Charging Storage to Higher Than Needed Temperatures Wastes Wood?

[jebatty]

I had developed a hypothesis that heating storage to a temperature higher than that needed to provide heat until the next scheduled burn results in burning of a material excess quantity of wood than would result if storage were heated only to the needed temperature. I also had developed a hypothesis that heating storage to temperatures above 185F was inefficient as compared to limiting storage heating to 185F.

The data I collected does not clearly support either of these hypotheses, and based on the data for the burns described below, I cannot conclude that charging storage to a higher temperature results in wasting or inefficient burning of wood.

In Gasser Burns I posted a Wood Loading Chart. I have complete three weighed wood burns based on the Chart, each time using the 80% efficiency table and targeting storage charge temperature of 185F. These were the burns:

.             tank start     tank target     tank end       lbs per
date    lbs      temp            temp           temp          *F

01/11    84      138             185            175 **       1.71 **
01/13   119      117             185            175 **       1.70 **
01/15   139      105             185            187          1.70

** During both of these burns, the radiant called for heat and drew from storage; not realizing target temp was the result of the btu draw on the tank during the burn. There was no call for heat during the third burn, and I am assuming that tank end temp during the first two burns also would have been 187F.

Prior to these Chart burns, I winged my burns, roughly aiming for a 185-190F tank temperature at the end of the burn. Although I thought I was pretty good at estimating the final wood load by "feel and sight" to reach this temperature without idling, pretty good proved to be by no means very good. The four prior burns, showing these results, indicates I tend to burn to a higher tank temperature than my mental target:

.             tank start     tank target     tank end       Chart     lbs per
date    lbs      temp            temp           temp         lbs        *F

01/03   187       84             185+/-       187           172        1.82
01/05   156      103             185+/-       194           142        1.71
01/07   150      112             185+/-       197           126        1.76
01/09   176      107             185+/-       198           135        1.74

Based on lbs of wood per degree of rise in tank temperature, I believe the data does not support the hypotheses, as about 1.7 lbs of wood was needed for each 1 degree rise in tank temperature, regardless of the end temperature. Perhaps you might read the data to draw a different conclusion or have different insights.

I have found one clear benefit of pre-weighing the wood based on the Chart. I can burn the weighed wood load free of an anxiety that the boiler will idle before the load is fully burned.

 

[jebatty]

Incidentally, 1.7 lbs of wood per degree rise in tank temp, for a 1000 gallon tank, works out to 81% efficiency (based on available heat energy of 6050 btu/lb of wood). If I add 55 more gallons of water for the boiler and plumbing, efficiency rises to 86%.

 

[DaveBP]

Jim, for the 1/09 burn that took the tank temp up to 198F, do you have flue gas temp data?

Have you found that trying to squeek the tank to a higher temperature (to get through the coldest night or whatever) results in a higher flue temp?
The pounds/ degreeF column sure doesn't imply it.

 

[sparke]

I find the point of diminishing returns to be around 190-195* for tank temp. I have no evidence to back this up just intuition...

 

[jebatty]

I didn't log the stack. 380-450F is very typical over burn of a load. 197-200F is about max, as the draft fan shuts down right around 190-194F, and then temp may rise to 200 or so, if return temps are high, before it starts to fall again. I don't try to get over 190-195 intentionally, and with my weighed wood burns, the target will be 185F unless good reason to do something else.

 

[wood thing]

I tried half way last fall weighing my wood. My intent was to tie the data to degree days in order to estimate the amount of wood needed for the season. As firing became more often, weighing the wood became a pain, so I abandoned the project. Not sure as what the data means but I made an estimate. Hope its close.

 

[RobC]

I have 2 X 500G stacked propane tanks. My temp sensors are about 18 inches in from the top and bottom of both tanks. Where the sensors are placed most of the volume of tank water is between the tank sensors on each tank.
What would be a good way to get a single number to use for degrees F to start tank charging from ?
I was thinking of using your chart as a base for efficiency comparison. Last night when I started to charge my tanks read.
T1, 181 / T2, 176 / T3, 149 / T4, 131. I was thinking I could average T1 + T2 + T3 + T4 = 637 divide 4 = 159 ? Tanks at end were 191 for a difference of 32 degrees.

 

[Rick Stanley]

What is the diameter of a 500 gallon propane tank? In your set-up T1 is down 18 inches from the top??

 

[slowzuki]

It does waste wood. Research entropy as it relates to thermodynamics. Also, in more plain terms, the hotter the storage, the more btu's out the stack at the end of charging and more btu's lost from the tank. Hotter tank is only helpful to store more btu's in a given volume.

 

[jebatty]

Rob - Looks like you have an excellent setup and are achieving good stratification with very little mixing. Do you know whether during the charge burn your system also was drawing btu's from the tank, because if so, then your charge burn was greater than just charging the tank.

If you could do a charge with no system draw, then you can determine quite accurately what it takes to just charge the tank. Based on your readings, I think an average would be a good start. Your 32* difference at 80% efficiency translates to 55 lbs of wood (32 x 1000 x 8.34 / 0.8 / 6050). And I'm sure you know that when doing a burn based on weight, species of wood is irrelevant; but MC does make a difference. Did you happen to weigh your wood before the burn?

The 6050 btu/lb of wood energy is based on 20% MC and stack temp of 400F. Somewhere in the past I had info that indicated a 100F change in stack temp = 2 to 3% efficiency change. But it takes quite a bit more btu's to wring out excess moisture, and quite a bit less if the moisture is less. I don't have any info on conversion factors based on MC.

As you might have done, based on your tank draw down you can determine exactly what your heat load is based on the conditions at the time of the draw down. Did you do a heat load calc at any time before or after installing your system? How close is the measured draw down heat load to your calculated heat load?

In the end, it's what works, is not too inconvenient or time consuming, and achieves the results you want that count. Have fun with this and report the results, PM if you wish.

 

[jebatty]

It does waste wood....

I don't dispute the theory. If you have insights which might help in making a more accurate determination, I would appreciate the input.

What I can say is that based on my data set I cannot reliably determine a "waste" factor. Obviously there are many variables at work, and I chose to measure only those variables which seemed to me to have the biggest impact. I also chose 185F because in my system, as the fire is burning down on the last load, I still have sufficient delta-T and water flow to prevent idling at the end of a tank charge without much attention to slowing down the draft and/or increasing flow, both of which I can do. In the circumstances on which I reported it may be that the "waste" factor in moving from 185F to 195F +/- is small and beyond my capacity to accurately measure, or within a reasonable margin of error. I think I did achieve the goal of testing my hypotheses, and based on the results, loading storage to 185F+, if that is the choice, does not result in measurable waste.

 

[Nofossil]

Jim, nothing like real data.

Looks like you're right on track with your 'no measurable loss' conclusion. I'll agree that there has to be some additional losses, but there may also be offsetting efficiencies.

Besides raw efficiency, there can be other factors:

1) For those of us with unpressurized storage, it's a lot more complicated. Higher storage temperatures mean more idling since there's no way to transfer the full output of the boiler into storage as the storage temperature gets higher.
2) For those of us without radiant floors, the minimum usable temperature can be much higher, especially on cold days. A few extra degrees in storage can make a big difference in how long we can maintain adequate heat transfer from storage into the living spaces.
3) For me, it's also about convenience. I really want to use storage to allow me to build the NEXT fire at a convenient time. I'll happily sacrifice some BTUs on the altar of self-indulgence.
4) If storage is inside, the added loss from storage at higher temps just heats the house. If storage is outside (like mine) that's not the case.

 

[RobC]

Rick, tanks are .... 36" dia X 10' long. The T1, T2 etc, are all set in about 10" as drawn.
Jim, There is a demand for heat and DHW while tanks are charging this time of year. I won't be able to really zero in on BTU's used during burn. But I burn about the same time every day.
No heat Calc. Tied into existing system.
Wood. This is what makes this interesting. Wood weight makes all things equal. MC aside. I'm fortunate to have unused greenhouse that I keep wood in. I burn mostly Red Oak. No moisture meter but the stuff is dry so the bark is falling off most pieces. It's 2 years old. I can conclude it's dry and it's consistent.
Weight. Last PM I put in 137lbs, two partial loads. Tank max temps @ 191F top to bottom were around 1AM ( could have been earlier ) and boiler was still cycling around 4AM.
I way over shot my full storage BTU capacity. This time of year I do overload boiler to burn past max storage capacity in order to gain a few more hours before I need to fire the following evening. Last PM I was way over. Tonight I put in one load @ 100lbs and had a average tank @ 144F. I'm going to see where that takes me.

The 6050 btu/lb of wood energy is based on 20% MC and stack temp of 400F

400F is about where I run with the primary air cranked down some.
When my sweet wife came home last PM and saw me weighing fire wood on the way to being burnt she knew I had lost it !

 

[jebatty]

When my sweet wife came home last PM and saw me weighing fire wood ...

The next time she asks you, "Does this make me look fat?", you can say with confidence, based on your new knowledge of weight of wood, "Definitely not! In fact it's very slimming."

Yesterday afternoon I overshot also. Floor was demanding heat, and I made as assumption that the tank would drop another 10F to meet demand, so I added another 17 lbs of wood. Half of that would have been about right. I did log stack temp, haven't checked the result yet, but I doubt any idling.

 

[heaterman]

It does waste wood. Research entropy as it relates to thermodynamics. Also, in more plain terms, the hotter the storage, the more btu's out the stack at the end of charging and more btu's lost from the tank. Hotter tank is only helpful to store more btu's in a given volume.

In a perfect world far far from here where trees grow only 20% moisture content wood..............

Following Jim's line of thought and what you are saying it would also hold true that any heating system that requires water temps of over.......say, 130* also wastes wood. Systems such as fan coils or baseboard which need 140* or higher water temps would also be deemed energy wasters compared to a radiant slab which will heat a structure with 100-120* water. When it comes to transferring heat from the fire side of any type of appliance to the water side it's just common sense that the greater the temp difference between the two mediums (fire and water) the faster and more efficient the transfer becomes.

The chart on page 2 of this PDF graphically shows what happens to boiler efficiency as water temps rise. This phenomena holds true regardless of what fuel is being burned.

http://www.viessmann-us.com/etc/med....Par.66169.File.File.tmp/Vitodens_200_TDM.pdf

Note that raw efficiency drops nearly 10% when the water side goes from around 100* to around 170*.

Systems that demand high water temps are a far greater source of efficiency loss than storage in and of itself.

 

[woodsmaster]

With my boiler I've noticed that with higher water temps I get a cleaner burn ( bluer flame. ) I wonder If this offsets some of the loses in heat transfer efficiency ?

 

[DaveBP]

Having the room thermostats set any higher than the outdoor temperature wastes wood, too.

And Rob, the scene where the wife walks in and finds her husband weighing the firewood???? That's the part of the movie where you know that guy's life will never be the same.

 

[jebatty]

With my boiler I’ve noticed that with higher water temps I get a cleaner burn ( bluer flame. )

Sometime in the past I submitted a post which evidenced that the Solo 40 increased in boiler btu output as return water temperature increased, at least to the point of 160F return water. This means that insuring return water protection up to 160F results in higher boiler btu output. At that time I had a flowmeter on my system so I could easily compute btu's. My rational, fwiw, is that hotter water around the firebox resulted in production of more wood gas (and less condensation in the firebox) than with colder water. This observation is consistent with your observation of a cleaner burn.

Maybe this summer I will reinstall the flowmeter.

 

[Nofossil]

There are a ton of interacting variables. Data is the only way to make sense of it. Whenever you start to collect real data, you will find something that doesn't fit - that doesn't make sense.

When that happens, you have some choices:

1) Ignore the contradictory data
2) Fudge the numbers to make the data match your preconceived ideas
3) Dig deeper and learn something

Human nature favors choices #1 and #2. I strive for #3, if only because I find perverse pleasure in discovering areas where most of what people believe is wrong. It's surprising how often that happens. Happy to have others along on the journey.

 

[DaveBP]

With my boiler I’ve noticed that with higher water temps I get a cleaner burn ( bluer flame. )

Sometime in the past I submitted a post which evidenced that the Solo 40 increased in boiler btu output as return water temperature increased, at least to the point of 160F return water. This means that insuring return water protection up to 160F results in higher boiler btu output. At that time I had a flowmeter on my system so I could easily compute btu’s. My rational, fwiw, is that hotter water around the firebox resulted in production of more wood gas (and less condensation in the firebox) than with colder water. This observation is consistent with your observation of a cleaner burn.

Maybe this summer I will reinstall the flowmeter.

Be interesting to know what the high-tech gassers with lambda controls and all are using as the recommended return temperature. Any of you pros have some insight?

 

[heaterman]

With my boiler I've noticed that with higher water temps I get a cleaner burn ( bluer flame. ) I wonder If this offsets some of the loses in heat transfer efficiency ?

Aha!! The paradox of burning solid fuel.
What you are observing is very true. The higher temps in the firebox are the better you gasification will be.
There's a very good reason that the manufacturers of typical gasification wood boilers recommend and design their units around minimum return temps of 140-150*. Flue gas condensation is a very bad thing with solid fuel and has to be 100% avoided. So we have equipment that demands a relatively high water temp for proper operation and heating systems that demand low water temps for maximum efficiency.

Getting back to the perfect world.................. The perfect wood burner/system would be able to modulate output from 0% - 100% thereby eliminating the need for storage. It would also be able to handle any water temp from 70* on up to 200* if required.
Such a thing does not exist, nor will it ever given the constraints of the fuel itself and the physics involved.

And to the point of your observation, yes you are obtaining a cleaner and more efficient burn at a very small cost in heat transfer efficiency.

 

[jebatty]

So we have equipment that demands a relatively high water temp for proper operation and heating systems that demand low water temps for maximum efficiency.

And so the circle is closing. This is precisely where storage/buffer capacity offers close to the best of both worlds. Burn the boiler hot, store the hot water, mix down to cooler water. Now the modulation thing ... to eliminate storage ....

 

[heaterman]

Having the room thermostats set any higher than the outdoor temperature wastes wood, too.

That is absolutely true LOL but it kinda denies the reason for heating in the first place. We'd all have to move to the Florida Keys or something like that.

 

[heaterman]

So we have equipment that demands a relatively high water temp for proper operation and heating systems that demand low water temps for maximum efficiency.

And so the circle is closing. This is precisely where storage/buffer capacity offers close to the best of both worlds. Burn the boiler hot, store the hot water, mix down to cooler water. Now the modulation thing ... to eliminate storage ....

You got it Jim. Given the impossibility of perfectly matching the load with solid fuel equipment we are left with the reality that well insulated storage is probably the best option.

 

[RobC]

My results from last PM's burn get tossed. A few weeks back I worked on closing down my Tremovar to the point of allowing 147+ mix water entering the boiler, with the idea at the end of burn I would be pumping more water away from boiler. Had been burning two half loads to charge tanks. I had been noticing that it seemed that it was taking longer to charge tanks since I closed down Tremovar from wide open. A second thing I noticed was that there was a bigger differential between Boiler temp and Supply ( hot out ) temps. Last PM I put in 100LBS or a full load of wood. Some where about 2 hours into burn, boiler goes into overheat. So after a few minutes when things cool down and I start up again, I start opening up Tremovar and the boiler / supply, differential goes from 10F down to 3F once I get back to Tremovar fully open. Boiler stops climbing and cycling. With the Tremovar open, it looks like I'm actually pumping more heat way from Boiler. Go figure ?
Gotta go see what everybody is doing in house, so I can weigh some wood, and hopefully..... not get caught ......