Electric fuel energy storage system: make charging as easy as refueling

Schematic diagram of commercial operation of electric fuel energy storage system. Photo courtesy of Xu Jianbo

Is it possible to charge new energy vehicles as simple as refueling? Electric fuel energy storage systems may turn ideals into reality.

Previously, Zhao Tianshou, an academician of the Chinese Academy of Sciences and professor of the Department of Mechanical and Aerospace Engineering of the Hong Kong University of Science and Technology, proposed the concept of e-fuel energy storage (e-fuel). Users can directly pour the electric liquid that has been charged at the charging station in advance. In electric fuel cells, it is used for discharge and subsequent use.

Since the electrode of the fuel cell on the car is dedicated to discharge, it has higher performance and longer life. Moreover, the system has no noble metal element catalysts, which can greatly reduce the investment cost of the system.

Recently, the team of An Liang, associate professor of the Department of Mechanical Engineering of the Hong Kong Polytechnic University published a paper in the "Cell Report-Physical Science", reporting an electric fuel cell that uses a highly reactive electric fuel and a catalyst-independent graphite felt electrode, which significantly improves the liquid The power density and energy efficiency of electric fuel cells. This also proves the feasibility of the idea of electric fuel energy storage.

Liquid can be charged

Hydrogen-oxygen fuel cells are considered as a potential energy supply method for new energy vehicles in the future. However, this type of fuel cell has many drawbacks. Not only does it require a precious metal catalyst for hydrogen production and use, there are also safety and cost limitations for hydrogen to achieve large-scale storage and transportation.

In response to this, Zhao Tianshou came up with an idea-can he design an energy storage system with a cyclic charge and discharge liquid as the medium?

"As early as 2016, I began to conceive of a fuel cell that can be used to charge liquids." Zhao Tianshou told reporters. "Take metal ion electric fuel as an example. Before charging, it is in a high-priced state. When it becomes a low-priced state, the electric fuel is charged. In the same way, organic molecular electric fuel or suspended particle electric fuel can also be charged and discharged by gaining and losing electrons."

According to Xu Jianbo, assistant professor of the Department of Mechanical and Aerospace Engineering of the Hong Kong University of Science and Technology, such a liquid electric fuel energy storage system includes three parts: electric fuel, electric fuel charger and electric fuel cell.

Zhao Tianshou said that unlike the traditional flow battery that must be able to charge and discharge, in the liquid electric fuel system, the charging and discharging units are independent of each other. The charging process is realized in a special power station, and the discharging process takes place on the dispersed discharge equipment. Only the liquid electric fuel can be transferred to complete the energy transfer in time and space.

"This not only realizes the electrode decoupling of charging and discharging, but also solves the problem of mutual restriction of oxidation and reduction reactions caused by the common electrode of charging and discharging in the past. Each part can be designed with special oxidation or reduction electrodes, which improves the energy storage of electric fuel. System performance reduces costs." Xu Jianbo said.

He also pointed out that the electric fuel energy storage system has a theoretical life of up to 20 years due to its high stability. From the perspective of environmental protection, electric fuel charging devices and electric fuel cells do not use harmful precious metals, and electric fuels can be recycled and reused, which overcomes the non-recyclable defects of lithium-ion batteries and lead-acid batteries.

Serving large energy storage to eliminate wind photovoltaic

In addition to being a power source for automobiles, the new electric fuel energy storage system can also serve large fixed energy storage facilities.

"In the past ten years, the cost of photovoltaic and wind power has been reduced by as much as 80%, but from a global perspective, the proportion of photovoltaic and wind power applications is less than 6%." Zhao Tianshou said, "The main reason is that the supply of wind and photovoltaic power is not sufficient. Stable and geographically dispersed."

Various energy storage facilities such as pumped water storage and compressed air energy storage provide a solution to the above-mentioned problems of wind and photovoltaics. However, pumped storage power plants require mountains and water, and compressed air storage relies on gas storage caves, all of which impose high requirements on geographic conditions.

Xu Jianbo introduced that the new electric fuel energy storage system can store wind or solar energy through electric fuel chargers, store electrical energy as chemical energy of electric fuel, and realize off-grid or grid-connected power supply through electric fuel cells.

"The energy storage cycle efficiency of this type of system is higher than 80%." Zhao Tianshou told reporters, "The efficiency of charging alone can reach more than 90%, effectively storing light and wind power into electric fuel; and during the discharge process, In addition to achieving high efficiency, it also achieves high power density."

Xu Jianbo emphasized, “The peak power density of electric fuel cells is twice that of traditional hydrogen-oxygen fuel cells.”

Not only that, there is no limit to the capacity of the electric fuel energy storage system, which is determined by the volume of the electric fuel. Electric fuel charging stations continuously convert renewable energy into electric fuels like an oil refinery, and electric fuel cells can provide users with clean power anytime, anywhere, with high efficiency and high power.

Clean and environmentally friendly

"In a broad sense, petroleum is also a liquid energy carrier, storing solar energy in petroleum over the course of millions and tens of millions of years." Zhao Tianshou said.

As a liquid energy-carrying medium, petroleum is very convenient to use, but there are two major problems. First, although oil burns quickly, the energy storage process is too slow, and the rate of oil regeneration is far behind the rate of consumption; second, the use of fossil fuels will also bring about carbon emissions and environmental pollution.

If electric fuel storage is combined with clean energy sources such as wind, light, and water, these two major shortcomings of petroleum can be avoided.

On the one hand, the energy stored by the electric fuel charger comes from renewable energy, which is clean and environmentally friendly. On the other hand, the charging process of this kind of energy storage system is very fast. Renewable energy can be charged into electric fuel efficiently and quickly, and then be transported and used flexibly and conveniently.

It can be said that the advantages of clean and environmentally friendly renewable energy and the flexible and convenient use of electric fuels have laid the broad application prospects of electric fuel energy storage systems.

According to Zhao Tianshou, they are speeding up research and development to promote the application of electric fuel energy storage systems in automotive power and large stationary energy storage facilities. For vehicle power sources, they will focus on energy density and power density, and make the electric fuel cell as compact as possible so that it can travel longer distances with a single refueling.

In terms of stationary energy storage, Zhao Tianshou's team is already experimenting with kilowatt-level energy storage facilities.

“For large-scale stationary energy storage, we can currently achieve better stability and lower cost than existing flow batteries, but the cost needs to be further reduced to be competitive in the market. Therefore, we plan to develop lower and more expensive Stable core materials to further reduce system costs." Xu Jianbo said that in the near future, electric fuel energy storage systems are expected to be widely used and play an important role in green travel, energy saving and emission reduction. (■The reporter's trainee Chi Han)