Smart curtailment: the little-known battle between nature and efficient wind power

Bats that need our help

Two species of bats that are most relevant to this discussion are the Indiana bat and the Hoary bat:

• The Indiana bat is a federally endangered species that has long suffered from impacts related to habitat loss. More recently, populations have been further devastated by a rapidly spreading disease called White Nose Syndrome—an emerging condition that’s crippling bat populations across North America. The potential risk to this species can effectively block the construction of a project or even shut down operations until impacts are minimized and mitigated.

• The Hoary bat is a long-distance migratory species that consistently accounts for the largest proportion of bat fatalities at wind projects. The existing buildout of wind energy in North America could threaten this species, highlighting the importance of reducing fatality rates as the industry expands.

It may come as no surprise, but bats are only at risk at night and when the turbines are spinning. Shutting the turbines down effectively avoids the risk to bat populations, but it also precludes our client’s ability to generate power. So, how can we minimize the level of bat casualties while maximizing energy production?

The power of wind

The amount of energy produced from wind ultimately varies with wind speed. Most commercial wind turbines don’t begin to produce electricity until the wind speed reaches about 3.5 metres per second (m/s)—or about 7.8 miles per hour. Referred to as the “cut-in speed” (indicated below by the blue dashed line), this is when a turbine’s blades start spinning to generate power.

What does this have to do with bats? Even though bats are more active during periods with lower wind speed, quite a bit of bat activity occurs when wind speeds are above the cut-in speed, which increases exposure to risk. So, how can reduce bat fatality rates?

By changing the cut-in wind speed, we can manipulate and manage the amount of risk to bats. This is known as curtailment. The US Fish and Wildlife Service (USFWS) has determined that if we curtail operations below 6.9 m/s (indicated by the red line) at night during the time of year when bats are active, we can avoid risk to federally endangered bats. This approach is effective and substantially reduces risk, but it is also very costly in terms of energy loss.

Accordingly, the space between the red and blue lines is the difference between unacceptable risk and manageable risk to listed bat species in the eyes of the USFWS. But, it can also be the difference between the economic success or failure for a wind project.

A recipe for respecting risk

Wind speed is only one of many variables affecting risk to bats. The aerosphere—the part of the atmosphere where bats interact with turbine blades—is a complicated habitat where other factors such as temperature and precipitation can come into play at any given moment.

While the complexity of bat behavior in the aerosphere can be challenging, Stantec has tools to help understand how bat activity and risk relate to these factors.

Acoustic bat detectors monitor the ultrasonic echolocation pulses that bats use while flying. By mounting detectors on top of the turbines, we can characterize bat activity in the rotor zone. Next, we align time-stamped bat recordings to temperature, wind speed, and turbine rotor speed recorded at corresponding turbines to determine conditions when bats are active and whether that activity is exposed to risk.

With this information, we can visualize bat activity as a spectrum. Red indicates the highest activity and the highest risk while blue indicates conditions with very little bat activity and very little risk.

The heat map shows the distribution of 60,000 bats recorded over six years at two commercial wind projects. It highlights that risk is concentrated during calm wind speeds but also during warm temperatures. By overlaying the normal cut-in speed of a wind turbine, you can see that quite a bit of warm color (and therefore risk to bats)—in this case 70% of activity—occurs to the right of the blue line. By raising the curtailment speed to 6.9 m/s, the risk is minimized with only 6% of bat passes now exposed to risk.

But, notice the large amount of blue (low risk) between the red and blue lines. This effectively represents wasted curtailment—energy loss during conditions when bats are rarely active. By redesigning curtailment to account for temperature as well as wind speed, we can safely operate turbines in a wider range of conditions and generate more renewable energy with a minimal increase in risk to bats.

Smart curtailment at Stantec

At its core, smart curtailment recognizes that the value of curtailment is proportional to the amount of avoided risk. In our experience, incorporating temperature and wind speed—as well as adjusting cut-in speeds monthly to reflect bat activity—can substantially reduce the cost of curtailment while continuing to minimize risk to bats.

If applied at a typical 50-turbine wind farm, this approach could save enough renewable energy to power over 500 homes for a year compared to blanket curtailment below 6.9 m/s. So, how do we convince the industry to adopt this approach on a broader scale?

Ultimately, we can boost awareness and acceptance of smart curtailment through:

1. A combination of innovative techniques to visualize and assess risk.
2. Finding forward-thinking partners within the wind industry to test smart curtailment approaches at existing wind projects.
3. A willingness by regulatory agencies to consider alternatives to their recommended curtailment programs.

At Stantec, we are uniquely positioned to understand not only the biology of bats, but how it intersects with the business of running a wind farm. By tailoring curtailment to fit site-specific risk patterns, we are essentially designing something that integrates biology and business. We can offer our clients a potential solution to a difficult problem while helping them meet their goals for renewable energy production. And, if we do it right, we can simultaneously provide important information on minimizing impacts to an ecologically diverse and important group of species while meeting these goals.