Given the thermal shortcomings of glass, windows can significantly undermine the sustainability of a building. Most conventional windows allow heat to escape during cold weather, increasing heating costs, while, during hot weather, they allow sunlight to stream in, thus driving up the use of energy-hungry air conditioning.

In a bid to overcome these limitations, Dr. Matyas Gutai, a lecturer in architecture at Loughborough University, has created a new type of window – one that’s filled with water.

Each “water-filled glass” (WFG) window contains a vertical sheet of water, sealed between two sheets of glass. As sunlight passes through the glass, it heats the water, thus keeping the room itself from getting as hot as it would otherwise.

Once it reaches a high enough temperature, that sun-warmed water is pumped out of the window, traveling through pipes in the wall to a storage tank elsewhere in the building. Cooler water is simultaneously pumped into the WFG, to replace that which was pumped out.

When the outdoor temperature drops, later on, the stored warm water is pumped back out of the tank and into the pipes, warming the room by radiating heat through the walls. Alternatively, warm water can also be used in the building’s taps, reducing the need to run the water heater.

Although some electricity is required to pump the water back and forth, computer simulations have shown that a WFG-equipped building would use up to 72 percent less energy compared to one with traditional heating systems.

“Glass is currently a liability in buildings as it compromises energy consumption, thermal comfort, acoustics, and other aspects,” says Gutai. “WFG changes this paradigm and turns glass into an opportunity for sustainable construction. It shows us that thinking holistically about buildings and building components leads to a more efficient and sustainable built environment.”