1. The role of hydrogen in the power grid
Intermittent renewable energy generation poses significant challenges to grid stability and supply-demand balance, and hydrogen can play an important role in mitigating these impacts. The scheme shown in Figure 1 consists of a hydrogen storage unit that acts as a buffer tank and supplies hydrogen to any end of the gas network. In addition to this, from an electricity perspective, fuel cell plants can use previously stored hydrogen to provide renewable electricity to the grid. In the case of a surplus of renewable energy generation, when it cannot be fed into the grid, electrolyzer plants can produce hydrogen and store it for later use. Similarly, when the grid's electricity demand peaks, the stored hydrogen can be used in fuel cell plants to balance the grid. (A typical scenario where you are connected to the grid, but do not draw power from the grid).
Figure 1: Integrating green hydrogen systems into smart energy systems. Because hydrogen, not just electricity, is incorporated into smart energy systems, the term smart energy systems is used in place of smart grids2. Typical process flow of alkaline hydrogen production plant
Figure 2 below shows the process of a typical industrial alkaline water electrolysis (AWE) plant. The device is composed of an electrolyzer and surrounding supporting components, namely: (1) a power supply system; (2) Horizontal gas-liquid separation vessel; (3) Centrifugal pump; (4) Shell and tube heat exchanger; (5) optional mixing schemes; (6) intermediate storage tank; (7) Hydrogen purification system. 
Figure 2: Using a typical alkaline water electrolyzer as an example, a process flow diagram is shown for an industrial-scale AWE electrolyzer that requires a few hundred volts of DC voltage and a few thousand amperes of DC current with minimal AC ripple to achieve higher electrolyzer efficiency. For larger-scale applications, to reduce voltage, these systems typically consist of transformers and large current rectifiers, combined with filters to convert the supplied AC to DC and improve power quality. The AWE electrolyzer receives a DC current and converts the water into hydrogen molecules at the cathode chamber catalyst plane and oxygen molecules at the anode chamber catalyst surface through a water cracking reaction, as shown in Table 1.
Table 1: The chemical reaction of alkaline electrolyzed water, the electrolyte solution and the resulting gas-liquid two-phase flow leave the electrolyzer and enter the horizontal gas-liquid separation vessel, and the gas is separated from the electrolyte liquid under the action of gravity. Next, the condenser cools the separated hydrogen until it reaches **, in order to remove some of the moisture. The buffer tank then stores the hydrogen in the middle and releases it into the purification system at certain intervals to eliminate the remaining moisture and oxygen impurities in the output hydrogen. As a result, the hydrogen produced is of very high purity. In terms of the anode cycle, the oxygen produced usually goes through the same process as hydrogen, but for the sake of simplifying the study, it is as if it were directly discharged.
At the bottom of the horizontal gas-liquid separation vessel, the separated electrolyte solution is recirculated back to the AWE electrolyzer with the help of a centrifugal pump. Before entering the electrolyzer, the shell-and-tube heat exchanger cools the electrolyte to balance and control the temperature of the electrolyzer so that it is in ideal working condition. In addition, in order to keep the concentration of potassium hydroxide the same between the cathode and anode, it is necessary to mix the electrolyte occasionally before the tank. Concentrations may vary depending on the amount of water in the anode chamber and the consumption in the cathode chamber.
For a complete electrolysis system, the dynamics of the heat flow, energy flow, mass flow, and plant process are modeled as shown in Figure 3. Each component has its own system block. These system blocks are solved using MATLAB object-oriented programming and are connected to each other in MATLAB Simulink. Each MATLAB System-on-Module receives a certain number of data signals as input and generates a certain number of data signals as outputs in time steps. These data signals contain the basic thermodynamic properties of hydrogen, oxygen, and electrolyte flows, and are transferred from one unit operation to the next. Within each time step, the DC current supplied to the stack is the primary transient force for the entire process.
Figure 3: MATLAB Simulink diagram and process model square**3. Alkaline off-grid hydrogen production
The research system shown in Figure 4 below consists of a solar photovoltaic installation, an onshore wind farm, an alkaline water electrolyzer, and a battery energy storage system (BESS). The hydrogen produced is purified and ready to be given to the end user. In the framework of a smart energy system, as shown in Figure 1, hydrogen can be pumped into the gas grid or stored. In the analysis below, it is assumed that the remaining electricity production is redirected to the grid. 
Figure 4: Configuration of an off-grid green hydrogen production unit. Surplus electricity generated from solar PV and wind is considered to be redirected to the grid** while optimizing the system's component capacity and control to minimize hydrogen production costs. The plant simulation is based on real-world solar PV and wind production data collected from installations in southeastern Finland in 2021. By replicating the annual data, the total time of the simulation is 30 years, and the time step resolution is 5 minutes. **Includes degradation of electrolyzers, photovoltaic cells, and lithium-ion batteries.
The column chart in Figure 5a shows the optimal capacity for each component. The installation of each component is estimated based on 2040. Modules from earlier years will result in neither solar PV nor batteries in the optimal system configuration, so use 2040** to demonstrate how the system will perform with all modules installed. In an optimized system, the optimal solar PV peak power is 125 for a fixed 100MW nominal alkaline water electrolyzer4MW with a nominal wind power of 1217MW with a BESS energy capacity of 419mwh。Full load hours (FLHS) is a measure of the utilization rate of certain units or equipment, which is defined as the maximum annual energy output or the energy consumed by the electrolyzer divided by its rated power. Currently, the cost of hydrogen production varies widely geographically, and its economic potential depends on changing factors such as fossil fuels and electricity**. Compared to hydrogen produced by steam methane reforming (containing carbon capture), water electrolysis requires 3,000 to 6,000 flhs to be cost-competitive. The histogram in Figure 5b shows that the average FLH of the alkaline electrolyzer is 4304 hours per year under the simulated scenario. In this case, the FLHS of solar PV plants is significantly lower than that of wind plants, mainly due to the low amount of solar PV generation in Finland in winter.
Figure 5: Optimal capacity of components (a) and (b) for average annual full load hours (FLH) over a 30-year simulation period Although power production is intermittent, hydrogen production from electrolyzer units can be optimized under these conditions. Figure 6 shows a 25-hour run time for 30 years of simulation. Batteries are used to maximize and stabilize the power supplied to the electrolyzer and to charge the battery when solar PV and wind energy are unable to sustain the electrolyzer at its minimum load (fixed at 20% of its nominal load).
Figure 6: Excerpt from 25 hours of a 30-year simulation of the plant. The time-step resolution of the electrolyzer is 5 minutes, and the battery utilizes all the available energy in the system to increase yield. Between approximately 2935 and 2936 hours, it can be observed that the electrolyzer is at maximum load while the battery is charged at the same time. Once the battery is fully charged in around 2937 hours, excess power is generated.
It is clear that the hydrogen production of off-grid systems varies greatly over short time intervals, depending entirely on the climatic conditions at the installation site. However, for longer time periods, total hydrogen production can be estimated more accurately. Figure 7 shows the percentage of remaining production and hydrogen production for each simulated year. Hydrogen production gradually decreases in the first 13 years due to the degradation of the electrolyzer, and then the electrolyzer is replaced in the 14th year. Similar behavior can be seen in the 27th year (this cycle depends on the electrolyzer replacement cycle).
Figure 7: The illustration is as follows: blue line (axis): Annual surplus energy divided by the percentage of total solar PV and wind generation (left axis). Red line (axis): Annual hydrogen production (right axis) The attenuation degradation of solar PV cells explains the decrease in surplus production over a 30-year period, as the PV cells were not replaced in the simulation. On the other hand, during the 2nd 11th, 13th 24th and 25th 30th years, the gradual increase in surplus was due to the degradation of the battery during operation, resulting in the attenuation of capacity and power. In the 11th 13th and 23th 25th years, the surplus suddenly decreased due to the replacement of the batteries.
There are certain challenges in the direct combination of green hydrogen production from water electrolysis and solar photovoltaic and wind power generation, mainly due to the intermittent power output of renewable energy. Depending on the initial cell temperature, the ramp-up of the unit may last up to 1 hour until the unit reaches its nominal rated capacity. Hydrogen production can be stabilized if batteries are installed in the system, but hydrogen storage is required to further improve stability and meet the requirements of certain consumers in sectors such as energy and industry.
In a smart energy system consisting of gas and grids, consumers and producers can influence each other for mutual benefit. For example, in the previous simulation, the excess power generation (about 72%) can be transferred to the grid for use by other users in the smart energy system or by external energy storage operators. Similarly, the hydrogen end-user shown in Figure 3 can be a gas & grid or any other hydrogen user in a smart energy system. 
2024 China International Hydrogen Energy and Fuel Cell Industry Exhibition(Abbreviation: 2024 China Hydrogen Energy Exhibition).March 26-28, 2024 (Tuesday to Thursday) Beijing · China International Exhibition Center (Chaoyang Pavilion) Exhibitor Consultation: Mr. GaoHeld concurrently2024 China International Clean Energy Expo (jointly held by the two exhibitions, sharing resources among exhibitors and participating merchants, and promoting the coordinated development of wind, solar, and hydrogen storage).The scale of the exhibition
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