The HVAC Factor: Keeping Steady With Geothermal
By Avery L. Monroe, P.E., LEED AP and Paul Harry
From the October 2013 issue of Today’s Facility Manager
Completed in December 2012, the University of North Carolina Coastal Studies Institute (UNC CSI) is a 52,000 square foot research facility located on a 200 acre site on Roanoke Island along the Croatan Sound on the Outer Banks near the Town of Manteo. The facility includes research labs for marine archeology, coastal processes, estuarine ecology, public policy, and engineering. The new building also comes equipped with teaching classrooms and labs as well as marine operations, administration, and research offices.
One of the goals for UNC CSI is to be a model for sustainability in the local and regional communities. As such, the facility was designed and constructed to achieve LEED Gold certification (pending), with a particular focus on low energy use and water conservation. The crux of the project, however, was temperature control. Since multiple functions would be performed in the facility, it was important to maintain the same level of space temperature control as would be achieved using conventional VAV systems. A geothermal heating and cooling system was designed and installed in an effort to achieve the greatest energy savings for the HVAC system.
The building is heated and cooled with a geothermal system in conjunction with a heat pump chiller/heater, pumps, and central VAV air handling units. The chiller/heater is modular, capable of simultaneously producing chilled water and heated water from a single source of condenser water. The heat pump chiller/heater automatically assigns modules for heating and cooling depending on space conditioning requirements.
Key to the success of the project was determining the most appropriate geothermal source for use as a heat sink. Initially, closed loop geothermal wells were considered and would have included over 180 wells, each 300′ deep, supplemented by a closed circuit cooling tower. This option had the greatest potential for energy savings; however wells at this depth would penetrate three aquifers, one of which supplies water to the county’s public water distribution system. The other two aquifers contain brackish water; therefore the geothermal wells may have compromised the water supply system, as the county’s water treatment plant is not capable of treating salt or brackish water.
An additional consideration was that based on the geothermal model, the ground temperature could potentially increase 10°F over the next 20 years. This would have reduced the overall efficiency of the system and required more wells in the future.
The good news was that the water supply aquifer had plenty of water available for use and the local and state authorities were willing to allow the use of an open loop geothermal (groundwater source) system if the wells were constructed to water supply well standards. The groundwater temperature at the site is typically 64°F year round, providing a consistent heat source/sink.
Three different open loop solutions were considered: a one pass system with water supply wells, with return water discharged into the Croatan Sound; injection wells used in conjunction with water supply wells; and using one of the county’s water supply wells as the geothermal source.
Ultimately, the third option was implemented—an open loop system using raw water from an existing county water supply well. Coincidentally, the project site is located within a quarter mile of the municipal water treatment plant and the raw water main passes by the site en route to the treatment plant, thereby making this solution the most feasible and efficient. The raw water is intercepted, passed through a heat exchanger and returned to the distribution system. Condenser water is then circulated through the condensers of the modular heat pump chiller/heater. The heat pump chiller/heater simultaneously produces 42°F chilled water and 120°F heating water.
Future plans for UNC CSI include dorms, an auditorium, and additional boat and research equipment storage.
Monroe, project manager at Baltimore, MD based RMF Engineering, served as the project manager for this CSI project. Harry is a project manager at RMF Engineering, serving as principal in charge and mechanical engineer on this project.