Types Of Ground Heat Exchangers Research Articles

Studying the air flow heating process in the vertical type ground heat exchanger

Studying the air flow heating process in the vertical type ground heat exchanger

Comparison of the Energy Contributions of Different Types of Ground Heat Exchangers Related to Cost in a Working Ground Source Heat Pump System

Geothermal systems face adoption challenges due to their high initial investment cost. Accurate cost analyses and a more precise understanding of updated prices could assist geothermal industry projects in obtaining investment financing and better money management with the right equipment. As the cost of geothermal installations can vary widely depending on case and location, it seems essential to clarify the factors and parameters that determine the cost of the system. These include the type of loop system, the ground conditions, the type of heat pump, the system size, and the geographical location. The scope of this study is to compare the operation of various types of ground heat exchangers (GHEs) present in a Ground Source Heat Pump (GSHP) system installed in the coastal area of the Mediterranean climate zone of Cyprus. The highlight of this work is that it presents real installation cost data as well as recorded total energy contributed by the GHEs to the GSHP system of a HP cooling and heating capacities of 101 kW and 117 kW, respectively. The input contribution from the GHEs to the HP is 85,650 kWh (308,340 MJ) in summer and 25,880 kWh (93,168 MJ) in winter. It is shown that, among the three groups of GHEs investigated, the open-well GHE complex has the lowest cost per kWh ratio (0.32 EUR/kWh), followed by the vertical GHE complex (1.05 EUR/kWh), and lastly by the helical coil GHE (2.77 EUR/kWh). This clearly suggests that when underground water is available, the open-well GHE is much more favorable than other GHE types.

Multi-Criteria Study on Ground Source Heat Pump with Different Types of Heat Exchangers

Heat pumps are currently one of the most frequently applied heat sources in residential buildings. Ground source heat pumps are more reliable than air source heat pumps in terms of energy efficiency, especially in colder climates. However, they are more expensive and involve increased material inputs; therefore, multi-criteria analyses taking into account environmental and economic aspects seem necessary for the green design of these systems. The aim of this work was to analyze the environmental and economic impacts of the ground source heat pump providing heating for a family house located in eastern Poland, cooperating with three types of ground heat exchangers (each in two sizing options): helix, vertical and horizontal. The multi-criteria analysis was based on the life cycle assessment methodology using IMPACT 2002+ and life cycle costs methods. The lowest environmental impact was reported for the variants with vertical ground heat exchangers, mainly due to their high efficiency in the operation stage. On the other hand, the lowest economic impact was observed for the horizontal heat exchangers, which are not demanding in terms of material and construction costs. Final recommendations based on multi-criteria analysis propose the vertical probes as a sustainable solution, with a weighted sum indicator in the range 0.085–0.297 on 0–1 scale.

Suggestion of a design method for horizontal spiral coil type ground heat exchangers

The need of geothermal energy is constantly increasing for economical and environmental benefits. Horizontal ground heat exchangers (GHEs) can reduce the installation cost and they can also compromise between increasing in efficiency and costs. There are many kinds of GHEs, and it is known that slinky and spiral coil type GHEs show high thermal performance in the horizontal system. However, there is no adequate guideline to design the spiral coil type GHEs even though there is a design program for the slinky type GHEs. Therefore, this paper presents a design module for the horizontal spiral coil type GHEs. Dried Joomunjin standard sand was filled in a steel box including a horizontal spiral coil type GHE of which the size was 5m x 1m x 1m and thermal response tests (TRTs) were conducted for 30 hours to evaluate heat exchange rates. Based on the experimental results, a modified design module, that includes a scheme to properly evaluate design parameters such as ground thermal resistance and borehole thermal resistance used in the Kavanaugh theory for designing a spiral coil type GHE, was suggested.

Experimental and Numerical Investigations on the Heat Transfer Performance of a New Type of Energy Pile

The ground‐source heat pump heating and cooling system is currently the most important development and utilization method of shallow geothermal resources, but its key component is the ground heat exchanger system. Herein, a new type of ground heat exchanger, the soil‐cement energy pile, is proposed. The similarity model comparison experiments of the soil‐cement energy piles and the soil energy pile in summer and winter were conducted respectively, and the 3D numerical models of heat transfer of the two kinds of energy piles are established, their related thermal properties are verified. Then the heat transfer calculation methods of the soil‐cement energy pile are presented and validated. The research has shown that the soil‐cement energy pile has good thermodynamic performance, which effectively reduces the largest thermal resistance in the heat transfer system of the energy pile, the thermal resistance of pile body material, and the heat transfer per linear meter of the soil‐cement energy pile is 21.20%–55.60% higher than that of the soil energy pile. The soil‐cement energy pile is of great significance to improve the thermal efficiency of the ground‐source heat pump system and can reduces the overall project cost.

Feasibility and investigation of residential HVAC system with combined ground source heat pump and solar thermal collectors in different climates of Iran

Geoexchange is one of the most energy-efficient, environmentally clean, and cost-effective space-conditioning systems available on the market. Seasonal storage of solar energy in geothermal boreholes is sharply interesting as a means of heating and cooling in buildings with different applications. In this study, a closed-loop geo-exchange and solar thermal system with two different heat exchangers are examined in an energy-efficient house. TRNSYS, a simulation software tool, was used to model the yearly performance of the hybrid ground-source heat pump (GSHP) system. Two different types of ground heat exchangers, e.g., tube-in-tube and U-tube were employed. The mass flow rate through the heat exchangers and the solar collector was examined to choose the best scenario for each of them. The economic benefits of a gray water heat recovery system and solar collector were studied; the results show that 517 and 536 US Dollars/Year can be saved from each of them, respectively. System performance analysis was carried out in different cities with various climates in Iran. Moreover, a complete economic assessment was evaluated for all cases. The results show that for the same inlet water temperature to GSHP, the tube-in-tube heat exchanger can pass 45% more flow than the U-tube heat exchanger for the same size as the HP. Furthermore, in the absence of the solar collector, a 2.7 × 108 kJ increase was conducted in the auxiliary heater component yearly. Finally, four different climates were evaluated for the launching of the GSHP system with a solar thermal collector. The hot and dry climate with 27 °C fluid inlet to a heat pump, a total borehole depth of 220 m, and an initial cost of $ 25,047 was selected as the most appropriate weather conditions for launching the system in Iran.

Modeling and techno-economic comparison of two types of coaxial with double U-tube ground heat exchangers

Among different types of ground heat exchangers (GHEs), multi-external chamber coaxial GHE is recently a promising design due to its geometrical characteristics of concentric tube-in-tube configuration and maximal heat exchange performance. Nevertheless, the characterization and commercialization of multi-external chamber coaxial GHEs need a deep understanding of the effect of different opt-geometrical and operational parameters on their thermo-economic performance. Therefore, accurate numerical modeling of the transient heat transfer proficiencies and the techno-economic feasibility of the multi-external chamber coaxial GHE and coaxial GHE compared to conventional double U-tube GHE is crucial to promise an optimal design of ground-coupled heat pump systems. This study introduces a comprehensive systematic and techno-economic analysis of the transient heat transfer characteristics of coaxial and multi-external-chamber coaxial GHEs compared to conventional double U-tube GHE. A three-dimensional numerical model was adopted and validated by field experiments. Moreover, a sensitivity analysis was conducted to investigate the impacts of several key operating and design parameters on heat transfer performance under heating and cooling modes conditions. Furthermore, various case studies, along with a comprehensive analysis of multi-objective optimization for minimization of the GHE number and energy consumption, were also carried out. In addition, a techno-economic analysis was also carried out using the total annual cost method to assess the economic viability of the proposed GHE designs. The results reveal that, compared to double U-tube GHE, coaxial GHE has the best thermal performance with maximum and average heat transfer rates of 127.54% and 17.67%, while the multi-external-chamber coaxial GHE is estimated to be 62.38% and 11.72%, respectively. Also, the corresponding total thermal resistance of coaxial GHE is the lowest by 14.86%, and for multi-external-chamber coaxial, it is 10.30%; also coaxial GHE has the lowest pressure drop by 76.45%, followed by multi-external-chamber coaxial GHE with 57.18%. In the case studies, replacing double U-tube GHE with coaxial or multi-external-chamber coaxial GHEs, the GHE number can be decreased by 13.3% for both the designs, or the energy consumption of the water pump by 33.91% and 30.97%, and the energy consumption of the entire system by 17.21% and 10.54%, respectively. The techno-economic analysis results indicate that replacing double U-tube GHE with coaxial GHE is the most feasible alternative, which can decrease the total annual cost by 13.58%, followed by multi-external-chamber coaxial GHE by 6.77%. Further discussion for the deep applications was performed, which concluded the optimal GHE design. The findings of this paper can be used as a guide for future research on GHEs for potential applications of ground-coupled heat pump systems.

Parametric optimization of a spiral ground heat exchanger by response surface methodology and multi-objective genetic algorithm

Horizontal spiral-coil type ground heat exchanger (HSGHE) has been increasingly used in ground source heat pump systems owing to its high heat transfer performance and low cost. This study proposes a method to optimize the design and operation parameters based on the combination of response surface methodology (RSM) and multi-objective genetic algorithm (MOGA). First, a 3D numerical model simulating the thermal–hydraulic characteristics of the HSGHE was built and the reliability of the model was validated through an indoor test rig. Subsequently, some parametric studies were performed to analyze the degree of influences that each input parameter on the outputs. On this basis, the Box-Behnken design method and RSM were used to establish response surface models between the input parameters (spiral diameter, pitch, and fluid velocity) and objective functions (net heat exchange rate and thermal performance capability) and determined the interactions between them. The results showed that the spiral diameter had the most significant influence, followed by the pitch, and fluid velocity. Finally, based on the developed models, the MOGA employing the Pareto optimum solutions were used to optimize the design and operation parameters and determine their optimal combination: spiral diameter of 0.4 m, pitch of 0.1 m, and fluid velocity of 0.4 m/s. The findings of this study may assist designers in the development of high efficiency ground heat exchangers.

Effect of tube-in-tube configuration on thermal performance of coaxial-type ground heat exchanger

Among various types of ground heat exchangers (GHEXs or GHEs), a coaxial-type GHEX exhibits outstanding thermal performance owing to the geometric characteristics of the concentric tube-in-tube configuration. In this study, the unique configuration of the coaxial-type GHEX was investigated through parametric studies, which provided the energy efficient configuration. A computational fluid dynamics (CFD) model was developed and verified by using the field test results for the coaxial-type GHEX constructed in a test bed. Then, the parametric studies were conducted using the developed CFD model for various influential factors (i.e., flow rate of a circulating fluid, hydraulic diameter, and thermal conductivities of inner pipe and bentonite grout), which were selected considering the tube-in-tube configuration. The thermal performance of the coaxial-type GHEX increased with an increase in the flow rate of a circulating fluid and the thermal conductivity of bentonite grout. In addition, the inner pipes of the coaxial-type GHEX should be composed of materials with a thermal conductivity lower than 0.025 W/(m⋅K) to relieve thermal interference between the outer and inner pipes. Notably, it was concluded that the coaxial-type GHEX should be designed to secure the sufficient heat exchangeable area and retention capacity of the circulating fluid.

Thermal performance analysis of an energy pile with triple helix ground heat exchanger

A Ground Source Heat Pump (GSHP) is a renewable energy-based HVAC system that extracts or supplies heat from/to the ground via a Ground Heat Exchanger (GHE). One of the most commonly used types of GHE in GSHP systems is the energy pile. In this realm, the GSHP system with a triple helix energy pile has become the focus of attention. To this aim, a comprehensive three-dimensional transient Computational Fluid Dynamics model of the energy pile with triple helix GHE and the surrounding soil is developed. The effect of several parameters, including helix pitch, helix diameter and pipe diameter, on the thermal performance of the system, is investigated. Simulated cases are chosen using the design of experiments methodology. To evaluate the thermal performance of GSHP, an objective parameter is defined as the ratio of the load provided to the load required by the building. The sensitivity of the objective parameter to the studied parameters and surface response curves are analyzed using the response surface model. Also, the optimization process was implemented to obtain the optimum system. It is demonstrated that, the tiple helix GHE enhance the objective parameter by 24% compared to conventional helix GHE. According to the obtained results, the helix pitch has the greatest impact on GSHP thermal performance. Furthermore, the pipe thermal interaction, in addition to the surface area between GHE pipes and the surrounding soil, plays an important role in thermal performance of GSHP.

Development of a Low-Depth Modular GHX through a Real-Scale Experiment

The global energy sector is aiming to rapidly transform energy systems into those less dependent on fossil fuels to reduce their harmful effects on the climate. Although ground source heat pump (GSHP) systems are more efficient than conventional air-source heat pump (ASHP) systems, the high initial investment cost, particularly for a vertical closed-loop type ground heat exchanger (GHX), makes it difficult to incorporate them into small buildings. This paper proposes a low-depth modular GHX for reducing cost and improving the workability of GSHPs. A modular GHX is a cubical structure comprising tubes and buried using an excavator at a depth 4 m below the ground surface. This GHX is manufactured at a factory, carried by a small truck, and then installed by a small lift or a backhoe such that it can be installed in small buildings or narrow spaces at low depths underground. In this research, the performance and feasibility analyses of modular and vertical GHXs were conducted via a real-scale experiment. The results demonstrate that the modular GHX influences the workability of GSHPs by 91% during the heating period and 70% during the cooling period. In contrast to the conventional HVAC, the modular and vertical GHXs could recover the initial investment costs in 4 years and 10 years, respectively.

Analysis of influence of pipe group arrangement and heat exchanger type on operation performance of the ground source heat pump

Arrangement of pipe group and type of ground heat exchangers (GHEs) are crucial engineering design factors affecting the ground source heat pump system (GSHPS) performance. Taking an office building as the object, based on the built three-dimensional simulation platform considering heat and moisture transfer, seepage and freezing, the influence of these two design factors on the GSHPS is simulated. Results show that the influence of moisture transfer on heat transfer of GHEs is smaller. When the pipe group is arranged in staggered and square arrangements and the type of GHEs is single-U, the soil freezing degree is more serious. After 5 years of operation, the effect of pipe group with linear arrangement and GHEs with W and double-U types on improving the system performance is more obvious. Average coefficient of performance (COP) with linear arrangement increases by 14.82% and 12.10% in cooling mode, and 20.62% and 16.98% in heating mode, compared with staggered and square arrangements. Average COP with W and double-U types increases by 20.30% and 25.49% in cooling mode, and 23.77% and 29.43% in heating mode, compared with single-U type. Moisture transfer, seepage and freezing can improve the heating COP, but only seepage can improve the cooling COP.

Heat transfer rate characteristics of two-phase closed thermosyphon heat exchanger

Two-phase closed thermosyphon heat exchangers (TPCTs) have considerable potential for use in ground source heat pump systems. This paper proposes a new metal-polyethylene TPCT that uses water as the working fluid and is composed of a galvanized steel pipe and a polyethylene pipe. The heat transfer rate characteristics of the metal-polyethylene TPCT were studied using a constant-temperature water bath test bench. The experimental results show that the metal-polyethylene TPCT has the best heat transfer rate when the vacuum degree and filling ratio are 1.00 kPa and 12.5%, respectively. The average heat transfer rate increased as the temperature of the heat source and the flow rate of the cooling water increased. Finally, a sandbox test bench was built, which provided seepage and non-seepage experimental conditions for the metal-polyethylene TPCT. In the sandbox experiment, the heat transfer rate capacity of the metal-polyethylene TPCT was initially studied and compared with that of a single U-tube heat exchanger, and the heat transfer rates were similar. When the evaporation section length was 0.5 m, the heat transfer rates were 34.89 W/m and 111.75 W/m under non-seepage and seepage conditions, respectively. The experimental results show that the metal-polyethylene TPCT is feasible as a new type of ground heat exchanger.

Prospects of shallow geothermal systems for sustainable heating and cooling of buildings

Net-zero-energy buildings have the potential to mitigate climate change through reduced energy use. Shallow geothermal systems use the thermal inertia of the Earth to provide a temperature gradient between ambient conditions and underground soils. This thermal inertia can be used by heat exchangers to provide space heating and cooling during winter and summer, respectively. This paper provides a brief but broad overview of the different active and passive technologies involved in the use of heat exchangers for heating and cooling to achieve near net-zero-energy buildings. Different types of ground heat exchangers (GHEs) and heat pumps are introduced along with relevant studies of significance in this field. It has been demonstrated that different types of heat exchangers can be integrated with thermally active building envelopes and renewable energy resources to minimise building energy use. A pathway is then presented for using GHEs to realise net-zero-energy buildings.

Application and Design Aspects of Ground Heat Exchangers

With the constant increase in energy demand, using renewable energy has become a priority. Geothermal energy is a widely available, constant source of renewable energy that has shown great potential as an alternative source of energy in achieving global energy sustainability and environment protection. When exploiting geothermal energy, whether is for heating or cooling buildings or generating electricity, a ground heat exchanger (GHE) is the most important component, whose performance can be easily improved by following the latest design aspects. This article focuses on the application of different types of GHEs with attention directed to deep vertical borehole heat exchangers and direct expansion systems, which were not dealt with in detail in recent reviews. The article gives a review of the most recent advances in design aspects of GHE, namely pipe arrangement, materials, and working fluids. The influence of the main design parameters on the performance of horizontal, vertical, and shallow GHEs is discussed together with commonly used performance indicators for the evaluation of GHE. A survey of the available literature shows that thermal performance is mostly a point of interest, while hydraulic and/or economic performance is often not addressed, potentially resulting in non-optimal GHE design.

Applicability evaluation of cast-in-place energy piles based on two-year heating and cooling operation

An energy pile is a type of ground heat exchanger (GHEX) that can reduce the initial construction cost by being installed in the existing foundation structure (i.e., pile). However, despite economic benefits, energy piles have not been globally employed owing to a lack of construction cases and appropriate design methods. Therefore, in this study, a design method applicable to large-diameter cast-in-place energy piles was developed and verified by conducting long-term monitoring on the heating and cooling operation. In addition, to evaluate the applicability of the cast-in-place energy piles, their thermo-mechanical behaviors and economic feasibility were analyzed using the long-term monitoring results. Although degradation of thermal performance and significant changes in the ground temperature were observed in the heating operation, it was concluded that the proposed design method could provide reliable design outcomes. The maximum thermal stress was estimated to be 2.38 MPa during the long-term monitoring, which is approximately 8.5% of the design strength criterion for cast-in-place concrete. Finally, compared with a conventional closed-loop vertical GHEX, the cast-in-place energy piles could reduce the initial investment cost by 25.31%, and the investment payback period was evaluated to be 1.61 years.

An investigation on the environmental impact of various Ground Heat Exchangers configurations

Ground Source Heat Pumps (GSHPs) are used for space heating and cooling, where the Ground Heat Exchangers (GHEs) are used to extract or reject heat from/to the ground. GHEs come in various configurations, vertical or horizontal. Compared to Air Source Heat Pumps (ASHPs), GSHPs, albeit having a higher installation cost, achieve a better coefficient of performance (COP) and, hence, electricity savings. This reduction in consumed energy is translated to a reduction in fossil fuels and environmental “harmful” gas emissions. As the environmental impact does not lie in a single aspect, it would be useful not to stop the discussion in terms of COP and cost, but to identify whether a GSHP system is indeed a sufficiently overall greener solution. Hence, a more comprehensive investigation on the environmental impact of different types of GHEs as part of a GSHP compared to an ASHP system is attempted in the current study. A case study of a residential building with a fixed heating and cooling load is considered for moderate climate conditions. Using GLD software, a GSHP system is studied for various GHE configurations. The system undergoes a Life Cycle Analysis (LCA), with the yearly heating and cooling load as functional unit, for a direct environmental impact comparison between the GHE configurations and an ASHP system. The openLCA software is used for the application of two different methods, namely CML2001 and Eco-Indicator99 for seven and three impact categories respectively. It is concluded that the ASHP system exhibits the highest impact for all the main categories, while among the GSHP systems the vertical coaxial GHE configuration exhibits the highest impact and the horizontal GHEs the lowest, at times significantly below the ASHP impact. The largest reduction among all categories for the vertical and horizontal GHEs respectively is about 22% and 24% for the CML2001, and 17% and 21% for the Eco-Indicator99.

Prospects of shallow geothermal systems in HVAC for NZEB

Shallow geothermal systems use the thermal inertia of the earth to provide a temperature gradient between the ambient conditions and the underground soil. This thermal inertia can be used by the heat exchangers to provide space heating and cooling during the winters and summers. This paper provides a brief but broad overview of the different active and passive technologies involved in the use of heat exchangers for HVAC in order to achieve a near net zero energy building. Firstly the different types of ground heat exchangers and heat pumps are introduced along with the relevant studies of significance in this field. It has been demonstrated that the different types of heat exchangers can be integrated with thermally active building envelopes and renewable energy resources to significantly minimize the building energy use. Finally a pathway has been devised for use of ground heat exchangers to realize a net zero energy building.

Heat transfer analysis of energy piles with parallel U-Tubes

An energy pile with U-tubes is a special type of ground heat exchanger (GHE) formed by setting U-tubes into a concrete pile during the construction of building foundations. The parallel U-tubes increase heat transfer area, thereby improving energy pile performance. The analytical and numerical heat transfer models of U-tube energy piles are established in this study, considering homogenous and heterogeneous thermal properties of the pile and surrounding soil. Analytical solutions are obtained using the Laplace method and superimposition principle. The solutions of the analytical heat transfer model are verified using the numerical model with finite element method. The results provide a method for designing pile heat exchangers with U-tubes in parallel.

CFD 시뮬레이션을 이용한 모듈형 지중열교환기의 성능 인자 검토

Recently, as a measure for global warming, zero energy building has become a major issue as a means of conserving energy and reducing carbon emission in the building. In this situation, the focus on renewable energy systems has intensified as well as ongoing technology development. Especially, the ground source heat pump system (GSHP) attains high efficiency by using the annual stable ground temperature. However, it is difficult to introduce it in small-scale buildings because of initial investment costs, such as excavation costs and the installation of the GSHP. In this study, the performance factors of the low-depth modular heat exchanger were reviewed for the development of the ground heat exchanger that is relatively inexpensive and can respond to small-scale buildings compared to the vertical type ground heat exchanger. Assuming an underground heat exchanger is installed in the ground, an analytical model was built and the circulating water temperature was calculated by using the CFD program. Additionally, to find out the factors affecting the performance of the low-depth ground heat exchanger, the prediction of heat exchange rate was performed.