Seaweed contains a lot of cellulose and starch, just like land-based plants. It can be processed with acids or enzymes to convert these into sugars, just as is done to convert corn starch into corn syrup. The sugary byproduct can be fermented to produce ethanol.
Dry distillation is heating biomass without oxygen. It produces a range of gaseous and liquid fuel byproducts, plus charcoal. Solar heat can of course be used.
I imagine this idea works, but to compare some orders of magnitude... solar cells may have efficiencies of 15%. 70% for electrolysis, unclear for desalination (depends on how pure it has to be), unclear for methanol production but probably >50%... maybe you get 3% net in the product?
It's not amazing, but special engineered algae in perfect environments do what, 10% to biomass in a miracle situation? Isn't a major energy input to dry distillation the drying part, especially in this case? And the energy lost during fermentation is more yes?
Maybe they are in the same range, but I think my bet would be on the solar system over a system requiring collecting algae from the open ocean.
The actual efficiency of algae growth is irrelevant in this case, because we're not paying for the sunlight and there's an excess of seaweed available free for the picking up.
Measures of the theoretical efficiency of solar panels generally don't include the costs of manufacture and maintenance. Until panels become a really cheap and disposable commodity, land-based customers will always be able to outbid someone who wants to put them on the ocean, because of lower maintenance costs.
Solar energy is perfect for dry distillation. It's free.
I can get on board with solar thermal for distillation and other heat inputs to chemical reactions. But collecting algae isn't free.
Past experience with pond algae growth is that the drying part is indeed extremely expensive (using solar). But if you throw it into a gasifier I like it a lot better since it doesn't need to be as dry, so I guess fermentation might be similar although you need cellulosic ethanol tech to work better first. Still didn't break even against solar at the time though (a decade past), and solar has gotten way better.
Maybe the algae tech has improved commensurately, but the tech for biomass handling is much more mature so I'd be a bit surprised if the rate of improvement wasn't much different.
The silliest proposal I ever had to write was for why it made sense to take electricity from a power plant, "free hydrogen" and make chemicals. It was so strange, just use the fuel as a precursor to make the chemicals directly instead of burning it...
Edit: I said as much in the proposal. We did not win.
It's true that collecting algae requires effort to collect and transport. But my ultimate point is that the total energy and resource costs of directly converting seaweed to fuel would be far lower than in OP's proposal. Just as in your power plant anecdote. And you achieve the same goal of removing CO2 from the ocean.
Drying pond algae with solar heat is tricky because you can't concentrate the sunlight too much or you get ash instead of dry algae. But with dry distillation the objective is to pyrolize the input, so you can efficiently use concentrated sunlight.
>you need cellulosic ethanol tech to work better first. Still didn't break even against solar
My proposal isn't competing with solar electricity (which doesn't remove CO2 from the ocean), but with OP's proposal to use solar electricity to produce methanol from CO2 extracted from seawater.
Biological processes are usually much less efficient than industrial processes, so it would be hard to believe that your proposal is better without lots of evidence. If we relied on legumes for nitrogen fertilization of agricultural soil, we would starve.
