The idea that energy can be converted (not created) from one form into another fits in our intuition. We are willing to accept that we can not get more energy than the energy contained in the source. We also know that during conversion some energy is lost to heat or friction (in case of moving systems). We aren’t however clear about the the logical conclusion of these two statements.
Energy is convertible (e.g. coal to heat, water dam to turbine’s spinning etc.) but it isn’t freely convertible. The process of energy conversion is governed by the second law of thermodynamics which states the entropy (a measure of ‘disorder’) of a closed system always increases during the conversion. Water that is neatly ‘arranged’ in a dam is flowing in a stream (less arranged, more disorderly than still water). We can not put it back in ‘arranged’ form in its original place without pumping it back or without putting more energy into it than what we got out of its running down. We can’t use the heat we obtained by burning coal to put it (the ash) back into the form of coal. The energy lost in the form of heat or friction corresponds to increase in disorderliness.
If we apply the above principles we can understand that energy of one kind cannot be regarded as equivalent to energy of another kind. For example, one joule of solar energy has a smaller ability to do work then one joule of energy contained in coal, since the coal energy is more concentrated than the solar energy. Whenever we try to extract energy from sun rays some energy gets lost as heat. Bigger solar panels catching more sun rays will generate more heat. There are thermodynamic and quantum limits to efficiency of solar panels.
‘Better’ solar cells too involve manufacturing that is dependent on and presupposes the existence of a complex global industrial manufacturing system. This comprises not only the fabrication systems typically accounted for in estimates of manufacturing energy, but the contingent mining, refining and global transportation systems, as well as other energy intensive critical support systems including finance, information, and security systems. The uncertainty of that energy component confers uncertainty on any estimate of payback times derived from that estimate, considered by some to be significant.
We will substitute one form of mining by another, we will re-distribute heat loss between point of manufacture and consumption if we substitute coal energy by solar energy.
Solar energy may seem to be clean (and even free) if we don’t take into account energy used in manufacture, transportation, installation, and regular cleaning of panels. But it isn’t. It also generates heat.
According to The second law of thermodynamics coal as an energy source is more efficient therefore more ‘green’ than the solar energy. Geological forces acting over forests buried in earthquakes tens of thousand of years have concentrated energy in coal in compact (lower entropy) form than the sun rays.
I will try to cover other issues (not directly related to Physics) related to use of coal or fossil fuels (or even solar panels) at some other time.
I know there will be protests by the greens over this. But I would welcome discussion based of physics. That will be enlightening.