Geothermal Heat Pumps with Underground Thermal Energy Storage

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ESTCP 2017 Project-of-the-Year Award for Energy and Water

Conventional Geothermal Heat Pumps (GHP) Heating, Ventilating and Air-Conditioning (HVAC) systems are considered one of the most efficient active HVAC systems. According to the Whole Building Design Guide 1 and the National Renewable Energy Laboratory (NREL) 2, GHPs use 25% to 50% less electricity and offer energy savings of up to 40% compared to the conventional heating or cooling systems. They are relatively quieter, last longer, need little maintenance, and do not depend on the temperature of the outside air. However, in the United States, the fundamental design of GHP systems has largely remained unchanged for decades. Conventional GHP ground-source designs are susceptible to performance deterioration in applications where annual heating and cooling loads are imbalanced. In facilities that are cooling dominant, which applies to most DoD installations, this load imbalance can lead to higher supply water temperatures over time and cause the operating efficiencies of the water-cooled GHP to decrease.

Mr. Charles Hammock from Andrews, Hammock & Powell, Inc, and his team demonstrated the performance and savings of an innovative system design, which couples a GHP system with underground thermal energy storage (UTES). This system demonstrates higher energy savings not only by capturing the waste heat of cooling systems and the waste cool of heating systems, but also capturing out-of-season winter’s “cold” or summer’s “heat” (from the air or via solar thermal collectors), if needed, in cooling-dominated or heating-dominated buildings, respectively. The demonstration of this project included installation of two types of GHP-UTES HVAC systems installed at two different locations: Borehole Thermal Energy Storage (BTES) System, installed at the Marine Corps Logistic Base in Albany, Georgia (MCLB) and the Aquifer Thermal Energy Storage (ATES), installed at Ft. Benning, Georgia.

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The BTES technology is a closed loop Ground Heat Exchanger (GHX) system that utilizes a bore field configured with radial sub-mains of 3 boreholes in series. They create a bullseye pattern of concentric thermal zones for maximum storage efficiency. Adiabatic dry coolers, and reversing valves redirect cold water flow into the perimeter or the core of the bore field depending upon the season. A principle operating mode for this technology is to utilize the adiabatic dry cooler in the dry mode during periods of cold outside air temperatures to efficiently dump heat from the building and bore field to the outside air and therefore “charge” the core of the bore field with “cold”. In the opposite season, the reversing valves change position to use the stored energy from the core of the bore field to cool the building. Given the geographic location of the demonstration site, the BTES is not designed to store heat for heating the building during the heating season as that load is adequately covered by the heat recovery mode of the GHP and the surrounding geology. If that building was located in a colder climate, the BTES could be designed as a “warm” BTES to store heat during the cooling season.

The ATES technology is similar to the BTES mentioned above, but utilizes an open loop system instead of a closed loop. With the ATES technology, energy is stored in the aquifer in either the cold wells or the warm wells. These wells are located approximately 200’-300’ apart. During the cooling season, cold water is pumped from the cold wells and used to cool the building. The heat rejected from the building is then injected into the warm wells. During the heating season, warm water is extracted from the warm well and used to heat the building (while simultaneous cooling the ATES water). An adiabatic dry cooler is then used to efficiently dump additional heat to the air during times of cold outside air conditions. This helps to balance the load between heating and cooling, and allows for additional cooling to be stored in the cold wells. After the water leaves the building or dry cooler, the cold water is injected back into the cold well where it is stored for use again in the cooling season.

The technology demonstration was successful, resulting in the reduction of HVAC energy by nearly 50% and the elimination of cooling tower water use, a reduction of 4.2 million gallons a year. Impressed by these results, the MCLB’s (Albany, Georgia) Installation & Environment (I&E) Division funded three new BTES systems to serve an additional 10 buildings at MCLB. In addition to the improved energy and water performance of the new BTES systems, the bid for these projects came in under the budget for the traditional GHP system designs. The construction contract was awarded in the spring of 2017 and the boreholes for BTES-2 and -3 have been completed with BTES-4 drilling starting the week after Thanksgiving 2017.

For this outstanding work, Mr. Hammock and his project team received the 2017 ESTCP Project of the Year Award for Energy and Water for their project titled Coupling Geothermal Heat Pumps with Underground Seasonal Thermal Energy Storage.

Project Team:

Andrews, Hammock & Powell, Inc.

  • Charles Hammock, PE
  • Stephen E. Sullens, PE, Co-PI

Marine Corps Logistics Base Albany

  • Frederick R. Broome Jr., PE

Fort Benning

  • Mark J. Fincher

1.  https://www.wbdg.org/resources/geothermal-heat-pumps

2.  http://www.nrel.gov/docs/fy99osti/26275.pdf

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Photo by Trish Gottesman

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