Distributed cogeneration: modelling of environmental benefits and impact

Radical changes occurred in the energy scenario in recent years, with a clear trend towards shifting part of the energy production from large centralized plants to relatively small decentralized systems. The growing diffusion of distributed generation systems for Combined Heat and Power (CHP) production represents a significant part of these changes. In particular, CHP generation could bring substantial improvements in energy efficiency and energy saving, as well as economic benefits, with respect to the separate production (SP) of electricity in the centralized power system and of heat in local boilers (Horlock, 1997). The development of CHP systems is particularly relevant for relatively small-scale applications (e.g., below 10 MWe) in urban areas, including potential coupling to heat networks for larger capacities as well as micro-cogeneration (Pehnt et al., 2006; Pehnt, 2008) for domestic applications. The adoption of CHP systems can even be more effective when it is possible to supply, in periods with little or no heat demand, absorption chillers to satisfy the cooling demand (for instance, for air conditioning), thus obtaining high-efficiency seasonal tri-generation systems (Meunier, 2002; Mancarella, 2006). Moreover, CHP systems can be conveniently used in a distributed multi-generation framework, to supply various types of chillers to better fit the overall characteristics of the demand of various energy vectors (Chicco & Mancarella, 2009a). Higher energy efficiency can also correspond to lower environmental impact in terms of CO2 emissions with respect to SP, mainly depending on the generation characteristics of the power system in the specific country (Mancarella & Chicco, 2008a), particularly where electricity generation prevailingly occurs from fossil fuels. On the other hand, distributed cogeneration could worsen the air quality on the local level, due to emissions of various hazardous pollutants such as NOx, CO, SOx, Particulate Matter (PM), Unburned Hydrocarbons (UHC), and further substances conveying the pollution into the human body. In particular, in urban areas the environmental pollution is more critical because of a host of reasons, among which: a) high concentration of background pollutants, in particular due to road traffic pollution; b) difficult dispersion in the atmosphere of the pollutants produced from small-scale generators located in urban sites, with respect to large power plants with high stacks; c) relatively high number of receptors, due to the population density; 1