Qualified Software for Tax Credits

Topic: Energy-Efficient Commercial Building Tax Deduction under Internal Revenue Service Code 179D. Reference IRS Notice 2006-52 dated June 2, 2006.

• IRS Press Release
  
  
• IRS Notice 2006-52
  
  
• Qualified Software

Consultants can help facility owners take advantage of the above tax deductions. As of this writing the qualified software list includes the latest versions of EnergyPlus, Trace 700, and Carrier HAP. There is a rumor on the boards that eQUEST is in in the process of becoming certified.
 

eQuest Displacement Ventilation

Topic: What adjustments to the baseline model are required to simulate a displacement ventilation system?

Energy Design Resources has a reference for doing underfloor and displacement ventilation systems with eQuest. While useful since eQuest (version 3.61 as of this writing) does not yet natively support displacement ventilation, the information is evidently based on an earlier version of eQuest prior to implementation of the 'lighting heat to adjacent space and return air' features shown in the following dialogue (which may be found by doing a 'detailed edit' on spaces in the component tree):

(click on the image to see a larger version)

Until eQuest is released with specific displacement ventilation modeling capability, the following workaround may be used to approximate a displacement ventilation simulation solution:

• Assign a higher fraction of the lighting heat load to the return air for displacement ventilation over the baseline case.
  
• If you've accepted the default of 100% lighting heat load to the occupied space for the baseline case, then for example a 50-50 split may be assigned for the displacement ventilation scenario.
  
• If there is significant occupant load per square foot, then assign some additional fraction of the lighting load to the return air to account for it, since there is not an explicit option for doing so.
• Continuing the example above, in school classrooms perhaps a split assigning 70% of the lighting load to the return air is a better approximation for the displacement ventilation scenario.
  
• Also adjust supply air delivery temperatures, supply air volumes, and drybulb economizer high limit per the proposed displacement ventilation design; consult with the mechanical engineer.
  
• Typically the supply air and drybulb high limit temperatures will be somewhat higher and the overall supply air volume somewhat lower than a baseline overhead distribution system.

If the project is fortunate enough to be located in a temperate area that does not require mechanical cooling in the spring and fall and is unoccupied during the summer (e.g. a school in western Washington State), then don't expect much in the way of reduced energy consumption. Displacement ventilation can only provide estimable savings by avoiding mechanical cooling.

Carrier HAP Displacement Ventilation

Not to be outdone, the folks at Carrier HAP software support also have some recommendations for modeling displacement ventilation systems:

1. TRACE has a feature that models the return air plenum as a separate space. This lets you define the size of the plenum and heat going into and out of it. The calculate a load for it as if it was a room and also calculate its temperature. The seem to be recommending you use this feature to model the upper stratified zone of the room as a "plenum space" and assign portions of the room loads like lights, etc.. to the plenum space.

In HAP we don't have an equivalent feature. But you could define a return air plenum and assign part of your lighting, wall and roof loads to the plenum as a way of derating the heat going to the occupied zone in the space. You still need a way to deal with people and plug loads. Perhaps use the schedules to derate these heat gains, or just specify derated sqft/person and W/sqft to begin with. What you'll miss is the portion of these heat gains that goes into the upper zone. And you'll miss the kWh consumption for plug loads if you're doing an energy analysis (could put those back in via miscellaneous energy in the building inputs). Don't know if TRACE lets you put people and office equipment inside a plenum space. Maybe they have a problem with people and plugs too.

2. On the system side it sounds like they recommend modeling a constant volume AHU that uses a bypass so you have part of the air going thru the cooling coil and part going through a bypass and the two streams mixing downstream. Something equivalent in HAP might be this:

a. Choose a 3-Deck Multizone system. In cooling mode it mixes neutral (untreated) with cold deck air in the proper proportions to get the proper SAT to meet the load.

b. Specify the central cooling with 55 F SAT and constant supply temperature control.

c. You'll want to use user-defined sizing on the sizing tab so you can directly specify the supply airflow. This airflow should be sufficient to meet the peak zone sensible load with a 60 to 65 F supply temperature at the supply diffuser. To figure out what this airflow for 60 to 65 F SAT is to begin with, you may need to run the system once with SAT specfied as 60 to 65 F. Then go back and edit the air system, change the SAT to 55 F and switch to user defined sizing to put in the airflow. Then run calculations a second time. The result is the supply temperature at the diffuser will be 60 F or above, and it will be a mixture of 55 F cold deck air plus untreated neutral deck air.

We recommend looking carefully at the results like the component loads on the System Design Load Summary and the system operating details on System Psychrometrics to get a feel for whether everything is working the way you intend it to, and is representing what you think displacement ventilation behavior should be.

Best Regards,
Carrier HAP Support