Integrated combination of concentrating solar thermal technologies and photovoltaics - the bifacial PV-Mirror

Kurzfassung

Concentrating solar thermal (CST) technologies produce renewable and sustainable heat up to high temperature levels. In this work four concepts are compared to integrate photovoltaic (PV) cells into CST systems to increase efficiency and decrease cost in comparison to stand-alone CST and PV systems. One of them - the bifacial PV-Mirror - is being investigated experimentally. The concepts are summarized in Figure 1. Fig. 1: Schematic overview of the analyzed concepts of combining CST and PV. In the first concept PV cells are mounted on the back of the solar mirrors (Rear-PV) to convert the diffuse radiation hitting the back of the mirrors or to be able to turn the mirrors around (if the design allows so) and produce electricity when no CST operation is possible. In the second concept, PV modules with spectrally selective mirrors on top are used to replace the solar mirrors (PV-Mirror). The spectrally selective mirror is used to split the incident spectrum between the PV and the CST part. The third concept is a PV-Mirror utilizing bifacial PV cells (Bifacial PV-Mirror). In the last concept the spillage of solar towers is utilized by high-flux PV cells (Spillage-CPV). Parameters are derived to quantify the energy production and investment cost of the concepts based on existing research. It is found that Rear-PV, PV-Mirror, bifacial PV-Mirror and Spillage-CPV can increase the overall energy produced by a CSP power plant by up to 23%, 29%, 40% and 36%, respectively (Figure 2). Fig. 2: Energy production of the different concepts compared to a reference CSP plant. Image taken from [1], label changed The Rear-PV, the PV-Mirror and the bifacial PV-Mirror might cost 3.0 times, 4.8 times and 5.7 times as much as the conventional mirrors per aperture area for a break-even of investment costs. If it is feasible to manufacture these components below the break-even price, the integrated CST-PV technologies will also have financial benefits compared to the stand-alone technologies. This is estimated to be achievable in the case of the normal and the bifacial PV-Mirror but not for the Rear-PV. The Spillage-CPV can produce electricity at lower investment costs than stand-alone PV if the spillage radiation flux is higher than ~350 kW/m2 at peak. Of the concepts the bifacial PV-Mirror was chosen for further investigations as it seemed to be the most promising candidate. Prototypes are being produced utilizing magnetron sputtering techniques. Several spectrally selective coating stacks are being deposited, determining the ratio of radiation reflected to the CST receiver and transmitted to the underlying PV cell. Commercial PV cells are used. The prototypes are tested under outdoor conditions in desert environment. The PV cells are operated in their maximum power point and the I-V-curves of the cells and their temperature are measured. The Pt100 temperature sensors are directly laminated into the modules. The goal of this is to eventually be able to validate models for PV under a spectrally selective mirror. References [1] M. Ruhwedel, K. Gehrke, E. Lüpfert, F. Sutter, P. Heller, R. Pitz-Paal „Integrated Concentrating Solar/Photovoltaic Hybrid Concepts-Technological Discussion”, Energy Technol., 2024, 2301181. https://doi.org/10.1002/ente.202301181, © 2024 Deutsches Zentrum für Luft- und Raumfahrt. Energy Technology published by Wiley-VCH GmbH under CC BY 4.0 DEED license, material may be freely shared and adapted if appropriate credit is given