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The Difference Between Thermodynamically Rich Heat and Heat as a Thermodynamic Degradant.
In a PM a correspondent here at DU wrote me thus:
what you mean by "Heat is the least thermodynamically degraded form of energy." (Point me to some text rather than explain if you wish.) You compare electricity to heat in another part of the post. I took three semesters of undergraduate physics, one being mostly thermo, and even taught a semester of P Chem, but my immediate puzzle is that I see heat as being both the source of useful energy if you have a lot (from nuclei or reduced carbon) and the useless end point ("heat death" back when) when its entropy is high. Do you mean that electricity is only a step along the way from low entropy/high G to a high entropy/low G state?
I actually never got around to answering the question, but was reminded of it when I came across this paper recently:
Solvent Effect on the Chemical Equilibrium of Ammonium Carbamate for Chemical Heat Pump Suzhou Dai, Yonggao Yin, Yikai Wang, Xuanlin Liu, Maurizio Peruzzini, and Francesco Barzagli Industrial & Engineering Chemistry Research 2023 62 (18), 7212-7223.
The device described in this paper is kind of a heat pump on steroids; it uses both compressional heat and the heat of chemical reactions to upgrade low grade heat to high grade heat, but to do this, it requires work, the expenditure of energy.
But let's cut to the chase. The correspondent is right about what I mean in the last sentence. It is fairly straight forward to show why this is true, if one understands that one statement of the second law of thermodynamics is "every process by which one form of energy is converted to another form will result in an increase in entropy," thus a loss of irrecoverable energy to the environment.
Before proceeding to the simple example, a little bit about entropy, what it is.
In general, the the increase in entropy is observed in the form of heat energy, although entropy can take other forms.
We are all familiar with irreversible processes. If we drop an egg off a table, it falls and breaks into pieces and goo. We never observe pieces and goo reassembling into the form of an egg. The shattering of the egg is an example of entropy. The development of statistical mechanics allowed Boltzmann to define what entropy is on a molecular scale, but before that, before Boltzmann entropy was known by the Clausius inequality:
The funny looking "delta" in the equality refers to the fact that this is an inexact differential; the way it integrates depends on the path. q is the standard variable used to describe heat, and S is entropy. If the equality holds rather than the greater than symbol, the process is reversible, but reversible processes in thermodynamics are idealized forms; they do not exist in real life.
As the nature of heat, then something of a mystery, was explored in the 19th century, largely the result of the invention of heat engines, in particular the steam engine, people realized that if you run a locomotive backwards, it never sucks all of the smoke back into the smokestack and reassemble the lumps of coal after sucking heat out of the environment. The locomotive is like a smashed egg. Clausius, writing in the middle of the 19th century - just as humanity was abandoning reliance on the weather for energy for a reason - basically was the first person to define entropy per se, building on the more intuitive work of the French engineer Carnot.
One does not need to have anymore than a knowledge of 7th grade algebra to see intuitively by looking at the Clausius inequality that a way to minimize entropy is to have the transfer of heat (by any path) is to do it at high temperatures. Thus having high temperatures is a route to minimizing entropy.
Now we are in a position to describe by example - I've done this before in this space - why I say electricity is thermodynamically degraded.
All heat engines of course run on primary energy. In a filthy dangerous fossil fuel plant this primary energy is the potential energy in the dangerous fossil fuel, coal, oil, or gas, but in order to recover this potential energy we combust the dangerous fossil fuel to make heat. Chemical energy is converted to heat energy. In a clean nuclear plant, the energy comes from the nucleus of a fissionable atom, usually uranium or plutonium, although neptunium, americium and curium would work too. Nuclear energy is converted to heat energy.
The common element is heat. Now let's look at the conversion of this common form of energy heat, and see what happens in making it into electricity, and moreover in storing it. The heat of the fuel is converted into steam energy (pressurized gas) at a thermodynamic loss - the boiler radiates heat to the environment. The steam is converted to mechanical energy at a thermodynamic loss, friction in the turbine, heat radiating from the housing. The mechanical energy is converted to electrical energy using a generator at a thermodynamic loss, friction, resistance in the copper wire, air resistance in the armature, etc. The electricity is then "shipped" (transported) over wires at a thermodynamic loss, the heat of electrical resistance.
Thus the electricity is thermodynamically degraded. Heat is converted to steam, steam is converted to mechanical energy, mechanical energy is converted to electrical energy, electrical energy is transported, four steps, each surrendering some of the value of the original heat to electricity.
A Rankine cycle (steam) power plant runs about 33% efficiency on average, depending on the temperature of the steam as well as the temperature of the surroundings: Powerplants are more efficient in winter than in summer. Thus right out fo the box, about 67% of the energy is wasted in just this step, steam to mechanical energy. This is really not surprising, there are lots of opportunities for steam to reject heat to the environment in these processes.
Now lets consider what happens in some of the more stupid fantasies that run around, batteries and hydrogen. The already thermodynamically degraded electricity is converted to chemical energy in the battery at a thermodynamic loss; most people can observe that lithium batteries get warm when they recharge; indeed under some conditions they'll burst into flame. Some of the stored electricity is degraded over time whether the battery is used or not, via internal diffusion. However, if the battery is used to provide electricity, chemical energy is converted back to electrical energy at a thermodynamic loss. If the electricity is used to drive a motor, the electrical energy is converted to mechanical energy at a thermodynamic loss. Four steps have now become (for an electric car) 7 steps. (It is true that mechanical energy can be converted back into chemical energy by electronic braking, but in this case 7 steps have become 9.)
Hydrogen is even worse, because in general, the hydrogen gas has to be compressed, and if liquified, cooled below its critical temperature, involving a huge amount of work (energy).
Almost all of the energy processes described release energy as heat, low grade heat. Low grade heat can be - often wisely - upgraded to high grade heat with heat pumps, but this also expends energy. Some heat pumps are better than others. The ammonium carbamate heat pump described in the paper referenced above is one that is new to me; on some level it should have been obvious, but I never thought of it. Industrially ammonium carbamate is a rather important compound, a key intermediate for example of some routes to urea for fertilizer and for SCR catalysts for diesel engines. Ammonium carbamate is synthesized by reacting ammonia gas with carbon dioxide gas under pressure. Both ammonia and carbon dioxide have been used as refrigerants, this process adds a little extra "kick" to their efficiency, but the second law of thermodynamics still applies to the system; it requires energy inputs to run, just like any other refrigerator.
It follows from the above that the most sensible way to store energy - if one must do so for load leveling for example - is as heat. Batteries and hydrogen are the worst, and most wasteful ways to store energy. I note that the direct conversion of heat to chemical energy, eliminating the degradation associated with electricity, is a thermochemical process. These are known, my personal favorite being the much discussed SI (sulfur iodine) cycle. There are others.
Have a nice day tomorrow.
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