Cape Town - The looming white giants stand in silence, their mighty wings still in the cool dawn. As the sun bursts orange over eastern mountains, they wait for the air to move, as if that would be the order for them to start their regular chore.
But on this particular day, this will not be. The Overberg air will remain still, the great wings motionless in the blue sky. May is not known for many days of high winds over Caledon and this year it is no different.
At the Biotherm Energy wind farm, Dassiesklip, outside Caledon, the nine huge wind generators are visible from miles away.
The tall tubular structures stretch 90m into the blue and their huge, three-blade variable pitch rotors are too big to fit on the pitch at Newlands rugby stadium. Each blade is 55m in length and the hub is 13m in diameter.
And yet, despite the size, one such generator tower can be put up in a day – if there is no wind, stresses site manager Gerald Francis.
The land on which the nine generators stand, is still being farmed. Later in the year, the towers may be surrounded by canola yellow, and sheep and cattle may graze on the stubble of wheat or other grain.
On the untilled patches among the towers, groups of blue cranes amble around slowly, calling their unique shrill calls.
Francis, who lived in Ireland and was trained to manage the construction and maintenance of wind farms in Germany, knows his nine charges like they are his children.
“The foundation of such a tower contains 60 tons of steel and 600m3 of concrete and is 20m deep,” he said.
“The tower is 90m high at, as we say, hub height, which is the centre point of the hub of the rotor. The nacelle, which houses the actual generator and planetary gearbox, is quite large and spacious, you can move around freely in it and it is about 6.5m high.”
To put up a tower, the tubular “body” is mounted on the foundation in four sections, after which the nacelle is mounted on top. The rotor is assembled on the ground and then lifted up and onto its axle.
“We brought a 900-ton crawler crane here and assembled it on site. You have to assemble and disassemble it for every lift in order to adjust its position,” Francis said.
At optimum wind speed the rotor, with its three huge blades, turn at 12 revolutions a minute. This gives the tips of the rotor blades a speed of just under the speed of sound, he pointed out.
“The generator starts generating power at a wind speed of about 3m per second. Mounted on the top of the nacelle is a wind speed and direction sensor which signals a programmable logic controller to start five motors which turn the turbine into the wind and feather the blades to the correct degree of pitch to start turning. The blades can be set anywhere between zero and 90 degrees, with 90 degrees being the brake position. As the wind get stronger, the degrees of pitch increase to keep the rotation at the speed required to ensure that the optimum voltage and frequency is supplied. The greater the degrees of pitch, the more difficult it is for the wind to turn the turbine and, at 90 degrees, the turbine will be still.”
Each tower has a capacity to produce three megawatts of electricity at an optimum 12.6m per second wind speed. The planetary gearbox has a ratio of one to just short of 99 to multiply the rotation of the rotor.
Inside the tower, a small two-person lift takes eight minutes and 10 seconds to lift maintenance crews to the nacelle, for maintenance, inspection and repair purposes.
“One of the biggest concerns people have about this method of generating electricity is that it will kill birds,” Francis said.
“We employ three environmental monitors who patrol the area and gather dead birds in the area. They store them in a freezer and the carcasses are later given a post mortem to determine cause of death. We have had bird strikes, mostly from birds being sucked in by the vortex created by the spinning blades, but we have not had a single blue crane yet and that is very good news,” Francis said.
It took about a year to set up the wind farm. Since the beginning (of full production in February 2014), Dassiesklip has produced 122 000MW of power at 3 000kw/h, which is roughly sufficient for 12 750 average homes, Francis said.
The first unit began feeding power into the grid from December 2013, said Biotherm Energy technical director Ludwig van Aarde.
“The company began operating in 2004 and initially focused on co-generation with a waste-to-heat energy project at Mossel Bay,” he said. Since 2010, we began focusing on renewable energy. Our total investment in the business now runs at just over R1billion, with about R500 million invested in the Dassiesklip project.
“Biotherm Energy is a completely South African private company, but with equity ownership from the US. We only sell into the Eskom national grid and essentially pay Eskom for the use of their network.
“We started with a 27MW wind and 10MW solar farm near Pofadder and Kenhardt and we have a 250MW project on the way,” said Van Aarde.
Cape Argus