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Experimenting Close to the Wind FLASHBACK_Aerodynamics Experimenting Close to the Wind Wind energy is a technology with a future – and with origins that can be traced far back into the past. One of its pioneers was Albert Betz, who was a Director at the Max Planck Institute for Fluid Dynamics in Göttingen from 1947 to 1956. TEXT MICHAEL GLOBIG “When, after the war, our economy was be allowed to flow through without reduc- Inspired by the sophisticated criteria for suffering acutely from the general shortage ing the air speed at all – in which case, aircraft propellers, in the mid-1920s Betz of coal, attention once again turned more once again, no energy would be “tapped.” turned his attention to windmill sails and eagerly to other sources of energy. In addi- discovered that these are subject to the tion to the development of hydropower, it GÖTTINGEN BECOMES THE same laws as propellers: a propeller driven was primarily recommended to make CENTER OF EXISTENCE by an engine creates air pressure, whereas greater use of wind energy. This interest windmills rotate in response to natural air continued even after the coal shortage was It can thus be assumed that, between pressure. Exactly the same aerodynamic overcome.” Albert Betz, who wrote these these two extremes, there must be an area laws apply to both and can be calculated lines, was himself one of the pioneers of in which mechanical energy can be extract- using the same equations. However, wind- wind power. It was he who postulated a ed by slowing the wind down. Upon closer mills with sails that exhibit an exact pro- law, now named after him, that no engi- examination, it becomes apparent that peller shape have a very low starting neer can afford to ignore. there is a very simple solution: the ideal torque: they do not attain their high effi- Betz’ law states that a wind turbine can wind turbine slows the wind by two thirds ciency until they reach nominal speed. convert a maximum of 59 percent of the ki- of its original speed. As the successor to renowned expert in netic energy of the wind into mechanical Albert Betz was born in Schweinfurt on fluid mechanics Ludwig Prandtl, from 1937 energy. Betz arrived at this theorem math- Christmas Day 1885. He studied philosophy to 1945, Betz headed the Aerodynamics ematically, but it can also be explained in Eichstätt, and later mechanical engineer- Laboratory, which belonged to the Kaiser thus: if an attempt were made to extract ing at the Technical University Munich. In Wilhelm Society at the time and was later all of the energy from the wind with a tur- 1905 and 1906, he worked at a shipyard, the taken over by the Max Planck Society. bine, the air speed behind the rotor would Germaniawerft Kiel, after which he studied From 1947 to 1956, Albert Betz was a Direc- be zero – in other words, it would never shipbuilding at the TU Berlin-Charlotten- tor at the Max Planck Institute for Fluid pass the wind turbine. In this case, there burg, earning his degree in 1910. From 1911 Dynamics (which was renamed the Max would be no energy to extract from the until 1918, he was an assistant at the Insti- Planck Institute for Dynamics and Self- wind, as no more air could flow into the ro- tute of Applied Mechanics at the Universi- Organization in 2004). In the early 1930s, tor. At the other extreme, the wind could ty of Göttingen – a position that left him Betz had also learned to fly a glider and time to pursue his mathematics and phys- qualified for his license in 1934 – a very ics studies. In 1918, Betz took charge of the practical way of coming to grips with the model trials department at the Göttingen subject of his research. aerodynamics laboratory. He received his He investigated numerous wing pro- doctorate from the university in 1919 and files and described in mathematical terms qualified three years later as a professor of the optimum width at any given point physics. In 1935, Albert Betz was appointed along the wing. professor at Göttingen University. From Using the latest discoveries in aerody- 1924 to 1937 he was a Scientific Member, and namics, he then went on to develop the from 1938, Director of the Kaiser Wilhelm optimum sails for windmills together with Institute for Fluid Dynamics, which was es- his colleague Kurt Bilau: they proved their tablished in Göttingen in 1925. superiority when tested and measured in This photo, taken in July 1950, shows the pioneers of aerodynamics, Ludwig Prandtl (left) and Albert Betz, by the shallow water tank at the Max Planck Institute for Fluid Dynamics, watching eddies form behind objects as water flows around them. Photo: MPI for Fluid Dynamics 96 MaxPlanckResearch 3 | 09 RÜCKBLENDE_Lockstoffe Albert Betz in 1930 standing beside a three-component balance. The object being measured is a rectangular wing. the wind tunnel. Betz also carried out ways to reduce their dependence on oil. speed is slowed by a variety of obstacles. wind tunnel research together with anoth- Here, at last, wind energy offered one pos- That, and the fact that the wind aloft is er prominent engineer, Ludwig Bölkow, sible alternative. more constant, is why wind turbines are al- who was working for Messerschmitt. ways erected on tall masts. These trials demonstrated that the flat ENERGY FROM ALTERNATIVE The design of such wind power instal- sails propagated by the Dane Poul La Cour SOURCES – THANKS TO THE lations generally follows a basic pattern: in 1890 as the ideal solution were in fact OIL CRISIS the rotor is shaped like a propeller and is no match for streamlined profiles. Once combined with a gearbox and generator to the rear section of such sails was stream- In parts of Europe, America and Asia, wind- form a single module. The module sits on lined, the performance almost doubled. mills were the principal sources of energy top of a tubular or lattice mast and is This was because turbulence ceased, air until well into the 19th century. They were turned into the wind with a fixed rudder. resistance fell and lift increased. By 1940, never quite forgotten, but it took the oil Windmills like this are a competitive op- sails of this shape had been successfully crisis to revive interest in wind energy. The tion wherever there is plenty of wind and fitted to 130 mills in place of the originals. governments of a number of industrialized few other energy sources available. How- ever, using the windmill to produce elec- tricity can be problematic: when demand Göttinger Presse, June 14, 1955 is high – in the evenings, for example, A wish long cherished by the Director of the Max Planck Institute when the wind drops – the windmill is un- » for Fluid Dynamics, Prof. Betz, is set to come true with the construction able to supply enough energy. Conversely, of a “flying wind tunnel.” when the wind is strong, there is insuffi- cient use for the power. Albert Betz died on April 16, 1968 in Göt- These and other improvements came at a countries encouraged the development of tingen. Three years prior to this, he was time when a public electricity grid was wind energy converters (wind-powered awarded the Carl Friedrich Gauß Medal in just being developed, and wind energy turbines for electricity generation). This honor of, among other things, his meritori- technology was in danger of being side- would soon bear fruit, for example in the ous achievements in formulating the basic lined. The successes achieved in theory US, where wind energy converters with a principles of aerofoil theory. Betz did not and in practice were all the more impor- capacity of up to several hundred kilo- live to see the boom in wind energy in Ger- tant in that they laid the foundations in watts were grouped together in wind many. In 1976, the author Felix von König physics and technology on which the re- farms. Some mega-installations were also came to the following conclusion in his finement of wind energy systems would built, with output running into thousands book “Windenergie in praktischer Nutzung” later be based. of kilowatts. (Wind energy in practical use): If, apart from The Second World War brought devel- Such projects were undertaken in Ger- water power, wind energy should one day opment to an abrupt halt: during the War, many, too: in 1982 the 100-meter tall be the only source available, wind power plans for new windmills were abandoned Growian (Grosse Windanlage, or Large Con- would certainly be inadequate to guarantee the world over, and the few installations verter) with a design capacity of up to a regular power supply in today’s sense, that were under construction were not three megawatts was built at the mouth of since the wind is too irregular. However, for completed. When the War was over, wind the River Elbe. Growian, however, didn’t a pre-industrial standard of living it should energy projects remained on the back measure up to the stress and strain. Cracks suffice – thanks to wind energy and our burner – oil was plentiful and cheap. It was soon appeared, as winds are stronger by knowledge of physics and technology, there not until the oil crisis of 1973 that numer- half at a height of one hundred meters will be no return to the Stone Age. Albert Photo: DLR-Archiv Göttingen ous industrialized countries began to seek than they are at ground level, where their Betz surely would not have disagreed. 3 | 09 MaxPlanckResearch 97.
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