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Smart Window Technology to Boost Energy Efficiency

Perovskite‐based solar cells have been at the centre of scientific interest for several years due to their low cost and high photovoltaic (PV) performance. In addition to their success in the PV sector, there have been growing efforts to employ perovskites in energy‐efficient technologies, such as smart windows and other building technologies because of their large absorption coefficient and colour tunability. Smart windows have the ability to convert sunlight into electricity, thus helping to create an energy efficient environment and enabling sustainable energy production. The major challenge, however, lies in integrating perovskite materials into windows and building facades and combining them with additional functionalities, while maintaining their power conversion efficiencies.

Now (2020), scientists at the Argonne National Laboratory have developed a solar cell technology which uses an optimisation algorithm to create a smart window prototype whose design covers wide range of criteria.The optimization algorithm is made up of comprehensive physical models and advanced computational techniques to increase overall energy usage while taking into account building temperature and lighting requirements in different regions and throughout changing seasons. This technique is called multicriteria optimisation and adapts the thickness of solar cell layers in the window design to cater to the needs of the user. If, for example, the window design is aimed at reducing the energy required to cool a building in the summer, it should minimize the amount and type of light passing through while providing the desired luminosity inside. On the other hand, if the window is required to save energy in winter, the design should let as much sunlight as possible through, which also reduces the energy required for heating the building. To determine the optimal design, the algorithm includes comprehensive physics-based models of the interactions between light and the materials in the smart window, as well as how the processes affect energy conversion and light transmission. The algorithm also takes into account the different angles at which the sunlight hits the window throughout the day in different seasons and different geographical locations. To demonstrate the feasibility of this highly customised smart window, the scientists designed a small prototype of the window with an area of a few square centimetres.The prototype was made up of dozens of layers of varying materials that controlled the amount and frequency of the light passing through as well as the amount of solar energy converted into electricity. One group of layers consisting of a perovskite included the solar cell which collected sunlight for energy conversion. The window prototype also comprised a set of layers called a nanophotonic coating. There, each layer was only tens of microns thick. The window was built using an aperiodic design which consisted of layers of various thickness and enabledthe scientists to accommodate the performance of the window to the user’s preferences.

Scientists have long been trying to find ways to efficiently harness the sunlight that hits surfaces, especially windows, so as to create more environmentally-friendly closed-loop energy systems. In 2011, scientists developed a solution type photovoltaic electrochromic (PV-EC) device. The device consisted of a semi-transparent silicon thin-film solar cell (Si-TFSC) substrate, an electrochromic solution, and a transparent non-conductive substrate. The electrochromic solution was placed between the transparent non-conductive substrate and the Si-TFSC substrate. When sunlight hit the surface, a portion of electronic current produced by a Si-TFSC was converted into ionic current and caused colour changing of the PV-EC device, while the integrated Si-TFSC module generated electricity to a connected load. The PV-EC device could both function as solar cell module and as self-powered smart glass, which has great advantages in green energy application.

In 2017, scientists designed a solar window which enabled an average of 68% of light in the visible portion of the solar spectrum to pass through when in a transparent or bleached state. When the window darkened, it generated electricity. The colour change was caused by molecules (methylamine) that were reversibly absorbed into the device. This technology bypassed the trade-off between solar cell and window because it reconciled both qualities. The only problem that remained was that in testing under 1-sun illumination, the 1-square-centimeter demonstration device cycled through repeated transparent-tinted cycles, but the performance was witnessed to decline over the course of 20 cycles due to restructuring of the switchable layer.

There are several advantages to the new window design: one of the most important is that the window design is customisable and can be integrated into any building. ​It is possible to adapt the algorithm to user needs and preferences because it can regulate the sunlight in a room to provide the desired luminosity while managing the amount of energy in the building used for heating and cooling. Also, the sunlight that does not pass through is captured by the solar cell in the smart window and converted into electricity. The computer model makes it possible to create millions of unique designs because the algorithm uses computational mechanisms that resemble reproduction and genetic mutation to make out the optimal combination of each design parameter for a certain scenario.

The scientists are convinced that the new smart window will open up new possibilities for energy-efficient architectural engineering because the synthesis methods they used to produce the window prototype were tailored to common industrial-level manufacturing processes. This allows for simple scaling ofthe window prototype to full size by existing commercial processes.