Our walls could double up as a store of electric energy, says S.Ananthanarayanan.

A disadvantage of green ways of generating electricity is that they are intermittent. Wind power, for instance, is there only while the wind blows and solar power is there only in the daytime.

The result is that we cannot do without coal or oil-based power plants. What is worse, these utilities need to be turned on and off, which lowers their efficiency and increases their damage to the environment. Hence arises the industry if batteries, or facilities to store electricity – so that power is available when needed, and the demand on the conventional sources stays steady.

Hongmin Wang, Yifan Diao, Yang Lu, Haoru Yang, Qingjun Zhou, Kenneth Chrulski and Julio M. D’Arcy, from Washington University at St Louis, Michigan, write in the journal, Nature Communications, of a process of enabling the common brick, which we use to construct buildings with, to store electricity. It is not a large energy store, but individual houses can now store in their walls some electricity, generated, say by rooftop facilities, to work low-power devices. The masonry available all over the city could also serve as batteries for off-grid services.

The principle, the paper says, is that the mud and soil used for making bricks contains some 8% of iron oxide, as an ore called hematite (which, incidentally, gives bricks their red colour). Hematite, which is freely available, is the starting point for making alloys, magnets, catalysts and so on. And also in the manufacture of special purpose batteries and electrical equipment, the paper says.

The authors refer to their earlier work, published a year ago, where they made use of ordinary iron rust, again iron oxide, to create fibres and films of conducting, organic chain molecules. In the current paper, they work on the hematite content of the clay, and the microstructure of the bricks, to deposit a layer of nano-fibres made of conducting chain molecules – to create conducting sheets on opposite sides of the bricks. When these bricks are used to construct a wall, the wall becomes a beehive of conducting surfaces, which can be manipulated to gather and store electric charge.

It is a property of conducting surfaces, when in pairs, separated by an insulator, that the surfaces can be charged to opposite polarities. What this implies is that the negatively charged surface has an excess of negatively charged electrons, which have been driven there when the pair was charged. As the two surfaces are insulated, the difference in charge can remain for long periods. When the two surfaces are connected by a conductor, like a copper wire, however, the charges equalize and there is an electric current through the wire. And rapid charging and discharging of such an arrangement is often used to control electrical circuits which carry variable or alternating currents.

Large pairs of conducting surfaces can be created, for instance with metallic coating on two sides of a strip of paper, which is rolled up, and these are used in electronic circuits. Arrangements with high capacity, even ‘banks’ where several devices are arrayed and combined, are used to control the effects of rise and fall in demand or supply in the public electricity network. The normal, storage battery, like a car battery, too, stores charge that is driven on to lead plates through a reversible chemical reaction. In the same way, the specially processed bricks created by the Washington University team are separated by a gel which can undergo a chemical change, like in the storage battery.

The picture shows how the conducting surface is coated on the bricks, and the bricks, which have the coat of the conducting film, are stacked, adjacent bricks being separated by the gel. And the columns of bricks are separated by a layer of cement, or an adhesive epoxy, to build a wall. And groups of bricks are arranged with the conducting surfaces connected so that the differences in charge add up. "A fired brick's open microstructure, mechanical robustness and 8% iron oxide content afford an ideal substrate for developing (the bricks) that readily stack into modules,” the paper says. Coated with epoxy, they are stable, stationary and waterproof, the paper says.

The battery, or the interconnected assembly of charged devices, can be charged, the paper says, to deliver power of 1.6 Watt hours of energy for every square meter of wall. This would not amount to a great deal of power, but it is possible to include different kinds of masonry – the bricks, concrete blocks, paver blocks – as storage resources, and this would add up to a large area. The current work is a proof of concept –we can expect improvements both in the coating and in design of bricks, for more effective area. The study also shows that the current coating stays 90% effective for 10,000 cycles of charging and discharging. If the cycle is once a day, this is about 30 years!

Another ‘green’ idea in building technology, recently proposed, is to go back to use of wood as a construction material. The idea is to save on reinforced cement concrete, as manufacture of steel and cement lead to huge CO2 emissions. Wood has strength comparable with RCC, has less weight and large members are fire resistant, whereas steel begins to buckle at high temperature. And then, the wood is a carbon store, while growing more trees would draw more CO2 out of the atmosphere.

Construction of buildings with wood as the material for the structure, while the walls are of brick treated to work as a reservoir of electric charge, would substantially reduce the carbon footprint of residential and commercial buildings. The efficiency of solar panels is rising and innovative methods of harnessing solar power are being developed. If the ideas were integrated by architects and town planners, living spaces could become more sustainable, apart from making the concepts a part of popular understanding.

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