Dear all, I have a question regarding an HVAC baseline model for systems 7 and 8. We have a project located in a very arid zone, where water is very limited. The project is contemplating a water chilled system with an air cooled chiller. We are doing an energy model, and according to ASHRAE 90.1 2007, in Appendix G, we must model the baseline as a water cooled chiller. Air cooled and water cooled chillers have very different efficiencies, and so far the results have shown that in no way will we be able to come close to demonstrating 10% in savings. (the whole building is 100% glass, we’ve tried having a reasonably priced but very efficient glass, lowered the LPD, used controls.. etc.) My question is, what can we do? Could we model the baseline chiller as an air cooled one? Could we use an alternative compliance path? Asking the client to install cooling towers, given the limited water situation seems a bit unreasonable. Have you ever had this kind of situation in a project? How do air cooled chillers demonstrate compliance? Thank you!

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Jorge Torres-Coto

Building Systems Commissioning Engineer

Empirical Engineering, LLC

LEEDuser Basic Member

14 thumbs up

Wed, 02/26/2014 - 02:27

I have done various e-models in Mexico in different climate zones. And yes air-cooled chillers are used extensively. They can also tend to be more economical to operate. I have found that utilizing an air-cooled chiller with primary-variable pumping system are more efficient overall than the ASHRAE baseline with a chiller, primary-constant/secondary-variable pumping, and cooling towers w/CDW pumps. The saving is generated by the reduced pumping, which offsets the lower efficiency of the compressors compared to water-cooled. You can also equate the additional reduced pumping, since you do not have to pump water from the basement to the roof for the cooling tower (MAKE-UP WATER), this is not a small amount of water and/or pumping energy. By the way, the owner ends up saving money on the initial installed equipment also, and reduced water usage fees. THERE IS NO OTHER WAY TO CALCULATE THIS, YOU ONLY HAVE APPENDIX G AS YOUR OPTION.

Jean Marais

b.i.g. Bechtold DesignBuilder Expert

LEEDuser Basic Member

581 thumbs up

Wed, 02/26/2014 - 06:57

1) I get downright annoyed when I hear of architects building glass houses. It's a bad idea in almost all climates. Both user comfort and energy savings suffer...for many reasons. Thank goodness there are systems like LEED now that will force them to reconsider. 2) the above point on chiller performance is well made. Because chillers are often oversized and running everywhere except at full load, the comparitive energy usage lay in the mostly unintelligable partload performance matrix consisting of a marad of parameters. The two chiller systems may be much closer than you think. In your situation I would say that the main problem on achieving 10% savings does not lay with the chiller type, it's the glass. And now you're left trying to "pollish the turd" as they say in Manchester.

Santiago Rodriguez

Revitaliza Consultores

LEEDuser Basic Member

47 thumbs up

Wed, 02/26/2014 - 15:30

We do achieve high energy savings in pump energy but this is corresponds to a very low percentage of the total energy consumption of the building, where as the cooling energy represents around 40%. So in this particular case there is no way to achieve a better overall performance when using an air-cooled chiller. The main problem as Jean says is the % of glass, but even with the same % of glass as the baseline design it is difficult to comply with the pre-requisite.

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