Renewable hydrogen electrolysis system, advice needed

[hypervalve]

Hello

I'm currently in the middle of a dissertation project, a feasibility study of a tidal/hydrogen generation system. At the moment I'm getting pretty bogged down trying to model the electrolyser. I'd like it to be more detailed than simply power in*conversion efficiencies. In particular I'd like to be able to comment on the ramifications of the transient nature of the tidal stream energy. From what I've read I would expect to see efficiency decrease as the input power increases.

I've been sifting through many research papers on the subject which attempt to mathematically model such systems with varying degrees of detail. I'm not sure if I'm more or less confused for it. If anyone has a grasp of the workings of electrolysers and is willing to answer a few of my (stupid) questions I'd be much obliged.

The technology I'm modelling is PEM. Which at the moment seems to be pretty small scale as far as production rates go (compared to alkaline). I'll basically be choosing an existing unit and scaling up to meet the power output of the tidal turbine. Which we'll say has a twice daily peak and minimum outputs of 500kW and 0kW respectively.

So I guess the first thing I'd like to know before I think of modelling the system, is the power conditioning arrangement. The electrolyser will require DC current, and it is my understanding that a stable voltage is also required? So I'm thinking some kind of double bridge rectifier could be used to supply stable voltage with varying current. The hydrogen output could then be calculated from he current flow if I'm not mistaken?

My apologies for the lack of understanding here, my background is in mechanical engineering so much of this is new to me!

Thanks in advance!

 

[Mike_In_Plano]

Might try researching submarines - nuclear subs need electrolysers to keep up with oxygen needs.

 

[Topher925]

So I guess the first thing I'd like to know before I think of modelling the system, is the power conditioning arrangement. The electrolyser will require DC current, and it is my understanding that a stable voltage is also required?

Although a stable power input into an electrolyzer is generally preferred, it is not necessarily required. PEM electrolyzers can operated under varying current input and not complain to much as long as the transients aren't to large.

The power scheme of your system should go something like this,

turbine w/inverter,controller -> rectifier -> DC bus w/large cap bank -> DC-DC converter/controller -> electrolyzer stack

Much of your analysis will be matching the output (with time) of the turbine with the demand of the electrolyzer and minimizing any transients with the capacitor bank. It sounds simple on paper but can actually be a royal pain in practice. Don't forget to do a WCA.

So I'm thinking some kind of double bridge rectifier could be used to supply stable voltage with varying current.

Electrolyzers don't work that way. Voltage and current have an explicit relationship which is described by the devices polarization curve. Here's an example,

[PLAIN]http://2hho.com/images/imgF.jpg

The relationship between voltage and current for an electrolyzer (or any electrochemical cell) can be mostly modeled using the Butler-Volmer equation and Ohms law.

The hydrogen output could then be calculated from he current flow if I'm not mistaken?

Yes, using Faraday's law of electrochemistry or

$$\dot{N}$$ = $$\frac{J}{nF}$$

Where $$\dot{N}$$ is the molar rate of production, J is the current, n is the number of electrons transferred in the reaction, and F is Faraday's constant.