Rating System

LEED BD+C: New Construction

Rating System Version

v2 - LEED 2.0

Inquiry

This inquiry pertains to Energy and Atmosphere Credit 1. The project is a 3 storied speculative office building in McKinney Texas, 60,000 sf, aiming for a Platinum certification. The mechanical cooling system under consideration is a 100 ton Single Effect Absorption Chiller served by a rooftop array of hot water parabolic collectors with a back up gas boiler. Combined with a hot water storage tank in the building, this system has a solar fraction of 89% for cooling and 93% for heating. The air delivery system will be under floor with a VAV system. There are 2 parts to this inquiry. (1) Since this is an atypical system and not covered by the ASHRAE ECB method for selecting a base HVAC system, we would like to use a rooftop air cooled direct expansion gas fired VAV system (this is the typical system for a 3 storied speculative office building of our size). Is that the correct system for our base system? (2) The energy model has been built in DOE2.1E. This program does not have the capability to model solar assisted absorption chillers. As a result, in DOE2 we model this system using a single effect absorption chiller and backup boiler along with our envelope and lighting measures. We then extract hourly loads on the chiller and boiler, outdoor temperatures and solar insolation (rate of delivery of direct solar radiation per unit of horizontal surface)out of DOE2. Then we post process these numbers in an hour-by-hour spreadsheet model that uses the insolation and outdoor temperatures to calculate the solar panel efficiency based on the manufacturer\'\'s efficiency equations. Thus an hour-by-hour output from the solar panel is added to a hot water storage tank that is depleted by the hour-by-hour heating and cooling loads from the building. We use this thermal balance calculation at the storage tank to size the tank and the solar array. The annual summation of the thermal balance calculation gives us the solar fraction as well as annual reduction in utilities. We would like to submit the DOE2 results (from the BEPS report) as result of the envelope, lighting, daylighting, and mechanical system (absorption chiller) compared to the energy cost budget model (the air cooled DX system), as shown in Table 7 (page 132) of the LEED Reference Guide (June 2001). We will then add energy savings as a result of the solar panels (arrived at from our hour-by-hour spreadsheet model) similar to the way renewable energy savings are added in that table. The building also has a significant photovoltaic array and the energy from this array will be added as another line item. Does this method of calculation and documentation seem plausible?