Study on combined optimization technology of wind-solar-storage hybrid power generation based on electricity hydrogen complementarity

The wind-solar-storage hybrid power generation system is mainly composed of wind turbines, solar cell arrays, electrolytic cells, fuel cells, battery packs, intelligent controllers, multi-functional inverters, cables, supports and auxiliary parts to supply power to the load. The system utilizes wind and solar energy for power generation without an external power supply, and an energy storage module composed of an electrolytic cell, fuel cell, and battery is used to assist the power supply. In this paper, a fuel cell model, a wind power generation model and a solar power generation model are respectively constructed, and a small experimental platform was built to validate the model. The pulse width modulation (PWM) control method, combined with the auxiliary regulation of the system by the battery, reduces the impact of the volatility of wind power generation on the electrolytic cell and fuel cell, and makes the load current more stable under different working conditions. The simulation results show that the system has good compression resistance, fast response and good stability.


Introduction
Energy is a major strategic issue that restricts China's economic environment.At present, the main energy used for power generation is still coal, but coal is a non-renewable resource.With the growing demand for energy supply, we urgently need some renewable energy to solve the need.Wind energy and solar energy widely exist in nature, are rich in resources, and are non-polluting green energy.Naturally, they have become the most valuable energy source for development and research.They have the characteristics of day and night complementation and seasonal complementarity, the system is stable and reliable, and the cost performance is high.The seasonal power generation characteristics of solar energy and wind energy determine that they can be combined to form a complementary power generation system, so that a relatively stable power generation can be obtained during four seasons and resources can be maximally and reasonably utilized [1].We can generate and store energy at the same time.The generated electricity can be directly stored in a large battery or fuel cell for later electricity demand.
The most critical part of the wind-solar hybrid system is the establishment of the model, because the accuracy and rationality of the model can directly affect the feasibility of the system, and the power generation efficiency and power will also be affected.In addition to using mathematical modeling methods, there are also many scholars and workers trying to optimize equipment through the analysis of system characteristics.The wind-solar-storage hybrid system is often divided into two states: grid-connected and off-grid.At present, the main research issues in China include considering the cost and maximum power tracking control according to natural light conditions and wind speed conditions, optimizing battery discharge and safety performance simulation.

Wind-solar-storage hybrid power system model
The wind-solar-storage hybrid power generation system constructed in this paper is essentially a wind power generation system and a solar power generation system connected in parallel to the energy storage system.In this paper, a fuel cell model, a wind power generation model and a solar power generation model are constructed respectively.
Referring to the existing literature and improvement, the following fuel cell model, wind power generation model and solar power generation model are obtained [2].Through these models, we can roughly understand the working principles and related features of wind-solar-storage hybrid power generation.

Proton exchange membrane fuel cell model
The fuel cell is a kind of power generation device that converts the chemical energy of fuel directly into electric energy.It is mainly composed of positive and negative poles and electrolytes.The oxidation reaction of the fuel occurs at the anode and the reduction reaction of the oxidizer occurs at the cathode.The electrolyte provides protons for the redox reaction to provide an electronic pathway.Since fuel cells do not go through a heat engine cycle, their efficiency is not limited by the Carnot cycle.Most of the discharges from the fuel cell are water, which is environmentally friendly.
Ideally, the output of the proton exchange membrane fuel cell should be thermodynamic electric potential (ENerst), but in practice, there are irreversible losses in the battery work, which is known as the activation polarization voltage (Vact), ohmic polarization overvoltage (Vohm), concentration polarization overvoltage (Vcon), as in Equation (1).
where T is temperature, Ci is the mass fraction of component i, I is the current, ρm, l and A are the resistivity, thickness and effective area of the proton exchange membrane, respectively, RC is the electronic impedance, and J and Jmax are the actual and maximum current density, respectively.

Wind power generation model
Wind power generation converts the kinetic energy of the wind into mechanical rotational kinetic energy and then converts the mechanical rotational kinetic energy into electrical energy.The whole process of wind power generation is shown in Figure 1.The wind energy is transmitted from the wind to the blades, the rotation of the blades generates mechanical energy to drive the transmission system, and the mechanical energy of the transmission system brings the wind turbine to finally generate electricity.
In order to build the model, the necessary assumptions must be made: First, the rotor must be regarded as an ideal rotor with countless blades; the wind blowing must be perpendicular to the rotor surface and be continuous and constant in size.
The wind power generation model is shown in Figure 2. S1 is the cross-sectional area of the front end of the wind rotor, S2 is the cross-sectional area of the rear end of the wind rotor, v1 is the average wind speed at the front end of the wind rotor, v2 is the average wind speed at the back end of the wind rotor, S is the actual area of the wind rotor, and v is the wind speed.The wheel passes the average wind speed.The wind turbine power (P) and the force on the wind wheel (F) are calculated as in Equation ( 2).
( ) ) The terminal voltage and current of wind power generation meet a certain regular curve.The terminal voltage and power of wind power generation also meet a certain regular curve.The curves can be seen in the following chapter.

Solar power generation model
Based on the theory of optoelectronics, photovoltaic panels convert solar light energy into electric energy through the photoelectric effect, thus realizing the utilization of renewable energy.It can be obtained from the relationship between the voltage and the current of the photovoltaic semiconductor, and the basic output characteristics of photovoltaic power generation are as follows [3][4]: where U is the output voltage, I is the output current, C1 and C2 are the correction coefficients, Um is the voltage corresponding to the maximum power, Uoc is the open circuit voltage, Im is the current corresponding to the maximum power, Isc is the short-circuit current.
Performance parameters modified by environmental impact can better meet the actual requirements, and photovoltaic output needs MPPT (Maximum power point following) control so that photovoltaic power generation works stably near the maximum power point.The improved disturbance observation method based on voltage stability is used in this paper.

Experiments of wind-solar-storage hybrid power system
In this paper, the effects of different factors on wind power, photovoltaic power generation and energy storage systems are studied experimentally to verify the accuracy of the mathematical model.

Experiment instruments
The instruments used in the experiment include solar panels, USB data collectors, fuel cells, wind turbines, hydrogen production and storage devices, including fuel cells and batteries, as shown in Figure 3.

Experiment of the wind energy module
The experiment points out the existence of the cut-in wind speed and through many experiments, the cut-in wind speed of this system is measured to be 3 m/s.In the process of increasing the current from 0 to 36 mA under the condition of high wind speed (about 7 m/s), the maximum power of 1430 mW is reached when the current is about 33 mA.At this time, the voltage across the wind turbine is about 720 mV.The overall trend of the obtained curve basically agrees with the voltage-current-power curve proposed by the previous model establishment, which means that the experiment is completely consistent with the designed model.The curves at different wind speeds are shown in Figure 4.In this paper, the variable-step tracking method (that is, to adjust the angular velocity of the wind rotor continuously and gradually smaller) is adopted to make the angular velocity of the wind rotor gradually approach the optimal angular velocity, so as to keep the wind power generation at the maximum output power.

Experiment of the solar power module
In the experiment, the power generation experiment under the light intensity of 54000 lux was selected to draw the voltage-current-power curve.During the process of the current from 0 to 600 mA, when the current reached 560 mA, the voltage dropped to 0 from 2000 mV, and the maximum value of the power 1120 mW dropped sharply to 0. The entire test curve fits perfectly with the voltage-currentpower curve proposed in the model shown in Figure 5.The greater the light intensity, the lower the experimental temperature and the higher the power generation.In this paper, the perturbation observation method (constantly changing the voltage value to find the perfect voltage) is used to make the voltage across the photovoltaic cell gradually approach the perfect voltage, and ultimately maximise the output power.

Experiment of the energy storage module
Whether it is wind power or solar power generation, in order to obtain more stable power, an energy storage module is needed.The specific operation in the experiment is to pass the generated electricity into a small electrolytic cell to electrolyze water to obtain oxygen and hydrogen.The generated hydrogen and oxygen come out from both ends of the hydrogen generator, respectively, and pass into the bottom of two identical containers filled with distilled water for drainage and storage.It can be clearly observed that the gas volume on one side is about twice that of the other side.The side with more gas should be hydrogen and the side with less gas should be oxygen.Then, the obtained hydrogen and oxygen are fed into the fuel cell to store electricity, and the electrical energy is completely converted into the chemical energy of the fuel cell.

Experiment of the wind-solar-storage power generation
In the experiment, the wind power generation device is connected in parallel with the solar power generation device.Since the electricity generated by the wind power generation device and the electric voltage and current emitted by the solar power generation device are not the same kind, they cannot be directly connected in parallel.Hydrogen is produced by electrolyzing water, and electrical energy is converted into chemical energy through fuel.The battery is then converted into electrical energy for use.The obtained current-power curve was found to be close to the solar power generation curve.The maximum power has only increased a little about 1130 mW.

Stability of wind-solar-storage hybrid system
As wind power generation is characterised by high volatility, which is prone to cause an unstable power supply to electrolytic cells, user loads and other power-using equipment, a better control method based on battery regulation is introduced.

Output stabilization method of wind-solar hybrid system
Considering the importance of load current stability, the PWM control method is used to control the load current.The main operation method is to add a sample resistor and connect it in series to the load circuit, and monitor the sample resistor, because they are connected in series, as long as the current through the sample resistor is stable, then the load current is also stable.As for the sample resistance, this paper controls the voltage at both ends of it and adopts the PWM feedback control method.
Through this process, the voltage across the sample resistor can be compared with the target voltage, and the difference between them can be adjusted proportionally (P) and integrally (I) through the PI regulator [5][6].Then the upper slope of the triangle is compared with the target voltage by a limiter and the pulse width is modulated to stabilize the voltage at the end of the sampling resistor [7].The process can be seen in Figure 6.

Wind speed increasing situation
In the face of a sudden increase in wind speed, the load completed the response within less than 0.1 s, and the operation was stable after that.The change curves are shown in Figure 7(a).It can be seen that at 1.5 s, the sudden change in wind speed does have a small effect on the load current.But the regulation is very fast, and the load current returns to normal operation in a very short time.This shows that the load response speed is very fast, reflecting the timeliness of the wind-storage complementary power generation system.Stable operation shows that the wind-storage complementary system has good regulation, the load current changes little to the sudden change of wind speed and the regulation speed is fast, indicating that the system has good regulation and pressure resistance.

Light intensity increasing situation
In the face of the sudden increase in light intensity, the curves are shown in Figure 7(b).Due to the power supply and storage of the battery and PWM control, the battery completes the response within 0.1 s and then keeps stable.In the face of a sudden increase in light intensity, the battery current was slightly disturbed at 1.5 s but quickly returned to a stable state.This means that the load response speed is very fast, reflecting the timeliness of the photovoltaic power generation system.Stable operation shows that the optical-storage complementary system has good regulation, and the load current is small for the sudden change of light intensity.The fast regulation means that the system has good regulation and pressure resistance.

Wind speed and light intensity increasing situation
In the face of the sudden increase in wind speed and light intensity at the same time, the load current completes the response in about 0.05 s and keeps stable after that.The change curves are shown in Figure 7(c).From left to right, the images are solar power, wind power and load current.The sudden change curves of wind speed and light intensity are the same as (a) and (b).When the wind speed and light intensity suddenly increase at 1.5 s, the load current has a slight disturbance, but the stable operation is quickly restored.This shows that the load response speed is very fast, reflecting the timeliness of the wind-solar-storage hybrid power generation system.Stable operation shows that the wind-solar-storage hybrid system has good adjustability.The load current has little change when light intensity and wind speed abruptly change.The rapid adjustment shows that the system has good adjustability and compression resistance.It also shows that the wind-solar-storage hybrid system designed in this paper is completely feasible and stable in theory.

Conclusion
In this paper, the construction and working process of the wind-solar-storage hybrid system is mainly carried out by means of experiments and simulations.A better control method based on battery regulation is introduced to reduce the impact of the volatility of wind power generation on electrolytic cells and user loads.After that, we simulate three working conditions, and due to the stabilizing effect of the PWM method, the current completes the response in about 0.05 s, and then is very stable.When the wind speed and light intensity suddenly increase at 1.5 s, there is a slight disturbance to the load current, but it quickly returns to stable operation, which shows that the system has good compression resistance, fast response and good stability.

Figure 1 .
Figure 1.The whole process of wind power generation.Figure 2. The wind power generation model.

Figure 2 .
Figure 1.The whole process of wind power generation.Figure 2. The wind power generation model.

Figure 3 .
Figure 3.The experiment instruments and the hybrid system.

Figure 4 .
Figure 4. Angular velocity-power curves at different wind speeds.

Figure 5 .
Figure 5. Characteristic curves of solar energy system: (a) and (b) at different light intensities; (c) and (d) at different temperatures.

Figure 6 .
Figure 6.The schematic diagram of PWM method.

Figure 7 .
Figure 7. Curves of power and current at different times in three working conditions. )