Author Topic: Small scale, efficient steam generator  (Read 1015 times)

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Offline elektrinis

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Small scale, efficient steam generator
« on: December 17, 2017, 06:15:55 am »
I have a theoretical problem on my hands. There is an industrial process, generating a byproduct in form of hot (25C) air with power of around 20kW. It's a real shame to see all that energy being wasted, very tempting to recover that in to electricity somehow.
I am obviously not going to build anything, just want to discuss this issue.

Now, currently the "industry standard" way of dealing with this is steam generators. Obviously, water will not work in this case, as temperature is only 25C. To pull this off, one would need a heat pump to up the temperature from 25C to above 100C to boil water and get enough steam to drive the turbine, or find other liquid, that boils at much lower temperatures, like a common r134 refrigerant. Now, you can't dump refrigerant to atmosphere, so you would end up with a closed loop. Which sounds good, as it would allow full energy recovery.
The more I think about this, the more closer I'm getting to a conventional heat pump, but with a turbine and alternator in place of expansion valve.

Again, obviously, if this were as easy, we would have many products by now, that do just that. So I am likely doing some false assumptions at a fundamental level.
I did find several patents on this matter (which does not mean much, but it's definitely not new idea), also a legend about a company offering such products in the '90s (that went bust). In is being referenced by some perpetuum mobile activists and I would certainly hate to end up in that mine field.
If there's someone here well familiar with heat pumps and steam turbines, I would love to hear some thoughts where my error is.
« Last Edit: December 17, 2017, 06:25:07 am by elektrinis »
 

Offline elektrinis

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Re: Small scale, efficient steam generator
« Reply #1 on: December 17, 2017, 07:44:22 am »
Well, it seems it's not impossible:
http://www.fujielectric.com/company/research_development/theme/heatpump.html

I consider Fuji a credible source...
 

Offline Gregg

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Re: Small scale, efficient steam generator
« Reply #2 on: December 17, 2017, 07:55:21 am »
If usable power could be extracted from 25 degree C air, otherwise useless politicians and other nefarious groups that blow a lot of hot air could actually provide some value for their presence.
Steam, small, efficient: you can only achieve two out of the three.  Steam is a wonderful energy transfer medium but it requires a large temperature differential and is only efficient when exhausted to vacuum.  Steam powered ships are reasonably efficient because they have large bodies of water to cool condensers.  Nuclear power uses steam because of the high reactor temperatures but the cooling towers are immense to get reasonable efficiency.  The problem with steam is all of the ancillary equipment and associated maintenance needed.
There are volumes written on the thermodynamics of steam; look at some steam tables for comparisons of temperature differentials and steam pressures.
A sterling cycle engine can be made to rotate with 25C heat, but it still requires a temperature differential to work.
 

Offline elektrinis

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Re: Small scale, efficient steam generator
« Reply #3 on: December 17, 2017, 09:46:40 pm »
Found a book on thermodynamics and gave it a good read. Now I get the complexity. Thanks for your input.
 

Offline SeanB

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Re: Small scale, efficient steam generator
« Reply #4 on: December 18, 2017, 12:37:42 am »
Is the hot air 25c above ambient, or is it 25C  above zero. Waste low grade heat can be used in either a Stirling engine ( reasonably efficient, though at 25c differential it will be poor) or a simple peltier thermal generator, which will give some power at abysmal efficiency, around 3% probably.  Will around 600W be useful power for this.
 

Online David Hess

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Re: Small scale, efficient steam generator
« Reply #5 on: December 18, 2017, 02:03:03 am »
Now, currently the "industry standard" way of dealing with this is steam generators. Obviously, water will not work in this case, as temperature is only 25C. To pull this off, one would need a heat pump to up the temperature from 25C to above 100C to boil water and get enough steam to drive the turbine, or find other liquid, that boils at much lower temperatures, like a common r134 refrigerant. Now, you can't dump refrigerant to atmosphere, so you would end up with a closed loop. Which sounds good, as it would allow full energy recovery.

25C is not hot.  The power extraction loop operates with a difference in temperature so unless you have a cold sink, 25C is not going to be a viable source no matter what working fluid is used.

OTEC (Ocean Thermal Energy Conversion) does what you are suggesting and has the same limitation.

 

Offline IanMacdonald

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Re: Small scale, efficient steam generator
« Reply #6 on: December 18, 2017, 08:32:44 am »
Indeed it's the difference that matters not the hot side temperature. However, there are fluids which will boil at 25C which would be more suitable than water.

Though, if the temperature difference is only 5C or so you would need vast quantities of air to get any useful energy out of it.

It's really the same situation as with hydro power; it's much easier to exploit a small but fast flow over a long drop than a large but slow flow over a small drop. For the latter to work you need some very well designed and very expensive turbines. For the former, a few tin cans cable-tied to a bicycle wheel will work.
 

Online rhb

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Re: Small scale, efficient steam generator
« Reply #7 on: December 18, 2017, 11:16:05 am »
How is the air being heated?  If it's by a heat exchanger, redesigning the heat exchanger to heat a working fluid to higher temperatures might improve things.
 

Offline elektrinis

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Re: Small scale, efficient steam generator
« Reply #8 on: December 18, 2017, 09:32:57 pm »
The air is being heated by industrial process, which can't be modified. Basically I need to keep the air at 25C, as it may go much higher than that, if not dissipated. Sadly, in my case the ambient air is also 25C.

It was pointed out already in this topic, but not very clear. So I will try to explain to future readers why this is so difficult.
This is a book I have read for the basics (1k+ pages): https://www.amazon.com/Thermodynamics-Engineering-Yunus-Cengel-Dr/dp/0073398179
It's available from TPB, of course. Really good and easy to follow book, I highly recommend purchasing it if you need it for actual learning, not just one evening like I did.

What I was trying to achieve is known as Rankine cycle: https://en.wikipedia.org/wiki/Rankine_cycle

After reading some more about thermodynamics, here are some basics. Basically you have a heat source (25C in my case), where your refrigerant is vaporized, then you have a turbine which generates electricity, after which your refrigerant drops in temperature and pressure and can be condensed back in to liquid form and pumped back for re-heating. The main issue is the condenser, of course, which will have to dissipate a LOT of energy to environment for this to work. In my case, since ambient temperature is also 25C, I would have to use another heat pump to extract heat from condenser and move it somewhere else, like back in to hot side... More on that later.

The problem is, energy can only by taken from a difference between hot and cold sides. Max theoretical efficiency of thermal engine is: ? = 1-Tout/Tin. T is in Kelvin.
This very well applies to steam engines and also to internal combustion engine. Let me explain, why efficiency of internal combustion engine is so low... Better yet, let's explore steam turbines in power plants. For example, max temperature of steam can be 565C (due to various technical limitations), and lowest possible condenser temperature is around 30C (if a lake or river is used to dump the excess heat). So max theoretical efficiency of such turbine is:
? = 1 - (303K/838K) = 63.8 %. That sounds not too bad, but in reality your steam turbine will not run at 30C... So you end up much lower, usually around 40% for single stage, in large scale, or closer to theoretical max if multiple turbine stages are used.

Now, let's calculate what we get at 25C. Let's say we use low temperature refrigerant, like R134A, and also let's assume we will be able to move the condenser down to -10C... So the max theoretical efficiency will be:
? = 1 - 263K/298K = 11.7 %

Here's an interesting simulation of Rankine cycle. Note the numbers.


So basically I feed 20kW heat in, take my 2.35 kW out, and dump the remaining 17.65 kW to somewhere.
2.35 kW "for free" sounds not too bad, but wait, there's more.

Remember, I assumed we will be able to keep the condenser at -10C. To do that, I would need another heat pump to take that 17.65 kW, up the temperature to at least [ambient+10C] and dump in to air. A COP of a typical REAL heat pump with this temperature difference (25+10+10) will be maybe up to 3. Sooo... To extract that 17.65kW, I would need to feed another 9kW or electricity to run the heat pump.

In the end I would get:
+2.35 kW from turbine
-9 kW to run heat pump
Which equals a loss of 6.65 kW.
So there you have it, it's a russian business.

Here's an example of industrial power plant, running from geothermal energy at 105C:
https://youtu.be/RAT6LK3aLNM?t=1m43s

They are proud to have efficiency of 10.9%. In their case, the cold side is likely around 30C, being dumped either to ambient air (or nearby lake), or another, much colder water bead underneath. Not sure how mother Earth feels about it though. Should be a warm furry feeling for sure.


On a further note on heat pumps.
Typical COP of an existing heat pumps varies between 2-4, depending on temperature difference and construction.
A theoretical COP, however, is much more optimistic and is determined by this equation, and in my case would be:
COPmax = Tcold / (Thot-Tcold) = 263K / (308-263K) = 5.84
So even if I would be able to achieve maximum theoretical efficiency of heat pump, I would still consume more energy to run it, than produced by the turbine.


Basically I have answered my own question and hopefully provided some basic information for someone else, googling for this in the future.
« Last Edit: December 18, 2017, 09:48:14 pm by elektrinis »
 


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