Alleviating freshwater shortages with combined desert-based large-scale renewable energy and coastal desalination plants supported by Global Energy Interconnection

1 Introduction

Energy,environmental,and freshwater resources are necessary for social sustainable development [1].Since the industrial revolution,fossil fuels such as coal have been the main energy sources used to promote the rapid development of human civilization,and the existing fossilbased energy structure supports the current social economy.However,excessive dependence on fossil fuels has led to serious pollution (including greenhouse gas emissions) [2],global warming [3-5],and energy crises [6].For instance,the pre-industrial global average atmospheric concentration of carbon dioxide never exceeded 300 ppm [3],however,it reached 407 ppm in 2017,which is the highest level during the past 800,000 years,and contributed to occurrence of the three hottest years in history [7].Thus,the international community must respond by controlling greenhouse gas emissions and actively promoting the transition to clean,low-carbon,renewable energy [8-10].

Global Energy Interconnection (GEI) is a bold venture for the generation,transmission,and use of clean energy,applying a smart grid as well as ultra-high voltage (UHV)transmissions to transport renewable energy over long distances and with minimal losses [11].It was conceived as a means of promoting clean,low-carbon renewable energy,with the intents of addressing global climate change and realizing energy sustainable development [8,9].In tandem with GEI,ultra-high-voltage (UHV) transmission is an important energy technology that guarantees largescale development of renewable energy and clean energy utilization.Actively promoting the use of large-scale renewable energy is both an urgent and inevitable trend which can be promoted through the realization of UHV and GEI [7].

As another global environmental problem,the global distribution of freshwater resources is extremely uneven.With the improved living standards and rapid growth of the social economy,an increasing number of countries are facing serious freshwater resource shortages [12].In addition,desertification is defined as land degradation resulting from both climatic-natural variation and human activities,such as the decreasing in surface precipitation and the over-exploitation of natural resources that aggravating the process of desertification [13,14].According to the estimation by the United Nations Environment Programme,approximately 167 countries are affected and 24% of land worldwide is threatened by land desertification [15].Therefore,solving water supplying problems could help to alleviate both freshwater shortages and land desertification.

Seawater desalination has attracted attention as an effective and strategic method for creating freshwater resources [16].However,traditional fossil-fuel-driven desalination technologies consume large amounts of energy and have negative environmental impacts.Therefore,abundant,cheap,and clean renewable energy is needed to supply power for modern seawater desalination operations.Renewable energy could reduce the reliance on fossil fuels for desalination.Compared with fossil fuel-derived energy,wind and solar energy are cleaner,have reduced environmental impacts,and are renewable.However,these energy resources have temporal fluctuations due to the randomness of wind and irradiation.Given the rapid development of renewable energy generation technologies,the use of large-scale wind and solar energy generation has become an attractive solution for energy development [17][18].As of 2017,global renewable energy investments exceeded 200 billion dollars annually for the previous consecutive eight years.So far,China is the largest investor in renewable energy in the worldwide.The investment in 2017 is 126.6 billion dollars,which increases 31% compared with that of in 2016 [19].And the global solar energy power generation increased 98 GW,which is increased about one third compared with 2016 (303GW) [20].Such advances depend not only on political support,but also on continuous reductions in the cost of solar power generation [18-22].

Among renewable energy sources,large-scale solar power stations in deserts have become an important direction for industrial-scale solar power development[23].Solar power can be obtained directly or indirectly from solar energy.For instance,coal,oil,natural gas,and other fossil resources formed after solar energy is converted and accumulated under high temperature and pressure.Meanwhile,solar radiation can be harnessed and used directly to drive power generation to generate electrical power.Different forms of solar energy have different intervals,or time scales,which presents that the solar radiation received or absorbed until it is converted into a usable form for energy production (Fig.1).From the perspective of smart grid development and energy utilization,fossil fuel will be replaced completely with renewable energy in the near future,supported by advanced technologies.Thus,solar energy,which is produced on a very short time scale,can be used to meet energy requirements without developing solar energy resources that are produced on very long-time scales.

In the face of large-scale energy,environmental,and freshwater crises,which may all be addressed with renewable energy,it is necessary to discuss the optimal methods for effectively producing and using large amounts of cheap,clean energy to solve these global problems.In the worldwide,solar power as an efficient,clean and cheap source for seawater desalination will alleviate freshwater crisis in most parts of the world [24],such as Saudi Arabia [25],Iran [26].However,most of seawater desalination is concentrated on the coast city [27].Longdistance transmission of large-scale renewable energy for desalination and then using it to solve water crisis is seldom discussed when the city is far away from the coast.

Fig.1 Schematic diagram of the time scales of solar power

Therefore,we analyzed the potential for using solar energy within a GEI framework to address these issues,with a focus on China.To this end,we estimated the potential amount of solar energy that can be developed in deserts in China in Section 2,and the electrical power necessary for seawater desalination and freshwater redistribution in Section 3.Then,the benefits of desalination and water redistribution for suppressing renewable energy power fluctuations were considered in Section 4,and the fundamental utility of GEI in realizing the energy transition was discussed in Section 5.Finally,conclusions are drawn in Section 6.

2 Potential of desert-based large-scale solar power generation as an energy alternative

Deserts are typically sparsely populated and abundant in solar radiation,both attributes highly suitable for the development of large-scale solar energy facilities.Fig.2 presents a map of the global distribution of deserts and vegetation.Deserts are mainly concentrated in North Africa,the Middle East and western Asia,central Oceania,and western North and South America.Previous studies have analyzed the feasibility of desert-based solar energy development,including the influences of large-scale solar power plants on local climate in desert areas [28],the global distribution of deserts and feasibility of large-scale solar energy development [29],and the challenges in realizing solar power generation in desert areas and the proposed solutions [30].Although the density of solar energy is high variability,large-scale solar power generation has a considerable energy potential.Large-scale desert solar power stations have become an important direction for the development of solar energy industry.

Fig.2 Map of the global distribution of deserts (source:NASA)[31]

In China,deserts are mainly concentrated in the northern and western regions (Fig.2).An estimate in 2002 suggested that equipping only 50% of the desert area with PV in China could meet the electricity consumption needs of the whole world [29].However,the actual amount of solar radiation that reaches the Earth’s surface varies greatly by season,weather condition,and time of day [32].Golmud is located in the west of Qinghai province and is on the mid-south edge of Qaidam basin.And the Qaidam basin is the main part of the urban area.The solar energy in the Golmud region of Qinghai province,China,is presented in Table1 based on measurement data from NASA.Using the radiation energy data,considering the conversion efficiency and the equipping area of PV,the average annual power generation of per square meter can be calculated by equation (1).

where Wave is average annual power(kWh),S is the area equipped with PV(m2),ηPV is conversion efficiency of PV.

Table1 Annual solar radiation in Golmud,China(source:NASA[33])

Annual Radiation energy (MJ/m2)2009 6950.97 2010 6925.77 2011 7025.90 2012 6875.32

Assuming that only 50% of per square in Golmud area is equipped with photovoltaic cells and the conversion efficiency is 15% [34],the average annual power generation per square meter is 144.7 kWh based on equation (1)[33].In 2012,the total annual power generation capacity of Mainland China was approximately 4.94E+12 kWh[35].Therefore,to meet the annual electricity demands of Mainland China,approximately 34,140 km2 of land would need to be equipped with photovoltaic.

According to this analysis,if 144.7 kWh is used as the average annual power generation of per square meter and all 2.61 million km2 of desertified land in China were equipped with PV [36],it would provide 76.5 times the annual electricity consumed in China in 2012 [35].Based on the average utilization hours of thermal power-generating units(5,500 h),desert-based renewable energy generation plants could potentially provide 69.3 billion kilowatts of installed capacity (converted into thermal power units).

With the rapid reduction of the cost of photovoltaic power generation and sufficient suitable land worldwide for generating larges amount of solar energy,desert-based solar energy is expected to help realize the complete transition away from fossil fuel-based power production.

3 Seawater desalination powered by renewable energy

The global freshwater crisis is steadily worsening.For instance,an estimated 3.6 billion people (nearly half the global population) live in areas that are potentially waterscarce at least one month a year,and this population could increase to 4.8-5.7 billion by 2050 [37].In particular,northern North America,the Middle East and western Asia,eastern Oceania,India,and northern China are facing water stress according to the United Nations World Water Development Report 2016 (Fig.3)[12].

Solar energy generation could represent a feasible method for powering desalination plants to successfully solve water shortage problems in most areas worldwide.For example,most of freshwater in Saudi Arabia,Israel,and other nearby countries facing serious freshwater shortages comes from seawater desalination [38].Consequently,seawater desalination is not only considered based on cost or technological advances,but is also to ensure national security and sustainable development.

Globally,the Middle East and North Africa (e.g.,Saudi Arabia,Israel,United Arab Emirates,and Kuwait) are the largest users of desalinated seawater,which currently have over 50% of the world’s desalination capacity,resulting from rapid population growth and extremely freshwater shortages [40].China is also facing serious water problems,including shortages and pollution.For instance,China has much lower per capita water resources than the world average [35],and some Chinese cities have per capita water resources far lower than the world average water demands standard (4,600 m3) [37,39].

China is abundant in renewable energy resources and seawater resources,with a vast land area and a long coastline.Therefore,it has great potential to provide large amounts of surplus clean and renewable electrical power for desalination and for redistributing water to solve the freshwater shortage crisis.To alleviate desertification in western China,we propose the redistribution of seawater from the Bohai Sea to western China.

Fig.3 Map of global water scarcity (source:United Nations World Water Development Report 2016[12])

Therefore,we estimated the power needs of seawater desalination and freshwater pumping.In mainstream desalination plants,electricity consumption represents more than half of the production cost.Assuming that if the water that then is used for seawater desalination is equivalent to the annual average runoff of Yellow River,the electricity demand W1 can be calculated by Equation (2).The time variables in all equations are used as 3600 seconds or 1 hour.

where Q is the annual average runoff (m3/s),Ptypical is the typical amount of energy consumed in desalination (kWh/t),and t is time (=1 h).

To represent the electricity demands for pumping water to redistribute freshwater from east to west,it is assumed that the desalinated seawater is raised to a given height that is assumed as a setting value.The efficiency of the whole system is η,and the electricity demand W2 can be calculated by Equation (3).

where m is the quantity of water (kg),g is gravitational acceleration (N/kg),h is height (m),and Pbase is the base energy of 1kWh(=3.6E+06J).

Therefore,the total electricity demand W in an hour for both seawater desalination and freshwater redistribution is as follows.

Because the electricity demands are estimated for 1 h,the electric power can be calculated by Equation (5).

where T is time,and is equal to 1 h.

For this analysis,we assumed two possible desalination locations and redistribution routes (Fig.4).In China,the distribution of deserts is roughly consistent with that of areas experiencing freshwater shortages.Therefore,the proposed redistribution routes were planned to run through desert areas.Constructing artificial lakes and wetlands in strategic areas to provide water for sand-break forests according to desert topography would help control deserts and offer environmental improvements.

According to Equations (2) to (5),if Ptypical = 4kWh/t [41,42],h = 2000 m,η = 70%,and Q = 1774.5 m3/s [39],the abandoned renewable energy power generation that cannot be absorbed by power grid in China every year would alleviate freshwater shortages for approximately 2 million people based on the international minimum standard (global threshold) [26].

Considering that the costs of renewable energy are expected to decline dramatically,preliminary seawater desalination projects could initially be carried out in some regions with suitable conditions to alleviate freshwater shortage problems for select cities.Then,the amount of freshwater generated could be increased gradually and redistributed further westward to improve environmental and promote groundwater recycling.

Fig.4 Diagram of the proposed freshwater diversion from desalination plants to arid western China

Because deserts are optimal sites for large-scale solar energy generation,construction of large-scale solar energy plants in northwestern China would result in the asymmetrical distribution of energy production (i.e.,solar plants) versus consumption (i.e.,desalination plants).Therefore,high-capacity,long-distance,and low-loss power transmission technologies are needed to transmit large amounts of energy eastward to support desalination.This need could be met by UHV transmission technologies and GEI,which can effectively use clean energy and solve the problems of asymmetric clean energy generation and consumption.

4 Seawater desalination systems for dealing with fluctuations in renewable energy sources

Solar and wind energies are subject to interruptions and fluctuations.Solar power generation mainly occurs during the daytime,whereas the output of wind farms is often greater at night.Thus,wind and solar integrated energy systems are complementary [43,44].To realize this complementary pattern,large-scale solar power plants and wind farms can be connected by an ultra-high-voltage direct current (UHVDC) system (Fig.5) [45].

In China,renewable energy bases,many with installed capacities of ten million kilowatts,are relatively concentrated in northern and northwestern regions.Interestingly,the distribution of large-scale renewable energy bases is roughly consistent with the distribution of deserts and the proposed water pumping route.Both seawater desalination plants and water pumping stations have huge potential load capacities.However,seawater desalination and pumping systems do not generally require constant operation at full load,and could be modulated according to the output of renewable energy bases to ensure that the output power transmitted to load center is as stable as possible.In addition,a number of pumped-storage power stations could be selectively built along the redistribution route and used as generating stations.Therefore,the electricity demands of desalination and pumping water can be viewed as an ultra-large capacity,low-cost interruptible load,and could be used as the spinning reserve for the largescale renewable energy bases.

Fig.5 Diagram of the connection of renewable energy bases using an ultra-high-voltage direct current (UHVDC) grid

As the major challenge of such a system,the output power of large-scale renewable energy bases in the desert areas would be much lower in winter.On an annual scale,the trends in solar energy output power in summer and winter are consistent with the trends of the total load.For example,the largest loads may occur in summer.Meanwhile,both solar radiation and the evaporation capacity in deserts are large (small) in summer (winter),which would increase (reduce) the electricity demands of the desalination system.In other words,the electricity demands of the seawater desalination system would be roughly consistent with the annual trends in solar radiation.On a daily scale,the output trends of solar power plants adapt to load characteristics,which increase in the morning and decrease in the evening.The power grids in North,Central,and East China have huge capacities,and the differences in the load during the daytime and at night are the same as the changes in the solar power plant output.

In terms of these energy characteristics,UHV power grids that optimize the allocation of large-scale electrical power throughout a country can be used as important support for large-scale renewable energy development,and can help address the problems arising from differences and changes in the output power of large-scale solar power plants at different times and seasons.

5 Role of GEI in global sustainable development

Clean energy is distributed unevenly around the world,and is subject to temporal interruptions and fluctuations.Therefore,it is necessary to integrate clean energy sources into a large-scale grid that connects power grids in different regions,countries,and even continents to ensure safe and reliable power consumption.Such a system could also allow for the efficient use of wind energy in the arctic and solar energy at the equator.To support the interconnection of regional grids,power transmission should have a greater capacity and higher voltage.UHV transmission technologies provide an ideal solution.

For instance,the hydropower UHV transmission project in Brazil was completed in 2017,which enables transmission of electric power to load centers in the southeastern region[45].Furthermore,HVDC transmission between China and Arabia to achieve energy interconnection has been proposed,which would use UHVDC transmission to transmit electric power from Kashgar (China) to Sukur (Pakistan) [47].Both China and the European Union are major energy-consuming centers,which have different renewable energy sources available and span eight time zones,and UHV transmission technologies could also support the interconnection between these two regions [48].Finally,large-scale and long-range HVDC transmission technologies also have important roles in the strategy of “west to east transmission and national interconnection” in China [49].Thus,UHV is expected to have a fundamental role in the energy revolution.

As noted above,solar and wind energies are subject to temporal interruptions and fluctuations.UHVDC grids can connect renewable energy bases spanning multiple regions,which can minimize the power fluctuations of the integrated energy bases.In China,with the gradual increase in the load from desalination plants and the phasing-out of coal-fired power generation in eastern regions,the uneven distribution of energy production and consumption will be further exacerbated.Therefore,UHV power transmission technologies could both reduce fluctuations and bridge the asynchronous distribution of integrated energy bases in northern and northwestern China and the load center in eastern China to support sustainable development.

At present,UHVDC power transmission technology with a range of ±800 kV is relatively mature,and China has achieved technological breakthroughs as high as ±1100 kV.Building integrated renewable energy bases,constructing strong receiving-end power grids for load centers,and accelerating the development of long-range,large-capacity transmission projects will represent a great achievement toward realizing the replacement of fossil energy,ensuring the use of clean electrical power,and improving the environment.To this end,UHV power transmission technologies will undoubtedly have an indispensable and fundamental supporting role.

6 Conclusion

Given that the problems associated with conventional fossil energy emissions and environmental pollution are becoming increasingly severe,clean renewable energies are seen as the main energy sources for the future.To prepare for the widespread use of clean energy sources,it is necessary to carefully consider the most efficient and rational methods for using clean power to promote and improve social and economic development,while also improving national security.

Using China as an example,this paper presents an approach for using large-scale clean,renewable energy sources to realize energy replacement,improve the ecological environment,and increase freshwater supply.In this paper,desalination systems have the potential to be used as the spinning reserve of large integrated energy bases to improve the stability of energy transmitted to the load center.Furthermore,to address the asynchronous distribution of power generation and consumption,UHV power grids are expected to transmit the electrical power of renewable energy bases to load center,which has an indispensable fundamental supporting role in energy transition and replacement and the development of largescale clean energy.It further promotes the improvement and realization of GEI.Large-scale development and utilization of clean energy is guaranteed through power interconnection,and such development and utilization will also promote the improvement and realization of GEI and then improve the environmental problem.