EES Blog

Scientists from China and the US have produced glass that responds to its environmental temperature, making it a promising material for energy efficient windows. On cold days, the windows would prevent heat escaping and on hot days, the windows would reflect infrared radiation, keeping the room inside cool.

Vanadium dioxide (VO₂) has long been recognised as a potential candidate material for making ‘smart windows’ because it can change from a transparent semiconductive state at low temperatures, allowing infrared radiation through, to an opaque metallic state at high temperatures, while still allowing visible light to get through. VO₂ has its drawbacks, however: it’s not a very good insulator and can only be made at very high temperatures of 420-500°C.

Zhong Lin Wang at the Georgia Institute of Technology, Atlanta, and co-workers at the   Shanghai Institute of Ceramics Chinese Academy of Sciences and the Graduate University of Chinese Academy of Sciences in Beijing, have created a system with improved insulating ability by combining a layer of VO₂ with a transparent fluorine-doped tin oxide (FTO) layer coated on glass. As an added benefit, the  FTO layer enhances the crystallinity of the VO₂ film – an important factor for improving the material’s performance and reducing its cost – and lowers the synthesis temperature to 390°C.

‘Buildings and maintaining man-made structures use 30-40 per cent of the primary energy supply, mainly for heating, cooling, ventilation and lighting. Effective control of the energy exchange between the interior and exterior of buildings through windowpanes is a key area in saving energy,’ says Yanfeng Gao, one of the researchers. ‘We conducted this research to create a film that can combine the functions of low emissivity and thermochromic coatings.’

Materials expert Seeram Ramakrishna, from the National University of Singapore, sees advantages to the group’s approach. However, he also feels that improvements could be made to optimise the system. The work ‘doesn’t attempt to bring down the transition temperature of VO₂ to room temperature, which would be desirable if it is to have practical technological applications,’ he says.

Gao now hopes to realise his dream of seeing the smart windows go into mass production.

Heather Montgomery

Read this HOT Energy and Environmental Science article:

Solution-based fabrication of vanadium dioxide on F:SnO2 substrates with largely enhanced thermochromism and low-emissivity for energy-saving applications
Zongtao Zhang, Yanfeng Gao, Hongjie Luo, Litao Kang, Zhang Chen, Jing Du, Minoru Kanehira, Yuzhi Zhang and Zhong Lin Wang
DOI: 10.1039/C1EE02092G

Limestone batteries could be the key to transporting energy across huge distances, according to chemists in Germany. The idea, which would be used to take solar energy harnessed in the African desert to cities in Europe, might be more efficient than power lines, and could even sequester carbon dioxide emitted by fossil fuel plants.

Researchers exploring greener ways to generate electricity have often looked to the deserts, where enough sunlight falls in six hours to power the world for an entire year. DESERTEC, developed by European scientists, economists and politicians, offers one way to tap this resource. In this concept, power plants would be located in northern Africa that concentrate the sun’s rays onto oil, which would boil water into steam to drive turbines. The electricity generated by these turbines would then be transported thousands of kilometres to cities in Europe via high-voltage, direct-current (HVDC) power lines.

HVDC lines can be more efficient than normal alternating-current lines, because they operate at less current and, therefore, suffer less resistance. But over 3000km, HVDC lines are still expected to lose 10 per cent of their power, dragging down the overall efficiency of the DESERTEC concept from 12 per cent to 10.8 per cent.

Now, Benjamin Müller and colleagues at the University of Erlangen, under the auspices of the Energy Campus Nuremberg research platform, have come up with an idea that could ameliorate this loss, and at the same time cut some of the CO₂ produced by fossil fuel plants. Rather than generate electricity at the African solar concentrators, says Müller’s group, engineers should direct the sunlight to reactors filled with limestone (CaCO₃). At around 1000°C, the limestone would convert to quicklime (CaO) and CO₂, which could be converted using other solar energy and hydrogen into useful fuel such as methane. Meanwhile, the lime could be transported over land and sea to European cities where, upon heating to 650°C in the presence of CO₂ from local fossil fuel plants, it would reconvert to limestone with a massive release of heat. This heat would then boil water into steam to drive turbines, generating electricity on-site.

 The Erlangen researchers estimate that, overall, their method would keep the DESERTEC efficiency at 12 per cent. What’s more, they calculate that the extra CO₂ sucked up at fossil fuel plants could reduce the total CO₂ output of Germany alone by up to 50 per cent.

‘The idea is sound, intriguing and is worthy of detailed investigation [because] it challenges existing thinking,’ says Magdi Ragheb, an engineer at the University of Illinois at Urbana-Champaign, US, who has studied the DESERTEC concept. However, Ragheb points out potential problems, such as the transport of solids. ‘Liquids such as petroleum are easy to handle as they can be pumped; solids are more difficult to handle,’ he adds.

Other environmental experts agree that the idea may suffer practical hurdles. ‘It was the first time I had seen this idea, so from that perspective it’s intriguing,’ says Sally Benson, an expert in carbon dioxide sequestration at Stanford University in California, US. ‘[But] you’ve got all this solid you need to react very quickly…you’d need to have it very finely ground, so now all the material handling becomes a big issue.’

Müller’s group itself points out another practical drawback – that, over time, the limestone’s crystal structure would change, so engineers wouldn’t be able to convert 100 per cent of it to lime. ‘Artificially synthesised materials [would instead be] the best option to achieve a high degree of conversion,’ says Müller. ‘But addressing this problem is a big issue in research at the moment.’ However, he adds that his group is exploring other candidates besides limestone for chemical storage.

‘The [idea] sounds interesting,’ says Michael Straub, a spokesperson for the DESERTEC Foundation. ‘But as we have just a few years left to slow down global warming, we are actually focusing on technologies that are available today. As the worldwide implementation of DESERTEC will take decades, this technology might be an option for future [solar concentrator] plants.’

Jon Cartwright

Read the paper from Energy & Environmental Science:

A new concept for the global distribution of solar energy: energy carrying compounds
B. Müller, W. Arlt and P. Wasserscheid, Energy Environ. Sci., 2011
DOI: 10.1039/c1ee01595h