Small-scale biomass power plant for distributed energy generation

Nowadays concern about climate change is gaining more importance not only in the scientific community but also in the public opinion, which is pushing for having policies oriented to encourage the use and the development of renewable energies. In order to contribute to the spreading and evolution of renewable energy, the approach to the distribution of the energy is changed from centralized to distributed generation (DG). In this frame, Combined Heat and Power (CHP) systems are becoming crucial, thanks to their capability to satisfy at the same time heat and power demand with a relatively high efficiency. One of the main advantages of these new solutions is that they can give a boost to the spreading of renewables and developments of green technologies. However, the selection and installation of the best performing renewable plants is never trivial due to the inherently intermittent nature of some renewables (as wind or solar energy) and so, their need to be integrated with energy storage and programmable generation systems in order to match energy demand. This thesis deals with the integration of energy produced by biomass, mainly woody, with other small-scale power plants, such as Organic Rankine Cycle (ORC), and Thermal Energy Storage (TES), in a DG approach that exploits cogeneration to reduce energy waste. Two different scenarios are the main line of the following work: a hybrid plant, where biomass integrates solar energy, and a pure biomass boiler fed system, where the biomass supply heat to different power generations technologies. Tools such as optimization algorithms and thermo-economics analysis have been used to quantify the 7 effect of some of the many variables on the efficiency and profitability of such systems. In the hybrid biomass-solar plant case, the effect of coupling this system with an external combustion gas turbine is analayzed. The turbine is fed by biomass to overcome the main limits of solar energy, i.e. energy intermittence. The coupling between the two systems is done by means of a TES that receives heat from both systems. In turn, the TES transfers heat to an ORC, which will be the element that supplies energy to the end-user. Concerning this layout, the performances, the effects of the use of different organic fluids in the plant ORC and the variation in economic profitability depending on the geographical location of the plant are analyzed. The second case analyzes different layouts all with a biomass boiler. The furnace is here coupled with systems such as TES, steam plants and ORC in different configurations. Real input data from a biomass boiler installation and heat demand have been applied to the systems. The analysis has been performed by implementing hourly energy costs and electricity feed-in tariff. The aim is to evaluate the effect of TES and/or ORC installation in a biomass system and eventually select the best performing size and operating conditions of the plant components. The two studies highlight that, in general, (i) coupling the biomass boiler with a TES allows the boiler to work at higher part-load conditions and at a higher global energy efficiency; (ii) profitability of ORC installation could increase with flexible plant, where the heat demand is satisfied also by the other component such as a TES: in this case, a smaller ORC size could be selected, increasing equivalent operating hours and reducing investment costs; (iii) investment profitability increases in the presence of a dedicated subsidy framework such as the one available in the Italian energy market. The future steps of this research will focus on the quantification of the techno-economic 8 advantages of the proposed system configurations in terms of higher generation flexibility and implementation of demand response strategies.