Using Tethered Aircraft to Harvest the Stronger Winds at 400 Metres
Airborne wind energy is a new development in the wind industry. While the regular wind turbine industry is now a mature industry, electricity produced by wind turbines is still not competitive with that produced from fossil fuels. The main problems are the large initial investment associated with the purchase of the wind turbine, and the low capacity factor. The high cost of a wind turbine is caused by the large amount of steel, composites, copper and concrete required for the construction. The low capacity factor is caused by the limited wind resource at 100 metres altitude. Airborne wind energy has the potential to generate power at lower cost and with a higher capacity factor.
By Bas Lansdorp, General Director, Ampyx Power, The Netherlands
The PowerPlane is a promising innovation developed by Ampyx Power, a spin-off of the Delft University of Technology. The PowerPlane is a straightforward machine that harvests the stronger winds at 400 metres altitude using a tethered aircraft.
The PowerPlane consists of three main systems. The first one is the plane at around 300 to 500 metres altitude. The second one is the ground station, containing a winch with a drum and a generator that can also work as an electric motor. And the third one is the cable connecting the plane to the ground station. During power generation the plane flies patterns downwind of the ground station. By doing so it is generating a high force in the cable and pulls the cable off the drum, hence extending the cable. The drum drives the generator, thus producing electricity. When the maximum cable length is reached, the plane flies towards the ground station so that the force in the cable is low. Simultaneously the winch retracts the cable. Since the force is low, reeling the cable in consumes less power than is produced in the reel-out phase, resulting in net power gain at the end of each extension-retraction cycle.
The basis of the PowerPlane facility is the power in the wind, and the physical objective of the PowerPlane is to extract this power. The power available in wind is expressed by the wind power density, Pw.
where P is the power, A the cross-sectional area, the air density and Vw the wind speed. In practice the power available is not readily accessible. The actual power producible by any device is described by multiplying the power density with the area swept (A) and furthermore a power coefficient, Cp.
Thus one machine can generate more power than another by being located in an area with a higher power density, by sweeping a larger area or by having a higher power coefficient. In conventional wind turbines the swept area is the rotor area and for modern wind turbines the power coefficient is a number around 70 to 80% of the Betz limit (about 0.59). The Betz limit is the theoretical physical maximum power coefficient.
The PowerPlane has to obey the physical limitations given by the Betz limit, and the power coefficient for the PowerPlane is likely to be lower than for modern wind turbines. However the advantage of the PowerPlane over wind turbines lies in the swept area, A. With a wingspan of 30 metres and a path of almost a kilometre wide, the swept area is larger than any wind turbine can ever achieve. Another advantage lies in the fact that at altitudes of 300 to 500 metres the wind power density is higher due to higher average wind speeds.
Advantages of the PowerPlane
Since the PowerPlane has no tower, it has lower installation and foundation costs, especially offshore. Also, because the PowerPlane is installed on the ground or a few metres above, no special cranes are required. Road transportation is easier because components are smaller. The use of materials for the PowerPlane is much smaller than for wind turbines. The PowerPlane requires less than 20% of the materials of a wind turbine, reducing the CAPEX significantly. The required material use for a 2MW wind turbine and a PowerPlane with equal energy output is shown in Table 1.
Table 1. Comparison required material
The PowerPlane operates at about 400 metres altitude where wind speeds and wind consistency is much better than at the hub height of a regular wind turbine. This results in a higher capacity factor of more than 50% and much better prediction of the output. Finally, the environmental impact of a PowerPlane is lower than that of a wind turbine, because of smaller size and lower noise.
10kW demonstrator
In the last two years, Ampyx Power has developed a 10kW demonstrator of the PowerPlane. The demonstrator has been tested in the Noordoostpolder in the north of the Netherlands with permission of the Dutch airspace authorities. The demonstrator has been connected to the grid and delivers power to the grid during tests. Current tests are focused on improving the control algorithms for autonomous flight. The 10kW PowerPlane operates at an altitude of 300 metres. The aircraft has a wingspan of 5.5 metres.
Commercial PowerPlanes
The PowerPlane can be scaled up rapidly, because all the hardware components of the PowerPlane are existing technology that can be purchased from partners. Ampyx Power has identified suppliers for the aircraft, the cable, the winch, the power electronics and the generator. The innovation that Ampyx Power contributes is the algorithms for autonomous power generation.
In 18 months Ampyx Power will have scaled up to a 100kW machine, which is also usable off-grid. The wingspan of the 100kW PowerPlane aircraft will be about 15 metres. The 100kW PowerPlane has a much higher capacity factor than small wind turbines, because the PowerPlane operates at 400 metres altitude where winds are stronger and more consistent. The 100kW PowerPlane fits into a sea container.
Within three years Ampyx Power will have scaled up to a 1MW PowerPlane, which has the energy output of a 2MW surface-based wind turbine. The 1MW PowerPlane will have a wingspan of about 35 metres, much smaller than the diameter of a 2MW wind turbine.
Table 2. Dimensions of the PowerPlane
PowerPlane Windpark
Permits
A PowerPlane wind park will always consist of three or more PowerPlane systems. The process of obtaining permits from the relevant airspace authorities for operating a PowerPlane wind park will probably require the same amount of effort, largely independent of the number of systems installed. It is therefore more practical to install several systems in one location.
Levelling power production
Furthermore, the power production of one single PowerPlane system is not continuous. No power is produced during the phase when the plane descends and the cable is reeled in. This rewind phase comprises about one-sixth of the cycle time, whereas the power production/pattern flying phase comprises about five-sixths of the cycle time. The phasing of several PowerPlane systems can be tuned so that the power production of the wind park as a whole can be kept steady over time.
Placement of PowerPlane systems
PowerPlane systems are placed at a distance of about half of their maximum cable length. In comparison, conventional wind turbines are placed at about six times their rotor diameter, which translates to a spacing of 300 metres for conventional 1MW wind turbines. Due to the higher capacity factor realised by PowerPlane systems, the megawatt hours produced per hectare of land is slightly higher for PowerPlane systems.
Strategic Partner
Work on the PowerPlane started in late 2008 with a small budget from a Dutch government subsidy and an investment from a private investor. In 2010, a second round investment was attracted from Statkraft, Europe’s largest producer of renewable energy, and from Byte Webhosting, a Dutch IT company. Also in 2010, Ampyx Power received a subsidy from the European Fund for Regional Development. At the current time , Ampyx Power is looking for a new strategic partner.
Biography of the Author
Bas Lansdorp is the general director of Ampyx Power. He has an MSc in mechanical engineering from Twente University. He worked at Delft University of Technology researching airborne wind energy for five years, before co-founding Ampyx Power.{/access}
Airborne wind energy is a new development in the wind industry. While the regular wind turbine industry is now a mature industry, electricity produced by wind turbines is still not competitive with that produced from fossil fuels. The main problems are the large initial investment associated with the purchase of the wind turbine, and the low capacity factor. The high cost of a wind turbine is caused by the large amount of steel, composites, copper and concrete required for the construction. The low capacity factor is caused by the limited wind resource at 100 metres altitude. Airborne wind energy has the potential to generate power at lower cost and with a higher capacity factor.By Bas Lansdorp, General Director, Ampyx Power, The Netherlands
{access view=!registered}Only logged in users can view the full text of the article.{/access}{access view=registered}Airborne wind energy is a name for systems generating energy from the wind with a structure that is supported by air instead of the tower of a typical wind turbine.
The PowerPlaneThe PowerPlane is a promising innovation developed by Ampyx Power, a spin-off of the Delft University of Technology. The PowerPlane is a straightforward machine that harvests the stronger winds at 400 metres altitude using a tethered aircraft.
The PowerPlane consists of three main systems. The first one is the plane at around 300 to 500 metres altitude. The second one is the ground station, containing a winch with a drum and a generator that can also work as an electric motor. And the third one is the cable connecting the plane to the ground station. During power generation the plane flies patterns downwind of the ground station. By doing so it is generating a high force in the cable and pulls the cable off the drum, hence extending the cable. The drum drives the generator, thus producing electricity. When the maximum cable length is reached, the plane flies towards the ground station so that the force in the cable is low. Simultaneously the winch retracts the cable. Since the force is low, reeling the cable in consumes less power than is produced in the reel-out phase, resulting in net power gain at the end of each extension-retraction cycle.
The basis of the PowerPlane facility is the power in the wind, and the physical objective of the PowerPlane is to extract this power. The power available in wind is expressed by the wind power density, Pw.
where P is the power, A the cross-sectional area, the air density and Vw the wind speed. In practice the power available is not readily accessible. The actual power producible by any device is described by multiplying the power density with the area swept (A) and furthermore a power coefficient, Cp.
Thus one machine can generate more power than another by being located in an area with a higher power density, by sweeping a larger area or by having a higher power coefficient. In conventional wind turbines the swept area is the rotor area and for modern wind turbines the power coefficient is a number around 70 to 80% of the Betz limit (about 0.59). The Betz limit is the theoretical physical maximum power coefficient.
The PowerPlane has to obey the physical limitations given by the Betz limit, and the power coefficient for the PowerPlane is likely to be lower than for modern wind turbines. However the advantage of the PowerPlane over wind turbines lies in the swept area, A. With a wingspan of 30 metres and a path of almost a kilometre wide, the swept area is larger than any wind turbine can ever achieve. Another advantage lies in the fact that at altitudes of 300 to 500 metres the wind power density is higher due to higher average wind speeds.
Advantages of the PowerPlane
Since the PowerPlane has no tower, it has lower installation and foundation costs, especially offshore. Also, because the PowerPlane is installed on the ground or a few metres above, no special cranes are required. Road transportation is easier because components are smaller. The use of materials for the PowerPlane is much smaller than for wind turbines. The PowerPlane requires less than 20% of the materials of a wind turbine, reducing the CAPEX significantly. The required material use for a 2MW wind turbine and a PowerPlane with equal energy output is shown in Table 1.
| Wind Turbine (tonne) | PowerPlane (tonne) | |
| Concrete | 575 | 40 |
| Steel | 210 | 25 |
| Glass Fibre | 19.5 | 3.5 |
| Copper | 20 | 3 |
Table 1. Comparison required material
The PowerPlane operates at about 400 metres altitude where wind speeds and wind consistency is much better than at the hub height of a regular wind turbine. This results in a higher capacity factor of more than 50% and much better prediction of the output. Finally, the environmental impact of a PowerPlane is lower than that of a wind turbine, because of smaller size and lower noise.
10kW demonstrator
In the last two years, Ampyx Power has developed a 10kW demonstrator of the PowerPlane. The demonstrator has been tested in the Noordoostpolder in the north of the Netherlands with permission of the Dutch airspace authorities. The demonstrator has been connected to the grid and delivers power to the grid during tests. Current tests are focused on improving the control algorithms for autonomous flight. The 10kW PowerPlane operates at an altitude of 300 metres. The aircraft has a wingspan of 5.5 metres.
Commercial PowerPlanes
The PowerPlane can be scaled up rapidly, because all the hardware components of the PowerPlane are existing technology that can be purchased from partners. Ampyx Power has identified suppliers for the aircraft, the cable, the winch, the power electronics and the generator. The innovation that Ampyx Power contributes is the algorithms for autonomous power generation.
In 18 months Ampyx Power will have scaled up to a 100kW machine, which is also usable off-grid. The wingspan of the 100kW PowerPlane aircraft will be about 15 metres. The 100kW PowerPlane has a much higher capacity factor than small wind turbines, because the PowerPlane operates at 400 metres altitude where winds are stronger and more consistent. The 100kW PowerPlane fits into a sea container.
Within three years Ampyx Power will have scaled up to a 1MW PowerPlane, which has the energy output of a 2MW surface-based wind turbine. The 1MW PowerPlane will have a wingspan of about 35 metres, much smaller than the diameter of a 2MW wind turbine.
| 10kW prototype | 1MW | |
| Wingspan | 5.5 metres | 35–40 metres |
| Weight | 25kg | 3,000kg |
| Operating height | 300 metres | 400 metres |
| Cable diameter | 4mm | 30mm |
| Aircraft loading | 4kN | 400kN |
Table 2. Dimensions of the PowerPlane
PowerPlane Windpark
Permits
A PowerPlane wind park will always consist of three or more PowerPlane systems. The process of obtaining permits from the relevant airspace authorities for operating a PowerPlane wind park will probably require the same amount of effort, largely independent of the number of systems installed. It is therefore more practical to install several systems in one location.
Levelling power production
Furthermore, the power production of one single PowerPlane system is not continuous. No power is produced during the phase when the plane descends and the cable is reeled in. This rewind phase comprises about one-sixth of the cycle time, whereas the power production/pattern flying phase comprises about five-sixths of the cycle time. The phasing of several PowerPlane systems can be tuned so that the power production of the wind park as a whole can be kept steady over time.
Placement of PowerPlane systems
PowerPlane systems are placed at a distance of about half of their maximum cable length. In comparison, conventional wind turbines are placed at about six times their rotor diameter, which translates to a spacing of 300 metres for conventional 1MW wind turbines. Due to the higher capacity factor realised by PowerPlane systems, the megawatt hours produced per hectare of land is slightly higher for PowerPlane systems.
Strategic Partner
Work on the PowerPlane started in late 2008 with a small budget from a Dutch government subsidy and an investment from a private investor. In 2010, a second round investment was attracted from Statkraft, Europe’s largest producer of renewable energy, and from Byte Webhosting, a Dutch IT company. Also in 2010, Ampyx Power received a subsidy from the European Fund for Regional Development. At the current time , Ampyx Power is looking for a new strategic partner.
Biography of the Author
Bas Lansdorp is the general director of Ampyx Power. He has an MSc in mechanical engineering from Twente University. He worked at Delft University of Technology researching airborne wind energy for five years, before co-founding Ampyx Power.{/access}
