Hydrogen, the new green energy resource

. The desire to reduce pollution and price instability generated by the major share of fossil fuel use requires firm solutions for green transition of economy, that must provide access to clean, safe and affordable energy. An important sector, seriously affected, being a competitive field with consistent limitations, is the transport system, where use of alternative fuels, the hydrogen, is perhaps a solution. Hydrogen can be used as a raw material, fuel, and can be stored with many other possible uses in industry sectors, not only transportation. Most importantly, it does not emit CO 2 and almost does not pollute the air when in use. It therefore provides a solution for decarbonising industrial processes and economic sectors where carbon reduction is both urgent and difficult to achieve. All this makes hydrogen essential to support the EU's commitment to climate neutrality, on the one hand, and to global effort to implement the Paris Agreement, on the other. The present paper presents all aspects, through simulation in MATLAB-Simulink, of the production, storage and use of hydrogen as an alternative fuel source.


Introduction
Transport of the future will look different: ecological, environmentally friendly, with minimal consumption and maximum efficiency.Already the first signs are visible on the European market.Sales of non-polluting cars are increasing, charging stations are multiplying, we have sustainable means of transport in cities.But it is not only the car industry manufacturers who are making efforts to reduce the climate impact of vehicles as much as possible.[1] Non-polluting cars have been in development for a long time, more than 100 years, going through several stages and periods of stagnation, and in the last decade there has been a new wave of enthusiasm, this it comes with rising oil prices, a new approach to climate policy, the development of storage technologies and charging infrastructure.Thus, only through a common conscious choice can we have a healthier way of life and a cleaner environment.
For the period 2030-2050, the headline climate targets require European states to set an intermediate target for 2040 to drive the transition to a climate-neutral economy.This proposal will consider input and evidence from scientific sources.Among them is the major interest of using hydrogen as a solution to climate change.[2] Perhaps the best-known use of hydrogen is in transportation, where automakers have launched a number of hydrogen-powered models that could compete with electric cars and capture a segment of the automotive market in the coming years.Hydrogen cars can use the fuel to generate electricity and then mechanical power.But with the exception of a few prototypes and a limited number of commercial models, figure 1, the mass release of these cars is extremely slow.Another important possible use for this fuel is energy storage.Storing energy has a number of major problems such as cost, mass and scarcity of raw materials used in making batteries.However, the real problem lies in the fact that affordable forms of storage offer possibility of storing energy for a relatively short term.
To make, the energy system, carbon neutral we need to overcome discrepancies between renewable supply and consumption demand.
In other words, popular renewable sources such as solar energy have a much higher usage rate in the summer months while energy consumption is higher in the winter months.Hydrogen solves these shortcomings by providing a tool to use the energy produced (preferably from renewable sources) during the summer to meet energy demand in the winter.
Another important aspect to consider is the cost of transporting hydrogen.Thus, infrastructure needed to transport hydrogen is 10-20 times cheaper than needed for electricity.Moreover, a possible overhaul of the current natural gas distribution system is planned so that hydrogen transportation costs would decrease even more.

The study of hydrogen production method
Hydrogen is a source of energy, but in order to take advantage of the energy it incorporates, it must first be produced.In general, there are two ways to produce hydrogen: electrolysis or from fossil fuels.[3] Electrolysis is the process that uses electricity and water to produce hydrogen.Electricity can be produced from renewable sources (e.g., photovoltaic or wind), in this way we can produce hydrogen in a sustainable way.Hence the name -Green Hydrogen.Hydrogen can also be produced from fossil fuels.Steam methane reforming is the most popular and cost-effective method of producing hydrogen using methane from natural gas as a feedstock.Hydrogen produced this way (with carbon emissions) is often called Gray Hydrogen.If hydrogen is produced from fossil fuels without carbon emissions, it is called Blue Hydrogen.Hydrogen production by catalytic reforming of methane with steam, while using carbon capture and storage techniques, is the most promising process at the moment, but alternative methods, such as methane pyrolysis, are being developed.Blue Hydrogen is not sustainable but has a low carbon content, figure 2.
It is not a single colour, but rather a very wide gradation, because it is not possible to capture 100% of the CO2 produced and not all means of storing it are equally effective in the long term.The main point is that by capturing much of the CO2, the climate impact of hydrogen production can be significantly reduced.[4] Fig. 2. Classification of hydrogen according to production method.
Depending on the production method we can have three types of hydrogen: • Green -Produced by electrolysis, based on water and energy from renewable sources; • Blue -Produced from fossil fuels with no carbon emissions; • Gray -Produced from fossil fuels with carbon emissions.Combination of widespread applicability in different socioeconomic sectors, the possibility of being used for energy storage, the fact that it does not produce harmful emissions (at least green and blue hydrogen) and the ability to reduce costs of energy transition are some of reasons why hydrogen it was dubbed "the missing link of energy transition".This achievement was basis of the EU's decision to formulate a Hydrogen Strategy.

Modelling and simulating process of hydrogen production and use in MATLAB/Simulink
The challenges for modeling this complex and at the same time extremely important process for transition to a green economy without dependence on fossil fuels in transport system are: • benchmarking the existing systems and determining the requirements for the hydrogen system; • selection of components; • dimensioning of the components; • performance analysis and optimization: • how do the selected components work together?• with the above component set, what is the best fuel economy.The solution is rendered only from the simulation-based approach.An application that greatly simplifies the creation of models is MATLAB -Simulink which contains a wide range of pre-built reference applications that can be used as an excellent starting point, and the blocks in the library help to customize the model of the tracked system.[5] Hydrogen is not a free element in nature, therefore it does not have characteristics of a (natural) fuel that can be collected and used directly for energy purposes.
A hydrogen electrolyser is an electrochemical device that uses electricity to split water into hydrogen and oxygen.Hydrogen electrolyzers are used for hydrogen production and are considered as part of a green energy production and storage distribution system when combined with a renewable energy source, a hydrogen reservoir and fuel cell systems, such as be electric vehicles with fuel cells.[6][7][8] The three main types of hydrogen electrolyzers-alkaline, polymer electrolyte membrane (PEM), and solid oxide-focus on the differences between the electrolyte materials.A PEM hydrogen electrolyser breaks down water into a semi-permeable membrane that allows the transport of protons and blocks the flow of electrons.Because of these characteristics, this type of hydrogen electrolyzer is also known as a proton exchange membrane electrolyzer.
As shown below, water enters the electrolyzer at the anode.When direct current (DC) electricity is applied between two electrodes, the negatively charged oxygen in the water molecule gives up electrons, resulting in protons, electrons, and oxygen at the anode.Protons pass through the membrane to the cathode, where protons combine with electrons to produce hydrogen, figure 3. PEM electrolysers are valued for their higher energy efficiency, wider operating temperature and easier maintenance compared to other types of hydrogen electrolysers.To build a hydrogen electrolyser model, one can start with the Simscape Electric Electrolyser Model.This model provides the amount of hydrogen produced and water consumed based on the electricity supplied and the water temperature.The hydrogen electrolyser model is an assembly of individual electrolyser cells connected in series with customizable configurations such as number of cells, water purge logic, transport area, electrode spacing, and number of electrode pairs.Additional options for parameterization include electrolysis efficiency, efficiency temperature, electrical resistivity, and pH value assumptions.[9] Simulating the behavior of the hydrogen electrolyser, figure 4, with different component specifications and under different operating conditions: • design controls for thermal, pressure and water management; • monitoring and managing key operational metrics such as electrolyzer assembly temperature, voltage, electricity consumption, water consumption and hydrogen production level.The simulation results of presented model are shown in figure 5. Figure 6, to the left, shows the electrical power consumed by electrolyzer.Electrical power is greater than the power required to produce hydrogen due to various losses, difference being the dissipated heat.Also, thermal efficiency of electrolyzer, which indicates the fraction of electrical power used to generate hydrogen based on thermal power of hydrogen, due to an efficiency of about 87% at a current density of 2 A/cm 2 .
Rate of hydrogen produced, the amount of water consumed at anode, as well as the water transported to cathode due to diffusion, electro-osmosis resistance and hydraulic pressure difference are shown in figure 6, in the right, as well as the need for a dehumidification stage to produce hydrogen at the desired purity.
Also in this figure, it can be seen that the total mass of hydrogen produced and the equivalent energy based on its higher thermal power are directly proportional.This fact gives an indication of the amount of energy available if hydrogen is used to generate energy in a fuel cell.Fig. 6.Variation of consumed power, dissipated heat and thermal efficiency of the system, to the left, and in the right, the amount of hydrogen produced, of water needed and water transported, but also correlation between the total mass of hydrogen produced and equivalent energy if H2 is used in the fuel cell.Another approach, for example, shows a DC islanded microgrid that provides power to an electrolyzer using a solar array and an energy storage system.Is can being use this model to evaluate the operational characteristics of producing green hydrogen over a 7-day period, by power from a solar array, or from a combination of a solar array and an energy storage system.The model includes electrical, thermal liquid, and thermal gas domains, figure 7. The simulation result shows, in figure 8, the amount of green energy needed to obtain hydrogen, the amount of hydrogen produced and variation in percentage of stored energy for a period of 180 hours.A last example to simulation model analysed in this paper is a hydrogen refuelling station, the model for simulation is shown in the figure 9.For simulation, hydrogen is stored in low-pressure storage tanks at 200 bar at the station.A 3-stage intercooled compressor maintains the necessary pressure in a cascade buffer storage system so that the station is ready to dispatch hydrogen to any connected vehicles.The buffer is divided into high-pressure tanks at 950 bar, medium-pressure tanks at 650 bar, and low-pressure tanks at 450 bar.To avoid wasting compression energy, the lowest pressure buffer that is greater than the vehicle tank pressure is used to dispatch hydrogen.Priority valves switches between the different buffer tanks to control which buffer tanks to fill and discharge from.
When dispensing, a reduction valve controls the flow between the cascade buffer storage and the vehicle tank.A precooler chills the hydrogen before it is dispatched to the vehicle to avoid excessive temperature rise in the vehicle tank.This example is modelled after an existing refuelling station type from fuelling protocols, which means that hydrogen is delivered at -40ºC and up to 700 bar.A Stateflow® chart is used to model the logic needed to control the operations of the station.It is determining when to turn on the compressor to recharge the buffer, when to dispatch hydrogen to the vehicle, and the switching logic of the cascade buffer priority valves.When the dispensing, the Valve Controller and the Precooler Controller subsystems are enabled.They contain PI controllers that maintain the flow rate and temperature of the hydrogen fuel being dispatched.
State of hydrogen fuel, in the vehicle tank is shown in left column of the scope plots, figure 10.At t = 2400 s, the dispenser is triggered and filling starts.The increase in pressure is maintained at a constant slope, called the Average Pressure Ramp Rate (APRR), by the Valve Controller.Filling stops once the target pressure is reached.The APRR and target pressure come from the fuelling protocols and depend on environment temperature and initial pressure in vehicle.In this example, they are 18.5 MPa/min and 74.5 MPa, respectively.While filling, temperature increases significantly due to the amount of compression that occurs in vehicle tank.That's how it is sets a maximum temperature limit of 85ºC.In this example, the temperature reaches 68.6ºC when full.This is achieved by precooling the hydrogen to -40ºC before it enters the vehicle, as shown in the bottom right plot.
The bottom left plots show the flow rate of hydrogen into the vehicle tank.Because the controller maintains an APRR, the mass flow rate decreases as pressure increases.The two spikes in the flow rate occur when it switches from the low-pressure buffer to the mediumpressure buffer and from the medium-pressure buffer to the high-pressure buffer.Further adjustments of the valve timing while switching buffers may reduce these spikes.Nevertheless, the flow rate is below the maximum limit of 0.06 kg/s set by the standards in force.
The top two plots on the right column shows the state of the cascade buffer storage.At the start of simulation, compressor is turned on to fill the buffer in order from the highpressure tank to the low-pressure tank.Dispatching hydrogen to vehicle causes a drop in the buffer pressure.Therefore, the compressor turns on again to refill the buffer tanks and maintain buffer pressure.
The graph in figure 11 shows a closer view of the state of the hydrogen in the vehicle's tank while it is being filled.It also shows the state of charge (SOC) and total mass of hydrogen in the vehicle.SOC is defined as the hydrogen density in the vehicle divided by the hydrogen density at nominal working pressure (NWP) and 15 °C.SOC should never exceed 100%.The graph shows the pressures and flows in the cascade storage buffer tanks.At the beginning of the simulation, the high-pressure tank, medium-pressure tank, and low-pressure tank are filled at a rate of about 0.034 kg/s.As you switch from one buffer tank to another, some hydrogen from the higher-pressure tank leaks into the lower pressure tank, resulting in an increase in flow rate.This can be reduced by adjusting the valve switching time.At t = 2400 s, hydrogen is delivered to the vehicle at a rate varying from 0.035 kg/s to 0.018 kg/s.The hydrogen initially comes from the low-pressure buffer tank.As the vehicle's tank pressure increases, it moves to the medium pressure buffer tank and then finally to the highpressure buffer tank.As the vehicle's tank is filled, the buffer pressure drops, so it must be refilled for the next vehicle.A larger buffer will allow more vehicles to be filled before the buffer needs to be refilled.

Conclusions
In recent years, the growing concern about climate change and the actions that need to be taken to preserve the planet we live on have begun to occupy the top places on the agenda of mankind.Among these, finding a replacement for fossil fuels, "classic" that reduces greenhouse gas emissions and controls pollution, is among the first problems to be solved.As you could see above, there is a variant that can help us and be of real use in the future: "green hydrogen".
Thus, this work presents the simulation of different models to highlight the essential parameters in the design, optimization and, last but not least, exploitation of hydrogen production, storage and use systems.Knowing that all these processes can generate critical situations, the minimum ignition energy being approximately 17 µJ, the analysis of existing pressures, temperatures and other parameters are of major importance for the evaluation of explosion risks in the context of safety and health and environmental protection surrounding.
Another conclusion resulting from looking at the whole "ideology" of green hydrogen is based on the need to ensure at any cost this clean energy resource for the highly industrialized EU countries.
In the above case, due to the high production costs, it could be interpreted that, according to the projected size of the export capacity to the West, for the excess hydrogen produced in Romania, our country will thus have a marginal role, since its role will only be that of a "resource provider (green hydrogen)" and by no means a large consumer.The production-export route provides exactly the same routes as natural gas today.

Fig. 4 .
Fig. 4. A simulation model that shows how a membrane electrode assembly (MEA) is connected to a thermal liquid network and two separate moist air networks to create a PEM electrolyzer system.

Fig. 5 .
Fig. 5. Rendering of the simulation result and variation of essential parameters such as voltage, power, stack temperature correlated with the amount of hydrogen produced.

Fig. 7 .
Fig. 7.A Simulink model that shows how an electrolyzer can be integrated into a DC microgrid green hydrogen production system.

Fig. 8 .
Fig. 8. Operational characteristics of a green hydrogen production microgrid over a 180-hour period.

Fig. 10 .
Fig. 10.Variation of parameters obtained after the simulation.

Fig. 11 .
Fig. 11.The variation of the state of hydrogen in the vehicle tank while it is being filled and the state of charge (SOC) / total mass of hydrogen in the vehicle and pressures and mass flow rate in cascaded storage buffer tanks.