# Alkaline Water Electrolysis Powered by Renewable Energy: A Review

^{*}

## Abstract

**:**

## 1. Introduction

^{−1}and for PEMEL from 1400 to 2100 €KW

^{−1}. Furthermore, the lifetime of alkaline water electrolyzers is higher and the annual maintenance costs are lower compared to a PEMEL system [15,20,22,23]. Often, PEMEL systems are preferred for dynamic operation due to the short start-up time and a broad load flexibility range. The shortcomings of AEL are gradually being overcome by further development [24]. Therefore, this review focuses on alkaline water electrolysis powered by renewable energy. To ensure safety and high efficiency, alkaline water electrolyzers must be optimized for dynamic operation. Hence, the process needs to be analyzed for how the dynamics will affect the system performance and what aspects should be considered when fluctuating renewable energy is used instead of a constant load [25]. Thus, this contribution shows model descriptions for alkaline water electrolysis, photovoltaic panels, and wind turbines to identify the limitations when combining all components into a hydrogen energy system. Furthermore, theoretical models can help to solve the existing problems using intelligent system design and suitable operation strategies.

## 2. Alkaline Water Electrolysis

^{−1}) [28].

## 3. System

## 4. Cell Design and Cell Voltage

^{™}Perl UTP 500 (AGFA) or dense anion exchange membranes can be used as the separator [33,34,35,36,37].

^{™}Perl UTP 500 is often used, anion exchange membranes are promising alternatives. For Zirfon

^{™}-based materials, experimental data of the resistance at a fixed electrolyte concentration for different temperatures are available [51].

^{−1}at 50 °C, NaOH reaches a value around 65 Sm

^{−1}. At a temperature of 25 °C, a similar effect can be seen. The conductivity of KOH is around 40 to 50% higher than the conductivity of a NaOH solution at the optimal weight percentage. Another aspect is the solubility of the product gases inside the electrolyte, as this influences the resulting product gas purity. In general, the gas solubility decreases with an increasing electrolyte concentration due to the salting-out behavior [53]. NaOH also shows a slightly higher salting-out effect than that of KOH. Hence, the product gas solubility is higher in a KOH solution [54,55,56].

## 5. Gas Purity

^{−2}and the system pressures range from 1 to 20 bar [64].

^{−2}, slightly above the safety limit of 2 vol.% H

_{2}in O

_{2}, this limit is already reached at 0.5 A cm

^{−2}for a system pressure of 10 bar. At 20 bar, no sufficient gas purity could be measured, as even a current density of 0.7 A cm

^{−2}results in a gas impurity of 2.5 vol.%.

## 6. Periphery

## 7. Renewable Energy

^{−1}, the significant solar radiation is only available during the daytime. Hence, the averaged value over the whole day is 233 W m

^{−2}for a sunny day and only 29 W m

^{−2}for a cloudy day.

#### 7.1. Solar Photovoltaic Power

^{−2}, in combination with a typical polarization curve of an alkaline water electrolyzer (10 cm

^{2}electrode area) from (8) in Figure 9a. The power–voltage curves for the photovoltaic cell are shown in Figure 9b. The maximal power point (MPP) for each radiation level is marked with a dot in both diagrams.

#### 7.2. Wind Power

^{−3}), the area spanned by the rotor blades A, and the wind velocity are needed [74,85].

^{−1}with rotational speeds from 6 to 17 min

^{−1}. The cut-in wind speed is 3 m s

^{−1}and the cut-out wind speed is 22 m s

^{−1}[74]. In comparison with the power characteristics of photovoltaic panels, the polarization curve of alkaline water electrolyzers can not be directly optimized towards the MPP trajectory, as the optimal operation point highly depends on the wind turbine design and weather conditions. Therefore, an efficient AC/DC converter is the best option for maintaining an efficient operation of an alkaline water electrolyzer [82].

## 8. Hydrogen Energy System and Power Grid Stabilization

## 9. Limitations and Solution Approaches

#### 9.1. Limited Operation Time

#### 9.2. Optimal System Design and Operation Strategies

## 10. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Abbreviations

AC | Alternating current |

AEL | Alkaline water electrolysis |

CFD | Computational fluid dynamics |

DC | Direct current |

HTEL | High-temperature electrolysis |

ILs | Ionic liquids |

MPP | Maximum power point |

PEMEL | Proton exchange membrane electrolysis |

PV | Photovoltaic |

SOEL | Solid oxide electrolysis |

## Appendix A. Correlations and Parameters

Parameter | Equation (7) [28,94] | Equation (8) [39] | Unit |
---|---|---|---|

${r}_{1}$ | 8.05 $\xb7{10}^{-5}$ | 4.45153 $\xb7{10}^{-5}$ | $\mathsf{\Omega}{\mathrm{m}}^{2}$ |

${r}_{2}$ | −2.5 $\xb7{10}^{-7}$ | 6.88874 $\xb7{10}^{-9}$ | $\mathsf{\Omega}{\mathrm{m}}^{2}$ °C^{−1} |

s | 0.185 | 0.33824 | $\mathrm{V}$ |

${t}_{1}$ | 1.002 | −0.01539 | ${\mathrm{m}}^{2}{\mathrm{A}}^{-1}$ |

${t}_{2}$ | 8.424 | 2.00181 | ${\mathrm{m}}^{2}$ °C A^{−1} |

${t}_{3}$ | 247.3 | 15.24178 | ${\mathrm{m}}^{2}$ °C^{2} A^{−1} |

${d}_{1}$ | – | −3.12996 $\xb7{10}^{-6}$ | $\mathsf{\Omega}{\mathrm{m}}^{2}$ |

${d}_{2}$ | – | 4.47137 $\xb7{10}^{-7}$ | $\mathsf{\Omega}{\mathrm{m}}^{2}{\mathrm{bar}}^{-1}$ |

Parameter | Equation (A2) [57] | Unit | Equation (A3) [52] | Unit |
---|---|---|---|---|

${K}_{1}$ | 27.9844803 | $\mathrm{S}{\mathrm{m}}^{-1}$ | −45.7 | $\mathrm{S}{\mathrm{m}}^{-1}$ |

${K}_{2}$ | −0.924129482 | $\mathrm{S}{\mathrm{m}}^{-1}{\mathrm{k}}^{-1}$ | 1.02 | $\mathrm{S}{\mathrm{m}}^{-1}$ °C^{−1} |

${K}_{3}$ | −0.0149660371 | $\mathrm{S}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$ | 3200 | $\mathrm{S}{\mathrm{m}}^{-1}$ |

${K}_{4}$ | −0.0905209551 | $\mathrm{S}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ | −2990 | $\mathrm{S}{\mathrm{m}}^{-1}$ |

${K}_{5}$ | 0.0114933252 | $\mathrm{S}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$ | 784 | $\mathrm{S}{\mathrm{m}}^{-1}$ |

${K}_{6}$ | 0.1765 | – | – | – |

${K}_{7}$ | 6.96648518 | $\mathrm{S}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ | – | – |

${K}_{8}$ | −2898.15658 | $\mathrm{S}\mathrm{K}{\mathrm{m}}^{-1}$ | – | – |

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**Figure 1.**The number of publications per year from 1990 to 2019 containing the specified keywords. Around 2010, the publication rate increases due to greater interest in the energy turnaround. While the topic is often discussed technology-independently (unspecified), more publications for low-temperature technologies, like alkaline water electrolysis (AEL) and proton exchange membrane electrolysis (PEMEL), are available than for the high-temperature technology solid oxide electrolysis (SOEL) [26].

**Figure 2.**A schematic flow diagram of an alkaline water electrolyzer. The electrolyte is pumped through the electrolysis cell where the gas evolution takes place. Adjacent gas separators split both phases, and the liquid phase flows back to the electrolysis stack. Heat exchangers ensure that the optimal temperature is maintained, and the product gases can be purified afterward.

**Figure 3.**Different cell designs for alkaline water electrolysis. Whereas (

**a**) shows a conventional assembly with a defined distance between both electrodes, (

**b**) depicts a zero-gap assembly where the electrodes are directly pressed onto the separator [38].

**Figure 4.**The calculated cell voltage of an atmospheric alkaline water electrolyzer at a temperature of 60 °C according to Equation (8). The overall cell voltage consists of the reversible cell voltage ${U}_{\mathrm{rev}}$, ohmic losses $I\xb7{R}_{\mathrm{ohm}}$, and activation overvoltages ${\eta}_{\mathrm{act}}$ [39,40].

**Figure 6.**Anodic gas impurity (${\mathrm{H}}_{2}$ in ${\mathrm{O}}_{2}$) in relation to the current density at different pressure levels for (

**a**) separated and (

**b**) mixed electrolyte cycles, at a temperature of 60 °C, with an electrolyte concentration of approximately 32 wt.% and an electrolyte volume flow of 0.35 L min

^{−1}[64].

**Figure 7.**Typical time-related profiles for (

**a**) solar radiation and (

**b**) wind velocity, measured by the weather station of the Clausthal University of Technology. Though solar radiation peaks around noon, wind velocity shows sinusoidal oscillations.

**Figure 9.**Example calculation results of the (

**a**) current–voltage characteristics of a photovoltaic panel at different solar radiation levels and the corresponding (

**b**) power–voltage curve. Additionally, a current–voltage characteristic of an alkaline water electrolyzer (AEL) is implemented. The intersections determine the possible operation points. For an efficient operation, the distance to the maximal power points (MPP) should be minimal [29,72,73].

**Figure 12.**The schematic process scheme of a hydrogen energy system. Photovoltaic panels and wind turbines generate renewable energy to power alkaline water electrolyzers, and stored hydrogen can be converted back into electricity by fuel cells. Therefore, either oxygen or air can be utilized. Additional energy storage devices can damp fluctuations, and the complete hydrogen energy system can be used for power grid stabilization [25,28,82,87].

**Table 1.**Parameters for the example calculation of the photovoltaic current using Equation (10). The number of serial n

_{s}and parallel n

_{p}connected photovoltaic cells, the short-circuit current I

_{sc}, the open-cell voltage U

_{oc}, the serial R

_{s}and parallel resistance R

_{p}, and the non-ideality factor m are setup-specific data. A constant cell temperature T

_{c}is assumed [29,72,73].

${\mathit{n}}_{\mathbf{s}}$ | ${\mathit{n}}_{\mathbf{p}}$ | ${\mathit{I}}_{\mathbf{sc}}$ | ${\mathit{U}}_{\mathbf{oc}}$ | ${\mathit{R}}_{\mathbf{s}}$ | ${\mathit{R}}_{\mathbf{p}}$ | m | ${\mathit{T}}_{\mathbf{c}}$ |
---|---|---|---|---|---|---|---|

– | – | A | V | Ω | Ω | – | °C |

9 | 2 | 5.98 | 4.615 | 0.099 | 20 | 1.6 | 48 |

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**MDPI and ACS Style**

Brauns, J.; Turek, T.
Alkaline Water Electrolysis Powered by Renewable Energy: A Review. *Processes* **2020**, *8*, 248.
https://doi.org/10.3390/pr8020248

**AMA Style**

Brauns J, Turek T.
Alkaline Water Electrolysis Powered by Renewable Energy: A Review. *Processes*. 2020; 8(2):248.
https://doi.org/10.3390/pr8020248

**Chicago/Turabian Style**

Brauns, Jörn, and Thomas Turek.
2020. "Alkaline Water Electrolysis Powered by Renewable Energy: A Review" *Processes* 8, no. 2: 248.
https://doi.org/10.3390/pr8020248