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What is the best way to model water cooled DX units in EnergyPlus

asked 2019-05-04 07:55:20 -0500

RCulham gravatar image

On my most recent energy simulation project, I had to model multiple water cooled DX fan coil units with hydronic heating coils, each serving a unique zone in a production facility. These zones had both high internal lighting and latent loads. OpenStudio and EnergyPlus were used to not only determine the baseline energy consumption, but also to assess the ability of the HVAC system as designed to control zone temperature and humidity.

The manufacturer of these fan coil units termed them to be heat pumps, but this is a misnomer. Each fan coil unit was configured with a DX cooling coil with its own refrigerant compressor along with a hydronic heating coil. The compressor does not provide reverse circuiting as in a true heat pump, hence the DX coil only provides cooling. Heating is accomplished by a hydronic heating coil within the fan coil unit.

These DX fan coil units are similar to distributed heat pumps in that the heat from the refrigerant condensing process is rejected into a common 2 pipe building hydronic loop. In order to effectively reject heat to the hydronic loop, the loop temperature is nominally set at 110°F [43.3°C]. In conditions where most of the DX cooling compressors are operating the loop temperature will continue to rise and when it exceeds a nominal temperature of about 120°F [48.9°C].fluid coolers reject the excess heat to the ambient outside air conditions. In conditions where most of the DX cooling compressors are not operating and there is a call for heating by the space thermostats, the loop temperature will fall below the nominal setpoint of 110°F [43.3°C]. In this case, a heating boiler is used to maintain the building loop set-point temperature.

The DX fan coil units are nominally configured to provide either cooling or heating, but not both simultaneously, hence they are normally set up to operate on space temperature. However, they could also operate the DX coil to provide space humidity control with the hydronic coil providing reheat. The ratio of cooling coil capacity to heating coil capacity is about 1.0 for these units.

The EnergyPlus HVACTemplate:Zone:PTHP could be considered for modelling these units by setting the Heat Pump Heating Coil Availability Schedule to Off and by defining the Supplemental Heating Coil Type to Hydronic. However, the availability of the Supplemental Heating Coil is overridden by the Maximum Outdoor Dry-Bulb which is limited to 21°C [69.8°F]. On this particular project, the primary strategy was for space humidity control. Hence, this limitation is unacceptable for space humidity control when reheat is required.

The Coil:Cooling:DX:SingleSpeed object only allows for the Condenser Type to be either AirCooled or EvaporativelyCooled. Water cooled is not an available option.

As there was no way to model these units directly in OpenStudio 2.5.0 or EnergyPlus 8.9, and given the time constraints available on the project, I had to use the following ... (more)

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Matt Doiron thank you for your answer. These systems may be come more prevalent on future projects.

RCulham gravatar image RCulham  ( 2019-05-16 19:22:33 -0500 )edit

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answered 2019-05-14 23:33:15 -0500

Hi, you say these systems are similar to distributed heat pumps? It may be worth trying to model them that way and test if they behave they way you want? There are objects "Coil:Cooling:WaterToAirHeatPump:*" which EnergyPlus uses when you use HVACTemplate:Zone:WaterToAirHeatPump. You can specify a supplementary heating coil that is hydronic and disable the heating side of the heat pump (two separate coils are always created, one for heating and one for cooling). I have done this in the past for what I have heard refered to as hybrid heat pumps.. These WaterToAirHeatPumps use a common condenser loop with boiler and chiller, similar your described system.

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Asked: 2019-05-04 07:55:20 -0500

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Last updated: May 14 '19