Energy from the Medium the Windmill and the Solar Engine

The Windmill and the Solar Engine – Motive Power from Terrestrial Heat – Electricity from Natural Sources

By Nikola Tesla, from the manuscripts titled “Problem of Increasing Human Energy” written in the year 1900.

Nikola Tesla is one of the greatest inventors ever known to the World, and one of the greatest contributors to the technology of modern civilization.  His writings below are an extract from his article “Energy from the Medium – The Windmill and the Solar Engine – Motive Power from Terrestrial Heat – Electricity from Natural Sources”.


“Besides fuel, there is abundant material from which we might eventually derive power.  An immense amount of energy is locked up in limestone, for instance, and machines can be driven by liberating the carbonic acid through sulphuric acid or otherwise.  I once constructed such an engine, and it operated satisfactorily.

But, whatever our resources of primary energy may be in the future, we must, to be rational, obtain it without consumption of any material.  Long ago I came to this conclusion, and to arrive at this result only two ways, as before indicated, appeared possible – either to turn to use the energy of the Sun stored in the ambient medium, or to transmit, through the medium, the Sun’s energy to distant places from some locality where it was obtainable without consumption of material.  At that time I at once rejected the latter method as entirely impracticable, and turned to examine the possibilities of the former.

It is difficult to believe, but it is, nevertheless, a fact, that since time immemorial man has had at his disposal a fairly good machine which has enabled him to utilize the energy of the ambient medium.  This machine is the windmill.  Contrary to popular belief, the power obtainable from wind is very considerable.  Many a deluded inventor has spent years of his life in endeavoring to “harness the tides”, and some have even proposed to compress air by tide – or wave – power for supplying energy, never understanding the signs of the old windmill on the hill, as it sorrowfully waved its arms about and bade them stop.  That fact is that a wave – or tide – motor would have, as a rule, but a small chance of competing commercially with the windmill, which is by far the better machine, allowing a much greater amount of energy to be obtained in a simpler way.  Wind power has been, in old times, of inestimable value to man, if for nothing else but for enabling him to cross the seas, and it is even now a very important factor in travel and transportation.  But there are great limitations in this ideally simple method of utilizing the Sun’s energy.  The machines are large for a given output, and the power is intermittent, thus necessitating the storage of energy and increasing the cost of the plant.

A far better way, however, to obtain power would be to avail ourselves of the sun’s rays, which beat the earth incessantly and supply energy at a maximum rate of over four million horsepower per square mile.  Although the average energy received per square mile in any locality during the year is only a small fraction of that amount, yet an inexhaustible source of power would be opened up by the discovery of some efficient method of utilizing the energy of the rays.  The only rational way known to me at the time when I began the study of this subject was to employ some kind of heat – or thermodynamic engine, driven by a volatile fluid evaporated in a boiler by the heat of the rays.  But closer investigation of this method, and calculation, showed that, nothwithstanding the apparently vast amount of energy received from the Sun’s rays, only a small fraction of that energy could actually be utilized in this manner.  Furthermore, the energy supplied through the Sun’s radiations is periodical, and the same limitations as in use of the windmill I found to exist here also.  After a long study of this mode of obtaining motive power from the Sun, taking into account the necessarily large bulk of the boiler, the low efficiency of the heat engine, the additional cost of storing the energy, and other drawbacks, I came to the conclusion that the “solar engine”, a few instances excepted, could not be industrially exploited with success.

Another way of getting motive power from the medium without consuming any material would be to utilize the heat contained in the Earth, the water, or the air for driving and engine.  It is a well known fact that the interior portions of the globe are very hot, the temperature rising, as observations show, with the approach to the center at the rate of approximately 1 °C for every hundred feet of depth.  The difficulties of sinking shafts and placing boilers at depths of, say, twelve thousand feet, corresponding to an increase in temperature of about 120 °C, are not insuperable, and we could certainly avail ourselves in this way of the internal heat of the globe.  In fact, it would not be necessary to go to any depth at all in order to derive energy from the stored terrestrial heat.  The superficial layers of the Earth and the air strata close to the same are at a temperature sufficiently high to evaporate some extremely volatile substances, which we might use in our boiler instead of water.  There is no doubt that a vessel might be propelled on the ocean by an engine driven by such a volatile fluid, no other energy being used but the heat extracted by the water.  But the amount of power which could be obtained in this manner would be, without further provision, very small.

Electricity produced by natural causes is another source of energy which might be rendered available.  Lightning discharges involve great amounts of electrical energy, which we could use by transforming and storing it.  Some years ago I made known a method of electrical transformation which renders the first part of this task easy, but the storing of the energy of lightning discharges will be difficult to accomplish.  It is well known, furthermore, that electrical currents circulate constantly through the Earth, and there exists between the Earth and any air stratum a difference of electrically pressure which varies in proportion to the height.”

As it Pertains to Modern Day Development of Alternative Energy Resources

Tesla is a clear advocate of wind turbines for generating useful electricity, though notes the drawbacks in the periodic availability common with wind energy.  Specifically he mentions the windmill as a far superior technology vs. tidal force machinery.

Although a big promoter of solar power, he appears to have little faith in large scale industrial solar power generation, which in his terms he considered the boiling of a fluid and the driving of a steam engine.  We do, in fact, have a couple small plants of this nature in existence in modern times in the United States.  They use large arrays of mirrors or collectors, take up large amounts of space, and are limited strictly to Desert locations with constant Sun.  The heat is concentrated to boil steam, which is then fed to a steam turbine coupled to an electrical generator.  For the most part Tesla is correct, however.  Most locations do not have the required clear sky radiation, and the amount of electricity that can be generated for the size of the mirror arrays does not compete well with modern combustion power stations.  That said, emissions and fuel consumption are zero, and so in certain local markets this style of solar power plant may make sense.

Regarding solar power and Tesla’s general affinity for making energy from the ambient medium, one can’t help but wonder his impression of our modern day free piston Stirling Engine – Linear Generator – High Temperature Parabolic Collector combination.  More than likely he would have been intrigued.  The modern free piston Stirling Engines are quiet, virtually maintenance free, and elegant in simplicity.  With the high temperatures obtained through solar concentrators they are achieving good efficiencies and making useful amounts of electricity.  The same argument applies, however, with the consistency of clear sky radiation that is necessary, and the large amount of collection area required to provide industrial scale power.  Regardless, for small scale energy generation and where the climate permits, they are an excellent modern development in renewable energy technology.

Geothermal energy is another topic with which Tesla shows interest.  Noting the increasing temperature gradient as one drills toward the center of the Earth, with the right equipment one could setup a thermodynamic engine based on this.  Such deep well drilling ideas have been under consideration in recent times.  Their practical use in the market place, however, has yet to bear fruit due to the lack of new age drilling technologies.

Geothermal stations are in extensive use in locations where the Earth’s thermal vents penetrate toward the surface.  The drilling depths required are not as extensive, and therefore more practical based on present day drilling technologies.  Geothermal stations produce steady amounts of electricity, however the constant battle at these power plants is with the corrosive nature of the hot aqueous fluids extracted from the Earth.  In general, geothermal fluids wreak havoc on pipes, even when special alloys are employed.  As with other renewable and alternative energy technologies, the physical output of a geothermal does not compete with the modern combustion power plant.  But once again the cost of the fuel is zero, and so in certain local markets they have some use.

Tesla continues with the topic of capturing the energy of a lightning strike.   This brings to mind the old Ben Franklin experiment from times of yore.  It is an interesting concept, as we know certain regions of the country and certain natural structures are more prone to lightning strikes.  If one were to build a heavy duty lightning rod and large capacitance storage bank, surely there would be a steady diet of electricity.  Perhaps modern super capacitance technologies will open the doors for such a heavy duty, high surge capability storage bank.  But for the moment, I am aware of no such large scale facilities attempted or in existence.

In addition to the great inventor that he was, and the father of AC electricity and wireless communication, Nikola Tesla was one of the original proponents of solar power, wind power, renewable, and alternative energy systems of almost every form.  Much time has transpired since his death, and much change has taken place in the realms of science and engineering.  However, it is still interesting to notice how is ideas are still ringing true in the modern age.

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