Located in Georgetown, Del., the Sussex County Emergency Operations Center contains some $4 million worth of electronic equipment. With the heat gain from all of that equipment, the 18,000-sq-ft facility requires air conditioning virtually year-round.
Along with a photovoltaic solar array located in an adjacent field, the Emergency Operations Center’s energy needs are met with a closed-loop geothermal system featuring 13 EC Series heat pumps from FHP Manufacturing, part of the Bosch Group, which range in capacity from 0.5 ton to 24 tons.
The geothermal infrastructure is comprised of 24 600-ft-deep boreholes. The system was designed to operate at a temperature of 60°F to 70°F. During the summer of 2011—three years after the center opened—however, the temperature of the fluid in the system’s wells began to climb. Interior temperatures began to range from 80°F to 85°F, jumping to 95°F more than once. Temperatures above 100°F had the potential to cause the air-conditioning system to fail, which could have caused failure in a range of electronics equipment and systems that tie the facility to essential public-safety and government agencies.
The county brought in a temporary cooling tower at $50 per day to handle the heat the ground could not absorb quickly enough. County officials then enlisted geothermal consultants and engineers for ideas for a more permanent solution, which included:
- Complete system replacement.
- Installation of a permanent cooling tower.
- Upgrade of various mechanical systems.
The Solution
Jay Egg, founder of Egg Geothermal, a mechanical-services company focused on geothermal consulting, engineering, and contracting, analyzed the problem on site, concluding the problem was the closed-loop infrastructure had too few loops and a cooling-dominant load; as a result, thermal retention—a condition by which fluid in a system’s boreholes becomes overheated—was occurring.
County officials accepted Egg’s analysis, opting to go with his recommendation: a pump-to-injection thermal-exchange well system that would allow for expansion and remediation of the thermal glide, increase overall system redundancy, and retain the heat-pump equipment intact.
Pump-to-injection is a method by which groundwater is pumped over a heat exchanger and injected back into the aquifer from which it came.
Installation
To add the pump-to-injection element to the closed-loop system, four 8-in. encased water holes were drilled to a depth of 400 ft. Two of the holes are for redundant supply water pumping, while the other two are for injection water discharged from plate-and-frame heat exchangers in the building’s geothermal circuit. Three-way valves were placed in the circuit to allow for the following operation modes:
- 100-percent pump to injection.
- 100-percent closed loop.
- 100-percent backup cooling tower.
- Varying combinations of the preceding modes.
Normally, the Emergency Operations Center operates on the closed loop. When temperatures become elevated, usually during summer, the pump-to-injection circuits cool the equipment and remediate underground and loop temperatures back to design parameters. Throughout construction, the 13 EC Series heat pumps continued to operate, requiring little attention and service.
Not ‘Pump and Dump’
One of the reasons pump-to-injection is not more popular among installers is that it often is confused with a practice called “pump and dump.” Before engineers started routing warmed or cooled water back to an aquifer, they would let it spill onto the ground or into a lake or pond. The problem with that is that most of the water would not make it back into the aquifer; instead, it would evaporate on the ground or flood adjacent property.
Additionally, some consider pump-to-injection dangerous from an environmental standpoint, thinking it could pollute groundwater, when, in fact, water passes over what should be a clean heat exchanger; nothing is added to or taken out of the water.
Outcome
The retrofit/upgrade cost about $250,000, but solved the thermal-retention problem and resulted in:
- Low equipment-replacement cost.
- Reduced water consumption.
- Greater loop-field capacity.
- Energy savings (less than $4 in electricity per day to operate).
- Elimination/reduction of combustion/electrical resistance heating.
- Prolonged HVAC-system life.
Just as important, the retrofit demonstrated how to remediate, without violating the integrity of, a closed-loop system using a decoupled open-loop secondary circuit. In this manner, the system operates most of the time on the closed loop, using the open loop to cool the closed loop as needed and adding a layer of redundancy.
“The fact is,” Egg said, “a properly engineered pump-to-injection system will cost one-third that of a closed-loop system. It will use almost no space, and the performance will improve.”
Editor’s note: Portions of this article appeared in National Geographic.
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