Research on the implementation of West Africa-North Africa grid interconnection using new electricity-water composite transmission technology

0 Introduction

For a long time,the world’s energy development has over-relied on fossil energy[1],which has led to increasingly prominent problems such as resource shortages,climate change,and environmental pollution[2-3].At the same time,the demand for energy for economic and social development continues to grow.There are still a large number of people without electricity in many countries and regions,and the sustainable development of energy is facing severe challenges[4-5].

According to the current mining intensity of fossil fuel,the world’s proven remaining coal and oil reserves can only be exploited for about 100 years and 50 years respectively.The long-term large-scale development and use of fossil fuel has caused serious pollution and damage to the ecological environment such as water quality,soil,and atmosphere[6].The cumulative emissions of sulfur dioxide,nitrogen oxides,and inhalable particulate matter from global fossil fuels have reached 9 billion tons,8.6 billion tons,and 5 billion tons,respectively,causing serious environmental problems such as air pollution and acid rain[7-8].At the same time,coal and oil mining caused ground collapse and vegetation destruction,resulting in a drop in groundwater level and damage to water quality.

At present,there are still 1.06 billion people without electricity in the world,of which more than 90% live in sub-Saharan Africa,South Asia,Southeast Asia and Latin America.Approximately 3 billion people in the world still use fuelwood,animal manure,etc.for cooking and heating.If everyone can enjoy clean and sustainable modern energy services,the energy demand will continue to grow in the future[9].

To cope with global energy challenges and accelerate the world’s energy transition,the key is to establish a global energy outlook,research and solve energy problems with a global perspective,historical perspective,forward thinking,and systematic methods,and promote the coordinated development of energy and economy,politics,society,and the environment.

Clean energy is the dominant energy in the future,and it is mainly converted into electricity.To vigorously develop clean energy power generation,it is necessary to shift from the traditional power development mode of local balance and self-sufficiency to the coordinated development of countries on all continents and the global configuration of power development,and accelerate the construction of clean-oriented,power-centric,interconnected,coconstructed and shared modern energy system,namely Global Energy Interconnection(GEI)[10].

It is technically feasible to build GEI,and high-voltage and ultra-high-voltage power transmission technologies can achieve tens of thousands of kilowatts and thousands of kilometers for power transmission[11-14].At present,there are 24 UHV projects in operation and under construction in China,realizing large-scale and long-distance transmission of clean energy.The difficulty in building GEI is scientific and reasonable planning,global consensus,worldwide investment and financing.In order to achieve the optimal allocation of large-scale clean energy resources on a global scale,and to achieve the dual optimization of economy and carbon emission reduction,it is necessary to comprehensively consider the distribution of resources,demand distributions,and different resources and load centers on all continents,regions,and countries around the world to achieve complementary characteristics of time zone and cross-season.

At present,regional grid interconnection planning has been carried out in some regions,and the construction of transnational backbone transmission channels in the region has been proposed.For example,the European Union proposed to build a pan-European super grid in 2008.Later,the European network of transmission system operators(ENTSO-E)take the lead in super grid planning,which covers 34 countries and 41 transmission grid operators in Europe,and proposes to build backbone channels through high-voltage direct current transmission technology to achieve interconnection between European countries and neighboring regions,including Kazakhstan,North America,and Turkey[15].The Desert Solar Energy Initiative(DII)proposed a desert science and technology project in 2009.It plans to build high-voltage DC channels to send solar energy from the Mediterranean coast and other desert areas to densely populated load centers[16].Southeast Asia Super Grid,Asian Super Grid,Nordic Super Grid,North America Super Grid,etc.have all proposed constructing power transmission channels to send clean energy electricity to load centers.

In the backbone grid of GEI[17],the idea of constructing a power transmission channel of approximately 8,000 kilometers across Africa and Europe and connecting the northern and southern hemispheres,from the Inga Hydropower Station to the European load center,is proposed.In order to realize this grand vision,in addition to solving the problem of the transmission line crossing the Mediterranean,how to traverse the Sahara Desert that spans between South and North Africa is also an important constraint.

The Sahara Desert is rich in solar energy resources,but due to severe water shortages,photovoltaic development conditions are severe;power shortages in North and West Africa have become increasingly prominent,but water resources are relatively abundant.In response to the abovementioned problems and needs,this paper proposes a combined electricity-water transmission technology to introduce water resources from West Africa and North Africa to the Sahara Desert photovoltaic base,benefiting people’s livelihoods along the route,and supporting the construction of photovoltaic bases.At the same time,the electricity from the Sahara photovoltaic power station is transmitted to North Africa and West Africa,to achieve optimal allocation of resources.

1 Overall plan

Because it is located in the desert,hot and dry,and blocked by the high mountains of North Africa and the north of West Africa,the Sahara region is severely short of water,and the natural and living conditions are severe.There are only two north-south highways crossing the Sahara Desert between West Africa and North Africa(Mali,Niger to Algeria).Due to severe water shortages,the towns along the route are sparsely distributed,with a small population and weak economic development capabilities.Solving the water problem is not only related to the people’s livelihood in the cities and towns along the route,but is also the key to improving the regional economic development capacity,and it is also an important measure for people to connect.In addition,the choice of tower bases in desert areas and the erosion of wind and sand make the construction of overhead lines difficult;the backward infrastructure,harsh living conditions and harsh natural environment of the Sahara and surrounding areas make the operation and maintenance of transmission lines extremely difficult.In order to realize the transmission of electricity from Central Africa Inga hydropower stations to Europe,it is necessary to overcome the onshore bottleneck of the transmission channel crossing the Sahara Desert.

Africa is rich in solar energy resources,about one third of which are mainly distributed in the northern and central Sahara Desert and surrounding areas,including Mali,Niger,Morocco,Tunisia,Algeria,Egypt and other countries.However,due to the severe water shortage and poor living conditions in the Sahara,it is difficult to convert the abundant energy resources into economic benefits.

West Africa and North Africa are densely populated(330 million and 220 million,respectively),and their economies are relatively developed(per capita GDP is 2,200 and 3,700 US dollars,respectively,twice and three times the average level of Africa),and North Africa has a large demand for electricity(per capita electricity consumption is 1,166 kWh,twice the average level in Africa),countries such as West Africa and Nigeria are seriously short of electricity,and electricity consumption has grown rapidly(the average annual growth rate in the past five years has exceeded 5%).At the same time,compared with the Sahara region,the Niger River Basin in West Africa is rich in water resources.The use of solar thermal power generation along the coast of North Africa can produce fresh water,which can provide necessary water resources for the economic and social development of the Sahara hinterland and the people’s living water.

Combining the characteristics of the reverse distribution of solar resources and water resources in North Africa,West Africa,and the Sahara,this paper aims to propose a West Africa-North Africa grid interconnection project,to realize the optimal allocation of desert solar resources and surrounding water resources,and lead water resources of West Africa’s Niger River and North Africa’s seawater desalination into the hinterland of Sahara desert,to improve the living environment and development conditions;build photovoltaic bases along the route to send clean electricity to North and West Africa to accelerate energy transition;overall,after the completion of the project,it will show that “electricity is sent to both ends and water flows to the middle”,as shown in Fig.1.In the long run,the interconnection project across the Sahara Desert will also become a key link in opening up the power transmission channel from the Inga Hydropower Station to Europe.

Fig.1 Schematic diagram of West Africa-North Africa grid interconnection containing power and water

2 Transmission channel path

The route consideration of the West Africa-North Africa grid transmission channel is shown in Fig.2.The starting point is Gao in Mali and the ending point is Bouzanib,Morocco.The length is about 1860 kilometers.The passage is mainly constructed along the road connecting Marigao and Adrar,Algeria.The starting point and ending point of the channel are higher in altitude,and the Sahara water receiving area is lower in altitude,which can make the water resources at both ends flow to the central water-scarce area with the help of potential energy,reducing engineering technical difficulty.

Fig.2 The path and altitude of the grid interconnection channel in West Africa and North Africa

3 Electricity-water composite transmission technology and engineering scheme

There are two implementation schemes for the transmission channel,as shown in Fig.3.One is the electricity-water separation transmission scheme,which uses the same route for different projects to transmit electricity and water separately;the other is the electricitywater composite transmission scheme,which uses the same route for the same project to transmit electricity and water at the same time.

Fig.3 Two possible schemes for the electricity-water transmission

For the electricity-water separation transmission scheme,considering the large amount of water evaporation in the canal transportation in desert areas,the transmission method of overhead lines + water pipelines is adopted.It is not only difficult to construct iron towers in desert areas,but also overhead lines and iron towers have to withstand wind and sand blows and sand and gravel impacts all year round,and the operating environment is very harsh.

For the electricity-water composite transmission scheme,inspired by the ultra-large-capacity transmission pipeline technology proposed by the State Grid Corporation of China(integration of transmission conductors and coolant water pipes in a pipe fitting,see appendix for details)[18-19],this paper further studies and proposes electricity-water composite transmission technology concept,the crosssectional structure of the pipeline is shown in Fig.4.

Fig.4 Sectional structure of electric-water composite transmission pipeline

The six electricity transmission pipes are used for power transmission,and the deionized cooling water in the middle is used to take away the heat generated by the electricity pipes and cool pipes.Fiber optic conduits are used for communication.The center of the entire pipeline is the cooling return pipe.A domestic water delivery pipe is coaxially arranged on the outside of the cooling return pipe to realize the efficient reuse of the transmission pipeline,reduce the difficulty of construction and improve the reliability of operation.

The electricity-water composite transmission technology proposed in this paper is based on the ultra-large-capacity transmission pipeline technology of the State Grid Corporation of China.A water delivery pipe is coaxially arranged outside the cooling return pipe.The water delivery pipe is used to desalinate water resources from the Niger River in West Africa and the light and desalination seawater in North Africa.It is sent to the Sahara photovoltaic base and areas along the line,and the electric power generated by the Sahara photovoltaic power station is sent to the powerdeficient areas of North Africa and West Africa using the conductive pipe of the transmission pipeline,so as to realize the efficient reuse of the transmission pipeline.

The working principle of the electricity-water composite transmission project is shown in Fig.5.The power transmission line and the water transmission pipe are integrated into a composite pipeline at the water intake point;after reaching the water consumption point and the photovoltaic base,the separation of the power line and the water pipe is carried out,and the power line is connected to the local power grid to realize power exchange.Part of the water is taken out to meet the local water consumption.Then the power line and the water pipe are integrated again to continue transmission.

Fig.5 Working principle of electric-water composite transmission project

In terms of project implementation,each section of pipe is prefabricated in the factory,which is similar to concrete prefabricated parts,and the electrical connection and installation are completed at the construction site,similar to the installation of oil and gas pipelines.

4 Analysis of water resources supply and demand conditions in the Niger River

The electricity-water composite transmission pipeline uses North African desalinated seawater and West Africa’s Niger River as water resources.Considering that seawater desalination in North Africa is mainly related with solar thermal power generation for artificial water production,and its water production can be adjusted according to regional power demand and water demand,this paper focuses on analyzing the supply and demand conditions of the Niger River in West Africa.

As shown in Fig.6,the Niger River is located in West Africa,with a total length of 4,180 km,a drainage area of 2.12 million km2,and an average annual flow of 5,589 m3/s into the sea.The upstream and downstream are rich in precipitation and large in flow.Around by the middle reaches of the river flowing by the marginal area of Sahara,there is little precipitation,strong evaporation,and reduced water flow.The average annual flow is only 1,020 m3/s.The minimum flow measured in the river section with the smallest flow between Mali Timbuktu and Gao is 22 m3/s.

Fig.6 Geographical location map of the Niger River basin

The total amount of water used to support the construction,operation and maintenance of photovoltaic bases along the electricity-water composite transmission channel can be defined as

In formula(1),sis the total amount of water,m3/year;cis water consumption per capita in the Sahara,m3/person·year,this paper takes its value as 500 m3/person·year;pis the increased population of water consumption,person.Considering the cascade development,operation and maintenance of 50 GW photovoltaic bases,10,000 people will be newly added.Then the total amount of water needed scan be calculated as 5 million m3/year,and that’s about 0.16 m3/s,which accounts for about 7‰ of the smallest flow in the middle reaches of the Niger River.

Taking the middle route of the South-to-North Water Transfer Project as a reference,the head flow of the Taocha Junction of Danjiangkou Reservoir is about 60 m3/s,the water level of Danjiangkou Reservoir is 156.78 meters,the inflow is 830 m3/s,and the discharge flow is 1,620 m3/s,which is 4% of the discharge flow.The maximum flow of the main stream of the Han River is 833 m3/s,the minimum flow of the main stream is 42.86 m3/s,and the head flow accounts for 7% of the maximum flow.

This proportion of 7‰ is much smaller than the ratio(7%)of the head flow of Danjiangkou canal to the inflow of the middle route of China’s South-to-North Water Transfer Project.Therefore,it is hydrologically feasible to divert the water of the Niger River along the transmission channel that crosses the Sahara Desert.

5 Economic and social benefit evaluation

The estimated investment of the Hami-Zhengzhou DC project line is 5 million yuan/km,the wire is 6×1000 mm2,and the wire current density is 0.8333 A/mm2.It is estimated that the project cost is three times the material cost.For the large-capacity transmission pipeline of ±800 kV and 8 million kilowatts,according to the current density of the Hami-Zhengzhou project,the unit cost of the two bipolar pipelines is 9297 yuan/m.Since each pair of pipelines contains 6 DC transmission lines,the cost of a single DC project transmission line is 1.55 million yuan/km,which is less than 1/3 of the overhead line cost;considering heat dissipation issues,such as the wire current density is 0.5 A/mm2,the cost of a single DC project line is 2.55 million yuan/km,which is about 1/2 of the cost of overhead lines.Therefore,super-capacity transmission pipelines have obvious economic advantages over overhead lines.

For the conventional overhead line + water pipeline transmission scheme,the water pipeline will bring additional costs,so the cost of the DC project line must be higher than 5 million yuan/km;for the electricity-water composite transmission pipeline scheme,the pipeline only increases the water pipe part,and the cost increase is not obvious.When the wire current density is 0.8333 A/mm2,the cost of a single DC project composite transmission line does not exceed 2 million yuan/km,and the wire current density is 0.5 A/mm2.The cost of DC engineering compound transmission line does not exceed 3 million yuan/km.Therefore,the electricity-water composite transmission pipeline scheme has obvious economic advantages over the conventional overhead line + water pipeline transmission scheme.

The Sahara Desert is rich in solar energy resources,but due to severe water shortages,photovoltaic development conditions are poor.The electricity-water composite transmission scheme proposed in this paper can lead the water resources of West and North Africa to photovoltaic bases in the Sahara Desert,benefit the livelihoods of the people along the route,and support the construction of photovoltaic bases.The operation and maintenance personnel of photovoltaic power plants in the Sahara region are calculated at 10 MW per person,and the photovoltaic installations along the Sahara Desert are calculated at 30 million kilowatts,and a total of 3,000 operation and maintenance personnel are required.The water consumption of operation and maintenance personnel is calculated at 500 m3/(person·year),and all operation and maintenance personnel need a total of 1.5 million m3 of water per year,which is approximately 0.05 m3 per second,which occupies 10,000th of the smallest flow of the Niger River(500 m3/year).

6 Conclusion

Research results of this paper shows that the use of electricity-water composite transmission technology to build a interconnection project that crosses the Sahara Desert and connects North Africa to West Africa can solve the problem of water use along the line to a certain extent and fully mobilize the enthusiasm of all parties.And the engineering technical solution is economically feasible.At the same time,the construction of an electricity-water composite transmission channel can achieve a transmission capacity of nearly 50 GW,and because the pipeline is underground,on the one hand,the pipeline operation and maintenance is less affected by natural climate and human factors,and on the other hand,the pipeline has nearly no impact on the surrounding environment and ecology,compared with overhead lines.Therefore,it will also have the promotion and application value for the construction of large-capacity power transmission projects in areas with harsh natural climate conditions such as the Arctic in the future.

Regarding the North Africa-West Africa interconnection project proposed in this paper,the next step is to jointly conduct in-depth research with relevant parties.First,further in-depth analysis of the overall water resources supply and demand conditions and economy of the project,reasonable allocation of water and power transmission costs,and calculation of local water prices.The second is to investigate and collect the hydrological,geographic,and geomorphological data of the Niger River Basin and the Sahara Desert,optimize the Niger River water intake point,and use the hydropower station with high water level and large storage capacity as the water intake according to the Niger River hydropower development plan as much as possible,combined with the path of the composite pipeline.If necessary,consider joint development of integrated hydropower and water conservancy projects at the starting point of the pipeline.The third is to refine the research on engineering technical plans and regulations and policies,including the prevention of electric shock for water intake at the intermediate site of the composite pipeline,technical issues such as the aquatic environment inside the water pipeline,pressure loss,and water hammer effect,as well as the utilization of water resources,policy and legal issues in relevant countries in the Niger River Basin.

Appendix A

This appendix mainly introduces the technology of ultralarge capacity transmission pipeline.

Compared with other energy transmission methods horizontally,the existing power transmission technology still has certain limitations,which are mainly manifested in the relatively low transmission energy density,relatively high transmission loss rate,and the transmission reliability is easily affected by the external environment.Transmission lines mainly come in two forms:overhead lines and power cables.The two main technical problems faced are insulation and heat dissipation.

The ultra-large-capacity transmission pipeline technology proposed by the State Grid Corporation of China has the characteristics of “conductors underground,rigid insulation,hollow cooling,and waste heat utilization”.The main body of the power transmission pipeline is metal aluminum,and the insulating material is polymer concrete PCC;structurally,the water inside the pipe is cooled in a closed cycle,and the heat generated by the conductor resistance is taken out and recycled by the heat exchanger;the outside of the conductor and the conductor are filled with inorganic.The mineral material plays the role of electrical insulation.The outermost layer is covered with a protective layer and a shielding layer;each conductor can be used as a pole(phase);each section of pipe is prefabricated in the factory,similar to the concrete prefabricated parts,electrical connection and installation are completed at the construction site,similar to the installation of oil and gas pipelines.

Fig.A1 Sectional structure of ultra-large-capacity transmission pipeline

If the metal pipes in the pipeline are arranged independently,the transmission power will be 6 times that of the conventional overhead line,and the transmission capacity will be greatly improved;the use of polymer concrete PCC is beneficial to increase the rated voltage of the line;the internal cooling system makes the heat dissipation under large currents less than a problem again;the waste heat recovery system is used to make use of the heat generated by the resistance of the metal conductor;after the pipe enters the ground,it is no longer affected by natural weather conditions and some human factors,and the reliability is improved.

Acknowledgements

This work was supported by National Natural Science Foundation of China(No.72131007):Agentbased modeling and analysis on evolution of the energy technology system for alternative fuel vehicles;Science and Technology Foundation of Global Energy Interconnection Group Co.,Ltd.(No.SGGEIG00JYJS2100027).

Declaration of Competing Interest

We declare that we have no conflict of interest.