Electromagnetic radiation from the Sun heats the air –manifested in air currents- which eventually powers, for example, windmills. Schematically, Sun->Air->Windmills. Question: Why isn't Sun->Windmills? What´s the need of intermediary like a air/gas to perform work?
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There is no need for an intermediate medium to do the work. Solar cells and photosynthesizing plants convert solar light energy directly into electrical work. However, windmills happen to use heated air as an intermediate.
Buzz
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1And that´s the quid of the question, Buzz, why is heated air necessary? If the energy from the sun imparts KE to the air molecules and they do the same with the windmills, then, what is the explanation of the need for air? It seems to me your answer is a paraphrase of my question. – Rene F. Gastelumendi Dec 18 '17 at 12:39
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@ReneF.Gastelumendi I don't understand what your question is asking for then. A windmill is, by definition, a device that extracts energy from moving air. It's like asking why there is water in the ocean; if there were not water, it wouldn't be an ocean. – Buzz Dec 18 '17 at 19:19
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Buzz, you posted that “A windmill is, by definition, a device that extracts energy from moving air” and I agree. Nevertheless, Buzz, the energy that moves the air (forgetting fictitious forces that arise from the rotation of the Earth), comes from the Sun mostly in the form of EM radiation. Question: ¿why can´t we have a Sunmill that directly transforms that EM energy into rotational movement? Why do we have windmills and not sunmills? – Rene F. Gastelumendi Dec 20 '17 at 12:39
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We do have "sunmills," but they are called "solar panels." – Buzz Dec 21 '17 at 02:06
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Buzz, a solar panel is not a mechanical device ... I'm trying to elucidate, understand, view in my mind, and eventually be able to quantify, using the contributions of this forum, how is it that heat (EM radiation) moves matter, in particular gases (and eventually also how it expands solids.). Thats all. I understand very well the concept of the transformation of energy, as a solar panel does. But then, that device that is also an intermediary bypassing again, the essence of my quest/query. – Rene F. Gastelumendi Dec 21 '17 at 17:41
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Windmills rely on mechanical energy to power them.
The mechanical energy directly from the sun's radiation is very limited (I think radiation pressure is essentially negligible in terms of windmill forces).
Instead, the sun's heat creates a heat engine, with the air and water as working fluids. The radiation heat causes the ground air and water to heat up in non-uniform ways, generating fluid flows which can be harnessed for mechanical energy. The most common examples are windmills for air, and turbines for water.
As mentioned in another answer, heat can do work in other ways; it's just not very good at doing mechanical work unless it has some sort of process to extract that work from the heat.
JMac
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JMac, could you elaborate "...it's just not very good at doing mechanical work unless it has some sort of process to extract that work from the heat..", for that is the essence of my question? Why is that? – Rene F. Gastelumendi Dec 18 '17 at 12:42
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JMac: moreover the Sun is doing excellent mechanical work pushing molecules of a gas inside a closed vessel and rising the pressure inside! – Rene F. Gastelumendi Dec 20 '17 at 12:59
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@Rene It's not really doing much mechanical work though. It requires a working fluid to do that work. It only supplies heat to the fluid; when heated; the fluid develops mechanical energy which you can extract. The point is, although the thermal radiation carries energy; without some sort of intermediate, you can't really get mechanical work out of it. – JMac Dec 20 '17 at 14:46
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JMac, and what is the mechanism by which the fluid "develops mechanical energy which you can extract?" Isn't the sun really which is really imparting mechanical energy on the gas? If not, how does the gas molecules move from one point of space to another? What moves those molecules atoms through space? Being a little "tongue-in cheek", is it that the gas molecules/atoms have a "motor" to transform sunlight energy into propulsion? I don´t think so, therefore I assume that it is the sunlight pressure that imparts that movement to that gas or fluid. – Rene F. Gastelumendi Dec 20 '17 at 17:51
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@ReneF.Gastelumendi You need the gas and something to extract the energy from the gas. The sun heats the fluid. It's this heated fluid that is able to provide mechanical energy, not the solar radiation itself. Your question was about why we can't go Sun > Windmill; it's because sun is not good at providing the forces needed to generate notable work. If you let the sun interact with a medium like air first; then the energy that was once solar radiation is now in a form that gives far greater mechanical work per unit of solar energy input. – JMac Dec 20 '17 at 18:39
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This is perhaps more a question of economics than physics.
The energy in wind or wave power, hydroelectric power and fossil fuels is enormously concentrated, both in terms of land area, and also in terms of time. Capturing the same amount of sunlight using photo-voltaic cells would be significantly more efficient, but it would require vast areas of solar panels, or thousands of years, to accumulate the same amount of energy.
The sunlight which powers the winds, or lifts water from the sea up into the mountains, has been collected over an enormously bigger proportion of the Earth's surface than the total area of all of our wind farms and hydro-electric power stations. The chemical energy in coal or oil has been extracted from sunlight over the lifetime of a tree or marine creature, then compressed over millions of years.
If we devoted enough resources to it we could reach the same goal far more efficiently without any intermediaries. Why don't we? 1. Convenience : It would be a project of global proportions. 2. Consequences : Because of its size it would have bigger disruptive effect on the climate and environment than our use of fossil fuels has had.
The same dilemma applies to food production. We could synthesize the nutrients we need directly from chemical elements far more efficiently, but it is very much more convenient to be at the top of a global food chain which has taken millions of years to become established and extends over a vastly bigger area than all of our factories. The nutritional value we get out of hunting and fishing, and even farming, vastly outweighs the effort which we put in.
sammy gerbil
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Sammy, your comments are indeed true, and very interesting, but here we are trying to elucidate the mechanism by which EM energy from the Sun eventually its transformed to mechanical energy through a gases media. – Rene F. Gastelumendi Dec 20 '17 at 18:02
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Do you mean that you are asking how sunlight creates the wind? Or how windmills work? Your title asks "Why intermediaries?" As Buzz and I are saying, they are not needed. JMac told you that radiation pressure is negligible. If these answers do not satisfy you, what kind of an answer are you looking for? – sammy gerbil Dec 20 '17 at 23:47
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If radiation pressure is negligible, then ¿what causes storms, window currents and other atmospheric phenomena? – Rene F. Gastelumendi Dec 21 '17 at 13:24
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In essence, I'm looking for the explanation of why gas molecules move when heated, that is, a description of the process of how EM radiation moves molecules (and in a sealed enclosure it augments the pressure.) The kinetic theory of gases relates kinetic energy with temperature and that's it. Well then, how does the amount of heat (EM radiation) propulses the molecules? Do they have a "jet pack" attached in there back? – Rene F. Gastelumendi Dec 21 '17 at 13:44
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That is a good question, but it is far from obvious from the question you have posted that that is what you are asking. – sammy gerbil Dec 21 '17 at 18:03
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Some answers are provided by What are the various physical mechanisms for energy transfer to the photon during blackbody emission? and Why do moving particles emit thermal radiation? Of course these discuss emission rather than absoption, but that is the same process in reverse. – sammy gerbil Dec 21 '17 at 18:19
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Sammy, I just learned in “Why do moving particles emit thermal radiation” which you referenced to me, that “molecules undergoing translational motion (which therefore have a temperature) emit energy in the IR range and the intensity of the radiation in that range is related to the temperature.” I wonder how can I reverse that to “energy in the XYZ range makes particles undergo translational motion” and then relate that fact to the Kinetic Theory of Gases. Would that be an appropriate transformation, i.e. a symmetrical reversal? Any ideas? – Rene F. Gastelumendi Dec 21 '17 at 19:54