|
OPEN LOOP
GEOTHERMAL WELL SYSTEMS ON LONG ISLAND Paul
K. Boyce, P.E., Doreen Fitzsimmons P.W. Grosser Consulting, Inc. 630 Johnson Avenue, Suite 7 Bohemia, New York 11716 Introduction The
use of groundwater for cooling and heating purposes on Long Island dates back
as early as the 1930's. Long Island’s
abundance of readily available groundwater supplies with constant water
temperatures allows for technologies relying on groundwater acting as a heat
pump to be employed cost effectively and efficiently. Numerous
methodologies exist to use this abundant natural resource as a renewable
energy source in an efficient and environmentally friendly manner for heating
and air conditioning industrial, commercial, institutional and residential
structures. The
more common systems utilized on Long Island are ground source heat pumps or
geothermal systems involving open or closed loops. Another type of system also found on Long Island, though not
quite as numerous as the others, is a standing water column system. This system is a hybrid of open and closed
loop systems, though more closely resembling closed loop systems. A
brief overview of open loop ground source heat pump or geothermal systems is
presented in this paper. The
operating principles, important design criteria, hydrogeology and operation and maintenance procedures
are introduced. Some field
experiences and lessons learned are also discussed. Different Types of Systems
Typically Found on Long Island - Overview Open
loop geothermal systems typically include one or more supply wells and one or
more diffusion, recharge, return or injection wells. In an open loop
geothermal well system, groundwater is withdrawn from an aquifer through the
supply well and pumped to a heat exchange device where it acts as a heat
source or sink in the heating or cooling process. A typical heat exchange device is a plate heat exchanger, in
which a non-contact, non-consumptive process takes place between the
groundwater and the building’s internal circulation water. Heat is transferred between the two waters
without ever physically coming into contact or mixing with one another. Once the groundwater passes through the
heat exchange device it is returned to the aquifer through a diffusion
well(s). The only difference between
the supply and return water is the temperature. The open loop system is one of the more common systems found on
Long Island and is utilized in both large and small scale applications. Closed
loop ground source heat pump systems do not involve wells but rather a series
of deep boreholes (usually 300'+) fitted with long narrow u-tube
configurations of piping. The
boreholes are drilled deep below the water table and rely on the soil mass
and groundwater to act as the heat source or sink for the water being pumped
through the tubes within the boreholes.
The water within the tubes never physically contacts the groundwater
and groundwater is not withdrawn from or recharged to the aquifer. The water within the tubes is simply
circulated between the boreholes and the building’s heat exchange
device. Again, the only exchange
between the water within the borehole tubes and the groundwater and soil mass
is a thermal one. The closed loop
type of geothermal system involving boreholes is gaining popularity on Long
Island due to its ease of maintenance and can be utilized in both large and
small scale applications. The major
draw back to this type of system is that a large number of boreholes are
usually required and spacings between boreholes can vary between 10 to15
feet, thus a large area of land is often required. A
standing water column system is generally a single deep well drilled into
bedrock. A casing is set from grade
down to bedrock and from there the well is essentially an open rock
well. The geothermal water in this
case is circulated within the same well.
Here, if the water is withdrawn from the bottom of the well the water
will be returned at the top and allowed to heat or cool as it traverses down
the well to where it is being withdrawn from. In most instances the bedrock is fairly impermeable and less
than 10% of the water being pumped from the well is “new” or “fresh” groundwater. Hence, this type of system behaves
primarily as a closed loop ground source heat pump. Due to the large depth of bedrock on Long Island these systems
are not very common, are generally limited to the north shore of the island,
and typically used in small scale applications such as residential projects. Open Loop Geothermal Well
Systems - Operating Principles The
groundwater side of an open loop geothermal well system operates in a fairly
simple and straight forward manner.
Groundwater is pumped from a supply well and recharged through
diffusion wells. Pumping ranges for
these types of systems on Long Island vary from tens of gpm up to hundreds of
gpm and in some large systems, over 1,000 gpm. What
happens to the groundwater between the supply and diffusion wells can vary
based on the complexity and importance of the system. Larger systems are generally found in
hospitals, schools, industrial and commercial buildings and apartment
complexes. In these instances the
system is relied on heavily for providing air conditioning and heating and
will require a more serious control and monitoring methodology. With the larger systems, after the
groundwater is withdrawn from a supply well it will be pumped through a
piping system where it will pass through various control devices, monitoring
equipment, instrumentation and a heat exchanger before being returned. Figure 1 depicts a schematic of a typical
large system with the different types of controls often necessary for an open
loop geothermal system to operate optimally. Groundwater
is withdrawn through the supply well using a pumping unit, usually a
submersible pump. An in line check
valve either integral to the submersible pump or installed in the discharge
column is often specified to prevent the system from partially draining on
shut down. This helps minimize air
from entering the system. Gate valves
are part of the piping system that can be used to isolate certain aspects of
the system depending on the circumstances.
For instance, if multiple supply or diffusion wells are part of the
system a gate valve may be closed to isolate one of the wells while
maintenance procedures are being performed on it. Thus allowing the rest of the system to continue to operate. Air relief and vacuum breaker valves are
generally installed in critical locations in the system such as at high and
low points and at well heads. These
types of valves help reduce the amount of air in the system and alleviate
many of the problems air can cause, such as poor heat exchange and entrained
air in the return water. Other important control devices include strainers and throttling valves. Strainers can be of particular importance if a supply has a tendency to pump
Figure 1 - Open Loop Geothermal Well System Schematic sand. The sand will ultimately end up in the diffusion well and likely start to plug the screen zone and reduce the overall capacity and efficiency of the diffusion well. An in line strainer will remove large particulate matter and prevent it from ending up in the diffusion wells. A throttling valve, such as a butterfly valve, can be used to control the flow from the supply well by increasing or decreasing the amount of back pressure the pumping unit is working against. This is particularly convenient for conducting pump tests to gage the overall efficiency of the pumping unit and if a variable flow is desired during system operation. Another way to vary the flow of the geothermal system is to use a variable frequency drive (VFD) on the pumping unit motor. Here the motor rpms are varied by changing the frequency of the electricity being used by the motor. As the rpms the motor and subsequently the pump impellers spin at varies, so does the flow rate through the system. Monitoring
how well the system is operating can be accomplished using different
instrumentation apparatuses. These
may include: flow meters, pressure/vacuum gauges, thermometers and water
level sensors. Design Criteria for Open Loop
Geothermal Well Systems Design
of open loop geothermal well systems requires detailed knowledge of the local
hydrogeology and an understanding of the hydraulics involved with the piping
and heat exchange portion of the system.
The most critical aspect of the design involves the aquifer system
from which groundwater is being withdrawn from and recharged to. Since the
wells are the key to tapping and utilizing the groundwater, they need to be
carefully sized and located. When
designing these types of wells the usual aquifer parameters such as porosity,
hydraulic conductivity, transmissivity, storativity and aquifer thickness,
are sought out through research of published works about the local area,
obtaining data from nearby existing wells or through conducting actual field
tests. Obviously, conducting actual
field tests is the most preferred and accurate method to obtain data specific
to the project, but is often economically infeasible especially for smaller
projects such as residential applications.
Depth to water and groundwater flow direction are also important
pieces of aquifer information to obtain. If
field tests are to be conducted, test wells are installed, split spoon
samples collected, water quality samples analyzed and the boreholes logged
both visually and geophysically.
Pumping tests are conducted and the results are analyzed to estimate
the key aquifer parameters. Borehole
logging helps establish the lithology, stratigraphy and depth to water. Once
the appropriate information is collected well design begins. The flow rate of geothermal water is
generally dictated by the acceptable temperature differential between the
supply and return waters and the cooling/heating loading of the
building. Lower flow rates will mean
a higher temperature differential.
Higher temperature differentials will require larger separation
distances between the supply and diffusion wells to avoid or reduce the
effects of thermal breakthrough.
This phenomena occurs when the supply water temperature begins to
change from ambient groundwater temperatures (commonly 50EF to 55EF for Long Island) because the
recharge water that is returned to the aquifer is now influencing the supply
well. Geothermal water requirements,
based on the cooling or heating demand of a facility, are typically expressed
as gallons per minute per ton of cooling or heating load (a ton being equal
to 12,000 Btu/hr). Temperature
differentials can be approximated based on the following simple linear
relationship:
The
faster water moves through the system the lower the temperature differential
between supply and return waters.
Thus if a site is limited in areal extent, the designer may need to
consider going to a higher flow rate system. After
the system flow rate is established, well location is addressed. Supply wells are recommended to be sited
upgradient and as far away as possible from diffusion wells, again to reduce
the potential effects of thermal breakthrough. If multiple supply and diffusion wells are to be implemented
the wells should be spaced as far apart as is economically feasible to limit
the effects of drawdown and mounding.
Diffusion wells spaced too closely together can have negative effects
on one another. They can create
excessively high mounds in unconfined aquifers or increase the potentiometric
surface so much in confined aquifers that they no longer behave as gravity
diffusion wells but become forced injection wells. This effect will create higher backpressures on the system
pumping units and ultimately cause lower flow rates, higher temperature
differentials and disrupt the system’s overall efficiency and performance
(i.e., no longer meeting the building’s cooling and heating needs). Therefore extreme care must be taken in
properly locating and spacing wells.
A good understanding of the local hydrogeology and well hydraulics is
necessary to properly locate open loop geothermal system wells. In many cases numerical groundwater
modeling is conducted during the design phase to estimate to what degree
impacts can be expected based on different well location and
pumping/recharging scenarios. With
the flow rate and general well locations determined the wells are then
designed as typical water supply wells.
The well screen diameter, length and slot size are based on the
required flow rate, desired screen entrance velocity and the grain size
distribution of the aquifer it is to be situated in. The overall depth of the well and the
screened interval are computed based on aspects such as static depth to
water, drawdown, required pumping unit submergence and the location of high
conductivity water bearing strata in the aquifer. A
difference between supply and diffusion well screen design is that typically
it is much more difficult to return water to an aquifer than it is to remove
it. Many theories have been posed as
to why this is the case, but none that the authors are aware of have been
proven to be the universal case. To
compensate for this effect it is generally recommended that two to three
times the amount of screen be provided on the diffusion end of the system as
opposed to the supply side.
Experience has shown that substantially more than this may be
necessary in some instances.
Additionally, diffusion wells have a greater tendency to become fouled
and require maintenance more frequently than supply wells. Often more diffusion wells, than are
theoretically necessary, are installed for redundancy and to keep the system
operational while maintenance procedures are conducted. Long
Island geothermal wells are required by the NYSDEC to be screened within 50
feet vertically of one another and be situated within the same aquifer
unit. In other words, the water
removed from an aquifer must be returned to that same aquifer and not by more
than a 50 feet vertically in either direction (i.e., above or below). Therefore, the bottom of a supply well can
not be more than 50 feet above the tops of the diffusion wells it is
discharging to and vice versa. Also
no confining units may exist between the screen zones of the wells. Taking these criteria into consideration
can further complicate system well design. Water
quality is a design variable that is often overlooked. Groundwater with high quantities of
dissolved inorganics such as iron, calcium and manganese can cause serious
diffusion well problems. As the
groundwater is returned to the diffusion well it cascades down the well
casing before reaching the water surface.
The water goes through an aeration process where many of the dissolved
inorganic constituents can precipitate out of solution and begin to
accumulate in the screen zone and reduce the well’s overall diffusion
capabilities. Also using the water as
a heat sink can cause scale deposits to form as a result of the presence of
excessive amounts of calcium and manganese.
The local water quality needs to be carefully evaluated during the
design process to ensure that the system will not be adversely effected be
these parameters. As the process is a
non-contact, non-consumptive one, treatment of the water is prohibited. Therefore, if unacceptable water quality
exists an open loop geothermal well system may not be appropriate. Hydrogeology and Open Loop
Geothermal Well Systems Hydrogeology
plays important part in the design of an open loop geothermal well
system. Fortunately much is known
about Long Island’s hydrogeology across most of the island. A significant amount of published material
exists and can be used as a preliminary screening tool in determining the
applicability of using an open loop type system. The significant parameters that need to be evaluated when
considering an open loop geothermal well system include: depth to water,
porosity, specific yield, hydraulic conductivity, specific capacity, and what
type of aquifer conditions are present (i.e., confined, unconfined, etc.). Depth
to water is important because generally the greater this distance the better
off the diffusion side of the system will perform. A large depth to water unfortunately increases the horsepower
size of the pumping unit motor, but allows for a large recharge head to be
developed in the casings of the diffusion wells. Also, if mounding is a concern in the diffusion well area,
larger mounds can be accommodated.
Generally, as the depth to water becomes more shallow, especially in
unconfined aquifers, open loop geothermal systems become less and less
desirable. For most Long Island
systems, a depth to water of 35 to 40 feet is usually adequate. Successful systems have been installed
with shallower depths to water, but, if the formation begins to plug or the
water table rises, localized problems such as aquifer saturation and flooding
have been known to occur. Larger or
higher flow rate systems will require larger depths to water to ensure operation
as a gravity type diffusion system. The
porosity of the formation from which water is being supplied and returned
needs to be investigated. The more
coarse or the more porous the material in which an open loop geothermal well
system is to be installed the better.
Coarser formations can provide and accept water more readily than
tighter, finer ones. As mentioned, it
is generally more difficult to recharge the groundwater than it is to remove
it, therefore a coarse medium with a high percentage of void spaces will lend
itself more readily to the diffusion process. Most Long Island open loop geothermal well systems are
installed in the Upper Glacial and Magothy aquifers. The Upper Glacial aquifer is generally
described as a sand and gravel aquifer with varying porosity. Porosities below 0.15 can make using open
loop geothermal well systems difficult.
Porosity
however should not be the only criteria considered when evaluating an open
loop geothermal system. Clays can
have high porosities but extremely low hydraulic conductivities. Hydraulic conductivity is an indicator of
the aquifer’s ability to transmit water.
Low hydraulic conductivity formations are obviously unsuitable for
open loop geothermal well systems.
Long Island open loop geothermal systems have been installed in
various hydrogeological settings with hydraulic conductivities that range
from on the order of hundreds of gpd/ft2 to over 2,000 gpd/ft2. The better performing or more efficient
systems have higher hydraulic conductivities associated with them (i.e.,
fewer diffusion wells, less diffusion well screen). The higher hydraulic conductivities associated with the Upper
Glacial aquifer, as opposed to the Magothy or Lloyd aquifers, make it the
most desirable aquifer for an open loop type of system. Specific
capacity is a basic measurement of a well’s efficiency. Specific capacity is dependent on several
variables that include: well screen length, diameter and slot size, gravel
pack size and the type of material that the well is situated in (i.e.,
gravel, sand, silt, etc.). This
parameter gives good insight into how well an aquifer, and the wells
installed in it, are interacting to produce and accept water. The higher the value of this parameter the
better. Assuming most wells are
properly and similarly designed and constructed on Long Island (i.e., using
the same methodologies, procedures and materials), this value can be assumed
during the design phase to be typical for the area where the system is to be
installed. Values for the Upper
Glacial aquifer are reported to range between 10 to 200 gpm/ft. Open loop systems may become difficult to
implement in areas where the specific capacity is less than 10 gpm/ft,
especially larger systems. What
type of aquifer the system is being installed in also plays an important part
in how the system is designed and how well it will perform once
constructed. Confined, unconfined and
semi-confined aquifers or portions of aquifers can be found on Long Island. Depending on the type of aquifer,
different properties will be associated with it and need to be taken into
consideration when designing and constructing open loop geothermal wells. Geothermal Well Rehabilitation An
effective maintenance program is essential in order to keep a well at its
maximum efficiency. As a well ages,
its ability to produce or diffuse water decreases as the screen slots slowly
become plugged. Well rehabilitation,
which is the restoration of a well to its most efficient condition, to any
well is eventually inevitable. The
necessity of rehabilitation will depend on how often and effectively the
wells are maintained. In some cases,
damage to a well may be so severe that screen replacement or lining may be
necessary. More often the well can be
chemically treated in the hopes to disrupt, dissolve, or remove deposits in
the well or the surrounding aquifer. Declining
well capacity may be the result of a number of different problems. Well yield may be reduced by chemical or
biological encrustation, silt or clay intrusion, and mineral deposits. The type of problem is often related to
the type of aquifer the screen is placed in or the quality of the
groundwater. For example, on Long
Island iron in the groundwater is very common and this can accelerate
deterioration of the well screen zone.
Mineral deposits in the screen zone will effect the well’s diffusion
capabilities as well as its ability to produce water. Before a treatment
process can be determined the problem must first be identified. In most cases a problem is indicated as a
result of a well’s declining well capacity.
Specific capacity is defined as the flow rate divided by the draw down
of the water column. Observing a
decreasing specific capacity will often provide the initial insight to
potential well problems that will likely require rehabilitation efforts to
correct. Increased power usage, i.e.,
amperage draw or voltage drop may also indicate a well related problem. The
first step to identifying the type of problem is performing a televised well
inspection. In this process, a
submersible camera is lowered into the well and the interior condition is
video logged. Encrustations and
slimes in the well or screen can usually be identified on the monitor and the
color can most times identify the type of problem. Samples of the formation immediately surrounding the well may
also be collected and analyzed to determine the type of buildup. Once
the type of problem is identified, the second step is deciding a treatment
process, which is mostly dependent upon whether the problem is geochemical or
biological. Types of rehabilitation
or cleaning agents include: wetting agents and surfactants which form
chemical “bridges” between solids and water helping it to suspend the solids;
dispersants which penetrate and interfere with the chemical attraction of
deposits; chelaters which combine with metal ions to keep them in suspension;
and disinfectants which kill microorganisms.
In a case where there is an inorganic buildup in the screen zone,
hydrochloric acid (HCl) may be used. Before
a well is treated the contractor will first want to brush the well with a
wire brush to remove and loosen debris.
Following this procedure the sump of the well should be bailed,
removing as much accumulated debris and sediment as possible. Chemical treatment will then be introduced
into the well. Each treatment should
consist of approximately three well volumes to allow for the solution to
disperse into the formation. A tube
called a tremie pipe should be used to deposit solution directly into the
screen zone, in turn reducing the dilution effect and also preventing the
solution from reacting with the casing above the water table. The solution should then be allowed to stand
at least overnight. There
are a number of other techniques that may be used in conjunction with
chemical treatment. Some examples
include: surge blocks, jetting, air bursting or blasting, high-pressure air
lifting, and sonar jetting. These are
all variations of physical methods meant to agitate the formation and break
up materials with the use of force. Upon
completion of treatment, the waste must then be pumped off and properly
disposed of. In general it is
necessary to pump off approximately ten times the volume of the chemical
solution that was put into the well in order to clear it out. Some chemicals can become a bacterial
substrate after dilution so clearing out as much as possible is essential. Prior to disposal it is important that the
waste is neutralized before being released into the environment. Once treatment and disposal is completed,
a second televised inspection will usually be performed in order to ensure
that most of the buildup has been removed. Once well rehabilitation procedures are completed a performance test should be conducted in order to confirm that the treatment was effective. The performance test should be carried out between four to 12 hours and involves pumping the groundwater from the supply well(s) into the diffusion well(s). The water can be either pumped directly into the diffusion well(s), or may be run through the heating/cooling system prior to recharging through the diffusion wells. Preference is for conducting the test utilizing the entire system to observe system operating temperatures and pressures. During testing, water levels in each well should be recorded periodically throughout the test. In addition, the pumping rate and well head pressures/vacuums should also be recorded. Observation of these criteria will allow the performance of the rehabilitated system to be observed and compared to specified operating values. Wells
are often allowed to deteriorate for such a long time that their specific
capacity may be impossible to restore completely. Rehabilitation procedures should be initiated before the
specific capacity has declined 25 percent.
For this reason, it is essential that the well owner keep good well
records so that and decline in performance will not go undetected. Conclusion Open
loop geothermal well systems are a proven, efficient, and reliable
alternative cooling and heating method that can be used in a wide array of
different applications. Long Island’s
vast natural resource, groundwater, is an excellent heat source/sink that can
be easily tapped and used in such systems.
Geothermal systems are often encouraged by local utility companies
because of their efficiency and ability to help reduce energy
consumption. Rebates of up to
$350/ton with a maximum of $100,000 per project have previously been offered
by LIPA to help encourage the use of these types of systems on Long Island.
The rebates are offered to help off set the often larger capital cost
associated with geothermal systems as opposed to the more conventional
systems (i.e., cooling towers, dry coolers, etc.). The increased efficiency of geothermal systems over
conventional systems helps quickly pay for them and savings are often seen
within a few short years after installation. References 1. Driscoll,
F.G., Groundwater and Wells,
Johnson Division, St. Paul, MN, 1986. 2. Franke,
O.L., P. Cohen, Regional Rates of
Ground-Water Movement on Long Island, New York, U.S. Geological Survey
Professional Paper 800-C, United States Government Printing Office, Pages
C271-C277, Washington D.C., 1972. 3. Freeze,
R.A., J.A. Cherry, Groundwater,
Prentice-Hall, Inc., Englewood Cliffs, NJ, 1979. 4. Kavanaugh
S.P., K. Rafferty, Ground-Source Heat
Pumps Design of Geothermal Systems for Commercial and Institutional Buildings,
American Society of Heating Refrigeration and Air Conditioning Engineers,
Inc., Atlanta, GA., 1997. 5. Lund,
J.W., P.J. Lienau, B.C. Lunis, Geothermal
Direct-Use Engineering and Design Guidebook, United States Department of
Energy, Idaho Falls, Idaho, 1998. 6. McClymonds
N.E., O.L. Franke, Water-Transmitting
Properties of Aquifers on Long Island, New York, Geological Survey
Professional Paper 627-E, United States Government Printing Office,
Washington D.C., 1972. 7. Thomas, Larry, How to Recover a Well’s Capacity, Opflow, Feb 2002: 11-14. |
