Development of a Concentrating Hybrid Solar Collector based on the Approach of Spectral Splitting: Concept Development, Modelling, Material Research, Prototyping and Performance Validation

The necessity of transforming the global energy supply from fossil into renewable sources is undisputed. As half of the worldwide final energy consumption is requested thermally, this enormous challenge of energy transition can only be successful, if the heat demand can be covered by emission-free technologies. This becomes even more difficult for the industrial sector, where process heat at high temperatures is requested. On the other hand, continual electrification can be observed, leading to an increasing electricity demand. The conversion of solar radiation into useful energy by the available solar technologies photovoltaics (PV) and solar thermal systems (ST) is one of the key measures to contribute to a more sustainable energy supply. The combination of both solar technologies in one component by photovoltaic-thermal (PVT) collectors can be advantageous depending on the application, however, the provision of heat at temperatures above 85°C is difficult with such hybrid systems so far.
The research project described by the present thesis aimed to develop a concentrating PVT collector (CPVT) capable of providing industrial solar heat and electricity simultaneously. The basis was a linear Fresnel concentrator with a mirror surface of 11 m², available at FH OOE in Wels, Austria. The main goal of the project was to develop a CPVT receiver suitable for this mirror field. The approach of “Spectral Splitting” was chosen for tackling the fundamental
discrepancy in such hybrid systems, that the thermal and the electrical part have contradictory temperature requirements. Based on the state-of-the-art in this research field, several novel receiver designs were conceived. The most promising design proposal was utilized as the basis for a comprehensive modelling phase, which revealed an optical model for the Fresnel concentrator, an electrical model for optimizing the Spectral Splitting configuration, and a thermal model for predicting the thermal performance of the CPVT collector. All models were developed in a numeric approach using MATLAB™. Beside the conducted theoretical investigations, experimental feasibility was a major requirement throughout the entire project. Therefore, material research was needed with a special focus on the heat transfer fluid, which also serves as the liquid part of the optical filter necessary for realizing the concept of Spectral Splitting. Subsequently, the developed CPVT receiver was implemented as a prototype and installed on the Fresnel mirror field for conducting performance measurements. The experiments confirmed the capability of the novel CPVT collector to provide industrial solar heat and electricity simultaneously. The thermal performance showed a distinct progress compared to the state-ofthe-
art, as a thermal efficiency of 33.28 % could be achieved at a fluid temperature of 150°C. The electrical part of the receiver reached a conversion efficiency of 1.73 %, which is expected to be improved by PV modules adapted for the concentrated radiation, as a part of continuing research activities with the developed CPVT system. Although the fluid temperature was 150°C, the temperature of the PV modules could be limited to 47.3°C in the best receiver configuration. This confirms the effectiveness of the novel approach for thermal decoupling, which can be seen as another important contribution for the further development of CPVT collectors.