Photoelectrochemical Demonstrator Device for Solar Hydrogen Generation Project Deliverable Report D2.3 Deliverable title: Deliverable nature: Dissemination level: Lead beneficiary: Contracted date of delivery: Actual date of delivery: Author(s): Report on the optimal design of large-area hybrid PEC-PV cells Report Public Technion - IIT Dec-16 Jan-17 Avner Rothschild, Hen Dotan, Roel van de Krol, Fatwa Abdi, Matthew Mayer, Michael Wullenkord, Bernd Stannowski, Adélio Mendes, Tânia Lopes PECDEMO is a Collaborative Project co-funded by FCH JU under the call SP1-JTI-FCH.2013.2.5. GA n : 621252. Start date: April 1 st, 2014. Duration: 36 months.
TABLE OF CONTENTS Executive Summary... 1 1. Core of the Report... 2 1.1. Optical coupling of the PEC and PV cells... 2 1.2. Electrical coupling of the PEC and PV cells... 3 1.3. Series resistance loss in large-area hybrid PEC-PV modules... 4 2. SUMMARY... 4 3. References... 5 EXECUTIVE SUMMARY In the efforts of further strengthening Europe s leading position in the field of solar fuels, PECDEMO aims at developing a hybrid photoelectrochemical-photovoltaic (PEC-PV) tandem device for solar water splitting. The project targets solar-to-hydrogen (STH) conversion efficiency of 8-10%, large scale devices ( 50 cm 2 ) and long-term stability (1000 h). To achieve these goals, the second work package (WP2) aims to guide the optimization efforts of hybrid PEC-PV cells (WP1) and modules (WP4) by identifying material degradation processes and efficiency losses; quantifying their effect on the long-term stability and efficiency; scrutinizing materials compatibility for stable longterm operation with minimal degradation and efficiency losses; and optimizing the optical and electrical coupling of the PEC and PV cells. To this end, we examined different designs for large area PEC-PV tandem cells, which include optical and electrical coupling with optimal photon and power management between the PEC and PV components. The different designs were presented in the Deliverable 2.2 report that was delivered last year, where the pros and cons of each design were discussed in details. In this report, Deliverable 2.3, we summarize our recommendations for the optimal design of large-area PEC-PV tandem cells. Page 1
1. CORE OF THE REPORT 1.1. Optical coupling of the PEC and PV cells In the last two and a half years, we simulated and experimentally tested different photon management schemes to optimize the optical coupling of the PEC and PV components. Detailed modeling of the optical coupling between the PEC and PV cells was carried out for the conventional stacked cells design as well as for advanced configurations that employ spectral splitting between the PEC and PV cells in order to improve the solar to hydrogen conversion efficiency of the tandem cell. Several spectral splitting schemes with passive or active light management were considered, enabling tailoring the degrees of freedom of the tandem system in order to make it more efficient (but also more complicated). The different optical designs are illustrated in Figure 1, and the details were described in Section 2.1 of Deliverable 2.2. Despite having potential to increase the solar to hydrogen conversion efficiency, the spectral splitting schemes complicate the device fabrication, which is especially challenging for large-area devices. Therefore, we decided to leave out these schemes and focus on the stacked cells configuration according to Figure 1 (a) for the production of largearea devices for our final demonstration system. (a) (b) Figure 1. Different configurations for optical coupling of the PEC and PV cells in PEC-PV tandem cells. (a) Stacked cells configuration (b) Spectral splitting using dichroic mirrors (reproduced from Deliverable 2.2); (c) Spectral splitting using distributed Bragg reflectors (DBRs) (reproduced from Deliverable 2.2). (c) Page 2
In addition, using concentrated solar irradiation enhances the performance of thin film hematite photoanodes (Ref. 2) and allows compact set-up of PEC/PV devices in large-scale application. Up to 17.5-fold concentrated solar irradiation provided by DLR s test facility SoCRatus (Solar Concentrator with a Rectangular Flat Focus) is employed in the final demonstration phase of the modular PEC/PV prototype developed in PECDEMO (http://pecdemo.epfl.ch/page-113308-en.html), Figure 2. Figure 2: Modular prototype in the focal plane of DLR s solar concentrator SoCRatus. 1.2. Electrical coupling of the PEC and PV cells The most straightforward electrical coupling between the PEC and PV cells is connecting them in series. With this design, the same current flows in both the PEC and PV cells and the photovoltages generated in both cells adds up together as in series connection of multijunction PV cells. In this configuration the operation point of the tandem cell is at the cross point of the J-V curves of the PEC and PV cells. This approach is described in detail in Section 2.2.1 of Deliverable 2.2. The main problem in this design is that the photocurrent is limited by the worse cell and the operation point is fixed and it cannot track the maximum power point of the tandem system. This leads to significant power loss and generation of excess heat. These problems can be rectified by connecting another electric load in parallel to the PEC cell, as illustrated in Figure 3. This allows dividing the overall power generated by the PV cell between the PEC cell and the parallel load, thereby leading to co-production of both hydrogen and electrical power. This solution significantly increases the overall efficiency of the tandem device. Furthermore, since the parallel load acts as a sink for excess power Page 3
generated by the PV cell, this approach enables tracking the maximum power point by using a DC/DC convertor, as discussed elsewhere (Ref. 3). (a) (b) Figure 3. Different configurations for electrical coupling of the PEC and PV cells in PEC-PV tandem cells. (a) Seriesconnected cells; (b) The power from the PV cell is divided between the PEC cell and another parallel load, with a power management unit (PMU) tracking the maximum power point and adjusting the output voltage to match the voltage requirement of the PEC cell. Adapted from Ref. 3. 1.3. Series resistance loss in large-area hybrid PEC-PV modules The non-negligible sheet resistance of the FTO transparent electrode that collects the current from the photoelectrode in the PEC cell, which is typically in the range of 10 to 15 /square, gives rise to a series resistance loss that shows up as a V = IR (V voltage, I current, and R resistance) voltage loss that adds up to the polarization overpotential losses of the electrodes. The IR loss is proportional to the current, therefore the larger the cell area the larger the current that is drawn from it and the larger the IR loss becomes. According to simulations presented in the Deliverable 2.2 report, the IR loss becomes significant for photoelectrodes larger than ~1 cm 2. In order to rectify this loss, a current collector metal grid must be integrated onto the FTO transparent electrode. This is especially critical for large-area devices that produce large currents. We recommend metal grid with 1 cm spacing. 2. SUMMARY In summary, for large-area devices we recommend the stacked PEC-PV tandem cell configuration, with a power management unit (DC/DC convertor and maximum power point tracker) and a parallel load sink for excess power from the PV cell. Page 4
Additionally, employing solar concentrators potentially improves system performance and allows a compact PEC/PV design. A metal grid current collector must be integrated onto the FTO transparent electrode of the PEC cell, with spacing of no more than 1 cm. 3. REFERENCES [1] Fatwa F. Abdi, Lihao Han, Arno H.M. Smets, Miro Zeman, Bernard Dam & Roel van de Krol, Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode, Nature Communications 4, 2195 (2013). [2] Gideon Segev, Hen Dotan, Kirtiman Deo Malviya, Asaf Kay, Matthew T. Mayer, Michael Grätzel & Avner Rothschild, High solar flux concentration water splitting with hematite ( -Fe2O3) photo-anodes, Advanced Energy Materials 6, 1500817 (2016). [3] Avner Rothschild & Hen Dotan, Beating the Efficiency of Photovoltaics-Powered Electrolysis with Tandem Cell Photoelectrolysis, ACS Energy Letters 2, 45 51 (2017). Page 5