Recently, Cell’s sister journal Joule (IF=41.248) published the latest joint research paper on carbon neutrality by Professor Chen Xinyu's team from the School of Electrical and Electronic Engineering, HUST, along with a team from Harvard University. The paper was titled, Pathway toward carbon-neutral electrical systems in China by mid-century with negative CO2 abatement costs informed by high-resolution modelling. The study, jointly conducted by HUST, Harvard University and Tsinghua University, was led by Professor Cheng Shijie from HUST, the academician of the Chinese Academy of Sciences, and Professor Michael McElroy from Harvard University, the academician of the American Academy of Arts and Sciences. Professor Chen Xinyu is the first author and leading corresponding author, Professor Wen Jinyu and Professor Michael McElroy are the co-corresponding authors, and a student, Liu Yaxing, is the first co-author. Other important collaborators include Professor Kang Chongqing from Tsinghua University and Professor Chen Xia from HUST.

At the General Debate of the United Nations General Assembly in September 2020, President Xi Jinping proposed that China will strive to achieve carbon neutrality before 2060. Being the world’s largest energy producer and consumer, China expects to require over hundred trillion yuan investment for carbon neutrality transformation, which will become an important engine for the green recovery of the world, and China's economy, in the post-pandemic era.

At present, however, the national top-level planning of the transition pathway towards the electrical system is still unclear. The local goals and plans of emission reduction lack coordination, enterprises with low energy efficiency were blindly shut down. Affected by the world's macroeconomic downturn, the price of imported thermal coal has soared while coal production has fallen sharply in China due to the shutdown of coal mines and natural disasters. As a result, the safe supply of domestic power faces grave challenges, with power restricted and even residential power cut on a large scale. The contradiction between carbon emission reduction and energy supply security is becoming increasingly acute. It is a matter of urgency to formulate a scientific and reasonable transition pathway so as to coordinate low-carbon development while ensuring secure energy supply and the technical and economic feasibility of electrical systems.

For the above reasons, Professor Chen Xinyu's team from the School of Electrical Engineering, HUST and Harvard University jointly develop a digital twin model for electrical energy systems after six years of breakthroughs. Comprehensive, specific, and precise data are included: assessment of more than 40 years of high-solution historical resources on onshore wind power, offshore wind power and photovoltaic power, detailed geographical locations and operational conditions of more than 5000 thermal power units, water inflow information and operational characteristics of all major hydropower units across China, the national data of trans-provincial transmission network frames, the hourly load demand in actual operation of all provincial grids, the performances and costs prediction of current mainstream energy storage technologies and the most promising technologies of producing hydrogen by water electrolysis. The model can simulate the operation of the electrical systems for 8760 consecutive hours throughout the year, optimize the provincial generation mix under the carbon-neutral transition and the investment of the national trans-provincial transmission grid, and plan a technically feasible and most cost-competitive carbon-neutral transition pathway for electrical systems in China. The model adopts acceleration methods such as fast unit commitment technology, pre-optimization technology for national transmission grid planning, overall optimization technology for electric vehicle groups’ charging behaviors, and cascade modeling technology for hydropower plants with larger reservoirs to overcome the difficulty of super-large computing capacity of coordinating macro planning and hourly operation clearing. The model structure is shown in Fig. 1. Based on this model, an optimal investment plan and an hourly simulation scheme for 8760 hours all year round and 60 different scenarios have been formed. Results indicate that the annual investment and operation costs for different transition scenarios can deviate by more than 2.3 trillion yuan. The optimal carbon-neutral pathway provided by the study is not only technically feasible, but also more cost-competitive than conventional energy systems based on fossil fuels. In contrast to provincial balancing scenarios, the scheme can reduce annual costs by 1.2 trillion yuan, roughly 2/3 of China's annual financial expenditure for health. The costs comparison among scenarios is shown in Fig. 2.

Fig.1 Analytical model structure of national carbon-neutral electrical systems

Fig.2 Decarburization costs across different scenarios for China in 2050 at 80% renewable energy penetration

The study analyzes the investment and operation results of the national, and provincial planning with the target of realizing 80% renewable energy penetration, emphasizing the importance of coordinated planning for the transmission line network and strengthening the connection among the provincial electrical systems. Results show that under the coordinate optimization, 1.2 trillion yuan can be saved from the annual investment and operation costs per year, amounting to 5% of China's total fiscal expenditure in 2020. In the national level, overall planning, increasing investment in cross-sector grids can effectively promote the integration of renewable energy, substantially reduce investment for energy storage by taking advantage of large spatial scopes to weaken the volatility of renewable energy, and, to a great extent improve the extremely uneven distribution of load and resources in China.

The paper presents provincial power generation mix and trans-provincial transmission structures across China in 2050 at 80% renewable energy penetration (see Fig. 3). Overall, achieving  80% renewable energy penetration by 2050 requires about 900 GW offshore wind power, about 1700 GW land wind power, and about 1350 GW photovoltaic power. Regionally, a large number of offshore wind power installations will be established in  eastern coastal China; onshore wind power, mainly in northeastern China, northwestern China, and northern China, with a total of 1500 GW; photovoltaic power, mainly in northwestern China (200 GW), northern China (500 GW) and eastern China (250 GW). In 2050, in the context of a high-proportion of renewable energy, the national transmission line pattern will undergo structural changes. For lack of quality solar and wind resources in central China, the overall transmission pattern will change from the current “west to east” pattern to a new “peripheral to central” regime.

Fig. 3 Provincial power generation mix and trans-provincial transmission structure across China in 2050 at 80% renewable energy penetration

The paper also analyzes the configuration and operation of thermal power units in the transition to carbon-neutral electrical systems. The analysis suggests that at 80% renewable energy penetration, the total installed capacity of thermal power units will still exceed 1,000 gigawatts. The results, however, suggest that the annual full load hours of these thermal power units will be only 1,400 hours (equivalent to 16% utilization ratio), which will decouple the installed capacity of thermal power units and their carbon dioxide emissions. With renewable energy in high proportions in the electrical systems, thermal power units will transform from the main electricity supply to vital support for capacity and adjustment flexibility, which requires regulation to the formation mechanism of on-grid thermal power tariffs accordingly.

Meanwhile, the study analyzes the crucial role the decline in energy storage prices, and the maturity and application of the technology of producing hydrogen by water electrolysis play in realizing carbon neutrality of the electrical systems. Results demonstrate that for every 10% decrease in the electrochemical energy storage prices in 2050, about 280 billion yuan can be saved on the total annual system operation cost. The decline in energy storage prices will increase the energy storage and the proportion of renewable energy capacity, enhance the capability of the electrical systems to integrate electric power generated by renewable energy, and thus raise returns of renewable energy construction. The maturity and application of the technology for producing hydrogen by water electrolysis enable the electrical systems to function much more flexibly. With an extremely high proportion of renewable energy, harnessing a large amount of surplus electric power generated by renewable energy to produce hydrogen can improve the overall benefits of carbon-neutral electrical systems.

In addition, the study optimizes the grid-connected charging strategy of a large number of electric vehicles under the carbon-neutral scenario and analyzes the impact of charging behavior on grid operation, which serves as key guidance for the joint decarburization of electrical systems and transportation systems. The study improves the charging strategy of electric vehicles, pointing out that taking the slow charging strategy can effectively harness the solar photovoltaic power to reduce the curtailment rate and the peak load in the course of charging (see Fig. 4). In the case of slow charging, there are shifts in charging times from the evening rush to the afternoon and spots from residential areas to destinations (working places or shopping malls) compared with the current situation. The result plays an important guiding role in planning charging installations for electric vehicles and formulating policies on charging prices.

Fig. 4 Electrification strategies of different electric vehicles and coupling operation of electrical energy systems

Based on the finding of the study, the following several suggestions are made for the formulation of subsequent policies related to carbon-neutral transition. First, it is necessary to strengthen the scientific top-level design of the transition pathway towards electrical energy systems. Annual costs differ by trillions of yuan for different transition pathways and a lack of scientific planning will bring challenges to the secure and stable operation of energy supply. Second, it is necessary to further boost investment, and research and development of offshore wind power. Offshore wind power will become the largest source of low-cost decarburization of load centers in China's southeast coast in the long term. Third, it is necessary to coordinate plans for trans-provincial grids across China, and attach importance to the changes of national grid patterns during the carbon-neutral transition. Developing intra-regional and inter-regional ultra-high-voltage transmission technology may sharply reduce the total investment costs for China. Fourth, it is necessary to optimize the layout of the charging infrastructure of electric vehicles. At present, charging stations are mainly laid out in residential areas. By 2050, with high renewable energy penetration, office space, shopping malls and other destinations will become mainstream charging spots. Therefore, attention should be paid to the construction of charging stations in these areas.

The project is supported by HUST-State Grid Future of Grid Institute, Harvard Global Institute, Energy Foundation and National Natural Science Foundation of China.

Full text (29 pages) link: https://doi.org/10.1016/j.joule.2021.10.006

Appendix (95 pages) link: https://authors.elsevier.com/c/1dxne925JEG7-P

Written by: Yu Quan

Edited by: Scott, Peng Yumeng