Optical and Thermal Analysis and Optimization of Hybrid Solar Roof Tiles

Building-integrated photovoltaic and solar thermal systems have great potential to save greenhouse gas emissions through improved area utilization of building envelopes while replacing building components and energy transfer losses. In addition, the application of hybrid technology to these modules enhances their overall performance. Despite these advantages, the investment costs are higher, there are no suitable building codes, and there is a lack of solutions to application-specific problems. In order to address these issues, an improved module design is presented. The design has the characteristics of a roof tile and meets application-specific requirements such as durability, accessibility, easy installation, replacement and maintenance, as well as high efficiency & aesthetics. Nevertheless, predictability of the energy yields, operation performance and effective heat utilization are also decisive factors for the commercialization of these systems. Another point to consider is the application of colored glasses, which can improve the aesthetic appearance and thus the acceptance of building-integrated solar systems. In order to contribute to these factors with regard to the improved module design presented, research gaps on three topics were identified through an analysis of the current state of this technology. These topics are: i) modeling and analysis of custom module designs; ii) identification and quantification of the optimization potential to maximize operational performance; and iii) experimental and numerical performance assessment of improved module designs.
This thesis focuses on optical and thermal analysis as well as optimization of an improved building-integrated photovoltaic and solar thermal module design with active rear ventilation. A simulation framework with suitable models to analyze optimization potential and predict the overall performance under real operating conditions is developed and validated by experimental field measurements. There-fore, optical, electrical and thermal models are set up and parameterized by small-scale laboratory measurements and simulations. In addition, a modeling approach using angular-dependent spectral responsivity to simulate the performance of colored glasses is developed and validated. Another focus of the work is to identify and quantify the optical and thermal optimization potential. Hence, angular-dependent optical parameters of nano-imprinted textured front covers are measured and used to quantify the annual increase in optical performance in a simulation study. Likewise, numerical simulations are used to determine the optimal mass flow rate as a function of system leakage using the effective thermal efficiency approach. The performance of a system with the improved module design is assessed by measurements in an experimental field test over a one-year period. In addition, annual performance is evaluated by a simulation case study with different climatic conditions and system configurations using the validated modeling framework. Finally, an air-source heat pump to utilize the preheated air was used in the field test and modeled in the simulation study.
A summary of the key findings related to the research topics is presented as follows: A time-series based simulation framework for the improved module design with a prediction accuracy of the electrical and thermal energy yield with a mean relative error of less than ±1% is developed. The angular-dependent spectral responsivity performance modeling approach for high-transmissive colored glasses shows a root mean square error between 1.9% and 2.5%. The improvements by optical optimization result in a maximum annual relative increase in optical performance of 2.2% for nano-imprinted textures. In contrast, the thermal optimizations revealed a maximum effective thermal efficiency of 24.3% for a module design with air leakage. Thirdly, the performance assessment of the experimental system shows a maximum thermal efficiency of 12%. In comparison, the simulated annual thermal efficiency for Cologne shows 11%, while the overall efficiency taking into account the energy for the rear ventilation system is 23%. Also, the field test revealed a relative increase in performance ratio of up to 16% in comparison to systems without active rear ventilation. The experimental measurement series showed a relative increase in the coefficient of performance of 20%on average, while the simulated maximum seasonal performance factor increased by 21%. Finally, the maximum module temperature was observed at 74°C. The maximum air outlet temperature in the rear ventilation system is 52°C in summer and 33°C in winter.
In conclusion, the accuracy of the developed simulation framework is acceptable for assessing the performance of the improved module design. The presented modeling approach is highly flexible and can therefore be adapted to custom module and system designs. The angle-dependent spectral responsivity simulation model has the greatest value for predicting the outdoor performance of colored glass fronts. Furthermore, it can be stated that the angular-dependent optical performance optimization potential is comparatively low, while the thermal optimization potential is high when considering various system design parameters such as mass flow rate and air leakage. The performance assessment revealed a significant improvement in the electrical performance, while the observed thermal performance is comparatively low. The operating temperatures of the modules are in an acceptable range, and therefore deterioration of performance is not expected. In summary, the observations in this work show that the module design studied has the potential to lower the levelized cost of electricity in addition to solving application-specific problems, paving the way for a standard building component.