[CliffordK]

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.

[neilep (OP)]

Thank ewe ALL for your continued very interesting links & responses here.

[peppercorn]

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 []

[damocles]

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.

[Geezer]

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.

[jgchemie]

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 newbielink:http://en.wikipedia.org/wiki/Phase-change_material [nonactive] that uses the newbielink:http://en.wikipedia.org/wiki/Latent_heat [nonactive] 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!

[peppercorn]

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.