Researchers from the City University of Hong Kong have developed a novel thermochromic bifacial photovoltaic (TC-BiPV) glazing system that integrates hydrogel-based thermochromic (TC) layers with bifacial PV modules. This system dynamically regulates solar transmittance and simultaneously harvests irradiation from both sides, while modeling and optimizing building energy use to reduce consumption, costs, and emissions.
TC technology enables buildings to self-regulate solar irradiation by altering their optical properties in response to temperature. Common TC materials include vanadium dioxide (VO₂), perovskites, and hydrogels; however, VO₂ and perovskites face limitations such as high transition temperatures, toxicity, and challenges in large-scale fabrication.
Hydrogel-based TC glazing is more practical, offering full-spectrum modulation, low cost, and scalability, the researchers explained. It transitions from transparent (hydrophilic) to translucent (hydrophobic) as temperature rises, reducing energy consumption while enhancing visual and thermal comfort. Nonetheless, in its hot state, hydrogel reflects a significant portion of solar irradiation, limiting energy utilization.
The bifacial design of the proposed TC-BiPV system addresses this limitation by capturing the solar irradiation reflected by the hydrogel in its hot state, effectively reducing energy waste.
The team also emphasized that while PV glazing alone efficiently harvests solar energy, it cannot adapt its optical properties, while hydrogel-based TC glazing can dynamically modulate solar transmission but fails to capture reflected energy. Integrating both functions into a single system is therefore 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 system consists of a bifacial PV (BiPV) glass pane, an air gap, and a hydrogel glass pane 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.
Image: City University of Hong Kong
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 the hot state, the hydrogel reflects light toward the rear side of the BiPV glass, enhancing rear-side electricity generation.
Spectral selectivity ensures the front BiPV glass receives the full solar spectrum, while the rear-side irradiation is concentrated within the PV response range, lowering cell temperature and improving efficiency. The hydrogel state responds to outdoor temperature, solar irradiation, and incidence angle, linking system performance to orientation and climate conditions.
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 for easier maintenance.
“We conducted prototype experiments and validated optical-thermal-electrical modelling show strong combined benefits,” the research's lead author, Chin Yan Tso, told pv magazine. “For example, we found 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.”
“We also found that, compared with conventional bifacial PV (BiPV) 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%,” he went on to say. “Annual simulations across tropical locations indicate 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.”
“Our 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. Sensitivity analyses identified PV coverage ratio and hydrogel transition temperature as key design levers,” Tso explained.
“The TC‑BiPV approach 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,” he concluded.
The system was described in “Experimental and numerical study of a novel thermochromic bifacial photovoltaic glazing system,” published in Building and Environment.
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