CBECC-Com: Control issue on variable speed condenser water loop

joesinger12 commented 5 years ago

Condenser loops can be configured with variable flow pumps and cooling towers. Such configurations take advantage of variable frequency drive (VFD) condenser loop pumps to significantly reduce condenser water pumping energy. For a condenser loops serving WSHPs, when a zone needs neither cooling nor heating the WSHP compressor turns off. When the compressor is off, a motorized valve shuts off water flow to that heat pump, and the condenser loop pumps reduce their flowrate in response.

Condenser loops specified this way in CBECC-Com, when simulated in EnergyPlus, failed to properly control the condenser water outlet temperature. The condenser water loop temperature was specified as dual fixed temperature, with the two temperatures set to 60°F and 85°F for heating and cooling, respectively. The CBECC-Com inputs are currently translated to place constant temperature setpoint managers on the individual pieces of plant equipment. In other words, the boilers get a setpoint manager set to 60°F, and the cooling towers get a setpoint manager set to 85°F. In addition, the condenser water loop has a single setpoint manager set to the cooling temperature, 85°F. When this configuration of controls is used with constant speed pumps, the controls work as expected. When used with variable speed pumps, however, the control does not work correctly.

In cooling, the condenser water loop temperature, or more specifically the cooling tower leaving water temperature, was not controlled to the 85°F setpoint. Figure 1 shows an example of improperly controlled condenser loop temperature, where the condenser water temperature was approximately 100°F for most of the day. This increase in condenser water temperature will result in a reduction in WSHP cooling efficiency.

A workaround for this control issue on condenser water loop is to replace the single temperature setpoint manager on the condenser water loop exit node with a dual setpoint setpoint manager, specifically the EnergyPlus object SetpointManager:Scheduled:DualSetpoint.

With this change, the supply water temperature is correctly controlled to be no higher than 85°F, no lower than 60°F, and allowed to float in between. The leaving water temperature with the corrected control is also shown in Figure 1.

Table 1 compares energy consumption results between WSHP systems with and without the dual setpoint manager on the loop exit node. The site energy consumption for heat rejection is reduced substantially (by nearly 76%) for the case where SetpointManager:Scheduled:DualSetpoint is placed on variable speed condenser water loop. The case with SetpointManager:Scheduled:DualSetpoint was able to maintain the condenser water loop below the setpoint temperature. This resulted in the cooling energy consumption reduction: the cooling energy consumption for the case with SetpointManager:Scheduled:DualSetpoint was reduced by 14%. The natural gas consumption for heating showed 6% difference between two cases. It was observed that the condenser water loop exit temperature in heating mode was different in two cases while the temperature doesn’t violate the setpoint temperature schedule. The total electricity consumption was decreased by 6% and the total natural gas consumption was increased by 3%.

Reported by: joesinger12

Original Ticket: cbecc-com/tickets/2949

joesinger12 commented 5 years ago

assigned_to: Nikhil Kapur --> Benjamin Park