Simulation Research of the Energy Storage System based on Unitized Regenerative Fuel Cell

The Unitized Regenerative Fuel Cell (URFC) is an advanced energy storage system that integrates a fuel cell and an electrolysis cell. In this paper, MATLAB/Simulink is used to construct mathematical models of URFC. Through the steady-state simulation, it is found that with the increase in temperature, the output voltage of fuel cell mode increases, and the electrolysis mode voltage decreases. The highest efficiency reaches 41.8% at 80°C. As the pressure increases, the performance of URFC improves but requires good packaging and gas sealing. 1 MPa is the optimal pressure value considering URFC operating efficiency.


Introduction
Compared with traditional thermal power generation technology, photovoltaic power generation has advantages of significance, environmental protection, and economic benefits.It is not only one of the best renewable energy but also a new type of power generation and comprehensive utilization of energy with broad development prospects.With the increasing demand for energy from end users, the traditional energy grid has too much load transportation and peak load regulation pressure, which is difficult to meet the growing demand.Therefore, it is urgent to upgrade the current grid system.
Distributed photovoltaic power generation is a kind of system that uses photovoltaic modules to convert solar energy into electric energy directly.The combination of distributed photovoltaic power generation and energy storage systems can form a microgrid for isolated or multi-users [1].At present, the battery is a commonly used microgrid energy storage equipment, but the charging time is long, and the capacity is limited.Due to the use of lead and other harmful metals in the battery, it will cause serious environmental pollution as well [2].Regenerative Fuel Cell (RFC) is the battery with the largest specific energy so far.Because its energy storage is realized by hydrogen storage, and the energy storage equipment is separated from the battery, its capacity is unlimited.At the same time, it is environmentally friendly and pollution-free.It is a high-quality choice of energy storage components in the microgrid system.RFC converts chemical energy into electrical energy through a fuel cell process, and then converts electrical energy into easily stored fuel by reverse reaction, so as to realize energy storage.There are three main components of RFC: fuel cell which generates electric energy, electrolytic cell which generates fuel and fuel storage.
The unitized RFC (URFC) is a kind of RFC that can realize the functions of fuel cell power generation and water electrolysis charging in the same device.It is also the energy storage battery with the highest specific energy at present (400~1000 W•h/kg) [3,4].URFC also has the advantages of a simpler system structure, longer life, and less capital investment.
Lawrence Livermore National Laboratory of the United States is the first institution to study URFC.In the 1960s, 50 W URFC was successfully developed.In the 1990s, Mitlitsky studied the stability of URFC with low catalyst and observed the degradation of battery performance after 2010 cycles [5].The URFC system modified by Proton Energy System greatly improved the performance of electrolysis and fuel cell mode [6].Hwang studied the effects of three different types of flow field plates on the performance of RFC, which were single channel serpentine flow plate, double channel serpentine flow field plate, and parallel flow field plate [7].
URFCs are different according to the electrolyte.In this paper, the research object is Unitized Regenerative Proton Exchange Membrane Fuel Cell (UR-PEMFC).Simulink in MATLAB is used to build the model of fuel cell and water electrolysis [8], and then simulate the influence of temperature, pressure, and other factors on UR-PEMFC performance.

Mathematical model of UR-PEMFC
Many scholars have conducted research on the simulation of UR-PEMFC.After the concept of combining fuel cells with solar photovoltaic cells and electrolytic cells was first proposed, Marangio et al. studied the dynamic model according to the principle of conservation of mole number [9].Salwan studied the theoretical model of each component used in the solar generation system [10].In this study, the mathematical models of electrolytic cells and fuel cells are established respectively.Since the UR-PEMFC model involves many subjects such as electrochemistry, mass transfer, and so on, this study simplifies the model appropriately and then establishes the steady-state and dynamic mathematical models according to various principles and laws.The working diagram of UR-PEMFC is shown in Figure 1 [11].Since the model of the fuel cell is similar to the electrolyzer, only the model of the water electrolyzer is shown here.

Open circuit voltage
When an electrochemical cell operates under reversible conditions, the cell voltage is the open circuit voltage, which can be calculated by using the Nernst equation, as in Equation (2).
In Equation ( 2), ΔG* is Gibbs free energy change in any temperature at standard pressure; ΔH* and ΔS* are enthalpy change and entropy change of chemical reaction; Tcell is the temperature of fuel cell; pH2, pO2 and pH2O are the reaction pressures of hydrogen, oxygen and water.

Activation overvoltage
The current density at the electrode and electrolyte surface can be calculated using the Butler-Volmer equation.Activation overvoltage is included in the equation and can be reversed.In Equation (3), i0,an and i0,cat are reference exchange current density in the anode and cathode; i0 characterizes the current density at a steady state.But this is an indeterminate value, and its determination is based on temperature and current density [12].

Ohmic overpotential
The ohmic overpotential can be divided into two parts, one is the membrane resistance and the other is the electrode and plate resistance.The ohmic resistance of the membrane, due to the resistance to ion passage, is greater than the conductance.
In Equation (4), Rcell is the ohmic impedance of the battery; σm is membrane conductivity, determined by temperature and water content of the membrane; λ is the water content of the membrane.Because the whole membrane can be considered to be fully wetted in the PEM water electrolysis, the value of λ in this paper is 21.

UR-PEMFC operating parameter analysis and dynamic simulation
Based on the established UR-PEMFC model, this study uses MATLAB/Simulink to carry out steadystate simulations on UR-PEMFC.According to the battery performance obtained from the simulation, the optimal conditions for battery operation were analyzed, which provided a theoretical basis for the experimental design and analysis of UR-PEMFC.

Influence of operating parameters in fuel cell mode
In this study, the steady-state parameters in the fuel cell mode, such as temperature and pressure, are simulated respectively.The influence of temperature and pressure is obtained from the mathematical model in the previous chapter through MATLAB programming.The simulation results are shown in Figure 2 and Figure 3.
Figure 2 is the polarization curve of the battery obtained by increasing the working temperature under normal pressure.It can be seen that the polarization curve of the fuel cell changes sharply when the current density is small, but changes slowly as the current density increases.The battery voltage increases as the battery operating temperature increases, which means that increasing the temperature can help improve performance.This is because when the temperature increases, the diffusion coefficient of the PEM increases, and the water generated by the cathode diffuses to the anode faster, so that the water distribution of the proton exchange membrane is uniform and the membrane resistance decreases.In addition, the increase in temperature helps the water to be discharged in a gaseous state without causing flooding.But the excessive temperature can affect battery life.So the temperature of the battery is usually set around 343 K.  Figure 3 is the polarization curve of the battery when the temperature is 343 K and the pressure changes from 0.1 MPa to 10 MPa.It can be seen from the that increasing the pressure promotes performance.From a theoretical point of view, increasing the gas pressure can increase the reversible electromotive force of the battery, increase the exchange current density, and reduce the activation overvoltage to improve the battery performance.However, it can also be seen that the pressure increasing from 5 MPa to 10 MPa does not help the improvement very much, and may also make the equipment unsafe and increase the cost of pressurization.Therefore, improving performance by increasing the pressure must be within a certain pressure range to ensure the economy.

Influence of operating parameters of electrolytic mode
Similar to the fuel cell model, the effects of basic operating parameters such as temperature and pressure on the performance of the electrolytic cell are studied through programming by applying the electrolytic cell mathematical model in the previous chapter.Because the electrolytic cell is a device that produces gas, the pressure here represents the back pressure of the gas at the outlet.Figure 4 shows the effect of temperature increase on the performance of the electrolytic cell under normal pressure.Under the same current density, the higher the temperature is, the lower the voltage is required by the electrolytic cell, which is consistent with the operating law of the electrolytic cell.However, the high temperature of the PEM electrolytic cell will aggravate the corrosion phenomenon, so the temperature of the electrolytic cell should not be too high, most suitable at 60~80°C.
Figure 5 shows the change of the polarization curve of the electrolytic cell under the pressure of the outlet gas from 0.1 MPa to 10 MPa at 343 K.It can be seen from the figure that the performance of the electrolytic cell can be improved by appropriately increasing the outlet gas pressure, but the excessively high pressure will affect the performance.At high pressure, improving cell performance has no significant effect.Because the hydrogen permeation phenomenon of the proton exchange membrane occurs under high pressure, which greatly weakens the performance of the battery, the pressure of the battery is controlled at about 1 MPa as the optimal operating pressure.
Combining the steady-state performance of the fuel cell and the electrolytic cell, it can be concluded that when the performance of the dual modes increases with the raising temperature, it can be obtained that the theoretical efficiency of URFC is 41.8% at 80°C.

Conclusion
Mathematical models of UR-PEMFC were established and MATLAB/Simulink was used to simulate and analyse.Then, the steady-state analyses were carried out, and appropriate operating conditions were proposed.UR-PEMFC has the highest dual-mode operating efficiency at 80°C, reaching 41.8%.Increasing the pressure has a positive impact on the performance, but requires good packaging and gas sealing, so the pressure should be controlled at about 1 MPa.

Figure 1 .
Figure 1.The working diagram of UR-PEMFC. act

Figure 2 .
Figure 2. The polarization curve of UR-PEMFC at different temperatures.

Figure 3 .
Figure 3.The polarization curve of UR-PEMFC at different pressures.