Until recently, wind turbine manufacturers and operators were challenged by the tasks of keeping machines operating reliably and improving energy capture. Although dramatic improvements have been made in both areas, there have been occasions when acoustic emissions proved so vexing they overshadowed performance and reliability issues. For example, some wind turbines suffer an unfavorable reputation for noise problems associated with high tip speeds, furling, or blade flutter. Because of the potential for installation near residences, noise may be even more important for small turbines than for large turbines installed in wind power plants. – 2004 NREL Report
Complaints (from customers or others) about sound characteristics are rare, but are usually related to turbines with flexible blades and side furling mechanisms. – Canadian Wind Energy Association, Small Wind Purchasing Guide
The market for small residential and rural wind turbines (up to 10KW) has been growing at double digit rates for several years and is now entering its young adult stage as venture capital enters the market and certification standards are being introduced in both the U.S. and U.K. markets. Wind energy is roughly on par with PV in return on investment, but faces much tougher zoning issues due to the visual impact of towers and the lack of reliable and easy to use acoustic criteria for use by zoning officials. As a result, the greatest market challenge for small wind turbine manufacturer’s and early adopters that want to take advantage of the technology, is getting zoning approval in the more 25,000 different zoning districts within the U.S.
In my view, zoning ordinances should focus on easing height restrictions to recognize the physics of wind power, building permits to assure adequate tower foundations and anchoring, and on protecting consumers from unwarranted sound pollution. Setbacks should be a function of both tower heights and the need for acoustic buffers. I’ve attached my version of a model ordinance based on a review of what I think are best practices from the U.S., the U.K., and Canada including a method of assuring that no unwarranted sound disturbance crosses a property line. The balance of this post will focus on the sound issue.
The American Wind Energy Association [AWEA] recommends the following noise provision in it’s model small wind ordinance:
For wind speeds in the range of 0-25 mph, small wind turbines shall not cause a sound pressure level in excess of 60 dB[A], or in excess of 5 dB[A] above background noise, whichever is greater, as measured at the closest neighboring inhabited dwelling. This level, however, may be exceeded during short-term events such as utility outages and severe wind storms.
In the sense that most small wind turbines operate below ambient noise levels under most conditions, this provision seems reasonable. However, it is filled with loopholes that serve the small wind industry, but fail to protect the public from unwarranted sound pollution. For example:
- Sound restrictions end at 25mph (presumably the definition of a “severe wind storm”) prior to the acoustic problems of furling and blade flutter start starting to become objectionable.
- When there is a blackout or “utility outage” and therefore no load on a grid-tied wind system, some turbine designs “free wheel” or spin a much higher rpm’s causing spikes in the acoustic emission. This may also be true of off-grid systems whose batteries reach a full charge.
- 1)According to British Standard 4142 a “noise rating level” (i.e. the noise level once corrected for the presence of tones or other noise characteristics) of 10 dB above the background noise is likely to give rise to complaint, [and] one only 5 dB above background would be marginal…”. The proposed “5 dB[A] above background noise” is therefore right on the edge of being complaint worthy. In addition, the “measured at the closest neighboring inhabited dwelling” takes the sound provision over the property line and does not account for future adjacent construction (i.e. the rights of future property owners).
In short, the AWEA model noise ordinance provision seems designed to protect legacy designs that have not been optimized for sound emission and to create the smallest possible acoustic buffer in an effort to open the market to smaller lot sizes. However, the concept of a provision based on some level of emission in excess of background noise is a step in the right direction. What that provision might be and how it might reasonably be enforced is another matter. Before I tackle that topic, I think it’s important to review the new Small Wind Safety and Performance Standards sponsored by the American Wind Energy Association [AWEA] and the British Wind Energy Association [BWEA].
The BWEA standard was released in February 2008 and the AWEA standard is in process and currently available as a “Draft Document”. Both standards will require a much needed third party verification of energy conversion performance and assure that minimum safety and quality standards are met. This will allow consumers to easily compare the performance of competing designs and trade that performance off against cost, warranties, and other features. It will also provide government incentive programs and zoning officials with an reasonable and acceptable list of “approved designs”. Both standards are virtually identical relative to the provisions for energy conversion, safety, and quality. However, language and requirements of the two standards diverge significantly relative to “Acoustic Sound Testing”.
The current AWEA “Rated Sound Level” states that:
The sound level that will not be exceeded 95% of the time, assuming an average wind speed of 5 m/s (11.2 mph), a Rayleigh wind speed distribution, 100% availability, and an observer location 60 m (~ 200 ft.) from the rotor center1, calculated from IEC 61400-11 test results, except as modified in Section III of this Standard.
In contrast, the BWEA “Rated Sound Level” provides for two conditions at higher wind speeds. One appropriate for larger turbines and one for smaller residential zone turbines:
BWEA Reference 60m Sound Level, Lp,60m. The sound pressure level in dB(A) re 20 μPa rounded up to the nearest dB, at an observer distance of 60 m [~197 ft] from the rotor centre (i.e. a slant distance) and calculated from the Declared Apparent Emission Sound Power Level when the turbine is subjected to a wind speed of 8 m/s [17.9 mph] at its rotor centre. The 60 m distance is representative of the closest observer distance expected for a turbine toward the larger size of small wind turbines.
BWEA Reference 25m Sound Level, Lp,25m. The sound pressure level in dB(A) re 20 μPa rounded up to the nearest dB, at an observer distance of 25 m [~82 ft] from the rotor centre (i.e. a slant distance) and calculated from the Declared Apparent Emission Sound Power Level when the turbine is subjected to a wind speed of 8 m/s [17.9 mph] at its rotor centre. The 25 m distance is representative of the closest observer distance expected for a micro or domestic size turbine.
Both standards provide for a method of determining sound levels at X distance from a turbine relative to ambient(background) sound levels. However, as currently written the AWEA standard only provides information to make this calculation at the 5 m/s [11.2 mph] rated wind speed which bypasses the issues of furling, blade flutter, high wind speeds, free wheeling, and power outages.
In contrast, the BWEA standard includes the provision for an Emission Noise Map in their labeling requirement which maps the sound emission of the turbine relative to the distance from the turbine hub and the wind speed. The map starts at the turbine cut-in speed and ends at the cut-out speed. However, for turbines that do not “cut-out”(i.e. stop operating), “but achieve protection to high winds by various mechanisms such as furling their blades or yawing the turbine to cross-wind. In these cases the red and orange zones of the Noise Map will continue up to the wind scale maximum”. This still doesn’t address free-wheeling and power outages, but I’ll get to that in a moment. It should also be noted that the BWEA standard assesses a noise penalty of 5dB[A] if the turbine emits any specific tones that may be objectionable to the human ear.
In the appendix, the BWEA standard provides a method of using the noise map to assess site suitability for U.K. zoning purposes based on the red, amber, and green zones. In addition, using the Sound Power, Noise Slope, and Noise Penalty the standard provides equations for calculating the noise level at any wind speed and any distance from the turbine. It also provides a conservative formula for estimating the ambient noise level in a “country field” as a function of wind speed. According to the BWEA standard, the values from this background noise formula “can be taken as a worst-case scenario (i.e. the lowest ambient noise) for a rural background.”
In all fairness, the AWEA standard is still in draft form and may end up addressing small wind turbine acoustic issues in a better way, however from a consumer viewpoint the BWEA standard is currently far superior. That said, how should zoning officials protect the public interest regarding small wind turbine sound emissions? I’ll start with a list of basic premises:
- What’s at issue is not so much the sound emitted by the turbine, but the sound emitted in excess of the ambient noise conditions, especially night time noise conditions.
- The noise or sound limitations should be established at the property line.
- The sound emission criteria should include all conditions including high winds (furling, yawing, and flutter) and power outages (free wheeling).
- For some turbine designs, the effect of the above will be to increase the effective setback (and indirectly the minimum lot size) by creating an acoustic buffer in excess of any setback based on tower height.
Under most conditions wind turbine sound emissions disappear into the background offering only a faint whoosing sound to the human ear. However, for zoning purposes an acoustic buffer should be required to encompass all operating conditions and to protect residents from turbine designs that have not been optimized for acoustic performance. The best way to achieve that is to establish a property line sound level based on ambient noise levels plus some standard.
European noise standards tend to be more developed, so I’ll use those as a baseline. In the U.K. the guideline is:
- Daytime: A background noise + 5 dB[A] or 43dB[A] whichever is greater
- Nighttime: A background noise + 5dB[A] or 35 to 40 dB[A] whichever is greater
- Plus a 2 to 5dB(A) penalty for tones
In France the guideline is:
- Background noise + 5dB[A] during the day
- Background noise + 3 dB[A] at night
Since 3 dB[A] over background noise is barely discernible outside of the lab, and sleep disturbance should be the key criteria for a small wind turbine sound provision, I believe that 3dB[A] over background noise should be the property line limit for a zoning ordinance. This is conservative in that any habitable building on an adjacent property will be setback an additional distance which provides additional acoustic buffer.
Language for the sound provision of my model Small Wind Turbine zoning would look something like this:
Acoustic Setback Requirement – The acoustic emission sound level of the Small Wind Turbine as measured at the property line, shall not be more than 3dB[A] over the background noise level under any operating conditions, including high winds, yawing, furling, and power outages. Background noise may be calculated using Equation A.1 of the British Wind Energy Association [BWEA], Small Wind Turbine Performance and Safety Standard (February 2008) as plotted in Figure 1 below.
Measured site specific background noise levels may also be used provided that they are verified via a survey and report prepared by a qualified engineer. The acoustic setback from the property line required to meet the 3dB[A] over background noise level requirement can be satisfied/calculated in two ways:
- For Small Wind Turbines that are certified to the BWEA Small Wind Turbine Performance and Safety Standard (February 2008), the wind turbine manufacturer shall provide calculations that use BWEA Equation A.2 to quantify the required acoustic setback.
- For Small Wind Turbines not certified to the BWEA standard, the wind turbine manufacturer shall provide certified data and calculations prepared by a qualified engineer that quantify the required acoustic setback. These calculations must include a 5dB[A] penalty for any tonality according to ISO 1996-2:2007 Annex D based only on 1/3rd octave band data as follows:
The turbine is declared tonal if any 1/3rd octave band (in any of the spectra from section 3.4.16) is higher than its adjacent bands by
15 dB in the low frequency bands (50 to 125 Hz)
8 dB in the mid-frequency bands (160 to 400 Hz)
5 dB in the high frequency bands (500 to 10000 Hz)
The maximum wind speed used for the submitted calculations shall be the cut-out speed or for wind turbines that do not have a cut-out speed no less than 50 mph.
A proposed model small wind turbine ordinance is attached below in pdf format which includes provisions for height limitations based on physics, turbine size limitations based on turbine blade size and lot size ( consideration of scale), requirements for building permits, and the acoustic provisions outlined in this post.