|
|
Post by wallybear on Mar 8, 2011 16:59:20 GMT -8
I posted this in another thread and got no response so I thought I might just give it, it's own thread and see what happens.
Here are some questions I have about "heat batteries".
#1 Is the absorption and release of heat strictly a linear equation based on mass and density?
Base your answer on efficiency rather than change points.
#2 If it is not a linear equation then what would be the most efficient material for a battery? Meaning which material would absorb heat the fastest while releasing it the slowest? I am asking this in the hopes that I am seeing this wrong and that maybe there is a material out there that works differently then I am aware of.
#3 Based on a linear equation answer. What about a mechanical solution. What if we were to insulate the battery while it was heating and then opened it as heat was needed?
Also in a related question; What about insulating the battery from heat sinks, such as building directly on the ground, exterior walls or other areas that could rob heat?
#4 What about surface area? Surface area being a direct link to density of the mass would seem to play a integral part to how fast heat was released.
Last thought/question: What would happen if we took material such as steel plates separated by space so that they heated faster, then pressed them together so that the heat was released back out of them at a slower rate?
|
|
|
|
Post by Donkey on Mar 10, 2011 15:49:52 GMT -8
I posted this in another thread and got no response so I thought I might just give it, it's own thread and see what happens. Here are some questions I have about "heat batteries". #1 Is the absorption and release of heat strictly a linear equation based on mass and density? Base your answer on efficiency rather than change points. Dunno that. I'm not absolutely sure of this.. But I think that materials treat heat the same coming AND going. All else being equal, heat should move to and from a material in the same way, at the same speed. Possible, though I tend to avoid moving parts in my solutions. As a general rule, you insulate the "battery" from the places you don't want heat to go, like the floor or an exterior wall. Though floors will tend to give the heat back eventually, a lot can be lost there. If you live in an extremely cold climate (sustained neg. zero, etc.) then you probably should insulate from the floor (if not UNDER the floor). Yep. Umm... Dunno.. Try it and let us know.  |
|
|
|
Post by vbarritt on Mar 15, 2011 8:21:00 GMT -8
Water is commonly used as heat storage - heat up an insulated tank and then pump it through heat exchangers as the heat is required.
The steel plate idea is a good one, but one has to look at the economics - All that steel plate could make a lot of water storage, and only require some valves and pumps.
There are waxes that are formulated to melt at certain temperatures. The chemical bonding (solidifying) releases heat, and breaking those chemical bonds (melting) releases heat. It's like boiling water - the water temperature goes up to the boiling point, then stays at that temperature until until it is all turned to steam, and then the steam can continue to go up in temperature (under pressure).
If you have cheap access to wax, it's something to experiment with. Keep in mind it is flammable, and when it solidifies it will block heat exchanger tubes - could have issues... Safety first!
|
|
|
|
Post by Donkey on Mar 15, 2011 10:29:20 GMT -8
I have to disagree with vbarritt here..
Not that what he says is wrong, just that those ideas (as a general rule) shouldn't be your first experiments.
The best policy is to keep it simple. Liquid heat storage systems have complications, some of which have deadly consequences. State change solutions are complex and generally not for home tinkerers. Simple solid (cob, stone, concrete, metals, etc.), thermal mass has few drawbacks, none of which (that I can think of right away) are highly dangerous.
Hunting for efficiency is a slippery business. It's a simple, relatively inexpensive matter to achieve fair levels of it, shaving up to the top 20% is where 99% of your troubles and expense will be found. With a well built rocket stove, made of mud and trash, for less than $100, you can have a device that rivals (and even beats) the alternative, which average around $20,000.
Unless you have some special circumstance or (like some of us) a deep seated urge to tinker, the simpler solution will be QUITE enough.
|
|
|
|
Post by artificer on Apr 9, 2011 8:48:12 GMT -8
I posted this in another thread and got no response so I thought I might just give it, it's own thread and see what happens. Here are some questions I have about "heat batteries". #1 Is the absorption and release of heat strictly a linear equation based on mass and density? Base your answer on efficiency rather than change points. The simplified answer is yes. On the heat-in side you'll have a flowing gas, and turbulence makes a difference. On the outside you'll have radiation as well as conduction, but... really simplified its linear. You have to define "efficiency." To me, heat transfer is 100% efficient. Your heater will be less than 100% efficient, but thats a matter of how much heat transfer you achieve before the gasses escape. Heat captured/heat in. Did you mean to ask how to get the highest heat transfer rate? #2 If it is not a linear equation then what would be the most efficient material for a battery? Meaning which material would absorb heat the fastest while releasing it the slowest? I am asking this in the hopes that I am seeing this wrong and that maybe there is a material out there that works differently then I am aware of. Once again... effiiciency. If you mean the material that can suck the most heat out of the hot gasses the quickest, I would have to argue water with a tube in tube heat exchanger. To maximize heat transfer from the gas to the storage material, you want the maximum temperature differential. Water's temperature will stay lower because you pump cool water through the heat exchanger. If you use a counterflow system the coldest water gets heated by the coolest gasses. If you use two tanks, the average temp of the second tank can be lower, and you would use that water to cool the gasses before they leave the system. If you mean "best bang for the buck" as efficiency, you can't beat cob. People get condensing exhaust gasses from their rocket heaters, so they've pull most of the useable heat from the flue gas. Its cheap, readily available, does a good job, if you use enough of it. Even with its higher interface temperature on the inside, it works if you give it enough tranfer area. So again... you need to define "efficiency." #3 Based on a linear equation answer. What about a mechanical solution. What if we were to insulate the battery while it was heating and then opened it as heat was needed? Also in a related question; What about insulating the battery from heat sinks, such as building directly on the ground, exterior walls or other areas that could rob heat? If you want heat control, once again water is a fantastic choice. The problem with it is all the problems with it. Much more design needed, power for pumps if not a thermosyphon system, dangers of explosion and leaks, much higher costs. For heat storage, I like a simple bell. You get a good amount of surface area for heat transfer, you can add as many or as large of one as you need, and they're reliable. (as long as it doesn't fall over, its working) Since they tend to be flat, you could also put on louvered insulation that would open and close to allow more heat out. All heaters should be insulated from heat sinks. It makes no sense to heat the outdoors or the ground. This is one reason that many new and old masonry heaters are installed in the middle of the house. Heat all around, and no losses to outside walls. Modern heaters a built on top of an insulating slab. Typically something like vermiculite and portland cement. #4 What about surface area? Surface area being a direct link to density of the mass would seem to play a integral part to how fast heat was released. Last thought/question: What would happen if we took material such as steel plates separated by space so that they heated faster, then pressed them together so that the heat was released back out of them at a slower rate? Surface area is directly proportional to the amount of heat transferred, as long as the surface temp is the same. What you find, however, is that a large surface area will tend to be cooler, if you have the same energy from the heater. Lower temps can still transfer the same amount of heat into the room, but you get less radiation heating, and more conduction/convection. Density of the mass is set once you choose the material. MASS of the... mass can be affected by the surface area. Do you maintain the same thickness, but make it larger? Mass increases. Do you take a large/thick section, and spread it out for more surface area? Mass can stay the same, but area is increased. The steel plate idea is a technical solution that probably will cause more problems than it solves. A simple blanket on the thermal mass will change its heat transfer characteristics, and its a whole lot simpler. Michael |
|
|
|
Post by livinontop on Apr 9, 2011 18:29:01 GMT -8
I'm new to this forum....been on other forums with this but no reply. Hoping someone will comment here on this. I want to put a rocket heater in my greenhouse. Going to use the "Hermon Heater" I think. www.youtube.com/watch?v=-jzKKIHhTU0 I want to run a cement or mortar U channel so the top is at ground level. (I'm doing aquaponics, so will be raising vege's in "raised" beds. U channel will be under my growbeds. I'm thinking of putting my 8" stove pipe coming out of the heater, in the U channel with WATER-FILLED 55 gallon barrels, lying on their sides, end to end, to absorb the heat, and then release it during the night. No pressure pipes, steam, etc to worry about this way. But hopefully I'll get hot water. Thinking of maybe filling in the channel around the stove pipe with 3/4" rock? Whatdya think? I'm about ready to pour foundation for greenhouse, and U Channel would be next. Thanks! |
|
|
|
Post by Donkey on Apr 10, 2011 8:49:24 GMT -8
I'm not really picturing what the u channel is for..
Other than that.. Why not?
Though I'd say your expecting an awful lot of heat from one little stove.
|
|
|
|
Post by livinontop on Apr 10, 2011 21:43:06 GMT -8
Sorry if I didn't make it clear. THANK YOU for responding!! I'm laying my exhaust 8" stove pipe inside a fire-clay mortar U channel. This top of this mortar U channel (or trough)containing my stove pipe is what the 55 gallon barrels will rest on. They will be on their sides, end to end, for the entire 20' length running parallel to the south wall of the greenhouse.
The 8"stove pipe resting on the bottom of an approximately 11" high U channel will rise to 3" short of the top of the U. The side of the barrels will dip down (due to it's round shape) below the tops of the U channel holding it, allowing the barrel sides to be very close to top of the hot pipe, hopefully heating barrel and water inside it. Does that make sense? I'm only trying to keep the greenhouse from freezing. (Zone 8, fairly mild winters) Water's thermal mass stores 4 times what mortar or stone will. Thats why I'm desperately trying to figure a way to heat the water in these barrels. Thanks again Donkey!!!
|
|
hpmer
Full Member
Posts: 240
|
Post by hpmer on Apr 11, 2011 6:07:19 GMT -8
You'll probably want to cob around your 8" pipe and up around your barrels as cob is a much better heat conductor than air (which is actually a good insulator). Others have had the experience that once they cobbed everything together the heat recovery went up dramatically. Or, said another way, just piling rocks on top of the 8" pipe didn't really do much in the way of heat storage UNTIL they established a link between the two by encasing the entire thing in cob.
|
|
|
|
Post by livinontop on Apr 12, 2011 6:42:59 GMT -8
Thank you hpmer!! I didn't want to have to do "cob". But I hear you, for sure! Maybe I'll "mortar" the stove pipe into the channel?? Won't that be the same thing??
|
|
hpmer
Full Member
Posts: 240
|
Post by hpmer on Apr 12, 2011 7:51:12 GMT -8
That'll help, as at least the channel will be part of the heat battery. But you'll likely need to link the barrels in as well to bring them into the equation. Cob doesn't have to be anything special, just some clayey dirt mixed with sand to prevent cracking. Others can weigh in with formulas but you want to have the entire heat battery as one combined piece if possible.
|
|
