|Adopted from Orio 1999|
Through the Virginia Department of Health I received a contact from a homeowner interested in installing a ground source heat pump utilizing his current drinking water well as both a water supply well and a standing column thermal exchange well for the geothermal heat pump. He apparently had run across this idea in some materials by Carl Orio. In truth, Mr. Orio is the man. Mr. Orio has been working with geothermal heat pumps since 1974 when he founded WESCORP. Since that time he has expanded his knowledge and experience about geothermal heat pumps leading the way and is the co-author of practically everything including the New York City "Geothermal Heat Pump Manual" and ASHRAE funded research papers.
Though standing column wells have been around since the advent of geothermal heat pump systems, only recently are they receiving more attention because of their superior performance in regions with suitable hydrological and geological conditions (Orio 1994, 1995, 1999; Yuill and Mikler 1995; Spitler et al. 2002) and limited horizontal space. According to a paper by Mr. Orio et al. in 2005 there were only about 1,000 standing column well-geothermal heat pump installations in the United States. Standing Column Well systems are groundwater heat pump systems that use groundwater drawn from wells in a semi-open loop arrangement. The ground heat exchanger in these systems consists of a vertical borehole that is filled with groundwater up to the level of the water table. Water is circulated from the well through the heat pump and back to the well in an open loop pipe circuit. The standing column can be thought of as a cross between a closed-loop vertical well system and an open-loop groundwater source system. The systems identified by the paper are in the colder climates of the northeast and west where air heat exchangers could not do the job, but a geothermal system can function in the extreme cold. In a survey of existing systems the most significant parameters were found to be well depth, rock thermal/hydraulic conductivity, and bleed rate.
During much of the year, the standing column systems operate by recirculating water between the well and the heat pump. The water is drawn from the bottom and returned to the top of the water column. However, during extremecold or hot periods they can “bleed” some water from the well to maintain thetemperature range of the standing column well by inducing groundwater flow into the well. This causes groundwater to flow to the column from the surrounding formation to make up the flow. This serves to cool the water column during heat rejection in the summer, and warms the column and surrounding ground during heat extraction in the winter, thus restoring the well-water temperature to the normal operating range and improving the system performance. A bleed is especially important in the colder climates where the loss of temperature in the well could result in freezing of the system in extreme cold. In warmer climates an excess of temperature in the well during the hottest days would only result in a loss of efficiency since the temperature increase would still result in water significantly below the ambient temperature.
Most of the existing standing column installations are located in the Northeast and Pacific Northwest in heating-dominated residential and light commercial applications. The vast majority exist in the northeastern Appalachian region including Maine, Massachusetts, New Hampshire, New York, and the northwestern states. Because air heat exchangers have improved in efficiency so much in the last few years and the heating demands in Virginia and the rest of the south are much less than in the Northeast, ground source heat exchangers have been less popular in the south.
The home owner who contacted me lives on the Occoquan Reservoir right around the fall line in the coastal plain of Virginia, to be entirely accurate, the homeowner actually lives just west of the fall line and within the harder bedrock of the Piedmont. The Fall Line so named because the meeting of the Piedmont and Coastal Plain is marked by a line of waterfalls. The Fall Line is really a zone rather than just a narrow line. The rapids and waterfalls characteristic of the Fall Line extend up to a mile wide in some locations. The waterfall on the Occoquan River near Lorton has been "dried out" by the construction of the dam that created the Occoquan Reservoir, but still marks the line. You can see the exposed rocks at the Fall Line by walking upstream from the town of Occoquan when the path is open. The geology at the home is most likely a layer of generally unconsolidated, inter-bedded sands and clays, underlain by bedrock.
The homeowner’s well is 450 feet deep with the pump at 400 feet. The well is probably that deep because that is what it took to find an aquifer just west of the Fall Line. That is not a good well for this water rich region of Virginia, but groundwater is not equally distributed throughout the region. The water level in the well is about 35 feet from the surface creating water storage within the well of over 2,000 gallons. The water storage has served to produce a consistent supply for the house since the well is reported to recharge at only 5 gpm- not really enough to run a household. This is a non-robust aquifer and they drilled 450 feet to reach it. This well may indicate the difficulty of locating a well on the property or replacing the well if it is damaged. As a Virginia Master Well volunteer, my first reaction is always that the drinking water well is sacred and should not ever be risked especially in a location where there was difficulty with drilling a producing well. The homeowner is aware of this concern.
The groundwater aquifer recharges this well at 5 gallons per minute. This aquifer is not robust, even a 10% bleed at 0.6 gallons per minute would consume over 800 gallon per day. However, a bleed is not necessary. The thermal demands are not as high in Virginia as Maine and other parts of cold New England and the actual use of water by the household is probably 100-150 gallons a day. The use of the water for the heat exchanger is non-consumptive (it will be returned to the well at the 4-6 gallons a minute that it is drawn) and since the water in the well will turn over in less than a week and be constantly diluted with recharge, any impact from the metal portion of the heat exchanger should be negligible. The only true potential contamination would be a leak in the heat exchanger allowing the gallon or so of ethylene glycol to contact the well water and that should be carefully watched.
An important design feature to convert the existing well to a duel purpose well is the pump for the well will have to be replaced with a variable speed pump that can deliver at least 12-15 gallons per minute. A variable pump is needed because the pump will now serve two purposes. When the heat pump is operating it requires flow at 4 or 6 gallons per minute and the well must also be able to respond to the household needs an simultaneously pump to the pressure tank. There will have to be a duel switching system to trigger the pump. The heat exchanger has to be able to trigger the pump and lower demand when it shuts off and the pressure tank needs to be able to trigger the pump to either turn on or increase flow. There also has to be a valve trigger to open or close the valves to the pressure tank or heat exchanger. Typically a standing column well needs to be 250-500 feet deep depending on the bleed. This well at 450 is perfectly sized for a 4-5 ton system and the recharge rate at 5 gallons per minute is really below the ideal level for a household, but this household has two residents so, their daily water demand is low.
The return line to the well should be buried at 6-8 feet below grade to reduce heat loss or gain when returning the water to the well. The return line should deliver the water within the water table in the well to avoid introducing turbulence induced mixing in the well column to allow the length of the well to provide adequate time for the temperature of the returned water to return to normal well conditions which for this well is about 50 degrees F. In addition, the homeowner will have to obtain an Underground Injection Control, UIC, Individual Permit from the U.S. Environmental Protection Agency, EPA. The EPA issues its UIC Permit under the Safe Drinking Water Act (SDWA), as amended, and implementing regulations at Title 40 of the Code of Federal Regulations, Parts 124, 144, 146, 147, and 148 and has some experience issuing permits for geothermal standing column installations in other parts of the country. In Virginia the EPA Region 3 manages the program and will walk the homeowner through the Class V permit process filling out the inventory data required for the homeowner. Their single requirement is a monitor to identify if there is a leak in the system coil that could release, according to the EPA, ethylene glycol into the homeowner’s well though they do not require regular reporting on the monitoring. The EPA representative explained that they are just trying to save the well owners from themselves. A Virginia Department of Health permit is not required, but plumbing permits are required on a county level.