Question:
You are given the full text of a scientific research paper.

    Identify and summarize the **policy problem** that this research addresses. Your response should:
    - Clearly state the societal, environmental, or strategic issue motivating the study
    - Be written in accessible, policy-brief style sentences
    - Focus on the core challenge the research aims to solve
    - Avoid excessive detail or technical jargon unless necessary for clarity

    Write a short paragraph suitable for inclusion under the “Policy Problem” section of a policy brief.

    Scientific Research Paper:
    Articles
https://doi.org/10.1038/s41560-020-00717-1
1Energy Economics and Regulation Group, Department of Technology, Management and Economics, DTU Technical University of Denmark, Kongens
Lyngby, Denmark. 2European Commission—Joint Research Centre, Petten, the Netherlands. 3National Renewable Energy Laboratory, Golden, CO, USA.
✉e-mail: lkit@dtu.dk
W
ind energy has emerged as a mainstream electricity pro-
duction technology. Although globally it accounts for
approximately 10% of electricity production capacity, it
now has a leading market share in some pioneering countries1,2. As
technology rapidly improves and operating facilities increase in age,
the modernization of wind facilities becomes increasingly relevant.
Repowering, that is, the combined activity of dismantling or refur-
bishing and commissioning of wind turbines, has been an active
segment of the industry since the 1990s, when the first commer-
cial wind farms (commissioned in the late 1970s and early 1980s)
began to require substantial reinvestments and modernization,
but the scale and impact of repowering on the global wind indus-
try is expected to grow substantially in the next decade. Moreover,
although repowering activity to date has been concentrated in a
few markets and driven by relatively unique policy incentives, by
the late 2020s repowering could become a primary source of busi-
ness activity for the wind industry across Europe, North America
and China, and a key to optimizing wind energy utilization in the
context of energy transition. Today, only a few countries have wind
energy fleets that support substantial repowering activity2. In this
context, empirically derived insights from repowering activities are
increasingly informative. Denmark is a prime case for analysis: wind
energy has emerged as the primary electricity production technol-
ogy, having reached a 47% market share in 20193, and it features the
oldest wind turbine fleet in the world1,4 (Table 1). Further factors
that contribute to a robust repowering market in Denmark are the
size of the country, with its consequent space limitations, and its
continued desire to expand wind energy production5.
Repowering has historically been viewed as a means to increase
the productivity of existing projects, with improved profitability
as the primary decision driver4,6. Although the motivation behind
much repowering activity is an improved business case for an exist-
ing facility, repowering offers an array of benefits when compared
with greenfield development, which include lower implementation
barriers through existing grid connections, long-term empirical
understanding of the available wind resource at a given site, rela-
tively well-known environmental impacts and accustomed neigh-
bours. Further, subsidy schemes and short-term market incentives
may encourage repowering, and include incentive programmes
directed at repowering (Supplementary Note 1), market premiums
for new projects6 or favourable tax policy. Recent repowering in the
United States, for example, was predominantly driven by the ability
to requalify for full value and tenure of the production tax incentive
programme5.
From a societal perspective, repowering may offer environ-
mental and social improvements through the replacement of older
(faster-rotating and noisier) turbines with fewer and more efficient
newer turbines—although the new (taller) turbines may intro-
duce new negative impacts, largely related to visibility over longer
distances7. It is further suggested that birds and bats benefit from
repowering, although challenges remain in the monitoring and
assessment of bird fatalities and collision rates before and after
repowering8. Repowering is also particularly relevant in offering the
potential to increase wind energy production from a limited area
of available land, so that even when “virtually all the good wind
resource sites have been taken” (using the words of a representative
from the Danish wind industry)9, increases in wind energy produc-
tion may still be facilitated.
Previous scientific studies on repowering can be broadly catego-
rized into five groups: feasibility-type studies on single projects4,10–15;
optimization of repowering decisions, for example, regarding
time and wind farm topology4,16–19; repowering statistics, such as
increased capacities or productivity, on a turbine-by-turbine com-
parison20,21; research on public acceptance6 and rejection factors
for repowering projects7,22 and impact assessments, which address
life-cycle emissions23, visual impacts24 and impacts on birds and
bats25–28. Few studies explore techno-economic and regulatory driv-
ers of repowering decisions5,6 and policy options16,29. All the studies
Multifaceted drivers for onshore wind energy
repowering and their implications for energy
transition
Lena Kitzing   1 ✉, Morten Kofoed Jensen1, Thomas Telsnig2 and Eric Lantz3
Wind energy is anticipated to become a backbone of the future energy system. Ageing wind turbine fleets, increasing land-use
constraints and rising relevance of societal factors make the deployment of land-based (onshore) wind energy ever more com-
plicated. Consequently, repowering is expected to become a rapidly growing point of focus for the wind industry. Here we
propose a more holistic and socially informed project-level approach to analyse repowering activity that enables a more robust
understanding of the process and potentials. We demonstrate that for wind pioneer in Denmark, only 67% of the capacity
removed in repowering projects was related to the physical space needed for a new turbine. Other factors that drive repowering
include regulation (for example, noise-related, 8–17%), development principles (for example, aesthetics, 7–20%) and political
bargaining (4–13%). The recognition of repowering as a negotiated process between host communities and wind developers
will probably be critical to unlock the full potential of wind energy in the future.
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focus their scope towards the replacement of turbines on existing
sites or consider a turbine-by-turbine replacement. We label this
dominant approach as an ‘on-site approach to repowering’, and
show that this is insufficient for a robust understanding of repower-
ing decisions, processes and outcomes.
We propose a more holistic and socially informed perspective
on repowering based on comprehensive project-level information.
Specifically, this approach considers multifaceted drivers for repow-
ering decisions and hence identifies dismantled turbines that are
located at the same site as the new development project (on-site) as
well as existing turbines in other locations that need dismantling so
new turbines can be installed, that is, we considered all the activity
that can be conditionally associated with the repowering project,
regardless of spatial or temporal proximity between the existing
and the new turbines, as illustrated in Fig. 1. We then identified
categories for dismantling reasons in repowering projects, espe-
cially for those turbines physically located outside the immedi-
ate project development site (off-site). Applying our approach to
Denmark as an empirical case, we quantified repowering projects
in their entirety. Through data collection from municipal plans30,31,
project publications, geospatial analysis and semi-structured inter-
views with developers covering 91% of all dismantled capacity in
repowering projects, we identified the full breadth of repowering
projects and determined dismantling reasons. We revealed sev-
eral notable differences between the traditional on-site and our
more holistic and socially informed repowering perspective. Our
work emphasizes that a pure technology perspective alone cannot
explain implementation pathways and must be supplemented with
the political and social dimensions. Ultimately, this work enables
more-informed estimations of the future global repowering mar-
ket. It also informs the challenges and opportunities presented by
repowering as a means to increase renewable energy production.
Repowering projects in Denmark and their market
significance
Our synthesis database contains 102 wind energy projects that were
developed in Denmark between 2012 and 2019. Table 2 provides an
overview of our database by greenfield projects, repowering proj-
ects and dismantled non-repowered turbines, which are turbines
dismantled without relation to any new project. Eight projects are
excluded from the analysis (Methods).
More than a third (38%) of all new wind energy developments
were repowering projects. Only 10% more capacity was developed
in greenfield projects than in repowering. In repowering projects,
the net capacity additions amounted to 576.8 MW, whereas, at the
same time, the net number of turbines decreased by 1. Overall, we
observe 1.3 GW of net capacity additions and a net reduction of 109
turbines, which considerably reduces the number of turbines physi-
cally present in the landscape. This shows that newer, more-efficient
turbines have replaced earlier, less-efficient turbines over time.
Turbines removed in repowering projects are, on average,
5.8 years younger than those dismantled on a stand-alone basis
(non-repowering). Interviewees confirmed that most of the dis-
mantled turbines in repowering projects had not reached the end
of their operational lifetime, but were dismantled prematurely
so that the new project could be executed. This emphasizes the
importance of distinguishing between end-of-life decisions and
repowering decisions, which involve multiple social and economic
considerations.
Further, dismantled repowering turbines are, on average, 3.4
times as large as dismantled non-repowered turbines. Although
repowering comprises 66% of the total dismantled capacity, it only
comprises 37% of the total number of dismantled turbines. This
size difference between dismantling in repowering and as a separate
activity meant we further analysed both the capacity and the num-
ber of turbines.
Repowering has played an increasingly important role in
Denmark since 2012, both in terms of capacity (Table 3) and the
number of turbines (Table 4). Repowering shares have steadily
increased for gross added capacity and the number of installed
turbines. The relatively low starting value in 2012, of 23%, may be
due to the removal of a pre-existing repowering incentive scheme
that was in place from 2005 to 201132, which left a weak market
for repowering at the beginning of the observed period. In the last
observed year, the repowering market share jumped to an unprec-
edented level of 86% of gross added capacity (Table 3), or 87% of
added wind turbines (Table 4).
Repowering shares in the dismantling of turbines were even
more notable, although they varied over time. Repowering was
the principal reason for capacity dismantling in five of the eight
years investigated, and the repowering share in capacity reductions
ranged between 17% in 2015 and 92% in 2013. Variations in repow-
ering shares for the number of dismantled turbines follow a differ-
ent pattern, as turbine sizes have increased in recent years.
More detailed results from our dataset, including on market
shares of project developers and turbine manufacturers, can be
found in Supplementary Note 3.
Categorizing dismantled turbines into repowering reasons
As argued earlier, more holistic and socially informed repower-
ing analysis must include all the dismantled turbines related to a
repowering project regardless of distance and time. For a deeper
understanding of the reasons behind dismantling turbines in
repowering projects, we conducted in-depth interviews with lead-
ing wind developers in Denmark, which covered 91% of all the
capacity dismantled in repowering projects in the investigated time
frame. From the interviewee responses, eight mutually exclusive
categories emerged for dismantling existing turbines in repowering
projects, as summarized in Table 5.
It is noteworthy that none of the interviewees named profitabil-
ity or other economic reasons as drivers to dismantle turbines in
repowering projects. On the contrary, talking about the market for
repowering, one interviewee stated that greenfield projects generally
provide the highest profits. Repowering is seen as a costly necessity,
not least because the existing turbines often have to be purchased at
a high cost before they can be dismantled. This is notwithstanding
the fact that some older wind turbines may not have been feasible
to operate much longer, and in general may not have fulfilled the
owner’s expectations, for example, in regard to production volumes,
failure rate and downtime.
Although we focused our study on repowering from the
developers’ perspective, others who investigated the attitudes of
local politicians towards wind energy projects and repowering, for
Table 1 | Turbine fleet age structure in leading countries for
onshore wind energy
Denmark
Germany
Spain
EU28
USA
China
Cumulative
capacity
installed in
2019 (GW)
4.4
53.2
23.5
160.7
97.7
206.8
Share of cumulative capacity
 >10 years
55%
43%
73%
39%
34%
7%
 >15 years
53%
26%
27%
17%
6%
0.4%
 >20 years
23%
4%
3%
3%
1%
0.2%
The table shows the cumulative capacity of installed onshore wind energy and the age distribution
of installed turbines in selected countries. Data from refs. 1,42,43. EU-28, European Union 28
countries.
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example, in the Czech Republic, found that they equally considered
economic value creation and displeasure of local residents and other
local stakeholders22. Concrete reasons that are likely to contribute
to dismantling requests in our context were found to involve the
opposition of local residents, local landscape disruption and impact
on well-being (noise)22.
Table 6 shows the results of our categorization effort. We found
that space is the strongest motivation for dismantling turbines in
repowering projects in Denmark, with 67% of the total capacity
and 63% of the total number of turbines, respectively. Space is, of
course, a well-known reason given that most repowering projects
are developed from existing wind farm sites. This category gener-
ally corresponds to what is typically captured by analyses using an
on-site approach.
The second major individual reason for dismantling turbines
in repowering is a violation of the cumulative noise emissions,
which comprises 13% of the total number of turbines and 8%
of the full capacity dismantled. Hence, noise regulation in Denmark
has a considerable impact on repowering projects. This impact
was also underlined by several of the interviewees. Here, it is rel-
evant to note that noise regulation differs from country to coun-
try. Interestingly, turbines removed because of noise have the
smallest average capacity size (435 kW) and highest average age
(22.1 years, Fig. 2b).
It was sometimes impossible for developers to point to a specific
reason for dismantling a particular turbine, especially when aes-
thetics and politics were involved. We thus analysed the remaining
categories jointly. The individual category aesthetics comprises 9%
Aesthetics
Political
bargaining
Noise
Noise
Dismantled turbine
Commissioned turbine
4 times the total height of
commissioned turbines
28 times the total height of
commissioned turbines
1.5 times the total height of
commissioned turbines
Cumulative noise violation
zone
Space
Settlements
Fig. 1 | Illustrative map of a typical repowering project that includes the on-site as well as off-site dismantling of turbines. This representative example
for a (fictive) Danish repowering project is based on the median repowering project information from our dataset, in which six 600 kW turbines
commissioned in 1997 are replaced by five new 3.3 MW turbines in 2016. Annual energy production increased by a factor of 6.5 and operations and
maintenance costs declined by about 20%, which results in an increased annual net cash flow of up to €0.9 million assuming a high electricity price
scenario. The overall levelized cost of energy is €38–42 MWh–1, approximately 70–78% of the original project overall levelized cost of energy (see
Supplementary Note 2 for more details). Note that wind farm layout and scales are chosen to aid visualization and not to represent a realistic case.
Table 2 | Onshore wind energy projects developed in Denmark 2012–2019
Number
of
projects
Total
number of
turbines
Average
number of
turbines
per project
Range of
number of
turbines
per project
Total
capacity
(MW)
Average
capacity
per project
(MW)
Average
capacity
per turbine
(MW)
Range of
capacity per
project (MW)
Average
turbine age at
dismantling (yr)
Greenfield projects
58
272
4.7
1−21
804.3
13.9
3.0
2−46.8
Repowering projects
(net)
36
−1
0
−14−16
576.8
16.04
0.15−65.9
Repowering projects
(gross additions)
220
6.1
1−22
731.9
20.3
3.3
0.2−77.4
Repowering projects
(gross reductions)
221
6.1
1−35
155.1
4.3
0.7
0.075−23.0
18.6
Non-repowered
dismantled turbines
380
78.8
0.2
24.4
See Methods for details on the derived numbers, and Supplementary Note 3 for more detailed results.
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of the total number of turbines and 7% of the total capacity dis-
mantled. However, aesthetics also play a role in three mixed catego-
ries (aesthetics or noise, politics or aesthetics and politics, aesthetics
or noise). Assuming that all the turbines in these three categories
were removed because of aesthetics, it would become the second
most pronounced reason, comprising 21% of the total number of
turbines and 20% of the total capacity dismantled. The individual
category politics comprises 3% of the total number of turbines and
4% of the total capacity dismantled. If all the turbines in the respec-
tive mixed categories (politics or aesthetics and politics, aesthetics
or noise) were dismantled because of politics, the category politics
would contain 10% of the total number of dismantled turbines or
13% of the removed capacity.
Major differences in distance, age and repowering factor
The distance between dismantled and newly installed turbines dif-
fers considerably across the categories (Fig. 2a), of which the cat-
egory ‘politics’ has the highest spread. With a dismantled turbine
located 4.6 km (31 times the total height) away from the nearest new
turbine, it is apparent that politics can target a broad range of exist-
ing turbines. As stated by one interviewee, politics seem spatially
unrestricted within the municipality because voluntary agreements
between developers and politicians can include the removal of any
existing turbines within the municipal area. Some interviewees
perceived that some municipalities had an informal one-for-one
requirement, and one interviewee confirmed that this was the very
reason why they chose to dismantle a specific older existing turbine
located in the municipality. Concerning the one-for-one require-
ment, another interviewee elaborated on a current project (to be
commissioned in 2022), in which, as part of a voluntary agreement,
they would dismantle several turbines located more than 10 km
away from the site of the new project.
In contrast to the informal nature of politics-related agreements,
formal regulations are in place that restrict the farthest impact
distances of noise and aesthetics: the relationship between existing
and planned turbines only needs to be evaluated within a radius
of 28 times the total height of the planned turbines33. Here, the
observed maximum removal distances are 2.3 km (15 times the
total height) for noise and 2.8 km (18.5 times the total height) for
aesthetics.
Turbine age at dismantling varies considerably across the catego-
ries (Fig. 2b). The space category shows some of the highest and
lowest dismantling ages, which range from 3.8 to 32.9 years with a
median value of 17.4 years. The average age for the noise category is
22.1 years, the highest of all groups, which suggests that older tur-
bines more commonly induce a violation of noise emission limits.
Those older turbines are mechanically louder and have less sound
insulation than newer turbines, as stated by one of the interviewees.
In seven of the eight categories, both the median and average
value for the operating turbine lifetime are <20 years. This is sur-
prising because turbines installed in Denmark between 1996 and
2008 are guaranteed government support for 20 years34. Hence,
many of these turbines forego support payments because of dis-
mantling. This reinforces the idea that repowering is seldom a tech-
nical end-of-lifetime decision or even an end-of-support decision.
One interviewee stated that they continuously monitor their fleet
of existing turbines and evaluate the turbines’ economic viability to
include them or not in a new repowering project; hence, the current
age of the existing turbine is, in itself, largely irrelevant, and merely
a parameter in the broader economic evaluation.
Our analysis reveals that turbines in repowering projects are dis-
mantled on average 5.8 years earlier than those separately disman-
tled (non-repowering) (Table 2). These ‘lost’ years of production
constitute foregone value creation. Of course, the lost production
Table 3 | Annual wind turbine capacity additions and reductions in megawatts
2012
2013
2014
2015
2016
2017
2018
2019
Total
Repowering, gross additions
30.0
122.4
40.5
92.9
104.8
126.9
108.3
106.2
732
Repowering, gross reductions
−0.8
−21.7
−30.8
−2.7
−21.4
−47.1
−8.5
−22.3
−155
Greenfield
103.2
167.3
57.5
110.9
94.3
174.9
79.5
16.8
804
Separately dismantled turbines
−1.0
−1.8
−6.2
−13.5
−30.0
−18.8
−3.0
−4.4
−79
Annual net capacity instalment
131
266
61
188
148
236
176
96
1302
Share of repowering: commissioned
(%)
23
42
41
46
53
42
58
86
48
Share of repowering: dismantled (%) 43
92
83
17
42
71
74
84
66
The data exclude test and small-scale (household) turbines (<25 kW) (Methods).
Table 4 | Annual number of wind turbine additions and reductions
2012
2013
2014
2015
2016
2017
2018
2019
Total
Repowering, gross additions
10
38
13
31
32
37
33
26
220
Repowering, gross reductions
−1
−26
−53
−6
−32
−67
−8
−28
−221
Greenfield
34
63
21
38
37
51
24
4
272
Separately dismantled turbines
−3
−9
−37
−70
−145
−94
−11
−11
−380
Annual net turbine instalment
40
66
−56
−7
−108
−73
38
−9
−109
Share of repowering: commissioned
(%)
23
38
38
45
46
42
58
87
45
Share of repowering: dismantled (%)
25
74
59
8
18
42
42
72
37
The data exclude test and small-scale (household) turbines (<25 kW) (Methods).
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is offset by the additional value created from the larger and more
efficient new turbines. However, it can be argued that for the tur-
bines that fall within the more intangible dismantling categories,
which include aesthetics and politics, the value of lost production
should be considered and weighed against the societal willingness
to pay for the turbine removal. Approximating the value of lost
production through electricity market prices, we estimate between
€0.27 and 0.43 million per turbine is lost as a result of premature
dismantling—the lower value reflects the average of all the cat-
egories and the higher one that of the three categories that capture
non-physical reasons for dismantling (that is, aesthetics, politics,
and aesthetics and politics). This value compares with estimated
operations and maintenance costs of €0.25 to 0.29 million per tur-
bine over a period of 5.8 years35. The general comparability of these
ranges points toward economic as well as societal factors as drivers
of turbine dismantling in repowering projects.
The presented empirical data demonstrate the difficulties in
analysing repowering strictly from an on-site perspective. Likewise,
simply broadening the scope to a larger radius around the new
development site comes with considerable limitations, as illustrated
in Table 7, which depicts the total number of dismantled turbines
and capacity as a function of proximity to the nearest new turbine
within each repowering project, along with the share of dismantled
turbines captured by the respective approach.
We observe that increasing the radius of investigation around
the newly developed turbine also increases the likelihood of captur-
ing a higher share of relevant dismantled turbines. However, even
a radius equal to ten times the total height of a new turbine fails
to capture 10% of the associated dismantled turbines. Furthermore,
such a simplistic radius approach bears the risk of including unre-
lated dismantling of turbines in the repowering statistics. Therefore,
it is still necessary to manually establish a causal relationship
between a commissioned and dismantled turbine within each
repowering project.
The on-site approach equally falls short of estimating a realis-
tic net repowering factor, which is the ratio between commissioned
and dismantled values (number of turbines or capacity), and hence
denotes the amount commissioned for every unit dismantled.
Including all the causally identified dismantled turbines in repow-
ering projects, we obtain a net capacity repowering factor of 4.72,
in contrast to 13.10 if using a radius derived from one times the
total height of the new turbines. If we only consider those turbines
that have been identified in the space category, the net repowering
factor would equal 7.05. Hence, project-level information and
causal relationships are crucial to determine the full extent of
repowering impacts.
Discussion and conclusions
This work proposes a more holistic and socially informed
project-based perspective on wind plant repowering that starts
from the development of new projects and explores the need for
and drivers of removing existing turbines to enable the realization
of new wind energy facilities. Our results suggest that to focus on
end-of-life considerations and assuming a simple on-site localized
replacement of existing turbines is insufficient to truly understand
the repowering impacts and opportunities. This is demonstrated
by an application to Denmark, in which more than a third of all
wind energy development projects between 2012 and 2019 involved
repowering. Only 67% of the capacity removed in repowering
projects was related to turbines that occupied the physical space
Table 5 | Reasons for dismantling existing wind turbines in repowering projects as derived from interviews
Space
Physical requirement for the installation of new turbines, such as a space requirement for new foundations, access roads,
setbacks and grid infrastructure. Aside from the stated reasons by the interviewees, we included dismantled turbines
within the radius of 1.5 times the total height of the new wind turbine also to be in this category, as a minimum distance
required to prevent the hazard from a collapse of turbine towers.
Noise
Violation of the cumulative noise emissions within a radius of 28 times the total height of the planned wind farms. A noise
emission difference less than 15 dB between existing wind turbines and planned wind turbines at an inhabited property
induces a cumulative noise calculation. If this calculation exceeds the noise limit, either noise reduction on the new
turbines or dismantling of the existing turbines are conceivable options.
Aesthetics
A disturbance of the scenic or aesthetic values in the landscape. The Environmental Impact Assessment (EIA) requires
a visual impact analysis that concerns disturbances of aesthetic values within a radius of 28 times the total height of
the planned wind turbines. Focusing on location, design and the interaction between existing turbines and planned wind
turbines, the visual impact analysis can result in the dismantling of turbines.
Aesthetics or noise
Either a violation of noise emissions or a disturbance of the scenic or aesthetic values in the landscape caused the
developer to dismantle existing wind turbines. Although the interviewees could not single out one reason for the
dismantling, they explicitly negated politics as a reason for turbines in this category.
Politics
During local development and planning dialogues, informal requests by local politicians, proactive suggestions by
developers or voluntary agreements by both parties are put forward to satisfy the agreement on the development of the
new project. All of these actions involve the dismantling of existing wind turbines located in the municipality based on
subjective intentions as a condition for the new project.
Politics or aesthetics
The interviewees could not point out precisely if the main driver was politics or aesthetics. Some mentioned that local
politicians tend to use the disturbance of aesthetic values in the landscape to argue for the removal of existing wind
turbines, which made selecting between aesthetics or politics difficult.
Politics, aesthetics or noise
For some turbines, a differentiation between the dismantling reasons is unobtainable. A combination of factors may
prompt the developer to plan the dismantling of existing wind turbines before the permit proposal. In the pre-permit
process, the developer identifies problematic wind turbines knowing that they will be chosen for dismantling as a result
of politics, noise or aesthetics, and thereby save time on wind farm design, noise calculations and the visual impact
assessment.
Unclassified
Unassigned reasons for dismantling wind turbines. This category contains those wind turbines for which our interviews or
the local development plans do not cover the motivations behind the dismantling.
See Methods for a more comprehensive description of the reasons.
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needed for a new turbine. We identified several other reasons for
dismantling, which included regulation (for example, noise-related,
8–17% of removed capacity) and development principles (for exam-
ple, aesthetics, 7–20% of removed capacity). Also, political bargain-
ing around the dismantling of unwanted older turbines was not
uncommon (4–13% of removed capacity).
The broader perspective offered by our approach highlights how
the repowering process is not a matter of turbine replacement but
a negotiation between the developer and the local host community
about how wind energy should coexist with the community given
the changes that the introduction of the new technology might
induce. There is no doubt that repowering can increase the total
contribution of wind energy from a given land area. However, we
demonstrate that this contribution is not as high as theoretically
possible based on physical considerations alone. Considering only
the on-site replacement of turbines might lead to the mispercep-
tion that a net repowering factor of 7.05 could have been achieved
in Denmark, that is, that the installed capacity after completing
an average repowering project would be 7.05 times higher than
the original capacity. In fact, by establishing causal relationships
between repowering projects and all dismantled turbines, which
include the additional off-site capacity removed, we observed that
the actual net capacity repowering factor per project was only
4.72. Although the repowering potential within a project may still
be considerable, it is weakened by the conditional dismantling of
unwanted off-site turbines and the associated land use and societal
considerations that this dismantling may serve. In addition, tur-
bines dismantled in repowering projects achieved an average oper-
ating age of 5.8 years less than those dismantled on a stand-alone
basis (non-repowering) and are often dismantled before reaching
the end of their operational life. Accordingly, repowering may arrive
earlier and with a stronger force than would be predicted when tak-
ing an end-of-life perspective.
Notwithstanding the relatively lower net repowering factor and
shorter lifetime of the dismantled repowered turbines, our analy-
sis also indicates that repowering, in combination with techno-
logical innovation, can help reduce the impacts of wind turbines
on neighbouring communities. In a sense, repowering provides
Table 6 | Wind turbines dismantled in repowering projects by dismantling reason
Space
Noise
Aesthetics
Aesthetics or
noise
Politics
Politics or
aesthetics
Politics,
aesthetics or
noise
Unclassified
Number of turbines
139
28
20
10
6
6
10
2
Capacity (MW)
103.8
12.2
11.3
6.7
6.3
5.9
7.4
1.5
Average capacity per
turbine (MW)
0.747
0.435
0.566
0.669
1.050
0.983
0.740
0.750
Share of number (%)
63
13
9
5
3
3
5
0.9
Share of capacity (%)
67
8
7
4
4
4
5
1.0
Space
Noise
Aesthetics
Aesthetics or noise
Politics
Politics or aesthetics
Politics, aesthetics or noise
Unclassified
0
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
4,500
5
10
15
20
25
30
Distance (m)
Years
Space
Noise
Aesthetics
Aesthetics or noise
Politics
Politics or aesthetics
Politics, aesthetics or noise
Unclassified
b
a
75%
25%
Min
Max
MedianMean
75%
25%
Min
Max
MedianMean
Fig. 2 | Distribution of wind turbines in repowering projects by dismantling category. a, The nearest distance between dismantled and installed wind
turbines. b, The age of dismantled turbines.
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dual benefits to society—an increased clean energy supply and
fewer turbines present in the landscape. Although these benefits do
not come without a trade-off (such as higher visibility over longer
distances), they are substantial in regions where space is limited and
an increased deployment of wind energy is desired. This situation
may be increasingly experienced with a growing reliance on renew-
able power to serve clean energy needs.
As Denmark is a global pioneer of wind energy and repowering,
the experiences observed here are anticipated to become increasingly
relevant for other countries and regions. Of course, we acknowledge
that many of the non-space-related reasons for dismantling turbines
are idiosyncratic and depend on the physical, political and social
landscape in a country. Nevertheless, the diversity of repowering
and dismantling drivers captured here illuminates the complex
and multifaceted decision process that constitutes repowering and
illustrates considerable implications for policymakers and planners
who try to understand the potential of wind power to serve their
energy needs.
Methods
Ethical compliance. We have complied with all relevant ethical regulations. The
interviews were conducted in Denmark by the Technical University of Denmark
according to the guidelines. All interactions followed Chatham House rules. We
have obtained informed consent from all interview participants.
Approach. We took a new wind energy development project as the starting
point of the analysis. From this lens, we considered all the turbines that need
to be dismantled to enable the undertaking of the new project (as a conditional
requirement). We used the term dismantling, but, in principle, our approach
can capture both the complete dismantling of turbines (also referred to as full
repowering4), and the installation of new equipment (for example, drivetrain
and rotor) on an existing tower and/or foundation (also referred to as partial
repowering4). We emphasize that the analysis must include the dismantling of
existing turbines not only at the location of the new project undertaking (on-site),
but also those located elsewhere (off-site). Furthermore, existing turbines may
be dismantled years before the new turbines are installed. Therefore, repowering
must reflect a combined action of dismantling existing turbines and establishing
new turbines, regardless of spatial or temporal proximity. We thus investigated
the conditional relationship between capacity reduction and capacity addition.
This enabled us to reveal notable differences between repowering projects
(comprising both commissioning new turbines and dismantling existing turbines)
and greenfield projects (only comprising the commissioning of new turbines) for
several key indicators, such as project size and lifetime.
Applying our approach on an empirical case, we created a synthesized database
of all the approved Danish wind projects between 2012 and 2019. We constructed
the database by using publicly available sources and semi-structured interviews
with leading wind developers in Denmark. Our database has two key advantages.
First, it is exhaustive on a project-level basis, whereas previous databases focused
solely on technical specifications or the physical planning of wind turbines.
Second, our database allows for a comprehensive overview of the extent of
repowering projects in Denmark, whereas most previous studies only provide a
general overview (in the form of ‘bulk’ sizes or turbine-per-turbine comparisons).
Our database is uniquely suited to identify the reasons for dismantling turbines in
repowering projects. We created our database between May 2018 and September
2019 in three major stages. In the first stage, we manually extracted data and
coded the database. In the second stage, we interviewed leading wind developers
to collect qualitative data on the motivations behind dismantling turbines in
repowering projects. In the third stage, we synthesized and analysed the rich data
from the interviews into our database.
Data selection. We limited our time horizon to the period between 2012
and 2019. First, we excluded the years before 2001 because of discontinued
regulations that may have distorted repowering decisions from today’s perspective.
Second, we also excluded the years from 2001 to 2011 as, during that period,
the Danish government provided policy incentive programmes for repowering
(Supplementary Note 1) that could have confounded our analysis. Third, we
excluded projects that involved research and test turbines, as these turbines
are subject to different regulations. Of the 102 identified wind projects, 8 were
experimental test projects, that is, turbines established and dismantled at test sites.
In total, 70 test turbines were dismantled during the investigated period. Fourth,
we excluded household wind turbines, that is, turbines with a generation capacity
of 25 kW or less, as there are different regulations for these types of turbines.
During the investigated period, 77 household turbines were dismantled. Fifth, we
excluded a dismantling project that was part of an infrastructure development
project, in which 26 turbines were dismantled because of the construction of the
Femern tunnel in Southern Denmark. Overall, we determined that 221 out of the
relevant 601 dismantled turbines were repowering related.
Note that we included all wind energy projects that were developed, approved
and project execution started up to the end of 2019. We included two repowering
projects, with a total of 106.2 MW, for which the commission date was rescheduled
Table 7 | Dismantled wind turbines in repowering projects as a function of proximity to the nearest commissioned new turbine by
capacity and number of turbines
Proximity to the nearest commissioned repowering turbine
Category
space
All
dismantled
turbines
<1 × total
height of a
new turbine
<2 × total
height of a
new turbine
<3 × total
height of a
new turbine
<4 × total
height of a new
turbine
<5 × total
height of a
new turbine
<10 × total
height of a new
turbine
Number of
dismantled
turbines identified
75
147
168
174
179
199
139
221
Share of
dismantled
turbines captured
(%)
33.9
66.5
76.0
78.3
81.0
90.0
63
100
Dismantled
capacity identified
(MW)
55.9
110.1
125.6
129.9
134.1
144.5
103.8
155.1
Share of
dismantled
capacity
captured(%)
36.0
71.0
80.9
83.8
86.4
93.2
67
100
Net number of
turbine repowering
factor
2.93
1.50
1.31
1.26
1.23
1.11
1.58
1
Net capacity
repowering factor
13.10
6.65
5.83
5.63
5.46
5.06
7.05
4.72
The bottom two rows denote the net repowering factor, that is, the ratio between commissioned and dismantled turbine capacity in repowering projects
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from 2019 to primo 2020. Decommissioning of turbines for one project
commenced in November 2019, whereas the other was postponed into 2020.
Data sources. We constructed the database by using three primary sources.
The first source was the master data registry of wind turbines from the Danish
Energy Agency31. The master data registry is a comprehensive dataset that
consists of postconstruction information. The dataset contains information, such
as identification number, technical specifications of the turbine and Universal
Transverse Mercator (UTM) coordinates, on all commissioned and dismantled
turbines in Denmark. The Danish Energy Agency periodically updates the dataset
under European Union directive 2001/77/EC36. The second source was local
development plans for wind projects, which we retracted online from the Danish
Business Authority30. Local development plans set out the framework for future
planned projects and describe the project; they comprise information on the
location, appearance, number of commissioned and, in most cases, also dismantled
turbines and an EIA. The third source was interviews with repowering wind
developers in Denmark.
Data collection by desktop research. We manually reviewed all the approved and
undertaken local development plans for wind turbines from 2012 to 2019. For each
local development plan, we proceeded as follows.
Using spatial analysis in the proximity of the local development plan, we
identified and connected commissioned and dismantled turbines from the master
data registry to the respective local development plan in our database. To identify
the commissioned turbines, the local development plan specifies exact locations
on maps. By translating these locations into UTM coordinates in Google Earth,
we were able to locate the matching commissioned turbines in the master data
registry of wind turbines. The local development plan often does not provide exact
locations for the dismantled turbines. Instead, the local development plan describes
the location of the existing turbines within the text. Consequently, using the
Euclidean distance formula with the UTM coordinates, we calculated the distance
between each dismantled turbine and the centre point of the local development
plan. Next, we sorted the distances according to the shortest value. Using the sorted
list of dismantled wind turbines, we used Google Earth to find existing turbines
that best matched the location of those mentioned in the local development plan.
This evaluation served two purposes, to identify the turbines mentioned in the
local development plan and to check if there were any unmentioned dismantled
wind turbines in the nearby area that had a possible connection to the local
development plan.
After the spatial analysis, we categorized the project from the local
development plan as either a greenfield, repowering or demonstration project.
Then, we searched through the local development plan and the EIA to identify
the reason for dismantling each turbine in repowering projects. If the local
development plan or the EIA mentioned a specific reason, we included that
information in our database, mostly for later verification in the interviews. Finally,
we searched through the local development plan to obtain any contact details
regarding the developer of the project. To account for missing details on the
developer, we also drew on web searches of property valuation settlements, news
articles and company announcements.
To ensure that we identified all the approved wind farm projects from 2012 to
2019, we consistently checked if we had assigned all commissioned wind turbines
from the master data registry of wind turbines to a local development plan in our
database. In the case of any unassigned commissioned wind turbines, we used the
associated UTM coordinates together with an interactive map provided by the
Danish Business Authority32 to detect if any local development plan existed in the
area. As a result, we identified all wind projects in Denmark from 2012 to 2019.
Data collection by interviews. We conducted five in-depth semi-structured
interviews with the leading wind developers in Denmark, which comprised
52.8% of all the repowering projects between 2012 and 2019 and 72.4% of all
the dismantled turbines. Additionally, we conducted eight informal interviews
with local wind developers, which comprised 22.2% of the approved repowering
projects in the same period, or 16.3% of the dismantled turbines. Overall, our
interviews cover 89% of all the dismantled turbines in repowering projects in the
designated time frame, and 91% of the dismantled capacity in repowering projects.
For each project covered, we received responses on all the turbines in the respective
project. Using our interviews and the special planning reports, we identified a
specific reason for 94% of the turbines dismantled in repowering projects.
We conducted the interviews by phone between March 2019 and September
2019. The in-depth interviews lasted 45–70 min. We recorded and transcribed the
interviews. The informal interviews lasted 10−25 min. Here, we took handwritten
notes. All the interviews were conducted in Danish. In both interview types, we
informed the interviewees that we would reveal neither their identity nor their
affiliation, according to the Chatham House Rule37.
Using a protocol, we structured the in-depth interviews into two main parts. In
the first part, we asked the interviewees five open questions:
	(1)	 How would you define a repowering project?
	(2)	 Can you delineate the motivations behind the dismantling of wind turbines
during a repowering project?
	(3)	 What would be a required minimum space between planned and existing
wind turbines?
	(4)	 Does your company have any guidelines in selecting existing wind turbines
for dismantling?
	(5)	 What happened with the dismantled wind turbines?
In the second part of the protocol, we asked project-specific questions to
establish the motivations behind dismantling each turbine in the respective
repowering projects. In the informal interviews, we only focused on the second
part of the protocol.
After the interviews, we analysed the transcripts and notes to identify and
classify the reasons for dismantling wind turbines in repowering projects. Based
on this, we found eight mutually exclusive categories for dismantling turbines in
repowering projects, as described below.
Space. Commonly, developers construct repowering projects at existing project
sites. Therefore, it is often necessary to dismantle existing wind turbines on that
site because of physical constraints. Developers reported that these physical
constraints cover the land use needed for the installation and operation of the
individual new planned wind turbine, such as a space requirement for access roads,
setbacks, foundation and grid infrastructure.
Aside from the statements given by the interviewees and for consistency
reasons, we categorized all the existing wind turbines dismantled within the
proximity of 1.5 times the total height of the new wind turbine in this category.
We consider the value of 1.5 as a minimum distance required to prevent a collapse
of the turbine tower, a crash of a wind turbine or a detached blade from the hub.
Note that this is our consolidated estimate of a minimum distance. When asked
about a required minimum distance between planned and existing wind turbines,
the interviewees gave no fully conclusive answer. In an additional legal analysis (in
which we reviewed primary sources of law), we found no state-level legislation or
statutes concerning a minimum distance between wind turbines. Additionally, we
examined legal statutes in all municipalities involved with repowering projects. We
only found guidelines that specified a distance that corresponded to 3−5 times the
rotor diameter between turbines when planning new wind farms38. These guidelines
do not consider the relationship between existing and planned wind turbines where
the hub height differs, and wake effects might be negligible. Another guideline
states a minimum distance of one times the total height of the wind turbine to main
roads and railroads, in case of crash down or falling ice debris38.
Noise. Wind turbines generate noise as a result of moving mechanical parts and
the aerodynamic effects of air passing over the blades. According to Danish noise
regulation, a developer must calculate the cumulative noise emissions from the
planned and existing wind turbines at a property if the difference in noise emission
from the planned and the existing wind turbines is less than 15 dB (ref. 39). The
cumulative noise emissions include outdoor open space areas near the property
and low-frequency noise indoors. If the cumulative noise emissions violate the
noise limits, the developer can either choose to reduce the noise from the planned
wind turbines, and thereby reduce the generation output, or dismantle the existing
wind turbines.
Aesthetics. Aesthetics are defined as the visible impact on the landscape of
individual wind turbines. The growing height and rotor diameter means wind
turbines are more visible over greater distances within the local community.
Some citizens feel affected by the presence of wind turbines with regard to a
‘disfigurement’ of the landscape. This feeling is subjective: some see wind turbines
as a positive element in the transition to sustainability, others may see them as a
disturbance to scenic or aesthetic values.
Using a visual impact analysis, the EIA includes an evaluation of the
disturbances of aesthetic values within 28 times the total height of the planned
wind turbines. The evaluation focuses on location, design and the interaction
between the proposed new wind turbines and the existing infrastructure. Several
developers mentioned in our interviews that the difference in height and rotational
speed between planned and existing wind turbines are two of the main aesthetic
factors that cause the dismantling of existing turbines in repowering projects.
Other aesthetic factors include the number of turbines in a location, the distance
between farms and the array layout in relation to other existing wind farms.
Guidelines exist to portray and evaluate the visual impact40; however, the final
decision rests with the municipality.
Aesthetics or noise. This category comprises those turbines that interviewees
could not place into only one of the two categories, but for which they
were certain it was one of these two reasons. Note that for a turbine to be placed in
this category, interviewees had to explicitly exclude politics as a possible reason for
dismantling.
Politics. Politics may have an impact on the dismantling of existing turbines during
the local development and planning dialogues, where informal requests by local
politicians, proactive suggestions by developers or voluntary agreements by both
parties come into play. In Denmark, wind project developers must obtain approval
from the majority of local politicians in the municipality. Therefore, the developers
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engage with individual politicians during the planning process. During these
negotiations, developers are sometimes met with informal requests to dismantle
existing turbines located in the municipality. Developers see these informal
requests as a requirement to achieve support for the project and the dismantling as
a necessary action to receive approval for the project.
Politics or aesthetics. This category comprises those turbines that are dismantled
either because of aesthetics or politics. Interviewees mentioned that local
politicians tend to use the disturbance of aesthetic values in the landscape to argue
for the removal of existing turbines. Using the visual impact illustrations from
the EIA, local politicians would address their view on the disturbance in aesthetic
values and argue for the dismantling of existing turbines. As no firm regulation
exists, but only guidelines for evaluating visual impact, it was often impossible
for interviewees to select between aesthetic or political motivations behind the
dismantling requirements of a specific turbine.
Politics, aesthetics or noise. For turbines in this category, interviewees excluded
space as a reason for dismantling, but could not point to any other individual
reason. Many of the interviewees noted that, during the early evaluation of the new
development site, potentially problematic existing turbines are already identified
on a more intuitive basis or from experience. Assuming the removal of those
existing turbines before entering the permitting process can save time on wind
farm design, noise calculations and the visual impact assessment. This makes
it difficult to (in retrospect) point towards a specific dismantling reason, as, for
example, the respective analyses for noise and aesthetics were not undertaken. For
turbines in this category, interviewees could not rule out politics as a reason, either.
Here, some noted that it was common to proactively offer dismantling turbines
that are known to be unwanted by politicians in the municipality to create a good
start for negotiation during the permitting process.
Unclassified. All the dismantled turbines that the local development plans or the
interviews did not cover are put into this category.
Having identified and defined the categories for dismantling turbines in
repowering projects, we analysed the interview transcripts and notes
and added categories to the identified dismantled turbines in our database.
Of the 221 dismantled repowering turbines, only two turbines remain
without category.
Value of lost production. We approximated the value of lost production
specifically for each turbine dismantled in repowering projects by multiplying
its annual production averaged over its lifetime (obtained from the master data
registry of wind turbines31) with the wind-weighted electricity price41 for Denmark
in each year from the dismantling of the wind turbine until the respective wind
turbine would have reached a lifetime of 24.5 years (which corresponds to the
average lifetime of non-repowered dismantled turbines). We approximated
operations and maintenance cost assuming €30–35 MWh–1 (ref. 35), an expectedly
high level because of the turbine age (which is between 18.6 and 24.4 years during
the lost years of production).
Reporting summary. Further information on research design is available in the
Nature Research Reporting Summary linked to this article.
Data availability
The dataset generated during the current study is published as Supplementary Data
1 (spreadsheet format). Although the dataset contains all quantitative data collected
from the different sources, we cannot disclose specific disaggregate information
from the interviews, which have been conducted under the Chatham House rules.
Interview transcripts and notes may be requested from the authors, but will only be
handed out after explicit consent from the interviewees. Source data are provided
with this paper.
Received: 13 March 2020; Accepted: 25 September 2020;
Published online: 2 November 2020
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Acknowledgements
This work constitutes a contribution to the research in the international working group
IEA TCP Wind Task 26 (Cost of Wind Energy). We thank all the members of the group
for their extraordinary collaboration and essential comments on our work that formed
this article. The work was in part funded by the Danish Public Energy Technology
Development and Demonstration Program (EUDP), project no. 64018-0577. This Article
constitutes a contribution from the European Commission to IEA Task 26 research.
The views expressed are purely those of the authors and may not in any circumstances
be regarded as stating an official position of the European Commission. This work
was authored (in part) by the National Renewable Energy Laboratory, operated by the
Alliance for Sustainable Energy, LLC, for the US Department of Energy under Contract
no. DE-AC36-08GO28308. Funding was provided by the US Department of Energy
Office of Energy Efficiency and Renewable Energy Wind Energy Technologies Office.
The views expressed in the Article do not necessarily represent the views of the DOE or
the US Government. The US Government retains and the publisher, by accepting the
article for publication, acknowledges that the US Government retains a nonexclusive,
paid-up, irrevocable, worldwide license to publish or reproduce the published form of
this work, or allow others to do so, for US Government purposes.
Author contributions
L.K. and M.K.J. conceived the study, developed the analysis and undertook the
interviews. M.K.J. led the data processing and analysis, with support from L.K. E.L. and
T.T. contributed with international data. All the authors contributed to data analysis and
interpretation. All the authors wrote and edited the paper.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information is available for this paper at https://doi.org/10.1038/
s41560-020-00717-1.
Correspondence and requests for materials should be addressed to L.K.
Reprints and permissions information is available at www.nature.com/reprints.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
© National Renewable Energy Laboratory under exclusive licence to Springer Nature
Limited 2020
Nature Energy | VOL 5 | December 2020 | 1012–1021 | www.nature.com/natureenergy
1021

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Corresponding author(s):
Lena Kitzing
Last updated by author(s): Aug 20, 2020
Reporting Summary
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in reporting. For further information on Nature Research policies, see our Editorial Policies and the Editorial Policy Checklist.
Statistics
For all statistical analyses, confirm that the following items are present in the figure legend, table legend, main text, or Methods section.
n/a Confirmed
The exact sample size (n) for each experimental group/condition, given as a discrete number and unit of measurement
A statement on whether measurements were taken from distinct samples or whether the same sample was measured repeatedly
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Only common tests should be described solely by name; describe more complex techniques in the Methods section.
A description of all covariates tested
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A full description of the statistical parameters including central tendency (e.g. means) or other basic estimates (e.g. regression coefficient)
AND variation (e.g. standard deviation) or associated estimates of uncertainty (e.g. confidence intervals)
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Give P values as exact values whenever suitable.
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Estimates of effect sizes (e.g. Cohen's d, Pearson's r), indicating how they were calculated
Our web collection on statistics for biologists contains articles on many of the points above.
Software and code
Policy information about availability of computer code
Data collection
We manually collected data from different online sources and interviews (see below).
Data analysis
n/a. Manual data processing, see below.
For manuscripts utilizing custom algorithms or software that are central to the research but not yet described in published literature, software must be made available to editors and
reviewers. We strongly encourage code deposition in a community repository (e.g. GitHub). See the Nature Research guidelines for submitting code & software for further information.
Data
Policy information about availability of data
All manuscripts must include a data availability statement. This statement should provide the following information, where applicable:
- Accession codes, unique identifiers, or web links for publicly available datasets
- A list of figures that have associated raw data
- A description of any restrictions on data availability
 We collected data manually from different sources:
1) master data registry of wind turbines from the Danish Energy Agency (Excel sheet, online available, regularly updated, version),
https://ens.dk/en/our-services/statistics-data-key-figures-and-energy-maps/overview-energy-sector, downloadable excel sheet, Version upladed on 25th of March
2020.
2) local development plans from the Danish Business Authority ('Searchlist over all plan proposals and plans adopted pursuant to the Planning Act', PDF documents),
http://kort.plandata.dk/searchlist/#/
3) semi-structured interviews with project developers (see below)

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nature research  |  reporting summary
April 2020
We make the final dataset available online together with the published paper. The data set contains the full project data and all data underlying the figures and
tables in the manuscript.

While the data set contains all quantitative data collected from the different sources, we cannot disclose specific disaggregate information from the interviews,
which have been conducted under the Chatham House rules. Interview transcripts and notes may be requested from the authors but will only be handed out after
explicit consent from the interviewees.
Field-specific reporting
Please select the one below that is the best fit for your research. If you are not sure, read the appropriate sections before making your selection.
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Behavioural & social sciences study design
All studies must disclose on these points even when the disclosure is negative.
Study description
Mixed-methods study with quantitative data collection in both manual online data search and interviews, as well as qualitative
classification of turbine dismantling reasons by interviewees.
Research sample
Our quantitative data sample contains all wind development projects in Denmark between 2012 and 2019.
Our interviews covered 27 out of 36 repowering projects in total (19 ( 52.8%) projects were covered by in-depth interviews;
8 (22.2%) projects were covered by shorter phone-conversations), corresponding to 91% of all dismantled turbine capacity in
repowering projects in Denmark between 2012 and 2019.
Sampling strategy
We excluded the years before 2001 because of discontinued support regulations that would have produced different and obsolete
repowering decisions. For the same reason, we also excluded the years 2001 to 2011, because political incentive programs for
repowering were in place.
We included all wind energy projects that have been developed, approved, and where project execution has started until the end of
2019. We have included two repowering projects, with a total of 106.2 MW, for which the commission date was rescheduled from
2019 to primo 2020.
The sample of developers we conducted interviews with was based on a major criterion: they needed to have developed at least one
repowering project in Denmark during 2012-2019. We conducted extended semi-structured interviews with all developers who are
operating nationwide and added shorter interviews with local developers until we reached a broad coverage of projects,
representing above 90% of all dismantled capacity in repowering projects.
Data collection
To ensure that we had constructed a comprehensive dataset, we consistently checked if the two primary data sources aligned, i.e.
that we had assigned all commissioned wind turbines from the master data registry to a local development plan. We ensured
alignment in an iterative process, in which we conducted an online spatial analysis or all still unassigned commissioned wind turbines
used the associated UTM coordinates (which we translated from the master registry using Google Earth) together with an interactive
map provided by the Danish Business Authority to detect if any local development plan existed in the area. We then checked the
respective additional local development plans from the search list. We continued until all turbines were assigned a project.
In the interviews, we asked specifically for each project about the association of specific turbines with the project and their individual
dismantling reasons. For each project covered, we received responses on all turbines in the particular project. We asked open-ended
questions about the reasons for dismantling each turbine and constructed the classification categories after the interviews had been
finalised. The interviews were conducted in Danish.
Timing
The main elements of the quantitative data were collected between May 2018 and January 2019, and then updated with concurrent
events until December 2019. In the end of March 2020, we implemented some adjustments that were made to the master registry of
wind turbines (version of 25 March 2020).
The interviews were conducted between March 2019 and September 2019.
Data exclusions
Out of the 102 identified wind projects, we excluded 8 experimental test projects comprising dismantling of 70 test turbines, as these
turbines are subject to different regulations. We also excluded 77 dismantled household wind turbines, i.e., turbines with a
generation capacity of 25 kW or less, as there are also different regulations for these types of turbines. Further, we excluded a
dismantling project that was part of an infrastructure development project, where 26 turbines were dismantled because of the
construction of the Femern tunnel in Southern Denmark.
Non-participation
All project developers we succesfully established contact with have agreed to participate in the interviews. We received responses on
all specific turbines in all discussed projects.
Randomization
not applicable
Reporting for specific materials, systems and methods
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system or method listed is relevant to your study. If you are not sure if a list item applies to your research, read the appropriate section before selecting a response.

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Materials & experimental systems
n/a Involved in the study
Antibodies
Eukaryotic cell lines
Palaeontology and archaeology
Animals and other organisms
Human research participants
Clinical data
Dual use research of concern
Methods
n/a Involved in the study
ChIP-seq
Flow cytometry
MRI-based neuroimaging
Human research participants
Policy information about studies involving human research participants
Population characteristics
Our research focused on business activities for which we collected information through human interaction (interviews) with
company representatives. The humans were, thus, information providers and not the subject of our investigation. We had 12
individual participants in the interviews and phone conversations. All participants are business professionals, working as
project development experts in their respective companies. 11 participants were male, 1 participant female.
Recruitment
We obtained information about the responsible project developers for each relevant company via the internet. Typically,
contact details were provided on project homepages. We approached potential interviewees by e-mail and by phone. In few
cases, the approached person established contact to a more suitable interviewee in the company.
Ethics oversight
Interaction with human participants was undertaken by DTU according to Danish regulation. All interactions followed
Chatham House rules. Informed consent was obtained from all participants. All data is anonymised and stored on Danish
institutional servers according to GDPR regulation.
Note that full information on the approval of the study protocol must also be provided in the manuscript.

    Policy Problem:

Answer: The combined activity of dismantling or refurbishing existing wind turbines and commissioning new ones (known as repowering) is a growing point of focus for wind energy deployment. Ageing turbine fleets, increasing land‑use constraints and the rising relevance of societal factors in siting decisions make the deployment of land‑based (onshore) wind energy ever more complicated. To unlock the full potential of wind energy, policies will need to target increasing clean energy supply as well as manage community impacts. Repowering will be an increasingly important element of these policies but it involves complex technical, social and political dynamics. Formulating adequate repowering policy will rely on a robust picture of the extent and potential of repowering activity and of how repowering decisions are made.