aperture reduces the short-circuits between helix pitch and the equivalent thermal resistance with the ground surface. However, obtained results are notably affected by the assumption of an isothermal surface temperature, which leads to a shallow/plain helix/spiral as the best configuration: different conclusions are expected when a time dependent or adiabatic boundary condition will be accounted in the model.

Assessment of Energetic, Economic and Environmental Performance of Ground-Coupled Heat Pumps

Ground-coupled heat pumps (GCHPs) have a great potential for reducing the cost and climate change impact of building heating, cooling, and domestic hot water (DHW). The high installation cost is a major barrier to their diffusion but, under certain conditions (climate, building use, alternative fuels, etc.), the investment can be profitable in the long term. We present a comprehensive modeling study on GCHPs, performed with the dynamic energy simulation software TRNSYS, reproducing the operating conditions of three building types (residential, office, and hotel), with two insulation levels of the building envelope (poor/good), with the climate conditions of six European cities. Simulation results highlight the driving variables for heating/cooling peak loads and yearly demand, which are the input to assess economic performance and environmental benefits of GCHPs. We found that, in Italy, GCHPs are able to reduce CO2 emissions up to 216 g CO2/year per euro spent. However, payback times are still quite high, i.e., from 8 to 20 years. This performance can be improved by changing taxation on gas and electricity and using hybrid systems, adding a fossil-fuel boiler to cover peak heating loads, thus reducing the overall installation cost compared to full-load sized GCHP systems.

Estimation of soil and grout thermal properties for ground-coupled heat pump systems: Development and application

Ground thermal properties, including the thermal conductivity and diffusivity of both soil and grout, are significant considerations for the design of a ground-coupled heat pump (GCHP) system. However, as a result of the limitations inherent in available response models, few in-situ thermal response tests (TRTs) can identify grout thermal conductivity and diffusivity. This paper proposes a new method to estimate the thermal conductivity and diffusivity of both soil and grout simultaneously using the recently developed infinite composite-medium line source (ICMLS) model. Firstly, a linear dependence analysis is performed on the aforementioned four parameters to ensure the feasibility of the proposed method, leading to an estimation of the minimum TRT duration. Secondly, uncertainty analysis is carried out to analyze the influence of U-pipe shank spacing, as it is considered a sensitive parameter in modeling the heat transfer of ground heat exchangers (GHEs). Thirdly, a genetic algorithm is used to identify these four parameters using the data collected from a TRT. The proposed method is verified using a well-designed sandbox experiment. Finally, its application is demonstrated and evaluated by applying it to the design of a GCHP system for an office building at Hunan University.

Computational methods for ground thermal response of multiple borehole heat exchangers: A review

Abstract Since ground-coupled heat pump system (GCHPs) has been more and more widely applied, it is vital to build an accurate method for sizing borehole heat exchangers (BHEs), and the methods for determining the heat transfer between the boreholes and surrounding ground under time-varying loads in different time scales are the kernel of optimizing design of BHEs, which is helpful to obtain accurate results and significantly decrease computation time consuming. This paper summarizes computational methods for the ground thermal response of BHE under time-varying loads and thermal interaction of multiple BHEs, compares the representative design tools in terms of their performance characteristics, and analyzes the direct applications of computational methods in thermal response test (TRT) and operating performance simulation of GCHPs. Finally, some recommendations for future development of computational methods are proposed.

Hybrid analytical model for composite heat transfer in a spiral pile ground heat exchanger

A spiral pile ground heat exchanger (PGHE) is a recently developed type of ground heat exchanger (GHE) used in ground-coupled heat pump system. Existing analytical models are not applicable to a spiral PGHE when the pile has thermal properties that are different from those of the soil. A novel hybrid analytical model to represent this situation was developed in the present study. The proposed model utilizes a decomposition algorithm to solve the problem of a coil line source embedded in a composite cylinder. The decomposition algorithm uses a composite cylindrical surface source solution to deal with the composite heat conduction along the radius and a coil line source solution to deal with the heat conduction between tube loops in depth direction. An assumption of homogeneity enables the solution of the hybrid model with only a slight error. The error of the hybrid model was investigated with respect to thermal properties and geometries of a spiral PGHE and compared with those of existing models. The error under normal cases was found to be acceptable for engineering applications. The proposed hybrid model is superior to existing analytical models, enabling simultaneous and reasonably accurate evaluations of the effects of the thermal properties of the pile and the coil pitch.

The observed effects of changes in groundwater flow on a borehole heat exchanger of a large scale ground coupled heat pump system

A study was completed comparing the effectiveness of a borehole heat exchanger (BHE) within a large scale closed-loop geothermal system to changes in groundwater flow, as a result of periodic pumping of a large municipal well. The BHE studied consists of 144 geoexchange boreholes drilled to a depth of 122 m into karstic bedrock of the Ozark Aquifer. Estimated groundwater flows rates range from 15 to 1.1 × 10−3 m/year were measured, and the effectiveness of the BHE during pumping was calculated to understand the effects of groundwater flow on the system. Changes in the BHE effectiveness under varying flow rates were compared to estimates from previous study estimates with changes in aquifer saturation. Overall, the variations in aquifer saturation tended to have the greatest influence on the effectiveness of the BHE.

A general distributed parameter model for ground heat exchangers with arbitrary shape and type of heat sources

The heat and mass transfer simulation model of a ground heat exchanger (GHE) directly affects the design and operation performance of a ground-coupled heat pump system. The GHE models based on the response function (like the Green function and g-function) can achieve a fast calculation speed. However, the heat sources in these models are limited to points or whole boreholes, leading to low calculation accuracy in heat transfer during a short time period and limitation to a certain GHE. A general distributed parameter model for a ground heat exchanger (RF model) is proposed based on the principle of response factors in this paper. A sandbox experimental platform is then built to test the temperatures of typical points in the double-layered soil and to validate the RF model. After that, the calculation of the RF model is simplified by determining suitable positions for the soil boundaries and the numbers of sub pipes and sub soil boundaries. Finally, the RF model is applied in different scenarios to demonstrate its characteristics. The results show that: (1) the RF model is suitable for different kinds of GHEs with arbitrary shape and type of heat sources releasing heat in arbitrary time steps; (2) the RF model has only 0.01 °C and 0.23 °C temperature response errors compared to those from numerical solutions and experiments, respectively; (3) the general RF model has similar accuracy to the numerical solution in calculating the distributed temperatures of the borehole and pipes, heat transfer in the short term, and heat transfer of borehole groups and the energy pile.

An hourly simulation method for the energy performance of an office building served by a ground-coupled heat pump system

Abstract Ground heat exchangers are key component of ground-coupled heat pump systems, and their thermal response is therefore very important for ground-coupled heat pump system design and operation. This paper proposes a new hourly simulation method, and uses it to study the performance improvement potential for the ground-coupled heat pump system. First, with an effective U-pipe shank spacing determined by the calculated and measured borehole thermal resistance, a reasonable and accurate fluid temperature prediction method is developed, and the hourly energy performance simulation method is also proposed accordingly with the Fast Fourier Transform superposition algorithm. This hourly simulation method is validated using experimental data collected from a well-designed ground-coupled heat pump experiment platform, which shows that the maximum absolute error for the predicted fluid temperature is smaller than 1.04 °C. Second, using the proposed hourly simulation method, a framework for the energy performance simulation of an office building served by the ground-coupled heat pump system is developed. Impact factors on ground-coupled heat pump system performance are systematically analyzed using this simulation method, and the results show that performance can be improved with shorter operation schedules and lower heat fluxes.

A comparison of numerical simulation methods analyzing the performance of a ground-coupled heat pump system

Ground-coupled heat pumps are increasingly being utilized to heat and cool buildings. Although it is difficult to size and to predict their behavior and performance, their design can be optimized via simulations. EnergyPlus is a popular energy simulation program for modeling building heating and other energy flows and, since it is organized to consider borehole heat exchangers via the well-known g-functions approach, it can be used advantageously for that purpose. The Capacity Resistance Model is another recent numerical simulation tool devoted to ground and borehole heat exchangers. In this work, two methods to calculate the g-fucntions were analyzed, using as case-study a real office building, whose imbalance between the heat extracted and injected into the ground was found to be appreciable. The energy imbalance involves a ground temperature drift affecting the system efficiency. The results of the EnergyPlus g-functions and the Capacity Resistance Model model approaches were compared. The capacity of the two methodologies to accurately simulate this phenomenon were analysed also with reference to the available building's long-term monitoring data. The analysis showed the importance of using g-functions suitable to reflect the layout of the borehole field, in order to correctly evaluate the energy performance of the entire ground source heat pump system.

A computationally efficient numerical model for heat transfer simulation of deep borehole heat exchangers

Being a promising technique for seasonal thermal energy storage, the deep borehole heat exchanger (DBHE) has also provided a new variance of ground-coupled heat pump systems especially for applications in cold-climate regions. Numerical simulations are recognized as the appropriate means in the study on heat transfer around the deep boreholes, which often features time-consuming computations. A model has been established in this study for DBHEs with coaxial tubes, considering coupled heat transfer in the tubes and surrounding subsurface. A software package is developed based on the finite difference method for thermal analysis of the DBHEs, and an algorithm for direct solution of resulted algebraic equation set is used so as to achieve very efficient computation. The model and the numerical algorithm are validated by comparison with simulation results from a reference using the finite element method. Performance of DBHEs is then assessed with parameter analyses. A diagram is presented to show relations of the nominal DBHE capacity to its key parameters such as the borehole depth, geothermal heat flux and thermal conductivity of subsurface, which is helpful in engineering practice. Studies also indicate that a high-resistance inner pipe helps in improving the heat transfer capacity of DBHEs.