Annualized exergoenvironmental comparison of solar-only and hybrid solar-biomass heat interactions with an organic Rankine cycle power plant

Abstract

This study is aimed at comparing the environmental performance of a solar-only and hybrid solar-biomass organic Rankine cycle (ORC) plant using the exergoenvironmental method. The system studied adopts the design features of a real ORC plant currently running at Ottana, Italy, rated nominally at about 630 kW. Established procedures of the exergoenvironmental methods were applied, which integrate the principles of exergy and life cycle analyses. The method quantifies the yearly environmental impact rates of each of the plant components. The eco indicator-99 impact assessment method was adopted to quantify impact rates. Results showed that implementing the biomass hybridization scheme would improve the annualized exergetic efficiency of the existing solar ORC plant by about 3 percentage points, from about 7% with solar-only to about 10% with hybrid solar-biomass heat sources, although a small increase in the relative irreversibility rate is observed (from 49% to 51%). It would as well improve the capacity factor of the plant. However, the environmental impacts would be impacted negatively. Particularly, the specific exergoenvironmental impact rates were obtained as 27.4 Pts/MWh for the hybrid solar-biomass ORC plant, as against 20.3 Pts/MWh obtained for the solar-only plant, implying that the hybridization strategy would increase impact rate by about 35%. Similarly, it was obtained that biomass hybridization would increase the overall exergoenvironmental impact rates of the ORC plant by about 92,000 Pts/year, due majorly to increased exergy destruction in the plant components and the polluting emissions of the combustor.