Storage - Size vs. Efficiency

[Clarkbug]

Hello All.

So after harassing some of the members here, and paying a visit to a few different vendors, the path to gasification is at least moving forward. My wife and I got the old forced air furnace out of the basement, and managed to get most of the mess cleaned up.

My question at this point is about storage. We went to look at a Tarm Solo 40, which can run with no storage (in fact the sales guy has one at his residence, and has run it without storage for years). This is appealing, given the cost of installing storage. Before this, I went to go look at the Varmebaronen boilers, which I like, but the factory storage is really expensive for what you get (~$5000 for 400 gallons or so). Now they are nice insulated tanks with gauges and air purgers and one of them has the domestic hot water tank in it, but its still a lot of outlay at first.

I have talked with Tom of Maine about some of his unpressurized tanks, as I cant get a big tank into my basement without some major excavation and demolition to remove a wall, and Im just not up for that. So we are limited to a knock down tank, or using something like 120 gallon propane tanks, if I can find some of them locally. So my question is about what would be the better solution, to use something like 3x120 gallon propane tanks for pressurized storage, or to use an unpressurized tank at ~420 gallons? Yes, I know its not much, and I may be able to fit 4 of the propane tanks, or have Tom make me a custom rectangular tank to fit in the space I have.

But assuming the volumes above, what is everyones thoughts on the better install? I like the Varm better from a cleaning standpoint, but the Tarm has a local dealer, and can run with no storage if I need to delay that part of the project.

All thoughts are welcome!

 

[henfruit]

Have you looked at the vigas line of boilers?It will be worth your time to look at and speak to mark.

 

[PassionForFire&Water]

We do recommend 10 gallons of water storage per 2600 BTU of heat output to the water (or 50 Liter per 1000 Watt heat output to the water).
So for a gasification boiler that has 100,000 BTU/hr output this is 100,000 / 2,600 * 10 = 385 gallons
This "rule" is also used in Europe - EN305
I don't now if there is a best choice, however there is a best "economical" choice.
I would see which one of the 2 works out better and having any form of storage is better then no storage at all

 

[Clarkbug]

I have not looked at the Vigas line as Im pretty happy with either the Varm or the Tarm. Also, it looks like the Vigas has a poured refractory in the combustion chamber, and I would like to avoid that for ease of future replacement if possible.

Thanks for the 10 gallon rule, I have not seen that previously through all of my various searches here on the forums. I do think that some storage is better than no storage, its just a matter of being able to afford it or not....

The rub is that if I go with storage, I think I would go with the Varmebaronen boiler, as I like its ease of cleaning, and the boiler itself cheaper than a Tarm Solo 40. However, the Tarm is an option since it can be run without storage, and that is cheaper than the Varm + storage. (I know that sentence seems to contradict itself....)

 

[henfruit]

What do you mean by a poured floor?

 

[Clarkbug]

By poured floor, I meant that the refractory was cast in place, as opposed to a removable brick.

I dont know if this is the case with the Vigas, just what it appeared from the pictures I have seen, so I may be incorrect on that point.

 

[dogwood]

Marc, what is the reasoning behind the ten gallon rule?

Mike

 

[PassionForFire&Water]

I got it out of one of the EN-standards for wood boilers. If I have the time i will try to find it back.
As Europeans don't put a 5 liter engine in a small car, the same logic they follow to determine the "optimal" size of storage tank in relation to the BTU's the boiler kranks out.
There is a break-even point to everything.
Bigger is better is a "concept" that is not used in Europe.

 

[dogwood]

Marc,

Jebatty has posted calculations on how many btus you can store in a given sized tank which you'd apply to your particular application, btu usage, and hoped for times between burns. I was wondering if results from his calculations would bear out the ten gallon rule you allude to. I hope he reads this because I'd be interested in hearing his thoughts on this rule of thumb.

Mike

 

[PassionForFire&Water]

Marc, what is the reasoning behind the ten gallon rule?

Mike

Below a figure from EN305-5 on how determine the water storage or accumulator tank size.
The maximum storage capacity in this figure is 3000 liters = 800 gallons
The minimum storage capacity in the figure is 300 liters = 80 gallons

When you take a boiler with a capacity of 100 kWh (=341,000 BTU), what is already a substantial unit, you will more or less end up with a maximum storage of around 1000 Liters according to this figure (= 270 gallons).

This is where my reasoning came from, storage between 100 and 200 gallons.
Please, somebody double check what I'm doing here.

This still does not explain my rule of 10 gallons per 2,600 BTU. I need to sleep over this ... it's a while ago that I came up with this by now "famous" rule.

 

[tom in maine]

If you are going to consider calculating heat storage properly, you have to first calculate your heat load.
This is usually done at design temperature (-10F here in Maine).
That might leave you with storage that is large, but it is a question of how often you are going to want to fire the system.

Then you need to figure the temperatures that your heating loop can work with. Ideally, a radiant slab or a hot air distribution system will allow you the maximum use of storage.

In theory we can look at my house. It is radiantly heated. The heat load is 6,000 btus/hr at design temp. X 24 hours this equals 144,000btus for a day when the average temp is -10F.

If we ignore the DHW that I also draw off the same tank, let's say that the cutoff temp is 110F.

If I heat the tank to 180F and draw it down to 110F, the 350 gallon tank that I have will hold 180-110= a 70 degree delta. 70 degree delta T X 8.32# per gallon of water X 350 gallons of water = 203,840 btus stored in the tank that is usable for heat.

In the dead of winter, this allows me to fire the unit once a day. As it gets slightly warmer outside, I can go every other day.
My determining factor is when the DHW gets below 110F. I can still get usable heat lower than that temp, but I still want to have good hot water.

The math is simple. Calculating the heat load is not too difficult, there are calculators of www.builditsolar.com.

We usually default to either 550g or 820g since these are the most commonly used sizes.
It is better to know ahead of time, though, since there is expense in time and/or money involved. Planning is cheap.

 

[kuribo]

The heat load is 6,000 btus/hr at design temp. X 24 hours this equals 144,000btus for a day when the average temp is -10F.

.

That is very hard to believe....

 

[tom in maine]

The house is only 1100 square feet and is all R-65+ foam insulation.

 

[kuribo]

If that is the case then you could heat your home with a hair dryer-what are you doing with a gassifier that must put out 25 times your heat load?

 

[tom in maine]

That's a reasonable question.

First, electricity is 18 cents a kw here in Maine. So no hair dryer or electric heat. Maybe a heat pump, but not right now.
Second, I like burning wood, no oil or gas.
Third, we are developing/testing a wood boiler and manufacture tanks and heat exchangers.

And we now have half of a drainback solar system in place so the cord to cord and a half a year that I burn will be cut down to something else.

Is it practical, yes and no. A pellet stove would've been a better fit. But I like to play with fire and water.

 

[jimbom]

Marc, what is the reasoning behind the ten gallon rule?

Mike

Below a figure from EN305-5 on how determine the water storage or accumulator tank size.
The maximum storage capacity in this figure is 3000 liters = 800 gallons
The minimum storage capacity in the figure is 300 liters = 80 gallons

When you take a boiler with a capacity of 100 kWh (=341,000 BTU), what is already a substantial unit, you will more or less end up with a maximum storage of around 1000 Liters according to this figure (= 270 gallons).

This is where my reasoning came from, storage between 100 and 200 gallons.
Please, somebody double check what I'm doing here.

This still does not explain my rule of 10 gallons per 2,600 BTU. I need to sleep over this ... it's a while ago that I came up with this by now "famous" rule.

I don't have a boiler, so this may be gibberish:

This graph has nominal boiler size in kW times burning period in hours for an abscissa scale of kWH.
The ordinate is storage volume in liters.
Heat load of the building divided by minimum boiler output for various sizes are depicted by the sloped straight lines.

An example might be a 30 kW boiler that is fired for 5 hours = 150 kWH;
Building heat load of 25kW and boiler minimum power output of 10 kW = 2.5;
Graph reads 560 liters of storage.

If in this example, the building heat load was 10kW, 1600 liters of storage would be required.

In other words, if your building can't use the boiler output, storage has to be available to absorb the boiler output. Does this make sense?

 

[Duetech]

To my understanding of storage that makes perfect sense JimboM unless you have a system designed to "idle" when the heat load is satisfied. People have to "learn" the characteristics of thier system to know when to load or stop loading their system in relation to the actual weather they are experiencing or anticipate. (sometimes that's why we love the weatherman and sometimes not). Some of the figures used in this thread reflect radiant systems and very ample insulation and many setups do not reflect idealism. My system doesn't reflect too well but is still eocnomically more efficient than strict fossil fuels and the labor needed to meet my needs. In the end a lot is determined by what your needs and life style dictate and that often means storage larger than what is finitely required so as to allow the greatest flexibility to meet your life style and still not so big that it changes your life style.

 

[kuribo]

That's a reasonable question.

First, electricity is 18 cents a kw here in Maine. So no hair dryer or electric heat. Maybe a heat pump, but not right now.
Second, I like burning wood, no oil or gas.
Third, we are developing/testing a wood boiler and manufacture tanks and heat exchangers.

And we now have half of a drainback solar system in place so the cord to cord and a half a year that I burn will be cut down to something else.

Is it practical, yes and no. A pellet stove would've been a better fit. But I like to play with fire and water.

If 6000 btu/hr is your worst case heat load, I would think on an average winter day, say when it is 10F outside, if you have more than 3-4 people in your home, on a sunny day, you will have to turn on airconditioning or start opening windows....

 

[PassionForFire&Water]

Marc, what is the reasoning behind the ten gallon rule?

Mike

Below a figure from EN305-5 on how determine the water storage or accumulator tank size.
The maximum storage capacity in this figure is 3000 liters = 800 gallons
The minimum storage capacity in the figure is 300 liters = 80 gallons

When you take a boiler with a capacity of 100 kWh (=341,000 BTU), what is already a substantial unit, you will more or less end up with a maximum storage of around 1000 Liters according to this figure (= 270 gallons).

This is where my reasoning came from, storage between 100 and 200 gallons.
Please, somebody double check what I'm doing here.

This still does not explain my rule of 10 gallons per 2,600 BTU. I need to sleep over this ... it's a while ago that I came up with this by now "famous" rule.

I don't have a boiler, so this may be gibberish:

This graph has nominal boiler size in kW times burning period in hours for an abscissa scale of kWH.
The ordinate is storage volume in liters.
Heat load of the building divided by minimum boiler output for various sizes are depicted by the sloped straight lines.

An example might be a 30 kW boiler that is fired for 5 hours = 150 kWH;
Building heat load of 25kW and boiler minimum power output of 10 kW = 2.5;
Graph reads 560 liters of storage.

If in this example, the building heat load was 10kW, 1600 liters of storage would be required.

In other words, if your building can't use the boiler output, storage has to be available to absorb the boiler output. Does this make sense?

This absolutely makes sence!
That's why, in my opinion, the heat load of the house should determine the size of the boiler. Better to have a slightly undersized boiler then a super big satorage tank for an oversized boiler. It saves on space it saves on money.

 

[ihookem]

Even @ 6000 btu per hr. is hard to believe. I have a 2200 sq. ft ranch with r 30 walls and r60 ceiling. Some heat calcs say I need 20,000 btu per hr @ -10* f. I do have 9' walls with big windows and basement but still.