Energy research in Nigeria: A bibliometric analysis

The scant supply of energy in Nigeria in relation to its energy demand has triggered interest in the scientific investigation of various energy research. This study reports on the bibliometric analysis of energy publications from Nigerian researchers from 1974 to 2019 (45 years) from the Elsevier Scopus database. The analysis includes publication types, languages of publication, institutions of authors and collaborators. Based on the analysis, the number of publications has increased in the 45 years period. With significant changes being from the period 2006–2015 by an average of 113 publications per year, to an average of 326 publications per year from 2016 to 2019. The contributions of institutional energy publications by region showed that the South-Western States region had the highest number of publications. A global map showing energy collaboration at international level Nigerian-authors mainly co-author energy publications with South Africa, Malaysia, the United States and the United Kingdom institutions. The publications are mainly in solar energy, wind energy and biomass energy and surprisely less in gas and hydro energy which are the main sources of electricity generation in Nigeria.


Introduction
Energy is identified as the key accelerator of economic and industrial development for a successful nation [1][2][3][4], its adequate supply is vital to human comfort and poverty alleviation [5][6][7]. The worldwide energy demand is increasing rapidly, arising from economic and population growth, particularly in emerging market economies, accounting for about 90% of the growth in energy demand by 2035 [8]. The shortage of energy has been a constraint to economic development in many developing countries.
Africa continent is naturally bestowed with a vast range of energy resources, outstripping its requirements [9]. However, most African countries have an energy deficit including Nigeria [10,11]. In Nigeria, the shortage of energy supply has been detrimental to economic development [2], despite the country being recognized as the most populous in Africa. The dismal performance in the Nigeria energy services has contributed immensely to the unemployment rate and exacerbated the poverty rate. Majority of the populace lives below $2 per day [10][11][12][13][14][15][16][17]. It is common knowledge that the living standard of a nation is related directly to the per capita energy consumption [2,14,18]. Consequently, the field of energy research is of paramount to a nation's socio-economic development, improvement in human welfare and health [19][20][21][22][23].
Nigeria indeed is considered as Africa's energy giant [2], the most prolific crude oil nation, along with Libya, accounting for 2/3 of crude oil reserves in Africa's [2,24]. Likewise, in the production of natural gas, Nigeria is rated second to Algeria [25] in Africa. Many of Africa's bitumen resources are located in Nigeria, making energy exportation the centrepiece of the country economy [2,26]. Due to rapid depletion of this fossil fuel resources (oil, natural and gas) and its environmental challenges [26][27][28][29], there is a shift in the energy mix towards the government implementation of the renewable energy resources (solar, wind and biomass) in curbing the severe global warming [5,[30][31][32][33][34][35][36][37][38][39]. The Federal Government of Nigeria has embarked on several initiatives to improve on the dire state of the energy challenges in Nigeria. In 2003, the Nigeria government approved the National Energy Policy (NEP) which main thrust is on sustainable development, environmental conservation and efficient usage of the energy sources [27,30,40,41]. One of the objectives of NEP is to guarantee the nation's energy resources development, with diversified energy resources alternative, for the accomplishment of national energy security and efficient energy delivery system with an optimal energy resources mix [42]. In 2007, the National Energy Master Plan (NEMP) sets the implementation framework of NEP, covering the entire aspect of energy resource and its usage [41,43]. Also, in 2015, the National Renewable Energy and Energy Efficiency Policy (NREEEP) presents an overall framework for Renewable Energy (RE) and Energy Efficiency (EE), serving as an umbrella policy for the various implementation measures [44].
Other initiatives include Electric Power Sector Reform (EPSR) Act 2005 which sought to deregulate and restructure the power sector and privatise electricity generation, transmission, distribution and supply [45,46]. Other forms of deregulation were the removal of a high percentage of the subsidy on diesel in 2009 and petrol in 2012 [44]. At the beginning of the Second Quarter 2020 (Q2/20), the Nigerian Federal Government (FG) abolished fuel subsidy regime as full deregulation sets in Ref. [47]. This comes as the global oil industry continues to grapple with the low demand and subsequent price slump that have been caused by the Coronavirus pandemic among others. The liberalization of the Nigerian National Petroleum Corporation (NNPC) sector and removal of subsidy will eradicate the incentive to smuggle petrol across the various borders of the country. Putting an end to the arbitrage which the smugglers used to enjoy [47].
Despite all the aforementioned policy initiatives by the FG, Nigeria is still a relatively poor country regarding energy self-provision. Nigeria has more than 100 Universities and research institutions across different fields including social sciences, humanities and engineering. Also, Nigerians have contributed to the international scene regarding global peace, economy and policy formation and technological solutions. Therefore, the gap between the research institutions and policies must be bridged to achieve energy sustainability for the future energy mix. Thus, knowing the status of and various distribution of the country scientific information concerning the energy landscape is of special interest.
Hence, there is a need to analyse efforts by the research community in Nigeria in formulating solutions for the energy problems in the country. This study aims to investigate the state of Nigeria energy research and its trends in the last 45 years, using a bibliometric approach in assisting the research communities towards understanding the present condition and predicting the dynamic changes that could occur in the energy field for future prospects. The results were used to determine active regions of energy research, the leading organisations and their collaborators. Thus, giving some feedback to the research communities.

Methodology
A detailed search was carried out in the Elsevier Scopus database using the "Energy" under Keyword and "Nigeria" under Affiliation Country. Thus, finding publications on energy topics where Nigerian researchers, organisations, institutions and their collaborators have contributed immensely. Scopus is a good tool for the extraction of energy search information (KEY, AFFILCOUNTRY AND PUBYEAR) and have previously been used by other researchers for energy research of different countries [48][49][50][51][52][53][54]. The search is from between the period of 1974-2019 where a total of 3803 documents was identified and scrutinized, but later reduced to 2791 documents. This was done by the exclusion of non-related keywords as illustrated in the search string in Table 1. Also, Table 2 shows the template from various obtainable features as extracted and computed from Elsevier Scopus database. This template has provided sufficient details on each publication recorded.
Equally , the Google Scholar was examined as an additional indexing  source on Nigerian energy scholarly documents within the custom range  1974-2019, as illustrated with the advanced search query of Table 3. When the keyword "Energy research in Nigeria" was searched without any filter applied, a total of 687,000 results were generated. After which the data was reduced to 3880 by the exclusion of non-related keywords in the search string. Also, the reduction was achieved by a careful review to eliminate duplicate entries (i.e. referring to the same document), incomplete/incorrect bibliographic details, and the inclusion of nonscholarly/non-peer-reviewed materials.
Although Google Scholar returned a total of 3880 using advanced search, it has a major technical limitation that hinders metadata-export for onward bibliometric analysis. This makes the data difficult to use with the Scopus results due to insufficient generated information in terms of the document by author, publication type, year of publication, the institution of author and collaborator. The data retrieved from Google Scholar was impractical for use with large data set, as many important data were missing. Also, 80% of the filtered data from Google Scholar database (3880) also appeared in the SCOPUS database. To this end, the extracted data from the Scopus database was used for further analysis. The hope is that these results can be of help in energy research areas and other stakeholders (local and international) for making informed decisions in respect of this study area.
An assessment of 78 Nigerian institutions in the last 45 years who have strictly published on energy and its related field were evaluated with the following features: the regional publication distribution, the language of publication, document by subject area, type, author and index keyword. The extracted data were compiled in an Excel format with the necessary graphing and analysis in OriginPro 2018.

Analysis of publications and languages
Fig. 1 depicts the result of 2791 documents related to energy from the Scopus database over the past 45 years and it was categorised into 13 document types. The prominent document type is "Article" with 69.8% (1949 records) of the entire publications, next far behind is "Conference Paper" with 23.5% (656 records), "Review" with 4.05% (113 records), "Book Chapter" with 1.83% (51 records), "Data Paper" with 0.22% (6 records), "Letter" with 0.11% (3 records), "Book" with 0.07% (2 records). The record of other documents accounted for a mere 0.04% (1 record), such as Business Articles, Editorial, Note, Short Survey and Retracted, having the lowest weight.
The predominant language of publication is English, reaching 2791 documents, followed far behind are French (1 document) and Portuguese (1 document). This is because the official language of communication among the citizen of Nigeria in the process of disseminating information, making most of the publications to be written in the English language. In percentage, English documents are 99.928% while French and Portuguese documents contribute 0.0358% each.

Evolution of energy research document
The publication trend in Fig. 2 and 2014 respectively. Following the incorporation of the NREEEP, 2015 that promote RE and EE towards diversifying the conventional energy resources (fossil fuel) and bringing to the attention of the policymakers the economic, social, environmental and political prospect of renewable energy resources (solar, wind, hydro and biomass) into the Nigeria energy mix thus, an exponential trend was verified in the number of publications from the beginning of the year 2015 to the recent year 2019. This was as a result of the National Renewable Energy Action Plan 2016, which targets 16% of its electric power consumption via RE by 2030 with an installed capacity of 13.8 Gigawatts [55]. Table 4 illustrates the four phases of the substantial increase in Nigeria energy publication concerning the policy framework, laws and regulations. The Federal Government is consistently improving the initiative of the framework for the energy sector, especially in regards to the non-renewables. Moreover, in recent years there has been positive development on this policy on energies which are renewable thus, extending the number of publications chronologically. Contrarily, most of the rural electrification strategy and policy documents, particularly off-grid and EE still stay in draft form that required development when taken into consideration the inherent realities related to the sector for sustainability [55][56][57]. However, there exists a gap in the policy framework and their operation through the plans effective implementation with a clear roadmap. The Vision 20:2020 set in the year 2010, targeted 40, 000 MW of electric power to position the country among the prominent 20 economies of the world by the year 2020 [58,59], the reverse is the case as the year elapse with the country installed generation capacity of 12,522 MW depending largely on gas-fired thermal power and hydropower sources; 10, 142 MW and 2380 MW, respectively [60,61]. Although the current availability of electricity for onward transmission to the final consumer is about 3500 MW to 5000 MW [61]. Based on this scenario the federal government target another electricity Vision 30:30:30 in achieving a technology-driven RE sector that can harness the nation's resources towards complimenting the rapid Table 2 The pattern of extraction and computation from the Elsevier Scopus database. consumption of its fossil fuel and guarantees energy security [60,62]. Fig. 2(b) also illustrated the same information on a logarithmic scale (log), providing the trend in the energy growth rate. It can be observed that the trajectory of the chart was rapidly increasing from the 1980s to present (2019): this can be attributed to the understudy of the initial crude oil production and exportation in the mid-1950s to the late 1970s that boom the number of energy publication output from the early 80s, whereby neglecting the role of agriculture that has been the traditional mainstay since inception in Nigeria's economy.
The obtainable Regression Coefficient, R 2 (R-Square) was 0.955, which denote a positive trend in energy research in Nigeria. This linear correlation model coefficient, R 2 (0.955) is comparable to similar work on energy research in other countries using bibliometric analysis techniques as shown in Table 5. Nigeria can be regarded as a developing nation in the research field of energy publications.

Institutional and regional publication distribution
The institutional energy publications in Nigeria has been very lopsided from its inception, with an existing region of great developmental impact such as the South-West estimated at 44.7% (1384 records), and others in the South-East, North-Central, South-South, North-West and North-East, estimated at 15.3% (473 records), 13.8% (427 records), 13.6% (420 records), 8.36% (259 records) and 4.33% (134 records) respectively, as detailed in Table 6. This one-sided publication distribution was expected to fade away with the establishment of 6 Energy Research Centres (ERC) at the designated institutions in Nigeria's six geopolitical zones and is funded by the Federal Ministry of Science and Technology(FMST). Although the ERC in South-Western Nigeria was established in 2008, it does not hinder the publications but also, shows the high level of commitment among the institutional researchers in the region.    Table 7. Closely behind are the FG institutions, some of which are the University of Ibadan (255/9.1%), Obafemi Awolowo University (225/8.1%), University of Nigeria (218/7.8%), and the University of Lagos (179/6.4%), while some State Government and few private institutions also featured.   (Table 1) as compared to the former that need not introduce an exclude word despite using the same KEY (Energy) AND AFFILICOUNTRY (Nigeria) thus, increasing the tendency of all form of energy documents in the Scopus database without any restriction. with the following number of energy published document 24, 24, 28 and 33 records, respectively. Their contributions are noteworthy for good productivity and high impact on the energy research field. Fig. 6 shows the global energy collaboration map between the Nigerian institutions and the international communities during the period 1974-2019. The extracted data in Table 7 provide Fig. 6 with the colour coded information and geo-localization, for a better interpretation of the considered publications on energy. The highest collaboration was with South Africa and it represent 7.13% of the total energy publications, closely followed by Malaysia, the United Kingdom and the United States with 5.11%, 4.23% and 3.25% respectively. While others are merely less than 2% as detailed in Table 8. It has been affirmed that a total of 88 countries are working jointly with the Nigerian researchers on the energy field in the past 45 years, accounting for about 1611 joint publications. The contributions of this foreign collaboration are equal to 36.6%, indicating that only 63.4% of the published documents are from Nigerian. Decisively, the Asia continent has played a significant role with 571 energy publications from 33 countries in collaboration with Nigeria. Other continents that are noteworthy for their contributions are Europe with 381 energy publications from 21 North-America with 194 energy publications from 4 counties, South-America with 21 energy publications from 4 countries, and Oceania/Australia with 32 energy publications from 4 countries.

Detailed analysis of the research keywords
The keyword is regarded as a term that captures the significance of the document, search and identify the trends in the branch of science and engineering [48,50,51,63]. This research identified a total of 160 different keywords in the energy field between 1974 and 2019. Fig. 10 shows the top 30 most pertinent keywords within the period of this analysis as retrieved from the energy document. The highest number of published documents is in the areas of solar energy (242 records), energy efficiency (227 records), activation energy (208 records), energy utilization (189 records), sustainable development (179 records) and renewable energy (144 records). While the minor research interest is focused on the following areas of fossil fuels (72 records), petroleum reservoir evaluation (69 records) and thermodynamics (67 records). Hence, solar energy in Nigeria demonstrated a significant interest in the energy field which is aimed at sustainable development of the Country's energy production.

Areas of energy research in Nigeria
Although Nigeria has a large reserve of fossil fuels such as crude oil, natural gas and coal [2,66,67], woody biomass is known for the largest source of energy consumption in the country [38,[68][69][70]. In accordance with the U.S. Energy Information Administration, the biomass type comprising of wood, crop residues, charcoal and manure, estimates for about 80% of the total primary energy consumed in the country [67,71]; while the crude oil, natural gas and hydro, account for 13, 6 and 1%, respectively [71]. The other sources of renewables are solar, wind, tidal waves and geothermal which are the main focus of energy diversification in recent time. The environmental challenges of fossil fuels coupled with the rate of depletion pave the way for green power technology in meeting the future energy demand and cater for the eco-system sustainability.
The analysis of the various energy sources in Nigeria has been carried out based on the keywords of various publications. Priorities have been given to some of the authors' keywords based on their predominant level of published documents in the energy field. Most of these documents were more focused on renewable energy (solar, wind, hydro and biomass) than the conventional energy sources (natural gas, coal and petroleum) for the sustainability of the ecosystem. This became noteworthy by Nigerian researchers to investigate the Renewable Energy Resources (RER) potential as the country diversifies from its full dependence on fossil fuels resources that is plagued with environmental challenges. The retrieved data from the Scopus was used to provide a comprehensive list of the topics. These topics were ranked in descending order and further analysed to provide insight as to why these topics have been prioritised, as detailed accordingly in the following subsections.

Solar energy
Solar energy is ranked first from the list of the 160 keywords documented from the search and the most researched RER amongst the energy researchers in Nigeria. Engineering and Energy are the two prominent subject areas outcome of the solar energy search, after following Table 1 search string by only replacing "energy" with "solar  energy" while still keeping all other parameters constant. Table 9 shows the top 5 authors and their respective published documents in the area of solar energy and Covenant University remains the leading institution in this field. Topics researched include the evaluation of solar resources in Nigeria (1981, 1983, 1988, and 1989), production of solar cells, the application of solar energy for lighting (1995), and heating of water and for refrigeration [72,73], with also a review of solar-assisted HVAC systems; its performance analysis using CO 2 as a refrigerant [74].
The use of solar PV/concentrated solar power system has made possible the utilization of solar energy in Nigeria as an alternative/ supplement to the nation's grid power generation in the urban and rural electrification. One of which is the 1 Megawatt (MW) solar photovoltaic mini-grid, situated at the National Hospital, Abuja. Also, some studies, surveys and pilot projects in this area have been undertaken by the NCERD and SERC as supervised by the ECN [75,76]. This includes the pilot water heater at UDUTH in Sokoto (SERC) and the 2-tonne solar rice dryer at Adarice Co. Enugu by NCERD.
Nigeria targets electric power production of 9.74, 18 and 30% of her electricity from renewable energy by 2015, 2020 and 2030, respectively [72,77]. Solar power is anticipated to generate 1.26, 6.92 and 15.27% of the electricity consumed by 2015, 2020 and 2030, respectively [72]. Most of the targets for solar electricity generation and thermal applications such as heating and drying have not been met as opined in a recent review [72]. Some policies recommendations such as tax incentives, capital grants and Feed-in-Tariffs (FiT) to meet the targets and increase solar energy penetration in Nigeria have been proposed [73]. Meeting these targets will drastically increase the proportion of the populace with access to electricity in the nearest future.

Wind energy
Wind energy appeared in the nineteenth position from the list of the author keyword's analysis and ranked the second of the RER in descending order of the published documents. Also, Energy and Engineering remain the top subject areas in the wind energy search. Table 9 shows the top 5 author documents in the publications considered, with the Covenant University having the most recorded published documents (47). The utilization of wind energy in Nigeria has been traceable to several applications, notable among which include electricity generation, water pumping and also for the milling of grains [67,76].
Earlier studies were mainly on the evaluation of the potential of wind energy in different regions in Nigeria [78][79][80]. Wind speed was characterized and was used to evaluate power density [78]. Plateau, Kano and Sokoto are some of the cities proposed for onshore wind power plants because of their excellent wind resources [80][81][82]. There has not been any study that looks at offshore wind applications. Also, researches have confirmed that there is no single offshore station amongst the 44 operated by Nigerian Metrological Agency, despite the reported potential of offshore regions in the country [83].  Some applications of wind energy that have evaluated are water cooling, provision of electricity in rural hospitals. Some of the noticeable installation sites are the rehabilitated windmill for water pumping at Kadawa village in the NW zone (Kano State), 5-kW aero generator at Gidan Gada area of the NW zone (Sokoto State) and also, the 10-MW wind farm at Lambatr, Rimi area of the NW zone (Katsina State) [76,84]. These illustrate the prospect of wind energy towards contributing to the country's energy mix. As inferred by Ref. [76], that this RER remains very low in the Nigeria energy mix, despite its large amount towards the coastal states (Ondo, Ogun, Lagos, Rivers, Edo, Bayelsa, Cross-Rivers, Akwa-Ibom and Delta) and in Northern Nigeria.
Some of the challenges that have been identified for lower penetration of wind energy in Nigeria are the absence of local manufacturing for wind energy components, the reluctance of government to encourage wind technologies, lack of offshore wind mapping, and low financing.

Biomass energy
Biomass energy is ranked twenty-two of the itemized keywords. The leading subject areas are Energy and Environmental Sciences. Table 9 depicts some of the leading subject areas and the authors' contributions in the publications. The University of Ibadan, University of Nigeria and Covenant University maintained the top list of biomass energy publication in Nigeria.
The energy discovered from organic materials, especially a manure, crops, scrap lumber, forest debris, and some kinds of waste residue, is attributed to biomass energy. The identification of biomass feedstock in the country includes shrubs, forage grasses, wastes and residues (agricultural, industrial and municipal solid wastes) by Refs. [5,85], and also the aquatic biomass [76]. Nigeria has a considerable figure of about 144 million tonnes per year of biomass potential [30,71]. Moreover, biomass is recognized as the major source of energy in Nigeria, accounting for approximately 80% of the total primary energy consumed in the country [71,86]. Energy from plant biomass could be utilized as fuel for Small and Medium Enterprises (SMEs) or fermented by anaerobic bacteria for the production of low cost and versatile biogas [67].
The classification of biomass feedstocks into four generations which are the first, second, third, and fourth generations [71]. The first generation applies to biofuels that are produced from the starch-based crops and oilseeds (agricultural products) such as palm oil to produce biodiesel or sugarcane for the production of bioethanol. However, this first biomass feedstocks generation can either be processed into biodiesel or bioethanol via trans-esterification or fermentation process respectively. Also, the biomass feedstocks are transformed into different fuels employing a series of processes for the production of liquid biofuels or biogas, generation of heat or electricity for the sustainability of the ecosystem. Investigations show that the first generation biomass feedstocks are the most commonly employed in Nigeria emerging bio-refineries [71]. The sources of the biomass feedstocks used are mostly food crops [71], causing ill prospects towards achieving the full potential of biofuel production due to environmental degradation and food insecurity in the country [86]. These challenges need to be addressed effectively for the country successfully implementation of its biofuel objectives in promoting low carbon emission.
To increase the supply chain of biofuel production, the bio-refining of the first generation is mostly driven by encouraging taxation and legislative Acts. Noteworthy is the 2007 United State Energy

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Independence and Security Act, the European Union directives (2009/ 28/EC), and the Nigeria Biofuel Policy Incentive of 2005 that provides legislative Acts for the establishment of biofuel industries in the country with an exemption from taxation [71]. The NCERD has worked on Biogas digester and several improved woodstoves with better combustion and low emission through the reduction of the heat losses thus, reducing the cooking period and also creating a proper channel for exhaust smoke [75,76]. However, research on the ethanol fermentation from sugarcane at different maturities levels was investigated by Rolz and de Leon [87]. The result of their work yields better ethanol after 300-325 days of planting and monitoring. Hayashida et al. [88] accounted for 20% (v/v) ethanol conversion from raw ground corn utilizing a mutant of Aspergillus awamori var. kawachi. Until the present, the production of the biggest volume of biofuel is in the form of ethanol, estimates for about 80% emanating from sugarcane and cone [89].
The apparent increase in the global food prices due to the implementation of the first-generation biomass feedstock has triggered the interest of scientific researchers into an alternative use of lignocellulosic biomass, likewise called the second-generation biomass feedstocks; such as wood residues, crop residues and other dedicated energy crops which can be cultivated annual, biennial and perennial, with the sole responsibility of producing biofuel. Moreover, as a sustainable alternative to fossil fuels usage, second-generation biomass feedstocks are gaining ultimate attention globally because of their non-competitiveness with food crops [90]. Research conducted by Kaparaju et al. [91] studied the production of biogas, biohydrogen and bioethanol from wheat straw in a biorefinery setting. The result compares the output energy from biogas production of untreated wheat straw with that of pre-treated wheat straw. The latter was considered the most energetically prolific and about 10% greater than the former. With the extracted sugar from wheat straw used for biogas, bioethanol and biohydrogen (biofuels) production in bio-refinery successfully. Also, Kalbande et al. [92] worked on the development of a biodiesel processor for the production of biodiesel from non-edible oil of Jatropha and Karanj. The produced biodiesel from these oils and its blends with diesel were capable for the generation of power in a 7.5-kVA diesel engine generator.
The third-generation biomass feedstock is the microbial cells [89], that composed of different processes of fermentative and photosynthetic algae and bacteria which are presently being investigated as biocatalysts for the production of biofuel due to their high oil/lipid contents [71]. Contrary to the previous biomass feedstocks generations, the sustainability of these microbial cells is not a factor for the nation's food crop competitiveness. Algae are known for their rapid growth rate with the tendency of growing in various liquid media such as wastewater streams which are mostly dominated in the Nigeria Niger Delta region [71]. The use of bioreactors aids the cultivation of algae in high yields [71]. For the prospect of biofuel production, microalgae could account for 10 to 300 times of biodiesel oil when compared to the dedicated energy crops [71,89]. Whereas, the immaturity in the technological know-how of the algal-based oil production in the country has been a major setback till date [71].
Dutta et al. [89] analysed the inadequacy of the third generation biomass feedstocks as a substitute for fossil fuels. These limiting factors include the reliance on sunlight, ecological footprint, geographical location and economic performance, making it insufficient for the full replacement of fossil fuels. As a result of these constraints, further research in the biofuel production from the metabolic engineering of algae is investigated [93], leading to the initiation of the fourth-generation bio-refining and thus having a great potential for the sustainability of the ecosystem in providing a clean source of energy. In a nutshell, the fourth generation biofuel production has presented to the field of study the concept of "cell factory" while shifting the research paradigm [93]. Some bottlenecks still exist in algae biofuel research and development which can solely be dealt with via using the post-genome tools on the photosynthetic organisms [93]. Conclusively, the first generation biomass feedstocks idea gained ground in the early '90s, after which several types of research was conducted within 2006-2010, with the commencement of the fourth generation biofuel research in late 2010 [89]. Dutta et al. [89] review the systematic evolution of various biofuel generations (first, second, third and fourth). In line with the published articles in the research field, it was shown that the third-generation biofuel received significant attention between 2011 and 2012 as compared to the first, second and fourth generation biofuel [89].

Fossil fuels
The identification of fossil fuels in the twenty-six ranking of the author keywords is based on the energy published documents that show the tremendous contribution of the researchers towards the diversification into RER in recent time. Sequel to this, Akuru and Okoro [94] investigated into the oil depletion and the need for renewables in Nigeria. The result shows an imminent trough in the country's oil reserve, accounting for how the rate of oil depletion will influence the domestic energy consumption in the future. Notable amongst the highlighted recommendations to tackle the dire effects are government investment in RER, as well as domesticating energy retail through the establishment of the SMEs with the sole responsibility of providing an alternative clean source of energy. This gradually turns the attention of the researchers from non-renewables to renewables, allowing the former to maintain the lower extreme as compared to the others as shown in Fig. 10.
Nigeria indeed is recognized as the largest producer of crude oil in Africa [2] with production merely influenced by sporadic supply interruptions, resulting to about 500,000 barrels per day (b/d) of unplanned outages [66]. The country ranks amongst the top 10 nations in natural gas reserves on the continent [104] and considered the 4th largest exporter of Liquefied Natural Gas (LNG) globally as at the year 2015 [66]. These fossil fuel resources are mostly dominated in the Niger Delta region of the country, comprising of 9 oil-producing states (Edo, Delta, Rivers, Cross Rivers, Bayelsa, Akwa Ibom, Imo, Abia and Ondo)  LGA's, that cut across over 800 oil-producing communities with a broad network of above 900 yielding oil wells and many related petroleum production facilities [101,117]. Following Cutis [118], a large percentage of the global oil and gas reserves are in Territory terrigenous fill on passive continental margins with major substantial hydrocarbon deposits located in the Gulf Coast/U.S. Gulf of Mexico, Canadian Beaufort-Mackenzie Delta and Nigerian Niger Delta. In the country Delta basin, the number of exploration wells drilled accounts for about 1182 till present, and 400 oil and gas fields of different dimensions are documented [119].
Nigerian merchandise export has been dominated by the oil and gas sector since the oil boom period of early 1973 to 1983 [120], after the civil war (Biafran war) ended on January 15, 1970 [121]. The exportation in the growth rate of oil revenue exploded from $718 million to $9.4 billion within the year 1970-1978 but decreased drastically from a peak of $25 billion in 1980 to $4.7 billion in 1986 due to the fall in the price of crude oil in the global market [104]. The economy vulnerability to the vagaries in the price of crude oil can be attributed to fiscal indiscipline, income distortions and revenue instability [104]. The receipts of the total oil exportation in the country as at the year 2008 was estimated at $75 billion, representing almost 98.8% of the year total exports. However, the oil and gas sector share of Nigeria's Gross Domestic Product (GDP) fell after attaining a peak each of about 47.7%, 25%, and 22% in the year 2000, 2005, and 2006, respectively [104]. The oil sector's contributions to GDP is still at a low rate of about 28.4% based on the World Bank estimate in recent time [104].
The state of the country oil sector is classified into 3 major sectors, including upstream, midstream and refining (downstream), and then broken further into individual businesses, as the country seeks to transform Africa's largest oil producer into a profit-making venture. The downstream sector has been noticed to be the major source of conflict among the other sectors, as it serves as a products distribution arm that connects to the final consumers of the refined petroleum (LPG, fuel oils, diesel oil, kerosene, petrol/gasoline, etc.) in the domestic economy [122]. In 2003 the federal government decided to take the bull by the hone towards deregulating the downstream sector [122], despite its economic impact due to the persistent conflict among the groups in the oil communities asking for the cut of the "national cake" (national wealth). Often leading to civil unrest with several attacks to oil infrastructure (pipeline vandalism/sabotage), prompting oil companies for the declaration of force majeur on shipments with immediate shut down of operation [66].
Countless tragedies have been witnessed in the Niger Delta region of the country from petroleum spills and accidental discharges by the oil sector due to blowouts and pipelines: Amongst others include, 1970 Shell-BP Bomu II well blowout, 1972 Safram/Elf Obagi 21 well blowout, 1980 Texaco Faniwa oil well blowout, and 1980 Agip Oyakama pipeline leakage [101,123]. Edoho [124] reported the occurrence of about 5733 oil-spill cases in the Niger Delta region within the stipulated the year 1976-2000, accounting for a total discharge of over 2, 567,966 barrels, of which the recovered estimates is about 549,060 barrels whereas 1, 820,411 barrels were lost to the environs. However, it has been recorded in the 1970s that the quantities of the cumulated oil spill are 10 times more than the 1989 Exxon Valdez disaster that took place in the United State [124]. It is no doubt that gas flaring and venting is another area of discharge employed mostly by the oil and natural gas sector, whereby the flammable gaseous/liquid by-products of crude oil are released (venting) and burnt (flaring) into the atmosphere through an elevated chimney. Norway as compared to Nigeria imposed limitations on this challenges using some strategic measures such as, the initiation of a carbon tax, gas flaring and venting reduction Act, and gas energy conversion process, which seems to be ineffective and weak in the Nigeria energy sector over time [125]. Over the past four decades of gas flaring pollution, the country has recorded above 123 gas flaring locations in the Niger Delta area and recognized as one of the highest Greenhouse Gas (GHG) emitter in Africa [126]. In the Niger Delta region, the total heat discharged into the atmospheric air is about 45.8 billion kW resulting from the gas flare of 1.8 billion ft 3 per day [126] thus, contributing significantly to the nation's global warming effect.
Ever since the first commercial oil discovery in Bayelsa State, the federal government has promulgated several policy frameworks, laws and regulations in guiding the onshore and offshore oil rigs, towards controlling and limiting the associated environmental discharge.  [101,127]. These federal government laws need to be strictly adhered to in the oil and gas exploration and production operation, for the ecosystem sustainability in the Niger Delta region and the country as a whole.

Hydro energy
As retrieved from the Scopus database, the word hydropower and hydro energy produced the same number of published document (21 records) during the search test. Energy has been the most outstanding subject area as compared to all other subjects in this category. The top affiliations under hydro energy are Covenant University, followed by the University of Ilorin which is known as the National Centre for Hydropower Research and Development (NCHRD) in the SW zone. It is a general knowledge that Nigeria is naturally endowed with numerous large rivers and several natural falls that favour the initiation of hydropower system [128]. The country as a whole has an abundance of water resources with a potential of approximately 14, 750 MW [129,130]. From this abundant potential, an estimated power explored for Kainji, Jebba and Shiroro hydroelectric stations is around 1930 MW leaving the remaining 12,220 MW unexplored [129]. This indicates that around 14% of the county's hydropower generation is in full utilization, which adds up 30% of the total installed grid-connected electric power capacity [128,129]. This is small in comparison with some other African countries such as Kenya, Burkina Faso, and Lesotho, which has developed her hydropower potential of about 34%, 46%, and 50%, respectively [130]. Also, Usman and Ifabiyi [131] estimates for about 45, 000 large dam globally, with the use of hydropower accounting for Nigeria energy production and the world total energy production of about 80% and 16%, respectively.  [132,133]. In comparing the cost between a diesel generator and SHP plants for remote areas electrification, the result of the analysis indicated the cost-effectiveness of the later, making it feasible for its exploitation as an alternative/supplement for the country power generation [76].
In 2002, the Economic Commission of Nigeria, together with the United Nations Industrial Development Organization (UNIDO) and some government parastatals organized a National Stakeholders Forum on RE-Technologies basically on Small Hydropower for rural electrification [69]. The goal was to devise strategies to rural/remote communities with access to a reliable and clean source of energy for the promotion of industrialization, which would tend towards rural development and job creation. At the time of the Forum, a Memorandum of Understanding (MOU) was signed among the parties. After which a training platform was initiated in 2003 on the development of Small Hydropower in the country. The focus of the ECN initiatives has been to create awareness on the potential of the Small Hydropower for electricity generation in adding to the energy mix of Nigeria. In 2004, the Federal Ministry of Power and Steel target 10,000 MW of National power (Power Vision 2010) using Small Hydropower that will contribute 1000 MW/10%, in which the development will be in phases [69]. This Power Vision 2010 was not fully met, due to change in government, weak policy framework implementation, economic instability, corruption, coupled with lack of "political will" that has led to the Roadmap for Power Sector Reform, Agric. and Bio.
Sciences (14) Landmark University (8) Madushele, N Chemical Engineering (12) University of Ilorin (7) Hydro Engineering (8) University of Ilorin (2) Abdullahi, S.A. Agric. and Bio. Sciences (3) Ladoke Akintola University of Technology (2) created in 2010 by the Presidential Task Force on Power (PTFP) which was later reviewed in 2013 with the responsibility of improving Nigerian power sector. In contrast to fossil fuel, hydropower was the type of energy resource employed in the country before the onset of the oil and gas resources. The vast deposits of fossil fuel in Nigeria create a sudden shift towards its focus while gradually neglecting the significant role of hydropower sector to the point that their performance was really below the installed capacity. It is now obvious to the government that the non-renewables had failed the entire country despite all the fossil fuel-based Independent Power Projects licensed as at February 2009 [128]. Ohunakin et al. [128] evaluate the development of SHP in Nigeria with regards to the EPSR Act 2005 and other policy implementation. The result shows that within the year 1971-2005 hydropower growth rate was around 360%, with about 5% tapped by some power sectors in 1923 and 1964. For rapid expansion and quick adoption of the hydropower plant, the government should incorporate Price Purchase Agreements (PPA) framework, FiT, and subsidies into policies to further strengthen and promote RER [128].

Geothermal energy
The geothermal RER development in Nigeria is yet to see the light of the day. There is a limited number of considered publications in this area, even though the internal heat of the earth is not a new process for generating electricity and space heating. In Italy, the development of geothermal energy was dated back to 1904 using the naturally bestowed internal heat and dry steam, after which the United States, Mexico, Iceland, New Zealand, Japan and Russia now generate electricity via the use of geothermal energy [5]. Nigeria is lagging in exploiting its available geothermal energy, even with the abundance of warm springs all over the country which includes Wikki Warm Springs, 32 • C (Bauchi State), Keana-Awe Thermal Springs, 34 • C (Benue Trough), Ikogosi Warm Springs, 37 • C (Ekiti State), Nike Lake (Enugu State) and Kerang Spring (Plateau State). While the others with high warm/thermal springs are Rafin Rewa Warm Spring, 42.5 • C (Plateau State), Akiri Warm Spring, 54 • C (Nasarawa) and Ruwan Zafi Hot Spring, 54 • C (North of Benue Trough) [134][135][136]. This abundance of natural resources should be critically looked into by the researchers, institutions, organisations and other international collaborators as an alternative for power generation to bolster the current effort of the government in ensuring stable electricity/energy supply. Fig. 11 shows the evolution in the scientific research publications on RER in Nigeria during the last 4 decades. It is observed from Fig. 11(a) that half of the published documents (242/52.8%) have been concentrated on the utilization of solar RER, after which wind (88/19.2%), biomass (85/18.6%), geothermal (22/4.8%) and finally hydro (21/ 4.59%) followed accordingly. The highlighted data shows that less attention has been given to the area of geothermal and hydro energy, even though the later (hydro) is one of the RER that generates most of the power to the country's national grid. Hydropower is the mainstay of the nation's electricity generation for the past decades but publications in recent years (2012-2019) showed an outstanding performance in the use of solar, wind and biomass as a supplement/alternative to the energy generation in the country. The possibilities for this could be that most of the LHP plants were old, established in the 60s, 80s and early 90s, which are the Kainji (1968), Jebba (1986) and Shiroro (1990), respectively. The lack of equipment maintenance culture, inadequate improvement and funding in the Research and Development (R&D) sector, and low investment in facilities have hindered the growth in the number of published documents since the inception of the hydropower plant as illustrated in Fig. 11(b). More energy research funding, government intervention on RER programmes, institutional energy awareness and international collaborations on renewable energies for sustainable development favoured numerous publications in solar field exponentially between 2006 and 2019, while a gradual increment is observed simultaneously in both wind and hydro as least attention given to it. The high threshold observed in 2006 from this trajectory chart was as a result of the Renewable Energy Master Plan (REMP) launched, with the responsibility of increasing the role of RER in all ramifications towards sustainable development.

Outlook of RER publications in Nigeria
The investigation into these renewable energy resources (solar, wind, biomass, hydro and geothermal) in Fig. 11(b), expressed an exponential trend after the initiation of the 2006 REMP. The increment in publications is relative to the researcher's areas of interest, most dominant is solar energy. Although there are research interests from some on geothermal energy, geothermal resources for energy generation has not been exploited in the country. The RER contribution to Nigeria power generation for long term includes LHP at 48,000 MW, SHP at 19,000 MW, Solar-PV at 500 MW, Solar-Thermal at 5 MW, Biomass at 800 MW, and Wind at 40 MW. All these RERs is estimated at 68,345 MW as compared to all energy resources at 192,000 MW [137]. Hence the percentage contribution of renewables is 35.6% in Nigeria.

Interdisciplinary nature of Nigerian energy researches
Most of the identified keywords in the research finding are interdisciplinary and, similarly, most of the research breakthroughs are products of interdisciplinary research [138]. Therefore, giving more opportunities to look into the following keywords area, as itemized in

Energy policy
Many researchers have published different policy documents investigating the role of renewable and non-renewable energy resources in Nigeria. 123 published documents on the keyword "energy policy in Nigeria" was identified. To wean from the usage of fossil fuel resources, policymaker must create inducements for the RER sector. In 2003, the Federal government approved the introduction of RER as parts of its National Energy Policy for sustainable development with the active involvement of the private companies [44,75]. Few amongst the anticipated policies for solar RER is that; the nation shall keep abreast with the global solar energy technological developments; for biomass RER, the nation shall enhance the effective utilization of biomass conversion technologies; and also, for wind, the nation shall develop its wind RER as a supplement to other sources of energy.
As part of the policy framework 2005, the ECN in collaboration with the United Nations Development Programme (UNDP) drafted a REMP from the NEP strategies. The REMP enables the development and implementation of RER policies [44]. This plan stressed the necessity for exploring RE in promoting sustainable development. The reflection of this policy framework in 2005 has yielded an improvement in the number of work done on renewable energy resources as shown in Fig. 11 (b). It can be generally concluded that the research on solar energy has been predominant in recent years as compared to all other renewable energy resources at the moment.

Energy costs
The "costs" of Nigerian energy had been identified as part of the research keywords, with 94 documents presented. The recent average cost of grid electricity in the country is ₦36.6/kWh (naira/kWh) or $0.10/kWh (excluding VAT) [139]. Other countries like Indonesia and South Africa have similar prices, while electricity consumers pay an average of $0.08/kWh in India for electricity [139]. In line with the Multi-Year Tariff Order (MYTO II), the DISCOs otherwise known as the distribution companies are anticipated to have gradually reduced tariffs, but the contrary is the case in the Nigerian energy sector. Since 2016, grid costs have largely remained the same, and in several cases, electricity consumers have been charged with higher costs via the estimated billing methodology [139]. Nigerian electricity consumers are divided into five categories based on the tariff classes under the 2012 Tariff Order: industrial, commercial, residential, special and street lighting. Although, the cost of electricity incurred by the end-user is dependent on the quantity of electric energy consumed in kWh, and tariff rate in ₦/kWh.
The power generation through the off-grid, specifically the off-grid solar PV systems in the country are known to be cost-competitive, costing an average of $0.20/kWh as against both the diesel generator and gasoline costing about $0.30/kWh and $0.60/kWh [140], respectively. Today, renewable energy in Nigeria is competitive especially when the costs to the society are considered [140]. To overcome this barrier, global innovations in the energy business models and innovations must be developed and encouraged as it remains underexploited in the country.

Energy efficiency
The process of using less energy to perform the same task in eliminating energy waste can be term as energy efficiency thus, the country's large energy potential needs to be further exploited. There are 227 documents reported for Energy Efficiency (EE) in the Nigerian energy research. The number of this published document is considerably higher as compared to the others in this category. This shows Nigerian research interest inefficient energy, to contribute towards the reduction of greenhouse emissions. More of the essential benefits brought about by EE is to reduce demand for energy imports, lower household costs (such as zero energy building) and overall economic development [76]. The changes in energy consumption on our existing infrastructures can easily be made by reducing energy usage and costs. The use of LED light bulbs and energy-efficient appliances, or incorporating some drastic measures such as weatherization of the buildings, are steps to be considered now and for the future energy prospect. The smart grid is another alternative that will significantly enhance the efficiency of electric generation, distribution, and consumption. There are vast opportunities for EE improvements in every sector of the Nigerian economy: industry, transportation, building, or energy generation.

Energy gap
The energy gap in Nigeria returned a total of 139 published documents in the Nigerian energy research on the Scopus database. A wide gap exists between the energy requirement of the country population and the amount generated [141]. The act of bridging this gap on a sustainable basis is arguably the single most important developmental challenge facing the country [142]. Some of the highlighted barriers to policy implementation that need urgent attention by the FG of Nigeria include ignorance and low-level awareness, lack/absence of R&D, poor level of government commitment, economic incentive, quality control and standards, inadequate data on consumers, non-availability of EE product, poor legal framework and enforcement, unreliable electricity supply, and corruption. Bridging this gap is seen as one of the major way-out for the country to achieve sustainable developments.

Conclusion
This work has outlined the use of Elsevier Scopus database to analyse the information regarding the contributions of Nigerian institutions and its international collaborators toward the scientific advancement in the energy field between 1974 and 2019. A total of 2791 published documents in various categories were recognized. This publication number increased from 8 documents in 1988 to 445 documents in 2019 as generally observed. Within this stipulated period, gradual increment in the number of publications became noteworthy from 2006 to 2015 with an average of 113 publications per year, to an average of 326 publications per year from 2016 to 2019. These documents were mostly published in journal articles (1949/69.8%) and conference papers (656/ 23.5%%), where the prevalent language of publications is written in English (99.9%). Nigeria SW region has been notable for most of the published document on energy, reaching about 44.7% of the country scientific outcome. The top 3 prolific institutions were suited in the following states of the country, Covenant University (Ogun), University of Ibadan (Oyo) and Obafemi Awolowo University (Osun), with the first being a private University while the remaining two Federal public University. South Africa (Africa), Malaysia (Asia), United Kingdom (Europe) and United States (North America) formed a high cluster of foreign collaboration with Nigeria, notable for higher publication shared in the field of energy.
Most of these research collaborations are have been in solar energy, wind energy, biomass energy however there are still some challenges in the optimum application of these forms of energy in Nigeria.
Most of the targets for solar electricity generation and thermal applications such as heating and drying have not been met. Some policies recommendations such as tax incentives, capital grants and Feed-in-Tariffs (FiT) to meet the targets and increase solar energy penetration in Nigeria have been propose but not fully implemented.
Some of the challenges that have identified for lower penetration of wind energy in Nigeria are the absence of local manufacturing for wind energy components, the reluctance of government to encourage wind technologies, lack of offshore wind mapping, and low financing. There have not been any policy programmes instituted to address these challenges.
Investigations show that the first-generation biomass feedstocks are the most employed in Nigeria emerging bio-refineries. The sources of the biomass feedstocks used are mostly food crops causing ill prospects towards achieving the full potential of biofuel production due to environmental degradation and food insecurity in the country. Second generation and third-generation biofuel research are limited in Nigeria, with nearly or no research on third-generation biofuel. These challenges need to be addressed effectively for the country successfully implementation of its biofuel objectives in promoting low carbon emission.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.