Coupling Geothermal Heat Pumps with Underground Thermal Energy Storage (EW-201135)

The Environmental Security Technology Certification Program (ESTCP) has successfully completed the demonstration of an innovative geothermal heat pump (GHP) system which includes underground thermal energy storage (UTES). This system generates higher energy savings with lower installation costs than conventional geothermal heat pump systems. Though this technology is widely used in Europe, this was the first installation in the U.S. and the ESTCP demonstration has helped validate the technology’s performance and savings for the U.S. market.

Conventional GHP HVAC systems are considered one of the most efficient active HVAC systems and offer energy savings of up to 40% and substantial maintenance cost savings [1] compared to other traditional HVAC systems.  However, 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. 

The GHP system coupled with an UTES 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-USTES HVAC systems installed at two different locations - Borehole Thermal Energy Storage System (BTES), installed at the Marine Corps Logistic Base in Albany Georgia (MCLB) and the Aquifer Thermal Energy Storage (ATES), installed at Ft. Benning Georgia (GA).

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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 BTES, it 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 this particular building were 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 demonstration of the technology was successful and the system achieved over 45% energy usage reduction over a conventional HVAC and 15% over a conventional GHP. It also achieved 100% reduction in water usage by the on-site conventional cooling tower (4.7M Gal./Yr.) and 100% reduction in on-site greenhouse gas emissions for HVAC space heating. With respect to cost, it achieved a reduction of over 30% in construction cost compared to conventional GHP system. Additional details on the demonstration and results can be found in the Final Report and Cost & Performance Report which will be posted on the product webpage soon.

The success of this demonstration has already generated positive interest among DoD installations. To learn more about the technology, please join the SERDP and ESTCP Webinar on June 15, 2017, where Charles Hammock, the Principal Investigator of this project, will present in detail the technology, demonstration and results. Register for this webinar at https://www.serdp-estcp.org/Tools-and-Training/Webinar-Series/06-15-2017

[1] http://www.nrel.gov/docs/fy99osti/26275.pdf

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