**2. Literature Review**

The use of renewable energy sources has been determined by legal regulations. They relate to legal conditions on the global and European Union (taking the form of Directives) levels and to individual solutions introduced by the Polish regulator. Let us mention some of them:


Renewable energy sources have been governed by legal regulations for many years. At the European Union level, these include:


Furthermore, it is worth noting the proposed changes in the regulations.

The current Renewable Energy Directive was published in the EU Official Journal only at the end of 2018. The European Commission has now signalled that changes are needed to it to bring it in line with the European Union's new, more ambitious CO2 reduction targets. The provisions that appeared in it were supposed to lead to a 40% reduction in CO2 emissions by 2030 compared with 1990 emissions. However, now, according to the EU, emissions reductions should be increased by 55%. This means adapting EU legislation. Moreover, EU officials stress the need to introduce additional criteria for the use of forest biomass.

The ongoing consultation will contribute to a regulatory change, which will undoubtedly be linked to EU countries' actions, aimed at further developing and increasing the use of renewable energy sources. Renewable electricity is a pillar of the transition to sustainable development, implemented through climate and energy policy strategies, and the European Green Deal provides a potential investment blueprint for this new phase of development [14]. Halkos and Gkampoura [15] analysed fossil fuel and renewable electricity generation and examined their links to CO2 emissions and economic growth in 119 countries around the world with different income levels. The publications by Szopik-Depczy ´nska et al. [16] and Szopik-Depczy ´nska et al. [17] are the examples of articles dealing with broader aspects of the implementation of the sustainable development goals, taking into account the role of RES. The problems of energy transformation, including the development of the use of renewable sources for electricity production, were studied by Pao and Fu [18] on the example of Brazil, by Lin, Liou, and Chou [19], who compared Taiwan and Japan, and by Pietrzak, Igli ´nski, Kujawski, and Iwa ´nski [20] in relation to Poland.

Recently, there have been a number of publications relating to various aspects of renewable energy generation and consumption in European Union countries, as well as the question of energy self-sufficiency in these countries.

A wide spectrum of motivations to switch to environment friendly green energy was described by Grosse [21] and Pietrzak, Igli ´nski, Kujawski, and Iwa ´nski [20] in relation to Poland, by Chovancová and Tej [22] from the point of view of four countries of the Visegrad Group, by Zielenkiewicz [23] and the Institute for European Environmental Policy [24] in the context of all the EU countries. The use of RES in the EU countries from the perspective of fuel and energy consumption data was presented by Piekut [25]. The share of renewables in power generation in EU countries was analysed by Yu et al. [26], in the context of both investment, production, and consumption aspects of power generation. The authors highlight the significant progress in the penetration of renewables in Western Europe against the background of Eastern Europe making progress, but much less within the EU. At the same time, they point to the significant, as ye<sup>t</sup> untapped potential for renewable development across the EU [26]. Markandya, Arto, Eguino, and Román [27] examined the impact of low-carbon technologies on the employment in the EU countries Kacperska, Łukasiewicz, and Pietrzak [28] used the clustering method to assess the use of renewable energy sources in EU countries with a focus on the Visegrad Group, i.e., the Czech Republic, Hungary, Poland, and Slovakia.

Simoes et al. [29] produced six climate projections to calculate indicators of future wind, solar, and hydro power generation capacity, as well as temperature impacts on electricity demand for heating and cooling for each EU Member State. The projected indicators for climate-dependent renewable energy sources showed relative stability in the ability to meet renewable energy production and emission reduction targets across the EU, but at the same time, significant variation at the level of individual member states, especially for wind and solar energy.

Mehedintu et al. [30] investigated the evolution and forecasting of renewable energy consumption in EU countries with a special focus on Romania, also estimating the development of renewable energy use in the energy sector. Incentives and barriers to renewable energy consumption in twelve net energy importing EU countries were presented by Marra and Colantonio [31] using panel vector autoregression, focusing on socio-technological rather than economic aspects. Chakraborty and Mazzanti [14] in a panel analysis of OECD countries showed the existence of a moderately significant positive long-run relationship between renewable electricity consumption and economic growth. According to them, economic growth per capita is a causal factor for total electricity consumption as well as for fossil electricity consumption. The links between energy transition and economic growth were addressed in their articles by Marinas, , Dinu, Socol, and Socol [32], who examined Central and Eastern European countries, by Belke, Dreger, and Dobnik [33], who examined OECD countries, and Overland [34] in the broader context of globalisation processes.

Remeikiene et al. [35] evaluated the progress in the development of the use of renewable energy sources in European Union countries for construction purposes by grouping countries according to a set of characteristics. They showed that the more developed EU countries use renewable energy sources to a greater extent than the others. Aklin [36] pointed out the danger of EU households associating the impact of renewables aggressively promoted by policymakers with the rising cost of purchasing electricity, resulting in declining public support for renewables.

The relationship between generation and consumption of electricity leads to a surplus of electricity for export or a shortage of electricity that has to be imported. The relationship between dependence on energy imports and carbon emissions in EU countries was investigated by Percebois and Pommeret [37], who showed that the best combination of low-carbon electricity production and independence from electricity imports is achieved by countries with a significant share of hydro and nuclear generation.

Much space in current publications is devoted to the proper functioning of power grids, which is necessary for the storage of electricity and the managemen<sup>t</sup> of its surpluses and shortages. The issue of the cost of managing the stock of renewable energy produced is often raised.

Schreiner and Madlener [38] made a macroeconomic assessment of planned investments in electricity grid infrastructure in Germany. By conducting a static input–output analysis, they showed how the multiplier effects of grid investments affect macroeconomic outcomes—in terms of output, value added, employment, and fiscal revenues. They highlighted the importance of the planned significant increase in investments in electricity grid infrastructure as an important element of a sustainable energy transition in Germany. These investments are intended to ensure the flexibility of the electricity system, which is increasingly based on renewable energy sources.

Hiesl, Ajanovic, and Haas [39] highlighted the problems of long- and short-term storage of solar and hydropower and energy from other renewable sources in the countries of the European Union. According to the authors, managing surplus energy from a wide range of renewable sources requires a flexible and case-by-case approach to choosing the form of energy storage. This, however, implies potentially higher storage costs, limiting the profitability of production and encouraging the search for a mix of renewable energy sources that is optimal mainly from an economic rather than an environmental point of view.

The problem of managing surplus energy from renewable sources, in this case solar and wind, in the UK was studied by Cardenas et al. [40], concluding that providing

adequate energy storage capacity from these two sources would require an investment equivalent to 7% of the country's GDP. Aqachmar et al. [41] performed an in-depth analysis of solar power generation technology options taking into account the environmental and financial performance of each option, creating global rankings of solar power generation. An important conclusion of their analysis is that many countries are not using their solar power production potential due to technological limitations, especially if the solar technology applied is accompanied by too high water consumption [41].

The prospects and cost effectiveness of combining biomass liquefaction with photovoltaics for energy storage and electricity production were presented by Perkins [42]. He concluded that the levelized cost of electricity (LCOE) of such production can be competitive with solar production combined with lithium-ion batteries in certain situations. This is another example of technical solutions to increase the economic viability of renewable power generation [42]. Based on the numerical data on solar modules, De Negri, Pezzutto, Gantioler, Moser, and Sparber [43] indicate a significant relationship between technological progress, the development of electricity generation devices, and their ever lower price, which translates into the universality of use and the amount of energy obtained.
