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Based on a quasi 3-D steady model inside and a 2-D unsteady model outside of the borehole of ground heat exchanger(GHE), the relationship between the heat exchange rate per depth of borehole and main parameters (center-to-center distance of two branch pipes, radius and depth of the borehole) has been analyzed. The rational values of those design parameters with considering the heat exchange rate per depth of borehole are obtained. The dynamic simulations of underground temperature field (UTF) of single-pipe and pipe-group of U-tube GHE have been conducted by ANSYS. The change rules of UTF are obtained when ground source heat pump(GSHP) has experienced a complete cooling and heating cycle during one year. The simulation results of a single-pipe show that the action radius of borehole is 1.9 m, i.e., it is reasonable that the borehole center-to-center distance is 3.8 m. The simulation results of pipe-group indicate that temperature of the soil center encircled by four boreholes at the beginning of heating in winter is higher than the initial temperature of soil, but it is in opposite for cooling in summer, which is beneficial to the efficient operation of GSHP. This work can provide guidance for the rational design and operation of GSHP. • Relation of heat exchange rate per depth of borehole(HERPDB) with design parameters. • Rational values of design parameters with considering HERPDB. • Dynamic simulation of underground temperature field (UTF) of a single U-tube GHE. • Dynamic simulation of UTF of pipe-group of U-tube GHE round one year. • Efficient operation of GSHP with borehole center-to-center distance of 3.8 m [ABSTRACT FROM AUTHOR]

The performance of a vertical ground source heat pump system (GSHPS) largely depends on the fluid temperature leaving the borehole heat exchanger (BHE) that may be affected by the short-term behavior of the BHE. Although considerable research has been carried out to analyze the short-term transient response of the BHEs, few studies have investigated its impact on dynamic simulation of GSHPS. Therefore, this paper presents a numerical approach based on a transient BHE model to evaluate the performance of a residential GSHPS over short and long timescales. The numerical results are compared with the results of EnergyPlus software. It is shown that the proposed model can appropriately predict the dynamic behavior of the system. Moreover, effect of borehole thermal capacity on the performance of the GSHPS is investigated in comparison with a quasi-steady state model. It is found that including the borehole thermal capacity substantially affects the design borehole length. Using the transient model instead of the quasi-steady state model leads to a 16% reduction in the required borehole length. [ABSTRACT FROM AUTHOR]

Large-scale commercial buildings served by ground source heat pump (GSHP) systems generally require a large land area for ground heat exchanger (GHE) installation. However it is difficult to provide such large area around the buildings, especially in urban area. Additionally, a large-scale GHE may consume more energy consumption compared with a medium or small GHE system because of the significantly large transmission energy consumption in the closed GHE loop. This study takes a commercial building located in northern China as a case study and proposes three different design schemes: chillers plus municipal central heating system, a single GHSP system and a hybrid GSHP system with chiller and municipal central heating as supplemental heat source and sink. The simulation models of the three systems have been established in the platform of TRNSYS. An optimal control strategy has been found with the aim of lowest annual operating cost. The results show that the municipal heating system can be put in operation when the outlet water temperature from GHE is less than 4℃ in winter; the cooling tower would be open to assist heat dissipation when the outlet water temperature from GHE is above 33℃ in summer. In this case the GSHP can provide 42% of the cooling and 83.5% of the heating loads, respectively. [ABSTRACT FROM AUTHOR]

Despite many studies that were found in the literature regarding the modelling of the short-time-step (STS) behaviour of the borehole ground heat exchangers, reported investigations of its effect on the long-term dynamic simulation of a ground-source heat pump (GSHP) system were sparse. Hence, in this study, the dynamic performances of a GSHP system applied to an office zone in Hong Kong over a period of 10 years were compared with both the long-time-step (LTS) and STS approaches for the boreholes. It was found that with the adoption of the more appropriate STS approach, the simulated fluid temperature leaving the ground heat exchanger borefield was lower by at most 1.62 °C over a range of ground and grout thermal conductivities investigated. This also allowed the design borehole length to be reduced by at most 14.4% while the resulting energy consumption was still better than those based on the LTS approach. In this regard, the STS approach should be employed in the long-term dynamic simulation of GSHP systems. [ABSTRACT FROM AUTHOR]

The European Union introduced the concept of "Net Zero Energy Building" (NZEB) to promote competitive, sustainable, eco-friendly, and "decarbonised" cities. To these aims, mainly with reference to highly urbanized places, it is necessary to renovate existing buildings to achieve the NZEB target. Thus, this paper analyses the energy retrofit of an existing building considering two low enthalpy geothermal systems in the HVAC plants: the Ground-to-Air Heat eXchanger (GAHX) and the Ground Source Heat Pump (GSHP). These two plants are used in different retrofit strategies and compared from an energy, environmental and economic point of view. A dynamic simulation software based on EnergyPlus calculation engine is used. The results are very interesting and demonstrate how by using a low enthalpy geothermal plant, a very low value of primary energy requirement can be obtained (about 60 kWh/m2, i.e., −71% compared to the existing building, characterized by a gas boiler). Moreover, substantial savings on annual energy bills and reduction of CO 2 emissions are obtained. • Retrofit strategies of an existing building to obtain a NZEB are proposed. • Two low enthalpy geothermal energy systems are analysed (GAHX, GSHP). • A technical-economic evaluation is made by using dynamic energy simulation. • The performances of the GAHX and the GSHP are evaluated in order to obtain a NZEB. [ABSTRACT FROM AUTHOR]

Abstract: Ground coupled heat pump systems can offer high energy efficiency for heating and cooling of buildings. The coupling of the ground loop with system and building represents a complex system and system performances can vary over long periods, in the range of 10–15 years due to the charge or discharge of the ground loop. In order to correctly concept, size and optimise these systems, numerical simulation is a perfect tool. The assessment of the global performance of such systems through simulation relies on several parameters, from the physical model validity to the operating conditions and control of the system. Typically, a GCHP system is simulated with a 1-h step. Transient phenomena inside the borehole are often neglected since ground models are mainly developed to characterise long term phenomena. However, detailed system simulations considering a realistic control of the system require much smaller time steps. In these cases, the validity of simulation models has to be verified. This paper proposes a hybrid approach combining experiments and simulation to highlight and quantify the impact of borehole short-time response on the system efficiency and system operation. First, a comparison between steady state and transient models as well as measurement is carried out. It reveals significant influence of dynamic effects inside the borehole. The comparison is then extended a parametric study with annual simulations for different European climates. [Copyright &y& Elsevier]

This article compares the performance of geothermal absorption and compression heat pump systems for space conditioning. The comparison is performed on an R-2000 residence for three Canadian locations representing three different climates and energy contexts: Montréal, Edmonton and Vancouver. The analysis is based on simulations carried out using TRNSYS. Results indicate that the required length of the geothermal heat exchanger is significantly different for the two types of heat pumps (compression and absorption). In Edmonton, geothermal absorption heat pump systems have a lower life-cycle cost and emit fewer greenhouse gases than geothermal compression heat pump system. In Montréal, both systems have similar life-cycle costs. Since electricity is mainly hydro-based the greenhouse gas emissions are much lower for the compression system. In Vancouver, the compression system has both lower life-cycle costs and lower greenhouse gases emissions. [ABSTRACT FROM AUTHOR]

• Comparison of thermal performance between single and double U-tube BHEs was made. • Numerical model that governs unsteady heat transfer in and outside BHE was applied. • Dynamic simulation was made to examine the effect of parameters on BHE performance. • Double U-tube BHE has better performance and lower resistance than single U-tube. • CaCl 2 (25%) solution showed better thermal performance than other working fluids. In the ground coupled heat pump system that delivers space heating and cooling to the buildings, often questions are raised as to which borehole heat exchanger configuration (single or double U-tube) provides higher thermal performance. To answer this question, in this paper, detailed comparative transient heat transfer analysis between single and double U-tube borehole heat exchanger (BHE) with two independent circuits (that can operate with the same or different mass flowrate and inlet fluid temperature) is conducted. Validated numerical heat transfer model was applied to perform the dynamic simulation and analysis. The two borehole heat exchangers are compared from the perspectives of their thermal performance in the heating and cooling mode of operation. Detailed sensitivity analysis of the BHE has also been performed by investigating the impact of the important borehole parameters, thermal properties and working fluids on the thermal performance of the double U-tube BHE. The simulation result revealed that the heat injected and extracted by the double U-tube BHE at steady state is 21.8 (77%) and 10.2 W/m (71.8%) more than that of the single U-tube BHE respectively. The simulation results also indicate that the thermal effectiveness of the single and double U-tube BHE at steady state are 0.24 and 0.31 respectively. The single U-tube BHE showed higher borehole thermal resistance than that of the double U-tube BHE with values of 0.47 and 0.31 m.K/W, respectively. The results of the study also revealed that among the studied working fluids, CaCl 2 (25%) solution showed better thermal performance than other anti-freeze solutions with thermal effectiveness of 35.6%. The simulation result of this work is crucial as an input for the optimization studies made to obtain a single set of optimal borehole paramaters and thermal properties that results in the best performance of BHE coupled to ground source heat pump system designed to provide space heating and cooling of buildings. [ABSTRACT FROM AUTHOR]