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Flooded Mine Water-to-Air Heat Pump System

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©2015 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 57, no. 8, August 2015

Richard M. Kelso, Ph.D., P.E., Fellow/Life Member ASHRAE; W. Stanley Johnson, Ph.D., P.E.

About the Authors
Richard M. Kelso, Ph.D, P.E., and W. Stanley Johnson, Ph.D., P.E., are professors emeritus at the University of Tennessee in Knoxville, Tenn.

A flooded, abandoned, underground zinc mine in Tennessee is used to provide the heating, ventilating and cooling at Samaritan House, a short-term shelter for families and single women. The building has a system of water-to-air heat pumps with its closed-loop piping lowered into the mine’s water. Using the mine shaft as the “borehole,” resulted in considerably lower initial costs than a conventional borefield and in a first-year annual energy use intensity of 27.5 kBtu/ft2 (312 kJ/m2). And, it shows that using flooded mines can offer opportunities to install ground-coupled heat pumps even in building projects with a limited budget. The primary objective for this article is to add to the existing performance database of ground-coupled heat pump systems.

Samaritan House is operated by Appalachian Outreach, a ministry of Carson-Newman University in Jefferson City, Tenn. When the shelter outgrew its first home, a former residence, Appalachian Outreach’s board and staff wanted to construct a sustainable and efficient building and recognized the potential for using the water in the flooded zinc mine as a heat source/sink.

The 12,100 ft2 (1124 m2) rectangular two-story building (Photo 1) was built with donated funds on 10 acres (4 ha) of donated land. The relatively simple form and conventional wood-frame brick veneer construction were chosen partly to enable volunteer labor to perform a significant part of the work.

Samaritan House is comprised of 13 sleeping rooms, kids’ play, family, computer, laundry and large multipurpose rooms and a commercial kitchen. Occupancy for the first year was typically 20 to 25 people.

 

The Mine

The mine on which Samaritan House was built opened in 1927. The shaft is 843 ft (257 m) deep and 13 ft (4 m) in diameter. Horizontal tunnels radiate from the shaft at three levels beginning about 300 ft (91.4 m) below the surface, with the most extensive tunnels at 478 ft (145.7 m). Figure 1 is a map of the tunnels in the vicinity of the shaft, indicating the large volume of water-filled passages.

To prevent flooding during mine operations, the operators had to pump 2,400 gpm (151.4 L/s) of water out of the mine. The mine was closed in 1995 and capped with a concrete slab approximately 1 ft (0.30 m) thick. Since the mine was abandoned, groundwater has flooded it up to about 130 ft (39.6 m) below the surface in the shaft.

 

HVAC System

The building is served by nine vertical unitary water source heat pumps ranging from one to five nominal tons and two console water source heat pumps. Heat gain and loss estimates and the heat exchanger design were obtained using proprietary software. Ventilation air is provided by individual direct intakes to the heat pump return air ducts. The total specified cooling capacity is 285,000 Btu/h (83.505 kW) or about 24 tons.

A 1750 rpm constant speed centrifugal pump is designed to circulate 60 gpm (3.79 L/s) of water at 45 ft head (134.5 kPa) through a closed loop to transfer heat between the heat pumps and the mine water. The water pump design efficiency is 59%, and it has a 2 hp (1.49 kW) motor. A constant speed motor was selected because of the small size and simplicity of operation. Flow control valves were not installed on each heat pump for the same reason. Each heat pump is controlled by a room thermostat.

To conduct a study of the performance of this unusual system, the Electric Power Research Institute (EPRI) located in Knoxville, Tenn., provided and monitored an instrumentation system. Data were collected from Jan. 20, 2012 through Jan. 19, 2013, at one-minute intervals.

 

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Photo 1

 

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Figure 1

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