Design Of Ground Source Heat Pump Research Articles

Fast segregation of thermal response functions in short-term for vertical ground heat exchangers

A detailed understanding of the short-term performance of the ground heat exchanger (GHE) is a major concern in the design of ground-source heat pumps (GSHP), as it has a significant impact in the efficiency and costs of the facility. The use of numerical models or artificial intelligence techniques may help in the short-term, although they are generally very time-consuming, becoming unpractical in numerous cases. The aim of this work is to obtain the thermal response factors according to the GHE, segregating the transient borehole behavior from the soil effect, based on a previous study which applies experimental data from thermal response tests (TRT) to the finite line source (FLS) model. The proposed method implies an extensive study of the fluid temperature as well as the development of new auxiliary thermal resistances related to the borehole and fluid, avoiding the implementation of numerical models. This procedure has been validated with the temperature outcomes of a 3D numerical model, obtaining low deviations, into a range of ±0.1 K, and mean absolute deviations below 0.01 K.

Numerical simulation and thermal analysis of ecological energy-saving ground source heat pump design

By constructing the heat transfer model of the buried tube heat exchanger, using the mathematical method and numerical calculation software, the temperature of the buried tube heat exchanger is calculated numerical, so as to better understand the temperature effect of the buried tube heat exchanger on the surrounding soil. This paper establishes a mathematical model of a vertically buried heat transfer system based on the principle of thermal equilibrium and the thermal conductivity differential equation. Conduct research on the temperature field around single pipes in different soil (geological conditions), temperature field under different drilling backfill materials, and the temperature around buried pipelines in both short-term and long-term operation of the system. The experimental results show that the simulation results of the system?s short time (10 days) and long time (90 days) performance show that the cooling and temperature cycles in different areas directly affect the heat radius of the buried pipe and the heat accumulation near the buried pipe. The thermal action radius of the short-term operation system is about 1.8 m, and the long-term operation reaches 4.5 m.

Uncovering the Efficiency and Performance of Ground-Source Heat Pumps in Cold Regions: A Case Study of a Public Building in Northern China

In cold regions, due o the impact of climatic conditions, the heat load in winter and the cooling load in summer are unbalanced. In the long-term operation of the ground-source heat pump (GSHP), the soil heat imbalance phenomenon has still not been successfully solved. Therefore, this study took the GSHP of a public building in the cold area of northern China as the research object. Based on the unit performance data of the system over 8 years and the measured data of the soil temperature field, the long-term operation efficiency of the GSHP in the cold region and the variation law of the soil temperature field were explored. In order to further study the problem of soil heat imbalance, the effect of heat exchange hole groups at different intervals on the underground soil thermal environment after 30 years of operation in the system was simulated, and the optimization scheme of heat exchange hole spacing was proposed. The research results support the improvement and optimization of GSHP design and construction, and have important practical significance for the popularization of GSHPs in cold regions.

Experimental work over borehole filling material to reinforce characterization and model validation of Ground Heat Exchangers

Ground Heat Exchangers (GHE) are part of Ground Source Heat Pumps (GSHP), employed as a renewable source in air conditioning systems. Models implemented are quite in tune to the real behaviour, enabling reliable designs and optimization, once several thermal properties of the surrounding soil and borehole are known. Since some values of those properties are evaluated from experiments, other ones are selected and approached from data base, leading sometimes to poor reproductions, due to the lack of their specific knowledge. This work demonstrates the effectiveness of the experimental evaluation of the filling material properties in a vertical GHE, to guarantee more reliable values to model its thermal behaviour: moisture content, density, and both thermal capacity and conductivity. Experiments must be easy and fast to perform, because their values must be known previous to the GSHP design. In addition, an identification process over a numerical model has been developed. In order to check its consistency, results have been compared with those determined experimentally. As well as a high model accuracy, it has been demonstrated the importance of the presented assays to be included in the previous experimental work to the design step of a GSHP system, providing a valuable borehole characterization.

Investigation for a novel optimization design method of ground source heat pump based on hydraulic characteristics of buried pipe network

In this paper, a novel optimal design method for ground source heat pump (GSHP) system based on hydraulic characteristics of buried pipe network (BPN) is proposed. The influence of BPN hydraulic characteristics and soil thermal balance on the operation efficiency of GSHP system are considered. This paper studies the GSHP system of a building in Changsha in the Central and South China Hilly Region. First, referring to the soil thermal property test, a simple simulation model is built using TRNSYS. Furthermore, the influence extent of various factors on soil thermal balance are studied by orthogonal experimental design method (OED). Then, the BPN model of this system is established and TRNSYS is employed to establish the GSHP system model of the building. A comprehensive coefficient of performance of the whole year transportation system (COPall) is presented as the evaluation index to optimize the borehole depth (Bd), borehole spacing (Bs), buried pipe length (Bl) and buried pipe form. These results indicate that, the resistance of vertical buried pipe accounts for 80% of the total resistance in a BPN partition. Reducing the Bl from machine room to BPN by 80% in the whole BPN, and the total resistance decreases by 31.8%. It is found that there is a power function relationship between the average temperature rises of soil (ΔT) and the BPN resistance (H). Considering comprehensively, Bd of this project should be designed within 60 m-100 m and Bs should be designed 4.0 m-5.0 m. When the Bl is certain, the COPall reaches its maximum when the Bs is 3 m. For any Bs between the two buried pipe forms, the Bl corresponding to the maximum COPall is almost the same. They are 25.2 km for single U- tube and 67.2 km for double U- tube respectively. When the Bl is the same, 24 km is the critical length of two buried pipe forms. Our work can provide theoretical basis and guidance for the optimization design of GSHP in similar areas.

Multi-objective evolutionary-based optimization of a ground source heat exchanger geometry using various optimization techniques

Ground Source Heat Pumps (GSHP) are commonly implemented to reduce energy consumption. This study seeks to delineate optimum operating conditions and borehole geometry configurations for GSHPs on the basis of multi-objective evolutionary algorithms. A thermodynamic heat transfer model is proposed to describe the GSHP behavior, and an economic model is proposed to assess the GSHP total cost for the amount of energy provided. An exergoeconmic optimization is employed to estimate the Pareto optimal solution for the GSHP, along with the optimum operating conditions and borehole configuration. Three prominent points of the Pareto frontier (equilibrium point, total cost rate objective and exergy efficiency objective) are used to isolate and illustrate each objective’s performance. Scatter density distribution plots of design parameters are also obtained to find their trends, which can give more insight into GSHPs design parameters. The solutions from five different Evolutionary Algorithms (EAs), NSGA-II, GDE3, IBEA, SMPSO, and SPEA2, are compared. Three different refrigerants (R134-a, R123 and isobutene) are also compared to find the most suitable working fluid for the decision space delineated. This study attempts to find the optimal configuration of ground heat exchangers (GHEs) based upon the most reliable multi-objective evolutionary algorithm.

Evaluation of thermal-mechanical properties of quartz sand–bentonite–carbon fiber mixtures as the borehole backfilling material in ground source heat pump

Ground source heat pumps (GSHPs) use sustainable technology and renewable geothermal energy to exchange heat between the ground and the interior of residential, industrial, and commercial buildings. The borehole grouting material is a key contributor to the stability and thermal transmission properties of GSHPs. To investigate the thermomechanical properties of quartz sand–bentonite–carbon fiber mixtures to assess their potential as a borehole backfilling material, the thermal needle and unconfined compressive strength (UCS) tests were conducted. The thermal conductivity of quartz sand–bentonite–carbon fiber mixtures increased with the sand particle size, and the thermal conductivity showed a parabolic relationship with bentonite fraction for all sand particle sizes. The thermal conductivity of the mixtures increased with increasing moisture content and degree of saturation, and increased linearly with increasing dry density. The optimal percentage of bentonite, by dry mass, was approximately 10–12%. A relatively high thermal conductivity (1.90–1.98 W/m·K) and an unconfined compressive strength of 124–200 kPa were considered acceptable for borehole backfilling material used for GSHP. The practicality of quartz sand–bentonite–carbon fiber mixtures as borehole backfilling material for GSHPs was verified using the TICA GSHP Design Program. This research confirms that quartz sand–bentonite–carbon fiber mixtures are viable new borehole backfilling material that enhance the thermal efficiency and reduce the costs of GSHPs.

New perspectives in thermal performance test: Cost-effective apparatus and extended data analysis

Two different experimental methods, namely thermal response test (TRT) and thermal performance test (TPT), have been used in the field of ground-source heat pumps (GSHPs) for different purposes. TRT is a well-established method for estimating the design parameters of ground heat exchangers (GHEs), whereas TPT has recently been adopted to examine the thermal performance of newly developed GHEs. Although both methods provide important information, they are rarely performed together because they require different experimental apparatus. To overcome this limitation, we developed a cost-effective TPT apparatus by adding a general proportional-integral-derivative (PID) controller and a solid-state relay to an existing TRT apparatus without a hot water tank. The apparatus showed sufficiently fast and accurate controllability: when the setpoint was 25 °C, the rise time was ∼7 min from the initial temperature of 16.9 °C, and the steady-state error was within ∼±0.1 °C. Two TPTs were conducted using the developed apparatus and a 50 m-long borehole heat exchanger with two different setpoints of 30 °C and 40 °C. We applied a Bayesian inference technique using the infinite line source model as a forward model to extend the TPT data to the estimation of the GSHP design parameters. Thus, the information usually obtained from independent TPT and TRT can be obtained using a single TPT. Moreover, a new index, namely, the unit heat exchange rate, was defined to facilitate a comparison among TPT results obtained under different TPT setpoints, GHE configurations, and ground conditions.

Bayesian inference of structural error in inverse models of thermal response tests

For the design of ground-source heat pumps (GSHPs), two design parameters, namely the ground thermal conductivity and borehole thermal resistance are estimated by interpreting thermal response test (TRT) data using a physical model. In most cases, the parameters are fitted to the measured data assuming that the chosen model can fully reproduce the actual physical response. However, two significant sources of error make the estimation uncertain: random error from experiments and structural bias error that describes the discrepancy between the model and actual physical phenomena. Generally, these two error sources are not evaluated separately. As a result, the suitability of selected models to correctly infer parameters from TRTs are not well understood. In this study, the Bayesian calibration framework proposed by Kennedy and O’Hagan is employed to estimate the GSHP design parameters and quantify the random and structural errors in the inference. The calibration framework enables us to examine structural errors in the commonly used infinite line source model arising due to the conditions in which the TRT takes place. Two in situ TRT datasets were used: TRT1, influenced by contextual disturbances from the outdoor environment, and TRT2, influenced by a strong groundwater flow caused by heavy rainfall. We show that the Bayesian calibration framework is able to quantify the structural errors in the TRT interpretation and therefore can yield more accurate estimates of design parameters with full quantification of uncertainties.

An Experimental Facility to Validate Ground Source Heat Pump Optimisation Models for the Australian Climate

Ground source heat pumps (GSHPs) are one of the most widespread forms of geothermal energy technology. They utilise the near-constant temperature of the ground below the frost line to achieve energy-efficiencies two or three times that of conventional air-conditioners, consequently allowing a significant offset in electricity demand for space heating and cooling. Relatively mature GSHP markets are established in Europe and North America. GSHP implementation in Australia, however, is limited, due to high capital price, uncertainties regarding optimum designs for the Australian climate, and limited consumer confidence in the technology. Existing GSHP design standards developed in the Northern Hemisphere are likely to lead to suboptimal performance in Australia where demand might be much more cooling-dominated. There is an urgent need to develop Australia’s own GSHP system optimisation principles on top of the industry standards to provide confidence to bring the GSHP market out of its infancy. To assist in this, the Queensland Geothermal Energy Centre of Excellence (QGECE) has commissioned a fully instrumented GSHP experimental facility in Gatton, Australia, as a publically-accessible demonstration of the technology and a platform for systematic studies of GSHPs, including optimisation of design and operations. This paper presents a brief review on current GSHP use in Australia, the technical details of the Gatton GSHP facility, and an analysis on the observed cooling performance of this facility to date.

A PCM Thermal Storage for Ground-source Heat Pumps: Simulating the System Performance via CFD Approach

Abstract The conventional design of ground source heat pumps (GSHPs) is based on the peak heating and cooling loads. A possible optimization in GSHP design, including a thermal storage device between the ground exchangers and the heat pump, was already realized and it was found that a reduced-size geothermal field (-66%) is still able to cover the energy demand. In this paper, the design of the prototype was used as a starting point to study the potentialities of two possible upgrades for the optimization of the energy performance (COP) of the system. In the first case, the thermal storing material is water, as in the working prototype, and the efficiency is improved removing the cylindrical heat exchangers that were designed to separate the ground side from the heat pump side. In the second case, a completely new and more compact thermal storage was designed using phase change materials (PCMs). Computational fluid dynamics (CFD) simulations were performed in a transient regime to validate the model against observed data and to assess the potentiality of the two improvements. The system behavior is studied in terms of driving energy input and output energy production. Significant improvements of the system COP are observed (up to +20%). In the first case (water thermal storage), the overall COP is 4.1 during winter and 5.7 during summer, in the second case (PCM thermal storage), the COP is 4.1 and 5.9, respectively. The PCM thermal storage, in particular, is approximately 10 times smaller than the original design, and could be easily placed within the technical room.

Modeling the Heat Regime of Thermal Boreholes with Pore Moisture Freezing/Melting

Modern computer programs modeling heat transfer near the boreholes of geothermal or ground source heat pumps (GSHP) ignore freezing and melting of pore moisture. This is so because most GSHP are deployed in regions with moderate climate, where ground temperatures are generally positive. To ensure efficient exploitation of GSHP under Russia's climatic conditions, we have to allow the heat carrier temperature, i.e., the ground temperature, to drop below zero. During the heating season, pore moisture freezes and melts releasing and absorbing the latent heat, which may affect the heat regime in the neighborhood of the thermal borehole. To assess the effect of latent heat on the borehole heat regime, we need a sufficiently simple and "fast" algorithm that can be used in engineering calculations during GSHP design. In this article, we present a pore moisture freezing/melting algorithm developed by the Insolar group of companies. The algorithm utilizes the concept of equivalent or "effective" ground thermal conductivity. Results of computer experiments are reported, confirming the essential dependence of the GSHP efficiency on ground-moisture phase transitions.

Thermal Property Measurements of Stratigraphic Units with Modeled Implications for Expected Performance of Vertical Ground Source Heat Pumps

Ground temperature and lithology influence ground source heat pump (GSHP) performance; however, typical design for residential systems uses estimated handbook values that are not necessarily representative of local geology. The incorporation of true ground temperature and thermal property measurements into design models would yield a more optimal design and thus decrease life cycle cost. A two-part study was conducted, first focusing on recording thermal properties of specific lithofacies in a particular region (Wisconsin, USA), then modeling how these measurements could change expected GSHP design and performance in a typical residential setting. Representative sedimentary, igneous, and metamorphic rocks from Wisconsin were characterized using guarded-comparative-longitudinal heat flow experiments (ASTM E1225), calorimetry, and weight-volume assessments. X-ray diffraction was also performed on some samples to assess the effect of mineralogy. Across all rock types, thermal conductivity ranged from 1.84 to 6.71 W m−1 K−1, and specific heat capacity ranged from 713 to 891 J kg−1 K−1. The indexed values were used to construct a hypothetical vertical heat exchange loop penetrating vertically consecutive Paleozoic strata. The hypothetical loop was examined using the rate of system temperature drop under full load as a measure of performance during the heating season. Expected system performance was compared between commonly cited thermal conductivity values of their general lithologies (e.g. sandstone, dolomite, shale) and the laboratory-measured thermal properties of the specific formations. Thermal conductivity indexed by general lithology proved to be insufficient as a design parameter to generate accurate assessments of GSHP performance.

Shallow geothermal energy application with GSHPs at city scale: study on the City of Westminster

Geothermal energy is an efficient low carbon solution for the heating and cooling of buildings. For many megacities such as London and Beijing, the amount of energy that can be stored in the urban local subsurface is greater than their annual heating and cooling demands. The ground source heat pump (GSHP) system – a shallow geothermal technology that provides heating and cooling for buildings by continuously replenishing the energy in the subsurface – has been used increasingly in recent years, but its application has been generally limited to single buildings. In this study, a geographic information system-based simulation model was developed to estimate how many GSHPs could be installed at the city scale without losing control of the ground thermal capacity and to evaluate the degree to which such a system could contribute to the energy demands of buildings in a city. The model was built by embedding a Python-based GSHP design code into ArcGIS software and was trialled on the City of Westminster, a borough in London, UK, as a case study under the two scenarios of boreholes placed under buildings and boreholes around buildings. Under both scenarios, the model produced borehole allocation maps and ratio of capacity to demand maps. The results show that a large proportion of buildings could support their own heating demands through GSHPs and, through a well-organised district heating system, GSHPs may be used efficiently to satisfy heating demands throughout an urban area.

A novel design method for ground source heat pump

This paper proposes a novel design method for ground source heat pump. The ground source heat pump operation is controllable by using several parameters, such as the total meters of buried pipe, the space between wells, the thermal properties of soil, thermal resistance of the well, the initial temperature of soil, and annual dynamic load. By studying the effect of well number and well space, we conclude that with the increase of the well number, the inlet and outlet water temperatures decrease in summer and increase in winter, which enhance the efficiency of ground source heat pump. The well space slightly affects the water temperatures, but it affects the soil temperature to some extent. Also the ground source heat pump operations matching with cooling tower are investigated to achieve the thermal balance. This method greatly facilitates ground source heat pump design.

Short-term behavior of classical analytic solutions for the design of ground-source heat pumps

Over the years several methods have been proposed to simulate and design the earth heat exchanger for a ground-source heat pump (GSHP) system. Some of these methods are based on numerical techniques while others rely on analytic solutions. Among the latter, two classical solutions have been extensively used over the years, the infinite line source (ILS) solution and the infinite cylindrical source (ICS). These solutions were known to overestimate the fluid temperature when the time scale is important and are valid only in a time range between a minimum and a maximum value which are often adequate for must design applications. It is usually accepted that for small Fourier numbers, the ICS solution should be used instead of the ILS. This paper revisits the short-term behavior of these solutions and we arrive at different conclusions than those usually accepted in the literature if the Fourier number is based on the borehole radius, which is normally the case. The reasons for these discrepancies are discussed and several options are proposed.

Impact of weather variation on ground-source heat pump design

This article presents an investigation on the impact of year-to-year weather variability on the optimal design of a system for heating and cooling a building using a ground-source heat pump. The designs of a boiler–ground-source heat pump hybrid and a cooling tower–ground-source heat pump hybrid were optimized using a typical meteorological year (TMY2) weather file and also using 15 years of actual weather data. The results indicate that a design based on a TMY2 weather file may be undersized for a severe weather year; this is particularly true if the severe weather year is encountered during the first year of system operation. A cooling tower–ground-source heat pump hybrid model was developed, which includes the use of a backup boiler placed on the building side (rather than the loop side) of the system. It was found that the use of a boiler backup mitigated much of the negative impact of a severe weather year. The boiler supplied the heating during periods of particularly severe weather so that the ground loop length could be maintained at a reasonable value.

Techno-economic and spatial analysis of vertical ground source heat pump systems in Germany

The objective of the current study was to assess the technical and economic factors that influence the design and performance of vertical GSHP (ground source heat pump) systems and to evaluate the spatial correlation that these factors have with geographic components such as geology and climatic conditions. The data from more than 1100 individual GSHP systems were analysed. The average capital cost of one GSHP system is about 23,500 € ± 6800 €; the large standard deviation is primarily caused by local market dynamics. In comparison to other countries such as USA, Austria, Norway, UK and Sweden, the highest capital costs for vertical GSHP systems are in Germany and Switzerland, which is almost certainly partly due to economies of scale. Although geological, hydrogeological and thermal conditions in the studied state considerably vary spatially and the evaluated specific heat extraction rates are heterogeneously distributed, no correlation between the subsurface characteristics and the design of GSHP systems could be identified. This outcome suggests that as yet subsurface characteristics are not adequately considered during the planning and design of small-scale GSHP systems, which causes an under- or oversizing and therefore a long-term impact on the maintenance costs and payback time of such systems.