They are. I would personally define desalination efficiency of 100% as a perfectly-reversible reaction that establishes an equilibrium between fresh water and

They are. I would personally define desalination efficiency of 100% as a perfectly-reversible reaction that establishes an equilibrium between fresh water and oceanic salt water. Since adding sea salt to fresh water until it has oceanic salinity represents a theoretical maximum of 0.810 Wh/L (a maximally efficient osmotic power plant, situated where a river empties into the ocean, could get about 0.75 Wh/L). So 100% efficiency would be adding 0.810 Wh to one liter of seawater to get one liter of fresh water back. 100% is an unachievable goal, thanks to the laws of thermodynamics. So to figure your solar desalination efficiency, from a solar panel that receives X Wh/m^2/day of insolation energy, you divide by 0.810 Wh/L to get L/m^2/day. Whatever fresh water you can produce per day, divide by that number to get your efficiency. An MIT roof gets mean 4.59 kWh/m^2/day of solar energy, so 100% efficiency for them would be 4590/0.810 = 5667 L/m^2/day. By the numbers given, their process is about 2% to 3% efficient (by my definition). They could be a lot more efficient if they didn't have to overcome the huge heat of vaporization that water has, which is exactly why reverse osmosis is so much more efficient than multistage flash distillation. They are stacking so that the evaporation energy can be recovered from the condensation, which deposits the same amount of energy on the next layer, but it's better off all around to just never evaporate in the first place.