In Chile, a wind farm is assessed within the Sistema de Evaluación de Impacto Ambiental (SEIA, the Environmental Impact Assessment System) on its worst-case scenario: the wind condition that produces the highest noise level at the most exposed receptor (downwind). This is what the specific guidelines of the Servicio de Evaluación Ambiental (SEA, the Environmental Assessment Service) require, and it is the right way to demonstrate that a project is viable. The difficulty comes later, once the wind farm is operating, because a wind farm does not run in its worst-case scenario: it runs all year round and under very different conditions, many of them with noise emission levels far below the downwind condition.
Why the SEIA assesses the worst case
The logic is conservative by design. If compliance is demonstrated under the most unfavourable condition, it is also demonstrated for all the others. Before the Resolución de Calificación Ambiental (RCA, the environmental approval) the question is whether the project can be built, and answering it requires the most demanding case, not the average.
The reference regulation is Decreto Supremo N°38/11 of the Ministerio del Medio Ambiente (MMA, the Ministry of the Environment), «norma de emisión de ruidos generados por fuentes que indica» (emission standard for noise generated by specified sources), and D.S. N°14/2024 of the MMA, which is gradually replacing it.
Real operation does not look like the assessed scenario
Wind speed and direction change from hour to hour, and the critical condition — the one used for the assessment — may occur for only a few hours a year. When the only assessment available is the worst-case one, the compliance measure adopted tends to apply to the whole wind farm and permanently, which unnecessarily restricts its energy generation: turbines running in reduced mode at hours and wind directions where there was no exceedance to avoid.
Assessing condition by condition
The alternative is not to assess less, but to assess more often. Instead of a single assessment on the worst-case scenario, compliance is assessed under each operating condition. That requires four things:
- Continuous noise measurements at the receptors, covering the range of wind conditions that actually occur at the site.
- Synchronised data on wind speed and direction, energy generation and the actual operating modes of each turbine during the campaign.
- Acoustic modelling of the wind farm under different operating conditions, by wind speed and direction. We work with CadnaA.
- Assessment of compliance under each of those conditions, not under just one.
If the wind farm is already operating, site conditions are measured rather than estimated. This is the most accurate situation for optimisation, because the turbines’ real data are available.
Fourteen days is the minimum, not necessarily enough
The SEA guideline on wind farm noise and the proposed measurement protocol for D.S. N°14/2024 set a minimum of fourteen days of measurement. We recommend at least one month per measurement point.
The reason is practical: fourteen days may not yield enough information on the wind farm’s noise emission to reach a correlation coefficient of r² ≥ 0.6 between the measured noise level and wind speed. The regulatory minimum and the technical minimum are not the same number, and the difference is paid for by having to assess regulatory compliance through modelling (if the r² value is not achieved). And, in general, we have found that wind farm noise prediction models tend to overestimate noise levels when compared with field measurement data.
What you get at the end
Not a compliance report, but an operating criterion: which turbines, at which wind directions and speeds and at which times should run in reduced mode, with the sound power level corresponding to each mode, and an estimate of the generation associated with each strategy.
The restriction stops being an all-or-nothing decision on the entire wind farm and applies only where and when it is needed.
When to do it
At three stages. At feasibility, while the investment decision still allows design changes. Before construction, to anticipate which conditions will turn out to be critical. And in operation, when the wind farm is already generating and the commitments made in the RCA must be met.
If you are assessing a wind farm at feasibility, during permitting or in operation, see our wind farm acoustic optimisation service. Would you rather talk it through? Book a 20-minute call: we will carry out a free preliminary diagnosis and send you a free technical proposal within 1 to 5 working days.
He leads Acústica Austral’s environmental noise and vibration studies for the SEIA across energy, port and infrastructure projects, and has contributed to reference work for Chile’s Ministry of the Environment.
Meet the team →Free resources for your project
Guide to the D.S. N°14/2024 and the acoustic checklist for entering the SEIA, in PDF. We email them to you.