Wind power

Wind power

The UK is the windiest country in Europe. Wind farms are therefore a great way for us to meet our energy needs without carbon emissions. Small-scale wind power can also be effective. However, a small turbine must be very carefully sited to be a cost effective and low-carbon option. For most of us, supporting and investing in large scale wind power is the more effective option.

CAT has been using wind power since we started in the 1970s, trying out different types of turbine. You can see examples and displays around our visitor centre, or learn more on one of our courses.

How big does a wind turbine need to be?

When harnessing the wind, there are two factors that make a big impact – the size of the rotor and the speed of the wind.

When you double the diameter of a rotor it makes the swept area four times as big. This is because the area of a circle comes from the square of the radius (A = π r²). A rotor with a 10 metre diameter will therefore capture one hundred times as much wind as one with a one metre diameter. This means that very small turbines can’t capture much wind.

The smallest turbines, costing a few hundred pounds, are fine for charging up a 12volt battery in a boat or caravan. However, to produce an output similar to that from a domestic solar PV roof, a much larger turbine would be needed.

A rotor diameter of about 1 metre could give you a few hundred kilowatt-hours (kWh) of electricity per year. This is about the same as one solar panel. To get as much energy per year as a whole solar roof array, you’d need a rotor about 4 metres across. For most houses this sort of turbine would not be feasible, but if you have enough land around your home it may be suitable.

How important is wind speed?

Doubling the wind speed will yield eight times as much power. This means that if you get half as much wind as expected, you’ll only have one-eighth as much power to harness. Because of this, putting a turbine in the windiest possible position is vital.

Many sites are not windy enough – especially at a small scale. Wind speeds increase significantly with height, so even small turbines need to be mounted on a high tower, away from buildings and trees. Those obstacles will create turbulence that reduces power output and increases wear and tear.

Where should I put a wind turbine?

An ideal site is a smooth hilltop with a clear open stretch – at least in the prevailing wind direction. Towns & villages don’t usually have open stretches like this, which is why they’re usually bad sites.

Before spending thousands of pounds on a wind turbine, we strongly recommend spending a few hundred on decent measuring equipment – such as a mast and data logger. Be wary of relying only on an online database that gives wind speeds for a given postcode or map reference. These don’t account for the local conditions that a small turbine will be affected by.

Can I put a wind turbine on my roof?

We strongly advise against mounting turbines on rooftops or buildings, as they will receive only weak and turbulent winds. Several field trials have found that building-mounted turbines perform poorly, producing only a small fraction of the energy they would produce if properly sited.

Clearly a turbine that does not spin will not save you money or reduce carbon emissions. In addition, turbines can resonate in the wind and produce vibrations, which can damage the structure of the building.

How much do small wind turbines cost?

A turbine plus tower and installation might cost between £2,500 and £6,000 per rated kilowatt (kW). When calculating installation costs it is important to factor in the tower and other system components.

Turbines are built to operate without much maintenance, but some moving parts of the turbine may have to be replaced during its lifetime of around 20 years. If an inverter is needed, then this is likely to need replacing within the life of the turbine.

How much will it pay back?

Whatever you can use directly will save you about 16p per kWh, based on current electricity prices.

You can now get payments back under the Smart Export Guarantee (SEG) scheme. This applies to any wind-generated electricity that you export to the grid rather than using directly. Therefore, to claim SEG payments you will need a type of smart meter able to measure exported electricity (which many first generation smart meters cannot do).

You should shop around and compare the prices offered for your exported electricity. Some companies may offer a decent SEG price of over 5 pence per kWh, but some might pay 1p per kWh or less. The Solar Trade Association (STA) have compiled a league table of tariffs. The STA says a fair price is between 5 and 6 pence per kWh. That sort of price is comparable to the wholesale market rates that large generators get for selling electricity.

Community Power

Large-scale wind turbines can harness the much stronger winds on hilltops, away from towns. Investing in a big community owned wind turbine is therefore usually the best way for most of us to benefit from wind power.

Find out more

See our questions and answers section below and the books we sell, such as Wind & Solar Electricity.

You could also come for a day visit to CAT or attend one of our short courses. Our short courses on DIY wind turbines are based on the robust designs of Hugh Piggott (Scoraig Wind).

To find suppliers you could use the Microgeneration Certification Scheme (MCS) listings of accredited installers and products.

This page was written by CAT’s Information Officer Joel Rawson. You can contact me with further questions (choose ‘free information service’ on the form) – for example if you’re looking for some specialist installers of small off-grid systems.

Related Questions

How much power will a wind turbine produce?

The ‘rated power’ of a wind turbine, given in kilowatts (kW) is the power produced at its ‘rated’ wind speed. This speed is quite high – often 10 or 12 metres per second. Different turbines have different rated wind speeds, so don’t just go by the rated power when comparing them.

The power output at lower wind speeds will be substantially less. For an overview of the performance of a turbine, look for a ‘power curve’ – a graph of power output against wind speed.

wind turbine power curve

Domestic wind turbines are typically rated between 1kW and 10kW. A very small turbine for use on a boat might be rated at only 100 to 200 watts.

Many factors affect how much energy you’ll get, but a conseravtive rule of thumb is that a 1kW turbine on a reasonably good site could generate 1000kWh over a year. On a better site, an annual output of 1500kWh per rated kW could be possible.

 

 

What is a windpump?

A windpump is used for raising water from a borehole to provide a water supply or irrigation. The design uses a fairly large multi-bladed rotor, mounted on a high tower.

Over the last 150 years or so, windpumps have been popular in America and Australia to provide water for agriculture. More recently, windpumps have been used in developing countries.

We’ve had a windpump at CAT for many years. We have no need to pump water from the ground, so ours moves a seat up and down to demonstrate the power available.

Windpump at CAT

They are quite site-specific, requiring a steady moderate wind to start up and keep turning. You’ll need to monitor and evaluate your site to properly size one. A 3.5 metre diameter rotor operating in a windspeed of 3 metres per second could pump 45,000 litres a day from a depth of 5 metres.

Windpumps have a relatively low capital cost and long lifetime (about 25 years). They’re good for off-grid sites, as they’re entirely mechanical – no need for electricity to run a controller, for example. However, they do require a 6-monthly service, and the pumping efficiency drops off in high wind speeds. A windpump may cost several thousand pounds, but could be a cost-effective solution for a large scale application.

For more information on a DIY option, CAT’s online store sells the book How to Build a Wind Pump. For commercial opions, the Poldaw Windpump is manufactured around the world but unfortunately the UK supplier (GB windpumps) recently ceased trading. The Bosman windpump is a design used in the Netherlands.

How can I pump water using wind or solar power?

A renewable energy technology could be ideal for pumping water where there is no mains electricity available, as a grid connection may be expensive and a diesel generator noisy and polluting.

Sizing a pump

The two main factors to consider when seeking a suitable pump are the flow rate – the amount of water that the pump will deliver, and the head – the height through which it will raise the water. These are related, as increasing the head will decrease the delivered flow. It’s important to minimise bends and other friction losses in pipework, as navigating these will require greater pressure, and as pressure and head are directly related, this effectively means a greater head.

Manufacturers’ technical data sheets will give the performance range of each pump, with graphs showing optimum combinations of flow and head. A pump sized properly to your needs will operate most efficiently. Suction pumps are limited to a depth of a few metres, so to draw water from a well or borehole, you’ll almost certainly need to lower in a submersible pump. Pumping wastewater or sewage necessitates one designed to handle drainage or effluent.

Small electric pumps for circulating water could cost tens of pounds, whilst those for drawing water from a well or borehole supply are likely to be a few hundred pounds. The main cost will be providing power to the pump, particularly when off-grid. Therefore, do first take all appropriate water-saving measures (such as spray-head fittings, mulches on plants to minimise water loss, etc) as these easily pay for themselves in the energy saved by reduced demand.

Off-grid electric pumps

Meeting a year-round water demand with a renewably-powered pump may require a combination of PV panels and a wind turbine, as this will balance energy production over the year. Sunshine and wind are naturally intermittent, so you may need some form of storage. Pumping water up to a tank (with demand then fed by gravity) during sunny or windy periods is more efficient than transferring the energy to batteries. If storing lots of water, you’ll need to balance the costs of a large tank (and supporting structure) against the costs of batteries (and their environmental impact and toxicity). An inexpensive control system can pump when needed, and otherwise divert power to batteries, giving extra backup facility.

The price of a small-scale renewable energy system will depend on the power and the maximum capacity needed. A very rough estimate is around £5 to £10 per installed watt. Siting generating equipment close to the pump minimises the cost and power loss incurred by cabling. As small turbines and PV panels usually produce power at 12 or 24 volts, a low-voltage pump would enable you to do without a costly inverter (for stepping up to 240 volts).

Mechanical pumps

For larger-scale pumping applications, you can avoid the losses in electrical systems by using mechanical power directly. See for example the question on our wind power page about wind pumps, or the question on our hydro power page about hydraulic ram pumps.

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