AWES Performance Definitions

Power

Average Cycle Power / Electrical Average Power

Symbol: Pe,avg

Definition: Energy over one Power Cycle trajectory divided by the Cycle trajectory time. [kW]

Notes:

Figure 1: The figure shows the instantaneous electrical power measured at the generator/motor of the AWES for a ground wind speed of 6 m/s (normally a wind speed given at pattern trajectory height would be better).

The blue areas indicate the reel out phase of the cycles (positive power) while the red areas indicate the reel-in phase of the pumping cycles. The average cycle power is indicated in the figure by the horizontal dashed line.

Figure 2: The figure shows the instantaneous mechanical power for an arbitrary 150 kW system during one pumping cycle.

  • Most important is the electrical power.
  • Measured on the output side of the individual AWE system, i.e. before the transformer (see definition for conventional wind turbines: Measured on the low voltage side of the transformer, i.e. it is measured on the turbine side.)
  • A cycle is mainly relevant for ground-generation systems.

TBD:

  • How to define for off-grid systems that are only connected to a battery pack.
  • Losses in the battery should be excluded because flattening with batteries is not mandatory, especially in parks and arrays
Rated Power / Nominal Power

Symbol: Pn

Definition: Maximum Average Cycle trajectory Power. [kW]

Notes:

  • The Rated Power refers to the Reference Power Curve
  • The Rated Power can be limited by the system or the generator
  • The Rated Power is the blue dot in the figure above.
  • This metric is the one that would be compared with a conventional wind turbine (e.g. “2 MW-turbine).
Electrical Peak Power during reel-out

Symbol: Pe,o,peak

Definition: Maximum power that a system can sustain over a certain time before failures are very likely. [kW]

Notes:

  • In general measured after the generator.
  • To be differentiated:
    • measured at the system side (just before feeding it from a single system into the park)
    • measured at the system side of the transformer (park towards grid) – this is important for the grid operator. Some measures may be applied so that peaks are avoided.
  • It depends on the rated power of the electrical machine.
  • If this value is exceeded for extended period of times, the generator may get hot or something may break.
  • The Peak Power at the transformer could be lowered through heat elements (resistor banks) or storage devices.
  • Focus should be on the relevant value from a grid operator perspective or what is the critical value to size the grid connection.
  • It might be important to specify values for different durations (1s peak vs. 30s peak for example), also the power factor would be important as well.
  • Graph with the different system components and measurement points:

Electrical Peak Power during reel-in

Symbol: Pe,i,peak

Definition: Maximum power measured during reel-in. [kW]

Notes:

  • Power should be measured where relevant to capacity that needs to be provided (either contracted as grid imports or through energy storage system).
Produced energy during reel-out

Symbol: Ee,o

Definition: Energy generated by the system during reel-out phase [kWh]

Notes:

  • “Internal” term, not so relevant for AWE clients.
  • Relevant mainly for ground-gen systems.
  • The produced energy is always equal or larger than zero.
Consumed energy during reel-in

Symbol: Ee,i

Definition: Energy consumed by the system during reel-in phase [kWh]

Notes:

  • “Internal” term, not so relevant for AWE clients.
  • Relevant mainly for ground-gen systems.
  • Important to determine efficiency of the system and size of the generator.
  • Consider that with a battery there would be no power needed from the grid.
  • Tbd if instantaneous or average

KPIs: Performance indicators / metrics

Availability factor

Definition

Factor obtained by the hours that the full AWES is available during periods with operational wind conditions divided by the total (cumulative) amount of hours in those periods. [%]

Notes and explanation

  • Important is the energetic availability, not the time-related availability
Annual Electricity Production (AEP)

Definition

The annual electrical energy production of a facility. Calculated as produced energy minus the consumed energy. [MWh/a]

Notes and explanation

  • A standard site may have to be defined to allow comparison between systems vs. versions of systems, or single systems vs. wind farms (wake effects, etc.)
  • Make distinction between gross-energy & net-energy-production. AEPnet=AEPgros* energetic availability
  • Make distinction between Measured AEP. vs. Calculated AEP.
Capacity factor

Definition:

Energy generated by a facility divided by the maximum amount of energy that could have been generated by that same facility when operating continuously at Rated Power during a specific time period (default: 1 year).  [%]

Notes:

  • If it is measured over the period of one year, the numerator is the Annual Electricity Production (AEP)
  • The Capacity Factor is site-dependent.
  • Important when comparing technologies from a system or market perspective.
Power Density

Definition: Power capacity installed above one square kilometer of land/sea (area) [MW/km2]

Notes:

  • This is asked frequently by policy makers and others; they want to know if you need more or less space for AWE compared to a conventional wind farm.
  • Tbd: Need to specify/define how to account the occupied area: circle with radius=cable? Include safety margins? And is it computed for a farm with possibly overlapping circles?
  • It depends on the park configuration.
  • Overlapping circles should be possible.
  • Rated power over outer limits of the ground stations (so the area that has to be owned – like an airport: the land over starting and landing corridors is not owned by the airport operators.).
  • Important for comparison with other technologies.
  • 35 MW/km2 may be possible.
  • Principles for conventional turbines: Neighbour must be able to put wind turbine up. Different for AWE because they point in the same direction.
  • “Energy density” may be better in comparison, because of the higher capacity factor / lower rated power.
Yearly Weighted Energy Average

Definition: Power obtained by summation of the power contribution over all wind speeds, by weighting each power contribution at a wind speed by its probability of occurrence, according to a wind speed bin using a Weibull distribution, during a year. [MWh/a]

Notes:

  • Simplified approach to the AEP, gives customer an idea of energy output by site (low, medium, high wind site)
  • It needs to be defined how this is done exactly. In the end it is a sort of weighted power curve
  • Probably a long way to a standardized calculation – ongoing discussions in other Working Groups
Production Predictability

Definition: Ratio of the actual parameter value (energy produced) divided by the simulated parameter value, i. e. it provides the simulation error (1-predictability). [%]

Notes:

  • “Strategic” KPI: Potentially an advantage for AWE because there may be less wake problems. Once companies sell systems, this will become important.
  • This is a challenge even for conventional wind turbines due to difficulty in assessing the wind field. more easily done on other variables than energy, but then it becomes a bit too technical.
  • Distinction between "wind resource does not match wind forecast" and "electricity generation does not match calculation/simulation”. Probably years away from meaningful values/data
Uptime / Annual Flight Hours

Definition: Time that Airborne component is Airborne [h]

Notes:

  • Similar to the Availability Factor.
  • There will be times when the kite is flying but not producing or even consuming energy.

Operational phases

Launch

Definition: Take-off phase of the kite

Notes:

  • Close to the ground
Reel-out

Definition: Phase in which the kite pulls out the tether from the winch

Notes:

  • There may be times of reel-out where the system produces very little power or even consumes onboard-power in case of propellor-driven aircraft.
  • Depending on the system, also during reel-out there may be phases where the system needs to be reeled-in to keep the tether tension and manoeuvre (technically the aim is to remove this).
Reel-in

Definition: Phase in which the kite is being retracted

Notes:

  • n/a
Park in air / hover / idle

Definition: Phase in which the kite is flying but not generating, either steady or carrying out flight paths with almost constant tether length

Notes:

  • State where the kite is meant to stay airborne
  • This usually applies mainly to systems with light flexible kites.
  • For fixed wing kites with onboard vertical axis propellers, it may be called “hovering” phase.
  • For systems with horizontal axis propellers (e.g. Ampyx), it may be called “idling” phase.
Trans-in

Definition: Transition from hovering to any other operational phase

Notes:

  • Term is also used in the Makani videos
Top Transition

Definition: Transitioning from reel-out to reel-in phase

Notes:

  • Only relevant for Yo-Yo systems
Bottom Transition

Definition: Transitioning from reel-in to reel-out phase

Notes:

  • Only relevant for Yo-Yo systems
Approach

Definition: Phase before landing

Notes:

  • n/a
Land

Definition: Phase during which the kite lands

Notes:

  • n/a
Operation

Definition: Phase which includes all airborne flight phases excluding Emergency Operation

Notes:

  • Depending on the system, it could include launch, reel-out, hover/parking in the air, reel-in, land.
Emergency Operation

Definition: Phase that includes all manoeuvres under emergency conditions

Notes:

  • There may be different Emergency Landings, e.g. with or without reel-in, landing close or further away from ground station, etc.
Parked on ground

Definition: Kite not aloft and systems functions turned off

Notes:

  • Different parking status are possible.
  • “Parkposition”, IEC
Stowed on ground

Definition: Kite secured when not in operation

Notes:

  • This include stowing for extended periods of time
  • Stowed on ground under extreme conditions includes e.g. additional fixation, moving to a hangar or other shelter to withstand extreme weather conditions.
Yo-Yo Mode / Pumping Mode

Definition: xxx

Notes:

  • Mode of ground-generation AWE systems which operate in two phases (reel-out and reel-in)

Trajectories

Pattern

Definition: A figure the tethered aircraft follows during power production (figure of eight or circle).

Notes:

  • n/a
Pattern Trajectory

Definition: A Pattern the tethered aircraft follows during power production

Notes:

  • Examples:
    • Multiple figures of eight in a cone shape
    • Multiple figures of circles, i.e. spiral
    • Single circle (e.g. Makani)
  • Ground-gen example (pumping mode):

  • Fly-gen example:

Cycle Trajectory

Definition: The unique trajectory of a single cycle running through all operational phases once (without launch and land).

Notes:

  • Ground-gen example (pumping mode)

  • Fly-gen example

  • Depends on the wind conditions, tether length.
  • This includes the pattern trajectory and a retraction phase where applicable.
  • One cycle may contain multiple patterns. In such a case, the first pattern starting from initial tether length is the Reference Pattern.
Pattern Trajectory Height

Definition: Expected or logged altitudes time-averaged height over the pattern trajectory

Notes:

  • For a fly-gen system this would be the center of the circle
  • For a ground-gen system this would be the average height of the trajectory during reel-out
Rated Trajectory Height

Definition: Pattern Trajectory Height at which Rated Power is achieved

Notes:

  • n/a

KPIs: Performance indicators / metrics

Reference / Rated wind speed

Definition: Minimum wind speed at pattern trajectory height at which an AWES’ rated power is achieved in the case of steady wind without turbulence.

Notes:

  • IEC 61400-1: "minimum wind speed at hub height at which a wind turbine's rated power is achieved in the case of steady wind without turbulence."
  • Default: When not explicitly stated when referring to wind speeds, one can assume it refers to the rated wind speed.
  • One point on the power curve (see graph in Power Curve)
  • Basic parameter for wind speed used for defining AWES classes
  • Can be measured with LIDAR and is standard. ERA5 provides wind speed at 100m as a standard output. Good data availability and comparable to conventional WTG hub heights.
Cut-in wind speed

Definition: Lowest 10 min average wind speed at pattern trajectory height] at which the AWES starts to produce power in the case of steady wind without turbulence

Notes:

  • According to IEC61400-1: “lowest wind speed at hub height at which the wind turbine starts to produce power in the case of steady wind without turbulence.”
  • Still, cut-in wind speed depends on system and operation strategy such as adapting operational altitude.
  • The kite may be launched at wind speeds lower than the cut-in wind speed, this depends on the company and operational/economic optimization.
Cut-out wind speed

Definition: Wind speed at pattern trajectory height at which wind speed above which the facility can no longer be operated without risking damage to the system and at which power production ends

Notes:

  • Measured at pattern trajectory height.
  • Term should be used consistent with definition applied for conventional wind: IEC 61400:2020: “highest 10 min average wind speed at hub height at which the wind turbine is designed to produce power in the case of steady wind without turbulence”
  • This doesn’t mean that the kite must land, it could be put in an airborne parking position, or fly to another altitude or out of the wind window.
Launching minimum wind speed / Launching Limit Low

Definition: Minimum wind speed at which system can be launched (at 10m)

Notes:

  • n/a
Launching maximum wind speed

Definition: Maximum wind speed at which system can be launched (at 10m)

Notes:

  • n/a
Forced Landing minimum wind speed / Drop-out wind speed / Flying Limit Low

Definition: Minimum wind speed (pattern trajectory height) at which system must land

Notes:

  • Wind speed below the Cut-in speed at which an operational system can no longer be operated or will be ordered to halt operation in order to reduce wear and power consumption.
  • The AWES can "search" suitable wind conditions in its operational range and not land immediately if a certain threshold is reached.
Forced Landing maximum wind speed / Blow-out wind speed

Definition: Wind speed at pattern trajectory height above which the facility can no longer be operated without risking damage to the system during landing.

Notes:

  • Maximum wind speed at which system must land.
  • Time period tbd
  • If the wind speed is above that threshold, the system must land to avoid risking damage and cannot stay in parking position for example.
Maximum (survival) wind speed in the air

Definition: Wind speed above which the AWES will take damage in the air.

Notes:

  • In the range between “blow-out” and “survival” it might be impossible to land the AWES in a regular way.
Survival wind speed on the ground in storage position

Definition: Wind speed above which the AWES will take damage when stored or parked on the ground.

Notes:

  • Conventional wind: Range 216 km/h normal survival wind speed, AWE could go up to 300 km/h

Wind speeds

Flight Geography / Flight Volume

Definition: Volume the kite can fly through during normal operation.

Notes:

  • In general, the SORA approach is being followed:

  • Staying within the Flight Geography may have to be secured through Contingency Procedures.
  • AWEurope suggest replacing the term “Containment Area” with “Containment Volume”. The term “area” would be used for 2D, “volume” for 3D.
  • This includes the volume spanned by the tether as well
Current Flight Geography / Current Flight Volume

Definition: Volume in which the kite is flying under current wind conditions.

Notes:

  • The Current Flight Geography moves with respect to the wind; this volume will be in downwind direction of the ground station.
  • Alternative term: Instantaneous Flight Geography, Operational Flight Geography
Sector

Definition: A Flight Geography that is deliberately limited.

Notes:

  • Examples
    • excluding sectors to avoid flying above certain structures,
    • power generation sector downwind
    • reel-in sector
Containment Volume

Definition: Volume that the kite must not leave, even in emergency situations

Notes:

  • Staying within the Containment Volume must be secured through Contingency Procedures or (Emergency Procedures?).
  • Includes instances when they system is malfunctioning)
Operational Volume

Definition: Volume that includes Flight Geography and Containment Volume, i.e. where the kite can potentially fly through while continuing normal operation.

Notes:

  • n/a
Operational Area

Definition: Area where the kite can potentially fly over under normal conditions.

Notes:

  • "Normal conditions" still tbd
Landing Area

Definition: The area where kite is supposed to land under normal conditions.

Notes:

  • n/a
Emergency Landing Area

Definition: The area where kite is supposed to land under emergency conditions.

Notes:

  • In most cases of failure, mitigating measures can prevent the AWES from crashing outside the emergency landing area (see “contingency volume”).
  • This includes situations where the kite can “land” (or crash) in case of tether failure during flight
Launching Area

Definition: The area where kite is launched.

Notes:

  • For certain systems the Launching Area may be the same as the Landing Area.
Hazardous Area

Definition: Operational Area plus the Ground Risk Buffer Area

Notes:

  • This could imply no residential areas
Restricted Area

Definition: Area where only authorized personnel is allowed.

Notes:

  • This could imply e.g. a fence or wall.
Ground Risk Buffer Area

Definition: Area outside the Contingency Area which provides additional ground risk mitigation. Entering the Ground Risk Buffer Area implies the application of the Emergency Procedures.

Notes:

  • Definition still tbd

Flight Volumes and Areas

Fixed-Wing Kites / Aircrafts

Definition: Kites that are designed like conventional aircraft, but take into account the higher wing loading.

Notes:

  • They are used with single tether without any bridle lines (e.g. Ampyx Power) or with bridle lines (e.g. Makani).
  • The term “rigid” is not useful because the larger the wing span, the more flexible the wings actually get.
Soft Kites

Definition: Kites that use inflatable membrane wings and are generally of the type leading edge inflatable (LEI) tube kite or ram air wing.

Notes:

  • LEI kites (e.g. Kitepower) for high wing loading, such as customary for AWE applications, often use rigid reinforcement elements for the inflated tubular frame or the textile canopy.
  • Ram air wings (e.g. Skysails) typically achieve higher wing loading by increasing the number of bridle lines and reinforcing the textile material used for ribs and wing surface.
Hybrid Kites

Definition: Kites that combines a rigid support structure with a textile membrane canopy.

Notes:

  • These can be delta kites (e.g. UC3M, or EnerKite's trainer kite) or the EnerKite swept wing.