**2. Literature Review**

The importance of renewable energy in the energy mix is increasingly recognized. Traditionally, literature reviews of the papers devoted to the issue have been based on the problem of sustainable development (SD) and strategies or policies of achieving it in the context of international or national goals [26,27]. SD is today a well-established term both in all territorial perspectives as it had been introduced already by 1970 by the United Nations and the most common definition was proposed by the Brundtland Commission of 1987 and the interdisciplinary discussions that followed the publication of *Our Common Future* [28–30]. They allowed SD policy to be distinguished from environmental policy as it can only be achieved by integration of environmental, social, and economic concerns and objectives, and it has given a fresh impetus for advancing both theoretical and practical aspects of SD. It became a guiding institutional principle and a policy goal, implemented also in the EU.

The advantages of renewable energy for the environment are undeniable and widely discussed in the world literature as RES are usually seen as an element that will contribute to mitigating climate change and the opportunity to achieve also other sustainability targets

as RES technologies can facilitate social and economic development [31]. Therefore, there are also analyses concerning issues related to climate change prepared for the EU [32] and the CEE countries [33]. In order to address the sustainability challenges the considerations connected with environmental concerns are not only concentrated on ecosystem complexity but accompanied by social and economic impacts of RES in territorial [34] or general perspective [35]. As mentioned, energy policy is often perceived also in the energy security context [36–38] also as RES has the potential to improve energy security by making the system more resilient to disruptions.

As the energy sector is often seen as a key to economic development, there are numerous studies on the link between RES and various macroeconomic variables [39,40]. They often focus on the impact of RES on economic growth. In their studies, Ohler and Fetters [41] identify a two-way relationship between aggregate renewable energy and real GDP. Simultaneously, they argue that biomass, hydroelectric, and waste electricity generation have the largest impact on real GDP in the long-run. The findings of Kula [42] also support the existence of a long-term balanced relationship between renewable electricity consumption and GDP, while Anwar, Arshed and Kousar [43] argue that the impact of renewable electricity consumption on economic growth is positive and significant. As for measuring SD it is necessary to go beyond the quantitative indicators, a set of aggregate indicators of SD is usually suggested and widely discussed in the literature [31,44,45]. They have gained a lot of criticism due to the arbitrary character of procedures used in their construction, but one of the advantages of the development of such indicators is that they allow international comparability.

An international perspective is important here as issues related to climate change call for cross-border cooperation. The cooperation in the EU connected with Green Deal described above is important but in terms of the expansion of renewable energy is also significant and results in a possible increase in the efficiency of individual countries. Nevertheless, the studies show that the majority of European countries are ye<sup>t</sup> to pursue any cooperation and wish to achieve the expansion objective in terms of renewable energy primarily through expansion within their own national borders [46,47]. On the other hand, history shows that new member states' energy security meant the pursuit of the highest self-sufficiency and independence compared to other entities. For several years, these countries have been transforming their economies at a different pace, with the aim of the marketisation of economic processes. Many changes are underway in the industry area, which is still energy-intensive and dependent on primary energy sources. Although transition economies are trying to become greener through the reduction of greenhouse gas emissions, they are doing so at different rates. Some of them, such as Estonia, Latvia or Lithuania, have undergone a drastic transformation, whereas others, such as Hungary or Slovenia, are making changes at a slower pace [48]. As noted by others, the large area of forestry land in Central Europe makes forest biomass and waste from the wood processing industry (sawdust, chips) or in a processed form (pellet, briquette) suitable for energy purposes, while the involvement of agriculture in building bioenergy production capacity, in particular the development and distribution of multiannual crops and their potential in new member states countries, is crucial for bioenergy in the long term [49].

The development of RES brings about challenges in terms of how to run energy policy as countries with a diversified level of RES offer incentives to advance RES [31] and obtain goals set at international, national, regional and local level. It must be more stable and, thus, more credible and clearer to citizens. This is the EU energy policy that is characterized by a high degree of social involvement in the ownership, management, and benefits of energy projects [50,51]. In addition, econometric analysis reveals a positive correlation between subsidies and the generation of motivated energy as well as installed capacity [24,52]. Bersalli, Menanteau and El-Methni [53] showed that promotional policies have a positive and statistically significant effect on RES investment.

Papers addressing the issue of renewable energy sources in the context of Europe mainly focus on the European Union [54–56]. Few of them focus on the uniqueness of Central and Eastern Europe countries [23], for which, for historical reasons, it is a challenge to change the regulatory system in order to allow entities not controlled by their governments to produce and transmit electricity. Recently, Pakulska [57] proved that for most of the CEE countries transformation towards a climate-neutral economy is challenging task. Papers focusing on the new member states countries often address a single country in terms of renewable energy for the near future [58,59] or a specific renewable energy source of a selected country [60].

What is proved in the research presented above is that energy transition, aimed at replacing conventional energy sources with RES, what is more, is a complex process that depends on many factors, and the reasons for the uneven distribution of energy production from RES in new member states countries are diverse and include social, economic, and environmental (like issues connected with climate, hydrology and geology). The results also support the need to conduct a study that should focus not only on the absolute values of renewable energy, but also on other factors that may shape the volume and structure of such RES production in the future. Therefore, such an approach to this analysis appears to be fully justified.

The analysis of new member states countries in this study fills a gap in the existing literature and contributes to the discussion on the European Strategy 2020–2030 for the whole EU and especially for the countries of the region of Central and Eastern Europe—new member states. The approach adopted in this paper gives a broader view and considers the uniqueness of each new member states countries.
