Thermal Management in Photo-Electrochemical Hydrogen Generation Devices Using Concentrated Solar Irradaiton

Human activity is overloading the atmosphere with global warming gas emissions which create significant and harmful impacts on our health, environment and climate. Utilizing the ample energy available from renewable sources like sunlight and water, and converting it into chemical energy by photoelectrochemical (PEC) processes, is one viable route to reduce these impacts as life cycle global warming emissions associated with renewable energy are minimal. This presents an effective path for renewable fuel processing and energy storage. Integrated PEC devices, which are composed of an integrated traditional photovoltaics (PV) component and an electrolyzer (EC) component, allow to circumvent some of the challenges imposed by solid-liquid interface in traditional PEC devices, and operate at higher efficiencies than externally wired (non-integrated) PV plus EC devices. To make the integrated devices economically competitive, we have employed concentration of irradiation. 

We proposed a novel integrated design, shown in fig. 1, combining EC, PV and concentrator [1].

We developed, a coupled 2D multi-physics model of the proposed concentrated PEC device. The model includes radiative heat transfer in the concentrator, electromagnetic wave propagation in the semiconductors (a triple/dual junction solar cell), charge generation and transport in the photoabsorbers and the integrated electrolyzer (polymeric electrolyte and solid electrode), electrochemical reaction at the catalytic sites, fluid flow and species transport in the channels delivering the reactant (water) and removing the products (hydrogen and oxygen), and heat transfer in all components.

The detailed simulation flow and coupling between different physical nodes have been shown in fig. 2. Detailed treatment was given for various heat source/sink term calculations for effective thermal management. Urbach tails and defect states in amorphous semiconductors have been incorporated using wxAMPS [2].We completely automatized the simulation flow using multiple interactions between MATLAB Inc., COMSOL and wxAMPS. Efficient computational power saving techniques have been rigorously employed making our model one of the most detailed yet computationally economical. The model proves to be a valuable design and optimization tool for integrated PEC devices working with concentrated irradiation and at elevated temperatures.

The model allowed to investigate and optimize the thermal management in the integrated PEC, highlighting that the operating point of the integrated device could be readily shifted to the maximum power point (MPP) of the PV. Thermal management therefore allowed for an efficient and cost effective way of tracking the optimum operating point (OOP), e.g. by controlling the flow rate of water in the water channel for a particular irradiation concentration, providing two distinct benefits: i) reducing the losses by utilizing the generated heat, ii) enabling simple tracking of OOP of the device. The model allowed to conclude that the OOP at a particular concentration was the MPP of the PV and that the OPP of the integrated system was decided by the EC’s current-voltage curve’s saturation part. The mass transport limitations in the EC govern the optical concentration for maximized solar-to-fuel efficiency. 

The top water channel above the PV not only helps in cooling the PV and supplying the heated reactants to the electrolyser, but it also enhances the optical absorption in the PV by reducing the reflections on top cell surface. The absorption in the water for comparable channel widths is found to be minimal within the absorption spectra of the PV. Parametric studies allowed to find the optimum channel widths governing the absorption and the cooling of device for varying irradiance concentrations.

We analysed triple/dual junction thin film solar cells as a cost effective alternative to the earlier studied Ga_0.51In_0.49P-GaAs-Si [1]. Smart thermal management allowed the design of an integrated device with a dual aSi-ucSi cell to operate at efficiencies comparable to those of Ga_0.51In_0.49P-GaAs-Si device under non-concentrated irradiation.

The model proves to be a valuable tool for the design of integrated PEC cells working with concentrated irradiation at elevated temperatures and highlights that the smart thermal management can help in achieving increased low cost production of solar fuel.

References:

[1] S. Y. Tembhurne, M. Dumortier and S. Haussener.  Heat transfer modeling in integrated photoelectrochemical hydrogen generators using concentrated irradiation. 15th International Heat Transfer Conference, Kyoto, Japan, 2014.

[2] Y. Liu, Y. Sun, and A. Rockett, "A new simulation software of solar cells--wxAMPS", Solar Energy Materials and Solar Cells, 2012