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Re: [EnergyPlus_Support] WindAC output
You should be aware of one other issue I have observed with the current DXACCoil model. The SHR is adjusted for coil performance at model input conditions, but there is apparently no degradation of SHR in relation to PLR.
I performed some AC total capacity change calculations for the same load conditions. I got different zone RH results depending on the fan cycle. I found that for my base model, my 1x system capacity was greatly oversized. A 0.1x system would run full time through most of my test period while maintaining close to set point temperature. I had 50 to 55% RH for a model that goes to 100% when there is no mechanical dehumidification is available. My assumption is that EnergyPlus reasonably models these PLR=1 conditions with the current equations. A 0.5x system and all larger systems (1x, 5x and 10x) had a PLR less than 0.1 for the same time period. With a cycle fan operation I found that all oversize systems degraded to exactly the same zone RH curve which was slightly higher than the constant run 0.1x system but less than 60% RH. When the 0.1x system did run less than PLR=1, it approached the same RH limits as all the other systems. The PLR for the 0.1x system did not go below about 0.7.
The constant fan models did not degrade to the same RH curve as the cycle fan or as other systems in the rest group. The RH curve of larger units typically crisscrossed the RH for the 0.1x system. However, the lowest zone RH curve belonged to the 10x system and it maintained an RH consistently below the 0.1x system. This performance is completely contrary to field observations. It is highly likely that short cycling, constant fan systems provide 0 dehumidification because coil evaporation will return all condensed moisture back to the zone space.
The EnergyPlus results in this study seem to be inconsistent with Henderson's observations (IAQ and Energy 98) that for residential and small commercial DXAC systems, SHR goes from the rated value to 1 as PLR goes from 1 to about 0.5. The existing EnergyPlus model makes some adjustments for SHR based on calculated time step total capacity, but there seems to be a need to have another PLR degradation variable to properly model these conditions. I suppose that it is possible that the current EnergyPlus model uses the PLF for this degradation which is limited to a minimum of 0.7, but the documentation does not describe the specific SHR degradation with PLR that Henderson recounts.
Consequently, any system using a DXACCoil is likely to have significant periods of questionable zone humidity prediction whenever systems operate at part load. This probably impacts comfort calculations and humidity control reheat calculations since these systems seldom reached a humidity control set point.
I suspect that a larger latent load would cause some of the systems to predict higher zone RH conditions because of reduction in total latent capacity due to PLF operation. However, this degradation is not the same as the SHR loss related to PLR that is found in oversized equipment.
Ned Lyon
Senior Staff Engineer
Simpson Gumpertz & Heger Inc.
41 Seyon Street
Building #1, Suite 500
Waltham, MA 02453
Phone: 781-907-9000
Fax: 781-907-9009
>>> alexander.beisteiner@xxxxxxxxxxxxxxxxxx 02/24/03 10:12AM >>>
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