INTRODUCTION
The issue of climate change is one of the greatest economic, social, and environmental challenges that the world has been exposed to. Countries, governments and individuals need to find ways of reducing the carbon footprint and one such option is the utilization of alternative, renewable energy sources. With new technologies in the field of the alternative energy sources being continually developed and the recognition of its important contribution earned by government organizations and the general public, implementation has recently become more feasible. Such an alternative renewable source of energy is the geothermal energy.
Traditional geothermal energy systems require interaction with kilometer-deep strata of rock, where thermal energy is much greater and can produce hot fluids to drive turbines for electricity. Its use is however weighed down by cost and practicality. More recently, encouraging developments are being achieved in the field of shallow geothermal energy systems. These systems show great potential, comparative to the traditional systems, in terms of long term sustainability, access, flexibility and economics. Shallow geothermal energy is based on the principle that the subsoil can be employed as a thermal energy source by using its natural potential and thermal storage capabilities. The use of these systems is however limited to heating applications due to the lower temperatures extracted. The benefits of this environmentally sustainable technology make it an attractive alternative to conventional heating systems, which require larger amounts of input energy and thus leads to increased greenhouse gas production.
Ground-source heat pump systems are a technology that taps into the shallow ground as a geothermal energy source. A subset of this system involves geothermal energy piles, also termed closed-loop systems. The most common application of these systems has been to achieve energy efficient space heating and cooling for both residential and commercial buildings of various sizes while satisfying load bearing requirements of the underlying foundation.
The following review will provide insight into many aspects of this technology. Technical background and environmental considerations will assist in understanding the various factors that govern the design of these systems. Experience and a wide range of applications that have been documented in past decades will be provided.
GEOTHERMAL ENERGY PILES
2.1 Geothermal energy
Geothermal energy is the second most abundant source of heat on earth, after solar energy. Geothermal energy can be defined as the energy derived from the heat within the earth. This heat is created through the constant movement of magma and tectonic plates on the surface of earth. When considering geothermal energy, it is thought that regions of volcanic activity are most suitable for optimal usage. Large geothermal power plants produce massive amount of electricity by pumping water into fissures in the hot bedrock below to create steam. This steam then turns large turbines which can produce electricity. Only certain regions in the world are capable of producing such massive amounts of energy. These regions are found at the boundaries between tectonic plates and therefore limit the usage of this technology.
Geothermal temperature increases with depth in the earth’s crust. Using the technology available at present, it has been found that the average geothermal gradient is about 3°C per 100m. According to Lund (2009) the approximate total thermal energy above surface temperature to a depth of 10 kilometers is 1.3×10²⁷ Joules, equivalent to using 3×10¹⁷ barrels of oil. As global energy consumption is equivalent to about 100 million barrels of oil per day, the thermal energy to a depth of 10km would supply all of mankind’s energy needs for six million years. However, based on current technology, only a fraction of this energy is available as a recoverable source.
2.2. Geothermal piles
Geothermal piles consist of pile foundations combined with closed-loop ground source heat pump systems. Their purpose is to provide support to the building, as well as acting as a heat source and a heat sink. In effect, the thermal mass of the ground enables the building to store unwanted heat from cooling systems and allows heat pumps to warm the building in winter. Generally, ground source heat pumps used in domestic situations extract heat from the ground, by way of underground pipes which are laid either horizontally or vertically in a hole in the ground. In geothermal piles, the pipe loops are laid vertically, in order for it to be possible for them to be incorporated into the pile foundations.
2.2.1 Construction of Geothermal piles
Structural piles are turned into heat exchangers by adding one or more loops of plastic pipes down their length. In the construction of geothermal piles, the pile diameter and length should be designed to resist the applied structural loads, and not increased to suit the geothermal requirements. When constructing the piles, initially the soil is bored out of the ground and a rigid, welded reinforcement cage is inserted. Several close-ended loops of high density polyethylene plastic absorber pipes (generally 25mm diameter and 2-3mm wall thickness) are then fixed evenly around the inside of the reinforcement cage for the full depth.
Loops are fabricated off-site and filled with heat transfer fluid (water with antifreeze or saline solution) and fitted with a locking valve and manometer at the top of the pile cage. Before concreting, the absorber pipes are pressurised for an integrity test, and to prevent collapse due to the fluid concrete. This pressure is maintained until the concrete hardens and reapplied before the absorber pipes are finally enclosed. When concreting, the tops of the pipes are held back to avoid damage and a tremie pipe is placed to the base of the pile. Concrete is poured through the tremie and it is raised up as the concrete fills the pile. Once the pile is finished, the absorber pipes are connected to a heat exchanger which is then connected to a secondary circuit of pipes in the floors and walls of the building.
fig.2.1: HDPE pipes after concreting
TECHNOLOGICAL BACKGROUND
3.1 The overall system
The conventional description used by many has involved dividing up the overall system, into three major components:
– the primary unit (ground heat exchanger),
– the heat pump and
– the secondary unit (pipe work for heating of the receiving infrastructure)
Fig.3.1: Process of the overall system heating only.
(Renewable & sustainable energy reviews14-2010)
Preene and Powrie (2009) proposed an alternative categorisation of the system into three main components: ‘the source side’, ‘the load side’ and ‘the heat transfer system’. This second classification represents a more function-based characterisation as compared to a component based approach conventionally adopted. The roles of some components will change when considering the use of these systems in heating and cooling modes. Ground-source heat pumps can be designed to operate as a single mode system only or can incorporate both heating and cooling operations.
Primary unit: Ground heat exchangers constitute the primary unit of the overall system with the task of extracting and injecting heat into surrounding soil. Conduction and convection are the two primary heat transfer mechanisms that take place between soils surrounding these subsurface elements as heat absorbing fluid is circulated through a loop of pipes embedded within these exchangers. A pump is responsible for driving the flow of heat transfer fluid through the subsurface pipes. In summer, the ground is used as a heat sink where thermal energy is stored in an effort to cool the infrastructure. This heat is extracted in winter for heating purposes as the ground changes its role to a heat source.
Heat pump: The heat pump is a mechanical device that functions like a reverse refrigerator. Its basic components include the compressor and expansion valve. The former compresses the refrigerant within the pump to a gaseous state of elevated temperature and pressure. In passing through the pump towards the expansion valve, heat is exchanged with the secondary unit leading to a decrease in temperature of the refrigerant. Upon reaching the expansion valve, the gas undergoes a change of state from gaseous to liquid and the cycle repeats itself. Primary and secondary units will supply and receive the thermal energy contained within the pump allowing for the cycle to run. For heating operations, the thermal energy obtained from the ground is transferred to the heat pump, which uses a compressor to elevate temperatures, the resulting thermal gradient is used to heat fluid or air within the secondary unit. During cooling mode, the surrounding environment transmits heat to the secondary unit (connected to the primary unit through the heat pump) and the reverse heat pump cycle conveys this thermal energy into the ground. Heat is transmitted from the secondary unit to receiving infrastructure in heating mode and vice versa in cooling mode. It is important to note that these systems do not completely offset the energy usage and that electrical energy input is still required to drive the heat pump. To quantify the temperatures, which need to be achieved by the pump, an example by Brandl (2006) suggests that initial temperatures of 10–15˚C that can be obtained from these shallow geothermal systems need to be elevated to a temperature range between 25 and 35˚C.
This component is regarded as the most energetic of the entire system. Careful design must ensure that the efficiency of the pump does not greatly affect the performance of the entire system. For systems, which are designed solely for the purpose of cooling, efficiency can be increased and energy saved by following the recommendation of omitting the pump and instead using plates for heat exchange.
Secondary unit: The secondary unit consists of pipes embedded in floors and walls of buildings, bridges, beneath roads and other infrastructure, which require heating. Its primary function is to utilise the extracted thermal energy for heating purposes during the winter and to receive unwanted heat from its surroundings in summer for transfer into the ground.
3.1.1 Types of ground-source heat pump systems
Ground-source heat pumps primarily consist of two types: Open and closed-loop systems. Open systems directly utilise the ground’s thermal storage medium. Groundwater is pumped from a well to the heating system to provide thermal energy to the secondary unit with the help of the heat pump. After the heating operation, water (at a significantly different temperature) is either injected back into the aquifer (using a second well) or disposed off in surface water bodies. Although these systems have been widely used and involve lower initial costs, long-term high financial, technical and environmental risks have become apparent.
Further limitations are restrictive regulations on groundwater use, required water quality (low iron content), limited availability of installation sites and required aquifer size. All these factors have caused a shift in preference to closed-loop systems. Closed-loop systems adopt a method whereby the heat transfer process occurs indirectly between soil and a heat carrier medium flowing through pipes. Energy piles are a form of closed-loop system, however alternative configurations exist using loops of pipes (without foundation elements or embedded in other structures such as diaphragm walls) placed horizontally (easier construction) or vertically (preferred configuration) beneath the ground. Vertical systems are generally preferred due to their lower surface area requirements, shorter pipe lengths, lower pumping costs and higher efficiency with less variability.
Horizontal loops are affected by shallow ground temperature fluctuations. Closed-loop systems are regarded to have a higher initial cost in comparison to open systems, but this is offset by greater versatility, long-term economic benefits and lower long-term risks permitting their installation under many types of ground conditions.
Fig.3.2: closed loop system
(Australian geothermal energy conference-2011)
Fig.3.3: open loop system
(applied energy101- 2013)
3.2 Energy pile materials
Geothermal energy piles fulfil two purposes, they are designed as both structural foundation elements and ground heat exchangers. These two purposes need to be considered during the design step of material allocation. The main materials used in the construction of bearing or friction piles that have been used in past geothermal energy pile studies include the following:
– Precast or cast in situ reinforced concrete
– Steel, and
– Grout
Reinforced concrete piles have been found to be advantageous due to the material’s high thermal storage capacity and enhanced heat transfer capabilities. Precast or driven piles are less favoured in comparison to the more subtle technique of cast in situ piles as the latter technique poses less harm to the integrated heat exchanger system. Steel had greatly assisted in the heat transfer between ground and circulating fluid due to its low thermal resistance and high thermal conductivity. There were two possible ways of incorporating ground heat exchange by use of a steel pile: direct water circulation (open system) or the more preferred option of using heat carrier pipes (indirect, closed-loop system). Variations of the above pile designs are described by Brandl (2006) and include steel tube piles filled with concrete and heat exchangers as well as vibrated concrete columns outfitted with heat absorber pipes.
Partially grouted stone columns have also been used, but have lower geothermal efficiency. This is primarily due to the grout, which has been an issue with other ground-source heat exchanger systems. Grout, however is an important component for borehole heat exchangers as it supports the pipes and protects groundwater from contamination as a result of pipe leaks. Consequently, specific measures need to be met to ensure successful thermal performance. The use of thermally enhanced bentonite grout or mixtures with sand will overcome issues of low thermal conductivity experienced with standard grout. It is known that increasing quartz content will improve thermal conductivity of the soil. In addition to these suggestions, it must also be ensured that the grout will maintain its thermal properties throughout the pile’s operation. Enhancing the geothermal efficiency of grouted energy columns can therefore be achieved and may potentially reduce the overall cost of the system by minimizing overdesign of the foundation (especially when piles are not required for bearing support). The choice of alternative heat exchanger systems may however be preferable from an economic and practical standpoint in such cases. The figure below shows HDPE pipes for pre casted piles. Here a protective cover is provided over the pipe to prevent its damaging while the pile is driven into the ground.
Fig.3.4: HDPE pipes for pre cast piles
3.2.1 Absorber pipe materials, shape and heat carrier fluid
Materials: Absorber pipes are commonly made of high-density polyethylene(HDPE), but PVC has also been trialed in the past. For concrete piles, the pipes are fixed to the reinforcement cage. Prior to placement of concrete, pipes are pressurised. This pressure is maintained to resist the external wall pressures imposed by the wet concrete and relieved only once the concrete has hardened after a few days. Pipe diameters range from 20 to 25 mm and their lengths will depend on several factors including pile length and performance requirements.
Heat carrying fluid: Heat transfer liquid, which is fed through the pipes serve the purpose of transmitting or receiving heat to and from the ground. For buildings where the cooling loads are much greater than heating loads, water may be sufficient. Its use is common, but not recommended in colder climates, where freezing of the fluid can occur resulting in damage to the pipes. For cooler climates, antifreeze solutions such as water and glycol mixtures, saline solutions, brine, potassium acetate or even methanol are possible substitutes.
Fig.3.5: Pile reinforcement with HDPE pipes
Shape: The installed pipes adopt the form of continuous loops of certain shapes. The choice of shape will affect the overall efficiency of the system. Common shapes, shown in 6, featured in several studies are:
– Single, double and triple U-shaped pipes
– W-shaped pipes
Single or multiple U-shaped or W-shaped pipes are the main types installed within the concrete piles. Single U-shaped pipes were regarded as the most efficient choice from an economic standpoint and in terms of workability. Experimental testing and numerical simulation results concluded that W-shaped loops were more effective than U-shaped loops, but its performance offset by high cost.
Fig.3.6: Four types of pipe configurations for energy piles.
(Renewable & sustainable energy reviews14-2010)
DESIGN CONSIDERATIONS
The following section will look at existing design standards for energy piles, the tools for prediction of system performance and present an appropriate design procedure along with current implementation issues to be aware of.
4.1 Current design standards
The German VDI 4640 design standard is the most comprehensive standard to date comprising of four sections published from 2000 to 2004. Section 2 in particular deals with ground-coupled heat exchangers, which include energy piles. Other guidelines include the IGSHPA guidelines, which are used by some contractors in both the United States and the UK and the guidelines for geo exchange systems in British Columbia, Canada.
4.2 Performance assessment
Methods allowing for performance assessment of ground-source heat pump systems during the design stage and after construction include:
– The coefficient of performance (COP)
– On-site testing
In the context of ground-source heat pumps, traditionally the most important performance assessment quantity is the coefficient of performance (COP), which is defined by
COP = Heat output(kw)/Electrical input(kw) (4.1)
The COP indicates how much heat can be gained for a unit input of electrical energy. Design often aims for values between 2 and 4.
Each individual component of the system has an effect on the overall COP value and thus optimizing the design of each individual component can ensure an appropriate COP. Selection of the heat pump has a significant impact on overall system efficiency and consequentially the COP. In operation stage, environmental factors can also affect the system performance. Michopoulos(2007) for example explain that injecting heat into an already high temperature ground will make the system inefficient resulting in a lower COP.
In order to assess the performance of the system once in operation, various tests are available. The thermal response test is probably the most popular choice and although a lack of literature documenting this test on energy piles has been found and the majority of reports have involved testing on borehole heat exchangers, use of the methodology can be easily transferred across different heat exchanger systems. The test involves applying a specific thermal load into the ground-source heat exchanger and measuring the resulting temperature changes of the circulating fluids. The results include graphs of fluid temperature development against time, thermal conductivity and thermal resistance of the exchanger, which provides the temperature drop between natural ground and fluid in pipes.
4.3 Design factors
Many important factors that need to be considered in the design of geothermal energy pile systems have been identified in past application. In the context of providing an alternative energy source, a general design procedure can be developed. Each of the following steps requires careful consideration of the pertinent aspects covered. Knowledge of the desired heating and cooling characteristics of the building is probably the most important first step to be
taken. These details determine whether a monodirectional (one operation mode only) or bidirectional (dual-operation mode) system is needed, what the preferred soil properties are for optimum performance, and whether any design is capable of providing the necessary energy requirements without additional assistance.
As with conventional pile design, the assessment of the systems requirements should then be followed up by an extensive geotechnical site investigation including (but not limited to) the following details:
– Geological strata (e.g. shallow profile as well as identification and depth of the foundation rock.)
– Geotechnical properties (e.g. water content, density, void ratio, hydraulic properties, strength parameters, etc.)
– Geothermal properties (e.g. thermal conductivity, specific heat capacity at different temperatures, in situ ground temperatures, the existence of thermal gradients, etc.)
– Hydrogeological properties (e.g. depth to groundwater, fluctuations of water levels, flow direction and velocities, etc.)
– Mineralogical and geochemical soil properties
The more information available, the more efficient the design can be thus achieving the most optimum performance and output of the system. Some of the above investigations may already be required if the geothermal energy piles have the existing primary purpose of structural stability. Pile dimensions and spacing dictate land and excavation requirements as well as system performance, but are in turn strongly influenced by several factors including: quantity of piles required satisfying both structural and thermal loads, project budget and piling depths if limited by site geology.
Absorber pipe material should be of thermally fused HDPE, which provides strength and reliability. Nowadays, special fittings are available for ground-source heat pumps. The size and number of pipes as well as shape of the embedded loop will affect installation and pumping costs as well as influence pipe friction loss and achievable flow rates. The choice of heat transfer fluid used is also important and Kavanaugh (1998) recommends the use of less antifreeze fluid. Choice of fluid should be based on availability, economics and non-corrosive properties. An adequate flow rate of the fluid through the system should also be selected. It has been established that low flow rates are preferred to allow for better delivery of heating and cooling requirements.
4.4 Implementation issues
In theory, geothermal energy pile foundations can work efficiently if properly designed. In practice, the performance of these systems is dependent on a lot more issues such as installation, component defects and proper use. Being relatively new, no definitive procedure exists for construction of these systems and this is an important factor that needs to be addressed as many stories exist of flooded construction sites, failed drilling jobs, and poorly performed systems. Amongst the design suggestions listed by Kavanaugh (1998) it is important that experienced contractors in this field are consulted if problems are to be minimized. The issue of apparent skills shortages is currently being addressed in the UK by the Ground Source Heat Pump Association (GSHPA).
APPLICATIONS
Ground-source heat exchangers have been found to have a range of applications in providing geothermal energy. This section will highlight the areas where energy piles are a feasible choice.
5.1 Different applications of ground-source heat pump systems
The most common use of this technology revolves around domestic and commercial space heating and cooling as well as the production of hot water. Rawlings and Sykulski (1999) list a number of commercial facilities, where these systems can be found including offices, schools, shops, hotels, sports centers, institutional buildings and military complexes. Adding to the list are other elements of infrastructure such as tunnels, green houses, roads, bridges and the agriculture industry.
Energy piles are suited for buildings that already require structural foundation piles. For tunnels and other structures, their use becomes less feasible, on an economic and practical level. Consequently, the choice of horizontal loops and diaphragm walls are attractive alternatives. Outfitting road tunnels and underground subway tunnels in urban areas can allow for profitable heat extraction as larger ground volumes are activated allowing for greater amounts of heat extraction and storage. Suggestions have also been made to use car parks as large collectors to supply nearby buildings for their heating and cooling needs.
The potential for this technology for the agriculture industry has been assessed by Tarnawski (2009) and Ozgener and Hepbasli (2005). Heating and cooling demands for growing vegetables in greenhouses, drying crops, heating water at fish farms and pasteurizing milk have all been considered. The farming sector has also been regarded to have fewer restrictions on ground availability.
5.2 Improving the thermal imbalance in colder climate
It has been found that solar panels aid in improving the thermal imbalance when heating is the predominant operation of the system. The idea of coupling the two renewable energy technologies was suggested in the 1980s and has become an interest in China. The underlying concept is to utilize solar collectors to assist in recharging the ground with thermal
energy during the summer months and to fulfill part of the required heating demand in winter. According to Zogou and Stamatelos (2007), this technology has shown its effectiveness in Northern Europe and is now under research in Asia.
General operation procedures would involve the collectors operating mostly throughout summer in an effort to recharge the ground and during the daytime in winter to supply the heating requirements. There are a few issues to be aware of when incorporating a solar collector into the system. Studies by Bi (2005) showed that care must be taken in the design as the collector’s COP has an effect on that of the overall system. Quoted values of COP showed the combined system was slightly less efficient (2.78) than the ground-source heat pump by itself (2.83). Despite the observed decrease in efficiency, there is still interest in this combination for three main reasons: the reduction in fossil fuel consumption, the use of non-polluting sources of energy and long-term environmental sustainability. Fig.5.1 shows a railway platform in Lauterberg, Germany which uses horizontal loops for heating and thereby clearing snow and ice from platform.
Fig.5.1: A railway platform in Germany
5.3 Improving the thermal imbalance in warmer climate
In the situation where cooling is the predominant operation of the system, the ground can be used as a cold storage to offset the otherwise long-term rise in ground temperature. Fan (2008) investigated this possible solution for systems installed in certain regions of China that lie in the middle and downstream of the Changjiang River. These regions exhibited warm climate conditions and as a result, the systems were required to output significantly higher cooling loads compared to heating loads. Due to the imbalance of the energy requirements, the soil temperature gradually began to increase over time due to the inability to complete the recovery process. This rise in temperature resultantly decreased the performance of the overall system.
In order to solve this problem, a new system, referred to as the integrated soil cold storage and ground-source heat pump system, was implemented. In order to reduce the imbalance caused by the significant cooling requirements of buildings, this system, charges cold energy to the soil at night in order to aid the system in producing chilled water to meet cooling needs during daytime. By injection cool night temperatures into the soil, the soil will exhibit lower temperatures during the day and hence its performance will significantly improve when required to provide cooling within the building. There is also the economic advantage of using the system at night because this is during the energy usage off-peak period and will result in a decrease in power consumption during the day since the system will perform more efficiently.
POSSIBLE CHALLENGES AND HOW TO OVERCOME THEM
There are some potential challenges that may have to be faced when constructing and using geothermal piles. Firstly there are issues related to the newness of this technology, namely that there is a severe skills shortage at all levels of the procurement chain. For example, there is difficulty finding good drilling operatives with the right kind of experience, leading to flooded construction sites, failed drilling, damaged pipes and poorly working systems. Design consultants also lack training which, along with a lack of UK design standards, leads to ‘open’ specifications and poor integration of ground source heat pumps into buildings. This leaves contractors with the opportunity to deliver lower quality equipment, materials and workmanship than may be expected.
There has also been significant concern about the effect of cyclical heating and cooling on pile performance. There have been two major studies into the impacts of this repeated heating and cooling: at the Swiss Federal Institute of Technology in Lausanne in 2006 and at Lambeth College in London in 2009. In Lausanne, thermal testing was carried out on a single geothermal test pile at intervals during the construction of the building: heating and recovery cycles were applied as increasing loads were added to the piles. This study indicated that the thermal loads on geothermal piles induce additional stresses on surrounding structural piles, causing a decrease of lateral friction. It confirmed that geothermal piles can be designed to absorb these thermal effects without causing undue subsidence of the foundations.
In the Lambeth College project there were a total of 146 piles at a depth of 25 metres. The study of the pile response to heat cycles was performed by Faber Maunsell, Skanska Cementation and Geothermal International. Pile-loading tests that incorporated temperature cycles while under an extended period of maintained loading were undertaken for seven weeks. It was found that concrete stresses in addition to those due to static loading were generated when the pile was heated. However, the shear stresses mobilised at the pile/soil interface during thermal cycling were not excessively large and it was concluded that the geotechnical capacity of the piles was unlikely to be affected and that minimal settlement occurred.
Another issue is the risk of long-term ‘below ground global warming’ or ‘below ground global cooling’, which is caused by an imbalance in the heating and cooling demands of the buildings above, especially as geothermal piles become more popular in densely populated areas. The solutions to this problem are to diversify the profile of buildings served by geothermal piles in the local area and to design buildings in such a way that the heating and cooling demand is balanced ( for example, if there is a high cooling demand, incorporate water heating into the system to balance this). However, if in the long term these strategies fail, the ground can be artificially helped back to its undisturbed temperature using dry coolers to cool the ground or waste heat recharge of the ground when the heating demand across the year is imbalanced.
ADVANTAGES & DISADVANTAGES
Advantages
Sustainable and Renewable Energy Source
Typical 50% reduction in building carbon emissions
Can achieve 16% renewable content
Gives fuel cost savings
No local emissions or pollution
No external vents or flues
Assists with planning approval
Significant reduction in plant room space
The efficiency of these systems is inherently higher than that of air-source heat pumps
Disadvantages
Efficiency of the heat pumps and absorber systems will reduce when substantial temperature fluctuations take place during the year
If the soil at the site consists mainly of dry sand or gravel, deeper piles and a larger area of absorbers are required which may significantly increase costs and reduce economic benefits
If the systems are installed in climates that may result in soil freezing, antifreeze solution is required, which may induce potential for leaks. This can however be avoided with the correct use of fluid within the system.
A heavier regulation is necessary to ensure long-term performance and prevention of short-circuiting of individual systems and impacts on nearby installations.
Improper setup, misuse and designs have been found to occur resulting in inadequate flow rates, leaks within the system, inadequate heat exchange with the ground and interference amongst components.
In terms of design, the major limitation that exists is that no theoretical model used to predict performance is able to consider all the parameters that affect the system.
CASE STUDIES
8.1 Main Tower – Frankfurt, Germany
The building reaches a height of 200 m. The building is founded on a combined pile-raft foundation. The thickness of the raft within the tower is between 3 to 3.8 m. A total of 112 piles with diameter of 1.5 m were installed. The length of the piles varies from 20 m to 30 m (fig.8.2). Power withdrawn from the ground was500 kW. The ground encountered consists of quaternary sands down to 10 m below the surface where it is underlain by tertiary sediments of the Hydrobien. These sediments (Frankfurt clay) consist of clay interbedded with sands and limestone bands (fig.8.2). The ground layers of the Inflaten (Frankfurt limestone) and Certithien (marl) were encountered beneath the Hydrobien. To ensure an economic design of the Main Tower two innovative ideas were put forward:
the bored piles of the retaining wall are part of the foundation system (combined pile-raft foundation). They transfer the loads in addition to the 112 foundation piles into the ground. Fig.8.1 shows the position of the piles of the foundation and of the retaining wall.
.
Fig.8.1: combined pile-raft foundation
(proceedings of the 1st intelligent building Middle East Conference, 2005)
Fig.8.2: Ground model
(proceedings of the 1st intelligent building Middle East Conference, 2005)
Apart from their static function the piles of the foundations and partly of the retaining wall are used for the environmental-friendly heating and cooling of the building. For this, the piles were additionally installed with heat exchanger tubes, so that the piles work as heat exchanging elements to create a closed system. Energy is transferred to the ground from the exterior (outside air) and stored until it is needed. The energy piles can load and unload the seasonal storage. In winter energy can be withdrawn, thus a cooling of the ground arises. In summer the cooled down ground can be used for cooling the building through the ceilings. For this, a very low groundwater velocity is essential
Fig.8.3: Frankfurt Main Tower
CONCLUSIONS
Geothermal heat pumps are highly efficient heating technologies that allow for reductions in CO2 emissions, the potential avoidance of fossil fuel usage and economic advantages. Heat pumps utilize significantly less energy to heat a building than alternative heating systems. Many variations of geothermal systems for heating exist, with different configurations suitable in different situations and most locations around the world. Heat exchanger or energy piles have the potential to reduce energy demand in built structures and tackle the ever challenging climate debate. Energy piles are increasingly used in various parts of the world today, and the benefits, experiences and opportunities gained from these experiences can be adapted and applied to the local conditions.
This review covered an extensive collection of literature, looking at the design, environmental impacts and operation experiences of geothermal energy pile systems. Developments in this technology have resulted in systems that are cost efficient, environmentally beneficial and socially acceptable. Furthermore, geothermal energy pile systems have been found to have great potential as an aid in tackling climate challenges and meeting legislation requirements. Research has found that over 80 countries have utilised shallow geothermal energy technology and gained the benefits and opportunities from it.
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