wind power: some home truths
Up front I will make a candid admission that I've always had a pragmatic view of renewable energy; it was okay if it could save me some money, but I wasn't on a crusade to save the planet. I suppose this stems from a long term involvement with the fossil fuel industry, where in my defence, I did what I could to raise awareness of the looming threat posed by the "Greenhouse Effect" long before it became a political football. I did this within the companies I worked for (including Griffin Coal's website) and the wider community. Needless to say, it wasn't always appreciated.
The debate still rages about the relative benefits of onshore vs offshore wind energy and considerations of visual amenity, noise, bird strikes, fires and inconvenience to migrating whales (which is a bit patronising to whales as they are more intelligent than a lot of people). Anyone who has any doubts about the levels of disinformation promulgated by the likes of Andrew Hastie MP, the anti-renewable lobby groups and the unfortunately conflicted pro-renewable (but not in my backyard) protesters, should watch ABC's 4 Corners episode "Wind Wars" from 2024.
I put "Home Truths" in the title, however, because I wanted to talk about small scale wind power. In the 80's and 90's I was very skeptical of solar panels and thought I wouldn't buy any until someone built an aluminium or silicon smelter powered by the things; they also cost over $10/watt, which was a lot of money then. Solar panels were added to our house only when the State Government of the day offered generous incentives to do so, and concerns about the invested energy in them had been largely dispelled. It was immediately apparent that there were huge gaps in the daily and seasonal availability of solar power, and that (where we were, at least, in the south west of WA) there was a roughly inverse availability of wind power. I set about doing a bit of research and was tempted by the availability of numerous small cheap turbines on eBay (and nowadays other platforms including Aliexpress, Temu etc).
To capture all of the kinetic energy of the wind in a turbine, you would need to achieve zero wind velocity on the downstream side; obviously that is never going to happen. A chap called Betz proved over 100 years ago that the theoretical maximum efficiency is actually 59.3%; most small scale turbines struggle to achieve 35%, with horizontal axis turbines better than vertical axis ones. Derivation of the kinetic energy of wind is complex, but close to sea level, it is around 0.61 x V3 x A in watts, where 0.61 is a factor representing the density of air, V is the wind velocity in m/sec and A is the swept area of the turbine in square metres. Divide that by 3 to get the maximum likely output wattage of your turbine.
It is essential to note that wattage is proportional to the cube of the wind velocity, which means that if you halve the wind velocity, you only get one eighth of the power. This means that a turbine rated for 2000 watts at 10m/sec is the same as one rated at 250 watts at 5m/sec. Now 10m/sec is a fairly strong wind (36km/hr) that you aren't going to experience very often or at best, for much of the time; 5m/sec might be more realistic in most of the country. It is also worth knowing that wind speed increases in roughly logarithmic fashion with height above the ground or other obstacles; if your Council won't let you put up a 20m tower you mightn't get much wind anyway.
But what about those small turbines on the Internet?
This is sold as a 1200W turbine, without any information as to the wind speed needed for such an output. It has a diameter of 1.2m, so has a swept area of 1.13 square metres. From the equation above we get an estimated output of 0.61x103x1.13x0.33=230 watts, if it is 33% efficient and in our fairly strong 36km/hr wind. To get 1200W out of it you would need a steady gale of over 60km/hr (if it didn't destroy the turbine).
These lantern-style turbines are also claimed to have a 1200W output, rated at a wind speed of 12m/sec (a bit fast, but thanks). They have a diameter of 900mm and height of 630mm, so have a swept area of a bit under 0.5 square metres. In a 12m/sec wind we would therefore expect about 0.61x123x0.5x0.30=158 watts (allowing for a slightly reduced efficiency because it has a vertical axis). To get 1200W out of it you would need a strong gale of around 85km/hr (bye bye turbine).
This smart-looking turbine is sold as a 3kW model but has a claimed output of over 2000W at a wind speed of 18m/sec (getting a bit silly now). It is 530mm wide and 750mm high, so has a swept area of just under 0.4 square metres and a likely output at 18m/sec of 0.61x183x0.4x0.30=427 watts. At a more realistic wind speed of 10m/sec you may struggle to get 75 watts.
Are we starting to see a pattern here? These are not wind turbines, they are garden ornaments, so buy a pretty one if you must (often you can get different colours) put it where you can see it, and enjoy the scene. If anyone has found one that works, please drop me a note.
There is a very useful and informative article on small wind turbines here, which discusses matters in a US context and makes my assumed efficiencies look fairly generous.