How The Use Of Steam Can Hinder Efficiency.

There exists a concept in Thermodynamics, which describes theoretical limits in the efficiency of all possible heat-engines. This principle states, that if we have a heat-source and a heat-sink, each has an absolute temperature. The ratio between these temperatures defines the highest-possible output of free energy from a heat-engine, as well as the lowest-possible consumption of free energy by a refrigeration-device.

The principle is based on the axiomatic assumption, that there exist no perpetual-motion machines, which simply convert ambient heat into free energy. If we could connect a heat-engine to an air-conditioner, and if these limits could be exceeded, we would have such a perpetual-motion machine.

This also explains why in practice, air-conditioners, refrigerators and heat-pumps can transfer heat from a colder source to a warmer sink, with the energy in heat far-exceeding the electricity consumed. They are all examples in which the ratio of absolute temperatures is close to 1.0 . Actually, what matters is the ratio of the temperatures of the working-fluid in each case, which is actually more oblique than the ratio for air temperatures, because heat-exchangers are never perfectly efficient. And the working-fluids used tend to be similar, because the temperatures at which those systems are designed to work, are also similar.

This also implies that if we wanted to build a heat-engine that uses small temperature-differences to generate electricity, large reserves of heat would be needed as a source, and sent to the sink, before even small amounts of electricity result – which might sometimes be available – but which constrains the system, regardless of what type of heat-engine is used.


Well, in Industrial Power Generation, the temperatures which the heat-source can be run at, depend firstly on what type of fuel is burning, but also depend on the range of temperatures at which water will boil. At 1 atmosphere of pressure, water only boils at 100⁰C, which is also 373K, while the external temperature tends to be around 273-300K .

Actually, by keeping the water boiling at much higher pressures, its boiling-point can also be increased. But it is generally not boosted beyond 200⁰C , which corresponds to about 473K . And so, according to basic principles, no power station based on water and steam, can be more than 50% efficient.

(Edit 05/12/2017 : Additionally, my late father, who was a professional Engineer, used to tell me, that something prevents a steam turbine from being more than 50% efficient. But, this is not a subject I know about, even though it would additionally limit the maximum efficiency of steam-turbine-based power-stations, to approximately 25%. )

In theory, if we could operate our heat-engine at 1000K, and its heat-sink still at 300K, we could achieve efficiencies closer to 70%. Mind you, that that point our heat-source might resemble a lightbulb, more than what we are used to, but this would still obey the rules of Thermodynamics.

My only point being, that the use of water, and its associated boiling-points, is an arbitrary decision. There is no magical reason why we must use it. We could use vaporous sodium if we knew how to work with it safely.

If one breaks out, a sodium-fire is a nasty hazard, much more dangerous in its nature than wood or oil-fires already are.

Dirk

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The Effect of Atmospheric Pressure, on our Industrial Revolution

My question has been:

Assume that there exists a species similar to Man, but on a hypothetical Earth, with an atmospheric pressure of 150 PSI – i.e. with 10x the actual atmospheric pressure. The atmosphere should still contain free oxygen, so that early, primitive tools and fire remain possible. But my Civ seems to have a very late-onset Industrial Revolution.

The reason for this seems to be, that internal combustion engines just wouldn’t work. An actual gasoline engine needs to achieve pressure-ratios of 1:8 or better, to be efficient, and should not need to be made from super-alloy. An actual diesel might seem prohibitively expensive already, just because it uses a pressure-ratio of 1:12 or so. You see, the fact that a simple, cheap solution already exists, can prevent our actual civilization from developing solutions that could ultimately be better. We still don’t see a lot of electric cars on the road, just because the internal combustion engines are so much more-affordable.

Flying-machines already exist, that use the infernal-combustion engines, so why try to invent (presumably more-expensive) flying machines that would be electromagnetic?

But on the hypothetical Earth, electric motors that spin propellers would work just fine. In fact, electric motors should work just as well, at 100x actual atmospheric pressure. So the obvious question should be, where would our counterpart-Civ derive its electricity, if not again, from an internal-combustion engine?

And so one observation which might be useful to me, is that drones such as our electric drones should work well for this hypothetical Civ. But then, that would also seem to be the earliest-possible onset of their version, of heavier-than-air flight. By that analogy, flight would have to wait, until they have invented a lightweight source of electricity, comparable in performance to our Lithium-Ion batteries.

But I don’t have my full answer yet. Our actual drones today are still not powerful enough, to lift a Man into the air and fly way with him. Apparently, Lithium-Ion Batteries are not lightweight and/or high-energy enough yet. Our Civ has gotten unmanned drones to fly, but no manned flight yet.

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