Rural electrification remains a common challenge for many developing countries and especially in Sub-Saharan Africa (SSA)   . Indeed, the rate of electrification in SSA is only 32% to 35%  of which only 14% are related to rural electrification  . Approximately 20% of the world’s population lack access to electricity   and they will not have access by 2030  . In SSA, only 290 million out of 915 million people have access to modern energy services and major party of rural populace lives in the darkness . Nowadays, household electrification rates of the Global South remain below 15% to 20%  while the continent of Africa consumption is only 3.2% of the world’s primary energy consumption . SSA is marked by a low power consumption (less than 1 kWh per capita per day) . Additionally, SSA is often characterized by insufficient power grids . Indeed, improving access to electricity in SSA rural areas is crucial, access to electricity occurs generally good result such positive health, education and income consequences and social welfare . Energy sufficiency is a prerequisite for achieving the economic development of a country   . Moreover, access to sustainable energy services allows the expansion of commercial and educational activities at night and will improve the standard of living of the local population . Therefore, a United Nations project named Sustainable Energy Project for All (SE4All) has been implemented with the aim of providing electricity to all by 2030, especially in SSA where electricity demand is often modest . However, planners and policy makers always faced many barriers to supply electricity in remote areas. Section 2 highlights the main barriers in electrifying embedded areas. Several studies were done to trace the effective solutions and to facilitate rural access to electricity. In this paper, some strategies and solutions implemented in SSA and in some other developing countries located in South Asia and Latina America are shown in Section 3. Section 4 presents the cost-effective schemes that research studies have occurred during the last decades.
2. Main Barriers to Rural Electrification in SSA
The main barriers to rural electrification in SSA are: the population affordability and low income  , rural inaccessibility due to the road difficulties   and remoteness    , low population densities and dispersed households   , low project profitability  , fiscal deficit, scarcity of energy resources   , population growth, lack of professionalism, and over-dependence on subsidies    .
Firstly, households in southern countries still suffer from the unequal distribution of electricity and low electricity generation capacity . For a long time, urban electrification has been prioritized over rural electrification    . This situation increased the rate of rural-urban migration in SSA .
Secondly, the fossil energy resources such as oil and natural gas will eventually be depleted and the environmental issues caused by their use  , are increasing day by day. Brownouts and blackouts are very common in developing countries, as supply cannot meet demand     . These are the main motivating factors to use renewable energy resources   . Project planners and policy makers strive to use renewable energy as alternatives due to the increasing fossil fuel prices. SSA is also marked by higher electricity prices, with an average of 26 US cents per kWh . However, renewable energy resources cannot fully guarantee demand    . In addition, the use of some renewable energy resources is highly dependent on the variability of wind speed and intensity of sunshine  . Supplying remote areas are complex because data on population distribution   and electric load demands and geographic location are lacking    . Hence, the use of software is not easy due to lack of data on electric load in SSA . Energy planners in SSA are often poorly informed and lack skills, knowledge, documented experiences and open source quality data on renewable mini-grids. Therefore, they must rely on grid extension projects for the electrification of rural communities . Moreover, off-grid and mini-grid system installations are often complex and expensive in SSA because of the administrative processes to follow .
Most of infrastructures present in developing countries such as SSA are constructed during colonial period . Indeed, the transmission lines are outdated and cannot handle the distribution of high voltage power. Much of the electricity distributed across the national interconnected grid is lost through transmission lines. With current infrastructure conditions, transmission to embedded zones is only increasing the loss .
Moreover, another barrier to rural electrification is the price difference between rural and urban areas. Customers in the rural area are engaged in high investments and high operational costs compared to those in the urban area. In addition, there are technological complexities which lead to sustainability issues.      . The regional disparity is another big dilemma for the electrification of rural and remote areas .
Furthermore, the combination of political instability and commercial risks discourages private investors from seizing opportunities in the field of decentralized rural electrification   .
In developing countries, measurement error and lack of precision have often shattered estimates of network coverage and network connectivity . Increasing of fuel prices is also an issue of rural electrification   .
Many strategies and solutions have been implemented in many different developing countries to overcome the barriers mentioned in Section 2. There are large scales of policy in developing countries especially SSA.
3.1. Software Use for the Implementation of PV and Renewable Energy
There are many approaches for rural electrification . Nowadays, several tools related to electricity supply in rural area are available commercially or for free. A comprehensive modeling of an electrification project is done using software such as HOMER Energy  , Network Planner and GEOSIM . Understanding the consumer profile, identifying the potential site and local resources are the first step in developing an electrification strategy  . Therefore, geographical analysis is required, the type of renewable energy to be implemented is obtained from an analysis of the structure of the terrain by using a satellite image  . The HOMER Energy modeling tool is a key element especially for the implementation of scenarios  . HOMER has been used by planners and decision makers to evaluate the design issue  and the techno-economic viability of renewable energy systems . Moreover, a certain number of load scenario must be designed in order to improve simulation . Nevertheless, additional research is highly recommended due the limitation of available software (HOMER, RET, LCOE) as Onyeka et al. show in their article . In order to locate the consumers and to know if their area is electrified or not, a spatial modeling and a GIS (Geographic Information System) database are applied . The zones with higher solar resources can also be identified by using maps of solar resource based on GIS database . In addition, the use of global solar radiation data from the NASA can be helpful while deploying renewable energy technologies .
Nowadays, the level of electrification can be measured by the night-time lights . The remote-sensing data or data obtained by this new process when administrative archives are absent or unreliable are auspicious resources .
3.2. The Decentralized System Based on Renewable and PV-Hybrid Technologies
As in many developing nations, power supply mostly depends on diesel generations  though the rate of carbon dioxide emission of Africa is less than 4% of the global emissions . However, climate change mitigation can now be considered as the essential factor that can influence the rates of renewable energy deployment in SSA . In fact, in developing countries, the consumption per capita might increase for the next decades . Indeed, in order to recover the demand forecasting, development of renewable energy technologies as alternatives to replace fossil fuels has become imperative  .
The continent of Africa has unlimited resources in term of renewable and nonrenewable energy sources . Despite its high potential  , the implementation of PV (Photovoltaic) electricity generation still remains non-considerable . Many papers show that researchers try to study the cost of PV system and reduce its cost to enhance the rural access to electricity. Otherwise, the result proposes that current cost of PV technologies is cost-effective for policy makers, besides PV has the ability to repay its financing loan . Now, the PV price reduction is about 22% according to findings of  .
Moreover, the decentralized system is efficient for a remote village   . In order to provide electricity in scattered sectors, the use of decentralized systems such as small diesel power generations and renewable technologies such as biomass, PV systems, small wind and small hydro power systems are currently used . The decentralized system uses some components like batteries, diesel generators, pedal generators and so on . Furthermore, by considering the PV-Wind system  , the wind and solar intensity variation can be mitigated  due to complementation between solar and wind system  . Sometimes hybrid system such as PV-wind-diesel systems can be used  . H. Kim and T. Yong  proposed in their research article several types of hybrid system that can be utilized, such as PV-diesel generator system, PV-ESS (Energy Storage System), PV-Hydropower system  . Otherwise, the use of the local resources available such as solar system    , wind system  , biomass   and micro-hydropower system  enhances the decentralized system deployment, however its nature depends on the area   .
Furthermore, over 90% of renewable energy in SSA is assumed by hydropower . Small hydropower is entirely considered as renewable energy and has more potential than large hydropower to contribute to poverty reduction . In SSA excluding South Africa, the use of Concentrating Solar Power (CSP) plants is also a promising way to increase rural electrification rate. CSP is a generating electricity system that exploits the heat from the sun . Another study suggests the use of biomass (charcoal, wood) coupled with wind and solar power systems such that the system capacity is up to 20 kW .
3.3. The Microgrid and Off-Grid System for Rural Electrification
In addition, microgrid is one of the strategies implemented to electrify rural areas . Microgrids are small electrical grids based on renewable resources. It is a kind of decentralized system that uses renewable energies and low carbon-resources . Indeed, microgrid plays a major role in improving access to electricity in embedded area  as it is also an autonomous grid system . In the Global South, central grid cannot reach remote areas   . Hence, SSA needs to establish an alternative for the grid extension. Off-grid solar system is estimated to have great potential in SSA . Paulo et al.  show off in their paper different off-grid system type that can be implemented in remote areas. Furthermore, installed renewable energy systems must be revisited about 12 years after installation to check if goals have been reached or not .
Moreover, the choice of off-grid system to be implemented in isolated area is made by considering certain conditions   . The opinion of the end-users  and their needs and wishes should be considered in order to improve rural electrification policy    . Technical maintenance and financial management are required within an installed decentralized network . Hence, each village in which technologies are installed should have an administrator and an operator who have received training .
3.4. The Energy Market in SSA
Researches about energy market in SSA have been spearheaded by Bensch et al.  , their study reveals that solar market has known significant revolutions in terms of technologies and business models since 2012. They also emphasize that energy market in SSA is entirely characterized by non-branded products. Otherwise, lower taxes will strengthen the distribution of renewable energy in the sub-Saharan African market .
3.5. The Role of Democracy and Government in Rural Electrification
Furthermore, in SSA, democracy promotes rural electrification . It has been found that it is at the level of public services such as education and health that democratic states invest the most. Therefore, multipartism accelerates rural electrification . Moreover, government must establish support schemes and regulatory frameworks to facilitate the share of renewable energy . Hence, institution also plays key role in rural electrification project. In each country, energy policy must be installed . Access in remote areas led by the government is important because government support coupled with strong political measures is important for the implementation of an electrification project  . Some countries such as Tanzania start the grid extension project by electrifying the headquarters and then the distribution line promotes the villages access to electricity .
3.6. The External Factors Influence on Rural Electrification
Moreover, external factors have a great influence on rural electrification projects, especially renewable energy projects . As an example, it can be cited the case of a fee-for-service model chosen by European Union to facilitate access for the poorest households to the energy technology . In order to double access to electricity in SSA, Power Africa is aiming to add 60 million new electrical connections. Additionally, the project supports both renewable and non-renewable energy projects and aims to increase the installed capacity by 30,000 MW by 2030 . Furthermore, to encourage private investors, policymakers and planners in developing countries need to design an attractive project in terms of lower investment costs .
Another approach is to create local cooperatives that implement a decentralized system and therefore offer attractive alternatives to the public sector or the private sector .
Table 1 summarizes the strategies and solutions provided in developing countries to overcome the barriers in rural electrification.
4. The Existing Cost-Effective Schemes for Rural Electrification
Researchers proposes that Africa requires between US$41 billion to US$55 billion annually until 2030 to achieve access on electricity for all  , findings show that there is a strong hope for SSA to attain the massive electrification within the next decades. Researchers agree that renewable option is economically viable in developing countries . By reducing renewable energy cost such as PV and CSP cost in the energy market and establishing regulatory frameworks and institutions, SSA’s rural electrification rate have increased. For instance, since 2012, the number of PV projects implemented has increased . Furthermore, solar technology has been accepted as a sustainable future alternative to replace fossil fuel  . Researchers also confirm that implementation of energy systems requiring higher investment than diesel-only generators will be rare . Diesel-only scenario is the most expensive system to generate electricity. They
Table 1. Strategies and solutions for rural electrification in developing countries.
also highlight that hybrid diesel/PV with the Energy Storage System (ESS) is the optimum scenario . PV coupled with ESS costs the highest  for a hybrid system but it is also a viable alternative for energy technology . Furthermore, in terms of cost-effectiveness, even non-branded products are the most used in SSA, non-branded products perform significantly better than branded products due to considerably lower prices . Moreover, democratic institutions have influences in rural electrification, reduce the inequality of rural and urban electrification in SSA . In addition, cooperation with international community and developed countries also improves energy access to rural areas. Chirambo et al.  highlighted the aid that China brings toward SSA, there is also Power Africa which is the U.S government initiative.
Rural electrification improves access to education, health, safety and water supply   . Access to electricity leads to improved quality of life and higher standard of living  . RESs (Renewable Energy Systems) have advantages such as: faster procurement, sometimes more economical than grid connection, and clean for the environment . The advantage of decentralized system is the utilization of the local resources, it avoids the inefficient transmission losses and it is often not expensive, and suitable for low-load factors projects . Moreover, the use of batteries provides fast-access to electricity, no pre-installation work needed, independent from either site-specific or source available, suitable for loads used frequently . Furthermore, biomass use for electrification reduces disparity in remote areas  . Moreover, microgrids ease maintenance compared to SHSs . Besides, exploitation of hydroelectric resources could alleviate chronic energy shortages and lower energy prices .
The decentralized model will be cheap if the consumption in the areas is low and the centralized model is cheap in high consumption region depending on the available resources and the model using a grid extension depends on the number of factors and geographical distributions of the target population . As an example, for a low consumption (up to 400 KW): the cost of electricity using microgrid is about 24.2742 EUR/MWh  while for the case of Madagascar the electricity cost is 122.5 EUR/MWh for grid extension .
6. Extension Scheme for Rural Electrification Model
The figures presented below show the existing rural electrification models. Figure 1 represents the decentralized model using both renewable and non-renewable energy. Microgrid systems which are designed to supply electricity for loads such as small village  are presented by Figure 2. Furthermore, the grid extension suitable for a high consumption is presented by Figure 3.
One finds that in SSA, only 290 million out of 915 million people have access to modern energy services  and major party of rural populace lives in the darkness. Many of those who do not have electricity are in remote and rural areas, consequently rural electrification is a serious subject for many governments around the world. Thus, there are two methods to extend the electrification in the rural areas: the decentralized model and the grid extension. The latter approach is more cost-effective for high consumption level and the decentralized model is more cost-effective for lower consumption level. Electricity in the
Figure 1. Example of a decentralized model.
Figure 2. Example of a decentralized model using microgrid system.
Figure 3. Grid extension schematic representation.
farming sector offers the possibility to improve the suppliers of welfare department, and economic directed advantages. Rural electrification and its associated impact on income and quality of life may not be sufficient to reduce rural-urban migration flows and other drivers of migration such as aspirations for the glamorous urban life depicted on television or for an elite education, need to be targeted and addressed to reduce stress on cities in developing countries today. It is debatable if renewable energy-based rural electrification programs are more feasible, cheaper, and less problematic solutions for improving electricity access in remote rural areas of developing countries than grid extension electrification programs based on fossil fuel, hydro, or nuclear.