Two major jet streams, the Sub-Tropical Jet and the Polar Front Jet exist in both Earth hemispheres. These enormous energy streams are formed by the combination of tropical region sunlight falling and Earth rotation. This wind resource is invariably available wherever the sun shines and the Earth rotates. These jet stream winds offer an energy benefit between one and two orders of magnitude greater than equalrotor-
area, ground mounted wind turbines operating in the lowest regions of the Earth’s boundary layer. In the USA, Caldeira and O’Doherty and Roberts have shown that average power densities of around 17 kW/m2 are available. In Australia, Atkinson et al show that 19 kW/m2 is achievable.These winds are available in northern India, China, Japan,Africa, the Mediterranean, and elsewhere. Various systems have been examined to capture this energy, and these include tethered balloons, tethered fixed-winged craft, tether climbing and descending kites, and rotorcraft. Our preferred option is a tethered rotorcraft, a variant of the gyroplane, where conventional rotors generate power and simultaneously produce sufficient lift to keep the system aloft. This arrangement, using a twin-rotor configuration, has been described and flown at low altitude by Roberts and Blackler (Fig. 1). More recent developments have produced a quadruple rotor arrangement (Fig. 2). Commercialization of the quad-rotor technology could significantly contribute to
greenhouse gas reductions.
Tethered rotorcraft, with four or more rotors in each unit, could harness the powerful, persistent jet streams, and should be able to compete effectively with all other energy production methods. Generators at altitude also avoid community concern associated with ground-based wind turbine appearance and
noise. Bird strike problems are also less. However, tethered generators would need to be placed in dedicated airspace, which would restrict other aircraft. Arrays of tethered
generators would not be flown near population centers unless and until operating experience assured the safety of such a configuration.
Fig. 1. Photograph of early two-rotor prototype in flight.
At this time, the best tether for the rotorcraft appears to be a single, composite electro-mechanical cable made of insulated aluminium conductors and high strength fiber. When operating as a power source, two, four, or more rotors are inclined at an
adjustable angle to the on-coming wind, generally a 40 degree angle. The wind on the inclined rotors generates lift, gyroplane-style, and forces rotation, which generates electricity, windmill-style. Electricity is conducted down the tether to a ground station.
The craft simultaneously generates lift and electricity. However, it can also function as an elementary powered helicopter with ground-supplied electrical energy, and with the generators then functioning as motors. The craft can thus ascend or descend from altitude as an elementary, tethered helicopter. During any lull periods aloft, power may be supplied to maintain altitude, or to land on a small groundbase. A ground winch to reel the tether could be used to retrieve the craft in an emergency.
Fig. 2. Rendering of Sky WindPower Corp.’s planned 240 kW, four-rotor
ELECTRICAL SYSTEM DETAILS
Flying electric generators need to ascend and remain aloft for short periods on grid-sourced energy. In low-wind conditions, only a small proportion of output rating as grid sourced energy is required to raise or maintain the craft aloft. Voltages at the terminals of both the generator/motor and at the grid interface need to be kept within designed tolerances and/or be adjusted by timely voltage regulation.
In a national regulated electricity market, such as that found in Europe and elsewhere, a System Impact Study (SIS) is required to connect a new generator to the grid if the generator’s capacity is above a minimum level, e.g. 5 MW. Even non-dispatchable “embedded generators“ require Grid System Impact Assessments. The generator proponent usually pays for the generator-to-grid network connection. Land and sea locations for generation from renewable energy sources, especially wind energy, are often remote from the existing grid, hence, connection costs are often 50% of the total investment for new generating capacity. Also where a renewable energy source generator is not n-1 reliable for availability, the Network Connection Contracts usually include the costs of back-up supply contingencies. These relate to
network charges when the renewable generator is not supplying.
Flying electric generators at altitude will have a relatively high availability, around 80%. Reliability and peak premium sales could be enhanced by a link to a pumped storage facility for off-peak filling/storage and peak-release energy sales and
delivery. Energy could be stored as hydrogen gas produced from electrolysis, or as water pumped-back and re-released for hydroelectric generation.
Conventional ground-based wind energy systems harvest only about 30% availability. Flying electric generators, in single units of 20 MW or more, can achieve about 80% availability with suitable sitting at land or sea locations. These generators at altitude involve power transmission over lengths of between 4 and 8 km. Flying generator/tether voltages between 11 kV and 25 kV ac could be used on units of 30 MW at the most extreme altitudes. Also there are recent modern innovations, which use powerformers/motorformers. The latter, being developed by equipment suppliers such as ABB, Siemens, Mitsubishi, etc., would allow polymeric cable stators and tether voltages at say 33 kVac or more. Grid interfacing would then be easier at bulk energy levels.
The jet-stream location can drift north and south, so seasonal mobility from one prepared site to another could be a feature of flying generators’ grid utilization and optimization. This could be advantageous in seasonal summer/winter
demand-side management through peak-matching generator placement or relocations. This would include matching seasonal peaks for rural industries, such as grape processing, cotton harvesting, and irrigation to urban air-conditioning etc.
Because arrays of flying generators could move north or south to follow seasonal shifts in wind patterns or power demand, it could be advantageous to have “plug-in” flying generators at pre-arranged sites along an existing grid 33 kV, or more, overhead feeder with minimal interfacing. This would use, for example, a HV Live Line HV Bypass cable, sometimes called Temporary cable, with a mobile or
transportable High Voltage Generator switchyard circuit
If the tether arrangement were to contain three conductors, two could form the single-phase circuit, while the third could be the ground wire and control cabling function. Three-phase balance is then achieved by adding other nearby generator
outputs to form single-phase combinations for grid connection. Alternatively, if necessary, a transformer with OLTC could be used, similar to that used for monoplex or 50 kVac duplex rail electric traction supply. This would be similar to a rail traction
supply transformer of 50 MVA and 132 kV three phase to 25 kV ac positive and 25 kV ac negative to centre tap earth.
When using a shipboard site, fixed ocean site, or a site adjacent to a water-reservoir which is remote from the desired FEG ground-surface connection location, then the use of HVDC on tethers, with surface/submarine cabling, should be
considered in combination with a HVDC voltage motorformer/powerformerTM design. In addition, a unit’s DC motor/generator commutation by conventional brushes might be facilitated by more modern electronic switching or by
triggered Vacuum Gaps (TVG).
Where an AC interfacing transformer, or a HV AC /DC Converter Station (usually with an included transformer) is required for grid interfacing connectivity, the economics of scale would encourage more multiple-unit connections.
A 60 MW to 150 MW grid connection composed of three 20 to 50 MW airborne units with a powerformerTM, or HVDC AC/DC connection, can perform as a synchronous condenser, thereby adding AC grid stability advantages in the SIS. This will depend on grid siting.
Starting and retrieval characteristics of flying units at specific grid connections could be an important SIS review item. A higher fault level at the connection site is desirable for a large motor start up. Generator and tether performance depend on a good lightning storm detection system. Surge protection schemes and hardening of the control systems are also under examination.
ADVANTAGES & DISADVANTAGES OF FEG
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