closed loop storage standby losses

[Piker]

I went on a short trip the last few days, which gave me the opportunity to do some standby loss testing on my storage tanks. My wife doesn't feel comfortable running the thermal storage yet, so we shut them off for 4 days while I was out of town and ran the boiler like a traditional wood stove/boiler.

The tanks have 4 sensors top to bottom, so I have a pretty good idea of what the average temperature was at both the beginning and the end of the experiment. Tanks are 2-500 gallon propane with 3" of closed cell polyurethane spray foam. Piping is such that the water cannot gravity feed out of the tanks when they are shut down. The 1.5" copper piping to the tanks is not insulated.

Here goes... enjoy...

Average tank temperature at the start - 126.8
Average tank temperature at the end - 111.4
delta t - 15.4
Tank volume - 1000 gallons
Elapsed time - 86 hours

[8.3(15.4)(1000)]/86 = 1,486 Btu's per hour

I realize these are pretty low water temps to be testing at, but that's what they were when I decided to run the experiment as I was dashing out the door on tuesday. If you take into account the mass of the steel tanks themselves, which have a very low specific heat of .12btu's/lb/*F, you can add another 45 or 50 Btu's per hour to that figure.

I thought some of you might enjoy this...

cheers

 

[sgschwend]

Those are amazing numbers around 1/6 of a degree per hour.

I always thought that was a bouncing insect.

 

[Piker]

Those are amazing numbers around 1/6 of a degree per hour.

I always thought that was a bouncing insect.

Nope, that is one happy elephant.


cheers

 

[Nofossil]

I'm assuming that these are indoors, do the temperature drop from the tanks to the environment was around 50 °F. Conductive heat loss (unlike radiant heat loss) is pretty much a linear function of delta T, so at full storage temp your loss would be about twice as great. Not bad for 3" of insulation. I'm outside with a LOT more insulation. My loss at around 150 degrees is between 1 and 2 degrees per day.

 

[stee6043]

You guys all kick my butt on heat loss, but I must say I've never tried it with such a low starting temp. I've left mine for 4 or 5 days before (indoor tanks). My starting temps were 180+ and I was seeing between 8-12 degrees per day standby loss. I used 11 rolls of R30 fiberglass to shore up my tanks. Since they are inside I guess I didn't get crazy surgical on insulating.

Nice job Piker

 

[Piker]

I'm assuming that these are indoors, do the temperature drop from the tanks to the environment was around 50 °F. Conductive heat loss (unlike radiant heat loss) is pretty much a linear function of delta T, so at full storage temp your loss would be about twice as great. Not bad for 3" of insulation. I'm outside with a LOT more insulation. My loss at around 150 degrees is between 1 and 2 degrees per day.

The tanks are indoors, but in an unheated crawlspace that's probably around 50* or 55* most of the time. The way the spray foam guy explained it to me was that 3" was the point of diminishing returns. I don't remember the numbers exactly, but 1" was something like 75% energy efficient, 2" was 90%, 3" was 94%, and it took 6" (or double the insulation and cost) to get to 95% or 96%.

I wonder if some sort of reflective radiant covering would gain much over what I currently have. When you're at 90+ percent, there's not a lot of room to improve.

I am sure there are a few btu's being lost between the 1.5" copper pipe that connects both the supply and return on the parallel install... maybe even some convection currents inside the pipe itself. Insulating these pipes might help a bit but I think the difference would be negligible.

1 or 2 degrees of temperature drop per day is pretty fantastic. Indeed, you must be insulated very heavily.

cheers

 

[Nofossil]

The tanks are indoors, but in an unheated crawlspace that's probably around 50* or 55* most of the time. The way the spray foam guy explained it to me was that 3" was the point of diminishing returns. I don't remember the numbers exactly, but 1" was something like 75% energy efficient, 2" was 90%, 3" was 94%, and it took 6" (or double the insulation and cost) to get to 95% or 96%.

I wonder if some sort of reflective radiant covering would gain much over what I currently have. When you're at 90+ percent, there's not a lot of room to improve.

cheers

From an engineering perspective, 'efficiency' is a pretty much meaningless concept when it comes to storage. However, the rule of diminishing returns holds true. At some point, the cost and/or bulk of insulation outweighs the additional reduction in heat loss. Each doubling of insulation cuts your heat loss in half. At some point that's a lot of insulation and not much heat loss.

I've never been able to find definitive data on radiant barrier effectiveness. I use several in my insulation, though.

 

[Donl]

I was watching a tv program that was showing the large indoor ski hill located in Dubai They commented on the engineering that went into determining how best to insulate such a large facility in the dessert. They said that of utmost importance was to maintain a large air gap between two layers of insulation. I wonder if this same philosophy can be successfully applied boiler water storage?

 

[Nofossil]

I was watching a tv program that was showing the large indoor ski hill located in Dubai They commented on the engineering that went into determining how best to insulate such a large facility in the dessert. They said that of utmost importance was to maintain a large air gap between two layers of insulation. I wonder if this same philosophy can be successfully applied boiler water storage?

Absolutely. There's a fine line, though. Too large an air gap or too large a distance between horizontal baffles and you get convection cells that carry heat from one surface to the other. Too small a gap and it becomes ineffective at blocking radiant loss. I really couldn't find hard data, though.

 

[DaveBP]

When it comes to radiant bubble roll type insulation, I think 100% efficiency is defined as the point at which you have extracted all the cash you're going to get from the customer.

Except in outer space, I think any reflective surface is going to get dusty and/or oxidized. It then becomes a non-reflective surface. And once the reflective surface is in contact with another surface it is no longer relflective.
Bubble wrap is sometimes the best insulation that is handy for some applications but I wouldn't put a lot of faith (or extra money) on the reflective aspect.