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Public Summary

This EngD thesis evaluates the performance and implementation potential of the innovative ZIEZO window system, developed by a consortium led by TNO. The ZIEZO system integrates bifacial photovoltaic (PV) cells with smart shading technology to enhance building energy efficiency while maintaining indoor environmental quality. The study employs comprehensive Building Performance Simulation (BPS) to analyze the system's daylighting, thermal, and energy generation characteristics across various configurations and operational scenarios.

The ZIEZO technology addresses specific challenges in the built environment sector, particularly in dense urban areas where building envelope space for conventional renewable energy systems is limited. It offers a practical solution for mid-to-high-rise commercial buildings seeking to meet increasingly stringent energy performance regulations while maintaining occupant comfort and reducing operational carbon emissions. This innovation is particularly relevant for regions with moderate-to-high solar radiation and buildings with significant cooling loads or stringent daylighting requirements.

The system features bifacial crystalline silicon solar cell strips and embedded Venetian blinds that reflect sunlight to the rear side of the PV cells, boosting energy generation by up to 25% on sunny days. The system targets commercial buildings undergoing new construction or major retrofits, offering a dual-purpose solution for energy generation and shading.

The design objectives were to develop an integrated simulation framework to assess the performance of ZIEZO windows at the building level, evaluate performance trade-offs through parametric analysis, generate a comprehensive performance database, and develop adaptive control strategies.

Building Performance Simulation (BPS) played a crucial role in this design, allowing for a detailed evaluation of the ZIEZO product's performance under various real-world conditions before physical implementation. BPS provides insights that component-level assessments cannot deliver, capturing the complex interactions between building systems and their overall impact on occupant comfort and building energy performance.

The simulation model was developed to analyze room-level performance, integrating daylighting, energy, and PV performance. This approach was essential to understand how different PV window configurations influence daylighting and indoor climate under various scenarios. The simulation results were thoroughly analyzed to verify accuracy before compiling a comprehensive performance database. This database captures the interdependencies between daylight availability, energy performance, and PV electricity generation across various ZIEZO configurations and operational modes.

Key findings from the simulations include:

• Daylighting Performance: ZIEZO windows show reduced daylighting compared to non-PV windows due to shading from PV cells. However, dynamic control strategies can slightly improve this.
• Energy Performance: The system requires slightly more energy for heating, cooling, and lighting due to reduced daylight harvesting, but in many configurations, these losses are offset by the energy generated by the PV cells.
• Bifacial PV Electricity Generation: The system achieves optimal performance in south-facing rooms with a 50% window-to-wall ratio, where solar exposure is maximized.

Two control strategies were evaluated: irradiance-based control and ISO/DIS 52016-3. The irradiance-based control strategy, which I developed, simplifies installation and operation, performing better in daylighting and comparably or better in reducing cooling demand. The ISO/DIS 52016-3 strategy, while more complex, offers better energy efficiency in some scenarios.

Recommendations for future development include enhancing transparency, exploring semitransparent PV technologies, quantifying CO2 emission reductions, developing new control strategies, and testing across diverse locations.

In conclusion, the ZIEZO window system represents a significant step towards integrating renewable energy into building envelopes while maintaining or enhancing indoor environmental quality. The study provides a foundation for further development, focusing on optimizing daylighting, energy efficiency, and user comfort. The simulation-based approach was instrumental in understanding the performance of this innovative product, highlighting the importance of BPS in the technological design process.

16 April, 2025

Co-funded by: RVO - HER+ project: ZIEZO