An Emergy Comparison of Different Alternatives for the Cogeneration of Heat and Electricity

The study deals with an emergy evaluation of the supply of electricity and heat to a large user, namely a public hospital in Northern Italy. We performed firstly a conventional LCA evaluation, in order to identify and quantify the material and energy flows involved in three cogeneration alternatives for the hospital. The evaluated cogeneration alternatives were: 1) an Internal Combustion Engine, 2) a Gas Turbine, 3) a Hybrid system composed of Molten Carbonate Fuel Cell unit (compact unit) + Gas Turbine. The structure of the system is based on the above mentioned cogeneration units and a 92% efficiency auxiliary boiler fuelled with natural gas, capable of supplying the additional thermal demand when demand is higher than the cogenerative unit is capable to meet. Based on LCA results, we focus in the present paper on the added value of including the Emergy method within the Life Cycle Impact Assessment (LCIA) framework. In fact, Emergy offers an overview of the demand for environmental support (total emergy and transformity of local electricity and heat production) and helps assessing the global performance of the three investigated cogeneration alternatives on the biosphere scale by means of selected emergy-based indicators (total emergy U, transformity Tr, ELR, EYR, ESI, Empower Density). Results indicate, as a general trend, system 3 (hybrid system with MCFC coupled with the auxiliary boiler) as the solution with the highest electric and thermal efficiency and correspondingly lower overall environmental impact. The study also helps understanding the importance of identifying the appropriate boundary of the assessment, in order to be able to shed light on crucial environmental, technological and economic aspects that affect the sustainability of the investigated system.