Status and challenges
Although China currently is the world’s largest CO2 emitter, the cumulative and per-capita-based emissions are far less than those of developed countries, resulted from a much shorter industrialization process of China. As shown in Fig. 6, the cumulative CO2 emissions between 1750-2019 from China is only about half of the values of the United States. As the world’s largest economy, the US has an average CO2 emission of 15.97 tons per person in 2019, more than twice that of China (Fig. 5).
Though with much lower cumulative and per-capita-based emissions, China still faces tremendous challenges in reducing carbon emissions, as the time remaining for the transition from its carbon peak toward neutrality will be much shorter than that of developed countries. As shown in Fig. 11, China’s carbon emission is currently about 10 Gt and has not reached its peak yet. And the EU-28 countries peaked the carbon emissions in the 1980s, reaching its net-zero target in 2050 from a much lower peak takes about 70 years. The time for the US to carbon neutrality from its peak at about 6 Gt in 2007 still lasts for more than 40 years. By comparing the 2019 emission level with 1990, Europe’s CO2 emissions have decreased by almost 20%; the US’s CO2 emissions declined from the 2007 peak to the same level as in 1990; while China’s CO2 emissions increased more than three times at the same time. If as have pledged that achieve carbon peak before 2030, China will only have about 30 years to achieve carbon neutrality after reaching a much higher peak than those of the US and EU. The longer China’s primary energy consumption and CO2 emissions continue to rise and the larger the peak values become, the more difficult it will be for China to achieve carbon neutrality in the future. In order to achieve the goal of carbon neutrality, China needs to make prompt adjustments to economic and social development and pay huge prices.
In addition, China’s energy structure is still dominated by high-carbon energy and China’s economy is heavily dependent on coal. China’s energy supply is dominated by fossil energy, accounting for as high as 85% of the primary energy consumption and 92% of the total CO2 emissions, and the energy utilization efficiency is relatively low. As shown in Fig. 12, coal accounts for 57.64% of the total primary energy consumption in China in 2019, while Europe and the United States only consumed about 13.54% and 11.98% of coal in the respective primary energy mix. According to the IEA’s prediction [28], China will consume 3,875 Mt of coal in 2021, which accounts for more than 52% of global coal consumption.
Figure 13 presents the share of primary energy consumption and share of electricity production by different sources of China in 2019. Fossil fuel-fired electricity generation accounts for about 67% of the total power generation, while coal-fired power generation alone takes more than 62%. On the contrary, only 17.5% Europe’s electricity production is from coal and that number is 23.2% for the US. In order to achieve carbon neutrality for China, coal-fired power generation is expected to be phased out by 2050. Coal has to be replaced by wind, solar, hydro, and nuclear energy. The share of fossil energy is expected to decline to less than 20% in 2060 (Fig. 14). Reducing dependencies on energy and electricity generation from high-carbon sources will be a major challenge in combating climate change because of its price advantages and the relatively young age of coal-fired power plants in China [15]. Furthermore, China is still in the industrialization process, the demand for energy and electricity will continue to rise and may plateau for a relatively long time. Currently, there are still strong coupling relationships among economic development, energy consumption, and carbon emissions in China.
The strong coupling of economic development and carbon emissions also means the transition from fossil fuel-based energy systems to renewables will also have significant socio-economic impacts. In a long run, it would enable faster growth of the economy, create more jobs, improve the overall welfare of a country, and has positive demographic and geopolitics impacts. But these socio-economic benefits require a rapid but smooth transition. Therefore, the realization of China’s carbon-neutral vision must be under the premise of sustained and stable economic growth, and a pragmatic path that can not only ensure the safe and reliable supply of energy and electricity but also achieve carbon emission reduction. This is a very serious challenge on the path of china’s carbon neutrality mission.
Energy transition pathways
To achieve the goal of reaching a peak in CO2 emissions before 2030 and carbon neutrality before 2060, the energy system would be reshaped by the energy transition in China. The low-carbon transition of energy systems is imperative to achieve the goal. Carbon neutrality can be realized by reducing CO2 emissions directly by reducing consumption of traditional fossil fuels and offsetting CO2 emissions by negative emissions produced by bioenergy, in conjunction with carbon capture, utilization, and storage (CCUS), and direct air capture of CO2 with storage [29, 30]. There are a lot of pathways for energy transitions involving different rates of change, different aspects of the transformation of the energy system and many uncertainties. To achieve a peak in CO2 emissions as soon as possible, the government of China controls the peak quantity of fossil fuels [31]. Therefore, different pathways have the same trend that the share of low carbon energy such as solar, wind and other renewable sources jumps from 15% today to over 80% in 2060 [32].
According to the IEA’s announced pledges scenario (APS) [32], coal makes up 57% of the primary energy system in 2020 among various primary energy sources, bringing large amounts of CO2 emissions. Coal reduction in the energy transition is of great importance [33]. The ratio of fossil fuels in the total primary energy systems will decrease to less than 20% in 2060; they will be used mainly as supplemented energy sources in 2060 and the emissions will be removed by CCUS technology to achieve net-zero emissions [34]. Among different renewable energies, solar energy is one of the dominant energy sources. Solar energy is utilized for both power generation and heating by technologies of solar photovoltaic (PV) and solar thermal, accounting for a quarter of total primary energy demand in 2060 [35].
China’s total energy consumption from 2020 to 2060 is as important as primary energy demand to achieve peak in CO2 emissions before 2030 and net-zero CO2 emissions before 2060. Coal demand in final energy shows the biggest decline, falling almost 85% between 2020 and 2060 while electricity and hydrogen increase the most in the final energy system [32]. Electricity consumption demand is almost doubled in 2060 compared to 2020. Final energy demand mainly lies in power generation, industry, buildings and transport sectors. Electricity is the leading energy source in 2060. Electricity generation increases by 130% by 2060. In the industry sector such as cement and steel, coal is replaced by low-carbon fuels including bioenergy and waste and hydrogen. In the transport sector, oil demand falls 60% while hydrogen, ammonia and synthetic hydrocarbon fuels cover almost 25% of all energy needs in 2060 and electricity around 55% [32]. In the buildings sector, fossil fuels are mainly replaced by electricity and renewable energy. It is obvious that electrification is a critical component in main sectors to realize decarbonization and net-zero CO2 emissions. Electrification refers to the process of replacing technologies that use fossil fuels with technologies that use electricity as a source of energy [36]. Electrification is realized in different sectors such as factory electrification and household electrification. In the goal of carbon neutrality, electrification performs a significant and irreplaceable role. In China, the share of electricity in total final energy consumption is more than 50% in 2060. Electricity is highly demanded in sectors of industry, buildings, transportation and hydrogen generation.
On the path of the energy transition for carbon neutrality, there are various types of emission reduction and low carbon technologies involved, including high-efficiency and low-emission combustion power generation technology, CCUS technology, high-efficiency catalytic conversion of carbon-based energy technology, and renewable energy utilization technologies such as hydrogen, solar, and wind energy. Figure 15 shows the three main energy transition paths for carbon neutrality in China. First, decarbonization, energy saving and CCUS are important energy transition ways to reduce carbon emission at an early stage since fossil energy accounts for 85% of the primary energy consumption. However, the CCUS technology is still immature and the cost is currently high, it is forecast to be widely used after 2035. With the increase of wind and solar power capacity, the combination of renewable sustainable energy and energy storage technology will become more and more important. The two energy transition paths will coexist for a long term, supporting and supplementing each other. At the same time, the energy internet and smart energy, which combines multiple renewable energy systems, data and information technology and carbon emission management, further contributes to the energy transition and emission reduction.
Different sectors: industry, transport and building
As described above, several key sectors contribute to most CO2 emissions so it is important to analyze energy transition pathways in key sectors including electricity generation, industry, transport and building sectors. Electricity generation has been discussed above. In this section, the industry, transport and building sectors are being reviewed. CO2 emissions in the industry account for 35% of China’s total energy sector emissions in 2020. There are several prominent changes in industrial CO2 emissions mainly from cement, steel and chemicals sectors between 2020 and 2060, showing a rapidly decline trend [38]. Industrial CO2 emissions decrease by 95% by 2060 due to electrification, energy efficiency improvements, hydrogen and CCUS [39] in industry. The decreasing trend of industrial CO2 emissions is consistent with the trend of final energy consumption in China with decreasing coal and increasing electricity consumption. Reducing coal used in industry results in a direct decrease in CO2 emissions while increasing electricity consumption results indirectly. Both two trends contribute to energy transition in the industry positively.
China has a great share in the global production of major bulk materials. Crude steel and cement produced by China account for more than half of global production in 2020 and both drop to 30% of the global share in 2060 during energy transition. In China, steel demand has increased rapidly over the past two decades due to infrastructure needs. In 2020, China’s steel production reaches a record of 1.1 Gt despite the Covid-19 pandemic. During steel production in China, coke and coal are used as reduction agents to cleave the oxygen from iron ore to produce molten iron. Except for the steel produced from carbon-based reducing agents, steel can be also produced by using electric furnaces which is typically used for scrap steel [40]. Scrap steel with low carbon emissions will grow rapidly in the coming decades. By 2060, electric furnaces play the main role in steel production, facilitating the steel sector’s transition to using electricity [41]. Besides scrap steel, hydrogen-based direct reduced iron is an important part in the steel sector, which uses hydrogen as a reducing agent substituting coal and coke [42]. This method offers advantages of high energy efficiency and low cost by using electricity from renewable energy. Crude steel production equipped with CCUS also contributes to energy transition in the steel sector, including innovative smelting reduction with CCUS and innovative blast furnace steelmaking with CCUS [43]. The abovementioned routes make up more than two-thirds of primary steel production and scrap-based electric arc furnace production is more than half of total steel production by 2060. China has the biggest share of cement in the global market. More than half of global cement is produced by China. The production of cement is responsible for one-third of China’s overall industrial CO2 emissions [44]. China’s cement production still grows in the near future due to demand for infrastructure and building and then begins to fall after peaking. Cement is produced from limestone in dry kilns with coal as energy input, thus emitting large amounts of CO2. By reducing the clinker-to-cement ratio, increasing material and energy efficiency and replacing coal with alternative fuels such as hydrogen and bioenergy blending, CO2 emissions decreases in 2030 and continues to decrease sharps with the help of CCUS [45]. More than 80% of China’s cement production is equipped with CCUS by 2060.
China’s transport sector emitted 9% of the total energy sector CO2 emissions in 2020 [46]. CO2 emissions from transport reach a peak in 2030 and then drop by 90% in 2060 compared to 2020. The peak in 2030 is predicted by spectacular growth in car ownership. The sharp decline of CO2 emissions is mainly from cars, buses and freight trucks, driven by policy efforts to adopt low-carbon technologies across various transport mode [47]. Decarbonization in all main transport modes in China is the key to achieving carbon neutrality. Gasoline and diesel used in most vehicles are substituted by electricity and hydrogen. Modal shifts from cars to public transport and non-motorized mode also play an important role in decarbonization.
Energy and carbon emissions in the building sector raise much attention nowadays. In China, buildings accounted for 20% of total CO2 emissions in 2020, resulting from providing heat and electricity for various buildings [48]. In the global view, China accounted for 17% of global energy consumption and 25% of CO2 emissions in the building sector in 2020. Residential properties including heating and cooling, cooking, electrical appliances and lighting are the sources of direct and indirect CO2 emissions in buildings [49]. The direct use of coal and traditional biomass has decreased rapidly in recent years. Electricity is the dominant energy in the building sector in 2060, propelling reduced emissions. Besides, renewable energy also promotes reduced CO2 emissions by employing solar PV in buildings, getting electricity from solar energy instead of public networks.
Key technologies
Solar energy
Solar energy is a promising and freely available renewable energy source for the energy transition. The PV and solar thermal are two kernel technologies for using solar energy as shown in Fig. 16 [50–52]. Solar thermal works by using mirrors to concentrate sunlight first. Then, the concentrated solar energy is transformed into stored heat to generate electricity [51]. The solar PV directly converts the sunlight into electricity using semi-conductors [52]. As the main and the most important usage of solar energy, solar PV technology has been developed rapidly in the past 20 years and this brought about a rapid reduction in the cost with price parity achieved on the grid. The solar PV power generation can be further divided into distributed and centralized power stations according to the scale of PV panel arrays. For the distributed style, the PV panels are usually located on rooftops, in rural and commercial areas. The centralized PV power stations usually occupy a large area of land to form a certain scale. In China, the centralized solar PV power plants now account for about 69% of the total installed PV capacity by June, 2020 [53].
In 2020, China increased 48.2 GW of solar power capacity, which is far ahead of any other country. Now China has the world’s largest cumulative installed solar power capacity, with 253 GW at the end-2020 compared with about 151 GW in the European Union and 93 GW in the United States, according to International Energy Agency data [55]. Thus, solar PV power generation is a key technological and industrial support for realizing the carbon neutrality commitment in China.
Driven by the target of carbon neutrality, the proportion of solar energy in China’s total energy will increase from about 2.7% to more than 25%. Considering that solar PV power generation involves many upstream and downstream industrial chains, this will provide a large number of jobs and open up a huge development space for the solar energy industry. In the past decade, the costs of solar PV have generally decreased since 2013 despite the seasonal effects and monthly fluctuations [56], as shown in Fig. 17. Although prices have rebounded this year due to the rising prices of bulk products and raw materials, there is still room for a further decline in the cost of PV power generation with the progress of solar PV power generation technology and the exponential development of the solar PV industry.
China is playing a leading role in the solar PV industry chain of the world, including the manufacturing capacity, completeness of the industrial chain, industrialization technology, manufacturing cost, and market scale. However, there is still great potential for China to make further progress in fundamental research related to solar energy. With the promotion and application of advanced solar technologies, China still needs to strengthen the capabilities of research and development, standards-setting, proficiency testing and verification. In the future, improving the utilization level of solar PV power generation is still the key to further increasing the proportion of solar energy consumption. This involves a series of issues including alleviating the impact of renewable energy on the grid, grid resilience, energy storage technology, etc.
Wind energy
Similar to solar energy, wind energy is another sustainable energy source for the energy transition in China [57]. According to the data released by the Nation Energy Administration (http://www.nea.gov.cn/), in 2020, China added 71.6 GW of wind power generation capacity to reach a total capacity of 281GW which is around 38% of the world’s total installed wind power capacity. Both China’s installed capacity and new capacity in 2020 are the largest in the world by a wide margin compared with the next largest market, the United States, adding 14 GW in 2020 and having an installed capacity of 118 GW. The installed capacity of onshore wind power in China reaches 278 GW by the end of 2020, which occupies 39.34% of the global onshore capacity and is more than two times of the United States as shown in Fig. 18. By the end of June 2021, China had increased its utility-scale offshore wind electricity generation capacity to 11.13 GW, rivaling the approximately 10.4 GW of installed capacity of the UK at the end of 2020, according to the data from China’s National Statistics Administration. By the end of 2030, China is expected to reach a cumulative grid-connected wind capacity of 689 GW, accounting for 67% of the global share.
Wind power generation integrates multiple technologies, including material research and development, blade design, hub/bearing/generator manufacturing, etc. The exponential growth of wind power generation will promote the rapid development of the wind power industry chain. Similar to solar PV power generation, the wind energy companies in China have formed a complete and internationally competitive industrial chain and occupy half of the global wind power industry. This is another guarantee for China to achieve the final carbon neutrality goal. Due to land constraints, the development of onshore wind power in China has gradually slowed down. Considering that China has a coastline of 18,000 kilometers, offshore wind power generation will increase a lot in the near future.
In 2020, the total installed capacity of solar PV and wind power generation in China reaches 533 GW (Table 4). Stimulated by China’s target of 1200 GW of wind and solar set for 2030, 408 GW of new capacity will be added from 2021 to 2030. In order to ensure the achievement of the carbon neutrality goal in 2060, China needs to increase the installed capacity of wind and solar power generation at least to 5000 GW by the end of 2050. Therefore, it is estimated that the wind and solar energy industries will keep developing rapidly in the next 30 years.
Table 4 Comparison of new installed and cumulative capacity of solar and wind energy since 2016. Data is from the National Energy Administration (http://www.nea.gov.cn/) Energy storage
Renewable energy technologies such as wind and solar always face the problems of volatility, intermittentness, and uncertainty during energy generation. Simply developing renewable energy technologies will inevitably bring about problems such as the vibrational impact on the power grid and a high rate of abandoned wind and solar energy. The energy storage technology can effectively alleviate the above-mentioned problems by storing and releasing the intermittent and fluctuating energy [57, 59]. The Chinese government has paid great attention to the development of energy storage technology and industry. The National Development and Reform Commission and the National Energy Administration in China have successively released documents and guidance on promoting the development of energy storage [60, 61]. According to the national plan, the scale of new installed energy storage capacity (excluding pumped storage) in China will increase from the current 3–4 GW to more than 30 GW in 2025, making China the world’s largest energy storage market. By the end of 2050, the global energy storage capacity in the world is expected to reach 1600 GW (5500 GWh), and the cumulative installed energy storage capacity will exceed 200 GW (700 GWh). With the rapid growth of renewable energy power generation, the energy storage industry will also leap forward at the same time. The combination of renewable energy and energy storage will become important components of future energy systems.
Energy storage is the charging and discharging of energy through a reversible process, including mechanical energy storage, electrochemical energy storage, electromagnetic energy storage, thermal energy storage, etc. [62–64] The broad concept of energy storage also includes the conversion of renewable energy to chemical energy for storage, including hydrogen energy (hydrogen production by electrolysis of water), synthetic fuel (reverse conversion of carbon dioxide into fuel), biomass energy, etc. The evaluation of energy storage technology usually includes the rated power, rated capacity, response time, charging/discharging efficiency and stability. Different energy storage technologies have their own advantages, disadvantages and application scenarios (Fig. 19). The main driving force for the development of energy storage technology in recent decades has been the large-scale application of electric vehicles, mobile phones and laptops. In the future, the high demand for stable, continuous and reliable new energy power generation will become another important driving factor for the development of large-scale energy storage. Among different energy storage technologies, the dominant energy storage technology is the electrochemical energy storage. On one hand, solar and wind energy will become the main energy supply in the near future. On the other hand, the terminal utilization of human energy in the future will be electric energy. All of those determine that future energy storage will be mainly based on electricity storage.
The advanced electrochemical energy storage includes lithium-ion batteries, sodium-ion batteries, flow batteries, etc. Lithium-ion batteries are widely used in mobile phones, laptops and electric vehicles due to the advantages of high energy density, rapid response, and high cycle times. Meanwhile, lithium-ion batteries can catch fire or explode if they have been improperly manufactured or damaged, or overheated. With the development of thermal management technology, this serious problem has also been alleviated to a certain degree.
Currently, more than 70% of the lithium supply in China depends on imports [65]. Therefore, the development of alternative technologies such as sodium-ion batteries has also become an alternative way for China to solve the bottle-neck problem since the reserves of sodium are much richer. Compared with lithium, sodium-ion batteries have the advantages of low cost, no over-discharge, and high safety due to their chemical characteristics. Meanwhile, the energy density of sodium-ion batteries is correspondingly lower than that of lithium-ion batteries because the molecular weight of sodium is higher than that of lithium.
The flow battery has the advantages of large capacity, high cycle times, and large-scale storage of new energy since the electrolyte can be separated from the positive and negative electrodes. However, the energy density of flow batteries is lower than that of lithium batteries, and the cost is relatively high. The current manufacturing cost of lithium batteries is between 1,000 and 2,000 \(\yen \)/kWh, while flow batteries are more than 2,000 \(\yen \)/kWh. With large-scale manufacturing and application, the price of flow batteries is expected to keep decreasing.
With the motivation of carbon neutrality, the future electrochemical energy storage has a huge development space. Take the lithium battery as an example, the small battery involves various industries, including positive and negative materials, electrolytes, dispersants, and films. The massive demand for lithium batteries has driven the rapid development of entire industry chains. By the end of 2019, the new installed capacity of electrochemical energy storage in China reaches 0.64 GW, and the cumulative installed capacity has reached 1.71 GW. In 2020, the cumulative installed capacity exceeded 2 GW. By the end of Chin’s 14th Five-Year Plan, the capacity of electrochemical energy storage power stations will reach more than 20 GW. With the large-scale application of various energy storage technologies, the cost has also been significantly reduced. Taking lithium batteries as an example, the energy density and cycle life have been both doubled, and the application cost has been reduced by more than 70% during the past five years.
In addition to electrochemical energy storage, the thermal energy storage technology also has broad applications in district heating, mobile heating vehicles, distributed solar heating, combined cooling, heating and power systems, and solar water heating systems, etc. The temperature range for thermal energy storage application varies widely from −160 ∘C to 1000 ∘C [54]. Thermal energy storage technology can be divided into three main types: sensible heat storage, latent heat storage, and thermo-chemical energy storage. The energy storage density for the three types is gradually increasing, but the technological maturity is gradually decreasing. The sensible heat storage has been used in industrial applications, such as solar thermal power generation technology which uses molten salt as the heat storage medium. The medium and low temperature latent heat storage has been used in heating scenarios, while the medium and high temperature latent heat storage has not yet been commercialized on a large scale. Compared with sensible and latent heat storage, thermo-chemical energy storage is much more complicated and still at the laboratory research level.
Hydrogen energy and synthetic fuel
Hydrogen energy is another important way of energy storage in a broad sense, and it is a clean, efficient, safe and sustainable secondary source of energy without CO2 emissions [66–68], and plays important role in energy systems as shown in Fig. 20. Hydrogen and fuels derived from hydrogen make up 6% of final energy consumption in China in 2060. Hydrogen can be used as fuel taking place of fossil fuels which is widely used in heavy industry, fuel-cell vehicles and making ammonia. If evaluated by unit mass, the calorific value of hydrogen is the highest among all fuels, which is three times that of oil and four times that of coal. As the product of hydrogen combustion is only water [69], the direct combustion of hydrogen to obtain energy is also an effective way to achieve a zero-carbon energy supply. According to different energy sources for hydrogen generation, hydrogen can be subdivided into gray hydrogen, blue hydrogen and green hydrogen. Gray hydrogen is produced through the combustion of fossil fuels, green hydrogen is produced by using renewable wind/solar energy to electrolyze water, and blue hydrogen generation is through the combination of fossil energy combustion and carbon capture technology to achieve carbon neutrality. The future path for hydrogen production is to produce hydrogen through the electrolysis of water using renewable energy [70, 71], which is based on the utilization of renewable energy and large-scale energy storage. This contributes a lot to decreasing CO2 emissions. The current hydrogen production in China is mainly based on coal, while it is by natural gas in other countries. At the current stage, the hydrogen production by electrolysis of water only accounts for about 4% of the total hydrogen supply.
Under high pressure, hydrogen molecules will pass through the metal walls, causing hydrogen leakage. Therefore, the storage of hydrogen needs high requirements. The current hydrogen storage is mainly divided into gas, liquid and solid style. High-pressure gaseous hydrogen storage and low-temperature liquid hydrogen storage have been widely used in the aerospace field. The organic liquid hydrogen storage and solid-state hydrogen storage technologies are still in the demonstration stage. Hydrogen can be stored and transported on a large scale, which is an important feature different from battery energy storage. The storage performance and transportation efficiency of hydrogen are current bottlenecks in the construction of hydrogen energy networks.
Hydrogen energy plays important role in fuel cells, which directly converts the chemical energy of hydrogen and oxygen into electrical energy. This technology has the advantages of no pollution, no noise, and high efficiency, but the high cost is still the main reason that limits its large-scale application. In the future, two problems still need to be solved for the large-scale hydrogen energy application. The first one is the cost of fuel cell stack manufacturing, and the other one is to improve the corresponding infrastructure constructions, including the hydrogen refueling stations, hydrogen pipelines, and hydrogen storage units.
Hydrogen production can be combined with the consumption of renewable energy to achieve large-scale storage of renewable electricity. Before the large-scale promotion of electrochemical energy storage, the production of hydrogen from renewable electricity will become an effective way to solve the problem of abandoning solar and wind. The future hydrogen production route will inevitably evolve from the current non-green or light green to the final dark green stage. The transition in the energy structure will inevitably bring evolutions in the production route of hydrogen. It is estimated that hydrogen energy will account for about 10% of China’s energy supply in 2050, and the demand for hydrogen will be around 60 million tons. Therefore, there is a large development space for producing hydrogen in the way of electrolysis of water using solar and wind energy.
In addition to using renewable electricity to produce hydrogen, the reverse synthesis of carbon dioxide to fuel has also become an important way for energy storage in a wide range. In 2017, Jaramillo et al. [72] reported the electrocatalytic reduction process to convert solar energy and wind energy into chemical energy for storage using nitrogen, carbon dioxide, water and other air components as raw materials, achieving controllable conversion and storage of clean energy. In 2018, Bai et al. [73] looked forward to the prospects and plans of using carbon dioxide as a raw material to convert intermittent solar energy into renewable liquid synthetic fuels. This technology route for storing wind and solar energy through renewable fuels has great potential applications. The European Union has announced the full use of synthetic fuels based on renewable energy by 2050. The one-way conversion process of fossil fuels to carbon dioxide has led to an imbalance in the distribution of carbon in the various layers of the earth. Through the reverse conversion process of carbon dioxide, including photocatalytic conversion, biochemical conversion, thermo-chemical conversion, electrocatalytic conversion, etc., not only energy storage can be realized, but also the purpose of carbon sequestration can be achieved, and finally a carbon-neutral cycle process can be realized.
The synthetic fuels using carbon dioxide also faces some technical challenges: poor selectivity for small molecule products, the low energy conversion efficiency of electrocatalytic carbon dioxide reduction, effective separation of liquid phase products, various ions transportation in a gas-liquid-solid three-phase environment, the thermal management issues, and the challenges of devices. Therefore, the synthetic fuels by electrocatalytic carbon dioxide reduction require the cross integration and collaborative innovation of multiple disciplines such as physical chemistry, energy materials, and engineering thermophysics.
Carbon capture, utilization and storage (CCUS)
The achievement of the carbon neutrality goal depends not only on the large-scale use of non-chemical energy, but also on the effective management of carbon dioxide emissions from steel, cement and chemical industries, as well as the combustion of fossil energy. The CCUS is currently believed to be an effective way to quickly neutralize carbon dioxide emissions [74]. However, the long-term impact of carbon dioxide storage on the ecological environment still needs to be further evaluated.
The key technical processes involved in CCUS are the CO2 capture, transportation, utilization, and storage, as shown in Fig. 21. The capture process is to separate CO2 from fossil fuel combustion, industrial production, or directly from air. The capture technology covers the pre-combustion, post-combustion, oxy-fuel combustion, and chemical looping combustion capture. The captured CO2 will be transported either by tanker, pipeline, or ship for further utilization, which mainly includes three categories: geological, chemical and biological utilization. The CO2 storage mainly stores liquefied carbon dioxide in abandoned coal seams or oil and gas fields, saline aquifers, and deep seabed. While achieving CO2 storage, injecting high-pressure liquid or supercritical CO2 into oil wells or natural gas fields can improve the oil and gas recovery rate. Besides the typical CCUS, the bio-energy with carbon capture and storage (BECCS) and direct air carbon capture and storage (DACCS) also become more and more attractive carbon dioxide removal technologies. The BECCS extracts bioenergy from biomass and captures and stores the CO2, thereby finally removing it from the atmosphere. The DACCS technology captures CO2 directly from ambient air and can be used with very low CO2 concentration.
According to the annual report on CCUS in China 2021 [75], the emission reduction demand via CCUS in China under the carbon neutrality target is 20 to 408 million tons in 2030, 0.6 to 1.45 billion tons in 2050, and 1.0 to 1.82 billion tons in 2060, as shown in Fig. 22. China has promoted CCUS technology and launched over 40 demonstration projects with carbon capture capability of 3 million tons/year. Those projects were mainly conducted in the oil, coal, and electricity industry with small-scale capture capability, lucking of large-sale, combinatorial of multi-technologies, and whole-process demonstration. The high technical cost limits the large-scale application of CCUS technology, which includes the economic and environmental costs. The economic cost covers the whole operation process of CCUS, including the capture, transportation, storage, and utilization. It is estimated that by 2030, the cost of CO2 capture will be 90-390 \(\yen \)/ton, and 20-130 \(\yen \)/ton in 2060. The pipeline will be the main transportation method for large-scale CCUS demonstration projects in the future. It is estimated that pipeline transportation costs will be 0.7 \(\yen \)/(ton ·km) in 2030 and 0.4 \(\yen \)/(ton ·km) in 2060. For storage, the cost will be 40-50 \(\yen \)/ton in 2030 and 20-25 \(\yen \)/ton in 2060. The high technical cost and financing also mean that the promotion of CCUS technology requires further finical policy support from the government.
Energy internet and smart energy
In the future, the deep integration of renewable energy and information technology will form the energy internet and smart energy system, which is a combination of distributed energy gathering devices, distributed energy storage devices and various types of loads, etc [76, 77]. The energy nodes are intelligently interconnected to achieve an energy reciprocal exchange and sharing network with a two-way flow of energy. Thus, the basic characteristics of the energy internet are renewable, distributed, interconnected, intelligent, open, and commercialized. From the architectural point of view, the energy internet can be generally divided into three levels as shown in Fig. 23, including the physical layer, the network/information layer, and the business/application layer. The key technologies involved in the energy internet include novel power generation technology, novel power transmission technology, power distribution technology, advanced energy storage technology and information and data technology.
With electricity as the mainstay, the distributed energy system involves multiple energy complements such as water grids, optical grids, and gas grids will be the development trend of future energy supply models. The distributed energy system uses wind and solar energy for the electrolysis of water to produce hydrogen, and it also combines biomass energy and municipal solid waste power generation technology. This system can intelligently dispatch, manage and recycle the supply of various forms of energy according to user-side demands, and finally achieves a high efficiency of energy conversion and usage.
The micro-grid is also an important part of the smart energy system. Compared with the distributed energy system, the micro-grid effectively connects the energy production side and the energy consumption side through big data, artificial intelligence, and information technology. It is foreseeable that the micro-grid will be a major way for human society to use energy in the future. The characteristics of decentralization and on-site energy production and consumption make the micro-grid different from the current energy networks. Inter-regional micro-grid interconnection which couples the cold, heat, and electrical loads between multi-regional micro-grid systems, can not only realize the horizontal multi-energy complementation between multiple micro-grid systems, but also realize the vertical source network. The interconnection and optimization of micro-grid between multiple regions can effectively increase the comprehensive utilization rate of energy. The individual in the micro-grid can first connect to the regional distributed energy supply system, and then further connect to the hub energy system of the town. Through this energy network structure, it is possible to effectively realize the ultimate goal and philosophy of everyone as an energy producer and energy consumer. This is of great significance for building an efficient, stable and safe energy network. China has also launched many demonstrations and promotions of Energy Internet/Smart Energy System, including Chongming Energy Internet, Suzhou Industrial Park, and Lingang Energy Internet Project [79].
We summarized the limitation and trends for the above-mentioned key technologies in Table 5. Generally, the high cost is one of the current main limitations for energy storage, hydrogen energy, synthetic fuel, and CCUS. The highly integrated CCUS and energy internet also depends on the construction of the infrastructure. This problem can be solved with the development of the industrial chain and as well as the support of carbon finance. Another typical trend is that those low carbon key technologies will develop mutually and promote each other, for example, solar/wind energy + energy storage, renewables + hydrogen energy, CCUS + synthetic fuel, etc. All those key technologies will contribute to the energy transition together.
Table 5 Comparison of the limitations and trends for different key technologies