Sodium Heat Engine

I’d like to describe this subject, according to memories I have, of how it was once described in material I read as a child.

In Physics and Chemistry, there often exist semi-permeable barriers, which are more commonly called semipermeable membranes. To call them membranes ignores the fact that pressure often exists across them, for which reason they need to be made thick. And thus, ‘barriers’ seems more appropriate to me.

Semipermeables can be such, that only certain ions are transported. For example, a barrier can exist that only allows sodium ions to pass, which are positive charge-carriers. What this means is that if we were to attach a pipe to each side of this barrier, which carries vaporous sodium in both cases, but with a difference in pressure, then the sodium will want to pass through the barrier. But the result will be that the pipe with the higher pressure, will also have a strong negative electrical potential. And the reason is the fact, that the only way in which sodium ions can pass through, is if electrons pass around the barrier.

By allowing electrons to flow, we allow sodium to flow from a higher-pressure source to a lower-pressure sink, where the ions recombine with the electrons.

I used the possibility of vaporous sodium to define the concept. But this is not strictly necessary. A solution of some kind, that bears sodium ions, should also work, as soon as their concentration is different on both sides of the barrier.

What I dimly seem to recall, was that ‘alpha-alumina’ would act as a selective carrier for sodium ions, at the temperatures of vaporous sodium. It would not allow free electrons to pass.



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.


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