Feasibility and performance of ground source heat pump systems for commercial applications in tropical and subtropical climates
Introduction
Harvesting affordable and clean energy is the seventh sustainable development goal (SDG) to be addressed by the year 2030 for making cities sustainable (United Nations, 2015) [63]. Energy from renewable resources like wind, water, solar, biomass, ocean (tide and wave), and geothermal is inexhaustible and clean. Therefore, many developed and developing countries are striving for renewable energies to address the ever-increasing demand for energy use. According to the Renewable global status report (GSR) (2018) [54], China, USA, Brazil, Canada, Iceland, Denmark, Sweden, Norway, Germany, Finland, Austria, Belgium, Spain, France, Italy, Russia, UK, Turkey, Japan, Australia, and India are some of the countries pioneering the production of renewable energies.
As per the projection of Renewable GSR, 2018 [54], major developments of modern renewables (e.g. hydropower, solar, wind, biofuel and geothermal resources) in the heating-cooling sector occurred in heating-dominated countries of Europe (Austria, France, Germany, Turkey, Italy, Netherlands, Romania, Sweden, Switzerland, and UK), North America (USA and Canada), and Asia (China and Japan). The countries in the tropical climatic regions, with very different socio-economic conditions, face enormous problems regarding energy sufficiency, and harvesting sustainable and clean energy may alleviate these problems. For example, India is one of the biggest energy consumers (in fact, sixth largest energy consumer in the world) with 3.4% global energy consumption (Kumar et al. 2014) [42] and the present modern renewables like hydropower, solar, wind, modern biofuels and geothermal are not sufficiently harnessed to mitigate the present and future energy demands of the country. According to the international renewable energy agency (IRENA, 2017) [35], the present share of modern renewable energy is 17% of the total energy consumption in India, which may decrease to 12% by 2030 because of rapid growth in energy demand. Clearly, increased use of renewables is required in tropical regions to mitigate the rising energy demand and growing energy shortage and to minimize emission of greenhouse gases.
Geothermal energy systems that extract the conventional, high-grade geothermal energy from the ground require a heat interaction with deep geological strata. Such deep geological strata exist either in the form of hot dry solid rocks or in the form of hydrothermal reservoirs where heat becomes trapped at certain locations in the earth (Gupta and Roy, 2007) [29]. Occurrences of the hydrothermal reservoirs are restricted to specific geographical locations where the surficial manifestation of geothermal activities such as hot spring, geysers, fumaroles, etc., exist. Typically, these geologic formations are found at depths of 1–4 km from the ground surface and have a temperature of 200–300 °C because of high heat flow from magmatic intrusions (Gupta and Roy, 2007) [29]. Extraction of geothermal energy from such hydrothermal reservoirs is suitable for small- to medium-scale electricity generation (Olasolo et al., 2016) [50]. The main limitation of hydrothermal reservoirs is that these geothermal sources can be used only for limited periods. Alternatively, recent enhancements in rock drilling technology have increased the viability of extracting intermediate-grade geothermal energy using the enhanced geothermal system (EGS). EGS is used to extract heat from poorly permeable hot dry rocks located approximately 4–5 km below the ground surface (Olasolo et al., 2016) [50]. Deep geothermal systems, EGS or hydrothermal reservoirs, need high installation and maintenance costs and, therefore, are suitable for large-scale projects.
Shallow geothermal energy systems can be a cost-effective and environment-friendly option for heating and cooling applications in place of the traditional air conditioning and heating systems. These shallow (<400 m deep), low-grade geothermal systems use the undisturbed temperature (10–50 °C) of soil mass and/or groundwater that typically exists 10–15 m below the ground surface. According to the working principle, shallow geothermal energy systems can be classified as (1) closed-loop system that consists of a closed-loop pipe with heat-carrying fluid buried beneath the ground and exchanges the heat with the underground soil mass or groundwater, and (2) open-loop system that consists of an open-loop pipe buried beneath the ground and uses the groundwater or surface water for the heat exchange process (Sass et al., 2016) [58]. Among these geothermal systems, the vertical and horizontal borehole heat exchanger, pond loop system, well system, and geothermal pile system (Fig. 1) are widely used for heating and cooling of buildings. Especially, the vertical ground source heat pump (GSHP) system and geothermal pile are potentially the most popular and widely used geothermal systems because these systems require less land footprint and present a higher efficiency in operational energy than other geothermal systems (Brandl 2006, Spitler 2005) [[13], [60]]. According to Yari and Javani (2007) [66], shallow geothermal energy systems were operational in 33 countries in the year 2007, which shows an upward trend in the usage of these systems from the year 2000 when installations were done in only 26 countries.
The shallow geothermal energy systems are successfully used in the heating-dominated countries of Europe, North America, and Asia but are not widely tested in countries with tropical and subtropical climates. In fact, several historical records are available that highlight the range of application of shallow geothermal systems for extracting low-grade energy in the western countries. The first GSHP system was recorded as a Swiss patent in 1912 developed by Heinrich Zoelly (Ball et al., 1983) [6]. However, commercial uses of such systems began in 1973 after the first oil crisis. Gradually, these systems gained ample attention and popularity in many heating-dominated countries, especially in Europe and North America, after various limitations like leakage (Sanner, 2001) [57] and undersizing (Kroeker, 1949) [41] problems were eliminated. More recently, countries in the southern hemisphere (e.g., Australia) and Asia (e.g., Japan, Iran, and China) have shown interest in this new technology because of environmental and economic advantages (Rybach and Sanner, 2000) [56]. However, the system requirements for extraction and utilization of geothermal energy in the tropical and subtropical regions may not be identical with the cold-region countries because of (a) higher cooling demands, and (b) differences in the built infrastructure. In the tropical and subtropical regions, the vertical GSHP system could be a promising option for providing space heating and cooling. However, there is a need for better clarity on the feasibility and performance of such a system under tropical and subtropical climates where operational problems may be encountered because of a higher cooling requirement than heating requirement. Thus, a comprehensive assessment of performance of vertical GSHP systems for tropical and subtropical climatic region is required.
The objective of the present study is to assess the feasibility and performance of the vertical GSHP system in tropical and subtropical climatic regions for different soil conditions. Ten cities are chosen for this study that belong to the hot dry climate (New Delhi, Jodhpur and Bengaluru), hot humid climate (Kuantan, Chennai, and Alexandria), warm marine climate (Melbourne and Bogota), mixed humid climate (London), and mixed dry climate (Srinagar). Different types of soil (clayey soil, silty soil, and sandy soil) are considered in this study based on the geographical location of the cities. A typical office building with a total floor area of about 9000 m2 is considered, and the heating and cooling loads of this building are simulated using the energy simulation software EnergyPlus considering the climatic conditions of the ten cities mentioned above. Based on the calculated energy loads, vertical GSHP systems are designed using the design software GLD for the office building with the appropriate ground conditions of all the aforementioned cities. The response and performance of the vertical GSHP systems, under different soil and climatic conditions are evaluated in terms of the parameters like system coefficient of performance (COP), heat extraction rate (HER), and heat rejection rate (HRR). Finally, the feasibility of the vertical GSHP system for all the cities is assessed by performing a cost analysis. It is observed that the installation and performance of the vertical GSHP system in the tropical and subtropical cities depend on several factors like local climatic conditions, ratio of heating and cooling loads, and ground conditions. The average installation and operational cost of vertical GSHP systems is much higher for tropical cities than for subtropical cities.
Section snippets
GSHP system
A conventional GSHP system, as shown in Fig. 2, consists of three major components (Brandl, 2006) [13], namely, (a) primary circuit, which is a closed-loop ground heat exchanger (GHE) placed inside the ground, (b) one or more heat pumps, either water-to-air or water-to-water, integrated with the primary and secondary circuits, (c) a secondary circuit, which is a closed-loop pipe with heat-carrying fluid (mostly water with anti-freezing), embedded inside the receiving infrastructure elements
Details of cities with climatic conditions and soil parameters
In the present study, ten metropolitan cities located in different parts of the world with tropical and subtropical climates and different soil conditions are selected. Table 1 lists the cities and summarizes the information about the climatic zones, global locations, elevations, cooling degree days (CDD) (i.e., the number of days in a year for which heating ventilation and air conditioning (HVAC) system for cooling is required), heating degree days (HDD) (i.e. the number of days in a year for
Office building energy simulations
Fig. 6 shows the peak heating and cooling loads of the office building for the different cities considered in this study for the summer and winter design days. The highest peak heating load of the building is captured for London (700 kW), followed by Srinagar (645 kW), Melbourne (551 kW), Bogota (452 kW), New Delhi (448 kW), Alexandria (434 kW), Jodhpur (361 kW), Bengaluru (170 kW), Chennai (50 kW) and Kuantan (14 kW). In contrast, the peak cooling load is maximum for Chennai (588 kW), followed
Conclusions
The feasibility of installation of GHSP systems from economic and performance points of view is investigated for ten cities in the world (New Delhi, Jodhpur, Chennai, Bengaluru, Srinagar, Kuantan, Bogota, Alexandria, London, and Melbourne) located in tropical and subtropical climates. The critical factors influencing the performance of the GSHP system are identified and evaluated for these cities using the EnergyPlus and GLD software packages. The EnergyPlus software is used to calculate the
Author contribution
Debasree Roy: Conceptualization, methodology, software, visualization, data collection, validation, investigation, writing- original draft preparation.
Tanusree Chakraborty: Conceptualization, methodology, supervision, writing- reviewing and editing.
Dipanjan Basu: Conceptualization, methodology, supervision, writing- reviewing and editing.
Bishwajit Bhattacharjee: Supervision, writing- reviewing and 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.
Acknowledgement
The authors gratefully acknowledge the financial support (DST No: TMD/CERI/BEE/2016/072(G)) provided by the Department of Science and Technology (DST), Ministry of Science and Technology, New Delhi, India.
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