Thermal performance analysis of helical ground-air heat exchanger under hot climate: In situ measurement and numerical simulation

Highlights

• Experimental assessment of helical ground-air heat exchanger performance under hot climate.

• The transient CFD model of the helical ground-air heat exchanger system was established and validated.

• The extent of the borehole thermal degradation and regeneration was estimated.

• The borehole temperature near the helical pipe strongly influenced by ambient air temperature.

Abstract

The ground air heat exchanger (GAHE) is a promising passive approach for cooling and heating buildings. In this study, thermal performance of helical ground air heat exchanger (HGAHE) has been experimentally assessed in summer season for arid climate. The findings revealed that, the outlet air temperature of the HGAHE is strongly dependent on the inlet air temperature. Furthermore, the air temperature drop and the heat exchange rate as high as 13.3 °C and 463.4 W respectively, are attained at the highest inlet temperature of 41.0 °C. Besides, a transient numerical model was established and validated through the experimental data to investigate the heat penetration into the borehole. The results acknowledge that, the borehole temperature distribution in axial direction is higher at the upper surface and then decreases with the HGAHE length. In the other hand, the borehole temperature distribution in the radial direction reduces rapidly with the distance away from the pipe surface. Moreover, when the ambient air temperature during the night shift is lower than the borehole temperature, the forced convection which helps to take the heat away via purge air circulating into the HGAHE pipe allowed the borehole to restore its cooling ability.

Introduction

The worldwide total population growth along with the annual raise in the energy usage, as well as the global warming generated from burning of fossil fuel resources, calls for more efficient energy use and leads to sustainable development. In this context, renewable resources appeared to be one of the most energy efficient and productive alternatives [1,2]. In recent years, attempts have been made to establish alternative sources of energy to supply the building's cooling and heating needs. One of the best possible solutions as alternative energy is to utilize geothermal resources, which is classified as one of the world's most efficient and effective sustainable energy. It is green, renewable, appropriate for energy storage and accessible throughout the day. Geothermal energy could be harnessed via the earth air heat exchanger systems (EAHE) [3].

Ground air heat exchanger (GAHE) can be deployed to cool or warm air for building air conditioning, therefore decreasing the energy requirement for ventilation purposes passively and contribute in more cleaner built environment. The GAHEs use the earth's undisturbed temperature as a heat source. When air passes across the ducts, thermal energy is transmitted from air to ground or from ground to air depends to the air temperature relative to the soil temperature which remains more than the outdoor air temperature during winter, and lower in summer, for air heating/cooling based on the seasonal energy needs [4,5]. Many researchers have conducted experimental or numerical modeling analysis to access the thermal efficiency of GAHE systems. Mihalakakou [6] assessed the efficiency of EAHE device experimentally as well as via mathematical modeling. Al-Ajmi et al. [7] investigated the cooling capability of GAHE for buildings air conditioning under Saharan climate. Givoni [8] evaluated the potential of the EAHE system under arid climate. Findings revealed that, the system could be improved by using different ground cooling techniques to decrease the natural underground temperature such as shading and surface treatment with pebbles and plants.

Bansal et al. [9,10] analyzed the performance of EAHE connected to a room and showed that the change in EAHE tube material has minor influence on the outdoor temperature. Misra et al. [11,12] carried out CFD simulations to analyze the operation time and ground thermal conductivity effect on the efficiency of an EAHE system for cooling and heating. Findings revealed that the efficiency of the system is mainly affected by the ground thermal conductivity and operation time. Mathur et al. [13,14] carried out numerical simulation to investigate the condition of the ground saturation versus long time operation of EAHE. Results showed that the ground saturation conditions were faster with soil having lower heat conductivity. Kaushal et al. [15] presented a EAHE model using the ANSYS Fluent and response surface method to improve the design parameters of the EAHE system. Mathur et al. [16,17] presented different operative strategies for prolonged use of EAHE system under arid condition. Khabbaz et al. [18] installed EAHE in residential housing for cooling purposes under the hot climate of Marrakech, Morocco. Findings showed that air temperature drop of 18.3 °C can be achieved on the outdoor of the EAHE system. Menhoudj et al. [19] investigated the efficiency of EAHE for refreshing buildings under Algerian climate and found that thermal efficiency of the EAHE is more profound in the southern part of the country compared to the northern part. Results revealed that, in one year, the system could supply 246.815 kWh of energy. Amanowicz [20] conducted CFD simulations to examine the effect of various geometrical parameters on the system efficiency of multi-tube EAHE.

Zeng et al. [21] implemented EAHE to avoid the infrared exposure of a generator room to decrease the temperature inside the room. Findings revealed that, by using the EATHE system in intermittent mode, the maximum infrared camouflage can be reduced up to 86.96%. Benhammou et al. [22] performed numerical research work of EAHE integrated with a residential building for cooling purpose under Algerian Saharan climate. The findings pointed out that, the maximum air temperature drop inside the building was more than 11 °C and the drop rate in the amplitude of the air temperature inside the building increments up to 91%. Yusof et al. [23] developed a mathematical EAHE model to determine the thermal characteristics of EAHE system in a warm climate condition, and showed that EAHE system proves real opportunity for application under such conditions. Liu et al. [24] conducted a parametric study on the thermal performance of EAHE considering the influence of insulation parameters, pipe parameters, mass flow rates, as well as soil types. Findings revealed that, the EAHE with a smaller pipe diameter exhibits a higher heat capacity, whereas a larger diameter of pipe can enhance the COP of the system.

Previous research has shown that GAHE systems have a significant potential for providing cooling effect as well as a viable solution for taking the advantage of the underground temperature. Yet, it is not being used extensively due to longer tubes required to obtain a high amount of thermal exchange. Hence, longer trenches are needed to install the required tubes. Besides, a sufficiently ground space is also required, which most likely is not available in densely populated places. Thus, various researchers have studied a non-straight pipe configurations in the aim of achieving extra tube length per unit trench length to assure the maximum heat exchange rate.

Zarrella et al. [25,26] evaluated the effectiveness of helical-coil shape compared to multiple U-pipes in a foundation pile. The helical GHE has been shown to deliver the highest efficiency in foundation pile installations. Gao et al. [27] conducted analyses study of single, double, triple U and W- shaped for a ground source heat pump (GSHP) system to identify the most energy efficient configuration for cooling/heating systems. Results showed that, among the studied configurations, the GHE with W-shaped has the highest effectiveness. Mathur et al. [28] investigated the heat efficiency of a spiral duct pattern compared to a U-shaped duct pattern of EAHE systems and concluded that thermal efficiency of spiral and U-shaped duct for EAHE are almost the same. Thus, if an enough ground space is not available to install the ducts in U-shaped layout, the spiral configuration could be implemented.

Naoufel et al. [29] conducted an investigation on thermal performance of a spiral layout GAHE operating for cooling mode under the arid climate of Algerian Sahara. Results showed that, by incrementing airflow velocity from 2 to 5 m/s, the GAHE system COP drops substantially from 2.84 to 0.46 and the efficiency of the spiral GAHE decreases from 60 to 33%. Agrawal et al. [30] conducted a numerical study to compare the heat efficiency of U-pipe, slinky-pipe, and helical-pipe tube configurations. Findings concluded that, the highest heat efficiency of the EAHE was found with the helical-pipe and slinky-pipe configurations. Mathur et al. [31] conducted a numerical simulation of helical-coil layout EAHE system compared to conventional U-layout EAHE for heating and cooling. Findings revealed that, the helical-coil layout EAHE system provides the highest effectiveness as well as required lesser land area compared to conventional U-layout EAHE system.

Above discussion on various studies conducted by many researchers reveal that among the main problems hindering the spreading of the GAHE is the large installation area as well as the excavation cost required for long pipes to attain the desired amount of heat transfer. According to the literature, it has been identified that the helical-pipe configuration requires a lesser ground space for installing longer pipes. The helical-coil layout has been extensively studied by various researchers in the GSHP system [32,33]. However, this configuration still has not been examined in real applications of GAHE system. So, the novelty of this paper is to assess the thermal behavior of helical-coil layout GAHE both experimentally and computationally. Experimentation was conducted on a real in-situ experimental set up for summer cooling under arid climate. Later on, the proposed helical shaped GAHE system was also numerically simulated using ANSYS Fluent. Findings of the experimental and numerical data were introduced to analyze the performance of the proposed configuration in terms of air temperature drop obtained, heat exchange rate, distribution of heat into the borehole as well as to investigate the borehole temperature recovery using cold ambient air for a night purging. These results are expected to be helpful to improve the thermal performance of the system and useful for providing valuable guidance for the design of GAHE operating under arid climate condition.