We are trying to account for a CHP plant near our project in our energy model. The owners of the plant are very secretive and treat efficiency data as a trade secret, so it appears that we would have to go for the Building stand-alone scenario in the USGBC's DES Guidance (i.e. Option 1).

Looking through the methodology, we would have to model the non-DES parts of the system as per ASHRAE App G. However, according to the guidance, the parts of the building supplied by the DES would have to use a "virtual DES rate", which is derived from the virtual energy rate by multiplying the fuel and electric rates by a weighting factor. I assume that the weighting factor reflects some sort of average CHP efficiency, though the guidance does not explain it any further where those weightings numbers come from.

So far I can make sense out of this, but the last sentence of the guidance for Option 1 states that the same virtual DES-rates should be used as inputs for both the Baseline and Proposed buildings, which is where it gets very confusing. If we use the same rates for both Baseline and Proposed cases, aren't we just modelling the same building as if it didn't have any CHP at all? It appears to be that this approach eliminates any efficiency improvements arising from the use of the CHP plant and we would just be modelling as we would have done without any CHP.

Could anyone help me shed some light on this? Thanks for any help in advance.