Coupling Geothermal Heat Pumps with Underground Seasonal Thermal Energy Storage

Chuck Hammock Jr. | Andrews, Hammock & Powell, Inc.

EW-201135

Objectives of the Demonstration

The objective of this demonstration was to fully maximize the inherent advantages of the geology and hydrogeology accessed by means of Ground Heat Exchanger (GHX) with closed loop systems or via direct ground water use with open-loop systems, which conventional Geothermal Heat Pump (GHP) systems in the U.S. are not designed to achieve. Deliberately engineered Underground Thermal Energy Storage (UTES) systems not only allow for the waste heat of cooling systems and the waste cool of heating systems to be captured, but also allow for the out-of-season capture of the 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.

This project was proposed because, in the United States (US), the fundamental design of GHP systems, with their underground GHX or water supply wells, has been largely unchanged for decades. In pursuit of reducing energy consumption, the heating, ventilating and air-conditioning (HVAC) industry is typically progressing to develop various types of HVAC technology to help reduce the energy consumption of an associated HVAC system. Historically, conventional GHP HVAC systems are considered one of the most, if not the most efficient active HVAC systems. Most Department of Defense (DoD) (and non-DoD commercial) facilities in many geographic regions are cooling dominated due to the consistent presence of cooling loads associated with people, lights and equipment (computers, copiers, monitors, etc.). Furthermore, most HVAC systems used in the Southeastern (SE) US (the region where both Environmental Security Technology Certification Program [ESTCP] projects were accomplished), are significantly cooling dominated. Typically, DoD facilities have imbalanced cooling versus heating loads, which in some applications, can have annual cooling loads that are as much as 5-10 times more than the annual heating loads. For a conventional GHP system, this load imbalance, over time, can lead to higher supply water temperatures and cause the operating efficiencies of the water-cooled equipment to decrease. In extreme cases, the supply water temperatures can increase up to the point where the water-cooled equipment can fail/fault due to high refrigerant pressure safeties. HVAC systems with UTES capability do not necessarily have to reject (or extract when heating dominated) heat during peak conditions. In retrospect, since the 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) both are capable of diurnal and seasonal storage, this project would more aptly be named simply Coupling GHPs with UTES.

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Technology Description

This demonstration project involved the implementation of high-efficiency GHP systems, coupled with an UTES system, at two locations in the SE U.S. to provide a sustainable infrastructure with higher energy savings than conventional geothermal systems, but with lower installation cost. The demonstration project at the Marine Corps Logistics Base Albany (MCLBA) coupled GHPs with a form of UTES commonly known within the international community as BTES. The demonstration project at Fort Benning, GA coupled GHPs with a form of UTES known internationally as ATES.

  • BTES
    GHP’s were connected to closed loop system comprised of 306 conventional underground grouted vertical boreholes with high-density polyethylene (HDPE) u-bends, but has an active outdoor adiabatic dry cooler. The dry cooler is optimized, from a construction and control/operational perspective, to capture the cooling energy of winter and store this energy in the underground formation for use in the peak summer months. This style of USTES is commonly referred to as a Seasonal Borehole Thermal Energy Storage system or simply “BTES”.
  • ATES
    The ATES system at Fort Benning includes an outdoor adiabatic dry-cooler, but utilizes an open loop system of four wells to directly extract/inject the native ground water from/to the local aquifer. The Fort Benning open loop is properly known as a seasonal Aquifer Thermal Energy Storage system or simply “ATES”. 

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Demonstration Results

As part of the demonstration project, several quantitative performance objectives were proposed and evaluated as part of the overall performance. All but one of the performance objectives were achieved as part of this demonstration project. Overall, the demonstration project illustrated a successful performance evaluation for the implemented technology and it is hoped it opens two new architectures of HVAC system that can create significant energy and water savings for DoD and others.

The objective of this project was to successfully demonstrate that a high-efficiency GHP System, coupled with an USTES system could provide truly sustainable infrastructure with higher energy savings than conventional geothermal systems, but with lower installation cost and thereby address DoD’s substantial building energy consumption issues in a widely deployable and more affordable manner. The specific performance objectives for this demonstration are as follows.

201135 table

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Implementation Issues

The challenges described herein are generally applicable for most ATES/BTES projects throughout the US. With both projects involving either boreholes (BTES) or water wells/injection wells (ATES) they fall under the jurisdiction of the GA Department of Natural Resources’ (DNR) Environmental Protection Division (EPD). They have been given the authority to rule on groundwater injection through a legal mechanism called “Primacy”. ATES projects are considered a “Class V Injection Wells” and therefore they fall under EPA’s jurisdiction and in this case, a UIC permit is required. In some states, this is not a complex affair. In GA it proved to be difficult.

Overall, in the US, BTES system are not generally difficult to permit as they do not physically remove or inject groundwater and therefore no UIC permit is required. BTES projects can be permitted easily in most states and in some locations (like GA), no State permit is required.

Due to the demonstration plan goal of an 80-100% water reduction for the ESTCP project, a rare (in the US) adiabatic dry-cooler (sometimes referred to as a hybrid dry-cooler) was chosen. While common in Europe and elsewhere, these are rare in the US. Nevertheless, there are multiple manufactures of this product and the selected units were made in North America.

In the US, open loop GHP system do not typically have high level ATES well injection valve designs or controls. After US manufacturers were investigated, the search turned to Europe and elsewhere. After extensive investigation, ultimately a firm in Switzerland was selected.  Ironically, this company was the European branch of a US firm, but with no demand yet for ATES valves in the US, this product is only manufactured in Europe and in metric dimensions and European electrical characteristics (230 VAC/50 Hertz [Hz]). These seemingly minor inconvenience created several delays, but through the use of US and Swiss piping adapters/fittings, and the ability of the hydraulic unit to be furnished at 120 VAC/60 Hz, the issues were ultimately resolved.

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Publications

Hammock, C. (2018). Borehole Thermal Energy Storage. Plumbing Engineer, 46(3). ( https://www.phcppros.com/articles/6989-borehole-thermal-energy-storage)

Hammock, C., J. Acuña, B. Caves, and A. Sequera. 2016. Fiber Optic Based Distributed Temperature Sensing (DTS) for Large Geo Installations. GeoOutlook Magazine, 13(1):22-26. ( https://www.geooutlook.org/epub/GO2016No1/22/)

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