Naked Science Forum
Non Life Sciences => Technology => Topic started by: neilep on 25/09/2011 11:38:47
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Greetings,
Take a look at this piccy of the Gemasolar power plant !
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Nice eh ?
Being delivered next Tuesday as the centrepiece for my kitchen table
The Gemasolar power plant near Seville in southern Spain is the world's first solar power plant that can generates electricity during the night. The solar power plant comprises 2,650 solar panels that spread over 185 hectares of rural land with heliostat mirrors for 95 percent of solar radiation on a receiver at the center of the giant plant. It manages to generate energy during the night by channelling heat from it's arrangement into tanks of molten salt and these tanks reach temperatures of up to 500C !
How does it do that ?...and why molten salt ?..what's so special (energy wise) about very hot molten salt ?
Do ewe know ?..I don't...
I'd like to know !
hugs and shmishes
mwah mwah mwah
Neil
"It's Salty" [;)]
xxxxxxxxxxxxxx
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The big problem with electricity generation from solar is, of course, that the sun only shines during the day [::)] To get a good price from the electricity grid the generating station should be able to track demand as best as possible - So, for solar, this means storing a large amount of it's available energy after the sun has set.
So the engineers are looking for the best material that can:
- absorb a lot of heat in the range that the solar station can use (at temps suited to raising superheated steam).
- can take-on or give-up it's stored heat quickly (a good heat transfer coefficient).
- weighs as little as possible and is as compact as possible - w.r.t. the energy stored.
- can evenly spread the absorbed heat throughout, ie. not a solid.
The reason Molten Salt is so suitable is it is a Phase-change-material (http://en.wikipedia.org/wiki/Phase-change_material) that uses the latent heat (http://en.wikipedia.org/wiki/Latent_heat) effect to store far more thermal energy per kilogram than the same weight of some medium that does not change state. The best P.C.M.s in most cases are those that go from solid to liquid due the expansion of gases being an practical limitation.
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The big problem with electricity generation from solar is, of course, that the sun only shines during the day [::)] To get a good price from the electricity grid the generating station should be able to track demand as best as possible - So, for solar, this means storing a large amount of it's available energy after the sun has set.
So the engineers are looking for the best material that can:
- absorb a lot of heat in the range that the solar station can use (at temps suited to raising superheated steam).
- can take-on or give-up it's stored heat quickly (a good heat transfer coefficient).
- weighs as little as possible and is as compact as possible - w.r.t. the energy stored.
- can evenly spread the absorbed heat throughout, ie. not a solid.
The reason Molten Salt is so suitable is it is a Phase-change-material (http://en.wikipedia.org/wiki/Phase-change_material) that uses the latent heat (http://en.wikipedia.org/wiki/Latent_heat) effect to store far more thermal energy per kilogram than the same weight of some medium that does not change state. The best P.C.M.s in most cases are those that go from solid to liquid due the expansion of gases being an practical limitation.
Thank ewe very much Peppercorn. Yes...I understand now....I have one of those PCM packs for use as a heat pack....and then I have to boil the thing to get it back to a liquid.
Thank you very much for your explanation !....Presumably molten salt is not just molten salt ?...in other words..are there other chemicals added to achieve this reaction ?
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Thank ewe very much Peppercorn. Yes...I understand now....I have one of those PCM packs for use as a heat pack....and then I have to boil the thing to get it back to a liquid.
Thank you very much for your explanation !....Presumably molten salt is not just molten salt ?...in other words..are there other chemicals added to achieve this reaction ?
Indeed, you are correct about the 'heat pack' - same principle [:)]
Re: the molten salt - it is a molten salt (but not the table variety!) - I believe it is in suspension and has to do with crystallisation (on the cooling phase) of the salt for the release of heat. <I expect one of our more chemically knowitalls astute brethren can give a better (correct) explanation>
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Of course, it should be mentioned that this method of capturing energy would be
bugger all use not terribly effective in locations like, for instance, Glasgow.
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Sodium Chloride
MP: 801°C
BP: 1413°C
Molecular Weight: 23+35=58 (g/mol)
Density (solid) 2.165g/cc [0.037 mol/cc]
Heat of fusion: 2.6kJ/mol [0.097 kJ/cc]
Specific Heat Capacity 36.79 J/K*mol [1.37 J/K*cc]
Lead
MP: 327°C
BP: 1749°C
Molecular Weight: 207 (g/mol)
Density(liquid): 10.66g/cc [0.051 mol/cc]
Heat of fusion 4.77 kj/mol [0.25 kJ/cc]
Specific Heat Capacity 26.65 J/K*mol [1.37 J/K*cc]
Tin
MP: 231°C
BP: 2602°C
Molecular Weight: 119 (g/mol)
Density (liquid): 6.99g/cc [0.059 mol/cc]
Heat of Fusion: 7.04 kj/mol [0.411 kJ/cc]
Specific Heat Capacity 27.112 J/K*mol [1.59 J/K*cc]
Ok,
I looked up Sodium Chloride, Tin, and Lead (as representative substances)
Assuming I got my conversion right, by converting to Cubic Centimeters, one gets about 4 times the heat of fusion (per cc) with Tin as one gets with Sodium Chloride.
Obviously different metals, salts, and alloys will give you somewhat different properties, but the heat of fusion alone wouldn't drive me to choose Sodium Chloride over Tin.
Many metals (Tin, Lead, Etc) would tend to oxidize, and would have dangerous vapors. However, they could easily be stored under an inert gas.
I don't believe that Sodium Chloride would oxidize. Does it release Chlorine Gas? Sodium Chloride itself isn't particularly dangerous.
Anyway, we need to know what "salt" is being used, especially since Sodium Chloride has a relatively high melting point (above the specified 500°C).
It is possible that Salt is cheaper than Tin, or other metals/alloys.
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The information that the original questioner is seeking is nicely provided and explained at this website:
http://www.reuk.co.uk/Molten-Salt-for-Heat-Storage.htm (http://www.reuk.co.uk/Molten-Salt-for-Heat-Storage.htm)
Sodium nitrate/potassium nitrate is the salt system used. This is readily available and moderately inexpensive -- saltpetre.
One reason the salt is used as an energy storage medium is that it can give the option of thermal or electrical energy offtake.
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Many metals (Tin, Lead, Etc) would tend to oxidize, and would have dangerous vapors. However, they could easily be stored under an inert gas.
I don't believe that Sodium Chloride would oxidize. Does it release Chlorine Gas? Sodium Chloride itself isn't particularly dangerous.
Anyway, we need to know what "salt" is being used, especially since Sodium Chloride has a relatively high melting point (above the specified 500°C).
It is possible that Salt is cheaper than Tin, or other metals/alloys.
An interesting comparison Clifford.
But it surely is primarily an issue that the melting point of Tin or Lead are just too low.
[How toxic is Tin? Nothing like as bad as Lead, I think, but .... ? ]
As an aside though:
I wonder if Tin would be a good candidate for a fast warm-up system on a motor vehicle? (I think the Prius has a device like this). The melting phase for the device would, of course, not be workable via a traditional coolant (water/glycerol) but temps conducted from the cylinder-head (via a headpipe for example) could deliver the required heat, I think.
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But it surely is primarily an issue that the melting point of Tin or Lead are just too low.
[How toxic is Tin? Nothing like as bad as Lead, I think, but .... ? ]
I'll try to look up the Nitrates later.
I chose to look up Tin/Lead because they have a low melting point.
The MP of Zinc is 420°C
The MP of Aluminum is 660°C
The MP of Iron is 1538°C.
One could also choose an alloy, although the freeze/melt cycling might tend to crystallize certain elements in the alloy, and thus degrade the mix.
Tin is used as a coating in Tin Cans, although I think I remember there was some concern about toxicity, so they now varnish the insides of tin cans.
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Many metals (Tin, Lead, Etc) would tend to oxidize, and would have dangerous vapors. However, they could easily be stored under an inert gas.
I don't believe that Sodium Chloride would oxidize. Does it release Chlorine Gas? Sodium Chloride itself isn't particularly dangerous.
Anyway, we need to know what "salt" is being used, especially since Sodium Chloride has a relatively high melting point (above the specified 500°C).
It is possible that Salt is cheaper than Tin, or other metals/alloys.
An interesting comparison Clifford.
But it surely is primarily an issue that the melting point of Tin or Lead are just too low.
[How toxic is Tin? Nothing like as bad as Lead, I think, but .... ? ]
As an aside though:
I wonder if Tin would be a good candidate for a fast warm-up system on a motor vehicle? (I think the Prius has a device like this). The melting phase for the device would, of course, not be workable via a traditional coolant (water/glycerol) but temps conducted from the cylinder-head (via a headpipe for example) could deliver the required heat, I think.
How about sodium? I think they use it in some nukeyoular nookilar newclearer bugger - atomic reactors.
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How about sodium? I think they use it in some nukeyoular nookilar newclearer bugger - atomic reactors.
Metallic Sodium?
The melting point is pretty low: 97.72 °C, so you couldn't use the phase transition to boil water (at or above standard pressure).
And, of course, it is very reactive.
I think I've heard that it can be used in conjunction with nuclear power, but I'm not sure why.
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How about sodium? I think they use it in some nukeyoular nookilar newclearer bugger - atomic reactors.
They use Sodium in high performance exhaust valves to allow more aggressive timing without burning out the seats/valves.
It's a pretty tiny amount but I don't fancy riding around in a car with lots of molten (or boiling!) Sodium in!
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I chose to look up Tin/Lead because they have a low melting point.
The MP of Zinc is 420°C
The MP of Aluminum is 660°C
The MP of Iron is 1538°C.
Zinc sounds quite interesting temp wise, esp. with:
7.32 kJ·mol−1 !
Q: How hot would the hottest part of an engine's head get if it were in contact with a heatpipe (one that could manage temps of 450degC) instead of a water-jacket?
NB: using (essentially) water to cool the head means a rather high heat-gradient between the metal walls of the cylinder and the metal in contact with the liquid coolant. Having a correctly designed heatpipe(s) interface instead should not (as far as I can tell) lead to over-hot cylinders internally.
<Sorry to Neil for going off-topic BTW> [8)]
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Zinc is not a good idea -- it is notoriously volatile, with quite a high vapour pressure well below its normal boiling point
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Zinc is not a good idea -- it is notoriously volatile, with quite a high vapour pressure well below its normal boiling point.
Would it be so risky if kept well below it's boiling point?
I'm not talking about vaporising it (or even close to doing so).
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Here's a useful chart:
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from
Screening of high melting point PCMs in solar thermal concentrating technology based on CLFR (http://subversion.assembla.com/svn/eeb781/Report%20Documents/Resources/screening%20of%20high%20melting%20point%20phase%20change%20materials%20in%20solar%20thermal%20concentrating%20technology%20based%20on%20CLFR.pdf)
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A high dosage of zinc: Neti’s group believes encapsulated phase-change materials (EPCMs) offer a more promising alternative. (http://www3.lehigh.edu/engineering/resolve5/reaching-for-the-sky-2.html)
Thoughts..., huh?
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I like the idea of encapsulated pellets, although I'm surprised they don't run into problems with thermal expansion/contraction. One would also have to have a fairly thick shell of the encapsulating material, especially if they have several tons of the materials packed together.
Personally, I think I'd design some kind of a radiator type of heat exchanger, possibly with heat distribution fins.
Copper has a melting point of 1084 °C which might be high enough for the heat transfer, Nickel at 1453 °C, or perhaps one could use a tungsten alloy for the heat exchanger.
Assuming the whole system is sealed, vapors wouldn't be a big issue unless containment is compromised, at which point it would all be shut down, and would just need cooling.
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I like the idea of encapsulated pellets, although I'm surprised they don't run into problems with thermal expansion/contraction. One would also have to have a fairly thick shell of the encapsulating material, especially if they have several tons of the materials packed together.
It's only going from solid to liquid, so not going to be too much expansion - as long as there's a little 'room-at-the-top' of each sphere, so to speak. I got the impression from the article that the loading on the balls has been well factored in.
-I had a similar idea previously, for using enclosed balls or 'pills' (shape of medicine capsules) containing a PCM that could be sent down a pipe in contact with a hot 'source' (Though it sounds a little too much like a suppository now I come to think about it!).
Personally, I think I'd design some kind of a radiator type of heat exchanger, possibly with heat distribution fins.
Copper has a melting point of 1084 °C which might be high enough for the heat transfer, Nickel at 1453 °C, or perhaps one could use a tungsten alloy for the heat exchanger.
This would seem to be a perfect application for a heatpipe.
Heatpipe technologies seem a still underutilised tech in many fields, but not least automotive engine cooling (possibly not great with sustained exposure to vibration). I can't work out why it's better to transport engine-block heat by water rather than at temps much nearer to those at which they are produced - as a heatpipe could.
The melting temps (1084°C and 1453°C) you're mentioning strike me as very high, even for exotic solar or ICE exhausts. What would be their purpose at those temps - I can;t quite see it.
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I can't work out why it's better to transport engine-block heat by water rather than at temps much nearer to those at which they are produced - as a heatpipe could.
Cos water has a high specific heat, and you need something that's a lot colder than the hot thing to cool the hot thing down.
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Water is used with engine blocks, as it is a natural thermostat... keeps the engine temperature around 100°C, which is low enough to protect the oil from burning up (at least in theory).
Copper, of course, is an excellent heat conductor. It has quite a bit higher MP than Zinc, so assuming one is targeting temperatures close to the phase transition point, then it may be adequate, as long as it doesn't distort too much. A tungsten alloy would give it significant heat stability.
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Thank ewe ALL for your continued very interesting links & responses here.
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I can't work out why it's better to transport engine-block heat by water rather than at temps much nearer to those at which they are produced - as a heatpipe could.
Cos water has a high specific heat, and you need something that's a lot colder than the hot thing to cool the hot thing down.
True - but if the metal walls of the cylinder are at, say, 700degC+ then plenty of heat transfer will still take place across a heatpipe (the 'wick' of the heatpipe will vaporise the transfer medium if the 'fluid' is correctly chosen for the heat range).
Water is used with engine blocks, as it is a natural thermostat... keeps the engine temperature around 100°C, which is low enough to protect the oil from burning up (at least in theory).
Water as a coolant can have negative results as well.
As we know, having a largish thermal mass of water in the block when it's cold extends the period before which the engine can be at it's most efficient.
In fact for an engine that operates at continuous fixed revs and load (a gen-set for example) then I'd argue an air-cooled motor (if purposely designed for the job) should be as good, or better.
+ It should warm up quicker (admittedly not so much of an issue on a genny that might run all day and/or night).
+ It can deal with changing ambient conditions as easily if fitted with a suitable clutched- or electric- fan.
- The biggest factor against would be a potential increase in noise.
- Keeping the oil cool enough, especially around the [overhead] cam would be a potential stumbling block I agree, but I think it's all in the design ultimately.
Incidentally, there has been a prototype air-cooled motorbike with addition cooling (specifically around the exhaust valves) supplied by a heatpipe circuit (can't find the link right now).
Thank ewe ALL for your continued very interesting links & responses here.
And thank EWE for putting with my blatant off-topic-ness [:D]
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As we know, having a largish thermal mass of water in the block when it's cold extends the period before which the engine can be at it's most efficient.
In fact for an engine that operates at continuous fixed revs and load (a gen-set for example) then I'd argue an air-cooled motor (if purposely designed for the job) should be as good, or better.
+ It should warm up quicker (admittedly not so much of an issue on a genny that might run all day and/or night).
+ It can deal with changing ambient conditions as easily if fitted with a suitable clutched- or electric- fan.
- The biggest factor against would be a potential increase in noise.
- Keeping the oil cool enough, especially around the [overhead] cam would be a potential stumbling block I agree, but I think it's all in the design ultimately.
Incidentally, there has been a prototype air-cooled motorbike with addition cooling (specifically around the exhaust valves) supplied by a heatpipe circuit (can't find the link right now).
I think there were a few air cooled automobile engines back in the 60s and 70s. The beetle type VW may well have been one of them.
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I think there were a few air cooled automobile engines back in the 60s and 70s. The beetle type VW may well have been one of them.
There were lots of them. Citroen, Fiat, VW, Porsche, Pahard spring to mind. Problem was, they were not entirely air cooled. They relied heavily on the engine oil to remove heat, which is why the manufacturers advised the use of non-multigrade oil.
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The big problem with electricity generation from solar is, of course, that the sun only shines during the day [::)] To get a good price from the electricity grid the generating station should be able to track demand as best as possible - So, for solar, this means storing a large amount of it's available energy after the sun has set.
So the engineers are looking for the best material that can:
- absorb a lot of heat in the range that the solar station can use (at temps suited to raising superheated steam).
- can take-on or give-up it's stored heat quickly (a good heat transfer coefficient).
- weighs as little as possible and is as compact as possible - w.r.t. the energy stored.
- can evenly spread the absorbed heat throughout, ie. not a solid.
The reason Molten Salt is so suitable is it is a Phase-change-material (http://en.wikipedia.org/wiki/Phase-change_material) that uses the latent heat (http://en.wikipedia.org/wiki/Latent_heat) effect to store far more thermal energy per kilogram than the same weight of some medium that does not change state. The best P.C.M.s in most cases are those that go from solid to liquid due the expansion of gases being an practical limitation.
Thank ewe very much Peppercorn. Yes...I understand now....I have one of those PCM packs for use as a heat pack....and then I have to boil the thing to get it back to a liquid.
Thank you very much for your explanation !....Presumably molten salt is not just molten salt ?...in other words..are there other chemicals added to achieve this reaction ?
Too bad the Gemasolar power plant is using sensible heat storage and not latent heat storage. There is no phase-change occurring in these salts. Thermal expansion would destroy the storage tanks and the thermal conductivity for a Na/K nitrate salt is much to low for the heat to be efficiently utilized during a phase-change. We are still many years off from utilizing phase-change materials in large, utility-scale electricity production. But the comments about using alloys, Zn, or other salts or encapsulated phase-change materials are worth investigating!
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Too bad the Gemasolar power plant is using sensible heat storage and not latent heat storage. There is no phase-change occurring in these salts. Thermal expansion would destroy the storage tanks and the thermal conductivity for a Na/K nitrate salt is much to low for the heat to be efficiently utilized during a phase-change. We are still many years off from utilizing phase-change materials in large, utility-scale electricity production. But the comments about using alloys, Zn, or other salts or encapsulated phase-change materials are worth investigating!
Thanks for the info @jgchemie.
I didn't think the expansion would be that great for these salts, but for advantages to going to PCMs I would have thought the mechanical hurdles would have been overcome. Of course thermal conductivity is a genuine stumbling block if it is really too low.
Perhaps molten-zinc would be too expensive to scale-up to a system like this, but it appears that it might be useful on the more everyday, personal scale.
The sort of temperatures that it could allow could easily warrant the application of an Organic_Rankine_Cycle (http://en.wikipedia.org/wiki/Organic_Rankine_Cycle).