Effect of Groundwater Flow and Thermal Conductivity on the Ground Source Heat Pump Performance at Bangkok and Hanoi: A Numerical Study

In recent decades, the fast-growing economies of Southeast Asian countries have increased the regional energy demand per capita. The statistic indicates Southeast Asian electricity consumption grows for almost 6% annually, with space cooling becoming the fastest-growing share of electricity use. The ground source heat pump technology could be one of the solutions to improve energy efficiency. However, currently, there are limited data on how a ground source heat pump could perform in such a climate. The thermal response test is widely used to evaluate the apparent thermal conductivity of the soil surrounding the ground heat exchanger. In common practice, the apparent thermal conductivity can be calculated from the test result using an analytical solution of the infinite line source method. The main limitation of this method is the negligence of the physical effect of convective heat transfer due to groundwater flow. While convection and dispersion of heat are two distinctive phenomena, failure to account for both effects separately could lead to an error, especially in high groundwater flow. This chapter discusses the numerical evaluation of thermal response test results in Bangkok, Thailand, and Hanoi, Vietnam. We applied a moving infinite line source analytical model to evaluate the value of thermal conductivity and groundwater flow velocity. While determining the ground thermal properties in a high accuracy is difficult, the moving infinite line source method fulfills the limitation of the infinite line source method. Further, we evaluated the five-year performance of the ground source heat pump system coupled with two vertical ground heat exchangers in Bangkok and Hanoi. The results suggest the importance of groundwater flow to enhance the thermal performance of the system.

Download Full-text

The Effects of Groundwater Flow on Vertical-Borehole Ground Source Heat Pump Systems

Volume 3: Engineering Systems; Heat Transfer and Thermal Engineering; Materials and Tribology; Mechatronics; Robotics ◽

10.1115/esda2014-20065 ◽

2014 ◽

Cited By ~ 1

Author(s):

Ayako Funabiki ◽

Masahito Oguma ◽

Taisei Yabuki ◽

Takao Kakizaki

Keyword(s):

Thermal Conductivity ◽

Heat Flux ◽

Groundwater Flow ◽

Flow Velocity ◽

Heat Pump ◽

High Thermal Conductivity ◽

Ground Source Heat Pump ◽

Ground Heat Exchangers ◽

Ground Source ◽

Gravel Size

Heat advection by groundwater flow is known to improve the performance of ground heat exchangers (GHEs), but the effect of groundwater advection on performance is not yet fully understood. This study examined how parameters related to groundwater flow, such as aquifer thickness, porosity, lithology, and groundwater flow velocity, affect the performance of a borehole GHE. Under the thin-aquifer condition (10 m, or 10% of the entire GHE length in this study), groundwater flow velocity had the greatest effect on heat flux. With a groundwater flow velocity of at least 10−4 m/s through a low-porosity aquifer filled with gravel with high thermal conductivity, the heat flux of a GHE was as much as 60% higher than that of a non-aquifer GHE. If the aquifer is as thick as 50 m (50% of the entire GHE length), the high thermal conductivity of gravel doubled the heat flux of the GHE with a groundwater flow velocity of at least 10−5 m/s. Thus, not only groundwater flow velocity, but also aquifer thickness and thermal conductivity were important factors. However, groundwater seldom flows at such high velocities, and porosity, gravel size, and aquifer thickness vary regionally. Thus, in the design of ground source heat pump systems, it is not appropriate to assume a large groundwater effect.

Download Full-text

Effects of Groundwater Flow on a Ground Source Heat Pump System

Journal of Thermal Science and Engineering Applications ◽

10.1115/1.4035502 ◽

2017 ◽

Vol 9 (2) ◽

Author(s):

Ayako Funabiki ◽

Masahito Oguma

Keyword(s):

Thermal Conductivity ◽

Heat Flux ◽

Groundwater Flow ◽

Flow Velocity ◽

Heat Pump ◽

Numerical Study ◽

High Thermal Conductivity ◽

Ground Source Heat Pump ◽

Pump System ◽

Ground Source

Heat advection by groundwater flow is known to improve the performance of ground heat exchangers (GHEs), but the effect of groundwater advection on performance is not yet fully understood. This numerical study examined how parameters related to groundwater flow, such as aquifer thickness, porosity, lithology, and groundwater flow velocity, affected the performance of a borehole GHE. Under a thin-aquifer condition (10 m, or 10% of the entire GHE length in this study), groundwater flow velocity had the greatest effect on heat flux. At a groundwater flow velocity of at least 10−4 m/s through a low-porosity aquifer filled with granite gravel with high thermal conductivity, the heat flux of a GHE was as much as 60% higher than that of a GHE in a setting without an aquifer. If the aquifer was as thick as 50 m, the high thermal conductivity of granite gravel doubled the heat flux of the GHE at a groundwater flow velocity of at least 10−5 m/s. Thus, not only groundwater flow velocity but also aquifer thickness and thermal conductivity were important factors. However, groundwater seldom flows at such high velocities, and porosity, gravel size and composition, and aquifer thickness vary regionally. Thus, in the design of ground source heat pump systems, it is not appropriate to assume a large groundwater effect.

Download Full-text

In-Situ Thermal Response Test for Ground Source Heat Pump Application

Volume 4: Energy Systems Analysis, Thermodynamics and Sustainability; Combustion Science and Engineering; Nanoengineering for Energy, Parts A and B ◽

10.1115/imece2011-63318 ◽

2011 ◽

Author(s):

Wenzhi Cui ◽

Quan Liao ◽

Guiqin Chang ◽

Qingyuan Peng ◽

Tien-Chien Jen

Keyword(s):

Thermal Conductivity ◽

Thermal Resistance ◽

Heat Pump ◽

Thermal Response ◽

Ground Source Heat Pump ◽

Thermal Response Test ◽

Response Test ◽

Ground Source ◽

Borehole Thermal Resistance

The design and performance optimization of ground source heat pump (GSHP) systems need the exact thermal properties of the soil, such as ground thermal conductivity and capacity, and the borehole thermal resistance of borehole heat exchanger (BHE). In-situ thermal response test (TRT) is the most widely used method to determine the overall thermal physical properties of the geological structure around the borehole. A TRT experimental apparatus has been developed and thermal response test was performed in Chongqing, southwest China. Both single-U and double-U borehole heat exchangers are studied in this work. The test duration is about 70 hours. Data direct fitting and parameter estimate method are both used to determine the soil thermal conductivity and the borehole thermal resistance. The results showed that the average ground thermal conductivity of the test region for single U and double U BHE conditions are 2.55 and 2.51 Wm−1K−1, and borehole thermal resistance are 0.116 and 0.066 mKW−1, respectively.

Download Full-text

Study of the sustainability of a ground source heat pump system by considering groundwater flow and intermittent operation strategies

Energy Exploration & Exploitation ◽

10.1177/0144598718800725 ◽

2018 ◽

Vol 37 (2) ◽

pp. 677-690 ◽

Cited By ~ 2

Author(s):

Wanli Wang ◽

Jin Luo ◽

Guiling Wang ◽

Xi Zhu ◽

Guiyi Liu

Keyword(s):

Groundwater Flow ◽

Heat Pump ◽

Thermal Response ◽

Continuous System ◽

Ground Source Heat Pump ◽

System Operation ◽

Pump System ◽

Heat Pump System ◽

Ground Source ◽

Intermittent Operation

In this study, the operation of a ground source heat pump system was investigated over a 25-year period with careful attention paid to the effects of groundwater flow and intermittent operation strategies. First, geological and hydrogeological investigations were conducted, after which ground thermal properties were determined by thermal response tests. In order to predict the heat transfer within borehole heat exchangers under a specific operating system, a numerical model was developed using finite element subsurface flow & transport simulation system (FEFLOW). The numerical model was validated with thermal response test measurements. Three operation conditions including continuous system operation without groundwater flow, continuous system operation with groundwater flow, and intermittent operation with groundwater flow were examined. Results indicate that ground temperature disturbance was effectively reduced during groundwater flow and the intermittent operation of the system. Compared with continuous system operation without groundwater flow, the borehole heat exchanger heat transfer rate increases by 10% with groundwater flow conditions and increases by 16% with further implementation of the intermittent operation strategy. Intermittent operation with groundwater flow is highly recommended for the sustainable operation of ground source heat pump system.

Download Full-text

A Model for a Geothermal Well in a Hydraulic Groundwater Flow for Ground Source Heat Pump Systems

ASME 2011 5th International Conference on Energy Sustainability, Parts A, B, and C ◽

10.1115/es2011-54679 ◽

2011 ◽

Author(s):

Kevin D. Woods ◽

Alfonso Ortega

Keyword(s):

Thermal Conductivity ◽

Analytical Solution ◽

Groundwater Flow ◽

Heat Pump ◽

Temperature Response ◽

Ambient Air ◽

Heat Pumps ◽

Thermal Reservoir ◽

Geothermal Well ◽

Ground Source

Heat pumps are mechanical systems that provide heating to a space in the winter, and cooling in the summer. They are increasingly popular because the same system provides both cooling modes, depending on the direction of the cycle upon which they operate. For proper operation, the heat pump must be connected to a constant temperature thermal reservoir which in traditional systems is the ambient air. In ground source heat pumps however, subterranean ground water is used as the thermal reservoir. To access the subterranean groundwater, “geothermal” wells are drilled into the formation. Water from the building heating or cooling system is circulated through the wells thereby promoting heat exchange between the coolant water and the subterranean formation. The potential for higher efficiency heating and cooling has increased the utilization of ground source heating ventilating and air conditioning systems. In addition, their compatibility with a naturally occurring and stable thermal reservoir has increased their use in the design of sustainable or green buildings and man-made environments. Groundwater flow affects the temperature response of thermal wells due to advection of heat by physical movement of groundwater through the aquifer. Research on this subject is scarce in the geothermal literature. This paper presents the derivation of an analytical solution for thermal dispersion by conduction and advection from hydraulic groundwater flow for a “geothermal” well. This analytical solution is validated against asymptotic analytical solutions. The traditional constant linear heat source solution is dependent on the ground formation thermal properties; the most dominant of which is the thermal conductivity. The results show that as hydraulic groundwater flow increases, the influence of the ground formation thermal conductivity on the temperature response of the well diminishes. The diminishing influence is evident in the Peclet number parameter; a comparison of thermal advection from hydraulic groundwater flow to thermal conduction by molecular diffusion.

Download Full-text