Commercial First Cost
(Ground-Coupled)
Geothermal heat pump systems are easily confused with the more widely used
water loop systems, often called water-source heat pumps. While the loop inside
the building is similar, there are important differences.
With the conventional water loop systems, the heat pump units only deal with
water in the narrow 60°F to 95°F temperature range. With geothermal systems,
the heat pump must be able to function properly with a lower inlet temperature.
In some cases using an antifreeze solution, this can be below 32°F in northern
climates. This results in a somewhat higher first cost for the heat pump.
Another difference is the need for a ground loop system, and its added first
cost.
In some cases a hybrid system, using a cooling tower to reduce the loop cost,
is used. Commercial, institutional, and educational buildings typically use
vertical ground coils.
In vertical ground loop studies done by Steve Kavanaugh of the University of
Alabama, some general cost parameters were identified.
The costs of vertical ground coils range from $2.50 to $7.00 per foot.
Required bore lengths range from 125 feet per ton for cold climate, high initial load
buildings to 300 feet per ton for warm climate installations. Hybrid designs
can reduce this.
 Polyethylene piping costs can be as low as $0.20 per foot of bore for 3/4 inch
pipe and as high as $1.00 per foot of bore for 1 1/2 inch pipe.
Drilling costs can range from $1.00 to $12.00 per foot. Typically $5.00 per
foot is the upper limit for drilling the small holes required for geothermal
heat pump systems, even in the most difficult systems. Note: The larger pipe sizes result in shorter bore lengths. The added costs of vertical ground coils typically range from $500 to $1,000 per ton. Using these numbers, we can combine this information with a comprehensive study of water-loop heat pumps reported by the Electric Power Research Institute in October 1991. The results are shown in this graph of the cost per foot of building floor area in the range of 100 to 800 tons
of cooling.
The geothermal heat pump systems are plotted against:
- 2-pipe VAV system with centrifugal chillers
- Unitary Roof-top VAV system
- 2-pipe Fan-coil system with centrifugal chillers, and
- Water loop heat pump system
Hybrid systems use ground-coupled vertical loops and a cooling tower. They
are used in buildings with low heating needs.
The total cost per ton is shown at several heating to cooling ratios. This cost includes the indoor
water loop system and heat pumps at the cost per ton shown in the lower curve.
Hybrid system costs include the installed vertical ground loops, header piping
to the building wall, and closed circuit cooling tower, and are based on
several heating to cooling ratios. This ratio is the feet of loop needed to serve
the cooling load. For example: If 10,000 feet is the loop length to meet the
peak cooling load, and only 45,000 feet is needed to serve the peak heating
load, the ratio is 0.4. The cooling tower costs are based on 75°F wet bulb design.
In all cases, the building loop temperature during the peak cooling load is
85°F supply water, and 95°F return from the heat pumps. Below 100 tons the costs
rise rapidly.
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