Liquid windows against sky with clouds

‘Liquid windows’ could reduce buildings’ energy consumption

Image credit: Foto 1763475 © Michal Bednarek |

Researchers at the University of Toronto have developed a squid-skin-inspired fluidic system that could reduce the energy costs of heating, cooling and lighting buildings.

What if windows were not solid? A research team at the University of Toronto has developed a window prototype that uses a thin layer of liquid pigment between two glass panes to affect how much sunlight gets through.

The scientists were inspired by the multilayered skin of organisms such as squid. Each of these layers contains specialised organs that work together to protect the animals from sunlight and other external factors. 

The objective of the prototype is to optimise the wavelength, intensity and dispersion of light transmitted through windows. In doing so, it could offer much greater control than existing technologies while keeping costs low. 

“Buildings use a ton of energy to heat, cool and illuminate the spaces inside them,” said Raphael Kay, one of the scientists involved in the research. 

“If we can strategically control the amount, type and direction of solar energy that enters our buildings, we can massively reduce the amount of work that we ask heaters, coolers and lights to do.”

Prototypes of a multilayered fluidic system designed by U of T Engineering researchers contain several layers of channels that contain fluids with various optical properties.

Prototypes of a multilayered fluidic system designed by U of T Engineering researchers contain several layers of channels that contain fluids with various optical properties. / Raphael Kay, Adrian So

Image credit: Raphael Kay, Adrian So

Currently, most  'smart' building technologies focus on automating processes, such as automatic blinds or electrochromic windows. However, these systems cannot discriminate between different wavelengths of light, nor can they control how that light gets distributed spatially. 

“Sunlight contains visible light, which impacts the illumination in the building – but it also contains other invisible wavelengths, such as infrared light, which we can think of essentially as heat,” he said.

“In the middle of the day in winter, you’d probably want to let in both – but in the middle of the day in summer, you’d want to let in just the visible light and not the heat.”

The prototype developed by Kay's team consists of a set of flat sheets of plastic that are permeated with an array of millimetre-thick channels through which fluids can be pumped.

Customised pigments, particles or other molecules can be mixed into the fluids to control what kind of light gets through and in which direction this light is then distributed.

These sheets can be combined in a multi-layer stack, with each layer responsible for a different type of optical function: controlling intensity, filtering wavelength or tuning the scattering of transmitted light indoors. By using small, digitally controlled pumps to add or remove fluids from each layer, the system can optimise light transmission.

“It’s simple and low-cost, but it also enables incredible combinatorial control. We can design liquid-state dynamic building facades that do basically anything you’d like to do in terms of their optical properties,” Kay said.

The research team also leveraged computer models to analyse the potential benefits of covering whole buildings with these types of materials.

The findings showed that buildings that were covered with one single layer focused on modulating the transmission of near-infrared light could save about 25 per cent annually on heating, cooling and lighting energy. 

If the building were to have two layers, it could reduce its energy consumption by 50 per cent, the researchers said. 

“Globally, the amount of energy that buildings consume is enormous – it’s even bigger than what we spend on manufacturing or transportation,” said Ben Hatton, another of the team's researchers. “We think making smart materials for buildings is a challenge that deserves a lot more attention.”

The findings of the research were published in the journal PNAS.

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