# A Roadmap for the Integration of Active Solar Systems into Buildings
## 1 Introduction  
The first oil crisis in the early 1970s forced the building industry to a sudden awakening, and the development of a concentrated effort, in order to reduce building energy needs, that focused initially on the active systems of the building, with the passive approach to the subject following in the mid-1970s [1]. It is interesting, however, that the reckless use of fossil fuels for energy production that led to the aforementioned environmental problems and oil crisis took place while Europe's average Global Horizontal Irradiance (GHI) was about 1200 kWh/m2/year [2]. Considering the above, it became clear that the use and exploitation of renewable energy sources, especially solar energy, during architectural planning is one of the most important parameters a building needs to meet in order to be considered viable. On the other hand, the direction towards solar energy may be the only viable proposition, since it is considered to be the alternative energy source with the greatest potential to reduce the dependence on fossil fuels [3; 4; 5]. At the same time, the European Union (EU)'s Climate Action and, in particular, the 20-20-20 program that is currently active, sets targets for a 20% reduction in EU greenhouse gas emissions compared to 1990 levels, for a 20% improvement in the EU's energy efficiency, and for 20% of the EU energy share to be produced from renewable sources [6]. Regarding the post-2020 era, the European Commission has already set new targets for the year 2030, which aim at a 40% reduction in the EU's greenhouse gas emissions compared to 1990 levels, a 27% improvement in the EU's energy efficiency and for 27% of the EU's energy share to be produced from renewable sources [7]. The Commission is also committed by 2050 to reduce its greenhouse gas emissions by 80-95% compared to 1990 levels [7]. Given that buildings are responsible for 40% of the total primary energy needs in the EU [8], the development of effective alternative energy solutions for buildings is imperative. This is complemented by the Energy Performance of Buildings Directive (EPBD) [9], as well as by the Directive (EU) 2018/2001 of the European Parliament and of the Council on the promotion of the use of energy from renewable sources [10], which require Renewable Energy Sources (RES) to be actively promoted against the conventional fossil fuels that are used in buildings. The increasing role and incentives given to renewable energy sources through the European legal framework, and the fact that solar energy accounted for the 21.9% of the primary renewables production in 2017 in the southern EU countries [11], mean that Solar Thermal Systems (STSs) and Photovoltaics (PVs) will play a key role in buildings, as they contribute directly to their heating and cooling loads, as well as to their supply of electricity and hot water [8]. At the same time, other more specific solar systems, such as polymethylmethacrylate (PMMA) fibers [12] and heliostats for the concentration of solar energy [13], could also contribute in that respect. Thus, the integration of active solar systems into buildings is coming to the fore, since, according to [PERSON] [14], buildings should be able to exploit solar energy for energy generation through the integration of active solar features. This results in an effective contribution to the reduction of energy needs in the building sector, through onsite production of energy [8]. The key role of building-integrated solar active systems in the architectural design of buildings is increasingly becoming evident, especially in countries where high values of annual solar energy are recorded [15; 16]. Based on the above, and on the fact that the European Union's legal framework encourages member states to require the use of energy from renewable sources in new and existing buildings [9; 10] in their building regulations and codes, several countries have included in their legislation an indirect obligation to integrate active solar energy systems in buildings.