ature and geological conditions of the
site where the well field will be located.
Once the information from the
campus loads and the test well are
known, they are used in geothermal heat
exchanger ground loop design software
to model the depth and number of bores
required for the campus. Ground loop
sizing parameters include the ground
temperature increasing approximately
20 degrees F in summer (above normal
ground temperature) and decreasing
approximately 10 F in winter (below normal ground temperature). This temperature range will allow for optimum heat
transfer in the ground loop heat exchanger and operation of the equipment in
the district energy station. The ground
loop design software will predict how the
ground loop will affect the temperature of
the earth over the next 20 years.
Figure 1 shows a 20-year model of
an ideal ground temperature fluctuation
in a well field. The initial temperature
of the earth is 54 F. The ground loop
heat exchanger temperature varies
from a low of 43 F to a high of 68 F.
The trend of the earth temperature is
stable, meaning the ground isn’t getting
steadily warmer or colder over years of
operation. That indicates a good balance
between heating and cooling loads, and
thermal saturation is not a concern.
Temperature (F)
Figure 1. Example of Well Field Ground
Temperature Fluctuation Over a 20-Year Period.
80
70
60
50
40
30
20
10
0 —
0
225 —
100 —
125 —
150 —
175 —
200 —
75 —
Time (months)
Source: Courtesy MEP Associates LLC.
25 —
50 —
3. Evaluating building HVAC systems
Large campus geothermal heat pump
systems usually require an array of building conversion approaches due to the
various vintages of individual building
heating and cooling systems across the
campus. Typically the design approach
should maximize the existing building
assets. This means that the design hot
water temperature for the heat pump will
be as low as possible to save energy but
not so low that every heat transfer device
(air-handling unit coil, finned tube radiation, cabinet unit heater, unit heater, etc.)
has to be replaced.
One recommended test for the
campus, for any building that has a hot
water distribution system, is to set the
hot water supply temperature down to
the anticipated level of the future campus geothermal system. This test gives
the campus the ability to identify buildings that are short of heat transfer area
prior to construction starting. If a building is short of heat transfer area, then
additional heat transfer equipment will
have to be added to the building.
Determining the
Distribution System
There are many options to consider
when converting a campus heating
and cooling system to geothermal. An
institution and its design team need to
evaluate many factors to determine the
optimal distribution system, including
the following:
• Existing infrastructure available for
reuse – Most large campuses use
steam for heating, in which case all
the hot water supply and return
piping will need to be new; but an
example of campus infrastructure that
can be reused is chilled-water supply
and return piping. If a campus chilled-water system is in place, it can easily
be reused in a four-pipe distribution
system. If a campus chilled-water
system does not exist, installing a
four-pipe system will probably be
cost-prohibitive.
• Space for equipment in existing
buildings – Equipment such as heat
pumps, expansion tanks and distribution pumps can be located in the central plant or other buildings. If the
existing buildings do not have space
for more equipment, a new energy
station may be required for a centralized geothermal system.
• Temperature of hot water supply –
Some existing campus buildings will
use all steam heat transfer media, but
Why Campus
Geothermal Systems?
Campus geothermal heat pump systems offer
the following benefits:
•;Energy;savings;–;The;coefficient;of;perfor-
mance (COP) for the geothermal system
will range from 3. 5 to 4. 5 in heating and
cooling mode and as high as 10 for simul-
taneous heating and cooling mode.
Compared to a COP of 0.65 to 0.80 for a
coal-fired steam system, the geothermal
system provides significant energy savings.
(See figure 2.)
•;Reduced;carbon;footprint;–;The;carbon
footprint of a campus can be significantly
reduced by eliminating coal boilers.
•;Lower;ash-handling;costs;–;In;addition;to
decreasing the campus carbon footprint,
the reduction or elimination of coal use can
also significantly reduce ash-handling costs.
•;Simultaneous;heating;and;cooling;loads;–
The geothermal system performs extremely
well when a campus has a large simultaneous heating and cooling load.
•;Reduced;maintenance;–
;Maintaining;geo-thermal systems is significantly easier than
boiler maintenance. The elimination or
reduction of coal- and gas-fired boilers also
allows for lower maintenance costs.
Figure 2. Coefficients of Performance:
A Comparison.
Fossil Fuels Conventional Systems
1 unit of
purchased
fossil fuel
80-90% of heat
to the building
10-20% of heat
up the chimney
COP = 0.8
Fossil Fuels Conventional Systems
1 unit of
purchased
fossil fuel
80-90% of heat
to the building
10-20% of heat
up the chimney
COP = 0.8
Fossil Fuels Conventional Systems
1 unit of
purchased
fossil fuel
80-90% of heat
to the building
10-20% of heat
up the chimney
COP = 0.8
Electric Heat
1 unit of
purchased
electricity
the building COP = 1.0 Electric Heat 1 unit of purchased electricity 1 unit of heat to the building COP = 1.0 Electric Heat 1 unit of purchased electricity 1 unit of heat to the building Free, Renewable Energy From the Earth
COP = 1.0
Source: MEP Associates LLC. COP = 4. 5
Free, Renewable Energy From the Earth
1 unit of
purchased
electricity
4. 5 units of heat
to the building COP = 4. 5
Free, Renewable Energy From the Earth
1 unit of
purchased
electricity
4. 5 units of heat
to the building
Plus 3. 5 units of
free energy
from the earth
COP = 4. 5
1 unit of
purchased
electricity
4. 5 units of heat
to the building
Plus 3. 5 units of
free energy
from the earth
free energy
from the earth
