2.1. Study Areas
Greece, a country located in southeastern Europe, consists of 13 regions with a total area of about 132,000 km
2. By 2023, according to statistics [
12], the total population of Greece was 10,497,595 inhabitants and, among the more than 2000 Greek islands, only about 170 islands were inhabited. Although many uninhabited islands host established nature reserves for flora and fauna, other areas that meet the designated standards are worthy of becoming sites of investment in renewable energy due to the abundance of wind and solar resources [
10]. Based on the Archaeological Cadastre of Greece [
13], the current inventory reveals the presence of 220 museums, 420 historical sites, 844 protected areas, 3100 archaeological sites, and 17,000 monuments in the entire territory of the country. In the established network of nature protection areas in the European Union, Greece has a total of 446 protected areas, including 239 Special Areas of Conservation (SAC) and 181 Special Protection Areas for birds (SPAs), with the remaining 26 areas falling into both categories [
14].
In this work, the operations were based on a shapefile of Greece [
15]. Before the land eligibility analysis, Natura 2000 areas were excluded first according to SFSPSD-RES [
16]. In order to determine the land position that can support the construction of wind turbines and PV power plants in Greece, the open-source tool GLAES [
17] and the Prior datasets containing typical criteria for variable RESs were utilized. A geographic analysis was conducted to assess the wind and solar potential in Greece. This analysis involved the utilization of the Digital Elevation Model of Greece obtained from the European Environment Agency Digital Elevation Model (EU-DEM) [
18]. Additionally, WPD data at a height of 100 m were obtained from the Global Wind Atlas [
19], while GHI data were obtained from the Global Solar Atlas [
20]. These datasets were employed to compare the approximate wind and solar potential in Greece using a Geographic Information System digital platform, specifically ArcGIS 10.8 (Esri, Aylesbury, UK).
2.3. Geospatial Land Availability for Energy Systems (GLAES)
Land eligibility is a process that evaluates the suitability of a land parcel for implementing a specific technology based on a predetermined set of exclusion constraints and serves as a fundamental and widely utilized procedure through which geospatial criteria shape the distribution patterns across a given region [
22]. Since not all open fields are eligible for the installation of wind turbines and PV power plants, land eligibility analysis based on geospatial data is an essential step before analyzing RES potential.
GLAES is based on Python 3 language, providing a simple and efficient way to analyze land eligibility using the Prior datasets [
17]. Ryberg et al. [
23] examined more than 50 literature sources that independently conducted a land eligibility analysis for prevalent variable RES technologies, documenting the approaches and frequencies used in defining the criteria. In this study, 28 typical criteria were identified, which included the distance from settlements and the distance from airports. Meanwhile, depending on the underlying motivations driving their exclusion, the identified criteria were divided into 4 distinct groups, namely, socio-political, physical, conservation, and economic. The typical criteria were further subdivided into multiple sub-criteria, for example, the exclusion distance from settlements is different for urban and rural areas. Finally, a collection of standardized datasets called Prior was developed [
23], which defined common criteria related to variable RESs in the European context. There were 46 Priors in total, each of which represented the values of a criterion or sub-criterion across Europe and was georeferenced using the EPSG:3035 spatial reference system at a spatial resolution of 100 m by 100 m. The comprehensive documentation of each Prior dataset can be found in the work of Ryberg et al. [
23]. The GLAES model and the Prior datasets can be obtained via GitHub [
17]. The threshold of each criterion in the Prior datasets was determined considering the social, technical, environmental, and economic factors specific to Greece. These are discussed in
Section 2.4 and
Section 2.5.
2.4. Onshore Wind Land Eligibility Analysis
For onshore wind energy, there are already many studies on land eligibility considering different series of land constraints. For instance, the European Environment Agency conducted a land eligibility analysis of onshore wind only excluding protected areas, such as Natura 2000, and found that the available area for onshore wind is 85.3% in Europe [
24]. In contrast, other studies opted to apply multiple land constraints to analyze land eligibility. McKenna et al. [
25] and Eurek et al. [
26] both selected agricultural areas, settlement areas, protected areas, forests, waterbodies, slope, and elevation as land constraints, although the set thresholds for each exclusion constraint were different. Eurek et al. [
26] found that 40% of the area in Europe is eligible for wind energy. In comparison, in addition to the land constraints mentioned above, McKenna et al. [
25] also excluded buffer areas around airports, harbors, roads, and railways and used higher resolution maps, revealing that 23% of land surface in Europe is eligible. Moreover, Ryberg et al. [
27] reviewed 53 land eligibility studies to develop a set of common land constraints for Europe considering social, technical, environmental, and economic factors and found that the total eligible area amounts to 1,352,260 km
2 for onshore wind turbines in Europe overall, which includes an eligible area of 28,326 km
2 in Greece. However, according to the specific circumstances of different countries, there will be certain differences in the formulated exclusion criteria and thresholds. At the national level in Greece, a Special Framework for the Spatial Planning and the Sustainable Development of Renewable Energy Sources (SFSPSD-RES) was formulated, which in addition to considering common land constraints, such as settlements and protected areas, also excluded archaeological reserves and considered visual factors, i.e., the esthetic impact of wind turbines on the landscape [
16]. Several subsequent studies on wind farms’ site selection in Greece were based on this framework. Tsoutsos et al. [
7] conducted a study in Crete and found that 2517 km
2 on the island are available. Latinopoulos and Kechagia [
8] also conducted a study in the region of Kozani based on SFSPSD-RES, but they excluded the areas where the average wind speed is below 4.5 m/s and the slope above 25%, concluding that there are 550 km
2 available for wind farms.
In this study, the series of land constraints that were used to analyze the land eligibility of onshore wind in Greece are summarized in
Table 1. These were based on SFSPSD-RES, a previous study conducted in Greece [
16], and a generalized land constraints list for onshore wind developed by Ryberg [
27]. In order to estimate the eligible area and distributions for onshore wind turbines, a reference wind turbine was used (
Table 2). The parameters of this reference wind turbine correspond to the Vestas V136 wind turbines available at present and to technology changes for future wind turbines by 2050 [
28].
In the socio-political group of criteria, SFSPSD-RES set a safe distance to urban (population > 2000 inhabitants) and rural (population < 2000 inhabitants) settlements considering noise and safety factors that could cause a negative impact on society [
33]. At the same time, it clearly stipulated that wind turbines must be installed at least 1500 m away from touristic areas. For safety reasons, wind turbines should be installed at a distance from airports due to the possible interference with aviation radar signals [
33]. Also, they should be installed at a certain distance from roads, railways, and power lines, but to reduce transportation and transmission cost, the distance should not be too large [
34]. Land use covers, such as agricultural, industrial, and mining areas, were excluded in related studies of Greece [
6,
7,
8]. However, this work sought to explore the RES potential of deserted and decommissioned lignite mines in Greece without excluding mining sites.
In the physical group, slope was assessed as the third most significant factor affecting the construction of wind turbines in the study of Karamountzou and Vagiona [
10]. Less steep land provides a better access to construct and maintain wind turbines [
33,
35]; therefore, the present study excluded areas with slopes greater than 17°. According to SFSPSD-RES [
16], wind turbines are not allowed to be constructed in the areas of sand, wetland, and woodland and should be constructed 1500 m away from the coastline. Waterbodies and rivers are often covered by protected area status and are important for the functioning of biodiversity and ecosystems [
36]. Therefore, waterbodies and rivers were excluded in this work.
Natura 2000 areas are explicitly excluded by SFSPSD-RES. However, Natura 2000 status only includes protected bird and habitat areas. Considering the cultural environment and heritage protection of Greece, it is worth setting a certain exclusion threshold around protected natural monuments, parks, and landscapes [
8].
In order not to reduce the performance and increase the cost of construction and maintenance of wind turbines, the study by Karamountzou and Vagiona [
10] combined technology and economics to evaluate the criteria, and found four important economic criteria, one of which was slope and the remainder were wind velocity, access distance (the distance from accessible roadways), and connection distance (the distance from a power line). Wind speed is an extremely important factor affecting the operation of wind turbines. The wind turbine starts to work when the wind speed reaches a certain value, at which the wind speed is called cut-in speed [
28]. According to the power curve of the reference turbine, the cut-in speed is 4 m/s [
28]. Therefore, areas where the average wind speed is below 4 m/s were excluded. Under the condition of ensuring that the wind turbines are at a certain safe distance from roadways and power lines, the distance should not be too large, because it will lead to increased construction, maintenance, and electricity production and transmission costs [
9,
35]. Finally, the thresholds for access and connection distance were determined based on the study by Ryberg et al. [
27].
2.5. Open-Field PV Land Eligibility Analysis
While there are many studies on the land eligibility analysis of onshore wind, studies on the suitable construction of open-field PV power plants are relatively few. However, the analysis of land eligibility for PV in open areas adopts the analysis method of multi-criteria exclusion, considering the factors of society, technology, environment, and economy as well as that of onshore wind. A detailed study of European wind and solar energy potential by Ryberg [
28] presented a consilient list that included 26 criteria that could be applied as exclusion constraints to select eligible areas for PV at the country level of Europe and found that the area eligible for open-field PV is 294,851 km
2 in Europe and the eligible area in Greece is 11,740 km
2. Although these established exclusion criteria cannot be fully generalized for open-field PV studies in a specific country, it lays the foundation for related studies in Europe. Based on exclusion criteria listed by Ryberg [
28], Tlili et al. [
37] conducted a literature review [
38] about the areas that need to be excluded for the construction of PV power plants in France and found that the potential area for PV in France is 40,694 km
2. Likewise, on the basis of the general exclusion criteria list, Maestre et al. [
39] reduced the threshold for some criteria, such as adjusting the distance from settlements areas from 200 m to 100 m, because the work was based on a hypothetical framework favorable to Spanish decarbonization goals between 2030 and 2050, and added some criteria in line with Spanish national conditions, such as historical sites. Finally, Maestre et al. [
39] found that there was 143,820 km
2 eligible for open-field PV panels in Spain. Few studies have been conducted on the suitable sites of open-field PV in Greece. Vagiona [
11] conducted a study on Rhodes Island (Greece), which considered 6 exclusion criteria, such as land cover, distance from protected areas, and altitude, based on SFSPSD-RES and showed that nine sites were eligible for open-field PV on Rhodes Island without mentioning the total eligible area. Unlike other studies [
37,
39], the criteria chosen for excluding land constraints in the study by Vagiona [
11] did not include the slope and northward slope factors, which could lead to a poor performance of the PV panels due to shading [
28].
Although the SFSPSD-RES of Greece provides exclusion criteria for wind turbines’ construction and many related studies provide detailed land constraints and thresholds based on this framework, the exclusion criteria for PV power plants have not been elaborated. Therefore, in this section, the analysis of open-field PV land eligibility for Greece was mainly based on some criteria for the site selection of PV parks mentioned in the SFSPSD-RES and the detailed study by Ryberg [
28] on analyzing PV potential in Europe (
Table 3). Meanwhile, as Greece is very close to Turkey and the climate of the two countries is similar, this section combined relevant studies conducted in Turkey to provide a more informative analysis. Additionally, a specific PV panel (
Table 4) was used as a reference to determine the eligible areas for open-field PV power plants and to model their distribution in decommissioned lignite mines. The selected PV panel offered the optimal representation of distribution, while ensuring the highest number of full load hours [
28].
Initially, according to SFSPSD-RES [
16], the construction of PV parks is prohibited in areas of agriculture, wetlands, forests, natural monuments, protected landscapes, national parks, and Natura 2000 areas.
In the group of socio-political criteria, PV power plants should be built close enough to residential areas to provide a better energy demand and lower costs of electricity transmission without affecting the lives of residents [
40]. In order to avoid accidents caused by the reflection of PV panels, based on the study by Vagiona [
10], airports and the surrounding area within 2000 m were excluded. Most studies [
11,
28,
41,
42] considered some land covers, such as operational industrial and mining areas, as exclusion criteria since activities on them can stain PV panels leading to an inefficient performance. The same applies for land eligibility for onshore wind; however, this study did not exclude mining sites.
In physical criteria, in addition to considering the overall terrain slope (steep terrain can increase the construction cost), the northward slope should be also considered, because the self-shading losses of PV panels can be significantly high even with only slightly north-facing slopes [
28]. Constructing PV power plants in high-altitude areas will increase the cost of the installation and transportation of materials; therefore, it is suggested to set the exclusion threshold of elevation to 1500 m [
11]. At the same time, in order not to pollute environmental water resources when constructing PV panels, Ryberg’s study [
28] suggested that waterbodies and surrounding areas up to 500 m should be excluded. According to several studies [
41,
42,
43], since the topography and geological structure of sands and beaches are not suitable for constructing PV power plants, sandy and beach areas were excluded.
Two significant economic indicators were considered in this section, namely, connection distance (the distance to power lines) and access distance (the distance to accessible roads). It is suggested that PV power plants should be constructed as close as possible to power lines, since the farther away from the power lines, the higher the cost and loss of electricity power transmission [
44]. Moreover, it is also necessary to ensure that the distance between PV stations and the road is not too far, because a longer distance will increase the transportation cost of construction and the cost of operation and maintenance [
45]. In summary, based on Ryberg’s study [
28], this section excluded the areas that were more than 20 km away from power lines and more than 10 km away from roads.
2.6. Deserted Lignite Mine Potential
According to the NECP, the Greek government has set a goal to completely eliminate coal-fired power generation in the country by 2028 [
46]. Based on this plan, some lignite power stations, such as Kaida I and II as well as Amyndeo I and II, have been shut down since 2019 [
47]. However, for Greece, a country long dependent on lignite for power generation, such an ideal and complete energy transition is difficult. In order to stabilize the electricity power supply system for a future entirely powered by natural gas and RESs, five lignite-burning power stations in Agios Dimitrios, Meliti, and Megalopolis are scheduled to have their operations extended to 2025 by the Public Power Corporation (PPC) in Greece [
48]. It is obvious that the phasing out of lignite power generation is inevitable based on the energy transition plan of the Greek government. In the context of the shutdown of lignite-fired stations, lignite mines will be decommissioned, with potentially large areas without conservation value becoming available for other uses, such as renewable power generation. Meanwhile, there is an excellent connection to the power grid at these locations, which can be used by the newly installed PV parks. Therefore, it is important to analyze the RES potential of former lignite mines in Greece.
In this section, a lignite mining site in Megalopoli (
Figure 2) and two lignite mining sites in Ptolemaida (
Figure 3) were chosen as the study areas. The Ptolemaida Mine of Western Macedonia located in the northern part of Greece is the largest lignite mine in the whole of Greece, followed by what is the largest lignite mine in the Peloponnese Peninsula of Southern Greece, the Megalopoli Mine [
49]. Due to the extension of operation at the Megalopoli lignite power station until 2025 and the expected closure of the Ptolemaida lignite power station in 2028, lignite mines in both regions are still in use. However, with the mandatory and inevitable shutdown of lignite power station in Megalopoli, the adjacent mine will be decommissioned in parallel, since lignite is not suitable for long-distance transport.
Without excluding mining sites using GLAES, this section aimed to explore the wind and solar energy potential in the mining areas of Megalopoli and Ptolemaida combined with WPD and GHI. Meanwhile, a reference wind turbine (
Table 2) and solar panels (
Table 4) were used to assess the renewable energy potential of these open-pit mines after their decommissioning. The separation distance for wind turbines was based on 8D × 4D, where D represents the rotor diameter of the turbine. The separation distance for PV parks was 1000 m.