To realize all of this, we need to progress from heavy, rigid structures, that are expensive to ship and install, to conformal, thin, lightweight photovoltaics. Then the market will segment into some that are very efficient because area is at a premium, some which is transparent so it even fits over windows and lighting and other versions, probably including stretchable and edible ones. 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 is 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 DSSC, 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 cells but that is 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 life is usually only 2-5 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 polarized 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, this providing enough energy for more that the "hotel facilities", such as air conditioning, currently served by heavy, square "afterthought" panels seen on the roof of a car today. These first appeared at least 20 years ago and the Toyota Prius, the best selling hybrid car at last 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 such a CIGS printed reel to reel can be made fairly transparent by applying it to say a sunroof glass in thin lines. Spray deposition of some chemistries is proving possible (work at NREL on spray painting organic solar cells and at the University of Texas at Austin in spray deposition of CIGS) and Fiat is working with Solarprint, an Irish PV company dedicated to the development of DSSCs, developing options for components on and in vehicles. There are also new ways of using rigid technologies. For example, tinted glass can contain photovoltaics that have 15 years life, 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. Electrodes,(back electrode and transparent ones) barrier layers for encapsulation, assuring electrical connections are issues that need to be tackled if one is to imagine a complete vehicle generating electricity from the sun from every part of its surface. This might push feasibility further into the future but does not in any way prohibit the development of intermediate applications that help achieve the incremental advances necessary for this final target to be achieved.
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 hand launched all solar aircraft for civil use and 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. Some offer solar panels on the wings as an extra plus solar panels by or on the hanger where they are stored. 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. Ltd and Ryobi Group put solar panels on a bus for the latter's 100th anniversary. The "Sorabi" SOLARVE is described in the Japanese language press release as 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.