Storing Heat - Don;t Read If You Don't Like Maths :p

[Doodlebug]

Its late and my heads spinning with all sorts of ideas which I may wake up in the morning and realise are stupid, but they are seeming pretty sensible to me at the moment!

 

But if your not interested please don't read it!

 

As mentioned before we have issues with the stove making out little boat too hot when its first lit, and then in the hours that follow the boat then becomes cold. This is purely due to the fact that the stove needs to get hot when first lit to get the coals burning nicely. About 15 mins with open vents does it. This really does overheat the boat, and being a small boat it doesn't need much heating.

 

Ive done some calculations which suggest that if I was to put a tank under the bed holding 300l of water, that I could pump the water through the stove, thus heating it up, and then use the heat from the tank to help to buffer the heating of the boat.

 

So heres my calculations!

 

300l water is 300kg

 

Calculation for finding the amount of KJ water can hold is:

 

E = c_p dt m (1)

where

E = energy (kJ, Btu)

c_p = specific heat capacity (kJ/kg^oC, Btu/lb ^oF) (4.2 kJ/kg^oC, 1 Btu/lb_m^oF for water)

dt = temperature difference between water stored and the surroundings (^oC, ^oF))

m = mass of water (kg, lb_m)

So in my case it would be

4.2 (specific heat of water) x 40 (assuming the room is 20c and i heat the water to 60c) x 300 (mass of water used)

That makes 50400 KJ of energy stored

50400kj = 14 KWH

If the stove was stocked up once every 12 hours then the 12kwh of heat, if run down over 12 hours can give out 1.2kw of energy.

From what I can see that will help prevent the boat overheating, and will allow the heat to be released gradually, and will take off the excess heat that we don't want.

Does anyone agree?!

[MtB]

Sounds broadly right but I have had too much wine to check yr calcs.

 

The potential big hole in yer plan is the heat collection capacity of yer back boiler. How do you know what it is?

 

 

MtB

[oasis]

Your theorem holds good for total heat required but getting to grips with the rate of heat absorption or dissipation (from your heat sink, the water tank) is a harder equation with a lot of variables (tank surface conditions, air movement, etc).

I think it is a great idea though and beats mine which is modifying the natural convection of hot air throughout the boat. I am thinking of having a larger roof vent in the back cabin ( to remain legal) and closing off 2 of the forrad roof vents to force the air flow down the length of the boat. I'll probably be shot down in flames for mooting this but I enjoy thinking these things out.

[Captain Zim]

As above water may lose the heat a bit too quickly. One thing that is being used nowadays is sand or salt storage. How about a tank filled with sand heated by water passing through? It's not too different from those electric storage heaters that use concrete I suppose. Anyway, you could pass the pipework through the sand easily enough. Fewer problems if it leaks as well. Now, I've no idea about the specific heat capacity of sand, it's probably low so maybe you'd need too much sand? this is the problem with answering posts first thing in the morning...

 

Oh yes, I just checked. It's only 0.8.

Edited August 1, 2013 by Captain Zim

[Doodlebug]

In terms of the specific heat of the material used, water seemed to be the most practical. Our boat is not the fastest thing on the canal (by which I mean its painfully slow) and adding 300kg of sand will only serve to slow it down. Water could be pumped out if you were doing a long journey to improve fuel consumption?

 

What I did look into that I liked the look of was paraffin wax. It holds 4 times the amount of heat that water does, because it takes so much energy to get it to melt. Only issue is the best price I have got is £1 a kg for scrap wax. But thats £300 which is pretty expensive. Then again it would deffo hold enough heat.

 

I would insulate the tank on all sides I think, and then get a radiator and thermostat, so that when the temperature drops it pumps the hot water around.

 

It may not be perfect but its still looking like it will help quite a bit.

 

I would also have thought that if our stove is far to powerful for the size of our boat, that I would be able to put in as much coal as I would need to burn in a 24 hour stretch and burn it all at the same time. Then as long as I have enough surface area to pick up the heat i should be able to store most of it.

 

The stove has no back boiler at the moment, I was thinking of using microbore pipe coiled around the stove and chimney.

 

I'm amazed people think its a good idea

[Justme]

I did a similar calculation to work out what size thermal store I needed & what temperatures it needed to be heated up too.

 

The heat sources were a 2kw back boiler in a wood fired Rayburn & a solar thermal array.

 

Obviously I needed to do summer & winter calcs as the solar would not give much in winter & the Rayburn non in summer.

 

So I started with a cold tank & mapped the input & output over a 24 hour period for each season. The end temp of one day became the start temp of the next.

 

I fitted the system about 2 years ago & it does seem to be following the mapping quite well. You do need to know the standing losses from the tank as they can add up over time to quite a bit.

[Doodlebug]

How large is the thermal store you have?

 

I figure that I wont have any losses, the reason being that the tank will be contained inside the boat, with an air gap on every side after the insulation. Any escaping heat will then rise out and into the boat.

[Doorman]

Its late and my heads spinning with all sorts of ideas which I may wake up in the morning and realise are stupid, but they are seeming pretty sensible to me at the moment!

 

But if your not interested please don't read it!

 

As mentioned before we have issues with the stove making our little boat too hot when its first lit, and then in the hours that follow the boat then becomes cold. This is purely due to the fact that the stove needs to get hot when first lit to get the coals burning nicely. About 15 mins with open vents does it.

 

This really does overheat the boat, and being a small boat it doesn't need much heating.

 

 

I'm struggling to understand your needs here when you say that the stove makes the boat too hot when first lit, but in the hours that follow the boat becomes cold.

 

Surely by adjusting the stove after initial start-up to a comfortable temperature, the boat will remain hot, or is this too simplistic an answer? That's all we do with our Morso Squirrel. Also, the fact that the boat soon becomes cold would suggest that your insulation leaves something to be desired.

 

During my time in industry, we had a new engineering manager fresh out of university with so many theoretical thoughts about how best to maintain the factory. He would muse over multiple calculations all day long then put them into practice. We, in turn, would untangle his complicated ideas at great cost and replace them with common sense alternatives. The storage areas were full of his expensive failures.

 

Sometimes the answers are staring you in the face.

Edited August 1, 2013 by Doorman

[Doodlebug]

No, the boat is very small. The stove has a large number of fire bricks to minimise the burning space.

 

If I load it up with about 10 large pieces of coal and light it, It takes about 15 minutes to catch alight. if I dampen it down before it just goes out.

 

At 15 minutes I dampen it down. 15 minutes later the boat is far far too hot and needs the door to be opened.

 

The boat is then nice and warm for about 2 - 3 hours. Then the temperature gradually drops.

 

By morning, its cold, although if you check the fire there are still lots of glowing embers.

 

The heat storage would take the excess heat from lighting the fire and smooth it over the 12 hours.

[Doorman]

No, the boat is very small. The stove has a large number of fire bricks to minimise the burning space.

 

If I load it up with about 10 large pieces of coal and light it, It takes about 15 minutes to catch alight. if I dampen it down before it just goes out.

 

At 15 minutes I dampen it down. 15 minutes later the boat is far far too hot and needs the door to be opened.

 

The boat is then nice and warm for about 2 - 3 hours. Then the temperature gradually drops.

 

By morning, its cold, although if you check the fire there are still lots of glowing embers.

 

The heat storage would take the excess heat from lighting the fire and smooth it over the 12 hours.

 

It sounds as though your flue might need cleaning out if the fire goes out so easily. We light our fire with either firelighters or Fir tree cones and gradually bring it up to heat in order to avoid damaging the cast iron fabrick of the stove. Then, once lit it will stay ticking over throughout the night until morning when I simply riddle the grate, add more fuel and away it goes merrily.

 

To utilise the heat more efficiently, we had a back boiler installed from new and that feeds four double rads, this is a simple form of heat store and works very well for us.

[smileypete]

Maybe try starting the stove on smokeless then continuing with coal. The latter has lots of volatiles that need burning off which makes for a hotter fire initially.

 

cheers, Pete.

~smpt~

[Doorman]

Maybe try starting the stove on smokeless then continuing with coal. The latter has lots of volatiles that need burning off which makes for a hotter fire initially.

 

cheers, Pete.

~smpt~

Hello Pete,

 

Morso advise against using coal in their Squirrel stoves, it's brilliant for ruining those expensive fire bricks that they charge a king's ransome for!

[smileypete]

Hello Pete,

 

Morso advise against using coal in their Squirrel stoves, it's brilliant for ruining those expensive fire bricks that they charge a king's ransome for!

Ooop, forget what I said then.

 

Reading in between the lines it does sound like Doodlebug is using normal coal; if so the premium for decent smokeless should be well worth it and not cost a lot when heating a small space.

 

cheers, Pete.

~smpt~

Edited August 2, 2013 by smileypete

[Chalky]

There's been some interesting work in this area on using phase change materials to store heat. Google schatz battery / heat store.

 

I've also heard of someone having an engine cooling circuit that ran through the ballast which was insulated at the bottom and sides and used the ballast as a storage heater.

Edited August 2, 2013 by Chalky

[oasis]

"During my time in industry, we had a new engineering manager fresh out of university with so many theoretical thoughts about how best to maintain the factory. He would muse over multiple calculations all day long then put them into practice. We, in turn, would untangle his complicated ideas at great cost and replace them with common sense alternatives. The storage areas were full of his expensive failures."

He probably was trying to fix things that weren't broken. Whenever we had an improvement request or a project I insisted that my engineers throw theory at it before they attempted to modify.this saved a lot of tears.

 

Going back to the thread: It's a discussion on heat storage which is interesting, radiators aren't heat storage devices, quite the opposite. The OP wants to store excess heat for slow release during the night. His idea is good and the posts show an interest in the subject. I was looking at forcing the air convection down the length of the narrow boat and use the linings etc as the heat source ( hold a tissue to the saloon vent when you have a fire lit, you will see how much hot air goes outside, then do this down the length of the boat - the air has to start circulating.but remove the tissues after experimenting).

Edited August 3, 2013 by oasis

[Ships Cat]

You get thermal stores for houses so a boat should be no problem in theory.

 

In practice, thermal stores in houses need their own room. They're necessarily large. Typically when one's full it will give three days of radiators and hot water. Used often when people have a decent sized wind turbine, as the wind can be unreliable.

 

The back boiler takeoff should be rated on the paperwork that came with your MFS (or someone else's) Think a Squirrel is about 1.5kW.

 

Basically, if you can't run your stove flat out - ie at it's most efficient - it's too big for you. So your plan would be very useful. Space is the overbearing issue here.

 

I've seen an NB with a concrete ballast thermal store, too. Bloody good idea!

Edited August 3, 2013 by Ships Cat

[Doorman]

Going back to the thread: It's a discussion on heat storage which is interesting, radiators aren't heat storage devices, quite the opposite. The OP wants to store excess heat for slow release during the night. His idea is good and the posts show an interest in the subject.

I beg to differ.

 

If I had an ordinary stove and in our case situated at one end of the boat, once I've dampened it own for the night it soon loses heat and the boat's ambient temperature will drop accordingly.

 

With a back boiler installed, some of the stove's heat is absorbed in the central heating coolant whilst it's burning during the evening and then slowly released during the night through the radiators. Not only achieving a form of heat store but distributing it evenly throughout the boat.

 

Somewhat like a battery absorbing electrical energy whilst the engine is running, then slowly distributing that energy when the engine has stopped.

[Chop!]

So, a while back, you posted that you were too hot on summer days, now you post that you're too cold overnight, definitely sounds to me like you have little or no insulation.

[Doorman]

So, a while back, you posted that you were too hot on summer days, now you post that you're too cold overnight, definitely sounds to me like you have little or no insulation.

Simples, even without all of the calculus!

[Justme]

How large is the thermal store you have?

 

I figure that I wont have any losses, the reason being that the tank will be contained inside the boat, with an air gap on every side after the insulation. Any escaping heat will then rise out and into the boat.

 

Me?

 

260L

[Doodlebug]

 

Me?

 

260L

 

Thats interesting, so pretty much the same quantity as what I am thinking.

 

 

 

So, a while back, you posted that you were too hot on summer days, now you post that you're too cold overnight, definitely sounds to me like you have little or no insulation.

 

Not at all, If your in a boat in the middle of a heatwave and your nice and cool then I will be amazed. All I was saying when I posted that was that was that (during a heatwave) has anyone found any methods to keep the boat cool. And some people had interesting ideas worth trying.

 

Our boat is better than a fair few other boats in terms of insulation. The bottom half is covered with 20mm polystyrene and 50mm celotex. The top half is 20mm polystyrene but I am going to be insulating this further in the future.

 

Part of the problem is the size. Most of the heat is held in the air. When we open the door 50% of the air goes straight out the door. Most boats are large enough to still hold onto some of the heat.

 

I have posted lots of times before about the stove and managing the heat and having tried everyones suggestions I haven't solved it.

 

If I try it I will report back on how it goes.

[smileypete]

Would have thought the best way to store the heat would be in the form of slowly burning smokeless fuel.

 

cheers, Pete.

~smtp~

Edited August 3, 2013 by smileypete

[Doodlebug]

I meant to mention I only ever have used smokeless fuel. Ive tried all sorts, big, small, etc but the fact remains that when getting it lit the boat gets too hot and by the morning its cold. i could make it warm in the morning by doubling the amount of coal used but then its far too hot all evening.

[nicknorman]

I meant to mention I only ever have used smokeless fuel. Ive tried all sorts, big, small, etc but the fact remains that when getting it lit the boat gets too hot and by the morning its cold. i could make it warm in the morning by doubling the amount of coal used but then its far too hot all evening.

 

If the stove goes out too quickly, its because the heat loss from the stove is too great for the combustion rate, and so the internal temperature drops below combustion temperature and it stops combusting. One answer is to have more fuel and more air, but then as you say the boat gets too hot. So the other answer is to reduce heat lost from the stove, so it remains hotter for the same combustion rate. That allows you to reduce the combustion air flow without the internal temperature dropping to the point that the stove goes out.

 

So try an insulated flue and fire bricks. It that is not sufficient you could try some limited amount of (fireproof) insulation, but be careful not to overdo it or the stove might melt!

[oasis]

"I beg to differ."

Radiators, with their large surface areas and fins are efficient heat emitters, they are not storage devices per se. My contention is that the majority of the hot air, once circulation to the radiators is shut down, quickly escapes through natural convection - rising hot air going through the air vents. The OP's idea is to have a thermal heat store which is very interesting and worth debating. The various posts regarding heat storage and phase change materials etc which may be suitable is pertinent to the thread, also very interesting.