Pumped hydro energy storage and 100 % renewable electricity for East Asia

1 Introduction

Increasing concerns regarding climate change and rapid cost reductions have led to the widespread deployment of renewable energy globally.One third of the global power capacity is now provided by renewables,mainly solar photovoltaics (PV) and wind.In 2018,the net new deployment of solar PV and wind reached 143 gigawatts (GW) and surpassed the other resources (Fig.1).This trend is expected to continue in the 2020s considering the continuously decreasing cost of solar PV and wind energy and the retirement of existing coal-fired generators.

Fig.1 Global net new generation capacity added in 2015-2018 by technology type [1]

East Asia comprises 22% of the world’s population and some of the world’s fastest growing economies.(In this paper,Republic of Korea is referred to as South Korea.Democratic People’s Republic of Korea is referred to as North Korea) The installed renewable capacity per capita and the renewable share of electricity generation in these regions are demonstrated in Fig.2.While China and Japan are leading the installed renewable capacity per capita (blue bars),deployment of renewable energy in East Asia in terms of renewables’ share in electricity generation (green bars) is below the global average (green line) [2].

Fig.2 Installed renewable capacity per capita in 2018 (blue,left axis) and renewables’ share of electricity generation (green,right axis) in East Asia [2-6].The horizontal blue and green lines represent global averages for per capita renewable electricity generation and renewable electricity share,respectively.The renewables’ share of electricity generation in North Korea is estimated based on average capacity factors

To facilitate the transition to renewable energy,a series of renewable targets for the next decade (by 2030) have been set in East Asia:20% renewable energy in South Korea,22-24% in Japan,30% in Mongolia,and 35% in China [2].Solar PV and wind are expected to dominate this transition in most regions as they are cheap,mature,and not constrained by fuel availability,environmental considerations,raw materials supply,water supply,or security issues.

The transition to renewable energy will be achieved through the electrification of the transport and heating sectors by replacing the internal combustion engines with electric motors,and replacing the current use of gas and LPG in space heating,water heating,and cooking with cleaner and more efficient electric heat pumps and electric cooking appliances.In the longer term,renewable electrification of industrial processes and aviation and shipping can nearly eliminate the use of gas,oil,and coal and remove 75-85% of greenhouse gas emissions [1].

Energy storage,strong interconnection over large areas,and demand management can support a highly renewable electricity system at a modest cost [7].East Asia has abundant wind and solar resources and off-river pumped hydro energy storage (PHES) capacity.

Australia sets a good example for the East Asian countries,as Australia’s energy systems are experiencing a rapid and large-scale transition to renewable energy.New solar PV and wind capacities are being installed at a rate of 250 W/yr per capita,which is 4-5 times faster than in China,Japan,the European Union,and the United States (Fig.3).The Australian experience demonstrates the feasibility of the rapid deployment of solar PV and wind at a low cost in an isolated industrialized country.Australia is expected to achieve 50% renewable electricity by 2024 and 100% renewable electricity by 2032 if the current installation rate (approximately 6 GW per year for a population of 25 million people) is sustained [1].The replacement of coal-fired generators with renewable solar PV and wind technology is also expected to contribute to the continuous decrease in greenhouse gas emissions.

Fig.3 Annual renewable energy deployment rate per capita by region.Data for Australia (2018 and 2019) are from the Australian Clean Energy Regulator [8]and data for other regions (2018) are from IRENA [9]

This study contributes to the discussions around achieving the goal of 100% renewable energy in East Asia as follows:

1.It provides a summary of available wind and solar resources.

2.It demonstrates that sufficient land is available to supply current East Asian electricity consumption from PV and wind.

3.It identifies a vast,mature,and low-cost solution to the problem of storage to support high levels of variable wind and PV energy supply in East Asia,in the form of PHES.

4.It estimates the storage requirements to support 100% renewable electricity in East Asia,and demonstrates that this requirement is a small fraction of the available PHES.

2 Solar and wind resources in East Asia

2.1 Wind

East Asia has abundant wind resources compared to the rest of the world.It is one of the regions with the fastest wind deployment,with an installed capacity of 120 W per capita by the end of 2018,compared to the global capacity of 74W per capita [9].China,with the largest installed capacity,is leading the global wind power generation with continued growth.In 2018,over 40% of the global net new wind capacity was contributed by China [9],mostly onshore but well-distributed across the provinces.Although the current share of wind generation in East Asia is low,Japan and South Korea are planning to make significant investments in offshore wind energy to utilize the abundant wind resources along the coastline [10-11].

2.2 Solar

East Asia also has abundant solar resources.The best locations for solar power are in northern and western China and Mongolia,where the population density is relatively low.Western China also has excellent solar,wind,and pumped hydro resources.

The significance of solar PV in future energy systems is well recognized in East Asia.Japan has a target of supplying 7% of its national electricity demand by solar PV by 2030,while China is aiming at 105 GW solar PV by 2020.

The total electricity consumption in East Asia is 7,300,000 GWh/yr.Assuming an average capacity factor of 18%,solar PV systems with a rated capacity of 4,630 GW are required to meet the entire electricity demand in East Asia.This translates to a combined panel area of 23,000 km² or 14 m² per person assuming a panel efficiency of 20%.

Solar PV can be deployed not only on the ground but also on rooftops and water surfaces.For example,over 20% of Australian households currently host 1.9 million smallscale PV systems,with a combined capacity of 10 GW [12].Deployment of rooftop solar systems is expected to continue in the next decade.The Australian Energy Market Operator is expecting the electricity generation from rooftop PV in Australia to reach 23 TWh by 2030 [13],supplying 11% of the annual electricity demand in the national electricity market.

As discussed in Section 2.1,wind energy systems are also highly prospective in East Asia.Onshore wind turbines alienate negligible land (only the towers and access roads) and offshore turbines alienate no land.If we assume that half of the electricity demand in East Asia is met through wind energy and roof-mounted PV panels occupying negligible land,while the other half is supplied from PV panels mounted on the ground and water bodies that have a spacing of 3:1,the total area required to supply 100% of East Asia’s electricity demand is 35,000 km²,equivalent to 0.3% of the total land area or 11.5% of the total water area (lakes,reservoirs,rivers).Detailed information by region is presented in Table 1.The region has a vast amount of land and water area that can host wind farms and rooftops for the large-scale deployment of renewable energy technologies that would supply most or all of the electricity demand in East Asia.

Table 1 Requirements for solar PV to supply 100% electricity demand in East Asian regions.Territorial sea areas are calculated from the coastline using a standard 12 nautical miles convention

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3 Pumped hydro energy storage

3.1 The role of pumped hydro in a highly renewable electricity system

Due to the intermittent nature of solar PV and wind,a large amount of storage capacity is required when the share of solar PV and wind generated electricity rises to 50%-100% [7].Pumped hydro energy storage (PHES) systems and batteries are by far the leading storage techniques.PHES systems store excess electricity by pumping water uphill to the upper reservoir.By releasing the water through the turbine,the stored energy is recovered.PHES constitutes 97% of global stored power (160 GW) and over 99% of stored energy due to its high round-trip efficiency (around 80%) and low cost.PHES can either be river-based or off-river.However,conventional river-based facilities are constrained by water availability,flood control efforts,and environmental considerations and therefore have limited potential for future development.Off-river PHES utilizes a pair of artificial reservoirs at different altitudes (100-1,200 m altitude difference or “head”) to circulate water indefinitely in a closed loop.The reservoirs are usually separated by a few kilometers and are connected by a pipe or a tunnel (Fig.4).As the reservoirs are not river-based,there is no need for flood control,which would otherwise incur additional costs.Moreover,less land is required for off-river PHES and the heads are also higher because of the larger number of available sites [1].

Fig.4 Presenzano (Italy) PHES system (500 m head,1 GW power,and 6 GWh storage) showing twin off-river reservoirs.(Google Earth image)

The storage potential of PHES is proportional to the volume of the upper reservoir,the head,and the round-trip efficiency.For example,a PHES system with twin 2,000,000 m3 reservoirs,a 700 m head,and 80% round-trip efficiency can store 3 GWh of energy and operate at 500 MW of power generation for 6 h.

Batteries are ideal for small-scale (residential and electric vehicles) and short-term storage (sub-seconds to hours),but they are expensive for balancing a large-scale highly renewable electric grid.One of the largest batteries in the world has a storage energy of 0.13 GWh and storage power of 0.1 GW [14],whereas the Snowy 2.0 pumped hydro project has a storage energy of 350 GWh and rated power of 2 GW [15].

3.2 Global pumped hydro atlas

The authors have recently carried out a global assessment of viable off-river PHES sites by analyzing the topography of every 1 degree which is from 60 degrees North to 56 degrees South [16]and identified 616,000 promising sites with a combined storage potential of 23 million GWh (Fig.5) using GIS algorithms described in [17].Each identified PHES system includes an upper reservoir,a lower reservoir,and a hypothetical tunnel route connecting the two reservoirs (Fig.6).Each pair is ranked from A to E according to the estimated costs [18].This is shown in the maps with red points representing Class A sites and primrose points representing Class E sites.The cost of a Class A pair is roughly half of that of a Class E pair.Generally,off-river PHES systems with a higher water/rock ratio (the volume of the rock needed to be moved to a dam a given volume of water),larger head,larger slope between the reservoirs (head-to-horizontal separation ratio),and lower storage power are more cost-effective and therefore are ranked higher.For each identified site,detailed zoomable maps and spreadsheets containing all site attributes are available:altitude,head,latitude,longitude,slope,water volume,rock volume,water area,dam wall length,water/rock ratio,energy storage potential,and approximate relative cost (classes A-E).

Fig.5 Distribution of global pumped hydro sites identified with GIS analysis.616,000 sites were identified with a combined storage capacity of 23 million GWh

Fig.6 Two pairs of class A (red dots/world-class) off-river pumped hydro sites in southern China.The upper and lower reservoirs are colored light and dark blue,respectively.The blue lines represent the hypothetical tunnel routes.The head for these two pairs is approximately 600 m.The storage potential is 150 GWh per pair with a storage time of 18 h.Image credit:Data 61 hosting and Bing Map background

3.3 Pumped hydro potential in East Asia

East Asia houses 123,630 promising sites with a combined storage capacity of 4 million GWh (Fig.7),which is equivalent to 2,400 GWh per million people.As a comparison,a study on Australia indicated that the storage required to balance a 100% renewable electricity system supplying 25 million people is 500 GWh [7].Using the Australian “rule of thumb,” these 123,630 identified sites are 120 times more than required to support 100% renewable electricity in East Asia.

The identified off-river PHES potential in terms of GWh per million people for regions in East Asia is shown in Fig.10.All regions have significantly more PHES capacities than required (blue bars).All regions except South Korea have sufficient world-class (Class A) PHES sites to support 100% renewable electricity (green bars).However,South Korea has 1225 GWh or 24 GWh per million people of Class B capacity as a substitute,which is only 25% more expensive.

Fig.7 123,630 off-river pumped hydro sites with a combined storage capacity of 4 million GWh identified in East Asia

Fig.8 Energy storage potential (GWh per million people in log scale) for East Asia.As a guide,the amount of storage required to support 100% renewable electricity in Australia is about 20 GWh per million people [7].Most regions have hundreds of times more storage than required to support high levels of variable wind and solar PV

Off-river PHES sites in China are well-distributed.Most of the sites are in the western and southern parts including Guizhou,Guangxi,Fujian,Gansu,Sichuan,Inner Mongolia,and Xinjiang.In western China,the solar and wind resources are also abundant.Many high voltage transmission lines (HVDC/HCAC) have been built or are under construction,allowing the load centers in the east and south to have access to energy storage facilities,and solar and wind resources in the west and northwest.Widespread solar PV,wind,and PHES across large areas allow stable and reliable power output as the local weather effects are smoothed out.

Pumped hydro systems also offer black start,rapid start (20-200s),and system inertia capability in addition to storage,allowing them to be utilized for ancillary services that were previously provided by coal and gas.

In contrast to storage energy,unlimited storage power capacity can be effectively added to off-river PHES systems.Storage power depends on the cross-sectional area of the water conveyance and the power capacity of the pump/turbine,generator,and switchyard.It does not depend directly on the reservoir volume,although a higher power output will cause more rapid depletion of the water in the upper reservoir.Thus,storage power and storage energy can be largely decoupled,and energy and power requirements can be met independently.

Mongolia has low population density and low energy consumption per capita,but excellent wind,solar,and PHES resources.It would be easy for Mongolia to achieve 100% renewable electricity by following the Australian pathway.It is,however,more difficult for other East Asian regions.The storage requirement per million people is likely to be higher in these regions due to the relatively small land areas and smaller solar and wind resources.

While local solar and wind resources can provide large amounts of energy,Other East Asian countries can also access the enormous wind and solar resources of western China and Mongolia via connection with HVDC submarine cables.Other clean energy sources,such as imported renewable hydrogen,geothermal,biomass,tidal,or nuclear may play a small role in the transition to zerocarbon economies.However,their cost is likely to be higher than that of PV and wind sourced locally or delivered from Central Asia by an HVDC cable.

4 Environmental considerations

The reservoirs in off-river PHES systems are much smaller than for conventional on-river systems.For one million people,20 GWh of storage is equivalent to 2-4 km2 of land required for both upper and lower reservoirs.This is negligible compared to the land requirements for the solar and wind systems supported by PHES [1].Reservoirs that impinge protected areas and urban areas have already been excluded from the atlas.The identified sites will also introduce no threat to existing river systems.

The amount of water required for a renewable electricity grid supported by solar PV,wind,PHES,and large interconnection is much less than that for a corresponding coal-based system as cooling towers are avoided [19].Water required by PHES are for the initial fill (approximately 20 GL per million people) and the occasional replacement of evaporation losses (negligible).Assuming PHES is phased in over the next 20 yr,then the water required for the initial fill is 3 L/d per person,which is much less than the current water use in the electricity and gas industries [1].

5 Conclusion

Solar PV and wind are leading the transition to renewable energy sources,accounting for over 60% of the global newly added capacity.Continued cost reductions will lead to increasing deployment of solar PV and wind globally.Pumped hydro is a low-cost,large-scale,and widely deployed storage solution with a known cost.The storage requirement for a 100% renewable electricity system is approximately 20 GWh per million people based on the analysis performed for Australia.While the current share of renewable generation is low,regions in East Asia have sufficient solar,wind,and PHES resources for the large deployment of renewables in the following decades.

Acknowledgements

Support from the Energy Transition Hub (https://www.energy-transition-hub.org/),the Australia Indonesia Centre (https://australiaindonesiacentre.org/),and the Australian Renewable Energy Agency (https://arena.gov.au/) is gratefully acknowledged.AREMI is supported by ARENA and Data61.Special thanks to Mats Henrikson of Data61 (https://www.data61.csiro.au/) for mounting the data on AREMI.We also thank to David Singleton.This work has been partially supported by the Australian Government through the Australian Renewable Energy Agency (ARENA).Responsibility for the views,information or advice expressed herein is not accepted by the Australian Government.