Hacker News Reader

Comment by riazrizvi | Hacker News Reader

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.

View on HN

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.

As a reminder, "Be kind. Don't be snarky." [1]

[1] https://news.ycombinator.com/newsguidelines.html

So that's how this is gonna be.

Good luck.

Reply on news.ycombinator.com