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.
While I know the risk is low, I'm not sure I'd like to run the risk of getting ethylene glycol into my drinking water. It's the kind of thing which could easily be missed. I'd rather just leave the geothermal heating system to itself.
ReplyDeleteThere is no eythelene glycol in a standing column well. Glycol( propylene) usually is used as antifreeze in closed loop systems, SCW's use potable water run right thru the heat pump, and the exchange is with refrigerant gas in the source coil .
ReplyDeleteDave Leonard Evergreen Geothermal www.evergreengeo.com
I've done, what I hope is, really thorough research on the SCW/closed loop debate. From the I read, the proponents of closed loop never mention the viscosity of the antifreeze changes with temperature and in the winter months in the Northeast the danger is the fluid in the closed loop thickens in the return line thus causing either slower recirculation (less efficiency) and/or higher circulating pump current to offset the viscosity. Further, they fail to mention the reason for the thick plastic tubing (an insulator) is to minimize the rupturing due to bedrock motion which would pollute the ground water. While there is an argument to be made to not commingle drinking water with heating/cooling fluid if a lightning strike causes a burnout of your water pump you won't have long before toilets won't flush and showers don't run. Consequently you want to ensure the circuit to the pump is surge protected anyway. Further, what I've been told is calcium and magnesium don't corrode your exchanger, electrolysis due to bad system grounding does (with high minerals are present) At a price difference of $2,000/ton I figure it is less expensive and, arguably, safer for the environment to go SCW.
ReplyDeleteAny thoughts or guidance for a homeowner that is no longer using a 480 foot well, and considering using it for a geothermal source for the home heating and cooling?
ReplyDeleteI would also like the answer to this question. I have the same idea, have an exsisting drinking well but am on city water and am not allowed to use the well for drinking anymore. For the most part they have concreted the wells in. Can you re-purpose and use this well?
DeleteI am the homeowner Elizabeth so generously advised back in 2012. I have been running my 4 ton Geothermal heat pump using my well as a standing column since early 2014 with absolutely no issues or concerns. My original plan was to replace the original 3/4 hp 7 gpm well pump with a newer pump, but it has been working great, even though it is now nearly 20 years old. I think my setup has worked well in large part due to the very good water quality of my well, and the 450 ft depth.
ReplyDeleteHello. How many SF is your home and are you using the geothermal system for both heating and cooling? Is it the only system you have? I have a 450 FT well that we abandoned 10 years ago when we hooked up to public water and sewer. I have oil-fired baseboard and could use this for winter only heating and would like to use geothermal for 3 season heating and cooling. Our old central air system has finally died so this seems like a no-brainer. Can you recommend a good designer/ contractor?
DeleteYour posts never disappoint! I always learn something new and appreciate the thoughtful approach you take in covering each topic. It's clear that a lot of effort goes into your writing, and it definitely shows. Keep up the fantastic work!
ReplyDeleteHalltronics
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