Refrigeration Air Cooled Condenser Curves - EnergyPlus

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I dug a bit into the source code, and found some things that might help out here.

First, this statement:

the condensing temperature is determined by a variable called Hrej divided by HrejRated rather than a constant dT when the outside air temp is warm.

isn't quite correct. the condensing temperature is calculated by

    } else { // air-cooled condenser
        // MinCondLoad and TempSlope came from condenser capacity curve, using curve backwards
        TCondCalc = OutDbTemp + (state.dataRefrigCase->TotalCondenserHeat - condenser.MinCondLoad) * condenser.TempSlope;
        SinkTemp = OutDbTemp;

i.e. OAT plus total condenser heat from compressor rack minus Minimum condenser load (determined from the condenser curve), multiplied by the inverse slope of the condenser curve (i.e. (x_max - x_min) / (Cap_max - Cap_min)).

Next, this statement:

When the outdoor air temperature rises above that minimum condensing temperature, the program forces the fans to run at 100% all the time.

is also not quite right. The fan operation is determined by comparing the calculated condenser temp (from above) with the input Minimum Condensing Temperature from the Refrigeration:System:

    // Fan energy calculations apply to both air- and evap-cooled condensers
    // Compare calculated condensing temps to minimum allowed to determine fan power/operating mode
    if (TCondCalc >= this->TCondenseMin) {
        this->TCondense = TCondCalc;
        ActualFanPower = RatedFanPower;
        AirVolRatio = 1.0;

    } else { // need to reduce fan speed to reduce air flow and keep Tcond at or above Tcond min
        this->TCondense = this->TCondenseMin;
        TCondCalc = this->TCondenseMin;
        // recalculate CapFac at current delta T
        if (condenser.CondenserType == DataHeatBalance::RefrigCondenserType::Air) {
            // current maximum condenser capacity at delta T present for minimum condensing temperature [W]
            Real64 CurMaxCapacity = CurveManager::CurveValue(state, condenser.CapCurvePtr, (this->TCondenseMin - OutDbTemp));
            CapFac = state.dataRefrigCase->TotalCondenserHeat / CurMaxCapacity;
            AirVolRatio = max(FanMinAirFlowRatio, std::pow(CapFac, CondAirVolExponentDry)); // Fans limited by minimum air flow ratio
            AirVolRatio = min(AirVolRatio, 1.0);

Essentially, if the calculated condensing temp is >= the minimum, fans are on at 100%, else they're reduced to some fraction to maintain the minimum condensing temp.

So basically I don't think the condenser model can be controlled to a specific delta-T, because the condensing temperature is calculated based on the compressor load, and because the condenser curve is what it is. However, I think you could still resolve the high fan power issue, because 1. The Minimum Condensing Temperature input value is available as an EMS Actuator, and 2. (at least for air-cooled condensers) it only really seems to be used to determine the fan operation. So you could write an EMS program to calculate the condensing temp and reset the minimum condensing temp to be above that, up to maybe the design DT of the condenser at design OA. Or something to that effect.

I also thought about trying to actuate the output of the Rated Effective Total Heat Rejection Rate Curve, but I don't thing that's going to work. The curve is evaluated to determine the min/max capacities to invert the slope (as above) to calculate the condensing temperature. Since I think an EMS curve output actuator would override these min/max checks, it would mess with that whole process.

Hope this ... (more)