Hydrogen Production from renewable sources?

Subsequently, Italy, Germany, the United States, Japan and other countries began to invest a lot of research in thermochemical cycle hydrogen production, and there are hundreds of thermochemical hydrogen production cycles. Among them, the more promising thermochemical recycling methods for hydrogen production are mainly based on the four-step thermal-chemical hydrogen production cycle (UT-3 cycle) of calcium-bromo-iron compounds and the three-step thermal-chemical hydrogen production cycle (SI cycle) based on sulfur-iodine compounds proposed by Kameyama and Yoshida et al., University of Tokyo, Japan. The sulfur iodine cycle is the most studied and most promising cycle in all the thermochemical hydrogen production so far.

In addition to meeting the temperature requirements in the process of choosing thermo chemical hydrogen production cycle, the following problems should be paid attention to: 1) the yield of each step reaction must be high, and the reaction steps should be as few as possible; 2) Circulating reagents A and B should be cheap and readily available in the existing industry; 3) The intermediate products of each step of the reaction are easy to deal with and do not produce by-products, and the processing of intermediate products will increase the complexity of the system and lead to an increase in cost; 4) The whole process will not cause any impact on the external environment, such as pollution.

4. Economic analysis of hydrogen production from renewable energy sources

Taking hydrogen production by electrolysis as an example, hydrogen production by electrolysis is the core link of the future renewable energy power generation system. In order to achieve large-scale green hydrogen energy storage and application, the cost of hydrogen production by electrolysis, as an important limiting factor, must be further reduced.

Using the Levelized Cost of Hydrogen (LCOH) formula, it is estimated that the electrolytic efficiency of 1MW alkaline water electrolytic hydrogen production system can reach 54.88kW·h/kg, equivalent to 4.9kW·h of 1m3(standard) hydrogen. The levelized cost (LCOH) of electrolytic hydrogen production is $4.2 /kg, and from the point of view of the price composition, electricity accounts for about 75% of the cost of hydrogen production. At the same scale, due to the use of rare precious metals Ir and Pt as catalysts for PEM electrolytic reactors, the fixed cost and variable operation and maintenance cost are high. Although the electrolytic efficiency of PEM (49.28kW·h/kg) is better than that of alkaline electrolytic water system, the levelized hydrogen cost of PEM electrolytic hydrogen production is about 20% higher than that of alkaline electrolytic water system. It is not yet economical. Combined with the on-grid electricity price of wind power and photovoltaic power stations announced by the National Development and Reform Commission in 2019, assuming that the on-grid electricity price of renewable energy power generation system is 0.40 yuan /(kW· h), it can be roughly calculated that the cost of hydrogen production by alkaline water electrolysis is 2.61 yuan /m3(standard), which is converted into an equivalent unit calorific value price of 0.206 yuan /MJ. Natural gas is calculated according to 4 yuan /m3(standard), and the equivalent unit heat value price of natural gas is 0.1 yuan /MJ. Therefore, if the electrolytic hydrogen is simply priced by heat, the hydrogen energy of the renewable energy power generation system is not economically enough for external sale in terms of hydrogen power generation and urban gas mixed with hydrogen for gas supply.

However, if high-quality hydrogen is sold for industrial production and transportation, the external sales price of industrial hydrogen is between 2 and 3 yuan /m3(standard), so that the electrolytic hydrogen production of renewable energy power generation system has a certain economy. Between 2010 and 2019, the power generation cost of photovoltaic and onshore wind power decreased by 82% and 39% respectively, and there is still a large room for decline in the future, when the cost of electricity is reduced to 0.1 yuan /(kW·h), the cost of electrolytic hydrogen production is equivalent to that of coal hydrogen production, about 12 yuan /kg. If the carbon tax is included, a carbon tax of 100 yuan /t will increase the cost of coal to hydrogen by about 6 yuan /kg.

5. Conclusions and Suggestions

According to the classification of carbon emissions in the hydrogen production process, hydrogen is divided into gray hydrogen, blue hydrogen and green hydrogen. In order to comply with the carbon emission reduction policy, the renewable energy hydrogen production method belonging to green hydrogen will be the mainstream way of hydrogen production in the future. There are a variety of ways to produce hydrogen from renewable energy sources, and the following is a list of several key points to pay attention to in the process of choosing hydrogen from renewable energy sources.

① Carbon emissions. The primary energy input for hydrogen production should all come from renewable energy sources (wind energy, solar energy, etc.), and the process of hydrogen production needs to consider the process carbon emissions, and the total carbon emissions should at least meet the green hydrogen standard (less than 4.9kg CO2e/kg H2).