Towards a better tinyhouse

Inventing to freedom?

Biomass burner idea

with 5 comments

Burning biomass, especially something like grass clippings which are really easy to get, is a lovely idea.

Solar is great but it’s not always the best way to get your energy, and if you need a fuel biomass is the cheapest and most readily available. Unlike waste veggie oil, it also scales up very well and is inherently low carbon.

The main problem that I was seeing when thinking about how you might design an easy to build biomass burner is:
1. Safety, a hopper of dried grass clippings is a fire hazard
2. Feed mechanism.
3. If you want to make electricity efficiently by burning fuel, you need to capture the heat produced with good efficiency using a heat exchanger of some sort.
4. Ash needs to be handled.

A biomass burner for electricity production is the hardest, and I just thought up an idea for it that seemed pretty good and I though I’d share it.

Basically the heat exchanger is vertical, and the fuel is dropped down the intake of it. The heat exchanger could be shaped in a slight helix shape if you wanted rather than straight, but you get the idea.

To increase the efficiency of the exchanger it would be as long as possible, and the walls could be textured in an appropriate way, or fluted or otherwise enhanced.

The interesting thing is that it could be used for a wide variety of different biomass types. With a large center tube chopped would could be used. Having metal flaps protrude radially into the center of the tube could help a bit to improve heat exchanger efficiency here maybe.

With an auger or other feed mechanism grass clippings or pellets could be used.

BTW the intake of air is still convection powered, and it might need some start up procedure or some more clever designing in some way to make sure the hot air goes up the right way, not up the intake. A one way valve on the exhaust?

It could also help with safety a bit because the high temperatures are normally kept away from the fuel input. Although potential reversal of the siphon could be a small problem, potentially releasing sparks, the exhaust gasses should be relatively cool in any case.

BTW I recently discovered the Harwell Thermo mechanical generator which sounds pretty cool. No bearings, no accurate machining (or I think hard to make flexures depends on how easy the flexure is to make) needed.

If you could make a more efficient one that didn’t have rubber in the engine (which limits the operating temperature and therefore efficiency) that could be awesome with a burner like this. I think we can afford some wire gauze or steel wool here.

Way cheaper than solar panels, and the waste heat can still be used.


Written by gregor

March 21, 2011 at 01:03

Posted in Uncategorized

5 Responses

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  1. i must insist that you not refer to such things as relatively low carbon…using the term “reatively” here does not make the statement “correct” by any means, if you catch my drift…



    March 21, 2011 at 09:00

  2. Biomass is not a low carbon fuel??? Even if you grow it yourself onsite? Seriously? I’ll post a correction if you want.


    March 21, 2011 at 12:40

  3. Biomass is all carbon but that’s not the issue anyway. The issue with carbon emissions is whether or not the carbon emitted is from a short term renewable source – so whether the carbon emitted can be reabsorbed into the existing vegetation ‘in real time’ or faster. Even oil is renewable but we burn it faster than nature can reproduce it, thereby creating a net increase in carbon into the environment.

    So, I think when you say low-carbon, you mean that it has a low net-carbon footprint, assuming the feedstock is not harvested at a faster rate than it grows.

    In any case, my comment is more about the ‘invention’. I wonder why a good old efficient woodstove is not suitable and appropriate and it is already well thought out and functional. I use one to heat my house and I draw on wood which is grown sustainably. Why reinvent the wheel?

    The shakers produced a wonderful design which has an afterburner chamber and recaptures heat from the exhaust as much as is practical.

    The other approach is a masonry stove which attempts to recover a very large amount of the heat through thermal mass storage in the masonry shell. None of these designs are new but centuries old.

    Pellets are expensive where I am ( and they have to manufactured). Chopped wood is still a good alternative IMO. My wood stove has an option to heat water ( a so-called ‘wetback’ ) and electricity generation from burning wood is inefficient anyway as you are converting energy twice.


    March 21, 2011 at 13:25

    • The problem with an existing wood stove for producing electricity is that it is very inefficient. You need the heat to stay at a high grade, a high temperature to achieve good Carnot efficiency. That is mostly what this idea is about.

      For heating a house, which this idea doesn’t help with much, the other problem is that they are expensive, if you have ever looked at the price of a new one.

      Secondly there is no automatic feed for cord wood available, only for pellets and the auto feed furnaces that are on the market are super expensive, several thou a piece.

      So cost, and efficiency.


      March 21, 2011 at 15:52

  4. With all due respect to your creative forces, I think producing electricity from biomass has never been a great idea. Plants convert sun energy into biomass ( wood or otherwise), then we burn it, converting carbon into heat, then we use the heat (in whatever fashion) to convert it again into electricity, only for the electricity to be converted into some kind of storage, like a chemical battery or used directly where the electricity is converted to some useful work, some of which becomes useful work and a lot of it becoming heat.

    So you have 4, maybe 5 conversions of the original sun energy source leaving a very low utility of the original energy which was collected over months or maybe years.

    Is the best way to use limited resources? Not likely.

    Some people are looking for biomass replacements for current coal or natural gas burning power plants, but this is just perpetuating what was already a poor idea which existed only because feedstocks were cheap and therefore ‘economic’ but never have they been efficient.


    March 23, 2011 at 13:55

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