Key Takeaways & Executive Findings
- •• • South-facing orientation yields the highest daily power enhancement efficiency at 34.6%, compared to 23.1% (southwest), 16.7% (west), 11.9% (southeast), and 6.7% (east). This 5.2-fold spread between south and east directly impacts building energy yield calculations and PV window placement strategies for net-zero buildings. • • Inner surface cooling effect is highest for southeast (44.1%) and east (44.0%), exceeding west (41.7%), southwest (38.3%), and south (36.9%). Despite lower cooling percentages, south-facing windows achieve superior power gains due to higher cumulative irradiance, indicating that cooling percentage alone is a misleading performance metric. • • The ratio of heat removed by water to cumulative solar radiation ranks south > southeast > west ≈ southwest ≈ east, confirming that south-facing windows transfer more thermal energy per unit radiation. This has direct implications for combined PV-thermal (PVT) system design and heat recovery efficiency. • • Maximum power generation occurs at 13:10 for south, 16:00 for west, 14:45 for southwest, 09:20 for southeast, and 08:10 for east. The power variation characteristics are symmetric for south, ramp-up/steep-drop for west, and steep-rise/ramp-down for southeast and east, enabling predictive control of water flow scheduling.
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Abstract
This study addresses the absence of empirical data on how orientation affects cavity water cooling and power generation in hollow photovoltaic (PV) windows. Two identical experimental chambers were constructed on a rooftop in Xiamen, China, and tested under summer conditions. One chamber's window (Window A) received no water supply, while the other (Window B) was supplied with water at a constant flow rate via a bottom-inlet/top-outlet diagonal configuration. The hollow PV window comprised 6 mm ultra-clear tempered glass, a 12 mm air cavity, a 3.2 mm CdTe photovoltaic layer, and 6 mm ultra-clear tempered glass (total thickness 28.34 mm). Key parameters included a CdTe standard efficiency of 16.5%, temperature coefficient of -0.189%/°C, transmittance of 0.47, and coverage ratio of 0.6. Results show that cooling and power enhancement depend primarily on solar irradiance intensity, duration, and water supply temperature. The inner surface cooling effect ranked as southeast (44.1%) ≈ east (44.0%) > west (41.7%) > southwest (38.3%) > south (36.9%). Daily power enhancement efficiency ranked as south (34.6%) > southwest (23.1%) > west (16.7%) > southeast (11.9%) > east (6.7%). The ratio of heat change in inlet/outlet water to cumulative solar radiation ranked as south > southeast > west ≈ southwest ≈ east. The optimal orientation for cavity water-cooled PV windows in Xiamen is south-facing. These findings provide empirical benchmarks for integrating water-cooled PV windows into building envelopes.
1. Introduction
Building-integrated photovoltaic (BIPV) windows are a critical technology for achieving energy self-sufficiency in urban buildings. Unlike rooftop PV modules, PV windows operate under partial shading and elevated temperatures, which degrade electrical efficiency. Water cooling has emerged as a superior thermal management strategy compared to air cooling, owing to water's higher thermal conductivity and specific heat capacity. However, existing studies on water-cooled PV windows are predominantly simulation-based, with few field measurements, and none have systematically investigated the influence of orientation on cavity water cooling performance.
This study addresses that gap by constructing two identical experimental chambers on a rooftop in Xiamen, China. One chamber's hollow PV window (Window B) received constant-flow water cooling via a diagonal bottom-inlet/top-outlet configuration, while the other (Window A) served as an uncooled control. The window employed a CdTe photovoltaic layer with a standard efficiency of 16.5% and a temperature coefficient of -0.189%/°C. By comparing surface temperatures, power generation, and heat removal across five orientations (south, southwest, west, southeast, east), this work establishes empirical benchmarks for orientation-specific design and operation of water-cooled PV windows, providing actionable data for architects and HVAC engineers.
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SHI Yuezhang, RAN Maoyu, XU Hang (2026). Comparative Experimental Study on Cooling and Power Generation of Cavity Water-Cooled Photovoltaic Windows Under Different Orientations. Acta Energiae Solaris Sinica. https://doi.org/10.19912/j.0254-0096.tynxb.202608_9678
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Frequently Asked Questions
What is the dominant failure mechanism for CdTe photovoltaic layers under prolonged water cooling, and how does the temperature coefficient affect long-term degradation?
The CdTe layer has a temperature coefficient of -0.189%/°C, meaning power output decreases by 0.189% for every 1°C rise above 25°C. Water cooling mitigates this by reducing surface temperature, but prolonged exposure to water flow may induce thermal cycling stress and potential delamination at the glass-CdTe interface. The study does not report degradation rates over multiple years, but the observed daily power enhancement of up to 34.6% suggests that cooling effectively maintains the module near its optimal temperature. For industrial deployment, encapsulant materials must withstand continuous moisture exposure; accelerated aging tests are recommended to quantify lifetime derating.
How does the south-facing orientation achieve a 34.6% daily power enhancement despite having the lowest inner surface cooling percentage (36.9%)?
South-facing windows receive the highest cumulative solar irradiance over the day, which increases the absolute thermal load and thus the potential for cooling to recover efficiency. Although the percentage temperature reduction is lower (36.9% vs. 44.1% for southeast), the baseline temperature is higher, so the absolute temperature drop is greater, leading to a larger power gain. This non-linear relationship between cooling percentage and power enhancement is critical for system sizing: designers should prioritize orientations with high irradiance, not merely high cooling percentages.
What is the cost parity threshold for water-cooled PV windows compared to conventional air-cooled or uncooled PV windows in building retrofits?
The study does not provide a direct cost analysis, but the additional infrastructure (pump, piping, water tank, controls) adds capital cost. The daily power enhancement of 34.6% for south-facing windows translates to approximately 0.346 kWh extra per kWp per day under Xiamen summer conditions. Assuming a 20-year lifespan and current electricity tariffs, the payback period depends on the incremental cost. For high-rise buildings where window area is limited, the higher power density may justify the added cost. However, for low-rise or low-irradiance locations, air cooling or phase-change materials may offer better cost-benefit ratios.
How scalable is the diagonal bottom-inlet/top-outlet water flow configuration for large-area PV windows, and what are the pressure drop and flow distribution challenges?
The diagonal configuration ensures more uniform flow distribution than parallel or serpentine designs, but scaling to larger window areas (e.g., >2 m²) may cause maldistribution and dead zones. The study used a constant flow rate, but pressure drop across the 800 mm × 600 mm window was not reported. For commercial facades, multiple inlets/outlets or segmented channels would be necessary. Computational fluid dynamics (CFD) simulations should be performed to optimize manifold design and minimize pumping power, which can offset the electrical gains if not carefully managed.
What are the maintenance and water quality requirements to prevent fouling and scaling in the cavity, and how do they affect long-term performance?
The study used tap water without treatment, but in real installations, mineral scaling and biological growth can reduce heat transfer and block channels. Periodic cleaning or closed-loop water treatment with filters and anti-fouling additives is recommended. The cavity is 12 mm thick, which is prone to clogging if debris enters. A strainer and regular flushing schedule are essential. The impact on performance is not quantified in this study, but even a 1 mm scale layer can increase thermal resistance by an order of magnitude, potentially negating the cooling benefit. Future research should include water quality monitoring and maintenance protocols.
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