Hi all,
I know they always say there's no such thing as a stupid question, but I've been looking through ASHRAE 90.1 table 6.8.1 as well as I know how, and I cannot find the required efficiency rating for a water source heat pump with a capacity greater than 135,000 BTU/hr. We're trying to model Multistack heat pump/chillers, and we're using the 70-ton model, which correlates to about 840,000 BTU/hr.
Has anyone ever modeled a water source heat pump larger than 135 MBH, and if so, which efficiency did you use? I know I'm absolutely horrible at where's waldo, so there's a good chance I've just overlooked it. Any help or advice is as always appreciated. Thanks.
Daniel I. Deese
No, you are right, there is nothing listed in the tables for water-source heat pump greater than 135,000 Btu/h. I think that's when you go to the water-chilling packages table and start looking at the minimum efficiencies for water-cooled chillers. That wouldn't cover the heating mode, though. Maybe someone else knows for sure.
Kelsey Van Tassel
Daniel,
Water-source heat pumps of size greater than 135 kBtu/hr are not regulated by 90.1.
Meaning there is no minimum efficiency required for such equipment.
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Demba NDIAYE
Daniel,
To answer your question directly, there is no baseline efficiency for that type of package (Just like Kelsey said). If you're doing this for energy modeling purposes (comparing a baseline case vs. a proposed case), none of the ASHRAE baseline cases (Systems 1 through 8 from Appx G) require water source heat pumps, and that's why you don't find any required efficiencies. Based on the type of system you're describing (70 ton multistack), I'd think your baseline system would be a System 5 or 7. We have some curves in Trace built up for larger capacity heat pump chiller, so if you want the efficiencies we have for those, let me know and I'll tell you.
Kelsey,
The problem with using the minimum efficiencies for water chilling packages is that the heat pumps are not nearly as efficient in cooling only as a typical centrifugal chiller...where they really excel is the combined mode when they produce chilled and hot water simultaneously...usually the COP you find manufacturers advertising is the sum of the cooling COP plus the heating COP.
Hope this helps,
Jeff Seger EIT
Hi Jeff,
Yes, you are right that heat pumps in cooling are not as efficient in cooling only as a chiller. The best system type/minimum efficiency combination to use depends on the purpose for the modeling. I do a lot of modeling where there is not as defined of guidelines for the baseline system as Appendix G, so my baseline tends to be what the building owner might choose to do instead. So a proposed Multistack heat pump system would get a bit of a penalty in cooling only mode, but would save energy in other modes against a chiller/boiler system.
Kelsey Van Tassel