Man has harnessed the forces of nature since prehistory. He still does. The technology might have changed, the scope widened by recent knowledge, but the principles remain the same. The wind has filled sails, both at sea and on windmills. Waterwheels have ground corn and powered primitive machinery. The sun dried early pots and bricks before kilns were invented.

And there’s something immensely satisfying in employing commodities that are free. It’s not often one gets something for nothing – nothing after you’ve paid for the gear, that is.

Wind turbines
Many early designs were ‘generators’ ­– basically electric motors running in reverse, producing DC currents by way of commutators. The modern trend, however, has been overwhelmingly towards 3-phase alternators, similar in many ways to engine types but using permanent magnets (rather than field windings) for excitation. An internal rectifier converts the AC current to DC

Although engineering quality might vary from model to model, claims of superior output can be questionable. This is because the factors that govern output are the same for all – the area swept by the blades and the rotation speed. In short, the longer the blades and the faster they whirl round, the more electricity you will get. In their efforts to out-gun their competitors, one or two manufacturers overcooked the speed option and a number of skippers found themselves receiving complaints from other boats – even banishment from marinas because, unfortunately, high revs inevitably produce lots of noise, and it has been irrefutably established that banshee wailing does not a good neighbour make. Thankfully, most wind turbines now incorporate some means, either mechanical or electrical, of taming their excesses before they become intolerable. Some are fitted with remote ‘stop’ switches that short out the stator coils, creating enough magnetic drag to bring them to a virtual halt.

Caution: Watch out for those blades. They can cause serious injury. Wind turbines should be mounted at a safe height, well above easy reach.

As can be seen from the graph below, the output from a powerful generator becomes substantial as wind speed increases. At a relatively modest Force 7 you would see 350W to 400W (around 30A at 12V) and it would climb further if allowed to run away unchecked.

Whereas engine alternators are regulated by adjusting the power to their field coils, this isn’t possible with permanent magnets. Yet some form of regulation is clearly necessary to defend the batteries from overcharge. Once the batteries are fully topped up, the most usual method dumps the excess power to a large ballast resistor (or two) which dissipates the energy as heat. Another makes use of magnetic drag to reduce the generator to a sedate spin.

Incidentally, don’t assume because your wind turbine is turning it’s actually charging. About 6 knots of apparent wind is necessary before the generator ‘cuts in’. Also don’t be disappointed if your wind turbine doesn’t quite come up to the manufacturer’s claims. These tend to assume optimum conditions and could even be mathematically derived outputs. Regard them as guides rather than absolute predictions.

Water powered generators
These use the same basic generator units as wind turbines, but the drive power comes from a turbine immersed in the water – either on a rigid strut or towed astern on a rope. These are popular for downwind passages where the apparent wind strength diminishes, but they’re obviously useless at anchor or alongside. Recognising, their lack of versatility, a few manufacturers offer convertible versions that can be converted from water to wind power.

What can they contribute?
Maximum outputs are all very well but it’s more important to know what can reasonably be expected over a 24 hour period. This depends on season and location. The mean wind speed for south-west Britain in July is about 10 knots. Referring to the wind turbine graph shows us that that particular generator would, in theory, yield about 45 watts – 3.75A at 12V, about 90Ah over the day. By contrast, if the same boat was in West Indies in December (the high season) the trade winds might deliver 75 watts – a healthy 150Ah – enough to sustain many cruising boats without running their engines at all.

Naturally, mean wind speeds relate to open water, and you would have to compensate for time spent in sheltered havens, but the fact remains that a generator’s contribution can be very useful indeed.

Solar panels
Considering the enormous commercial and environmental potential tantalisingly presented by solar energy, it’s not surprising that the field of ‘photovoltaic’ (PV) technology enjoys vigorous development.

The majority of PV cells use silicon as their main constituent. In its pure form silicon is a poor conductor but, when deliberately ‘doped’ with impurities – phosphorus and boron being common – it becomes a ‘semi-conductor’. The phosphorus doped silicon has a negative charge while the boron bestows a positive charge. All PV cells have at least two layers of each. When exposed to sunlight, ‘photons’ (light particles) nudge electrons loose from the negative layer and electricity flows across the junction to the positive.

Silicon cells are those familiar iridescent dark blue ones and these divide into ‘monocrystalline’ (small individual cells) and ‘polycrystalline’ (larger cells). The first type is the most efficient – 15% as opposed to 13% but the bar is rising on these figures almost daily. Both are pretty good in sunny conditions.

However, crystalline silicon cells are expensive to produce, so manufacturers are looking at more economical alternatives. Non-crystalline amorphous silicon is used on flexible PV panels – relatively cheap, but only about 6% efficient.

A very promising newcomer doesn’t use silicon at all, instead relying on copper indium diselenide (CuInSe2 or ‘CIS’). CIS cells don’t have quite the same peak output as the best silicon types but they respond to more of the light spectrum, meaning they function better over a broader range of conditions – dawn, dusk and cloudy, for instance. Judged over a variable day’s output, they could actually be more productive. More importantly, they lend themselves to high-volume production, with all the cost savings that brings.
Obviously, the larger the array the more energy you can harvest. For a good quality panel, quoted figures indicate about 120W per square metre (much less for flexible panels) meaning a 10A current at 12V. But this is under ideal conditions, with a cloudless sky, a temperature of 25°C (output drops as PV cells become hot) and the panel pointing directly at the sun. You would probably only see this sort of performance at lower latitudes where the solar radiation is strongest (see below). However, summer days do lengthen the further north or south you stray so this at least partially compensates. As with wind generators, all but the smallest units should be regulated.

Overall, the average array packs nothing like the punch of most wind turbines but they can serve a very useful purpose. Working unattended and not prone to mechanical failure, they will maintain the battery charges levels for you when you’re not on board.