There is a growing global trend to utilize alternative energy resources mainly driven by rising energy demand, depleting natural resources and adverse effects of carbon emissions from fossil fuel consumption. The earth's near surface contains a huge potential of stored geothermal energy that can be used for heating and cooling. Below a depth of about 10 m, seasonal ground temperatures remain stable compared to outside air temperatures, typically lying between 10-18° C. This near surface heat energy can be efficiently accessed using heat exchangers consisting of buried pipes (i.e., "geothermal loops") filled with a glycol-water mixture that is circulated between the structure and the subsurface using a ground source heat pump. Over the past 20 years, this ground coupling concept has been expanded from mainly residential applications to large-scale projects. Of particular interest is the application where geothermal loops have been integrated into deep foundation elements, such as piles, piers, or drilled shafts. These systems, referred to as Energy Piles, are used in soft soils where deep foundations are already needed for structural support. The integration of the geothermal loops comes at little additional cost. Energy Piles have the advantage of being applicable in any climate or region, including those where wind and/or solar power have limited effectiveness. In ideal conditions, Energy Piles can significantly reduce carbon footprint and lower heating and cooling costs and energy use for buildings by as much as 80%.
Although Energy Piles have seen exponential growth in Europe and Japan over the last decade, they have received little attention in the US. This is partly due to a lack of awareness about their benefits, along with a lack of US case studies that demonstrate their cost-effectiveness. There are also questions related to their thermal and thermo-mechanical behavior that must be answered before their designs can be fully optimized. The main issues involve a lack of standardized field testing procedures and a lack of long-term operational studies that cover a wide range of climatic and soil conditions. The main purpose of this study is to investigate long-term Energy Pile behavior and to develop new Energy Pile design guidelines. The project will involve field testing at five sites across the US, along with advanced numerical modeling, and technology transfer and outreach. During the field tests, the tested Energy Pile will be loaded to simulate the combined effect of the building load as well as the temperature changes due to heat exchange operations. In addition to field tests on individual piles, the heat exchange behavior of a group of 9 Energy Piles will be tested at one of the sites to better understand the group efficiency of Energy Piles. Experimental results from the field tests will be used to calibrate the advanced numerical models that simulate long term Energy Pile behavior in a series of parametric analyses.
This study is conducted in partnership with several industrial participants as well as non-profit organizations and government agencies. Several industrial participants are providing in-kind contributions by building full-scale field tests, donating materials, instrumentation and heat pumps. Three foundation engineering contractors (Berkel, Thatcher and Layne GeoConstruction) are installing Energy Piles and assisting with the field testing. Piping for the geothermal loops and heat pumps for the field tests will be provided by REHAU and WaterFurnace, respectively. Geo-Instruments, Inc. is assisting with field instrumentation and data acquisition, and providing the necessary sensors for the field tests. Several non-profit organizations and government agencies such as the US Green Building Council, the Deep Foundations Institute, and the Federal Highway Administration are participating by reviewing the findings, providing guidance, and helping to broaden dissemination of the results to the engineering community and beyond.
The main intellectual merit is that, this university-industry collaboration will answer key questions about long-term thermo-mechanical and Energy Pile group performance, and provide new industry design guidelines and field testing procedures. In terms of broader impact, the work would help promote the use of Energy Pile technology in the United States, and thereby help reduce fossil fuel consumption, lower greenhouse gas emissions, lower the cost of heating and air conditioning, and decrease our reliance on foreign energy.
