Thermodynamic analysis of small-scale externally fired gas turbines and combined cycles using turbo-compound components for energy generation from solid biomass

This paper proposes the implementation of cost-effective commercially available automotive components in small-scale power plants for the energy generation from carbon-neutral biomass. Specifically, a turbocharger and a power turbine of turbo-compound systems are proposed to be coupled with an external combustor and a high temperature heat exchanger in order to obtain a cheap externally fired gas turbine capable of producing about 30 kW of electrical power from the combustion of pruning residues. The externally fired gas turbine cycle can be combined either with a final heat exchanger to generate useful thermal power or with a bottoming cycle to generate useful thermal power and an additional electrical power of about 15 kW. Two plant configurations are proposed for the bottoming cycle: the first is a water Rankine cycle employing the “green steam turbine” as the steam expander, whereas the second is an organic Rankine cycle using an axial turbine and toluene as the working fluid. The results of the simulations, obtained through a detailed thermodynamic model, show that the use of a combined cycle is fundamental to maximize the primary energy savings of the power plant. In the case of negligible pressure losses, the use of a bottoming water Rankine cycle leads to a maximum second law efficiency of about 0.25 and maximum primary energy savings of about 0.23. Instead, a bottoming organic Rankine cycle employing a single stage turbine can increase the second law efficiency and the primary energy savings up to about 0.27 and 0.26, respectively. It is also demonstrated that the use of a two-stage turbine for the organic Rankine cycle can further enhance the plant performance. The effects of the pressure drops in the system are investigated in detail to point out that the minimization of the pressure losses is fundamental to improve the performance parameters of all the proposed configurations.