Mass Thermal Storage

[Menessis]

Hello from Canada

 

I like to tinker.  I have a decent work shop to build things.  I heat the shop with a Rocket Stove I built for instance.

 

I am intrigued with the sand battery technology.  I may end up building a WSHP.  We used to install WSHP years ago.  I know they work well.  So expensive if you need to hire a contractor though.

 

So I am working out mass storage (water) for heat pump but I want to play around with a sand battery too!  LoL 

Here is what I googled up trying to learn how to calculate heat storage.  Maybe some one here can check if I'm doing it right or not.

 

Thanks

Menessis

 

 

[SteamyTea]

Hello

 

We use some different terminology over here, and try very hard to stick to SI units throughout.

 

A couple of decades ago I did my BSc thesis in this very field.

While there is some space saving to be made, compared to using water, once insulation, extra floor reinforcing, pumps etc is included, there is no real benefit.  Water is cheaper than sand as well (I used granite chippings as it was a forced air design).

 

Which part of Canada are you in, it is a large country.

 

 

[Russell griffiths]

Who was the bloke who had a wood stove in his workshop with about 3 tonnes of concrete blocks built around it. 
 

cannot find the picture. 
anybody. 

[joe90]

26 minutes ago, Russell griffiths said:

Who was the bloke who had a wood stove in his workshop with about 3 tonnes of concrete blocks built around it. 
 

cannot find the picture. 
anybody. 

Reminds me of going ski ing and restaurant had a wood fire in a huge masonry “lump” with pockets all over to dry gloves in.

[Originaltwist]

It's not just about quantity of storage. Water is very portable energy ... much harder to pump sand.

[SteamyTea]

1 hour ago, Originaltwist said:

much harder to pump sand.

So true. Even passing air though 20mm granite chippings, inside a 100mm pipe in a experiment was hard.

 

 

[Menessis]

WoW a lot of replies.  But no feed back about my math.

I'm asking if I am doing the math right.

 

The same math works for water or sand or granite......

 

I'm asking if I am doing the math right.

 

The wood stoves with a huge mass works well because you burn that fire full throttle.  The heat sinks into all that masonry.  Once it's hot the room will stay plenty warm long after the fire is out.....   I didn't go that route in my little work shop.   Just the stove.  No room for all the mass.  And I don't want to wait for it to warm up out there.  It goes from -20'c to + 20'c in an hour.   An airtight stove might run around  250'f all day long.  A rocket stove retains the heat in the firebox to allow the combustion to reach a temp where the smoke will burn.  My stove goes to 240'f for about 10 minutes then off it goes.  The top will exceed 600' f.   I welded on 12 - 1  1/2" flat bar 18" long verticly all around the stove the second season. Helps quite a bit.  Also the 4" vent comes out the bottom at one side.  You can hold your hand on the vent.  Which means that stove is working pretty good.  BTW  the "fire pot" it only one brick square.

[Alan Ambrose]

It’s a BH tradition that we don’t answer the exact question that’s been asked. Instead, we riff on the theme like a seasoned jazz band 😂. Think of it as a more creative process than straight Q&A … akin to ‘art’.

Edited Saturday at 18:47 by Alan Ambrose

[SteamyTea]

1 hour ago, Menessis said:

But no feed back about my math

Terms are important.

 

Specific Heat Capacity of Materials, with the 'specific' being mass, as opposed to Heat Capacity which is by volume.

Often a subscript is used to denote if it is at constant pressure or volume so c_p and c_v.

 

So for water c = 4184 J.kg^-1.K^-1 where J is joule, the SI unit for energy, kg is kilogram and K is temperature.  Best to stick with kelvin as multiplication and division may take place.

Sand (quartz) is 830 J.kg^-1.K^-1

 

Then you have to get the energy into and out of the material, with water this is easy, sand not so easy as the thermal conductivity comes into play.

Thermal conductivity has the unit k and is measured in W.m^-1.K^-1 where W is a watt, which is a joule per second, J.s^-1, m is metre and K is kelvin.

Sand has a thermal conductivity k = 0.25 W.m^-1.K^-1.

 

Now you mention 'thermal mass'.  There is some debate about this term as it is a bad term.  When it comes to storing and releasing energy, the correct term is Thermal Inertia or Thermal effusivity.

This has the symbol e and the units are (kpc)^0.5 with p being density.

This washes out, by dimensional analysis as J⋅m⁻²⋅K⁻¹⋅s^−1/2 where s is time in seconds.

Now after over thirty year of thinking about this, I still do not understand the square root of time, but you will notice that there is a m^-2 which is area, which makes shape important as this affects the surface area.  The surface area is where the energy transfer takes place.  This means that there is no general formula for 'thermal mass'.  So don't use that term, ever again.

 

To get to the important part, which is for how long can you get energy out of a store, you can rearrange the above to make time, s, the subject.

 

s = (e / J.m^-2.K^-1)^0.5

 

Now the above is just about the potential energy levels

 

You then have to think about power delivery.

Water is easy, you just put it in a pipe, pipe it to where you want and then use gravity and density difference (thermo-syphon) to move the fluid, or pump it.

 

With a sand store, you have to introduce a heat exchanger, the design if which will change depending on temperature and power delivery.  This is not unusual in a water based system, but is much harder with a higher temperature store.

What fluid will you use at 573 K (300°C), not water.

As it will be a pressurised system, safety becomes important.  Especially if you are working close to a phase change temperature.

 

Now you may have got this idea from some recent developments where high temperature sand storage is being proposed.  These are industrial units, designed by experienced engineers in thermodynamics, materials, safety compliance, etc.  They are not small either.

Thermal losses are a function of area (A) and temperature difference ΔT.  This means that a lot of insulation is needed the higher the temperature and the smaller the store, negating any space saving advantages on a small unit.

 

Finally, and maybe this should have been the opening statement, temperature is not energy.  Don't confuse the two.

 

 

Edited Saturday at 19:01 by SteamyTea

[Alan Ambrose]

And why on earth would you imagine that the British use British Thermal Units? 😀 I guess we did in the 1900s.

 

They are useful though - see the following equivalents:

 

One Btu is approximately:

• 1.0551 kJ (kilojoules)
• 0.2931 W⋅h (watt hours)
• 252.2 cal (calories)
• 0.2522 kcal (kilocalories)
• 25,031 to 25,160 ft⋅pdl (foot-poundal)
• 778.2 ft⋅lbf (foot-pounds-force)
• 5.40395 (lbf/in²)⋅ft³

A Btu can be approximated as the heat produced by burning a single wooden kitchen match or as the amount of energy it takes to lift a one-pound (0.45 kg) weight 778 feet (237 m).^[16]

 

^{p.s. BH members were mostly all brought up on a healthy diet of Monty Python.}

[SteamyTea]

6 minutes ago, Alan Ambrose said:

And why on earth would you imagine that the British use British Thermal Units

We gave Canada away after we decimalised and agreed to use the SI system (1982, 1972)

Edited Saturday at 19:06 by SteamyTea

[joe90]

Sorry @Menessis, I have no idea about the maths and this lot are like trying to herd cats at times. I do however like the idea of your rocket stove and sounds ideal for my workshop. Can you link to your plans? 

 

come on guys surely someone knows if his maths is correct (whatever units he is using over the pond ) 🤷‍♂️

Edited Saturday at 19:17 by joe90

[saveasteading]

54 minutes ago, SteamyTea said:

Finally

I didn't read it but may do. Gets a heart for effort whatever.

[Menessis]

I may have a 3d cad drawing of the stove still....I look through the old pc and see what I can find.

 

The higher temps in the sand don't allow for water to be used to move the heat.  The industrial ones use an air heat exchange.  The air in turn heats water.  The water is pumped away to do its job.

 

In my example I came up with 69 million jules.  And then converted to BTU's.... over here we conmanly use BTU's when describing heating and cooling equipment.

 

So If I change my formula from sand to water by changing the "c" value to that of water it should work?

[SteamyTea]

6 minutes ago, Menessis said:

So If I change my formula from sand to water by changing the "c" value to that of water it should work

Well not at 573 K, it would be a steam engine then.

 

But yes, you can store energy effectively in water.  The big advantage is that everything is easily purchased.

 

I will say that if energy storage in fluids and solids was effective, we would have been doing it for centuries, burning fossil fuels is relatively recent.

[Menessis]

Ha ya I overlooked the delta T....my bad.

 

[Menessis]

Here is a pic of the woodstove.    I have a 3D cad drawing also.....must be on the really old pc!

 

[Menessis]

OK I had to google myself because I could not find the pics that I promised.

 

Here is a link to the forum where I was asking for help.  Most people told me it wouldn't work because the tube was too small.  So now 10 years later they can kiss my entire butt!  LoL 

 

And here is the guy that inspired my build ZeroFossilFuels

 

 

 

 

[MikeSharp01]

9 hours ago, Menessis said:

And here is the guy that inspired my build ZeroFossilFuels

Great - made perhaps before wider concerns about wood stoves but a good watch all the same. 

[SteamyTea]

30 minutes ago, MikeSharp01 said:

Great - made perhaps before wider concerns about wood stoves but a good watch all the same. 

Not great really.

 

I have had an early morning rant about it already.

 

 

[G and J]

16 hours ago, Menessis said:

WoW a lot of replies.  But no feed back about my math.

I'm asking if I am doing the math right.

I like to be helpful but this early on a Sunday morning the phsyics leave me empty and yearning for more coffee.  
 

So instead I’ll offer my two-penneth of help in another area.  I think in Blighty it’s either maths (plural) or arithmetic. Or are they calculations?    OK, I give up, I’ll go make coffee. 

[SteamyTea]

1 hour ago, G and J said:

I think in Blighty it’s either maths (plural) or arithmetic

I prefer arithmetic for most of it.

The geometry, trigonometry, statistics and calculus can wait for another day.

[saveasteading]

1 hour ago, G and J said:

. Or are they calculations

Sums. When it is +/-/×/÷.

Called calculations to impress those who can't do it.

Maths when pythagorus is involved, and beyond.

Then at some stage it becomes philosophy.

[G and J]

28 minutes ago, SteamyTea said:

I prefer arithmetic for most of it.

The geometry, trigonometry, statistics and calculus can wait for another day.

I think he’s nonplussed.   😉

[SteamyTea]

1 hour ago, saveasteading said:

at some stage it becomes philosophy

Mathematics is applied philosophy.

 

55 minutes ago, G and J said:

nonplussed

That will be subtraction then.