Accurate wind speed data crucial for best results

Too many turbines are being installed on the basis of unreliable wind speed data, says a senior Met Office consultant.

Under- or overestimation of wind speeds can make the difference between a windfall and a serious shortfall of projected revenue. The bigger the investment, the bigger the risk, warns Steve Norman.

“Bigger is not necessarily better. It’s more a case of getting the right turbine in the right place. For that you need site-specific wind data,” he says.

Overestimation of wind speeds is the most common reason for installations not performing to the business plan and can lead to the wrong choice of turbine, to borrowings possibly being higher and payback periods being longer than necessary.

An under-realisation of just 0.1m/sec from an initial estimate over an entire project period of 20 years could lead to a drop in profits of about 10% in cash terms, says Mr Norman. Discrepancies of 1m/sec and larger can lead to much bigger shortfalls of 40% or 50%.

Virtual Met Mast reports

  • Offers site-specific wind speeds
  • Measured in m/sec
  • Long-term average wind speed at chosen hub height
  • Gives a measure of wind speed uncertainty, turbulence intensity and wind shear (difference between pressure at top and base of blades)
  • Maximum wind speeds (gusts, extreme winds)
  • Takes about two weeks to organise
  • Requires site-specific details such as OS reference, hub height and blade radius

However, underestimation is also an issue, leading sometimes to a site not being developed at all or to the wrong turbine being installed and to revenue being missed as a result.

To ensure your project does what it says in your business plan, you first need to know exactly how much wind you have, says Mr Norman.

“As FiTs reduce and the costs of planning, developing and operating a turbine increase, margins will fall and people need to make sure their plans will be profitable.”

He maintains that wind data is often the poor relation in a turbine business plan, with people often spending very large sums of money on planning, consultants and the equipment itself but relying on free data for their wind speed.

While there is free information such as the Numerical Objective Analysis of Boundary Layer (NOABL) data on the Department of Energy and Climate Change (DECC) website, this does not give the site-specific information needed for accurate planning, he says.

“This is the most commonly used data in turbine planning but the DECC’s site also warns the information is not warranted as suitable for any particular purpose or use.

“A family in the South West came to us for a second opinion after getting a NOABL wind speed rating of 6.5m/sec for their plans for a turbine. Our data showed that, in their location, the average speed was not even 5m/sec, so there was no point in them continuing with the plan.”

The Met Office runs its own Virtual Met Mast (VMM) reports, which are desk-based but are site specific and backed by its own wind speed data collection. These include more measures than simple wind speeds (see above) and can help make the decision not to proceed at all, says Mr Norman. The typical cost for one of these reports for a medium wind turbine is £820 plus VAT.

“VMM provides a fast, low-cost and accurate assessment of the long-term wind conditions, without installation of a site mast and is capable of assessing wind at lower hub heights, typical of the small and medium wind assets farmers might install. In developing the VMM, we have to be able to test and verify it against real sites every year.”

Lenders increasingly require this site-specific information to complete their due diligence procedures, which are the checks they have to carry out on business proposals.