Wood energy: calculation volume of a thermal buffer

[Christophe]

Here is a method for determining the volume of a thermal buffer for a wood energy installation. It's empirical.

A) Estimated average power.

- Measure and calculate the volume of "chamber" of your boiler
Example with our DEOM: V = 0.05 m3

I use sawdust briquette so:

- 1 m3 of fuel = 0.66 T and 1 kg of wood = 4.9 kWh
(see https://www.econologie.com/pouvoirs-cal ... bustibles/)

- From where 1 m3 of fuel = 3230 kWh

- wood filling rate of the boiler volume during filling: 50%

- energy contained in the wood: 3230 * 0.5 * 0.05 = 80.9 kwh

- average power if the boiler is running at 10 a.m. (loading in the morning at 9 a.m. and recharging at 19 p.m.): 80.9 / 10 = 8.1 kW average

- the stove is given for a maximum power of 15kW ... so it sticks since the filling rate is 50% ...

B) Recoverable energy

- estimated combustion efficiency 90%
- heat exchanger efficiency estimated at 90%
- overall efficiency of the boiler therefore: 81%
- energy recoverable over the "day": 80.9 * 0.81 = 65.5 kWh.

C) Storable energy: what volume required?

- maintains the temperature at 80-90 ° C
- maximum working temperature delta: 50 ° C (therefore 40 or 30 ° C minimum temp.)
- we would like to be able to store 2 days of heating, i.e. 65.5 * 2 = 131 kWh (the equivalent of burning 15 to 16 L of fuel oil for info)
- thermal balance:

Energy = Volume * delta * capacity or Volume = Energy / Delta * Cp

Cp = 4.18 kJ / ° C. L of water
Energy = 131 kWh = 131 * 3600 kJ = 471 600 kJ
Delta = 50 ° C

Either volume = 471600 / (50 * 4,18) = 2250 L

Conclusion: it's not small ...

This is a large volume for a power that is ultimately quite low, but:

a) it is in the theoretical case where no calorie is taken for 2 days and no loss on the circuit before the tampon

b) it is for 2 full days (2 * 10 h) of heating which is quite important!

c) it is therefore necessary to take this value as a greatly increased value! Indeed: generally we do not start our wood boiler when there is no need ...

-> In short, the method needs to be refined with empirical tests. I could do a few with the editing I'm in the process of making ... so to be continued!

Edit after 14 months of use: in practice we are very far from this necessary volume. The factors to correct are I think:
- lower wood PCI
- Filling rate less than 50%
- Deom efficiency lower than 81% (60 to 65% I would say)
- Various losses in non-insulated pipes: - 5% let's say

To estimate the real yield (pkoi I have not thought about it before?) I will weigh a load of wood and I will watch the evolution of the T ° of the DHW tank over the same period without drawing a liter of hot water of course. Thus we will have an idea of ​​the actual effective return!

[david adv]

shouldn't we take the bp in the other?
"I have 100m² to heat, when it's cold outside, no sun, I need XXkwh (20-30-40?) per day to heat my house. I want x days of autonomy without running the stove."

what do you think?

[Capt_Maloche]

I do not quite understand your question, but in principle on a heating installation, a buffer volume is only used to avoid short cycles of the equipment.

For energy storage, there is only the inertia of the building when it is well insulated, the balloons but there it is huge, and the underfloor heating (the best system)

This is why with a house isolated from the outside, the whole mass of the house is used for storage and the inertia can be several days !!!

[the middle]

Is there no way to reduce the volume by putting refractory bricks in water for example?
It's just an idea like that ...

[Capt_Maloche]

No, the best calorific value is water, after concrete, there must be mercury but good

[the middle]

No, the best calorific value is water, after concrete, there must be mercury but good

So ... why, in electric storage heaters, they put refractory bricks, not water?
I don't know anything about it, it's just a question

[Christophe]

shouldn't we take the bp in the other?
"I have 100m² to heat, when it's cold outside, no sun, I need XXkwh (20-30-40?) per day to heat my house. I want x days of autonomy without running the stove."

what do you think?

Yes it is another approach. I think it's probably your method that is used when building a buffer heater by the pros.

Only here, in my case, the boiler we already have it so I preferred to use a method that the "source" rather than the "demand" ... since my reasoning was to make the minimum intervention at the level of the stove possible!

Can you not check with your method if it sticks?

We assume our needs at 50 kWh / m2.year (semi-passive house) or 7500 kwh / year or 20 kWh per day.

The 131 kwh given by a loading of the stove can therefore be enough for more than 6 days!
If we only want 2 days, the volume of the buffer can be divided by 2 ... but beware: the stove must be loaded accordingly ... I think that the "source" method is therefore more " secure "because the calories must be dissipated in all cases.

[Christophe]

So ... why, in electric storage heaters, they put refractory bricks, not water?
I don't know anything about it, it's just a question

Well, there have long been electric radiators with inertia in oil bath ...

For water I have already cogitated a little about the problem (improvement of the thermal capacity of wood stoves) and I see several reasons in the case of an electric heating not to do it:

a) security: water and electricity have never gotten along
b) held over time (tightness, leakage)
c) rise in water pressure and evaporation (filling): T ° limited to 100 ° C

There must be other good reasons not to do it ...

This idea reminds me a little of the independent inventor who wanted to replace the concrete counterweights of washing machines with ... water tank (more variable filling so much better than concrete if I remember correctly) ) ... it's been almost 10 years and we still sell concrete ... Industrialists don't like too good ideas it looks like ??

[Christophe]

Keep in mind, the heat capacity of the cast iron is around 0.5 kJ / kg.C ° to compare with 4.18 kJ / kg.C ° of water (i.e. 8 times more for water).

I found the value in this doc http://www.jeunesreporters.org/action/t ... ysique.pdf
which poses some funny energy problems ...

[Capt_Maloche]

So ... why, in electric storage heaters, they put refractory bricks, not water?
I don't know anything about it, it's just a question

because refractory brick with poor conductivity
suddenly it stays hot for a long time