Application of the hottest hydrogen energy in fuel

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The application of hydrogen energy in fuel cell city buses

[Abstract] the pressure of environmental pollution, global warming and energy shortage makes the traditional internal combustion engine vehicles face unprecedented challenges in the 21st century. Automobile manufacturers are trying to use new energy sources to improve the economic and environmental protection characteristics of vehicles. Governments and major automobile manufacturers have invested heavily in the research and development of proton exchange membrane fuel cell (PEMFC) electric vehicles. Fuel cell vehicle technology has the following advantages compared with traditional vehicle and pure electric vehicle technology

[Key words] hydrogen energy, fuel cell city bus

0, project background

the pressure of environmental pollution, global warming and energy shortage makes the traditional internal combustion engine vehicles face unprecedented challenges in the 21st century. Automobile manufacturers are trying to use new energy to improve the economic and environmental protection characteristics of vehicles. Governments and major automobile manufacturers have invested heavily in the research and development of proton exchange membrane fuel cell (PEMFC) electric vehicles. Fuel cell vehicle technology has the following advantages compared with traditional vehicle and pure electric vehicle technology

(1) high efficiency

the working process of the fuel cell is a process of converting chemical energy into electrical energy, which is not limited by the Carnot cycle. The energy conversion efficiency is high. In 1999, Daimler Chrysler launched the fuel cell concept car necar 4, with an energy efficiency of 37.7% from the fuel tank to the wheel, While the efficiency of high-efficiency gasoline engine and diesel engine vehicles is 16-18% and 22-24% respectively.

(2) long driving range

the fuel cell system is used as the energy source, which overcomes the shortcoming of short driving range of pure electric vehicles, and its long-distance driving capacity and power performance are close to those of traditional vehicles

(3) green environmental protection

fuel cell has no combustion process, uses pure hydrogen as fuel, and only water is produced, which belongs to zero emission. Hydrogen produced by other hydrogen rich organic compound truck integrators is used as the fuel of fuel cells. In addition to water, the products may also contain a small amount of CO2, which is close to zero emission

(4) strong overload capacity

in addition to having high efficiency in a wide working range, the short-time overload capacity of fuel cells can reach 200% or more of the rated power

(5) low noise

fuel cell is a static energy conversion device, and there are no other moving parts except air compressor and cooling system. Therefore, compared with internal combustion engine vehicles, the noise and vibration during operation are small

(6) convenient and flexible design

fuel cell vehicles can be designed according to the idea of X-by-wire, change the traditional automobile design concept, and can be flexibly configured in terms of space and weight

under the background of the international development of fuel cell vehicles, according to the strategic needs of the development of China's automotive industry, the Tenth Five Year Plan of China's national high technology research and development plan (863 plan) lists fuel cell city buses as an important sub subject of the major special projects of electric vehicles. The fuel cell city bus project was launched in early 2002. The State Key Laboratory of automotive safety and energy conservation of Tsinghua University is the main research unit. The first prototype vehicle has passed the national acceptance at the end of 2002

1. Selection of complete vehicle hydrogen source and power system structure

in the design of 863 fuel cell city bus, the hydrogen source scheme and power system structure suitable for China's economic and road conditions must be selected first

1.1 comparison of hydrogen source schemes on-board hydrogen supply system can be divided into on-board hydrogen production and on-board pure hydrogen

1.1. L on board hydrogen production

on board hydrogen production requires an internal high-temperature fuel processor to obtain hydrogen from the fuel through reforming or partial oxidation. The fuel used for on-board hydrogen production can be alcohols (methanol, ethanol, dimethyl ether, etc.) and hydrocarbons (diesel, gasoline, LPG, methane, etc.). Other substances such as ammonia, metal or metal hydride can also be used as raw materials for hydrogen production

technically, the temperature of hydrogen production from alcohol fuel is low, and the hydrogen production reaction is easy to realize. Methanol is generally considered to be the most suitable vehicle fuel for hydrogen production. Necars of DaimlerChrysler completed the feat of crossing the East and west of the United States on June 4, 2002, with a journey of more than 3000 miles, which fully proved the technical feasibility of methanol on-board reforming to produce hydrogen fuel cell vehicles

it is more difficult for hydrocarbons to produce hydrogen than alcohols, which is mainly reflected in the high reforming temperature and sulfur removal. Among hydrocarbon fuels, natural gas, as a gaseous fuel, is difficult to store and transport on-board. In addition, on-board reforming is the most difficult and the operating temperature is the highest. Generally, it is not used as the fuel for on-board hydrogen source of fuel cells

among other hydrogen production methods, ammonia is not suitable for on-board hydrogen production because of its high cost as raw material for hydrogen production, high Corrosivity in article 6.3 of the national standard gb/t 2611-2007 general technical requirements for laboratory machines, and high complete cracking temperature of ammonia. However, the hydrolysis of metal or metal hydride to produce hydrogen can only be used in special occasions because of its high energy consumption, high cost and high emissions in the process of raw material preparation, and is not suitable for large-scale automobile industry

the on-board hydrogen production avoids the technical problems and investment of fixed infrastructure such as hydrogen production, transportation and filling and on-board hydrogen storage system. However, in the initial stage of China's independent development of fuel cell vehicles, it is more reasonable to choose the on-board pure hydrogen solution. First of all, on-board hydrogen production requires a complex high-temperature fuel processor, which is less mature than the on-board pure hydrogen scheme. Secondly, studies have shown that the on-board pure hydrogen scheme is better than the on-board hydrogen production scheme in terms of vehicle energy efficiency, expected total cost (including infrastructure, fuel and vehicles), reduction of pollution and greenhouse gas emissions, reduction of oil dependence and sustainable development

1. l. 2 on board pure hydrogen

the fuel chain of on-board pure hydrogen scheme includes centralized hydrogen preparation, separation and purification, storage, transportation and distribution. In the design of fuel cell vehicle, the storage technology of on-board pure hydrogen is mainly considered. The on-board pure hydrogen storage methods mainly include high-pressure hydrogen storage, liquid hydrogen storage, metal hydrogen storage, activated carbon adsorption hydrogen storage and carbon nano material hydrogen storage

(1) high pressure oxygen storage

it is the simplest and most commonly used on-board pure hydrogen storage method to compress and fill hydrogen into the vehicle carrying pressure vessel with an oxygen compressor. Among the existing fuel cell bus demonstration projects in the world, this on-board hydrogen storage method accounts for the majority. High pressure resistant hydrogen storage pressure vessels and materials are the key to this method

the main problems of high-pressure hydrogen storage methods are: ① small capacity. Pressure vessels made of ordinary steel are widely used in China. When the hydrogen storage pressure is 15MPa, the weight of hydrogen only accounts for 1% of the total weight, and the volume capacity is about 0.008kgh2/l. However, when vessels made of special high-strength austenitic steel materials are used, the hydrogen storage weight can reach 2-6% of the total weight. ② Poor safety. The high-pressure vessel itself needs special care and maintenance. Moreover, the vessel contains flammable, explosive and leaky hydrogen. A car accident may have serious consequences. ③ Implementation issues. The higher the pressure of the vessel, the higher the cost of the construction and compression operation of the hydrogen charging station. Moreover, it takes about 0.5 kwh to fill 1 cubic meter of hydrogen, while only 2 kwh can be obtained from 1 cubic meter of hydrogen generated by fuel cell

(2) liquid hydrogen storage

the necar3 and NECAR4 models developed by Daimler Chrysler and the "hydrogen power 1" fuel cell electric vehicle developed by General Motors all use liquid hydrogen as fuel. In theory, among various hydrogen storage formulas that play a late developing advantage, the highest storage density can be achieved only when hydrogen is stored in liquid state, whether from the perspective of volume density or weight density. At present, the weight ratio of liquid hydrogen storage is about 5% - 7.5%, and the volume capacity is about 0.04kgh2/l. However, due to the heat leakage of cryogenic vessels, the production, storage, transportation and injection of liquid hydrogen, and the large amount of energy consumed by hydrogen liquefaction, it is not feasible to carry liquid hydrogen on a large scale

(3) metal hydride hydrogen storage

this method first makes hydrogen and metal form metal hydride. After heating, the metal hydride decomposes and dehydrogens to obtain hydrogen

compared with pressure vessel hydrogen storage, metal hydride hydrogen storage: ① the oxygen storage capacity per unit weight is not high. After adding the container to the hydrogen storage material, the hydrogen storage capacity per unit weight is lower than that of the pressure vessel made of high-performance materials, and the hydrogen storage weight is less than 1.5% of the total weight. ② The hydrogen storage capacity per unit volume was increased to 0.05kgh2/l. ③ The hydrogen storage pressure is 1-zmpa, which is much lower than the pressure vessel, which improves the safety, and reduces the requirements of hydrogen charging station and hydrogen charging energy consumption. ④ Metal hydrides have higher sensitivity to a small amount of impurities in hydrogen such as O2, H2O, Co, etc., which is higher than the sensitivity of fuel cell electrode catalyst, thus improving the quality requirements of raw hydrogen. ⑤ There are some problems such as the mechanical strength of metal hydride and the crushing after repeated charging. At present, metal hydride can be charged and discharged repeatedly for a few times and is expensive, so the operation cost of using metal hydride as a hydrogen storage method is very high. ③ The container for storing oxides shall be able to withstand high pressure and have sufficient heat exchange area to quickly transfer the heat released or required during oxygen absorption and hydrogen release reactions

(4) hydrogen storage by activated carbon adsorption

low temperature adsorption of activated carbon has quite good hydrogen storage capacity. At -196 ℃, 4.2mpa, the hydrogen storage capacity of activated carbon is about 5% of the total weight. However, considering the low temperature of -196 ℃ and the pressure of 4.2mpa, as well as the weakness of high-pressure vessel method and liquid hydrogen method, it is not a feasible method on the vehicle

(5) carbon nano materials for hydrogen storage

carbon nanotubes are considered as a very potential high-capacity hydrogen storage material. However, the reported high hydrogen storage capacity is controversial. In addition, carbon nanotubes are expensive. At present, the method of large-scale preparation has not been solved. In addition, carbon nanotubes are difficult to dehydrogenate, with low capacity and rate of dehydrogenation, and difficult to apply in practice. Therefore, the development of their technology is difficult to predict, and it is impossible to apply in practice, at least in a short time

different hydrogen storage methods have different requirements for hydrogen purity. Table 1 shows the requirements for hydrogen purity of various hydrogen storage methods. High pressure hydrogen uses pure hydrogen, while other hydrogen storage methods require high-purity hydrogen or ultra pure hydrogen. The purification and separation of hydrogen costs a lot of energy, so the higher the purity of hydrogen, the greater the cost of storage methods

in general, considering the technical difficulty, cost and energy consumption of developing fuel cell vehicles in China at this stage, high-pressure hydrogen storage has certain advantages on fuel cell city buses with low volume requirements for hydrogen storage containers. Therefore, 863 fuel cell city bus adopts the scheme of hydrogen storage with high-pressure gas cylinder

1.2 structural analysis of vehicle power system

there are many kinds of power system structures of fuel cell vehicles. At present, the fuel cell buses studied in various countries mainly include pure fuel cell (PFC), fuel cell and auxiliary battery combined drive (FC + b), fuel cell and super capacitor combined drive (FC + C), fuel cell and auxiliary battery plus super capacitor combined drive (FC + B + C)

pure fuel cell vehicles have only one energy source, and all power loads of vehicles are borne by fuel cells. In this structure, the fuel cell has high rated power and high cost, which has a negative impact on the cold start time, the number of start-up cycles and

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