> the team’s demonstration device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation.
I need an explanation with pictures for that, because it seems like the author is using ‘efficiency’ incorrectly.
With a heat pump, for instance, you might be able to produce 5BTUs per watt-hour, because you're extracting heat from the environment (a very large heat source), which would be 140% efficient.
The paragraph you excerpted has your answer:
> The key to the system’s efficiency lies in the way it uses each of the multiple stages to desalinate the water. At each stage, heat released by the previous stage is harnessed instead of wasted. In this way, the team’s demonstration device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation.
You can't drink steam (and even bottling it is futile), so that heat has to go somewhere.
With counterflow systems, like ventilation systems for houses, the inlet and outlet temperatures are pretty close to each other, and the temperature on the 'inside' of the system is either much higher or much lower.
When distillation came up a month or so ago, several people pointed out that modern systems do not operate at ambient air pressure, so the temperature delta may be less than you'd imagine.
I really enjoy artful displays of mental gymnastics, so thank you. A watt is a unit of power, a BTU is a unit of energy, and a watt hour is also a unit of energy. If you can show me how solar watt hours are converted into BTU's at an equivalent rate of 3.85 to 1, I will short sell all the energy stocks using my children as collateral.
A watt hour is actually 3.41 BTUs, not 3.85. A BTU is the energy to raise a pound of water by one degree Frankenstein. (An avoirdupois pound, not a Tower pound, a troy pound, an apothecaries' pound (which happens to be equal to the troy pound), a merchant's pound, an Imperial Standard pound, or a pound sterling or pound of paper, which aren't even units of weight.)
The specific heat of water is one calorie per gram per kelvin, so a BTU works out to one pound, times a calorie per gram, times a degree Frankenstein per kelvin.
A calorie has been defined as having various different values, since water's specific heat varies with temperature (and pressure!) but they're all about 4.18 to 4.19 joules, except for the food "calorie", which is actually a kilocalorie.
Pounds have also been defined as having many different values, even avoirdupois pounds; the values used include 6992 grains, 7000 grains, 7002 grains, and 6999 grains. (Troy grains, not metric grains, which are different.) The currently most popular pound is 7000 grains, but by international agreement it is now defined as 453.59237 grams, previous definitions in terms of the metric system having included 453.59265 grams and 453.59243 grams.
Finally, one degree Frankenstein is precisely defined as 5/9 of a kelvin, although Dr. Fahrenheit's original definition was rather different.
Working all of this out, a BTU turns out to be about 454 g · 4.18 J/g · (5/9)K/K, which is about 1054.3 J. A watt hour is of course 1 W hour · 60 s/min · 60 min/hour = 3600 W s = 3600 J. 3600/1054.3 is about 3.41.
I'll be here all week. Don't forget to tip your servers.
Solar watt-hours are not converted to BTUs at a rate > 1, however, the device effectively uses each watt-hour multiple times, as some of the energy is recovered after each cycle. Needless to say, if the goal were to produce energy, this would be impossible, but much like with heat pumps (which can exceed 100% efficiency by moving heat instead of converting electricity), the device is not producing energy from solar power. The baseline 100% efficiency they are comparing against is that of convertible the water to steam, i.e. the maximum efficiency of a solar desalinator that does not reuse any waste heat.
If we're talking about distillation, the civil engineer probably wants to think about rates (liters per minute per $1000), but for a civilian just trying to figure out how this could work in a laboratory situation?
I find quantities easier to comprehend (and relate). Tell me how many AA batteries or days of full sun it would take to convert a big beaker of salt water into a smaller beaker of distilled water.
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.
I need an explanation with pictures for that, because it seems like the author is using ‘efficiency’ incorrectly.