Cogeneration Micro Turbine Fuelled by Solid Biomass: A Technical-Economic Study for Italy

ABSTRACT The paper presents part of the results of two studies, the European “Radar” (Raising Awareness on renewable energy Developing Agro-eneRgetic chain models) Project and the “Energy and environmental plan for the consortium of the municipalities of the Esino-Frasassi mountain area”, conducted in an area in central Italy. The area is characterized by huge forestry biomass resources and by substantial amounts of agricultural residues. The work presents a technical-economic study of a cogeneration plant using a solid biomass-fuelled micro turbine as the prime mover. The energy conversion of solid biomass can be achieved with different technologies, e.g. organic Rankine cycles, micro turbines with an external combustion chamber, or Stirling engines. The choice of the conversion system depends mainly on biomass availability and on the level of user demand. Of the conversion technologies mentioned above, the micro turbine is suitable to meet the requirements of the cogeneration plant examined here, which is applied to a low thermal demand public building. The work describes a micro turbine based on a regenerative Brayton cycle endowed with an external combustion chamber. The inlet air, after being compressed, passes through a regenerator and then through an external furnace fuelled by solid biomass, where it is further heated, and finally expands through the turbine. The outlet air of the turbine, before being funnelled through the chimney, passes through the regenerator and subsequently through a dry kiln, thereby reducing the humidity of the solid biomass. The micro turbine studied produces 75 kWe and 300 kWt. The biomass is made up of olive tree prunings. After the technical analysis, an economic study stresses the critical role of incentives systems (herein provided by the Italian legislation) in making the technology appealing to investors in renewable energy solutions. The energy and economic analysis considers different combinations of three different amounts of annual operation hours, of two operating modes (with/without cogeneration) and three purchase prices of the solid biomass. The incentives mechanism considered is the Feed-In Tariff (FiT) granted by the Italian legislation for plants < 1 MWe. The economic analysis highlights some influential factors for solid biomass-fuelled systems: contract with fuel suppliers, biomass price, availability, transportation, storage, and processing, and plant location. In particular, the purchase price of solid biomass is substantially negotiated between the manager of the energy conversion plant and suppliers. The work demonstrates the crucial role of the incentives mechanisms for economic sustainability; the strong influence of biomass price on investment profitability; and the role of cogeneration in further shortening the payback period.