NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Small Wind Site Assessment 

Guidelines

Tim Olsen

Advanced Energy Systems LLC

Robert Preus

National Renewable Energy Laboratory 

Technical Report

NREL/ TP-5000-63696

September 2015
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Small Wind Site Assessment 

Guidelines

Tim Olsen

Advanced Energy Systems LLC

Robert Preus

National Renewable Energy Laboratory 

Prepared under Task No. WE14.2B01

Technical Report

NREL/ TP-5000-63696

September 2015
NOTICE

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or favoring by the United States government or any agency thereof. The views and opinions of authors 

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Acknowledgments

The authors would like to acknowledge the many people who contributed to the development of 

this document. Mick Sagrillo, Lisa DiFrancisco, Roy Butler, Charles Newcomb, Rich 

Hasselman, Roger Dixon, Steve Woll, Ken Kotalik, and Mark Mayhew contributed comments, 

suggestions, and reference material. Their extensive contribution of their time and expertise was

vital to the development of this document. 

This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-

08GO28308 with the National Renewable Energy Laboratory. The authors would also like to 

thank the staff of the Wind and Water Power Technology Office, specifically Mark Higgins, Bret 

Barker, and Patrick Gilman for their support of this activity. 
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Table of Contents

Acknowledgments ..................................................................................................................................... iii

Table of Contents....................................................................................................................................... iv

List of Figures ............................................................................................................................................. v

List of Tables ............................................................................................................................................... v

1 Introduction........................................................................................................................................... 1

2 Background and Importance............................................................................................................... 2

3 Small Wind Site Assessor Qualifications .......................................................................................... 3

4 Site Evaluation and Description.......................................................................................................... 4

4.1 Land-Use Considerations.............................................................................................................. 4

4.2 Interconnection and Utility Policies.............................................................................................. 5

4.3 Site Information and Administrative Data .................................................................................... 5

4.4 Safety and Environmental Concerns............................................................................................. 5

4.5 Topography ................................................................................................................................... 5

5 Wind Resource Data and Production Estimates............................................................................... 7

5.1 Wind Data Sources........................................................................................................................ 7

5.2 Adjustments for Topography......................................................................................................... 9

5.3 Adjustments Based on Wind Shear............................................................................................. 12

5.4 Wind Shade Adjustment.............................................................................................................. 15

5.5 Turbulence Intensity.................................................................................................................... 16

5.6 Gross Annual Energy Production Estimate................................................................................. 16

5.7 Net Annual Energy Production Estimate .................................................................................... 16

6 Other Site Issues ................................................................................................................................ 18

6.1 Zoning and Permitting................................................................................................................. 18

6.2 Federal Aviation Administration Considerations........................................................................ 19

6.3 Environmental ............................................................................................................................. 19

6.4 Other Siting Issues ...................................................................................................................... 20

7 Site Assessment Report .................................................................................................................... 21

7.1 Energy Efficiency........................................................................................................................ 21

7.2 Turbine and Tower Options ........................................................................................................ 21

7.3 Minimum Acceptable Turbine Tower Height for a Site ............................................................. 22

7.4 Economics and Incentives........................................................................................................... 22

7.5 Operation and Maintenance Expectations................................................................................... 22

7.6 Off-Grid Systems ........................................................................................................................ 22

7.7 Building Integration .................................................................................................................... 22

7.8 Electrical Load Usage Analysis .................................................................................................. 23

8 Conclusions ........................................................................................................................................ 24

9 Glossary .............................................................................................................................................. 25

References................................................................................................................................................. 30

Bibliography .............................................................................................................................................. 31

Links for Further Information................................................................................................................... 31

Appendix A ................................................................................................................................................ 32

Sample Checklist.................................................................................................................................. 32

Sample Report Outline ......................................................................................................................... 33

Appendix B. Case Studies and Lessons Learned ................................................................................. 35

Case Study #1: Comparison of Two Similar Turbines: Bassetti and Grim.......................................... 35

Case Study #2: Comparison of Two Turbines: Kittery and Vosters.................................................... 39

Case Study #3: Hempstead Project....................................................................................................... 44
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List of Figures

Figure 1. Acceleration of wind over a ridge ........................................................................................... 10

Figure 2. The effects of ridge orientation and shape on site suitability for wind generators........... 10

Figure 3. Vertical profiles of air flowing over a cliff .............................................................................. 11

Figure 4. Sample wind rose...................................................................................................................... 12

Figure 5. Effect of surface friction on low-level wind............................................................................ 13

Figure 6. Formation of a new wind profile above ground level............................................................ 13

Figure 7. Zone of disturbed flow over a small building. ....................................................................... 16

Figure B-1. Bassetti turbine in Klickitat County. ................................................................................... 35

Figure B-2. Grim turbine in Peshastin, Washington.............................................................................. 36

Figure B-4. Turbine in Kittery, Maine ...................................................................................................... 39

Figure B-5. Vosters turbine in Appleton, Wisconsin............................................................................. 40

Figure B-6. Kittery wind rose ................................................................................................................... 42

Figure B-7. Vosters wind rose ................................................................................................................. 42

Figure B-8. Map of MERRA data sites and turbine location ................................................................. 45

Figure B-9. Hempstead, New York, NW100 wind turbine...................................................................... 45

Figure B-10. Hempstead aerial photo before (left) and after (right) installation................................. 46

Figure B-11. (Left) total wind energy rose using long-term data from two nearby MERRA sites. 

(Right) total wind energy rose using data from two nearby MERRA sites for the period 

March 27, 2012 to March 26, 2013..................................................................................................... 48

Figure B-12. The synthesized mean monthly averaged wind speed from two nearby MERRA sites 

at 37 m for the period 1999‒2013 (green) and for the period since the NW100 was installed 

(2012‒2013, in blue); the dashed overlay is the 12-month period used for the wind analysis 

summary.............................................................................................................................................. 50

Figure B-13. The actual monthly energy production of the NW100 at Hempstead and the estimated 

monthly energy production for the same time frame using synthesized hourly averaged wind 

speed from two nearby MERRA sites at 37 m; the dashed overlay is the 12-month period used 

for the wind analysis summary......................................................................................................... 50

List of Tables

Table 1. Textbook Versus Measured Wind Shear by Terrain Type...................................................... 15

Table B-1. Case Study #1 Turbine System Details ................................................................................ 37

Table B-2. Case Study #2 Turbine System Details ................................................................................ 40

Table B-3. Case Study #3 Turbine System Details ................................................................................ 46
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1 Introduction

Site assessment for small wind energy systems is one of the key factors in the successful 

installation, operation, and performance of a small wind turbine. A proper site assessment is a 

difficult process that includes wind resource assessment and the evaluation of site characteristics. 

These guidelines address many of the relevant parts of a site assessment with an emphasis on 

wind resource assessment, using methods other than on-site data collection and creating a small 

wind site assessment report.

The small wind site assessment report should include recommendations on suitability of the site, 

best locations for the installation, and appropriate turbine and tower options. In many cases, the 

recommendation will be that the site is not suitable for a wind generator. As a result, supporting 

information should be provided. In addition information on energy efficiency, incentives, 

economics, operation and maintenance (O&M) requirements, and off-grid or back-up power 

systems should be provided. Guidelines for the compilation and analysis of information for the 

report and sample reports follow. Several Case studies are also provided at the end of this 

document.

On-site data collection is always preferable to the simplified methods addressed here, but for 

small residential wind systems (usually less than 20 kilowatts [kW] in size) the time and cost is 

often not justified by the risk reduction available from this type of data collection and analysis. 

In cases in which this approach may be justified, these simplified methods can be useful as a 

preliminary filter to qualify a site for further evaluation.

A glossary of terms used as part of a small wind site assessment are provided at the end of this 

document along with several case studies that demonstrate the importance of proper site 

assessment in the implementation of any small wind project.

This document serves as a resource that describes the elements of a small wind site assessment

and highlights the guiding principles using currently available tools and experience. The methods 

presented were compiled from leading experts in small wind site assessment.
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2 Background and Importance

Historically, there has been a large variation in the accuracy of energy projections for small wind 

generators. There were two basic factors that impacted the accuracy of the energy estimates 

produced in site assessments performed before 2000, inaccurate power curves and wind resource 

assessments.

The first factor was that there were no certified power curves for small wind generators, and 

manufacturer-supplied power curves were often “optimistic.” To help resolve these issues, the 

American Wind Energy Association’s (AWEA’s) Standards Committee developed a U.S.

certification standard (AWEA 9.1) that included the International Electrotechnical Commission 

(IEC) 61400-2 standard with minor modifications. Part of this standard covered the third-party 

certification of power curves and specifications on how the data must be gathered and processed 

to produce a certified power curve. To date, 13 small wind generators have been completely 

certified, with many more in process. Most of the wind generators that are currently in the 

certification process have certified power curves available.

The second factor was that existing resource assessment efforts were often unrealistic. Although 

various nonprofit organizations had been offering site assessor training since the early 1990s, 

there were no consistent and systematized resource estimate processes widely published, few

training programs for site assessors, and limited certification programs. The exceptions to this 

were the Midwest Renewable Energy Association in Wisconsin, Solar Energy International in 

Colorado, and various other nonprofits that offered site assessor trainings. In 2002, Wisconsin’s 

Focus on Energy funded formal site assessor training and certification, which was housed at the 

Midwest Renewable Energy Association. At the time, small wind installation rebates in 

Wisconsin required that the application include a site assessment from a certified site assessor. In 

2004, the small wind industry worked with the North American Board of Certified Energy 

Practitioners (NABCEP) to establish a national program of site assessor certification. Much of 

the required preparation was done before NABCEP decided that certifying site assessors was not 

a profitable business opportunity and halted work toward creating a certification program. To 

date, no sponsor for a site assessor certification program has been found. 

The guidelines in this document are the result of conversations within the U.S. small wind 

industry, attempting to synthesize a diverse set of experiences into common, effective practices. 

It is not intended to serve as a training program or comprehensive text on the subject, but as a 

guiding framework to help site assessors, state program managers, and the general public better 

understand what to expect when conducting or reviewing a small wind turbine site assessment.

For the purpose of this discussion, small wind turbines are defined as having a power rating 

ranging from 1 kW up to 100 kW, as determined by the U.S. Department of Energy (DOE). It is 

important to note that the levels of detail and analysis in site assessment typically increase with 

turbine size and cost. On the upper end of even the small wind generator range, a site assessment 

based on a simplified wind resource assessment might be followed by a more detailed wind 

analysis and feasibility study that includes on-site data collection to reduce risk. In the end it is 

usually a balance between the cost of assessments, the cost of the turbine installation and 

potential risk profile of any error in the prediction of long term energy production.
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3 Small Wind Site Assessor Qualifications

A practicing site assessor is expected to go well beyond the guidelines in this document in skill 

development, including formal training and experience, usually through apprenticeship and/or 

extended field practica.

Many practicing site assessors have taken 40 hours or more of site-assessment-specific training, 

plus field work under the mentorship of experienced practitioners. Such training should include 

the range of challenges, practices, and tools for site assessment, including an examination of 

ethics and safety, with the goal of ensuring effective assessments for clients that will serve to 

solidify the reputations of practitioners and the industry.

According to NABCEP’s Small Wind Site Assessors Job Task Analysis [1], a site assessor must 

be competent in the following areas:

• Basic mathematics, including basic algebra and geometry

• Spreadsheets (e.g., wind speed calculators, output calculators, and economic calculators) 

• Computer skills (e.g., Microsoft Excel and Microsoft Word) 

• Basic documentation, photography, and drawing

• Listening, writing, and verbal communication 

• Organization

• The Internet (e.g., Google Earth, wind maps, and incentives) 

• Map interpretation (e.g., topography, location, and terrain)

• Compass and Global Positioning System (GPS) use 

• Erection techniques for towers (including the anchors and foundations) and wind systems

• Report interpretation (e.g., utility bills, wind reports, and manufacturers’ specifications) 

• Elements of basic electricity (e.g., AC, DC, and power conversion systems)

• Fundamentals of wind energy and wind behavior 

• Basic identification of tree types and mature heights

• Height and distance estimations

• Local, state, and federal energy policies and regulations

• Zoning ordinances and permitting and inspection requirements

• Wind turbine technology, performance, operation, and economics 

• O&M practices and basic turbine/tower repair

• Basic financial knowledge 

• Small wind installation process.
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4 Site Evaluation and Description

Because the site evaluation and wind resource evaluation are interrelated, it is valuable to 

understand as much as possible about both before the initial visit. Assessors need to obtain the 

average wind speed and direction information for the area, because it is essential in determining 

which micro sites to document and analyze based on terrain and obstacles. A site assessor should 

also understand local zoning and permitting regulations if available to help understand local 

siting concerns.

Although understanding both site characteristics and wind resource is essential to successful 

wind turbine siting, site characteristics will be addressed first, because that information will help 

determine which site locations to analyze for the wind resources. The site assessor will guide the 

client through a rough optimization of land-use issues, constructability, good wind exposure, and 

the client will help the assessor understand their personal needs, motivations, expectations, and 

goals. One of the most important aspects of a site assessment is a physical visit to the location, 

allowing for an accurate evaluation of potential obstructions to the wind resource, including 

ground clutter (e.g., surrounding buildings and trees), as well as an overall grasp of locationspecific characteristics, such as overhead and/or underground utilities, the best location for 

electrical interconnection, and site access.

4.1 Land-Use Considerations

The ability to site a wind turbine and tower in an ideal location that is completely free of 

obstacles and has access to unobstructed wind flow often clashes with the realities of land-use 

limitations, such as:

• Property boundaries 

• Zoning setbacks and height limits

• Owner or neighbor view impact

• Soil conditions

• Construction access

• Interconnection requirements and wire run routing

• Safety.

These issues tend to be non-negotiable, and they must be addressed for any project that is 

expected to meet legal, technical, and owner requirements.

The zoning and permitting processes seek to address safety, aesthetics, and community interests 

and concerns. Some of these concerns might include sound level, visual impact, wildlife impact, 

TV/radio interference, ice shedding, or broken equipment. Although some of these issues may be

perceived rather than actual, the installer and owner must be ready to address them all—and the 

site assessor needs to understand them well enough to provide sound advice on the turbine 

location.

Additionally, the site assessment must cover roads, obstacles, and site accessibility for the 

delivery of the wind turbine, tower, and construction equipment, as well as for the actual 
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installation. Although the site assessment does not normally include a soils test, it is important to 

include at least some information about obvious soil issues that could affect foundation design 

and construction, such as intermittent water, sand, unstable slopes, rocks, expansive clay, and 

frost depth. A first estimate of soil type and condition can be found at the U.S. Department of 

Agriculture’s Natural Resources Conservation Service at http://soils.usda.gov. This information 

is focused on the agricultural value of the top few feet of soil, yet most tower footings extend 

deeper than this characterization. A project may still need actual soils testing if required by the 

turbine or tower manufacturer or permitting authority, or if there are potential issues with the

subsoil.

4.2 Interconnection and Utility Policies

In most cases, it is quite advantageous to interconnect a small turbine with the customer’s utility 

service, thereby using the utility for backup power to cover the variability of the turbine’s energy 

production as well as storage of excess energy. Such interconnection typically requires utility 

permission, which is usually in the form of an interconnection agreement. This agreement will 

address metering and billing arrangements with the utility, and may include requirements for 

additional safety equipment or procedures, protection devices, and inspections. The site assessor 

should be knowledgeable about these requirements and help educate the client. As described 

below, an off-grid-capable system may be recommended if no utility is available, the cost to 

bring grid power to the property is too expensive, the utility refuses interconnection, or the 

customer requires back-up power or utility independence.

4.3 Site Information and Administrative Data

As a basic framework, it is useful to note the site address, GPS coordinates, location maps

(general area, or macro scale, and close-in, or micro scale), a site plan, and site photographs. 

Panoramic shots of the proposed wind turbine sites will help to document topography, landscape, 

and obstacles (typically start facing north and rotate 45° for each subsequent photo).

4.4 Safety and Environmental Concerns

Of critical importance is an assessment of potential hazards to project success, such as safety or 

the environment, including overhead and underground utilities, which affect excavation and 

trenching, and uneven terrain. Environmental concerns include potential avian and wetlands 

impacts and the potential impact of erosion on the installation. Again, the site assessor should 

understand these issues well enough to steer clear of a major problem with the turbine location.

4.5 Topography

Hills and many other topographic features alter air flow. The changes in air flow can increase, 

decrease, or even reverse flow, and intensify turbulence. In addition, uneven ground features can 

cause a wide range of local wind flow effects. As a result, if the surrounding area of a potential 

site is not relatively flat for several miles, then a description of the main topographic features is 

necessary, both nearby (macro siting) and at the proposed turbine site (micro siting). The 

topographical description should include shape, height, length, width, and distance and direction 

away from the proposed turbine site of any landforms. “Nearby” could include influences from 

large objects such as hills, groves of trees, or high wind breaks up to a mile away, and smaller 

objects could include single trees and buildings, especially within 500 feet (ft) of the proposed 
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turbine location. If wind data used for the site evaluation are from a location several miles 

away, then it is important to have topographic maps of the area around both sites and between the 

sites. This information is essential in evaluating if the reference wind data are likely to be wellcorrelated with the wind resource on the site and if it needs to be adjusted up or down.
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5 Wind Resource Data and Production Estimates

The following comprehensive set of steps address the most difficult cases of remote and/or lowresolution wind data and development of an estimate for energy production at a chosen project 

site. Background information for each of these steps is provided in the following sections:

1. Collect wind data. Use map estimates or actual wind data sets as described in Section 5.1.

2. Calculate the adjustment for topographic impact on speed and direction.

3. Adjust wind speed. Derive a sheer factor (alpha) and rationale to adjust the wind speed 

and direction from source data. Apply displacement height, vertical wind shear exponent

(alpha), data height, and hub height to adjust wind speeds.

4. Adjust for obstacles. Set up a wind direction sector analysis to determine the effects of 

each nearby obstacle by applying its shade factor with a weighting for the amount of 

wind in that direction from the obstacle to the turbine. Summarize the composite results 

of the above steps to produce new estimates for the average wind speed, range, 

distribution, Weibull parameters, and other parameters at the selected site.

5. Estimate turbulence intensity. Use roughness and obstacles by sector to estimate the 

turbulence intensity of a site.

6. Estimate gross annual energy production. Apply the adjusted estimated wind distribution 

to the wind turbine power curve or several if different turbines are being considered and 

adjust for expected turbulence intensity to estimate the monthly and annual energy 

production.

7. Estimate net annual energy production. Determine other factors that could further reduce 

the actual energy produced and calculate the net annual energy production. Provide the 

customer with a range of expected values for wind speed and energy production to 

account for the uncertainties that may come from the previous steps in this process and 

the variation of wind speeds from year to year.

5.1 Wind Data Sources

Although there may be many methodologies for understanding the wind resource at a specific 

location, when conducting a site assessment, gathering on-site, measured wind data is typically 

preferred. Sources of local wind resource information include wind maps, wind data collected for 

other projects, data from local airports or weather stations, and modeled resource data which are 

based on sophisticated public and private weather assessment tools. Search for nearby data in 

online public or low-cost data banks intended for wind resource monitoring. Sample links to 

such data sources are provided in Section 5.1.3.

The best data sets for wind resource assessment tend to be those that underlie the state wind 

maps. Contact the state energy offices to learn how to access these data sets.

Local airport or weather stations are also attractive locations to find local data but often provide

wind data that are less reliable than actual site data. If airport data (typically recorded at 30 ft or 

10 meters [m] above ground) or weather station data (typically recorded at 5‒20 ft above ground 

level) are used, the installation of the monitoring equipment should be checked. This equipment 
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is not primarily intended for wind resource assessment; it is not always at an appropriate height 

or in a location that is free of obstructions. As a result, it is important to inquire not only about 

the site’s current equipment and location, but also if it is historically consistent with the data 

collection equipment and siting. Unfortunately, airport and weather stations are usually far from 

the site of interest, with considerably different orography, tree cover, and monitoring height, 

making these data of questionable usefulness. Given the expertise required to effectively 

establish and correlate wind resource data, the data provided by airport and weather stations may

only be useful to provide a rough screening assessment.

5.1.1 Wind Maps

Site assessors often use state wind maps to conservatively estimate the wind resource at turbine 

hub height by extrapolation using a conservative wind shear. Unfortunately, wind maps lack 

critical information such as wind speed distribution, direction distribution, and turbulence 

intensity, which is necessary for making accurate adjustments to account for the local site

features. Despite this, multiple resources can be combined to improve data details and 

confidence in projection accuracy.

Wind maps continue to evolve and improve with better data and modeling techniques. A rough 

estimate usually starts with a state wind map, available for free at WINDExchange website

(http://apps2.eere.energy.gov/wind/windexchange/windmaps/) and the Canadian Wind Atlas

(http://www.windatlas.ca/en/index.php), and through a number of commercial providers for a 

fee. These maps can provide a general indication of good or poor wind resources, but may have

very low site-specific accuracy, as they do not provide high enough resolution or include

information on complex terrain, ground cover, and other local effects. Free wind maps are 

helpful, but maps or services that must be paid for can often provide higher resolution and more 

flexibility with zooming, orientation, and additional features. Specifically, attention should be 

given to a map’s height above ground as it relates to the potential project’s tower height. 

Adjusting the wind speed for the height difference between the map and the turbine height adds a 

potential source of error depending on the wind shear exponent that is selected, and the greater 

the height difference the greater the potential error. Therefore, for small wind generator 

applications, 30- to 40-m wind maps are far more useful than 10-, 60-, 80-, or 100-m wind maps. 

It is also important to understand the resolution of the wind map or model-generated data set. If 

the resolution is lower than the terrain features, adjustments will be needed to account for local 

terrain effects.

5.1.2 Measurement

On-site data measurement adds a new layer of confidence to the techniques discussed above, but 

with substantial additional costs, effort, and time, especially when the preferred methodology is 

to match turbine hub height and collect data for a minimum of 1 year. Obtaining several years of 

data is better, or 1 year that can be referenced to a longer-term data set if there is good 

correlation with the on-site data. A number of small, affordable wind data collection systems are 

available for on-site measurement and are best run for at least 1 year. These systems include

anemometers, wind vanes, and temperature sensors that are mounted as close to hub height as 

possible. Calculating the wind shear exponent requires taking data at two different heights. 

Having wind shear data is essential for conducting an accurate analysis of the cost versus 

benefits of taller towers. In addition, analysis must be performed to determine wind speed
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averages and extremes, wind distribution, Weibull parameters, the wind direction rose, 

turbulence intensity, vertical wind shear exponent, and associated uncertainties. Low-to-medium 

cost measurement systems can be found at suppliers like, but not limited to Etesian, APRS 

World, Vaisala, Oregon Scientific, and Hobo, and advanced systems and tall towers can be found

at suppliers such as SecondWind, NRG, MetOne, and several European and Canadian suppliers.

A helpful guide for setting up a monitoring tower and conducting a meteorological tower wind 

study is the Wind Resource Assessment Handbook—Fundamentals for Doing a Successful 

Monitoring Program, available at http://www.nrel.gov/wind/pdfs/22223.pdf.

5.1.3 U.S. wind maps and services

The following links can provide a starting point to find wind resource maps, data, and services to 

support the site assessment process.

• DOE-supported, National Renewable Energy Laboratory (NREL)/AWS True Powergenerated 30-m state wind 

maps: http://apps2.eere.energy.gov/wind/windexchange/windmaps/residential_scale.asp

• AWSTruePower: www.awstruepower.com/ (Option of purchasing a day pass)

• 3Tier By Vaisala: www.3tier.com/en/ (Offers subscription services and data reanalysis)

• WindLogics: www.windlogics.com/ (Includes consulting services, but no apparent 

product offerings)

• Distributed Wind Site Analysis Tool: http://app.dsat.cadmusweb.com (annual 

subscription)

• National Aeronautics and Space Administration: https://eosweb.larc.nasa.gov/sse/

• University of Utah, national data sets from multiple sources: http://mesowest.utah.edu/

• Western Regional Climate Center national data sets: www.raws.dri.edu/

• Alternative Energy Institute: www.windenergy.org/datasites/

• University of Massachusetts Wind Energy 

Center: www.umass.edu/windenergy/resourcedata.php

• Wisconsin Wind Data: www.uwex.edu/sco/windex.html

• New Roots Energy Wind Report: http://www.newrootsenergy.com/

• Video of U.S. current flows: http://hint.fm/wind/.

5.2 Adjustments for Topography

This section includes a discussion of the effect of a variety of terrain features on wind flow,

adapted from A Siting Handbook for Small Wind Energy Conversion Systems [2]. The site 

assessor should refer to this report and related references for more comprehensive information

about the impact of terrain features on turbulence and wind resource with respect to turbine 

siting. Although many of the images included in the report help give a better sense of the wind 

flow patterns around various terrain elements, they do not quantify exact dimensions, distances, 

and amounts of wind impact. As a result, they are useful as general guidelines to help better
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understand wind flow, turbulence, and siting options, but the value of actual site data should 

become more apparent through this section.

Landforms, or orography, can influence wind speed, and as a result, the amount of electricity that 

a wind turbine can generate. Elevated areas not only experience increased wind speeds because 

of their increased height in the wind profile, but, given the size and shape of the landform, may 

cause local acceleration of the wind speed. Idealized cases have been shown to double the wind 

speeds over a ridge, but uncertainty is great without high-resolution computational fluid 

dynamics modeling or on-site data collection. An example of how wind flow is accelerated over 

a ridge, is illustrated in Figure 1.

Figure 1. Acceleration of wind over a ridge. Illustration adapted from [2]

Note in Figure 1 the potential area of turbulence behind the top of the ridge as well as in the lee 

of it. These are locations that should be avoided when siting a small wind turbine tower. Wind 

prospectors looking for sites for utility wind plant development are keenly aware of this 

phenomenon, and seek out elongated ridges perpendicular to dominant wind flow. The 

orientation of the ridge relative to the prevailing wind direction is critical to optimizing 

accelerated wind flow over the ridge. Note from the diagrams in Figure 2 that ridges are better 

than hills at accelerating wind flow.

Figure 2. The effects of ridge orientation and shape on site suitability for wind generators. 

Illustration adapted from [2]
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Unique to elevated landforms are bluffs and cliffs (Figure 3), which create turbulence, including 

back eddies, as the wind passes up and over them. As a result, siting the tower to avoid the zones 

of turbulence created by the landform is critical. At minimum, the site assessor needs to 

understand the qualitative nature of these terrain effects, and perform more detailed modeling or 

site testing before siting a wind generator near a bluff.

Figure 3. Vertical profiles of air flowing over a cliff. Illustration adapted from [2]

Free topographic maps are available from Google Earth and the U.S. Geological Survey 

at www.nationalatlas.gov/mapmaker

and http://store.usgs.gov/b2c_usgs/usgs/maplocator/(xcm=r3standardpitrex_prd&layout=6_1_61

_48&uiarea=2&ctype=areaDetails&carea=%24ROOT)/.do, as well as www.listsofjohn.com

(with some clever searching for mountainous areas). Free topographical data (often called digital 

elevation model data) are also available from the U.S. Geological Survey 

at http://data.geocomm.com/catalog/ and http://viewer.nationalmap.gov/viewer/, but these data 

require some proficiency with geographic information system (GIS) software to be useful.

5.2.1 Wind Rose 

Knowing the prevailing wind direction(s) is essential to determining the impact of obstacles and 

landforms when seeking the best available site location and estimating the wind resource at that 

location. To help with this process, a wind rose can be used, which shows the wind direction

distributions of a given area. The wind rose divides a compass into sectors (usually 8 or 16) and 

indicates the average wind speed, average percentage of time that the wind blows from each 

direction, and/or the percentage of energy in the wind by sector. Wind roses can be generated 

based on annual average wind speeds, or by season, month, or even time of day as needed. An 

example of a typical wind rose is shown in Figure 4.
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Figure 4. Sample wind rose

In this wind rose, for any of the 16 sectors, blue conveys the total wind energy available from 

that direction, whereas green indicates the total time the wind blows from that direction. It is 

important to recognize that sectors where the wind blows frequently may not provide the most 

energy if the winds from those sectors are relatively weak. For this reason, it is important to

examine the wind roses for both the wind time and energy production.

Both common types of wind roses (percent of time and average wind speed or percent of time 

and percent of energy) help site assessors understand where to site a tower relative to landforms. 

The information from a wind rose is especially useful in determining a location for a tower that 

is upwind of any obstructions on a site and well exposed to the sectors that can produce the most 

energy.

In addition, if the wind rose data available is from a nearby site, it is important to review the 

topography at both sites and between the sites to determine if the wind direction data needs to be 

adjusted.

5.3 Adjustments Based on Wind Shear

5.3.1 Friction and surface roughness

Moving air experiences friction with the ground, ground cover and also between one air layer 

and another. This friction results in wind shear, which is the difference in wind speed at different 

heights above the ground. This change in wind speed with height is depicted in a wind profile,

shown in Figure 5. The horizontal arrows represent wind speeds at their respective heights in the 
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wind profile. Wind shear is an important factor in estimating wind speeds for turbine hub heights 

that are different from measured or modeled data heights.

Figure 5. Effect of surface friction on low-level wind. Illustration adapted from [3]

Rough topography and ground cover increases friction, adds turbulence to the air, and can even 

displace the effective ground level upward (called “displacement height”). Figure 6 shows how 

the wind profile is displaced upwards from the ground level by the grove of trees. This 

displacement height is called the “level of effective zero wind” since the wind below that level is 

nearly zero. Displacement height (d) is defined as a percentage of canopy height depending on 

vegetation or forest density, and often is approximated at 67% of canopy height for dense 

deciduous forests and 75% of canopy height for dense evergreen forests.

Figure 6. Formation of a new wind profile above ground level. Illustration adapted from [3]

The effects of surface roughness from ground cover can extend as much as 1,650 ft above 

ground, strongly impacting the first 60 ft—which can have a significant effect on residentialscale wind turbines. 

5.3.2 Calculating Wind Shear Effect on Wind Speed

Analysts typically use the power law equation to characterize the measured wind shear profile.
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Power Law

The power law equation is: V = Vref * ( h / href ) α

V = wind speed at height of interest (e.g., hub height)

Vref = wind speed measured at height href

h = height of interest (e.g., hub height)

href = height of measured data

α = wind shear exponent

The wind shear exponent, α, defines how the wind speed changes with height. When wind speed

data are available at multiple heights, the wind shear factor can be calculated using the power 

law equation.

The wind shear exponents from several heights with known wind speeds are used to estimate the 

wind speed at other heights of interest (e.g., turbine hub height). Depending on the type of terrain 

and surface roughness features, typical wind shear exponents may vary from 0.2 to 0.5.

Much work has been done to quantify the wind shear exponent based on various levels of surface 

roughness. As a result, the site assessor should be able to estimate an appropriate wind shear 

exponent from the surrounding land cover. A sample of these factors can be seen in Table 1. 

Note the two columns for wind shear exponent: one based on classic textbook calculations, and 

another based on extensive empirical data collected by several sources tied to Wisconsin’s Focus 

on Energy and the Wisconsin Energy Bureau described in [4].
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Table 1. Textbook Versus Measured Wind Shear by Terrain Type

Terrain Type Wind Shear 

Exponent α

(Textbook)

Wind Shear 

Exponent α 

(Wisconsin)

Ice 0.07 0.20

Snow on flat ground 0.09 0.20

Calm sea 0.09 0.20

Coast with onshore winds 0.11

Snow-covered crop stubble 0.12

Open, smooth surface (i.e., concrete) 0.20

Cut grass 0.14 0.25

Short-grass prairie 0.16 0.25

Open agriculture without hedges/fences 0.30

Crops, tall-grass prairie 0.19 0.30

Agriculture with homes, hedges at 1,250 m 0.35

Hedges 0.21

Scattered trees and hedges 0.24 0.35

Agriculture with homes, hedges at 250 m 0.40

Trees, hedges, a few buildings 0.29 0.45

City suburbs, villages, scattered forests 0.31

Larger cities with tall buildings 0.60

Woodlands 0.43 0.50

Very large cities, skyscrapers Measure

Source: Adapted from [4].

5.4 Wind Shade Adjustment

Strong wind “shadows” are present behind obstacles such as buildings and trees. For example, 

for a 20-ft building, turbulence bubbles (wind shadows) can extend up to 40 ft above the ground, 

up to 400 ft downwind, and 40 ft upwind of the obstacle, as illustrated in figure 7. The prevailing

simplified recommendation is to ensure that the lower blade tip of the wind turbine rotor is 

installed at least 30 ft above obstacles within 500 ft. A more complex analysis requires 

information on the height, width, type, distance, direction, and porosity of individual obstacles, 

and then for multiple obstacles in different directions. The Danish Wind Shade Calculator is a

free tool that can be used to run this analysis. It is available

at http://www.motiva.fi/myllarin_tuulivoima/windpower%20web/en/tour/wres/shelter/index.htm. 

Although the calculator provides a good picture of the obstacle impact on wind flows, it is still 

an approximation, and is not accurate for close obstructions (which should be avoided in any 

case).
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Figure 7. Zone of disturbed flow over a small building. Illustration adapted from [3]

5.5 Turbulence Intensity

Experience indicates that turbulence intensity is a major issue for small turbines because of their 

tower height and location around ground clutter. Turbulence can reduce the annual energy output

estimate anywhere from 15% to 25% because wind turbine power curves are typically developed 

based on measurements taken at sites with relatively low turbulence intensity compared to 

typical small wind project sites. Most assessors include a reduction for turbulence in their annual 

energy production estimates, and explain why. Sagrillo and Taivalkoski have developed 

guidelines for turbulence intensity that are based on terrain cover, available

at www.smallwindtraining.org/wp-content/uploads/2011/12/12-Turbulence-Intensity.pdf. 

5.6 Gross Annual Energy Production Estimate

The gross annual energy production estimate is developed by applying the adjusted estimated 

wind distribution to the wind turbine power curve (and possibly multiple options), and then 

adjusting the energy production for expected turbulence intensity to estimate the monthly and 

annual energy production. The process of adjustment should be done by sector so that the wind 

shade and turbulence intensity factors for each sector can be applied.

Turbine manufacturers usually provide energy production curves that are based on average wind 

speeds and Weibull shape factors of 2.0 (a special case called a Rayleigh distribution), and these 

energy production values should use power curves validated by third-party agencies. These 

energy production values can be used as a first approximation; however, deviations in the 

Weibull shape factor can have strong impacts on energy production. If possible, measurements 

or estimates of the wind speed distribution should be used. If there are no data available to 

establish a site- or area-specific distribution, a strong written caveat should be provided in the 

assessment report and include an estimate of the potential impact on annual energy production.

5.7 Net Annual Energy Production Estimate 

To calculate the net annual energy production estimate, it is necessary to first determine or 

estimate other factors that could further reduce the actual energy produced, including downtime 

for faults and servicing, blade soiling, turbulence intensity causing control error/hysteresis and 

yaw error losses, icing losses, grid outages, wire losses, and inverter losses. In most cases, wire 

and inverter losses for small wind turbines are included in the certified power curve because 

power delivered is measured at the connection to the breaker panel. Likewise, control and yaw 
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error losses are also included in measured power curves unless the site under consideration has 

extreme turbulence. The other losses are very dependent on the turbine or local environment with 

a likely range of 5% to 15%. The high end of this range would be for remote sites in which a

long lead time can be expected for service. To estimate the net annual energy production, reduce 

the gross annual energy production by the estimated losses.
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6 Other Site Issues

Issues other than wind resource and obstructions can affect site selection, suitable turbines and 

project cost.

6.1 Zoning and Permitting

Zoning refers to the general local regulations that allow and restrict various types of projects, 

whereas permitting refers to acquiring permits for a specific project within the scope of those 

zoning rules. By drafting a project site plan, an element of a site assessment report that provides 

a pictorial representation of the potential installation of the turbine at the preferred location, the 

site assessor provides an important first step to developing the permit package.

Zoning and permitting practices vary dramatically across the country so it is very valuable for 

the site assessor to be familiar with the local regulations, authorities, and general requirements. 

In some cases, zoning and permitting expectations are consistent and straightforward, allowing

the assessor to provide the customer with a clear understanding of what is required. In other 

cases, hearings may be required and the process is uncertain. When zoning and permitting

practices are clear, a project designed within the existing limitations will experience a much 

smoother permitting process and will be more likely to be granted a permit. But for a project that 

falls outside of defined limits, the customer will usually be sent through a special review process 

to obtain a variance from the existing rules and regulations—a potentially expensive and timeconsuming process that often involves at least one public hearing and has no guarantee of 

success.

For more information on state and local wind zoning policies and permitting issues, visit:

• DOE’s Database of State Incentives for Renewables & Efficiency: www.dsireusa.org

• WINDExchange – Wind energy ordinances: 

http://apps2.eere.energy.gov/wind/windexchange/policy/ordinances.asp 

• Windustry – Planning a Small Wind 

Project: http://www.windustry.org/small_wind_toolbox.

If no zoning ordinances exist for a specific location, it might be worthwhile to help create one. 

Several DOE sponsored Regional Wind Resource Centers 

(http://apps2.eere.energy.gov/wind/windexchange/regional.asp) have developed regionally 

appropriate model ordinances and the WINDExchange provides a database of existing 

ordinances (http://apps2.eere.energy.gov/wind/windexchange/policy/ordinances.asp). 

Additionally the Distributed Wind Energy Association provides some resources 

at http://distributedwind.org/assets/docs/PandZDocs/dwea-model-zoning-ordinance-passed-01-

07-12.pdf, and Wisconsin’s model Small Wind Energy System Ordinance developed by Focus 

on Energy, is available at http://www.renewwisconsin.org/wind/ToolboxZoning/Small%20Wind%20System%20Model%20Ordinance%2012-06.pdf .

Although the permitting process generally falls in the purview of the installer, the site assessor 

can provide a great service by apprising the client of permits that are required with local, state, 
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and federal agencies, such as for proximity to military bases, shoreline management areas, 

designated scenic areas, stream protection zones, or sensitive government lands.

6.2 Federal Aviation Administration Considerations

If a project is anywhere near an airport, then a Federal Aviation Administration (FAA) review 

and “no hazard determination” may be needed, which requires knowledge of the distance and 

direction from the site to the airport, and the associated FAA class. Frequently asked questions 

about the FAA and wind turbines can be found

at https://oeaaa.faa.gov/oeaaa/external/searchAction.jsp?action=showWindTurbineFAQs. 

Information on FAA notification can be found via the FAA Notice Criteria Tool 

at https://oeaaa.faa.gov/oeaaa/external/gisTools/gisAction.jsp?action=showNoNoticeRequiredTo

olForm. If FAA notification is required, then a Form 7460-1 must be filed online after setting up 

an account 

at https://oeaaa.faa.gov/oeaaa/external/userMgmt/permissionAction.jsp?action=showRegistration

Form.

6.3 Environmental

It is essential to consider the environmental impacts and benefits of a small wind installation to 

the extent required by permitting agencies. Such impacts could include but are not limited to 

ground restoration after excavation and trenching, and avian and wildlife impacts. The U.S. Fish 

& Wildlife Service now requires voluntary compliance with their screening process for all wind 

turbine installations and has provided guidance on the tiered approach. Distributed wind projects 

are expected to complete only the first two tiers: land-scale level assessment and project level 

assessment.1 Water runoff and erosion control are important to both minimize environmental 

impact and safeguard the structural integrity of the turbine foundation. Avoid building in obvious 

wetlands for environmental reasons, tower foundation stability, and structural and electrical 

integrity. Most foundation designs preclude use in standing water or potential flood situations so 

proper site drainage should be understood.

Providing an objective assessment of impacts includes reviewing development benefits as well

documenting the potential impacts of the project. Calculators for estimating emissions and 

pollutants avoided by using a wind generator can be found at the following websites:

• www.epa.gov/cleanenergy/energy-resources/calculator.html#results (Carbon dioxide

only)

• www.cleanerandgreener.org/resources/pollutioncalculator.html

• www.abraxasenergy.com/energy-resources/toolbox/emissions/

• www.csgnetwork.com/elecpowerpolcalc.html. 

 1

For more information, visit http://www.fws.gov/windenergy/wind_training/wind_training.html, 

http://ecos.fws.gov/ipac/, http://www.fws.gov/endangered/, http://www.wind.tnc.org/, and http://www.natureserve.org/.
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6.4 Other Siting Issues

Although the available wind resource at a specific site is a critical element of site assessment, 

there are many other factors that will impact the development process. If a site has access to a 

viable wind resource, the considerations described in this section will influence where a wind 

turbine could be located and what additional restrictions may be placed on the project.

The site assessment should include notes on the following:

• The location of the utility meter, main service panel, and any subpanels

• The voltage of the main service panel and whether it is single or three-phase

• The rating of the panels or main breakers and whether or not there is room for a breaker 

or breakers for the wind turbine system, or if a subpanel needs to be installed to 

accommodate the breaker(s).

The site assessor needs to have sufficient understanding of these issues to recommend 

compatible wind turbines, as modifying the electrical infrastructure can incur considerable costs.
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7 Site Assessment Report

After compiling all of the information provided in the previous sections, the resulting site 

assessment report should present two key recommendations:

1. A simple yes, no, or maybe statement on the efficacy of installing a wind turbine, with 

explanation

2. If yes, the preferred (and an alternate) location for the turbine, with justification.

The site assessment should conclude with one or more wind energy production estimates, based 

on the expected wind resource, tower height options, and turbine selections. Turbine 

recommendations should be appropriate to the customer’s needs and goals as well as available in 

the area with O&M support. If there is no data available to establish a site- or area-specific 

power production estimate, a strong written caveat should be provided in the assessment report 

and include an estimate of the potential impact on annual energy production.

Examples of good site assessments can be found here:

• http://www.smallwindtraining.org/wp-content/uploads/2014/05/Faller-Residential-WindAssessment.pdf

• http://www.smallwindtraining.org/wp-content/uploads/2014/05/Taivalkoski-BusinessWind-Assessment.pdf

• http://www.smallwindtraining.org/wp-content/uploads/2014/05/West-Agricultural-WindAssessment.pdf.

Appendix A of this document also provides several example check lists that may be helpful in 

the development of a site assessment report and appendix B provides some project case studies 

that highlight different aspects of the site assessment process. Additional information to be 

included in a site assessment report includes, but should not be limited to, the following:

7.1 Energy Efficiency

Although an energy efficiency audit is a different project, it is certainly appropriate for a small 

wind site assessor to recommend taking this additional step. It is often most cost-effective to 

address energy efficiency issues first, and then provide wind generation for the essential energy 

needs that remain. Despite the fact that the site assessor is not expected to be an expert in this 

area, he/she should be ready to acknowledge its relevance and recommend consultation. It is also 

prudent to discuss replacing existing natural gas, propane, or fuel oil appliances with electric

appliances if the wind turbine can be sized to serve those loads.

7.2 Turbine and Tower Options

The assessment report should provide turbine and tower recommendations that meet site and 

owner requirements. Recommendations should be for products that are third-party certified, such 

as from the Small Wind Certification Council, or Intertek, or that are recommended by the 

Interstate Turbine Advisory Council. The site assessor should be able to explain the various 

tower types, what is appropriate for the site under consideration, and why.
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7.3 Minimum Acceptable Turbine Tower Height for a Site

Based on the obstructions and ground clutter within at least 500 ft of the site or a tree line in the 

surrounding area, the site assessor should specify the minimum turbine tower height with a 

minimum of 30 ft additional clearance for the lowest point of the rotor. This specification should 

also include estimated tree growth for the 20+ year life of the wind system. Although this is the 

minimum acceptable height, the report should consider the economics of higher towers that will 

provide more energy.

7.4 Economics and Incentives

A preliminary financial analysis may be included in a site assessment based on typical installed 

costs. Site-specific quotes are usually done by the installer, sales person, or feasibility assessment

consultant(s). Financial analysis should include upfront and ongoing costs, such as maintenance, 

performance, electricity rates, incentives, and economic factors (e.g., interest, discount rates, and 

inflation) to estimate simple paybacks and internal rates of return. The site assessor needs to 

check www.dsireusa.org for federal incentives, such as the Investment Tax Credit and U.S.

Department of Agriculture grants, loans, and loan guarantees, and state incentives, such as netmetering, rebates, feed-in-tariffs, Renewable Energy Credits, and be able to understand and 

explain them. In addition, state incentive programs may be limited to a list of specific turbines as 

listed by the Interstate Turbine Advisory Council. More subtle incentives are also very 

important, including laws permitting the use and interconnection of wind, and net- or dualmetering rules that allow for a financial exchange of excess energy with a utility.

7.5 Operation and Maintenance Expectations

Ultimately, O&M is the responsibility of the turbine installer and owner, but the site assessor 

should be able to advise the customer that maintenance is required. Turbine and tower 

manufacturers should provide their own O&M plan; however, turbine owners should be aware 

that all rotating equipment will require some maintenance. Many turbines require periodic 

lubrication, oil changes, and replacement of wear surfaces such as brake pads. 

7.6 Off-Grid Systems

If a customer wants or needs full independence from the utility or battery backup in the event of 

power outages, the site assessor should be able to discuss the extra challenges, costs, and O&M 

requirements of a battery backup system and potential hybrid blends with solar, hydro, fossil, or

other energy resources. Design and sizing of the components for an off-grid system is beyond the 

scope of site assessment.

7.7 Building Integration

Most experts in the wind industry discourage building integration because of insufficient wind 

resource at building height, potentially damaging turbulence, and excessive costs from additional 

building engineering and reinforcement. Building integration has strong implications for 

aesthetics, very difficult wind resource prediction challenges, and building-scale structural 

impacts that may require extensive engineering, building reinforcement, noise management, and 

system safety analysis. Given these complications a site assessor should avoid conducting an 

assessment for a building-integrated wind generator.
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7.8 Electrical Load Usage Analysis

For a potential small wind site, turbine size, performance, and project economics must relate to 

the customer’s energy needs and expenditures. Often customers have very little understanding of 

those needs and costs, so they should be included in the site assessment, along with some 

customer education on their energy needs. For residential customers, the bill is based on 

kilowatt-hours (kWh), or energy consumption, and may have charges that are fixed (e.g., a meter 

charge, billing charge, or customer service charge). Residential utility bills may also note the 

energy cost (retail rate for the electricity itself) as well as the delivery cost (retail rate to deliver it 

to the site). In some locations the energy billing rate will vary based on the level of consumption 

(tiered rates) or time of day. Commercial customers—usually, but not always, using three-phase

electric service—may have other charges, with the most common being a demand charge. The 

demand charge can be a significant part of the bill and is commonly based on the peak demand in 

any 15-minute period of the month or quarter. Demand charges are not reliably reduced by 

adding a wind generator. For existing facilities, the quickest way to determine the energy 

consumption is to review the electric bills. For new facilities or off-grid installations, a more 

involved estimate process is needed and will require tallying up all of the electrical equipment

and expected operating hours.
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8 Conclusions

This document is intended to provide guidelines in conducting and writing a small wind site 

assessment, highlighting common guiding principles using currently available tools and 

experience. These guidelines present a basic set of expectations for site assessors, customers, 

local agencies, and incentive programs, and are not intended as a substitution for site assessor 

training, which is strongly recommended and available from many sources.
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9 Glossary

The following provides a glossary of terms commonly used in the site assessment process. An 

expanded online glossary of small wind specific terms can be found as part of the small wind 

guidebook at http://en.openei.org/wiki/Small_Wind_Guidebook/Glossary_of_Terms.

Anemometer: An instrument that measures wind speed, which usually produces an electrical 

signal for logging data over time.

Beaufort scale: A scale of wind forces, described by name and range of velocity, and classified 

from force 0 to 12, with an extension to 17. The initial (1805) Francis Beaufort wind force scale 

of 13 classes (0 to 12) did not reference wind speed numbers but related qualitative wind 

conditions to effects on the sails of a frigate, then the main ship of the Royal Navy, from “just 

sufficient to give steerage” to “that which no canvas sails could withstand.” Although the 

Beaufort scale has little use in site assessments, a system of tree flagging observations has been 

used to estimate prevailing wind directions and levels on the scale over time.

Displacement height: The height above ground level where wind speed is theoretically zero 

based on the effects of ground cover.

Diurnal: Having a daily cycle or pattern. It may be useful to average many daily cycles of wind

speed or wind energy production to understand a typical daily pattern, by month, season, or year.

Downwind: To the lee, as in located behind an obstacle relative to the prevailing wind direction. 

A downwind turbine has wind passing the tower first and then through the blades.

Dual-metering: Buying electricity from the utility and selling it to the utility with two different 

energy rates, typically retail (buying) and wholesale (selling).

Energy curve: A diagram showing the annual energy production at different average wind

speeds, typically assuming a Rayleigh wind distribution (with a Weibull shape factor of 2.0).

Flagging: Is the deformation of local vegetation toward one direction, indicating the prevailing 

wind direction and relative strength (more formally called Krummholtz formation). Flagging is 

sometimes used with the Beaufort scale to generate an initial estimate of local site conditions.

(Note: flagging does not determine the wind resource, but is a confirming indicator of it. For 

example, sometimes flagging is the result of sunlight availability, or trimming of tree branches 

near electrical lines. The assessor needs to understand when flagging is relevant, or when it is a 

confirming indicator of another condition at the site.)

Frequency distribution: A statistical function presenting the amount of time at each wind speed 

level for a given data set and location, usually in percent of time or hours per year.

Electric cost adjustment (sometimes referred to as fuel cost adjustment): An energy charge 

(dollars per kilowatt-hour) on a utility bill in addition to the standard rate in the tariff, which is 

associated with extra costs to purchase fuel, control emissions, construct transmission upgrades, 

and so on. These various costs may be itemized or rolled into one electric cost adjustment rate.
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Geographic information system (GIS) software: GIS software is used for managing map-based 

information and data. It may also be used to visualize the relationships between terrain, wind 

data, land-use boundaries, obstacles, and potential wind turbine locations.

Global Positioning System (GPS): GPS is a satellite-based navigation system that uses handheld 

receivers to identify the location of the receiver and coordinate with mapping system locations.

Gross annual energy production: The amount of annual energy (usually in kilowatt-hours) 

estimated for a given wind turbine at a given location, before adjusting for losses (see net annual 

energy production).

Horizontal-axis wind turbine (HAWT): A wind turbine designed with the axis of rotation around 

a horizontal shaft, typically with a propeller-like configuration. 

Hub height: For a HAWT, this typically refers to the height above ground of the center of the 

rotor that consists of blades mounted to a hub connected to the turbine’s main shaft.

Interannual variability: The variation from year to year in average wind speed, distribution, and 

patterns. 

Micrositing: The process of selecting a wind turbine location and determining the likely wind 

resource available after considering all of the possible impacts, such as topography, ground 

cover, and obstacles, and their location relative to the tower’s location, turbulence, and land-use 

restrictions. 

Microturbine: A very small wind turbine, usually under a 1,000 Watt rating, which is appropriate 

for small energy needs (e.g., for cabins, campers, sailboats, very small communication stations, 

or other small off-grid loads).

Net annual energy production: The amount of annual energy (usually in kilowatt hours) 

produced or estimated for a given wind turbine at a given location, after subtracting losses from 

the gross annual energy production. A variety of losses may be estimated for obstacle wind 

shadows, turbulence, turbine wake effects, turbine availability, high-wind hysteresis effects, 

electrical efficiency, blade icing, blade soiling and surface degradation, idling parasitic losses, 

control errors, low temperature shutdown, utility system maintenance, and other issues specific 

to a given turbine installation.

Net-metering: Buying electricity from the utility and selling the excess generation to the utility at 

the same energy rate, typically retail, which includes the generation rate, delivery cost, and all 

additional costs. Demand charges, if applicable, are not included. Credits may or may not be 

carried forward from windy months to calm months and excess energy may be paid at a lower 

rate than retail—or not at all—depending on the local utility and state regulations.

Peak demand (otherwise known as “demand”): The maximum electricity consumption level (in 

kilowatts) reached during the month or billing period, usually for a 15- or 30-minute duration.

The definition of peak demand may vary by electric utility. This is a simplified definition of a 

complex topic.
27

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Permitting: The process of obtaining legal permission to build a project, potentially from a 

number of government agencies, but primarily from the local building department (i.e., the city, 

county, or state). During this process, a set of project plans is submitted for review to assure that

the project meets local requirements for safety, sound, aesthetics, setbacks, engineering, and 

completeness. The permitting agency typically inspects the project at various milestones for 

adherence to the plans and building safety standards.

Power curve: The table or graph showing the expected power versus wind speed for a given 

turbine design and air density.

Prevailing wind direction: The direction or directions from which the wind blows most 

consistently, most often, or with the most energy. 

Reactive power: When the voltage and current waveforms for AC power are out of phase the 

resulting instantaneous power flow is modeled as real power and reactive power. The presence of 

reactive power increases the instantaneous current flow required to do work. The increase in 

current flow results in additional line losses. The utility tariff for larger customers may include a 

charge for reactive power compensation, measured in kilo-volt-amp-reactive. 

Shadow flicker: A moving shadow that occurs when rotating turbine blades come between the 

viewer and the sun. 

Site assessment: The act of evaluating a site to determine a favorable location for a wind turbine,

which includes assessing the expected wind resource and potential turbine performance at that 

location.

Siting: See micrositing and site assessment.

Small wind turbine: A wind turbine that has a rating of up to 100-kilowatts, and is typically 

installed near the point of electric usage, such as near homes, businesses, remote villages, and 

other kinds of buildings.

Tariff: An official schedule of rates or charges from a utility, usually with different rate 

schedules by customer classification (e.g., residential, commercial, industrial, farm, or other 

designation) and/or a service or meter rating for the customer.

Topography: The surface configuration and relief features of an area, such as hills and bluffs,

and the detailed mapping and description thereof.

Turbulence bubble: A region of excessive turbulence around an obstacle.

Turbulence intensity: A basic measure of turbulence that is defined by the ratio of the standard 

deviation of the wind speed to the mean wind speed. For wind energy applications this is 

typically defined as a 10-minute average wind speed and standard deviation based on 1-second 

samples. Turbulence intensity is important for wind energy applications because it has 

implications for both power performance and turbine loading. Experience indicates that it can be
28

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a significant issue for small turbines because of their tower height and location around ground 

clutter, which puts them in the most turbulent area of the atmospheric boundary layer. The 

effects of turbulence on distributed wind turbines can be seen in both power production and 

loading.

Upwind: Toward the windward direction, as in located in front of an obstacle relative to the 

prevailing wind direction. An upwind turbine has wind hitting the blades first and then the tower.

Vertical-axis wind turbine (VAWT): A turbine designed with the axis of rotation around a 

vertical shaft, with two prevalent configurations: Darrieus, which is an egg-beater style with 

primarily lift aerodynamics, and Savonius, a split-barrel style with primarily drag-based 

aerodynamics. 

Vertical wind shear exponent: The parameter that defines the rate at which the wind speed 

increases with height above ground (see equation in Section 5.3.2).

Weibull scale factor (or parameter): Parameter of a wind distribution as approximated by the 

Weibull statistical function that is closely related to the average wind speed. 

Weibull shape factor (or parameter): Parameter describing the shape or “fatness” of a wind 

distribution as approximated by the Weibull statistical function. A fairly median value of 2.0 

defines a special case called the Rayleigh distribution; lower values indicate a wider, flatter 

distribution, and higher values indicate a narrower, high-peaked distribution.

Wind direction: The direction from which the wind is blowing and a wind vane points. (This is 

sometimes confusing because it is not the direction the air is moving in).

Wind shadow: A turbulent and/or low-wind-speed region downwind of (behind) an object such 

as a building, tower, or trees.

Wind turbine: A machine that harnesses the energy of the wind, also called a wind generator,

when used to produce electricity. 

Wind rose: A diagram that indicates the average wind speed, average percentage of time that the 

wind blows, and/or the percentage of energy in the wind from different directions, on a monthly 

or annual basis.

Wind shear: The difference in wind speed and direction over a relatively short distance in the 

atmosphere. Wind shear can be broken down into vertical and horizontal components, with 

horizontal wind shear seen across storm fronts and near the coast, and vertical shear seen 

typically near the surface (though also at higher levels in the atmosphere near upper-level jets 

and frontal zones aloft). 

Wind vane: An instrument that measures the wind direction, usually producing an electrical 

signal to log data over time or provide input to the yaw control system for a wind turbine with an 

active yaw system.
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Zoning: Most land has been delegated to various zones by a region’s local government and 

building department officials (at the city, county, or state level [occasionally]). The zones control 

types of land use, such as agricultural, residential, commercial, and industrial, and include

subcategories. Each type of zoning carries its own specific permitting restrictions, such as 

building height and property line offsets (required separation distance). 
30

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References

1. North American Board of Certified Energy Practitioners. Small Wind Site Assessor – Job 

Task Analysis, 2011. Accessed April 5, 2015: http://www.smallwindtraining.org/wpcontent/uploads/2013/09/NABCEP-SWSA-JTA-2011-5-16-final.pdf.

2. Wegley, H.; Ramsdell, J.; Orgill, M.; Drake, R. A Siting Handbook for Small Wind 

Energy Conversion Systems, Pacific Northwest Laboratory/Battelle Memorial Institute, 

Contract DC-AE06-76 RLO 1830, 1980.

3. Chiras, D. Wind Power Basics, New Society Publishers, 2010.

4. Taivalkoski, A.; Sagrillo, M.; Hasselman, R. Revised Wind Shear Values, 2010. Accessed 

April 15, 2015: www.smallwindtraining.org/wp-content/uploads/2011/12/16-RevisedShear-Values.pdf. 
31

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Bibliography

1. Spera, D. “Characteristics of Wind,” chapter by Walter Frost and Carl Aspliden, Wind 

Turbine Technology, 1984.

2. Anderson, E. et al. “A Broad Overview of Energy Efficiency and Renewable Energy 

Opportunities for Department of Defense Installations.” Technical Report NREL/TP7A20-50172. NREL/DOE-SERDP interagency agreement IAG-10-1779.

3. Gipe, P. Wind Power: Renewable Energy for Home, Farm, and Business, Chelsea Green 

Publishing Company, 2004.

4. Sagrillo, M.; Butler, R. Small Wind Site Assessor Training Syllabus, IREC, 2011. 

Accessed April 5, 2015: http://www.smallwindtraining.org/?page_id=6.

5. Frost, W.; Nowak, D. Handbook of Wind Turbine Generator Siting Techniques Relative 

to Two-Dimensional Terrain Features. Prepared for Battelle, Pacific Northwest 

Laboratories by FWG Associates, Inc., Tullahoma, TN 37388, November 1979.

6. Van Eimern, J.; Karschon, R.; Razumova, L.A.; Robertson, G. W. Windbreaks and 

Shelterbelts. Technical Note 59, World Meteorological Organization, Geneva, 

Switzerland, 1964.

7. Meroney, R. N. “Wind in the Perturbed Environment: It’s Influence on WECS.” 

Presented at American Wind Energy Association Conference, Boulder, Colorado, May 

11‒14, 1977, Colorado State University, 1977.

8. Park, J.; Schwind, D. Wind Power for Farms, Homes, and Small Industry. Prepared for 

the Energy Research and Development Administration by Nielsen Engineering and 

Research, Inc., Mountain View, California, 1978.

Links for Further Information

1. Distributed Wind Energy Association: http://distributedwind.org/

2. Renewable UK: http://www.bwea.com/index.html

3. Wind Works: http://www.wind-works.org/articles/index.html

4. Wind Shade Calculator, 

Danish: http://www.motiva.fi/myllarin_tuulivoima/windpower%20web/en/tour/wres/shel

ter/index.htm

5. Met Office: http://www.metoffice.gov.uk/energy/vmm

6. Wind Shear: http://www.engineeringtoolbox.com/wind-shear-d_1215.html

7. Microgeneration Certification 

Scheme: http://www.microgenerationcertification.org/installers/installers

8. Small Wind Training Resources: http://www.smallwindtraining.org/?page_id=6

9. RENEW Wisconsin’s Small Wind Toolbox (useful information for zoning and permitting 

issues): http://www.renewwisconsin.org/wind/windtoolbox.htm.
32

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Appendix A

Sample Checklist

Wind Energy Site Assessment (Customer Name/Location)

1. Motivation

____Specify customer goals and expectations

____Reviewed electricity usage and costs (including rate structure)

2. Site Description

____Documented location(s)

____Reviewed topography, landscape, surface roughness, and ground clutter

____Estimated displacement height (if applicable)

____Documented obstacles

____Reviewed construction and delivery access

____Estimated soil conditions

____Documented potential safety hazards

3. Wind Characteristics

____Acquired wind maps and wind data

____Considered wind measurement

____Estimated wind characteristics (e.g., speed, direction, Weibull, turbulence, shear, 

uncertainty)

4. Micrositing and Analysis

____Completed micrositing

____Adjusted for topography 

____Adjusted wind direction

____Adjusted for wind shear 

____Adjusted for displacement height 

____Adjusted for obstacles and wind shading

____Completed adjusted wind speeds

____Completed energy estimation

____Adjusted energy estimation for losses

5. Other Requirements

____Reviewed interconnection and examined service panel 

____Checked zoning and permitting requirements (to the extent established for the area) 

____Checked for Federal Aviation Administration/aviation impact

____Checked environmental requirements (if established for the area), observed potential issues

____Prepared U.S. Fish & Wildlife Service materials for review

6. Report

____Recommended turbine options and tower options with estimated net annual energy 

production for each scenario

____Recommended prime location with secondary options

____Completed checklist and report information

____Included educational section.
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Sample Report Outline

Wind Energy Site Assessment (Customer Name/Location)

1. Motivation

Customer goals: __Energy __Independence __Marketing __Save World __Other _______

Electricity usage: ____Low ____High ____Avg kWh/mo ____Peak demand (kW) ____kVAR

Electricity rate (as appropriate): ____Tariff ____Base $/kWh ____Electric cost adjustment/Fuel 

cost adjustment $/kWh ____$/kW-mo ____$/kVAR ____Fixed costs ____Taxes ____ Other

2. Site Description

Address: ___________________________________________________________________

GPS: _____________________

Location maps: (insert here: general area, close-in, site plan)

Photographs: (insert here – i.e., Dermandar panorama)

Topography features: ___________________________________________________________

Main features: ______Shape______Height______Length ______Distance______Direction

Landscape description: __________________________________________________ 

Displacement height: ______Mature Canopy Height (*0.67 or 0.75 =) ______Displacement 

Height

Obstacles: (e.g., number, height, width, type, distance, direction) List here:

Access: (e.g., roads, obstacles, installation layout) ___________________________________

Soils: ______Class ______Strength ______Water Depth___________________Description

Potential Hazards: (e.g., overhead, underground, terrain, weather, wetlands, agricultural 

operations) ________________

Existing utilities: (location, overhead, underground: water/well, electric, sewer/septic system, 

natural gas/propane, cable television, phone, other.)

3. Wind Characteristics

Wind maps: (source, insert here)

Wind data sources: _________________________________________ (see Section 5.1)

Wind measurement: ___Needed (yes/no)?______Scope ________Duration ________Cost

Wind speeds: ______Average _______________Weibull parameters (insert details here)

Wind directions: __________Prevailing __________Secondary (insert wind rose, energy rose)

Turbulence: ________Intensity

Wind shear: ________Exponent

Uncertainties: __________________________List, combine: ____________Wind-speed range

Others: ________________________________________________________
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4. Micrositing and Analysis

Micrositing: (describe selected location and height, rationale, benefits, challenges, other options)

Topography adjustment: 

____ (Factor to adjust wind speeds from source data, rationale) ____New average wind speed

Wind direction adjustment:

____ (Degrees to adjust wind rose from source data, rationale) ____New prevailing direction

Wind shear adjustment: ____displacement height ____ exponent ____ data height 

____hub height ____New average wind speed

Wind shade adjustment: (Obstacle #, shade factor, direction (%), weighted factor)

List here:

Adjusted wind speeds: ____New average wind speed _______Range ____ Weibull parameters

Energy estimation: ____Avg __________Range (apply turbine power curve; multiple options)

5. Other Requirements

Because zoning and permitting requirements are local, there can be large variations in procedures

and the amount of information available to the site assessor at the time an assessment is 

prepared. Reporting on these factors will have to be adjusted accordingly. Environmental 

requirements may also vary. 

Interconnection: ______Utility ____Allowed ____________Location 

_____________Requirements

Service Panel Specs: ____Voltage____Phases ____Current ____Power ____Spare breakers

Zoning: ______Zone ____Height Limit ____Setback__________________Other restrictions

Permitting: ______________Authority Having Jurisdiction (AHJ) ______Fees ______Lead time

Federal Aviation Administration (FAA): ________________________Distance, Direction to 

nearest Airport ______FAA class

Environmental requirements: (Any requirements by AHJ, state, or federal agencies?)

Environmental observations: (Any obvious wetlands, other issues known to customer or by the 

site assessor?)

U.S. Fish & Wildlife Service report for the area

6. Report

Recommendation: ____ (Yes/No, Explanation)

Preferred location: _______________________________Describe, add to site plan

Checklist: (Include checklist developed above)

Education: (Include discussion of energy efficiency, turbine and tower options, horizontal-axis

and vertical-axis wind turbines, micro/small/large/multiple units, incentives, economics,

operation and maintenance expectations, off-grid systems, and building integration).

__________________________________________________________________________
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Appendix B. Case Studies and Lessons Learned

The following case studies are intended to highlight the various site selection and assessment 

choices that can affect turbine performance, without regard to specific turbine types, sizes, 

certification, and installation. 

Case Study #1: Comparison of Two Similar Turbines: Bassetti and 

Grim

Case Study #1 compares two similar turbines from the same manufacturer (Bergey Windpower 

Company) that were both properly installed. The primary factor contributing to a significant 

difference in the productivity of these turbines was the site selection. To better understand the 

potential impact that site selection can have on a given project, this case study looks into projectspecific details to determine the contributing positive and negative siting impacts associated with 

the projects. Figure B-1 shows the Bassetti turbine installed in an open, agricultural setting, and

Figure B-2 shows the Grim turbine installed in a forested setting. Table B-1 provides the system 

details for both the Bassetti and Grimm installations. 

Figure B-1. Bassetti turbine in Klickitat County. Photo by Gwen Bassetti, NREL 26429
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Figure B-2. Grim turbine in Peshastin, Washington. Photo by Abigail Krich, 

NREL 13495
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Table B-1. Case Study #1 Turbine System Details

BASSETTI GRIM

Location Klickitat County, Washington (Figure B-1) Peshastin, Washington (Figure B-2)

Date Installed September 2004 October 2002

Project Goal Customer decided to install a turbine 

after seeing their neighbor’s installation

Homeowners wanted to participate in the 

Sustainable Natural Alternative Power incentive 

program in the Chelan County Public Utility District 

service area, which would pay up to $1.50 per 

kilowatt-hour (kWh) for green electricity

Assessment 

Information

No formal site assessment prior to 

installation2

Assessment done in 2001, but it relied too much on 

the owner's estimate of the site wind speeds, 

which turned out to be incorrect.3

Met installation was suggested, but owners did not 

want added expenses; they believed site was 

windy. Installer observed landscape and agreed.4

Turbine Information

 Model Bergey Excel 10 Bergey Excel 10

 Capacity 10 kilowatts (kW) 10 kW

 Tower Height 120-ft guyed tower 100-ft lattice tower

Site Characteristics

 Elevation 632 meters (m) 610 m

 Latitude/Longitude 45.83/-120.65 47.547/-120.639

Surrounding 

Terrain/Obstacles

Turbine installed in an open field behind 

the ranch

Site situated on the top of a hill, surrounded by 

trees5

Performance

 Estimate (kWh) 13,000 kWh annually6 8,000 kWh annually

 Actual (kWh) ~13,000 kWh/year (yr)

7 (100% of 

estimate)

500 kWh during first year of operation (less than 

10% of estimate)

Maintenance Issues Inverter cut-outs caused by extra output

from new turbine blades; no other 

maintenance issues8

Faulty inverter

Current Operating 

Status

Still operating; owner believes the 

turbine has paid for itself since the

installation9

• After 4 years of operation, the turbine was 

taken down, sold, and relocated to a 

property in Ellensburg, Washington

 2 Jennifer Grove email. August 23, 2013.

3 Email correspondence with original installer. September 3, 2013.

4 Woofenden, Ian. “A Second Wind.” Home Power 130/April & May 2009. 5 Sinclair, K. (2005). Regional Field Verification – Case Study of Small Wind Turbines in the Pacific Northwest. 

NREL/CP-500-38166. Golden, CO: National Renewable Energy Laboratory. Accessed approximately April 2013: 

http://www.nrel.gov/wind/smallwind/pdfs/sinclair_rfv_case_study.pdf. 6 Sinclair, K. (2005). Regional Field Verification – Case Study of Small Wind Turbines in the Pacific Northwest. 

NREL/CP-500-38166. Golden, CO: National Renewable Energy Laboratory. Accessed approximately April 2013: 

http://www.nrel.gov/wind/smallwind/pdfs/sinclair_rfv_case_study.pdf

7 Owner interview. June 5, 2013. 8 Sinclair, K. (2005). Regional Field Verification – Case Study of Small Wind Turbines in the Pacific Northwest. 

NREL/CP-500-38166. Golden, CO: National Renewable Energy Laboratory. Accessed approximately April 2013: 

http://www.nrel.gov/wind/smallwind/pdfs/sinclair_rfv_case_study.pdf. 9 Owner interview. June 5, 2013.
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Wind Rose

Unlike Case Study #2 and #3, Case Study #1 does not include a wind rose because of the chosen 

assessment methodology for each project. 

Lessons Learned

Prior to installing a small wind turbine, it is vital to assess the wind resource to reduce 

production uncertainties that can be associated with a project. Case Study #1 highlights the 

Bassetti and Grim installations as a paired example, because both installations used a minimal 

resource assessment methodology, yet had decidedly different outcomes. 

Bassetti

The primary decision criterion for the Bassetti project was a neighbor’s successful wind turbine 

installation. Though no formal site assessment was conducted for this project, it did end up 

producing at a high level. 

The success of the Bassetti installation can likely be associated with not only a strong wind 

resource, but also a location that is free and clear of ground clutter, which would obstruct the 

wind resource. When deciding between locations on a property, sites with higher elevations and 

fewer obstructions are considered superior.

For the Bassetti case, not doing a wind resource assessment resulted in a successful installation;

however, this approach results in high levels of uncertainty in terms of how the turbine will 

perform at a particular site. The Grim project that follows illustrates this risk. 

Grim

The Grim project was heavily reliant on the owner’s belief that the site had a strong wind 

resource. Project installers did use a wind resource map in their assessment, but the 2001 wind 

resource maps had limited resolution when compared to more recently developed versions. A 

2009 article highlighting the Grim project indicated that one of the contributing factors to the 

project’s failure was the lack of advanced wind resource maps. At the time of the article’s 

publication, new resource maps indicated that the Grim site had an average wind speed of 4.8 

mph at a 60-m height, which is insufficient for adequate production.10

The access to reliable wind data through higher resolution resource maps could have factored 

into whether or not the Grim turbine should have been installed. Though the installer suggested 

that collecting data using an on-site anemometer would provide a higher level of production 

certainty, the owners declined because of the added expense. Had an anemometer been utilized, 

the added cost to purchase the necessary equipment would have been between $500 and $1,500. 

This method of assessment would have shown that the location was not ideal for a wind turbine 

installation.

Another factor that may have contributed to the project’s eventual failure and subsequent sale 

and relocation is the lack of clearance at the original location. Ground clutter creates an 

obstruction that can impact the strength of a site’s wind resource. As a result, it is absolutely 

 10 Woofenden, Ian. “A Second Wind.” Home Power 130/April & May 2009.
39

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essential to fully consider the surrounding ground clutter of a potential project site, including the 

possible removal of trees, shrubs, or other vegetation to provide unobstructed wind flow. 

Case Study #2: Comparison of Two Turbines: Kittery and Vosters

Case Study #2 compares two wind installations that feature similar sized turbines in which

comparable resource assessments were conducted using wind resource maps. The primary factor 

contributing to a significant difference in the productivity of these installations was that one site 

had access to a relatively strong and uninterrupted wind resource, whereas the other did not. To 

better understand the potential impact that site selection can have on a given project, this case 

study examines specific project details to determine the contributing siting impacts, both positive 

and negative, associated with the projects. Figures B-4 through B-7 and Table B-2 and the 

following information provide insight into both of the projects and their results. 

Figure B-4. Turbine in Kittery, Maine. Photo by Donald Doval,

NREL 28427
40

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Figure B-5. Vosters turbine in Appleton, Wisconsin. Photo by Kettle View Renewable Energy, LLC, 

NREL 28428

Table B-2. Case Study #2 Turbine System Details

KITTERY VOSTERS

Location Kittery, Maine (Figure B-4) Appleton, Wisconsin (Figure B-5)

Date Installed September 2008 2011

Project Goal Town of Kittery installed the turbine 

near the transfer station and local 

middle school in an attempt to reduce 

energy costs

Reduction of costs and clean energy 

production

Assessment 

Information

Project developers conducted a site 

assessment using a 50-m map with the 

best available resolution at the time;

11

prior to contracting with Entegrity, the 

town installed an anemometer and 

began data collection in November 

2006, which the client used as

validation to move forward with the 

wind installation process;

12

it is unknown how the data were used 

by the developers in the final site 

evaluation for this project

Assessment was conducted in May 

2010 using a 60-m wind resource 

map from 2007; the wind rose was 

provided through AWS Truewind13

 11 Project developer interview. (May 3, 2013). 12 Seacoastonline.com. (2008). “Kittery wind turbine pays dividends.” Accessed June 2013:

http://www.seacoastonline.com/article/20081223/BIZ/812230370

13 Email correspondence with original site assessor. (August 2013).
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KITTERY VOSTERS

Turbine Information

 Model Entegrity EW50 Endurance E 3120

 Capacity 50-kilowatts (kW) 177-m2 swept area 50-kW 290-m2 swept area

 Tower Height 125-ft lattice tower 140-ft tower

Site Characteristics

 Elevation ~26.8 meters (m) 221 m

 

Latitude/Longitude

43.119/-70.749 44.3382/-88.3149

Surrounding 

Terrain/

Obstacles

Developed on the crest of a hill, above 

some trees at the town transfer station 

with a big gap facing the principal wind 

direction down by the river (see wind 

rose B-6); trees are 70 ft high in the area

The site had excellent access to the 

prevailing western winds with open 

farmland in that direction for over 2 

miles (see wind rose B-7); open 

farmland stretches to the south and 

southeast for 3/4 mile14

Performance

 Estimate (kWh) 58,000 kilowatt-hours (kWh) annually15 111,830 kWh annually

 Actual (KWh) ~35,000 kWh/year (yr.)

16 (approx. 60% 

of estimate)

100,000 kWh/yr (approx. 90% of 

estimate)

Maintenance 

Issues

Main brakes malfunctioned, locking the 

blades in place

Minimal downtime for various issues 

that are not considered to have had a 

significant impact on production

Current Operating 

Status

Turbine is still located at the transfer 

station, but does not currently generate 

energy17

Still operating

Note: The point here is not the difference in production between the two turbines. These are very 

different machines and even though they have the same rated power, they would generate very 

different annual energy even on the same site. The point is the difference between the expected 

and delivered energy—with one site experiencing impacts from obstructions that were not 

effectively accounted for and one experiencing fewer impacts from obstructions. 

 14 Vosters site assessment. (May 18, 2010). 15 Project developer interview. (May 3, 2013). 16 Project developer interview. (May 3, 2013). 17 Email with Cameron Wake, member of the

Kittery Energy Committee.
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Wind Rose

Figure B-6. Kittery wind rose

Figure B-7. Vosters wind rose
43

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Lessons Learned

Case study #2 highlights the Kittery and Vosters installations as a paired example because both 

installations used a minimal resource assessment methodology, installed equal kilowatt-sized 

turbines, and had decidedly different outcomes.

Kittery

In the case of Kittery, project developers encountered a low but steady wind resource that was 

not adequate for a wind turbine installation. The estimated wind resource for the Kittery 

installation at hub height (~125 ft) was 4.6 m/s, or 10.39 mph, a resource that is characterized as 

a Class I wind site. In hindsight, with such a limited resource at that height, the project 

developers believe they could have potentially used a taller tower (~200 ft) with a larger turbine 

to reach a stronger wind resource.

An additional aspect of the wind resource that potentially affected the installation was that the 

project developers incorrectly overestimated the amount of flow recovery at the site. Flow 

recovery is the revitalization of the wind resource at a specific location after it has been

interrupted by an obstacle. Seventy-foot trees were in the vicinity of the Kittery project and 

presented a level of ground clutter that was likely to play an important role in the 

underproduction of this project. In addition to the surrounding trees were buildings located to the 

west and northwest of the turbine, which may have also interfered with the wind resource. It is 

critical that both ground clutter and other potential wind resource obstacles be considered during 

site evaluation and selection. 

In addition to the impact on wind speeds, ground clutter can create higher levels of turbulence 

that can influence a project’s annual energy output. High levels of turbulence also have the 

ability to decrease a turbine’s expected life span because of additional stresses on various 

components. 

Projects that could be impacted by ground clutter should take additional steps during a resource 

assessment to understand how the obstacles will affect the wind resource. It is suggested that 

these projects follow the best practice of installing an on-site anemometer for data collection to 

reduce wind resource uncertainty and increase the likelihood of a successful project. 

Vosters

Similar to the Kittery project, the Vosters installation used a 2007 wind resource map as a major 

part of the assessment. The map indicated that the proposed location had an average wind speed 

of 14 mph at a 60-m height. Although the project hub height was ~42 m, the project proponents 

predicted an average wind speed of 12.4 mph at that height and assumed the wind resource was 

adequate for the site. 

The Vosters’ assessment noted that the open space from the predominant wind direction made 

the site an ideal location. If there had been ground clutter in the form of large trees or man-made 

structures located in this area the site might not have had access to the necessary wind resource 

for a successful installation. Five hundred feet east of the installation is a barn with three 80-ft

silos. The height and mass of the barn and silos were factored into the assessment.
44

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Case Study #3: Hempstead Project

Case Study #3 examines a wind project in which various parties took different approaches in 

completing a wind resource assessment that used wind resource maps and supporting data sets, 

an online small wind assessment tool, publicly available data sets, and nacelle anemometer data 

each with slightly different values for Weibull shape and scale factors, vertical wind shear 

factors, losses, and so on. The different wind resource assessment techniques resulted in varied 

annual energy production projections which, of course, impacted the turbine performance 

evaluation and project economics.

The manufacturer’s preliminary annual energy production estimate provides a range of Weibull 

scale factors, wind shear factors, wind speeds, and annual energy production (with and without 

estimated losses). This is a prudent approach as no on-site hub height wind data were collected 

and analyzed. The wind will vary year-to-year and so will the energy production; however, the 

rotor diameter (two sizes are available for this turbine) is not included in this document and has a 

significant impact (~13%‒14% at these wind speeds) on the expected annual energy production.

For projects that are the same size as the one used in this case study (100 kW), the customer is 

often not willing to pay for a year-long wind resource assessment with a meteorological tower, 

so these multiple resource assessment techniques are prudent approaches to better frame the 

realm of possible outcomes and help reduce uncertainty in annual energy production projections.

The different energy production projections are provided in Table B-3 along with site details. 

New York State Energy Research and Development Authority (NYSERDA)18 sponsors the 

Small Wind Explorer, an online tool linked to AWS Truepower’s system, which can be used to 

estimate small turbine performance at various sites throughout New York. A free report can be

generated that provides mean annual wind speed at three heights (80, 100, and 120 ft), annual 

energy range (kWh) for generic 5-, 10-, 20-, and 50-kW turbines19 at each height, a color-coded 

wind speed map at 100 ft, and a wind energy rose. The site also provides a professional report

that includes wind speeds at 60 and 140 ft.

During Case Study #3, NREL used long-term hourly data from Modern-Era Retrospective 

Analysis for Research and Applications (MERRA)20 for two nearby sites (14 and 22 miles away) 

at both 10 and 50 m to generate synthetic wind speed and direction data at 37 m (hub height). 

Windographer21 software was used to analyze this data, which has several advantages: it 

provides hourly data that can be used to improve the accuracy of energy production calculations; 

it provides estimated wind speeds at two heights (10 and 50 m) so synthetic data at other heights 

can be extrapolated; and modeled data are adjusted by actual recorded data so that a reasonable 

long-term data set can be created. Figure B-8 shows the locations of the MERRA data sets 

relative to the turbine locations. 

 18 Small Wind Explorer, NYSERDA. http://nyswe.awstruepower.com/ 19 Full turbine list can be found at: http://www.cleanenergystates.org/projects/ITAC/itac-unified-list-of-windturbines/

20 Cullather, Richard & National Center for Atmospheric Research Staff (Eds). Last modified 25 Feb 2013. “The 

Climate Data Guide: NASA MERRA.” Retrieved from http://climatedataguide.ucar.edu/reanalysis/nasa-merra.

Originally posted on September 30, 2011, and last modified February 25, 2013. 21 Mistaya Engineering, Windographer Wind Assessment Software: http://mistaya.net/windographer/overview.htm
45

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure B-8. Map of MERRA data sites and turbine location. Image from Google Maps with notes 

added by NREL 

The photos in Figure B-9 and Figure B-10 show the site from the street and above before and 

after installation. Table B-3 provides turbine and site details along with the results of the four 

different site assessment approaches. 

Figure B-9. Hempstead, New York, NW100 wind turbine. Photo by the Town of Hempstead, NREL 

28963

~22 miles

~14 miles

NW100 turbine 

at Hempstead

MERRA data 40.5N 73.33W

LT mean WS @ 50m: 7.05 m/s

LT mean WS @ 37m: 6.84 m/s

MERRA data 40.5N 74.0W

LT mean WS @ 50m: 5.96m/s

LT mean WS @ 37m: 5.74m/s
46

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure B-10. Hempstead aerial photo before (left) and after (right) installation . Left photo by the 

Town of Hempstead, NREL 32728. Right photo from Google Earth. 

Table B-3. Case Study #3 Turbine System Details

HEMPSTEAD

Location Hempstead, New York (Figure B-9) 

Date Installed December 2011

Project Goal Town of Hempstead installed the turbine to provide power to Long Beach Island's only 

hydrogen-fueling station

Turbine Information

 Model Northwind 100

 Capacity 100 kilowatts (kW)

 Tower Height 37-meters (m), or 121-ft, monopole tower

Site Characteristics/ 

Surrounding 

Terrain/Obstacles

Long Beach Island is about 15 kilometers (km) in the east-west direction, and 1 km northsouth. The elevation range is 1‒3 m. The turbine is located in the eastern portion just 

north of Lido Beach and west of Point Lookout.

Reynold’s Channel is to the north and is ~200 m wide as it winds through a series of 

islands in the northwest-through-northeast arc. The densely packed residential 

neighborhood of Point Lookout is to the east-through-southeast arc. There is a mix of 

open grassy fields, small stands of trees (8‒15 m), parking lots, and 1 to 2 story buildings 

in the south-through-west arc. 

There is a 1.5-story building about 80 m to the southwest of the turbine.

The turbine site had a number of trees removed in preparation for the wind turbine, 

photovoltaic system, and hydrogen-fueling station.

There is clear fetch across the water of the Hudson Shelf Valley to the southeast-throughsouthwest of the island. Winds from the west-through-north blow across the island.

 Elevation ~2 m

 Latitude/Longitude 40.59307N /73.58834W

Performance Estimates

Wind Resource 

(m/s) and Energy 

Estimates (kWh/yr)

Wind Project Installer Performance22 Estimates

Mean wind speed at 37 m: 5.13 meters per second (m/s)

Assumed net losses: 8%

Weibull k: 2.0 Weibull a: 5.79 m/s

Annual energy production: 134 megawatt-hours per year (MWh/yr) 

Rotor diameter: 21 m

 22 Project Installer Estimates: Aegis, Alteris Renewables, Annual Energy Calculation and NW100 Power Curve.
47

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

HEMPSTEAD

Manufacturer Performance23 Estimates:

Mean wind speed at 37 m: 6.14 m/s 

Wind shear factor: 0.22 

Assumed net losses: 7%

Weibull k: 2.391 Weibull a: 6.921 m/s

Annual energy production: 209 MWh/yr 

Rotor diameter: not stated 

NYSERDA Small Wind Explorer24 Performance Estimates

Mean wind speed at 37 m: 6.22 m/s 

Wind shear factor: 0.22

Assumed net losses: 20%

Annual energy production range 

(Two generator 50-kW wind turbines: 168‒216 MWh/yr

Rotor diameter: not stated

MERRA Data with Windographer Performance Estimates Using Two Nearby 

Sites

Average mean wind speed at 37 m: 6.26 m/s 

Wind shear factor: 0.121 

Assumed net losses: 8%

Weibull k: 2.3 Weibull a: 7.07 m/s

Annual energy production: 235 MWh/yr Rotor diameter: 21 m 

Annual energy production: 267 MWh/yr Rotor diameter: 24 m

Operational Issues Operators of the Hempstead site indicated that there was a car accident that 

caused the turbine to be shut down from August 9 to August 14, 2012, (5 days),

and a post-Hurricane-Sandy inspection caused a precautionary safety shutdown 

from January 12, 2012, to March 4, 2012 (52 days). Expected annual energy 

production was adjusted accordingly for those 57 days.

Measurement 

Issues

Measurements taken at the turbine have consisted of very inconsistent time 

intervals and random gaps. As a result, it is not statistically possible to use these

data for verification of the mean wind speed for any meaningful interval. 

Consequently, the wind speed cannot be correlated to monthly or annual energy 

production. 

Cumulative kilowatt-hours produced are recorded in the manufacturer’s data set. 

Despite the inconsistent intervals, the cumulative tracking generally appears to be 

working, though some monthly totals do not appear to align with MERRA wind 

speed data during that month. 

Actual Energy 

Production

Wind Project Installer Actual Energy Production Report 

For a 6-month period (March 27, 2012 to September 27, 2012), the Town of Hempstead

reported energy production of 142 MWh. This appears to be the cumulative energy 

produced since the turbine began operating, not since March 27, 2012. Using the 

Northern Power Systems spreadsheet, the correct amount appears to be 99 MWh/6 mo.

Per Manufacturer Data, Energy Production for March 27, 2012 to March 26, 2013 

Note: Turbine off-line for 57 days during this time; 186 MWh/yr 

 23 Manufacturer Preliminary Annual Energy Production Estimate. 24 Small Wind Explorer, NYSERDA. http://nyswe.awstruepower.com/
48

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

HEMPSTEAD

MERRA Estimated Energy Production Using Averaged Hourly Mean Wind Speed for 24-

m Rotor: 

Mean annual energy production estimate: 267 MWh/yr

Mean annual energy production estimate for March 27, 2012 to March 26, 2013: 237 

MWh/yr

Mean annual energy production estimate for March 27, 2012 to March 26, 2013, with an

adjustment for Hurricane Sandy and the car accident: 189 MWh/yr

MERRA Estimated Energy Production Using Averaged Hourly Mean Wind Speed for 21-

m Rotor: 

Mean annual energy production estimate: 235 MWh/yr

Mean annual energy production estimate for March 27, 2012 to March 26, 2013: 210

MWh/yr

Mean annual energy production estimate for March 27, 2012 to March 26, 2013, with an

adjustment for Hurricane Sandy and the car accident: 165 MWh/yr

Maintenance Issues Nearly 2 months of production lost to Hurricane Sandy safety precautions and grid issues

Current Operating 

Status

Turbine is currently operating

The wind rose on the left in Figure B-11 shows the long-term average wind direction 

distribution. The wind rose on the right shows that for the year in question, more energetic winds 

came over the land to the west and north and less from the southwest than in more typical years.

Figure B-11. (Left) total wind energy rose using long-term data from two nearby MERRA sites.

(Right) total wind energy rose using data from two nearby MERRA sites for the period March 27, 

2012 to March 26, 2013. 
49

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

MERRA Data Sources

There is a relatively new climate data capability available from the National Aeronautics and 

Space Administration (NASA) called MERRA.

25 MERRA is a climate analysis data set 

generated by NASA’s Global Modeling and Assimilation Office using the Goddard Earth 

Observing System atmospheric model and data assimilation system.

The aspect that makes the MERRA data particularly useful is that it can be adjusted “after-thefact” to reflect the actual atmospheric conditions, not merely the predicted ones. MERRA data 

from two locations are used for this analysis. Spatially, these locations are separated north-south 

by 0.5 degrees, which equates to roughly 55.6 km (34.5 mi), east-west by 0.67 degrees, which 

equates to roughly 52.5 km (32.6 mi). The MERRA data set locations can be seen by the yellow 

push pins shown in Figure B-8.

Fifteen years of hourly data at 10 and 50 m were downloaded for both of these sites. 

Collectively, these sites provide insight into the long-term wind trends in the local region and 

allow for a determination of how well the wind data measured during the period of interest at 

Hempstead compares with the long-term wind trends reflected in the MERRA data. The power 

law was used vertically to extrapolate a synthetic data set for each site at 37 m. The average of 

these two data sets was 6.26 m/s, which compared reasonably well to the manufacturer’s 

estimate of 6.14 m/s and NYSERDA’s Small Wind Explorer’s estimate of 6.22 m/s. Figure B-12

shows a comparison between the long term wind speed data and the 2012 through 2013 data. 

Figure B-13 shows the energy production expected based on the 2012-2013 wind speed 

distribution and the actual energy production during that time period. 

 25 Cullather, Richard & National Center for Atmospheric Research Staff (Eds). “The Climate Data Guide: NASA 

MERRA.” Retrieved from http://climatedataguide.ucar.edu/reanalysis/nasa-merra. Originally posted on September 

30, 2011, and last modified February 25, 2013.
50

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Wind Speed Estimates

Figure B-12. The synthesized mean monthly averaged wind speed from two nearby MERRA sites 

at 37 m for the period 1999‒2013 (green) and for the period since the NW100 was installed (2012‒

2013, in blue); the dashed overlay is the 12-month period used for the wind analysis summary

Figure B-13. The actual monthly energy production of the NW100 at Hempstead and the estimated 

monthly energy production for the same time frame using synthesized hourly averaged wind 

speed from two nearby MERRA sites at 37 m; the dashed overlay is the 12-month period used for 

the wind analysis summary
51

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Lessons Learned

Purchasing data from a reputable wind assessment company can be an effective way to obtain 

data. Free online data from automated surface observing system weather stations 

(http://www.nws.noaa.gov/asos/), MERRA, NREL (http://www.nrel.gov/gis/data_wind.html), 

and others can also be effective. In the absence of on-site data, using multiple sources of existing 

wind data is a good way to cross-reference any data set, though the wider the variation between 

the data sets, the lower the confidence in the resultant energy predictions. 

As seen in the Hempstead case study, the findings of four resource assessments using different 

methodologies resulted in four different annual energy production estimates. The variation 

proved to be significant, especially when trying to verify turbine performance in a given wind 

year. 

Concerns

The Hempstead turbine manufacturer provided an estimate of the wind resource and annual 

energy production for one their turbines, but neglected to specify which turbine (21- or 24-m 

rotor diameter) the prediction was for, thereby reducing the confidence in the performance 

verification calculations. The estimate provided was challenging to verify in that the 

performance most closely aligned with the estimated performance of a NW100 with a 24-m rotor

diameter. The only report that included a rotor dimension cited a 21-m rotor, thus there is 

uncertainty of the rotor diameter for the subject turbine. 

The original wind speed estimate that was conducted by the eventual project installers reported 

the lowest estimated average speed for the site of 5.13 m/s.26 The low input led to an estimate 

that was significantly less than the installation’s actual production. As a result, inaccurate inputs 

can distort a project’s potential (positively or negatively). 

 26 Project installer annual energy calculation.