Thermo‑Exergo‑Economic and Environmental Optimisation of a Solar–Biogas Hybrid Brayton–Rankine Trigeneration System

ABSTRACT The growing demand for low‐carbon electricity, low‐carbon hydrogen, and freshwater has increased the need for integrated energy systems that efficiently utilise renewable resources while minimising environmental impacts. However, the energy, exergy, exergoeconomic, and environmental performance of hybrid solar‐biogas trigeneration systems has not yet been comprehensively evaluated. To address this research gap, this study proposes and assesses a solar‐biogas hybrid Brayton–Rankine trigeneration system for the simultaneous production of electricity, hydrogen, and desalinated water using an integrated 4E framework. A detailed thermodynamic model is developed for the solar receiver, Brayton and Rankine power cycles, steam methane reforming and water–gas shift reactors, and reverse‐osmosis (RO) desalination subsystem. Validation against data available in the literature yields deviations of less than ±5%, supporting the reliability of the developed model. The energy analysis shows that increasing solar irradiance from 400 to 900 W/m 2 increases the Brayton‐cycle power output from 65 to 205 kW. Meanwhile, the power output of the Rankine bottoming cycle increases from 15 to 55 kW because of enhanced waste‐heat recovery. Hydrogen production increases from 7.2 × 10 −4 to 1.97 × 10 −3 kg/s as the reformer temperature rises from 600 to 950 K, whereas freshwater production increases from 0.6 to 3.0 m 3 /h through condenser‐assisted RO desalination. Increasing the turbine inlet temperature from 850 to 1150 K raises the overall exergy efficiency from 0.68 to 0.84, corresponding to a 23.5% relative improvement, while reducing the levelised cost of electricity by 13%. Multi‐objective optimisation using the non‐dominated sorting genetic algorithm II (NSGA‐II) identifies a Pareto‐optimal operating region characterised by an exergy efficiency of 80% and a levelised cost of hydrogen (LCOH 2 ) of $1.8/kg. A comparison with recently reported solar organic Rankine cycle and biomass‐based systems indicates that the proposed configuration can improve exergy efficiency by up to 36% and reduce the hydrogen production cost by up to 27%, subject to the adopted system boundaries and economic assumptions. Overall, the results indicate that solar–biogas integration can enhance thermodynamic and techno‐economic performance, demonstrating the potential of the proposed trigeneration system as a low‐carbon platform for the integrated production of electricity, hydrogen, and freshwater.

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