Justify whether the eco-cooler as shown in the link can cool the air. State all your assumptions and list the relevant thermodynamic equations to support your arguments.
The Second Law of Thermodynamics says that no matter what happens, as long as things are dynamic, the entropy of the universe increases. If entropy, a form of energy, is always increasing, how are we supposed to make things cooler? Turns out that we can cool things down, but at a cost to the rest of the surroundings: you always put more energy into the process of cooling something than you remove from the something being cooled. Refrigerators run pretty hot!
The most basic model of a refrigerator is called a heat pump, which takes thermal energy from a "cold" source and transfers it opposite the heat gradient to a "hot" reservoir. This takes mechanical work to accomplish. Most often, the work involved is done with a condenser. A gas is compressed, which heats it up. The ambient temperature cools the compressed gas down inside the compressor. The gas is then re-expanded very quickly, causing it to cool down to below its initial temperature.
Answer and Explanation: 1
The unpowered refrigeration system shown in the video claims to reduce the temperature indoors by as much as 5 degrees C, which can make a big difference if the ambient temperature is 45 degrees C (though 40 C is still sweltering!). The idea is to affix a series of plastic bottles to a board with holes cut out. The air blows from outside to inside through the bottles, which narrow towards the aperture that opens into the house.
The fundamental idea behind this is expansional cooling. The air coming through the bottle gets compressed slightly as it goes towards the bottleneck, and once it pushes its way though, it comes out cooler. The external work is done by the wind, which pushes air through the static system in bursts whenever the wind gusts.
The claim of a 5 degree cooling effect seems far-fetched based on the design of the device. It seems more likely that the wind would just flow though slowly compared to an open window, since the wind would be forced to go through these small holes in the board. The direction of the wind must line up with the board for any cooling effect to occur, so over half the time, if the wind direction is random, the board won't even work. It takes a significant amount of compression to get a noticeable cooling effect, but with this design it seems like the air would just go around or away from the bottles more easily than be compressed inside the bottles, unless it's a particularly blustery day.
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fromChapter 10 / Lesson 9
You can choose from a number of different heating and cooling systems to make your home comfortable to live in. Watch this video to learn about the advantages and disadvantages of forced air, radiant heat, baseboard heat and geothermal systems.