As global energy demands continue to rise, along with the production of harmful greenhouse gases, there is a critical need for research to advance innovative renewable green energy technologies. Geothermal energy, for instance, is a renewable energy source whose use in the US has been limited mainly to western regions where high ground temperatures near the surface have been exploited for residential heating and production of electricity. With the development of new technologies, we can exploit geothermal energy much more efficiently and broaden its applicability to almost any geologic and climatic condition, including areas where there is no geothermal heat source. That is, there is tremendous potential in simply utilizing the relatively constant temperature of the ground (i.e., "geothermal energy") in the upper 30 m of the soil profile, about 55F (13C) in most areas, to help regulate the temperature of buildings.
This research will investigate the use of Energy Piles, a new geothermal energy concept designed to efficiently access the constant temperature of the ground for heating and cooling of buildings. In this new concept, the foundation piles that are already in place for support of the building are used conjunctively as geothermal cooling/heating elements. The piles, typically 20 to 30 m long, are installed with circulation tubes that act as heat exchangers where heat energy is circulated through the tubing with water or antifreeze. Heat energy from the building is fed into the ground for cooling in the summer and withdrawn from the ground for heating in the winter. The fluid circulation is performed via a heat pump similar to those used in conventional residential and commercial applications. Cost savings for heating and cooling could be as much as 80% for buildings outfitted with Energy Piles, especially in extreme climate regions.
Pilot tests and limited applications for major buildings have been recently completed in Japan and Europe, respectively, but the Energy Piles concept is largely unknown. No major systems of this type are currently being used the US. This research will develop the data and expertise needed for wide-scale and efficient implementation of this promising new technology. The project will involve development of a full-scale field test section, advanced numerical modeling and a detailed cost-benefit feasibility analysis to study key aspect of Energy Pile systems, such as what factors affect their conjunctive performance as heat exchangers and as load bearing foundation elements, how their performance varies with differing ground and climatic conditions, and how they can be installed and operated most cost-effectively. The main intellectual merit is that the findings could lead to wide-scale implementation of a new alternative energy technology that is currently being underutilized.
In terms of broader impact, the study represents an important new step toward the sustainable design of "green" buildings that use near-zero energy for heating and cooling. In addition to being an alternative renewable energy resource that reduces greenhouse gas emissions, Energy Piles offer the added advantage of being applicable in any climate or region, including those where wind and/or solar power have limited effectiveness. The findings of this study will be disseminated to a broad audience via journals, the Internet, and incorporated into academic courses and the PIs' frequent professional shortcourses for ASCE, FEMA, FERC, and USACE. Finally, there will be a diverse research team, including an ethnic minority, a disabled person, senior and junior faculty and an undergraduate student researcher.
There’s a developing trend around the world to explore alternative energy sources. The main driving forces are growing global energy demand, depleting natural resources and the potential effects of greenhouse gas emissions from fossil fuel consumption. Geothermal energy is one of the promising renewable sources that can be utilized to offset such trends. To date, the use of geothermal energy has been limited mainly to certain localized areas where it is used either for district heating and/or electricity generation. However, the constant temperature and heat storage capacity of near-surface soils in any region represent a tremendous potential of stored energy that can be used for heating and cooling of structures. Ground temperatures below a depth of about 20 feet remain stable compared to outside air temperatures, typically lying between 50-70° F in most U.S. regions. As an example, the ground temperature profile in Blacksburg, VA is shown in Figure 1. Temperatures near the surface fluctuate with seasonal ambient temperatures. However, the temperatures at deeper levels remain stable at 60° F as the overlying ground acts as an insulator. This relatively constant temperature and the thermal storage capacity of the ground can be exploited for heating and cooling purposes. Traditionally, geothermal boreholes have utilized this concept for space heating and cooling. In this system, a circulation loop is placed in a small-diameter borehole typically extending to a depth of 200-300 ft. The hole is then backfilled with a mixture of sand, bentonite and/or cement. The loop is connected to a geothermal heat pump and the fluid inside the loop is circulated. The heat energy is fed into the ground for cooling in the summer and withdrawn from the ground for heating in the winter. Because geothermal heat pumps use the ground as a constant temperature source which serves as a more favorable baseline compared to the ambient air temperature, these systems work much more efficiently for space heating and cooling compared to air-source heat pumps. Over the past 20 years, this ground coupling concept has been expanded from geothermal borehole systems to the use of building foundation elements as heat exchangers. Energy piles in particular are one innovative technology that combines geothermal heat exchange and structural foundation support (Figure 2). In this hybrid system, geothermal loops are integrated into the deep foundation elements, such as piles, piers or drilled shafts that are already in place to provide structural support (Figure 3). In this project investigated the use of Energy Piles, a new near-surface geothermal energy harvesting concept designed to efficiently access the ground for building heating and cooling. In specific, this study focused on thermo-hydro-mechanical processes that governed the behavioral trends of Energy Piles. Research activities involved full-scale field testing and advanced numerical modeling that helped us identify important phenomena related to pile-soil interaction during heat injection and extraction. The test setup installed by an industrial partner was the first full-scale field test in the US. Project activities established collaborative links with academia and industry and aided the development of several successful research grants. This project involved 1 PhD, 4 MS and 1 undergraduate student researchers, including two female researchers, one of whom later pursued doctoral studies with the PI. An undergraduate student used the field test facility to investigate the use of energy piles for bridge deck deicing who was awarded an NSF Graduate Research Fellowship and is pursuing a PhD. Findings from the project resulted in 7 journal papers, 8 conference papers and a book chapter. The project led to an international NSF workshop on thermo-active geothermal systems where future research needs were discussed.
