New Solar Window Designs Improve Efficiency and Visibility—But Mind the Ghosts
Integrating solar cells with buildings is increasingly important in densely populated areas. Architects and designers have long expanded their applications beyond standard rooftop power plants. Buildings with innovative, power-producing facades and windows have become flagships of modern architectures.
As in all devices incorporating solar cells, solar windows incorporate electrical contacts that generate power. Generally, this task is performed using metallic contact grids or transparent conductive oxides. The former leads to power losses through reflection of sunlight at the metal contact, and the latter results in power losses from unwanted absorption and poor conductivity.
There is a trade-off between these two options for applications involving solar windows: metallic grids are typically combined with transparent conductive oxides for good conductivity and as little sunlight loss as possible. However, there are two major issues with this approach: 1) 4-6% of the sunlight is still lost, and 2) the metal grids obscure the view. The latter disadvantage is not desirable, especially when aesthetics matter.
Microcontacts address this issue and offer a good solution. Due to their small scale (on the order of a few micrometers, corresponding to 1/10 thickness of an average human hair) these contacts are invisible to the naked eye. A very special class of microcontacts, called effectively transparent contacts (ETCs), can also offer a solution to the issue of sunlight losses. ETCs feature triangular mirror-like silver microlines resembling the iconic shape of the Toblerone chocolate bar. ETCs redirect sunlight towards the solar cell instead of reflecting it back to the sun, and thereby enhance the power output of solar cells—typically by 4-6%.
Schematic of an ETC window: triangular cross-section microsilver grids are integrated with a glass window.
In a paper published in the Journal of Photonics for Energy, L. Myers, H. A. Atwater, and R. Saive studied the visual appearance of conventional flat metal microgrids and ETCs. They found that when looking straight through a window with either type of microcontacts, the view does not get obscured [Fig. 1(a) and (c)] and landscapes only appear marginally darker. However, if looking through windows with ETCs under an angle (e.g., 50 deg, as reported in the current work), ghost images can appear-depending on pitch and aspect ratio of the microstructures [(Fig. 1(b) and (d)].
Furthermore, all microgrids produce diffraction patterns that need to be carefully considered when an undisturbed view is crucial, such as in cars and airplanes. Optimization of the geometric parameters of the microgrid contacts must be tailored to the requirements of the application in question. The ETC micrometal grids offer an aesthetically pleasing solution to the electrical contact problem in solar windows that can enhance the power output  for future smart and energy efficient windows.
The same scene observed in four different ways: (a) through an ETC window under normal incidence, (c) through a window with flat contacts under 50 deg viewing angle with a height of 10 µm and 80 µm periodicity; (b) and (d) through ETC windows under 50 deg viewing angle with ETCs, with (b) 10 µm height and 80 µm periodicity and (d) 15 µm height and 40 µm periodicity.
Read the original research article in the Journal of Photonics for Energy. L. J. Myers, H. A. Atwater, and R. Saive, "Visual appearance of microcontacts for solar windows," J. Photon. Energy, 9(2), 027001 (2019).
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