Emmanuel Hache
email: emmanuel.hache@ifpen.fr

Angélique Palle

Renewable energy source integration into power networks, research trends and policy implications: A bibliometric and stakeholders survey analysis ☆

Keywords: Variable renewable energy, bibliometric analysis, scenario, survey, power network, policy JEL Classification: Q42, Q48, Q55

This article studies the integration of variable renewable energy sources (RES) into power networks. The main goal is to confront the contents and trends of scientific literature with the eyes and projects of researchers on future topics and issues to be solved, especially in terms of modeling of electrical systems. The analysis relies on a bibliometric study of the Scopus database on the topic and on an online survey sent to the corresponding authors of the identified papers. The paper analyses the dynamics of publication, clusters of collaboration and main studied topics. It then identifies potential research leads, among which unresolved challenges regarding technical aspects, markets and financing issues and social aspects. The paper concludes on the policy implications of the mentioned results. The disparity of models and results is still a necessary evil as research is not mature enough to integrate in one model all the very complex parameters of VRE integration into power systems. Some recurring lacks though, such as the impact of emergent technologies or the development of substitute low carbon emitting technology (other than solar and wind), need to be addressed. The paper also advocates the need for a systemic vision, for both research and policy makers that goes beyond the sole power system.

Introduction

The United Nations Environment Programme's (UNEP) latest report on Global Trend in Renewable Energy Investment in collaboration with Bloomberg New Energy finance (BNEF) recorded an increase in renewable energy (RE) investment of 2% between 2016 and 2017 to about $280 billion, or cumulative amounts since 2010 of about $2.2 trillion. Nearly 157 GW of new renewable electricity generation capacity was added, representing nearly 70% of new electricity generation capacity. The solar sector (38% of new electricity production capacity) and China (45% of investments in renewable energy) are at the forefront of the changes observed in the global electricity mix. In 2017, solar, wind, geothermal, biomass, waste, marine energy and small hydro technologies accounted for 12.1% of global electricity generation, compared with 11% in 2016. Developed volumes are based on OECD countries excluding Mexico, Chile, and Turkey. Details can be found at the following internet address: http://fs-unep-centre.org/sites/default/files/publications/gtr2018v2.pdf For the past two decades, a strong interest has emerged in favor of the integration of renewable energy sources in the energy and electricity mix, both in order to ensure energy security within the framework of energy transition policies and to address climate change consequences. Transitioning towards renewable energy sources seems all the more relevant because they allow the state to earn double dividends, as their diffusion actually reduces the volume of imported fossil energy [START_REF] Criqui | European climate-energy security nexus: A model based scenario analysis[END_REF]. For instance, in France, the 2015 law on energy transition imposes a 30% diminution objective in the consumption of fossil energy. This would in turn reduce by about 30% the country's energy dependence (given that France imports 99.9% of its fossil energy), induce a decrease in commercial deficit (and thus, of their financing) and could disrupt certain geopolitical balances or relationships with other importing countries. In order to assess relevant energy policy in this context, governments and states are leading energy transition scenario, based on different tools such as model, storytelling or expert based study. One of the key questions raised by these foresight exercises concerns the evolution of the electricity supply and the conditions for integrating a growing share of variable renewable energy sources (RES) into power networks. This gradual increase must indeed take into account several factors: costs of the different technologies (wind, solar, etc.), public policies supporting variable RES, constraints due to the intermittency of these resources, productions in the network, regulation and political body expectations and evolutions, etc. All society stakeholders (state, company, consumer, non-governmental organizations) are now concerned by these various challenges and the multiple actions undertaken by politicians, citizens' associations, businesses and other institutional and noninstitutional actors have led research -whether public or private -to work on the concrete, technical, social or political impacts of the evolution of our energy systems. Both governments and citizens are nowadays associating the fight against pollution and climate change with the development of renewable energy sources.

The profusion of scientific research along with the numerous reports issued by government agencies reflect the interest and the concern of politics and society on this topic. The number of research papers or studies on the integration of variable RES into power networks has drastically grown since the beginning of the 2000 decade. In these numerous work, hypotheses differ depending on the space and time scale which are studied. The technical and complex aspect of the topic combined to its social and political offshoot often make it difficult to develop a global and synthetic approach of the research carried in this domain.

This paper seeks to provide this global synthesis, through a bibliometric and bibliographic approach combined with a survey among actors of the energy research field and especially modelers.

Methodology

This article is based on a bibliometric study of the Scopus1 database on the topic of the integration of variable RES along with a review analysis of different government or regulation body reports on this topic. To confront the bibliometric and statistical results to the current and prospective research trends, an on-line survey has been proposed to the researchers of the domain between June and July 2017.

Bibliometrics : recent state of the art

Bibliometric analyses are used to explore the research trends and the evolution of researchers' interests in a field. In the recent published research (2015-2018) we analysed papers in the field of energy and environment [START_REF] Han | Bibliometric indicators for sustainable hydropower development[END_REF][START_REF] Wei | Climate policy modeling: An online SCI-E and SSCI based literature review[END_REF]Wei Li et al., 2015 ;[START_REF] Chen | Chinese energy and fuels research priorities and trend: A bibliometric analysis[END_REF][START_REF] Gao | A bibliometric analysis based review on wind power price[END_REF][START_REF] Yu | Assessment on the research trend of low-carbon energy technology investment: A bibliometric analysis[END_REF][START_REF] Zhang | A bibliometric analysis of research on carbon tax from 1989 to 2014[END_REF]Wang et al., 2017 ;Merigo and Yang, 2017 ;Wang et al., 2017 ;[START_REF] Zhang | Mapping of water footprint research: A bibliometric analysis during 2006-2015[END_REF]Geng et al. 2017 ;[START_REF] Chen | Recent progress on energy research: A bibliometric analysis[END_REF]Geng et al., 2017 ;Fabianne de Paulo and Silveira Porto, 2017 ;Aleixandre-Benavent, et al., 2018 ;[START_REF] Mao | Research on biomass energy and environment from the past to the future: A bibliometric analysis[END_REF][START_REF] Tiana | A bibliometric analysis on trends and characters of carbon emissions from transport sector[END_REF] , explicitly referring to using bibliometric analysis. An analytic table of the different papers, their scope and the keywords and methods they use is provided in Appendix. These papers address different topics related to various energy questions : greenhouse gas emissions or climate policy assessment (Geng et al., 2017;[START_REF] Li | Bibliometric analysis of global environmental assessment research in a 20-year period[END_REF][START_REF] Tiana | A bibliometric analysis on trends and characters of carbon emissions from transport sector[END_REF][START_REF] Wei | Climate policy modeling: An online SCI-E and SSCI based literature review[END_REF]; low carbon development or technology development [START_REF] Gao | A bibliometric analysis based review on wind power price[END_REF]Fabianne de Paulo and Silveira Porto, 2017;[START_REF] Han | Bibliometric indicators for sustainable hydropower development[END_REF][START_REF] Yu | Assessment on the research trend of low-carbon energy technology investment: A bibliometric analysis[END_REF]Wang et al., 2017;Wang et al., 2017;); carbon tax [START_REF] Zhang | A bibliometric analysis of research on carbon tax from 1989 to 2014[END_REF]; water footprint [START_REF] Zhang | Mapping of water footprint research: A bibliometric analysis during 2006-2015[END_REF]; deforestation or biomass issues (Aleixandre-Benavent, et al., 2018;[START_REF] Mao | Research on biomass energy and environment from the past to the future: A bibliometric analysis[END_REF] or life cycle assessment (Geng et al., 2017). The two most common methods used are content analysis and citation analysis.

Content analysis uses either keyword analysis, which provides stats on the frequency in the literature of some chosen concepts, methods, technical terms, etc., or co-word analysis that searches for the strength of relations between words and by mapping them thus identifies relations and interactions between topics and emerging research trends (Wang et al., 2017).

Citation analysis maps the relations between the different parts of the literature -mainly who cites whom -and thus identifies core literature, papers, journals, countries, authors, etc. [START_REF] Gao | A bibliometric analysis based review on wind power price[END_REF]. Social network analysis (SNA) through softwares such as Bibexcel and Gephii which are popular ones used currently (five papers over seventeen studied mention using both softwares), derives from citation analysis and assesses the relations between researchers, institutions or countries exploring the links between them (co-publication, partnerships, coaffiliation, etc.). In most of the cases, results come out as clusters of relations between concepts, authors, research institutions, etc. Papers usually choose between the two approaches, they either focus on the content of the literature through keyword analysis or study the publication dynamics assessing production or collaboration status. We combined here the two methods as we needed both a mapping of production and collaboration statistics, to link it to the energy policy led by different countries or regions, and an assessment of the trending research topics to build our online survey. The most commonly used database is the Web of Science (6 papers out of 17 mention using it), which includes Science Citation Index Expanded2 (7 papers out of the 17 studied mention using it). Finally 13 papers out of 17 mention the Web of Science Core collection databases. It is the most diversified database covering various scientific topics (from Agriculture to Medicine and Economics). It encompasses more than 20 000 journals and 1.4 cited reference3 .

Bibliometric study and on-line survey

In this paper we used both content analysis and SNA along with some other indicators such as geographic scope and time frame. The list of papers included in the bibliometric study is provided in Appendix along with elements used for the analysis. The on-line survey was designed and conducted after the first results of the bibliometric analysis were obtained. It aimed at identifying further research issues, prospective trends and gather some verbatim on the current research state of play. It was carried through a Google Form and contained three different parts: 1. Semi-directive questions regarding variable RES integration (most important conditions, players, technology need, etc.); 2. Open questions regarding research trends, remaining problems and suggested solutions; 3. Questions addressed to economic modelers about their models, tools and issues. The respondents were asked to provide name, email and affiliation and the results were aggregated to ensure the confidentiality of answers.

The survey was sent first and foremost to the corresponding authors of the papers previously collected for the bibliometric study. We also spread out the survey to some European industrial (TSOs, electricity providers, smart grids start-ups), institutional (World Bank, IEA, OECD) and Non-governmental organizations (NGO) actors. They represent less than 10% of the answers all come from the European Union. The lists of questions and of respondent's institutions are available in Appendix. Sixty researchers and actors involved in the domain have answered the survey. Researchers mostly come from the European Union but also from the United States (U.S.), Australia, South-Africa and China. Through both the bibliometric and the survey approach, the objectives of this study were first to identify key players among states and research institutions or nodes of collaboration; second to identify key concepts, how they are linked together and what types of scenarios and models they bring; and third to identify potential research leads on the topic.

Dynamics of publication on variable renewable energy (VRE)

Since 1970, according to the Scopus database, around 10 000 papers (articles, conference papers or other peer-reviewed documents) have been published on the integration of variable renewable energy (VRE) integration into power networks (Figure 2). Among this wide database we have led bibliometric analyses, using the text-mining software Intellixir4 . Through statistics and graph of links, they show the relations between the concepts used in the publications and between the authors and institutions that produce them. A subbase has been made on the literature dealing with the scenario modelling of the energy transition and the integration of variable RES into power networks. Around 150 peerreviewed documents have been published on the topic, all of them during the last ten years.

Publication dynamics : mapping of acdemic collaborations and geographic trends

Variable RES integration: states, reasearch centers and main colaborations

The main publisher on the integration of variable RES into power networks remains the U.S.

with a steady level publication of around 250 peer-reviewed documents per year since 2012 (Figure 3). They have held this position since world research took interest for the subject. The American rate of publication was the first to take off and started growing between 2007 and 1) nor with the level of variable RES on the electricity mix (Figure 4). Because the topic of variable RES into power network is wide, multidisciplinary and very complex to tackle and because it is of interest for the entire international research community, collaborations between researchers and between research institutions are essentials to solve the problems and questions it brings about. We have approached and mapped the collaborative dimension of international research on the integration of variable RES into power networks through the analysis of co-publications.

From 1970 until 2017, the U.S. polarize the great majority of international collaborations (Figure 6). A second cluster of exchanges within the European Union (EU) is also visible, member states collaborate between themselves but, apart from the UK and Germany, they have few links with countries outside of the EU. Other states outside of the EU where the level of publication is high (India, Japan, Canada, Iran, Brazil, China or Australia) publish in a quite islanded manner. They all have collaboration links with the U.S. but collaborate very little between them or with the EU cluster. If we focus on the scenario building publications, the cartography seems a little different.

Collaborations seem more driven by geographic proximity, which could be explained by the fact that most of the integration scenario are made at a national or macro-regional level. The most important clusters of collaboration are European. North-American teams work together and one cluster breaks this general rule and brings together teams from China, Japan, Germany, Sweden and the UK.

The most influential actors for a fast integration of variable RES

The problem of the integration of high levels of variable RES into power networks is not just a technical one or an economic one. Because of the impacts it can have on markets, consumption behavior, supply policy for States and society at large, it goes beyond disciplinary barriers and calls for a multidisciplinary and multi actor approach. 90% of our survey respondents consider themselves as researchers. They were contacted and asked to fill in the survey as they are the corresponding authors of the papers considered in the two subbased we built from the Scopus database on RES integration scenarios. Among the 10% remaining respondents are some European policy makers, NGO members, market analysts or industry-oriented profiles. As they provided few verbatim and were not enough of a consistent group we chose not to consider them separately from the rest of the respondents. We have asked the respondents to choose and rank the actors whose implication they consider the most important for a fast and smooth integration of variable RES into power networks (Figure 10). Politics come first in the ranking of the most important players that have to be involved in the process of variable RES integration, which would call for a deeper study of policy impact from the research community. Transmission system operators (TSO), regulators and industry are mentioned immediately after politics, they represent the technical and economic part of the issue. Research comes then, along with local actors, which would make a case for the idea that this integration is not necessarily a matter of technological breakthrough brought about by new research but more a matter of political, economic and industrial choices. Many of the respondents commented their answer with the idea that communication and strong cooperation between all the above-mentioned actors would be necessary to allow high rates of integration.

Scenario and modeling tools

Policy makers are most interested in a few key questions: the delimitation of the amount of variable RES it would be possible to include into the power networks without threatening their stability, what the cost would be for society and how long it would take to implement.

We focus in this part on both scenario building and modeling tools in the scientific literature, as they are the main aspects the research community uses to address these questions.

key concepts

The bibliometric analysis allows for the mapping of key concepts used in scientific literature on the integration of variable RES into power networks and gives some insight on how they are linked together. One of the main results of this analysis shows the importance of scenario building and modeling (Figure 11).

Time horizon.

2050 seems to be the major reference for scenario projections and is used by around two third of the papers. However, half of the papers that model the integration of 100% of variable RES into the power network they study choose not to develop their scenario within a time frame, whereas only 20% of scenarios that postulate lower levels of integration do so. This difference of projection in time might be explained by a political aspect underlying the different types of scenarios. 100% integration scenarios often consider integration of RES as a goal per se and try to demonstrate it will be possible to reach it eventually, whereas other scenarios with lower levels of integration go the other way round and tend to test practical options more or less immediately applicable. In both cases, the choice of 2050 as a target remains political as a "mid-term", "mid-century" commonly accepted time reference for energy and environment policy making.

Scale

When asked about the most accurate scale for the integration of variable RES into power networks most of the respondent researchers put forward the local and regional level. Even though they do not discriminate strongly between the different scales and most of them comment on their answer saying cross-scale dynamics are essential to the process, the result is interesting when compared to the statistics of publication. More than half of the published papers that propose a scenario focus on the national level. The second most studied scale is the macro region (15% of the papers) and local and regional levels both make respectively for less than 10% of the published papers. This discrepancy between the scales that are deemed more accurate to study and the ones that are effectively studied, and the predominance of scenarios at the national level may be the result of policy makers' interest (and financing) for studies and scenarios at their own level of action, which is often a national one.

Geographic dimension

The geographic span of scenarios on the integration of variable RES into power networks is rather limited. Three poles are particularly studied: the U.S., the EU and an Asian pole (mainly China, South Korea and Japan). Australia also appears in the 100% RES integration scenarios. Most of the papers published come from the same areas and researchers tend to publish on the areas where they work. However, the lack of research on developing areas where the electrification process is scarce, ongoing or structured very differently from the large and highly meshed networks of industrialized countries, seems problematic from a global and long term perspective. The World Energy Council (WEC) foresees the African demand for electricity will triple between 2015 and 2025 but research scenarios show a remarkably low interest for this continent even though it might be a game changer from a global perspective.

In our survey their comments mention four key elements for the future of variable RES integration modeling: (i) an integrated vision that would need to consider all the impacts without a "silos" approach; (ii) the need for a constant link between models and scenarios;

(iii) a collaboration between energy system modelers and grid modelers is required; (iv) the challenge of flexibility, necessary for the integration of intermittence need to be addressed. happen in a predefined situation. In contrast, optimization models assume the definition of an objective function: it is a quantity to be minimized or maximized (sum of costs, collective well-being, etc.). Finally, the recursive approach is at the intersection of the two previous renewable energy (https://www.irena.org/-/media/Files/IRENA/Agency/Articles/2012/Jul/6_Koos_Theron.pdf?la=en&hash=AAA89C654FEB87FD550E0 3F2AAFB9CEA7DEBC142) 21 The TIMES (The Integrated MARKAL-EFOM System) model generator was developed as part of the IEA-ETSAP's methodology for energy scenarios to conduct in-depth energy and environmental analyses (Loulou et al., 2004). The TIMES model generator combines two different, and complementary, approaches to modelling energy: a technical engineering approach and an economic approach. In a nutshell, TIMES is used for, "the exploration of possible energy futures based on contrasted scenarios" (Loulou et al., 2005). (https://ieaetsap.org/index.php/etsap-tools/model-generators/times) 22 The TIMES Integrated Assessment Model (TIAM-WORLD) is a multi-regional and inter-temporal partial equilibrium model of the entire energy/emission system of the World, based on the TIMES paradigm (http://kanorstest1.kanlo.net/models/tiam-world) techniques. Because of their size, these models can be constructed by simplifying the decision-making process: economic agents optimize an objective function recursively.

Scientific literature proposes various classifications of models used by energy, economy and environment researchers [START_REF] Percebois | Energie : Economie et politiques, Ouvertures économiques[END_REF]Bhattacharrya and Timilsina, 2010;[START_REF] Vieille Blanchard | Les limites à la croissance dans un monde global -Modélisations, prospectives, réfutations[END_REF][START_REF] Nicolas | Robust energy and climate modeling for policy assessment[END_REF]etc.). They all differ but nevertheless agree on some key elements that would need to be included in any typology:

-the analytical approach chosen: engineer or economic model, bottom-up or top-down approach, partial or general equilibrium -the numerical methodology: optimization or simulation Computer tool recensions [START_REF] Connolly | A review of computer tools for analysing the integration of renewable energy into various energy systems[END_REF][START_REF] Sinha | Review of software tools for hybrid renewable energy systems[END_REF] conclude that none of the existing tools allow for a complete vision on all the integration challenges and that every modeler selects or builds one tool to answer some specific questions. Tools are thus scaled to answer integration questions on very diverse objects (a building, a local, national or theoric system) and some of the other parameters included in the models (sector description, time step, etc.) prevent the comparison of results.

The disparity of models and of the results they propose is often considered as a problem, especially by policy makers for whom it is difficult to read these different results and who then struggle to transform it in effective policy. According to our survey results, modelers consider that the difference in starting hypothesis is key to this disparity. Databases used on technology costs can differ but a more structural point is that models are built to answer different kind of questions and the current research state of play do not allow for the building of a model that would integrate the entire ongoing questioning. The ways models are structured differ according to their goal:

-Some choose between the physical aspect (network modeling) and the monetary aspect.

-Some are efficient to observe the behavior of a network in one given year but struggle with the projection of a system evolution over a longer period of time.

-The time granularity of a model affects both its efficiency and its results: traditionally, networks are modeled hourly, infra-hour models remain mostly the prerogative of transmission system operators for short term simulations because of the computing power needed for such projections. Whether it is appropriate or not to build infra-hour models outside of this type of exercise is debated among the scientific community: it often means that some "average weeks" are modeled and processed without assurance that critical moments are being taken into account and detected by the model.

-The spatial features of networks (size, configuration, meshed aspect or not, obsolescence, etc.) and of the areas considered (sunshine, population, land use) affect the dimensioning of models and their results. The two extreme cases of a power system in a developing country of the equatorial belt, with few connections, small extensions and many islanded parts and a very large European network in a temperate zone, highly meshed and interconnected, cannot be directly compared.

Two conclusions can be derived from these observations. First it is impossible, given the current research state of play, to obtain an immediate global convergence of results. Both models and questionings are not mature enough on these quite recent objects and the discrepancies one can find in the results precisely show the wide remaining uncertainties.

Second, such a convergence is not currently desirable as the models and scenarios purpose is precisely to answer and reflect the diversity of the ongoing questionings, through different approaches. It would be structurally impossible to ask for different questionings to bring one single same answer, even though it would be easier for policy making or communication.

However, a consensus among respondents points some recurring lacks in current modeling dynamics. The impact modeling of emergent technologies is still fragile and must be improved to strengthen the pertinence of projections. The possible development of substitute low carbon emitting technology (other than solar and wind) is seldom considered. Some of the basis scenario data would benefit from being harmonized and more systematically shared (economic databases, technologies features such as costs, technology readiness level). From a more general viewpoint, a lack of transparency and sharing of both data and methodologies used is often highlighted.

Potential research leads

The value chain for the integration of variable RES into power networks is long and particularly complex. Each of its elements brings up specific challenges (whether technical or not) and opens a specific research field. The interactions and interdependencies between these elements are also important, which makes a global vision on the topic difficult. What could be the potential research leads that would benefit some attention from the research community?

We do not pretend to enter into very technical details, the point here is rather to account for the critical perspective on ongoing research dynamics that derived from both the bibliometric and survey analysis.

Current important topics

The panel of survey respondents defined and ranked the factors for a good integration of variable RES (Figure 13). The following results show that there is once again no single and easy answer but rather a panel of solutions that need to be combined. Political and social aspects of the problem appear rather weak in this classification which could also be explained by the under-representation of social sciences among the respondents. The same identification and ranking has been done for important technologies and the same pattern shows. If storage and flexibility appear on top of the list, most of the comments made point at the impossibility to achieve this integration without considering at the same time all the technologies evolutions. The optimal balance between them will then vary depending on local conditions in every network (Figure 14).

Unresolved challenges a) Technical aspects

Modeling exercises either choose between a focus on network stability, economic dimension, consumer behavior, market design etc. Up to our knowledge there is no existing model that takes into account all of these elements at the same time and that integrates them. The following results derive from the verbatim brought by the survey, they present the challenges that the scientific community considers as unresolved or in progress.

On the technical side, network stability (and frequency stability) is one of the key elements for variable RES integration which often acts as a disturbing factor. This stability challenge covers frequency control, planning and building of new lines and the management of the network's kinetic energy and reserves. Studying all these complex factors and how they interact in one model is difficult. For example short-term storage, demand side response and storage dimensioning (intraday or seasonal) are aspects which are mentioned in certain studies but are seldom quantified or their dimensioning remains qualitative which does not allow for them to be integrated in currently developed models. The market structure regarding the tools for this stability is also rarely taken into account: should it be centralized or are we going towards a decentralized market with, for example stabilizing and monitoring tools installed directly in private individual residences or in industrial facilities? Flexibility and seasonal aspects are a second type of challenge that needs research development. From a pure technical perspective, some of the existing optimization options are way less studied and taken into account than others (transforming substation carrying capacity for variable RES, margin of security of old power station in the current evolving conditions of monitoring, etc.). The development of flexibility services would also benefit from a larger perspective. Often only a few flexibility options are considered (certain types of battery storage, demand side management, production flexibility, interconnexion, etc.): the focus is thus only placed on the energy sector when other opportunities could be found in related sectors such as transportation and heating. The impact of these operational aspects on long term decision investment is often not considered: the challenge of flexibility is not an upstream concern but rather a downstream one that comes up when the investment decision is made. In the same perspective, most of the studies use a static demand/supply balance, thus underestimating the networks' or the production's operational margin and reserves used to operate the system in a dynamic way. The dynamic functioning of the system is often not addressed, along with the information and monitoring systems of production facilities and the market design challenges it brings up. Distribution networks are less studied than transport networks, even though they absorb a large part of RES decentralized production which frequency response is very quick. Storage is conversely widely studied but some challenges call for deeper studies without limitations to the mere question of flexibility. Storage during low-production episodes remains mostly unstudied because of its complexity and its cost (but market studies should envision every situation) and the role batteries could play, especially associated with vehicle fleets, calls for a better understanding.

b) Markets and financing aspects

The market design that would be most suited for the integration of variable RES into power systems is still a grey area for research. This uncertainty creates an investment risk for the different players of the value chain. The market design is a large question for researchers as it includes a vast number of concepts: risk management, types of contracts, adaptability and adaptation of the network, time horizon, etc. The regulatory aspects and their evolution are particularly important in this context, both at the national level but also in their local aspects specific to each area. Their evolution and impact are still poorly studied in the literature.

The cost impacts at all levels of the value chain are still debated. At the production level, the level of financing and investment required for the development of variable RES is unclear and depends on the political vision of the evolution of the electricity supply in the short, medium and long term. At the level of the transmission and distribution network, the question of pricing arises: is it necessary to charge the consumer the access service to the network in addition to the demand for power to compensate for the losses associated with a new category of consumer-producers who only use the network as a backup of their own production? And if so, in what proportions? At the level of consumption, how to model the evolution of the price of electricity? The variable RES production means, which are currently the first in the "merit order" of production sources, are pushing out of the conventional power plants market which are unprofitable in the current market design and are dismantled or mothballed without guarantee that the original production intermittent renewable energy can ultimately ensure the satisfaction of the entire demand. Conversely, some wind or solar projects with almost zero margins can be threatened if the price of electricity drops, except to maintain in time a guaranteed price system as is the case in current tenders.

Generally speaking, the economic dimension of the problem of integrating variable RESs into electrical networks is still poorly taken into account and the models are insufficiently detailed, particularly as regards:

-the additional cost of storage in the final cost of energy; -the cost of the environmental, health and social impact of the new high-voltage lines; -the cost of intermittency of the variable RES; -the economic value of the MWh of the additional variable RES capacities. One often imposes a level of development (share in electrical energy at a precise date) without worrying about the gradual decline in the value of the MW of renewable energy as installations progress; the possible technological breaks announced; -the good representation of the various stages of the electricity markets to develop the flexibility services trade (reserve market, adjustment market, intraday market), as well as all the electrical equipment with its flexibility character (equipment ramping) conventional, production hazards on PV and wind types) on a model market in unit commitment; -the demand and its possible flexibility, which is generally much less studied in detail than the "supply side" (means of production and storage, and transport infrastructure, etc.).

c) Social aspects

The behavior of citizens and consumers in the development of large sources of variable RES production are not taken into account in the modeling exercises or in the scenario. Considered as a field of research specific to the social sciences, this topic is still not enough taken into account by the scenario designers. Yet the issues of social acceptability by citizens (for the establishment of new production or transport infrastructure) and the change in consumer behavior (the ecological choices made by individuals or the responsiveness to a given price signal on the electricity) partly condition the results. Despite the general support of the population for this type of policy, the cost to consumers or the taxpayer of the transition to an electricity system with a high rate of integration of variable RES may in some cases lead to a "fiscal fed up" of populations that may lead, as for some Americans, to a mistrust regarding the need to reduce the global carbon footprint. On the other hand, it is interesting to examine the influence of crowdfunding on the deployment and localization of renewable energy [START_REF] Lam | Crowdfunding for renewable and sustainable energy projects: An exploratory case study approach[END_REF][START_REF] Vasileiadou | Three is a crowd? Exploring the potential of crowdfunding for renewable energy in the Netherlands[END_REF]. Financing is an important barrier for variable RES and the new systems put in place (including crowdfunding platforms) are currently being developed to encourage the placement of local savings, whether in developed or in developing countries. The financial aspects of the development of variable RES are rarely taken into account in energy transition models. As key element for the integration of variable RES in the power networks, they should be studied more because they cover geographic issues (criterion of residence for the investors and reserve of savings, to favor the local anchoring of the projects) and issues related to the participation of the citizens. Eventually all the questions related to social acceptance of the projects and the way to address the well-known "Not In My BackYard" (NIMBY) or "Build Absolutely Nothing Anywhere Near Anything" (BANANA) problems need further addressing. The issue of governance for the management of these new sources of intermittent production needs to be further explored. The installation of variable renewable energy infrastructures has local impacts (land, landscape, etc.) and is associated with a particular experience of the populations concerned. It may be a feeling of "take-back", local control over the sources of energy production and supply, or a feeling of imposition by a centralized and distant power of infrastructure that does not serve the interests of the local population (impact on crops or tourism, for example). These questions relating to the local specificities of projects require upstream consideration of investment decisions if we wish to establish a constructive dialogue between stakeholders around the common objective of promoting and facilitating the development of variable RES. These are important topics for the deployment of variable RES that should be integrated into the modeling exercises in order to cope with the objectives of public policy recommendations for the political body [START_REF] Ribeiro | Modelling perception and attitudes towards renewable energy technologies, Renewable Energy[END_REF].

d) The need for a systemic vision

The scenarios (or the model) often study one aspect of the question of the integration of variable RES (adequacy of the production means, reliability of the electricity grid, power markets design, etc.) without presenting a systemic vision of the question (which would take into account technical, legal, behavioral issues, etc.). The complexity of the questions and the organization of the research (in silos) is explained by the multiple nature of the questions asked. It's currently difficult to work on the global picture of the problem which would require to integrate all of its components, insofar as interactions and feedback effects exist between them. More broadly, the integration of variable RES into electricity grids is influenced by elements that lie outside the power system. Taking into account other energy needs than electricity (heat, cooling, water, energy for transport ...) is necessary to consider all aspects of the problem. Similarly, it is necessary to take into consideration the other energy vectors that electricity, and the possible interconnections between networks and possible inter-conversions between energy vectors (especially storage).

Conclusion and Policy Implications

Integration of variable RES into power networks has been a trending research topic for the last decade. The complexity of the envisioned problems often leads to very specific papers focusing on one aspect of the problem in a silo approach. The comparison between trends revealed by the bibliographic analysis and needs expressed by the respondents to our survey among researchers in the field thus shows some discrepancies. More than half of the respondents mention the political body and the network operators as the most important actors for a fast integration of the variable RES to the electricity networks whereas the research and the technological breakthrough it might bring appear in only 5 th position. The geographic scope of current scenarios (let at a national level) is somehow contradictory to the need expressed for local studies. Among the interviewees, many modelers answered our questions and show a real diversity of models and methodologies used to treat the subject. The diversity and disparity of the research models and the results they propose is often presented as a problem. However, this disparity can be explained by many factors: lack of common databases on the costs of technologies and their evolution for example, but also more structurally, diversity of the questions asked through these models. Therefore, in the current state of research, it is not possible to build a model that integrates all the current questions.

The majority of the comments made by the researchers interviewed thus state not only an impossibility to converge the results of the various models but they also underline that this convergence is not desirable in the current state of research. The modeling exercises should not be intended to give the same results but to answer the diversity of the questions that research raises or to bring different and complementary insights, according to the qualities and limits specific to this or that model, on the same question. The disparity in the results of models currently used is therefore the result of both a lack of maturity of research on these issues, which are relatively recent in their study (and which require complex tools as well as considerable computing power) but also of a structural impossibility to see different questionings bring similar answers. However, some of the baseline data, such as economic databases and technology characteristics (costs, technology readiness level (TRL), etc.), would benefit from being harmonized and more widely shared. In general, a lack of transparency and sharing of methodologies and data is sometimes emphasized.

The integration of high variable renewable energy rates into electricity networks impacts the entire value chain of the power system and affects generation, transmission, distribution and consumption. This also calls for some integrated approach. For each part of the value chain, there are specific technical issues but research should also focus on transversal question such as governance issues, social, legal or political. Generally speaking, the question of the cost and the economic profitability of the integration of variable RES arise at every part in the chain, but it is difficult to define an economic optimum taking into account the combination of the different evolutions of this value chain.

Current research on the integration of variable RES into power networks would benefit from leading the following actions: building a common database for modeling, including cost of technologies and technologies specifications, opening international collaboration, conduct researches with a global approach including social sciences and other sources of energy consumption (transport, heating etc.). Researchers should also keep in mind that the topic stays very political: current market designs include wide subsidies and public opinion prevails over economic rationale in many cases.

Figure 1 :

 1 Figure 1: Global New Investment in Renewable Energy* (Billion $US)

Figure 2 :

 2 Figure 2: Number of publications related to integration of variable renewable energies (VRE) into power networks per year

  2008. China started publishing later and has been progressively catching up with the U.S. trend during the last few years. It gained second position over Germany in 2016, almost doubling its number of publication compared to 2015 and reaching around 200 papers per year.

Figure 3 :

 3 Figure 3: Number of publications on the integration of variable RES into power network per country

Figure 4 :

 4 Figure 4: Number of publications and % of variable RES in the electricity production in 2016

Figure 5 :

 5 Figure 5: Number of publications on the integration of variable RES into power network per research institutions (2000-2016)

Figure 6 :

 6 Figure 6: Collaboration network among the different countries

Figure 7 :

 7 Figure 7: Chinese research cluster on variable RES

Figure 8 :

 8 Figure 8 : European research cluster on variable RES

Figure 9 :

 9 Figure 9 : Japanese research cluster on variable RES

Figure 10 :

 10 Figure 10 : Most influential actors for a fast integration of VRE into power networks

Figure 11 :

 11 Figure 11: Map of the main concepts used in the papers published on VRES integration

Figure 12 :

 12 Figure 12: Types of model used

Figure 13 :

 13 Figure 13: Most important factors for the integration of variable RES into the power network

Figure 14 :

 14 Figure 14: Most needed technology development

Table 1 : Statistics on R&D, GDP, publication and researcher per country

 1

Source: Scopus, World Bank

Scopus is the database of academic journal articles and other peer-reviewed publications related to the Elsevier publisher. It covers about 55 million entries.

The web of science consists of 7 databases the Conference Proceedings Citation Index, the Science Citation Index Expanded, the Social Sciences Citation Index, the Arts & Humanities Citation Index, the Index Chemicus, the Current Chemical Reactions and the Book Citation Index.

https://clarivate.com/products/web-of-science/web-science-form/web-science-core-collection/

Intellixir is a text mining tool for statistical and bibliometric analysis of the articles found in the Scopus database (concepts used, origin of publications, links of collaborations between authors and institutions, etc.).

https://www.energy.gov/

☆ This study received the financial support of the French Alliance for the Coordination of Energy Research (https://www.allianceenergie.fr/) and the authors are very grateful to Marie Françoise Chabrelie and Cécile Dumas for their support. We are also grateful to François Kalaydjian, Philippe Ménanteau and Jérôme Sabathier for insightful comments and suggestions.

Model types

Respondent modelers used a large variety of models (Figure 12). The respondent modelers cited among others the following models 6 : Antares 7 , Dispa-SET 8 , EnergyPLAN 9 , H2RES 10 , Imaclim-R 11 , and In-house model based on GAMS 12 , LEAP 13 , LUT Energy System Model 14 , NEMO 15 , Plexos 16 , OEMOF 17 , OSeMOSYS 18 , Poles 19 , Power Factory 20 , Times for different countries 21 , TIAM World 22 . 6 All the footnotes related to model are based on Model presentation webpage. 7 ANTARES is a Monte-Carlo software for power systems analysis (https://antares.rte-france.com/wpcontent/uploads/2016/09/160913-Antares_public_short.pdf) 8 The Dispa-SET model is an open-source unit commitment and optimal dispatch model focused on the balancing and flexibility problems in European grids.( http://www.dispaset.eu/en/latest/) 9 EnergyPLAN simulates the operation of national energy systems on an hourly basis, including the electricity, heating, cooling, industry, and transport sectors. It is developed and maintained by the Sustainable Energy Planning Research Group at Aalborg University, Denmark. (http://www.energyplan.eu) 10 H2RES is a balancing tool that simulates the integration of renewable energy into energy systems. The model is developed by the Instituto Superior Técnico, Lisbon and the Faculty of Mechanical Engineering and Naval Architecture at University of Zagreb, Croatia in 2000. (http://www.energyplan.eu/othertools/island/h2res/) 11 IMACLIM-R is a multi-sector multi-region dynamic recursive growth model (12 sectors and 12 regions). It provides a macroeconomic framework which analyses the relations between the economy and energy sectors. (http://www2.centre-cired.fr/IMACLIM/Description-des-modeles-IMACLIM/IMACLIM-R/article/IMACLIM-R) 12 The General Algebraic Modeling System (GAMS) is a high-level modeling system for mathematical programming and optimization. (https://www.gams.com/products/introduction/) 13 LEAP, the Long-range Energy Alternatives Planning System, is a widely-used software tool for energy policy analysis and climate change mitigation assessment developed at the Stockholm Environment Institute. (https://www.energycommunity.org/default.asp?action=introduction) 14 The LUT-model is an hourly resolution model of the power generation system based developed the Lappeenranta University of Technology (LUT) in Finland (https://www.lut.fi/web/en/school-of-energysystems/modelling-energy-systems/studies) 15 The National Electricity Market Optimiser (NEMO) is a chronological dispatch model for testing and optimising different portfolios of conventional and renewable electricity generation technologies. It was first developed by Ben Elliston in 2011 at the Centre for Energy and Environmental Markets, University of New South Wales. (https://nemo.ozlabs.org/) 16 PLEXOS® Integrated Energy Model (PLEXOS) is tried-and-true simulation software that uses state-of-the-art mathematical optimisation combined with the latest data handling and visualisation and distributed computing methods, to provide a high-performance, robust simulation system for electric power, water and gas that is leading edge yet open and transparent. (https://energyexemplar.com/software/plexos-desktop-edition/) 17 OEMOF is a modular open source framework to model energy supply systems. Oemof addresses current and future challenges in energy system modelling by being cross-sectoral, multiregional, Time-Step-Flexible and community driven (https://oemof.org/ ) 18 OSeMOSYS is an open source modelling system for long-run integrated assessment and energy planning. (http://www.osemosys.org/) 19 Prospective Outlook on Long-term Energy Systems (POLES) is a world energy-economy partial equilibrium simulation model of the energy sector until 2050, with complete modelling from upstream production to final user demand and greenhouse gas emissions. POLES is used and developed by Enerdata in collaboration with the European Commission's JRC IPTS and University of Grenoble-CNRS (EDDEN laboratory). (https://www.enerdata.net/solutions/poles-model.html) 20 PowerFactory combines extensive modelling capabilities with advanced solution algorithms, thereby providing the analyst with tools to undertake the full range of studies required for grid connection and grid impact analysis of wind parks, photovoltaic (PV) plants and all other kind of power park modules using