Comment from Roos, Carolyn
This comment comes from CHP discussions emerging from within the RTF as a way to analyze CHP botrh both its efficiency gains and its power generation: Proposal to the Northwest Power Plan 1. Recognize that CHP systems legitimately serve as both conservation and generation resources. 2. Determine the percentage of a CHP project that serves as conservation and the percentage as generation using the “efficiency method”. Combined heat and power (CHP) systems are treated in the Northwest Power Plan strictly as generation resources and not as conservation resources. However, CHP systems have two well-defined final energy streams – heat and power. The heat generation is a conservation resource and the power production is a generation resource. The “efficiency method” is an internationally accepted calculation method that has been used to allocate greenhouse gas emissions between the heat and the power portions of CHP systems. This method can be used without modification to allocate other benefits and costs of CHP systems between the two final energy streams. The efficiency method is the preferred method for this purpose recommended by the California Climate Action Registry (Registry), WRI/WBCSD, UK Emissions Trading Scheme (ETS), and U.S. EPA Climate Leaders. The following description of the efficiency method is from “Cogeneration: Proposed Approach for Mandatory Greenhouse Gas Emissions Reporting” by the California Air Resources Boards available at http://www.arb.ca.gov/cc/ccei/presentations/ARBCogenApproach.pdf: Actual efficiencies of thermal energy and power vary between the two most common cogeneration systems; steam boiler/turbines and combustion turbines. A steam boiler/turbine can generate up to 5 times more thermal energy than electric energy. A combustion turbine can generate from 1 to 2 times more thermal energy than electric energy. The Registry, U.S. EPA, and WRI/WBCSD recommend cogeneration facilities identify actual thermal energy and electricity production efficiencies. If actual efficiencies of heat and power production are unknown, they allow for the use of default values of 80% for steam and 35% for electricity. Basic Steps to Allocate Emissions Using the Efficiency Method 1. Determine the total direct emissions from the cogeneration facility 2. Determine output flows of thermal energy and electricity expressed in BTU 3. Estimate the efficiencies of steam and electricity production 4. Determine the fraction of emissions allocated to thermal energy and electricity In life cycle cost analyses, the efficiency method is recommended in allocating costs to the final energy streams of CHP systems. The method is described in the user manual of RELCOST Financial as follows: For a CHP plant that provides both steam and electricity as outputs, one way of allocating costs is the efficiency method. The efficiency method allocates costs of CHP plants between electric and thermal outputs on the basis of the energy input that would be used to produce the steam and electricity products separately. To use this method, you must know the total steam production Es and electricity production Ee of the plant in consistent units (such as MMBtu) and the steam efficiency ηs and electricity efficiency ηe of the facility. Calculate the percent S allocated to steam production by and the percent P allocated to electricity production by To determine the energy output Es attributable to steam, use published steam tables that provide the enthalpy of steam at different temperature and pressure conditions. Then determine the net heat production by subtracting from this the enthalpy of the return condensate. If actual efficiencies of the CHP plant are not known, default values of 80 percent for steam efficiency ηs and 35 percent for electricity efficiency ηe are often used.