Windows switch seasonally between heating and cooling homes

Domestic heating is a significant contributor to carbon emissions, with maintaining of indoor temperatures accounting for 20 to 40 per cent of national energy budgets in wealthy countries. The type of window in a house is a major factor to heating and cooling efficiency, with smart windows which adapt between heating and cooling effects being proposed as a tool to help improve energy efficiency.

This new smart window design is a step above previous iterations, harvesting the Sun’s energy in the winter to warm the house while reflecting it in the summer to keep it cool.

“The major innovation is that these windows can change according to seasonal needs,” explained Professor Nathan Youngblood, an electrical and computing engineering expert at Pittsburgh and first author of the study. “They absorb near-infrared light from the Sun in the winter and turn it into heat for the inside of a building. In the summer months, the Sun can be reflected instead of absorbed.”

The film comprises of an optical stack of materials less than 300nm thick, with a very thin active layer made of phase change materials that can harvest near-infrared energy in the winter and reflect it in the summer; this would be impossible for materials for fixed thermal and optical properties. The material – which switches between amorphous and crystalline states – can modulate the amount of solar energy reflected while maintaining neutral colouration and constant transmission of visible light.

“Importantly, visible light is transmitted almost identically in both states, so you wouldn’t notice the change in the window,” said Youngblood. “That aesthetic consideration is critical for the adoption of green technologies.” 

The material could even be controlled so that, for instance, 30 per cent of the material is turning away heat while 70 per cent is absorbing and emitting it, allowing for precise temperature control. The researchers estimate that using these windows – including the energy required to control the film – would save 20 to 34 per cent in energy usage annually compared to the double-glazed windows typically found in homes.

Professor Harish Bhaskaran, of the University of Oxford department of materials, commented: “Here, we exploit tuning how invisible wavelengths are transmitted or reflected to modulate temperature. These ideas have come to fruition with the aid of our long-standing industrial collaborators, and are the result of long-term research.”

In order to create and test their prototypes, the researchers worked with ultra-thin-film specialists Bodle Technologies and Plasma App, and engineering firm Eckersley O’Callaghan. Bodle Technologies CEO Peiman Hosseini said: “This work demonstrates yet another interesting optoelectronic application of phase change materials with the potential to significantly improve our everyday life.

“The commercialisation of [phase change materials]-based tuneable low-e glass panels still has a number of significant challenges left to overcome. However, these preliminary results prove that the long developmental road ahead is certainly warranted. I believe this technology should be part of any future holistic policy approach tackling climate change.”