My Thermal Storage Project

[Geno]

My cogen system is on it's 4th year and has been working well. Cleaning the exhaust heatex is a PITA but that's how it is. I can provide enough hot water to meet all my heating and domestic hot water needs down to about 20°F.  The biggest problems I have is that by morning the water is just luke warm and in the summer I have to dump millions of BTUs to waste.

I scrounged 2, 165 gallon barrels. They will be hooked together with a 1" hose on top and bottom for temperature equalization. The hot water from the engine will heat the water in the barrels on it's own loop. When I need hot water for a shower, laundry, etc. I'll turn on a second loop that will heat the 10-12 gallons in the furnace. That's enough water for most jobs. If I need more I'll just leave the pump on. If it's not hot enough I can top it off with the furnace. If I need to I can bypass the storage barrels and bring it back to today's configuration. One barrel has 2, 1125 watt, hot water heater elements. I often have watts to spare. Heat exchangers are automobile heater cores. There will be 2 in the bottom (parallel) to heat the water in the barrels and one on top to move it to the furnace. They have about 315 sq. inches of surface area each. The heater cores will go in from the existing hole in the top and I'll use heater hose there instead of pex because of its curl.

I'll build a wood frame with a heavy duty bottom insulated to about R-18. The sides and top will be a light duty wood frame with walls made of 2.5" fiberglass insulation board. I have a bunch of cellulose insulation laying around and that will go around the barrels. The sides and top will have a minimum of 12" insulation, which gets me R-45 minimum and the average will be even higher.

I'll need a tube on the outside to check water level and top it off when needed.
I'll need a drain on the outside in case something goes wrong or it needs maintenance.
I have a thermometer on a wire, which will be close to the top.
I'm going to put a 2nd pump on the loop that goes to the engine room. There is close to 200' of  ¾" pex with a number of elbows, T's, valves and heat exchangers in it.

12/6/10 The barrels are going through an overnight leak test and are looking good. I hope it still passes when the water is hot.

Questions:
1, If I put thermal chimneys on the heater cores how much more efficient will they be?
2, I don't want to spend the $ on an expansion tank for the loop that goes to the furnace. It has less than a gallon. Do I need one? What's an easy DIY fix?
3, Additional thoughts or suggestions are welcome.

Thanks, Geno

[mike90045]

Corrosion control ?
Cast iron block,  steel drums, aluminum heater core, SS hose clamps ?  Will drums be 50/50 anti-freeze w/corrosion inhibitor ?    Maybe add some outboard motor zinc's ?

[Ronmar]

+1 for corrosion protection.  If using antifreeze, be carefull that leaks cannot pass antifreeze to the domestic system.  What pressure is your domestic water system at?  Heater cores are not really pressure rated, and only designed for cooling system pressures of less than 30 PSI or so...  What temperature are you trying to store at?  The higher the temp the greater the heat transfer/loss.  I would say add another layer of high density foam underneath to up that R-18.  If I were doing it like that, I would try and put the barrels in a tray with a small drain line outside the enclosure to alert me to any leaks before the insulation got saturated.  A small chimney on the bottom heat exchangers should help with the thermosiphon thru them when heating.  What is your backup heat dissipation scheme?
   

[Crofter]

I was wondering about whether or not the heater cores were lead soldered but likely the heating fluid here is well separated from potable water.

For expansion tank for small circuits I have heard of tapping on to a small trailer tire wheel assembly.

[Geno]

I obviously should have explained my current setup in detail. Most of your completely legitimate concerns have already been addressed. Currently the system has 3 loops. Loop 1 in the engine room to cool the engine. Loop 2 to move the hot coolant into the house and loop 3 which gets me domestic hot water. Loops 1 and 2 have poisonous antifreeze. Loop 3 is water with two levels of physical protection and one level of pressure protection before hot water comes out of the faucet (town water pressure is higher than loop 2 pressure). Loops 1 and 2 are at atmospheric pressure and loop 2 has an additional 15' of head on it. All loops are connected with heat exchangers. Loop 2 to 3 is a flat plate rated at way more pressure than it will ever see

Crappy hand drawn diagram one shows the engine room and garage side. Crappy hand drawn diagram two shows the house side. The engine room is attached to the garage and the garage is attached to the house. Once you understand what I have, imagine this. Cut loop 2 in the basement, put a heatex on each side and create another loop. Put the engine side heatex in the bottom of the barrel and the furnace side heatex in the top of the barrel. The water in the barrels doesn't go anywhere, it just stores the BTU's. I "think" I read that the barrels are rated up to 160°F, their very thick and I doubt the water will ever exceed 160°F. My current excess heat dissipation schemes are 2, 5" boxer fans on the rad on loop 1, an automotive heater core with a fan on loop 2 (which also serves as a garage heater in the winter) and my last resort is to run domestic hot water for a while. A thermal chimney on the heatex's in the barrel is now on the list as well as ss hose clamps.

Thanks, Geno

[Geno]

Another Pic or 2

For expansion tank for small circuits I have heard of tapping on to a small trailer tire wheel assembly.

I got some of those laying around. Good idea! Thanks

I would say add another layer of high density foam underneath to up that R-18.   

That's pretty easy at this stage and will increase the BTU value by around 10.

[mbryner]

Interesting setup.    Thanks for the pics.   The diagrams help a lot.   Great work!

[Geno]

The pics pretty well show what I did. One improvement I did to prevent leaks was to cut a hole in the top big enough for me to get in so I could seal the fittings from the inside. I used some flexible silicone and smeared it on heavy. The epoxy I used on the outside worked well but will crack if you flex the fittings.

Thanks, Geno

[Geno]

More pics. The barrel on the left shows the top being glued back on.

Thanks, Geno

[sailawayrb]

A sort of related thought .... has anyone explored heat storage in molten salt ? Is there a readily available affordable salt that changes for solid to liquid in the 60C to 80C range ? How much additional heat can you store in a given volume of liquid salt compared to the same volume of water ? 

Well, I am exploring heat storage using sodium acetate that changes from an ambient temp liquid to a 54C (130F) solid.

http://www.microcogen.info/index.php?topic=1492.msg17694#msg17694

You can't really store additional heat energy in sodium acetate than same volume water, but you can store this heat energy forever in sodium acetate without using any additional energy, and you can release it when needed via the exothermic reaction.  If the energy is surplus and/or free (e.g., from surplus hydro/solar power), the efficiency of "putting it into storage" becomes less significant.

[sailawayrb]

Hi Jens,

These links explain the sodium acetate approach fairly well:

http://en.wikipedia.org/wiki/Sodium_acetate

http://www.howstuffworks.com/question290.htm

You would start with solid sodium acetate (at ambient temp) and heat it up to 100C at which point it would become a liquid.  It could then be stored in this liquid state (without insulating the storage container and without using any additional energy to maintain some temp) at ambient temp indefinitely.  This "supercooled" sodium acetate liquid is very stable and will stay a liquid at ambient temp indefinitely until you mechanically initiate the exothermic reaction as desribed in the first link.  The exothermic reaction will then create solid sodium acetate at 54C (130F).  This cycle can then be repeated indefinitely.

The process in inefficient in that you have to heat the sodium acetate solid to 100C in order to only obtain a future 54C solid. However, the storage requirements are easy and the heat can be released when needed.  Not all that different from storing electrical energy in a typical battery...hence my term "heat battery".

Bob B.

[mobile_bob]

now isn't that the coolest stuff ever!

one question though, how would you do it so that you can be assured the stuff is fully melted, so that it can be on hold until you are ready
for it to go back to 130 degree's?

is there a link to this salts use as a large thermal battery, large enough to be useful in our applications?

bob g

[sailawayrb]

Hi Bob G.,

I'm not aware of anyone having built a practical "heat battery" like this.  I don't see a big market for it either.  The patent app was submitted couple years ago, about same time we submitted this one which only published last month, so IP has been covered.

http://www.freepatentsonline.com/y2010/0292869.html

You would ensure fully melting the sodium acetate modules by continuing to heat the primary battery core fluid at 100C for a pre-determined sufficient time (which is a function of the sodium acetate module volume) pretty much like one does for small store bought heat pad.  The exothermic reaction is initiated remote from the battery electromechanically.  The sodium acetate modules then heat the central battery core which also contains the typical heat exchanger plumbing for transferring the heat to wherever it is desired.

Bob B.

[Geno]

I read up on Sodium Acetate the other day and it sounds like neat stuff. Even found a video of "hot ice"

I don't know how many of you freeze jugs of water for coolers (I do it a lot) but the problem I've had is that after a while you get an insulating layer of water around the block of ice and your beer gets warm. If you add about cup of salt to a gallon of water the ice melts evenly throughout the block and also gives up its BTUs faster. I built this highly insulated cooler out of a box someone was going to throw away and some scrap insulation I had laying around.

Thanks, Geno

[Geno]

Where most people see a dust bin I see a thermal chimney. This is the second one I built and it's a bit better than the first.

Thanks, Geno