[Bldg-sim] Optimizing a DOAS

[Jim Dirkes]

Dear Eric,
I know enough to be dangerous with EnergyPlus and can say this much:

*         Your questions are good ones about DOAS operation.  They can be very complex and all of the details you mention have an impact.

*         Don't forget the impact of all that pressure drop for various coil and heat exchanger components.  I modeled something in E+ with similar complexity (indirect evap cooling with economizer, heating and DX coil) not long ago and found that using a constant speed fan for variable airflow loads negated most of the savings for some climates.  VSD fans proved to make a big difference.

*         Energy Plus can probably model anything you wish, especially when you include its EMS capability.  That does not mean it's easy!  DesignBuilder and Simergy are making the learning curve become easier, but it is still no small task.

*         Finally, I know that Semco has a bin-based spreadsheet which is used for modeling performance of their equipment with both single and double energy recovery wheels and almost any heating / cooling configuration you might like.  Consider checking in with your local rep.  I have used that in the past and it's pretty thorough for constant airflow applications.

James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com<http://www.buildingperformanceteam.com/>
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Dennis Knight
Sent: Friday, March 22, 2013 8:33 AM
To: Eric O'Neill
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Optimizing a DOAS

Eric,
I have been looking at similar situations in our warm and humid climate for about three years trying to optimize both DX and water cooled heat pump DOAS with various combinations of heat recovery (sensible and total) and have not found anything that can do a very good job of analyzing these units under all oa conditions and varying return/exhaust air conditions. I've played around with a spreadsheet solution as well using TMY3 weather data for all 8760 hours and columns for heat transfer/temperature/humidity across each device node in the process at each oa condition with fixed return/exhaust conditions.
I will be very interested to hear/see if we get many responses to this. These units are a big portion of the energy consumption in k-12 schools and institutional occupancies in the warm and humid southeast. Thanks for kicking this discussion off.
Dennis

On Thursday, March 21, 2013, Eric O'Neill wrote:
All,

I'm working on a retrocomissioning project trying to optimize a DOAS system as best as I can. This particular system has (in this order) an enthalpy wheel, preheating steam coil, cooling coil, sensible wheel, and heating coil. Downstream there are 4-pipe fan coil units. Hot water is provided by a steam to hot water converter and a chiller with a closed circuit tower that can be switched over to free cooling when ambient conditions allow.

I've made myself an excel model, which has worked to some extent, but when I want to ask some of the more challenging questions it's going to require a not-so-modest commitment to developing it (and circular references are driving me nuts). I'm fairly confident I can't do what I want to do with DOE2 (with which I'm most familiar). Is E+ capable of modeling to this level of detail? Some of the types of questions I'd like answers to:


*         When it's extremely hot outside and the enthalpy wheel and cooling coil are running to dehumidify, is it more efficient to run the sensible wheel (near) full speed (thus lowering the exhaust temperature, allowing some additional cooling from the enthalpy wheel, and lowering the cooling coil load) or to do more modest reheating to balance the downstream cooling and heating demands? My gut says it's going to be best to run the sensible wheel only as fast as is necessary to keep all the downstream heating coils closed, but I'd like some numbers.

*         When temperatures are mild, but not sufficient for full water-side free cooling, what's the right balance between the enthalpy wheel and terminal cooling and heating? Is the chiller operating at a part load that's more cost effective than the heating, and how does the enthalpy wheel strike the right balance? Is there a sequence that is optimum, or would it be something that needs to be continually optimized by the controls system?

Obviously I'd need to go up the E+ learning curve, but I think that's something I'll need to do eventually (unless the fabled DOE2.3 engine comes out and can do this type of thing), so if it's between that and continuing to develop a complicated excel model, it may be worth it for me. I just don't want to go down that path only to find out I can't get some of the answers I'm looking for...

Any thoughts (or references) are appreciated.

Cheers,





 Eric L. O'Neill, P.E., LEED AP

 Managing Engineer - Implementation |  Michaels Energy

 Phone 608.785.3328 | Cell 608.792.7721

 www.michaelsenergy.com<http://www.michaelsenergy.com>     <http://www.facebook.com/michaelsenergy>
MichaelsNRG




--
M. Dennis Knight, P.E.
Founder & CEO
Whole Building Systems, LLC
P.O. Box 1845
Mt. Pleasant, SC 29465
Phone: 843-437-3647
Email: dknight at wholebuildingsystems.com
Website: www.wholebuildingsystems.com


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/bldg-sim-onebuilding.org/attachments/20130322/2d0dd344/attachment-0002.htm>