The project is a large mixed use complex which will include several buildings: 1. Building A, which consists of a retail base of 101,000 square feet (SF). It is connected to the other buildings only thru a utility corridor below grade. 2. Building B, which consists of a retail base of 88,000 square feet (SF) under a 175,000 SF high rise residential tower. It connects to the loading dock thru a small area on the lower level. 3. Building C, which consists of a retail base of 125,000 square feet (SF) and a 80,000 SF theatre/entertainment complex. It connects to the loading dock thru a small area on the lower level. 4. Building P, which consists of a 403,000 SF, 7 level, aboveground parking facility and a 30,000 SF below ground loading dock with ancillary support spaces. Building P shares a common wall with building B and another with building C. The design team has determined that it is feasible to certify buildings A, B and P. Because building P is adjunct to building B it is proposed to include this building with building B since there is no provision to certify an un-heated parking structure independently. This was accepted in a previous correspondence with USGBC. Thus there will be 2 separate certifications, one for building A and one for building B and P. The proposed HVAC system for the majority of the complex is a closed loop heat pump system. Each space will be provided with condenser water and will provide its own heat pumps. Because heat pumps are not effective at treating outside air it is expected that they will be used primarily to offset skin losses, while electric resistance heat is used to temper ventilation air. A central plant will be located in building C, with cooling tower, pumps, heat exchangers and auxiliary heat source to circulate water for use by heat pumps in building A, B, C and the lower level of P. Building D is not planned to be served by the central system due to the nature of the occupancy. The question to be verified is as follows: The central condenser water distribution does not constitute a district energy system, since it provides only a heat sink for the heat pump units which generate the heating and cooling for the spaces. It does have one similarity to a district energy system in the sense that there is energy consumed at the central system which is serving multiple buildings. However, unlike a district energy system, the energy consumed at the central system is only a small portion of the energy used for heating and cooling. Based on the above, it is requested to use the approach of modeling downstream energy equipment in each building based on using condenser water from the central system and modeling the portion of energy consumed by the central system based on a virtual central system sized only for the building modeled. The central condenser water system for the proposed design would be modeled based on electric resistance as the auxiliary heat source. The baseline buildings would be modeled for system 6 for the commercial areas (less than 150,000 SF, all electric) and system 2 for the residential areas. The owner would consider using gas boilers for the auxiliary heat source at the central plant, reducing total energy cost for the project, but would likely not do this if it was interpreted that this requires modeling the baseline systems as systems 5 and 1 respectively. We request that this decision of improving the central system energy cost not impact the energy modeling of the individual buildings, since the decision of how to construct the central system is separate from the design of the individual buildings which are being certified. Again, the requested modeling approach is for the proposed design would use electric resistance heating for the auxiliary heat source in the virtual central system to provide a fair comparison against systems 6 and 2 in the baseline model. Please confirm acceptability of the above approach.
The project team is requesting guidance on a proposed modeling approach, as well as baseline system type selection. As stated, the project should treat the central system much in the same way as a District Energy System, in which a virtual central system is modeled for each building onsite. For more guidance on modeling procedure, please see http://www.usgbc.org/ShowFile.aspx?DocumentID=4176. This document also gives guidance on appropriate baseline systems. Regarding the question of baseline system selection, an earlier NCv2.2 EAc1 CIR dated 9/27/2006 allows for a Baseline system to be specified under the "Electric and Other" category though equipment uses natural gas under some circumstances, as long as the modeling does not "attempt to take credit for natural gas heating versus electric heating." If electric resistance auxiliary heating is used in the proposed virtual system, systems 2 and 6 can be used for comparison. Applicable Internationally.
Related Addenda (Corrections & Interpretations)