Energy From Thorium Discussion Forum

It is currently Jan 20, 2018 11:35 am

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 21 posts ]  Go to page 1, 2  Next
Author Message
PostPosted: Feb 20, 2016 12:26 pm 
Offline

Joined: Jun 19, 2013 11:49 am
Posts: 1493
Well we all know that industrial steam is often required at temperatures in excess of 650 Celsius in the oil and chemical industry, amongst other uses.
Unfortunately current reactors are unable to produce steam at much above 250C and even advanced reactors will likely struggle to get much above 550C in many cases.

Would it be possible however to build a heat pump that could utilise 250C steam as a heat source for producing these very high temperature steams?

Would require a suitable working fluid but it seems like there is nothing physical that prevents this from being done. After all the saturated steam of an LWR or similar reactor releases almost all of its contained heat before the temperature drops much below the outlet temperature as the steam condenses.


Top
 Profile  
 
PostPosted: Feb 21, 2016 4:51 pm 
Offline

Joined: Sep 02, 2009 10:24 am
Posts: 507
How good are heat pumps anyway?

Currently, they're useful for a temperature increase of about 50C - at which point the COP is already down. Intuitively (been a long time since I studied this) I'd guess that's proportionate to absolute zero, so maybe they add 20% to the Kelvin temperature at a COP of 3. So 550K -> 660K?

But if you want to go from 550K -> 950K, you're COP is going to be well below 2, so it may not be worthwhile.

If that's good enough, then we have the technology to build compressors operating at 900 Kelvin - especially if you can find a non corrosive working fluid. But the market might be pretty small.


Top
 Profile  
 
PostPosted: Feb 21, 2016 10:56 pm 
Offline

Joined: Dec 22, 2015 8:07 pm
Posts: 3
So the idea is to take 250C steam from a LWR, and using a heat pump (powered by the same plant, presumably), take heat from the steam and upgrade the heat to 650C or thereabouts, with the sink being the industrial steam output? Another feed of 250C steam could be used as the starter source of the industrial steam, cutting down on the heat required to get to 650C...

Theoretical maximum COP* on a heat pump operating between 250C and 650C is only about 2.3, which doesn't look attractive. On the other hand, if you have 550C steam, then a heat pump to upgrade it to 650C has a theoretical maximum COP over 9, so if your heat pump isn't too expensive, a molten salt reactor could be a good source of 650C steam without having to push reactor temps beyond what's already been demonstrated.

I don't have any sense on the economics of this though; currently I'd assume some kind of fuel is used to get steam at these temps...


*Coefficient of Performance = units of heat moved per units of work used; I've occasionally found the use of acronyms without definition maddening (LWR seems a safe one to use on this board though).


Top
 Profile  
 
PostPosted: Feb 22, 2016 6:59 am 
Offline

Joined: Jun 19, 2013 11:49 am
Posts: 1493
Even with a COP of 2.3 over 95% of typical industrial steam boiler operating costs are the fuels required.

Considering the alternative is a high temperature electrode boiler with a COP of 1... it might be plausible to beat it significantly.
Even a COP of 1.5 would be quite something.


Top
 Profile  
 
PostPosted: Feb 22, 2016 10:53 pm 
Offline

Joined: Nov 14, 2013 7:47 pm
Posts: 569
Location: Iowa, USA
E Ireland wrote:
Would it be possible however to build a heat pump that could utilise 250C steam as a heat source for producing these very high temperature steams?

Would require a suitable working fluid but it seems like there is nothing physical that prevents this from being done.


I think finding a suitable working fluid is going to be the hardest part of all of this. You are asking for some crazy temperature swings and having the material be something that won't want to eat through your pipes.

I think that there is an easier way to do this. Use the 250C steam to drive a turbine, run a generator, and hook that up to some high temp resistive heating elements.

I got a heat pump in my house a few years ago and I spent some time figuring out how to make the best gain on my investment. Living in Iowa a heat pump is impractical for much of the year as the temperatures can get too cold for the heat pump to work efficiently. I looked over the COP of the heat pump given the different outside and inside temperatures. I also made note of the comparable cost of natural gas and electricity. I saw that if the outside temperature gets a few degrees below freezing I'd be better off with electric resistive heat, the COP becomes near one. The dollars and cents of it comes in at a much higher temperature, natural gas beats everything except in a very narrow window. While my house does not have any where near the temperatures you are talking about I believe the concept is similar.

If you have a nuclear reactor that can achieve 250C and you need 650C to drive your processing systems then I'd think you need to buy some natural gas or a new nuclear reactor. Even if you get a COP over 2 the amount of steam you are going to need to both preheat the final stage and drive the heat pump is going to be incredible. The math on the heat pump is just half of this heat engine equation, you are also going to need a low temperature heat sink to turn that 250C steam into mechanical power to drive your pump. What is the temperature of your heat sink? If you are lucky you got some 0C water. If you are not so lucky you got some 40C air.

A side note on my residential heat pump experiment, I found out it was not such a wise investment. I didn't know natural gas was going to get so cheap. I don't feel too bad about it though, it was a gamble and I didn't lose much on it. I just consider it an educational expense.

_________________
Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.


Top
 Profile  
 
PostPosted: Feb 22, 2016 11:45 pm 
Offline

Joined: Dec 22, 2015 8:07 pm
Posts: 3
Kurt Sellner wrote:
I think that there is an easier way to do this. Use the 250C steam to drive a turbine, run a generator, and hook that up to some high temp resistive heating elements.

Yeah if the heat pump ends up requiring exotic materials then the efficiency savings of it could be swamped by the capital costs. And in the current environment I'd guess that natural gas- or oil-fired boilers win out economically speaking.

Kurt Sellner wrote:
If you have a nuclear reactor that can achieve 250C and you need 650C to drive your processing systems then I'd think you need to buy some natural gas or a new nuclear reactor. Even if you get a COP over 2 the amount of steam you are going to need to both preheat the final stage and drive the heat pump is going to be incredible. The math on the heat pump is just half of this heat engine equation, you are also going to need a low temperature heat sink to turn that 250C steam into mechanical power to drive your pump. What is the temperature of your heat sink? If you are lucky you got some 0C water. If you are not so lucky you got some 40C air.

I think Ed's idea is to use 250C steam as the heat source, with most of its heat energy in the form of the heat of vaporization. After this is grabbed by the heat pump the resulting condensed water could be used for preheating, but wouldn't be used in power generation. Of course you need cooling for the reactor, and a low temp source greatly helps the efficiency of electricity production using the steam not diverted for use with the heat pump, but that consideration applies for any nuclear reactor. An advantage of this scheme compared to a residential heat pump is you can optimize for a single input temperature (250 C) and output temperature (assuming the industrial side can always use 650C steam or whatever you end up outputting); no idea how close you can get to the theoretical maximum COP though...


Top
 Profile  
 
PostPosted: Feb 23, 2016 3:15 pm 
Offline

Joined: Nov 14, 2013 7:47 pm
Posts: 569
Location: Iowa, USA
Matthew wrote:
Yeah if the heat pump ends up requiring exotic materials then the efficiency savings of it could be swamped by the capital costs. And in the current environment I'd guess that natural gas- or oil-fired boilers win out economically speaking.


Yes, you worded my concerns better than I did. If this turns out to be a Rube Goldberg contraption then that will show in cost, by just burning oil and gas the costs are known and likely much cheaper.

Matthew wrote:
I think Ed's idea is to use 250C steam as the heat source, with most of its heat energy in the form of the heat of vaporization. ...


Okay, I took thermodynamics a long time ago and I had to look up what you meant. I see that I was incorrect to account for the temperature of the heat sink and ambient air (both likely to be the same thing) since that will come into play regardless of what power source is used since all options must operate in the same environment. Even so I still believe that the complexity of the heat pump, and its inherent losses, will make such a system unprofitable. I'd think that nuclear powered electric heating, a new high temperature reactor, or just burning fossil fuels, will prove to be more viable options.

_________________
Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.


Top
 Profile  
 
PostPosted: Feb 23, 2016 4:50 pm 
Offline

Joined: Jun 19, 2013 11:49 am
Posts: 1493
The power cycle's hot reservoir at 250c would be the cold reservoir of the heat pump cycle.
Industrial heat pumps used to produce low pressure steam from warm waste water are commercially available.

Some heat flows from 250c to ambient and provides work needed to pump heat from 250c to 650c


Top
 Profile  
 
PostPosted: Feb 24, 2016 10:44 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5057
Hi Ed,

The theoretical COP with the Carnot assumption is 1.3 (not 2.3 as someone else mentioned). A real system would be under 1.2.

What are you planning to use as working fluid? It'd have to evaporate at 250C at reasonable pressures, and then condense at 650C again at reasonable pressures.


Top
 Profile  
 
PostPosted: Feb 25, 2016 2:39 am 
Offline

Joined: Jun 19, 2013 11:49 am
Posts: 1493
Are you sure Cyril?

Carnot efficiency of a heat engine operating between 650C and 250C is 400/(273+650), which comes out at 0.43.

The reciprocal of that value gives you the COP of 2.3


Top
 Profile  
 
PostPosted: Feb 25, 2016 6:12 pm 
Offline

Joined: Dec 22, 2015 8:07 pm
Posts: 3
Not sure if this is contributing to the confusion, but I realized my definition of COP above referring to "units of heat moved" was ambiguous, since heat is not conserved in a heat pump; it would have been better to say "units of heat delivered".

An ideal heat pump operating between 250C and 650C would, for each unit of work provided to it, remove 1.3 units of heat from the 250C reservoir and deliver 2.3 units of heat to the 650C reservoir (the work having been converted to heat in the heat pump's operation).


Top
 Profile  
 
PostPosted: Feb 25, 2016 11:43 pm 
Offline

Joined: Jun 19, 2013 11:49 am
Posts: 1493
How about mercury?

It boilts at 250-270C at roughly 0.15 atm and condenses at 650C at something like 10 bars as far as I can tell.


Top
 Profile  
 
PostPosted: Feb 26, 2016 1:30 am 
Offline

Joined: Nov 14, 2013 7:47 pm
Posts: 569
Location: Iowa, USA
I was also thinking about what could be used as the working fluid as I suspect the fluid used can affect the performance of the heat pump. I'll use my house heat pump as an example because I know some of the numbers, please forgive the use of non-SI units.

On a cool 60F fall day and 72F interior my heat pump operates with a COP of about 4 and can produce something like 32,000 BTU/hr. As the temperature falls outside so does the COP, obviously. When the outside temperature gets to about 30F the heat output of the heat pump drops to about 15,000 BTU/hr. When the temperature gets to about 0F the heat pump is still producing heat but with a COP less than one. To even operate the heat pump at such a low temperature I'd have to defeat the low temperature safety. At -20F the heat pump would not be pumping heat, at least not from outside to inside. At -40F the "working fluid" is more like a "non-working solid". I imagine that at this temperature, if I were to defeat the safety and run my heat pump, the compressor would be damaged.

Having the working fluid boil and condense at the right temperatures and pressures is certainly required for this heat pump idea to work. There's other properties to consider, such as will the working fluid freeze and expand if the steam isn't hot enough? What kind of safety features would there have to be to prevent pipes bursting? Perhaps I am preaching to the converted here but these are some of the issues that came to my mind while pondering this.

E Ireland wrote:
How about mercury?

It boilts at 250-270C at roughly 0.15 atm and condenses at 650C at something like 10 bars as far as I can tell.


How does mercury act at 10 bar and 50C? I honestly don't know but I imagine this would be important during a loss of coolant event.

_________________
Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.


Top
 Profile  
 
PostPosted: Feb 28, 2016 9:20 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5057
Matthew wrote:
Not sure if this is contributing to the confusion, but I realized my definition of COP above referring to "units of heat moved" was ambiguous, since heat is not conserved in a heat pump; it would have been better to say "units of heat delivered".

An ideal heat pump operating between 250C and 650C would, for each unit of work provided to it, remove 1.3 units of heat from the 250C reservoir and deliver 2.3 units of heat to the 650C reservoir (the work having been converted to heat in the heat pump's operation).


Thanks for pointing that out, that's what threw me off.

Yes, its 1.3 units moved and if we consider no losses on the electric-thermal conversion it would be a COP of 2.3.

What would a real system get in terms of efficiency? Maybe half the theoretical COP, 1.15?

Mercury sounds pretty nasty as a working fluid. The chemists are always looking for new ways to pollute the environment, eh?


Top
 Profile  
 
PostPosted: Feb 28, 2016 11:51 am 
Offline

Joined: Jun 19, 2013 11:49 am
Posts: 1493
Could probably do better than that - after all we have a system that can be optimised for precisely one input and one output temperature.

Might make 1.5 on a good day, if the unit is large enough to benefit from energy recovery turbines on the liquid return and such things as that.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 21 posts ]  Go to page 1, 2  Next

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group