# Have: excess heat, want: tricity



## Adios Pantalones (Feb 22, 2011)

Once a quarter I fire my kiln, burning about 2 cord of wood.  A huge amount of heat goes to waste; it would be nearly impossible to recover the heat for my house as the kiln is away from the house for safety reasons, plus- maybe 2 times a year it's in heating season and lasts less than 2 days- so big deal, I'm not gaining so much for my effort.

Are there any interesting, affordable external combustion generators that can be plugged into existing heat sources like this?  Seems like there's a ton of waste heat sources, but a full Stirling engine is pretty expensive.  A lot of thermo-chip methods are too small.  I need a commercial steam system or something that can operate on an external heat source  Any ideas?


----------



## benjamin (Feb 22, 2011)

Abysmal utilization rate, and capitol intensive equipment.  

How about a seasonal heat store along the lines of Annualized Geo Solar?  Insulate a large mass of soil and pipe your kiln exhaust through to heat it up every few months.  Use this mass for hot water and low temp radiant heat.


----------



## midwestcoast (Feb 22, 2011)

Before considering how to generate, what are you going to do with the electricity during & after generation. Will you be able to feed power to the grid? Consider the cost of equip, permit... to do that.  If no grid tie, you're looking at battery storage?
A big problem I see here is you're looking at quite a large generation capacity (therefore equip costs are high) at a very low utilization (~8 days/year). 
I wouldn't want to waste all that heat either.  Maybe it'd be easier & cheaper to focus on ways to improve the kiln efficiency & reduce waste heat?


----------



## Adios Pantalones (Feb 22, 2011)

I'm mostly interested as a project to look at waste heat in general.  There are massive kilns that are fired for 9-10 days at a time reasonably often, and one in Japan being built that may operate for a month.  Production kilns operate on a daily basis.

Efficiency is not the name of the game with this kiln style, as the amount of wood burned directly translates to effects on the pots- so I need to burn a certain amount at a minimum.


----------



## Adios Pantalones (Feb 22, 2011)

earlll said:
			
		

> Not waste heat as much as a promoting factor for global warming? How better to heat the Earth than to simply waste heat!



... or I could fire with non renewable resources...  I come here looking for advice on improving my footprint and get this.  At least I think you're being a jerk; it's hard to tell what you're saying.


----------



## Adios Pantalones (Feb 22, 2011)

benjamin said:
			
		

> Abysmal utilization rate, and capitol intensive equipment.
> 
> How about a seasonal heat store along the lines of Annualized Geo Solar?  Insulate a large mass of soil and pipe your kiln exhaust through to heat it up every few months.  Use this mass for hot water and low temp radiant heat.



This could be interesting.  It could also power a Rankine engine (they make them with low-boiling point fluids) for extended periods.


----------



## midwestcoast (Feb 22, 2011)

Adios Pantalones said:
			
		

> Efficiency is not the name of the game with this kiln style, as the amount of wood burned directly translates to effects on the pots- so I need to burn a certain amount at a minimum.


Okay, I figured you probably had some constraint like that with firing.
The direct heat storage is the most practical application I see off the top of my head, so +1 on that. Apart from soil you can also look at using rocks, water or both. I'd heat the storage mass indirectly with air or water & not directly from flue gasses due to creo condensation issues... 
I'd be concerned that the amount of mass you can heat-up in 2 days would not be enough to store a significant amount of heat for months before it's used. A few calcs on the thermal mass & expected btu's delivered would suss that out.

Sounds like you're interested in an R & D project for electric generation rather than an easy, practical system though. More power to you & good luck!


----------



## Adios Pantalones (Feb 22, 2011)

Midwest- Solar has been stored in latent heat of melting asphalt, and other materials.  Water is a great heat sink, but it has practical limitations once boiled. 

I would like to look at systems that could be implemented n a small scale in say a university ceramics program.  There are thousands of kilns out there getting to well over 2200F every day, and that heat just goes up a stack.  The question was spurred by a recent discussion about waste heat recovery from kiln exhaust.  If there were simple methods, I could look at them in my own kiln, maybe write an article, see who could advance the idea.  In fact- waste heat being used to power other kilns would be a winner.


----------



## midwestcoast (Feb 22, 2011)

earlll said:
			
		

> Not waste heat as much as a promoting factor for global warming? How better to heat the Earth than to simply waste heat!


The guy posts a question trying to figure out how to use the heat that is a by-product from his kiln & you flame him for wasting heat? Nice.
Please do some reading on the Greenhouse Effect. Wikipedia would be a good enough place to start: http://en.wikipedia.org/wiki/Greenhouse_effect
The kiln's waste heat is not a 'promoting factor for global warming'.


----------



## peakbagger (Feb 22, 2011)

If you really want to go in this direction take a look at the carnot efficiency which is the maximum theoretical efficiency to convert a thermal heat difference to shaft power. 

Carnot Eff = 1 - (Cold temp absolute degrees/ Hottest Temp in absolute degrees). For english units add 460 deg R to deg F to convert to absolute. THe hot temp is the average exhaust gas temp from the kiln and the cold temp is the where you can reject heat (usually the ambient air temp or if use a cooling tower somewhere between the dry bulb and wet bulb temps

So now you need to know how much exhaust gas flow and its specific heat. Exhaust flow will require a pitot traverse of the stream to get an average flow, and specific heat can vary but usually approx values can be looked up in tables. Multiply those two and convert to Kw/hr and then multiply by the carnot efficiency. If you come up with a number that you want to proceed with, for arguments sake say you can buy commercial equipment that can get you 50% of carnot efficiency then multiply by $10,000 per KW hour for installed cost of equipment. Large scale Biomass power plants are around $4,500 per net KW so I bumped it up to $10,000 per net KW for a small installation. The only somewhat viable technologies that will work are Organic Rankine Cycle units, they are around 10% efficient but they need a fairly cold source to reject heat to. Pratt and Whitney sells ORC units and they are partnered with a Italian company that does wood pellet plants that generate power with waste heat so the equipment is out there. The capital cost for a small setup only used occasionally is going to sink the project. 

Another very rough way to look at it is look at the btu content of the wood being fired, assume a combustion efficiency of the kiln, subtract a value for the heat of fusion for the glazing process and probably 10% for radiant losses and then multiply by 10%. That will give you how many KW's potential. 

One off the wall way would be to partner up with a maple sugar producer and work out a deal to install a evaporator on the kiln exhaust to boil sap. Most of the big producers use oil fired evaporators and this would displace oil without the carnot cycle getting in the way. Obviously not practical unless you only want to fire in early March in vermont. Alternatively route the kiln exhaust through a fire wood kiln and sell seasoned fire wood. I expect it would upset the glazing process but why not install an air to air heat exchanger and preheat the combustion air going back into the kiln to reduce the fuel input? I expect it would screw up the process as the atmosphere would probably change but just curious. 

Realize that once you get up over a certain firing limit, you will need particulate control and possibly NOx and CO control along with environmental permits for your kiln. 

Overall, if you can avoid taking the hit on converting thermal to electricity and concentrate on using the waste heat for a process that normally would need heat there is a better benefit. One hint is to keep you exhaust gas temps after a heat exchanger well up over 250 deg F to ensure that you dont have any condensation of tars on the ductwork, otherwise you have to go to expensive metalurgy. On big industrial kilns, the exhaust gas can be routed through a heat recovery steam generator but for a small operations its just not practical. 

Sorry for the bad news, we all are looking for the "holy grail" of small electric power generation. 

If and when Dean Kamen goes into production with his small sterling cycle generators, you may be in luck, but Carnot will still prevail. Alternatively Sterling Energy Systems is going into production with small (25 KW) Sterling engines to be used on concentrating dish solar generators (currently planning to build a 790 MW plant in CA) . In theory as the production rate increases, the unit cost will decrease, plus they will work out the bugs which inevitably crop up. I expect your kiln would easily drive a couple of 25 KW units. Unfortunately I have been keeping an eye out for these technologies for more than 10 years and to date I haven't seen anything go remotely commercial.


----------



## Adios Pantalones (Feb 22, 2011)

I had thought about wood drying and a couple of other processes that could benefit.  We do have sugaring here, in fact my brother does some in the burbs near Boston- he could bring it up.  A friend that does small scale forging has talked about stopping by.  Some of this is just almost trivial use (though I would like to dry wood).  

Direct use of the heat, rather than conversion, would obviously be more efficient.  In a setting where it's being used all the time, the uses would be different than what makes most sense for my sporadic use.

On a similar note, I have been daydreaming about a woodfired boiler with storage for years now.  I had thought- what about a protection loop that runs through a cabinet where I keep my next charge of wood?  During heating season, it would put a little heat in the basement (where I spend time), dry some wood, and humidify.  Much better use than inefficient idling.  Alternately, build a rack over a plumbing area with known or designed-in inefficiency.


----------



## Jags (Feb 22, 2011)

I am guessing that ANY form of energy conversion from the waste heat of the stack to another source will have wild inefficiencies UNLESS you restrict the output (which will more than likely have adverse affects on the kiln firing).  Hmmm...dunno.


I know that you are looking at tricity as a final product, but any conversion from one energy type to another has inefficiency built in.  Stack that on top of the likely probability that extracting the heat to convert will also be inefficient and it brings me back to storage, not conversion.  Just thinking out loud.  I should probably stop that.


SMART PEOPLE - keep posting.  Im'a gonna wanna learn more.

Dang it AP - you posted before I hit my enter key.


----------



## jimbom (Feb 22, 2011)

Can a bunch of food.  Dry fruit, dry grain, bake bread for the year, heat the whiskey mash, etc.  Distill some fuel alcohol from biomass.  Direct use seems most practical.  Hearth.com BBQ party.


----------



## Jags (Feb 22, 2011)

JimboM said:
			
		

> Distill some fuel alcohol from biomass.



Yeah, yeah - we will call it umm...fuel...

I'll just leave this here:
http://www.moonshine-still.com/page2.htm


----------



## Delta-T (Feb 22, 2011)

i would like to suggest the 30 second pig roast....set up a lil lunch cart.. dangle a pig over the chimley and "pow" super fast BBQ piggie.... could be used for 3 second squirrel too.


----------



## fishingpol (Feb 23, 2011)

AP, as far as stirling engines go, It would have to be pretty substantial.  I have built a very small one and viewed many others on Youtube, but they do not seem to produce much power.  If your kiln was being operated several times a month, maybe something could be worked up that would be beneficial.  The only thing I could see would be some sort of heat exchanger for supplementing plumbing, heating a shed, heating a hot tub or an outdoor shower of some sort.  Storage would be important, but once the kiln temps die down, so do your temps.  It is an intriguing idea to harness that waste heat, it would begin with tinkering and seeing what results you could get.

I did make a kiddie pool heater out of my Weber grill and copper piping using the pool pump to push the water through the heat exchanger.  Got lots of laughs from the neighbors, but it did raise the pool temp up a few degrees, as it was in the shade.  

Sometimes that is all it takes to start an idea rolling. 

Enjoy firing the kiln.  I was at Old Sturbridge Village last fall, and the potter had an semi-open barn just full of pieces waiting to go into the kiln.  The OSV website had night photos of their kiln being fired.  It was awesome.


----------



## semipro (Feb 23, 2011)

I'm not exactly sure how kilns work but it seems that if you insulated the kiln to prevent heat loss and then were able to recover (and recycle) the heat from the exhaust you'd at least vastly improve efficiency and therefore the carbon footprint.   Is exhaust really required? Is there sufficient outgasing from the pottery to require this?   Or is the exhaust just because you're using a combustible as a fuel?  

How about a super-insulated electric kiln with no exhaust?  


There really is no economically viable way to make use of waste heat other than as heat.  Conversion is just too inefficient and expensive.


----------



## dougstove (Mar 2, 2011)

I really like the food drying/maple sugaring/wood drying ideas.

Could you time your firings around harvest periods?

Or perhaps there is some heat-driven scrap recycling process;  accumulate the scrap until firing time, then melt it?
Or some industrial scale washing procedure that needs lots of hot water?
    (Potter's Pantless Carwash?)
Or, water de-salinization, in batches?

As a general thought:
We used to do things when energy was available (grind grain etc.).  Now the magical switch gives energy at any time.  But we could go back to a more coordinated production ecosystem where complementary processes are better coordinated to minimize inputs.


----------



## Dune (Mar 2, 2011)

Here is one way you could do it, COTS. http://victorygasifier.com/

Not cheap, you need the gassifier, as well as a water cooled internal combustion engine generator and a decent sized water/water heat exchanger. 

Use the heat from the engine coolant, through the heat exchanger to heat your house (hydronic or hot air, either will work). 

In order to take full advantage, you will need net metering. Size the generator to your yearly electricity load divided by your heating months. Bank a years worth of electricity every winter.

My plan (presently in construction phase) involves an underground bunker for a generator room, primarily for noise abatement.

If you want to get theoretical, pm me. I have a project in the works, but need some serious math help.


----------

