Reduced Scale Experimental Modelling of Distributed Thermal Response Tests for the Estimation of the Ground Thermal Conductivity

The knowledge of the ground thermal properties, and in particular the ground thermal conductivity is fundamental for the correct sizing of the Ground Coupled Heat Pump (GCHP) plant. The Thermal Response Test (TRT) is the most used experimental technique for estimating the ground thermal conductivity. This paper presents an experimental setup aimed to realise a suitable scale prototype of the real borehole heat exchanger (BHE) and the surrounding ground for reduced scale TRT experiments. The scaled ground volume is realised with a slate block. Numerical analyses were carried out to correctly determine suitable geometric and operational parameters for the present setup. The scaled heat exchanger, inserted into the block, is created with additive technology (3D printer) and equipped with a central electrical heater along its entire depth and with temperature sensors at different radial distances and depths. Present measurements highlight the possibility to reliably perform a TRT experiment and to estimate the slate/ground thermal conductivity with an agreement of about +12% with respect to measurements provided by a standard commercial conductivity meter on proper cylindrical samples of the same material and onto 10 different portions of the slate block.

Performance Analysis of Solar-Assisted Ground-Coupled Heat Pump Systems with Seasonal Thermal Energy Storage to Supply Domestic Hot Water for Campus Buildings in Southern China

The supply of domestic hot water (DHW) on college and university campuses is indispensable and is also one of the main components of campus energy consumption. The density of residential patterns and similar occupancy behavior of college students make it economical to use centralized systems to cover the DHW demand, and utilization of solar energy can make the systems more economical. Seasonal thermal energy storage (STES) is a promising key technology that can minimize the imbalance between the availability of solar energy and thermal energy demand. In this paper, a solar-assisted ground-coupled heat pump (SAGCHP) system that meets the DHW demand of 960 students was investigated by means of dynamic simulation and energy-economic analysis. The simulation results in terms of the underground heat balance are compared with a standalone GCHP system and a SAGCHP system without STES. Results show that heat recharging operations during university summer and winter breaks (when there are minimal students on campus) lead to improved underground heat balance and energy performance. Finally, a sensitivity analysis on system performance was carried out by varying solar collector arrays. It was found that there exists an optimal value of solar collector area to achieve the lowest system lifecycle cost (LCC).

A 3-Dimensional Numerical Thermal Analysis for A Vertical Double U-Tube Ground-Coupled Heat Pump

The ground heat exchanger plays a major role in the thermal performance and economic optimization of the ground-coupled heat pump. The present study focuses on the effect of the borehole size and the grout and soil thermal properties on the thermal assessment of these heat exchangers. A double U-tube heat exchanger was studied numerically by the COMSOL Multiphysics 5.4 software in a 3-dimensional discretization model. The double U-tube was circuited as a parallel flow arrangement and situated in a parallel configuration (PFPD) deep in the borehole. The grout and ground thermal conductivities were selected in the range of (0.73-2.0) W/m.K and (1.24-2.8) W/m.K respectively. The results revealed that the ground thermal conductivity showed a more pronounced influence on the thermal performance of the ground heat exchanger and with less extent for the grouting one. Increasing the grout filling thermal conductivity from (0.73) W/m.K to (2.0) W/m.K at a fixed ground thermal conductivity of (2.4) W/m.K has augmented the heat transfer rate by (10) %. The heat transfer rate of the ground heat exchanger exhibited marked enhancement as much as double when the ground thermal conductivity was increased from (1.24) W/m.K to (2.8) W/m.K at fixed grout thermal conductivity range of (0.78-2.0) W/m.K. It has been verified that increasing the borehole size has a negligible effect on the ground heat exchanger thermal performance when a grout with a high thermal conductivity was utilized in the ranged of examined configurations. The steady-state numerical analysis model outcomes of the present work could be implemented for the preliminary borehole design for a ground heat exchanger.

Impacts of hydrogeological characters of fractured rock on thermodynamic performance of ground-coupled heat pump

Ground-coupled heat pump (GCHP) is used to recovery shallow geothermal energy, a widely distributed green energy source. Due to the imbalance between heat rejection and extraction, heat buildup underground is commonly associated with the long-term operation of GCHPs, which undermine system performance. Heat buildup intrinsically results the irreversibilities (entropy production) in subsurface heat sink, in which thermodynamic and transport properties are largely influenced by hydrogeologic properties, especially the existence of fractures and groundwater. This study investigates the influence of water flow in fractures on the thermodynamic performance of a single borehole heat exchanger (BHX) and heat buildup in the underground heat exchange zone (UHXZ). Potential influence factors were screened out, and new terms were proposed to quantify the scale of fractures and available heat and cold in the heat sink. Governing equations were established to calculate the impacts of vertical and horizontal fractures on the heat exchange rate in BHX as well as on the heat flow across the UHXZ. The analysis results show that water flow in fractures can significantly enhance heat transfer, reduce required number of boreholes, mitigate heat buildup and reduce irreversibilities underground. The results also suggest that the role of fracture scales and water velocity in GCHP operation should be carefully evaluated. Therefore, detailed hydrogeological survey is necessary. The study results provide a guide on more accurately evaluating the risk of heat buildup and how to take advantage of hydrogeological characters to improve the performance of GCHPs.

Accurately predicting heat transfer performance of ground-coupled heat pump system using improved autoregressive model

Nowadays, ground-coupled heat pump system (GCHP) becomes one of the most energy-efficient systems in heating, cooling and hot water supply. However, it remains challenging to accurately predict thermal energy conversion, and the numerical calculation methods are too complicated. First, according to seasonality, this paper analyzes four variables, including the power consumption of heat pump, the power consumption of system, the ratios of the heating capacity (or the refrigerating capacity) of heat pump to the operating powers of heat pump and to the total system, respectively. Then, heat transfer performance of GCHP by historical data and working parameters is predicted by using random forests algorithm based on autoregressive model and introducing working parameters. Finally, we conduct experiments on 360-months (30-years) data generated by GCHP software. Among them, the first 300 months of data are used for training the model, and the last 60 months of data are used for prediction. Benefitting from the working condition inputs it contained, our model achieves lower Mean Absolute Error (MAE), Mean Absolute Percentage Error (MAPE), Root Mean Square Error (RMSE) than Exponential Smoothing (ES), Autoregressive Model (AR), Autoregressive Moving Average Model (ARMA) and Auto-regressive Integrated Moving Average Model (ARIMA) without working condition inputs.