# Quo Vadis Solar Energy Research?
## 1 Introduction  
The increase in energy consumption is a direct consequence of improvements in our living standards, the growing worldwide population and the general increase of nations wealth [1]. However, extensive energy consumption is associated with climate change and global warming, due to greenhouse gases emissions and other pollutants from the massive use of traditional energy resources based on fossil fuels [2]. Climate protection awareness to mitigate energy consumption consequences has led to stringent pollutant emissions legislation and extensive research in energy conversion technologies seeking higher conversion efficiencies. However, also to a global awareness about the importance of energy-saving, energy efficiency and renewable and sustainable energy resources [3; 4; 5; 6]. In that context, solar energy is seen as one of the best alternatives to meet climate change actions, whether for electricity generation utilizing solar photovoltaics (PV) and concentrating solar power (CSP), or for heat generation, whether for the domestic or industrial application using different solar thermal energy technologies [7]. In the last few decades, solar energy technologies have experienced a significant cost reduction through new materials developments, technology improvement and wider commercial experience [8]. Between the years 2010 and 2015, solar photovoltaics (PV) reduced the global weighted average levelized cost of electricity (LCoE) from an average 0.30 USD/kWh to an average 0.13 USD/kWh, mainly thanks to PV panels' new materials and larger operational experience. Predictions estimate that by 2025, installed cost reductions together with O+M costs will decline and the increase in the global weighted average capacity factor will result in 0.055 USD/kWh. Furthermore, the 5 th to 95 th percentile LCoE cost for individual utility-scale solar PV projects could fall to between 0.03 USD/kWh to 0.12 USD/kWh [8]. Cost reduction has contributed to the technology deployment evolving from 40.277 GW nominal power installed worldwide in the year 2010 to 580.159 GW by the year 2019 that represented an increase of 1340% of the installed power [9]. In the case of concentrating solar power, levelized cost of electricity (LCoE) has been reducedfrom 0.346 USD/kWh average in the year 2010 down to 0.182 USD/kWh average in the 2019 year. This represented a reduction of 47% during the last decade, while the auction database for 2021 reflects LCoE of 0.075 USD/kWh [10]. Regarding power installation, its fast deployment in 2008-2014 was slowed down due to policy changes in relevant CSP countries such as Spain [11]. However, CSP installation has sped up again during the last few years, with the large projects in the Middle East and North of Africa region leading to 6.275 GW of total installed capacity by 2019 compared to 1.266 GW of installed capacity by the year 2010, what represented an increase of 396% in the last decade. Furthermore, a large interest in CSP technologies has arose based on their potential for large thermal energy storage that enables thermal energy harvesting and electricity production being decoupled. This is offering a cost-competitive solution for large energy storage, which is contributing to grid stability and the deployment of non-dispatchable energy resources [12; 13]. Cost reductions on solar energy technologies and their growing relevance in current energy and environmental context have been supported by extensive research that can be found in scientific journals, conference proceedings, books and other materials. Based on a large number of publishing sources and the existing materials on this topic, a systematic methodology is required to analyze this big data and to derive research trends and conclusions from the study of all those publications. One of those methodologies is known as bibliometric analysis and consists of the statistical study of the existing publications in a field by using networking maps and semantic analysis [14].