Improving PV-powered water electrolysis with external fields

Australian researchers have analysed different ways to improve the efficiency of PV-powered water electrolysis for hydrogen generation. They include the use of magnetic fields, light energy, ultrasonic fields, and pulsating electric fields. Energy costs remain prohibitive, but molecular movement and the redistribution of molecules in water during electrolysis could open a path to viability.

Image: Jason Jacobs, flickr

Scientists from Southern Cross University in Australia are trying to understand the extent to which the use of magnetic fields, light energy fields, ultrasonic fields, or pulsating electric fields could help to improve the efficiency of water electrolysis in solar-powered hydrogen generation plants.

“This study has taken a closer look at these applied fields due to the potential that their efficacy can be further enhanced by considering the molecular dynamics of water molecules,” the researchers said in “Increasing the efficiency of hydrogen production from solar powered water electrolysis,” which was recently published in Renewable and Sustainable Energy Reviews. “This study also advocates the use of accurate and effective test equipment to help develop a universal approach which would allow future research to be easily evaluated.”

Green hydrogen is currently more expensive the blue hydrogen, and the efficiency of commercially available solar-hydrogen hybrid systems ranges from a minimum of around 2.3% to a maximum of approximately 12%. This low efficiency is generally attributed to efficiency losses in PV generation and the electrolysis cell, power consumption by electronic controls, temperature moderation and ineffective coupling systems between the solar panels and the electrolyser.

One way to increase the efficiency of solar-powered electrolysis, regardless of the effectiveness of coupling, is to improve the efficiency of electrolysis through the four proposed approaches, which are already known to assist in reducing the formation and flow of oxygen and hydrogen gas bubbles in electrolysers.

“The main issue reported with bubble evolution is the gas bubbles sticking to the electrodes which causes the bubbles to build-up between the electrodes, resulting in an increase in the ohmic resistance within the electrolyser,” the Australian group explained, adding that more power consumption is needed to compensate for this resistance.

Magnetic fields

Magnetic fields are produced by electric currents that can be either macroscopic currents in wires or microscopic currents associated with electrons in atomic orbits. These fields are a well-known solution for enabling the desorption of hydrogen from the electrodes and the resulting reduction of the bubble size of the produced gases. Although different explanations have been given for the increased flow rate of hydrogen out of the system, when these fields are applied, the scientists believe that these contradictory findings are possibly the result of different electrolyser designs.

“Such a difference in design would result in a different interaction between the externally induced magnetic field, the electrodes and the electric field generated within the electrolysis chamber,” they said.

However, the current issues with this approach are the difficulty of using bar magnets in large-scale applications, and the high cost of producing rare earth magnets and generating a magnetic field from an energy source. More research is needed to assess the true efficiency of this approach in improving electrolysis, the researchers said.

Light energy

Green lasers have already been used at the research level to enhance hydrogen generation through light energy, which creates an induced electric field. The lasers have proven to be efficient enough to provide considerable enhancements in the production of the fuel, but the costs of the energy needed to power them may still be too prohibitive to make this technology viable.

Instead, focused sunlight does not need any additional input energy and might represent an interesting alternative.

“The utilisation of sunlight energy to enhance hydrogen production in solar-hydrogen hybrid setups would complement such a system if setups incorporated sunlight receivers into photovoltaic panels to induce sunlight into the electrolysis chamber,” the academics said, adding that the real potential benefits of the application of light energy must be thoroughly investigated.

Ultrasonic fields

Ultrasonic fields have been applied with success in improving the efficiency of electrolysers, showing potential for a 20% increase.

“For example, higher ultrasonic power inputs in some electrolyte concentrations caused gas bubbles to break up into smaller bubbles, resulting in greater bubble adhesion to the electrodes, higher system impedance and an increase in energy consumption,” the researchers said.

However, the energy needed to create the fields is less than the power saved due to the increased efficiency, which currently makes their application to hydrogen unviable. Furthermore, several researchers have shown that the efficiency of the electrolyser is not always proportional to ultrasonic power input.

“For example, higher ultrasonic power inputs in some electrolyte concentrations caused gas bubbles to break up into smaller bubbles, resulting in greater bubble adhesion to the electrodes, higher system impedance and an increase in energy consumption,” the scientists said, noting that only a few studies have investigated this and other issues related to this external field.

Pulsating electric fields

Pulsating the applied electric field at different intensities and frequencies is another way to improve hydrogen generation. Different theories were outlined to explain why this technique effectively produces an increase in electrolysis efficiency, although no conclusive findings were established.

“As with all applied fields, a disadvantage of applying a pulsating electric field is the cost of system required to generate the pulsating electric field, especially for high frequency electric fields with high power requirements,” the academics explained. “The cost of such equipment also rapidly increases as the power requirements of the equipment increases to meet the needs of scaled-up systems for industrial applications.”

They said that the application has yet to be comprehensively studied. More work is needed to assess the effects of pulsating the applied electric field for water electrolysis.

Water molecule

A look at how these four applied fields interact with molecular movement and the redistribution of molecules in the water during electrolysis could bring us closer to proving their viability, the researcher claimed. In particular, they have looked at the ortho-para conversion of water molecules to understand the movement and reactivity of water.

“Ortho H₂O is a term for water molecules that have a spin state due to the energetic moment of each of the hydrogen elements working in conjunction with each other,” they explained. “While para H₂O is a term for water molecules that have no spin state due to the energetic moments of the each of the hydrogen elements working in opposite directions.”

The scientists concluded that a deeper understanding of the molecular dynamics of water in electrolysis will be crucial to bringing the proposed four approaches closer to viability.

Reference: https://www.pv-magazine-australia.com/2021/01/22/improving-pv-powered-water-electrolysis-with-external-fields/