Energy Payback Time Modeling of Integrated Photoelectrochemical Devices Using Concentrated Solar Irradiation

The photoelectrochemical (PEC) production of hydrogen offers a direct pathway for the conversion of solar energy into an energy dense and transportable fuel. In an integrated PEC device, solar radiation is absorbed by photoactive components which provide the necessary potential for water electrolysis to take place in the integrated proton exchange membrane electrolysis cell (PEMEC). The produced current density in the photovoltaic component (PV) increases with increasing solar irradiation, i.e. concentration. This results in higher hydrogen production rates for a PEMEC with small overpotentials, i.e. good ohmic conductors and efficient catalysts, compared to non-concentrating systems. We also expect further reduction in energy and material requirements of a concentrated PEC (CPEC) device and its fabrication and, consequently, its energy payback time (EPBT). We compared the EPBT of PEC and CPEC devices for water electrolysis. For the latter, we compared existing concentrating technologies - lenses, mirrors, and Cassegrain reflectors - and a novel integrated CPEC device using a self-tracking wave-guide concentrator.

We developed a model that assesses the energy requirements of the different component (concentrator, electrolysis cell, photovoltaic cell, external components) by coupling the energy and material inventory of devices to a detailed performance model under different concentrations. The model uses the Shockley-Queisser limit to assess the performance of the multijunction photovoltaic cell, and an equivalent circuit model was used to assess the performance of the electrolysis cell, including kinetic overpotentials via Butler-Volmer expressions, ohmic resistances via Ohm’s law, and mass transport overpotentials [1].

The results show that the EPBT of all concentrating technologies are in the same order of magnitude (5-10 years). However, the integrated self-tracking concentrator design shows a lower EPBT than Fresnel lenses-based concentrating systems due to a lower primary energy demand of the former (1600 MJ/m² for integrated, self-tracking concentrator, and 2800 MJ/m² for external Fresnel concentrator coupled to PV-electrolyzer). A sensitivity analysis showed that the most influential parameters for the reduction of the EPBT are the yearly solar irradiation, the primary and operating energy demand of the concentrator, the performance of the concentrator, the exchange current density of the catalysts, and the fill factor of the photovoltaic cell. The EPBT could be reduced by at least 6% with a variation of 10% of one of these parameters for all the technologies studied.  

This developed model provides a useful tool for device design to compare and optimize the energy requirements of integrated PEC and CPEC devices

[1] P. K. Das, X. Li, and Z.-S. Liu, “Analytical approach to polymer electrolyte membrane fuel cell performance and optimization,” J. Electroanal. Chem., vol. 604, no. 2, pp. 72–90, Jun. 2007.