Project Description Our request is for guidance on the energy modeling protocol for a building that has both a connection to a district thermal energy system and a stand-alone plant within the building for LEED CS EA prerequisite 2 Minimum Energy Performance and EA credit 1 Optimize Energy Performance. The project was registered for LEED CS after May 28, 2008 and is subject to the USGBC paper titled "Required Treatment of District Thermal Energy in LEED-NC version 2.2 and LEED for Schools version 1.0, May 28, 2008". The project is a high-rise speculative office building. The total area is just over 550,000 ft2 and includes office space, lobby, cafeteria and retail at the ground level. A combination ground source heating and cooling system and an ice storage chiller and closed-cell cooling tower primarily comprise the building heating and cooling systems. The district connection includes chilled water and heating hot water service from the centralized chiller and boiler plant. The engineer has sized the system to operate primarily using the plant within the building (downstream equipment), but it would need to draw from the district system for a fraction of the year. The intent is that the use of the district would be to provide redundancy and peak shaving. Our first question is whether the ruling for the Required Treatment of District Thermal Energy applies to this project. The mechanical engineer estimates 15-25% of the annual heating and cooling energy will be provided by the district system with the remainder provided by the on-site ice storage chiller and ground source heating and cooling. Does the percentage of heating and cooling provided by the central plant determine the applicability of the ruling? Our second question is how to follow the procedures outlined in the ruling, if it applies. We see two options for modeling the baseline building in Step 1. These options are described below. Step 1 - Stand Alone. Proposed Building: Model the ground source heating and cooling systems and ice storage as per the installed capacity. Model the connection to the district system using chilled water and steam meters. Sequence the energy model to draw on the district heating and cooling system as needed to satisfy the load. Using the actual purchased energy rates for electricity and natural gas, we will calculate the cost of generating chilled water and steam. These rates will then be applied to the chilled water and steam meters in the Proposed Building energy model. Baseline Building option 1: Model the baseline systems with an on-site code-compliant chiller and boiler, as per ASHRAE Standard 90.1-2007 Appendix G Table(s) G3.1.1A and G3.1.1B in lieu of ice storage chiller and ground source heating and cooling system. Model the connection to the district system using chilled water and steam meters. Sequence the energy model to draw on the district heating and cooling system as needed to satisfy the load. Using the actual purchased energy rates for electricity and natural gas, we will calculate the cost of generating chilled water and steam. These rates will then be applied to the chilled water and steam meters in the Proposed Building energy model. Baseline Building option 2: Model the baseline building with a connection to the district chilled water and steam with meters. Using the actual purchased energy rates for electricity and natural gas, we will calculate the cost of generating chilled water and steam. These rates will then be applied to the chilled water and steam meters in the Proposed Building energy model. Step 2 - Connection to District System. Proposed Building: Model the ground source heating and cooling systems and ice storage as per the installed capacity. Model the central plant using a virtual plant to provide the hot water with a 70% Higher Heating Value efficient boiler and a cooling plant with a COP of 4.4 (as per page 8 of the aforementioned USGBC publication). Baseline Building: Model the building with one ASHRAE compliant chiller/boiler plant as ASHRAE Standard 90.1-2007 Appendix G Table(s) G3.1.1A and G3.1.1B.
This project falls under the purview of District Energy System (DES) and is required to follow the guidance stated in this document found on the USGBC website at http://www.usgbc.org/ShowFile.aspx?DocumentID=4176 It must be noted that the percentage DES contribution is not relevant - it still must be modeled in conjunction with the on-site plant as per the DES guidance. With regard to the DES plant energy costs that include overhead, capital costs, etc., it will not be allowed to model a portion of the system using DES rates and then offset with internal utility costs. The proposed case should be modeled as designed. The baseline case should be modeled as outlined in the CIR for the connection to the District System: "Proposed Building: Model the ground source heating and cooling systems and ice storage as per the installed capacity. Model the central plant using a virtual plant to provide the hot water with a 70% Higher Heating Value efficient boiler and a cooling plant with a COP of 4.4 (as per page 8 of the aforementioned USGBC publication). (Note: Using documented efficiencies of the DES plant in lieu of these default values is also acceptable.) Baseline Building: Model the building with one ASHRAE compliant chiller/boiler plant as ASHRAE Standard 90.1-2007 Appendix G Table(s) G3.1.1A and G3.1.1B." Applicable internationally.