Science Buddies: "Ask an Expert"

Can acetone—a volatile liquid with a boiling point of 56.5 degrees Celsius-- be put in an enclosed vacuumed environment and boil at body temperature? If so, when the acetone is boiled, can it travel up tubing to with enough force to turn a turbine? If the turbine can be turned, how much energy can be produced? Will this energy be enough to give your cell phone an emergency charge? Can the complete circuit be woven into a jacket, to charge your i-pod or mp3 player? Theoretically, can it be scaled up to take up the heat from your entire body in a bed like form, and turn a larger turbine? Could these bed like machines change the way energy is made?

The acetone will begin in a reservoir and be brought to a boil. From there, it will pass up through a tube, turn a turbine, be cooled back down to a temperature below 37 degrees Celsius, and be recycled back into the reservoir. From there the process should hopefully repeat. Even though acetone has a low boiling point of 56 degrees Celsius, the body stays at only 37 degrees Celsius. So in order to be able to boil the liquid at 37 degrees I would need to create a very low pressure environment for the acetone, through the formation of a vacuum. If all goes to plan I hope to create, a machine that can convert thermal energy of the body, in to kinetic energy of moving/boiling liquid, in to electrical energy through the use of a turbine: with 0 emissions.

I will begin my project, by first getting the acetone in a vacuum, and attempting to boil it. I have already found that the boiling point of acetone at 37 degrees would be 395 Torres using the Clausius-Clapeyron equation: ln(Pv,T1/Pv,T2) = DHvap/R + (1/T2 - 1/T1) .
If I get it to boil at 37 degrees Celsius, and receive the permission from the Contra Costa County Science and Engineering fair judges, I will build my machine. There will be a reservoir for the main body of acetone that will be brought to a boil by the heat given off by the human body. The boiling liquid will pass up a copper tube until it reaches a miniature Keplin style turbine. After the liquid turns the turbine it will circulate to a cooler part of the body and eventually return to the reservoir, for the process to be repeated.

I would like to know if my project sounds like it's a good one and if it is plausible. If it is, then i would also like to know the location of any nearby labs with vacuums that i might possibly be able to use.

If you can get a vacuum gauge ( anyone who has worked on a car will have one in his/her toolbox) to measure what you are doing, getting a vacuum is no problem. Aspirator pumps (look up the term) are sold at most hardware stores. They hook up to a water faucet and are usually sold to pump water but pump air just fine. Hand-operated vacuum pumps with gauge are available for about $20 at auto supply stores. However, before you buy anything, think about what you are doing. Does the pressure rise when the liquid boils? does the heat of vaporization cool the liquid? Look up thermodynamic efficiency of heat engines. After you do a bit of arithmetic, you may be able to decide whether the idea has a chance.

If the vapor were to re-condense after moving away from the heat of the user's hand, wouldn't the pressure in the circuit go back down?

You need to consider evaporation and condensation. Lets first think about something you have more first hand experience with, water. The triple point of water is around 0 degrees which is where water can be a solid (ice), a liquid (water), and a gas (water vapor) all at the same temperature. On a warm day, if you get wet, you will feel cold as the water evaporates. This is the heat of vaporization. When water vapor hits a cold surface, it condenses and warms the cold surface. This is again the heat of vaporization, just in reverse. Acetone has a flash point of -17 degrees C hence its high degree of volitility. At anything over -17 degrees C at standard pressure, it is going to evaporate until its gas pressure equalizes and the rate of vaporization equals the rate of condensation. Any increase in temperature or decrease in pressure will increase the relative amount of acetone vapor. In a closed space or system, this quickly becomes an explosive mixture and not something that should be experimented with.

There are much safer ways to generate the small amount of electrical power required to run a cell phone or radio. Turning a hand crank generator can easily produce enough electrical power for this task.

Bestsciencebuddy,
You have a very interesting idea, which has been the topic of some previous thought and design. In addition to the suggestions already given, I recommend you try an internet search for "closed heat pump system producing electrical power" - you will find a patent for a device like the one you are describing, but much larger in scale.
Here is another example of this idea in use, with some good information about these types of generators:
http://www.yourownpower.com/Power/#How%20it%20Works%20...

Good luck!
Chris

Thanks so much for your help and on the link to the site Chris_G. The Chena Geothermal Power Plant is very near my idea exactly. But ofcourse my project would be a huge scale down, and the source of energy would change from the heated water of a hot spring in Alaska to heat given off from my body, and the volatile liquid would be acetone instead of R-134a. Craig_Bridge believes that the project would cause an unstable reaction. Can you explain to me the difference between the Chena Geothermal Power Plant and my idea, that causes the Chena Power Plant to be stable, while my idea is explosive?

Thanks so much for your help Craig_Bridge. The reason why I like this idea much more than a crank, is that I feel like it is important to be able to convert heat (the most simple source of energy) which is wasted once it is released into the universe into usable energy, whereas the kinetic energy used to turn a crank can be used for many other things.

"Can you explain to me the difference between the Chena Geothermal Power Plant and my idea, that causes the Chena Power Plant to be stable, while my idea is explosive?"
In your proposed design, mixtures of acetone vapor and air could be explosive, and the liquid acetone would be highly flammable:
http://www.ccohs.ca/oshanswers/chemical ... g_ace.html
http://cameochemicals.noaa.gov/chemical/8
At Chena, the R-134a is non flammable.
http://www.refrigerants.com/msds/r134a.pdf

I do not want to discourage you from this topic, because it seems like something that interests you, and it is a very timely topic in science and engineering. On the other hand, I think that your particular design of a miniature device to be used on human skin is probably too restrictive for a first attempt. I would recommend
(1) Consider alternatives to using humans as a heat source. People go to great lengths to isolate themselves from temperatures above or below 37C and this device would do the opposite. You would want to consider heat sources and sinks that currently are not used in any way, such as as hot pavement and rooftops or cold air and water. For a first attempt, you would be justified in using a laboratory setup without an immediate commercial application.
(2) Allow yourself some flexibility in the dimensions and materials used so that you increase your chances of success.

Good luck! I'm looking forward to hearing more about the project.