Numerical simulation of a ground-coupled heat pump system with vertical plate heat exchangers: A comprehensive parametric study

doi:10.1016/j.geothermics.2020.101913

Geothermics

Volume 88, November 2020, 101913

https://doi.org/10.1016/j.geothermics.2020.101913 Get rights and content

Highlights

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The thermal performance of vertical plate GHEs is computationally scrutinized.
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The effects of five parameters on the performance of plate GHEs are comprehensively studied.
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The optimum distance between two adjacent GHEs is found to be about 4 m.
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Soil type has the most critical effect on the thermal performance of plate GHEs.
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Selecting a proper value for buried depth is finding a compromise between performance and cost.

Abstract

Plate ground heat exchangers (GHEs) are renowned for having the highest heat transfer rate per unit land area and could be of interest when the accessible land area is limited. In this study, a 3-D numerical model is developed to investigate the thermal performance of vertical plate GHEs at the real-scale level. The proposed model accounts for ambient temperature fluctuations and building cooling load variations and is such developed to couple the GHE to the heat pump. The effects of different parameters, including GHE spacing, buried depth, the height of GHE, soil type, and climate on the thermal performance of vertical plate GHEs, are comprehensively investigated for the first time. The optimum distance between two adjacent GHEs is obtained to be about 4 m to avoid the adverse effect of thermal interference. It is demonstrated that increasing GHE spacing dramatically increases cooling load per unit GHE area for values below 4 m while decreasing cooling load per unit land area. As a case in point, increasing GHE spacing from 2 m to 4 m increases cooling load per unit GHE area by 30.1 % while lowering cooling load per unit land area by 34.9 %. It can be inferred from the simulation results that soil type has the most critical effect on the thermal performance of vertical plate GHEs, and better thermal performance can be achieved when GHEs are buried in a soil type with higher thermal conductivity as well as higher heat capacity. The results also indicate that selecting proper values for buried depth and height is in fact finding a compromise between thermal performance and excavation cost. Investigating the effect of climate reveals that the maximum cooling load per unit GHE area is decreased by 37.7 % when the GCHP system operates in Ahvaz (hot climate) rather than Tehran (mild climate). However, when the GCHP system is designed to operate in Tabriz (cold climate), the maximum cooling load improves by 17.4 %.

Graphical abstract

Introduction

Population growth, together with global economic development, has underscored the need for more energy resources. Today, most of the world energy demand is supplied by non-renewable resources, which has led to greenhouse gas emission and environmental degradation at a remarkable pace (Ritchie and Roser, 2019). On the other hand, we humans are using up fossil fuel resources, and soon, we will be confronted with the depletion of these non-renewable energy resources. Thus, it is essential to replace fossil fuels with renewable alternatives first to avert the detrimental effects of these resources and second be able to supply our energy demand in the future. Residential and commercial buildings account for 40 % of world energy consumption (Ghadiri et al., 2013), and their energy demand is usually supplied by Heating, Ventilation, and Air Conditioning (HVAC) systems. As a result, developing state-of-the-art technologies for increasing building energy efficiency and ameliorating building energy performance has garnered the attention of many researchers throughout the world. In this respect, Ground-Coupled Heat Pumps (GCHPs) have come to the fore for several decades. Their high Coefficient of Performance (COP) has made them a viable choice to be used in HVAC systems; however, exorbitant installation cost hinders their widespread use in these systems (Kavian et al., 2019).

A typical GCHP system consists of five major components, evaporator, compressor, condenser, expansion valve, and Ground Heat Exchanger (GHE). GHEs are generally categorized into horizontal and vertical arrangements (Tang and Nowamooz, 2020; Yang et al., 2020). Horizontal Ground Heat Exchangers (HGHEs) are often buried in trenches with a depth range of 1−2 m, while Vertical Ground Heat Exchangers (VGHEs) are often embedded in boreholes with a depth range of 15−120 m (Kavanaugh, 2015). As utilizing VGHEs necessitates drilling deep boreholes, their installation cost is much more than that of HGHEs. On the other hand, less required land area is assumed as the advantage of vertical GHEs over horizontal ones. HGHEs themselves are classified into four different configurations, namely linear, slinky, spiral, and plate. Thus far, several research papers have studied the effects of various parameters on the thermal performance of GCHP systems with VGHEs (Brunetti et al., 2017; Habibi et al., 2020a; Hakkaki-Fard et al., 2015; Miglani et al., 2018) and HGHEs (Asgari et al., 2020; Cui et al., 2019; Kayaci and Demir, 2020; Larwa et al., 2019; Mirzanamadi et al., 2020) in detail.

Congedo et al. (2012) carried out a numerical study to evaluate the thermal performance of three different types of HGHEs during a twelve-month operating period. Their results demonstrate that soil thermal conductivity and fluid velocity inside the pipe are the primary parameters that affect the thermal performance of these GHEs, while buried depth is of little significance. Also, their results reveal that spiral GHEs have the best performance among different types of HGHEs. Pu et al. (2018) numerically investigated the in-line and staggered arrangements of linear GHEs. They found that the staggered arrangement can outperform the in-line one when the relative offset displacement is appropriately selected. They also concluded that thermal interference is of considerable significance and can noticeably affect thermal efficiency. It was demonstrated that selecting a suitable value for pipe spacing is of paramount importance when it comes to designing a GCHP system, while the effect of buried depth is not as significant as that of pipe spacing.

In another study, Al-Ameen et al. (2018) experimentally and numerically investigated the feasibility of using waste materials as potential backfills in HGHEs. They concluded that using metallic particles in lieu of sand to backfill the trench could lead to a 77 % improvement in the thermal performance. They also studied the effect of particle size on the thermal performance of HGHEs and found that using medium particles (1.18−2 mm) instead of fine and coarse particles could result in a considerable enhancement of about 92 % in the thermal performance. Habibi and Hakkaki-Fard (2018) proposed a novel design concept of applying secondary soil around HGHEs in order to enhance their thermal performance. Their results revealed that using secondary soil could reduce initial installation cost up to 40 %.

It is well-established that plate GHEs yield the highest heat transfer rate per unit land area and, as a result, could be a viable choice when the accessible land area is limited. Generally, plate GHEs are categorized into two configurations, horizontal and vertical. Horizontal plate GHEs are typically buried in the foundations of the building as they occupy only a small volume of soil. To date, only a small amount of research is dedicated to horizontal plate GHEs. Zukowski and Topolanska (2018) experimentally compared the thermal performance of a plate Ground-Air Heat Exchanger (GAHE) with that of a tube GAHE. The results of this study demonstrated that the plate GAHE could supply twice the cooling load that could be supplied by the tube GHE in the cooling season. They also observed that the plate heat exchanger was more efficient when it operated in cooling mode rather than heating mode. Habibi et al. (2020b) conducted a numerical study to draw a comparison between horizontal flat-panel GHEs and linear ones. Their results demonstrated that the maximum building cooling load that could be supplied by the flat-panel GHE was approximately 27 % higher than that of the considered arrangement of linear GHEs. In addition, they studied the effects of various parameters including buried depth, soil type, and plate width on the thermal performance of flat-panel GHEs.

Vertical plate GHEs are often embedded vertically in parallel horizontal trenches. They show better thermal performance in comparison to their horizontal counterparts. Various studies have been devoted to the investigation of vertical plate GHEs so far. Bottarelli and Di Federico (2012) developed a numerical model to compare the thermal performance of a vertical plate GHE with that of a GHE named radiator. Their simulation results revealed that as the plate GHE affected a more expanded volume of soil, the surrounding soil was of lower temperature as compared with that surrounding the radiator leading to higher COP values.

Bortoloni and Bottarelli (2014) investigated the thermal performance of a vertical flat-panel GHE using an analytical solution based on the line source method. In their study, they assumed the flat-panel GHE to be an equivalent slinky GHE having the same heat transfer surface per unit trench length. The results of the proposed method were compared with those obtained from a 2-D finite element model. They found that the two sets of results were closely in consonance with each other testifying that their solution could be a reliable one for investigating plate GHEs. In another study, Ciriello et al. (2015) developed an analytical model to predict the soil temperature distribution induced by a plate GHE. Their model accounts for several important parameters that play crucial roles in the thermal performance of plate GHEs, including but not limited to ambient temperature fluctuations at the ground surface, anisotropy of soil thermal properties, and soil water content. Their proposed model facilitates the design of plate GHEs and helps assess if they comply with environmental regulations.

Based on the preceding literature review, previous pieces of research concerning vertical plate GHEs were, to the best of the authors' knowledge, limited to only the investigation of their thermal performance and making comparisons between these GHEs and horizontal linear ones or so-called radiator GHEs. The literature lacks a comprehensive study that is dedicated to investigating all the involved parameters in the thermal behavior of a vertical plate GHE. Moreover, a real-scale 3-D analysis is of the utmost importance to predict the thermal performance of vertical plate GHEs as accurately as possible as the above-mentioned studies were all performed at the small-scale level (the GHEs considered in these studies were all less than 20 m long).

The present work aims to tackle the aforementioned gaps and presents a comprehensive numerical model that accounts for ambient temperature fluctuations and building cooling load variations. The model is also capable of coupling the GHE to the heat pump. The effects of several involved parameters, including GHE spacing, buried depth, height of GHE, soil type, and climate, are evaluated in this piece of research. The model is validated against experimental data to ensure that the results are reliable.

Section snippets

Theoretical model

Fig. 1 illustrates a schematic of the GCHP as well as the configuration of the vertical plate GHEs. A Computational Fluid Dynamics (CFD) model is developed in OpenFOAM and utilized to solve the governing equations using the finite volume method based on the SIMPLE algorithm. It is noteworthy that the CFD model simulates only the vertical plate GHEs and the surrounding soil, and the heat pump is coupled to the vertical plate GHEs through the inlet and outlet water temperatures.

The numerical

Grid independence study

A grid independence study is carried out to ensure that the obtained results are independent of grid resolution. In this piece of research, the total pressure loss of the fluid through a typical vertical plate heat exchanger (L×H×W = 100 × 1×0.02 m³, d = 4 m) for a mass flow rate of 0.145 kg/s is monitored for different grid resolutions, as depicted in Fig. 4. It can be deduced from this figure that the pressure loss does not change significantly when the grid consists of more than 102,000

Model verification

To ensure that the simulation results are reliable and can accurately predict the thermal performance of vertical plate GHEs, it is pivotal to verify the numerical results against experimental data. Therefore, the experimental data obtained by Metz (1983) for a conventional horizontal ground heat exchanger is used for the sake of validation. The working fluid in this experiment was 80/20 water-methanol mixture. The involved parameters in this study are listed in Table 1.

Fig. 6 depicts the

Results and discussion

This section is devoted to a parametric study on vertical plate GHEs taking different parameters into account, including GHE spacing, buried depth, height of GHE, soil type, and climate. The effect of each parameter on the thermal performance of vertical plate GHEs has been investigated. Table 2 and Table 3 present the standard conditions considered for this parametric study. It is worth mentioning that all the required geometrical and material properties are obtained from these tables, and

Conclusion

In this study, a 3-D real-scale numerical model is proposed that can help optimally design vertical plate GHEs and assess their thermal performance to make sure that designed GCHP systems are in compliance with environmental regulations. The model accounts for ambient temperature fluctuations and building cooling load variations and is also capable of coupling the GHE to the heat pump through the inlet and outlet water temperatures. The effects of different parameters, including GHE spacing,

CRediT authorship contribution statement

Ali Amadeh: Software, Visualization, Writing - original draft. Mohammad Habibi: Conceptualization, Methodology, Validation, Software, Visualization, Writing - review & editing. Ali Hakkaki-Fard: Supervision, Funding acquisition, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors would like to express their gratitude to Sharif University of Technology for their financial support via the quality grant program.

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