Page 16 - North American Clean Energy May June 2018 Issue

wind power
Bat Smart Curtailment
Suitability Assessment
and Modeling
by Christine Sutter
IN THE US, TWO MITIGATION METHODS HAVE PROVEN
to reduce bat fatalities by wind turbines: standard curtailment (see Arnett et al. 2008) and smart curtailment (see Sutter et al. 2017). e rather discouraging results from recent deterrents tests (Table 1) suggest these may continue to be the only fatality minimization options for the foreseeable future.
Table 1: Summary of results from three acoustic deterrent studies conducted during 2017
Study Name and (state)
BWEC/DOE (OH)
NRG/DUKE (TX)
GE/INVERNEGY (IL)
Summary
Ine ective in reducing overall bat fatalities Ine ective in reducing fatalities for any single species Increased fatalities for eastern red bats
Ine ective in reducing overall bat fatalities
E ective in reducing fatalities for Brazilian free-tailed bats only Increased fatalities for eastern red bats
E ective in reducing overall bat fatalities E ectiveness for individual species was not reported Increased fatalities for eastern red bats
Both curtailment strategies pitch out wind turbine blades to reduce rotation rates to ~2 rpm, and both are shown to be e ective in reducing bat fatalities. Smart curtailment has the added bene t of being the only strategy that signi cantly reduces fatalities of Myotis species. e strategies di er in their e ect on energy yield. Standard curtailment typically has a larger negative e ect on energy yield because curtailment is determined solely by climatic conditions, and much of the curtailment (25 to 75 percent) occurs when no bats are present. In contrast, smart curtailment has a lesser negative e ect on energy yield because turbines are curtailed only when bats are present in the rotor-swept zone (RSZ).
The optimal curtailment strategy at a wind farm may also be influenced by considerations such as the likelihood of
the taking of a federally endangered species, state wildlife regulations, project economics, and the organizational culture of the wind farm proponent. There is no one-size-fits-all solution; instead, each site should seek an optimum solution based on all of these considerations.
Developing the optimum solution(s) starts with determining if smart curtailment is suitable for the site, then nding a scenario that balances the energy loss and the bat conservation bene t. Smart curtailment suitability and scenario modeling accomplishes both of these. e bat activity within the rotor-swept zone is analyzed to determine if the activity is spatiotemporally clustered. If so, then smart curtailment may be suitable at this site. Scenario modeling creates a variety of potentially optimum scenarios that can be judged based on all of the above optimizing factors. ese scenarios estimate reductions in fatalities and reductions in energy yield (Table 2).
Table 2 contains a series of scenarios developed for a US wind farm. e inputs used were a single season (May to Oct) of bat exposure data from the rotor-swept zone, and meteorological data.
Table 2: Standard and smart curtailment scenarios for a proposed US wind farm
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Input Value
Results
Scenario
Bat activity level to trigger smart curtailment
Expected bat conservation
(% reduction in fatalities/facility)
Expected reduction in annual energy yield (MWh/ turbine)
Expected percent reduction in annual energy yield (%)
Expected reduction in annual revenue ($/turbine)
Wind speed ≤ 3.5 m/s
1 (standard)
NA
16.7
Wind speed ≤ 5.5 m/s
0.73
0.0
$14
2 (standard)
3 (smart)
NA
43.7
43.7
38.2
0.3
$725
1 or greater
19.7
0.2
$375
Wind speed ≤ 6.5 m/s
4 (standard)
NA
55.9
92.1
0.8
$1,744
5 (smart)
6 (smart)
1 or greater
55.9
54.5
Wind speed ≤ 7.0 m/s
47.5
0.4
0.3
$900
2 or greater
38.0
$721
7 (standard)
NA
61.3
143.4
1.2
$2,717
8 (smart)
1 or greater
61.3
69.2
0.6
$1,311
Wind speed ≤ 8.0 m/s
9 (standard)
NA
71.2
322.8
2.7
$6,117
10 (smart)
1 or greater
71.2
149.4
1.3
$2,832
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Summarizing the data presented in Table 2, the following has been observed for this site:
• e standard curtailment and the smart curtailment scenarios are expected to conserve roughly equal number of bats, but smart curtailment reduces the negative economic impact by nearly half of that observed under standard curtailment (e.g. compare scenario 4 to 5).
• Using smart curtailment, turbines can be curtailed at higher wind speeds (potentially conserving more bats) for the same economic cost of standard curtailment at a lower wind speed. (e.g. compare scenario 2 to 6).
• Four scenarios are expected to reduce bat fatalities
by around 50% at a cost of less than 0.5% of annual energy production. ree of these are smart curtailment strategies - scenarios 3, 5, and 6.
Smart curtailment scenario modeling can allow operators and regulators to identify and achieve win-win scenarios that reduce bat fatalities, in exchange for a reasonable reduction in energy yield.
e quantitative nature of the scenario modeling supports more nuanced stakeholder discussions on the costs and bene ts of potential curtailment strategies. It also generates a cost estimate (in lost energy yield) that assigns a value to the minimization e orts. And it provides crucial information on the likely timing and duration of curtailment events, which can be incorporated into energy forecasts, and power purchase agreements (PPA).
Christine Sutter is Head of Environment at Natural Power North America. She provides advice and guidance on natural resource issues to clients in the wind and solar industries throughout the US and Canada.
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