CVT for WIND applications
Reducing the production cost for renewable energy is the main priority to improve their market penetration. Such a reduction can only be achieved through real technological advancements allowing the fall of plant production costs and contemporaneous bringing an efficiency improvement to the same plants.
An effective front-line to work on is surely the improvement in the features of turbines' axes line. The first generators used in wind machines were conventional electrical motors employed as generators normally connected to the mains. In spite of their very low cost this solution exhibited the drawback that the efficiency curve had a quite peaked maximum at the motor nominal power. When the turbine worked at power levels different from the nominal one the efficiency fell to very low values, thereby negatively impacting on the overall energetic performance. It has been tried to solve the problem by introducing huge inverters, but the cost for such power electronics is so high that other roads have been preferred to be trodden. Thus, to solve the problem above, the industry has developed several systems.
Double Polarity Generator Fixed Speed Machines
In these machines the generator is connected directly to the mains, see Figure.
Variable Speed Machines
Nowadays almost all the new installed generators are equipped with this type of machines, see Figure. By means of proper power electronic devices the rotor's rotation speed can be controlled (slipping control), thereby regulating the generator's, so that it is able to work at maximum efficiency adapting to the wind conditions. In this case the frequency converter handles only the rotor current while the stator current is connected directly to the mains. This has allowed to reduce the costs for the power electronics with respect to the old inverter models where all the generator current flowed through the inverter itself.
No Revs-Multiplier Multi-polar Generator Machines
This type of solution occurs to be very reliable in terms of reduced maintenance in time, see Figure. The drawback of these systems is however the high manufacturing cost due to their complexity and to the small standardization of the generators, as well as the high cost for the control electronics.
No Revs-Multiplier Permanent Magnets Multi-polar Generator Machines
As the previous machines the main advantage is the modest maintenance necessary, see Figure. The drawback remains the excessive cost for manufacturing the turbine.
SOLUTION
The above problems can be solved by employing a revs multiplier/variator carried out with overall absence of toothed gears. Such multiplier is able to maintain an efficiency in all its working conditions higher than 95%.
A first application of our patented CVT (Continuously Variable Transmission) can be the one represented in the scheme below and which we attach PDF format.
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The wind turbine with asynchronous generator normally mount a gear rpm multiplier , Rd 1:62, allowing the rotating motion of the blades, 15-30 rpm, up to 1000-1950 rpm, ideal rotation for a current generator that provides power at 50 Hz
Such transmissions have not high efficiency and about 8% of the power impressed by the wind to the blades is dispersed. These fixed ratio transmissions limit the use of the turbine within a fairly narrow rpm range, so as to allow the alternator to function properly. The fixed transmission ratio then requires the use of special inverters/converters aimed at balancing the changes in frequency of the current produced by the generator. As it was said above there are winded rotor generators where the sliding of poles is managed, but also in this case the electronica is very expensive.
The use of a rpm multiplier with Rd 1:50 combined with one of our CVT Rd 1:2 allows producing a turbine with a standard 4-pole generator connected directly to the network and able to maintain the generator at constant rpm (1500 rpm) while allowing the rotor blades to move from 15 to 30rpm. Then one gets a system with variable blade speed , +/- 30%, particularly economic (slightly more the cost of a normal rpm multiplier) and with energy efficiency in excess of 95%.
Another application of our CVT is to integrate it into a multipolar generator. In this case, due to lower rpm from the multipolar generator, a two-stage CVT without the rpm multiplier can be used. The advantages in terms of efficiency are high as the CVT does not use sprockets inside. At the same time maintenance costs are drastically reduced, as it happens in generators without rpm multipliers.
