Groundwater Effects - Bosch Geothermal Heat Pumps Applications Manual

Applications Manual
Vertical loops require a contractor to use well-drilling
equipment to normally bore a 4-6 inch diameter vertical
hole in the ground. These vertical boreholes are typically
anywhere from 150-450 feet deep. Sometimes the depth
could be outside this range depending on the speci c
situation and ground conditions. Next, a single pipe loop
with a U-bend at the bottom is typically inserted into the
drilled hole (Fig. 55). Occasionally multiple loops are
inserted into the same borehole. After the pipe (or pipes)
is/are inserted, the hole will typically be lled from bottom
to top with grout.
Fig. 55 Single loop with U-bend
Each vertical pipe is then connected to a horizontal pipe
(header), which is also buried underground. The horizontal
pipe then carries uid in a closed system to and from
the Bosch geothermal heat pump unit. Vertical loops are
generally more expensive to install, but require less piping
than horizontal loops because the earth at deeper depths
is cooler in summer and warmer in winter.
The vertical ground loop should be designed using the
appropriate ground thermal characteristics (conductivity
and diffusivity) for the site. These parameters may be
determined from existing information (local well logs,
United States Geological Survey (USGS), state geology
department data), a test bore, or a loop test. The thermal
characteristics of the native material must be adjusted for
the borehole diameter, the type of grout/ ll, and the pipe
diameter.
When dealing with vertical loops, many factors affect the
thermal resistance of the ground loop. These include the
pipe properties, ow rate, back ll and grout properties,
soil properties and uid properties.
Bosch Thermotechnology Corp.
Bosch Geothermal Heat Pumps | 65

10.1 Groundwater Effects

Groundwater movement through the borehole eld can
have a large impact on its performance. Groundwater
recharge (vertical ow) and groundwater movement
(horizontal ow) can all carry away large amounts of heat.
Evaporation can also cool the surface soil and improve
horizontal loop performance.
The presence or absence of groundwater also in uences
total borehole length requirements. Groundwater
movement assists in heat diffusion and can help overcome
an imbalance in the annual thermal loads (cooling
dominated loads) to prevent long term temperature
buildup in the ground around the loops. The loop installer/
designer should account for the presence or absence of
groundwater in the loop design.
It is assumed that a smaller borehole diameter is also
less likely to permit aquifer contamination by water
movement through the borehole. Long-term changes in
localized ground or groundwater temperatures can occur
if the system heating and cooling loads are not balanced.
For borehole-to-borehole spacing, the installer/designer
should consider the depth of the borehole, the loop
eld arrangement, drilling method, drilling and geologic
conditions, the annual thermal loading and land surface
restrictions.
Larger systems with larger loads require more space
between boreholes. The annual thermal loading should
be considered for larger systems for long-term thermal
changes in the subsurface. Subsurface thermal changes
can negatively impact the ef ciency of the system.
Increases of groundwater temperatures of neighboring
property are highly unlikely with a properly operating
closed loop geothermal heat pump system. The drilling
contractor may also be concerned about drilling into
other boreholes at depth, which is more likely with closer
borehole spacing. Larger system loop elds may be
divided into separate clusters or circuits to accommodate
ushing, purging and leak detection/repair. The number
of boreholes per circuit will depend on borehole depth,
spacing, heat extraction or rejection load, and site layout.
Headers should be designed to maintain uniform uid
velocities and to facilitate ushing and purging during
construction and balanced ow during normal operation.
The use of close coupled header designs instead of
extended or reduced header designs will generally
eliminate the need for reverse return piping. However,
reverse return is common for the industry. Headers may be
eld fabricated or prefabricated.
Data subject to change