Design and experimental validation of a high voltage ratio DC/DC converter for proton exchange membrane electrolyzer applications
Introduction
Over the last decades, reserves of fossil fuels (e.g. oil, natural gas) have been depleting due to the growing demand [1]. Indeed, fossil fuels are widely used for diverse applications, especially in transportation and electricity production [2]. Their increasing use all over the world leads up to the large release of greenhouse gases, responsible for global warming and hazardous meteorological phenomena [1]. Based on the current reserves and fossil fuels production [3], oil and natural gas should be depleted during the 21st century (forecast exploitation time around 50 years). Hence, new energy solutions must be developed to face the depletion of fossil fuels and global warming.
Among the envisaged solutions, hydrogen is considered a promising solution for a sustainable future by providing a clean and efficient energy carrier [4]. Hydrogen can supply or store energy. Furthermore, it has a very high energy content (i.e. 120 MJ/kg) and can be used in fuel cells (FCs) to generate electricity or power and heat as byproducts. Although hydrogen is not naturally present in nature, it can be found in fossil fuels, biomass, and water. At the present time, hydrogen can be produced in many ways such as by thermochemical, electrolytic, direct solar water splitting, and biological processes [5]. Among these different hydrogen production processes, water electrolysis is a promising option. The operation of water electrolysis is mainly based on the use of electricity to split de-ionized, pure or distilled water into hydrogen and oxygen. The chemical reaction occurs in a system called electrolyzer (EL). On one hand, given that electricity is needed to start hydrogen production, different energy sources can be used. On the other hand, in order to drastically minimize environmental impact, the use of renewable energy sources (e.g. wind, solar) is mandatory [6]. Besides, over the last years, wind energy has gained a growing interest in hydrogen production, as demonstrated in the literature [7], [8], [9], [10], [11], [12], [13], [14]. In particular, micro-wind energy conversion systems (μWECSs), involving power levels under 10 kW, are attractive for their low cost and easy installation.
Different types of ELs can be distinguished by their electrolyte and the charge carrier: (1) alkaline ELs; (2) proton exchange membrane (PEM) ELs; and (3) solid oxide (SO) ELs [15], [16]. Table 1 indicates the main features of each technology; whereas Table 2 presents the advantages and disadvantages of each technology. From Table 1, Table 2, alkaline and PEM ELs are currently the two main technologies, which are commercially available. Alkaline ELs are the most mature and widespread compared to PEM ELs (under development). As highlighted in Table 1, Table 2, alkaline ELs have a longer lifespan than PEM ELs. However, PEM ELs have several advantages over alkaline ELs, such as compactness, fast system response, wide partial load range and high flexibility in terms of operation. As a result, this technology is an attractive option for integration into the grid including renewable power generating systems [16]. For this reason, a PEM EL has been considered for carrying on this work.
Like for FCs, DC/DC converters are mandatory to interface the DC voltage grid and the EL. Generally, the EL required a very low DC voltage in order to generate hydrogen. Indeed, at nominal power, the cell voltage of an EL is equal around to 2.5 V [17]. With the aim to optimize the reliability of the EL, the number of cells has to be limited. As emphasized in a previous work [17], DC/DC converters for EL applications must meet several requirements from voltage ratio, energy efficiency, and low output current ripple point of view. Among these issues, the most important requirement expected from the DC/DC converter is a high conversion ratio. Indeed, for hydrogen production via water electrolysis from WECSs, the DC bus voltage is very high (i.e. between a hundred and a thousand volts) [17]. Given that the voltage of the electrolyzer is quite low (i.e. around 10 V) [17], DC/DC converters for EL applications must have a high conversion ratio ability.
Based on the current literature survey carried out in a previous work [17], three types of DC/DC converters are mainly used for these applications: (1) buck converter; (2) half-bridge converter (HBC); and (3) full-bridge converter (FBC). On one hand, classic buck converters are widely used within hydrogen production systems based on WECS [7], [8], [9], [10], [11], [12], [13], [14]. In Ref. [7], Şahin et al. have proposed a synchronous buck converter by replacing the diode with a power switch. Therefore, the diode reverse recovery issue can be removed. In the other related papers [8], [9], [10], [11], [12], [13], [14], a simple buck converter is used since the study is mainly focused on energy management and cost analysis. By comparison, in Ref. [18], Monroy-Morales proposes a system composed of a rectifier and a classic buck converter to supply an electrolyzer. The rectifier is used to convert an AC grid to a DC grid; whereas the buck converter is used to adjust the required voltage to supply the electrolyzer. On the other hand, classic buck converters present several disadvantages, particularly from voltage ratio and output current ripples reduction point of view. These topologies must be particularly used for applications requiring a low voltage ratio as reported in the literature (i.e. for very small, low voltage wind turbines) [7], [8], [9], [10], [11], [12], [13], [14]. On the other side, isolated DC/DC converter topologies (e.g. HBC, FBC) provide more perspective, especially in terms of voltage conversion ratio due to the use of a transformer. In Refs. [19], [20], Andrijanovitš et al. have developed a zero-voltage switching half-bridge DC-DC converter to interface the DC grid and the electrolyzer within a stand-alone power supply based on renewable energy sources. By comparison, in Ref. [21], Blinov and Andrijanovitš have developed a half-bridge phase-shifted active rectifier for electrolyzer applications. Finally, full-bridge DC-DC converters have been proposed in Refs. [22], [23], [24], [25], [26], [27], [28]. The advantages and disadvantages of each topology have been provided in a previous review work [17]. If the transformer is well designed for a given application, these topologies are particularly suitable for hydrogen production via water electrolysis including WECS. However, so far, they have not been considered in the literature for these applications [17].
Accordingly, the main purpose of this paper is to present a suitable high voltage ratio DC/DC converter for EL applications within a hydrogen production system based on a WECS. In this application, the DC bus voltage is quite high (i.e. around a hundred volts) and is very representative of the usual DC bus grid voltage met in wind power systems; consequently, a DC/DC converter with high conversion ratio is required. Another important aspect treated in the paper is the control of the DC/DC converter. In this paper, model-based control techniques are used. To this end, a mathematical model is obtained, assuming the behavior of the transformer as ideal. Using this model, two controllers are designed, one consisting of integral action, and the other consisting of an integral action together with a proportional action. Experimental tests have been carried out in order to validate the concept of the developed converter and its control for applications requiring a high conversion ratio.
This paper is divided into four sections. After this Introduction presenting the current state-of-the-art and motivations to carry out this work, Section The investigated system and the chosen converter topology describes the investigated hydrogen production system (i.e. from the wind turbine to the EL), and particular attention is given to the proposed DC/DC converter. Then, in Section Converter design and development of the control laws, details about the design and control laws for the DC/DC converter are provided. Finally, Section Experimental test bench and validation presents the experimental test bench used to validate the proposed DC/DC converter and its control, as well as the obtained results.
Section snippets
The investigated system and the chosen converter topology
Within the framework of this work, a hydrogen production system based on a μWECS is investigated and is shown in Fig. 1. This system is composed of the following components: a horizontal axis wind turbine, a three-phase bridge rectifier, the proposed DC/DC converter, and finally a PEM electrolyzer and metal-hydride hydrogen storage. In the following, further details are provided for each component being part of the studied hydrogen production system.
Design and sizing of the proposed converter
In general, it is possible to size the output filter of the FBC ((4) in Fig. 4) considering that the output waveforms of an FBC with unity turn ratio, controlled with duty ratio D at a switching frequency Fsw, are the same as those of a buck converter operating at 2D and 2Fsw [31]. In the case under study, the FBC is operated at a constant duty ratio equal to 0.5. As a result, the output waveforms should have no ripples at all. However, the leakage inductance of the high-frequency transformer
Developed experimental test bench
In order to validate the high voltage ratio converter and its control, experimental tests have been performed. The experimental test bench set up for this work is shown in Fig. 10. It is composed of an auto-transformer which substitutes the wind generator, a 3-phase diode rectifier combined with a buck converter and LC filters, a full-bridge converter followed by a transformer, a diode bridge rectifier including an LC filter, a PEM EL, a metal-hydride hydrogen storage system, de-ionized water
Conclusion
The main objectives of the work were to design and develop a high-voltage ratio DC/DC converter based on buck and full-bridge DC/DC converters. Indeed, based on the current literature, it was demonstrated that current hybrid renewable energy systems with a hydrogen buffer storage are limited to low-power applications. This can be explained by the use of classical DC/DC converters (buck converters for electrolyzer applications) which present several drawbacks from voltage ratio and output
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