# Quo Vadis Solar Energy Research?
## 1 Introduction
Bibliometric analysis has been traditionally applied to social sciences and humanities. Nevertheless, its application to engineering topics and energy research is of great interest to discover connections between different institutions, collaborations among countries, and to obtain a wider picture of research trends in the field of study. They also allow identifying hot emerging topics within the research field by analyzing the change in the number of publications and citations for the given keywords. Compared to a traditional literature review approach that is mostly based on scholar experience and institutional access to different editorial repositories, bibliometric analysis benefits from big data extraction and processing tools to get a better view of a research topic and to identify main contributors in the field, as well as trending and hot topics of research [15]. They can also be used to identify gaps in the research field, helping scholars to fulfil deeper studies and reviews to cover that research gap [16]. Bibliometric methods together with networking map layouts are a good complement to perform systematic literature reviews, as they enable one to find connections and research trends by extracting large data contained in whole corpus publication. Bibliometric analysis has been applied recently to investigate energy research topics, both from trends and thematic areas point of view. Some examples of the existing literature are referring to the use of bibliometric mapping to identify areas of research on sustainable supply chain management and how hot topics in that field changed over the last decade [17]. There are studies as well about the global transition to low-carbon electricity systems based on research hot spots and trends analysis for the last three decades [18]. Other bibliometric studies have been focused on the relationship between energy, economy and environment, based on different subject areas and by performing a clustering study to analyze the evolution of most common keywords along two decades to conclude those which were the research trends and hot topics in the field area [19]. Other works analyzed thermal energy storage perspectives based on their most referenced publications [20] and detected the research trends [20]; others studied the relevance of distributed cogeneration of power and heat within an energy management strategy [21] or studied the research contribution of a particular country in wind energy research [22]. Regarding bibliometric studies on solar energy research, some examples analyze the existing solar energy literature between 1992 and 2011 [23], the analysis of solar power-related publications between 1991 and 2010 [24], or the analysis of open innovation in solar energy research between 2000 and 2014 [25]. There are some works about more detailed studies regarding the integration of combined cycles with concentrating solar power plants [26], others referring to the state of the art of solar cooling technologies [27] and the analysis of research tendencies on photovoltaic solar energy management [28]. This paper aims to identify the main areas of research in solar energy that have emerged over the last few decades by analyzing indexed publications metadata. The paper is of relevant interest for scholars and researchers in solar energy to understand the evolution of research hot topics and which new ones have appeared. That analysis is performed by comparing most common research keywords and their interdependence through networking and heat maps over the last few decades. First of all, it is reviewed about the growing number of publications in the solar energy field, which are the most productive countries and which ones are gaining or losing relevance in the international research picture in terms of number of publications and publishing sources. Secondly, changes in thematic areas according to the different research categories are discussed to detect modifications on research interest.