Converting sea water directly into hydrogen is an interesting tack, but would require a lot of new technology. I found one example of an existing marine desal unit (presumably for a yacht): the MECO MMRO1500: 1500 gallons of fresh water per day (enough for 30 people, assuming 100% duty factor; at 5 hrs/day, this is only 6 people.) input is 4.2 gallons/minute of seawater power input is 2.2kW (requires 15m^2 = ~161 sqft of solar panels) unit weighs 580 pounds Clearly this unit is optimized for a seagoing vessel, where discharging a huge amount of slightly-more-saline seawater isn't an issue. This unit probably wouldn't be a candidate for home use, where the major fraction of the input water is also discharged into the sewer. Wasting discharge water is also true of existing home RO units, although I suspect that this marine unit is even more profligate about the percentage of discharge in order to optimize other things (cost, weight, power, reliability). So the unit above could be built into a seagoing solar-powered module taking up about 1/3 the area of a minimum 2-car garage (400 sq ft). At 03:42 AM 4/23/2015, Mike Speciner wrote:
What if you used solar to just split off the hydrogen? Then transport just the hydrogen and use it to reconstitute water wherever needed, simultaneously retrieving at least some of the energy? How efficient could that be made? I know there's work being done to produce hydrogen directly from sunlight and water (though I don't know if it works for salt water), and of course there's work being done on hydrogen fuel cells. If you aren't worried about Hindenburgs, you could also piggyback other transportation on the hydrogen.
On 23-Apr-15 00:23, meekerdb wrote:
On 4/22/2015 8:07 PM, Henry Baker wrote:
Thanks, Gene for the terrific analysis.
Let's say that RO requires 5 kWh/m^3.
'A typical "150 watt" solar module is about a square meter in size. Such a module may be expected to produce 0.75 kWh every day, on average, after taking into account the weather and the latitude, for an insolation of 5 sun hours/day.'
It's not clear why you should convert to electrical power in order to desalinate water. You need some for pumping the water around, but you can use solar energy directly to heat and distill water in imitation of the atmospheric rain cycle. It'll take some research to figure out how efficient this would be, but I think it's a lot more efficient than going through the solar photons to electricity step.
Brent
http://en.wikipedia.org/wiki/Photovoltaic_system
1 cubic meter = 264.172052 US gallons
Each person requires ~ 50 gallons = ~1/5 m^3 of water per day, or about 1kWh per day for energy for desalinization.
So each person requires about 2 m^2 = ~21 sqft (~ size of a large 3'x7' bed) of solar cells just to power his/her desalinization needs.
Clearly, there is more than enough roof area on most (California) homes to provide enough power to desalinize the water for its occupants. So even if we're off by a factor of 2-4x, this is still reasonable.
So perhaps the answer is piping seawater to people's homes and doing distributed desalinization?
At 06:40 PM 4/22/2015, Eugene Salamin via math-fun wrote:
According to [ http://en.wikipedia.org/wiki/Desalination ], reverse osmosis requires 3-5.5 kWh/m^3, just for the reverse osmoses, and excluding the energy needed to operate the facility.