In summer, the ground loop acts as a "heat sink," rejecting the unwanted building heat. In winter, it acts as a "heat source," absorbing heat from the ground to heat the building.
Consequently, the ground loop is typically sized to do the largest job. In geographical areas where the summer cooling load is much larger than the winter heating load, this can result in loops being much oversized during the winter.
Also, in many commercial buildings, the cooling load exceeds the heating load, even in northern climates.
A hybrid system with supplement heat rejection is an alternative for commercial applications that may not have the land area for a full-sized ground coil. This system would be a viable option to reduce the size of the ground coil, which would be sized to meet the building's heating needs.
Also, the installation cost can be reduced, since the cooling tower typically has a lower installation cost than the comparable ground loop for the same heat rejection duty.
When sized for the heating needs, the ground loop is typically shorter than required to meet the cooling needs for these reasons:
- In heating, only 70 percent comes from the ground loop. The remaining comes from the power to heat pump.
- In cooling, the ground loop must transfer both the building load and power to the heat pump.
- Cooling loads in buildings with high lighting and internal loads often exceed the heating loads.
- Heating of commercial buildings is often a morning warm-up spike followed by a reduced load. Due to earth mass and loop water volume, earth coils can handle the spike.
The ground loop is sized for the heating requirement, and a cooling tower is added to help meet the summer load. When needed in the summer, both the ground loop and the cooling tower are operated to reject the excess heat.
Since the cooling tower is normally an open water circuit, an intermediate plate heat exchanger is used to prevent contamination of the clean and inhibited loop water with dirt from the tower.
For example, a government building in Louisiana had a cooling load of 110 tons and heating load equivalent to 50 tons. The ground loop was installed to meet the heating load and a 60-ton tower added.
In this case, the installation cost was reduced by $30,000. The payback on the lower installation cost compared to the added operating cost of the tower made this design the best economic selection.
Core areas often require cooling well into the heating season. These heat pumps can also use free cooling with economizer coils. When the outdoor temperature is low enough, cold tower water is used without operating the compressors.
The loop temperature could be reduced to about 50 degrees if the water bypasses the ground coil through a three-way valve. The core zones could be cooled without using the compressors and the perimeter heat pumps could operate in the heating mode. Even with near 50-degree water to the perimeter heat pumps, the heating COP is about 4.0.
Another alternative to the cooling tower in the hybrid system is the closed circuit evaporative cooler. In this design, the heat exchanger is eliminated and the loop water flows directly through the cooler's internal coil. Where water is a problem, use a dry cooler instead.