Multi energy complementarity focuses on achieving multi energy complementarity and integration from the energy supply side, user demand side, and energy transmission and distribution side. According to the different supply and consumption characteristics of energy forms, it optimizes the comprehensive complementarity of production and utilization of various types of energy, solves problems such as wind, solar, water, and electricity restrictions, and achieves coordinated development of energy, economy, and environment.

At the end of January 2022, the National Development and Reform Commission and the National Energy Administration jointly issued the "14th Five Year Plan for Modern Energy System", proposing for the first time the construction plan for a "modern energy system". In the planning, the mention of "multi energy complementarity" up to 12 times is sufficient to demonstrate the important position of "multi energy complementarity". The following are the contents related to multi energy complementarity mentioned in the plan:
1. In areas with good wind and solar resource endowments, superior construction conditions, continuous integrated development conditions, and compliance with regional ecological environment protection requirements, we will orderly promote the centralized development of wind and photovoltaic power generation, accelerate the construction of large-scale wind and photovoltaic base projects with a focus on deserts, Gobi, and desert areas, actively promote the construction of multi energy complementary clean energy bases in the upper reaches of the Yellow River, Xinjiang, and northern Hebei, optimize the operation mode of multi energy complementary scheduling on the power side, and fully tap the potential of power peak shaving.
2. Actively promote the construction of clean energy bases with multiple complementary energy sources, scientifically optimize the proportion of power sources, prioritize the use of existing conventional power sources to implement multi energy complementary projects such as "wind, solar, and water (storage)" and "wind, solar, and fire (storage)", vigorously develop new energy sources such as wind and solar power, and maximize the utilization of renewable energy.
3. Relying on existing and newly added cross provincial and cross regional power transmission channels, as well as point to grid thermal power transmission channels, we will promote the complementary development of wind, solar, water, and thermal energy storage, with a focus on building clean energy bases such as the Yellow River "Ji" bend, the Hexi Corridor, and Xinjiang.
4. Based on the development of clean energy bases and the electricity supply and demand situation in the central and eastern regions, a batch of construction projects will be completed and put into operation, a batch of construction projects will begin, and a batch of multi energy complementary transmission channels will be studied and demonstrated.
5. Adapt to the development requirements of digital, automated, and networked energy infrastructure, build an intelligent scheduling system, achieve source network load storage interaction, multi energy coordination and complementarity, and intelligent regulation of energy demand.
6. Based on new models and formats such as clean energy bases with multiple complementary energy sources, integrated projects of source network load storage, comprehensive energy services, intelligent microgrids, and virtual power plants, we will carry out smart energy system technology demonstrations such as intelligent scheduling, energy efficiency management, and intelligent load regulation.
7. Regional (provincial), municipal (county), and park (residential area) level integrated demonstration of power grid, load and storage, multi energy complementary construction of integrated demonstration of wind and solar energy storage, wind and solar water (storage), and wind and solar fire (storage), as well as demonstration of smart energy use in smart cities, smart parks, and beautiful rural areas.
8. Multi energy complementary integration and intelligent optimization, intelligent regulation of energy demand, smart energy production services, smart energy system digital twins and other platforms and data center demonstrations.
9. Innovate the management mechanism for integrated power source, grid, load storage, and multi energy complementary project planning and construction, and promote the integration of project planning, construction implementation, operation regulation, and management.
10. Establish a coordinated operation and benefit sharing mechanism for integrated source network, load storage, and multi energy complementary projects.
From the planning, it can be seen that the development of "multi energy complementarity" revolves around two aspects: the user side and the power supply side. According to the Implementation Opinions of the National Development and Reform Commission and the National Energy Administration on Promoting the Construction of Multi energy Complementary Integration and Optimization Demonstration Projects in 2016, the user side is aimed at the various energy needs of end users, such as electricity, heat, cold, and gas, tailored to local conditions, coordinated development, and complementary utilization of traditional and new energy, optimizing the layout and construction of integrated energy supply infrastructure. Through natural gas cogeneration, distributed renewable energy, and energy intelligent microgrids, it achieves multi energy collaborative supply and comprehensive cascade utilization of energy. On the power side, emphasis is placed on utilizing the advantages of resource combinations such as wind, solar, hydro, coal, and natural gas in large-scale comprehensive energy bases, and promoting the construction and operation of wind, solar, hydro, thermal, and multi energy complementary systems.
According to the list of projects selected for the first batch of multi energy complementary integration and optimization demonstration projects released by the National Energy Administration in 2016, a total of 17 terminal integrated energy supply systems and 6 wind solar water thermal storage multi energy complementary systems were selected. The six projects facing the power supply side are: Zhangjiakou Zhangbei Wind Power Thermal Storage and Transmission Multi energy Complementary Integration Optimization Demonstration Project in Hebei Province, Hancheng Longmen Development Zone Multi energy Complementary Integration Optimization Demonstration Project in Shaanxi Province, Haixi Prefecture Multi energy Complementary Integration Optimization Demonstration Project in Qinghai Province, Shenhua Shendong Electric Power Wind Power Thermal Storage Multi energy Complementary Integration Optimization Demonstration Project in Gubaotou City, Inner Mongolia, Hainan Prefecture Water Light Wind Multi energy Complementary Integration Optimization Demonstration Project in Qinghai Province, and Yazuihe River Basin Light Water Animal Husbandry Multi energy Complementary Integration Optimization Demonstration Project in Muli County, Liangshan Prefecture, Sichuan Province. As of March 2023, there are 198 proposed projects related to China's multi energy complementary industry, of which 11 were announced in 2022. Multi energy complementarity has become an important strategy for energy development, integration, and the construction of a modern energy system.
The dual guarantee of "natural resources+infrastructure" is an important prerequisite for the development of multi energy complementarity. Regions need to have abundant solar, wind, hydro, thermal, energy storage, and other resources, as well as a strong power grid system infrastructure covering most areas as a guarantee. In fact, ultimately, complementary power generation relies on intelligent control of multiple power generation methods, utilizing energy storage systems to store electrical energy, and finally converting direct current into alternating current through inverters for power supply. Among the first batch of six national level multi energy complementary projects announced, there is one fully implemented project, two partially implemented projects, and three projects that have not yet started construction.
The Golmud Multi Energy Complementary Integration and Optimization Demonstration Project in Haixi Prefecture, Qinghai Province is an innovative project that integrates wind, light, heat, storage, regulation, and load into a pure clean energy comprehensive utilization. It fills the gap in intelligent scheduling technology for wind and solar thermal storage and load regulation in China. It mainly uses wind power, solar thermal power generation, and energy storage power stations as regulating power sources, and implements flexible and intelligent regulation of multiple energy sources through a smart energy system to reduce output volatility and achieve the effect of "1+1>2". The total investment of the project is about 6.37 billion yuan, with a total installed capacity of 700MW. It has invested in the construction of 200MW photovoltaic power generation project, 400MW wind power project, 50MW solar thermal power generation project, and 50MW battery energy storage power station. The current average annual power generation of the project is about 1.137 billion kilowatt hours, which can supply the total electricity consumption of urban and rural residents in Qinghai for one quarter. The specific implementation is that wind power, photovoltaic power, and solar thermal power are connected to a 110 kV booster station through a 35 kV collection line and transmitted to a 330 kV collection station for grid connection; After the on-site voltage of the energy storage power station is increased to 35 kV, it is transmitted to the 330 kV collection station through the 35 kV busbar of the photovoltaic power station, as shown in the diagram [4].

Demonstration project diagram

The Zhangbei wind solar thermal storage and transmission multi energy complementary integration and optimization demonstration project is a renewable energy project that integrates wind power, photovoltaic power generation, energy storage and transmission engineering. The total investment is about 4.35 billion yuan, and the total installed capacity is 475MW, including 150MW of wind power, 250MW of photovoltaic power, 50MW of solar thermal power, and 25MW of energy storage. It has been promoted for implementation. After the completion of the project, the annual power generation will be about 9.3 million kWh, exploring a world first wind solar energy storage and transmission combined operation mode, which achieves combined power generation through seven combined power generation methods of wind, solar, and storage.

The Hainan Prefecture Water, Light, and Wind Multi Energy Complementary Integration and Optimization Demonstration Project in Qinghai Province is the world's largest single unit water, light, and wind energy complementary integration and optimization demonstration project. The project includes 4.16 million kilowatts of hydropower, 4 million kilowatts of photovoltaic power, and 2 million kilowatts of wind power. After completion, the three power sources will be bundled and sent out through a multi energy complementary 750 kilovolt collection station to achieve coordinated control of water, light, and wind power. After multi energy complementarity, they will be sent to the power grid. Among them, the 320 MW Longyangxia Water Light Complementary Photovoltaic Power Station has achieved the use of controllable hydropower to supplement and regulate unstable light and wind energy. After being regulated into stable and high-quality electricity, it is then uniformly transmitted to the power grid.
In terms of energy storage systems, the energy storage project in the Golmud Multi Energy Complementary Integration and Optimization Demonstration Project in Haixi Prefecture, Qinghai Province adopts lithium iron phosphate batteries. Due to the decrease in the cost of energy storage batteries in recent years, the battery cost of energy storage power stations is much lower than that of newly built thermal power plants with the same frequency regulation capacity, and the benefits are significant. The selection of energy storage batteries needs to fully consider safety, operational performance, and economy. In common energy storage technologies (such as lead-acid batteries, lithium batteries, flow batteries, sodium sulfur batteries, supercapacitors, etc.), although lead-acid batteries are cheap, their cycle life is short, their environmental friendliness is poor, and their energy density is low; Supercapacitors are also constrained by energy density; Currently, sodium sulfur batteries and flow batteries are mainly limited by cost, so a more suitable and comprehensive lithium iron phosphate battery has been chosen [4]. At present, the preferred configuration for energy storage on the power generation side is lithium iron phosphate and other lithium batteries. With the continuous expansion of domestic energy storage project development scale, new energy storage technologies such as flow batteries, compressed air, flywheels, and thermal storage batteries have made varying degrees of progress [5].
In summary, multi energy complementarity is an important development direction for both energy and user sides in the future, and it is currently in a period of emerging development. However, there are still many technical problems that need to be addressed, and mechanisms for efficient coordination between different types of energy need to be addressed. In terms of energy storage, lithium iron phosphate is currently the main focus, mainly due to comprehensive considerations of energy density and cost. In the future, new energy storage technologies such as flow batteries and sodium batteries will also emit light and heat in the field of multi energy complementarity after cost reduction.
[1] Chen Zhengxi, Yu Yi, Liang Caihao, etc. Clean energy multi energy complementary and collaborative development under the framework of the Global Energy Internet [J]. Global Energy Internet, 2023,6 (02): 126-138
DOI: 10.19705/j.cnki.issn2096-5125.2023.02-004
[2] Liu Shubo, Yang Bin, Zhong Chunlin, et al. Research on demand response strategies for energy storage systems based on multi energy complementarity [J]. Automation Technology and Applications, 2023,42 (11): 137-141. DOI: 10.20033/j.1003-7241. (2023) 11-0137-05
[3] Xiaoyu Visiting the world's first multi energy complementary demonstration project [N]. Xining Evening News, 2023-09-14 (A02)
DOI: 10.28899/n.cnki.nxnwb.2023.002414
[4] Li Jianlin, Wang Jianbo, Ge Le, Yuan Xiaodong, Zhou Jinghua, Li Chunlai Inspiration from the Construction and Operation of Multi energy Complementary Demonstration Projects [J] Hubei Electric Power, 2019, 43 (03): 49-56
[5] Kang Junjie, Zhao Chunyang, Zhou Guopeng, et al. Research on the Development Status and Implementation Path of Wind, Solar, Water, Fire, and Multi energy Complementary Demonstration Projects [J]. Power Generation Technology, 2023,44 (03): 407-416