Our project incorporates microturbines and high efficiency condensing boilers. Modeling the high efficiency boilers is straightforward, but our system and controls are setup to use the microturbine waste heat first (from storage tanks) and then use the high efficiency boilers. Since this can\'t be modeled easily, we plan to perform an Exceptional Calculation Method calculation. The microturbines produce power and waste heat. If we assume that all electricity produced is the same rate as from the utility and the waste heat recovered is used as the first stage of heating at an effective cost less than the boilers, the microturbine recovered energy and energy cost are known and can be calculated as a discrete measure and the cost savings deducted from the DEC\' in the same fashion as renewables. Our procedure to show these savings would be to take the final interactive model (DEC\') heating energy and subtract the recovered microturbine heat, and then apply the condensing boiler efficiency on the remaining heating energy. Then, the heating energy cost savings of the microturbines and condensing boiler would be subtracted from the DEC\' . Please confirm that this approach is acceptable as an Exceptional Calculation. Thank you.
The narrative raises two EAc1 related issues 1) establishment of a rate for micro-turbine generated electricity and 2) appropriate accounting of recovered/waste heat being used to offset high efficiency boilers. Page 158 of the LEED-NC v2.1 Reference Guide explains the virtual rate computation methodology. This method should be used to establish the cost of the electricity generated by the microturbines. There appear to be several ways to account for the savings associated with baseloading with recovered heat. The project team should note that there are several energy modeling tools capable of simulation waste heat recovery systems directly (without the need for an Exceptional Calculation Method) but an exceptional calculation method can be used to quantify these savings. The methodology proposed to account for the use of the recovered/waste heat is acceptable with minor revisions. The exception to ASHRAE 90.1-1999 Section 11.2.3 states that site-recovered energy is considered no-cost. The methodology proposed to quantify the cost savings generated through the use of the recovered/waste heat should be modified to estimate the efficiency of the heat recovery process and the amount of boiler fuel usage this process offsets. This fuel usage should be converted to cost at the appropriate boiler fuel rate. The recovered/waste heat cost savings should then be directly subtracted from the final DEC\' modeling run.