Comments on "Generic Air-Gen Effect in Nanoporous Materials for Sustainable Energy Harvesting from Air Humidity", authored by Xiaomeng Liu, Hongyan Gao, Lu Sun, and Jun Yao, at the Univ of Mass and published in Advanced Materials on 5 May 2023:
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In any system or device used to transform energy to do useful work, there are two well-established principles in our description of nature that should be considered: They are
1. The Law of Energy Conservation. It is also call the First Law of Thermodynamics, and can be shown to be deeply rooted in nature due to nature’s symmetry in time translations.
2. The Law that the Entropy of a Closed Macroscopic System Cannot Spontaneously Decrease. Entropy is a measure of the disorder in a system. This law is also called the Second Law of Thermodynamics, and can be derived from the statistical behavior of interacting particles.
Both laws have never been found violated in any careful observation of nature over the last one hundred and fifty years.
Liu, et al’s paper does not mention either, even though they describe getting energy in a sustainable process with no depletion of its source and with an accompanying order generation without loss of order elsewhere.
It is no surprise that water in the air (determining the air humidity) contains energy. In fact, all matter contains energy. If one wishes to produce work from any other form of energy, a system should be devised that converts energy from that other form to mechanical energy, such as the energy a piston supplies in exerting a force over a distance, or the kinetic energy of an electric current. It is also desirable that one is able to bring the system back to its original state in a cycle, so that the process can be repeated.
When water vapor changes from a gas phase to a liquid or bonds with another molecule or adsorbs to a surface, the entropy of that water necessarily decreases. If one surrounds the materials taking part in the process with a boundary impervious to energy or material transfer, then somewhere in that closed system the entropy must increase to a value at least as large as the amount that the water lost. This entropy most often is derived from the heat release in the process. A useful example is a hurricane. Hurricanes are formed over warm water. That warm water heats the air above, and releases water vapor. Because the air in the atmosphere several thousand feet above is colder than the warmed air below, and because warmed air is lighter than colder air, the warm air rises, carrying water vapor with it. As that moist warm air rises into the colder air above, the cooling causes the water vapor to condense into liquid water (droplets). In the process, heat is released, warming the air, which then rises even faster. In the meantime, the water droplets drop back to the ocean, tending to cool its surface. But rising moist air draws more moist air from the surrounding water surface, intensifying the hurricane. The net result is that heat flows from the warm surface air and ocean into the colder stratosphere while work is done in the motion of the air, i.e. there is a conversion of heat energy into some work. Air conditioners take advantage of the inverse process by changing the phase of a liquid into a gas to produce cooling. None of these processes violate the first or second law of thermodynamics.
Note that extracting energy to produce just work in a non-cyclical process does not violate the laws of thermodynamics. A good example is a very long insulated cylinder containing a gas, with one end wall being a piston. One can let the gas push the piston while expanding. The piston can do mechanical work, all coming from the kinetic energy of the gas. (For an ideal gas in an adiabatic (no heat exchange) process, the entropy of the gas does not change during an expansion of the gas, so the overall entropy change is zero. Neither the first nor the second law is violated. But this expansion is not a sustainable (i.e. cyclic) process.
Now, in Liu et al’s work, some water vapor presumably adheres to the surface of a material (inside small channels). This reaction must reduce the entropy of the water. In the process, it may also produce free electrons which could cause an electric potential gradient along the channels. The freed charges can make a current. However, once all those charges are removed, the process stops. Liu noted that other research groups had observed this loss of current in similar setups. However, if Liu and workers had a much larger effective surface, the total in all the channels, then they may not have waited long enough to see the current drop (i.e. longer than 48 hours). Again, since the charge depletes, the process is non-sustainable. One could rejuvenate the system by purging the channels of water. But this would require an energy input. As the initial adsorption of water molecules into the channels required a production of entropy, the efficiency of this now thermodynamic engine will be less than 100%.---------------------------------------------------------------------
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