We are working on a higher ed. project building connected to a campus-wide district heating/cooling and power plant. The plant consumes gas for direct steam production as well as in a CoGen turbine operation for electricity generation. We were given data for monthly electric and natural gas consumption and the corresponding steam, chilled water and electricity generation to be used to calculate the total heating and cooling plant efficiencies. Here are questions on how we should calculate the credit and penalty associated with this new project adding to the central plant’s needs:
1. How do we need to calculate the ratio for the electricity generation that the project can claim credit for? Do we need to calculate it as:
a. Ratio of the modeled building’s heating energy consumption (MMBtu/yr) to the plant’s current steam generation (MMBtu/yr) or
b. Ratio of the modeled building’s heating energy consumption (MMBtu/yr) to the current steam generation (MMBtu/yr)+ the modeled building’s heating energy consumption (MMBtu/yr).
2. A portion of the natural gas consumption in the data is used directly by the turbine for electricity generation. How should we factor this in to the efficiency and building ratio numbers? As our calculation stands, the turbine natural gas factors in to the overall natural gas consumption of the central plant, and the resulting effect is a reduction in overall plant efficiency in steam generation. We have calculated the overall steam generation efficiency as: (steam sent to building loop MMBtu)/[(turbine natural gas) + (steam boiler natural gas MMBtu)+(heat recovery steam generator)] = efficiency. It’s a simple in/out = efficiency calculation. We are wondering if this is an accurate representation of plant efficiency and the CHP, or if we should be breaking out the natural gas for the turbine separately from the efficiency calculation, and perform a separate calculation using the proposed building ratio calculated in (1) to determine how much turbine natural gas to count as a “penalty” for the CHP electricity generation we are taking credit for in the proposed LEED model.