A reporter walked past a flow battery device in Japan.
With the gradual decrease of the reserves of fossil fuels such as coal, oil and natural gas, the greenhouse effect caused by burning fossil fuels is becoming more and more serious, and governments all over the world have put the development and utilization of new energy sources on an important agenda. With the improvement of people's attention and the progress of related technologies, the proportion of renewable energy such as solar energy and wind energy in power supply is increasing.
However, compared with traditional thermal power generation, these new energy forms have an inherent defect, that is, their normal operation is limited by natural conditions, which often leads to the mismatch between the output of electric energy and the needs of users. For example, using solar energy to supply electricity to a residential area, most residents are not at home during the day, and the electricity generated by solar cells exceeds demand and is wasted; At night, when residents come home from work, the electricity consumption increases sharply, but at this time, solar cells can't supply power normally.
In order to solve the problem of instability of new energy, we usually need to provide a certain energy storage system to cooperate with it. When the power generation capacity exceeds the actual needs, we can convert the redundant electric energy into other forms of energy for storage, and when the power generation capacity cannot meet the needs, we can convert the stored energy into electric energy again. It can be said that the development of energy storage technology supporting new energy is as important as the development of new energy technology itself.
So how do you store excess power? By building a pumped storage power station, we can use surplus electric energy to move water from a low place to a high place, that is, convert electric energy into gravitational potential energy of water, and then let the water fall from a high place when the power supply capacity is insufficient, and convert the potential energy of water into electric energy. We can also use surplus electric energy to compress the air, and release the compressed air when the electric energy supply is insufficient, so that it can drive the generator to generate electricity.
However, although these technologies can convert electric energy into other forms of energy for storage, there are still some limitations in practical application. For example, the construction of pumped storage power stations usually depends on certain terrain. Therefore, people prefer to use batteries to accomplish this task, that is, to convert electrical energy into chemical energy and store it. In particular, the secondary battery, which can be repeatedly charged and discharged, is the best choice to assist new energy.
But in practice, people found that the "old revolution" encountered new problems, but many mature battery technologies had shortcomings. So what is the reason? This should start with the structure of common batteries.
Take lead-acid batteries that are often used in automobiles as an example. Its basic structure is to insert an electrode made of lead dioxide and metallic lead into dilute sulfuric acid solution. When the circuit is connected, lead dioxide at the positive electrode gains electrons and becomes lead sulfate, while lead at the negative electrode loses electrons and becomes lead sulfate. When the solid of lead and lead dioxide turns into lead sulfate, we will find that the battery is dead.
If we connect the lead sulfate on both sides to an external power supply at this time, under the action of current, the lead sulfate connected to the positive pole of the power supply loses electrons and becomes lead dioxide, while the lead sulfate connected to the negative pole of the power supply gains electrons and becomes lead. If the external power supply is removed, lead and lead dioxide can react chemically to release electric energy, that is, the battery is fully charged again.
In addition to lead-acid batteries, nickel-cadmium batteries, lithium-ion batteries and other common batteries, carbon-zinc batteries (dry batteries) and other disposable batteries, the materials constituting the positive and negative electrodes not only serve as conductors to transmit current, but also participate in electrochemical reactions. In other words, the solid electrode constitutes the carrier of battery energy storage. This benefit is obvious: the battery can be designed to be small and compact, which is very suitable for portable devices with "land and money".
However, it is very troublesome to apply the traditional storage battery to the energy storage of new energy: the instability of new energy supply means that the supporting energy storage equipment needs to be able to flexibly adjust the total amount of energy to be stored and the power to provide energy. However, for the traditional battery that relies on solid electrodes, how much power the battery can store and how much power it can release has been fixed at the moment of packaging and leaving the factory, so it is difficult for users to adjust it as needed.
So how to overcome this defect? The solution is that the solid electrode is only responsible for transmitting current, and the task of storing electric energy is undertaken by the liquid reactant, which is the basic principle of the flow battery. For example, the all-vanadium redox flow battery, which has been completely developed at present, has a basic structure of a cavity separated by a selectively permeable membrane, and solid electrodes are arranged on both sides of the cavity. If we add acidic solutions containing two different vanadium ions to both sides of the cavity, when the battery is discharged, such reactions will occur in the two stages respectively:
The overall result is that two compounds containing vanadium become the other two compounds of vanadium, and the electric energy generated by the reaction is continuously output to the external circuit through the electrode. When the battery is charged, the above reaction is the opposite.
Basic principle of flow battery: inside the battery, the reactants of anode and cathode are separated by semi-permeable membrane. No matter charging or discharging, all chemical reactions are carried out in solution. After the reaction, the solution can be pumped out of the battery by the pump, and the new solution to be reacted can be injected into the battery from the storage tank. The picture is quoted from the reference [1]
Whether charging or discharging, as long as the reaction is complete, the energy conversion will come to an end. But different from the traditional battery, for the flow battery, we can extract the converted solution from the battery and then inject the unreacted solution into the battery, so that the charging or discharging process can continue. The energy conversion of this battery no longer depends on solid electrodes, but on flowing liquid, hence the name flow battery.
Compared with the traditional battery using solid electrodes, the flow battery has an obvious advantage, that is, it gives users more freedom to adjust the performance of the battery. If it is necessary to increase the energy storage capacity of the battery, there is no need to change the structure of the battery, only need to connect more storage tanks filled with vanadium ion solution or increase the concentration of vanadium ion in the solution. What if I need to increase the output power of the battery? It is not difficult to do it, just connect several identical batteries, increase the contact area of the two solutions, and let more solutions react chemically in unit time. The flexibility of use enables the flow battery to better play its energy storage function.
All-vanadium flow battery combined with solar cell
Compared with the traditional battery, the flow battery also has a significant advantage, that is, the battery becomes easier to maintain. As mentioned earlier, the materials that make up the solid electrodes of traditional storage batteries are always transformed into another substance when discharging, and return to the original substance when charging, such as the transformation between lead in lead-acid storage batteries and lead sulfate, and the transformation between metal cadmium and cadmium hydroxide in nickel-cadmium batteries. However, what happens in actual operation is often not as simple as what is written in the chemical reaction formula. For example, after a charge-discharge cycle, the solid material that constitutes the electrode returns to its original chemical composition, but its structure may change, which will inevitably affect the performance of the battery and may even lead to safety accidents. On the contrary, in the flow battery, the chemical reaction is carried out in solution, and the solid electrode is only responsible for transmitting current, which is less disturbed by various side reactions. Therefore, the flow battery can often withstand more charge and discharge cycles than the traditional battery, and keep its performance basically unaffected.
Having said so many advantages of flow battery, we should also talk about its limitations. Because energy is no longer stored in solid materials like traditional batteries, one of the inevitable disadvantages of flow batteries is that the electric energy per unit mass will be greatly reduced, because no matter how high the concentration of the solution is, there will still be a lot of solvents that will not contribute to the storage of electric energy. Like a wallet of the same size. One person put money in it, and the other person put toilet paper in addition to the money. When two people go shopping together, it goes without saying that who buys more things. For example, the all-vanadium flow battery mentioned above, the lithium-ion battery per unit mass can only provide about 20% energy [1]. Even if the flow battery is not used in portable equipment, people still want it to be as small as possible. In addition, how to use cheaper materials to reduce the cost of flow batteries is also a major direction for researchers at present.
Of course, having shortcomings is not terrible. The key is how to overcome these shortcomings step by step through technological progress. In fact, the flow battery technology actually appeared as early as the 1970s, and it was only in recent years that people realized its value. At present, the research of flow battery has become a very hot field. I believe that in the near future, this unique battery technology can play a greater role in solving human energy problems.
refer to
[1] Bruce Dunn, Haresh Kamath, Jean-Marie Tarascon, "Electric Energy Storage for Power Grid: Selection of Battery", Science, 201,334,928.
(Author: yuyu)