Coordinated Control of a Wind-Methanol-Fuel Cell System with Hydrogen Storage

: This paper presents a wind-methanol-fuel cell system with hydrogen storage. It can manage various energy flow to provide stable wind power supply, produce constant methanol, and reduce CO 2 emissions. Firstly, this study establishes the theoretical basis and formulation algorithms. And then, computational experiments are developed with MATLAB/Simulink (R2016a, MathWorks, Natick, MA, USA). Real data are used to fit the developed models in the study. From the test results, the developed system can generate maximum electricity whilst maintaining a stable production of methanol with the aid of a hybrid energy storage system (HESS). A sophisticated control scheme is also developed to coordinate these actions to achieve satisfactory system performance.


Introduction
In order to curb global warming, international efforts have been devoted to developing renewable energy sources as well as utilizing conventional energy in a cleaner way.Wind energy, as a renewable source of energy, attracts much attention from research community and industry.As the largest CO 2 emitter, China had installed 150 million kW of wind turbines by the end of 2016 [1].However, wind power is intermittent and its increasing penetration in the power network can create power stability issues.Therefore, much research is focused on the integration of wind energy in the power system for a resilient energy supply [2][3][4][5].On the other hand, China is also the world's largest coal producer.Coal is a primary source of energy and accounts for 90% of China's CO 2 emissions from fossil fuels [6].How to use coal in a cleaner way is always a challenge facing the successive Chinese governments.
Methanol is a clean-burning liquid that requires only minor modifications to existing engines and fuel-delivery infrastructure.Manufacturing it could even make use of carbon dioxide, which is the main cause for global warming.While methanol's benefits have long been understood, recent advances in methanol synthesis and methanol fuel cells could make this fuel even more attractive [7].Conventionally, methanol is a by-product of mining coal through a hydrocarbylation reaction.In this process, 1.526 times of CO 2 could be produced with one unit of methanol.This has led to a significant increase in CO 2 emissions to the environment.In order to tackle this issue, a new representative process has been developed [8] to produce methanol from hydrogen, oxygen and coal which is used in this study.
Energy storage is important to maintain the stable operation of energy systems.Reference [9] presented a new energy storage design to support energy flow in a hybrid wind-hydrogen system.Reference [10] proposed a hybrid energy system consisting of a wind turbine generator, a fuel cell with a water electrolyser and a battery energy storage system.Typically, these energy components are divided into two groups and two PI controllers were used to manage energy flow.By combining wind energy with fuel cells, reference [11] discussed the feasibility of a hybrid system satisfying residential demands when connected to the power grid.Reference [12] proposed an optimal method for sizing a grid-connected wind farm with a hydrogen system.Fuel cell systems with wind energy were also investigated in references [13][14][15][16] in order to find an optimal structure in the hybrid system.New concepts for optimal applications for wind energy were proposed [17,18].These hybrid energy systems were integrated with integrating wind energy and coal-based methanol production, so as to provide an effective method to utilize local energy resources.Energy management and control methods in multi energy sources are critically important also important.Fuzzy logic control is a popular option for hybrid energy systems.Reference [19] presented a particle swarm-based fuzzy logic control for a hybrid wind-fuel cell-battery system to decrease total harmonic distortion of the power sources.A maximum power point tracking (MPPT) algorithm was used in a wind-fuel cell system [20] to meet variable load.Reference [21] adopted a probability distribution function to manage a wind-fuel cell-turbine with energy storage.The test results for different scenario of load or demand uncertainty.These systems are effective but have their limitations.It is the author's review that using systematic approaches to integrate the developed techniques will increase multiple functions in the loop of energy generation, utilization, operation, clean environment and so on.
The research project uses Xinjiang, the large territory province in China, as a case background.It is an important energy base in China, it connects advanced provinces in East China eastward, and links to the Central Asia countries westward.Xinjiang is regarded as a strategic hub on the "Silk Road Economic Belt".It is rich in coal resources and wind energy, which is a perfect match for the project as generated electricity from wind can be used for water electrolysis to produce hydrogen and oxygen.Xinjiang has been one of the five national integrated energy bases [22,23].Therefore, how to effectively utilise coal sources and wind energy in Xinjiang is one of the crucial issues in China's national agenda.In national perspective, China has been a major contributor to global warming and thus is committed to play a significant role to cut down its CO 2 emissions.In June 2014, China proposed a radical measure for managing its energy consumption and for developing renewable energy technologies.China has become the largest renewable producer in 2016, according to British Petroleum Statistical Review of World Energy [24].
This study proposes a hybrid system including a wind turbine, hydrogen/oxygen storage tanks through water electrolysis, a coal-base methanol device and a fuel cell.In this multi-source system, energy is managed into different forms: kinetic energy in a turbine; chemical energy in hydrogen/oxygen and fuel cells; electrical energy in the wind turbine/generators and the fuel cells.The hydrogen production through water electrolysis and the charging/discharging of the fuel cells provide flexibility when the intermittent wind power cannot match the energy requested.Meanwhile, methanol production with the new technology reduces the CO 2 emission for the system [17,25] which also in turn increase the revenue from the carbon trading scheme according to the national policy [26,27].In this system, energy flow is managed by a coordinated control strategy.It determines the electricity generation and methanol production, with a target of maximising utilisation of wind energy.The feasibility and the performance of the strategy are tested through modeling and simulation in the MATLAB/Simulink environment.The novelty and contribution of the study are listed below.
(1) Integration wind energy into the grid has resulted in degradation of the inertia response, which in turn seriously affects the stability of the power system's frequency.This problem can be solved by using an active power reserve to stabilize the frequency.Hydrogen/oxygen fuel cell technology is one of solution as a power reserve for this purpose.(2) Hydrogen/oxygen production from water electrolysis could provide the resource for fuel cell applications.As typical electricity-to-hydrogen conversion devices, electrolysers are regarded as deferrable loads with the ability to operate under a flexible schedule on the demand side.(3) The coal-based methanol production is one of research challenges of clean energy applications of fossil fuels.This study adopts a "greener" process.The oxygen is fed to the gasifier as the gasification agent and then the hydrogen is mixed with the CO-rich gas to adjust H 2 /CO so as to produce methanol.However, it requires local supplies of hydrogen/oxygen continuously.Therefore, this kind of applications are limited.However, specific geometry advantages of Xinjiang province is perfect for this kind of applications.(4) This innovative application integrates wind power, hydrogen from water electrolysis, hydrogen/oxygen fuel cells and a coal-based methanol production by a "greener" process.
The system proposed has advantages of both the renewable sources and sustainable coal industries in China.Methanol receives hydrogen/oxygen to balance the local loads and to increase the overall profits.However, the hybrid construct would increase the system complexity.Therefore, it is necessary to investigate the possible energy management strategy and control mechanism before physical complicated system is build up.
The rest of the main studies are arranged as follows: Section 2 introduces the configuration of the hybrid system; Detailed discussion associated with the control scheme is carried out for energy management of this system; Section 3 provides the simulation outcomes and analysis; Section 4 summarises the main findings and suggests future work.

The Hybrid System and Its Control Scheme
The schematic diagram of the system is shown in Figure 1.The system includes a doubly-fed induction generator (DFIG) of a wind turbine, coal-based methanol production sub-system, a water electrolyser for hydrogen production, hydrogen and oxygen storage tanks, fuel cells, a DC link, DC/AC or AC/DC converters.The coal chemical process comprises gasification, mixing, purification, reaction, separation and rectification.
Energies 2017, 10, 2053 3 of 21 (2) Hydrogen/oxygen production from water electrolysis could provide the resource for fuel cell applications.As typical electricity-to-hydrogen conversion devices, electrolysers are regarded as deferrable loads with the ability to operate under a flexible schedule on the demand side.
(3) The coal-based methanol production is one of research challenges of clean energy applications of fossil fuels.This study adopts a "greener" process.The oxygen is fed to the gasifier as the gasification agent and then the hydrogen is mixed with the CO-rich gas to adjust H2/CO so as to produce methanol.However, it requires local supplies of hydrogen/oxygen continuously.Therefore, this kind of applications are limited.However, specific geometry advantages of Xinjiang province is perfect for this kind of applications.(4) This innovative application integrates wind power, hydrogen from water electrolysis, hydrogen/oxygen fuel cells and a coal-based methanol production by a "greener" process.The system proposed has advantages of both the renewable sources and sustainable coal industries in China.Methanol receives hydrogen/oxygen to balance the local loads and to increase the overall profits.However, the hybrid construct would increase the system complexity.Therefore, it is necessary to investigate the possible energy management strategy and control mechanism before physical complicated system is build up.
The rest of the main studies are arranged as follows: Section 2 introduces the configuration of the hybrid system.Detailed discussion associated with the control scheme is carried out for energy management of this system; Section 3 provides the simulation outcomes and analysis; Section 4 summarises the main findings and suggests future work.

The Hybrid System and Its Control Scheme
The schematic diagram of the system is shown in Figure 1.The system includes a doubly-fed induction generator (DFIG) of a wind turbine, coal-based methanol production sub-system, a water electrolyser for hydrogen production, hydrogen and oxygen storage tanks, fuel cells, a DC link, DC/AC or AC/DC converters.The coal chemical process comprises gasification, mixing, purification, reaction, separation and rectification.This system is operated on the following basis: This system is operated on the following basis: i.
Wind power is used to generate electricity for supplying the grid and for producing hydrogen through water electrolysis.ii.Normally, the generated electricity is fed into the power grid.If the wind power is plenty, the additional power is used for water electrolysis and the energy is stored in hydrogen.iii.Hydrogen and oxygen produced are stored in separate tanks and are controlled to be within a range (upper limit 90% and lower limit 20%).iv.Fuel cells absorb hydrogen and generate electricity to supply the grid.v. Methanol is produced constantly (from coal, hydrogen and oxygen) at a set value regardless of wind power input.
Figure 2 shows the operation of the proposed system.
Wind power is used to generate electricity for supplying the grid and for producing hydrogen through water electrolysis.ii.
Normally, the generated electricity is fed into the power grid.If the wind power is plenty, the additional power is used for water electrolysis and the energy is stored in hydrogen.iii.
Hydrogen and oxygen produced are stored in separate tanks and are controlled to be within a range (upper limit 90% and lower limit 20%).iv.
Fuel cells absorb hydrogen and generate electricity to supply the grid.v.
Methanol is produced constantly (from coal, hydrogen and oxygen) at a set value regardless of wind power input.
Figure 2 shows the operation of the proposed system.

State Index of the Hybrid Energy Storage System (HESS)
In this paper, the HESS consists of a water electrolyser, a hydrogen and an oxygen tank, and fuel cells.The energy flow in the HESS is bi-direction.However, methanol production is single direction and cannot be fed back.Therefore, coal-based methanol production is not involved in the storage system in this study.In the HESS, the equivalent state of charge (ESOC) is evaluated by the state of the hydrogen and oxygen tanks: where p vre and p cap are the current pressure and full pressure of the tanks, respectively.Based on the average weighting method, the ESOC is calculated as follows: where ESOC H is the ESOC level of the hydrogen storage tank, p Hre is the pressure of the hydrogen tank, p Hcap is the full pressure of the hydrogen tank, ESOC O is the ESOC level of the oxygen tank, p Ore is the pressure of the oxygen tank, p Ocap is the full pressure of the oxygen tank, V Hcap and V Ocap are the volumes of hydrogen and oxygen tanks, respectively.In order to operate the HESS effectively, the indicators of ESOC H , ESOC O , ESOC S should be controlled within reasonable range.Basically, they are divided into two intervals and three situations, as shown in Figure 3.Each interval can be determined by Equation ( 5): where ESOC X_min represents the minimal of ESOC and X represents hydrogen, oxygen and HESS states, respectively.ESOC X_max is the maximal of hydrogen, oxygen and HESS.N H 2 O is the hydrogen flow rate in water electrolyser.N f c is the hydrogen flow rate in the fuel cell.N mhg is the hydrogen flow rate used in coal chemical system when producing methanol.

State Index of the Hybrid Energy Storage System (HESS)
In this paper, the HESS consists of a water electrolyser, a hydrogen and an oxygen tank, and fuel cells.The energy flow in the HESS is bi-direction.However, methanol production is single direction and cannot be fed back.Therefore, coal-based methanol production is not involved in the storage system in this study.In the HESS, the equivalent state of charge (ESOC) is evaluated by the state of the hydrogen and oxygen tanks: where and are the current pressure and full pressure of the tanks, respectively.Based on the average weighting method, the ESOC is calculated as follows: where is the ESOC level of the hydrogen storage tank, is the pressure of the hydrogen tank, is the full pressure of the hydrogen tank, is the ESOC level of the oxygen tank, is the pressure of the oxygen tank, is the full pressure of the oxygen tank, and are the volumes of hydrogen and oxygen tanks, respectively.
In order to operate the HESS effectively, the indicators of , , should be controlled within reasonable range.Basically, they are divided into two intervals and three situations, as shown in Figure 3.Each interval can be determined by Equation ( 5): where ESOCX_min represents the minimal of ESOC and X represents hydrogen, oxygen and HESS states, respectively.ESOCX_max is the maximal of hydrogen, oxygen and HESS.
is the hydrogen flow rate in water electrolyser.
is the hydrogen flow rate in the fuel cell.Nmhg is the hydrogen flow rate used in coal chemical system when producing methanol.When the tank or ESOC level is in the normal range, the coal chemical system operates normally to produce methanol.The ideal operation state of the system is that hydrogen generated by the water electrolysis is equal to the sum of hydrogen consumption in the methanol production and in the fuel cells discharging while the ESOC of the HESS is still within the normal range.
In the control scheme, ESOC S is considered as the priority factor in the operation.And the relationship among the wind power output P wind , the cluster power dispatch demand P jh , local load demand P load is defined as follows: (1) When ESOC S > ESOC S_max , let i = 1 and P (i)

Operation Scheme of the Hybrid System
The system converts energy between electrical, chemical and mechanical.Hydrogen/oxygen generation is an integral part of the energy flow.The processes involve several reactions as follows [28,29]: Water electrolysis: Electrochemical reactions in a fuel cell: For the constant production of methanol at set power P mhg , the hydrogen and oxygen are the same.The flow rate of hydrogen N mhg and power demand P (i) re are given in Table 1.When P H 2 O and N H 2 O are in interval γ 1 , hydrogen from water electrolysis is not sufficient to sustain the required methanol production, as a result, the stored hydrogen in the tank is also consumed and ESOC S slowly reduces.When P H 2 O and N H 2 O are in interval γ 2 , the generated hydrogen is more than the needed for methanol.Extra hydrogen is consumed by fuel cells to generate electricity.In γ 3 or γ 4 , when the hydrogen is much more than needed, extra hydrogen is used for fuel cells and methanol and increasing tank levels.Table 2 gives detailed explanations.
ESOC S is over the upper limit.The wind power is insufficient to satisfy both local demands and cluster power requested by the grid.
The total wind power is fed into the grid.The fuel cells discharge.Water electrolysis suspends, P H2O = 0.
ESOC S is over the upper limit.The wind power is sufficient to satisfy both local and cluster power, the rest of the wind power is equal to 0.
The wind power is completely transferred to the grid.Water electrolysis suspends, P H2O = 0.
ESOC S is over the upper limit.The wind power is sufficient to satisfy both local and cluster power.The rest of the wind power is within Y 1 .
The wind power fed into the grid is set (P load + P jh ), P H2O = P (i) re .

Indicator
Scenario Action ESOC S is over the upper limit.The wind power is sufficient to satisfy both local and cluster power.The rest of the wind power is within The wind power is used for water electrolysis which provide the hydrogen for methanol production.P H2O = P (i) re and the rest wind power is transferred to the grid.
ESOC S is within the normal range.The wind power is sufficient to satisfy the local demands.The rest of the wind power is within The fuel cells suspend.Wind power is used for water electrolysis which provide the hydrogen for methanol production with P H2O = |P Both The fuel cell and water electrolysis process suspend; No wind power is fed to the grid.
ESOC S is below the lower limit.The wind power is within All wind power is used for water electrolysis ESOC S is below the lower limit.The wind power is within Y 4 .
The wind power used for water electrolysis P H2O is equal to 4P mhg .The rest wind power is fed to the grid.

Electrical Power and ESOC Control
Based on the control priority and the operating state of the system, a power distribution strategy is proposed.Its flow chart is shown in Figure 4.The power consumed by water electrolysis is obtained as follows: re ≤ 2 (i−1) P mhg 2 (i−1) P mhg P (i) re ≤ 2 (i−1) P mhg (7) The fed-in wind power to the grid is given by: All hydrogen is generated by water electrolysis and is consumed by either fuel cells or methanol production.Thus, the hydrogen distribution can be expressed by: where n H 2 O_H and n H 2 O_O are the amount of the hydrogen and oxygen produced by electrolysis of water respectively.n ∆H and n ∆O are the amount of the hydrogen and oxygen consumed in the tank respectively.n rH and n rO are the amount of hydrogen and oxygen that fed into the fuel cell respectively.The hydrogen flow rate N H 2 from the electrolysis of water power P H 2 O is obtained: where η e is the electrolysis efficiency of water electrolyser, which is generally between 60% and 80% [30].N e is the number of electrolytic cells in series.F is the Faraday constant.Ie is the operating current of the cell which is given by dividing the consumption power P H   Thus: Assuming the operation process is ideal, the amount of hydrogen and oxygen consumed in the coal chemical system during ∆t can be written as: where M W is the wind turbine output power (MW), and V m is the hydrogen molar volume 22.4 L/mol.From Equation (8), n rO can be derived from n ∆H and n ∆O , and vice versa.Thereby the state of the hydrogen storage system at the end of each cycle can be obtained.Therefore, the hydrogen distribution control is simplified to study the operation state of the fuel cells.The cases for the fuel cells discharging can be summarised as the follows: (1) If P (1) re ≥ 0, reduce ESOC S by consuming more hydrogen and oxygen for electricity generation, Let flag1 = 1.Logical order is to derive n rH , n rO (P f c ) from n ∆H , n ∆O .
At this moment, the amount of hydrogen and oxygen can reach the upper limits of the tank pressure n ∆H , n ∆O : The calculations of the parameters n ∆H , n ∆O are shown in Table 3.
The molar flow rate of the hydrogen and the oxygen in the fuel cell is: i.
The actual stock of the hydrogen is calculated by: ii.The actual stock of the oxygen is calculated by: The partial pressures of hydrogen and oxygen p f c_H , p f c_o are obtained: where N is the number of the fuel cell units, V a and V ca is anode and cathode volumes per unit hydrogen fuel cell respectively, T is the operating temperature of the fuel cell, I f c is the current of the fuel cell, K H 2 and K O 2 are the molar constant of hydrogen (anode) and oxygen (cathode) respectively.
Based on a fuel cell model, the power of the fuel cell P f c (P f c ≥ 0) can be obtained: where U f c is the output voltage of the fuel cell, η f is the power generation efficiency of the fuel cell, P f c_max and P f c_min are the upper and the lower limits of the fuel cell stack, respectively.Based on double electric layer phenomenon, the output voltage of the hydrogen fuel cell is calculated as follows [31][32][33]: where U cell is the output voltage of a single fuel cell, E nernst is the thermodynamic potential, U ohmic is the ohmic polarization overvoltage, and U c is the equivalent voltage.
The thermodynamic potential can be calculated by Equation ( 21): The ohmic polarization overvoltage is obtained by: where Z m is the equivalent membrane impedance; Z c is the impedance which represents the proton through the exchange membrane; ρ M is the resistivity; l is the film thickness; A is the activation area of proton exchange membrane; and ϕ is the proton exchange membrane moisture content.The cell equivalent voltage can be given by: where B is the equation coefficient; J is the active current density of the cell; J max is the maximum current density; C is the equivalent capacitance; and ε 1 , ε 2 , ε 3 , ε 4 , are the empirical parameters of fuel cell.
(2) If (P re < 0), ESOC S level is operated within ideal range but the wind power cannot meet the demand.The hydrogen fuel cell discharges to assist wind power with flag1 = 1.n rH and n rO can be obtained by the P fc and then n ∆H and n ∆O can be derived.
The above equation can be simplified as: where N rH_min is the flow rate of the hydrogen when the fuel cell operates at minimum power output: (3) If (P re ≥ 0 or P re ≥ 0, the hydrogen fuel cell suspends with flag1 = 0.
According to the above control rules, the HESS pressure p Hre is obtained: where p H_0 is the initial pressure of the hydrogen tank; ∆p H_1 is the pressure increment of the hydrogen caused by water electrolysis; ∆p H_2 is the pressure increment of the hydrogen caused by incomplete consumption from the fuel cells.∆p H_3 is the pressure decrement of the hydrogen caused by methanol production.∆p H_4 is the pressure decrement of the hydrogen caused by the fuel cell discharging.k' is the hydrogen recovery rate.
The pressure of the oxygen tank p Ore is calculated by: where p O_0 is the initial pressure of the oxygen tank.

Overall Control Scheme
This study is focused on the optimal operation of the hybrid system.When the cluster power is required by the cluster control center, the power from the hybrid system is sent to the grid after the local demand is satisfied.In order to communicate with the cluster control center, a specific indicator "flag2" is used.
By changing flag2, the operation states and the request of the local hybrid system are recorded.The specific actions are as follows: If P s + P fc < P load + P jh , flag2 = −1, the local system does not satisfy with the grid plan and the local system requests the cluster control center to provide power assistance; If P s + P fc = P load + P jh , flag2 = 0, the local system operates in an ideal state and the established target is realised; If P s + P fc > P load + P jh , flag2 = 1, the local system requests assistance from cluster control center and the more power from the local system is sent to the grid.
In order to prevent the amounts of hydrogen and oxygen from reaching their tank limits, an additional control action is developed, as shown in Figure 5. i.
As shown in Figure 5a,c, when ESOCH or ESOCO is lower than ESOCH_min or ESOCO_min, the hybrid system produces the hydrogen/oxygen in maximum and consumes the hydrogen/oxygen in minimum.∆ta should select the smaller one between ∆ and ∆ : ii.As shown in Figure 5b,d, when ESOCH or ESOCO is over than ESOCH_max or ESOCO_max, the hybrid system produces the hydrogen/oxygen in minimum and consumes hydrogen/oxygen in maximum.∆tb should select the smaller one between ∆ and ∆ : Therefore, the time interval ∆t is controlled to be:

Simulation Results
The simulation of a local hybrid system with 10 MW wind power has been carried out by using MATLAB/Simulink.The key parameters of the system are shown in Table 4. i.
As shown in Figure 5a,c, when ESOC H or ESOC O is lower than ESOC H_min or ESOC O_min , the hybrid system produces the hydrogen/oxygen in maximum and consumes the hydrogen/ oxygen in minimum.∆t a should select the smaller one between ∆t aH and ∆t aO : ii.As shown in Figure 5b,d, when ESOC H or ESOC O is over than ESOC H_max or ESOC O_max , the hybrid system produces the hydrogen/oxygen in minimum and consumes hydrogen/oxygen in maximum.∆t b should select the smaller one between ∆t bH and ∆t bO : Therefore, the time interval ∆t is controlled to be:

Simulation Results
The simulation of a local hybrid system with 10 MW wind power has been carried out by using MATLAB/Simulink.The key parameters of the system are shown in Table 4.  28) and ( 29), ∆t is less than 3.75.Therefore let ∆t = 0.5 s.The input and the output power curves are shown in Figures 6 and 7, respectively.The wind power data are from a real 50 MW wind over a 15-h duration.In order to validate the proposed models, the data are scaled down to fit the 10 MW wind turbine model and the time duration is reduced from 15 min to 1 s.ESOC S exceed the upper limit (0.9) at 11 s as shown in Figure 7, the fuel cell discharges and the ESOC S reduces.After a control period, ESOC S restores normal.From 26 s to 27.5 s and from 31.5 s to s, the ESOC S is in the normal range and the storage state is normal.The fuel cells discharge to assist wind power integration, and ensure the output power of the hybrid system to meet the tied-grid demand.28) and ( 29), ∆t is less than 3.75.Therefore let ∆t = 0.5 s.The input and the output power curves are shown in Figures 6 and 7, respectively.The wind power data are from a real 50 MW wind over a 15-h duration.In order to validate the proposed models, the data are scaled down to fit the 10 MW wind turbine model and the time duration is reduced from 15 min to 1 s.ESOCS exceed the upper limit (0.9) at 11 s as shown in Figure 7, the fuel cell discharges and the ESOCS reduces.After a control period, ESOCS restores normal.From 26 s to 27.5 s and from 31.5 s to 34.5 s, the ESOCS is in the normal range and the storage state is normal.The fuel cells discharge to assist wind power integration, and ensure the output power of the hybrid system to meet the tied-grid demand.Therefore, actual tied-grid power and tied-grid demand curves of the hybrid system are shown as Figure 8.It can be seen from Figure 8 that when actual tied-grid power of the hybrid system does not meet the tied-grid demand, that is Ps + Pfc < Pload + Pjh, flag2 is set −1 and the hybrid system requests the cluster center for assistance.Otherwise, flag2 is set 1, the local hybrid system power is excess.If the system power equates the tied-grid demand, the flag2 is set 0. The control objective is achieved.In the above process, the states of the hydrogen/oxygen tanks and HESS are shown in Figures 8-11   Therefore, actual tied-grid power and tied-grid demand curves of the hybrid system are shown as Figure 8.Therefore, actual tied-grid power and tied-grid demand curves of the hybrid system are shown as Figure 8.It can be seen from Figure 8 that when actual tied-grid power of the hybrid system does not meet the tied-grid demand, that is Ps + Pfc < Pload + Pjh, flag2 is set −1 and the hybrid system requests the cluster center for assistance.Otherwise, flag2 is set 1, the local hybrid system power is excess.If the system power equates the tied-grid demand, the flag2 is set 0. The control objective is achieved.In the above process, the states of the hydrogen/oxygen tanks and HESS are shown in Figures 8-11   It can be seen from Figure 8 that when actual tied-grid power of the hybrid system does not meet the tied-grid demand, that is P s + P fc < P load + P jh , flag2 is set −1 and the hybrid system requests the cluster center for assistance.Otherwise, flag2 is set 1, the local hybrid system power is excess.If the system power equates the tied-grid demand, the flag2 is set 0. The control objective is achieved.In the above process, the states of the hydrogen/oxygen tanks and HESS are shown in Figures 8-11  Therefore, actual tied-grid power and tied-grid demand curves of the hybrid system are shown as Figure 8.It can be seen from Figure 8 that when actual tied-grid power of the hybrid system does not meet the tied-grid demand, that is Ps + Pfc < Pload + Pjh, flag2 is set −1 and the hybrid system requests the cluster center for assistance.Otherwise, flag2 is set 1, the local hybrid system power is excess.If the system power equates the tied-grid demand, the flag2 is set 0. The control objective is achieved.In the above process, the states of the hydrogen/oxygen tanks and HESS are shown in Figures 8-11     It can be seen from Figure 7 that drops rapidly since 26 s.From 26 s to 36 s, is less than Pmhg and even reduces to zero at 31 s.From 26 s to 27.5 s and from 31 s to 34.5 s, the fuel cells discharge, and the ESOCH, ESOCO and ESOCS decrease rapidly as can be seen in Figure 11.= 0 and the hydrogen/oxygen for the coal chemical process are provided by the tanks.ESOCS decreases slowly.From 50 s onwards, rises slowly.However, it does not reach 2Pmhg until 60 s.Therefore, ESOCS is seen decreasing in this period.
In Figure 13, when the hydrogen for the methanol production is zero, ESOCS decreases 0.0004 within 15   The simulations when ESOCS below the low limit is carried out based on this model.The hydrogen tank and the ESOC of the HESS are shown in Figures 14 and 15.In Figures 14 and 15, the ESOCS reach the lower limit and the control priority changes.The wind power is used for the water electrolysis at 1 s.The ESOCS recovers to the normal range at 3 s.After that, the power for the water electrolysis increases and maintains 2Pmhg until 5.5 s.The ESOCH, ESOCO and ESOCS increase slowly.The simulations when ESOCS below the low limit is carried out based on this model.The hydrogen tank and the ESOC of the HESS are shown in Figures 14 and 15.In Figures 14 and 15, the ESOCS reach the lower limit and the control priority changes.The wind power is used for the water electrolysis at 1 s.The ESOCS recovers to the normal range at 3 s.After that, the power for the water electrolysis increases and maintains 2Pmhg until 5.5 s.The ESOCH, ESOCO and ESOCS increase slowly.The simulations when ESOC S below the low limit is carried out based on this model.The hydrogen tank and the ESOC of the HESS are shown in Figures 14 and 15.In Figures 14 and 15, the ESOC S reach the lower limit and the control priority changes.The wind power is used for the water electrolysis at 1 s.The ESOC S recovers to the normal range at 3 s.After that, the power for the water electrolysis increases and maintains 2P mhg until 5.5 s.The ESOC H , ESOC O and ESOC S increase slowly.The simulations when ESOCS below the low limit is carried out based on this model.The hydrogen tank and the ESOC of the HESS are shown in Figures 14 and 15.In Figures 14 and 15, the ESOCS reach the lower limit and the control priority changes.The wind power is used for the water electrolysis at 1 s.The ESOCS recovers to the normal range at 3 s.After that, the power for the water electrolysis increases and maintains 2Pmhg until 5.5 s.The ESOCH, ESOCO and ESOCS increase slowly.

Conclusions
This paper has presented a new hybrid system involving wind turbines, hydrogen energy storage, water electrolyser, and fuel cells.The novelty lies in the coordinated control scheme to maximize the utilisation of wind power with a constant methanol output whilst still reducing CO2 emissions.All the symbols used in the paper can refer to Table A1 in Appendix A. The main contributions of the study can be concluded as follows: (1) This work takes advantage of wind power for electricity generation and for energy storage.In addition, coal-based energy systems for methanol production in a cleaner manner.This is critically important for China as well as many coal-dependent economies.(2) It integrates some interdisciplinary techniques into a multi-functional dynamic system to effectively manage various energy sources to enhance stable power supply stability, increase energy efficiency in utilization, and reduce CO2 emissions.(3) It proposes a concept prototype and then implements the structured system in a simulation environment, which significantly reduces the real test cost.(4) The simulation results have confirmed the technical feasibility of the proposed system.It paves the way for next stage progression in small scale real tests and future commercialization of the technology.(5) Because of the optimal design and control of the hybrid system, energy efficiency and cost efficiency will be improved.This has a significant economic implications.Moreover, the reduction in CO2 emissions will have an additional benefit from Carbon Trading Scheme in China.
As this is just a theoretical study in pioneering stage in China, there are several pertinent aspects need to be fully investigated before real system implementation and applications.These are: (1) Development of a full-scale simulation with hardware-in-loop real-time simulator for feasibility investigation.(2) Development of a full scale demonstration experimental setup of the system.(3) Conduction economic analysis, costs and gains, energy trading and carbon trading.(4) Carrying out a study on other social, environmental, legal impacts to minimize the uptake of the proposed technology.

Figure 1 .
Figure 1.Schematic diagram of the proposed system.

Figure 1 .
Figure 1.Schematic diagram of the proposed system.

Figure 2 .
Figure 2. The proposed scheme with coordinated control.

Figure 2 .
Figure 2. The proposed scheme with coordinated control.

Figure 3 .
Figure 3.The capacity range of the HESS.Figure 3. The capacity range of the HESS.

Figure 3 .
Figure 3.The capacity range of the HESS.Figure 3. The capacity range of the HESS.
ESOC S is within the normal range.The wind power is insufficient to satisfy the local demands.The total wind power is fed into the grid.The fuel cells discharge with P fc = |P (i) re |.Water electrolysis process suspends, P H2O = 0.
P H2O = 2P mhg .The rest wind power is fed to the grid.i= 3 P (i) re = P wind P (i) re = 0ESOC S is below the lower limit.No wind power provided.

Figure 4 .
Figure 4. Control diagram of electric energy allocation.Figure 4. Control diagram of electric energy allocation.

Figure 4 .
Figure 4. Control diagram of electric energy allocation.Figure 4. Control diagram of electric energy allocation.

Figure 5 .
Figure 5.The extreme cases of hydrogen energy storage system operation.

Figure 5 .
Figure 5.The extreme cases of hydrogen energy storage system operation.

Figure 6 .
Figure 6.Output power, load and scheduled power curves.Figure 6.Output power, load and scheduled power curves.

Figure 6 .
Figure 6.Output power, load and scheduled power curves.Figure 6.Output power, load and scheduled power curves.

Figure 7 .
Figure 7.The wind power and the fuel cell power.

Figure 9 .
Figure 9.The ESOC of the hydrogen tank, oxygen tank and HESS.

Figure 7 .
Figure 7.The wind power and the fuel cell power.

Figure 7 .
Figure 7.The wind power and the fuel cell power.

Figure 9 .
Figure 9.The ESOC of the hydrogen tank, oxygen tank and HESS.

Figure 7 .
Figure 7.The wind power and the fuel cell power.

Figure 9 .
Figure 9.The ESOC of the hydrogen tank, oxygen tank and HESS.Figure 9.The ESOC of the hydrogen tank, oxygen tank and HESS.

Figure 9 .
Figure 9.The ESOC of the hydrogen tank, oxygen tank and HESS.Figure 9.The ESOC of the hydrogen tank, oxygen tank and HESS.

Figure 10 .
Figure 10.The ESOC from 0 s to 26 s. From 27.5 s to 31.5 s, the fuel cells suspend and the hydrogen produced by water electrolysis is less than the hydrogen used for coal chemical process.The ESOCH, ESOCO and ESOCS decrease slowly.At 34.5 s, ESOCS < ESOCSa and the fuel cells suspend.The downward trend of the ESOCH, ESOCO and ESOCS has been curbed.It can be seen from Figures 7 and 12 that ESOCS_min < ESOCS < ESOCSa between 35 s and 60 s while the fuel cells suspend.From 35 s to 50 s,

Figure 13 .
Figure 13.The power and the flag bit curve beyond lower limit.

Figure 14 .
Figure 14.The curve of the ESOCs below the low limit.

Figure 12 .
Figure 12.The ESOC from 35 s to 60 s.

Figure 13 .
Figure 13.The power and the flag bit curve beyond lower limit.

Figure 14 .
Figure 14.The curve of the ESOCs below the low limit.

Figure 13 .
Figure 13.The power and the flag bit curve beyond lower limit.

Figure 12 .
Figure 12.The ESOC from 35 s to 60 s.

Figure 13 .
Figure 13.The power and the flag bit curve beyond lower limit.

Figure 14 .
Figure 14.The curve of the ESOCs below the low limit.Figure 14.The curve of the ESOCs below the low limit.

Figure 14 .
Figure 14.The curve of the ESOCs below the low limit.Figure 14.The curve of the ESOCs below the low limit.

Figure 15 .
Figure 15.The curve of the ESOCH and ESOCO below the low limit.

Table 1 .
Interval scheme of power P re with the flow rate N H 2 .

Table 2 .
Operational regulation for different scenarios.

Table 3 .
Parameter selection for the calculation.

Table 4 .
Key parameters of the hybrid system.In the simulation, ESOC Sa is set 0.31.P mhg and N mhg are set 2.48 MW and 5.134 mol/s respectively.According to Equations (

Table 4 .
Key parameters of the hybrid system.In the simulation, ESOCSa is set 0.31.Pmhg and Nmhg are set 2.48 MW and 5.134 mol/s respectively.According to Equations ( It can be seen fromFigures9-10 that the ESOCS is in the normal range at the beginning.equates 2Pmhg from 0 s to 4 s and from 6 s to 11 s.The ESOCH and the ESOCS rise slowly and the ESOCO remains unchanged.From 4 s to 6 s, from 11.5 s to 16 s and from 17.5 s to 23 s, < < 2 .The ESOCH and the ESOCS rise slowly and the ESOCO decreases slowly.At 11 s, ESOCS exceeds the upper limit, and the control priority changes.The fuel cells start, and ESOCH, ESOCO and ESOCS drop rapidly.The ESOCS restores to the normal range.From 23 s to 26 s, < Pmhg.The ESOCH, ESOCO and ESOCS decrease slowly.
s.It is at 1 s that ESOCS decreases from ESOCsa to ESOCS_min under unchanged condition.The ESOC from 26 s to 35 s.The ESOC from 0 s to 26 s.It can be seen from Figures 9 and 10 that the ESOC S is in the normal range at the beginning.P H 2 O equates 2P mhg from 0 s to 4 s and from 6 s to 11 s.The ESOC H and the ESOC S rise slowly and the ESOC O remains unchanged.From 4 s to 6 s, from 11.5 s to 16 s and from 17.5 s to 23 s, P mhg < P H 2 O < 2P mhg .The ESOC H and the ESOC S rise slowly and the ESOC O decreases slowly.At 11 s, ESOC S exceeds the upper limit, and the control priority changes.The fuel cells start, and ESOC H , ESOC O and ESOC S drop rapidly.The ESOC S restores to the normal range.From 23 s to 26 s, P H 2 O < P mhg .The ESOC H , ESOC O and ESOC S decrease slowly.It can be seen from Figure 7 that P H 2 O drops rapidly since 26 s.From 26 s to 36 s, P H 2 O is less than P mhg and even reduces to zero at 31 s.From 26 s to 27.5 s and from 31 s to 34.5 s, the fuel cells discharge, and the ESOC H , ESOC O and ESOC S decrease rapidly as can be seen in Figure 11.From 27.5 s to 31.5 s, the fuel cells suspend and the hydrogen produced by water electrolysis is less than the hydrogen used for coal chemical process.The ESOC H , ESOC O and ESOC S decrease slowly.At 34.5 s, ESOC S < ESOC Sa and the fuel cells suspend.downwardtrend of the ESOC H , ESOC O and ESOC S has been curbed.It can be seen from Figures 7 and 12 that ESOC S_min < ESOC S < ESOC Sa between 35 s and 60 s while the fuel cells suspend.From 35 s to 50 s, P H 2 O = 0 and the hydrogen/oxygen for the coal chemical process are provided by the tanks.ESOC S decreases slowly.From 50 s onwards, P H 2 O rises slowly.However, it does not reach 2P mhg until 60 s.Therefore, ESOC S is seen decreasing in this period.In Figure 13, when the hydrogen for the methanol production is zero, ESOC S decreases 0.0004 within 15 s.It is at 1 s that ESOC S decreases from ESOCs a to ESOC S_min under unchanged condition.The ESOC from 0 s to 26 s.It can be seen from Figures 9-10 that the ESOCS is in the normal range at the beginning.equates2Pmhgfrom0sto4sandfrom 6 s to 11 s.The ESOCH and the ESOCS rise slowly and the ESOCO remains unchanged.From 4 s to 6 s, from 11.5 s to 16 s and from 17.5 s to 23 s, < < 2 .The ESOCH and the ESOCS rise slowly and the ESOCO decreases slowly.At 11 s, ESOCS exceeds the upper limit, and the control priority changes.The fuel cells start, and ESOCH, ESOCO and ESOCS drop rapidly.The ESOCS restores to the normal range.From 23 s to 26 s, < Pmhg.The ESOCH, ESOCO and ESOCS decrease slowly.It can be seen from Figure7that drops rapidly since 26 s.From 26 s to 36 s, is less than Pmhg and even reduces to zero at 31 s.From 26 s to 27.5 s and from 31 s to 34.5 s, the fuel cells discharge, and the ESOCH, ESOCO and ESOCS decrease rapidly as can be seen in Figure11.From 27.5 s to 31.5 s, the fuel cells suspend and the hydrogen produced by water electrolysis is less than the hydrogen used for coal chemical process.The ESOCH, ESOCO and ESOCS decrease slowly.At 34.5 s, ESOCS < ESOCSa and the fuel cells suspend.The downward trend of the ESOCH, ESOCO and ESOCS has been curbed.It can be seen from Figures 7 and 12 that ESOCS_min < ESOCS < ESOCSa between 35 s and 60 s while the fuel cells suspend.From 35 s to 50 s, = 0 and the hydrogen/oxygen for the coal chemical process are provided by the tanks.ESOCS decreases slowly.From 50 s onwards, rises slowly.However, it does not reach 2Pmhg until 60 s.Therefore, ESOCS is seen decreasing in this period.In Figure13, when the hydrogen for the methanol production is zero, ESOCS decreases 0.0004 within 15 s.It is at 1 s that ESOCS decreases from ESOCsa to ESOCS_min under unchanged condition.The ESOC from 26 s to 35 s.The ESOC from 35 s to 60 s.
H ESOC S ESOC O ESOC Figure 11.H ESOC S ESOC O ESOC Figure 10.H ESOC S ESOC O ESOC Figure 11.The ESOC from 26 s to 35 s.Figure 11. S ESOC Figure 12.
The ESOC from 35 s to 60 s.