Review: Integrated Roof Systems for Green Building Energy Efficiency
A critical review published in the journal City and Built Environment in August 2025 examines the potential of integrating rainwater harvesting with photovoltaic-thermal systems on building rooftops. The study, led by Handan Fan and colleagues, analyzes how this coupled approach, termed the Rainwater Photovoltaic-thermal System (RPS), can address specific inefficiencies inherent in separate green building technologies. The primary objective of the review is to determine whether installing these coupled systems is more energy-efficient than maintaining photovoltaic-thermal (PV/T) and rainwater harvesting (RWH) systems independently. The authors conclude that the integration significantly contributes to improving building energy efficiency, reducing overall consumption, and advancing the development of sustainable green buildings.
Green buildings are designed to reduce reliance on conventional energy sources and lower carbon emissions by harnessing renewable resources such as solar and wind energy. Despite these goals, persistent challenges remain in the sector. Building energy consumption accounts for 49.3% of a city’s total energy use, with energy systems alone responsible for over 30%. A major technical hurdle for PV/T systems is the overheating of photovoltaic components, which reduces conversion efficiency. During peak sunlight hours, photovoltaic modules can experience an operating temperature increase of 10 to 15 degrees Celsius compared to ambient conditions.
Simultaneously, rainwater harvesting systems face distinct operational challenges, particularly regarding water supply and demand mismatches. While a typical RWH system can intercept 85% to 92% of roof runoff and achieve a rainwater utilization rate of 65% to 78%, these systems often depend on external power supplies for water pumps. This dependency increases energy consumption and operational costs. Furthermore, both PV/T and RWH systems compete for the limited space available on green roofs, a constraint that is particularly acute for small and medium-sized buildings. The review identifies this spatial competition as a key factor impeding the further enhancement of building energy efficiency in urban environments.
The proposed RPS system integrates these two technologies to create a mutually beneficial infrastructure. In this configuration, rainwater pipes are designed to circulate through the PV/T modules, serving a dual purpose. First, the water cools the photovoltaic panels, effectively addressing the overheating issue that degrades electrical performance. Second, the electricity generated by the photovoltaic panels powers the energy-intensive components of the rainwater system, such as pumps and purification equipment. This integration allows for the mutual utilization of resources, where the specific inclination angle of the photovoltaic panels is optimized to enhance rainwater harvesting efficiency while maintaining optimal solar energy capture.
Experimental data cited in the review supports the efficacy of water-cooling mechanisms for photovoltaic panels. One study demonstrated that channel-based water cooling reduced the average surface temperature of photovoltaic panels by 9.7 degrees Celsius and the back temperature by 14.5 degrees Celsius. Another comparison between monocrystalline and polycrystalline silicon panels showed that water cooling resulted in an average temperature reduction of 13.6% and an average power generation efficiency increase of 13% for monocrystalline silicon. The review notes that active cooling systems, which create dual circulations above and below the panels, can achieve electrical efficiencies of up to 22%, compared to 15.5% for passive cooling systems. These temperature reductions directly correlate with improved thermoelectric conversion efficiency and extended component lifespan.
The RPS system also incorporates advanced materials and design features to maximize performance. The review describes a modular design that includes perovskite tandem photovoltaic panels with double-glass encapsulation, achieving a conversion efficiency of at least 28%. These panels are paired with copper-aluminum composite heat-collection tubes that utilize aluminum oxide-based nanofluids to improve thermal conductivity by 40% over traditional fluids. The surface of the photovoltaic panels features a hydrophobic coating with a contact angle greater than 150 degrees, which boosts photoelectric efficiency and enhances rainwater runoff. This design ensures that the panels remain clean and that water is efficiently channeled into the collection network, achieving a runoff coefficient of 0.85 to 0.92.
Water management within the RPS system is designed to handle both cooling and domestic needs. Collected rainwater is filtered through a multi-layer system comprising coarse filters, activated carbon, and UV disinfection, achieving over 90% harvesting efficiency. The water is then split via a diverter valve into two streams. One stream is directed to non-potable building uses, such as toilet flushing and irrigation, reducing reliance on municipal water supplies. The other stream is used to cool the photovoltaic panels through a micro-channel cooler. This process reduces panel temperature by 8 to 12 degrees Celsius, which in turn boosts power generation efficiency by 7.2% to 14.5%. The cooled rainwater, having absorbed heat and increased in temperature by 5 to 8 degrees Celsius, returns to the heat-collection systems, creating a positive feedback loop that enhances thermal collection efficiency.
Energy storage is a critical component of the RPS architecture, ensuring stability in power and heat supply. The system utilizes lithium-ion battery packs with a cycle efficiency greater than 95% and phase-change thermal-storage tanks with a thermal-storage density of at least 180 megajoules per cubic meter. These storage devices facilitate multi-energy complementation, allowing the system to prioritize selling power to the grid on sunny days and switching to storage-powered mode during rainy periods. The thermal storage material, often a wax-based composite, stores photovoltaic waste heat for winter frost prevention and domestic hot water supply. This integration allows the system to supply domestic hot water to the building, with experiments verifying that such configurations can increase the overall comprehensive efficiency of the system to an average of 86.6%.
Despite these technical advantages, the widespread adoption of RPS systems faces significant barriers. The initial investment for RPS systems is higher than that of traditional photovoltaic solutions, with comprehensive costs estimated to be 1.5 to 2 times that of independent photovoltaic systems. The investment payback period is generally between 6 and 9 years, which is longer than the 4 to 6 years typical for standalone photovoltaic systems. Additionally, the technical integration is complex, requiring precise matching between the tilt angle of the photovoltaic panels, recommended to be between 15 and 30 degrees, and the design of the rainwater tanks. Mismatches in these parameters can lead to a decrease in water collection efficiency by 30% to 50%. In low-temperature environments, there is also a conflict between the need for anti-freezing measures for rainwater and the efficient operation of the solar thermal system, which can increase material costs by approximately 12%.
Institutional and standardization challenges further hinder the promotion of RPS systems. Unified technical standards for these integrated systems have not yet been established, leading to inconsistencies in component selection and installation procedures. Existing green building evaluation standards, such as LEED and GB/T 50378, do not currently include water-energy coupling efficiency as a core indicator, making it difficult to fully reflect the comprehensive value of RPS systems. The review highlights that the penetration of distributed photovoltaics in buildings is currently less than 15%, which restricts the large-scale application of RPS. Future research is recommended to focus on constructing thermodynamic models to optimize system parameters and conducting pilot projects in multiple climate zones to verify applicability and accumulate practical experience.
The review emphasizes that the RPS system represents a cutting-edge approach to green building innovation by doubling the spatial value of the roof through three-dimensional design. The tilt angle of the photovoltaic array is optimized to serve as a rainwater diversion structure, while the hydrophobic coating reduces dust accumulation. This dual benefit improves photovoltaic power generation efficiency and optimizes the rainwater recovery rate. By reducing reliance on municipal water supply and providing energy support for the building, the system offers a structural change in life cycle benefits. The integration of photovoltaic power generation, solar thermal conversion, and rainwater harvesting allows for the frequency-divided utilization of the solar spectrum and the temporal-spatial allocation of rainwater resources.
Looking forward, the RPS system has the potential to increase a building’s energy self-sufficiency by 40% to 60%, reduce the urban heat island effect, and ease the burden on drainage systems. The review suggests that digital twin technology could be used to build multi-objective optimization models for photovoltaic array tilt, rainwater harvesting efficiency, and thermal output power. These models could verify optimal configuration schemes for different climate zones, including humid, arid, and transitional regions. The authors recommend that future efforts should identify the optimal coupling ratio of the system, improve overall operation technology, and develop market promotion strategies. By addressing the technical and economic challenges, the RPS system offers a reliable solution for sustainable building development and a solid data foundation for the promotion of roof PV/T rainwater integrated systems.
Subject of Research: Prospective of roof rainwater photovoltaic-thermal systems for combined cooling, heating, and power generation in green buildings: a critical review
Article Title: Prospective of roof rainwater photovoltaic-thermal systems for combined cooling, heating, and power generation in green buildings: a critical review
Article References: Fan, H., Chen, W., Wei, X., Liu, Z., Chang, Z., Li, Q., & Wang, J. (2025). Prospective of roof rainwater photovoltaic-thermal systems for combined cooling, heating, and power generation in green buildings: a critical review. City and Built Environment, 3(1), Article 15. https://doi.org/10.1007/s44213-025-00055-6
Image Credits: AI Generated
DOI: 10.1007/s44213-025-00055-6
Keywords: Prospective, roof, rainwater, photovoltaic-thermal, systems, combined, cooling, heating, power, generation, green, buildings
News Source: Denise Maddox. (November 12, 2026). Review: Integrated Roof Systems for Green Building Energy Efficiency. Scienmag.
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