Keywords

1 Introduction

The core problems of wind hydrogen are how to configure the nominal power and number of the electrolyzer and formulate operating strategies of multi-electrolyzers based on the operation law and characteristics of the alkaline electrolyzer. Varela et al. [1] solved a mixed-integer linear program to find the optimal number of electrolyzers and production schedules in a wind-hydrogen system. The results show that the optimal objective value and total energy absorption are obtained when 13 electrolyzers are configured in the system. Matute et al. [2] presented a multi-state model of electrolysis systems, optimal dispatch of hydrogen production plants is obtained, and the model was tested using real data from a wind farm, the results demonstrated that the model is a support tool to operate hydrogen production plants more profitably. The current studies mainly focus on the total capacity of the wind-hydrogen system [3, 4], few researchers make efforts to the comprehensive management between multiple electrolyzers, even if the multi-electrolyzers system has been considered, the nominal power of every electrolyzer is the same in most of the works [2,3,4].

2 Methodology

2.1 Multi-electrolyzers and Historical Data of Wind Turbine

The multi-electrolyzers have shared Bop (Bop, Balance of plant), and every electrolyzer has an independent power supply so that the state of the electrolyzer could be switched independently and flexibly. The control system will decide the operating state of electrolyzers depending on the Pout (Pout, output power of wind turbine), in this way water electrolysis system is operated safely and efficiently.

The distribution of Pout over time is shown in Fig. 1. The total capacity of electrolyzers in the system is subject to the historical and future Pout. Generally, the hydrogen production power of the electrolyzer shall be equal to the maximum Pout, while the maximum Pout is about 2.2475 MW, and the time that Pout exceeds 2 MW just is 442 h during 6000 h. Considering the balance between investment costs and energy absorption rate, the total capacity of electrolyzers is set to 2MW preliminary.

Fig. 1.
figure 1

Power distribution of wind turbine during 250 days.

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2.2 Assumptions for the Alkaline Electrolyzer

In this work, the alkaline water electrolyzer is selected as hydrogen production equipment due to alkaline electrolyzer is currently the most mature and durable [5, 6].

There is an operating point with the lowest energy consumption (eop, operating point with the lowest energy consumption), which approximately is 60% of the nominal power of the electrolyzer [7]. When the Pin (Pin, input power of electrolyzer) increases from lop to eop, the energy consumption decreases gradually and once the Pin exceeds the eop, the energy consumption will continue to increase. When the Pin reaches the highest operating point, the electrolyzer has maximum energy consumption. Based on this fact, it’s ruled that when the Pin is in the operating range of 20–60%, the energy consumption is 48 kWh/kgH2, while the Pin is in the operating range of 60–100%, the energy consumption is 52 kWh/kgH2 [8].

Generally, the lop of the electrolyzer is 20% of the nominal power, which is the reason why this work needs to be done. The lop could be reduced to 11% relying on some novel control methods [8, 9]. The following work will be based on that lop is 20% of the nominal power. Simultaneously, the impact of reducing lop to 11% of nominal power on the research results will also be explored.

2.3 Configurations and Control Strategy of the Electrolyzers

Considering the standardized manufacture of alkaline electrolyzers, it’s assumed that the nominal power of the electrolyzers includes 0.5MW, 1MW, 1.5MW, and 2MW. And nA, nB, nC, and nD represent the number of electrolyzers with 0.5 MW, 1 MW, 1.5 MW, and 2 MW respectively. For hydrogen production system with a total capacity of 2MW:

$$ 0.5{\text{nA}} + {\text{nB}} + 1.5{\text{nC}} + 2{\text{nD}} = 2{ } $$
(1)

To solve Eq. (1), there are five configurations in this work. The configurations are listed in Table 1.

Table 1. Configurations of four electrolyzers.
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2.4 Equal Load Strategy

In this work, an equal load strategy is proposed. For the sake of explanation, the electrolyzers are numbered EL1, EL2 until ELn according to the nominal power of electrolyzers from small to large, by the way, n represents the number of electrolyzers in the system. For this strategy, the controller needs to judge whether Pout exceeds lop of EL1, if the answer is yes, EL1 is started. Then the controller needs to calculate the result of Pout minus lop1 and if the calculation result exceeds lop2, EL2 will be started, and so on. The controller needs to repeat the calculation and judgment process after Pout changing. The core of strategy is to keep as more electrolyzers in operation as possible even if they can’t be operated at nominal power. Figure 2 shows a schematic diagram of power distribution for three electrolytic cells as an example.

Fig. 2.
figure 2

Schematic diagram of equal load strategy.

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3 Results and Discussion

3.1 Comparison and Analysis of System Performance Under Different Configurations

Based on the characteristics of multi-electrolyzers systems, the theory of the Equivalent load lower limit (El) is proposed in this paper. EL can be represented by Eq. (2).

$$ El = \frac{Pmin*20\% }{{Powertotal}}{ } $$
(2)

The \(Pmin\) is nominal power of electrolyzer with lowest power, \(Powertotal\) is the total power of system.

The performance of hydrogen production systems can usually be measured by the following indicators: lower load limit (L); Hydrogen Production (P); Energy absorption rate (Ea); Average working hours per electrolyzer (Aw); Average times of starts and stops per electrolyzer (At). The operational indicators within 6000 h are listed in Table 2. This result shows that all indicators of the system with single electrolyzer are the worst. Multi-electrolyzer system has significant advantages in both hydrogen production and energy absorption rate. According to definition of Equivalent load lower limit, the energy absorption rate and minimum load of the system depend on the electrolytic cell with the lowest power.

When Pout (Output power of wind power) is the same, more electrolyzers could be started and the electrolyzers have the same operating point under this strategy, which results in electrolyzers being operated at a relatively low operating point with high efficiency. Though the #2, #3, #4 has same energy absorption, the hydrogen production show some differences. The hydrogen production is positively correlated with the number of electrolyzers with small nominal power under, the reason is that increasing the number of electrolyzers with small nominal power leads to the more chances for the electrolyzers to be operated at the operating point with high efficiency.

As for the average working hours per electrolyzer, the #4 has greater performance, which means that the electrolyzer will have a higher utilization rate using this configuration. In fact the difference of Aw of different configurations is small, the multi-electrolyzer system also couldn’t solve the Relatively low utilization rate of electrolyzer in the scenario of wind power hydrogen production.

Frequent startup and shutdown of electrolyzer have a certain negative impact on the lifespan of the electrolyzer, so this indicator is also worth paying attention to. The #3 has minimal At, and the At will deteriorates as the number of 0.5 MW electrolyzer increases. However, Similar to Aw, the difference in At performance between different configurations is also relatively small.

The more electrolytic cells in the system, the higher the difficulty and cost of control, meanwhile, Bop also needs to be optimized accordingly. Thus # 3 maybe an ideal configuration based on a comprehensive comparison. Only two electrolyzers are configured in the system, and the overall performance is also very good.

Table 2. Hydrogen production in 6000 h and total costs of electrolyzers.
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4 Conclusion

This work proposed configurations and operating strategy of electrolyzers for an Off-grid isolated stand-alone wind hydrogen system. Based on the historical data of Pout, the total nominal power of the alkaline electrolyzer is determined to be 2 MW, and considering the electrolyzer will be manufactured standardized, five configurations of electrolyzers are obtained. According to the operation characteristics of alkaline electrolyzer, two operating strategies are proposed.

The single electrolyzer is a bad choice in the wind-hydrogen system due to its relatively high lop, but the multi-electrolyzers system performs well in the aspect of hydrogen production and energy absorptivity. Under equal load Strategy, the number of electrolyzers with small nominal power is positively correlated to hydrogen production.

By utilizing this configuration and control strategy, the system achieved an energy absorption rate of over 99% while working safely.