MYC RIGGULSFORD continues his series on renewable energy by taking a look at wind power
Our near neighbour, Bob Boothby, and his partner, Kate, run a successful holiday cottage business based around an old water mill, and they have already put in a woodchip boiler for heating, a water turbine and solar panels. Now they are trying to erect a domestic wind turbine as well (see case study, next page). When they put in the planning application, they got an immediate reply from the planners wondering whether Bob knew that he was proposing to site the turbine on top of a hill and asking if perhaps he could put it down in the valley next to his holiday cottages where it would be less visible.Since ancient times people have been harnessing the wind for work – to separate wheat from chaff, to power sailing boats and drive windmills. Wind turbines work by intercepting a flow of air and turning the sideways force of the wind into rotational energy, which is converted to electricity by a generator, usually using a system of gears to step up the speed.Some of the sun’s energy falling on earth is absorbed in the atmosphere, causing different parts of the world to heat up at different rates. When air is heated it expands, causing different pressures around the world, resulting in winds as the air moves, and adding to the winds caused by the earth spinning. This gives us our global wind pattern. On top of this are local wind patterns, such as sea breezes and mountain or valley winds. The UK has the most wind in Europe – the Energy Saving Trust claims that we have 40% of Europe’s wind resource, but it varies across the country, with many of the best sites in high mountain areas. Onsite monitoring will help you assess whether it’s worth investing in a wind turbine at your smallholding. However, recent results of an extensive wind turbine trial by renewables consultancy Encraft have shown that the same turbine can generate 20 times more electricity in one site than another, and that urban sites are probably not worth the investment involved.Bigger wind turbines are more efficient since wind speeds increase hugely with distance off the ground, so most commercial wind farms monitor at a height of 25 metres. The Government expects that offshore wind will be the most significant renewable energy source for the UK in the long term though, in spite of all Britain’s claims of climate leadership and our complaints about other irresponsible countries, the USA has now installed the most commercial windpower in the world, with China a close second. In the UK we have around 14,000 freestanding domestic turbines and 14,000 building-mounted turbines although, following Encraft’s survey results released in January this year, the DIY chain B&Q has announced that it will no longer stock the �1,900 Windsave turbines that went on sale in 2006 and which were claimed to generate 1kW of electricity when wired directly into a house ring main.Most of the modern wind turbines in the UK are horizontal axis with two or three blades, rather than the four paddles of a traditional Dutch windmill, but newer designs include vertical axis turbines. The latter have some advantages since the gearbox and generator can be sited directly on the ground, and so the mounting pole or tower does not need to support their weight.
Trouble with turbulenceBut all turbines are affected badly by turbulence, and this is usually much greater at ground level, around buildings, in urban areas and near trees, which is why the usual horizontal axis turbines are mostly mounted on tall poles or towers, held in place by guy wires, and ideally at least twice the height of any nearby obstructions. Domestic wind turbines range in size from 1kW to over 2MW (2,000 kW). An average wind speed for a site could be made up from mostly calm weather with occasional gale force winds – neither of which will allow you to generate power. Manufacturers’ ratings are calculated at winds of about 10-12 metres per second, but 4-5 metres per second is more realistic. Imagine the turbine is cutting out a cylinder from the air as wide as the blades and as long as the wind speed per second. The power depends on the mass of that cylinder of air.“The kinetic energy intercepted every second (the power) is proportional to the cube of wind velocity. Hence power rises very rapidly with wind speed,” says Dr Patricia Thornley, from Manchester University’s Tyndall Centre for Climate Change. “Every wind turbine has a wind speed-power curve which shows how power output varies with wind speed. Generally there is a cut-in speed below which the machine will not generate, and then rising up to the rated power. Further increases in wind speed do not achieve any higher power output and, in very high winds, the machine will automatically shut down.”I’ve been trying to get to the bottom of the rumours that most wind turbines fail to live up to manufacturers’ claims and have much longer payback periods than you expect. Shutdowns for plant maintenance are usually necessary only one or two days each year and, as Britain has 40% of Europe’s wind, our supply cannot be that intermittent. So why don’t wind turbines generate the power you expect?
Why so little power?I asked renewables supplier Andrew Moore, of British Eco, why turbines produce so little power. After all, if you have a 6kW turbine, and the wind blows half the time, that should generate 6kW x 12 hours x 365 days = 26,000 kWh or 26MW of electricity which, at current prices of 10-15p per electricity unit, should pay you at least �3,000 per year. This would make the payback on a typical 6kW wind turbine costing �15,000 only about five years – but the reality seems to be twice or three times longer.“There are a number of reasons why the outputs of wind turbines fluctuate, but the main factor is wind speed,” says Andrew Moore. “The usual misconception is that a 12kW turbine is twice as powerful, or will generally produce twice the amount of energy, as a 6kW turbine. This is not necessarily true.“It is very unlikely that a wind turbine, even in an ideal situation, will constantly experience its optimum working conditions and therefore will not continually produce the maximum amount of energy that it is capable of, as outlined in the manufacturer’s guidelines,” says Andrew. “Realistically speaking, no small turbine – less than 12kW – is more than 30 per cent efficient. This is based on the models that are currently available in the marketplace which operate at full capacity at 10-12 metres per second in high winds – approximately 27mph winds. With a good site match and assuming good maintenance, we can estimate an average 30 per cent or more mean output over five years.”Based on Andrew’s assessment, that would mean cutting my calculation by a third, but that would still give you a theoretical annual income of �2,000 per year from a 6kW turbine, if the electricity companies paid retail rates for the power you generate, although some only pay the wholesale rate of about 5p/1kW unit. Andrew’s figure of wind speeds of 10-12 metres per second seems a bit high too – the British Wind Energy Association reckons that speeds of 4-5 metres per second should make installing a turbine worthwhile, and claims that a 5kW machine will produce 13,000kW per year on average, nearly three times the electricity used by an average household.
A rule of thumbPaul Baker, of Devon Association of Renewable Energy, explained that the rated output of a wind turbine can be thought of a bit like the fuel efficiency of a car. Just because the top speed is 120mph and the manufacturer claims that it will do 35 miles to the gallon, it doesn’t mean that we can realistically expect to hurtle round the country using minimal fuel the whole time.“As a rule of thumb, if you instal the turbine in a decently windy site, you can expect to generate 25-30% of the manufacturer’s rated output,” says Paul. “Commercial wind farms, with much bigger scales, reckon on a load factor of 28%.”That would drop my calculation for a 6kW turbine to an electricity income equivalent of around �1,000 per year, which sounds much closer to reported levels. Paul also tells me that the smaller turbines are much more badly affected by turbulence, cutting their efficiency still further, so turbines in towns are particularly badly affected.However, the good news is that, from this April, domestic energy producers will qualify for double Renewable Obligation Certificates – previously worth about 4.5p/1kW unit of electricity generated, which from this month will now be worth 9p per unit. “If you use as much of the electricity as you can yourself, and get paid by the electricity companies for your ROCs, you can multiply the value. Up to 50kW domestic installations should now be able to get deals from their electricity supply companies,” says Paul Baker. “Up to April this year, for under 5kW turbines, EDF were offering 5p/kWh for everything you produced while Scottish & Southern were offering 18p/kWh for electricity exported to the grid.”In February, it was rumoured that the Government will introduce a feed-in tariff, which would replace the complicated ROC system, and provide even better incentives for microgeneration using wind turbines. If we adopt the German system (which I favour) this could mean payments of up to four times the retail price of electricity for any power you generate. So, instead of paying 10p per unit, you could receive 40p per unit or more for each 1kW you generate. Personally, I’ll wait to see what the Government actually offers before investing. So far in Britain it has just been talk and, in the current downturn, many of the most polluting companies in the UK are now cashing in the free carbon credits they were given, generating cash windfalls at taxpayers’ expense. Personally I also cannot see any reason why a hedge fund should be allowed to own any carbon credits at all, or cream off profits for overpaid vulture bankers.The Encraft survey looked at several different models and manufacturers of wind turbines ranging in cost from �1,900- �5,400. Encraft found that many of the turbines performed at less than one tenth of the manufacturer’s predicted energy output, and some only generated one twentieth, or 5%, of the expected electricity. In fact, one performance was so bad that the electronics of the turbine equipment used more power than it generated. Average energy generation was 628Wh of electricity per day when the turbines were switched on, dropping to 214Wh of electricity per day if every day of the year was included, while energy consumption of the turbines averaged 80Wh per day.
Smart electronicsAs well as the turbine itself, you are probably going to need some smart electronics to monitor, start and shut down the equipment, and an inverter to connect your direct current output to the grid, so that you can simply use or export any surplus electricity you generate to the grid. Otherwise you will need a massive, expensive and short-lived bank of batteries to store your electricity, and you’ll have to equip your farm with yacht technology designed to run on 12vDC current.Installation is relatively easy as most turbines come ready assembled from the manufacturers; you often just need small digger and delivery van access to the site. Pole and tower mounted turbines need a solid concrete block about 3m by 3m by 1m to support the pole, and another smaller block about 1 metre cube nearby for a winch anchor, which will also allow the turbine to be raised and lowered for annual maintenance. Your biggest headache by far is likely to be getting permission for a sufficiently high pole past the local planning department. All turbines vibrate, especially in turbulent air, so building mounted kits could shake older houses to pieces, and buildings create their own turbulence. You’re also going to get noise concerns from the neighbours, usually unfounded.