Estimating the amount of energy a wind turbine will generate in a year is a key factor in determining the viability of any wind development project. To do that responsibly, it is important to understand wind characteristics, especially its variable nature. Equally important, however, is an understanding of site and operational issues that affect the ultimate, bottom-line net usable energy production from a wind turbine installation. This document outlines the issues and describes how we address those issues at Northern Power.

WIND CHARACTERISTICS When it comes to generating electricity, the single most important characteristic of wind is its speed. A wind turbine turns kinetic energy, a function of mass and velocity, into electrical energy. The faster the wind, the more energy it contains. The denser the wind, the more energy it contains. In fact, the kinetic energy in the wind increases in relation to the cube of the wind speed. Doubling wind speed increases its energy by a factor of eight.

WIND SPEED DISTRIBUTION

While average speed is arguably the single most important parameter, how the speed varies about that average also has a significant effect. Wind speed is nothing if not variable. To estimate how much energy we are liable to pull from the wind, we need to know how much time the wind spends at each wind speed. So we think about and talk about wind in terms of wind speed distribution. The probability distribution that is most often used to characterize wind speed distribution is the Weibull. A Weibull distribution has two parameters besides the distributed variable—a shape factor, generally represented as “k,” and a scale factor, generally represented as “λ.” Given a mean and a k factor, λ can be calculated. In order to estimate a distribution of wind speeds, that is, to estimate the amount of time at each separate wind speed, we need an average and a Weibull k factor. Given those, we can calculate the number of hours each year that the wind speed will fall within each wind speed range.

ESTIMATING WIND SPEED

Developers of wind farms have traditionally relied on site-specific studies involving collecting meteorological data from multiple towers for two years or so. The data then get reduced and interpreted, ultimately resulting in wind speeds and times – a distribution – that is used to support siting decisions for individual turbines and ultimately to estimate energy production.

Engineering Bulletin

Energy Production Estimating Estimating annual energy production from a Northern Power® NPS 100™ wind turbine

NORTHERN POWER SYSTEMS 29 PITMAN ROAD

BARRE, VERMONT, 05641 USA T: +1 877 90 NORTH

NORTHERNPOWER.COM

AWST windNavigator™ Web Site & Wind Speed Distribution

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

For smaller-scale wind projects, the time and cost involved in such a rigorous data collection and reduction exercise is cost prohibitive. Moreover, nowadays excellent wind maps exist that allow a capable wind site assessment professional to estimate wind speeds and distributions very well. Project funding agencies around the country allow the use of rigorously developed wind maps to support their granting decisions. At Northern Power, we work with AWS Truepower®, www.awstruepower.com, an organization with deep roots in wind resource analysis and modeling. AWST has developed windNavigator, www.windnavigator.com/cms, an online service that provides estimates of average annual wind speed at multiple heights above grade along with estimates of distribution parameters, direction frequency and monthly variation, among other parameters. These data, coupled with clear estimates of uncertainty allow us to define a range of expected energy production for a specific site.

ESTIMATING ENERGY PRODUCTION In addition to the wind speed and distribution, the last bit of information necessary to estimate energy production from a wind turbine is the turbine’s power curve. The power curve describes the power (kW) the turbine generates at each wind speed. Manufacturers publish power curves for their products. It is important to understand the veracity of manufacturer-published power curves. Many are verified by independent third-party testing labs, as is true for the NPS 100 wind turbine. Many are not.

NPS 100 Wind Turbine Power Curve

NOMINAL ENERGY PRODUCTION ESTIMATE

Calculating the theoretical energy output of a wind turbine at nominal conditions, then, is as easy as multiplying the power output at each wind speed by the number of hours each year the wind blows at that speed and summing the results.

Calculating Nominal Energy Production

 

 

 

   

DE-RATING OUTPUT

The theoretical output must be adjusted for real-world operating conditions at the site. These adjustments generally involve reducing the estimated output. Conventional thinking in the world of small wind turbines is to derate nominal output calculations by 15% - 30% to account for a variety of issues. This section addresses the various elements of energy output derates.

Air Density Because the kinetic energy in wind is directly related to the mass of the moving air, it is important to adjust energy production estimates for variations in air density. Both pressure and temperature affect density. Nominal conditions define pressure as that at sea level and temperature of 15°C (59°F). The air at a site at an elevation of 100m (330’) above sea level is 1.2% less dense than it is at sea level at the same temperature; at 1,000m (3,300’), it is 11.3% less. Similarly, a 10°C (18°F) increase in temperature reduces air density by 3.3%.

While adjusting for average annual temperature does not rigorously account for combined behavior of wind speed and temperature – in many places, the highest wind speeds occur during the winter when the air is denser, giving energy output rather a boost – it is important to consider. Elevation should always be adjusted; average annual temperature is a reasonable additional adjustment to include.

Turbine Availability If a wind turbine is down for maintenance or faulted and offline, it produces nothing, regardless of how hard the wind may be blowing. Wind turbine manufacturers may or may not have hard data to support assertions of turbine availability. Small-wind industry conventional wisdom holds 95% to be excellent and 90% probably closer to an average, though hard data does not exist. The NPS 100 wind turbine fleet had a median availability for 2010-2014 of just over 98%. We advise using 97% as a long-term average for an NPS 100 wind turbine installation.

Site Availability A number of external conditions affect the ability of a site to accept power from a wind turbine. Utility grids operate within defined tolerance limits for voltage and frequency. When grid conditions fall outside those limits, the turbine will disconnect, potentially losing energy production. Likewise, temperature extremes may exceed operating conditions for the turbine. Wind turbines will also disconnect during extreme wind conditions. And then there are times when an individual just turns the turbine off.

All these conditions fall outside the bounds of turbine availability, but must nevertheless be accounted for when developing an energy production estimate. We have seen sites with very poor power quality, where brownouts and blackouts were common. Other sites have shown no grid or environmental downtime at all. In the absence of any additional information, we recommend allowing a similar value for site availability as we use for turbine availability for an NPS 100 wind turbine: 97%. Sites with a history of brownouts and/or blackouts may be 90% or less; those with rock-solid power quality may be better than 99%.

Site Losses The amount of energy that is actually available at the point of use will be less than the total produced by the wind turbine. Any transformation of electricity – for example changing voltage through a transformer – and its distribution throughout a site both result in energy losses. A meter at the base of the turbine will show a different value than a meter across the site. Depending on the particulars of the site, specifics of the interconnection, and expectations of the user, some allowance should be made to account for losses from gross production to net usable energy.

It is also important to consider the effect of excessive turbulence. Small and community wind turbine sites are often constrained enough that ideal locations are unavailable for building. Such sites may well be subject to more turbulence than one would like. The effect of turbulence is manifest in two ways: loss of energy production and either increased maintenance cost or decreased turbine lifetime. Estimating the effect of turbulence on energy production requires an understanding of both site conditions and the source of the manufacturer’s power curve. A power curve that is not third-party tested may require more of a turbulence derate than one that has been validated in real-world conditions. The NPS 100 wind turbine’s power curve was validated in conditions that included a turbulence intensity of 15.4% at 15m/s. That means that an NPS 100 wind turbine’s output need not be derated for turbulence at sites with modest turbulence and that it be derated less for more-turbulent sites than turbines without an independently validated power curve in real-world conditions.

In the absence of site-specific information, we recommend allowing 4% for site losses. This accounts for a site with modest turbulence and short or higher-voltage distribution runs. Longer, lower-voltage distribution runs can add several points to that value, as can more-turbulent locations downwind from forests or near the turbulence zone of upwind obstacles.

 

 

 

 

AWST WindNavigator Web Site & Wind Speed Distribution

SUMMARY While the principles described here apply equally to any small- or community-wind installation, the specific numbers to use for derates will vary according to site conditions, interconnection requirements, and the wind turbine being used. Totaling our recommendations for derates to NPS 100 wind turbine output results in knocking down its nominal energy output by about 10% in addition to adjustments for air density. We feel this to be reasonable to expect for most sites over a 20-year lifetime. You should be sure to substantiate assumptions you make for other equipment and quantify as best you can the specific allowances you make for specific site conditions.

Of paramount importance are the expectations in an end user’s mind. While overly optimistic estimates may make front-end sales activities easier, they are seldom an appropriate path to long-term customer satisfaction. On the other hand, arbitrarily choosing a derating number, be it 20% or 30% or something else, can result in a potentially unfair evaluation. While you should never knowingly overestimate production, neither should you arbitrarily underestimate it.

 

NORTHERN POWER SYSTEMS 29 PITMAN ROAD

BARRE, VERMONT, 05641 USA T: +1 877 90 NORTH

NORTHERNPOWER.COM