1. Geothermal Energy Piles<br />Allen Berber<br />Clarkson University 2010<br />Civil and Environmental Engineering<br />Dr. Kerop Janoyan, Associate Professor<br />Michael Gangone, Graduate Research Assistant<br />Department of Civil & Environmental Engineering<br />The use of geothermal energy techniques incorporated with structural pile foundations are an energy efficient solution in sustainable civil engineering projects. These systems consist of closed loop heat exchangers (geothermal heat pumps coupled with vertical polyethylene tubing) embedded in reinforced concrete piles. By circulating a heat transferring medium through high density polyethylene tubing, heat can be extracted from below ground and provide the space heating and cooling needs for multi-story buildings. Due to the relatively steady-state temperature underground, a constant temperature can be maintained within the building throughout the year. There have been many case studies to understand the heating and cooling performance of geothermal energy piles. Little work has been done to understand the thermo-mechanical behavior (i.e. heat transfer processes from the ground and between the soil/pile interfaces) of this system when subjected to heating and cooling cycles (Bourne-Webb et al., Laloui et al.). More studies must be done to understand the effect of thermal cycles on the structural performance of the concrete piles. This project investigates such effects with the use of analytical tools to obtain an ideal working model. The program Pro/ENGINEER will be used to create a 3-D model of a simplified energy pile system and a finite element model will be generated thereafter. To obtain an accurate model, average ground water temperatures of the United States will be utilized. This information will be used to examine the model under real field conditions to provide a better understanding of geothermal energy piles performance. <br />Figure SEQ Figure ARABIC 1 Movement within the geosphereIntroduction<br />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 the earth (figure 1). When considering geothermal energy, it is thought that regions of volcanic activity are most suitable for optimal usage. For example, the use of large geothermal power plants as shown in figure 2, massive amounts of electricity are produced by pumping water into fissures in the hot bedrock below to create steam. This steam them 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. For this particular research in energy pile foundations, the study of thermogeology is more applicable. Thermogeology can be defined as 'the study of the occurrence, movement and exploitation of low-enthalpy heat within the relatively shallow geosphere' [1]. When considering the 'shallow geosphere', typically thermogeology deals with depths up to 200 meters below ground and at temperatures of 30 degrees Celsius. Ground source heat is the 'mundane form of heat found in the ground at normal temperatures' [1]. Energy pile foundations have the ability to use ground source heat and raise the relatively low temperature to a usable level.<br />Figure 4 Components lowered into Borehole [3]Figure 3 Arrangement of Steel reinforcement and Tubing [3]Figure SEQ Figure ARABIC 2 A Geothermal Power Plant in Iceland<br />Geothermal energy piles are typically laid out in a pattern suitable to support the structure above and the typical cross section consist of concrete, steel reinforcement and high density polyethylene tubing. The tubing and steel reinforcement are arranged together (Figure 3) before being lowered into a drilled shaft or borehole and filled with concrete (Figure 4). Once all piles are in place, the tubing is connected to a ground source heat pump which circulates a heat transferring medium throughout. As it is pumped down into the pile the medium is heated by the ground through heat transfer processes and it is then pumped back into the building and distributed throughout. Energy pile systems are conventionally used for multi-story buildings as they have high design loads and require more energy efficient systems to operate. <br />Figure 5 A Typical Geothermal Energy Pile System <br />Some advantages of geothermal energy piles are that they allow the structural as well as heating/cooling needs of a building to be met within one system. This creates a more sustainable design requiring fewer components to provide safe and comfortable living and working spaces. Since they derive most of their energy from the environment they have a reduced need for fossil fuels and have shown statistically that they reduce CO2 emissions by 50% as compared to other heating/cooling systems. Finally, since they require the heat derived from the ground to operate, they can be used in almost any location. A great advantage of ground source heat pumps is that they derive most of their energy from the environment and they can therefore operate in many locations. Ground source heat pumps are rated by a coefficient of performance (COP) which is expressed as:<br />COP = energy output / energy input<br />As stated by H. Brandl [3], a value of COP = 4 means that from one portion of electrical energy and three portions of environmental energy four portions of usable energy are derived [2]. Disadvantages of this system are that they generally require some electrical input to operate the heat pump. They risk leakage of the heat transferring medium, usually consisting of saline or antifreeze solution, into the surrounding soil causing severe environmental hazards. Lastly, due to the renewed interest of this technology within the past few years and among the scientific community, there is very limited information regarding the impact of these heating and cooling cycles on the structural performance of the pile foundations. <br />The objective of this research project is to understand the thermo-mechanical effects of ground source heat pump systems on structural pile foundations. It is important to understand what effect heating and cooling cycles have on the behavior of concrete foundations in order to determine their structural stability. To investigate the thermo-mechanics of energy piles, comprehension of the interface between the soil and pile surface must be understood. Pile foundations can find their stability by the use of frictional forces acting along their length and at their tip. Questions arise about the structural integrity of the pile when subjected to heating and cooling cycles since the pile will expand when heated and contract when cooled. This constant change will hinder its structural capabilities and will eventually fail. Therefore, engineers must understand these effects and research possible solutions that will help to create better design practices and standards. <br />Figure 6 3-D model of Geothermal Energy PilesDue to the variability in geologic conditions and the wide array of regions in which this technology can be applied, researchers should be mindful of these differences. Slight variations in the placement of piles can significantly affect their performance. As shown in Energy pile test at Lambeth College, London by Bourne-Webb et al., different end restraints in combination with thermal loading can have a considerable impact on the forces imposed within the concrete piles. In their experiment, test piles were subjected to heating and cooling cycles under maintained loading periods. They utilized an optical fiber system as well as other instruments to measure the temperature and strain induced on the piles. They were able to compare the load transfer carried by the pile under separate cooling and heating cycles. As seen in figure 6 different loading conditions are explained. It shows a pile being subjected to a load only, heating or cooling only as well as a combination of both. It can be seen that when under a load only, the pile resists this force by the frictional forces along its length (pile/soil interface) and at its tip. When only a cooling cycle is applied, the pile is free to move and contracts. If the pile had been restrained at one end, tensile strain would develop and excessive shear stresses would cause failure. The same holds true when a heating cycle is applied except that compressive strain would develop instead of a tensile strain. Under a combined loading and thermal cycle there was some increase in the tensile axial forces within the pile. <br />Experiment<br />To understand the thermo-mechanical effects imposed upon the pile, a computer generated model was created using the program Pro/Engineer. This program will be used as a platform to perform a thermal analysis of the energy pile. An essential part of this research project is to be able to create a three dimensional model that can be subjected to certain boundary conditions. These conditions include but are not limited to temperature, soil characteristics, ground water levels and temperature, heat pump types and capacities, loading and end restraints of piles, etc. Knowing these conditions before designing the system an accurate model depicting the true field conditions of the project site can be created. Afterwards, thermal analysis will predict what will most likely occur to the pile when subjected to these conditions. <br />Thermal analysis in combination with finite element analysis is a powerful tool in computer modeling since it has the ability to numerically solve complex engineering problems [2]. Many trials can be done before any full scale models are constructed. This allows designers to save time and makes for a more economical design. <br />Conclusion<br />Geothermal energy pile foundations have the potential to become a new source of renewable energy in conjunction with either wind or solar technologies. A vast amount of regions in the world have the capability to use geothermal energy and the use of energy foundations can help to make this technology available for commercial use. With the development of computer simulations and finite element analysis, a better understanding of the possible issues of this technology can be easily predicted before any construction has occurred. Further development of these computer generated models and the application of the finite element thermal analysis will help to increase knowledge of this technology, its application, design and construction.<br />References<br />Banks, David. An Introduction to Thermogeology: Ground Source Heating and Cooling. Oxford: Blackwell Ltd, 2008. Print. <br />Chandrupatla, Tirupathi R., and Ashok D. Belegundu. Introductionto Finite Elements in Engineering: Second Edition. Upper Saddle River: Prentice Hall, 1997. Print. <br />Brandl, H. quot;
Energy foundations and other themo-active ground structures.quot;
Geotechnique 56 (2006): 81-122.<br />