Ask The Sietch – DIY Combined Heat And Power (CHP)?

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I got this question the other day and I have to say its a really interesting one (it doesn’t hurt that they butter me up a bit).


I think your website is great! Just to keep your taste buds alive I have a question…

My friend has a fridge and a freezer, he lives in the middle of nowhere and powers them from a geny. In the whole house the fridge and freezer are the life support systems, they are more important than lights, TV, radio etc.

He has just ordered a new wood stove that is able to output 31Kw of hot water power that can heat his whole stone walled house and more. He lives in a forest so he has a large supply of wood.

Do you have any idea on how we can go about converting the fridge and freezer to be powered from the hot water? We both can make anything but need some ideas and guidance…

Kindest regards,

Very interesting idea Scott. First I would not try and convert your appliances to run on hot water, this would take a long time, and be very hard to do. What you can do however is use your heater to create electricity and then run your appliances on that.

What you are trying to do is called Combined Head and Power (CHP) specifically in your case you would be trying create microCHP (as its just for home use). CHP is nothing new, many people over the years have needed both heat and electricity.

There are a lot of CHP resources on the net, here are a few.

There are even several commercially available microCHP products available on the market today, all of them that I could find use natural gas, and not wood. So perhaps they would not be the right match for what you have in mind…

Here are the ones I found that are selling a commercial product.

Many of these units boast a 90% (or higher) efficiency rate, which is great, but if your friend lives in the middle of the woods, chances are they don’t have a gas line, and may not be interested in getting a propane or oil tank (they are nasty, expensive, and not very green).

What all of these units have in common is that they take extra heat that would have been wasted up the chimney and turn it into electricity. How they differ is how they turn that heat into electricity. Some use a steam engine to turn a turbine, others use a sterling engine, they are all pretty neat.

If you are interested in building a DIY project there are a couple of things you should think about before you start.

  1. How will you safely get the heat from your wood stove to your generator
  2. How will you turn the heat into energy
  3. How will you use the energy

The key aspect of the first point is safety, if you use a heat exchanger (coil of metal tubing filled with water) in the chimney to move heat to your steam engine and in the process back up the flue and kill yourself with carbon monoxide poisoning, or starts a flu fire, or your heat exchanger explodes because of too much pressure, or the hot pipes char the wood its left on, or burns someone, or, or, or, you get the idea. Please whatever project you come up with make sure you do it in the safest possible way.

steam turbine

So enough research lets brainstorm some simple DIY MicroCHP. My first thought would be to build a steam engine and use it to turn a turbine. Here are some directions on how to build your own steam turbine, looks a little complicated but what a cool (or rather hot) idea. You would build some sort of boiler, divert the steam to your turbine (you may be able to build one out of an old generator, or alternator), and your off to the races. You could also buy one from this place, or this place, and then work out some way to use the power from the engine to turn a turbine. The first shop even has a page about using steam to make electricity, I found this part interesting.

Here are some specifics about our engines and what you might expect to do with them. Our largest engine, the single-cylinder “M” engine, will produce about 4 kilowatts per hour (kwh) of electricity when connected to a generator, the compound about 6,000 kwh. In a compound engine, the steam goes into the smaller cylinder, then exhausts to the larger cylinder to expand further. This gives more work for the same amount of steam. Contrary to what most people say, it does not double the horsepower of a single-cylinder engine; it’s more like 2/3 more. The compound engine can also be built as a double simple (i.e., both cylinders receive high pressure steam) and produces about 8,000 kwh.

All of these engines can be used up to 150 psi boiler pressure, and 450 degree steam (this is mild superheat). You will need to evaporate about 5 gallons of water per hour per horsepower. The “M” and the compound will need a boiler with a minimum of 30 sq. ft. of heating surface, and the two-cylinder double simple would need about twice that amount.

Cost. The “M” single-cylinder engine ready to run is about $3,600. A 30 sq. ft. boiler would probably run about $7,000. To this you will need to add at least two water feed pumps, as well as safety valves (only if attended to at all times, two if automatically fired), water gauge and other plumbing for about $1,000. The generator itself will be $500 to $1,000. So all told, you’re looking at least $12,000 to get started with a 4,000-watt system.

You could also use a stirling engine. A Stirling engine works on a temperature difference. So if you had one hot end, and one cool end (say outside) you may be able to hook the engine up to a generator or old alternator and make energy.

After you figure out how to make the electricity you are going to have to figure out how to use it. You could use a battery system and charge batteries and then run your appliances off of the batteries. This would have the advantage of being able to store the energy when it is created, and use later. It would have the disadvantage of being really expensive and a lot of work to create.

Depending on if you make DC or AC electricity and what kind of power your appliances run on you may need to buy an inverter (inverters convert DC power to AC) Sunnyboy makes one, so does Outback but both are for solar panels and may not work for what you want. Sunnyboy seems to have something that says it can hook a CHP plant to, the sunny island.

Overall it looks like there is not a commercially available home scale wood-fired CHP. That however does not mean that you couldn’t build one. Perhaps you could be the one that invents one and gets rich!

If you look this whole post over and feel that you just don’t have the time/desire/money to do this you could also do some of the following.

Solar panels
Wind turbine
Biodiesel for your genny.

Good luck and I hope this helps you on your way.

9 thoughts on “Ask The Sietch – DIY Combined Heat And Power (CHP)?”

  1. Back in the 30’s you could buy an iceball. Heat one end in a fire and make ice on the other end. Single stage amonia absorber. I’ve seen plans to build one. No moving parts.

    I’ve also seen third world solar ice making plants using the same technology.

  2. The “bladeless” steam turbine concept seem fascinating. I’m trying to see if one could use a solar collector design to power the steam turbine, thus not having to burn wood or maintain the unit. It would be self-operating.

    My main interest is in trying to lower our utility bills, by augmenting our power with free alternatives.

    At worst, maybe the solar collector could “pre-heat” the water, so you wouldn’t need to use much electricity or other fuel to get it up to steam pressure?

    How big of a turbine do you need to generate a given amount of power/electricity? I’d like to make use of the roof of my workshop shed or the heat generated inside our small metal shed or something.

    Can you gear reduce the RPM of a spinning heat vent (which I have on top of my wood shed) and make it useful for powering a generator or something?


  3. i need a way to connect between 8-16 bladeless turbines to a dattery and create energy(usefull) say to power a car

  4. just stumbled across you conversation. you know you can make
    gas from wood. they run cars on them in sweeden etc.duing the
    secound world war alot of cars trucks ran on wood gas. any way
    just thought your freind living in the wood with no gas surply might
    find it helpfull. he could power a generator with it.

  5. Great question and thought engaging reply. Thank you. Hopefully the links still work, after all I’m getting to the discussion a little late.

    Adding to Ian’s comment; converting wood to gas would definitely be an option. Do some internet or video searches for wood gasification stoves, or wood assification boilers.

  6. A couple of comments need commenting.

    1( the energy from rising hot air out of a roof space vent will be so tiny that it is pointless to convert to any other form. However ; the heat itself is very valuable when used to heat water. A good way to do this with little risk of running out of hot water is to have a “Pre Heat water heater tank just in front of your regular water heater. Rig up a circulating pump 5 GPM or less on a thermostat that will circulate water through your roof collector coils when the roof box reaches about 80 deg F. you will almost always reduce your water heating bills and sometimes completely eliminate them.

    2( Wood GASIFICATION furnaces and boilers only momentarily produce wood gas and then immediately burn it. What you need it s GASIFIER that puts out “wood gas” which is also “Producers gas” AKA ” SYNGAS” AKA “SYNTHETIC NATURAL GAS”

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