Solar Power Results
Readers may recall the launch of my solar vehicle experiments in the last issue. At the time – having made a number of purely theoretical calculations – I concluded that a 20 watt solar panel might just be viable on an electric bicycle. In the event, for reasons of panel size and cost, it was necessary to settle on a pair of five watt panels, reducing the potential gains, but keeping the apparatus down to a manageable size.Why two panels? It’s simply that most small solar panels provide a 12 volt output, but the Lafree (the subject of my experiment) is, like many other electric bicycles, 24 volt, requiring two smaller panels connected in series.
Fitting solar panels to the bicycle brings a number of advantages: greater range, a less traumatic life for the battery (thanks to the constant trickle charge, reduced peak loading, and ‘shallower’ charge cycle), plus greater efficiency, because of the direct motor supply. Batteries are around 90% efficient when charging and much the same when discharging. That’s pretty good as these things go, but a direct supply will still give a bonus.
The disadvantages are the weight and cumbersome nature of even the smallest panels, and the potential screening effect of roadside buildings and trees, the rider, and of course vehicle orientation. In other words, if the panels are not angled towards direct sunlight for most of the journey, there is little point in carrying them around.
Static panels are remote from the vehicle, so range is unimproved. On the positive side, the panels can be positioned to capture most of the available solar radiation from dawn to dusk and they’ll charge all day, every day, and even in the rain, when the bicycle might be under cover. A spare battery is not obligatory with this system, but it does mean charging can be undertaken continuously – more effective if the bicycle is away for most of the day.
The Roof Option
After a few measurements, and long hours weighing up the options, it became clear that even the smallest solar panels would be unsuitable for use on a conventional bicycle that has to be wheeled through narrow gaps, leant against shop windows, dumped on the ground, and so forth. A recumbent trike would be a different proposition, of course, and I hope to investigate this option in a later issue.
The A to B long-term test Lafree spends most of its life towing a trailer.The roof of a child trailer would make a good location for solar panels, but one would need to plug and unplug the panels whenever the trailer was connected, and charging would not be possible while riding solo.Trailer mounting could make sense for a long hilly touring holiday, or for a commercial bike/trailer outfit left connected all day, but not for typical ‘school-run’ or commuter applications.
…a roof of this size receives a steady eight kilowatts of power from the sun…
Taking all these factors into account, it became clear that static ‘base station’ panels were the best option, and such was the persuasiveness of this argument that no attempt was made to rig the panels to the bike or trailer, even on a temporary basis.
In this case, we were fortunate to have a building aligned east-west, giving eight square metres of south-facing roof at 20 degrees to the horizontal.That’s a good compromise in British latitudes, although a steeper angle would be more productive in winter, and a flatter roof slightly more efficient in summer.
A roof of this size receives a steady eight kilowatts of power from the sun on a bright day, which could generate around 800 watts, working on our 10% efficiency formula from the last issue. But would our tiny five watt panels provide enough power?
The answer depends on the time of year, the amount the bicycle is used, and the cloud cover.The solid line on the graph indicates solar charging on a typical June day. Recorded just a week before the summer solstice, daylight hours are obviously very long, although one should not assume that solar radiation is necessarily stronger in June than at other times in the summer.
Note that the panels begin to provide a small charge well before sunrise, and continue to work after sunset, albeit at a low rate of charge.The real current flow begins when the sun first strikes the panels at 9am, climbing rapidly to peak at 2pm, before falling equally rapidly until 6pm. Note the pronounced troughs in production during the afternoon, when the sun is obscured by passing clouds. Readers will hardly need reminding that this is a fairly typical pattern in the United Kingdom, as clouds bubble up in the heat of the day.
In ideal conditions, we can expect our two 5 watt panels to produce about 70 Watt/hours (Wh) of electricity, and in practice, our representative English early summer day produces just under 60 Watt-hours. One could expect to do a little better, or much, much worse, according to conditions. Angling the panels to follow the sun would produce a little more power, but certainly not enough to justify the complication involved.
Sixty Wh is not much, but it’s enough to propel the efficient Lafree for about seven miles with gentle pedal-assistance. In other words, if your daily range does not exceed that amount (or, for example, 14 miles every second day), a ten watt power station should keep you moving in fine summer weather without the need to plug into the mains supply.
Our test bike travels further than this – about ten miles four days a week towing a child trailer (an effective solo mileage of 13.3 miles per day), plus smaller variable distances on the other three days, so a conventional supplement is required.
And the weather is not always fine! The figures below cover a typical 16-day period in late June. Early summer 2003 will be remembered as a very dry period, but cloud cover was about average, so the figures can be regarded as typical:
| Average Daily Mileage | Daily Conventional Charge | Daily Solar Charge | Solar Charge % |
|---|---|---|---|
| 9 miles* | 28Wh | 36.6Wh | 57% |
* The actual mileage of 6.8 miles per day has been multiplied by 1.33 to make allowance for the trailer. Nine miles would be a more realistic solo mileage.
In the summer, and with modest mileage, this basic system works well. By doubling the number of panels, we could either produce 100% of our power requirements, or double our daily mileage. Conventional rigid roof-panels are somewhat cheaper than the flexible panels we have used in this experiment, so for example, £300 should buy two 12 volt, 15 watt panels, providing a surplus of power under most conditions.
Rather than directly charging an expensive spare electric bike power pack, another option is to charge a cheap-and-cheerful 12 volt car battery direct from a single 12 volt panel, topping up the bike power pack through an inverter, which converts the 12 volts from the panel to 240 volts to run the conventional charger, which converts it back to the voltage used by the bicycle. One large panel is cheaper than two small ones; a second bike battery is unnecessary; and the car battery provides capacity of 500 watt/hours or more to carry the system through gloomy days. However, the efficiency of this multi-stage operation can prove alarmingly low, and there’s the extra cost to consider.
A 30 watt solar power station will cost around £300 for the solar panel and regulator, plus £100 for a basic car battery and inverter. In fine weather, such a system would easily generate enough power to give a daily solar range of 20 miles or more.This would also be the best solution for charging a 36 volt machine such as the Powabyke, for which three 12 volt solar panels would be too expensive and cumbersome. Note, though, that the Powabyke is a less efficient machine, so mileage would be rather less.
The Bottom Line
Sceptical readers may have noticed the accent on weather conditions throughout this article.The problem is that a ten watt panel will indeed provide a peak output of ten watts and a mean figure of five watts or so on a sunny day in June. But on an overcast day, these figures can be halved, and in really grim weather – even in high summer – output can fall to 20% of the rated figure, or even less. Note our average daily figure of 36.6Wh, which equates to about 3.6 watts over ten hours. And don’t expect to save any money: our ten watt system might produce eight kilowatt/hours per year, but at a cost of over £2 per Kwh (compared to £0.06 from the mains), assuming a ten-year panel life.
But if you have a hankering to run a solar vehicle, the means is clearly available.The Lafree/solar panel combination is relatively cheap, simple to use, and should provide 1,000 solar miles a year, even in temperate Britain, although we have yet to verify the winter figures… Despite all the talk of sustainability, no other vehicle or power system can match this sort of result for such a modest outlay.
Solar panels, inverters and other equipment are available from a number of specialist suppliers, principally yacht chandlers, such as Compass Watersports: www.compass24.com A guide to watts, amps and volts can be found on our web site: www.atob.org.uk
