Airborne Wind Energy Systems with Fast Motion Transfer

AWELabs advances airborne wind energy systems with a ground generator and a tethered wing, which flies crosswind. Energy transfer is accomplished by a separate belt that reels out with a speed at least comparable to the speed of the wing (i.e., 30 – 100 m/s).  The higher speed translates into lower force, which allows for a thinner belt.  The high linear speed also translates into a high rotational speed of the receiving sprocket.  The simplest of many possible schemes implementing this idea is shown in Fig. 1.

Scheme of Simple Airborne Wind Energy System, showing separate tether and belt

Fig. 1. Scheme of a simple airborne wind energy system with fast motion transfer

The wing is tethered to the ground.  The motion transfer belt trails the wing.  The electrical energy is generated when the wing moves up and the belt reels out.  The motion transfer belt rotates a sprocket on the ground, which in turn rotates the rotor of an electrical generator.  The rotational speed matches the synchronous frequency of the generator, thus, eliminating the need for a gearbox.  The wing can either be a flexible airfoil with an airborne steering unit (a kite) or a rigid wing with ailerons and an empennage (a glider), or an airfoil combining qualities of both (a ‘kiteplane’).

The system works in cycles.  Each cycle consists of two phases:  the working phase and the returning phase.  In the working phase, the wing harvests wind energy and pulls the belt while the belt unwinds from a drum.  This process transfers the mechanical energy to the generator. In the returning phase, an electrical motor on the ground rotates the drum back and pulls the belt back while the wing is set to fly in the opposite direction with minimum drag.  The returning phase starts with the wing doing a U-turn, then continues as the wing flies in the opposite direction and ends with the wing doing another U-turn to arrive at the position at which the working phase began.  The energy that is expended in the returning phase is only a small fraction of the energy generated in the working phase.

The system can accommodate changes in the wind direction by moving either an effective attachment point (Fig. 2) or the platform with the generator (Fig. 3).

Airborne Wind Energy System w/FMT, moving attachment - scheme

Fig. 2. Scheme of an AWECS w/FMT and a moving effective attachment point

 

Fig. 2 shows a system, in which the tether is attached by two cables of variable lengths.  A control system can change the effective attachment point by pulling in or letting out these cables.  Three cables allow accommodating winds from all directions.

Airborne Wind Energy System w/FMT, offshore, moving generator - a scheme

Fig. 3. Scheme of an offshore AWECS w/FMT and a moving generator

 

The system from Fig. 3 is practical only in offshore environment.  In it, the tether is ultimately anchored to the bottom, and the generator platform is installed on a buoy.  The buoy is controlled to move with the wind and to always stay downwind from the anchor.

One of the pictures on in the slide show on the home page illustrates an AWECS according to this scheme.  A slightly more complex construction allows attaching the tether to a rotating generator platform.  Such system is illustrated in another system on the slide show.

 

Economic Advantage

Three factors contribute to the reduced cost of the airborne wind energy conversion system with fast motion transfer compared to the conventional wind turbine: (1) a smaller amount of required components; (2) a lower cost of the components; and (3) a higher capacity factor.

In Table 1, the AWES column contains zeroes for components that are present in conventional wind turbines and are absent in the AWES.  The low cost of the wing area per kW is due to the wings’ high mechanical efficiency and the AWES’ ability to use flexible wings.  The costs for the conventional wind turbine components are taken from [3] and [4]. The AWES generator costs less because it is a mass-produced generator rather than a specialized wind-turbine generator.  The cost of the pitch system, rotor brake and coupling (i.e., rotor PBC) for the wind turbine is replaced by the cost of the control surfaces of the wing for the AWES.  A conservative estimate is presented in Table 1 for the additional costs of AWES’ components that are not present in conventional wind turbines, such as the tether and subsystem for launch and recovery.  The total cost estimate is $19/kW of peak power for AWES w/FMT.  Notice that AWES w/FMT can be entirely manufactured using existing and widely available materials, components and technologies.

 

Table 1.  Costs of AWECS w/FMT vs. conventional wind turbines, per kW

Component

Conventional Wind Turbine

AWES w/FMT

Blades / Wings

$177

$20

Hub

$77

$0

Gearbox

$143

$0

Generator

$76

$50

Yaw System

$19

$10

Nacelle cover & structure

$60

$0

Tower

$219

$0

Variable Speed System

$73

$0

Rotor PBC / wing control surfaces

$52

$20

Shaft

$41

$0

Other (incl. control system)

$63

$90

Total Components Cost

$1,000

$190

 

Table 2.  Costs ratio, taking into account capacity factors.

Component

Conventional Wind Turbine

AWES w/FMT

Components Cost, $/kW

$1,000

$190

Wind Energy Capacity Factor

35%

70%

Costs Ratio

1.0

0.095

 

Thus the airborne wind energy system is more than 10 times less expensive than a conventional wind turbine with the same average power output.

 

The amount of power that a single AWES can provide:

In all types of the crosswind kite power systems, the useful power can be estimated using the Loyd’s formula:

Loyd Formula of Airborne Wind Power, from lift and drag coefficients, air density, wind speed and wing area

where P is power; CL and CD are coefficients of lift and drag, respectively; ρa is the air density at the altitude of the wing; A is the wing area and V is the wind speed. This formula disregards tether drag, wing and tether weights, change of the air density with altitude and the angle of the wing motion vector to the plane, which is perpendicular to the wind.  More precise formulas are derived in [1] and [2].  With typical for a kite wing values of CL=1.2, CD=0.2, V=10 m/s and the wing area 500 m2, the produced power is 2 MW.  Multiple AWECS systems can be combined in wind farms similar to the conventional wind farms.

To learn AWELabs’ technology deeper, read the publications and technical documents, or write to info@awelabs.com.

 

Remarks

AWES is an abbreviation for Airborne Wind Energy Conversion System.  Same as AWECS (Airborne Wind Energy Conversion System).

Airborne Wind Energy is sometimes referred to as High Altitude Wind Energy (HAWE) or Kite Power.