Emergy evaluation of combined heat and power plant processes

An energy-focused environmental accounting method based on the embodied solar energy (emergy) principle was used for evaluating biomass and coal-based combined heat and power (CHP) cogeneration processes. The emergy method expresses all the resources needed (fuel, investment, labor etc.) as solar energy equivalents. The method looks at sustainability from the point of view of the biosphere. In fact, emergy aims to be a ’memory’ of how much work the biosphere has done to provide a product. Biomass and coal-based CHP alternatives were compared with independent production of heat & power. It was found that biomass-based cogeneration is 3.3 times more emergy-efficient than coal-based independent production; i.e. the biosphere needed to work 77% less for biomass CHP produced heat & power compared to that produced independently from coal. Cogeneration from the same fuel was in all cases 0.3 times more emergy-efficient than independent production. In general heat and power production from biomass is 2.3 times more emergy-efficient than that from coal in a similar process. The emergy sustainability index shows a similar trend, e.g. the sustainability index of a biomass CHP plant is 15 times higher than that of a coal CHP plant. The fuel, its transport, and the oxygen in air used for burning account for over 80% of the emergy in biomass CHP, whereas in a coal-based process the share is over 90%. The share of capital is quite small in terms of total emergy.