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CI-2009 EAc1.3:Optimize Energy Performance—HVAC

Savings calculations

Our client is occupying the 8th floor of a G+8 building. The central plant is a combination of Air cooled and water cooled chillers. The conditioned area of the project area alone is 45,000 sqft and segment area is 322,000 sqft. I simulated using the project area-segment area method. I used the existing conditions for segment area in both base case and proposed case. I modeled the project area as per ASHRAE 90.1 2007. Now the problem is with the savings calculation. As per LEED, the formula is (Base case-Proposed Case)/(Base case*(project area/segment area)). Since the project is occupying only 1 floor, the project area/ segment area ratio comes up to 0.139 and because of this the savings goes higher than 100%. In other words, we will be multiplying the savings by 7 times if we calculate the savings by the usual formula, (Base case- Proposed case)/base case. Kindly guide how to proceed. Thank you

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Paul Conrad
Energy Engineer
CLEAResult Consulting
LEEDuser Basic Member
222 thumbs up

Wed, 02/09/2011 - 14:09

Sundararaj, This is a very interesting case, but can I ask where you got that equation? It doesn't seem like it should work, because in CI, your project area would always be much less than your segment area. You may want to check that the formula is applied properly. Also, if you link to where you found it, we can go through it together. Paul

Wed, 02/09/2011 - 19:05

Sundararaj, Your formula is correct. Because your building is served by a central plant, the entire building must be modeled. Did you model the floors outside of your project area as existing for both proposed and baseline cases? This would mean that the central plant, lighting, systems, etc. is modeled identically in both cases for all floors excluding the project area. Because you are modeling the entire building, your segment area would be the total building floor area. Then, with your formula only accounting for cooling, heating, and fans/pumps you shouldn't see something in excess of 100%. I have reviewed some energy models that were incorrectly applying ASHRAE baseline systems to areas outside of the project area.

Thu, 02/10/2011 - 06:48

Thank you for the reply. The formula is in page 172 EA cr 1.3 of LEED Reference Guide for Green Interior Design and Construction, 2009 Edition. As per the reference guide, i have modeled the entire portion of the building sharing the central plant. Project area is the Eighth floor of 44,734 sqft conditioned area. The segment area is from 1st floor to 7th floor of 322,373 conditioned area. I have considered identical parameters (i.e. existing conditions) for segment area in both base case and proposed case. The savings calculations are done excluding the lighting and equipment load and considering only the HVAC loads. Also, the Base case as per ASHRAE 90.1 2007 is Packaged variable air volume system for project area. Project area is fully modeled as per ASHRAE 90.1 2007. The HVAC Energy consumption in base case is 2,353,462 KWH and proposed case is 1,787,825 KWH. Project conditioned area/segment conditioned area = 0.138. I have considered the segment area from 1st floor to 7th floor and have not included the 8th floor which is the project area. The savings achieved with the above results would be 178% which is not logical. Kindly help.

Sat, 03/05/2011 - 04:57

We are working on a couple of projects of this kind and again the savings achieved is high. But not going beyond 100%. However, i want to know whether the methodology for savings calculation is correct. Also, requesting you to give a solution for the previous post (regarding 178% savings).

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