3TIER has noted with interest the discussion regarding the upcoming Journal of Geophysical Research paper by Sara Pryor and coauthors entitled “Wind speed trends over the contiguous USA”. The links between climate change and variability and trends in wind speed have important potential implications for the wind energy industry, and have also been a topic of study at 3TIER.

3TIER’s primary impressions of the issues raised by the Pryor et al., paper are the following:

• Studies like this are an important step toward understanding the effects of climate change and variability on wind energy resources, but this and previous studies have highlighted unresolved discrepancies between observation-based and model-based estimates of wind speed trends. These discrepancies illustrate that large uncertainties remain regarding the nature, source(s) and magnitude of any trends present in the observational record.
• Further study of the longer station records and comparison with simulated wind speeds would add important historical context to the last few decades of observational records that are assessed in the Pryor paper, and provide insight into the extent to which the observed wind speed trends may include components of decadal-scale variability together with climate system changes.
• The relationship between wind speed changes and power output depends on a turbine’s power curve, and, in practice, this relationship tends to be more linear than expected from the cubic relationship between wind speed and power density.
• Studies indicate that the effects of climate change on wind speed are likely to vary from region to region, so location-specific studies of the potential effects of climate change will be valuable for informing decisions about wind projects.
• The presence of trends and/or decadal variability greatly underscores the need for diligent estimation of project risk, including extended retrospective assessments of the type 3TIER has been providing the industry for years.

The remainder of this document provides a more detailed discussion of related research on trends in wind speed, and on the results of the paper itself.

PREVIOUS 3TIER STUDIES ON CLIMATE CHANGE

At WINDPOWER 2004, 3TIER scientists presented projected changes in wind project capacity due to greenhouse gas-induced climate change, estimated using a set of regional climate model simulations. For a hypothetical wind project located near Kennewick, Washington, our results show that the annual wind project capacity is projected to decrease 1.3% by 2050. We noted, however, that climate change projections exhibit strong geographic variations, so the results from one location cannot necessarily be translated to other locations.

Last year, in a North American Windpower paper, 3TIER scientists analyzed projected changes in wind speed from 14 different climate simulations that were performed in support of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4). Our results show that the predicted ensemble-mean changes in annual mean wind speeds are expected to be modest. However, seasonal changes and changes predicted by individual models are large enough to affect the profitability of existing and future wind projects. The majority of the model simulations reveal that near-surface wind speed values are expected to shift poleward in response to the IPCC A2 emission scenario, particularly during the winter season. This response is likely associated with the poleward shift of the jet streams and associated storm tracks. In the United States, most models agree that the mean annual wind speed values will increase in a region extending from the Great Lakes southward across the Midwest and into Texas. Decreased values, though, are predicted across most of the western United States. However, these predicted changes have a strong seasonal dependence, with wind speed increases over most of the United States during the winter and decreases over the northern United States during the summer.

RESPONSE TO MEDIA COVERAGE OF THE PRYOR ET AL. STUDY

Much of the media attention to the upcoming Pryor et al. paper has focused on the paper’s report of a decrease in observed wind speeds since 1973 over the eastern United States, particularly the Midwest. However, another important conclusion of this study is that there are discrepancies between wind speed trends estimated from observations and wind speed trends estimated using regional climate models and regional and global reanalysis datasets. Both observed and simulated wind speeds have potential drawbacks. Observed wind time series can suffer from inconsistencies that include changes in instrumentation, nearby buildings, and vegetation; many in the wind industry no longer use ASOS stations as reference time series because of changes in anemometer instrumentation. On the other hand, the model simulations used in the study represent approximations of the wind on spatial scales of 20 to 200 miles, and require wind speed to be calculated from the north-south and east-west components of wind on these large spatial scales. The discrepancies between observation-based and model-based estimates of wind speed trends are consistent with earlier studies for Australia and Europe. In light of these unresolved discrepancies, 3TIER strongly agrees with the final sentence of the Pryor et al. paper: “Given the importance of the wind energy industry to meeting Federal and State mandates for increased use of renewable energy supplies and the impact of changing wind regimes on a variety of other industries and physical processes, further research on wind climate variability and evolution is required, as are detailed analyses focusing on reconciling the discrepancies illuminated herein.”

We would also like to address two misconceptions from the June 10 AP story on the Pryor et al. paper. First, the story reports that “a 10 percent change in peak winds would translate into a 30 percent change in how much energy is reaped.” Although the power density of the wind itself varies with the cube of the wind speed, the amount of power produced by a wind turbine is dictated by the shape of that turbine’s power curve. This curve relates wind speed at the turbine to power production and has three general sections. For low wind speeds, zero to minimal power is produced. For intermediate wind speeds, power increases dramatically with increasing wind, up to the full turbine capacity. Between intermediate wind speeds and a high “cut-out” speed (at which a turbine is shut down to protect itself), power production is constant at the turbine capacity. Although power production is very sensitive to changes in intermediate wind speeds, where a 10% change in wind speed can indeed result in a 30% change in power production, most wind projects are built at sites where typical wind speeds are above this intermediate, sensitive range. In practice, we find that the annual mean power output for a typical wind power plant has a nearly linear relationship with the annual mean wind speed.

Second, the AP story suggests that decreasing surface wind speeds are a logical consequence of polar temperatures warming more quickly than the rest of the globe. However, most of the climate change simulations performed for the IPCC AR4 actually produce stronger surface winds in response to increasing greenhouse gases. This is most likely because increasing greenhouse gases produce a larger temperature contrast in the upper atmosphere, which tends to increase storminess and wind speeds near the surface. This increase in storminess also tends to shift the regions of strong surface winds towards the poles, suggesting that there are likely to be some regions that experience increasing wind speeds, while other regions experience decreasing wind speeds.

SUMMARY

In summary, 3TIER lauds the authors for their contribution to our understanding of wind resources in the United States, and emphasizes the following take-away messages.

• The Pryor et al. study underscores the need to better understand the effects of climate change and variability on wind and account for these effects in decisions that affect our energy infrastructure.
• Resolving the discrepancies between observation-based and model-based estimates of trends in wind speed is of key importance for informing such decisions.
• Location-specific studies of the effects of climate change and variability on wind energy projects are also important for informing such decisions.
• 3TIER is well-equipped to provide information to place the expected performance of wind projects in a long-term context, which is important for due diligence activities such as estimation of performance expectation and risk over the lifetime of a wind project.

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