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# Heat Absorbing Materials

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Are there materials who can trap/absorb heat and become "supehot"? I need to trap heat and accumulate it to run a Stireling Engine.

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One can look up the heat capactance of materials to get that answer. The CRC is a good source of info. I believe the best materials are going to be metals, with heavier metals better for holding a lot of heat at high temperature? If you wanted a fixed temperature, maybe a phase change of a material might be useful. For example, some low temperature waxes can be melted with solar heat, and give off their heat of fusion as they lose the solar heat.

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Well it's for my fuel cell system - I want to trap the waste heat from the fuel cell. I have been looking at the heat capacity but I need some help on how to figure some stuff out.

To figure out the heat required to move x substance up y degrees the formula is,

Heat Required = (mass)(specific heat)(change in temperature).

Now I want to use it on, let's say, beryllium. I want to move 10 g of it up 10 degrees C.

Heat Required = (10.0)(1.824)(10.0) = 182.4 J

I think this is right. The problem is when I want to use the formula on something with the specific heat that is lower than one, like aluminum.

Heat Required = (10.0)(0.9)(10.0) = 90 J

Is that right? It seems too low to be right. But you never know. The brain plays weird tricks.

My last question. If I want to absorb the heat from the fuel cell, which heat capacity do I want, higher or lower? If the heat capacity is higher it takes longer to heat, but of it's lower, it can absorb less heat. What do I want?

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well I would say that depends - when you say you want to trap it do you want it to use for something else or as an insulator?

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I want it to absorb the heat and keep it... then let it go and trap some more. Like it acts as a sponge for the heat. Maybe a bit like insulating.

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then you want something with a higher specific heat capacity

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Metals will absorb and release the heat at the fastest rate. Whereas, something like a low melting temperature wax will absorb and release at a much slower rate. Between the two one should be able to get intermediate properties. Picture a honeycomb matrix of high conducting metal filled with low temperature wax. This would give one maximum flexibilty. Fast turn aronud via the metal matrix, and a reservoir for excess heat, that can even keep the temperature constant.

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Thanks a lot. That's the info I needed.

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• 2 years later...

Along these lines, what type of material do you think would be best for absorbing heat to utilize as a heat source for a heat engine. In other words to make a heat type battery, that would stay sufficiently hot long enough to generate pressure through steam or some other system. You would utilize this heat "battery" to create steam to turn a generator. Instead of burning something, it would just stay hot enough to create steam for a period of time before it cooled off, to possibly be re-heated and re-utilized at a later time.

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What's up Doc,

Solar concentrators, such as solar power towers, use molten sodium for the same purpose as you describe. Hope that helps. :bwa:

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Thanks Freestar, would molten sodium be a practical substance to handle by a lay person, either using special equipment or a special loader? Or is it too volitile to be practical, this is including the building of special equipment to handle it. Reason I"m asking is I'm curious in energy storage and I was wondering if heat could be stored and utilized for the same purpose in essence in utilizing a bunch of coal burning to produce heat/steam/pressure etc., thanks again.

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Thanks Freestar, would molten sodium be a practical substance to handle by a lay person, either using special equipment or a special loader? Or is it too volitile to be practical, this is including the building of special equipment to handle it. Reason I"m asking is I'm curious in energy storage and I was wondering if heat could be stored and utilized for the same purpose in essence in utilizing a bunch of coal burning to produce heat/steam/pressure etc., thanks again.

I would definitely recommend NOT attempting this. While it is possible, the risks far outweigh any benefits. I'll take a look in my CRC tonight and see if I can find something practical for you.

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Thanks! What I was thinking something along the lines of a large bar of some type of metal or other material, that could be heated to extreme temperatures, then placed in a compartment for the heat storage area to utilize in a heat engine, preferably something that could be used over again, after its cooled down, you could place it in a special heater and re-heat it, trapping the heat in again, kind of like a battery, but this would be essentially a heat battery.

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1st Law of Thermodynamics: You cannot win.

2nd Law of Thermodynamics: You can only break even on a very cold day.

3rd Law of Thermodynamics: It never gets that cold.

All you can grab is efficiency (co-generation). You confabulate heat (energy) with temperature (one of the things heat does). If you want a large thermal reservoir at a given temperature you need a first order phase transition to store energy as something other than temperature. Typically one melts a storage medium then gets back the heat as it solidifies (large latent heat of fusion, enthalpy of fusion).

However... First order phase transitions are typically accompanied by substantial volume changes - look up the densities and enthalpies of fusion of molten and solid paraffin; photographer's hypo; sodium acetate trihydrate, calcium chloride hexahydrate, sodium sulfate decahydrate; ice, water, steam). All schemes to store cold (freeze a big tank of water in winter then refrigerate in summer) or heat (molten nitrate-nitrite eutectic) look much better on paper than in steel. Such systems are remarkably clever about destroying themselves in large scale installations. The real world is non-linear.

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I don't know if this makes a difference, and I'm well aware of the laws of thermodynamics, including experiments I've seen that seem to violate those laws, but that's another conversation.

Point was is to heat something portable by conventional electricity (plugged into the wall) granted it would take a lot of juice to store your heat in your substance, but who cares about taking power from the grid. Point is to store it in a substance to then transport to a vehicle. Use the heat at one end to create steam/pressure whatever to spin a turbine to spin a generator, just like steam from burning coal, except your not necessarily burning anything, just heating something to extreme temperatures that creates a similar amount of heat as burning coal or what not. If that makes any sense, and I do appreciate you indulging a non-engineer.

Also, would it serve an efficient enough purpose to also utilize an extremely cold end (with say dry ice, a substance readily available) to draw an extreme at one end to help with the pressure?

Also, I realize your heat source would eventually cool down and the dry ice would eventually evaporate, but if you could get enough working time out of it, it may be worth it.

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• 3 years later...

what is the material which is most efficient in absorbing heat directly from sun rays?

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• 5 weeks later...

1st Law of Thermodynamics: You cannot win.

2nd Law of Thermodynamics: You can only break even on a very cold day.

3rd Law of Thermodynamics: It never gets that cold.

Yes indeed. Instead of saving heat, in a "heat battery," I would save fuel (or electric energy) from which heat can be generated when needed.

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