Dependable outcomes

The power characteristic, or power curve, is a key factor in engineering that helps determine the economic success of a wind turbine. To verify the guaranteed energy yield, the operator should survey the proposed site and take wind measurements according to a comprehensive specification.

Before a prospective wind farm project is ready to enter the planning stage, a number of essential assessment criteria, such as wind conditions, turbine types, infrastructure and efficiency, must be evaluated to determine the feasibility of the project. At this point, it is not necessary to conduct a full-scale, detailed analysis, which would involve significant time and effort. Instead, assessing certain key criteria summarily will provide meaningful and entirely adequate information. The results of this preliminary assessment form a solid basis for deciding whether the project should proceed. Once the actual planning process has been initiated, an extensive, in-depth study of all relevant aspects of the project will follow.

Focus on the power characteristic

One key aspect in determining the technical requirements for a prospective wind turbine and in assessing its long-term economic feasibility is the power characteristic of the envisioned system. The power characteristic reveals the power yield to be expected under specific wind speed conditions. It serves as an indicator of the output behaviour of any specific wind turbine type. It allows an operator to compare various machine types and select the type best suited for the given site in terms of annual energy output. The annual energy yield can be calculated by integrating the power curve over the wind speed distribution at the site.

Ignoring the power characteristic can have severe consequences for the operator’s return, since deviations in the operating behaviour will directly affect the financial feasibility of the power plant.

To mitigate this risk, purchase agreements usually contain a clause guaranteeing that the manufacturer will compensate the operator for any damages caused by deviations of the power characteristic from the type specification. Typically, such clauses cite ‘95 per cent of Annual Energy Production (AEP)’, referring to the relevant technical specifications.

Produce evidence

In wording such warranty clauses, operators should make sure the contract provides for an appropriate method of verifying conformance with the power characteristic once the power plant is in operation. Without a contractually-agreed form of evidence, the guarantee will be essentially worthless. The only way to prove non-conformance once the wind farm is in operation is to take measurements on single turbines located at the perimeter of the site.

The specifications set clear, tight limits for ambient conditions at the site under study, in particular regarding existing obstacles in the path of the wind (such as buildings, trees, turbines) as well as elevations and uneven terrain.

It is therefore essential to assess the measuring conditions at the site as early as possible, so as to ensure the validity of the warranty clause. In the event that the standard conditions as defined by the specifications cannot be met due to the particular situation at the site, the guarantee clause in the contract should be adapted to reflect the actual conditions.

Options include:

• Perform a site calibration pursuant to the IEC standards prior to constructing the power plant
• Negotiate site specific deviations from FGW TR2 and/or IEC 61400-12-1
• Agree on one specific wind turbine that can be measured as stipulated in the standards, to be used as a reference unit for the entire wind farm
• Take measurements based on the current draft version of IEC 61400-12-2.

It should be stated explicitly in the guarantee clause that the power characteristic of the entire wind farm should be used as a reference (wind farm power characteristic dependent on the wind direction, in analogy to the draft version of

IEC 61400-12-3), rather than relying on the power characteristic of individual turbines.

Measure the power characteristic

Both the site itself and the surroundings of a wind turbine play a critical role in determining power characteristics. In flat terrain free of obstacles, it can be assumed that the wind speed measurement on a measuring tower will be equivalent to the wind speed at the centre of the rotor area (hub). This basic assumption does not apply in complex terrain, however, which is why the site under investigation must be calibrated prior to building the wind farm.

The resulting factors will then be added to the wind speed readings associated with the actual power measurements.

When choosing a suitable site for a measuring tower, every attempt should be made to avoid the lee side of obstacles and adjacent wind turbines – apart from when purely practical considerations such as property ownership or the existence of firm ground conditions to support the tower base are necessary. This will ensure that the free geometric sector available for evaluation will be as large as possible. This sector must be oriented towards the prevailing direction of the wind. Following these rules will help minimise the measuring time and therefore the associated costs.

Select the criterion

IEC 61400-12-1 offers the user two criteria for establishing a complete set of data.

The first criterion involves measuring up to 1.5 times the wind speed at 85 per cent of the nominal power output. For a unit with a rated speed of 12 metres per second, this would be around 17 metres per second.

Since this criterion may not be practical for follow-up measurements, the AEP criterion was defined as an alternative. It stipulates that the data set is considered complete when the AEP, based on the readings, reaches a minimum of 95 per cent of the AEP extrapolated up to the cut-out speed. This is usually easy to achieve for lower to medium wind speeds since the contribution of high winds to the overall frequency distribution, and therefore to the extrapolation, is minimal. It makes sense to continue taking measurements until the AEP criterion has been met, up to the site-specific medium wind speed. This should be defined in advance by mutual agreement.

To allow the measurement of power characteristic to be compared to the guaranteed characteristic it is advisable to agree on a suitable method of comparison, if possible based on the AEP comparison. The standards leave some room for interpretation. Contracts should therefore contain appropriate stipulations to provide a definitive reference framework.