Many mining regions in the United States contain extensive areas of flooded underground mines. The water within these mines represents a significant and widespread opportunity for extracting low-grade, geothermal energy. Based on current energy prices, geothermal heat pump systems using mine water could reduce the annual costs for heating to over 70 percent compared to conventional heating methods (natural gas or heating oil). These same systems could reduce annual cooling costs by up to 50 percent over standard air conditioning in many areas of the country. (Formatted full-text version is released by permission of publisher

Ackman, T.E.

Watzlaf, G.R.

Watzlaf, George R.

Ackman, Terry E.

UNT Digital Library

IntroductionMany mining regions in the United States contain extensive areas of flooded underground mines. The water within these mines represents a significant and widespread opportunity for extracting low-grade, geothermal energy. Based on current energy prices, geothermal heat pump systems using mine water could re-duce the annual costs for heating to over 70 percent compared to conventional heating methods (natural gas or heating oil). These same systems could reduce annual cooling costs by up to 50 percent over standard air conditioning in many areas of the country.BackgroundLord Kelvin first developed the concept of heat pumps in 1852 (Lund et al. 2004). In the 1940s, Robert Webber modified the concept by using the ground as the source of heat (Lund et al. 2004; IGSHPA 2005). These ground source or geother-mal heat pump systems gained popularity in the 1960s and 1970s due to oil short-ages, and many alternative types of en-ergy systems were developed (Bloomquist 1999). Today, 500,000 geothermal units are used for residential heating and cool-ing in the United States and Canada with an additional 400,000 units in Europe (Manitoba Budget Papers 2004).  Geo-thermal heat pumps are one of the fastest growing types of renewable energy in the world, with annual increases of 10 percent in approximately 30 countries in the last 10 years (Lund 2001). Right now, interest is very high due to the high prices of natu-ral gas, heating oil and propane. The cost effectiveness of geothermal heat pump sys-tems is directly related to the ratio of the cost of conventional heating fuels (such as natural gas, heating oil and propane) to the cost of electricity (needed to drive the heat pump).  Currently, this ratio is the highest it has ever been.A heat pump moves heat from one place to another. It can be used for either heat-ing, by moving heat into an area, or cool-ing, by moving heat out of an area. A re-Flooded Underground Coal Mines:  A Significant Source of Inexpensive Geothermal EnergyBy george r. waTzlaF and Terry e. ackMan U.s. deParTMenT oF energy, PiTTsBUrgh, Pennsylvania Figure 1. Schematic of a heat pump system in heating mode. Heat source is in contact with the evaporator.4 reclamation matters   •   ISSUE	1			•			2007frigerator is an example of a heat pump. It moves heat from inside the box to outside the box. Within the heat pump, a refriger-ant is used that absorbs heat when going through a phase change from a liquid to a vapor. A compressor is used to compress this vapor, thereby increasing its tempera-ture. Then, an expansion valve allows for the vapor to be converted back to a liq-uid. Figure 1 shows a diagram of a heat pump system. A heat pump is made up of two heat exchangers, a compressor, an expansion valve and a reversing valve. In heating mode, the refrigerant – in a cold liquid form – gains heat from the outside source (air or ground) in a heat exchanger (evaporator), where it is converted into a cold vapor. After the liquid absorbs heat and is converted to a vapor, it is then compressed (requiring an input of electri-cal energy), converting it to a hot vapor. The hot vapor is sent to another heat ex-changer (condenser). Here, the hot vapor gives up the heat that was gained from the source in the evaporator and, in the process, is condensed to a hot liquid (the heat given up is used to heat the interior space). The hot liquid goes through an expansion valve where the drop in pres-sure converts it to a cold liquid and the process is repeated. In heating mode, the evaporator is placed in contact with the heat source. In cooling mode, the above process is reversed with the use of a revers-ing valve (Figure 2).Ground source or geothermal heat pumps use the near-constant temperature of the earth (in soil/rock, groundwater or deep surface waters). During winter months, the earth is at a higher tempera-ture than the outside air and, therefore, acts as a heat source. In summer, the earth is at a lower temperature than the outside air and, therefore, can act as a heat sink. Because of this, geothermal heat pump systems are much more efficient than air source systems for both heating and cool-ing. For heating, the amount of heat gen-erated divided by the amount of energy needed to operate the heat pump is known as the coefficient of performance (COP). A typical COP value for an air source heat pump is about 2, while geothermal systems have COP values commonly be-tween 3 and 4, with values as high as 6 re-ported in the literature (Sound Geother-mal Corporation 2003; O’Connell and Cassidy 2003). Geothermal heat pumps typically use about half the energy needed for cooling with air source systems.A ground source heat pump can be designed in a variety of styles based on groundwater access, land availability and drilling costs. The two main categories of ground source heat pumps are closed loop and open loop systems (U.S. DOE 2001). In a closed loop system, no fluid is ex-tracted or discharged to the environment. Pipes, which are filled with an antifreeze solution, are buried in the ground in either a horizontal or vertical format. The anti-freeze solution is pumped through these pipes exchanging heat with the ground. An average-sized house in the northeast-ern United States may require over 1,500 linear feet of pipe. The horizontal format requires a significant amount of area to bury the pipes. If there is not sufficient area for use of the horizontal piping, a ver-tical system must be used. In the vertical format, 100- to 400-foot deep boreholes must be drilled and pipes are placed in a U-shape within the boreholes. An average home may require two to eight boreholes. The cost of installing the closed loop pip-ing system is the most significant cost of the geothermal system and can exceed $10,000 for an average-sized home. An open loop system eliminates the expense of loop installation. In the open loop sys-tem, groundwater or deep surface water is extracted from the environment and subsequently discharged back into the environment. The typical flow rate for an open loop system is about 1 to 3 gal-lons per minute per ton (1 to 3 liters per minute per kilowatt-hour) of heating and cooling (PADEP 2001) (One ton of heat-ing and cooling is equivalent to 12,000 Figure 2. Schematic of a heat pump system in cooling mode. Heat sink is in contact with the condenser.Figure 3. Costs for using these typical heating systems based on average U.S. energy prices from 1990 to 2006. Furnaces using propane, natural gas or fuel oil, were assumed to be moderately efficient (84 percent). Coefficient of performance (COP) of geothermal heat pump was assumed to be 3.5.ISSUE	1		•			2007   •   reclamation matters British thermal units per hour). If the quality of the water is such that it could cause scaling or corrosion within the heat pump, an additional heat exchanger filled with antifreeze solution may be used.Cost	Effectiveness	of	Geothermal	Heat	Pump	SystemsThe cost effectiveness of geothermal heat pump systems for heating is directly related to the cost of electricity (to operate the heat pump) compared to the cost of the other conventional fuels: natural gas, heating oil and propane. Figure 3 shows the costs over the past 50 years for electricity, natural gas, heating oil, propane, as well as the cost for the electricity required for a geothermal heat pump system. To get a true representation of the actual cost to the consumer, the effi-ciencies of each system utilizing the fuel/en-ergy source must be taken into account. In Figure 3, an assumed 84 percent efficiency is used for furnaces/boilers burning natural gas, heating oil or propane, and a relatively conservative COP of 3.5 is used for a geo-thermal heat pump (Sound Geothermal Corporation 2003).Use	of	Mine	Water	in	Geothermal	Heat	PumpsThere have been a few examples of the successful use of mine water in geother-mal heat pump systems. Systems using water from abandoned mines have been installed in Canada (Jessop et al. 1995), the United States (GHPC 1997) and the United Kingdom (John Gilbert Ar-chitects; John Gilbert Architects 2002).  Potential	 of	 the	 Pittsburgh	 Coal	Seam	Mine	Pool	 for	Heating	 and	CoolingA significant volume of the Pittsburgh coal seam in Pennsylvania, West Virginia and Ohio is currently flooded. As shown in Figure 4, the availability of under-ground mine water in the Appalachian coal region is very widespread. Approxi-mately 5,000 square miles (13,000 km2) have been mined in the northern portion Figure 4. Overburden depths above the Pittsburgh coal seam in Pennsylvania, West Virginia and Ohio and location of existing water treatment facilities in Pennsylvania.Energy source Formula for cost per 106 Btu US$/106 Btu*Propane (11.1 x cost/gallon) / efficiency 32.11Electrical Resistance 293 x cost/kWh 20.80Fuel Oil (7.25 x cost/gallon) / efficiency 20.28Natural Gas (970 x cost/cubic feet) / efficiency 20.81Geothermal Heat Pump (COP = 3.0) (293 x cost/kWh) / COP   6.93Geothermal Heat Pump (COP = 3.5) (293 x cost/kWh) / COP   5.94Geothermal Heat Pump (COP = 4.0) (293 x cost/kWh) / COP   5.20Geothermal Heat Pump (COP = 6.0) (293 x cost/kWh) / COP   3.47Table 1. Energy costs for use of typical heating systems in southwestern Pennsylvania.*Cost of fuels and electricity were based on actual delivered cost to the Pittsburgh, Pennsylvania area during the winter of 2006. Propane = $2.43/gallon, electricity = $0.071/kWh, fuel oil = $2.35/gallon, and natural gas = $0.01802/cubic feet. Furnaces using propane, natural gas or fuel oil were assumed to be moderately efficient (84 percent). Most geothermal heat pumps operate at a coefficient of performance (COP) between 3.0 and 4.0 with values as high as 6.0 reported in the literature. in addition to the electricity cost to operate the geothermal heat pump, there would be a cost to pump the water to the system. To pump the water from a discharge to the system from depths of 100, 250, 500 and 1000 feet would add an estimated $0.46, $0.92, $1.69 and $3.23 per million Btu, respectively.6 reclamation matters   •   ISSUE	1			•			2007of the Appalachian coal fields and nearly 2,000 square miles (5,000 km2) are cur-rently flooded (Donovan et al. 2004). The heating and cooling capacity of this underground mine water is an extremely valuable resource that is not currently be-ing utilized. Throughout the Pittsburgh coal basin region, the water is easily acces-sible and maintains a constant tempera-ture of 50 F to 55 F (10 C to 13 C) (US-DOE 2001). The total volume of water estimated to be stored in the Pittsburgh coal seam is 1.36 x 1012 gallons (5.15 x 1012 liters) (Donovan et al. 2004). About 4 percent of this volume is discharged at the surface each year by treatment plants and abandoned discharges, which total about 5.3 x 1010 gallons per year (Dono-van et al. 2004). This current amount of discharged water could potentially be used to heat and cool up to 40 million square feet (3.74 million m2) of interior space, roughly equivalent to 20,000 homes. As the mines in this area continue to fill with water and with new voids being created by active mining, the volume of stored and discharged water from these underground mines will continue to increase into the future.Table 1 shows the cost for generating 1 million Btu of heat for geothermal heat pump systems compared to conventional heating technology using actual energy costs in the southwestern Pennsylvania area. Electricity and natural gas costs were calculated using the actual residential util-ity bills in southwestern Pennsylvania by dividing the total cost (including distribu-tion, taxes and other incidental charges) by the amount of the commodity received (kilowatt-hour for electricity or cubic feet for natural gas). Heating oil and propane prices were based on actual delivered cost to a consumer in the area, again dividing the total cost by the volume received.Because mining companies are required to treat mine water, it has always been considered a liability. If the technology of using mine water in geothermal heat pump systems proceeds, clarifications of legal rights for mine water may need to be addressed. Given the geothermal po-tential of mine water, non-mining entities may be enticed to use mine pool water for heating and cooling capabilities. If mine water were brought to the surface, the water would be either returned directly back into the mine pool or treated and discharged at the surface.Summary and ConclusionsUse of underground mine water in geo-thermal heat pumps could be extremely cost effective, particularly at existing mine water treatment sites where the mine water is already being pumped and treated. Op-erational costs for geothermal heat pumps are much lower than that of conventional heating and cooling options. Costs per unit of heat for geothermal heat pumps (COP=3.5) using underground mine wa-ter are only 29 percent, 29 percent, and 18 percent of the costs incurred using fuel oil, natural gas or propane, respectively. Cooling costs using mine water and geo-thermal heat pumps should be less than 50 percent of the costs associated with conventional air conditioning systems.The availability of mine water in the Appalachian coal region is widespread. The heat content of the mine water is a valuable resource that is not currently be-ing utilized and is simply being discharged with the treated mine water to a receiv-ing stream. The amount of water that is currently being discharged from under-ground coal mines in just the Pittsburgh coal seam, could potentially be used to heat and cool up to 40 million square feet of interior space, roughly equivalent to 20,000 homes. Using the additional water stored in the mines could conser-vatively extend this option to an order of a magnitude of more homes. Because most mines are currently filling, the vol-umes of discharged and stored water will continue to increase in the future. Research is needed to demonstrate and develop this extremely valuable resource. References:•  Bloomquist, R.G. (1999) Geothermal heat pumps: four plus decades of expe-rience. GHC Bulletin 20 (4): 13-18.•  Donovan, J., Leavitt, B., Ziem-kiewicz, P., Vandivort, T., Werner, E. (2004) Flooding of abandoned underground Pittsburgh coal seam mines. In WV173 Phase IV EPA Region III Mine Pool Project. Final Report for DOE contract DE-AM26-99FT40463. pp 380-382.•  Geothermal Heat Pump Consortium (1997) Municipal Building, Park Hills, Missouri. Ghpc #CS-064; www.geoexchange.org/pdf/cs-064.pdf•  International Ground Source Heat Pump Assoc. (2005) What is IGSH-PA?  www.igshpa.okstate.edu/about/about_us.htm•   Jessop, A.M., Macdonald, J.K., Spence, H., (1995) Clean energy from abandoned mines at Springhill, Nova Scotia. Energy Sources 17: 93 – 106.•  John Gilbert Architects (2002) Proj-ects 02 – Ochilview, Lumphinnans  http://www.johngilbert.co.uk/pdf_files/JGA_Lumphinnans.pdf•   John Gilbert Architects. Sustainable housing, Glenalmond Str, http://www.johngilbert.co.uk/ pdf_files/in-novation/9_Shettleston.pdf•   Lund J., Sanner, B., Rybach, L., Curtis, R., Hellström, G. (2004) Geothermal (ground-source) heat pumps, a world overview. GHC Bull 25 (3): 1-10.•   Lund, J., Freeston, D., Boyd, T. (2005) Direct application of geo-thermal energy: 2005 worldwide review. Geothermics 34: 691-727.•   Lund, J.W. (2001) Geothermal heat pumps – an overview. GHC Bull 22 (1): 1-2.•   O’Connell, S., Cassidy, S.F. (2003) GHC Bull 24  (4): 8-12.•   Pennsylvania Department of En-vironmental Protection (2001) Ground source heat pump manual, Document ID 383-0300-001, 47 pp.•   Sound Geothermal Corporation (2003) Fuel Cost Comparison. Sound Geothermal Corp, www.soundgt.com/costcomparison.htm•   U.S. Dept of Energy (USDOE) (2001) Ground-Source Heat Pumps Applied to Federal Facilities – 2nd Edit, DOE/EE-0245, Fed Energy Management Program, Washington, DC, 44 pp. nISSUE 1  •   2007   •   reclamation matters 7

Flooded Underground Coal Mines: A Significant Source of Inexpensive Geothermal Energy

https://digital.library.unt.edu/ark:/67531/metadc889446/m2/1/high_res_d/912831.pdf

Flooded Underground Coal Mines: A Significant Source of Inexpensive Geothermal Energy

Abstract

Similar works

Full text

Available Versions

UNT Digital Library