The thesis focuses on producing renewable fuels by direct conversion of concentrated solar energy via photoelectrochemical (PEC) approaches, which is one viable route for renewable fuel processing and energy storage. The aim of the Ph.D. thesis is to assess and support the development of solar fuel production and in particular to develop the practical design and operational guidelines in order to stabilize the performance, maximize hydrogen production, energetic effï ciencies, and to minimize the cost of solar-fuel generation systems. The thesis presents a novel integrated photoelectrochemical device design, i.e. composed of an integrated traditional photovoltaic component and an electrolyzer component, which allows to circumvent some of the challenges imposed by solid-liquid interface in traditional PEC devices, and has the potential to operate at higher efficiencies and lower cost than externally wired (non-integrated) photovoltaic (PV) plus electrolyzer (EC) devices. Further, the concentration of irradiation is considered reducing the usage of expensive photoactive/catalytic materials and making the device cost-effective.

A fully automated coupled 2D multi-physics non-isothermal model, which uses finite element and finite volume methods to predict the performance of the concentrated integrated PEC (CIPEC) devices, is developed. This validated model is the most complete yet computationally economical model of its kind. Further, the model is exploited formulating thermal management strategies and design guidelines; and showing that the thermal management is the rationale for CIPEC devices. Additionally, the collaborative work is performed on techno-economic and sustainability analysis showing that CIPEC devices can be economically competitive as well as sustainable.

The extensive learnings from the previous modeling work have been deployed to implement a lab-scale CIPEC prototype (PV area = 4 cm2, EC area = 25 cm2, Concentration = 474). This resulted in a successful implementation and demonstration of high performance (17.2% solar-to-fuel efficiency) at an operating electrochemical current density of 0.88 A/cm2 (with 6.04 A/cm2 PV current density) and output power of 27 W, the first demonstration of such high current density/power operation and a step towards production of cost-effective solar fuels. The dissertation further exploits the developed model which led to the design and development of controller/controlling strategies utilizing only the water mass flow to actively track the optimum power point of the system during the day and in turn counteract the adverse effects of frequent and sustained disturbances like weatherly irradiation changes and device degradation.

The thesis shows that concentrated photoelectrochemical approaches can provide a competitive solar fuel processing pathway. The detailed multi-physics model and techno-economic-sustainability model prove that operation of these devices is feasible and dynamic, and can be cost-effective as well as sustainable. The reactant flow based controlling approach shows device's ability to produce in a stable, reliable and robust way in spite of sustained/fluctuating disturbances. Finally, the experimental demonstration strengthens the case for the competitive performance and scalability of these devices. The thesis helps in bridging the gap between academia and practical implementation, and this work may lead to fast realization of these systems.