Practical and accurate measurement of cogenerated power

Abstract

The performance of CHP activity is measured by the amount of cogenerated electricity (ECHP) during a considered period. It is the proper yardstick because it combines attributes of quality (power-to-heat ratio σ) with those of quantity (recovered heat QCHP), given that ECHP = σ × QCHP is a commonly accepted identity. In practical applications of the formula, problems arise in finding the appropriate numerical values of power-to-heat ratios. The EU Commission, expert groups, and published literature expose circular logics, concealed by flawed approximations. The enigma is most relevant for extraction-condensing steam turbines, which mix cold condensing with one or more cogeneration activities, making the power flow ECHP not directly observable. This paper presents a generic and neat solution to the ECHP measurement problem. It starts with a clear problem statement. Then, the components of the solution are exposed. First, in a Mollier diagram the unit mass flow expansion path of a Rankine steam cycle with backpressure heat extraction(s) ahead of the cold condenser is noted. Second, the characteristic points on the expansion path provide the contours of the (Electricity E – Heat Q) production possibility set of the steam power plant. Third, the real capacities of the steam flows of the power plant are mapped on the possibility set expressed in electricity and heat capacity (Watt). It shows how limits on extracted steam flows truncate a significant part of the theoretical possibility set in an extraction-condensing turbine. By merging the extraction capacities with design characteristics of a plant, the accurate measurement of cogenerated power becomes self-evident. The method is documented with numerical cases. Applying the presented, transparent and accurate, method is prerequisite for regulations being effective in promoting optimal CHP plant investments and operations. Promotional support may imply subsidies for energy efficiency, priority ranking of cogenerated power in merit orderings of integrated power systems, among others.