Tri-gen

Hi, I am QA'ing a model for a building that will be fed via a cogeneration plant (with absorption chillers) and therefore is a little complicated. The cogen plant is owned by the utility (which is technically owned by the municipality which owns the building being built). The building will purchase the electricity from the utility at a reduced rate and get the waste heat for free. It has been modeled as: - The proposed purchases the electricity generated by cogen at the reduced rate with the remainder being purchased at standard rate (15% more) - The baseline is modeled with standard systems and uses the grid supplied gas rate and standard rate for all electricity - The proposed pays for gas at the grid supplied rate for the purpose of the model Does this approach sound valid? Key Q's I think are: 1) is it ok to use different fuel costs for the electricity generated vs. consumed (and therefore proposed and baseline blended rates will be different) if this is as per the contract 2) Should the proposed pay for gas (even if free in reality) and if so should it be a direct purchase amount for the load or should it take account of the CHP efficiency (E.g. 40% thermal) Thanks Victoria