**1. Introduction**

The population of Southeast Asia was almost 640 million in 2016 and is expected to increase to 760 million by 2040, assuming 0.7% annual population growth.

Urbanization is an essential factor that affects total energy consumption [1–3]. The residential sector accounts for the second-highest electricity demand after the industrial sector, growing by an average annual rate of 7.5%, owing mainly to the increasing number of appliances. The introduction of energy-efficient products can restrain household energy demand. The Japan Refrigeration and Air Conditioning Industry Association (JRAIA) reported that in 2016, Vietnam and Thailand were the second and third countries with the highest air conditioner demand in Southeast Asia, with 1.98 million and 1.56 million units per year, respectively. Indonesia ranked first with 2.3 million units in the same year [4]. Annual regional air conditioner demand increased from 12.2 million units/year in 2011 to 16.4 million units/year in 2016, equivalent to an average of 6.1% increase per year.

Researches have been focused on the possibility of introducing the Ground Source Heat Pump (GSHP) in the region. Even though GSHP is a mature technology, the application of GSHP in the tropical climate, such as Southeast Asia, faces several problems. The use of GSHP is mainly for cooling, eliminating the balance between heat rejection and heat extraction. The ground temperature is relatively higher and within the range of air temperature.

Yasukawa et al. conducted underground temperature surveys by measuring vertical groundwater temperature from several monitoring wells in Thailand and Vietnam [5]. They concluded that despite the differences between ground temperature and air temperature were low. However, there are still advantages of utilizing GSHP for space cooling. Further, they remarked that space heating might be possible for a short winter period in Hanoi, Vietnam.

In their subsequent study, Yasukawa et al. presented the pilot study of GSHP at Kamphaengphet province, Thailand. They confirmed the applicability of the system with series of experimental tests [6].

Several studies have focused on providing further information on GSHP applicability in the regions. Widiatmojo et al. evaluated the performance of GSHP systems coupled with horizontal/shallow Ground Heat Exchanger (GHE). They also performed cost analysis to estimate the payback time against Air Source Heat Pump (ASHP). Shimada et al. examined the different operational conditions based on field experimentation and numerical simulation. While in another publication, Sasimook et al. presented the experiments and performance comparison of GSHP and ASHP. They highlighted the GSHP advantage, especially in higher thermal load [7, 8]. Although most of the studies above remarked the possibility of GSHP application in Southeast Asia, none of these addresses the effect of groundwater on the performance of GSHP.

This chapter discusses numerical simulation results to evaluate the Thermal Response Test (TRT) conducted in Bangkok, Vietnam, and Hanoi, Vietnam. The numerical simulation uses the Moving Infinite Line Source (MILS) analytical method to account for thermal conductivity and groundwater flow. Further, we extend the simulation to estimate the GSHP performance for five years of operation considering different parameters obtained using the Infinite Line Source (ILS) and the MILS methods.

### **2. Thermal response test**

The Thermal Response Test (TRT) is a standard method to determine the ground thermal conductivity. From the TRT result, the apparent thermal conductivity of the ground surrounding the GHE can be calculated. A standard method to evaluate the apparent thermal conductivity from the TRT result is the Infinite Line Source (ILS) method [9, 10]. The ILS approach is based on the Kevin line source

*Effect of Groundwater Flow and Thermal Conductivity on the Ground Source Heat Pump… DOI: http://dx.doi.org/10.5772/intechopen.99061*

theory. This method calculates the temperature response of an infinite constant heat source analytically, assuming an infinite, isotropic, and homogeneous soil medium. This method also neglects the axial (vertical) heat transfer along the borehole. Considering the relationship between average fluid temperature, *T <sup>f</sup>* (C) at a time *t* (s) with the borehole wall temperature at a radius *rb* (m), constant heat–transfer rate per unit length of borehole, *q* (W m�<sup>1</sup> ) and borehole heat resistance, *Rb* (mK W�<sup>1</sup> ), the ILS solution is written as follows:

According to Carslaw and Jaeger [11], the temperature increase of a medium at a radial distance *r* (m) from an infinite line source with a constant heat exchange rate, *q* (Wm�<sup>1</sup> ), is expressed as:

$$T(r,t) - T\_0 = \frac{q}{4\pi\lambda} E\left[\frac{r^2}{4Dt}\right] = \frac{q}{4\pi\lambda} \bigcap\_{\substack{\nu\\ \frac{r^2}{4Dt}}}^{\infty} \!\!u u \tag{1}$$

*T0* (C) is the initial ground temperature, *D* (m<sup>2</sup> s), the thermal diffusivity, and *E* is the exponential integral function. Assuming that the following condition is satisfied:

$$t > 5\frac{r^2}{D} \tag{2}$$

Eq. (1) can be re-written as:

$$\overline{T\_f}(t) - T\_0 = \left(T\_f(r\_b, t) + qR\_b\right) - T\_0 \cong qR\_b + \frac{q}{4\pi\lambda} \left[\ln\left(\frac{4Dt}{r\_b^2}\right) - \gamma\right] \tag{3}$$

where, *Rb* (mK W�<sup>1</sup> ) is the borehole thermal resistance, *rb* (m) is the borehole radius λ (W m�<sup>1</sup> K�<sup>1</sup> ) is the thermal conductivity, *D* (m2 s) is the thermal diffusivity, ϒ (�) is Euler constant, and *T0* (C) is the soil temperature at initial (*t* = 0). *T <sup>f</sup>* (C) is the average circulation fluid temperature calculated by:

$$\overline{T\_f} = \frac{(T\_{bh-in} + T\_{bh-out})}{2} \tag{4}$$

where,*Tbh-in* and *Tbh-out* (C) are GHE fluid inlet and outlet temperature, respectively. Eq. (3) can be re-arranged into the linear form of fluid temperature against the natural logarithmic value of time as:

$$\overline{T\_f}(t) \cong \frac{q}{4\pi\lambda} \ln\left(t\right) + \left(qR\_{bh} + \frac{q}{4\pi\lambda} \left[\ln\left(\frac{4Dt}{r\_{bh}^2}\right) - \gamma\right] + T\_0\right) = m\ln\left(t\right) + c \tag{5}$$

From the fluid temperature gradient against the natural logarithmic value of time, *m* (C), obtained from TRT measurement, the value of apparent thermal conductivity, *λapp* (W m�<sup>1</sup> K�<sup>1</sup> ) can be calculated as:

$$
\lambda\_{app} = \frac{q}{4\pi m} \tag{6}
$$

In the ILS method, the effect of convective heat transfer as a result of groundwater convection is not considered. The value of apparent thermal conductivity represents both diffusive and convective heat transfer. Accordingly, it is recognized that the value of apparent thermal conductivity is larger than the value of effective

thermal conductivity, λ*eff*. The heat transfer due to convection of groundwater flow and heat conduction are two different physical phenomena. Thus, the use of apparent thermal conductivity (heat conduction) for calculating the thermal performance of vertical ground heat exchangers can lead to some serious errors, especially for the longer time-scale and high-velocity groundwater flow.

While groundwater flow is an important parameter, measuring the groundwater velocity is practically difficult. Besides, the ground layers are inhomogeneous. The practical way to measure the groundwater velocity is the pumping test. However, the pumping test is expensive as it requires an additional borehole for the observation well.

The TRT measurements were carried out in Bangkok, Thailand and Hanoi, Vietnam. These were the first and second measurements to be carried out in Southeast Asia [12]. The measurements were conducted in the existing GSHP systems. In Bangkok, TRT measurement was performed in an installed GSHP system at Chulalongkorn University, Bangkok campus. The measurement was also performed in the GSHP system installed at the Vietnam Institute of Geosciences and Mineral Resources (VIGMR), Hanoi. The measurement procedures were similar for both sites. **Figure 1** shows the measurements at Bangkok and Hanoi.

Both measurements applied the constant heating rates *q* = 39.72 Wm�<sup>1</sup> and *q* = 35.91 Wm�<sup>1</sup> for Bangkok and Hanoi, respectively. By evaluating the TRT results using the ILS method, the apparent thermal conductivity was calculated as λ*app* = 1.82 W m�<sup>1</sup> K�<sup>1</sup> and λ*app* = 1.42 W m�<sup>1</sup> K�<sup>1</sup> for Bangkok and Hanoi, respectively. The data regarding the effective thermal conductivities and groundwater velocities in both GSHP sites are unavailable. Further details on the measurements can be found in another publication [12].

**Figure 1.** *TRT measurement at the GSHP sites: Chulalongkorn University, Bangkok (left) and VIGMR, Hanoi (right).*
