Driving with solar cells

Harry Zervos, PhD

Most of the companies developing photovoltaics have been obsessed with reaching grid parity, meaning, to become competitive with conventional grid electricity generation which is often highly-subsidised; more recently, it has been realised that the market for disposable photovoltaics on consumer goods is at least as large. Even the potential on electric vehicles by land, water and air could be an enticing billion square metres a year, comparable to that for buildings.

To realise all this, we need to progress from heavy, rigid structures that are expensive to ship and install, to conformal, thin, lightweight photovoltaics. On vehicles, life needs to be 15 years – approaching the 20 years of building photovoltaics. That is, unless it is so low cost and easy to fit that it can be replaced during a service call.

Rethinking the on-road vehicle is long overdue and biomimetics – copying nature – tells us that we have many options for pleated, unfolding and unfurling solar cells on vehicles as well, particularly when they are parked. Indeed, solar fabric on seats and as internal body and floor covering will also generate useful amounts of electricity, even though it is behind glass. It will also be possible to make a “rag top” convertible generate electricity from its fabric.

Lightweight flexible photovoltaics are already available from several companies, and it’s good for evolution that they use different materials and assembly techniques, including roll-to-roll printing. The technologies include organic ink, dye-sensitised solar cells or DSSCs, which employ organic and inorganic materials, amorphous silicon and fourthly copper indium gallium diselenide CIGS.

So far, they tend to be about one-tenth of the weight of conventional crystalline silicon solar cell, but that is being offset by having about one-tenth of the efficiency. They are not yet cheap – a flexible panel similar to the one on a solar bag that charges your mobile phone currently costs about $30. They cannot be tightly rolled or folded, and lifecycle is usually only two to five years.

None are yet transparent so there is work to be done because most of them are potentially transparent, low cost, tightly rollable and foldable, and even lighter in weight. Indeed, new photovoltaic technologies such as the so-called quantum dots can also achieve all of this and harvesting of infrared and even ultraviolet has even been demonstrated with some options.

RETHINKING VEHICLE DESIGN

Already, some photovoltaic options are very good at converting polarised light reflected from snow or glass, low-level light and light at narrow angles of incidence. It is, therefore, reasonable for the electric vehicle industry to plan for solar conversion from the whole vehicle and unfurled panels, thus providing enough energy forair conditioning. These first appeared at least 20 years ago, and Toyota Prius, the best-selling hybrid car, has a solar panel as an option. It can drive a fan to keep the car cool when parked.

One car manufacturer has been looking at solar curtains in the car. An opaque, long-life, flexible technology like CIGS (copper indium gallium selenide) that’s printed reel-to-reel can be made fairly transparent by applying it, say, to a sunroof glass in thin lines. There are also new ways of using rigid technologies. For example, tinted glass can contain photovoltaics with a 15-year lifecycle, removing the need for expensive encapsulants.

Developments are already happening in photovoltaics technologies that are making new types of components possible, and the prospect of integration into a vehicle manufacturing line is not considered science-fiction anymore. It is important to note though that similar advances have to be made in the deposition of all other layers of a PV cell, not just the active material.

A few very lightweight vehicles use solar power entirely already, including some rickshaws, golf cars and record-breaking road vehicles in very sunny countries. Add to that the huge unmanned surveillance aircraft in the upper atmosphere and the recent record-breaking 24-hour flight of the all-electric Solar Impulse plane. Some autonomous underwater vehicles, AUVs of the type called gliders, cruise the oceans for years, coming to the surface to charge their batteries from solar and wave energy.

At ETH Zurich in Switzerland, they are designing a hand-launched, all-solar aircraft for civil use as well as larger, unfolding unmanned solar aircraft to traverse Mars. A solar-powered airship has been proposed. Pure electric manned aircraft burst onto the scene in the last year as suitably light and powerful traction batteries became available. A sport plane in the open for a week before it is used for a few hours at the weekend can garner an appreciable amount of traction power from the sun.

Sanyo Electric Co has a “futuristic hybrid bus” with two different types of high-efficiency but relatively heavy Sanyo solar-electric panels mounted on its roof – 420W HIT cells and 378W Amorton cells generating 798W. This can drive accessories, but not make a significant contribution to range.

The author is technical analyst at the UK-based consultancy IDTechEx.