The direct conversion of solar energy and water into a storable fuel via integrated photo-electrochemical (IPEC) devices is numerically investigated. Here, we focus on a device architecture which uses concentrated solar irradiation to reduce the use of rare and expensive components such as light absorbers and catalysts. For such devices, it is of particular importance that the device operates with stable efficiency and production rates. The degradation of the materials over the lifetime of the device as well as the daily and seasonal changes in incoming irradiation pose a challenge for the stable and secure supply of fuel from such devices. To tackle these issues, we develop performance optimization strategies utilizing device design, component and material choice, and adaptation of operational conditions.

We used a 2D coupled multi-physics model using finite element and finite volume methods to predict the performance of the concentrated IPEC device [1]. This device model was used to predict the performance of the device exposed to changes in operating conditions (short term transients in irradiation, longer term transients due to degradation) and, subsequently, to predict how controlling of the operating conditions (mass flow rate of reactant, irradiation flux and active areas) can be used to counteract performance decrease. Maximum power points, V_op and I_op which are calculated from the output of the CIPEC device, serve as the inputs for the proportional–integral–derivative (PID) controller. The mapped data of operating points for varied range of input irradiation and water mass flow rates (or flow velocities) is calculated using our detailed 2D multi-physics model. This data is used in the PID controller’s algorithm. The PID controller is connected to the water (reactant) mass flow rate controller, irradiation shade, and flow channel ports in order to control input water flow rate, irradiated area and electrochemically active area of the IPEC device and, correspondingly, to control the performance of the device.

The steady state results predict that the current-voltage curves of the electrochemical component (EC), with varying mass flow rates, intersect in a small potential region (figure 1.a), giving rise to two distinctive regions of operation [2]. Region I, corresponding to a low operating current density zone, is affected by temperature effects, leading to increasing slopes of the current-voltage curves of the EC for smaller temperatures. Region II, corresponding to a large operating current density zone, is characterized by mass transport limitations which lead to increased saturation currents of the EC with increasing mass flow rates. The formation of these two characteristic operational regions leads to specific trends and tradeoffs in the objective functions of solar-to-hydrogen (STH) efficiency and H₂ production rates when increasing the irradiation concentration and consequently require opposite controlling actions. These characteristic operating zones provide the basis for the formulation of controlling strategies. These strategies enhance the performance of the device and help in alleviating effects on the performance due to device degradation. Utilizing a simple PID controller, IPEC devices can be operated without significant decrease in performance for timescales relevant for degradation. This is even possible at very high irradiation concentrations, provided the input water flow velocity is at least 0.2 m/s. The effect of degradation is particularly pronounced if the operation is close to the maximum power point of the PV. The device degradation and the corresponding decrease in performance over the lifetime of the system can be controlled. For operation of an example device made of Si-based PV and rare catalysts at C = 707, a 12% degradation in STH efficiency can be compensated with a seven-fold increase in the water mass flow rate (figure 1.b). However, only if the operating point remains in region II, i.e. the degradation overpotential in the EC is limited to about 250 mV.

Our simulation frameworks in combination with a simple controller model provides evidence that a concentrated IPEC device can always be operated at optimized performance even though there are short and long term transients affecting the operation of the system. Consequently, controlling strategies show to be a simple approach to alleviate the effects of device degradation over the lifetime and short-term transients occurring due to e.g. irradiance fluctuations.

References:

[1] S. Y. Tembhurne and S. Haussener. Integrated Photo-Electrochemical Solar Fuel Generators under Concentrated Irradiation- Part I: 2-D Non-Isothermal Multi-physics Modeling, in J. Electrochem. Soc. 2016, 163 (10) H988-H998.

[2] S. Y. Tembhurne and S. Haussener. Integrated Photo-Electrochemical Solar Fuel Generators Under Concentrated Irradiation- Part II: Thermal Management a Crucial Design Consideration, in J. Electrochem. Soc. 2016, 163 (10) H999-H1007.