Wind is better than solar. Discuss

[Na Mara]

So I’m still trying to work out what the best power system for Na Mara is. I thought I had it when I decided on a 4kW generator coupled with a big Lithium bank and a fat inverter/charger but then my Dad went and fitted a new silentwind pro onto his boat and now he’s raving about it so I’m having to reconsider. Being a big nerd I decided to crunch some numbers and I thought I’d share this with the community.

To figure out whether a silentwind pro would work for me I closely approximated the output curve with an exponential function. I then found the area and season that I am likely to cruise in that has the lightest winds. That happens to be the Baltic Sea during the summer. The daily average winds there at that time have a mean of 5m/s (10kn) with a single standard deviation of 1m/s (2kn). Assuming that the wind speeds are normally distributed, I then calculated the average output of a silentwind pro by calculating the average value of the output curve for winds between 0 and 15m/s (Integrating the product of the output curve and pdf for the wind distribution between 0 and 15). The result was 36W, which translates to an average daily energy input of 860Wh or 72Ah at 12V. That is just enough to cope with our needs at anchor but no where near enough to cope with them underway. However, two silentwind pros would cover all our needs with a bit of spare capacity even on this worst case scenario.

In this worst case scenario - from a wind power perspective - of being in the Baltic during summer the capacity factor of my wind generators would be less than 9% and the capital cost per daily Wh would be in the region of 3.5€ (silent wind pro plus mast, controller and installation 3000€)

Now compare that with solar. Average monthly solar radiation in the southern Baltic in summer is around 130kWh/m2. Assuming 30 days in a month gives you a daily average of around 4.5kWh/m2. Assuming 20% efficient panels gives you average daily energy input from each meter squared of panels aboard of 900Wh. So as near as makes no difference 1m2 of solar will give me the same energy input as a silentwind pro in this worst case for wind. Again, to meet our needs we would need to double this so we would need about 2m2 of solar. To mount 2m2 of solar - about 350W - on Na Mara I would either need davits or an arch. Either of those plus panels and sundries comes to at least 5000€ installed and so the capital cost per daily Wh is at best 2.8€.

The Baltic in summer is a best case for solar and a worst case for wind. High pressure systems from Siberia tend to move in over the Baltic during the summer and settle there. These together with the high latitude lead to long cloudless days in summer with very little wind. Sailing pretty much anywhere else around these parts, such as Skagerrak or Kattegat or off into the North Atlantic, there will be more cloud and more wind. As output from wind goes up by the cube of wind speed, the above calculation rapidly shifts in the wind turbine’s favour if we sail anywhere else than the Baltic in high summer.

Of course an arch that carries 3m2 of panels isn’t much more expensive than one that carries two, so if Na Mara were a more hungry power consumer, then the calculation above would be more in solar’s favour with a capital cost per daily Wh of a bit less that 2€. However, that extra power would be wasted on Na Mara and the bigger arch would just increase windage, ugliness and capital cost to no practical end.

All in all it would seem that a pair of wind generators charging a very large bank - to smooth out the feast and famine nature of wind energy - would do the trick and for Na Mara better than solar. Basically, unless my calculations are way off, I have to disagree with all the “solar is all you need” crowd on this forum. Modern wind is virtually silent, easy to regulate through Bluetoothed apps, perfectly capable of more than meeting all the energy needs of a moderate cruiser even in relatively low wind zones, and price comparable on a capital cost per energy produced basis (at least this is so for those of us not sailing boats already sporting suitable structures for mounting panels).

One concern with this reasoning is that I, like manus herr, am also wanting to do an Atlantic circuit and everyone keeps saying that the reduction in apparent wind on wind generators makes them borderline useless on such trips. But the trades run at an average of 10m/s with a single standard deviation of 2m/s when yachts do the crossing so even running dead downwind at 6kn (3m/s) the average apparent wind will be 7m/s with 2m/s SSD. That gives an expected average output for the wind genny of about 66W for the crossing producing an average of 1.5kWh a day. That is almost enough to meet my needs on the crossing on its own. Add another wind generator - which I would need in the Baltic anyway to meet my needs summer cruising - and I would luxuriate in over 3kWh a day from the pair. So this concern about wind and power on downwind legs is, according to my calculations, also without foundation.