I. Hydrogen Fuel Cell Industry Chain
In the hydrogen fuel cell industry chain, the upstream involves the production, transportation, and storage of hydrogen, and the refueling of hydrogen fuel cell systems at hydrogen refueling stations; the midstream involves the production of key components such as fuel cell stacks, integrating the fuel cell stack and accessories to form a hydrogen fuel cell system; and the downstream applications mainly include transportation, portable power sources, and stationary power sources.
II. Comparison of the advantages and disadvantages of power batteries
Currently, there are four main technological approaches for power sources in transportation: lithium-ion batteries, hydrogen fuel cells, supercapacitors, and aluminum-air batteries. Lithium-ion batteries, supercapacitors, and hydrogen fuel cells are widely used, while aluminum-air batteries are still in the laboratory research stage. Regarding energy refueling, lithium-ion batteries and supercapacitors are suitable for pure electric vehicles but require external charging, while hydrogen fuel cell vehicles require external hydrogen refueling, and aluminum-air batteries require the addition of aluminum plates and electrolyte.
Comparison of the advantages and disadvantages of the four technical routes
1. Characteristics of hydrogen fuel cells
(1) Good environmental compatibility
Hydrogen fuel cells provide highly efficient and clean energy, emitting not only small amounts of clean water but also eliminating water pollution issues. Furthermore, unlike engines that convert thermal energy into mechanical energy, fuel cells directly convert chemical energy into electrical and thermal energy, resulting in high energy conversion efficiency and low noise.
(2) Good operational performance
Hydrogen fuel cell power generation does not require complex and bulky equipment; the fuel cell stack can be modularly assembled. For example, a 4.5MW power generation unit can consist of 460 fuel cell modules, and its power plant footprint is much smaller than that of a thermal power plant. Hydrogen fuel cells are suitable as distributed power generation devices. Furthermore, compared to thermal, hydroelectric, and nuclear power generation, hydrogen fuel cell power plants have shorter construction cycles, are easier to expand, and can be built in phases according to actual needs. At the same time, hydrogen fuel cells exhibit high operational quality and excellent characteristics in handling rapid load changes (such as peak loads), switching from low power to rated power within seconds.
(3) High-efficiency output performance
Hydrogen fuel cells convert the energy stored in the fuel into electricity and heat when they are working, with an efficiency of over 40% in converting electrical energy into electricity, while steam turbines can only convert one-third of the energy into electricity.
(4) Flexible structural characteristics
Hydrogen fuel cells offer great flexibility in assembly and easy power adjustment. Compared to traditional engines, their modularity allows for easy adjustment of output power and voltage by simply adding or removing individual cells without increasing infrastructure investment. This makes them easy to build and facilitates grid regulation. This characteristic of fuel cells enhances system stability.
(5) Hydrogen has a wide range of sources.
Hydrogen, as a secondary energy source, can be obtained through various methods, such as coal-to-hydrogen, natural gas reforming, and water electrolysis. When fossil fuels are depleted, hydrogen will become the world's primary fuel and energy source. Furthermore, hydrogen production via solar-powered water electrolysis produces no carbon emissions, making hydrogen arguably the ultimate energy source.
(6) Existing bottlenecks
From the current development perspective, the widespread adoption of hydrogen fuel cells has encountered certain bottlenecks, such as the high cost of the batteries themselves and the lack of widespread infrastructure.
2. Characteristics of Lithium-ion Batteries
(1) Voltage plateau
Due to the different positive and negative electrode materials used, the working voltage range of a single lithium-ion battery is 3.7~4V. Among them, the working voltage of the lithium iron phosphate single battery, which is widely used, is 3.2V, which is 3 times that of nickel-metal hydride batteries and 2 times that of lead-acid batteries.
(2) High specific energy
The current energy density of lithium-ion power batteries for passenger vehicles is close to 200Wh/kg, and it is expected to reach 300Wh/kg in 2020.
(3) Short battery life
Due to limitations in the properties of electrochemical materials, the cycle life of lithium-ion batteries has not seen a breakthrough. Taking lithium iron phosphate batteries as an example, a single cell can achieve over 2000 cycles, but when assembled into a battery pack, the cycle life is only over 1000 cycles. This cannot meet the requirement of an 8-year service life for public transportation.
4) Has a significant environmental impact
Lithium-ion batteries use the light metal lithium, and although they do not contain harmful heavy metals such as mercury and lead, they are considered green batteries with less environmental pollution. However, because their positive and negative electrode materials and electrolytes contain metals such as nickel and manganese, the United States has classified lithium-ion batteries as batteries containing toxic and harmful substances such as flammability, leaching toxicity, corrosivity, and reactivity. They contain the most toxic substances of any type of battery. Furthermore, because their recycling process is relatively complex and costly, the current recycling rate is low, and discarded batteries have a significant environmental impact.
(5) Costs remain high
The initial purchase cost of lithium-ion batteries is high. Taking lithium iron phosphate batteries, the mainstream power batteries used in buses, as an example, the price is approximately 2,500 yuan/kWh. With the popularization of electric vehicles, this price is expected to drop to below 1,000 yuan/kWh by 2020. Due to the limitation on the number of cycles after individual battery cells are assembled into a battery pack, buses typically need to replace their batteries every three years or so, resulting in significant cost pressures for operating units.
(6) Has a significant impact on the power grid
Firstly, the large-scale application of pure electric vehicles will lead to significant harmonic interference from charging equipment to the power grid due to the high charging demand, thus affecting the power supply quality. Secondly, during fast charging, the charging power is high (around 50kW for passenger cars and 150-250kW for buses), which puts a significant load on the power grid.
Therefore, based on the current technological level of lithium-ion batteries, their application in electric vehicles is mainly in short-distance pure electric vehicles with a driving range of less than 200km.
3. Characteristics of Supercapacitors
(1) Extremely high charge/discharge rate
Supercapacitors possess high power density, capable of discharging hundreds to thousands of amperes of current in a short time, and charging rapidly, completing the charging process in tens of seconds to minutes. Supercapacitor buses and trams utilize this characteristic to quickly charge and propel the vehicles.
(2) Long cycle life
Supercapacitors have minimal losses during charging and discharging, so theoretically their cycle life is infinite, and in practice it can reach more than 100,000 cycles, which is 10 to 100 times longer than that of batteries.
(3) Good low-temperature performance
Most of the charge transfer that occurs during the charging and discharging of a supercapacitor takes place on the surface of the electrode active material, so the capacity decays very little with temperature, whereas the capacity decay of a typical lithium-ion battery can be as high as 70% at low temperatures.
(4) The energy density is too low
One of the bottlenecks in the application of supercapacitors is their low energy density, which is only about 1/20th that of lithium-ion batteries, approximately 10Wh/kg. Therefore, they cannot be used as the main power source for electric vehicles and are mostly used as auxiliary power sources, primarily for fast start-up devices and regenerative braking systems.
4. Characteristics of aluminum-air batteries
(1) Low material cost and high energy density
The negative electrode active material of aluminum-air batteries is abundant metallic aluminum, which is inexpensive and environmentally friendly. The positive electrode active material is oxygen from the air, and the positive electrode capacity is virtually unlimited. Therefore, aluminum-air batteries have the advantages of being lightweight, small in size, and having a long service life.
(2) Key technologies have not been broken through and have not yet left the laboratory.
Problems such as air electrode polarization and aluminum hydroxide deposition are significant obstacles hindering the commercialization of metal-air batteries, and improvements in the performance of aluminum-air batteries have encountered considerable bottlenecks. Currently, they are still in the laboratory stage and are quite far from commercialization.
