Techno-economic analysis of hydrogen production systems based on solar/wind hybrid renewable energy

With the popularity of renewable energy, more and more countries and regions are utilizing renewable energy to produce hydrogen. However, renewable energy hydrogen production systems are often large in capacity and equipment, and require rational design as well as performance analysis of the system to satisfy the technicalities while ensuring the economics. In this paper, 12 different renewable energy systems for hydrogen production, including 6 grid-connected systems and 6 off-grid systems, are designed considering solar and wind energy as the main renewable energy sources in a place in China. Subsequently, simulations were carried out using HOMER software to compare and analyze the economic and technical performance of different schemes of renewable energy hydrogen production systems, and the best grid-connected and off-grid renewable energy hydrogen production system schemes were selected based on the comparison results.


Introduction
Renewable energy sources such as solar and wind are widely used internationally, but their fluctuating and intermittent nature leads to the need for the use of energy storage technologies to store the energy generated by the system in a timely manner [1].Currently, the energy storage technology that is in line with the concept of green development is hydrogen energy storage technology.Renewable energy hydrogen production technology utilizes electricity generated from renewable energy power generators for electrolysis of water to produce hydrogen, realizing the conversion from electricity to hydrogen.Since renewable energy hydrogen

System component model
Renewable hydrogen production systems are feature-rich, providing both electricity and hydrogen, and consist mainly of power generation components (PV panels, WT, etc.), electrolyzers, hydrogen storage tanks, FC, and the user end (electricity users, hydrogen users, etc.).
In this paper, monocrystalline silicon PV cells are used.In practical engineering applications, the PV cell output current   and voltage   are usually calculated by the empirical equations shown below [9]: where,   is the short-circuit current, A;   is the open-circuit voltage, V;   is the maximum power point current, A;   is the maximum power point voltage, V.
The relationship between the output power of the WT and the wind speed can be expressed using a segmented function with the expression: 2    3 ,  1 ≤  <     ,   ≤  ≤  2 where  1 ,   and  2 are the cut-in wind speed, rated wind speed and cut-out wind speed of the WT, m/s, respectively.When the wind speed is less than the cut-in wind speed or more than the cut-out wind speed, the WT will not work.  is the rated power of the WT, W. The hydrogen production rate of an alkaline electrolyzer can be calculated by obtaining the U-I characteristic curve of the alkaline electrolyzer under normal operating conditions.The expression for the rate of hydrogen production is: where   2 is the rate of hydrogen production, mol/s;  is the Faraday constant with a value of 96485C/mol;   is the electrolyzer operating current, A;   is the electrolyzer current efficiency.Based on practical engineering experience,   is expressed as: where   is the surface area of the electrode plate of the electrolyzer, m 2 ;  1 and  2 are both current efficiency parameters of the electrolyzer.A proton exchange membrane fuel cell is composed of   individual cells connected in series and its output voltage   can be expressed as: ) where ,   ,  ℎ , and   are the thermodynamic potential, activation overvoltage, ohmic overvoltage, and concentration overvoltage, V, respectively.

Renewable Energy Hydrogen Production System Program Design and Parameterization
In this section, a place in China is taken as the study site, and based on the distribution The basic structure of different schemes of renewable energy HPS is shown in figure 1. Renewable energy HPSs are categorized into grid-connected and off-grid systems based on whether they are connected to the grid or not, including: grid-connected/off-grid PV hydrogen production systems (G1/S1),grid-connected/off-grid PV/FC hydrogen production system (G2/S2), grid-connected/off-grid WT hydrogen production system (G3/S3), gridconnected/off-grid WT/FC hydrogen production system (G4/S4), grid-connected/off-grid PV/WT hydrogen production system (G5/S5), and grid-connected/off-grid PV/WT/FC hydrogen production system (G6/S6).production systems for different scenarios.For a grid-connected system, when the power generation of the system is less than the electrical load, the system takes power from the grid to meet the electrical load and hydrogen load, and when the power generation of the system meets the electrical load and hydrogen load, the remaining power is delivered to the grid.For off-grid renewable energy HPS, when the power generation of the system is greater than the electric load, the excess electricity will be used by the electrolyzer to produce hydrogen.When the power generation of the system is less than the electrical load, the hydrogen in the hydrogen storage tank can be used to generate electricity for the electrical load for the system configured with FCs components.
The input parameters for the simulation need to take into account weather data, load (electrical and hydrogen load) data, and system component parameters.Weather data are based on data compiled by the National Aeronautics and Space Administration.In this paper, the average monthly solar radiation intensity and clearness indices are used as the main basis for the evaluation of solar energy potential, and the average monthly wind speed is the main basis for the evaluation of wind energy potential.The average solar radiation intensity and clearness indices for different months in a year are shown in figure 2 (a).The annual average clear sky index is 0.572, and the largest and smallest monthly average clear sky index are January and June, with values of 0.654 and 0.535, respectively, indicating that the proportion of sunny days throughout the year is high and the region is sunny.In addition, the average annual solar radiation intensity in the region is 4.311kWh/m 2 /month, and the maximum and minimum monthly average radiation intensity are 6.205kWh/m 2 /month and 2.028kWh/m 2 /month in June and December, respectively.The monthly average data of wind speed and temperature are shown in figure 2(b).The annual average wind speed is 3.758m/s, the maximum and minimum monthly average wind speed are 5.020m/s and 2.110m/s in May and December, respectively, and the wind speed fluctuates significantly throughout the year.The annual average temperature is 10.19 °C, and the maximum and minimum monthly average temperatures are 25.19 °C and -7.55 °C in July and January, respectively.The electrical load data comes from the average monthly electricity demand of a small office building from 0-23 hours provided by HOMER software.As shown in figure 3, electrical load demand is highest in the month of August and the peak time period for electrical load demand occurs from 8-16 pm each day.The average daily electrical load is 172kWh and the average hourly electrical load is 7.2kWh.The hydrogen loading data was obtained from the literature [10], as shown in figure 4, the highest hydrogen loading of 4 kg per hour was observed at 8am -9pm.The daily hydrogen loading was 75 kg, with an average hydrogen loading of 3.13 kg per hour.
The economic parameters of each component of the system are shown in Table 1.The PV panels were selected as general-purpose flat panels, setting the upper power limit to 1200kW and the lower limit to 0. Considering that the minimum monthly average wind speed in the area is about 2m/s, the model selected for the WT is Enercon, with five power ratings including 500kW, 660kW, 800kW, 1000kW, and 2000kW.The electrolyzer was selected as a generalpurpose electrolyzer, and the upper power limit was set to 1000 kW, the lower limit to 0, and the working efficiency was set to 85%.The capacity of the hydrogen storage tank is set to an upper limit of 1000 kg and a lower limit of 0. The initial hydrogen mass in the hydrogen storage tank is set to 10% of the capacity of the tank.The fuel cell is set to have an upper power limit of 100kW and a lower limit of 0. The converter is set to have an upper power limit of 1,000kW and a lower limit of 0. Both the inverter and the rectifier operate at 95% efficiency.In addition, for grid-connected renewable HPSs, the cost of the grid is not considered, and the actual role of the grid is equivalent to an infinitely large energy storage device   off-grid and grid-connected scenarios.Firstly, with the objective of minimizing the NPV of the system, the capacity configurations of different scenarios to satisfy the electrical and hydrogen loads of the system are calculated and the results are shown in Table 2.Among them, scenarios S1, S3, and S5 are infeasible, and even an infinite increase in the rated power of the system components will not satisfy the load demand of the system.The main reason for this is that the fluctuating and intermittent nature of solar and wind energy results in systems that do not generate electricity at all times, leading to excessive capacity shortages when the system is not generating enough electricity.After calculating the appropriate capacity configurations, a comparative analysis of the different options of grid-connected and off-grid renewable energy HPSs will be performed in terms of system economics and technicality, respectively.The economic analysis is to compare and analyze the investment cost, operation cost, NPV, and energy cost of different scenarios respectively.According to the simulation results of HOMER software, the investment cost, operating cost, salvage value, NPV, and energy cost of the grid-connected and off-grid renewable energy hydrogen production system for different scenarios are shown in Table 3.It can be seen that among the grid-connected and off-grid renewable energy systems for hydrogen production, the investment cost, operating cost, NPV and energy cost are smaller for the systems containing PV panels (G1, G2, G5, G6, S2, S6), while they are larger for the systems that do not contain PV panels (G3, G4, S4).The main reason for this is that solar energy is more abundant than wind energy at this location, and when PV panels and WT produce the same amount of electricity, the WT is larger and the system is more costly, while at the same time the difference in actual electricity consumed by the electric load is not significant, resulting in excessive energy costs for the system.In addition, the hydrogen production systems without FCs (G1, G3, G5) have smaller investment costs, operating costs, and NPVs than the hydrogen production systems with FCs (G2, G4, G6), provided that the other components of the gridconnected hydrogen production systems are the same.Grid-connected (G2, G4, G6) have a smaller NPV than off-grid (S2, S4, S6) hydrogen production systems with the same components of the hydrogen production system, mainly due to the fact that the grid-connected hydrogen production system can deliver excess power to the grid to increase the system revenue.
In summary, analyzing from the perspectives of investment cost, operation cost, NPV and energy cost, for the grid-connected renewable energy HPS, the PV hydrogen production system (G1) and the PV/WT hydrogen production system (G5) are economically better; for the offgrid renewable energy hydrogen production system, the PV/FC hydrogen production system (S2) and the PV/WT/FC system (S6) are economically better.

Technical analysis
Technical analysis typically analyzes the output performance of a system over its full life cycle.This section compares and analyzes the generation, hydrogen production, hydrogen use, capacity shortage rate, and excess power rate (off-grid) of HPS for different scenarios.The power generation of the renewable HPS for different scenarios is shown in Table 4.For the grid-connected system, it can be seen that the total power generation of scenarios G3 and G4 is significantly larger than the other scenarios.Meanwhile, it can be found that the hydrogen production system configured with FCs has a larger total power and a smaller grid-supplied power than the hydrogen production system without FCs, when the other components of the system are the same.For the off-grid system, the total generation of Scenarios S4 and S6 is significantly higher than the total generation of Scenario S2.Therefore, from the point of view of system power generation, WT/FC (G4) and PV/WT/FC (G6) hydrogen production systems are technically better for the grid-connected type of hydrogen production system, and WT/FC (S4) and PV/WT/FC (S6) hydrogen production systems are technically better for the off-grid type of hydrogen production system.The amount of hydrogen produced and used by the renewable energy hydrogen production system for different scenarios is shown in Table 5.It can be seen that for the grid-connected hydrogen production system, the hydrogen production system equipped with a FCs (G2, G4 G6) produces more hydrogen and uses less hydrogen at the same time than the hydrogen production system without FCs (G1, G3 G5), provided that the other components of the system are the same.This is mainly due to the fact that the fuel cell utilizes some of the hydrogen to generate electricity, resulting in a reduction in the actual amount of hydrogen used by the user.In the off-grid type hydrogen production system, S2 produces more hydrogen than S4 and S6 hydrogen production systems, but at the same time the fuel cell uses more hydrogen.For Scenario S6, although the amount of hydrogen produced is smaller, the amount of hydrogen used by the fuel cell is also smaller, so the actual amount of hydrogen used by the user is larger.Therefore, from the perspective of system hydrogen production and consumption, for the gridconnected hydrogen production system, the G3 and G5 hydrogen production systems are technically better in terms of larger actual hydrogen consumption by users.For the off-grid hydrogen production system, the S2 and S6 hydrogen production systems are technically better.The capacity shortfalls of renewable HPSs for different scenarios are shown in Table 6.Due to the presence of the grid, only a shortage of hydrogen capacity is possible in a grid-connected system.In off-grid type systems, both electric and hydrogen capacity shortages may exist.From the table, it can be seen that for the grid-connected hydrogen production system, the hydrogen capacity shortages of the systems are closer and the shortages are all larger.In comparison, Scenarios G3 and G4 have smaller hydrogen capacity shortfalls.For the off-grid hydrogen production system, Scheme S6 has the smallest electric/hydrogen capacity shortfall in both cases.The main reason is that Scenario S6 utilizes the complementary characteristics of solar and wind energy in time, which has less impact on indirectness and volatility and enhances the stability of the system.Therefore, from the point of view of system capacity shortage rate, WT (G3) and WT/FC (G4) hydrogen production systems are technically better for grid-connected hydrogen production systems, and PV/WT/FC system (S6) hydrogen production systems are technically better for off-grid hydrogen production systems.For off-grid systems, after all loads are satisfied by the power generated by the system components, if there is any remaining power it will be discarded.For the three off-grid hydrogen production systems (S2, S4, S6), the excess power for the different scenarios was 200,892 kWh/year, 1,369,712 kWh/year, and 821,721 kWh/year, with excess power rates of 13.1%, 73.2%, and 38.6%, respectively.The PV/FC (S2) and PV/WT/FC (S6) hydrogen production systems are analyzed from the point of view of the system excess power rate, which is small and technically better.

System Selection
Based on the above simulation results, different schemes of renewable energy HPS have their own advantages in terms of economy and technology, and through the analysis of economy and technology, the optimal grid-connected and off-grid renewable energy hydrogen production system schemes when considering different evaluation indicators are shown in Table 7. From the economic point of view, Scenarios G1 and G5 are the best solutions in the grid-connected system, and Scenarios S2 and S6 in the off-grid system have smaller costs for each of them, especially Scenario S6 is more advantageous in terms of energy costs.From a technical point of view, for the grid-connected system, schemes G4 and G6 are the best ones when considering the power generation.When considering the amount of hydrogen produced and the amount of hydrogen used, the actual amount of hydrogen used by the users of Scenarios G3 and G5 is larger and technically better.Scenarios G3, G4 are optimal when considering the hydrogen capacity shortage rate of the system.For the off-grid type hydrogen production system, schemes S4 and S6 are the best scenarios when considering the power generation of the system.Scenarios S2 and S6 are technically better when considering the amount of hydrogen production, the amount of hydrogen used, the capacity shortage rate and the excess power rate.
Usually, for grid-connected renewable energy hydrogen production systems, economic considerations dominate and the main consideration is the energy cost of the system.For offgrid renewable hydrogen production systems, economics and technology have equal status, and the main considerations are the capacity shortfall of the system and the energy cost.Therefore, for the grid-connected renewable energy hydrogen production system, the optimal solution is the PV/WT hydrogen production system (G5); for the off-grid renewable energy hydrogen production system, the PV/WT/FC hydrogen production system (S6).

Conclusion
Based on the basic mathematical models of the system components, this paper designs 12 different renewable energy HPS scenarios, including six grid-connected and six off-grid systems, using a location in China as the study site.Subsequently, the HPSs of the different scenarios are compared and analysed from both economic and technical perspectives in conjunction with the weather data and load data of the location in order to select the optimal grid-connected and off-grid HPS.When considering the economic indicators such as investment cost, operating cost, NPV, and energy cost of renewable energy HPSs, the results show that the system containing PV panels has the lower of the four economic indicators for both grid-connected and off-grid renewable energy hydrogen production systems.Meanwhile, for a grid-connected hydrogen production system, other components being equal, a hydrogen production system without a FC has a smaller investment cost, operating cost and NPV than a hydrogen production system with a fuel cell.When considering the technical indicators such as power generation, hydrogen production, hydrogen use, capacity shortage rate and excess power rate of the renewable energy hydrogen production system, the actual hydrogen use of hydrogen loads is higher in Scenarios G3 and G5 of the grid-connected system.The actual amount of hydrogen used by the hydrogen loads in the hydrogen production systems of Scenarios S2 and S6 in the off-grid system is larger, while the electric capacity shortage rate, hydrogen capacity shortage rate and excess power rate are smaller.
In practice, for the grid-connected HPS, the economy is dominant and the main consideration is the energy cost of the system, and the optimal grid-connected hydrogen production system scenario is the PV/WT hydrogen production system with an energy cost of HEET-2023 Journal of Physics: Conference Series 2723 (2024) 012002 IOP Publishing doi:10.1088/1742-6596/2723/1/01200212 0.152$/kWh.For the off-grid hydrogen production system, the economic and technical status are comparable, and the main considerations are the capacity shortage rate of the system and the energy cost, and the optimal off-grid hydrogen production system solution is the PV/WT/FC hydrogen production system, with an energy cost of $0.162/kWh and an electricity/hydrogen capacity shortage of 0.15% and 4.42%, respectively.

Figure 1 .
Figure 1.Schematic illustration of the basic structure of renewable energy hydrogenproduction systems for different scenarios.For a grid-connected system, when the power generation of the system is less than the electrical load, the system takes power from the grid to meet the electrical load and hydrogen load, and when the power generation of the system meets the electrical load and hydrogen load, the remaining power is delivered to the grid.For off-grid renewable energy HPS, when the power generation of the system is greater than the electric load, the excess electricity will be used by the electrolyzer to produce hydrogen.When the power generation of the system is less than the electrical load, the hydrogen in the hydrogen storage tank can be used to generate electricity for the electrical load for the system configured with FCs components.The input parameters for the simulation need to take into account weather data, load (electrical and hydrogen load) data, and system component parameters.Weather data are based on data compiled by the National Aeronautics and Space Administration.In this paper, the average monthly solar radiation intensity and clearness indices are used as the main basis for the evaluation of solar energy potential, and the average monthly wind speed is the main basis for the evaluation of wind energy potential.The average solar radiation intensity and clearness indices for different months in a year are shown in figure2(a).The annual average clear sky index is 0.572, and the largest and smallest monthly average clear sky index are January and

Figure 2 .
Figure 2. Weather data for the region: (a) monthly averages of solar radiation intensity and clearness indices; (b) monthly averages of wind speed and temperature.The electrical load data comes from the average monthly electricity demand of a small office building from 0-23 hours provided by HOMER software.As shown in figure3, electrical load demand is highest in the month of August and the peak time period for electrical load demand occurs from 8-16 pm each day.The average daily electrical load is 172kWh and the average hourly electrical load is 7.2kWh.The hydrogen loading data was obtained from the literature[10], as shown in figure4, the highest hydrogen loading of 4 kg per hour was observed at 8am -9pm.The daily hydrogen loading was 75 kg, with an average hydrogen loading of 3.13 kg per hour.The economic parameters of each component of the system are shown in Table1.The PV panels were selected as general-purpose flat panels, setting the upper power limit to 1200kW and the lower limit to 0. Considering that the minimum monthly average wind speed in the area is about 2m/s, the model selected for the WT is Enercon, with five power ratings including 500kW, 660kW, 800kW, 1000kW, and 2000kW.The electrolyzer was selected as a generalpurpose electrolyzer, and the upper power limit was set to 1000 kW, the lower limit to 0, and the working efficiency was set to 85%.The capacity of the hydrogen storage tank is set to an upper limit of 1000 kg and a lower limit of 0. The initial hydrogen mass in the hydrogen storage tank is set to 10% of the capacity of the tank.The fuel cell is set to have an upper power limit of 100kW and a lower limit of 0. The converter is set to have an upper power limit of 1,000kW and a lower limit of 0. Both the inverter and the rectifier operate at 95% efficiency.In addition, for grid-connected renewable HPSs, the cost of the grid is not considered, and the actual role of the grid is equivalent to an infinitely large energy storage device

Figure 3 .
Figure 3. Average electrical load data for a small office building from 0-23 hours per month for one year.
energy in this place, 12 different scenarios of renewable energy HPSs are established, including six grid-connected systems and six off-grid systems.
4 characteristics of renewable

Table 1 .
System component parameters.In this paper, HOMER software is used to simulate and analyze renewable energy HPSs for 12

Table 2 .
Capacity configuration of renewable energy hydrogen production systems for different scenarios at minimum NPV.

Table 3 .
Data on economic indicators of renewable energy hydrogen production systems for different scenarios.

Table 4 .
Electricity generation from renewable hydrogen production systems for different scenarios.

Table 5 .
Hydrogen production and use in renewable energy hydrogen production systems for different scenarios.

Table 6 .
Capacity shortfalls of renewable hydrogen production systems for different scenarios.

Table 7 .
Optimal renewable energy hydrogen production system scenario for different performance metrics.