A combination of flexible solar cells and low-energy lighting provides a way to bring electric light to isolated communities

A bag full of sunshine
Mar 6th 2008
From The Economist print edition

IF YOU live in a remote area, particularly in a poor country, obtaining electricity can be a problem. It is probably too expensive to connect you to the grid, so you are left reliant on generators and batteries—and even these have to be wired to the points, such as light bulbs, where power is wanted. But Kennedy & Violich Architecture of Boston, Massachusetts may have the answer. In collaboration with Global Solar Energy of Tucson, Arizona, it has developed a cheap, practical and portable way to capture the sun's rays by day and release them by night as useful light, wherever it is needed.

The idea, dubbed “portable light”, combines solar cells with light-emitting diodes attached to the surface of a fabric that can be made into bags, and thus carried around during daylight hours. In sunlight, the cells generate electricity that is stored in batteries stitched into the material. When it gets dark, the batteries power light-emitting diodes that are also sewn onto the cloth.

The solar cells themselves are made from a substance called copper indium gallium diselenide, Global Solar Energy's speciality. This is not quite as good at capturing sunlight as silicon, the material from which solar cells are usually made, but it is less rigid and easier to work with. Crucially, a working cell can be made by spreading a thin layer of the stuff on another material, such as a sheet of plastic. The result is flexible and fairly robust.

Storing the electricity generated by the solar cells involves small batteries that are also woven into the fabric, along with plastic-coated wire connectors. Rechargeable lithium-ion batteries—the sort used in mobile phones—can store more energy per unit weight than other types, and do not lose their charge too rapidly if they go unused for long periods.

These batteries then power hundreds of light-emitting diodes a few millimetres across that are sewn into the fabric on the opposite face from the one occupied by the solar cells. Until recently, such diodes were expensive and did not shine very brightly. But they have improved enormously over the past few years and are now used in mainstream lighting. (A town in Italy recently became the first to switch its street lighting over to such diodes.) Light-emitting diodes are well suited to providing portable illumination, because they are robust, they are not heavy and, unlike traditional bulbs, they do not become too hot to touch.

So far, so good. But there was room for further improvement. As the person carrying the bag moves around, parts of it will be in the shade while others are overwhelmed with sunlight. So the engineers devised a way to direct the energy where it is most needed. To do this, they have woven sensors and switches into the fabric. These ensure that energy from a fully illuminated photovoltaic cell is sent to batteries that are not already receiving electricity at the highest rate they can store it.

In this way, the engineers have created a device that can stash away enough electricity to power the light-emitting diodes for ten hours after three hours in full sunlight. Unlike conventional light sources, the fabric can be spread out to provide background lighting for an entire room or rolled up to generate more concentrated light for a particular task. Moreover, many pieces can be joined together to produce light for larger meetings, and also to power devices such as mobile phones.

The first beneficiaries of this technology are the Huichol, a group of Amerindian people who live in the Sierra Madre mountains of Mexico, but if all goes well, Kennedy & Violich hopes to sell its invention to rural communities in Africa and Australia as well. And at $50 per bag—less than the average Huichol spends on candles and torch batteries each year—it seems a bargain.