Storage heater bricks - max temp?

[Simon Cee]

Any idea what temp they get up to? Max temp for long term use? Investigating heat-store possibilities. Doing some sums.

tia.

[Dave Liquorice]

At a guesstimate well over 100 C.

Water at 4.187 kJ/l/K is hard to beat and being a fluid distributes the the heat through it's body by convection. Heat transfer into and out of is probably easier (less lossy) as well.

[ratsack]

But isn't what is used with the commercial heat stores used in houses etc.

Heat transfer into and

Its harder than using air with a fan which is what is normally used with the commercial ones. And the problem with leaks is eliminated too.

[tabbypurr]

They're cooked at somewhere around 1000C. In operation not that hot usually :) The limit is going to be insulation temp limit & loss.

NT

[Simon Cee]

Yup, but volume is a prob. 1000L of water wll store about 5kWh at my required temps. Bloody big tank! Hoping maybe bricks will make the whole package smaller*.

*Yes.. and then a safe way to extract the heat... ideas?*

[ratsack]

Yes, that and the lack of a leak problem is why the commercial ones don't use water, they use metal bricks in my case.

The usual thing is air moved with a cylindrical fan.

[tabbypurr]

water, oil, fan

I assume you're already storing what you can in the already present large masses eg floor slab

NT

[Dave Liquorice]

distributes

What are your "required temps" 5kWh doesn't sound like a particulary large big delta T for 1000l of water.

Wanders off to pre-existing water heating spread sheet:

1000 l, 5 K delta T, = 4187 * 1000 * 5 = 20,935,000 J = 5.65 kWhr

Only 5 K? Water based store would normally work over about 50 K (40 to 90 C) thus 1000 l stores nearer 60 kWhr.

Only a 1 m cube, or 2m high 80 cm dia, will weigh a tonne (literally) but rocks are denser... Dismantled a night storage heater a while back the blocks are surprisingly heavy for their size

You need to know their thermal capacity, and your operating delta T. The dismantled heater would have stored a maximum of 21 kWhr (ie 3 kW for 7 hours on E7) in a box overall 30 x 24 x 9" but with inadequate insulation as it would be fooking hot in the morning with room also well warm. So it should have been shut down and only leaking a small amount of heat.

With hot (2 to 300 C maybe) rocks convecting or forced air is about all I can think of.

[Martin Brown]

Their working temperature was never hot enough and their heat capacity woefully inadequate at least for houses when electric storage heaters were common. Phase change eutectic salt mixtures are the least bad option if you want to get optimum performance of heat capacity per unit mass. The snag is that they tend to degrade or separate with time.

[tabbypurr]

You can alays heat water if you limit the rate of heat flow to said water.

NT

[The Natural Philosopher]

I built a barbecue out of night storage blocks.

I got them to dull red heat once

[Tim Watts]

600C

[Dave Liquorice]

Phase change is the way to go if you can handle it. 1 kg of liquid water @ 100 C to water vapour at 100 C requires 2,270,000 J compared to heating 1 kg of liquid water from 0 to 100 C of 418,700 J

[Brian-Gaff]

I think the most inefficient part of my storage heaters is insulation. In my view all around the sides and bottom of the heat store should be much better insulated than it is, and th vents when closed should also use some kind of insulation to keep the bricks warm without wasting it. Brian

[Robert]

Useful article. Bottom line magnetite heat storage bricks have a similar Volumetric heat capacity to water.

[spuorgelgoog]

There are fan-assisted storage heaters which operate in the way you describ e, but they're very bulky and expensive.

Height: 66cm Width: 99m Depth: 26cm

Weight: 223kg

costs £620 plus VAT.

Owain

[Tim Watts]

I expect the bricks could go a lot hotter - but you are going to impair the heating element at some point and safe containment is going to become more of a problem. Last think a storage heater needs to do is rupture and drop red/white hot bricks all over its combustible surroundings!

[ratsack]

But are easy to use over a much bigger temperature range and so occupy much less volume and don?t have a leak problem.

[ratsack]

That gets red hot and works fine like that, just like it does in an oven.

and safe containment is going to

You certainly need better insulation.

They don?t rupture. The bricks are basically blocks of metal and even if they crack they aren't going anywhere.

[Tim Watts]

I know "they" don't.

I am merely pointing out that designing for say, white heat, is going to make the whole containment problem *much* harder. And let's say you do design for white heat - and the insulation fails locally creating a hotspot. You now have the conditions to warp or even melt the load bearing structure. So yes, it could rupture.

There's clearly a reason 600C is a popular design choice, and 1000C is not.