Rise of Danish Concept
It was the Johannes Juul, a former student of la Cour, who introduced several innovations in his wind turbine designs that became the reason for the success of Danish concept wind turbines. He introduced the concept of stall regulated wind turbine blades that operates on constant speed and controls the rotor power at high wind speeds along with pitchable blades tips to control the rotor speed during gusts and storms.
To make a simple and rugged construction, he used struts ans stays for the long durability of rotors. He also reintroduced asynchronus genrators that proved to be a significant success.
The 200 kW wind turbine at Gedser was the landmark achievement of Juul's wind turbine success.
Source: windmillstech.com
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One of Juul's homemade anemometers. Photo: Energy Museum
In 1948, Johannes Juul started wind measurements in the SEAS-electricity network area of South Zealand and Lolland-Falster.
Juul used his own measuring equipment, for example, as seen on photo above, mounted on a power pole. Later, his self-built anemometers were supplemented by equipment loans from the Electricité de France in Paris and the Electrical Research Association in London.
Source: "Vindkraftens historie i Danmark, ch. 6". Jytte Thorndahl.
The Gedser location was selected for the wind measurements. The site is located near the open sea, surrounded by flat landscapes, without any protection from the wind. This makes Gedser comparable to the west coast of Jutland, which is considered Denmark's best location for wind turbines.
Source: "Analysis of Gedser Wind Turbine Data 1977-79".
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Photo: Energy Museum
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1954. The sketch (above) shows Juul's design for a multi-rotor system. The cost price per kWh produced would be lowest with three rotors on the same tower. Juul's second sketch shows the Gedser wind turbine on a lattice tower, which was eventually replaced with a concrete tower.
Source: "Vindkraftens historie i Danmark, kap. 6", Jytte Thorndahl.
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Multi-rotor demonstrator at DTU Risø Campus. Photo: Risø/DTU Vindenergi
2016-2018 - Risø and Vestas establish multi-rotor demonstrator
"It's been a little more than two years since the somewhat different multi-rotor wind turbine was set up on the DTU Risø Campus. The wind turbine, which has as many as 4 rotors, stood out from the usual wind turbine that most people know.
"Now the era is over. Because these days the iconic windmill is removed. Vestas, which is responsible for the installation of the wind turbine, has, in cooperation with DTU, completed the testing of the concept. For that reason, the wind turbine will be taken down," explains Erik Carl Lehnskov Miranda from Vestas:
"With the dismantling of the multi-rotor ends an interesting and profitable project collaboration between Vestas and DTU Wind Energy. We look forward to publishing the test results with DTU Wind Energy Science in 2019," says Erik Miranda, Director, Mechanical, Loads & Control Technologies at Vestas."
Source: farvel-til-multirotoren
Publication: 2019, "Power curve and wake analyses of the Vestas multi-rotor demonstrator" -
orbit.dtu.dk/
en/publications
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Photo: Energy Museum
1957. Gedser wind turbine. Upwind. Rotor diameter: 24 m. Performance: Self-starting at 5 m/s. 200 kW at 15 m/s. Typical annual production: 350,000 kW/year.
The image (above) shows Juul's final design of the tower. Here we see how the rotor blades with their stays and wires, as well as the machine cabin, are mounted on the tower. The vertical pipe (1) is made of prestressed concrete, with the buttresses (2) and the foundation (3) made of reinforced concrete. The measuring cylinder (4) is placed between the tower and the cabin. The service platform (5) is accessible via an inner and outer ladder (6). Near the tower there is a sheet steel transformer house (7).
Stiffening the three rotor blades with stays and wires is probably why the rotor has survived continuous operation for a decade without any problems.
Source: 1977-1979- DTU report regarding NASA and DOE's test runs of the Gedser Wind Turbine for the American energy program.
"Analysis of Data from the Gedser Windturbine - 1977-1979" 
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Rotor blade. Photo: Energy Museum
Quote from Johannes Juul's speech "Design of Wind Power Plants in Denmark" at the UN Conference, Rome, 1961:
"The brake flaps (1) make up 12 percent of the surface of the rotor blades and are an integral part of the rotor blades under normal operating conditions of the system. Each flap is fixed to a tubular carrying rod (2) which will, when actuated by the automatics of the mill, travel about 300 mm along the longitudinal axis of the blade and move in a link-motion (3) in the fixed part of the blade. By this movement, the brake flap is twisted about 60° out of the plane of the blade, thus counteractring the remaing part of the blade and bringing the mill to a dead stop. The brake flaps are operated by a hydraulic servo motor (5) in connection of automatics. When the mill starts, the servo-motor will pull the brake flaps into working position. When it stops, the centrifugal power will force the flaps out into braking position. The rotor blade beam (4) is square and consists of 16 mm and 10 mm welded steel plates.
(...) Wooden strips are attached to the front edge and near the back of the sheet. When designing the rotor blades as described above, it should be remembered that production options were limited. In future industrial series production, the rotor blades can undoubtedly be made more cost-effectively from other materials than steel, e.g. glass fiber reinforced plastic."
Source:
Johannes Juul's speech "Design of Wind Power Plants in Denmark" at the UN Conference, Rome, 1961:
"Design of Wind Power Plants in Denmark"
Juul brought a crash helmet in fiberglass to a meeting with the Wind Power Committee in 1955
"Perhaps the Danish Wind Power Committee considered fiberglass not yet sufficiently tested. But we know that fiberglass has been debated because at a meeting of the committee in 1955, Juul brought a crash helmet made of fiberglass and reported that an English company wanted to coat their rotorblades with fiberglass. The Wind Power Committee's technicians then decided to line the brake flaps of Gedser Wind Turbine with fiberglass instead of aluminum, but that never became reality."
Self-starting at 5 m/s
"The rotorblade had a tilt of 3 degrees at the tip of the wing and 16 degrees at the start of the wing. It was this tilt, which Juul had ascertained in his windtunnel tests for SEAS in 1948, made the rotorblades self-starting at 5 m/s."
Stall control - one of Juul's ingenious control devices
"Gedser Wind Turbine had to have a fixed speed in the sense that the output had to increase with the speed until a maximum value was reached and then it had to remain constant. The maximum output had to be 200 kW at a wind speed of 15 m/s. If the wind speed exceeded this number, the performance would not increase further due to "stalling".
Stalling occurs when the angle of incidence of the wind on the wings becomes too great. This makes it harder for the air to get away from the wing and the rpm will not
increase.
Juul had used the stalling phenomenon for regulation on his two previous SEAS turbines. A very simple method to prevent overloading in storms and - compared to the foreign experiments - something quite new and unique in the field of wind turbines.
Stall regulation is one of Juul's ingenious regulation devices. It could be difficult to calculate the significance of the stall, but repeated experiments with different wing profiles and observations of the effects of changing wings provided the solution."
Source: "Kapitler af vindkraftens historie i Danmark. 2." Jytte Thorndahl, Museumsinspektør, Energimuseet.
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Photo: Energy Museum
The rotor and machine cabin on the Gedser Wind Turbine was taken down in 1993 and brought to the Energy Museum in Jutland. The blades in particular were in poor condition and could not be exhibited. Only in 2004 did the Energy museum manage to collect money to renovate two blades (the 3rd has been deposited) and thus inaugurate Johannes Juul's masterpiece as one of the museum's most valuable treasures.
Museum inspector Jytte Thorndahl, who initiated the renovation and installation of Juul's rotor and machine cabin at the Energy Museum, states that each blade is constructed with up to 3000 screws. As a member of the "Operation of Gedser Wind Turbine" project group, it is Jytte Thorndahl's recommendation to meet Juul's previous request in 1957 to the SEAS board, for the manufacture of the rotor in glass fiber.
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2005. - Construction and exhibition of Juul's rotor and machine cabin. Photo: Energy Museum
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The Machine Cabin. Photo: Energy Museum
Quote from Johannes Juul's speech "Design of Wind Power Plants in Denmark" at the UN Conference, Rome, 1961:
"The design of the machine cabin is shown in the photo above. The blade beams (1) are bolted on the blade hub (2) which has two built-in ball bearings (3) and (4). The latter (4) cannot be replaced without removing the blades and hub. Its dimensions are therefore very ample so, that according to calculations it should have a very long life.
Ball bearing (3) is the axial pressure bearing and can be replaced without removing the blades and the hub which are mounted on a spindle (5) fixed in the cabin and bored.
The bore (6) serves as an oil pressure pipe for the service motors which are mounted on the blades for pulling the brake flaps into flush positions."
For more technical details:
Johannes Juul's speech "Design of Wind Power Plants in Denmark" at the UN Conference, Rome, 1961:
"Design of Wind Power Plants in Denmark"
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Bogø-mill's FLS cogwheel gear too expensive to establish in Gedser
The gear between rotor and generator had two stages. The first slow motion step from the rotor consisted of a chain drive with 2-piece sprockets with 2 roller chains. The inner high-speed stage (cf. photo above) had a triple roller chain drive for the generator.
Originally, Juul would have liked a Cogwheel gear, because Juul had satisfactory experience with that kind of gear from the Bogø mill. The cogwheel gear at the Bogø mill was designed by FLS with inspiration from the cogwheels FLS used in their large kilns for cement production. The same gearing had also been used on Juul's Vester Egesborg test mill. But the cogwheel gear proved too expensive to install on the Gedser Wind Turbine.
Source: "Kapitler af vindkraftens historie i Danmark. 2." Jytte Thorndahl,
Energy Museum.
Jytte Thorndahl recommends the installation of a newer, sustainable gear system in the recreated operational Johannes Juul Prototype. Although the design as a whole must resemble the original, it is important to adapt the operation to the extreme weather conditions of our time. In addition, Juul's prototype will be subject to applicable safety requirements and, like other wind turbines in Denmark, is already subject to an annual service visit by certified wind turbine companies.
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1957 - Juul's protection against over-speeding included: Two braking systems and relays
"The Gedser Wind Turbine was equipped with two braking systems: the brake flaps on the rototblades and a mechanical brake. An ingenious system of relays ensured that the mill could be braked at too high a speed and in other situations where it was needed.
When the tower was exposed to violent shaking, a falling weight ensured that a rocker relay tripped and activated the mechanical brake, so that the turbine stopped. The various safety systems locked themselves, so that the turbine could only be started after a manual operation. All safety and relay constructions were the work of Juul."
Source: "Kapitler af vindkraftens historie i Danmark. 2." Jytte Thorndahl, Energy Museum.
1979 - After several accidents, the Danish Organization for Renewable Energy (OVE) issues a safety leaflet based on Juul's brake concept
"It has often been mentioned that the Gedser Wind Turbine is the mother of Danish wind turbines. But on a single point, the new generation of turbine builders broke with the origin. They ignored or forgot to install aerodynamic brakes on the turbines - i.e. the most important part of the safety equipment. Nor did the authorities provide requirements for this, even though research was carried out in the Gedser Wind Turbine with the participation of Risø with American funds.
Until 1978, the new generation of wind turbine builders believed that it was possible to make safe mechanical brakes which, under all foreseeable conditions, could prevent the wind turbines from spinning out of control, which everyone who works with wind turbines rightly fears. But several accidents with the Danish company Økær's 5 m rotorblades in 1978 seriously put the brakes on the agenda.
The blade and wind turbine manufacturers quickly set about changing the technology so that the turbines corresponded to OVE's safety regulations. The wind turbine owners' organization, Danske Vindkraftværker (established in 1978) directly advised its members against buying wind turbines without aerodynamic brakes - which were also retrofitted to existing turbines.
The rules were more or less adopted as a norm by Risø in the approval of the wind turbines and unconditionally helped to secure the good reputation of Danish wind turbines."
Source: "The wings are the core of it all", Preben Maegaard, director of the Nordic Folkecenter for Renewable Energy. 2005
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