Ask Me Help Desk - Some questions about energy efficiency

I want to know if the known laws of physics, esp second law of thermodynamics, rules out any hope of making either a solar cell or an electric lamp that is 100% energy efficient or at least 99% energy efficient.

It is my understanding that the second law of thermodynamics rules out turning a less 'useful' type of energy into a more 'useful' type of energy without generating even more 'less useful' energy usually in the form of waste heat so that there cannot be any net increase in the overall 'usefulness' if the total energy from an energy conversion.

BUT, purely within this narrow thermodynamic context, is electric energy any more or less 'useful' than visible light energy?
Because if the answer is “no”, then surely it IS possible ( at least in theory ) to make BOTH a solar cell and an electric lamp that are both 100% energy efficient! -right?
But, even if the answer is “yes”, then which type of energy is the more 'useful' type? Electric or visible light?

If visible light is more 'useful' type of energy than electric then does that mean it IS possible ( at least in theory ) to make a solar cell that is 100% efficient but not an electric lamp that is 100% efficient?
And if electric energy is more 'useful' than visible light energy then does that mean it IS possible ( at least in theory ) to make an electric lamp that is 100% efficient but not a solar cell that is 100% efficient?

I also want to know if how 'useful' ( in this narrow context ) visible light is is partly dependent on how broad the range of frequencies that light consists of.
Is, for example, light that just consists of one wavelength, say 550nm only ( which is monochromatic green light which I assume can only be produced by a laser ) , more 'useful' ( in this narrow context ) than white visible light consisting of many wavelengths but with an average wavelength of 550nm?

It is my understanding that visible light energy of lower frequencies i.e. longer wavelengths is less 'useful' ( in this narrow context ) than the same amount of light energy consisting of higher frequencies -is that correct?

Quote:

Originally Posted by humy

It is my understanding that the second law of thermodynamics rules out turning a less 'useful' type of energy into a more 'useful' type of energy without generating even more 'less useful' energy usually in the form of waste heat so that there cannot be any net increase in the overall 'usefulness' if the total energy from an energy conversion.

By "useful" here you mean an energy form that you want - for example electrical energy is useful to humans for running appliances, whereas the kinetic energy in a falling raindrop is not, right? So whether one form uis more "useful" than another is purely judgmental based on what you want to do with it.

Quote:

Originally Posted by humy

BUT, purely within this narrow thermodynamic context, is electric energy any more or less 'useful' than visible light energy?

There is no such thing as inherent "usefulness" in any form of energy - from a thermo point of view all energy is equivalent. And all conversions of one form to another are inherently less than 100% efficient. So no - you cannot make a 100% efficient light source or a 100% efficient solar cell.

Thanks for that clarification of the subjectiveness of how 'useful' energy is.

I apologise for this but I should have made it clear in my questions that what I am really interested in is if such energy conversions can occur at OR very close to 100% efficiency ( lets say “very close to 100% ” means “more than 99% ” ) . It is my understanding that, by using superconductors as well as magnetic bearings, it is theoretically possible to construct both an electric motor and an electric generator that are 100% energy efficient or at least, say, 99.9% energy efficient ( this ignores the energy costs of cooling the superconductor but if above room-temperature superconductors are used, assuming such a thing can exist, then there should be no cooling energy costs ) .

Now, an electric motor converts electrical energy to mechanical energy while an electric generator converts mechanical energy to electrical energy and in both cases that energy conversion can be made to occur at more than 99% energy efficiency ( at least in theory ) so clearly the laws of physics do not rule out the possibility of either conversion occurring at more than 99% energy efficiency.

So, what I really want to know is does the same apply for solar cells and electric lamps i.e. does the laws of physics rule out the possibility of either a solar cell or an electric lamp operating at more than 99% energy efficiency?

Note: I understanding that the current “theoretical maximum limit” for energy efficiency for solar panels is somewhat less than 80% ( roughly about ~70% if I remember correctly ) but that is based on the constraint that the solar panels either must use conventional semiconductors in the conventional way or solar reflectors are used to convert solar energy into heat energy which is then converted to electric energy.
But, it is my current understanding that these theoretical maximum limits for energy efficiency do not apply outside that constraint so you may achieve greater efficiency by making a solar cell that uses some other quantum effects other than that in conventional semiconductors.
There are also some current “theoretical maximum limit” for energy efficiency for electric lamps ( also about ~70% if I remember correctly ) but that is also based on some constraints and, again, if I understand correctly, you needn't adhere to those constraints.

I don't know of any theoretical constraint to the efficiency of solar cells or lamps. LEDs are already pretty efficient and in theory can be close to 100%. Solar cells are much less efficient - if you put them out in the sun they get hot (like your car gets hot sitting in the sun), which tells you that much of the energy of the suns rays are going to heating the material, not in producing electricity. Here's an article that suggest that achieving 21% efficiency was a recent record:
Canadian Solar Sets New Solar Cell Efficiency Record - CleanTechnica