Abstract
A techno-economic feasibility analysis of the thermal pile group is performed for a reference building considered at different climatic locations in India. The space-cooling potential of the thermal pile group is numerically investigated using PILESIM2 software over 50 years of thermal operation. A parametric study is conducted to assess the effect of parameters such as (i) average ground temperature, (ii) pile length, (iii) pile spacing, and (iv) configuration of high-density polyethylene (HDPE) pipes on the thermal performance of pile group. Analysis results indicate that space-cooling through the thermal piles strictly depends on the geographical conditions of a region. The space-cooling percentage increases with an increase in pile length, pile spacing, and the number of HDPE pipes in the piles and their configuration. It is recommended that geo-cooling must be supported with mechanical cooling for the buildings in summer-dominated regions of India. A comparison study reveals that the heat exchange rate in the case of thermal boreholes is higher than the thermal piles, and the evolution of ground temperature over the long term is notably lower in the case of thermal boreholes. Overall, the cost-effectiveness of the thermal pile group seems plausible for the cases considered herein.
Introduction
Worldwide, there is a pressing need for the use of renewable energy as the ever-increasing demand and supply for electricity may impose severe threats to the global environment. About three decades ago, thermal piles were introduced first in Austria. Recently, thermal piles have been recognized as an efficient solution for space-conditioning of buildings in Europe, the United States, Australia and China [1], [2], [3], [4], [5], [6], [7]. The thermal piles serve the combined purpose of bearing the superstructure load and cater the space-heating and cooling for thermal comfort inside the building. The significant space-heating of terminal E at Zurich airport, supported by 306 thermal piles [8] and a building at Lambeth College, London, supported by 143 thermal piles [2], are examples of the successful application of geothermal heat exchange technology. A residential house in Hokkaido, Japan, is supported over 26 thermal piles providing efficient space-heating [9]. Space-heating and cooling of a building in Shanghai, China, via 5500 thermal piles is also a notable application of thermal piles [10]. The average temperature remains constant at a shallow depth of 8–10 m below the ground surface, enabling heat exchange between the ground and the building via the thermal piles. A ground-coupled heat exchange system comprises ground heat exchangers embedded in boreholes or structural piles, a heat pump and a heating/cooling distribution system installed in superstructure components. In the thermal pile technology, a primary network of high-density polyethylene (HDPE) pipes is fitted in conventional piles. A heat carrier fluid (water mixed with antifreeze) is circulated through these pipes, with the help of a pump. A secondary circuit of HDPE pipes is embedded in the floors of the building, and the heat carrier fluid is circulated through the secondary circuit. Primary and secondary circuits are connected via a heat pump, known as a ground source heat pump. Fig. 1 shows the working mechanism of geothermal piles for heat rejection into the ground during summer.
In summer, the building’s heat can be released into the pile concrete and then to the ground for space-cooling via the heat-carrier fluid. In winter, heat can be extracted from the ground and supplied to the secondary circuit for space-heating during winter. A schematic diagram of the functioning of the ground-source heat pump system during geo-heating mode is shown in Fig. 2. During the winter season, GSHP works on the principle of a vapour compression cycle which comprises four steps; (i) the low-pressure liquid refrigerant in the evaporator absorbs heat and converts into gas, (ii) the super heated vapour enters the compressor where its pressure increases, (iii) in a condenser, the high-pressure super heated gas cools down, and (iv) the expansion valve reduces the fluid pressure and controls fluid-flow into the evaporator.
Current literature offers many studies on the thermo-mechanical behaviour of the piles, and studies on their heat exchange efficiency are mainly concerned with the building’s space-heating requirements, as reported in a few studies. The studies on the thermal performance of the piles show that the heat exchange capacity of thermal piles depends on stable ground temperature, the flow rate of the heat carrier fluid, pile dimensions, the number of piles, pile spacing, soil thermal conductivity and specific heat capacity of the soil. Average ground temperature is considered an essential factor in deciding the heat exchanger’s size and the heat pump’s performance [11], [12], [13], [14]. The heat-carrier fluid flow rate in HDPE pipes also affects the heat exchange efficiency of the piles [10], [15], [16], [17], [18], [19], [20], [21], [22]. An increased number of HDPE pipes and increased pile length offer higher thermal efficiency through these piles [23], [24], [25], [26], [27], [28], [29]. Higher diameter piles can enhance the heat transfer rate of piles [27], [30]. In-situ soil thermal conductivity is also a key parameter for assessing the thermal performance of piles [18], [22], [31], [32], [33]. The groundwater flow is advantageous for improved heat exchange rate through the thermal piles [17], [22], [33], [34], [35], [36], [37], [38].
According to the Bureau of Energy Efficiency Report 2019 [39], 33% of total electricity is consumed in residential and commercial buildings in India, where HVAC (heating, ventilation, and air conditioning) has the most significant share of 64% [40]. Moreover, the average annual energy consumption in the residential sector is expected to rise by 4.6% during 2018–2050 [41]. Thus, there is an urgent need to adopt renewable and sustainable energy technologies to accommodate the buildings’ space-cooling requirements, particularly in summer-dominated regions of India. Energy Information Administration (EIA) 2010 [42] report indicates that as compared to air source heat pump (ASHP), the use of a ground-source heat pump (GSHP) can result in a cost savings of 18–56% and a reduction in carbon dioxide (CO2) emission by 45–80%. As the technology of thermal piles has not been implemented yet in India, the database/guidelines for installing thermal piles are currently unavailable. In the available literature, studies on the thermal efficiency of piles are rare in the case of summer-dominated regions where the unbalanced heat injection into the ground and extraction from the ground may negatively affect the long-term operation of the thermal piles. Therefore, this study aims to assess the heat exchange rate of the thermal pile group and to check their techno-economic feasibility in India. For a sound thermal design of piles, the heat exchange rate is evaluated through in-situ thermal response tests on the piles [22], [29], [31], [43], but these tests are expensive and time-consuming. In the present study, the heat exchange capacity of thermal piles is quantified using a simulation tool, PILESIM2. Morrone et al. [44] modelled a seven-story building in Naples and Milan using PILESIM2. They noted 98% of space-cooling and 100% space-heating with the thermal operation of the piles in Naples. Fajedev and Kurnistki [45] simulated the heat exchange through the thermal piles and boreholes for a single-storey commercial building in Helsinki, Finland, using the IDA-ICE software package. They noted a higher performance of thermal piles for space-heating than the thermal boreholes due to different ground temperature boundary conditions. In the present study, a comparison of the thermal efficiency of piles and boreholes is also explored for a building in Indian geographical and climatic conditions. Finally, a parametric study is presented to investigate the effect of initial ground temperature, pile length, pile spacing, and HDPE pipe configurations on the heat exchange capacity of the piles for a reference building considered at New Delhi location.
Acknowledgements
The authors acknowledge the financial support (Sanction no. 22/0721/17/EMR-II) provided by Council of Scientific and Industrial Research (CSIR), HRDG, Government of India for performing this research work.
References
Energy pile test at Lambeth College, London: geotechnical and thermodynamic aspects of pile response to heat cycles
Energy from earth-coupled structures, foundations, tunnels and sewers
