Improving Insert Efficiency, Electricity Production?

[jdlev]

We've been very happy with the results after installing a Century CW2900 last season - especially with our savings on propane! I'm looking for ways to improve the heat transference to the house, because it seems like the unit sends a lot of heat up the stack. I'm also interested in what energy the unit could be produce.

Improving Insert Efficiency

The unit makes pretty efficient use of the fuel it uses wasting little (if any) wood based on the fine/powdery ash it produces. The Century CW2900 came with a 130 CFM blower that circulates air around the firebox along with ceramic/volcanic bricks for heat reflection/absorption in the fire box. The bricks surround the firebox on all sides but the front, and has (what I believe) is a secondary combustion/recirculation system that comes through vented pipes that run just below the brick at the top of the unit.

The question is how do you improve the heat transferred into the home? Obviously, the main issue is the ventilation and air draw of the unit. The hotter the fire, the faster the air moves up and out the flue and the more fuel the unit uses. Theoretically, the faster moving air would offer less contact time w/ the unit decreasing heat transference to the house, and sending the heat energy up and out the pipe.

Heat Sinks

In looking at the model, you would think that perhaps adding a few heat sinks to the surface of the fire box might improve heat transference to the home? Heat sinks that contact the surface at variable intervals might be a good option to avoid a 'cold fire' scenario or to bring the insert up to temp.

Storing Heat Energy

This would be a tough nut to crack. If you're absorbing the heat, say with a coil of some type wrapped around the flue pipe or something like that, you'd restrict the rate of air flow up and out the unit which would produce draft/smoke problems. I suppose it could be done with a water jacket or other heat absorbent material beyond the bricks the unit comes with (I've heard silicone is great at storing heat energy), but that creates all types of questions about heat dissipation and utilization. I suppose an air turbine could be added to the flue, but you'd have air flow issues yet again.

I guess the real question is, how can you transfer more heat from the system to the home without negatively affecting things like air flow and the operation of the unit?

Steam Turbine

I thought a really cool idea would be a water circulation and steam generation system. The secondary ignition pipes in the fire box would provide the ideal location to send a few smaller copper lines through - sort of like a radiator. The system could produce either hot water for the home or generate steam to drive a small turbine. The pressure created by the steam could draw in cold water from a tank in our crawl space based on a pressure valve. When the steam pressure is high enough, the valve would close to push the steam through the turbine (and you'd naturally throw in a few emergency pressure relief valves).

Electricity Production

I love tinkering with electronics, microboards, and alternative energy, but am just an amateur at best. Thermoelectric generators caught my eye a few months back, so I ordered a few of the el-cheapo pelletier tablets for like $4/ea. Each is a 12v 8a thermogenerator that (I'm guessing) would produce about 96w of power - just shy of 300w at peak efficiency with all 3 units. I know you need to include things like a computer fan and heat sink to keep the peltier's from getting damaged, but beyond that, all this is new to me. Wondering if anyone else has tried anything similar?

 

[dBrad]

I've not done anything like this but I love the idea. I wish there was an easy way to use the residual heat for things like hot water too. I know there are some expensive, complicated systems out there - seems like there should be a simple method though.

 

[jetsam]

A minimally-engineered peltier solution will probably have real trouble keeping up required temperature differentials, unless you think your wife will be okay with a tractor radiator and fan running in the living room. Here's a sample of that.

A highly-engineered solution is quite expensive (and also struggles with temperature differentials, requiring high stovetops to work, so it's kind of a winter-only thing). Here's a sample of that.

If you're able to put a big water radiator inline (or use your house's hydronic baseboards for this purpose), you could probably come up with something functional- though probably not as functional as a wood fired boiler plus a small solar panel.

 

[maple1]

The question is how do you improve the heat transferred into the home? Obviously, the main issue is the ventilation and air draw of the unit. The hotter the fire, the faster the air moves up and out the flue and the more fuel the unit uses. Theoretically, the faster moving air would offer less contact time w/ the unit decreasing heat transference to the house, and sending the heat energy up and out the pipe.

Not quite. More CFM will transfer more heat to the house. The temp of the air the fan sends to the house might be less, but that is trumped by the volume increase. Total BTUs into the house will increase. The catch is that you don't want to transfer so much heat that you reduce the temp of the insert so much that it hinders combustion efficiency. I have no idea where that point would be in your case.

I would start with some simple temperature measuring - all relevant surfaces I could measure, along with internal flue pipe temps with a probe type thermometer. Along with flue draft, with a manometer. The pipe temp & draft will show or determine how much heat is going up & out, some surface temps might indicate say if insulation around or behind might help (knowing nothing about the install). How hard or possible all that is with your install might be another story, likely not so easy with an insert.

 

[Squirrel]

Trying to get the most out of my tiny morso squirrel I came up with the following:

Improving efficiency - Running the stove with as little primary air as possible while maintaining a high firebox temperature increases the residual time in the stove to its designed maximum. Practice makes perfect.

Heat sinks - I have a cast iron cricket that sits on the stove, does that count?

Storing heat energy - I have a 20 gal stainless water tank beside the stove that pre-heats the water going into my water heater, if the warm water from this is not used for the hot water tank it heats the room as the stove cools.

Steam turbine - I think my insurance people said "NO"

Electricity production - Small ecofan on stovetop produces 100CFM at 650°f and 50CFM at 230°f evening out temperatures nicely.

Here's a bad pic to prove it happened

 

[electrathon]

Trying to get the most out of my tiny morso squirrel I came up with the following:

Improving efficiency - Heat sinks - I have a cast iron cricket that sits on the stove, does that count?

Storing heat energy - I have a 20 gal stainless water tank beside the stove that pre-heats the water going into my water heater, if the warm water from this is not used for the hot water tank it heats the room as the stove cools.

View attachment 220973

The cricket only gets points if you snuggle with it under a blanket as you are watching movies. I have an old flat iron on my stove that falls into this same category.

Your water tank looks to be about 4' from your stove. What is your incoming water temperature? Average outgoing temperature? How long does it take the tank to gain 20 degrees when the stove is running? On average to satisfy hot water demand you would need about 60 degrees minimum heat gain to satisfy hot water needs. What did you use to control expansion? A 20 gallon tank would have about a quart of water more when hot than when cold. You are so far away that I doubt that you need to control circulation like you would have to with a coil heating the water at the stove, I doubt that overheat is an issue.

 

[jetsam]

Just being in the room with the stove would give the tank a ~35 degree rise in my house right now. Being in the basement with the water heater would be a ~20 degree rise. I never thought of that before. :/

 

[Squirrel]

The tank is only two feet from the stove. I picked a 20 gal tank because it was available and free. I have never checked the actual temperatures involved but the tank goes from ground water temp to warm to the touch in about three hours with the stove burning. It also works as a tempering tank in the summer of course. Any expansion is minimal compared to the real water heater (40 gal) Since installing it my annual water heating cost has almost halved (for just two adults).

 

[begreen]

There are products on the market using TEGs on a stove to create electricity. Caframo fans were the first. They have been followed by many Chinese knock-offs. There are also some TEG generators on the market. In general these are expensive and only make sense for off grid homes that can benefit from overnight charging when there's no sun on solar panels.
http://www.tegmart.com/thermoelectr...-wood-burning-stove-thermoelectric-generator/
(broken link removed to http://firevoltsystems.com/wood-stove-surface-thermoelectric-generator-devil-watt-100-watt-water-cooled/)

There's also a developing product based on a TEG in a stack-robber with a catalytic convertor. This may be ok with stoves on a well insulated chimney system that have stoves running a high flue temp, but I'd be concerned about creosote production. Not sure if this is a marketed product yet or still in development.
(broken link removed)

Another way to convert wood stove heat is via a stirling engine. These actually work pretty well at moving air. It's a bit noisier than a TEG, but they are cool in a steampunk sort of way. Seems like a woodstove stirling powering a generator might be an interesting project.
https://warpfivefans.com/products/US/stove-fans/stove-fans
(broken link removed to https://www.stirlingengine.co.uk/d.asp?product=VULCANSTOVEFAN)

 

[jetsam]

If you want practical ways to turn wood into electricity, forget the wood stove. Wood gasification and good old steam turbines are both within the reach of home tinkerers. Gasification is probably a lot safer to play with. I think I'd start by buying a cheap old gas generator (since they're very common), and converting it to propane.

 

[electrathon]

. Any expansion is minimal compared to the real water heater (40 gal) Since installing it my annual water heating cost has almost halved (for just two adults).

Just ran some numbers: 20 gallon tank, 30 degree rise (from 50 to 80 degrees) equals a , .42 gallon addition to the tank. It must go someplace, either a backflow to the city system or into an expansion tank in your house (you should already have one buy many times they are left out).

In the winter time I turn off my insta hot water heater when my stove is running hard. I have a 50 gallon preheat tank and a coil in the stove. I was getting a little too much heat in the preheat tank (was hitting about 140 degrees in the tank at the hottest) so I added 5' of baseboard heater in the basement.

 

[jetsam]

Man... I have an extra expansion tank sitting around, and plenty of room for a preheat tank.... and the water heater is in the room under the stove....

My wife is going to be deeply concerned about all this.

 

[electrathon]

Just being in the room with the stove would give the tank a ~35 degree rise in my house right now. Being in the basement with the water heater would be a ~20 degree rise. I never thought of that before. :/

Any heat in the water is gained by cooling the room. That is OK, you have a woodstove that is possibly producing too much heat.

 

[electrathon]

Man... I have an extra expansion tank sitting around, and plenty of room for a preheat tank.... and the water heater is in the room under the stove....

My wife is going to be deeply concerned about all this.

You will like the end results. Free hot water, sort of.

 

[begreen]

If you want practical ways to turn wood into electricity, forget the wood stove. Wood gasification and good old steam turbines are both within the reach of home tinkerers. Gasification is probably a lot safer to play with. I think I'd start by buying a cheap old gas generator (since they're very common), and converting it to propane.

Would you put the gasifier in the house for co-generation capturing waste heat?

 

[jetsam]

Any heat in the water is gained by cooling the room. That is OK, you have a woodstove that is possibly producing too much heat.

I view the stove's output as 'free' heat despite the fact that I harvest the wood, buck it, haul it, split it, haul it, stack it, age it, and then haul it again. Maybe I need a psychiatrist more than a water preheater...

 

[maple1]

Would you put the gasifier in the house for co-generation capturing waste heat?

There isn't any waste heat. It is pretty well all captured. So any energy you pull out for electricity would have to come out of the energy you captured for heat.

On second read, I guess you were perhaps talking the other way around? Electricity being the primary output? If that's the case, yes to co-gen. Although the heating load would be a lot more than the other electricity load for a typical household I suspect. And there would likely be a lot of waste heat when not heating, unless you could somehow capture all potential electricity while burning, for later use - and burn in batches. Or do something grid tied. Which would still produce quite a bit of waste heat when not heating.

(That was rather circular....)

 

[electrathon]

I view the stove's output as 'free' heat despite the fact that I harvest the wood, buck it, haul it, split it, haul it, stack it, age it, and then haul it again. Maybe I need a psychiatrist more than a water preheater...

That was what I was implying to. But the BTUs that are being lost to that tiny preheat tank is not much of a percentage compared to what is radiating out into the room.

 

[yooper08]

The question is how do you improve the heat transferred into the home? Obviously, the main issue is the ventilation and air draw of the unit. The hotter the fire, the faster the air moves up and out the flue and the more fuel the unit uses. Theoretically, the faster moving air would offer less contact time w/ the unit decreasing heat transference to the house, and sending the heat energy up and out the pipe.

I'm no expert at thermodynamics but convective heat transfer generally increases as fluid velocity increases, meaning how much heat is transferred is dependent on the movement of the fluid's molecules. More molecules moving, more heat is gained/released by the fluid. Just think wind chill.

So, the faster your fan runs, the more BTUs that are extracted off the insert. The opposite, I think, would then occur inside. The faster the air flows inside the insert, the more heat that transfers to the insert's walls. There does become a point where you're pulling more BTUs off the insert than the fire is providing, thus having a net cooling effect. If the firebox is cooled too much during the stage of the fire where secondaries are firing, you then lose that efficiency. You then need to increase the burn rate and/or decrease the fan speed.

 

[yooper08]

Also, have the same insert as you. I'm not familiar with your install, but what's your current setup? Do you have items such as a block off plate and an insulated liner?