“I’ve always been interested in the idea of solar-powered transport, but no-one seems to have built anything practical yet. Is a solar vehicle a practical proposition in the UK?”

Jonathon Crouch
King’s Lynn

Professor Pivot replies:

Clean inexhaustible solar power has been a transport dream since photovoltaic cells first began converting light directly into electricity, but the reality seems as far away as ever. Solar cells have many applications these days, from lighting remote telephone boxes to powering satellites, but for transport, the problems are two-fold: cost and energy efficiency. At £5 to £20 per watt (bigger panels are much cheaper), the cost has changed little for some years: It’s the classic Catch 22 of high technology produced in low volumes, but increased demand for other high technology products has seen prices tumble, so the same is bound to happen to photovoltaics eventually.

Energy efficiency is a more taxing problem. Photovoltaics are improving rapidly, but most convert only 10 to 15% of the light energy hitting their surface into electrical power. Specialist cells of 25% efficiency are becoming available, and 35% or more is possible in the laboratory, but to avoid disappointment, we should work on a performance of a little over 10%.

Solar Cars?

Cover the horizontal panels of a typical car in photovoltaics, and you might cram in six square metres, trapping around 3 kilowatts of energy, but giving a peak output of only 0.4 kilowatts of electricity.To reproduce internal combustion performance, you’d need 30 kilowatts or so.Thus, with today’s technology, under ideal summer conditions, we could expect to generate around 1% of the peak power required. In practice, cars spend far more time sitting in the sun than moving, but even with the best panels charging a battery all day long, range would be very limited, and practically nil in winter.

Solar cars have achieved some amazing feats, of course, but the successful machines utilise a large surface area of priceless aerospace-grade panels to optimise power input, with sophisticated motors and lightweight construction to optimise performance. And it’s no coincidence that the annual solar challenge takes place in the Australian desert… But as we all know, cars are notoriously energy-hungry machines, and we can do a great deal better with other modes. Motorcycles and planes are even worse in the power requirement/surface area stakes, but low-speed motor boats are feasible, and a few have been produced. Best of all, though, is the humble bicycle – true, it offers a modest surface area, but it has an even more modest power demand.

…bicycles, like cars, spend a lot of time sitting in the sun, going nowhere…

The power requirement of electric bicycles varies a great deal, but we know from experience that the Panasonic-equipped Giant Lafree is the most efficient currently available, with a mean consumption of less than 100 watts under typical conditions.We could generate 100 watts from a panel measuring about 1.5 square metres, which would certainly be feasible on a faired recumbent. However, with a battery on board, there’s no need to generate all the power, particularly as bicycles, like cars, spend a lot of time sitting idly in the sun, going nowhere. In practice, a much smaller panel generating 20 watts would more or less recharge the Lafree battery during a long sunny day, giving a daily solar range of nearly 20 miles. If one were to start the day with a full battery, the solar boost might extend the non-stop range from 20 to 24 miles, or anything up to 40 miles spread over the course of a day, provided the bike was left in the sun between rides. A solar charger of this kind might also enhance the battery life, thanks to reduced current drain on hills and a steady trickle charge, rather than a daily boost. Suddenly the technology looks more practical.

In Practice…

The bad news is that solar panels are generally unsuited to use on bicycles. Although the panels themselves are light, they’re fragile too, so they’re usually housed in a heavy rigid frame.A few lighter, flexible panels are produced for boats and mobile homes, and although these are not particularly space-efficient, we must concentrate our search in this area.

Taking weight, space constraints and price into account, a pair of Uni- Solar flexible USF5 panels look like a good compromise.The two panels weigh 1.1kg, measure 44cm x 54cm and cost around £180 in the UK. Rated output is 300mA at 33 volts, or 10 watts – only half the power required to refill the battery during the day, but enough to provide some reliable data. By comparison, a spare battery for the Lafree would give twice as much mileage (in all weathers, of course), but cost a little more and weigh three times as much.

I’ll be fitting the panels to A to B’s long-term Giant Lafree in the next few weeks, and reporting back in subsequent issues. In terms of practicality, this is cutting edge stuff:We The Uni-Solar USF5. The border is unproductive, but gives aim to find whether panels are the panel some protection from inevitable knocks practical in our gloomy climes, or whether the weight and bulk cannot be justified for everyday use.

Perhaps carefully angled panels on a south-facing roof would be more effective? Or a combination of the two systems? More in the next issue.