Análise exergética e termoeconômica de um ciclo Rankine orgânico acoplado a um ciclo Rankine convencional operando com a biomassa do coco verde

Abstract

Biomass is a resource that can be used to generate energy through combustion. According to the 2023 Brazilian energy matrix provided by the Energy Research Company (EPE), the main biomass residues used are: sugarcane bagasse with 16.8%, agricultural residues with 8.6%, and other sources—such as industrial waste, rice husks, and food waste—accounting for 7.2%. In this study, the green coconut husk is presented as an alternative biomass source for application in a conventional Rankine cycle coupled with an organic Rankine cycle. Green coconut is cultivated across approximately 190,000 hectares in Brazil, with an estimated annual production of around 1.8 billion fruits and a low rate of husk reuse. Thus, this research aims to study the application of green coconut husk bagasse for energy generation in a cogeneration plant. The analyses were carried out using the energetic potential of coconut husks reported in the literature, and three working fluids—R245fa, R134a, and R123—were applied to the organic Rankine cycle. The comparison among the cycles using the three fluids was conducted to determine which provides better exergetic efficiency under the proposed operating conditions. Furthermore, a thermoeconomic analysis was performed following the UFS exergy disaggregation model. The software used for the study was MATLAB. Upon examining the results, it was found that green coconut husk bagasse, with a Lower Heating Value (LHV) of 17,000 kJ/kg, demonstrates good energy recovery potential and can be used to supply a Rankine cycle. The R134a fluid showed the best performance for the proposed plant, with energy and exergy efficiencies of 33.15% and 43%, respectively. Meanwhile, the R123 and R245fa fluids reached exergy efficiencies of 34.65% and 35.85%, respectively. In addition to providing the highest exergy efficiency, R134a also presented the lowest unit exergy and monetary operating costs in the thermoeconomic analysis, with values of 46.99 kW/kW and 142.7 R$/MWh, respectively. In comparison, R123 showed an exergy cost of 424.8 kW/kW and a monetary cost of 1,259.5 R$/MWh, while R245fa achieved 183.45 kW/kW and 545.88 R$/MWh, respectively.