[EnergyPlus_Support] Heat recovery and economizer

["adamsrackes" <adamsrackes@xxxxxxxxx>]

Hi all,

I'm trying to model a variety of different ECMs for minimizing energy for conditioning OA. In particular, I'm trying to model a single zone system with economizer and heat recovery. The control I would like to achieve is that the exhaust air heat exchanger is bypassed when both of the following conditions are true: the enthalpy of the OA is less than the enthalpy of the RA, and the system is in cooling mode.

There are a few places where this is controlled. In the Controller:OutdoorAir object, the default for the field Heat Recovery Bypass Control Type is BypassWhenWithinEconomizerLimits. My understanding from the I/O Reference is that this means the heat exchanger is bypassed anytime the economizer status is 1, which occurs any time the first condition is met (h_OA < h_RA) regardless of the second condition. This means in effect that the heat exchanger is bypassed the entire winter, when h_OA < h_RA nearly always, negating much of the benefit that heat recovery might have during the heating season. Looking at the hourly .csv output, this indeed seems to be the case.

The other option for the Controller:OutdoorAir object field Heat Recovery Bypass Control Type is BypassWhenOAFlowGreaterThanMinimum. From the I/O Reference, this seems like exactly the ticket, since it requires both the first condition (h_OA < h_RA) and the second one, namely that the SA setpoint manager calls for greater than minimum OA flow, meaning that it is calling for cooling. However, when I use this control I find that OA intake is much lower than the required minimum at many time steps in the winter, often as low as zero. This is obviously not the desired behavior.

I should point out at this point that, in the Controller:OutdoorAir object I have the minimum OA flow set to zero. This is because I'm controlling minimum OA through a Controller:MechanicalVentilation object, which should enforce the min OA  design specification for the zone (which is very much not zero). My interpretation of the I/O Reference is that the Controller:OutdoorAir object takes as the minimum OA the maximum value specified by itself, the Sizing:System object (if Autosize is selected), the Controller:MechanicalVentilation object (if defined), or the OA needed to satisfy the SA setpoint (if economizer is active). This behavior has always worked correctly before, but seems to fail when BypassWhenOAFlowGreaterThanMinimum is selected in the Heat Recovery Bypass Control Type field in the Controller:OutdoorAir object. One possibly hypothesis would be that the "Minimum" referred to in BypassWhenOAFlowGreaterThanMinimum is the minimum explicitly defined in the Controller:OutdoorAir object, not the (maximum) minimum that that object otherwise normally calculates.

One other possible complication is in the HeatExchanger:AirToAir:SensibleAndLatent object, where there is a field Economizer Lockout, which I currently have set to Yes. It seems like maybe this should be set to No, but I'm not sure the order of precedence in terms the heat exchanger calling for lockout or the economizer calling for bypass. It also doesn't seem to explain the ventilation falling below min.

Has anyone accomplished the type of control I'm trying to achieve? It seems relatively simple and like the bare minimum of complexity that one would always want to incorporate into a system with heat recovery and that sometimes calls for heating.

Thanks for any advice.

Adams Rackes
Building Science and Engineering Group
Drexel University





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