How a new generation of 'smart windows' could keep you warm in winter and cool in summer

World energy demand is continuing to soar as cities grow, technology advances and industries develop. Buildings make up about 30%-40% of the total — even more than industry or transport. This comes largely from heating, cooling and ventilation systems, with air conditioning especially energy-hungry.

Windows are a significant part of the problem. They allow heat to escape in winter and enter in summer, forcing temperature systems to consume more energy and drive up emissions. The challenge is to control this heat transfer without compromising on windows' transparency and the amount of daylight they let in, both of which are essential for people's wellbeing and productivity.

The answer is smart windows. Most of the current versions on the market are what is known as electrochromic (EC), meaning they work by applying electricity at the touch of a button to layers of particles or crystals inside the glass.

This causes a reversible molecular transformation which turns the window either opaque or dark, depending on the product. This blocks out the majority of infrared light, which is what makes rooms uncomfortably warm. This drastically reduces the need for air-conditioning in hot countries, keeping some 60% to 70% of heat outside at peak temperatures. They can also reduce heat loss from rooms by about 40% in colder weather.

For a few years, these windows have been selling fairly well both for commercial and residential properties. The total global market in 2023 is estimated to have been worth US$6.6 billion (£5.2 billion).

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Yet they have several important limitations. Though the windows don't use very much energy, they only operate with a power source. This can be challenging in locations that are remote or have unreliable electricity. And to the extent that renewable options from the grid aren't available, users need to install an alternative like solar panels to make these windows carbon neutral.

With many varieties — though there are exceptions — you can only toggle between full restriction and full transparency. This means you're losing the benefits from having windows when the weather is hot, and rooms will probably need artificially lit. And as previously mentioned, EC windows do a great job of keeping out heat in hot countries, but they're a bit more limited in colder climes.

The future is thermochromic

One alternative which at least negates the need for electricity is known as photochromic. These use a layer of either tiny silver halide crystals or compounds known as naphthopyrans, both of which react to rising levels of ultraviolet (UV) light, causing glass to tint in brighter conditions. It's exactly the same material that is used in light-reactive sunglasses.

Compared to EC windows, they have the additional benefit of creating a barrier to UV light. UV light is not only carcinogenic, but damages everything from furniture to paintings to EC coatings.

However, photochromic windows are very expensive, at least if they use silver. They are highly sensitive to weather, which can reduce their reliability in cloudy or rainy conditions. They are also not as good at blocking infrared light and have no manual control, so are more really useful for privacy than regulating room temperatures.

Many would argue that a more promising variety of smart window for the future is a third one known as thermochromic, meaning they use a coating of particles that react to temperatures instead of light. Again, this means there's no need for electricity.

They are much cheaper than photochromic windows, still block UV light and have the potential to be comparable to EC windows in blocking infrared. They can also progressively tint darker as outside temperatures rise, meaning you can have more transparent windows than those on/off EC products.

But while thermochromic glass already exists, it's not feasible for windows yet. This is because the vanadium dioxide layers in today's versions only fully reflect infrared at around 67°C, which is much hotter than even the all-time highest temperature in the world.

Many researchers around the world are looking into how to improve thermochromic glass. This includes our project at the University of Exeter's Environment and Sustainability Institute, which is partly involved in testing other coatings to try and find one which is effective at reducing infrared light at more realistic outside temperatures.

Uniquely, we're also looking into combining this with several other types of capabilities that currently can exist in other varieties of thermochromic glass besides those that can reflect infrared light. These include making the windows more useful in colder climates by enabling them to work as an insulator when temperatures are low so that rooms don't lose their heat to the outside, and also storing energy so that it can be used to help heat rooms.

It's difficult to predict an exact timeline, but maybe five or ten years from now, this kind of research should bring smart windows to market that will be just as useful in cold countries — as well as both in the daytime and at night. This is the key to the widespread rollout of a single type of window around the world.

It should make a significant difference not only to aircon requirements but also to the need for heating and radiators. My rough guess would be that by installing five smart windows in an apartment in a colder country, this might enable the owners to reduce the number of radiators from, say, five to two. And besides buildings, these technologies could also be used in airplanes and cars.

In the meantime, there's every reason to assume that the market particularly for EC windows keeps growing. According to one projection, it should increase by nearly another US$4 billion or around 60% by 2028. With the right mix of research success and policy support, in both developed and developing countries, the next generation of smart windows should then be able to take this forward and make a big difference to the carbon emissions of buildings a decade or two into the future.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.