Water Well Criteria And Terminology - Bosch Geothermal Heat Pumps Applications Manual

Applications Manual
6.4 Open-Loop Geothermal Heat Pump
System Design
The design of an open-loop system must consider
the performance of the system based on the power
requirements of the well pump and geothermal heat pump.
With the open-loop system, the greater the groundwater
ow (GPM) the more favorable are the temperatures
at which the geothermal heat pump will operate. As
the groundwater ow is increased, the improvement
in geothermal heat pump performance is increasingly
compromised by rising well pump power. At some point,
increasing well pump power overshadows the gains in
geothermal heat pump performance and the total system
performance begins to decline. The open-loop system
pumping power is normally a much stronger function of
ow rate than with closed-loop systems because not only
must the pump overcome the circulating head, it must also
overcome the head due to the static level in the well and
any reinjection head requirements.
The task in open-loop system design is to gather enough
information about the well pump and geothermal heat
pump to identify these trends and to select the optimum
system performance point. It is the system cooling
ef ciency rating, the Energy Ef ciency Ratio (EER), and/
or the system heating ef ciency rating, the Coef cient of
Performance (COP), that should be the focus of the open-
loop system design, not simply the performance of the
geothermal heat pump.
The typical industry procedure is to evaluate the well
pump power required to produce a range of groundwater
ows and combine that with the geothermal heat pump
performance at those same groundwater ows. The
optimum relationship between pumping power and
geothermal heat pump performance is established at the
design condition and system performance at off peak
conditions is maintained by accurate well pump control.

6.5 Water Well Criteria and Terminology

Groundwater wells are the foundation of open-loop
systems and certain key terms should be understood
for proper design. In any well there will be a water level
at which the groundwater stands in the well under non-
pumping conditions (Fig. 27).
This level indicates the water table level in uncon ned
(or water table aquifers), or the "Piezometric" level (the
artesian equivalent of the water table) in con ned (or
artesian) aquifers, and is known as the "Static Water
Level" (SWL). When the pump is started, water level
will normally drop to a new, lower level referred to as
the "Pumping Level". The pumping level is a function of
the rate at which the well is being pumped, the greater
the rate the lower the pumping level. The optimum
groundwater ow rate is a function of many variables,
but the greatest in uence will typically be the static
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water level. If the water must be pumped from greater
depths then pumping power per unit of water produced is
increased.
The difference between the SWL and the pumping level
is referred to as the "Drawdown". Drawdown at a given
pumping rate, divided by the rate, results in a value known
as "Speci c Capacity" measured in Gallons per Minute per
Foot (GPM/Ft). The drawdown at a given rate is a function
of the "Cone of Depression" that forms in the aquifer
around the well during pumping. The size and shape of the
cone of depression and the depth of the drawdown are a
function of the aquifer and its ability to deliver water. At
the return well, there is a "Cone of Impression" that forms
in the aquifer around the well during discharge.
Fig. 27 Water well terminology
Speci c capacity is a useful value for indicating the ease
with which the aquifer produces water. A high speci c
capacity value, say, 10 GPM/Ft. typically indicates a "good"
well; whereas, a low speci c capacity value, say, 0.5 GPM/
Ft., would typically indicate a "poor" well. For artesian
aquifers, speci c capacity will be a constant value over
a broad range of ows. In water table aquifers, speci c
capacity will diminish as pumping rate increases.
The construction of a well is also a function of the
aquifer. In rock formations, often the bottom of the well is
uncased. This is referred to as "open-hole completion". In
formations in which there is a tendency to cave, a slotted
casing or screen may be used. In very ne sands and in
thinly strati ed formations, it may be necessary to place
a "gravel pack" around the screen to provide additional
ltering and to increase the permeability of the near well
materials.
A key part of the design process is the determination of
the well pump power required for a range of ground water
ow rates. To calculate these values it is necessary to
know something about the performance of the production
well in terms of the head (static water level plus
drawdown) it imposes on the pump to produce the water.
The best source of information is the results of a pump
Data subject to change