Materials with a high heat retention

[mhasson]

Dear all,

 

I am a design student, studying in South Africa and need help on a current project.

 

I am designing a barbecue that doubles as a small fireplace. the idea is that once the fire is out the fireplace can be taken indoors to warm the house.

So far my design can be simply put as 2 metal cylinders one inside the other - now I want to fill the gap with a material that has a high heat retention. the idea being that when the fire is burning (cooking food) the heat will be absorbed by this material, then once the fire is out the material will start to radiate the heat.

 

I have had a few suggestions from my lecturers including, sand, brick, stone, salt (NaCl) and ceramic grog.

 

I was wondering if there was any other material that could be better at this.

 

I have also heard that water is the best at this but I don't think that it will work for my application as the area I will be putting the material is sealed and if water will boil and possibly explode.

 

Any help will be appreciated!!!

 

Thanks

 

Mik

Edited May 16, 2011 by mhasson

[swansont]

You want a material with a high heat capacity. Another possibility is one which also has a large latent heat of fusion/melting, which can be even larger, which you would put inside of a container. That material will maintain a constant temperature as it solidifies. Lead, for example, has L = 871 kJ/kg but c = 0.13 kJ/kg-K. I think most high-heat-capacity materials are around around 1 kJ/kg-K, so finding the appropriate material to melt will be a benefit.

[ajb]

I have had a few suggestions from my lecturers including, sand, brick, stone, salt (NaCl) and ceramic grog.

 

These are the sort of things that spring to my mind, as opposed to anything metallic which would conduct the heat very well and cool quickly.

 

I don't know if you are interested in the physics, but the idea of heat capacity may be helpful.

[michel123456]

If you are using charcoal in your barbecue, the fire can remain latent for hours. You'll need a chemney and get problems when moving into a closed space.

 

The other way round is to enclose hermetically the fire immediately after cooking. You will get heat by radiation, but your barbecue-recipient will be under pressure. After cooling, you'll need a valve to make the air in, because it will be almost impossible to open simply by removing the cover.

 

-------------------------

You need to invent a portable version of a http://en.wikipedia.org/wiki/Masonry_heater.

 

In more recent applications, like electrical storage heaters , they use refractory bricks (fire bricks).

Edited May 16, 2011 by michel123456

[mhasson]

I am trying to keep the project as simple as possible and therefore wanted to use a simple material that I can cast or pour into the cavity. If the material were a solid this would also prevent me from having to have an airtight / watertight seal.

 

Would brick, sand, stones or ceramin grog be the best then?

 

 

[michel123456]

If you are using charcoal in your barbecue, the fire can remain latent for hours. You'll need a chemney and get problems when moving into a closed space.

 

The other way round is to enclose hermetically the fire immediately after cooking. You will get heat by radiation, but your barbecue-recipient will be under pressure. After cooling, you'll need a valve to make the air in, because it will be almost impossible to open simply by removing the cover.

 

-------------------------

You need to invent a portable version of a http://en.wikipedia.org/wiki/Masonry_heater.

 

In more recent applications, like electrical storage heaters , they use refractory bricks (fire bricks).

 

------------------------

edited

 

Here is another stove, a traditional belgian "poele de Louvain".

 

From bottom to up:

_four metallic legs upon ceramic or glass "shoes" for insulation

_a chamber with drawer for dust

_a spherical chamber in ceramic (same material as brick) which is the container for the burning coal.

_a horizontal plate. The plate is hollow (it is the chemney) and full of fire bricks. The plate is used for cooking. this one has also a lateral drawer, I suppose an oven.

_a chemney (build-in the back plate, hidden in the wall).

 

Your device would be the upper part (the sphere). You still need the chemney.

 

Note: the sphere gets really impressive because with a good fire, it gets completely red and radiates heat & light like a little sun.

Edited May 16, 2011 by michel123456

[ajb]

Would brick, sand, stones or ceramin grog be the best then?

 

Given their cost and availability I would suggest you experiment with these first. You want low thermal conductivity and high heat capacity.

[michel123456]

I remeber also how a traditional bread oven is made: it is an oven (like a pizza oven), a small dome made of bricks, in which the fire is ignited. when the oven is ready, the fire is taken out and the fresh dough is place to be heated by the bricks.

 

What you need to do, instead of putting the bread in, is to turn the walls of the oven inside out in order to radiate the heat outside.

[mhasson]

Thanks!! So from this I think that brick would be the way to go - apparently ceramic grog is basically fired ceramic which is then crushed might use this

[ewmon]

A concrete brazier. With sturdy concrete, metal cylinders may not be necessary. I also think wooden skids/skis/runners and a rope would help to drag/tow this stove/heater into and out of the house.

[michel123456]

Precedent link broken (thanks Ewmon)

http://en.wikipedia.org/wiki/Masonry_heater

Edited May 17, 2011 by michel123456

[mhasson]

Hi again,

 

I was wondering over my above problem if there was any material that was both light (in terms of weight) and had a high heat capacity - if such a thing exists.

 

I bought a whole lot of silicate pebbles which apparently is the best however my barbecue is now way to heavy for realistic use, any ideas??

 

Thanks

 

Mik

Edited June 8, 2011 by mhasson

[chilled_fluorine]

I've heard ruthenium is great for heat retention... But it might be a little too cheap for your purpose... LOLRemember, after whatever you use is as hot as it will get, it essentially becomes a heat sink, wasting your fuel. I would reccomend you put it in later on in the burn. Porous things (aerogel) tend to be good for extended heat retention, more dense ones (osmium) for capacity of heat per volume. The trick is to find a balance between the two. Iron powder could work, but it can be flammable. Lead powder would probably be too toxic. I would reccomend a very course iron or copper powder, but plain quartzite sand would probably be the best in my opinion.

[John Cuthber]

"I've heard ruthenium is great for heat retention"

I doubt that.

 

http://en.wikipedia.org/wiki/Dulong%E2%80%93Petit_law

[Enthalpy]

Aluminium is but better than sand, and graphite is worse.

 

Solar thermal power plants use molten nitrate salts. But for sure, a hot liquid if more dangerous than a hot solid, and hot nitrate spilled over a combustible material...

 

What if you heat soil, and let air flow through it later, to harvest the heat? Use separate tubes to avoid bad smells.

 

Or heat the walls of the home.

 

Or heat stones of reasonable size, which you can carry individually in the home. Then, technology brings only the suitable tweezers and the insulated container. You still need to avoid bad smell.