Thermo-economic evaluation of actively selected siloxane mixtures in a hybrid solar-biomass organic Rankine cycle power plant

This study aims to evaluate thermo-economic implications of implementing siloxane mixtures as organic Rankine cycle (ORC) working fluids for hybrid solar-biomass applications. Next to active selection of suitable mixtures and their thermodynamic evaluations, ORC components were sized under process conditions of different fluid studied. The main paper contribution concerns integrated off-design economic analysis of working-fluid mixtures in ORC. By comparing the studied mixtures with pure MM (hexamethyldisiloxane), results showed that 0.9MM/0.1MDM (octamethyltrisiloxane) and 0.8MM/0.2MDM increase net ORC power by 2% and 1.4% respectively, while 0.1MM/0.9MDM reduces net power by 1.3%. Also, specific exergy cost of power production increases by 3.7%, 1.9% and 1% for 0.1MM/0.9MDM, 0.8MM/0.2MDM and 0.9MM/0.1MDM, respectively. Furthermore, after implementing off-design models, increase in annual net energy production of 2.3%, 3.2% and 5.2% were obtained for 0.1MM/0.9MDM, 0.8MM/0.2MDM and 0.9MM/0.1MDM, respectively. However, these thermodynamic advantages do not translate to better economic performance. Exemplarily for the most thermodynamically efficient mixture (0.9MM/0.1MDM), levelized cost of electricity and specific payback period increase by 8% and 13% respectively, while net present value reduces by 15.7%, relative to pure MM. Thus, caution should be exercised when considering the use of selected siloxane mixtures as ORC working fluid in high- temperature applications such as solar/biomass.