Ground-source heat-pump system
A common variation of the water-source heat-pump system uses the earth as both a heat source and a heat sink. Known as a ground-source (GSHP) system, this variation takes advantage of the relatively constant 45°F to 65°F temperatures that exist 20 to 30 feet below the surface. When buried in the ground, the high-density thermal plastic pipe acts as a heat exchanger. During the cooling season, it transfers the heat absorbed by the heat pumps to the ground for storage. When heating is needed, the heat exchanger recaptures the heat from the ground and returns it to the building.
If properly designed, an applied GSHP system does not require a cooling tower. Aesthetically, this means that all of the heat from the building can be rejected without any visible sign of a cooling system. A boiler is also unnecessary if the heat pumps can satisfy the entire heating load, which saves initial cost and floor space.
Figure 3–19 Applied ground-source heat-pump system
4–6 ft
100–200 ft
to additional grids ground level pumps
water heaters water-to-water
heat pumps other
HVAC equipment
Temperatures at this depth typically range from 45°F to 65°F.
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Ground-source heat-pump system
■ GSHP systems are more expensive to install, but less expensive to operate, than conventional water-source heat-pump systems.
Perform a life-cycle cost analysis to determine the economic viability for a particular application.
■ Installation requires excavation, trenching, or boring by a qualified contractor.
■ Ground-coupled loops can be installed in a horizontal, vertical, or spiral configuration. Available land, soil conditions, and excavation costs will determine the appropriate choice for a given
application.
Note: A boiler will still need to be input for a GSHP model in TRACE 700. However, if the water loop is properly sized (input as the thermal storage capacity), the minimum and maximum condenser operating temperatures are correct, and the load profile is realistic, the boiler may not activate.
Related reading
■ Water-Source Heat-Pump Systems Air Conditioning Clinic, one of the systems series (Trane literature number TRG-TRC015-EN)
Sample scenario
An applied, ground-source heat-pump system (one heat pump per room) provides comfort cooling and heating for a multistory commercial building. The pump that circulates water through the common condenser loop is rated for 50 feet of static head. A gas-fired boiler and an auxiliary pump rated for 20 feet of static head serve as a backup if the condenser loop is unable to satisfy the entire heating load.
Note: TRACE 700 generates a special report, Thermal Storage, for HVAC designs that include thermal storage. The report provides an hourly profile of ambient conditions and plant-level cooling loads, as well as heat-pump loads, heat-pump energy consumption, and condenser-loop temperatures for hot and cold storage applications.
The following procedure demonstrates how to model the air-distribution, cooling, and heating functions of the GSHP system and assign the coil loads.
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To model an applied GSHP system, begin by defining air distribution.
1 Pick Water Source Heat Pump as the system type.
Then click Apply to save your entries.
2 On the Fans tab, select Hydronic in heat pump fan and enter 0.5 for the static pressure.
Next, describe the cooling and heating plants represented by the GSHP system and backup boiler.
3 Drag the appropriate icons from the Equipment Category section to define each plant. Rename the cooling plant as Ground source heat pump and the heating plant as Backup boiler.
4 Select the cooling plant and click the Cooling
Equipment tab.
5 Choose the ground-source heat pump that best matches the target performance.
6 Enter the full-load consumption of the pump that serves the common water loop—the primary chilled-water pump, in this case.
7 Select the backup heat source and select the ground loop for reject condenser heat.
8 Click Controls.
This entry defaults to Ground loop so that any excess heat from the heat pump is rejected to the ground loop.
This entry identifies the heating plant that will serve as a backup if the heat pumps cannot satisfy the load.
Click to save the displayed entries.
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Ground-source heat-pump system Geothermal Controls.
10Choose the geothermal loop type by specifying the temperature of the loop entering the building in the TLoop Ent Bldg field.
Note: IGSHPA should only be used for horizontal borefields.
Vertical bore requires a separate IDF file to be used in conjunction with the project.
The Custom selection in this field requires a separate Geothermal schedule defined in the Schedules Library.
When using either IGSHPA or Custom geothermal loop types, plant- or equipment-level thermal storage should be defined to reflect the capacity of the borefield loop.
11Enter the geothermal loop flow rate, geothermal loop pump type and associated full-load energy rate.
Enter fully mixed as the flow scheme.
12On the Heating Equipment tab, pick the boiler that most closely matches the anticipated performance.
13Enter the full-load consumption of the pump that circulates hot water from the boiler.
14 Finally, assign each coil load to the appropriate plant.
Note: The minimum operating condenser temperature of the selected heat pump (refer to page 3–67) determines when the boiler turns on to maintain the condenser water
temperature. To view or alter this entry, use the Library/
Template Editors program.
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