Experimental Verification to Estimate Ground Thermal Conductivity Using Spiral Coil Type Ground Heat Exchangers

This paper presents an experimental study on the evaluation of thermal response of a spiral coil type GHE （ground heat exchanger）. This GHE was installed on partially saturated landfill ground that was composed of silt and clay in the runway area of Incheon International airport. TRT （thermal response test） was conducted for more than 65 hours under continuous operation conditions. Ground thermal conductivity was derived based on line source theory, which has usually been found to be appropriate for line type GHEs such as U, W and 2U types. A reasonable method to derive ground thermal conductivity using the infinite line source theory for a spiral coil type GHE was introduced. Ground thermal conductivity from the TRT using spiral coil type GHE was compared with those from the analytical equivalent model of ground thermal conductivity.

Advances in ground heat exchangers for space heating and cooling:Review and perspectives

As a renewable energy source,geothermal energy has been widely used to provide space heating and cooling for buildings.The thermal performance of ground heat exchanger(GHE)is significant for the operating efficiency of the ground source heat pump(GSHP)systems.This paper presents a comprehensive review of developments and advances of three kinds of GHE,including vertical borehole GHE(VBGHE),Pile GHE(PGHE),and deep borehole GHE(DBGHE)which are currently popular in larger GSHP systems.Firstly,analytical models proposed to ana-lyze heat transfer process of VBGHE with different geological conditions are summarized,such as homogenous or heterogeneous ground,with or without groundwater advection.Numerical and short-time step models and measures to improve GHE thermal performance are also reviewed.Secondly,a summary of research advances in PGHE is provided,which includes the heat transfer models of PGHE,the effects of geometric structure,oper-ation modes,pile spacing,use of phase change material(PCM),thermal properties of PCM,thermo-mechanical behavior and/or thermal performance of PGHE.The effects of groundwater flow direction and velocity on PGHE are also summarized in brief.Lastly,models of three kinds of DBGHEs,i.e.,deep coaxial GHE(DCGHE),deep U-bend GHE(DUGHE)and super-long gravity heat pipe(SLGHP),are reviewed.The physical bases of the dif-ferent analytical models are elaborated and also their advantages and disadvantages are described.Advances in numerical modelling and improving numerical model calculation speed of DCBHE,DCBHE array,and DUBHE are summarized.The review provides a meaningful reference for the further study of GHEs.

Arrangement Strategy of Ground Heat Exchanger with Groundwater

The orientation strategy of side pipe and the heat transfer performance of six ground heat exchangers(GHEs) were optimized by numerical simulation,with soil being treated as a porous medium.An experiment on the heat transfer of four GHEs was carried out in 2010.Results indicate that the velocity field is disturbed by GHEs.The optimal orientation strategy of side pipe is that the upward pipe is located upstream and the downward pipe downstream.The space between GHEs should be appropriately adjusted,depending on the direction and flow velocity.Groups of GHEs should be installed perpendicular to the mainstream in a single row,but if the acreage does not meet the requirements,GHEs should be installed in staggered multiple rows.Fewer GHEs parallel to the mainstream strengthen the heat transfer.Moreover,numerical results agree well with the test data,with the maximum relative error being less than 7.7%.

Numerical Simulation of Heat Transfer Characteristics of Horizontal Ground Heat Exchanger in Frozen Soil Layer

A simplified numerical model of heat transfer characteristics of horizontal ground heat exchanger (GHE) in the frozen soil layer is presented and the steady-state distribution of temperature field is simulated. Numerical results show that the frozen depth mainly depends on the soil′s moisture content and ambient temperature. The heat transfer loss of horizontal GHE tends to grow with the increase of the soil′s moisture content and the decrease of ambient temperature. Backfilled materials with optimal thermal conductivity can reduce the thermal loss effectively in the frozen soil. The applicability of the Chinese national standard “Technical Code for Ground Source Heat Pump (GB 50366-2005)” is verified. For a ground source heat pump project, the feasible layout of horizontal GHE should be determined based on the integration of the soil′s structure, backfilled materials, weather data, and economic analysis.

Field evaluation of a ground-coupled heat pump in hot-summer and cold-winter regions

The experimental performance of small-sized ground-coupled heat pump (GCHP) is researched intensively. However, there are little data documenting the operation performance of existing large-sized GCHP system. We presented the actual performance measurement of a GCHP installed for apartment buildings in Wuhan, Hubei province, P. R. China. The system was constructed with a closed vertical typed ground heat exchanger with a total pipe length of 32 000 m. During one year, various operating parameters were monitored, including the outdoor temperature, the flow rate, the electrical consumption, and the water temperature. The seasonal coefficients of performances of the heat pumps and the system based on the measured data were found to be 4.01 and 2.96 in the cooling season, and 3.54 and 2.86 in the heating season, respectively. The GCHP system was more economical than the air-source room air conditioner in the energy efficiency which was increased by 29% in cooling mode and 50% in heating mode. There was an obvious heat imbalance of soil between the injection rate and the extraction rate in the residential GCHP system operation.

Thermal performance of a single U-tube ground heat exchanger:A parametric study

In this research,the thermal performance of a single U-tube vertical ground heat exchanger is evaluated numerically as a function of the most influential flow parameters,namely,the soil porosity,volumetric heat capacity,and thermal conductivity of the backfill material,inlet volume flow rate,and inlet fluid temperature.The results are discussed in terms of the variations of the heat exchange rate,the effective thermal resistance,and the effectiveness of the ground heat exchanger.They show that the inlet volume flow rate,inlet fluid temperature,and backfill material thermal conductivity have significant effects on the thermal performance of the ground heat exchanger,such that by decreasing the inlet volume flow rate and increasing the backfill material thermal conductivity and inlet fluid temperature,the outlet fluid temperature decreases considerably.On the contrary,the soil porosity and backfill material volumetric heat capacity have negligible effects on the studied ground heat exchanger’s thermal performance.The lowest inlet fluid temperature reaches a the maximum effective thermal resistance of borehole and soil,and consequently the minimum heat transfer rate and effectiveness.Also,multilinear regression analyses are performed to determine the most feasible models able to predict the thermal properties of the single U-tube ground heat exchanger.

Research on ground heat exchanger of Ground Source Heat Pump technique

Ground Source Heat Pump technique and its operating principle are described in this paper. Ground heat exchanger is the key technique of ground source heat pump and its patterns are discussed. Software is helpful to design ground heat exchanger. A project of Chinese Ground Source Heat Pump is introduced and its market is more and more extensive.

Numerical simulation for thermal response test performance in closed-loop vertical ground heat exchanger

In this study, a series of numerical analyses was performed in order to evaluate the performance of full-scale closed-loop vertical ground heat exchangers constructed in Wonju, South Korea. The circulating HDPE pipe, borehole and surrounding ground formation were modeled using FLUENT, a finite-volume method (FVM) program, for analyzing the heat transfer process of the ground heat exchanger system. Two user-defined functions (UDFs) accounting for the difference in the temperature of the circulating inflow and outflow fluid and the variation of ground temperature with depth were adopted in the FLUENT modeling. The thermal conductivities of grouts (cement vs. bentonite) measured in laboratory were used as input values in the numerical analyses to compare the thermal efficiency of the cement and bentonite grouts used for installing the closed-loop vertical ground heat exchanger. A series of numerical analyses was carried out to simulate in-situ thermal response tests performed in the construction site. From the comparison between the in-situ thermal response test results and numerical simulations, the average thermal conductivity of the ground formation in the construction site is back-calculated as approximately 4 W/mK. This value can be used in evaluating the long-term performance of the closed-loop vertical ground heat ex changer.

Measurement investigation on the feasibility of shallow geothermal energy for heating and cooling applied in agricultural greenhouses of Shouguang City: Ground temperature profiles and geothermal potential

The use of electrical energy for heating and cooling systems to control the temperature in greenhouses will lead to high production and product costs.To solve this problem,shallow geothermal energy as a local source of energy could be applied.In this study,a measurement model,the distribution profiles of temperature,and a preliminary assessment of the geothermal potential in the shallow zone at depths of 0.1 m to 3.6 m in Shouguang City,Shandong Province,eastern China were presented.The measurement results showed that the annual average temperature at depths of 0.1–3.6 m ranged from 13.1℃ to 17.6℃.Preliminary assessment results of the geothermal potential showed that the daily average temperature difference between the air and at depths of 1.5–3.6 m was mainly from 10℃ to 25℃ during the winter months and between-15℃ and-5℃ during the summer months.Therefore,the heating systems could operate during January,February,November,and December.In May,June,and July,the cooling systems could be applied.Moreover,the measurement model gave good stability results,and it could be used in combination with the monitoring of the groundwater table,a survey of the thermal conductivity of the soil,climate change studies,which helps reduce unnecessary time and costs.

Potential of GSHP coupled with PV systems for retrofitting urban areas in different European climates based on archetypes definition

According to the recent policies regarding energy use in buildings and the need of retrofit strategies,the aim of this work is to support policies concerning the installation of ground source heat exchangers in urban and historical areas,raising the awareness on the potential energy saving achievable with optimal sizing and limited impact on the urban environment.Archetypes have been developed distinguishing among existing and historic buildings,focusing on single-family terrace houses,which are the typical residential buildings in European historic centres.A methodology for the optimal sizing of ground source heat pumps,eventually considering dual-source system or air system has been developed combining simulations of a photovoltaic system to estimate the self-sufficiency and the self-consumption for five orientations of the building.Extreme results have been obtained for warm cli-mates,with negligible heating energy demand and possibly free cooling systems rather than traditional cooling systems needed in wintertime.Penalty temperature was acceptable despite unbalanced energy demands.With proper inclination,photovoltaic systems could provide up to 40%of self-sufficiency share also in northern cli-mates.An energy-economic analysis was carried out obtaining a variety of cases representing a general overview of the European building stock and the potential benefits achievable in terms of renewable energy share,energy savings and economic investments needed to be extended to simulations at urban scale.

Application of the ground cooling system for rural houses air conditioning in northeastern China

This study developed an appropriate ground cooling system that can be used for indoor cooling during summers in rural houses in northeastern China.To apply this system,a traditional rural house was selected and a heat exchanger was designed(PE pipe,32 m in length),manufactured,and installed according to the cooling load(2206 kJ/h)of one room of the experimental house.According to the experiment,the temperature of Room 1 equipped with the cooling system was 3℃ lower than that of Room 2 without the ground cooling system during the same time period.The temperature reduction effect of this system was confirmed through this study.When the ground cooling system was used in rural houses in Northeast China,it was found that a burial depth of 1.5-2.0 m for the ground heat exchanger is appropriate for considering workability and frost line.The use of the ground cooling system for indoor cooling during summers for rural houses in the northeastern region of China would reduce not only environmental pollution and the use of fossil fuels but also ease farmers off a certain amount of financial burden.