PTJ potential project - Rocket Electricity

I'm posting this in the hopes that some wonderful PTJ instructor will want to take it on.

Paul loves Rockety things.  They've heated up about every sort of thing at WL.  Except electrons...  Or at least they haven't gotten them to travel down a wire on purpose yet...  

Wouldn't it be sweet if they could come up with 3 different ways to get electricity from a rocket?  Or a RMH?  

Some things that come to mind are:

TEGs: Thermo Electric Generators.  They're small solid state devices that make electricity when you have one side hot and the other side cold.  They have ones you pump water through for the cooling side and it makes hot water as it cools the TEG.  The TEGs are broadly available, coupling them to a Rocket without melting them would be the challenge.  Plus three other challenges that don't come to mind :)

Stirling engine: Basically it's a different engine design that works off of a heat differential.  Supply heat with a rocket or rmh, supply cold with the ambient air or some water and it makes electricity.  These are not broadly available so the engine part would be the hard part of this one...

Steam engine: Beware the boom squish...  But a rocket could heat the water in a steam engine to make electricity.  Just need a safe steam engine and it would be pretty doable

Mike Haasl wrote:
TEGs: Thermo Electric Generators.  They're small solid state devices that make electricity when you have one side hot and the other side cold.  They have ones you pump water through for the cooling side and it makes hot water as it cools the TEG.  The TEGs are broadly available, coupling them to a Rocket without melting them would be the challenge.  Plus three other challenges that don't come to mind :)

I found an  instructable on generating some electricity for battery charging using a small rocket stove, that could be a starting point.

from above link

Simple circuit for a small charger.... The link they gave to the Thermoelectric panel source suggests that the maximum operating temperature is 200 degrees, so you'd have to locate the panel carefully. It would probably work best in winter, since it needs a temperature difference. This one uses an old heat sink....I wonder whether it could be built into a water heater?  Hmm, electricity and water don't mix.....

If you're able pressurize the boiling water system to ~15-20psi using a few valves you could run one or a stack of Tesla turbines (there's a nice diy design that's use 4'' to 6'' disks)

Heating a sealed barrel of wood over the rocket stove, with a pipe coming from the barrel, condensing the wood vinegar and tar, then running a generator off the wood gas would be my vote. Safest and least expensive in my opinion. Also pretty simple and you get wood tar (anti rot post preservative, good for wofatis), wood vinegar (many uses) and charcoal (from barrel, which can be further gasified in a dedicated charcoal gasifier to run a generator if so desired, bbq on, make biochar, etc).

See 'crankenstein1' channel for demonstration.

Why not just use a wood gasifier to convert the wood into electricity you may ask?
- wood chunks needs to be sized perfectly
- very complex to build properly
- high risk of tarring up engine if you are inexperienced/something breaks
- byproducts (wood tar and vinegar) consumed as part of process
- steep learning curve and difficult to initially comprehend

Using a rocket stove to generate wood gas breaks down the component concepts of the more complicated wood gasifiers into obvious sections.
You are showing
- how a rocket stove works
- how a charcoal kiln works
- how a condenser works
- how a bubbler/bubble filter works
- how a gasometer works (buffer for the gas production before going into the engine)
- how a air mixing valve/carburetor works (woodgas must be mixed with oxygen)

Hey all.
I’m really interested in this.

Is the scope to get as much electricity as possible or integrating some electric output into a rmh that would be in a normal persons home?

As to the different available technology

Sterling engine: would be the way I would most likely go. I don’t know what sterling generators are available commercially. As with TEGs you would probably have to buy one. Big ones are definitely available

TEGs: Thermo Electric Generators. Probably the easiest route, maybe slightly boring and involving electronics. Definitely have to buy them, no idea on how long they run.


Steam engine: apart from water, there’s also ORC organic Rankine cycles, operating at lesser temperature than water vapor cycles.
I think running any turbine is a lot of involvement in upkeep etc. apart from a heat source you also need a cooling system.

I've spent a bit of time thinking about TEG's but never thought of combining it with a rocket stove.

It might be well suited to putting the TEG on a mass that is heated, the other side of the TEG needs to be kept as cool as possible as electricity is made by a temperature difference from one side of it to the other.

So one side would be thermally attached to the heated mass and the other side needs to be cooled as efficiently as possible. Possible ideas are large heat sinks with fins to catch airflow, possibly in contact with the ground to let the ground cool it. Pump cool water over it,

I wonder if the flow of hot air from the fire could be captured somehow—probably not to any great effect though? Imagine a rocket stove with a turbine akin to a water wheel, that is pushed along by the movement of hot air through the chimney, while the heat is used for other purposes.

Another probably wildly inefficient idea—using the evaporation and condensation of water? Water condenses at the top of a tower and flows down, turning a turbine; then it flows down and boils in contact with a surface heated by the fire.

My thoughts on thermoelectric generator + rocket:

1 ) Expect ~4% efficiency. This could be justified if the fuel was going to be burned anyway for heat (aka “CHP”), otherwise it’s probably not efficient enough to be justified.

2 ) If you’re actually going to invest the $2-3/W, you might as well keep it running continuously if possible for the entire heating season.

3 ) RMH don’t burn much fuel. Say 1.5 cords/cold-season. Really not much thermal power to work with, especially at ~4% efficiency. To me this implies that all (or nearly all) of the heat should pass through the TEG. This is non-trivial to design, given the small area of the TEG device and the max temperature limitations.  Not saying it can’t be done, just non-trivial.

4 ) For max efficiency (already rather low), and max power, it should be run near the max temperature of the device.

5 ) Given these assumptions (moderately high temperature and constant temperature, continuously, I then look to the RMH properties and operation. Fluctuating temperatures and often not a lot of thermal storage at higher temperatures.

This makes me wonder: how could RMH temperature be made more constant?  

I was envisioning semi-automatic fuel conveyance. For instance a woodchip system, or perhaps a bundle of few sticks, several feet long, with a weight on top to gravity-feed the J-tube, and refilled once a day.

The batch-box style might work but could require more high-temp thermal storage and additional temperature regulating systems for optimal function.

6 ) From there, them one could think about ways to limit peak temp and heat exchanger design. Due to the high heat-flux requirement, liquid heat transfer on one or both sides of the TEG might well be needed or appropriate, (as mentioned above).  

7 ) Much like with solar cells (a very similar semiconductor device), clearly the TEG is enticing for its simplicity, solid-state reliability and low maintenance; that is, if it’s lower efficiency can be justified.  

8 ) The niche application would be for cold-climate off-grid use, especially in areas where solar and/or wind aren’t particularly abundant.  

9 ) The availability of off-the-shelf TEG components is excellent (especially compared to steam or Stirling heat engines).  The safety is probably also considered much better.

10 ) The TEG also makes essentially no noise, unlike an engine; an advantage that perhaps should not be underestimated if it will be used in your living room.

In my estimation, perhaps the next most accessible way to generate electricity from wood may be to use a woodgas gasifier running a typical internal combustion engine (normally gasoline/petrol or diesel engine).  

There is a long history of vehicle use going back to at least WWII, and many online resources including from “All power labs”.

A system typically consists of: 1 ) a low-tar downdraft gasifier, vibratory shaker-grate. 2) Multi-stage filtration, 3 ) woodgas cooling heat-exchangers. 4 ) Carburetor, and optional automation at varying levels sophistication.

I suppose the advantages are that a lot of design & build resources are available and anyone can do it if they really want. It is relatively energy efficient compared to alternatives.  Disadvantages are that a large, multi-component system is required and significant operator labor (or expensive, complicated automation), and significant maintenance labor.  Cutting corners on this tends to gum-up the engine with tar.  

Lots of cool ideas! Here's a contrarian approach -- sometimes it's easier to define a specific problem and then consider the sustainable tech that can solve it. Also, sometimes the problem we are conditioned to think of as "requiring electricity" can be solved with more efficient means. I still like the convenience of grid power, but it's a worthwhile thought exercise, because grids can and do fail.

An advantage of Peltier (TEG) modules is that they bypass Carnot theorem efficiency limitations https://en.wikipedia.org/wiki/Carnot_cycle
so more useful power is potentially available, although 4% efficiency may be more limiting than even internal combustion engines.

Since rocket stoves provide quick intense bursts of heat and these modules might best be used with steady near-constant heat sources, let's consider the near-constant heat output of TLUDs [Top Lit Up-Draft]: ( See:
https://en.wikipedia.org/wiki/Top-lit_updraft_gasifier
https://www.drtlud.com/
https://www.engineeringforchange.org/solutions/product/tlud-champion/)

Here's a related article:
https://solar.lowtechmagazine.com/2020/05/thermoelectric-stoves-ditch-the-solar-panels/

Brian
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What about a RMH that heats a large phase change material so it spends most of it's time at a set temperature.  Say 200 or 300 degrees?  As soon as the tank is up to temp, power is generated.  Keep adding fire and the TEG enjoys that same temperature.  When the phase change is complete and the temp goes higher, stop adding wood.  It would probably take a consistent amount of time to phase change back to a solid (several hours) at which time the RMH could be re lit to keep the heat battery at the phase change temp.