Hello modelling gurus,
I am modelling a restaurant space located in Canada serve by PSZ rooftop
units. I notice something weird when I increase the OA intake for the
space. Space heating energy use goes up as expected, but cooling energy
use went down a tad bit. This doesn't make sense as I'm bringing in
more hot/humid OA to the space. I've already setup dual enthalpy
economizers for the PSZ units, so I don't think it's a problem with free
cooling.
Anyone can point out what's going on?
Regards,
VICTOR YEUNG, BASc, LEED(r) AP BD+C
Hi Brian,
Thanks for the quick reply. I understand where you're coming from, but
100% free cooling was always available before I increased the OA intake.
The free cooling credit for both scenarios should be the same. If the
building had no economizers, then yes, bringing in more OA will help
with the cooling. There are definitely humid days where free cooling is
not available, and bringing in more OA on those days should increase the
overall cooling energy use.
VICTOR YEUNG, BASc, LEED(r) AP BD+C
Victor, I agree with Brian that it is probably a free cooling issue. The dual enthalpy economizer control will not always pick the lowest-energy airstream. The theoretically optimum economizer highlimit control strategy is close to a mix of dual enthalpy and dual drybulb. Dual enthalpy does better when there is latent cooling and dual drybulb does better when there is only sensible cooling. I suspect that the dual enthalpy control is missing some good free cooling opportunities. So, by increasing the minimum ventilation, there are probably some peak summer condition hours where this causes an increase in the cooling load but it is still better from an annual energy standpoint because you are increasing the amount of free cooling for many more hours of the year. You should be able to test this by comparing the hourly reports from the two sets of runs. For more background info, see the article in the November 2010 ASHRAE Journal which provides a detailed review of economizer high limit control strategies. Comparing the different strategies graphically on psych charts helps explain it so much better than words can alone. Hwakong Date: Thu, 3 Nov 2011 13:24:23 -0400