Relative energy production determines effect of repowering on wildlife mortality at wind energy facilities

ABSTRACT

1.   Reduction in wildlife mortality is often cited as a potential advantage to repowering wind facilities, that is, replacing smaller, lower capacity, closely spaced turbines, with larger, higher capacity ones, more widely spaced. Wildlife mortality rates, however, are affected by more than just size and spacing of turbines, varying with turbine operation, seasonal and daily weather and habitat, all of which can con-found our ability to accurately measure the effect of repowering on wildlife mor-tality rates.

2.   We investigated the effect of repowering on wildlife mortality rates in a study conducted  near  Palm  Springs,  CA.  We  controlled  for  confounding  effects  of weather and habitat by measuring turbine-caused wildlife mortality rates over a range of turbine sizes and spacing, all within the same time period, habitat and local  weather  conditions.  We  controlled  for  differences  in  turbine  operation  by  standardizing mortality rate per unit energy produced.

3.   We found that avian and bat mortality rate was constant per unit of energy produced, across all sizes and spacings of turbines.

4. Synthesis and applications. In the context of repowering a wind facility, our results suggest that the relative amount of energy produced, rather than simply the size, spacing or nameplate capacity of the replacement turbines, determines the relative rate of mortality prior to and after repowering. Consequently, in a given location, newer turbines would be expected to be less harmful to wildlife only if they produced less energy than the older models they replace. The implications are far-reaching as 18% of US and 8% of world-wide wind power capacity will likely be considered for repowering within ~5 years.

CONCLUSIONS

Concern  for  impacts  of  wind-power  generation  on  wildlife  has grown over the last 10–20  years and can be expected to increase as  higher  proportion  of  the  power-generation  portfolio  is  con-tributed by this energy source. Understanding impacts to wildlife caused by turbines is a central problem confronting both global expansion of wind energy and the increasingly prevalent process of repowering. Replacement of older and smaller wind turbines with newer, larger machines generating the same amount of en-ergy will have little consequence for rates of wildlife mortality. To date, studies of this problem have been qualitative (Hunt, 2002), focused  on  atypical  turbines  (Smallwood  &  Karas, 2009),  and failed to account for energy production (ICF International, 2016). Our study is the first to compare modern, horizontal-axis wind turbines of different ages in a study with high carcass detection rates, accurate accounting for energy production and a state-of-the-art analytical design.

Measuring  the  impacts  of  repowering  is  easily  confounded  by variability in space, time and operational constraints. Our study is the first to control for these features and it illustrates that benefits to wildlife of replacing older turbines with newer ones in the same location will depend largely on the relative amount of energy pro-duced, not simply on the size or spacing of the replacement turbines. As wind technology continues to improve and modern turbines con-tinue to increase in height, RSA and MW capacity, future research will be required to understand how this broader range of sizes and energy production might affect the impact of repowering on mortal-ity rates of birds and bats.