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Low Temperature Geothermal Resources: Ground Source Heat Pumps 193

it would be easy to undersize or oversize a ground loop system if care is not taken to adequately
characterize the thermal properties of the soils at a site.
Establishing the design length importantly constrains the design implementation. For a vertical
system that will utilize boreholes, less land is required but because drilling costs can be significant, the
initial investment expenses can be high. For a horizontal loop system, the amount of land required for
excavation and loop emplacement can be significant, but the installation costs are less than for vertical
boreholes. If there is sufficient land available for a horizontal loop system, the equations above can be
used to size the loop length and assist in determining what loop configurations would be appropriate.
Open-loop systems differ from closed-loop designs in that the working fluid is directly extracted
from the groundwater system, passed through the heat pump heat exchanger, and either rein-
jected into the subsurface or released in the surface environment. Such systems rely only on the
fluid temperature of the groundwater that is extracted, and thus do not depend on knowledge of
the thermal properties of the soil or bedrock at the site. Instead, it is necessary to obtain reliable
data on the temperature variation that the groundwater system historically has exhibited. In general,
such temperature swings will be minor, compared to that seen in the near-surface soils, usually
ranging within 5°C of some median value. These systems do not require a specified loop length,
thus making sizing of the system easier. However, disposal of the groundwater, whether by reinjec-
tion or by release into a surface catchment system, requires special consideration and care in order
to avoid potential water contamination. Due to that concern, special permits are often required to
install such systems.

case sTUdy: weaVerVIlle
Weaverville Elementary School is a 50-year-old public school located in northwestern California.
In July and August, the hottest months in Weaverville, the average daytime high temperature is
about 34°C (93°F). In winter, the coldest months are December and January, when the daytime high
temperature averages about 8°C (46°F). Heating and cooling are thus required for both time periods.
For reasons of economy and building maintenance efficiency, it was decided to retrofit the building
complex with ground source heat pump heating and cooling capability. The information presented
in this case study is from GRDA 2007.
Prior to installation of the ground source heat pump system, the building complex utilized 20
air source heat pumps, two diesel boilers, one propane boiler, two propane heaters, 11 evaporative
coolers, and four dual-fuel units.
For purposes of sizing the ground source heat pump system for the building complex, it was
calculated that the necessary heating capacity needed for the site was about 47 kW. The calculated
cooling capacity that was needed was estimated to be 35 kW. The amount of time required for
HVAC was calculated to be 700 hours/year.
The computed borehole length for the stipulated heat pumps and ground loop piping was 1005
meters (3320 feet). In this calculation, it was assumed that the subsurface temperature would be
about 11.5°C (53°F), the COP of the heat pumps would be about 4.8 and the EER would be 23.5.
In the final configuration, 11 separate borefields were put in place, with between eight and 22 indi-
vidual boreholes per field, each borehole being 90 meters (295 feet) deep. A total of 38 ground
source heat pumps were installed in classrooms and office spaces. The system was brought online
in September 2003. At the time the system was completed, an additional 371 square meters (4000
square feet) of classroom space was add to the school, and all of the classrooms were provided year-
round HVAC, which was an upgrade for 14 classrooms.
In Figure 10.8, the amount of oil and propane consumed before and after installation of the ground
source heat pump system is shown. Figure 10.9 shows the change in electricity consumption over the
same time period, and the total dollar cost for energy production is shown in Figure 10.10. These
changes in energy source amount to a reduction in the emissions of greenhouse gases as shown in
Figure 10.11. By eliminating oil consumption and reducing propane and electricity consumption,