Southeast Asian countries have experienced rapid economic growth, at an average rate of 5.2% per year since 2000. The rapid growth has been followed by an increase in the energy demand. In 2016, the total primary energy consumption in the region reached 643 million tons of oil equivalent. While Southeast Asian countries may not be considered a major global CO2
contributor, the CO2
emissions of those countries rose from 711 MT in 2000 to 1288 MT in 2015 [1
]. The total generation of electricity in the region increased from 370 TWh in 2000 to 868 TWh in 2015. By 2015, 83.4% of electricity was generated by burning fossil fuels (i.e., coal, natural gas, and oil). Serious action must therefore be taken immediately, in order to reduce the fossil fuel dependency [3
Thailand accounts for 21.7% of the primary energy demand in Southeast Asia [1
]. By 2015, the national primary energy consumption and total electricity generation were respectively 135 million tons of oil equivalent and 178 TWh, with fossil fuel accounting for 80.7% and 91.6%, respectively. In 2017, the generation of electricity emitted 96.035 MT of CO2
]. Air conditioners consume much of a household’s electricity demand. According to a report published by The Japan Refrigeration and Air Conditioning Industry Association, Thailand’s domestic total air conditioner demand in 2016 was 1.56 million units, the third largest demand in southeast Asia after Indonesia and Vietnam [4
]. Data published by the Ministry of Energy of Thailand suggest that the residential sector consumed 20.4% of national electricity, 46% and 17% of which were used for air conditioning and refrigeration, respectively [2
]. These sectors have high potential energy savings [2
]. Governments of Southeast Asian countries are aware of this problem, and are thus considering several actions that promote the higher efficiency of air conditioners in this region [7
]. The residential energy growth in Thailand is greatly determined by the increasing number of households, as well as an increasing income per capita. The use of energy-efficient products is an important way of restraining the household energy demand. However, the market prices of energy-efficient products, such as five-star-rated energy-saving air conditioners, tend to be higher than those of regular products [9
Meanwhile, it has been shown that the increase in the energy demand in Thailand accelerate climate change and the urban heat island (UHI) phenomena [11
]. Arifwidodo and Chandrasiri have estimated that the annual increase of average temperature in an urban area (Bangkok) and a sub-urban area (Pathumthani) are increasing as a result of UHI. The UHI severity index has been found to be higher than those of other major cities in the world, such as Shanghai, San Diego, and San Francisco, and within a similar range as Tokyo [13
]. On the other hand, other studies have found possible countermeasures of UHI in the Tokyo area by utilizing a district heating-cooling system and ground-source heat pumps (GSHPs) [14
The government of Thailand begun to improve the energy efficiency of air conditioners in the 1990s, and started labeling the ratings of air conditioners by the mid-1990s. End consumers are thus supposed to be well aware of the labeling system. The efficiency of air conditioners was improved by the introduction of an inverter, although its market penetration rate and market share remain low. Meanwhile, the high demand for air conditioning units has offset efficiency improvements [6
Among various alternative energy sources, the ground-source heat pump (GSHP), which utilizes a relatively constant ground temperature, is widely applied for the cooling and heating of spaces. Instead of exchanging heat with the outdoor environment, as in the case of a normal air-source heat pump (ASHP), the GSHP uses the ground as a heat sink (for the cooling of spaces) and a heat source (for the heating of spaces).
GSHP systems are generally classified as open-loop and closed-loop systems. Closed-loop systems can be further classified into systems having vertical and horizontal arrangements of the ground heat exchanger (GHE). The vertical closed-loop system has higher thermal efficiency, and the heat transfer rate can be further improved through the convective heat transfer of groundwater flow. Although the required site area for this system is small, the system has a high initial cost for drilling and installation of the GHE. Meanwhile, the horizontal (shallow) closed-loop system is relatively inexpensive, as it requires no vertical borehole and just shallow trenches that can be dug through manual (human) labor or mechanical means, such as the use of a mini-excavator [16
]. However, this system requires a larger footprint for installation. The drilling cost of a vertical GHE in Japan is USD 6700 for a 50 m borehole, while the same borehole costs USD 3000 in Thailand. In Thailand, meanwhile, the average wage for manual (human) labor is around USD 13 per day. The horizontal system thus has a great advantage in terms of the initial cost.
In most cases, however, the thermal performance of the horizontal closed system is lower than that of the vertical closed system, as the soil temperature at a shallow depth fluctuates and is strongly affected by the ambient temperature and near-surface heat flux [18
]. Thus, for a high cooling load demand, such as in the case of the central cooling of an office or public building with a GSHP, a horizontal heat exchanger may not be adequate.
The horizontal closed system is increasingly being studied. Several studies have compared shallow linear, helical, and slinky GHEs in numerical simulation, and have found that the helical configuration has the best performance [16
]. They have also found that the thermal conductivity surrounding a GHE and the flowrate of the heat transfer fluids are the most important parameters. Fujii et al. performed numerical simulations of slinky GHEs installed at different depths, simplifying the GHEs as thin flat plates [20
]. Their simulation results agreed well with experimental data. In subsequent work, they presented the results of a slinky GHE field test during heating (winter) and numerical simulations of double- and single-layer arrangements, and found that the double layer has a lower energy cost per unit of site area, owing to its better performance [21
]. Recent studies have also remarked the importance of soil properties, moisture content, environmental parameters, and installation design to heat-pump performance [22
Recent research on GSHPs also has focused on the hybrid system. The GSHP hybrid system incorporates another thermal system, i.e., a desiccant or solar thermal energy. The hybrid GSHP–desiccant system allows better and more effective means of controlling space humidity and air temperature [27
]. By taking direct solar heat energy, the efficiency of a GSHP during heating can be significantly improved. The hybrid GSHP–solar system offers higher thermal performance for applications, such as water heating, heat storage, or drying [30
Unlike the case of most GSHP applications in a four-season climate, the cooling load is predominant in a tropical climate. The application of the GSHP in a tropical climate thus uses the ground mainly as a heat sink to remove heat from a building. Furthermore, the difference between ground and air temperatures is negligible.
To the extent of our knowledge, only few studies have focused on the application of the GSHP in tropical climates. One study showed that GSHP are expected to replace underperforming air-cooled condensers in Singapore [34
]. Permchart and Tanatvanit used the ground as a heat sink for direct-expansion GSHP, by directly burying the refrigerant piping exiting the compressor of the heat pump [35
]. Yasukawa et al. identified regional variation in the subsurface temperature by investigating the vertical temperature variation in several observation wells around the Chao Phraya Plain in Thailand and the Red River Plain in Vietnam [36
]. In several areas, they observed subsurface temperatures lower than the monthly mean air temperature, while in other areas, subsurface temperatures were higher, but still lower than the monthly maximum air temperature. Furthermore, the authors emphasized that the application of GSHPs in these areas could take advantage of advective heat transfer due to groundwater flow. Following their study, a GSHP system was installed in Kamphaengphet, Thailand [37
]. The system uses a single 56 m borehole with a double U-tube GHE. Long-term performance results show that an average coefficient of performance (CoP) of 3 can be achieved. Additionally, during successive operation, the borehole temperature increased, but recovered to its initial temperature after a week, and there was no long-term increase in the subsurface temperature after more than a year of operation. Uchida et al. conducted subsurface groundwater surveys and a stable isotope evaluation on the Chao Phraya Plain [38
]. Their results show differences in the subsurface thermal gradient between lower and upper plains, due to thermal conduction by regional groundwater flow. Furthermore, they remarked that GSHPs installed in areas with different groundwater thermal conduction characteristics may have different performance efficiencies. The most recently published research on the application of GSHPs in Bangkok with vertical boreholes and a single U-tube configuration showed advantages in terms of energy savings compared with a normal ASHP [39
Various economic analyses can be applied to assess the economic value of GSHPs—e.g., present value (PV) analysis, internal rate-of-return analysis, net-benefit analysis, payback analysis, and benefit-to-cost ratio analysis [40
]. Noorollahi et al. performed a numerical simulation and economic evaluation using the PV for the application of GSHPs to the energy supply of greenhouses in Iran [41
]. The annual cost has been used to evaluate the feasibility of GSHP application in Turkey [42
]; it was concluded that the GSHP system is economically preferable to the ASHP system for cooling purposes. Esen et al. [43
] conducted a techno-economic assessment of GSHPs for heating in Turkey. Go et al. [44
] evaluated the economic feasibility of various configurations of the shallow spiral coil loop heat exchanger, using the PV, internal rate of return, and savings-to-investment ratio. Zu et al. analyzed the economic application of GSHPs in hot and humid climates using the PV [45
Owing to aforementioned concerns, the objectives of present work are (i) to demonstrate the applicability of GSHPs, using shallow heat exchangers compared to the ASHP, through the experimental results in the hot tropical climate of Thailand; and (ii) to highlight important financial considerations by analyzing the prevailing factors that must be considered in order to make GSHP application in Thailand and other Southeast Asian countries economically attractive.