Chinese Scientists Invent Photovoltaic Window with Heat Flow Control

A group of scientists from China has developed a new smart photovoltaic (SPV) window that generates electricity while calibrating the amount of solar radiation entering a building.

“Our study presents a special-purpose window and its operation control strategy to simultaneously improve building energy efficiency and grid friendliness,” said corresponding author Yutong Tan. “The results of the study show that the SPV window with a heat flow control strategy significantly reduces the excessive daylight-to-light ratio, peak load, average daily peak-to-valley difference, and annual net energy consumption compared with conventional low emissivity (Low-E) windows.”

The system combines crystalline silicon cells and electrochromic films. The proposed photochromic device (PECD) combines a photovoltaic function, which converts sunlight into energy, and an electrochromic (EC) function, which changes the transparency of the window and measures the solar radiation hitting the building.

The window consists of a transparent glass cover, a functional layer, a transparent glass substrate, an argon gas chamber, and a sheet of Low-E glass.The EC film is sandwiched between the second and third surfaces of the glass, and the Low-E coating is deposited on the fifth surface. The area under effective daylight was equipped with a 3-mm-thin strip of 11.6% efficient crystalline silicon.

The research team explains, “The EC film is sandwiched between two transparent substrates with five layers, including two transparent electrodes, an ion storage layer, an ion conduction layer, and an electrochromic layer.” When no voltage was applied, the EC film was in a bleached state with maximum visible transmittance. When voltage is applied to the EC membrane, lithium ions from the ion storage layer rush to the electrochromic layer, and the tungsten trioxide in the electrochromic layer begins to color upon encountering the lithium ions. The higher the voltage applied, the darker the color of the electrochromic film, resulting in a lower visible light transmission rate.

The structure was built in a Window program and then exported to the building simulation software EnergyPlus. The dimensions of the building are 50.0 meters long, 4.6 meters deep, and 2.7 meters high, with a window-to-wall ratio of 77%. The buildings were simulated in the cities of Fuzhou, Xiamen, Hong Kong and Haikou during the cooling season from May to October. The monthly average hourly solar radiation maxima were 482 W/m2, 444 W/m2, 468 W/m2 and 534 W/m2, respectively.

Two control strategies were created for the proposed smart window, i.e., solar radiation control (CtrlRad) and heat flow control (CtrlFlux). In CtrlRad, the coloring state varies according to the incident solar radiation threshold, while in CtrlFlux, the coloring state varies according to the density of heat flow through the window. In addition, a normal Low-E window is simulated as a reference.

The academics' analysis showed that the SPV window with the CtrlFlux control strategy was able to achieve reductions in excessive insolation ratio, peak load, average daily peak-to-valley difference, and annual net energy consumption of 81.6% to 93.1%, 49.3% to 54.5%, 54.7% to 65.8%, and 49.1% to 69.2%, respectively, when compared to the Low-E system. In the case of the CtrlRad control strategy, the reductions were from 92.1% to 96.6%, 50.9% to 57.3%, 44.0% to 54.2%, and 44.0% to 54.2%, respectively, when compared to the Low-E system.

“Considering effective daylight utilization, peak load, peak-to-valley difference, and net energy consumption, the average overall performance improvement of SPV windows with heat flow control strategies is 55.5%,” the team concluded. Dr. Chen added, “In the future, we will also investigate how to create indoor daylight and thermal environments using smart PV windows.”

The system was published in the article “Evaluating the energy efficiency and grid-friendliness of smart photovoltaic windows combining crystalline silicon cells and electrochromic thin films” in the journal Applied energy. The study was conducted by scientists from Hunan University in China and the Key Laboratory of Building Safety and Energy Conservation of the Ministry of Education of China.