Wikipedia:Reference desk/Archives/Science/2021 September 13

= September 13 =

Environmental chem questions.
1. Can't we rebuild into the ozone layer, by flying up there and releasing ozone? Or does that take thousands of years?

2. What % is fuel efficiency for gasoline, is it like 99%? Meaning 1% is left as exhaust gas? Say, how much has it changed going back to the '70s, I'm hoping it went up gradually? Thanks. 67.165.185.178 (talk) 22:32, 13 September 2021 (UTC).


 * No. Over 99% becomes exhaust, but up to several tens of percent of the energy released becomes useful work, it has gone up gradually. Sagittarian Milky Way (talk) 23:14, 13 September 2021 (UTC)


 * 1. The rate at which ozone is formed and destroyed naturally in the ozone layer is so high that it would be very hard to make an impact by adding ozone from an aircraft. Releasing catalysts that accelerate formation or destruction of ozone can have a significant effect. As it happens, the exhaust gases of combustion-powered aircraft contain catalysts destroying ozone, so the net effect of a high flying aircraft releasing ozone would be depletion of the ozone layer. Also see supersonic transport.
 * 2. All atoms of the fuel end up in the exhaust gas; nothing remains in the engine. But that's not what fuel efficiency is about. Somewhere around 30% of the energy released by burning the fuel is turned into work done by the engine, the rest is waste heat in either the exhaust gas or engine cooling. That's what fuel efficiency of an engine is about: the fraction of the heat released by burning the fuel that gets applied as work at the driveshaft. It has improved a bit since the 1970s, but this is also offset somewhat by the increasing weight of our cars (a heavier car wastes more fuel on moving the car instead of its occupants), so the change in fuel efficiency of the car (fuel used to move a person from A to B) hasn't been very dramatic. PiusImpavidus (talk) 09:07, 14 September 2021 (UTC)
 * Unless you have soot deposits, those are gasoline atoms that didn't reach the air. Sagittarian Milky Way (talk) 13:34, 14 September 2021 (UTC)
 * Gasoline is a compound, or rather a mixture of compounds, not an element, it consists of molecules, not atoms. --CiaPan (talk) 17:55, 14 September 2021 (UTC)
 * Everyone knows that. Gasoline-derived mass that stuck to the vehicle parts. In this case, some of the carbons. A sign of imperfect stoichiometry. Sagittarian Milky Way (talk) 18:22, 14 September 2021 (UTC)

Are photovoltaics made for sunlight inefficient in low light?
Or is the watts per lux a constant if spectrum is sun-like?

2. Do incandescent, fluorescent, CFL, HPS, and LED lightbulbs and candles yield more watts per lux than sunlight or less?

3. What happens if you try to charge a battery with light and it's too dim to make the normal charging volts, amps or both? Is there a non-zero watt minimum to reverse the discharge chemical reaction? Sagittarian Milky Way (talk) 23:09, 13 September 2021 (UTC)


 * 1. For the same spectrum and the same temperature, I don't think efficiency changes much with intensity. I wouldn't be surprised if there's some saturation effect at very high intensities. The temperature matters though; at higher temperature the efficiency drops. Under high light intensity, the panel tends to get hotter, so the efficiency is usually lower in brighter light.
 * 2. It depends. If the band gap of the PV semiconductor is tuned to the spectrum of the light source, the efficiency could be higher. If not, it could be lower. Generally, turning electricity into light, then back into electricity is a very inefficient method of power transfer, so people don't really try to make this more efficient. It will never work well.
 * 3. With less light, the PV panel will provide less current. The battery will charge slower. PV panels behave like a current source with a voltage cap. The current is proportional to the available light. PiusImpavidus (talk) 09:41, 14 September 2021 (UTC)