Utah Engineers Journal 2021 Issue

39 Continued on the following page For this article, the “Large Office” and “Medium Office” prototypes are chosen, and the different HVAC system strategies listed below are evaluated using EnergyPlus. Scenario Large Office Medium Office Base Case - Water-source heat pumps are tied to a fluid cooler for the data center - Uses VAV air handlers and VAV-reheat boxes for offices - Has a gas-fired boiler - The DX air conditioning unit is packaged with the gas furnace - Uses VAV reheat boxes with electric reheating coil Central Plant - Has water-source heat pumps tied to a fluid cooler for the data center - Uses VAV air handlers and VAV-reheat boxes for offices - Uses two water-cooled centrifugal chillers for building - Has a gas-fired boiler Central Plant - Uses water-to-air heat pumps for all zone - Has a central condenser loop with a gas-fired boiler and cooling towers Electrification - Has an air source VRF condenser - Has fan coil units Has an air source VRF condenser Hybrid - Has water-to-air heat pumps for all zones - Uses a central condenser loop with a gas-fired boiler and cooling tower - Has an air-source VRF condenser The template files can be simulated without modification to evaluate the base case. The large office and medium office HVAC systems are changed to air-source VRF to evaluate the electrification scenario. Lastly, the large-office template is modified for the central plant scenario so that the ratio of office space and data center square footage is equivalent to the total ratio of the eight buildings. This modification is done by adjusting the number of occurrences for each zone or adding floors to the buildings. Lastly, the HVAC system type is changed to water-to-air heat pumps with a central condensing loop. Figure 1. Large Office Energy Use Intensity and Energy Costs Building simulation results (see Figure 1) show that for the large office, the traditional HVAC system, or base case, has a higher energy use intensity (EUI) compared to either of the alternative system types. It is interesting to note that for the large office, an Air-Source VRF system has a lower EUI, but the annual operating cost is higher. The higher operating cost is due to wintertime electricity charges for running the VRF system in heating mode and the reduced efficiency with using Air-Source VRF to cool the data center compared to heat pumps tied to a fluid cooler. The lowest EUI and operating cost system for the Large Office is the heat pump system. A water-to-air heat pump system is well suited for large offices in the Salt Lake City climate because the data center’s waste heat can be recovered and used to heat office spaces. Furthermore, when the heat from the data center is insufficient to heat the entire building, the central boiler can operate in the condensing mode since the return water temperature is much lower for a heat pump condensing loop than a standard heating water system. Finally, the heat pump system has a lower EUI than the air-source VRF because the cooling tower rejects heat at the wet-bulb temperature, whereas the air-source VRF system rejects heat at the dry-bulb temperature. In the Salt Lake climate, the wet-bulb temperature is significantly lower for the majority of the year.