>As an aside, waste heat from electical applainces and lights does offset the requirement for heat from the furnace during the winter.
This is wrong. The waste heat from appliances and lights that you use indoors exactly offsets the heat requirement. Whether it offsets the cost of the heat depends on what you pay for oil heat (that is, what you pay for oil divided by the efficiency of the furnace) versus what you pay for electricity. But that’s a different statement.
It may not be obvious here, but if you use the heat moving from your furnace to your house to run a turbine or something, less heat goes to the house. It’s not like a water wheel, where you get energy as the water falls but all the water still comes out. The turbine converts some of the heat to mechanical energy.
Power plants do a pretty good job of turning heat to electricity. You’ll be hard pressed to do better, I think even after you account for the transmission losses in electricity. After all, distributing heating oil has transmission losses, too.
I’ve been thinking of adding a 30 galon drum of water to act as a heat sink for my woodstove. I’d run copper pipe out the bottom, snake it back + forth between the back of the stove and the heat shield, and run it back into the top of the drum. I’d leave the top of the barrel vented, of course, to prevent a nasty explosion. Hot water rises, so the water would circulate.
Could I put a turbine in the pipe and generate enough power to charge batteries? Perhaps by running a drill pump backwards and attatching a small electric motor?
I know this depends on a lot of factors, such as pipe diameter + length, temperature differential, etc, but I can’t be the only one who has ever thought of this. Someone has to have tried it.
I’ve also been thinking about ripping apart a dehumidifier I have to get out the peltier heat exchanger to experiment with. My thought is that possibly I can generate current off of the heat differential near the stove without frying the heat exchanger. Anyone know if this would work?
It would boil down (heh) to how much the addition of electricity extraction costs versus how much you are paying for electricity. Because after all, nearly all the electricity you are using in your house gets transferred to heat anyway, so by not using the power company’s electricity you are losing out on that source of heating, so you will have to use extra fuel to produce this heat.
I saw this idea years ago, in Italy. FIAT was making engines running on natural gas. The engines would substitute for a burner, and the electricity produced would be fed back into the grid. If you assume that about 30% of the engine’s energy input would get turned into electricity, and you reover the waste heat to heat the house, you still burn more fuel than you need for direct heat. Seems like a bad idea.
It’s not so much making the furnace more efficient, but getting more useful “stuff” out of the fuel. “Stuff” would be work and heat. Most of us in our daily routines don’t need heat at 1000+ degrees. We need it at around 100-500 degrees (assuming you’re going to cook with it). For hot water and space heating, you can easily get by with something around 120. But the combustion process takes place at well over 1000 deg. So by installing an engine or a microturbine, you get out useful work (which most people would want to turn into electricity), then you can use the 300+ exhaust temperature to do your routine heating. It greatly increases the efficiency of the fuel use.
This is what’s called cogeneration, as someone pointed out above. The laws of thermodynamics say that, to be able to run a heat engine (turbine, internal combustion engine, etc.), you can’t use all the energy that goes in - you have to throw some of it away. That “throw-away” energy typically amounts to quite a bit - around 60% or more - but it tends to be relatively low temperature and not terribly useful for industrial applications. But it’s very often just what is needed to heat rooms and water.
It’s theoretically a great idea. The downside is that it takes attention and maintenance and capital investment.