[QUOTE=Stranger On A Train]
First of all, most commercial power is generated via a steam-based thermodynamic cycle. typically the Rankine cycle or some variation therefrom. There are other thermodynamic cycles and other media that can be used instead of water steam, but water is cheap, only very moderately corrosive (in pure form), and operates very predictably at a pressure-temperature range which is suitable for use with common metals and alloys. Of course, steam generation plants tend to be big, because the efficiency of steam cycles increases (up to a limit) with size. Also, because of the fixed nature of the installation and the bulk amount of heat-energy within it, even small losses are worth recovering via a regeneration or cogeneration cycle. (The BMW steam turbine cited previously is an example of a regeneration cycle, albeit one based on an Otto cycle process of the main engine.) A very small steam-driven generator has a lot more inefficiencies in terms of energy lost to heat and a lot less margin for recovering the small amount of lost heat; thus reciprocating piston and gas turbine generators are more effective for portable applications.
The problem with using waste heat from other processes to generate steam is that you have to have a sufficient temperature–>100°C at standard atmospheric pressure–to generate it; otherwise, you just have hot liquid water, which tends to just sit there and doesn’t expand very much at all. Steam is an effective power conversion media because of its expansion properties, which cause it to move at the high speeds that can efficiently drive a turbine. All hot water is good for is washing clothes or washing dishes. If you can’t concentrate the lost heat sufficient to create steam, you might as well just let it disappear into the environment. (To be fair, there are heat pump generation systems which can use a few thermodynamic tricks to magnify the heat in a liquid water system up to a useful temperature or otherwise provide cogenerative recovery, but the complexity is rarely worth the effort except in limited applications.)
Now, if you have an externally-heated process and a cycle with a partial vacuum system, you can create steam at much lower temperatures, albeit also with much lower power density. Something like a Sterling cycle writ large could, in theory, be very efficient even at temperature that are only modestly above the ambient heat rejection temperature. In practice, however, these systems are rarely practical given the current state of the art because of mechanical losses from friction and drag, and thermal conductive choke points that limit how much heat you can force into the recovery system. If you make a system that is really thermodynamically efficient, but it is too heavy or too bulky to fit within the form of a vehicle then it may offer great savings but be completely impractical to build and use.
So, we use steam all the time, albeit where it is most practical and efficient, i.e. large fixed installations. Everything you ever wanted to know about steam power, by the way, can be found in the classic Babcock and Wilcox Steam: Its Generation and Use, the traditional reference for all things technical about steam and steam-based thermodynamic cycles.
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Sure. I just think that being the 21st century, probably we could look once again at things that weren’t viable just a few years ago.
I think part of what I’m talking here is also about being able to design multi-source systems. If they can be produced it at a reasonable prices. They may not necessarily be disposable. But perhaps even useful for multiple things in some instances rather than a single task.
I think the notion of small steam is more than just that. It’s describing a mindset for to throwing everything we can, even in small increments, towards energy efficiency. Do we have to use just water for steam? Is something better available? I think if we have all kind of things like steam or whatever looked at very closely again, we might find just the right scale where different things work.
Thanks for the information and resources to my question.
[QUOTE=Stranger On A Train]
The problem with using steam in pre-Industrial societies is that they had no way to contain steam at high pressure. Without being able to contain or direct it in a vessel, you are limited by ambient pressure and natural transmissability to air for the amount of work you can have it do; essentially, you can use it to make a pig bladder fly or steam food, but making it drive a mechanical cycle requires reciprocating or rotating mechanisms with high tolerances not possible with hand-built agrarian tools.
Stranger
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True, it was the only the bronze age I believe. Still, again I wonder about the effects of continuous long term scaling up even over centuries. How long might it have taken, again with a determined effort?
