Power-to-hydrogen-and-methanol model based on collaborative optimization of energy flow and material flow

0 Introduction

Advancing a green and low-carbon energy transition and building a secure,efficient,and clean renewable energy system are some of the key tasks needed to achieve ‘‘carbon neutrality” [1,2].To accomplish their ‘‘dual-carbon”goals, China has proposed that by 2030, non-fossil energy will account for approximately 25 % of primary energy consumption, and CO2 emissions per unit of GDP will drop by more than 65 % compared with 2005 levels.By 2060,non-fossil energy consumption will account for over 80 %, and a comprehensive green and low-carbon cyclical economic system and a clean,low-carbon,secure,and effi-cient energy system will be established [3].At present, the development of renewable energy is rapid.As of the end of 2023, China’s cumulative installed power generation capacity is approximately 2.92 billion kilowatts, of which hydropower, wind power, and solar power collectively account for 50.4%,historically surpassing that of thermal power (47.6 %) [4].

However, although renewable energy sources such as wind and photovoltaics are undergoing leapfrog development, their power output is characterized by randomness and variability,posing challenges for the large-scale development of renewable energy in terms of consumption and transmission.Coal-fired power is China’s main power source at present, playing the role of a ‘‘pillar” and ‘‘ballast”in ensuring a stable power supply.Thus far,bundling wind-photovoltaic-thermal power with ultra-high voltage transmission has become the main development model for renewable energy bases in western and northern China[5-7].However,coal-fired power is characterized by high carbon emission intensity,and thus,the problem of capturing and utilizing emitted carbon dioxide needs to be addressed in the future.

Carbon capture,utilization,and storage(CCUS)is one of the technologies for achieving low carbon emissions, of which carbon capture is the key to reducing the carbon emissions of conventional thermal power units [8-10].The main carbon-capture technologies for thermal power plants include post-combustion capture, pre-combustion decarbonization, and oxygen-enriched combustion [11].Post-combustion capture has a number of problematic aspects,such as high energy consumption and low capture efficiency, whereas pre-combustion decarbonization requires significant modifications to the units, resulting in high costs [12-14].By contrast, oxygen-enriched combustion capture combines the advantages of precombustion and post-combustion capture and is expected to address the aforementioned problems [15,16].Oxygenenriched combustion refers to a combustion technology where the oxidizer used for combustion has a higher oxygen concentration than that found in air.The flue gas produced by oxygen-enriched combustion has a CO2 concentration of over 80 %, which, after compression and purification, can even reach over 95 %, facilitating the recovery and liquefaction of CO2 [17].Literature [18]indicates that oxygen-enriched combustion capture is more economical compared to air combustion capture.However, oxygen-enriched combustion capture generally requires additional air separation-oxygen production systems, increasing the equipment investment costs to up to twice as high as those for post-combustion capture.Moreover, the high energy consumption of air separation-oxygen production equipment also leads to a high proportion of self-used electricity in power plants (up to approximately 15 % in total, 2-3 times that of a typical coalfired power plant), restricting any further improvement to its economic efficiency [19,20].Currently, the cost of oxygen-enriched combustion capture is 300-480 yuan/ton,with the cost of air separation equipment and energy consumption accounting for up to 50 % [21,22].

The co-development of wind and solar energy with hydrogen is another important approach to promoting the accommodation of renewable energy sources.A typical wind-solar-hydrogen system generally consists of three components: renewable energy generation, electric-tohydrogen conversion, and hydrogen storage [23-25].Electrolytic hydrogen production provides a flexible and adjustable load capacity, offering flexibility on time scales ranging from seconds to minutes based on different technological pathways [26-29].Furthermore, hydrogen storage facilities can provide long-term flexibility, potentially addressing the scarcity of long-term flexible resources caused by seasonal fluctuations in renewable energy generation [30].In recent years, numerous ‘‘wind-solar-hydro gen” projects have been implemented in various regions,leading to accelerated development within the green hydrogen industry[31].However,the current cost of green hydrogen remains relatively high, and downstream industries are not well-established.As a result,many completed projects operate at utilization rates that are lower than expected[32].Expanding into downstream chemical industries represents a new trend in the development of windsolar-hydrogen projects.By utilizing produced hydrogen for further synthesis processes such as ammonia or methanol production or refining applications, integrated windsolar-hydrogen-ammonia-methanol projects are gaining attention.Dozens of such projects are currently underway in China [33,34].The synthesis of green hydrogen into methanol enables the transformation of hydrogen energy into liquid fuels or chemical raw materials with broad application prospects.However, this process currently faces challenges related to high costs and limited carbon sources.

In constructing new power systems and building renewable energy systems, we need to ensure safety and reduce CO2 emissions.Therefore, addressing the problems of the large-scale development,accommodation of renewable energy sources,and the transformation of coal-fired power plants into low-carbon-emission, regulating power sources, are necessary.Among the 12 major problems and challenges selected by China Electrical Engineering Society for 2023, three are closely related to this problem[35].Power-to-hydrogen and power-to-methanol technologies are believed to play an important role in realizing carbon neutrality [23,36].For example, literature [37]proposed a power-to-hydrogen system based on deep learning to reduce wind and solar power curtailment.Literature [38] analyzed the fluctuation characteristics of solar power and optimized the capacity of the battery and hydrogen storage tank, which reduced the fluctuation of the hydrogen supply and lowered CO2 emission.Literature [39] proposed a variable load dispatching strategy for a power-and-biomass-to-methanol system.Literature[40] analyzed the relationship between electricity and methanol based on a linear model.However,research that combines the by-product oxygen of water electrolysis,oxygen-enriched combustion carbon capture, and powerto-methanol technologies is still lacking.

Based on the research on wind-solar-hydrogen-metha nol cooperative development and coal-fired power plant oxygen-enriched combustion carbon capture, this article proposes a new power-to-hydrogen-and-methanol model based on the collaborative optimization of energy flow and material flow, and presents a case study.This new model utilizes flexible hydrogen-production-by-electrolysis technology to absorb surplus renewable energy power to produce green hydrogen;the oxygen produced by the electrolysis is then used for oxygen-enriched combustion of coal-fired power plants to reduce the difficulty of carbon capture.Green hydrogen and carbon dioxide emitted by coal-fired power plants are further synthesized into chemical products such as methanol,achieving the organic combination of renewable energy generation, hydrogen production by electrolysis, oxygen-enriched combustion carbon capture, and synthesis of methanol.This new model is expected to solve simultaneously the two major problems of renewable energy accommodation and high carbon emissions from coal-fired power plants.

1 Power-to-hydrogen-and-methanol model based on collaborative optimization of energy flow and material flow

1.1 Schematic design

The overall strategy proposed in this article aims to integrate green electricity, green hydrogen, and green hydrogen-based products with carbon dioxide reduction and utilization,oxygen-enriched combustion through technological integration,and supply-demand alignment.This approach is intended to promote the stable and reliable supply of green electricity, efficient absorption of green electricity and hydrogen, reduction of carbon emissions from existing thermal power plants, and the development of emerging industries based on green hydrogen.The design of the model is illustrated in Fig.1.Green hydrogen produced by water electrolysis using wind and solar power serves as a raw material for methanol synthesis.The byproduct oxygen is directly utilized for oxygen-enriched combustion in thermal power plants, eliminating the need for conventional air separation equipment and reducing energy consumption.Additionally, carbon dioxide generated from thermal power plants acts as a raw material to react with green hydrogen for methanol synthesis, effectively turning waste into a valuable resource.This model ensures the effective accommodation and transmission of renewable energy sources while addressing challenges such as the single-mode development of green hydrogen production models, high carbon emissions from supporting thermal power plants, and elevated costs associated with carbon capture.

From the perspective of the large-scale development of renewable energy sources, the power-to-hydrogen-andmethanol model fully utilizes the load flexibility of largescale hydrogen or methanol production, and is complemented by the storage of electricity,hydrogen,and methanol; hydrogen power generation; and existing thermal power plants.In this way, the model can provide zerocarbon flexible adjustment resources for a renewable energy base over short-term to long-term full time scales,solve the problems of a lack of zero-carbon adjustment resources in the vicinity of the renewable energy base and the need for carbon reduction of the regulating coalfired power plants, improve the stability and controllability of the base’s power generation,and ensure safe and reliable supply of electricity.From the perspective of the carbon-neutral transformation of industries, the electrichydrogen-methanol co-production model promotes the application of green electricity and hydrogen in traditional chemical industries, and helps drive the low-carbon transformation of high-carbon-emission industries such as the petrochemical and coal chemical industries.

1.2 Energy and material flow of power-to-hydrogen-and-methanol model

The optimization of energy flow is aimed primarily at ensuring that wind power and photovoltaic power can utilize the flexibility provided by thermal power,energy storage, and electrolysis equipment to achieve a stable supply of electricity,hydrogen production,and methanol production.Wind power and photovoltaic power are the primary sources of energy input for the system and serve as the main sources of energy for water electrolysis in hydrogen production.Thermal power functions mainly as a regulating power source to ensure the stable transmission of renewable energy electricity while guaranteeing the necessary electrical supply for chemical processes such as methanol synthesis and distillation.Electrolysis serves as an adjustable load and provides an important flexible adjustment resource.It has the potential to reduce the scale of energy storage in renewable energy bases when combined with hydrogen storage and hydrogen power generation equipment,thereby providing adjustable power for renewable energy bases.Once the proportions of wind power,photovoltaic power,thermal power,hydrogen production, and hydrogen storage have been optimized, then achieving hydrogen production using surplus renewable energy and stable transmission of renewable energy while maintaining economic feasibility becomes possible.In green hydrogen, the electrical output from wind power,photovoltaic power, and thermal power is converted into chemical energy.Hydrogen can be used downstream in transportation or metallurgy heating applications or even converted back into electricity via fuel cells, and also serves as a raw material for methanol production.Traditional methanol production is a continuous and stable chemical production process that requires a high power supply stability.The synergy between green electricity,green hydrogen, and methanol production needs to address the problem of coordinating fluctuating renewable energy with the continuous and stable methanol synthesis process [41].In addition to providing carbon elements,coal-fired power plants also serve as an important regulating power source to ensure power supply for chemical production.Energy storage and hydrogen storage serve as an important buffer between renewable energy generation,electrolysis of hydrogen, and the synthesis of methanol.With the progress in the development of flexible chemical engineering technologies, methanol synthesis in the future may be able to apply adjustments within a certain range,thus making it better aligned with fluctuating renewable energy generation.

Fig.1.Schematic diagram of power-to-hydrogen-and-methanol model based on collaborative optimization of energy flow and material flow.

However, the optimization of material flow aims primarily to achieve the circularity and integration of hydrogen, oxygen, and carbon elements along with their respective compounds within the production processes.The primary material inputs for the entire system consist of water and coal utilized in thermal power generation.Hydrogen produced through water electrolysis serves as the hydrogen source for methanol synthesis, whereas the carbon source for methanol synthesis is derived from carbon capture in thermal power generation.Current codevelopments of wind and solar energy with hydrogen often overlook the potential application of oxygen generated as a by-product during water electrolysis.This model utilizes the surplus oxygen from water electrolysis as a raw material for oxygen-enriched combustion in thermal power generation, thereby enhancing existing thermal power generation efficiencies.Furthermore, without the need for additional air separation systems to produce oxygen, carbon capture could be achieved at a reduced cost,thus exhibiting an evident economic advantage.When hydrogen and carbon dioxide react to synthesize methanol, by-product water will be produced, which can also be recycled for further usage.

The main chemical reactions in the entire system are as follows:

Chemical equation (1) illustrates the reaction of electrolysis of water to produce hydrogen and oxygen, chemical equation(2)the main reaction of coal combustion in an oxygen-enriched environment, and chemical equation (3)the main reaction of carbon dioxide hydrogenation to produce methanol.The carbon element in coal in Eqs.(2)and(3)is ultimately converted to the carbon element in methanol, whereas the hydrogen and oxygen elements in water ultimately end up in the product methanol and the byproduct water.

For the methanol synthesis reaction in Eq.(3),

where MCH3OH is the yield of methanol;MH2 and MCO2 are the masses of hydrogen and carbon dioxide, respectively,required for synthesizing methanol; MrH2, MrCO2, and MrCH3OH are the relative molecular masses of hydrogen,carbon dioxide, and methanol, respectively; and ηH2 and ηCO2 are the conversion rates of hydrogen and carbon dioxide, respectively,in the carbon dioxide hydrogenation synthesis of methanol.

When the entire system achieves zero carbon emissions,the carbon dioxide produced by the coal-fired power generation in oxygen-enriched combustion is fully captured and converted into methanol.The required amount of oxygen can be calculated according to Eq.(2):

If the hydrogen produced by electrolysis is entirely used for methanol production, then according to Eq.(1),

where and represent the masses of the hydrogen and oxygen gases, respectively, produced by the electrolysis of water.

Calculation yields

Therefore,the oxygen produced as a by-product of electrolysis can fully satisfy the oxygen demand for oxygenrich combustion in a zero-carbon hydrogen methanol coproduction system, with excess oxygen still available.The excess oxygen can be sold for utilization in industries such as metallurgy, chemicals, and healthcare, thereby yielding additional economic benefits.

2 Cooperative optimization model for power-to-hydrogenand-methanol model based on 8760-hour-time-series operation simulation

This article proposes a planning model for the cooptimization of energy flow and material flow in the power-to-hydrogen-and-methanol model, based on an 8760-hour-time-series operation simulation.The model aims to optimize the optimal capacity of each device in the entire system by taking the overall system economic efficiency as the goal.It establishes the material conversion relationship between hydrogen, oxygen, and carbon based on chemical reaction Eqs.(1)-(3), and further establishes the conversion relationship between materials and electricity using the heat values of hydrogen and carbon.Electrolyzers, hydrogen fuel cells, and thermal power plants are the devices that realize the conversion of each element,and are also the key points of the model.Based on the characteristics and actual conditions of the various technologies involved, the operating range and ramp rate of each process are set, with energy storage and hydrogen storage equipment serving as a buffer between the process stages.

2.1 Model of renewable energy

This model considers the stochastic nature of wind and solar power output, allowing the system to curtail a portion of renewable energy output when the flexible adjustment capacity is insufficient, while imposing constraints on the minimum utilization rate of wind and solar power.

1.Constraints on wind power output

2.Constraints on photovoltaic power output

In these equations, and represent the power outputs of the wind power and photovoltaic units, respectively, in the time interval t; and denote the proportions of the power outputs of the wind power and photovoltaic units, respectively, in the time interval t;and and indicate the installed capacities of the wind power and photovoltaic power units, respectively.

3.Constraint on renewable energy utilization rate

In this expression, δi represents the annual utilization rate for type i of renewable energy power generation,and κ is a given threshold.

2.2 Model of electrochemical energy storage

The model takes into account the charging and discharging efficiencies of the electrochemical energy storage and simulates the energy loss during the charging and discharging processes.

where and represent the remaining battery energy at time intervals t and t 1, respectively; is the net charging power of the battery, with positive values indicating charging and negative values indicating discharging; and denote the power for charging the battery from the grid and receiving discharge from the battery, respectively; and ηBattery Charge and ηBattery Discharge signify the efficiencies of battery charging and discharging, respectively.

2.3 Model of electrolyzer and hydrogen fuel cell

The electrolyzer and hydrogen fuel cell are important nodes for converting electricity to hydrogen and oxygen.Eq.(18) simulates the hydrogen production rate of the electrolyzer,whereas Eq.(19)calculates the hydrogen production rate based on the water electrolysis equation (1)and the atomic weights of hydrogen and oxygen.Meanwhile, Eq.(20) models the electricity generation rate of the hydrogen fuel cell.

where ,ηHydrogen,and denote the hydrogen production rate, electrolysis power, electrolysis water efficiency, and oxygen production rate of the electrolyzer,respectively;LHVHydrogen is the lower heating value of hydrogen; and ηFuelcell represent the hydrogen consumption rate and electricity generation efficiency of the fuel cell, respectively; and and are the rated powers of the electrolyzer and fuel cell, respectively.

2.4 Model of thermal power plant

As a critical link in the conversion of carbon dioxide and oxygen into carbon dioxide and electricity, coal-fired power plants are modeled in Eq.(24), whereas their minimum ramp rates are considered in Eqs.(25) and (26).Meanwhile,the electricity generation from the combustion of carbon and oxygen is calculated using Eq.(27), which establishes a relationship between electricity generation and oxygen consumption based on the mass ratio coeffi-cient of oxygen and carbon in the combustion process.Substituting Eq.(27) into Eq.(28) yields the calculation for the mass of carbon dioxide captured by the carbon capture and storage(CCS)equipment.Eq.(29)establishes a relationship between the captured volume of carbon dioxide and electricity generation using the mass ratio coefficient of carbon and carbon dioxide in the combustion process.Finally,Eq.(30)shows that the amount of carbon dioxide captured by coal-fired power plants is equal to the amount of carbon dioxide required for the synthesis of methanol, indicating the absence of additional carbon emission.

where ,and denote the output power of coalfired power plants at time intervals t, t 1, and t τ,respectively;is the rated power of the coal-fired power plants; is the maximum change in power output within a unit time interval; indicates the oxygen consumption of the coal-fired power plants;μO - C and μC - CO2 are mass ratio coefficients for oxygen-to-carbon and carbon-to-carbon-dioxide conversion based on chemical equation (2) for carbon combustion, respectively; LHVC is the lower heating value of carbon; ηFG denotes the effi-ciency of coal-fired power generation; represents the carbon capture rate of the CCS equipment at time interval t; and ηCCUS signify the capacity and effi-ciency of the CCS equipment,respectively;and represents the synthesis rate of methanol at time t.

2.5 Model of methanol synthesis

The model considers the minimum output and ramp rate constraints of the methanol synthesis equipment.Eq.(31) establishes the mass proportion relationship between carbon dioxide, hydrogen, and methanol based on Eq.(3) and their respective molecular weights.

where and indicate the rates at which carbon dioxide and hydrogen are consumed to synthesize methanol at time interval t; and represent the rates at which methanol is synthesized at time intervals t and t 1,respectively;ηMethanol is the efficiency of methanol synthesis; and , and are the rated rates, minimum rates, and upper limits of rate changes per unit time interval of the methanol synthesis equipment, respectively.

2.6 Coordinated optimization of power-to-hydrogen-and-methanol model

For the electric system, Eq.(34) simulates the overall electricity balance, where the generation equals the consumption.For the hydrogen system,Eq.(35)simulates the hydrogen balance at the hydrogen storage tank node,where the hydrogen produced by the electrolyzer is supplied to the methanol synthesis, the export of hydrogen products,and consumption by the fuel cell.For the carbon system, Eq.(37) simulates the carbon dioxide supply-anddemand balance at the CO2 storage device node,where the CO2 captured by the power plant through CCUS is supplied to the methanol synthesis.For the oxygen system,Eq.(39)simulates the oxygen supply-and-demand balance at the oxygen storage tank node, where the oxygen generated by the electrolyzer is supplied to the power plant for power generation or exported as oxygen products.Electricity, hydrogen, carbon dioxide, and oxygen are converted between each other through the electrolyzer, fuel cell, and power plant.

1.Electric system balance

where is the power transmission, and is the power used for methanol synthesis and plant operation, respectively.

2.Hydrogen system balance

where and denote the remaining hydrogen volume in the hydrogen storage tank at time intervals t and t 1, respectively; is the production rate of output hydrogen; and is the capacity of the hydrogen storage tank.

3.Carbon system balance

where and denote the remaining amounts of carbon dioxide in the carbon dioxide storage facility at time intervals t and t 1, respectively; and is the capacity of the carbon dioxide storage facility.

4.Oxygen system balance

where and denote the remaining oxygen levels in the oxygen storage tank at time intervals t and t 1, respectively; is the production rate of output oxygen; and is the capacity of the oxygen storage tank.

2.7 Objective function

This article focuses on an integrated system for coproducing hydrogen and methanol.In optimizing the capacity configuration of each component, the objective is to minimize the total cost of the integrated system while considering constraints related to curtailed wind and solar power.Specifically, it includes investment costs and operating costs, which are defined as follows:

where Cinv and Cope represent the annualized investment cost and annual operating cost of the system,respectively;represent the annualized investment costs of the coal-fired power plant,wind power plant, photovoltaic power plant, electrolyzer, hydrogen fuel cell,hydrogen storage,oxygen storage,carbon capture and storage, and methanol production equipment, respectively; represents the annualized investment cost of equipment N (where N represents the aforementioned equipment); d is the discount rate; YN is the service life of the equipment; cosN is the unit investment cost of the equipment; is the capacity of the equipment;denote the unit operating costs of the coal-fired power plant, wind power plant,photovoltaic power plant,electrolyzer, hydrogen fuel cell, hydrogen storage, oxygen storage, carbon capture and storage, and methanol production equipment, respectively; and T is the simulation run time.

3 Case study of power-to-hydrogen-and-methanol model based on collaborative optimization of energy flow and material flow

3.1 Boundary conditions

This research study selects a renewable energy base in the sand and desert area of western China as a case for analysis.Currently, the wind-solar-hydrogen model is becoming a new trend in the large-scale development of renewable energy bases.Combining this model with carbon dioxide capture to produce methanol can effectively turn waste into treasure.Through the optimization of the energy and material flows in the production process and the utilization of by-product oxygen for oxygenenriched oxygen combustion capture, the economic feasibility can be further improved.

Based on the research findings of the Global Energy Interconnection Development and Cooperation Organization on clean energy generation, green hydrogen, and green chemical industries, this study conducts research with the current economic and technical parameters of the studied region as boundary conditions.The specific parameters are detailed in Appendix A.The case study ensures a 10GW capacity,with a minimum utilization rate of 5,000 h of output electricity per year while producing 1 million tons of hydrogen (4.6 million tons of methanol).The ratio of output electricity and production electricity(including hydrogen production and methanol production)is approximately 1:1.

3.2 Analysis of results

The power-to-methanol production process can be divided into three stages: renewable energy generation,electrolysis of water to produce hydrogen, and methanol synthesis.With the advancement of hydrogen electrolysis technologies, the adjustability of electrolyzers has gradually improved.Demonstrative projects have been initiated for the flexible electrolysis of water using renewable energy.This allows the electrolysis stage to achieve good alignment with fluctuating renewable energy generation.However,the downstream process for synthesizing methanol from electricity has traditionally been a continuous and stable chemical production process.The key technical challenge for electrically synthesizing methanol lies in achieving coordination between fluctuating renewable energy sources and continuous stable methanol synthesis.Here, thermal power plants provide important flexibility in reducing the system’s demand for hydrogen storage and energy storage while serving as a carbon source for methanol synthesis.The results of the example analyzed in the case study are listed in Table 1; herein, the benefits of excess oxygen are not considered.The unit costs are as follows: electricity at 0.196 yuan/kWh, hydrogen at 15.3 yuan/kg, and methanol at 3,698 yuan/ton.

The proposed model is then compared with other modes of handling the material and energy sources,as follows.If the oxygen produced by electrolysis is not used for oxygen-enriched combustion capture, and postcombustion capture is adopted, the cost of methanol will increase to 3921 yuan/ton under the same boundary conditions.If the thermal power generation is removed and carbon dioxide is purchased as the carbon source, more hydrogen storage or even hydrogen power generation is needed to regulate the seasonal fluctuations of wind power and photovoltaic power.Under the assumption that the cost of the purchased carbon dioxide is referenced to the cost of concentrated carbon dioxide from the flue gas(200 yuan/t)[42],the cost of methanol will increase to over5000 yuan/ton.By contrast, the power-to-hydrogen-andmethanol model based on the collaborative optimization of energy flow and material flow fully utilizes the flexibility resources of each link and greatly reduces the cost of carbon capture through oxygen-enriched combustion, thus exhibiting significant economic advantages.

Table 1 Results of case study.

Case titlePower-to-hydrogen-and-methanol model based on collaborative optimization of energy flow and material flow Installed wind power capacity (MW)33,220 Wind power utilization rate91 %Installed photovoltaic capacity (MW)17,000 Photovoltaic utilization rate91 %Installed thermal power capacity (MW)6520 Thermal power generation (GWh)8900 Installed energy storage capacity (MW)3880 Hydrogen storage capacity (t)5840 Oxygen storage capacity (t)37,000 Ratio of hydrogen storage capacity to annual hydrogen production0.6 %Hydrogen power generation capacity (MW)0 Utilization hours of hydrogen power generation (h)Installed capacity of electrolyzer for hydrogen production (MW)9900 Total investment (billion yuan)225 Cost of transmitted electricity (yuan/kWh)0.196 Cost of hydrogen (yuan/kg)15.3 Cost of methanol (yuan/t)3698

The typical power balance of the case analyzed herein is shown in Fig.2.The hydrogen production load is matched to the output of wind and solar power as closely as possible to reduce the system’s energy storage requirements.During periods of insufficient wind and solar power,coal-fired power generation and a small amount of stored energy are utilized to meet the electricity demand for chemical production and power transmission.

The case study of the power-to-hydrogen-and-methanol model based on the collaborative optimization of energy flow and material flow achieved effective buffering between hydrogen and methanol production, oxygen production,and oxygen-enriched combustion with carbon capture through the use of hydrogen storage systems and oxygen storage systems.The states of charge (SOC) of the hydrogen and oxygen storage systems throughout the year are shown in Fig.3.

The costs of hydrogen and methanol produced by this model are highly correlated with the costs of wind power and photovoltaic power.In the example, and at current technological levels, the production cost of methanol is 3,698 yuan/ton, which exceeds the market price of methanol (approximately 2,500 yuan/ton) and the coal-tomethanol production cost at a coal price of 700 yuan/ton(approximately 2,000 yuan/ton).With the decline of wind and solar power generation costs, potential exists for improved economic efficiency of the product.The future cost parameters for various technologies have been determined based on research findings from the Global Energy Interconnection Development and Cooperation Organization regarding clean energy generation, green hydrogen,green chemicals, etc., as detailed in Appendix A.Based on calculations using future cost parameters for each year up to 2030, when the carbon emissions peak stage is expected to be reached, the reduced production cost of methanol from the model is projected to be 2,835 yuan/-ton.Under the carbon tax policy in the coal chemical industry,the methanol cost will be comparable to the market price of coal-based methanol when the carbon price reaches 130-170 yuan/t.If post-combustion capture without oxygen-enriched combustion is used instead, the carbon price needs to reach 190-240 yuan/ton for the methanol product to be economically viable under the same conditions.After the power-to-hydrogen-andmethanol model based on the collaborative optimization of energy flow and material flow is adopted, the methanol product is expected to have market competitiveness 3-5 years earlier than that expected for conventional power-to-methanol production.By 2050, the cost of methanol is expected to be reduced to 1583 yuan/ton,with significant economic advantages.The curves of the methanol cost resulting from this model and that from the power-to-methanol model using post-combustion capture are shown in Fig.4.

Fig.2.Power balance during typical times.

Fig.3.Yearly SOCs of hydrogen storage and oxygen storage.

Fig.4.Cost curves of methanol for two cases.

To further demonstrate the advantages of the proposed model, we also conducted studies on two comparative cases: the electricity-hydrogen synergy case and the electricity-hydrogen-carbon synergy case.In the electricity-hydrogen-carbon synergy case, the utilization of by-product oxygen and the capture of oxygenenriched combustion is not included, and postcombustion capture is used to provide a carbon source for methanol synthesis.In the electricity-hydrogen synergy case, thermal power and CCS are not included, and methanol is synthesized from green hydrogen and purchased carbon dioxide at a price of 300 yuan/ton.The results, listed in Table A2, show that the power-tohydrogen-and-methanol model based on the collaborative optimization of energy flow and material flow provides the best economic benefits.

3.3 Future outlook

The economic efficiency of the power-to-hydrogen-andmethanol model based on the collaborative optimization of energy flow and material flow is closely related to the costs of wind and solar power generation and to the technology costs associated with hydrogen and methanol production.Wind and solar power generation, serving as the primary energy input, are directly related to the overall costs of hydrogen and methanol production.As technology advances, unit investments in wind and solar power generation equipment and hydrogen production electrolyzers are expected to decrease, leading to an overall reduction in project investment.In addition, the traditional methanol synthesis process is a continuous and stable process, requiring highly flexible resources for the system.With advancements in flexible chemical process technologies, such as intelligent reaction control and the development of catalysts capable of withstanding a wide range of load fluctuations, flexible methanol synthesis within a certain load fluctuation range can be achieved.The application of flexible methanol synthesis technologies can better align the entire production process with wind and solar power generation while further reducing system requirements for energy storage and hydrogen storage.This will enhance the utilization rates of wind power and photovoltaics while improving the economic efficiency of the products.Simultaneously, it will provide strong support for renewable energy development and transmission.

The methanol produced by this model can be further synthesized into methanol fuels and products such as plastics,rubber,and fibers,which can help consume renewable energy and drive the transformation and upgrade of the local industry.In addition to methanol, producing highvalue fine chemical products,such as formic acid,is possible, thereby enhancing the overall economic performance of the project even further.

4 Conclusions

In the context of the rapid development of renewable energy and the accelerated construction of large-scale wind and photovoltaic power bases in the desert and Gobi areas,the challenges of promoting the utilization of renewable energy, configuring regulatory resources, and managing existing thermal power have become increasingly important.This article proposes the concept of a powerto-hydrogen-and-methanol model based on the collaborative optimization of energy flow and material flow, and establishes a model for electric hydrogen and methanol coproduction and system operation based on technologies such as power-to-hydrogen, power-to-methanol, and oxygen-enriched combustion power generation technologies.To quantitatively analyze the economic value of the proposed model, a case study is conducted on renewable energy bases in western and northern China.The powerto-hydrogen-and-methanol model based on the collaborative optimization of energy flow and material flow has the potential to become a new form of a new quality productive force,enabling electricity,hydrogen,and other energy sources to play important roles in more scenarios,accelerating the formation of an integrated energy system centered on electricity and utilizing hydrogen as a carrier.This comprehensive system covers major sectors including energy supply, transportation, and industry, contributing to the efficient use of clean energies while fostering the sustainable development of various industries.

CRediT authorship contribution statement

Zehong Liu:Writing-review&editing,Funding acquisition, Conceptualization.Jinxuan Zhang: Writing -review & editing, Writing - original draft, Methodology,Investigation.Zedong Zhang: Software, Formal analysis,Data curation.Yuanbing Zhou: Supervision.Jinyu Xiao:Project administration.Jinming Hou: Writing - review &editing, Validation, Resources.Yu Ni: Visualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors gratefully acknowledge the financial support provided by the Major Program of Xiangjiang Laboratory (No.23XJ01006).

Appendix A Boundary conditions

1) Renewable energy generation

The wind and solar power output characteristics of a typical desert and Gobi area in the western and northern regions of China for renewable energy development are illustrated in Fig.A1.The number of wind power utilization hours is 2790 h, whereas the number of solar power utilization hours is 1780 h.

Fig.A1.Characteristics of wind power and photovoltaic resources of typical regions in western and northern China

2) Characteristic curves of transmitted power

The scale of transmitted power is set at 10 million kilowatts, with a minimum utilization time of 5000 h.The characteristic curves for typical daily and annual transmitted power are shown in the following figure.

Fig.A2.Daily and yearly load curve of outing power

3) Cost forecasting

Based on the research findings of the Global Energy Interconnection Development and Cooperation Organization on clean energy generation, green hydrogen, and green chemical industry,the cost parameters for each level year at present and in the future are shown in Table A1.With an 8%discount rate,the annualized costs for power generation, electrolytic hydrogen production, and chemical equipment are calculated based on a service life of 25 years, while the annualized costs for electrochemical energy storage equipment are calculated based on a service life of 10 years,and the annualized costs for hydrogen and oxygen storage equipment are calculated based on a service life of 40 years.These annualized costs are used as cost parameters for the case study.

Table A1 Cost parameters of dierent cases in dierent level years.

Level yearPresent203020402050 Power generationWind power (yuan/kW)3500250018001500 Photovoltaic power (yuan/kW)3000220014001000 Hydrogen power generation (yuan/kW)5000400035003000 Coal power (yuan/kW)3000300030003000 Energy storageElectrochemical energy storage (yuan/kWh)1200800600500 Hydrogen storage (yuan/kg)2500200015001200 Oxygen storage (yuan/kg)20202020 Power to hydrogenAlkaline electrolyzer (yuan/kW)180013001000900 Proton exchange membrane electrolyzer (yuan/kW)6000350025002000 Carbon capturePost-combustion capture (yuan/t)330 Oxygen-enriched combustion capture without air seperation (yuan/t)230185150130 Chemical industryConventional (rigid) methanol synthesis (yuan/ton)2500 Flexible methanol synthesis (yuan/ton)330030002750

4) Comparative case study

Table A2 Results of cases in the present level year.

Caseselectricity-hydrogen synergyelectricity-hydrogen-carbon synergyThis case Total investment(billion yuan)354.0245.7224.9 Cost of electricity(yuan/kWh)0.2780.2120.196 Cost of hydrogen(yuan/kg)21.816.315.3 Cost of methanol(yuan/t)514039213698