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,
[cid:image41d3e6.PNG at 389a20b0.47b7ac81]
Eric L. O'Neill, P.E., LEED AP
Managing Engineer - Implementation | Michaels Energy
Phone 608.785.3328 | Cell 608.792.7721
www.michaelsenergy.com
MichaelsNRG
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
--
M. Dennis Knight, P.E.
Founder & CEO
*W*hole *B*uilding *S*ystems, LLC
P.O. Box 1845
Mt. Pleasant, SC 29465
Phone: 843-437-3647
Email: dknight at wholebuildingsystems.com
Website: www.wholebuildingsystems.com
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
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653
With Excel you can enable circular references to enable the calculations to converge. If you have 8760 rows then you can also run a simple macro to reseed a column with a calculated result so you in effect get convergence.
One challenge with these types of systems is what is optimal under one set of ambient and internal conditions may not be as optimal in another set of conditions. So coming up with one set of controls will not optimise for all conditions. None of the software that I have used over the years would allow you to properly have different controls for different ambient conditions. So testing one set of controls and looking at annual results you would lose what is really happening under different conditions unless you look at hourly results but then your ability to adjust controls for different conditions is limited unless you go to something more open and flexible.
At the end of the day being able to model different controls would be useful to understand what the sensitivity and thus opportunity or risk is. If the differences are small then there is a strong argument to keep it simple and find that balance of simplicity and efficiency so that long term efficiency can be sustained rather than having something no one knows how to fix or operate ...
Regards,
Graham
Part of the attraction of the DOAS system with the fan-coil units is to reduce the amount of reheat needed in the building. You would have to be very efficient to provide simultaneous heating and cooling in a way that uses less energy than eliminating the simultaneous heating and cooling.
Generally you will be better off when you can reduce a load, rather than trying to provide that load through use of extra equipment. Understanding that the ability to program the sequence may not be infinite either, I think you are correct with your first thought to use the energy recovery equipment at little as possible while eliminating the need for reheat.
I?m not sure if I understand your second item about the chiller, but I think the same thought applies. Do you mean that the energy recovery system would false load the chiller to make it operate more efficiently, but at a higher load? The change in efficiency would have to be pretty dramatic to offset the additional MMBtu. Maybe there is some combination where it works out.
The other thought I had was if the control system has enough flexibility in the sequence of operations to account for some of the less frequent conditions. Particularly if the building operators are hands-on, it could be better to have a generically efficient scheme which might miss on a few infrequent conditions but over the course of the year provides a reliable energy savings.
Have you looked at TRNSYS as an option to simulate the components? Many of the flexibilities of EnergyPlus, but without the need to model the whole building. Unless you were going to model the building anyway.
David
David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP
Grumman/Butkus Associates