Why can't you extract heat from your attic and use it to supplement charge to a Tesla wall battery, while also making your house cooler?
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2 Answers
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Turning heat into work requires a heat engine. Unfortunately, the efficiency of a heat engine is determined by the temperature difference between the heat source and the heat sink. When temperatures are expressed in absolute terms (e.g., Kelvins), the temperature difference between the hot air in your attic (the source temperature), and the outdoor temperature (the sink temperature) is so small that the efficiency of such a device would be VERY low. For details, see http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html
answered Jun 27, 2017 at 16:38
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$\begingroup$ I'm not so much concerned with the output, as the ability to absorb the heat (thus cool the attic). Thank you for the response by the way! $\endgroup$– AustinCommented Jun 28, 2017 at 19:29
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$\begingroup$ @Austin, the most direct way to do this is to install an attic fan that can pull hot air out of the attic and replace it with cooler air from outside the attic. $\endgroup$ Commented Jun 28, 2017 at 21:53
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$\begingroup$ I get it. It just seems barbaric to use so much power to run an AC unit, when you're basically removing energy. It's like if twenty dollar bills where scattered around your yard and you hired a landscaper to mow them up. I just don't see how there hasn't been a huge breakthrough in this technology, considering how much money gets spent on cooling homes. I guess I should be careful what I wish for though, if there was a simple explanation, that would make one hell of a weapon. $\endgroup$– AustinCommented Jun 29, 2017 at 14:30
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$\begingroup$ @austin, more insulation would reduce your electric bill. You may also have the option of finding an electric service provider who is cheaper than the one you are currently using. $\endgroup$ Commented Jun 29, 2017 at 15:53
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$\begingroup$ I recently did change providers and it did greatly reduce my bill. I was mostly just curious about the science in this area and if there is currently any promising technology to simplify cooling homes. $\endgroup$– AustinCommented Jun 30, 2017 at 16:27
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The same reason why you can't leave your refrigerator open to cool the room. To cool an object you need to make the surroundings even hotter. The total energy you spend to do this will always be more than the total energy you can harness.
If you do make an ideal machine that'd do this for you, the maximum efficiency is given by: $$1-\frac{T_{surroundings}}{T_{attic}}$$
Let's take $T_s$ to be the room temperature, which on average is $293 {K}$. Let's make this attic $20 {K}$ hotter. (which is a lot)
This gives you a maximum efficiency of just $0.063$ or $6.3\%$.. Pretty bad if you want to do something practical with this.
answered Jun 27, 2017 at 16:46
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$\begingroup$ But I'm not talking about using energy to cool. If heat is energy, then why can't you just remove it through absorption? The amount of energy this would produce isn't as important as how well the device could absorb the heat energy. $\endgroup$– AustinCommented Jun 28, 2017 at 19:23