**6. Discussion**

On one hand, the EU members are obliged to increase the share of renewable sources in the total energy consumption, but on the other hand, they have certain freedom in shaping the energy policy and selecting the sources according to their own possibilities. This is why the EU countries differ significantly in terms of the types of renewable sources used to produce electricity. The presented data clearly show that all EU countries are increasing the share of renewable sources for electricity production, which is in line with the guidelines contained in EU directives. Furthermore, the diversity of these sources is increasing, which is indicated by the decreasing values of the Gini coefficients (for the vast majority of countries). It is also worth noting the fact that the EU-15 countries have already started this transition process at the beginning of the 21st century, the new EU member states followed only after their accession.

In 2005, at the beginning of the analyzed period, it is noticeable that the energy produced by hydroelectric power plants is of the highest importance in the production of electricity from renewable sources. Its share in the production of renewable electricity (*GEP\_RES*) accounted for over 80% and is recorded for as many as half (14) of the analyzed countries, and in the case of 21 countries, the share is greater than 50%. During the next 14 years, the importance of hydroelectric power plants in the production of electricity did not increase, although this method is said to have a high potential [96].

Although hydropower has an established position in the production of electricity and belongs to the so-called renewable sources, the amount of electricity produced by this method (in GWh) in the scale of the entire EU (EU-27) increased by only 1.4% in the period 2005–2019. In 15 countries (out of 26 analyzed), its production is even reduced (Malta and Cyprus are not included in this list, as, in 2005, electricity was not produced by this method in these countries). This is justified by the fact that this type of electricity production is not environmentally neutral [97–99], and the degree of its impact depends on the scale of the production [96,97]. There is little chance of a large-scale hydropower plant in the European Union, mainly due to the fact that most of the areas have already been taken into use [96]. Nevertheless, hydropower plays an important role in providing flexibility to the electricity system [100]. It is indicated that the technology used in the hydropower plants allows meeting sudden fluctuations in supply or demand of other renewable sources, such as solar and wind power. Therefore, the EU support hydropower innovation. It is worth noting, the EU legislation stands in the way of a freer choice of RES and restricts the development of certain technologies, including hydropower (e.g., References [99,101]). In 11 countries, the increase in production by this method is mainly not significant. Only five countries recorded an increase in production by more than 15%: Portugal (increase in production by around 100%), Slovenia (by around 35%), Spain (16.7%), Ireland (16.1%), and Lithuania (by 15.6%). It is worth mentioning that, in these countries, in 2005, hydropower accounted already for a significant share in *GEP\_RES*. In 2019, these shares decreased significantly due to the fact that these countries have been developing other technologies for obtaining electricity from renewable sources to a greater extent. Only in Slovenia, in 2019, the share of hydropower in the generation of *GEP\_RES* stayed significant (89.3%), and in the other countries mentioned above, it became less than 36%, while in Ireland, it was less than 10%. In Portugal, the largest increase in hydropower production is recorded, as it is a country with one of the highest possible potentials to exploit this area [99]. In addition, in 2007, the Portuguese governmen<sup>t</sup> approved the National Program of Dams with High Hydroelectric Potential [99]. As previously mentioned, the development of hydropower may be restricted by the EU legislation (e.g., see Reference [101]), due to negative environmental effects. However, as emphasized by Reference [102], the acceptability of the side effects of RES in terms of benefits related to climate protection and socioeconomic benefits lies with the national policy pursued by states as part of the development of RES and environmental protection. Portugal, as one of the few countries, has decided to invest in this type of energy on a large scale, as the contractor (Iberdrola, Bilbao, Spain) has been awarded €650 million by the European Investment Bank [103] for the expansion of the hydroelectric power plant in Portugal. This does not confirm the thesis by Reference [96] that indicates that only small projects (generating a capacity of no more than 10 MWh) can count on support from EU bodies. In the case of Slovenia, the work of Reference [104] indicates that energy needs will be best met by a mix of nuclear, water and gas technologies. The only source of renewable energy in this list is hydropower. Therefore, it is not surprising that the energy policy in Slovenia also focuses on the development of this technology. Another study [105] considered many criteria related to electricity generation, such as environmental protection and institutional—political, economic, social, and technological. The researchers indicated that hydropower, biomass, and nuclear power are the most effective RES investments. Hence, it is not surprising that the production in hydroelectric power plants increased by over 15%. However, this potential is not fully used in Lithuania. Lithuania has one of the most restricted environmental regulations related to the introduction of this type of technology, even to a small extent [106].

Wind energy is gaining importance in the renewable energy mix used for electricity production in the EU. The data we present in this paper confirm a significant intensification of electricity production, using wind during the analyzed period. Currently, it constitutes the largest share in the production of *GEP\_RES* (36.5%). The amount of electricity produced in this way increased in the period 2005–2019 by almost 440%. This is not a surprise, as the literature indicates this technology as the most competitive compared to other RES [13]. That competitiveness is examined by using a balanced scorecard based on four types of variables: the perspective of the consumer, internal business processes, the development aspect, and the financial aspect. Kapitonov and Voloshin [13] describe the advantages of this technology as "the cost of electricity, safety, minimum possible power, productivity, and performance development aspect and financial aspect". It is worth mentioning that, as in the case of hydropower, also wind energy can affect the natural environment. Wang and Wang [107] and Pecesila et al. [77] mention the following effects: noise pollution, change the landscape, and impact on local to regional weather and climate if the area of turbines is large enough, and it may affect the local populations of various species of birds. Nevertheless, this type of energy is indicated as the least harmful to the environment [108]. Therefore, it is strongly supported (in addition to solar energy) by the EU bodies as a mean of achieving the sustainable development goals, and in particular achieving the so-called climate neutrality planned for 2050 (see References [109,110]. At the same time, wind

energy is mentioned as the one to support these goals to the greatest extent [111]. The European Commission notes that, thanks to the pan-European efforts to reduce greenhouse gas emissions, in 2016 (compared to 1990), it succeeded in reducing these emissions by 22% [109]. This is due to the significant increase in the share of RES in energy production, in particular wind and solar energy. It is directly linked to a significant reduction in costs related to the production of solar and on- and off-shore wind energy in the recent years (European Commission, 2018). Currently, offshore wind energy receives particular interest from the EU bodies. A strategy for the development of this type of energy was formulated, an EU Strategy to harness the potential of offshore renewable energy for a climate neutral future [12], referred to as the EU Strategy on Offshore Renewable Energy. This strategy assumes an increase in the Europe's offshore wind capacity from 12 GW (level from 2020) to at least 60 GW by 2030 and to 300 GW by 2050. Additionally, the development of several new technologies (by 2050), such as floating wind and solar, is expected.

Currently, the leaders in wind energy production are (according to data from 2019) Germany (125,894 GWh), UK (64,334 GWh), Spain (55,647 GWh) and France (34,721 GWh). As for the share of wind energy in the production of *GEP\_RES*, the largest is recorded for Ireland (83%), Denmark (70%), Poland (58%), Spain (54%), UK (53%), Germany, and the Netherlands (51%). Countries with shares between 40% and 50% are Lithuania, Belgium, Portugal, Cyprus, and Greece. In the 2019 cluster classification, all these countries are classified into three clusters: #7, #9, and #10 (see Table 6). These three clusters are characterized by a high share of wind energy in *GEP\_RES*, and they are differentiated by significant shares of other sources. The UK, Sweden, Denmark, and Ireland are considered the most efficient countries in terms of wind energy use [112]. It is worth noting that the countries from the abovementioned clusters have favorable conditions for the development of this type of energy because they are either large in terms of area or have the possibility of developing offshore wind farms. At the same time, the countries using offshore wind energy are considered to be the most effective [112]. Therefore, large-scale investments are being made in many countries to develop offshore wind farms. An example is the support system for the construction of offshore wind farms in Poland, which is approved by the European Commission in May 2021 [113]. According to the assumptions of the Polish energy strategy, offshore wind farms are to be the main pillar of the energy system in Poland. Government support systems for offshore wind energy can be found also in the UK ([114]), Ireland ([115,116]), and Denmark ([117]).

Another renewable source used for electricity production that has gained in importance in recent years is solar energy. In the EU-27, the production of this type of electricity increased from 1458 to 125,717 GWh, i.e., by over 8500%. Solar technology is relatively new and in 2005 it is the least used resource for the production of renewable electricity (its share is only 0.3% in *GEP\_RES*). However, in 2019, it is the third most important source, and its share in *GEP\_RES* is 12.5%. The increase in popularity of this source may be due to several reasons. Firstly, solar energy is the second technology, after wind energy, considered the most competitive in the group of renewable energy technologies used for energy production [13]. Secondly, the development of this technology has contributed to a significant reduction in the cost of electricity production, and therefore it will continue to be of interest to the EU bodies as a technology to be supported. As such, more investments are planned for its further development, e.g., in the form of offshore solar energy (floating solar panels) (European Commission, 2020). Thirdly, this type of energy is included in the EU strategies for reducing CO2 emissions and ensuring energy security for the EU area.

There are different technologies for using the sun to produce energy [118]. Eurostat's data for gross electricity production include two technologies: solar photovoltaic and solar thermal, with electricity production using the latter in 2019 only in Spain (its share was 5.5% in *GEP\_RES*).

The environmental impacts are discussed at the level of the photovoltaic panels' production technology [118]. The first issue being the use of allium arsenide or cadmium telluride to produce more energy-efficient photovoltaic panels. In the event of a leak,

those compounds are said to not be harmful to the environment. Silicon used for the production of photovoltaic panels is said not to be harmful to the environment, however characterized by relatively lower energy converting efficiency. In the case of a technology called concentrated solar power techniques, coolant and lubricant are harmful, also in the event of a leak. The methods of neutralizing the harmfulness of these substances are included to be considered in the further development of these technologies.

The country which, in 2019, produced renewable electricity almost entirely by using solar technology was Malta (97% share in *GEP\_RES*). It is also the only country where this share is greater than 50%. Therefore, in the 2019 cluster classification, it is assigned to a separate cluster. The second country in this respect is Cyprus, where 42.7% of *GEP\_RES* is produced using this technology. Cyprus is classified in one cluster together with Germany, and this is due to the similarity of the use of other sources at a similar level (wind and biogases). The third country in this ranking is Hungary with 31.9% of solar energy in *GEP\_RES*. For five other countries, the share is greater than 20% (but less than 30%): Greece, the Netherlands, Belgium, Czech Republic, and Italy. Belgium and the Netherlands (cluster #7) have similar levels of solar energy use, but this is not surprising, as these countries are adjacent to each other and have similar climatic conditions for using RES to produce GEP.

All in all, the increased importance of solar energy in the production of electricity is recorded in many countries. However, only in the case of Malta is it considered as the main source. In other countries, where this share is also significant (but not leading), this source can be described as complementary to the energy mix.

The paper of Reference [119] presents the possibility of developing solar technologies for energy production with respect to geographic location. Without a surprise, the countries of Southern Europe are characterized by the greatest potential. By comparing these results with those presented in this paper, it can be concluded that so far only few of the EU countries are developing their solar energy potential. These are Malta, Cyprus, Italy, and Greece, as well as Hungary. On the other hand, Spain, Portugal, and Romania are examples of countries with significant potential for the development of this technology, but only to a limited extent (favoring wind and hydropower).

It is worth noting that the use of solar energy is quite important in countries that have not been previously named as those with such a high potential in this particular technology. These are the Czech Republic, Germany, the Netherlands, and Belgium. In their electricity mix, the share of solar energy accounted for at least 18%. In the case of the Czech Republic, support systems, which Reference [120] define as generous, are of grea<sup>t</sup> importance for the development of photovoltaics. They see this as the cause of the massive boom for the construction of the photovoltaic power plant in the period of 2009–2012. However, it is pointed out that these systems are only slightly in line with the potential of solar resources in the Czech Republic. Furthermore, in Germany, support systems and national strategies for the development of solar energy played a significant role in its development, despite the limited domestic potential (compared to the countries of Southern Europe). In the case of Germany, the most important factor is the feed in tariffs (preferential tariffs) [121].

Biofuels, biogases, and renewable municipal waste are collectively classified under the biomass category. Currently, it is estimated that biomass contributes as much as 60% to total renewable energy production in the EU, including electricity, heat, and energy used in transport [122]. In the production of renewable electricity, this share is lower, at the level of 15.9%. Thus, in its production, these sources play a smaller role than in the case of other types of energy. Of these three sources, biofuels and biogases are used to the greatest extent. In 2019, biofuels accounted for 8.55% of *GEP\_RES* production, biogases—5.47%, and renewable municipal waste only 1.9%. In the period 2005–2019, there was an increase in electricity production with these sources, by 102.9% and over 580% and 80%, respectively. These numbers show that the use of biofuels is already well established in the production of electricity in the EU, while the importance of biogas has grown significantly.

Biofuels used for electricity production according to the Eurostat category are divided into primary solid biofuels (fuelwood, wood residues, wood pellets, animal waste, and vegetal material), pure biodiesel and other liquid biofuels. The latter two are of marginal importance. Solid biofuels are used to the greatest extent in heating energy, but they have also gained popularity in the production of electricity. In this matter, biofuels are most often used in Estonia (share of 58.8% in *GEP\_RES*), Finland (38.5%) and Hungary (37.7%). Estonia and Hungary in the 2019 classification are classified as single-element clusters, due to, among others, such a dominant share of biofuels in the energy mix used for the production of electricity from renewable sources. Furthermore, Estonia is the only country among the analyzed countries where biofuels are the basis for the production of *GEP\_RES* (share greater than 50%). Finland, despite such a significant share of this source, was classified in the second cluster, as in addition to biofuels, hydropower, and wind power are used to a large extent. Other countries that use this source to a large extent are Poland (24.6% shares in *GEP\_RES*) and the Czech Republic (21.4%). For the following nine countries, this share ranges from 10% to 20%: Bulgaria, Denmark, Latvia, Belgium, Slovakia, the Netherlands, Sweden and Lithuania, and the UK. All in all, apart from Estonia, for the other countries mentioned, biofuels complement the portfolio of renewable sources used in electricity production as one of three (or four as in the case of the Czech Republic and Bulgaria) sources. The presented results also indicate that this is the source typical for countries located mainly in the northern part of Europe, where solar energy does not have as high potential as in the case of southern countries. Bulgaria is the southernmost country on this list, followed by Hungary. In the case of Hungary, the share of biofuels has significantly decreased—from almost 85% in 2005 to almost 38% in 2019 It is due to the fact that nowadays in Hungary more use is made of other sources (solar and winds). The production of electricity from biofuels in this country in the analyzed period slightly increased—by about 12%.

Biogases are reported to be as less common source than biofuels. They are used to the bigger extent only in the Czech Republic (22.5%) as well as in Germany, Cyprus, and Latvia (share between 11% and 14%). While in the case of the Czech Republic, Germany and Cyprus, the share of biogas in the energy mix distinguishes them from other countries, in Latvia, greater shares of hydro and biofuels meant that it was assigned to cluster #2.
