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We run the ceiling fans in our home constantly to keep the air circulating, which helps keep the temperature stable thru the house.
They don't use much power, and I wonder if a Stirling Engine system could drive them, using the temperature differential between the interior of the house and the attic.
The attic gets very hot in summer, while the interior of the house is cool. In the winter, the attic is cold and the interior is warm.
The fans would be direct-drive, with the engines mounted to the ceiling, with one heat exchanger in the interior and the other in the attic.
We run the fans blowing down in the summer and up in the winter. You flip a switch to change direction. If you set up the Stirling system properly, the seasonal shift in temperature would cause the fans to change direction.
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Welcome to the 'Bakery, [jdlaugh]. It seems your first idea is a winner. |
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Thank you. Of course, as soon as I put the idea into words I thought of several reasons why it wouldn't work... Now comes the fun part, putting together a model. Maybe in a month or two I'll know if the idea is half-baked or fully cooked :) |
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Researched this and experiments show a temperature differential of only 7° can produce power. Seems that you would have much more than that between the attic and occupied spaces. |
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I've been trying to work out ways to use the fan blades as the heat exchanger surface. They've got air moving over them and all, so it seems elegant, but it may be too hard to do the plumbing. |
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The only potential problem I see with this
is that the stirling engine will move some
of the heat (during the summer) from the
attic into the house. This may not be
enough of a problem to offset the
advantages of getting the air moving. [+]
anyway for a good concept. |
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Yes, it would. I wonder if the heat that the Stirling engine would bring in would be more than the waste heat off an electric motor doing the same work? |
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Great idea [jdlaugh] [+], and great points from [MechE] and [baconbrain]. Raises three sub-ideas;
) conventional electric fan with above-ceiling motor and extended drive shaft
) stirling engine ceiling fan where cooling area is in a third location (i.e. a stairwell)
) evaporative -cooling for stirling cooling coils (fine mist over coils, coils in path of fanned air) |
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When one sees an idea like this with the obligatory one bone, belief in the autoboner grows ever stronger. |
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[vincevincevince], that was me, not the autoboner. |
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For every watt of air circulation in the winter, your house heating system will have an increased load of perhaps 10 watts or more. Maybe OK if your heating is free... In the summer, you pay 10x or 20x heat load for your air circulation. So you swelter or pay for A/C. Compare an electric fan: in the summer you have an additional heat load of maybe 1.5x air circulation power, in the winter the fan power contributes to heating. |
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Iron Horse, are you talking about the heat-exchange effect caused by the Stirling engine? I wasn't aware it would be that large.... |
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The main idea of air circulation is comfort -- it makes the temperature in the house more uniform, which in turn allows for a more conservative setting on the thermostat. However, if the energy penalty is that large, then it's not practical. |
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The two main issues I saw with the idea: It would stop running when the temperature differential was too small, which would be cool summer nights, cloudy days, etc. Of course, those are times when you least need a ceiling fan. The system also would have to be self-starting -- and most stirling engines are not. |
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[jdlaugh], I believe the correct syntax is [iron_horse] |
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the efficiency of an engine is the fraction of the total energy input that gets converted to work. Whatever is left over gets dumped. In an IC engine, thats the heat in the exhaust stream. In a stirling, its the heat that gets carried away from the 'cool side'. So if your engine is 10% efficient, 90% of the heat input is dumped, and the total heat required is 10x the fan power. If the engine is 5%, then its 95% & 20x respectively. |
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The problem is that the maximum theoretical efficiency depends on the temperature difference between hot and cold sides. The greater the temperature difference, the greater the possible efficiency. The cold side is practically limited to ambient temperature, roughly 300 degrees Kelvin. The hot side is limited by what you have for heat and what your engine can withstand. So a Stirling engine can be as good as 40% or 50% efficient with a temperature difference of 1000 K or something, but in this example the maximum theoretical efficiency is low, about 10%. & thats not accounting for friction and heat leaks. |
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But, there might be circumstances (e.g. ample wood heat, no electricity) where this is useful, so lemme change my vote |
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