Bosch Geothermal Heat Pumps Applications Manual page 42

4 2 | Bosch Geothermal Heat Pumps
If a return well is used, it must be able to handle the
volume of water that passes through the Bosch geothermal
heat pump (Fig. 26). Ground aquifer characteristics such
as the transmissivity of the area surrounding the well
should be considered. Hydrogeological characteristics can
be estimated based on the geology of the well area.
The installer should consider other factors including:
1) Distance from existing wells
2) Volume of discharge water
3) Length of the well available for injection of the water
4) Design of the well screen (if used)
5) Local water quality
6) Local and state well construction codes
The return well must be isolated to allow the discharge
water to reach the ambient temperature of the aquifer
before being withdrawn again. The wells typically should
be isolated at distances greater than 100 feet (horizontal
distances). Larger capacity wells or wells in thin or
poorly transmissive aquifers should have greater isolation
distances.
The construction of the return well is critical to the
effectiveness of this type of water disposal. A well that
is not constructed properly can at some point cause
the entire system to fail. Problems often consist of well
clogging or slowing because of either poor construction or
the development of mineral precipitate that clogs the well.
Several actions can help to prevent precipitation of
minerals, although results cannot be guaranteed. The
well must be of suf cient diameter and depth to accept
the maximum discharge from the Bosch geothermal heat
pump system. The screened or open rock portion should
be greater than that of the supply well. A return well
constructed in rock typically requires twice the capacity
as the supply well. An extended pumping test (12-24
hours) is recommended to help determine hydraulic
characteristics of the return well. The use of a back valve
can help to prevent pressure differences that could result
in precipitation of minerals.
Also, the return well should be properly developed to
remove nes and stabilize the borehole. Mechanical
surging and some types of chemical treatment can
promote a stable well, or successfully treat a clogged well.
The rate of discharge acceptance for a return well can be
estimated based on values determined from a pumping
test of the well. A pumping test is accomplished by
pumping the water well at a constant rate for at least an
hour. Speci c capacity is the observed rate of yield (in
gallons per minute) divided by the drawdown resulting
from the pumping test. For example, if a well pumps at 10
GPM and the water level drops 5 feet, the speci c capacity
is two GPM/ft of drawdown.
Data subject to change
To push water back into the ground aquifer, the pressure
on the column of water must be greater than the pressure
resisting ow back into the aquifer. The discharge water
will "mound" inside the well until enough pressure is built
up to force the water into the aquifer. The term "head" is
used to denote the height of the water in the well. The
"injection head" is how far the water rises before water
will be moved into the aquifer. Injection head depends on
many factors including the construction of the well, the
discharge rate of the return water, the aquifer's ability
to transmit water and how much water is held in the
borehole area.
The theoretical value of the injection head can be
calculated by dividing the discharge rate by the speci c
capacity. For example, if 10 GPM is returned to the well
that has a speci c capacity of two, then the injection
head will be ve feet. If a positive number results when
the injection head is subtracted from the depth to water
(static water level) then the water will not over ow at the
surface. A speci c capacity value of less than 1 GPM/ft
may cause over ow at a well that is only 20-25 feet deep. A
water injection test can be performed to see how the well
reacts. The installer should consider that any decrease
in the aquifer transmissivity will result in an increase in
injection head. Clogging of the open spaces in a well will
eventually bring the water to the surface.
Fig. 26 System water disposal
Courtesy of International Ground Source Heat Pump Association/Oklahoma
State University
Applications Manual
Bosch Thermotechnology Corp.