[tlotman]

Running a DC current in salt water releases hydrogen gas and oxygen gas, which can be burned as fuel. If one was to build a generator deep in the ocean, the gases would already be compressed. Pipelines would deliver to a surface station to produce electricity.

To generate the DC current, solar collectors on the surface could power the generator.

Is this a viable alternative to burning fossil fuels?

[Enthalpy]

Hi Tlotman, welcome here!

This would be a route, but at what cost? Cost is (among others) what prevents renewable energies to replace coal, gas, oil. Fossil fuels have taken so much importance because they're dirt-cheap: bore a hole in the ground, obtain full cargo ships of gas, light it and you have heat.

So my suggestion is to put cost figures on the photovoltaic generator. Prices are public for smaller generators, you could get a first estimate. Even without converting to hydrogen, solar electricity is much more expensive than the one produced from coal - and electricity sells for more money than fuels.

Electrolysis wastes some power (often 2/3 efficient, can improve with an effort) but is cheap. Compressing the gases is cheap, don't invest time in that, nor decide the plant's location for it.

Transporting gaseous hydrogen (nor any gas) in a pipe is nothing obvious, I suggest that you put figures on the flow losses. Transporting it liquid isn't trivial neither, alas; it would be possible in a big boat, but I guess impossible in a pipe, because of heat leaks.

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The price of energies presently tells that
- Coal is dirt-cheap, gas very cheap as well, even refined oil products are;
- Taxes on some refined oil products are expensive, like car gasoline, but heating oil or plane kerosene far less;
- Electricity is more expensive than heavily taxed refined oil.

To replace fossil fuels the big way, alternatives must compete with untaxed ones to have a chance, because governments want their taxes. Presently:

- Untaxed biofuels are as cheap as taxed gasoline;
- Wind electricity (still without its storage) is cheaper than nuclear electricity but more expensive than coal electricity.
- Solar thermal electricity is storable; could compete with nuclear electricity?
- Photovoltaic electricity is seriously expensive. So are marine turbines.
- Geothermal heat seems competitive, geothermal electricity maybe but isn't developed.