Underground Heat Exchanger Research Articles

Closed loop low enthalpy geothermal systems for underground heat exchange: public records, available data, implications for planning

[Article in Italian] Impianti geotermici a bassa entalpia a circuito chiuso per il geoscambio nel sottosuolo: registri pubblici, dati disponibili, implicazioni per la programmazione

Badanie efektu dławienia w pożarze tunelu

As per the emergency ventilation strategy, air velocity of 3 m/s in case of the longitudinal ventilation is sufficient for smoke control in all fire conditions. Numerical experiments were carried out with FDS software to estimate the numerical value of the critical velocity. Numerical models were realized in 0-6% slope tunnels with a 1% step for 5, 10, 20, 30 and 50 MW fires for four types of fuel: gasoline, diesel fuel, oil and firewood. The paper notes that the dynamic pressure induced by a strong fire is much higher than the static pressure of tunnel jet fans. As a result, following the algebraic summation of positively-directed ventilation flows and the negatively-directed flows induced by fire, an intense back layering occurs, which casts doubt on the suitability of the specified emergency ventilation strategy when designing the fire ventilation. The critical ventilation speed of 3 m/s cannot cope with the traction caused by fire, expressed by the ascending movement of the high-temperature and low-density combustion products. The paper discusses the numerical modelling results with an adiabatic underground heat exchange model and presents typical tunnel fire modelling plans, which correspond to an inclined tunnel for ascending and descending ventilation flows as well as a horizontal tunnel. The article gives the regularities obtained by the numerical models of changes in the variables of average air temperature and density, average carbon monoxide, average carbon dioxide and soot concentrations. According to the emergency ventilation strategy, critical velocity is an important value and a major determinant of back layering prevention in sloping tunnels. Although many papers have been devoted to this problem, the obtained results differ much. The present paper shows that strong fires induce much greater dynamic pressures than the static pressures of the tunnel jet fans are. Consequently, the flows caused by these forces, as they move in different directions, following their algebraic summation, cause a strong back-layering in case of positive ventilation flows, i.e., when the ventilation flow is descending and the fire seat is found at a lower point compared to the air supply portal. The new results can be used to develop fire ventilation plans as well as life-saving and emergency control solutions in the operating tunnels for personnel and rescuers.

Configuration method for medium-deep ground source heat pump system considering renewable energy consumption and smart grid interaction

Medium-deep ground source heat pump (GSHP) systems have the advantages of low carbon, high heat exchange intensity, good thermal storage capacity, and intermittent operation characteristics, which are conducive to renewable energy consumption and grid demand response. The source-side parameters affect the configuration of underground borehole engineering, heat pump units, and other equipment. However, current research mainly focuses on the underground heat exchanger modeling, underground heat exchange, and system operation control, while there are few studies on multi-factor ground source-side parameter design and optimal configuration. In this study, the indicator analyzing the unsteady features and thermal balance state of medium-deep geothermal resources was introduced, and the source-side water temperatures were optimized based on the dynamic characteristics of the geothermal and building sides. A simulation configuration methodology for the source-side flow rate and storage capacity was developed considering the electricity prices, life-cycle cost (LCC), and grid interaction. The optimization of the ground source-side design temperature, flow rate, and capacity parameters of residential and office building heating scenarios were analyzed in northern China, respectively. The results showed that the borehole inlet and outlet temperature can be designed to 5.5 °C/17.5 °C under the annual sustainable state, and the optimal flowrate can be set to 65.77 m3/h, to achieve the lowest cost and highest heating season guarantee rate for the residential scenario. For the office building with optimal system and storage capacity, the average annual LCC is 192,000 yuan/year, with the cumulative peak shaving of 55.8 %, and an increase of 103.3MWh renewable electricity consumption.

Thermal Properties of Backfill Material for Underground Heat Exchange Applications

We are investigating the effect of the thermal properties of backfill material on the efficiency of underground thermal energy systems. We have used aluminum to increase the thermal properties of clay-bentonite and to investigate its potential as a backfill material in underground heat exchange applications. The measurements of thermal properties were made at room temperature using the Transient Plane Source (TPS) technique. The measurements were based on an infinite homogeneous medium using different composites of aluminum and clays in amounts of aluminum concentration ranging from 6 to 25 percent of the sample’s weight. The results confirmed an overall relative increase of 170% and 135% in the thermal conductivity and diffusivity, respectively as the Al content increased by 25%. The estimated average of the thermal transmittance (U-value ) of these samples was in the range from 0.40 to 1.1 W/m2K, which was directly proportional to the relative increase in the measured properties. Our findings can be applied to underground thermal energy systems, ground source heat pumps, and solid dielectric underground transmission lines.

Energy and exergy analysis with environment benefit of the underground cooling system of the chiller plant

Using an underground heat exchanger as an alternative to a wet cooling tower can offer several benefits, particularly in terms of energy efficiency and environmental impact. Heat exchangers and cooling towers are chosen depending on various factors, such as the application size, local climate conditions, and economics. The purpose of this study is to compare the use of an underground heat exchanger with a wet cooling tower for cooling purposes. The underground heat exchanger and wet cooling tower provide cooled water for vapor compression chillers. The comparison is conducted on technical, economic, and environmental grounds. Research has been conducted in the Mashhad region, where water resources are limited and cooling demands are high during the summer. In this context, Ferdowsi University of Mashhad is selected as a case study. Energy and water consumption are calculated and compared by simulating and modeling both methods of producing cooled water and the refrigeration cycle. However, improving the alternative mode's exergy efficiency was discussed for all system components. The findings reveal that using the underground heat exchanger results in a 14% reduction in energy consumption and eliminates water consumption. The current water-cooled compression chillers consume approximately 1.95 cubic meters of water per square meter of the building daily.

Design of a cooling system from underground thermal energy storage (UTES, Underground) Thermal Energy Storage) based on experimental results

Geothermal energy is a renewable and clean source that has been used for electricity generation in some countries since the 50s, the main characteristic to be used in this application is that the subsoil must have a high temperature geothermal resource (+150 °C). However, it can also be used in applications such as air conditioning in places where the temperature is around 30°C; In Europe alone, there are more than one million thermal installations operating by harnessing geothermal energy. The objective of the work was to design a cooling system from the storage of underground energy, for that, it is essential to know the variation of subsoil temperatures during a certain period of time. For this purpose, sensors were used that were installed at different depths and by means of an Arduino, information of a whole year was stored; so that these data are as representative as possible of the energy storage conditions and the changes depending on the seasons that pass. Additionally, the characteristics of the soil (conductivity, humidity and composition) were taken into account, where the equipment is intended to be installed in subsequent works. For the determination of the necessary cooling load, the design requirements of the ASHRAE standard were used and for the design of the underground heat exchanger, references of designs recommended through experimental tests in other research works are included, together with internal fluid methodology and one-dimensional heat transfer. It includes elements that can help improve the dissipation of energy into the subsurface and maintain transfer properties as stable as possible. This design is designed for the air conditioning of a classroom of normal dimensions that are used in the University and therefore avoid the energy consumption of conventional air conditioning equipment.

Seasonal performance of an energy pile heat pump system and prediction of building thermal load

Ground source heat pumps (GSHPs) are widely used for cooling and heating buildings, which can reduce the burning of fossil fuels and contribute to the reduction of carbon emissions. Energy piles, as underground heat exchangers, have advantages over conventional buried borehole pipes, saving drilling costs and accelerating the construction period. However, few studies have focused on the actual operation of an energy pile heat pump system, and the prediction of the building thermal load can reduce the instrument arrangement and test recording time. Hence, a heat pump system was built in this study to serve a single lab using energy piles with a cap structure as the energy supply, and summer cooling and winter heating tests were conducted. Under the test conditions in this study, the GSHP had a faster start-up speed and less power consumption than the air source heat pump (ASHP), and the average coefficient of performance (COP) of the energy pile heat pump system was 3.78, which is 40.0 ∼ 51.2% higher than that of the corresponding ASHP (the normative reference COP is 2.50 ∼ 2.70). The methods of higher-order surface fitting and an artificial neural network model were both suitable for predicting building thermal load.

Numerical simulation of temperature field around buried pipes of ground source heat pumps based on mathematical models

The author established a physical and mathematical model for the heat exchange of a ground source heat pump buried heat exchanger under the co-operation of heat and seepage, including the soil and fluid inside the pipe surrounding the heat exchanger. Using ANSYS finite element APDL language for programming, based on the line heat source model, simulate the temperature field around the vertical double U-tube underground heat exchanger, the effects of soil thermophysical properties, temperature outside the pipe, soil type and backfill material on soil temperature field were obtained through simulation analysis. The experimental results indicate that, the changes in soil temperature are also significant with different backfill materials. Therefore, it is necessary to conduct serious research and optimization on backfill materials, develop new types of backfill materials, improve backfill construction techniques, and conduct in-depth research by combining theoretical analysis with practical engineering to ultimately find efficient and economical backfill materials. The change in equivalent pipe diameter has little effect on soil temperature, and the linear heat source model is used for calculation without causing significant errors. It has been proven that the soil itself has strong resilience and has reference value for the design of buried heat exchangers in practical engineering.

Research on thermal performance of ground source heat pump based on artificial neural network predictive model

The long-term operation of ground source heat pump systems can affect the soil environment, causing a decrease in heat transfer performance of underground heat exchangers. Based on a real project, this study aims to determine the actual soil thermal property and propose optimization schemes. A combination of 1D-3D heat transfer model and artificial neural network model was proposed. This research confirms the possibility of using the outlet water temperature as input to predict the soil thermal property parameters. Through model calculations, the soil comprehensive thermal conductivity of 1.78 W/(m·K) and comprehensive heat capacity of 1909.8 J/(kg·K) was determined. On this basis, the impacts of burial depth, spacing, and initial ground temperature on the heat transfer of the underground heat exchangers was investigated. The results showed that the heat exchange is logarithmically related to the depth and spacing of the boreholes, but the heat exchange decreases as the initial ground temperature increases, following a quadratic function relationship. For the current hourly cooling demand, it is recommended to have the pile spacing of 5.5 m and the depth of 110 m. With the addition of a cooling tower that takes on 2/3 cooling load, the system's average coefficient of performance can reach 5.62, resulting in a 19.6 % improvement.

Improving thermal storage of energy screw pile groups with phase change materials

To achieve affordable housing in a carbon-neutral society, new buildings require a dual-purpose approach that comprises efficient construction and a green energy supply. Energy screw piles [1, 7] meet this demand as they combine the agility of screw pile drilling with the capability of extracting clean shallow geothermal energy. Moreover, the screw piles can be filled with phase change materials (PCM) to provide latent thermal storage. Studies regarding the use of PCMs as borehole backfill in a ground source heat pump system (GSHP) conclude that PCM implementation can improve GSHP performance. However, most commercially available PCMs have low thermal conductivities (λ) which undermine heat exchange rates [5, 10]. Besides, using PCMs as a composition of a concrete energy pile can reduce the pile structural performance since PCMs usually have a low mechanical capacity [2, 8]. The authors recently introduced PCM-sand mixtures as a core in the central hollow part of an energy screw pile, which does not impact the pile structural capacity [6]. The mixtures with a higher PCM content benefit the pile heat exchange through its latent heat, but only when the PCM does not reduce the mixture’s λ. To overcome this problem, this work tests a new underground heat exchange system where instead of mixing the PCM in the energy screw pile filling material, regular screw piles (i.e., without the heat exchange tubes) are filled with pure PCM, acting as thermal storage piles. A numerical model built via COMSOL [4] is used to evaluate how this combination of screw piles performs thermally when supplying/rejecting heat for a GSHP system operating for a whole year.
 This work uses a validated numerical model [3, 9] to simulate a grid of evenly distributed screw piles, where Energy Piles (EP) and Thermal Storage Piles (TSP) are positioned interspersed, evenly spaced 0.7 m apart. Inside the EPs, an U-loop pipe is inserted in the pile steel case and the remaining is filled with grout. In contrast, the steel case of the TSPs is filled with only PCM [11]. All other material properties and the screw pile geometry are based on [1]. The thermal load is based on the design of a building located in Melbourne, Australia (Figure 1(a)). The model considers the hourly operation of a GSHP for one year, calculating the GSHP Coefficient of performance (COP) at every time step and adjusting the ground thermal load. For comparison, two simulations (one where the TSPs are filled with PCM and a reference where they are filled with only air) are undertaken. Besides the energy stored, the COPs of both cases are compared as a relative increase/decrease percentage (COPchange = (COPPCM / COPReference) – 1).
 Figure 1(b) presents the energy stored by the PCM and its corresponding percentage in a solid state over the simulation. As the EPs reject heat underground due to GSHP summer operation for cooling, the TSPs temperature rise and store sensible heat, until they reach their phase change temperature (Tpc), when the PCM melts to store significantly more heat energy in a shorter time window due to latent heat. Conversely, the PCM solidifies when EPs extract more heat than reject due to GSHP winter operation (heating) for a certain period. The PCM melts again on the following summer, restarting the whole process. The peak energy stored in the TSPs is 190.3 MJ/m3. Even though the thermal load varies hourly, the PCM phase change process happens without immediate responses to the load variation. Figure 1(c) presents the resulting change in the hourly COP considering cooling (CCOP) and heating (HCOP) from implementing PCM in the TSPs, compared to the reference scenario (empty TSPs). The extra heat stored by the PCM lowers the circulating fluid temperature while the latent heat is engaged (months 2 to 7 and 12), which results in a higher CCOP and a lower HCOP. By plotting a monthly average of the COPchange (grey markers), it is clear that the impact of the PCM on the COP is positive when cooling is dominant and slightly negative when heating becomes dominant while the latent heat is engaged. When the PCM is implemented in the EP [6, 10], the heat exchanged by the EP drops once the phase change process is over due to the low λ of the PCM, but by positioning it outside the EP the higher CCOP is sustained even after the phase change ends. However, lowering the fluid temperature increases CCOP while lowering HCOP at the same time, which can harm thermal performance if not designed properly. The results underlie the thermal storage potential available not only in screw piles, but also in any hollow pile foundation, by simply implementing PCM in the hollow case.

Analysis of a Hybrid System Solar Chimney-Air Soil Heat Exchanger for Natural Ventilation

The current study looks at a hybrid passive cooling system that combines a solar chimney with an earth-to-air heat exchanger (EAHE) usually called Canadian Well. Numerous experimental and numerical examinations with various applied radiation heat fluxes were carried out to evaluate its ability to cool a room. Glass temperature, wall temperature, air flow mean temperature, hourly rate of air exchange (ACH), outlet airflow velocity, and rate of air mass flow were determined experimentally and numerically, and validated against previously published experimental and analytical works. It was found that the chimney's operation is dependent on the radiation intensity. The EAHE has reduced the room's temperature by improving exchanges with the solar chimney. The comparison of experimental and numerical data for different radiation intensities reveals that the best diameter of the tube of the underground heat exchanger for the proper operation of our system is d = 0.04m. The efficiency of our system increases as the radiation increases, causing an increase in the temperature of the absorber, which influences the air temperature in the chimney.

Cooling of a PVT System Using an Underground Heat Exchanger: An Experimental Study.

In the recent decades, the researchers have been focused on the use of photovoltaic thermal (PVT) systems that provide the best performance and cooling for the photovoltaic panels. In this study, a PVT system consisting of a monocrystalline PV panel and a spiral heat exchanger was connected to an underground heat exchanger that is buried at a depth of 4 m below the surface of the earth. The procedure of the current study can be considered the first of its kind in the Middle East and North Africa region (based on the researchers' knowledge). The study was carried out on agricultural land in Baghdad-Iraq during months of July and August-2022, which are considered the harshest weather conditions for this city. The heat exchanger consists of a copper tube with a length of 21 m and formed in the shape of 3U, and it was buried in the earth and connected with a PVT system. The results of the study showed that the site chosen to bury the heat exchanger (4 m deep) has a stable soil temperature at 22.5 °C. From various volumetric flow rates, a flow rate of 0.18 l/s was selected which is considered the highest flow rate that can show vibration in the PVT system which may harm the system. The practical measurements showed that the largest difference in the surface temperatures of standalone PV and PVT was around 20 °C in favor of the latter. The electrical efficiency of the studied PVT system also increased to outperform the standalone PV system by 127.3%. By comparing the results of the current study with studies of water-cooled PVT systems from the literature, it is clear that the proposed system is feasible and has an acceptable efficiency in such harsh weather conditions tested during the experiment.

Development of CFD simulation model of earth air heat exchanger for space cooling of a 36 M2 house in tropical climate Banda Aceh, Indonesia

The global warming makes the ambient temperature hotter and greater efforts are made to reach a comfortable temperature. The continuous use of air conditioners that consume electricity is also unsustainable for the surrounding environment. Several studies on thermal comfort have been conducted by various researchers. Earth-air heat exchangers (EAHE) with air-working fluids can be used as a passive contribution to reduce building energy requirements for heating or cooling purposes. It should be noted that there is very little information in the literature on the development of a CFD (Computational Fluid Dynamic) simulation model of an EAHE for space cooling of a 36 m2 house in a tropical climate, such as Banda Aceh, Indonesia. Therefore, this study aims to examine the performance of EAHE with several variations in design parameters, such as pipe length, pipe diameter, number of pipe bends, and the type of soil where the EAHE is installed, as well as the thermal regime of a 36 m2 house either with or without the use of EAHE. The simulation in this study was conducted with CFD ANSYS Fluent software. The inlet air temperature of EAHE was set to be the same as the ambient air temperature, namely 31.4oC. The simulation results reveal that for variations in pipe length, the highest drop in outlet air temperature was yielded by the 47 m pipe length, which is 26.8°C. In which an increase in pipe length causes a decrease in air outlet temperature. The variation in pipe diameter does not significantly affect the outlet air temperature. Where the average air temperature drop at the EAHE exit is 0.046oC. The variation in number of turns shows that the drop in outlet air temperature is identical, namely 28.2°C, despite the fact that their pressure drop values are different. In addition, it was found that the performance of EAHE buried under different types of soil is distinct. The highest drop in outlet air temperature was generated when the EAHE was buried in silty soil, namely 26.1°C. A case study on a 36 m2 house shows that the utilization an underground heat exchanger can reduce the house’s indoor temperature by 2°C, with an average house temperature of 30.4°C compared to that with a natural ventilation.

Thermal analysis of the hybrid HVAC unit using interpolant function

This study aims to cover the thermodynamic aspect of a geothermal site using the Hermite interpolant. The energy distribution, determination of the system exergy and the energy ratio are some of the factors investigated thoroughly. The steady flow energy equation was used to determine the design parameters related to the hybrid heating, ventilation, and air condition (HVAC) system. For the underground heat exchanger, it was assumed that the heat capacity ratio, R, was small in magnitude. The energy ratio estimated in the corridor of the lecture hall increased by 17% as compared to the hall area. The exergy destruction of the system was estimated to be 25%, whereas the maximum exergy gain computed at the secondary side of the geothermal unit, followed by the second fan coil, the first fan coil, and the radiator. The calculated heating rate was found to be 25.273% lower than the required sensible heat gain. The installed unit could attain the maximum energy ratio of 4.031 if the convective losses across the building material are minimised.

The Green’s function based thermoelastic analysis of spherical geothermal tanks in a semi-infinite domain

When an underground heat exchanger is subjected to a surface load on the ground and temperature change inside, the stress transfer between the thermal tank and the earth may cause the deformation and destruction of the tank. The bi-material thermoelastic fundamental solution of two-jointed dissimilar half-spaces is applied to elastic and thermal analysis of spherical heat storage tanks, where the continuity equations at the bi-material interface are satisfied. Using Hadamard’s regularization in the x3 direction, the two-dimensional bi-material thermoelastic fundamental solution can be obtained. By changing the material constants, the fundamental solution for a semi-infinite domain or an infinite domain with a single material can be recovered. In general, the storage tanks and soil exhibits different thermal and mechanical properties. A dual equivalent inclusion method (DEIM) is proposed to simulate the material mismatch of thermal conductivity and elasticity with continuously distributed eigen-temperature-gradients and inelastic eigenstrains on the tanks, respectively. Using the analytical domain integrals, no mesh is required for inhomogeneities. Due to the boundary effects and inhomogeneity interactions, the eigen-fields are expanded at the center of each inhomogeneity using the Taylor series with tailorable accuracy. The DEIM is verified by the finite element method and demonstrated by the geothermal applications using uniform, linear, or quadratic orders of eigen-fields. For a spherical heat exchanger in an infinite homogeneous domain, DEIM provides the exact solutions of the thermoelastic fields for a uniform heat source and a uniform far-field heat flux field.

Performance Analysis of Low-Capacity Water–LiBr Absorption–Cooling Systems Using Geothermal Heat-Sinks in Hot Climates

This paper addresses the use of a geothermal heat-sink to remove the heat released in domestic-sized single and double-effect water–LiBr absorption chillers operating in hot climates. This study is the continuation of a previous work, which demonstrated the operational constraints of these absorption chillers working in hot Algerian climate-zones. After localizing the non-operation zones for both systems, the thermo-physical properties of the soil at several depths are investigated for the implementation of the underground heat-exchanger. This heat-exchanger is connected to the condenser and the absorber of both systems, to supply cooling water at inlet temperatures of 33 °C in hot climate conditions, with ambient temperatures varying from 38 °C to 42 °C. The results show a steady operation for both absorption chillers in climate conditions which had not previously allowed the two systems to operate in water or air-cooled modes. A maximum coefficient of performance of 0.76 and 1.25 is obtained for single- and double-effect absorption cycles, respectively, with chilled water at 7 °C. The underground-tube length required is between 4.5 and 18 m, depending on the absorption-cycle configuration and the temperature of the chilled water.

Laboratory assessment of corrosion rate of carbon steel ground heat exchangers

Abstract. The materials used in the manufacture of geothermal heat exchangers for shallow geothermal applications play an important role in the overall system performance, especially if grout is not being used to seal the boreholes in which the heat exchanger is installed. The subject of this study is the durability evaluation of a vertical coaxial ground heat exchanger made of steel that is coupled directly to the ground. This solution minimizes the thermal resistance between the heat exchanger and the ground, but presents the important drawback of removing any protection toward the surrounding environment Among the materials proposed for manufacturing such vertical geothermal heat exchanger, carbon steel is suitable and have potential, due to its low cost and high thermal conductivity. The main disadvantage of this material is that it is strongly subject to corrosive attack, according to the chemo-physical properties of the underground. This study investigated the corrosion behaviour of carbon steel used in an experimental underground heat exchanger and assessed its durability over time. Corrosion rate of steel samples were measured in the laboratory by weight loss method after exposure over a specified period in a selected ground medium. Different ground conditions were tested, resulting in different densities and moisture contents of ground samples collected on the field. Based on the results, the corrosion rate of carbon steel is evaluated as a function of water content and rate of ground compaction. This information has allowed to advance more accurate quantitative forecast of the expected operational life of installed geothermal exchangers and their safety over time.

Efficiency Improvement of Photovoltaic Solar Modules by Cooling Using an Underground Heat Exchanger

Abstract Overheating of solar cells under normal operational conditions highly reduces their energy harvesting efficiency and produces additional problems related to thermal cycling and performance degradation of the modules. In this paper, a novel cooling system for solar photovoltaics, using the underground as a heat sink, is proposed, theoretically described and experimentally validated. A prototype of the technology (including a single-axis sun tracking mechanism) has been designed, manufactured, and rigorously tested in outdoor conditions during summer 2021 in Spain, under different environmental conditions. The excess heat is removed from the backside of the solar module by a close-loop and single-phase cooling system and then dissipated in the underground, which is at a constant temperature of about 16 °C at relatively low depths at the location where tests were performed. A single U-shaped copper tube, 18 mm in diameter, immersed in a 15.5-m-deep borehole naturally filled with water, is used as an underground heat exchanger. As a consequence of the reduction of the cooled module temperature, its net power generation is significantly increased. A promising improvement of the net power generation of the cooled solar module up to 12.4% has been measured for a coolant flowrate of 1.84 l/min per square meter of solar module, proving the technical feasibility of the approach. In addition, a dependency of the power gain with the pump efficiency, the global radiation, and ambient temperature has been observed.

Temperature damage regularity of granite based on micro-inhomogeneity

Enhanced geothermal system (EGS) is the primary means during Dry Hot Rock development. It is necessary to build an underground heat exchange area during its construction, and the temperature of underground rock will change significantly, thus, the mechanical properties of those rocks underground will be affected. In order to judge whether the mechanical properties under temperature are related to the crystal structure of granite, we firstly used the crystalline rock heterogeneity coefficient H to describe the crystal structure of granite. Then, the discrete element software was used to construct the GBM equivalent crystalline model and the thermal temperature field coupling model. Finally, the temperature effect test was carried out to explore the law of heterogeneity coefficient H and damage and fracture development. The results show that: 1) the variation of granite heterogeneity coefficient H and temperature will lead to the decline of mechanical properties of rock samples. 2) At the same temperature, the damage value D increases with the increase of the H value. This phenomenon is more apparent when the temperature is greater than 400°C. 3) The microcracks caused by temperature change are mainly tensile. The H value increases the number of microcracks in the crystal. 4) The damage phenomenon caused by temperature change will be affected by heterogeneity. When the temperature is high, the crystal will denature, and the stress concentration caused by heterogeneity is easier to be reflected.

Operation optimization of the coaxial deep borehole heat exchanger coupled with ground source heat pump for building heating

• The transient numerical model of the coaxial heat exchanger is established. • Operation optimization method for the heat pump system is developed. • The underground heat transfer model is validated using test data from literature. • The operation of the system before and after optimization is compared. Geothermal is considered to be one of the most promising renewable sources for district heating. Ground source heat pump systems coupled with the coaxial deep borehole heat exchanger have been widely applied because of their high efficiency. However, in order to reduce the total energy consumption of the system, there are few studies on the optimization of the water flow rates in the coaxial deep borehole heat exchanger by time to meet the load change during the heating season. In this paper, an optimization method for the two flow rates which are set at two time periods respectively every day in the coaxial deep borehole heat exchanger during the operation of the ground source heat pump system is proposed. The application of the method is to determine the applicable flow rates when the temperature of the next day is predicted. Space heating of a building (located in Tianjin, China) is taken as a scenario, comparisons were carried out on system operation before and after optimization, which shows that the method has a good effect on the energy-saving operation of the system. After optimization, the total power consumption of the system is reduced, the total performance coefficient of the equipment as well as the temperature difference between the inlet and outlet of the underground heat exchanger is increased, and the trend of flow rates changes with time is the same as the indoor and outdoor temperature difference.