Proposal for a new approach to assess the efficiency of combined heat and power installations.

Proposal for a new approach to assess the efficiency of combined heat and power installations. Combined heat and power (CHP), i. e. cogeneration of thermal and mechanical energy from fuels, is regarded as one of the key technologies for reducing primary energy consumption. Due to the energetically different qualities of the produced heat and the electricity generated usually directly from mechanical energy, it is difficult to determine the efficiency of specific CHP processes exactly. In the context of energy accounting of buildings, e. g. for the purpose of Energy Passports for new or existing buildings, the Building Energy Conservation Ordinance stipulates the use of algorithms according to DIN V 18599 and DIN 4108-6 in conjunction with DIN 4701-10, which also include a technique for determining the quality of CHP. EU Directive 2004/8/EC of 11 February 2004 "on the promotion of cogeneration based on useful heat demand in the internal energy market" stipulates a fundamentally different approach for determining the efficiency of CHP plants. When used for specific installations, the two methods can lead to results that may differ significantly and could even be contradictory for comparative assessment of different CHP processes. This article demonstrates that both assessment approaches are inadequate for describing the efficiency of CHP processes. A new method for determining efficiency is proposed, in which the quantity of fuel required for compensating the electricity loss as a result of heat utilisation is taken into account in the heat production calculation.