Researchers at the City University of Hong Kong have revealed a major thermochromic bifacial photovoltaic (TC-BiPV) glazing system that combines hydrogel-based thermochromic layers with bifacial PV modules. This system dynamically regulates solar transmittance and simultaneously harvests solar irradiation from both sides, aiming to reduce energy consumption, costs, and emissions in buildings.

How the System Works

The system features a bifacial PV glass pane, an air gap, and a hydrogel glass pane, arranged from exterior to interior. The BiPV glass contains PV cells sandwiched between two clear glass panes, while the hydrogel glass encloses a thermochromic hydrogel layer between two glass panes. The hydrogel transitions from transparent to translucent as temperature rises, reducing solar gains while maintaining visual and thermal comfort.

Below its transition temperature, the hydrogel is transparent, allowing solar irradiation for indoor lighting. Above the transition temperature, it becomes translucent, reducing solar gains. In its hot state, the hydrogel reflects light toward the rear side of the BiPV glass, enhancing rear-side electricity generation. This design effectively captures energy that would otherwise be wasted, improving overall system efficiency.

The system also incorporates spectral selectivity, ensuring the front BiPV glass receives the full solar spectrum, while the rear-side irradiation is concentrated within the PV response range. This lowers cell temperature and improves efficiency, as the hydrogel state responds to outdoor temperature, solar irradiation, and incidence angle, linking system performance to orientation and climate conditions.

Performance and Benefits

The prototype was fabricated with BiPV cells arranged in a 6×6 matrix with around 45% coverage and a 1 mm-thick hydrogel layer sealed between glass panes. The assembly includes a 5 cm air gap for wiring, mounting, and independent replacement of hydrogel and PV glass, facilitating easier maintenance.

Lead author Chin Yan Tso shared results from prototype experiments, stating that on a summer test day, the TC-BiPV glazing reduced direct solar heat gain by around 30% compared with thermochromic glazing alone, lowering the test-box air temperature by up to 4.8°C. Compared with conventional bifacial PV glazing, TC-BiPV reduced direct solar heat gain by about 62.6%, produced test-box temperature reductions up to 15.1°C, and increased electricity generation by approximately 16.5%.

Annual simulations across tropical locations indicated that the TC-BiPV bifacial gain ranges from 9-18% for skylights and 6-14% for vertical windows, versus 4-5% and 5-7% for BiPV. The analysis also showed that for skylight installations, TC-BiPV reduces annual indoor heat gain by 27.7% relative to BiPV and 38.4% relative to TC glazing; for façade windows, the reductions are 9.1% and 40.1%, respectively.

Why It Matters

The TC-BiPV system offers a scalable, passive pathway to reduce cooling loads while enhancing on-site PV generation, with practical promise for energy-efficient building envelopes in warm climates. According to Tso, the system is designed to be a practical solution for reducing energy consumption in buildings, particularly in tropical regions where cooling demands are high.

Traditional thermochromic materials such as vanadium dioxide (VO₂) and perovskites face limitations including high transition temperatures, toxicity, and challenges in large-scale fabrication. Hydrogel-based TC glazing, on the other hand, offers full-spectrum modulation, low cost, and scalability, making it more practical for real-world applications.

The integration of both thermochromic and bifacial PV functions into a single system is highly desirable for advanced glazing applications. Previous hybrid solutions, such as PV blinds or tracking PV modules, relied on manual or mechanical adjustments, increasing operational complexity and cost. The TC-BiPV system overcomes these limitations by combining passive and active solar technologies in a single, efficient design.

The system was described in an article titled “Experimental and numerical study of a novel thermochromic bifacial photovoltaic glazing system,” published in the journal Building and Environment. The research highlights the potential of the TC-BiPV system to revolutionize building energy efficiency by reducing reliance on conventional cooling systems and increasing the use of renewable energy.

The study also identified key design levers, including PV coverage ratio and hydrogel transition temperature, which can be adjusted to improve system performance based on specific building needs and climate conditions. This flexibility makes the system adaptable to a wide range of applications, from residential buildings to commercial structures.

As the global demand for energy-efficient building solutions continues to grow, the TC-BiPV system represents a promising innovation in sustainable architecture. With further development and testing, the system could become a standard feature in buildings aiming to reduce their carbon footprint and lower energy costs.