I think I made an Active Heat Battery

After a year of noodling on this, I recently put together a heat battery system for my greenhouse.

Passive thermal mass is often used in greenhouses to store heat during the day and release it during the night.  Often the mass is painted black to absorb sunlight.

I hope to have a full overstory in my greenhouse so I won't have sunlight on my tanks.  And I have a fair bit of elevation between the floor and the roof (17') to my advantage.  When it's sunny in January the greenhouse can hit 100F but the barrels don't get very warm.  I aim to change that.

The plan is to take hot air from the ceiling and blow it down to a radiator at knee level.  The radiator will circulate water from the tanks and actively heat the water.  The "cooled" air will help keep temps down a bit during the heat of the day.  Then on cold nights, the fan and circulating pump will come on (same direction of flow) and take cold air from the ceiling and blow it through the radiator with warm water circulating through it to heat the air.

I got a furnace fan from one friend and a water heat exchanger from another.  I took four 55 gallon drums and plumbed them together and added a Taco 007 circulation pump.  The fan is ducted to the ceiling with two 10" flex ducts (only one is hooked up so far).

Purging air from the barrel array was interesting but there was only one leak  I put standpipes in each tank so that the water enters the top of the tank and then leaves up a straw that starts near the bottom of the tank.  I'm using the two standard bung fittings on top of the blue barrels.  This is to ensure good mixing of the tank.

Turning it on with warm greenhouse temps gave some good results.  The air down low was 86 degrees and up high was 98.  One side of the fan was hooked to its ceiling duct, the other was sucking in knee level air.  Let's say the average was 92F.  The water going through the radiator was clearly being warmed up.  With my fancy non-contact thermometer it showed a temp increase from 75 to 81F.  The air coming out of the radiator was 85F.

So I got about 7 degrees of air cooling and about 6 degrees of water heating.  And the air wasn't even that warm.  I'm guesstimating the circulating pump was moving about 20 gpm of water.  If anyone knows more about those pumps and their actual flow rate, I'm all ears.  The head is under two feet but the main restriction is the heat exchanger.  That has 3/8" pipes but with the plenums I don't think the resistance is that great.

If my math is correct I was storing about 1000 BTU per minute (8.33 lb/gal * 20 gal/min * 6 degrees).  The full contents of the tanks should have increased by 6 degrees in 11 minutes.  Their rate of heat intake will drop off as the temperature difference reaches zero but it seems like they would have reached max temp (say 90F) after two to three more passes through the barrels (22-33 minutes).  So in well under an hour the tanks would be holding 27,500 BTUs.  If the tanks started at 40F, they'd increase by 50 degrees and end up storing 91,600 BTUs.  I'd have to dust off my calculus book to figure out if they could get to max temp during a short January day but my intuition says they will.

The current plan is to control the fan/pump with two thermostats, one up high to run in "heat" mode and one down low on "cooling" mode.  An arduino/raspberrypi controller would be even better but I'm too mechanical for that sort of stuff

Wiring for the thermostats and connecting the second duct is on the to do list but I'm happy so far.

Mike, that is a pretty ingenious idea.  The only part I don’t like is that I didn’t come up with it!

Well done,

Eric

Thanks Eric!  I didn't invent it though, I just adapted from other things I'd seen around. Gilbert Fritz has something similar he's been working on: My fan-coil active thermal mass greenhouse.  I saw a youtube video of a box fan and a radiator in a dome greenhouse somewhere as well.

hey mike

Great idea.

I find myself wondering about the amperage needed to run a fan like that. could you share what it is? Along with the amperage of the water pump.
I see you saying you are wondering what this will perform like in January. And being that i am off grid. using electricity for a fan and a water pump for x amount of hours a day would require me to run a generator to offset the bonuses of this system. maybe the fan is operated from a solar panel directly?

my simple mind says depending on the amperage of the fan. could the tanks be heated with the same amount of power? meaning minus the fan and continue to use the pump with a heater of the equivalent power wattage as the fan? I am thinking a large aquarium heater.

cool design

NICE! I've been wanting to build one of these for years... I like the cascade setup on the tanks.

I've also done some experimenting with a large tank buried in a compost pile, and after fill-up with 55*F water, I tried running my submersible pump hanging sideways and without a discharge hose for a few days to circulate the water around (the round) tank.
In a week the temperature climbed to over 90* (the compost was over 120*F).
I retold this to an engineer friend, his first reply was "How many watts is your pump?" (suggesting that the heat was in large part due to the pump... it was not, but also not insignificant (in the 5-10% range?))

Which brings us to Jordan's comment about your fan and pump.

It would be good to calculate what heating/cooling load the equipment contributes.
For heating, you'll get a more accurate picture of how the storage tanks perform.
For cooling, it might change your "break even" point of running the system vs. venting to outside (need for night-time heating vs. daytime cooling) Maybe there's even a summertime "reverse mode" where you run at night and store "coolth" in the tanks.

The fan came from a buddy so I'm not intimately familiar with its specs but I'll take a look.  The pump is 1/25 hp so I'm guessing it's pretty low on the amps.

My only goal with the system is to store heat in the winter and deliver it back in the evenings.  Any cooling or dehumidification I get from it is purely a bonus.  It will definitely sit idle from April til October.  I can easily ventilate the greenhouse in summer but I wanna keep all that heat in during winter.

By bringing all the hot air near the cool floor, you are also warming up the surrounding soil not just the fan coil. You are also de-stratifying the air too. All good things for the plants.

@Kenneth
I would love to see you redo the experiment by just burying the tank of 55F water into the compost pile with no heat exchange pipe or pump. I think after a week the temp will be the same 90F

Moving heat with electricity  is in general more efficient than making heat with electricity, so I doubt using the same energy to directly heat the water will be more efficient.
That said,  it would be informative to find out.

I like this build a lot.
For my own greenhouse I can see using insulation freewater heaters, painted black, a heater core and fan,a couple of temp controllers and a solar panel.
The tanks would simply radiate heat at night, the fan would create a thermal siphon during the day, and the plumbing would be very easy

Adding other heat sources could be fairly simple,  which is one of the benefits of liquid  thermal storage.

S Bengi wrote:By bringing all the hot air near the cool floor, you are also warming up the surrounding soil not just the fan coil. You are also de-stratifying the air too. All good things for the plants.

@Kenneth
I would love to see you redo the experiment by just burying the tank of 55F water into the compost pile with no heat exchange pipe or pump. I think after a week the temp will be the same 90F

Me too, in fact, you may be close enough to visit!
That was my second attempt, the first time I only got it buried halfway, and learned just how many more leaves it would take. As well as how to better arrange the fencing that contained it all (still a design in progress...)
It is a big undertaking. 2200 gallon tank, 5 foot thick "blanket"  of autumn leaves (about 70 cubic yards).
I need to get/install the plumbing between the greenhouse and compost site (about 100' apart) before I will do it again.

I tried to get the amp rating for the fan but I can't see the label.  It's on the motor under a clamp and I don't really want to tear it apart.  I'm guessing it's somewhere between 1/4 and 1/2 hp.

A 250W fan that effectively is a heater running say 10hrs a day for 2,500WHr.  

2,500 WHrs equals 8,530 BTUs.  So in 8.5 minutes I store the amount of heat that the fan would generate on its own in 10 hours.  I think we can say that the fan's energy input is negligible in relation to the overall heat storage of the system

*if my math is correct

What a beautiful design for an active heat battery!

Mike Haasl wrote:I'm guesstimating the circulating pump was moving about 20 gpm of water.  If anyone knows more about those pumps and their actual flow rate, I'm all ears.  [...]
If my math is correct I was storing about 1000 BTU per minute (8.33 lb/gal * 20 gal/min * 6 degrees).  The full contents of the tanks should have increased by 6 degrees in 11 minutes.  

The math seems far off to me. Hmm... Oh, that flow rate seems way too high. A high pressure garden hose delivers about 5 gallons per minute. With the small diameter of the plumbing in this gadget, I'd guess that the pump is circulating less than 3 gallons per minute perhaps as low as a gallon per minute. You could measure the actual heat rise in the barrels to get a better idea of how fast they are heating up, or measure the actual flow rate through the system.

Joseph, according to Taco’s pump flow chart, and 20 gpm seems right for the head Mike has. I’m not sure about the 1/2” vinyl tubing vs. 3/4” copper or 1” iron pipes common in hydronic systems.
Mike, a test of flow/time might make for more accurate calculations. Maybe time how long to empty your barrels with circulation pump? Measure the water that comes out, or how much to refill... but it would give a closer figure for your exact setup. Empirical data!

20 gpm does seem a bit high but when bubbles went past they were moving quite fast.  The display at the store said it could do 250 feet of 1/2" radiant floor tubing.  The only restrictions in my system is 15 feet of 1/2" piping and the radiator.  In the radiator it goes into the header and is then split into four 3/8" lines that do the back and forth.  

I'd love to tear it apart to check the actual flow but the purging process was enough of a struggle that I think I'll pass.  Once I get the second duct hooked up I'll run it for an hour and see how much the tank temps increase to calculate actual BTUs over a longer period.

Ha ha, yeah, you don"t have any bleed valves in those siphons. Getting the air out is tricky.
Job I'm doing at work is similar, filling a vessel completely with water/glycol fluid (no bubbles allowed). We hold the fluid mixture in a tank under vacuum to de-gas it. In the final product, one or two small bubbles (sesame seed size) will get dissolved in a day or two.
The final move is to remove a fitting and replace it with a plug... without losing much fluid, or letting air in. (the trick is to have fluid leaving )

The way I designed it was pretty slick (if I do say so myself).  I purged by pumping the water in the drop tubes that reach near the bottom of the barrels and out the tops of the barrels.  That way the air at the top of the barrel would be pushed on to the next barrel.  After they were all purged, I swapped the line from the pump and the radiator (fittings are at the upper left edge of the left barrel).  So in operation the water comes in the top of the tanks and leaves up the straw.  So any future bubbles will get trapped in the barrels where they can't cause any problems.

The bleed valve (if you can call it that) is the standpipe at the right end of the arrangement.  Several times I bled air out when the garden hose was topping up the system.  But due to the stretch/flex of the barrels, it was just as likely to suck air in if I wasn't careful.

Great stuff, Mike!
I'm curious about your Winterizing plans. Will you have to drain the system to stop it from freezing up, or will you run it continuously ?

Good question Chris.  I think I'll do some sort of antifreeze.  Unless I get wood heat in there this fall.

Last two winters the coldest it got in there was 20F.  I doubt this heat battery will keep it above 32 but if it mellows out the coldest nights it might let me grow avocados.

I'm open to other suggestions though.  The barrels were frozen the past two years so they can handle turning into ice cubes.

20F is warm enough for avocado trees. But if you want fruits you will have to bring up the min temp to 32F
https://onegreenworld.com/product/mexicola-seedling-2/

Joseph got me thinking of a way to check the flow without introducing air.  Good news is that it barely introduced any air.  Bad news is that I'm closer to 2.5 gpm  Doh.  So much for trusting the advertising.

So that would put the math at 125 BTU per minute when the air is at 92 and the water at 75 (Delta T of 17F).  I'd reason that I'll get more heat transfer if the air is warmer and/or the barrels colder.  So if the barrels were actually at 50 I think I'd get 2.5x the heat transfer, putting me up at 312 BTU/min or 18,742 BTU/hr.  For three hours of sunny weather in January that's 56K BTU.  Nothing to sniff at.

I guess the proof is in the pudding now.  How fast can it heat up the tanks and how quickly is their heat depleted.  I guess we'll find out this fall.  

You are on the right track.  I would say, dump the barrels and get totes.  You need volume.  I did some calculations for a system that I wanted to build, and also based on stuff I read in the 70’s and 80’s (yes Im OLD!!) where guys made entire walls of their house with barrels as passive ambient heat storage.  HOWEVER, that is just not a lot of heat.  1000 gallons (4 totes) is the same heat carrying capacity as 40,000 lbs of sand.  Stack the totes and put them in an insulated box.  Then you have some heat — equivalent to 6-8 gravel truck loads of sand.  As well, in a box, you don’t have to hook up every tote, as the heat will transfer from one to another and equalize all of them.  In cold temps you can lose 25-50000 btu’s per hour.  
Those are my thoughts anyhow.  Keep at it.  

I could easily add more barrels.  I'm thinking I'll run it through this winter to see if my limiter is the battery capacity or the amount of heat available to charge it.  I might convert them to a phase change material if the temperature swing crosses a convenient phase point.  That would give me the effect of more volume without needing more volume.

Totes would be tricky to fit in through the door

I am planning to make a zero pressure boiler like the rocket stove pizza oven.  Saw another one like it using evacuated glass tubes, which I already bought, and the two together should heat a house for 2 weeks to a month at a time, with 1000 gallons or so.  You could easily make a litte rocket stove for the barrels.  The down side to rocket stoves is the time and management of the fire, so the heat needs to be stored better than cob.  

Nifty project, enjoying the progress.

I'm curious about whether a fully liquid based system would work as well, or better, with less electrical load. Ie, absorb heat through radiators at roof level.

If I ever build one, it will need to have a pretty low electrical draw, since I'm off grid... obviously one would need more radiator surface area to absorb enough heat without major airflow but old truck rads can be had reasonably cheap...

This might also work nicely with summer ventilation; duct the rads below solar chimney style venting. Hottest air, and some airflow..



Barrels are not a very space efficient way to hold heat, but the upside is they have a lot of space around them to tuck vulnerable plants in close; I have seen visible improvement in marginal plant survival based on barrel proximity without any sort of heat collection for the barrel, so would hope you'll see good results!

Dave Mathia wrote:I am planning to make a zero pressure boiler like the rocket stove pizza oven.  Saw another one like it using evacuated glass tubes, which I already bought, and the two together should heat a house for 2 weeks to a month at a time, with 1000 gallons or so.  You could easily make a litte rocket stove for the barrels.  The down side to rocket stoves is the time and management of the fire, so the heat needs to be stored better than cob.  

Ooh. Please document! I totally agree that a RMH should be able to be a better thing when using a superior mass. Plus, once your mass is liquid, you have potential for moving that mass around in useful ways..

Yes truck rads, was going to suggest.  Put the barrels outside in an insulated shed.  Run pipes and low flow 12 or 24 v pump.  Rocket stove will fill tanks with hot water rising.
This is the guy i was telling you about.  Only 200 gallons.  Earlier he said it would las a week or two to heat his house.  This is my plan but with an outdoor shed and 1-2000 gallons of water.  I will eventually have 40 or 50 evactuated tubes as well.  They can catch an amazing amount of heat.  Anyhow.  Over and out.  Thanks for the chat

Oops...-forgot the link— https://www.latitude51solar.ca/solar-water-heating-projects/terry-fenwick-harwood-ontario

D Nikolls wrote:Nifty project, enjoying the progress.

I'm curious about whether a fully liquid based system would work as well, or better, with less electrical load. Ie, absorb heat through radiators at roof level.

I think that would work very well to collect the heat.  Either put a bunch of radiators up high, or a hundred feet of black poly pipe for the water to circulate through as it picks up heat.  I think you'd want/need to pump the liquid through the system and not rely on a thermosiphon.

The reason I didn't go with that is the heat delivery side of the equation.  I wanted to be able to get the heat out of the barrels "on demand".  Maybe that's silly and I would do just fine by letting the barrels bleed heat into the room.  But I thought if I forced the heat out of the barrels it might work better.  My footprint is 20'x40' so I wanted the heat to spread out a bit as well.

Mike Haasl wrote:

D Nikolls wrote:Nifty project, enjoying the progress.

I'm curious about whether a fully liquid based system would work as well, or better, with less electrical load. Ie, absorb heat through radiators at roof level.

I think that would work very well to collect the heat.  Either put a bunch of radiators up high, or a hundred feet of black poly pipe for the water to circulate through as it picks up heat.  I think you'd want/need to pump the liquid through the system and not rely on a thermosiphon.

The reason I didn't go with that is the heat delivery side of the equation.  I wanted to be able to get the heat out of the barrels "on demand".  Maybe that's silly and I would do just fine by letting the barrels bleed heat into the room.  But I thought if I forced the heat out of the barrels it might work better.  My footprint is 20'x40' so I wanted the heat to spread out a bit as well.

I'd definitely need to pump the water, given that it would be working backwards from how a thermosiphon would run, but have hopes that a very slow rate of flow would still be effective, especially with a really big reservoir.

My guess is that you will run out of heat very quickly forcing it out of the barrels this way, but it will certainly let you get more heat quickly if you need that much in a quick burst.

Are you happy with the footprint? It's huge!



I don't much like the idea of IBC totes as heat storage as they are pretty expensive around here... 200-250 bucks a piece unless you get a great deal, or they had something nasty in them. I'm starting to think there is a case for cement tanks for heat storage...

I will admit that the IBCs seem like a good compromise in terms of radiant surface area, especially when stacked and insulated as a group... hmm.

Dave Mathia wrote:Yes truck rads, was going to suggest.  Put the barrels outside in an insulated shed.  Run pipes and low flow 12 or 24 v pump.  Rocket stove will fill tanks with hot water rising.
This is the guy i was telling you about.  Only 200 gallons.  Earlier he said it would las a week or two to heat his house.  This is my plan but with an outdoor shed and 1-2000 gallons of water.  I will eventually have 40 or 50 evactuated tubes as well.  They can catch an amazing amount of heat.  Anyhow.  Over and out.  Thanks for the chat

Thanks for the link!

I have seen another setup with an insulated shed and big tanks, too... will try and find some links.


I don't much like the insulated shed idea, though. It seems more efficient to build the tankage into, or at least against, the house At the very least you ought to save one wall worth of.. wall.. but also:

1) Less piping cost.
2) No trenching.
3) Potential to get heat from the tanks by simply opening up the insulated area.
4) Potential to more easily connect with in-house heat sources. IE, you could have a RMH that heats water & a bench, a rocket cooker that also heats water... even a water blanket on a conventional woodstove. Other than solar water heating, IMO most of these things are best of *in* the house.
5) Heat leakage into house is not wasted, as long as it is heating season. If you chill the water in summer, ditto cold leakage..

I think I'm happy with the footprint.  It keeps evolving as I get more experience with it.  I tried to heat it with compost but that didn't work.  I'm hoping to do a batch box RMH but the insurance company may have other ideas.  This system is one of my last options to harvest heat to moderate the temps.

I do suspect that the barrels will cool off too quickly once the fan comes on.  My current minimum temp in winter is around 20F, compared to -30F outside.  If this can bump me up to 25F it will be great.  If my greenhouse was in balmy Chicago it would never freeze...

Food grade IBC totes are about $75 around here.

I store steel drums filled with water in my greenhouse. They pick up radiant heat during the day. At night, or during super cold weather, I may place a propane radiant heater next to them. Being steel, they don't melt...

I great project