**1. Introduction**

The consumption of resources in the last century has experienced an exponential growth in all fields, as indicated in the Special Report on Climate Change and Land of the Intergovernmental Panel on Climate Change (IPCC) [1], boosted in a synergetic cycle by the exponential increase in population, growing up from 1 billion at the beginning of the 20th century to the current 7.7 billion, with perspectives of reaching between 9.5 (the most optimistic scenario), to 12.5 million in 2100 (the most pessimistic) [2]. The IPCC report clearly outlines that the increase in Greenhouse Gas (GHG) emissions (mainly CO2, N2O, CH4, O3, CFC, H2O) produced by human activity are responsible for the acceleration of the current climate change. The IPCC have estimated an average increase of 1.5 ◦C as a safe limit to avoid catastrophic and irreversible global changes for the planet. Above 2 ◦C, the consequences can have unpredictable effects on life on Earth [3]. Primary energy consumption continues expanding (1.3% last year) [4]. This process has also intensified the generation of solid, gaseous, liquid and radioactive wastes [5]. This path has brought us to the record level of GHG in the history of the planet, raising from the 300 CO2 ppm maximum historical level to more than 415 ppm nowadays [6].

The European Union (EU), signatory of the Paris Agreement, assumed a leadership role in promoting measures to restrict it to 1.5 ◦C [7]. European policies have been establishing frameworks for action, first until 2020 and then for 2030 [8]. The European Green Deal presents an action plan to make the EU's economy sustainable [9], and a proposal for the first European Climate Law (EUR-Lex, 2020) establishing the framework to achieve climate neutrality and amend Regulation (EU) 2018/1999 [10]. The Draft Law 121/000019 on Climate Change and Energy Transition submitted in May 2020 in Spain, aims to achieve, by 2050, climate neutrality and an electricity system based exclusively on generation of renewable sources [11].
