Vertical Ground Heat Exchanger Research Articles

Numerical simulation of a ground-coupled heat pump system with vertical plate heat exchangers: A comprehensive parametric study

Plate ground heat exchangers (GHEs) are renowned for having the highest heat transfer rate per unit land area and could be of interest when the accessible land area is limited. In this study, a 3-D numerical model is developed to investigate the thermal performance of vertical plate GHEs at the real-scale level. The proposed model accounts for ambient temperature fluctuations and building cooling load variations and is such developed to couple the GHE to the heat pump. The effects of different parameters, including GHE spacing, buried depth, the height of GHE, soil type, and climate on the thermal performance of vertical plate GHEs, are comprehensively investigated for the first time. The optimum distance between two adjacent GHEs is obtained to be about 4 m to avoid the adverse effect of thermal interference. It is demonstrated that increasing GHE spacing dramatically increases cooling load per unit GHE area for values below 4 m while decreasing cooling load per unit land area. As a case in point, increasing GHE spacing from 2 m to 4 m increases cooling load per unit GHE area by 30.1 % while lowering cooling load per unit land area by 34.9 %. It can be inferred from the simulation results that soil type has the most critical effect on the thermal performance of vertical plate GHEs, and better thermal performance can be achieved when GHEs are buried in a soil type with higher thermal conductivity as well as higher heat capacity. The results also indicate that selecting proper values for buried depth and height is in fact finding a compromise between thermal performance and excavation cost. Investigating the effect of climate reveals that the maximum cooling load per unit GHE area is decreased by 37.7 % when the GCHP system operates in Ahvaz (hot climate) rather than Tehran (mild climate). However, when the GCHP system is designed to operate in Tabriz (cold climate), the maximum cooling load improves by 17.4 %.

Dynamic Simulation and Energy Economic Analysis of a Household Hybrid Ground-Solar-Wind System Using TRNSYS Software

The adoption of micro-scale renewable energy systems in the residential sector has started to be increasingly diffused in recent years. Among the possible systems, ground heat exchangers coupled with reversible heat pumps are an interesting solution for providing space heating and cooling to households. In this context, a possible hybridization of this technology with other renewable sources may lead to significant benefits in terms of energy performance and reduction of the dependency on conventional energy sources. However, the investigation of hybrid systems is not frequently addressed in the literature. The present paper presents a technical, energy, and economic analysis of a hybrid ground-solar-wind system, proving space heating/cooling, domestic hot water, and electrical energy for a household. The system includes vertical ground heat exchangers, a water–water reversible heat pump, photovoltaic/thermal collectors, and a wind turbine. The system with the building is modeled and dynamically simulated in the Transient System Simulation (TRNSYS) software. Daily dynamic operation of the system and the monthly and yearly results are analyzed. In addition, a parametric analysis is performed varying the solar field area and wind turbine power. The yearly results point out that the hybrid system, compared to a conventional system with natural gas boiler and electrical chiller, allows one to reduce the consumption of primary energy of 66.6%, and the production of electrical energy matches 68.6% of the user demand on a yearly basis. On the other hand, the economic results show that that system is not competitive with the conventional solution, because the simple pay back period is 21.6 years, due to the cost of the system components.

Extending capabilities of Thermal Response Tests in vertical ground heat exchangers: An experiment-based local short-time temperature response factor

This work presents a fast and practical method to determine a characteristic function that predicts a local ground response of vertical ground heat exchangers (GHE) in a short term, which is mandatory when intermittent operation modes are modelled. It expands the use of a Thermal Response Test (TRT) that is recommended previous to the design of a borehole field into a vertical Ground Source Heat Pump (GSHP). These tests are used to provide some ground thermal characteristics, such as ground conductivity or borehole effective thermal resistance. The same device can also be used to measure the undisturbed ground temperature along depth. From the measurements, it has been determined a temperature response factor function that characterizes the ground behavior in a short term as a consequence of a heat pulse. This function is then included into a finite line-source model to simulate the average temperature of the fluid that flows into the borehole pipes along time. In order to validate this method, several additional tests have been performed by the same device used for TRT’s, with intermittent operation modes. For each test, both experimental and simulated average fluid temperatures have been compared. Results present an excellent accuracy; thus, they demonstrate de effectiveness of the method, as well as other advantages: i) fast and accurate; ii) prevents high precomputing times; iii) several uncertainties from measurements disappear when it is used.

Critical Review on Efficiency of Ground Heat Exchangers in Heat Pump Systems

Use of ground source heat pumps has increased significantly in recent years for space heating and cooling of residential houses and commercial buildings, in both heating (i.e., cold region) and cooling (i.e., warm region) dominated climates, due to its low carbon footprint. Ground source heat pumps exploit the passive energy storage capacity of the ground for heating and cooling of buildings. The main focus of this paper is to critically review how different construction and operation parameters (e.g., pipe configuration, pipe diameter, grout, heat injection rate, and volumetric flow rate) have an impact on the thermal efficiency of the vertical ground heat exchanger (VGHE) in a ground source heat pump (GSHP) system. The published literatures indicate that thermal performance of VGHEs increases with an increase of borehole diameter and/or pipe diameter. These literatures show that the borehole thermal resistance of VGHEs decreases within a range of 9% to 52% due to pipe configurations and grout materials. Furthermore, this paper also identifies the scope to increase the thermal efficiency of VGHE. The authors conclude that in order to enhance the heat transfer rate in VGHE, any attempt to increase the surface area of the pipe configuration would likely be an effective solution.

A new analytical model for short-time analysis of energy piles and its application

An energy pile is a special form of vertical ground heat exchanger that couples the roles of structural support and heat transfer. Modeling the transient heat transfer process inside an energy pile has importance; however, available analytical models either have insufficient calculation accuracy or are computationally demanding. Based on three existing models, this paper proposes a novel short-term hybrid composite-medium line-source (HCMLS) model, which is not only efficient in computation but also more accurate than most traditional analytical models. The model is suitable for ground heat exchangers of various radii. Comparisons between the hybrid analytical model and a numerical model are made for energy pile cases with different parameters, including the thermal properties, borehole radii, relative positions of tubes, and number of tubes. In general, the hybrid composite-medium line-source model gives credible prediction after 100 min. The new model is further validated by the infinite composite-medium line-source (ICMLS) model, which is currently the most theoretically complete short-term model. Moreover, the new model is applied to thermal response tests (TRTs). The least dimensionless test duration for interpretations based on the modified hybrid composite-medium line-source (C-HCMLS) solution is Fo > 1.7. This study renders the application of in situ TRTs to energy piles with large diameters feasible.

A New Analytical Model for Calculating Transient Temperature Response of Vertical Ground Heat Exchangers with a Single U-Shaped Tube

The transient temperature response is of great importance for evaluating the thermal capacity of ground heat exchangers (GHE). Based on the composition line source theory and superposition principle, we have developed a novel analytical model in Laplace space for calculating the temperature transient response. In comparison to the existing models, this proposed model can account for the fluid thermal storage effect and heat rate difference between the two legs of the single U-tube. With the aid of this proposed model, we conduct a thorough sensitivity analysis to investigate the effects of different influencing factors on the thermal transient response. The calculated results show that fluid thermal storage and the rate difference can significantly influence the thermal response during the early studied period. Therefore, the effect of fluid thermal storage should not be neglected when the early-time thermal response is investigated. The thermal interference between the two legs will reduce the heat capacity of GHEs. A large distance between these two legs can be favorable for practical use.

A novel analytical multilayer cylindrical heat source model for vertical ground heat exchangers installed in layered ground

This paper presents a new analytical multilayer cylindrical heat source model for vertical ground heat exchangers (GHEs) installed in layered ground using the new integral-transform method. The analytical model was validated by model degradation, numerical simulation, and a laboratory-scale experiment. Results indicate that temperature profiles of vertical GHEs in layered ground are quite different from those in homogeneous ground, and that temperature differences increase with time. Thermal property differences between ground layers were found to result in additional vertical heat transfer across layer interfaces, which is not observed in homogeneous ground. Cross-layer thermal interaction was found to be stronger when thermal property differences are larger. The new cylindrical heat source model was also compared with the multilayer line heat source model, and it was found that differences between the two models decrease with time. Further, larger GHE thermal loads and smaller ground thermal conductivity values led to larger error of multilayer line heat source model. The new multilayer cylindrical heat source model was found to be suitable for quickly considering the effects of ground stratification on the design of vertical GHEs.

A New Simulation Model for Vertical Spiral Ground Heat Exchangers Combining Cylindrical Source Model and Capacity Resistance Model

Considering the heat capacity inside vertical spiral ground heat exchanger (VSGHEX) in the simulation is one of the most noteworthy challenge to design the ground source heat pump (GSHP) system with VSGHEXs. In this paper, a new simulation model for VSGHEXs is developed by combining the ICS model with the CaRM. The developed simulation model can consider the heat capacity inside VSGHEX and provide dynamic calculation with high speed and appropriate precision. In order to apply the CaRM, the equivalent length was introduced. Then, the equivalent length was approximated by comparing the results of the CaRM and the numerical calculation. In addition, the calculation model of the VSGHEX was integrated into the design and simulation tool for the GSHP system. The accuracy of the tool was verified by comparing with the measurements. The error between supply temperatures of the measurements and calculation is approximately 2 °C at the maximum. Finally, assuming GSHP systems with VSGHEXs, whose spiral diameter was 500 mm and depth was 4 m, were installed in residential houses in Japan, the required numbers of VSGHEXs were estimated. The results showed a strong correlation between the total heating or cooling load and the required number. Therefore, the required number can be estimated by using the simplified approximate equation.

A new analytical model for short vertical ground heat exchangers with Neumann and Robin boundary conditions on ground surface

While ground surface conditions have been counted accurately in numerical models of ground heat exchangers (GHE) by defining a Neumann or Robin boundary condition, current analytical models of vertical GHE still commonly adopt a Dirichlet boundary condition. Using a new integral transform method, this paper developed a new analytical model of vertical GHE with three different (Dirichlet, Neumann, and Robin) top boundary conditions. The new model was validated analytically and numerically. Using the new model, the effect of different ground surface boundary conditions on temperature responses of vertical GHE is studied. As an example of application of the proposed model, a case study of soil borehole thermal energy storage (SBTES) systems where the borehole top is covered with insulation material was presented. Results show that the calculated average temperature along the depth of vertical GHE by using the Dirichlet boundary condition would be 14.1% and 8.5% less compared with using the Neumann and Robin boundary condition respectively in the long term. The percentages may in turn quantify the improvement in thermal energy storage by placing insulation cover over boreholes in practical engineering.

Thermal performance of the vertical ground heat exchanger with a novel elliptical single U-tube

Ground-Coupled Heat Pumps (GCHPs) are known as a promising technology for building climatization and domestic hot water production. The main concern to employ these systems in urban areas is their high drilling and initial cost. This cost could be rebated by lowering the borehole thermal resistance and by shortening the borehole depth. In the present paper, a novel and simple design of the Ground Heat Exchanger (GHE), namely the vertical GHE with an elliptical U-tube, is proposed to improve thermal performance of GCHP systems. To prove that; a series of 3D finite element simulations are performed to study the thermal comportment of elliptical U-tubes and to compare their performance with typical single U-tubes. A dimensionless shape factor γ is introduced to evaluate effect of the elliptical U-tube geometry on thermal performance of the GHE. Furthermore, influential parameters on thermal performance of elliptical U-tubes are investigated and the decrement percentage of the borehole thermal resistance, with reference to the typical U-tube, are compared for each parameter. The results show that elliptical U-tubes could enhance heat transfer to significant extent and could decrease the borehole thermal resistance by more than 17 %, compared with typical single U-tubes. It is shown that the higher value of the shape factor γ leads to the better heat transfer enhancement. It is concluded that elliptical U-tubes are able to decrease the borehole thermal resistance and to boost the coefficient of performance (COP) of GCHP systems.

The Effect of Shank-Space on the Thermal Performance of Shallow Vertical U-Tube Ground Heat Exchangers

One parameter that may affect the performance of a ground source heat pump is the shank-space, the center-to-center distance between the two branches of a vertical U-tube used in a ground heat exchanger. A 3D steady-state computational fluid dynamics (CFD) model of a U-tube ground heat exchanger was used to investigate the influence of varying shank-space on the thermal performance of two isolated vertical shallow U-tubes, one 20 m deep and the other 40 m deep, given that most existing research focuses on systems making use of deeper boreholes. The models adopt an innovative approach, whereby the U-junction at the bottom of the U-tube is eliminated, thus facilitating the computational process. The results obtained show that, although the temperature drop across the U-tube varies for different shank-spaces and is lowest and highest for the closest and the widest shank-spaces, respectively, this temperature drop is not linear with increases in shank-space, and the thermal performance improvement drastically diminishes with increasing shank-space. This indicates that, for shallow U-tubes, the temperature drop is more dependent on the length of the pipework.

THE SIMULATION ANALYSIS OF A PROPOSED ‘HYDRONIC RADIATOR’ SYSTEM TOWARDS LOW COST HOUSING OPERATION IN TEMPERATE AND HOT TROPICAL CLIMATES

ABSTRACT Fuel poverty is one of the global concerns affecting not only users' financial capacity or affordability for maintaining housing operation but also the occupants' health and wellbeing. Space heating and cooling require a relatively large amount of domestic energy use in housing. Therefore, this study was formed with the aim to propose an innovative approach to utilising free, clean renewable sources of energy applicable to the space heating and cooling of housing in both cold and hot regions. Accordingly, housing test facilities based in Melbourne, Australia, and Kuching, Malaysia, were selected and used for this study that examined the thermal performance of a proposed ‘hydronic radiator’ (HR) system through simulation and onsite measurements. The geothermal heat capacity of a ‘vertical ground heat exchanger’ (VGHE) installed in the house in Melbourne was examined previously by the authors and the VGHE measured data was also applied to this HR performance simulation. The water that circulates through the HRs is heated by sunlight and VGHE or cooled by night sky radiation. This study drew conclusions that the sole utilisation of renewable sources through these proposed HR space heating and cooling systems can provide thermally accessible or comfortable indoor living environments in both heating or cooling dominant regions. Thus, fuel poverty issues may be alleviated through HR system application. The HRs can remove a ‘sensible’ portion of metabolic heat, but they cannot effectively contribute to the ‘latent’ heat removal. Thus, the future potential use or effect of ‘flow-through’ HRs, which are integrated into a underfloor air distribution (UFAD) plenum, was also dsicussed in this study. In the test house located in Melbourne, the flow-through HR UFAD system is currently under development. Therefore, the performance will be measured once the system has come into operation for further testing.

Externally heated geothermal bridge deck: Performance analysis of the U-tube ground heat exchanger

In recent years, the geothermal heat pump de-icing system (GHDS) is introduced as a sustainable solution for bridge deck de-icing, which utilizes renewable geothermal energy. Existing GHDS designs mostly rely upon hydronic loops embedded in concrete decks. To extend the GHDS for existing bridges, a new external hydronic deck has been developed recently, which employs a hydronic pipe being attached to the bottom surface of the bridge deck. In this study, the performance of the externally heated geothermal bridge deck is investigated through winter deicing and summer recharging tests with the focus on the ground loop heat exchanger (GLHE), a key component of the GHDS. The test results show that the de-icing system was successful in maintaining the deck surface temperature above freezing in all winter tests. The soil temperature measurements indicate, the 132.5 m vertical U-tube ground heat exchanger is benefited from the undisturbed soil temperature of around 21 °C. The overall average hourly heat extraction of 0.67 kW during winter operation and average hourly heat injection of 0.69 kW during the summer operation were observed. Also, the ground thermal recharge test showed the increase of undisturbed soil temperature at 1.5 m from the geothermal borehole by 0.36 °C after 50 days of system operation.

Simulation of temperature and moisture fields around a single borehole ground heat exchanger: effects of moisture migration and groundwater seepage

This paper aims to investigate the effects of moisture migration and groundwater seepage on the heat transfer capacity of ground heat exchangers in stratified soils. A three-dimensional unsteady groundwater flow and heat transport model was established using finite volume method. Sixteen cases with different model considerations and initial soil conditions were simulated based on the proposed model. A group of 8 cases considering only transverse moisture migration and another group considering both transverse and longitudinal moisture migration were compared. The heat and moisture fields after 30 days of operation reveal that considering the change of saturation caused by vertical moisture transfer, the soil temperature field will be affected, but borehole outlet temperature was less influenced. The absolute value of outlet temperature difference between corresponding cases in the two groups is only about 0.2 °C. The position of groundwater seepage and arrangement of unsaturated soil layers with different degrees of saturation on heat transfer capacity of vertical ground heat exchanger were further explored. The results show that the longitudinal moisture migration would be made more influential by the existence of seepage layer, because the average relative deviation of inlet and outlet temperature difference between the corresponding cases of Group 1 and Group 2 was 1.34% when setting seepage layer and was 0.44% when without seepage layer. Heat transfer performance of borehole heat exchanger is also affected by the location of seepage layer. The average relative deviation of inlet and outlet temperature difference between the reference case and cases with seepage in the top, middle and bottom layers is 34.18%, 25.08% and 16.82%, respectively. The arrangement of unsaturated soil layers also has a certain effect. When the soil layer with low degree of saturation is located in the upper layer of soil, heat transfer capacity is better.

Un-stationary thermal analysis of the vertical ground heat exchanger within unsaturated soils

The efficiency of heat exchange in ground heat exchangers (GHE) in many projects decreases throughout their service life. Large temperature fluctuations were observed during the ground heat exchanger loading and during its natural cooling. This problem stems mainly from the influences of heating and humidity migration on the operation of the ground heat exchanger in unsaturated soil. To find the answer to the problem numerical model of a porous medium with multi-component fluid flow was applied. The mathematical description was expanded by additional streams describing the exchange of thermal energy within the backfill material and the structure of the porous model. Ground porosity was mapped geometrically and mathematically. The definition of total temperature was introduced. The results obtained from proposed model were compared with another model results which was based on solving only the classical heat exchange equation. The model was also verified through the measurement data read from 3 sensors installed at different depths of one borehole and at different time intervals. The parameters of the model result from local climatic conditions of Jabłonna near Warsaw. 24-hour operation of a single borehole was simulated numerically. The results of the model proposed by the author were of a greater convergence with the real data than those obtained for the classical heat exchange model. The critical point of the model was the selection of the coefficients describing the flow resistance of i-components in the porous medium and the particular terms of the total temperature adopted definition. The proposed model allows for a more accurate estimation of the available thermal power in the future and more accurate analysis of the degree of the effort of the structure in the context of its exploitation and the destruction time.

Thermal characteristics in a spiral-tube buried in sand/bentonite/graphite under cooling and heating modes

ABSTRACT Thermal characteristics in a vertical spiral-tube ground heat exchanger were addressed. The influences of backfill materials (sand/bentonite/graphite blend) and operation modes were examined. In the heating and cooling modes with Reynolds number of 2500, the integrated evaluation factors of tube in blend rose all by about 20% compared with in sand. The longer recovery time was helpful to improve the heat transfer of ground tube. In cooling mode, the surface heat transfer coefficients, ranged from 225 to 610 W m−2 K−1. The average surface heat transfer coefficient under heating mode was 11 percent higher than that in cooling mode.

Experimental validation of a hybrid model for vertical ground heat exchangers under on-off operation conditions

This work presents an experimental validation of a two-dimensional hybrid model for vertical single U-tube ground heat exchanger. It is based on the use of the electrical analogy to model heat transfer within the borehole and thermal response factors (short and long time-step g-functions) to estimate heat flow to the surrounding ground. The substitution of ground nodes with short and long-term g-functions allows the simulation with short time steps keeping the possibility of simulating periods of several years. The model was experimentally validated by using reference data sets drawn from different tests at GeoCool plant, located at the Polytechnic University of Valencia (Spain). Validation process took into account the stop-start intervals of the geothermal heat pump operation. In a test of one week with 5-min time steps, model results agree very well with experimental data achieving RMSE values smaller than 1 °C for output fluid temperatures. The capability of the model to reproduce vertical distribution of borehole temperature was also studied. Furthermore, a comparison with the simulated results from a refined CFD model was made.

Thermal simulation of the ground source heat pump used for energy needs of a bioclimatic house in Tlemcen City (western ALGERIA)

ABSTRACT This paper presents a validation by simulation of feasibility analysis for the installation of the ground source heat pump systems that have been proposed the modeling study for use in Algeria. We design and calculate the energy needs of an example building with the COMSOL Multiphysics software and Geo TSOL Basic. The study addresses heating needs for the climate conditions of western Algeria, in one of North Africa’s biggest cities. The houses energy needs are assumed to be covered by a ground source heat pump (GSHP) system coupled with solar panels. The GSHP consists of a vertical ground heat exchanger and water-to-water heat pumps and is analyzed using an in-house developed and validated code. Energy consumption was calculated and validated by a simulation of the building’s properties. Results show that installation of the ground source heat pump system substantially reduces the building’s energy consumption. Finally, an economic analysis of the proposed system for construction practices in Algeria was carried out. The main novelty of this article, being that the study was conducted to seek a solution to reduce the energy bill of a house located in a city of a country Natural Gas Producer. Algeria is one of the 10 largest producers of natural gas in the world. More than 80% of the energy bill of a house located in Algeria, is due to the consumption of gas (Heating, Hot Water and Cooking), the electricity is mainly used in summer for air conditioning. The use of renewable energy will cost more than the use of natural gas. It is for this main cause that renewable energies are struggling to develop in Algeria, the cost of gas is very low. The economic analysis shows that in countries like Algeria, where the cost of conventional energy is heavily subsidized, solar thermal is not competitive with natural gas. It is only by coupling the GSHP with solar collectors that solar can take off and attract households. As a result, the only renewable way to compete with natural gas is the combination of GSHP with solar collectors. The only solution adopted to reduce the energy bill in a house located in Algeria, is the solution presented in this article. This is the first technical–economic study of how a GSHP could contribute to the energy needs of buildings in the city of Tlemcen ALGERIA.

Fast computation approach for parameterized design and simulation of vertical ground heat exchangers and GCHP systems

Vertical ground heat exchanger (GHE) is an expensive part of vertical GCHP systems. Fast and accurate computation of fluid and ground temperatures is desired for the long-term energy performance prediction of GCHP and optimal sizing of bore field. Hourly simulations and iteration of parameterized design in time domain with thermal response factors is quite time-consuming. In this paper, a new thermal response factors, δ-function is proposed for computing fluid and ground temperatures of bore field. 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. Computation of δ-function is quite fast due to its quite small integral range. The validity and accuracy of δ-function are examined by the simulation results of g-function. Through comparing hourly simulation of various periods between g- and δ-functions in both time and spectral domain methods, the combination of δ-function with Fast Fourier Transform spends the least computing time and is suitable to quickly and accurately compute fluid and ground temperatures in parameterized design and long-period hourly simulation of vertical heat exchanger and GCHP systems.

A workflow for bedrock thermal conductivity map to help designing geothermal heat pump systems in the St. Lawrence Lowlands, Québec, Canada

An accurate knowledge of the subsurface thermal conductivity is essential to design geothermal heating and cooling systems, specifically ground-coupled heat pumps. The subsurface thermal conductivity has a direct impact on the length of vertical ground heat exchangers needed to fulfill building energy needs. However, mapping the distribution of the subsurface thermal conductivity is a significant challenge due to the ground heterogeneity and the limited radius of influence associated with thermal conductivity assessment methods. Data from 79 thermal response tests, 90 thermal conductivity analyses conducted in the laboratory, and geophysical well logs from 72 oil and gas exploration wells were combined and analyzed together in an attempt to map the bedrock thermal conductivity distribution of the St. Lawrence Lowlands, Québec, Canada. Results from laboratory and well log analysis were adjusted for field scale-effects using thermal response tests to properly define a statistically reliable thermal conductivity for each thermostratigraphic unit of this sedimentary basin. The thermal conductivity obtained from thermal response tests and adjusted laboratory analyses was interpolated with sequential Gaussian simulations to generate a two-dimensional bedrock thermal conductivity map, which can be used by geothermal system designers to better understand the geothermal heat pump potential of the St. Lawrence Lowlands.