One step closer to self-powered switchable glazing?

Windows that change transparency could be powered by photovoltaic solar cells, say Irish researchers.

Look around your nearest city and you’ll see glass everywhere. In most large buildings, that glass is coated, so that it traps heat inside during the winter, and keeps it out in the summer. But anyone who has sat beside a window on a sunny day knows that it’s rarely that simple – without air-conditioning, a glass-clad room will only get hotter. And, when temperatures drop outside, coated windows aren’t enough to keep a room warm. Managing these temperature swings is a big challenge for building owners, and one that comes with an ever-growing energy bill.

A paper, published in a recent issue of Solar Energy Materials and Solar Cells [DOI: 10.1016/j.solmat.2016.05.013], looks specifically at one type of switchable glazing – glass that can adapt its transparency in response to external stimuli, to manage solar heat gain. For a suspended particle device (SPD), that stimulus is an alternating current – applying it causes randomly-oriented polyhalide particles (suspended between two panes of glass) to align, rendering the panel transparent. But, it means that a large area of glazing can consume a significant amount of electricity.

So, researchers from the Dublin Energy Lab connected a commercial SPD to a photovoltaic (PV) array, and tested it in an outdoor environment. They found that when connected to a well-matched DC-AC inverter, a 40 W PV array could continuously power a 0.07 W SPD glazing, even on a day with intermittent sun. A 12 Ah battery added to the circuit was also found to be full charged by this set-up, thus providing uninterrupted power for use on overcast days.  In addition, switching some SPDs off (i.e. leaving them opaque) during the day would offer a double advantage of reducing solar heat gain, while harvesting solar energy for later use.

The authors estimate that a 1 m² SPD glazing panel would consume 10.42 kWh of electricity in a year, producing the equivalent of 4.8 kg CO₂. This suggests that switchable glazing could add to a building’s energy costs, if plugged directly into the grid. But, these results show that when used in tandem with a photovoltaic array, SPDs could offer a self-powered route to reducing a building’s energy footprint.

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A. Ghosh, B. Norton, A. Duffy, “First outdoor characterisation of a PV powered suspended particle device switchable glazing”, Solar Energy Materials & Solar Cells 157 (2016) 1–9. DOI: 10.1016/j.solmat.2016.05.013