Rapid population growth

Rapid population growth

1.0: Introduction

1.1: The Global Crisis

The worldwide energy pursuit as a result of rapid population growth and global urbanisation and has led to energy shortages and rising energy costs. Since 1950, the global demand for power has escalated to 10,000 million tons of oil per annum (Walisiewicz 2002, p.6). According to the World Energy Council, energy consumption is set to increase by 50% by the year 2020 (Walisiewicz 2002, p.6). Fossil fuel that we have been utilising for decades has finally come to a depletion crisis. It is difficult to predict how much fuel is left to feed the world's growing population.

The need for greener energy has been at the forefront of the global agenda as fossil fuel poses a threat to human health and environment. The carbonaceous fuel has brought about shift in global weather patterns and has caused climate change unprecedented in human history. Many countries such as the UK and the US have already embarked on several renewable energy projects to generate power, however, they are still finding difficulty in harnessing them. For instance, solar energy is difficult and expensive to collect as sunlight spreads over a vast area. On the other hand, hydroelectric power generation erects giant reservoirs that displace local residents, concentrate pollution and disrupt aqua life. As for wind farms, they create visual intrusion and noise pollution, and like, waves and tides, do not generate consistent energy.

1.2: Ground source :

Ground source heat, extracted from the Earth's heat reservoir, has received considerable attention in the recent decades. Energy pile technology is dubbed as “killing two birds with one stone”. This is because, in geotechnical situations, buildings are constructed on drilled reinforced concrete piles and energy piles are these piles that are incorporated with GSHP systems. Thus energy piles provide structural support while supplying ground heat energy for domestic space heating and cooling applications, and do not pose any threat to the environment, with great flexibility to meet energy shortage and substitute the exhausting supply of fossil fuel.

The GSHP system has been around for over 50 years now, nevertheless, market penetration of this technology in UK, is still at its infancy, at levels far lower than in USA and other European nations.

1.3: Scope of Project

The UK energy policy aims to invest in new energy technologies with increased energy efficiency, security of supply and a shift towards diversifying renewable energy resources (DECC 2009b).This projects aims to look into the potential of ground-source heat energy in the form of energy pile. The project structure is as follows:-

Scope of project

summary of project stages, tests carried out, range of parameters investigated etc.

summary of research and contribution

UK policy - 10% renewable energy

2.0: Background

2.1: History of Energy

Human civilisation and energy have been inextricably associated throughout history. Before ..BC, people were found to produce salt from sea water and pipe heated hot water from springs for domestic heating, (NEED 2009, p.9). Natural resources such as solar, geothermal and wind energywere their main fuel.

The world went through a great change when Industrial Revolution spread throughout Britain in the 18th century. The modern steam engine, invented by James Watt (1736-1819) when he became an Industrial Revolutionary, was used to pump water from the mines and later used in textile factories (FI 2009). The fossil fuel era gradually emerged after the development of the steam engine. The use of oil spread from factories to private people when cars became a reality for ordinary people. Towards the end of 18th century, electricity was introduced, however, has certain drawbacks with regard to storage. Furthermore, many became aware of the damages to environment that fossil fuels are causing and therefore searched for an alternative energy, consequently led to nuclear energy. Nevertheless, safety issues have been brought to the fore due to unresolved solution to disposal of radioactive nuclear waste which deferred its expansion.

Consequently, energy history is now back at the beginning with the rise of sustainable energy age. Renewable energy sources, such as wind and wave, now use far more developed technique then when man initially focused on these sources. The cleaner, low-carbon energy sources are most likely to become the dominant energy source in the future.

2.2: Low-Zero Carbon (LZC) technologies

Since there has been more awareness of the seriousness of global warming, UK legislation, building regulations, planning consent and corporate environmental policies have driven the industry to deliver low carbon plant room solutions. In addition, UK commitment to the Kyoto Protocol, an international agreement linked to the United Nations Framework Convention on Climate Change, drives the industry to reducing green house gases. Low and zero carbon (LZC) system is one of the solutions and thus has been increasingly installed in buildings, e.g. small-scale and micro-CHP units. Carbon emissions reduction will contribute to slowing-down of global warming. These systems are also attractable to householder and contractors for public sector building or charitable body who want to meet the grant requirements in the low carbon building programme. These systems are practicable for different uses dependent on thermal load demand, geological conditions, installation and maintenance cost. The advantages and disadvantages of various low- or zero-carbon (LZC) technologies for generation of heat is justified in table ??

Comparison of low and zero-carbon (LZC) technologies in terms of carbon savings, cost effectiveness and local impact. (From Throne)


Primary output

Carbon savings

Cost effectiveness

local impact

Combined heat and power, fuelled by:


Heat and electricity








Solar thermal systems

Heat and electricity




District heating





Biomass boiler





Ground energy systems

Open loop


Depends on building type

Depends on building type


Closed loop





Justification of various low or zero carbon technologies.





Combined heat and power, fuelled by:


heat is captureed in electrical generation and supply to local buildings

-make use of the heat waste which increase efficiency of the power generation system

-low conversion efficiencies compared to heat only systems
-great initial investment is required
-maintenance cost s high

Solar thermal systems

make use of the sun's energy to heat the water

- the systems can fit into existing buildings. It does not affect land use.
-free of greenhouse gases

- large area of collectors is required.
- dependent on geography
- limited to daytime hours and non-cloudy days.

District heating

burn biomass to feed energy from a centralised location to a number of units

-an established and tested technology
-more efficient than individual domestic boilers
-do not require individual fuel storage
-maximises dwelling space

-require long term planning
-around 10% of heat is dissipated in distribution
-agreement between mutilple private users can be complex
-Boilers are large and expensive
-the systems is still being developed, reliability has not been proven

Biomass boiler

usually burn wood chip fuel in a small boiler to generate heat for domestic scale.

-an established and tested technology
-installation is relatively easy
maximises dwelling space

-Boilers are expensive
-require more space than gas boilers
-flue require regular cleaning
-wood burning can lead to pollution issue
-feed supplay blockages to the boiler can be common

Ground energy systems:
Open loop
Closed loop

make use of the constant temperature underground to provide heating and cooling to buildings

-long life-time
-visually unobstrusive
-high COP(coefficient of performance)
-no toxic gas emmision
-energy from the ground is constant and free
-low-maintenance system
-the system is multi-functional
-reliable and cost-effective

-good building insulation is required.
-installation is costly
-dependent on geography
-boreholes can destablize the ground surface
-repairing can be complex
-refrigerants used can leak into the atmosphere and pollute the environment

2.3: Ground Source Heat Energy

The energy shortages and step change in fuel costs gave a new dimension to ground source heat energy as an alternative source for residential and commercial space heating and cooling. Ground-source heat energy, from solar radiation, external air, ground and ground water, is now mainly used in heat pumps for homes initially and presently in commercial buildings, industrial sites as well as to generate electricity (DECC 2009a).

As a form of solar-geothermal energy, it makes use of the stable low-grade heat of 10-20°C found near the earth's surface for cooling and heating buildings. The present estimate is that there are about 5000 units of them installed in the UK every year and for industrial sites, 1550 units could be installed with an average capacity of 800kilowatts of thermal power. (DECC 2009a)

Ground energy systems are worldwide and there is a wealth of successful applications from domestic to major public sector buildings. Under successful design and operation, the systems can have far more benefits than other LZC systems. There is a significant potential to apply ground energy systems to improve environmental performance of buildings.

2.3.1: Open and Closed Loop Systems

Ground energy systems have two distinctive approaches dependent on site conditions; they are open-loop and closed loop systems. Both systems have been widely used in other European countries. The technologies have been established and tested for years. The advantages and disadvantages of open-loop and closed-loop systems are summarised in table ? The choice of choosing such systems in the initial design appraisal plays an important part in the success of setting up the GSHP systems. First time consumers can understand the concepts and practicability of these systems by comparing them in different aspects of characteristic.


Open-loop systems

Closed-loop systems

Requirements for groundwater abstraction and reinjection

All open-loop systems include abstraction and discharge of groundwater. This process is unsustainable as the waste water discharged can pollute the groundwater reservoir.

Groundwater abstraction and reinjection are not required.

Regulatory constraints

The abstraction and discharge of groundwater is strictly regulated by the Environment Agency. Such legislation contrains the feasibility of these systems in some locations.

There liitle or no regulation for closed-loop systems. One relevant concern is to ensure that the boreholes are adequately sealed or grouted to prevent the leakage of the toxic refrigerant from the ground loop pipes.

Dependence on favourable hydrogeological conditions

Open-loop systems are only practicable when significant water-bearing strata,eg. aquifers are present beneath or near the site.

Closed-loop systems do not require the presence of an aquifer and are practicable in a wide range of geological conditions.

Number and capacity of boreholes

Relatively small numbers of abstraction boreholes can supply large demands under favourable hydrogeological conditions where groundwater yields are constant. For example, a borehole yielding 24 litre/sec can provide a peak thermal output of 500 kW.

Closed-loop systems typically require a much greater number of boreholes to yield the same peak thermal capacity compared to open-loop system. A typical 100 m deep closed-loop borehole can yield a peak thermal output of 4-7 kW.

Requirements for heat transfer system

Dependent on the heating or cooling demand. Open-loop systems can operate in cooling mode without a heat pump. Energy efficiency is improved as there is no additional energy requirement to power the heat pump compressor.

Closed-loop systems almost always use heat pumps to enhance the heat transfer as heat extration is relatively low.

Ability to handle annually imbalanced thermal loads

As open-loop systems can control the rate of groundwater disposed of to the ground, it can operate successfully with very unbalanced thermal loads. It doesn't matter where heating or cooling demand dominates the annual cycle. The reinjection of water benefits it from co-ordinating the balance of thermal load. However, the discharge water can migrate and affect the temperature of the injection water which reduces system efficiencies.

Closed-loop systems work best where annual heating and cooling load is approximately balanced. If the thermal load is unbalanced there is a long-term risk in ground temperature change. The recovery of the ground temperature has to be concerned.

Potential for off-size thermal impacts

There is potential that the warmer/cooler water can migrate to the surface water source and casue environment impacts. This impact is due to the continuous aquifer reinjection of cooler/warmer groundwater over extended periods. It can affect the habitat of wildlife and also ecology.

Heat flux in the ground is predominantly by conduction. Therefore, migration of ground heating and cooling is very slow. The off-site thermal impacts is significantly low.

Constraints on location of boreholes making up the ground element

Although open-loop systems typically require small numbers of boreholes, boreholes are preferably spaced as widely apart as possible to minimise interference between themselves. The distance between the abstraction and reinjection boreholes has to be especially considered to avoid thermal disturbance.

A large number of boreholes are required for closed-loop systems. They need to be arranged on a grid pattern to maintain a minimum horizontal separation between boreholes. The recovery conditions of the ground temperature has to be taken into account.

3.0: Literature review

3.1: An Overview of Energy Pile

Energy piles are building foundation piles that act as ground heat exchangers in a closed loop system. They are incorporated with heat exchange systems buried in the pile that will bring a remarkable contribution to reduction in carbon emissions. They have a dual function in which they provide structural support and sustainable energy. The principle of GSHP in foundation pile is simple; it capitalizes the stable and consistent thermal capacity of the ground for heat exchange process.via a heat pump system.

This GSHP-incorporated pile system was patented by the Austrian engineering company Naegelebau, which was commonly marketed as Enercret system by the company and has been widely used since 1980. It utilises building substructure to extract take heat from the earth. The system allows heating in winter and conversely, cooling in summer by dissipating excess heat into the ground in an economical and environmentally friendly way.

This technology is widely used in new build public and commercial buildings as its efficiency in delivering heating effect is higher than conventional heating systems by at least a factor of 2. Another reason is that it can meet the 10% renewable energy target set by the Merton Rule (Environment and Regeneration 2009).

Energy pile comprises three major parts, bore pile, reinforcing cage and heat exchanger. They can be used for any buildings. Its heat pump utilises electrical energy to power its compressor. The system also requires a small amount of electricity for fluid circulation in the pipe network. Energy pile system does not cause much pollution considering solar-geothermal energy is the only source, however not entirely renewable because electricity is usually fossil-fuelled. (REN 2009) (STENT 2008)

Energy piles construction consists of two main phases, GSHP design and pile construction. GSHP design begins with site investigation which includes identifying soil composition, local ground temperature, groundwater level, flow and direction, as well as thermal properties of the soil. They are important in determining the feasibility of energy pile at a particular site. Furthermore, impacts on the pile load capacity and heat recovery of ground-source energy throughout its operating lifetime has to be taken into account as well. A wide range of foundation piles such as bored piles, pre-cast concrete piles, continuous flight auger (CFA) piles can be integrated with GSHP systems and the choice of energy pile design usually depend on the cost, pipe work installation, integration and construction programme. The installation of GSHP may affect the structural pile design, causing additional cost for and this may raise economic concern. (Ebnöther 2008) (Tomlinson & Woodward 2007, p.474)

Local council, government departments and the European Union are concern about the performance of energy pile or GSHP. This notion has been proven in terms of energy savings and zero carbon emissions. The way it contributes to reducing pollution and dampening the global warming raises awareness and they now offer grants for installation of heat pump systems, for example, the Low Carbon Buildings Programme Scheme (LCBP), with a budget of £30 million (DECC 2009c), tax credit is eligible under the Enhanced Capital Allowances (ECA) programme for those equipment which meets published energy-saving criteria (DEFRA 2009). Such schemes are encouraging and there is no doubt that this technology will become increasingly desirable. (need to rewrite)

3.1.1: Benefits of Energy Pile

Energy pile is almost a zero-carbon technology considering the small quantity of electricity needed to power its heat pump unless it is generated from a renewable source. The proportion of renewable heat energy utilised for heating is determined by the system efficiency, termed Coefficient of Performance (COP). COP of an electrical heat pump used in an energy pile is averagely rated as 4 which means renewable energy utilised is of 75% of the total energy used. (Further discussed in Chapter??

In the interests of groundwater protection, energy pile is a closed loop GSHP system that does not cause as much pollution as an open loop GSHP system. The advantage of having low noise operation and almost no space requirement outweighs several other renewable energy sources.

Energy pile, with no potential of being vandalized, has a long life expectancy, ranges from 20 to 50 years. This is also because it is installed in the subsurface hence there is no exposure to weather. The self- generating energy source has low dependence on conventional power.

3.2: Main factors affecting feasibility of EP

3.2.1: Ground and Surface Temperature

Heat is transfered by three mechanisms, conduction, convection and radiation. Conduction describes the process by which heat is transfered through a gas, liquid or gas by molecular interaction. Thermal conductivity describes how good the ground material is at conducting heat and will be discussed in next session. Convection describes the heat transport by fluid, e.g groundwater.Groundwater flow can affect ground temperature in different manner and will be disscused in next session. Radiation in this case, is dominant by the suns' radiation. The estimated anuual mean net incoming radiation is typically in the range 40-80 Wm-2 for Europe temperature (Linacre & Geerts 1997). Specific Heat Capacity

Ground thermogeology is important to the GSHP systems. We need to know which part of the Earth's subsurface store sufficient heat (geothermal energy) for extraction. Rocks, soils and groundwater all have various specific heat capacities (JK-1kg-1), the abilities to store heat. They are the major sources of open-loop GSHP systems. We can also express specific heat capacity per unit volume, termed the volumetric heat capacity (JK-1/m3). Volumetric heat capacity varies with temperature partly due to changes in density of the material. The Earth as Ground Source Heat Reservoir

The shallow subsurface of the ground is filled with layers of rocks and sediment. They have high values of volumetric heat capacity but modest values of thermal conductivity. Therefore they have low values of thermal diffusivity. These characters result in a low heat flow throughout the subsurface since at low diffusivity, heat pulse propagate very slow, unless there is presence of convection by groundwater.

The Earth's subsurface acts as a huge energy collector. In summer, the Earth's surface is heated by the intensified solar radiation and elevated air temperatures. This energy propagates a few to dozen metres down into the subsurface. GSHP make use of the enthalpy energy stored in the earth's subsurface.. The temperature difference between the ground and the circulating fluid in the ground heat exchanger drives the heat transfer.. Ground temperature vary both with daily and seasonal cycles However, at depths of about 15 m the temperature is remarkably stable and equal to the mean snnual air temperature (Rybach & Sanner 2000). This means that in winter, ground temperatures are generally slightly higher than the air temperature and in summer, ground temperatures are lower. This nature is beneficial to GSHP heating and cooling as heat can be extracted from and dumped to the ground due to difference of air and ground temperature. Geothermal Heat Flux

Geothermal gradient describes the ground temperature change to depth. It is a function of heat flux derived from the earth's interior. It varies considerably between different locations. In the UK, the measured geothermal gradient is of the order 0.015-0.004 oC m-1(1.5-4.0 oC per 100m). The average gradient is believed to be around 0.026 oC m-1. The British Geological Survey stores up maps of geothermal heat flux for the whole of the UK. The mean geothermal heat flux calculated is in the range of 60 ±28 Wm-2. The Fourier law allows us to estimate the conduction of heat flow through a given material assuming no heat is lost by convection or radiation by where Q= heat flow (W), = thermal conductivity (Wm-1K-1) of soils or rocks and grad T is geothermal gradient(Km-1). This equation can not be used estimate the actual ground temperature at depth because temperature raise of subsurface due to solar radiation aborbed by the ground surface is not taken into account in this eqaution. Measurement of exact temperature in the UK subsurface has always been a difficult task to surveyor. Site investigations have to be carried out to provide a temperature profile to determine the feasibility of applying |closed-loop GSHP systems, Looking at the ground surface temperature would be an approach to estimate the subsurface temperature. Overview of the temperature and heat flow in UK

The UK meteorological Office (UKMO) stores various climate temperature data of the UK. The mean annual air temperature measured at sea level in mainland UK varies from north to south from about 8 to 12 °C. The January-July average air temperature in the UK swing less than 15 oC. (Perry & Hollis 2005b). In South-west England, there is a high general decrease eastward and northwards. The mean annual ground surface temperature is approximately the same as the mean annual air temperature as the magnitude of the interior heat flow from the Earth's core is very small, the variation is within 1°C (Rollin 2002). Position and elevation has the major effect on air temperatures. However, the aspect of the site, exposure of the site and slope of the local terrain can have minor effect on it but it is generally neglected. The UKMO annual long-term average date can provide a resonable estimate of the mean annual air temperature if a location and height is given. The UKMO model is established and tested with a root mean square error of 0.75°C and a mean maximum and minimum errors of ±3.5°C. In fact, the heat energy stored in ground subsurface is relatively modest, compared to the open source (e.g. hot spring, underground hot water reservoir). Therefore, areas with higher heat flow is where GSHP systems is more likely to be installed. These areas are found where radiogenic granite is concentrated. They include SW, northern England, especially the batholith in the Eastern Highlands of Scotland.

3.4.2: Thermal properties of soil

For design of ground loop heat exchangers in ground source heat pump systems, the knowledge of the ground thermal properties is critical. The thermal properties determine the rate at which heat can be transferred to the ground heat exchangers (ground loop) from the ground, or to the ground. They include thermal conductivity and thermal diffusivity. Thermal conductivity is the capacity of a material to conduct or transmit heat, whereas thermal diffusivity shows the rate at which heat is conducted through a medium. Thermal diffusivity of the ground beneath the site is greatly considered before adopting the GSHP systems. It is related to heat capacity, density and thermal conductivity and can be represented by a equation where α is thermal diffusivity (m2s-1), λ is thermal conductivity (Wm-1K-1), Cp is specific heat capacity (J kg-1K-1) and ρ is density (kgm-3).

Generally, thermal properties of superficial deposits are functions of porosity and degree of saturation while rocks are degree of saturation, porosity and mineral content. Rocks usually possess lower porosities than soil since they originate under higher heat and pressure environments. These result in rock having higher contact area between grains and thus higher thermal conductivities than soils. Also, rocks with higher quartz content usually have higher thermal conductivities. For saturated materials, higher porosity results in increased heat capacities and therefore lower thermal diffusivities. Degree of saturation can affect the thermal conductivity since water has a higher thermal conductivity of 0.6 Wm-1K-1 and air a thermal conductivity of 0.0252 Wm-1K-1. It is dependent on the groundwater condition. Therefore, groundwater can have a significant effect on the thermal properties and will be discussed later. Typical values of thermal conductivity and volumetric heat capacity of various rocks and minerals are summarised in T?

The thermal conductivity and volumetric heat capacity of various rocks and minerals.

Thermal conductivity (Wm-1K-1)

Volumetric Heat Capacity

Rocks and sediments










Wet Clay






















































































Expanded polystyrene



Data from inter alia Halliday and Resnick (1978), Sundberg (1991), Clauser and Huenges (1995), Eskilson et al. (2000), Banks and

Robins (2002), Banks et al. (2004), Waples and Waples (2004a), and Lienhard and Lienhard (2006). Italics show recommended

values cited by Eskilson et al. (2000). Estimation of Thermal properties

In order to obtain the thermal diffusivity of the ground, we need to estimate the thermal conductivities and volumetric specific heat. One traditional method of estimation is started with ascertaining the types of soil or rock that surrounds the proposed locations of the boreholes. The thermal conductivity can then be estimated from tabulated data. The data can be found in the Soil and Rock Classification for the Design of Ground-Coupled Heat Pump Systems Field Manual (EPRI, 1989). However, thermal conductivity values for complex ground formation types usually vary. Therefore, an in-situ experimental method for more accurately measuring the effective ground thermal conductivity using a test borehole is utilized.

The ground thermal conductivity cannot be directly measured. It must be inferred from measurements of temperature and heat flux. In 1983, MOGENSEN) proposed and described the concept of using such measurements. Subsequently, such experimental apparatus was developed in 1995 at Oklahoma State University and a similar but independent one by EKLOF and GEHLIN (1996). Two approaches are used to estimate the thermal conductivity, they are the line source approach (Morgensen 1983) and the cylinder source method (Kavanaugh and Rafferty 1997). The principles of these approaches are about the same, both estimate the ground thermal properties from the performance. A heat pulse (hot water) is continuously imposed to the ground to analyse the temperature response. However, this method is not accurate enough as it requires several assumptions (e.g. the heat transfer to the ground loop is constant) which have significant effect on the actual result. A detailed numerical model is then established, tested and come over all the uncertainties is described below. Parameter Estimation Methodology

This model uses the theory of NELDER and MEAD simplex algorithm (1965). An alternative parameter based estimation approach was described by SHONDER and BECK (1999) to estimate the ground thermal conductivity. This technique is based on statistical principles that provide quantitative estimates of measurement accuracy. It can reduce uncertainties by providing a detailed representation of the borehole geometry and thermal properties of the fluid, pipe, grout, and ground. Conceptually, the parameter estimation model uses a transient, two-dimensional numerical finite volume model of the vertical borehole to estimate the temperature response of the ground to a known time-varying heat flux input. This estimation method provides a more accurate estimate of the ground thermal conductivity. Description of the Test Procedure

The experimental apparatus is housed in a trailer which contains the apparatus, two generators and a purge tank. A simplified schematic of the test system is illustrated below. It is an in-situ field test. Firstly, a test well is drilled to a depth on the same order as the expected depth of the heat pump exchanger; a U-tube exchanger is then inserted into the borehole. Secondly, water is heated and pumped through the U-tube. Thirdly, temperatures of the inlet and outlet water are measured as a function of time. At regular intervals (typically 1 to 15 mins), data on inlet and outlet temperature, power input to the heater and pump, and water flow rate are collected. The duration of the experiment is suggested to be 50 h for a better, accurate result. A computer program, the Geothermal Properties Measurement (GPM) is developed to determine thermal properties using data from short-term in-situ field tests. Once values of thermal conductivity and specific heat capacity are obtained, the thermal diffusivity can be calculated by the equation.

Delivering the potential

in-situ measurement

3.4.3: Groundwater flow

In practice, structural piles are usually 20-30m long and partially penetrate several geologic layers. The incorporation of GHSP in these piles raises the subject of the significance of groundwater flow on system performance. As heat transfer is influenced by groundwater flow velocity and thermal properties of the soil or rock (as mentioned in Chapter ??), Darcy's groundwater flow equation can be applied to initially assess flow through porous media, given by

where q = specific discharge (ms-1), K = hydraulic conductivity and h = hydraulic head

Heat, in the ground, can be transported by three main mechanisms

Conduction via soil or rocks and groundwater

Convection or advection via groundwater

Solar radiation Convection as a heat transfer mechanism

Convection in this study is described as heat transported by forced motion of a fluid. When significant groundwater flows across GSHP's ground loop, it transports about 4.18 kJK−1 L−1 of heat with it. Heat transfer by groundwater throughflow, hypothetically, can be seen as beneficial to a single borehole system for it offers moderating effect on pile temperatures when operating in both heating and cooling mode. If, however, downward groundwater flow is present, flowing from high head to low head under gravitational forces, groundwater will carry heat stored (UTES) away or cooler water to depth. Upward groundwater flow will have the reversed adverse effect.

Despite its heat-balancing capability, groundwater flow can cause thermal interference between several energy piles or systems when operating simultaneously in close proximity. Consequently, effects of thermal interference could have a significant impact on the system performance. Nevertheless, this can be overcome by only fitting GSHP to adequately-spaced piles. Furthermore, groundwater can alter the level of saturation of sedimentary deposits and rock. The lower the saturation of sediments and rock, the less conductive or diffusive it becomes. A study of Groundwater Effect on Great Britain's geology??

The mechanism of groundwater flow is by and large via secondary fractures of rocks and sediments.

Type of rock




- Low saturation, highly permeable

- Extensive unsaturated zone thickness, especially at higher ground

- Significant primary intergranular and secondary fracture

- Substantial seasonal water-level fluctuations

- Thermal conductivity is lowered and thus less efficient for GSHP scheme, for instance, the Lincolnshire Wolds and the Cotswolds


- Low saturation, highly permeable

- Small pore throat size resulting in similar water content in both saturated and unsaturated zones and therefore exhibiting similar thermal properties

- does not have significant effect on GSHP scheme


- High saturation, low permeability, irrespective of elevation

- Therefore high thermal conductivity promotes heat conduction

Table ??? Law of Conservation of Energy

The application of the law of conservation of energy to a control volume gives a partial differential equation to describe heat advection in groundwater, expressed as

where vi = velocity and T = temperature of rock/water matrix.

Complications in the GSHP design process, due to the presence of groundwater throughflow, bring about many numerical modelling codes considering that the ground-loop heat exchanger designs as well as the current in-situ thermal conductivity test data interpretation methods are solely established from models that only consider conductive heat transfer. In this event of one parameter analysis, conductivities deduced from the in-situ tests may seem higher than they should be and hence resulting in over- or under-designed GSHP.

ground surface, as the presence of porewater in the unsaturated zone means that its thermal properties are similar to those of the saturated zone. Numerical Solution

In view of complexity of heat flow equations, numerical solution and modelling is commonly employed to evaluate the significance of groundwater throughflow. One of the methods, Peclet number for preliminary assessment, is by weighing the relative impact of heat advection against conduction on the system performance. Peclet number is a dimensionless parameter that indicates the order of magnitude of heat advection to conduction, defined by

where ρw = density of water (kgm-3), cw= specific heat of water (Jkg-1K-1), L = length scale(m), and κeff = thermal conductivity of the aquifer(Wm-1K-1).

In principle, a large Peclet number indicates that heat advection is more significant or dominant in contrast to conduction transport and vice versa. This means that extracting heat energy from the geological bedrock and deposits with a large Peclet number will be less efficient for heat will be transported away through convection. The exact value of Pe, at order one which advection becomes dominant, is slightly dependent on the choice of characteristic length. This length scale could denote any length dimension, so long as it is consistent with other parameters, for instance, pile depth which corresponds to the thickness of the aquifer penetrated.

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