“Energy is a crucial factor that governs our lives and promotes civilization. The social and economic health of the modern world depends on sustainable supply of energy in most of the cases” [1
]. While most parts of the world have to deal with the increase of energy consumption, Germany has to meet a different challenge. Shortly after the Fukushima Daiichi nuclear disaster in 2011, the Federal Government decided to put an end to nuclear energy conversion. According to the 13th Amendment of the German Atomic Energy Act, all nuclear reactors in the country will be shut down until 2022. The 18% of electricity currently obtained from nuclear energy has to be converted otherwise in the near future [2
]. Therefore, the Federal Government agitates for a consequential development of renewable energies and thus structural changes concerning energy supply [4
]. In addition to the nuclear phase-out, other factors like the reduction of CO2
emissions and of the dependence from oil producing countries also speak in favor of a shift in this direction [6
According to the energy concept published in 2010, the German government nowadays targets a share of 80% of gross energy consumption to be provided by sustainable means until 2050. In 2012, a portion of 22.9% was reached [7
]. Next to other renewable energies, onshore wind energy contributes the major part of renewable energy production, with an 8% share of German gross electric power consumption in 2011 [8
]. Furthermore, it stands out due to its cost-effective development potential in a short and long-term perspective.
Next to feed-in tariffs, the economic feasibility of wind energy plants (WEPs), local restrictions and social acceptance, as well as the availability of useful sites, play a restrictive part in the future expansion of wind energy conversion [9
]. Thus, the success of renewable energy systems depends on sufficient availability of suitable sites for power plants [11
]. Therefore, the goal of the presented research is the determination of areas suitable for onshore wind energy use in Germany, as it constitutes the major part in renewable energy conversion in the country [12
]. Due to simplification, areas suitable for wind energy conversion are referred to as “potential areas”.
This article focuses on technical potentials regarding suitable areas for wind energy use on the scale of Federal States. The scale of states was chosen due to homogeneous legal situations within states (that may drastically differ from nationwide laws), their dominant use as second level administrative entities in Germany, and wide data availability at state scale. As solid political decisions with long-term effects should not just be based on the knowledge of the potential area size today, but also on the area which might remain in the future, the main focus of this work is placed on scenarios describing certain coming developments up to the year 2030.
In our approach, the determination of current and future potential areas is separated into two steps. In a first step, unsuitable or unavailable areas for wind energy use in 2011 are located. These areas are then excluded from the data set which represents the German territory. In a second step, the development of the potential area is calculated based on scenarios representing possible future trends. The scenarios concerning the future are based on the expansion of settlement and traffic (S&T) areas and on the increase of protection areas.
Our research does not focus on the estimation of economic potentials, which can be generated per site, because a well-informed forecast over 20 years until 2030 would be virtually impossible as the future development of economic factors (national economy, energy prices, subsidizing, etc.) cannot be predicted accurately enough. More, our research does not include the calculation of energy potential as there are a variety of approaches available, which can be applied to our research results. Furthermore, our research does not focus on visibility analysis as a number of well-established approaches are available in this area, but they are mostly not suitable for large-scale approaches (e.g., Light Detection and Ranging (LiDAR) point clouds would be too laborious to process on a country scale).
1.1. General Criteria for the Estimation of Potential Areas
The detection of potential areas for wind energy conversion has been subject of several analysis approaches to define and determine exclusion areas. To name some examples, Lüdkehus et al.
(2013) did research on the potential area of wind energy in Germany based on a number of technical and ecological restrictions like infrastructure, protection areas, water bodies, certain forest types and noise issues, leading to a potential area of 13.8% of Germany. Based on their assumptions, a capacity of 1.190 GW wind energy could be installed in this area [12
]. However, their study only focuses on current potential areas and does not assess their future development. Bofinger et al.
(2011) specifically distinguished between areas which can be used as sites for wind energy plants without any restrictions, potential areas within forests and potential areas within protection areas. Thus, they proved that a main part of potential areas are located within forest and protection areas rendering their usage more difficult and thus more unlikely than the usage of areas without restrictions [13
]. Next to potential calculations on a national scale, research was also conducted on smaller scales. In the context of the elaboration of a climate protection concept and a concept to make use of renewable energies, the Institute for Applied Material Flow Management (IfaS) and the Transferstelle Bingen (TSB) (2013) assessed potential areas for various kinds of renewable energy for several administrative districts in Rhineland-Palatinate [14
]. Also, most Federal States authorized the estimation of their potential areas for wind energy generation. In practice, the site assessment process is performed on a local basis. In contrast, this paper focuses on a large-scale approach, based on Federal States.
Potential areas represent the remaining space, where the installation or operation of a WEP does not compromise human well-being or flora and fauna habitats [15
]. A running rotor can cause optical emissions, light reflections, ice-throw and noise pollution. To avoid the disturbance of people living close by, minimum distances have to be kept between WEPs and settlements as well as certain types of infrastructure. To determine such distances, shadow and noise calculations are part of wind farm planning and licensing procedures. As small-scale analysis cannot be directly transferred to a large-scale approach, offsets are applied instead. In 2012, the Federal and State Commission on Wind Energy in Germany published a summary of the state-specific recommendations on these offsets [16
]. Due to its uniformity, completeness and up-to-dateness—the summary represents the status of January 2012—it is widely used in our approach.
Protected areas play an important part in the conservation of the characteristics, diversity and beauty of nature [17
]. Thus, protected zones are generally not regarded as potential areas. Moreover, in practice, every potential site is examined in advance concerning its significance to local flora and fauna. The final approval of every single project depends on the outcome of these examinations, which is not part of our research. The size of potential areas can therefore be regarded as slightly overestimated in our approach.
Due to their predominant location in protected sites and their limited suitability for the erection of wind turbines (lacking access roads, missing line network connection, high maintenance costs, etc.
), forests are not primarily considered potential sites. Yet, unprotected forests are initially regarded as suitable sites, the increasing of hub heights of modern WEPs allows for the installation of wind turbines without too much intervention in the forest ecosystem [12
]. Furthermore, noise pollution and optic emissions are rarely an issue in forests. Finally, municipalities or Federal States can profit from the leasing income of wind parks within woodland as they are often the owners of the real estate property [18
Furthermore, potential areas have to meet a number of criteria concerning certain wind conditions because adequate wind frequency and speed on a yearly average do not only influence the electricity yield generated per WEP, but also the governmental subsidies and the profitability of WEPs in general. Usually, a mean wind speed threshold of 5.5 m/s is chosen in recent studies. One example is the study on the current energy situation in the Palestinian Territories and the potential of renewable energies, wind being one of them, in meeting part of the energy demand [19
]. Palaiologou et al.
(2011) also applied the threshold of 5.5 m/s when investigating the wind characteristics of the island of Lesvos, Greece, with the objective of providing the necessary data for identifying the wind power production capabilities of the island [20
]. This wind speed is required at a height of 130 m above ground as the hub height of modern power plants is at about that height. However, the German Weather Service (DWD) provides wind speed information only up to a height of 100 m, superior heights are not available. More, due to a lack of data on surface roughness, an extrapolation of the mean wind speeds according to their logarithmic profile was not computed in our analysis. Hence, in our approach, this threshold is lowered to 5 m/s to avoid underestimation of potential areas, i.e.
, zones with an average annual wind speed of less than 5 m/s at a height of 100 m above ground are excluded in our study.
Another crucial parameter to consider for the assessment of suitable WEP sites is the terrain’s slope. This is due to the obvious fact that is more complicated to erect and operate WEPs with an increasing angle of slope because of structural engineering reasons. Following the approach of the German Federal Environment Agency (UBA) in 2012, terrain with a slope of more than 30° is therefore excluded from the potential areas [12
1.2. Research Context and Scientific Contribution
In the course of developing climate concepts, Federal States work on studies to determine potential areas. The results of these studies vary depending on the applied approach, i.e.
, the exclusion criteria. A central reason for these varying definitions is the difference in objectives (policy-making, wind farm planning, nature conservation, etc.
), which also influences the emphasis of the single research projects and their outcomes [12
]. On average, forests (4%) and protected zones (10%) constitute a large portion of potential areas of the country, while the remaining areas only comprise about 8%. This approach shows that the actual potential is large, but a great part of it exists in zones that face strong opposition factors.
In the energy network study performed by the German Energy Agency (DENA) [21
], no such classification is made. In fact, protection areas are defined as “mainly unavailable” for wind energy use due to their conservation purposes. Forests, however, are regarded as generally suitable in our approach as more and more WEPs are currently built within woodlands as long as they are compatible with the needs of forestry use and environmental protection.
Some studies, including the above mentioned DENA study [21
], only take areas into consideration that are declared to be usable as WEP sites by local planning agencies. A central issue with this approach is that these data only provided by local or regional planning agencies (not in a single country-wide dataset), making it hardly usable in practice. This is particularly true as such data are subject to regular changes and rearrangements, which makes the estimation of future developments virtually impossible. Also, the permission to install a WEP can be legally obtained as long as it does not object public interests or existing restrictions [22
McKenna et al.
apply simplified exclusion criteria when determining the technical potential of the state of Baden-Wuerttemberg and do thus not distinguish between different counties and regions [23
]. Such a generalization can possibly lead to inaccurate results. According to the German Weather Service (DWD), the overall usage of an offset of one kilometer to settlements can lead to a reduction of suitable areas by 60%, which in some cases results in an overestimation of exclusion areas. This is of major importance as generalized offsets are considered a dominant factor on potential limitations of onshore WEPs [24
]. In our approach, generalized distance recommendations are applied on the scale of Federal States, but not nationwide.
Finally, apart from research on current potential areas, research on potential areas of the past and future is still rare. Lüdkehus et al.
make assumptions on potential areas in 2050 in order to model the development of electricity supply [12
]. Yet, they do not specifically do research on the parameters influencing the development of potential areas over the period of the study. In contrast, our work focuses on their development over time.
As to the scientific contribution of this work, the main focus differs from most previous studies. So far, current research mainly emphasizes the calculation of today’s potential areas. Calculations of future trends
of those areas, which are based on geospatial data, are still rare, especially for the country of Germany. De Vried et al.
(2007) investigated the potential for wind energy for the first half of the 21st century at a global level by means of a scenario analysis. The analysis used four land use scenarios based on four qualitative storylines that were developed in the context of the Intergovernmental Panel on Climate Change (IPCC). This approach provides an overview of development possibilities which can be expected on a global scale. Yet, detailed information like region-specific offsets and concrete regulations that influence the resulting potential areas were not taken into account, in contrast to our paper [25
Another approach generated an overview of renewable energy potentials on a global scale [26
]. The authors based their calculation on earlier studies, which various authors had conducted for different, mainly European countries. The calculation of long-term potentials was based on mid-term potentials that were revealed in those studies. The findings were also transferred to countries, where no primary mid-term calculation on renewable energy potentials had been conducted. In addition, our approach estimates the potential area for wind energy generation of the future based on the assumption that conditions concerning the area itself change over time.
Xia and Song (2009) predicted and analyzed the future perspectives of wind energy development in China. Based on the installed wind capacity in China over the past 18 years and the technical potential of wind energy resources, the growth pattern was modeled for the purpose of prospect analysis, in order to obtain projections concerning the development potential. While spatial information was not taken into account, the future development of wind energy potentials was based on assumptions made on the growing political support for wind energy. Xia and Song did not focus on the maximum potential area, which might be available in China in the future, but on the actual growth of wind energy that can be expected in the People’s Republic [27
], which considerably differs from our approach.
1.3. The Development of Settlement and Traffic Areas
With a 17% share of the German total area, the influence of changes in Settlement and Traffic (S&T) areas is of major importance [28
], particularly as this land use category shows the highest growth rate of all [29
]. In other words, the size of S&T areas in total keeps growing, mostly at the expense of agricultural areas [29
]. Therefore, the development of S&T areas is regarded as crucial to potential areas in our work.
The German Federal Institute for Research on Building, Urban Affairs and Spatial Development (BBSR) and the German Ministry of Transport, Building and Urban Development (BMVBS) expect that the majority of new settlements will be established in the urban hinterland as more and more people move to cities. At the same time, settlements in rural areas demand more space per person than urban areas [32
]. Additionally, space occupied by traffic areas has been constantly growing for years, currently making up for one quarter of the overall S&T increase [30
]. According to prognoses concerning the future road capacity, a large portion of traffic-related land use can be expected to persist [33
The expansion of S&T areas is influenced by various factors that change over time. Thus, a number of models have been created for analyzing different scenarios of future S&T development. The Panta Rhei Regio (PRR) model, which is used in our research for estimating the development of S&T areas, was developed under the advancement initiative “Research on the reduction of land use and sustainable land use management” (REFINA) between 2006 and 2009 on behalf of the German Federal Ministry of Education and Research as part of the National Sustainability Strategy. This economic model estimates interdependencies of relevant demand and supply parameters concerning land use on a county scale [33
]. The well-proven model has previously been used in various national analysis efforts. For instance, Distelkamp et al.
] analyze the probability of achieving the government’s goal to limit S&T area increase to 30 ha daily until 2030 [31
]. Hoymann et al.
use the PRR model to calculate the most probable development according to the current state of knowledge [30
]. According to the results derived from the model, the authors assumed that the daily additional S&T area uptake is about to decrease and reach 50 ha in 2030. Furthermore, they estimate that the S&T area will occupy nearly 5.2 million hectares in 2030. Due to its timeliness and consideration of crucial influencing factors, the results of the BBSR's research [30
] provide a solid basis for the scenarios presented in Sub-Section 3.2