Most lightweight method for storing heat?

[some bloke]

Hello everyone,

I'm aiming to produce a lightweight steam engine, which will essentially have a coil of copper tube inside the walls of a tube made of >your suggestions here<. this tube is the exhaust for a blowtorch.

The principle is that this tube gets very hot, then transfers heat to the water flowing through the coil of copper embedded in it, which becomes superheated steam. I want to store as much heat as possible, but with as little weight as possible.

I know that there are a lot of properties relating to heat, like specific heat capacity, but I don't fully understand them. I need some insight to help me to select the ideal material for this high temperature exhaust heat store.

What are the ideal properties I should be looking for? is the amount of heat energy it can store directly related to mass? is it an impossible task, and it's simply "the denser the better"?

Any feedback will be greatly welcomed!

 

[Keith_McClary]

Specific heat is defined as energy/mass, so you want to maximise that. Here is a table:
https://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html
The materials with the highest values may not be practical (hydrogen ).

 

[russ_watters]

I'm aiming to produce a lightweight steam engine...

I want to store as much heat as possible...

I don't understand; what does storing heat have to do with a steam engine? What is the purpose of the heat storage?

 

[anorlunda]

Temporarily closed for moderation.

 

[anorlunda]

The OP provided the following additional explanation. Thread re-opened.

The design is a tabletop model. The "boiler" is going to be open at all times, and is restricted by turning off the water feed to it, and not by closing it off, where it could create dangerous pressures.
it's essentially just a coil of tube, which has a flame blowing down the middle of the coil and water flowing through the pipe. the other end will blow steam through a small turbine and then the steam is released. I just wanted to maximise the heat energy captured by the device to increase it's efficiency.

 

[anorlunda]

The principle is that this tube gets very hot, then transfers heat to the water flowing through the coil of copper embedded in it, which becomes superheated steam. I want to store as much heat as possible, but with as little weight as possible.

I know that there are a lot of properties relating to heat, like specific heat capacity, but I don't fully understand them. I need some insight to help me to select the ideal material for this high temperature exhaust heat store.

What are the ideal properties I should be looking for? is the amount of heat energy it can store directly related to mass? is it an impossible task, and it's simply "the denser the better"?

What you describe is called a "subcritical once-through boiler." They were used in power plants for a while, but abandoned in favor of supercritical once-through boilers. The reason was unstable surges as the water boils. The surge might leave some portion of the tubes dry for short periods and they would melt holes in the tubes.

I don't think your propane torch will melt the copper. However, expect your steam flow and pressure to be unstable with surges in both.

You must also control the water flow in. If you feed to much water, a surge can put water into the turbine; that's bad. Such an occurance in a power plant turbine can destroy the turbine. But if you have a tiny home-made turbine and lower temperatures, it probably won't damage the turbine. Still, power out from the turbine will be very unsteady.

Unless you increase the pressure, there is a limited amount of extra energy you can store by superheating. Do you know how to use steam tables? If so, you can easily look up the numbers.

Maximum heat transfer you get by maximizing the length of the tube. The traditional spiral coil is the solution to that. I don't understand your point about density. If the tube is not pressurized, the density of steam is determined by the steam properties.

What are you planning to do with the steam exiting the turbine? If you discard it, efficiency will be very low. If you condense it, efficiency will be higher, but complexity of your project goes up.

I'm moving this thread to DIY. You may get better answers there.

Edit: Your use of the word storing in the title and the thread is confusing. Do you mean put as much energy into the steam as possible?

 

[some bloke]

My aim was to put as much energy around the steam generation pipes as possible - in the same way as a mass heater warms a room.

If (throwing in some random numbers here) the copper pipe and surrounding heat-retaining medium is at 300°C, it would help in transferring that heat into the water, and I'm thinking that this will produce a more stable method of steam generation, as it will level out any fluctuations in heat from the fire and cooling of the water. pumping water through 300°C copper pipe in heat retaining medium would cool the pipe down a lot less than pumping it through bare copper pipe, I thought.

My reference to density was about the heat retaining medium, whether it's possible to get an efficient one without it also being the heaviest option.

 

[anorlunda]

just a coil of tube, which has a flame blowing down the middle of the coil and water flowing through the pipe.

the copper pipe and surrounding heat-retaining medium

Those two statements are contradictory. Where is the heat-retaining medium? Between the pipe and the flame? Perhaps you can attach a sketch.

Were will you get the turbine? If it a commercial product, do you have the tech specs?

How will you regulate the flow of water in?

 

[Rive]

pumping water through 300°C copper pipe in heat retaining medium would cool the pipe down a lot less than pumping it through bare copper pipe, I thought.

Actually it matters only on short term. Adding buffer mass to such system helps only if your heat source has unacceptable fluctuations. But for a blow torch in a DIY demo system without much efficiency or power requirements - it'll just give you problems, no benefits. You want all the energy of the flame transferred to the water. Focus on that and keep everything else at the bare minimum.

Also, in an open system you might do better with the mass of steam maximized instead of the temperature maximized. Expecting some spill water on the hot end might help.