Exergy and environmental impact assessment of solar photoreactors for catalytic hydrogen production

In this paper, a new photo-catalytic energy conversion system is analyzed for continuous production of hydrogen at a pilot-plant scale. Two methods of photo-catalytic water splitting and solar methanol steam reforming are investigated as two potential solar-based methods of catalytic hydrogen production. The exergy efficiency, exergy destruction, environmental impact and sustainability index are investigated for these systems, as well as exergoenvironmental analyses. A Compound Parabolic Concentrator (CPC) is presented for the sunlight-driven hydrogen production system. This study shows that an optimum water flow rate exists, where the exergy efficiency of the photo-catalytic hydrogen production is maximized. The amount of CO2 emissions that are reduced by this process increases at higher flow rates. The light intensity is one of the key parameters in design optimization of the photo-reactors, in conjunction with light absorptivity of the catalyst. The results show that a trade-off exists in terms of exergy efficiency improvement and CO2 emissions of the photo catalytic hydrogen production system. The optimal working condition of solar methanol reforming to satisfy the exergy–environmental considerations is found to be light intensity range of 530 W m−2 < J < 600 W m−2 and water–methanol mole ratio of 1.5–2. An optimum methanol feeding rate associated with CO2 emissions of methanol steam reforming is determined in order to establish the required solar flux for photo-catalytic conversion of methanol to hydrogen.

► We studied the performance of a new photo-catalytic energy conversion system to produce hydrogen.
► Two methods of photo-catalytic water splitting and solar methanol steam reforming are investigated.
► An optimum water flow rate exists for maximum exergy efficiency.
► A trade-off exists in terms of exergy efficiency improvement and CO2 emissions of the photo catalytic system.
► A light intensity range of 530 W m−2 < J < 600 W m−2 is found to be the optimum light intensity criteria.