Ground Source Heat Pump Applications Research Articles

Performance of vacuum-insulated central pipes for deep borehole heat exchangers in geothermal systems

Abstract Geothermal energy is considered a promising future energy prospect, with the geothermal well outlet temperature being one of the important parameters affecting possible utilization options. For ground source heat pump applications or direct district heating, using lower temperatures can be acceptable. However, efficient electricity production requires a higher enthalpy gradient, which cannot be achieved without high temperature at the wellhead. The selection of the dry co-axial close-loop deep borehole systems (DBHE) may be, in some cases, very beneficial. The operating performance of co-axial DBHE can be optimized if the undesired heat transfer between the central pipe and annular fluid zones is minimized. Therefore, the operational performance of such a system depends strongly on the high thermal resistance of the central pipe. The most common option would be a low thermal conductivity material, such as high-density polyethylene (HDPE). In addition, vacuum-insulated tubing (VIT) used as the central pipe could be considered. The article presents results from the study aimed at the comparison of the homogeneous central pipe made of HDPE material and the gap-insulated central pipe. In the study, various air pressure levels as well as variations of surface emissivity were examined to reveal the effect on the heat transfer between the fluid channels. The simulation has been performed using a new purposely developed WellTH simulation software. A coaxial heat exchanger system using a VIT outperforms significantly the heat exchanger with an HDPE for deep geothermal wells. However, this advantage diminishes for shallow wells and therefore this tendency should be considered in the design stage.

Research on multi-objective optimization of control strategies and equipment parameters for a combined heating system of geothermal and solar energy in cold and arid regions based on TRNSYS

Focusing on the multiple objectives of maximum solar heating capacity, maximum COP of heat pump and minimum soil heating capacity, a model of one typical building and two heating strategies was established in the TRNSYS. Non-dominated Sorting Genetic Algorithm (NSGA) and Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) based on entropy weight are used to optimize operation parameters. Optimized control strategies and equipment parameters of a combined geothermal and solar system in cold and arid regions were taken as the research objects. The final optimized strategy and the original strategy of the single ground source heat pump heating system were compared. Research results show that the optimized control strategy of the geothermal solar heating system transforms solar hot water from the storage tank to heating terminals in the heating season and transforms solar hot water to the ground heat exchanger in the non-heating season. Compared to GSHP, the optimizing strategy saves 16.8 MWh annually, and the energy saving rate is 20.59%. Soil temperature drop increases from 2.74 °C to 0 °C. The annual electricity saving cost intensity is about 4 yuan/m2, and the CO2 emission reduction intensity is 4.3 kg CO2/m2. This study provides a solution for soil temperature reduction in one year and low-cost operation in cold and arid regions during ground source heat pump applications.

Is Ground Source Heat Pump the Future of Space Cooling in Southeast Asia?

The rapid economic growth of Southeast Asian countries is followed by accelerated population and energy demand growth. On the other hand, the region still relies heavily on fossil fuels as the primary source of electricity. As a result, there is a growing concern about future regional energy sustainability. Space cooling has become one of the fastest-growing energy sectors. Introducing a new space cooling technology capable of providing a higher thermal efficiency can be an alternative solution. The Ground Source Heat Pump (GSHP) is a proven technology to reduce the required energy for space cooling and heating in four-season countries. However, its performance in the tropical climate is still questionable. This paper describes the challenges of GSHP application in tropical climates, such as in Southeast Asia. The research conducted to investigate the suitability of GSHP system application in Southeast Asia is also discussed. While more data are being gathered from several GSHP pilot tests in Vietnam and Thailand, the short to medium-term performance data analysis shows its advantages over the conventional Air Source Heat Pump (ASHP) or standard air conditioner. However, extensive research is still required to evaluate its long-term performance and the financial advantages over the ASHP.

Calibration and Uncertainty Quantification for Single-Ended Raman-Based Distributed Temperature Sensing: Case Study in a 800 m Deep Coaxial Borehole Heat Exchanger

Raman-based distributed temperature sensing (DTS) is a valuable tool for field testing and validating heat transfer models in borehole heat exchanger (BHE) and ground source heat pump (GSHP) applications. However, temperature uncertainty is rarely reported in the literature. In this paper, a new calibration method was proposed for single-ended DTS configurations, along with a method to remove fictitious temperature drifts due to ambient air variations. The methods were implemented for a distributed thermal response test (DTRT) case study in an 800 m deep coaxial BHE. The results show that the calibration method and temperature drift correction are robust and give adequate results, with a temperature uncertainty increasing non-linearly from about 0.4 K near the surface to about 1.7 K at 800 m. The temperature uncertainty is dominated by the uncertainty in the calibrated parameters for depths larger than 200 m. The paper also offers insights into thermal features observed during the DTRT, including a heat flux inversion along the borehole depth and the slow temperature homogenization under circulation.

STUDY OF BOREHOLE HEAT EXCHANGER HEAT TRANSFER ENHANCEMENT PARAMETERS

This paper discusses the study of parameters for improving the heat transfer of a borehole heat exchanger for a ground source heat pump application. The study of efficiency parameters was carried out based on an experimental prototype of a ground source heat pump developed by the authors. A mathematical model has been developed for calculating the efficiency of a ground heat exchanger based on three-dimensional equations of heat and mass transfer in a porous medium. The numerical solution was carried out using the COMSOL Multiphysics software. The numerical calculation algorithm was verified by comparison with experimental data from the created prototype. Calculations were made of the efficiency of a borehole heat exchanger with various geometric configurations of the pipes in the well. With an increase in the tube diameter, the heat transfer increases. With a tube diameter of 40 mm, the thermal efficiency of the heat exchanger was 42.4 W/m in the heat charging mode, which is 24% more with a diameter of 20 mm. With increasing well depth, the heat transfer efficiency increases. The influence of the thermal conductivity coefficients of the pipe material, grout material and various types of ground on the heat transfer efficiency was also studied. It was shown that with an increase in the thermal conductivity coefficients of grout and ground, the heat flux increases, but above 6.0 W/m K, the heat flux practically does not change. When the coefficient of thermal conductivity of the pipe material is higher than 1.0 W/m K, the heat fluxes almost do not change. In general, materials containing plastics are used for piping of ground heat exchangers, the thermal conductivity coefficients of which vary between 0.24-0.42 W/m K.

A Review of Ground Source Heat Pump Application for Space Cooling in Southeast Asia

Ground source heat pump (GSHP) systems have been used worldwide in buildings because of their advantages of highly efficient performance in terms of energy and environment for space cooling and heating; however, cooling demand is predominant in tropical climates. This paper reviews of the GSHP systems applications in Southeast Asia; several applications of GSHP in Thailand, Indonesia, Malaysia, Singapore, and Vietnam have been addressed. Experiments were initiated in 2006 in Kamphaengphet; the latest experiment found in the Scopus searching tool is the GSHP simulation in Kuantan in 2019 using EnergyPlus using the ground loop design software. GSHP systems have the potential to be used in Southeast Asia despite the dominance of cooling demand, leading to a thermal imbalance within the subsurface. This imbalance can reduce the performance of the system; however, groundwater flow is considered as a key factor in preventing the effect of thermal distribution owing to GSHP operation. These results suggest that the GSHP has the potential to reduce emissions and electricity consumption within areas having tropical climates, such as Southeast Asia, for sustainability and future generation.

AHP-Based Evaluation of the Suitability of Shallow Geothermal Energy Utilization in GSHP System

Shallow geothermal energy (SGE) is a part of geothermal resources and is mainly used through ground source heat pumps (GSHP). However, the potential of SGE varies from region to region due to different geological conditions. There is a lack of regulations and codes for assessing SGE, which makes the design and planning of GSHP restricted. In this study, an evaluation system of the suitability of GSHP in a region of Qingdao by using Analytic Hierarchy Process (AHP) is proposed, and the test area is divided into three suitability levels based on suitability scores. The evaluation system contains property indicators, elemental indicators, and their weights. The result shows that the highly suitable area for the application of GSHP in the test area is 110.04 km2, accounting for 41.8% of the whole test area. The area of moderately suitable area is 65.02 km2, accounting for 24.7%, and GSHP should be developed and utilized on the basis of full demonstration in this level. The unsuitable area for GSHP is 88.19 km2, accounting for 33.5%. The indicator weights in this article may only be applicable to the Qingdao area and cities with similar geological conditions to Qingdao. However, the indicators within this evaluation system can be applied to the vast majority of locations where GSHP are to be developed, as it provides a method of assessment in terms of geological conditions, groundwater conditions, construction conditions, and ecological aspects.

Study on the best heat transfer rate in thermal response test experiments with coaxial and U-pipe borehole heat exchangers

This paper concerns the modeling of vertical Borehole Heat Exchangers (BHEs) for Ground Source Heat Pump (GSHP) applications. Focus is devoted to the analyses of Thermal Response Test (TRT) simulations aimed at understanding the main factors that influence the ground thermal conductivity and the effective borehole thermal resistance estimations. The conventional infinite line-source (ILS) model does not include any influence of the external heat transfer rate on the BHE/ground property evaluation. Analyses of numerically simulated TRTs show this omission can sometimes produce an error in the estimate of the ground thermal conductivity. The error may be between ±10% and ±22% for coaxial boreholes (800 m depth), if the ground has a significant geothermal gradient. On the other hand, for single and double U-pipe BHEs the error is less than ±5% under similar conditions. The parameter qratio is identified as an indicator of when the error is significant. This parameter is equal to the external heat rate (injection or extraction) divided by a natural heat rate that is related to the geothermal gradient. Errors greater than ±10% tend to occur for coaxial boreholes with a center-pipe fluid inlet when |qratio| < 1. Under the same conditions but with the annulus as the fluid inlet, the error is less than ±6%.

Design and Simulation Analysis of Solar-coupled Ground Source Heat Pump System

Solar heating technology and ground source heat pump (GSHP) technology are green technologies which are mainly promoted and applied in our country. Based on the comprehensive analysis of domestic research results on solar energy utilization, GSHP applications and their coupled systems, this paper designs a set of solar-coupled ground source heat pump (SGSHP) system which can operate independently and coupled. Aiming at the characteristics of solar heating and GSHP, the operation mode and control strategy of the system are proposed. Taking a building in Qingdao as an example, the coupled system and the single GSHP system were simulated and analyzed to verify the energy saving advantages of the SGSHP system.

Exploring the Impact of a District Sharing Strategy on Application Capacity and Carbon Emissions for Heating and Cooling with GSHP Systems

To meet long-term climate change targets, the way that heating and cooling are generated and distributed has to be changed to achieve a supply of affordable, secure and low-carbon energy for all buildings and infrastructures. Among the possible renewable sources of energy, ground source heat pump (GSHP) systems can be an effective low-carbon solution that is compatible with district heating and cooling in urban areas. There are no location restrictions for this technology, and underground energy sources are stable for long-term use. According to a previous study, buildings in urban areas have demonstrated significant spatial heterogeneity in terms of their capacity to demand (C/D) ratio under the application of GSHP due to variations in heating demand and available space. If a spatial sharing strategy can be developed to allow the surplus geothermal capacity to be shared with neighbors, the heating and cooling demands of a greater number of buildings in an area can be satisfied, thus achieving a city with lower carbon emissions. In this study, a GSHP district system model was developed with a specific embedding sharing strategy for the application of GSHP. Two sharing strategies were proposed in this study: (i) Strategy 1 involved individual systems with borehole sharing, and (ii) Strategy 2 was a central district system. Three districts in London were selected to compare the performance of the developed models on the C/D ratio, required borehole number and carbon emissions. According to the comparison analysis, both strategies were able to enhance the GSHP application capacity and increase the savings of carbon emissions. However, the improvement levels were shown to be different. A greater number of building types and a higher variety in building types with larger differentiation in heating and cooling demands can contribute to a better district sharing performance. In addition, it was found that these two sharing strategies were applicable to different kinds of districts.

On the ground thermal conductivity estimation with coaxial borehole heat exchangers according to different undisturbed ground temperature profiles

This paper concerns the modeling of vertical coaxial heat exchangers for Ground Source Heat Pump (GSHP) applications. Vertical coaxial borehole heat exchangers (CBHEs) can be buried at depths which are even higher than the conventional ones. In this case they are referred as Deep Borehole Heat Exchangers (DBHEs). As it is known, there is indeed a strong recent tendency, especially in the Scandinavian regions, to use high depth (500–1000 m) underground heat exchangers for the GSHP applications. This study is aimed at the analysis of the BHE behaviour in the early period, say for Fourier numbers typical of the Thermal Response Test (TRT) measurements. The novelty of the present numerical results is related to the applicability of standard TRT methods when referred to DBHEs and different geothermal gradients can be found. To this aim a Fortran code has been developed for describing a 2D transient conduction and convection problem able to provide the fluid and ground temperature evolution as a function of a series of boundary conditions, including the initial and far field ground temperature distribution along the depth. The application of the present model is related to coaxial BHEs for the assessment of the effects of the undisturbed ground temperature profile and the direction of the carrier fluid on the ground thermal conductivity estimation in TRT experiments. It is here demonstrated that different BHE depths (ranging from 150 to 800 m) and different undisturbed temperature profiles (including zero and positive geothermal gradients) can severely affect the TRT ground conductivity estimation (errors up to 25%) if the flow direction is based on the annular pipe or the central pipe inlets.

THERMAL PERFORMANCES OF DIFFERENT TYPES OF UNDERGROUND HEAT EXCHANGERS

Globally buildings are responsible for approximately 40% of the total world annual energy consumption. Most of this energy is for the provision of lighting, heating, cooling and air conditioning. An increase in awareness of the environmental impact of CO2, NOx and CFCs emissions triggered a renewed interest in environmentally friendly cooling and heating technologies. Under the 1997 Montreal Protocol, governments agreed to phase out chemicals used as refrigerants that have the potential to destroy stratospheric ozone. An approach isneeded to integrate renewable energies in a way to achieve high building performance standards. However, because renewable energy sources are stochastic and geographically diffuse, their ability to match demand is determined by the adoption of one of the following two approaches: the utilisation of a capture area greater than that occupied by the community to be supplied, or the reduction of the community’s energy demands to a level commensurate with the locally available renewable resources. Ground source heat pump (GSHP) systems (also referred to as geothermal heat pump systems, earth-energy systems and GeoExchange systems) have received considerable attention in recent decades as an alternative energy source for residential and commercial space heating and cooling applications. The GSHP applications are one of three categories of geothermal energy resources as defined by ASHRAE and include high-temperature (&gt;150°C) for electric power production, intermediate temperature (&lt;150°C) for direct-use applications and GSHP applications (generally (&lt;32°C). The GSHP applications are distinguished from the others by the fact that they operate at relatively low temperatures.

Prediction of the ground temperature with ANN, LS-SVM and fuzzy LS-SVM for GSHP application

Ground source heat pump (GSHP) system has received more and more attentions for its energy-conserving and environmental-friendly properties. Acquisition of the undisturbed ground temperature is the prerequisite for designing of GSHP system. Measurement by burying temperature sensors underground is the conventional means for obtaining the ground temperature data. However, this way is usually time consuming and high investment, and also easily encounter with certain technical difficulties. The rapid development of intelligent computation algorithm provides solutions for many realistic difficult problems. Basing on a great number of the measured data of the ground temperature from two boreholes with 100m depth located in Chongqing, ground temperature prediction models basing on artificial neural network (ANN) and support vector machine based on least square (LS-SVM) are established, respectively. And then, two kinds of validation works, i.e., holdout validation and k-fold validation are conducted toward the two models, respectively. Furthermore, a new method that correlating fuzzy theory with LS-SVM is proposed to solve the big computation burden problem encountered by LS-SVM model. By comparing with the above two models, it is concluded that the newly proposed model can not only improve the calculation speed obviously but also be able to promote the prediction accuracy, especially superior to the single LS-SVM model.

A Review of Heat Pump Systems and Applications in Cold Climates: Evidence from Lithuania

Similar to other cold climate countries, space heating and domestic hot water (DHW) accounts form the largest share of household energy demand in Lithuania. Heat pump technology is considered to be one of the environmentally friendly solutions to increase energy efficiency and reduce the carbon footprint of buildings. Heat pumps have been finding their way into the Lithuanian market since 2002, and currently there are many good practice examples present in the country, especially in the residential and public sectors. Heat pump use is economically advantageous in the Baltic Region, and the market share of these systems is growing. Studies have reported seasonal performance factor (SPF) ranges within 1.8 and 5.6. The lower SPF values are typically attributable to air source heat pumps, whereas the higher efficiency is achieved by ground or water source heat pump applications. While the traditional heat pump techniques are well established in the region, there is a slow uptake of new technologies, such as solar-assisted heat pumps, absorption heat pumps and heat pumps integrated into foundations, tunnels or diaphragm walls. This paper provides a critical review of different heat pump technologies, using Lithuania as a cold climate case study to overview the market trends within the European context. Potential trends for the heat pump technology development in terms of application areas, cost-benefit predictions, as well as environmental aspects, are discussed.

A novel shallow bore ground heat exchanger for ground source heat pump applications—Model development and validation

A novel ground heat exchanger design, the Underground Thermal Battery (UTB), was proposed as a low-cost alternative to the conventional vertical bore ground heat exchanger (VBGHE) for the application of ground source heat pumps (GSHPs). The UTB is designed to be installed in a shallow borehole (less than 6 m deep) and thus could cost much less than the conventional VBGHE, which usually is installed in vertical bores 60 m deep. By utilizing natural convection of water and phase change materials, the UTB tempers its temperature change in response to thermal loads, which helps improve the efficiency of GSHPs. A one-dimensional (1D) model of the UTB has been developed and validated with the measured performance data of a small-scale UTB, as well as the simulation results of a more detailed three dimensional (3D) numerical model, which accounts for both heat transfer and fluid dynamics in the UTB. This 1D model is computationally much more efficient than the 3D model and thus can simulate the annual performance of the UTB with acceptable computation time. The 1D model has been used to evaluate the performance of the UTB so that it can be compared with the conventional VBGHE.

Experimental and numerical investigation of a thermal storage medium for ground source heat pump applications

As a means of improving performance and alleviating thermal imbalance issues associated with ground source heat pump systems, ground thermal energy storage is becoming increasingly appealing. Moreover, an efficient means of transferring energy from the borehole to the surroundings is crucial. In this paper, an experimental and numerical investigation of the performance of a thermal storage medium for ground source heat pump applications is presented. A thermal storage tank of 1 m diameter and 1.5 m height equipped with temperature sensors at different radial positions and depths was used. It incorporates a 150 mm-dimeter steel pipe in which a 25.4 mm u-shaped pipe is placed to emulate a borehole. A numerical model was developed using a finite element-based approach and validated with the obtained experimental data. Different bentonite-sand mixtures with varying thermal conductivities were considered inside and around the emulated borehole. A reduction in the tank temperature difference of about 57% was achieved by using bentonite-sand mixtures inside the borehole and around the borehole (in the tank) compared to using only bentonite inside the borehole and sand around the borehole. Results further show that the heat transfer performance increases by about 320% with the use of the bentonite-sand mixtures compared to the separated bentonite and sand arrangement. Moreover, the potential for energy storage with phase change material around the emulated borehole has been demonstrated.

Experimental Study on Thermal Conductivity of Sand Solidified by Microbially Induced Calcium Carbonate Precipitation

Soil thermal conductivity is relatively low in low saturation ranges in arid regions, resulting in low heat transfer efficiency of ground source heat pump (GSHP) systems. In this study, thermal conductivity of sand solidified by microbially induced calcium carbonate precipitation (MICP) was measured at different saturation levels. The test results reveal that the thermal conductivity of MICP treated sand increases with the increase in saturation degree (Sr), and the increasing rate gradually decreases. Use of the MICP technique not only increases the thermal conductivity of sands, but also reduces the influence of saturation degree on sand thermal conductivity when Sr is less than 0.22. This study provides a new research direction for the MICP technique and a technical reference for GSHP applications.

Assessment of three types of shallow geothermal resources and ground-source heat-pump applications in provincial capitals in the Yangtze River Basin, China

The evaluation of geothermal resources and ground-source heat-pump systems has been performed for a wide variety of individual buildings and cities. However, the assessment of ground-source heat-pump systems for multiple cities with different climatic and ground conditions has rarely been performed. The feasibility of ground-source heat-pump installation strongly depends on the local climatic, geological and geothermal conditions. This paper first investigates the shallow geothermal resources, including the thermal resources of open reservoirs (lakes and rivers, surface water), underground aquifers and geological materials. Then, the potential for ground-source heat-pump system applications in eleven provincial capitals in the Yangtze River Basin in China is assessed. The Yangtze River Basin covers a wide range of territory with dramatically different climates and geological conditions. The potential of ground-source heat-pump systems is evaluated by modeling the energy consumption of a certain conventional building with defined thermal and geometric characteristics under various climatic conditions. The results indicate that three ground-source heat-pump systems, namely, surface-water heat pumps, groundwater-source heat pumps and ground-coupled heat pumps, show different potentials among the investigated cities. The findings from this study are meant to optimize the high capital costs of ground-source heat-pump systems in the study area.

Ground-Source Heat Pumps with Horizontal Heat Exchangers for Space Cooling in the Hot Tropical Climate of Thailand

The cooling of spaces in tropical regions, such as Southeast Asia, consumes a lot of energy. Additionally, rapid population and economic growth are resulting in an increasing demand for space cooling. The ground-source heat pump has been proven a reliable, cost-effective, safe, and environmentally-friendly alternative for cooling and heating spaces in various countries. In tropical countries, the presumption that the ground-source heat pump may not provide better thermal performance than the normal air-source heat pump arises because the difference between ground and atmospheric temperatures is essentially low. This paper reports the potential use of a ground-source heat pump with horizontal heat exchangers in a tropical country—Thailand. Daily operational data of two ground-source heat pumps and an air-source heat pump during a two-month operation are analyzed and compared. Life cycle cost analysis and CO2 emission estimation are adopted to evaluate the economic value of ground-source heat pump investment and potential CO2 reduction through the use of ground-source heat pumps, in comparison with the case for air-source heat pumps. It was found that the ground-source heat pumps consume 17.1% and 18.4% less electricity than the air-source heat pump during this period. Local production of heat pumps and heat exchangers, as well as rapid regional economic growth, can be positive factors for future ground-source heat pump application, not only in Thailand but also southeast Asian countries.

Spatial Data Assisted Ground Source Heat Pump Potential Analysis in China, a Case of Qingdao City

Nowadays, China faces challenges of further implementing heat pump technology for meeting building heating and cooling demand. The utilization of heat pumps, especially ground source heat pump (GSHP) is associated with a number of geological, hydrological as well as meteorological criteria. Thus it is essential to systematically address the feasibility of ground source heat pumps application using quantitative evaluation. Spatial data analysis is a method widely used in energy field to investigate renewable energy potential. Therefore, this study strives to provide an estimation of electricity driven GSHP’s potential in north China using spatial data assisted tools. Followed by a case study using the methodology recommended, a spatial data assisted GSHP potential evaluation model is built for Qingdao city in north China. The evaluation model is constructed and analyzed through spatial data processing software visualized by ground source heat pump potential maps. The result maps show that, places with most potential of GSHP application locate in south Qingdao close to the sea. Such places have a higher ground extractable heat and relatively low drilling cost.