A fluid dynamics expert in the US has demonstrated a way to optimise energy from wind turbines by taking advantage of constructive hydrodynamic interference.
John Dabiri, head of the California Institute of Technology (Caltech) Biological Propulsion Laboratory, studies water- and wind-energy concepts that share the theme of bioinspiration: that is, identifying energy-related processes in biological systems that may provide insight into new approaches to – in this case – wind energy.
’I became inspired by observations of schooling fish and the suggestion that there is constructive hydrodynamic interference between the wakes of neighbouring fish,’ said Dabiri, associate professor of aeronautics and bioengineering at Caltech. ’It turns out that many of the same physical principles can be applied to the interaction of vertical-axis wind turbines.’
The biggest challenge with current wind farms is a lack of space. The horizontal-axis wind turbines most commonly seen – those with large propellers – require a substantial amount of land to perform properly. ’Propeller-style wind turbines suffer in performance as they come in proximity to one another,’ said Dabiri.
In the Los Angeles basin, the challenge of finding suitable space for such large wind farms has prevented further progress in the use of wind energy. However, with help from the principles supplied by schooling fish, and the use of vertical-axis turbines, this may change.
Vertical turbines have no propellers; instead, they use a vertical rotor. As a result of this, the devices can be placed on smaller plots of land in a denser pattern. Caltech graduate students Robert Whittlesey and Sebastian Liska researched the use of vertical-axis turbines on small plots during a research project supervised by Dabiri.
Their results suggest that there may be substantial benefits to placing vertical-axis turbines in a strategic array and that some configurations may allow the turbines to work more efficiently as a result of their relationship with others around them.
In current wind farms, all of the turbines rotate in the same direction. However, while studying the vortices left behind by fish swimming in a school, Dabiri noticed that some vortices rotated clockwise, while others rotated counter-clockwise.
Dabiri therefore wants to examine whether alternating the rotation of vertical-axis turbines in close proximity will help improve efficiency. The second observation he made studying fish – and seen in Whittlesey and Liska’s simulation – was that the vortices formed a ’staircase’ pattern, which contrasts with current wind farms that place turbines neatly in rows.
Whittlesey and Liska’s computer models predicted that the wind energy extracted from a parcel of land using this staggered placement approach would be several times that of conventional wind farms using horizontal-axis turbines. Once they have identified the optimal placement, Dabiri believes it may be possible to produce more than 10 times the amount of energy currently provided by a farm of horizontal turbines. The results are sufficiently compelling that the Caltech group is pursuing a field demonstration of the idea.
To do so, Dabiri has purchased two acres of land north of Los Angeles, where he is establishing the Caltech Field Laboratory for Optimized Wind Energy (FLOWE). The pilot programme at the site will feature six vertical turbines on mobile platforms.
Dabiri and his team will systematically move the turbines around, testing various configurations to find the most efficient patterns.
’Our goal is to demonstrate a new technology that enables us to extract significantly more wind energy from a given parcel of land than is currently possible using existing methods,’ he said.
’We want to take advantage of constructive aerodynamic interference between closely spaced vertical-axis wind turbines,’ added Dabiri. ’Our results can potentially make better use of existing wind farms, allow for wind farms to be located closer to urban centres, reducing power transmission costs, and reduce the size of offshore installations.’
Three of Dabiri’s turbines are being provided in partnership with Windspire Energy. In exchange for the use of the turbines, Dabiri will share his research results with the company. Each Windspire turbine stands approximately 30ft (9m) tall and 4ft wide and can generate up to 1.2kW of power. Three turbines from another manufacturer have also been purchased; in total, the six turbines give the pilot facility a total power capacity of 15kW.