Photovoltaic voltage regulation with power storage for DIY hydrogen generation cell

tovoltaic cell and provides a steady voltage output for a hydrogen generati on cell.

as you can give it (for the purposes of the electrolytic cell).

light. Because the energy from the solar cell is precious, I'd like to stor e any excess energy in capacitors.

ylight. If you use a series voltage regulator, part of the voltage will be soaked up by the regulating device resistance (Rv) with a power loss associ ated with that (Rv*I^2) I would prefer it if this power otherwise lost is s hunted to a capacitor bank.

Sorry, I should have clarified, the SC/Mean values given are for the photov oltaic cell.

Sorry, I should have clarified, the SC/Mean values given are for the photovoltaic cell.

You are in way over your head. Stop digging.

There is an optimum load voltage to get the most power from a solar cell at any instant. Solar cell controllers usually have a processor that finds that point, and tunes a switching regulator to operate at that peak. It's fairly complex.

Capacitors can't store much energy... thousands of times less than an equal cost or size battery.

If you want to make hydrogen, why store electric energy?

Making hydrogen electrolytically from solar cells will be very inefficient. What would you do with the hydrogen?

I am just ordering the basic cells. I am not sure if they come with a controller.

Understood.

If you can conceive of a voltage regulator that supplies a constant output voltage and just varies the current to the electrolytic cell, without storing or wasting the electrical energy, it would be even better.

It is just a home science experiment. The last time I did electronics was many years ago, and I would like to rekindle the hobby.

Just double checked with the supplier. They don't come with a controller, just +ve and -ve output terminals.

The supplier doesn't even have graphs of voltage/current output vs ambient light.

I don't think you actually want a constant anything from the output, you just want to convert all the power you can get from the photovoltaic cells into current through the electrolysys cell.

the voltage that goes into the electrolysys cell will automatically be fairly constant due to the chemistry of the cell.

running a electrolysis cell is much like charging a lead-acid battery that never gets charged, but the yoltages are different. if it can be modified to suit the voltage of the electrolytic cell (or several in series) a solar lead-acid charge controller cuircuit could provide a good starting point.

You can create a two-way regulator.

Use an H-bridge to drive an inductor that connects to a battery or ultracap. Create a PWM signal such that the duty cycle maintains a ratio between the unregulated side and the battery/ultracap.

For example, you could map a solar voltage of 18 to 21 volts to a sealed lead acid battery voltage of 11 to 14 volts. Vbat = Vsolar - 7.

You could also map 18 to 21 volts solar to 3 to 10 volts on four series ultracaps. Vcap= 7/3 * Vsolar - 39.

You can trick some synchronous buck regulators into doing this. Disable all of: discontinous mode, power save, soft start, under voltage, over voltage, and reverse current detection. You'll need an op-amp or some transistors to implement your in-out ratio. I've gotten an ADP2164 to maintain a smooth 4-6 V on 5V solar cells using a pair of 100F caps driven to a range of 0.5-5.2V. It's actually all on a hat with a tiny centrifugal fan for hiking in very hot weather.

Compensation is for this is hard and I can't help you with that.

DC to DC converter. The input voltage from the solar panel varies from zilch to whatever. The output is whatever your electrolysis rig likes best.

However, before you dive into hydrogen generation, I suggest you read some of the papers at:

Thanks, happy to see a OTS component that does what I need.

Thanks, I am familiar with the horrible inefficiencies of hydrogen generation as a "fuel" (or rather, energy storage medium). The device is not intended to be efficient but just a home science project.

Thanks for the information. Useful to know that I might not need any voltage regulation, as I was planning on doing a direct PV cell -> electrolytic cell and seeing what happens at a first instance.

I will be receiving the parts in about 1-2 weeks and will see what happens. At the moment I am speculating on what I might need because I can't wait to get my new toys :)

Or try the electrolysis cell out with a small battery to at least have some understanding of what you are doing. Making small amounts of hydrogen and oxygen this way is easy but also hopelessly inefficient.

Already did that with bare wires in brine connected to a bench D/C power su pply. The wires got eroded in no time but it worked. A bench power supply o f course controls voltage/current to the specified values, not quite the sa me as voltage/current from a PV cell.

Using PV cells + professional electrodes would be a first for me.

Very bad idea to use brine - chlorine isn't nice at all. You want sodium sulphate or dilute sulphuric acid.

Then you get hydrogen and oxygen in a 2:1 ratio assuming that you collect the gasses off each electrode.

Cheapest reasonably inert electrodes are the carbon rods out of classic zinc carbon cells. It is very *much* cheaper than platinum foil.

Very bad idea to use brine - chlorine isn't nice at all. You want sodium sulphate or dilute sulphuric acid.

Then you get hydrogen and oxygen in a 2:1 ratio assuming that you collect the gasses off each electrode.

Cheapest reasonably inert electrodes are the carbon rods out of classic zinc carbon cells. It is very *much* cheaper than platinum foil.

There are a number of issues with this.

You can't store any meaningful amount of energy at low voltage in any reasonably sized/priced capacitor bank.

You can't have a steady output voltage and variable current. For fixed voltage, the current is determined by the load. Not much you can do to increase it from the supply side.

Memory is vague, but, as I recall, your cell should operate somewhere around 3V and size the cell to use the available current. More voltage goes up in heat, not producing hydrogen...but look it up.

Solar power varies all over the map. And it's typically far less than the spec, unless you live in a very sunny climate and only use it at noon.

Assuming your cell is sized correctly, I believe that the output increases with current...so maximize that current.

I have a system that's been working on a mountain top charging batteries. The output voltage is always between 10 and 14V, so you may have to make adjustments due to the variable voltage of your hydrogen cell...

Basically, you use a buck switching regulator topology. I used the PWM output of a PIC processor, a Hall Effect current sensor and the A/D input to measure the current in the load.

The duty factor of the PWM is the ratio of the output voltage to the input voltage. Pick that ratio for typical operating conditions and program it at power on. Measure the current. Increase the duty factor. If current goes up, do it again. If current goes down, reverse direction to reduce the duty factor. You dither the duty factor at the peak current into the load. It tracks cloud occlusion, daytime variability, etc.

The simplest hydrogen storage I've heard about is a small barrel inverted inside a larger one filled with water. The small barrel floats as hydrogen is introduced. It's low pressure and relatively fail-safe, unless you're above the barrel (or in the landing zone) when it explodes.

Suggest you not run the thing when you're not watching it. Local guy built a Brown's gas generator into the trunk of his car. Accidentally left it on overnight and blew the valve covers off his engine when he tried to start it. He was lucky.

Have fun with your science experiment, but don't waste a lot of money on it. Unless your objective is to create hydrogen where solar is the only available energy source, there are more efficient ways to do what you want...and it doesn't depend much on what you want.

I think you're misreading something. "Short-circuit", by definition, means zero volts. I assume this is for the solar cells, in which case you probably mean _open_ circuit voltage.

For some specific illumination, no doubt.

As mentioned, capacitor storage is impractical. I think a little bit of homework will tell you this for yourself -- just start with the fact that the energy stored in a capacitor is equal to W = V^2 * C / 2, where W is the energy in Joules, V is the capacitor terminal voltage, and C is the capacitance. From that, and the power and time requirements, you can determine the minimum C necessary -- after that, a little bit of shopping will tell you why it's an issue.

You could also charge batteries during peak periods and let them discharge during off times -- but why?

I think a far better way to do this would be to investigate how to make an efficient hydrogen generator that works with a widely varying input power, then drive it with the peak power available at any given moment. By doing so, your hydrogen and oxygen tanks become your energy storage device -- and they'll be much more compact than capacitors or batteries.

I'm not sure if this means one humongous pair of electrodes that you sometimes drive very lightly, or if it means a number of electrodes that increase in size logarithmically that you switch in as the available current rises, or a whole bunch of same-sized electrodes that you cycle through (for wear leveling).

As mentioned, most photovoltaic arrays have some sort of maximum power point tracking algorithm. The idea behind this is that for any given illumination, the voltage-current curve of the array is unique, and there is a unique operating current (or voltage -- one determines the other) at which the array will generate the most power. The algorithms are complicated, but mostly based on the simple idea that you try it at one current and measure voltage, then try it at another current and measure voltage, and drive the current set point in the direction of maximum power (computed as voltage * current). If you stick with slow tracking your array may not be at its best with sudden illumination changes, but it'll be safe and stable.

I went digging for something useable. You don't need a regulated output, but something to get you in the range might save some effort. My guess(tm) is something that produces 3.3VDC should be in the ballpark.

That leaves the size of your "photovoltaic cell". If you're starting with individual 0.45v cells, then just string them in series until you get the desired 3v output, maybe insert a series resistor, and drive the electrolysis rig directly. If your electrolysis rig produces power (i.e. acts like a battery), you might need a series diode to protect the solar cells.

If your "photovoltaic cell" is a packaged 12v solar panel, my guess(tm) is that the maximum output voltage will be about 20VDC. You'll probably need to measure it: So, what you'll need is a DC-DC converter with a 0-24v input and a 3v output. One of these perhaps:

It's tempting to just modify a common 12v solar controller to "charge" four electrolysis cells in series which should yield about 3v per cell. Just remove all the over voltage protection, current control, and EOC detection circuitry. That leaves essentially a DC to DC converter with a 12v output.

However, if you want to just run one cell, you could probably use the solar controller to drive a 12v to 3v DC to DC converter or buck converter board. Such boards are all over eBay for cheap.

Efficiency might be a problem since each conversion strage looses some power. I'm fairly sure the solar controller takes efficiency into consideration. I'm not so sure efficiency is all that important on eBay DC to DC converters.

Good luck

Already did that with bare wires in brine connected to a bench D/C power supply. The wires got eroded in no time but it worked. A bench power supply of course controls voltage/current to the specified values, not quite the same as voltage/current from a PV cell.

Using PV cells + professional electrodes would be a first for me.

Use KOH instead of NaCl. Use Pt electrodes.

The optimum working voltage is just enough to split the hydrogen-oxygen bond anything much beyond that ends up as waste heat. Obviously you can trade more current through the cell higher output for lower efficiency.

Not convinced it will be very compact.

A 6Ah 12v lead acid battery stores about the same total energy. (and is physically much smaller but heavy)

Hydrogen in bulk has an irritating tendency to diffuse through materials like mild steel (even more so when under high pressure). And it is somewhat unforgiving - pure hydrogen flames are invisible and the gas mixture with air is explosive in a wide range of compositions.