Ya, Dan
(2025)
Novel use of miniature CPC in photovoltaic/thermal and radiative cooling systems.
PhD thesis, University of Nottingham.
Abstract
The rapid development of modern society has significantly increased energy consumption, particularly in the building sector. More than 80% of the primary energy supply still relies on non-renewable sources such as fossil fuels, raising concerns about energy shortages and environmental pollution. To promote energy conservation and transition to a sustainable society, research and development of sustainable energy technologies is becoming increasingly imperative. Among various strategies, photovoltaic thermal (PVT) systems, which use solar energy (~5800K) to provide heat and electricity to buildings, and passive radiative sky cooling (RC) technology, which utilizes the cold outer space (~3K) to cool buildings, are seen as attractive approaches. However, RC technology currently faces the challenge of inherent low cooling power, limiting its ability to meet the cooling demands of buildings. Therefore, enhancing the cooling performance of RC technology is urgently needed. Additionally, standalone building-integrated PVT system and RC technology encounter challenges such as seasonal adaptability and single functionality. To effectively integrate PVT and RC systems within a limited building envelope, exploring new configurations is crucial. In this thesis, a concentrated RC system based on a compound parabolic concentrator (named CPC-RC module) is deeply studied and continuously optimizes its structure to enhance cooling performance. Additionally, a hybrid system integrating CPC-RC modules with PVT modules is proposed. This system can simultaneously collect solar energy and cold energy for heating and cooling buildings, aiming to reduce energy consumption and offer an innovative solution for promoting net-zero carbon buildings.
This thesis first proposes a modelling approach to accurately characterize the cooling performance of RC modules, addressing the issue in existing theoretical analyses that often overestimate of cooling power. Additionally, utilizing this approach evaluates the cooling performance of CPC-RC module on different typical days. Numerical analysis demonstrates that CPC-RC module exhibits excellent cooling performance in summer, with an average nighttime cooling power of 130.10 W/m2, which is 5.7% and 18.1% higher than that of inverted trapezoidal concentrator-based RC module and flat RC module, respectively. Experimental results also show that the temperature reduction capacity of CPC-RC modules is 30% greater than the flat RC modules. However, during the daytime, CPC-RC module cannot achieve self-cooling due to the solar concentration characteristic by the CPC. To address this issue, the mirror CPC structure in the CPC-RC module has been replaced with a CPC made of dissimilar materials, with one side transparent and the other mirrored. This new module is referred to as the DCPC-RC module, features a unique CPC structure that allows part of the solar radiation to pass through the transparent side, achieving excellent daytime cooling performance. Experiments have shown that the module can achieve a cooling effect of 0.95 °C below ambient temperature during the daytime. This new configuration provides a design concept for integrating RC modules with PVT modules by using a transparent infrared reflective film (TIRF) to direct solar radiation toward the PVT modules while effectively reducing the solar radiation reaching the RC emitter. This thesis further proposes and develops an innovative system that integrates PVT and RC modules based on a transparent CPC covered with TIRF (referred to as TIRF-TCPC-PVTRC system) to achieve simultaneous heating and cooling, thereby meeting the dynamic energy demand of the building throughout the year. The experimental results show that the TIRF-TCPC-PVTRC system achieves an 88% improvement in cooling capacity compared to the PVTRC module without the TCPC structure, demonstrating excellent cooling performance. However, due to the weighted average solar transmittance of the used TIRF only being 0.69, the conversion efficiency of the TIRF-TCPC-PVTRC module decreased by 36% compared to the PVTRC module. This novel system integrates two different energy collections modules into a single unit, maximizing the use of renewable energy from both the sun and outer space. This integrated approach is expected to address the challenges faced by individual RC and PVT modules, such as low efficiency and intermittent operation.
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