Miniaeolic and Microaeolic
The words "miniaeolic" and "microaeolic" mean those small plants that can produce electric energy from wind, ideal for domestic use of as integration of the electric consumption in small economic activities. Usually the miniaeolic indicates plants with a nominal power between 20 kW and 200 kW, while the microaeolic stays for plants with nominal power lower than 20kW. These plants can exploit the specific conditions of the location in which there is the plant. They are adaptable plants, they can use both weak and strong winds and they can intercept even sudden gusts of wind.
They can be distinguished in two great types of aerogenerators:
- at vertical axis: characterized by a reduced quantity of mobile parts in its structure, that gives it a high resi stance to the strong gusts of wind, and the opportunity to use every direction of the wind without a frequent orientation. It is a very versatile machine, ideal for the domestic use as well as the production of electric energy in gigawatt (a single turbine fulfil the electric need of an avarage of 1000 houses).
- at horizontal axis: equipped with a steel tower high between 60 and 100 meters on which top there is a shell (gondola) which contains an electric generator activated by a rotor with blades long about 20 meters (usually 2 or 3). It generates a very variable power, tipically 600 kwatt, that corresponds to the daily electric need of 500 families or to 1000 houses.
The kinetic energy of wind is captured and transformed in a rotation movement that can feed the electric generator, working by the principle of electromagnetic induction.
A rudder placed at the bottom grants the best position of the turbine in connection with the directional point of the wind.
As for the dimensions of the small turbines, as already mentioned, they go from a few ten of watt up 20 kW.
The smaller turbines ideal for the lowest applications, mainly nautical, are characterized by diameters of the rotor equal to about 1 m or little more, up to 8 – 10 m for the sizes from 10 to 20 kW. Beyond that limit, it makes no sense anymore to talk of miniaeolic.
The supporting tower can have different heights, however as a general rule the lower part of the blade must not be at a height lower than about 10 m from the soil. Obviously, the more powerful is the turbine, the better the performances at higher levels, up to a maximum of about 30 m for turbines of 20 kW.
The energetic production of small turbines depends on the anemometric features of the location in which the system is installed. Normally, the most part of the manufacturers provides an estimation of the annual production made in wind standard conditions (generally 5,5 m/s that corresponds to a site with a good wind level). On average, in these conditions, the medium annual production sets at 2 – 2,5 MWh a year for 1 kW of installed power, that is the quantity of
electric energy necessary every year for a domestic use of average dimensions.
For such an energetic production it is also said that the particular site is characterized by a number of hours correspondent to 2.000 – 2.500 hour. The corresponding hours are the result of the relation between the energy produced (in kWh) and the power installed (in kW) and, to ensure enough production from the installation and its economic sustainability, it is ideal to never go under 1.500 – 1.800 corresponding hours.
USE OF THE ENERGY PRODUCED
In order to exploit the electric energy produced by the aerogenerator it is necessary that the machine is connected to the electric network or, when there is no network, to a collection system that includes a set of batteries.
SYSTEM NOT CONNECTED TO THE NETWORK:
In systems not connected to the electrical network, and for this equipped with batteries, it is necessary to provide a regulation system that can properly manage the charge level. The energy can be immediately at disposal for the charge, in the case that this is equipped with devices at direct current or, on the contrary, an inverter should transform the direct current in alternate current before this latter can be used. It is important to precise that all the devices placed after the turbine reduce, partially, the performance of the energetic transformation, so that only the 70% of the electric energy actually produced from the turbine arrives at the charge for its use.
In this configuration it is possible to choose two different methods of energetic valorization: the exchange on place or the direct sale.
In the first method, the owner of the installation uses the electric net as a great storage in which he brings the energy produced, then he takes it when necessary.
The owner self-produce the electric energy he needs with the opportunity, thanks to the net, to use it even in another moment. With this method, it is better to act so as to produce a quantity of energy not higher than the quantity needed.
In fact, the excess is not paied, but it makes a credit for the next year and, in case, for the following years.
With the method of the direct sale, instead, the owner of the installation sells the energy produced directly to the Operator of the electric system (GSE), that gives to the owner the choice of incentivation: all-inclusive tarif, or the green certificates.
The first is valid only for wind plants smaller than 200 kW (1 MW for the other renewable sources) and it corresponds and grants to the owner 30 euro cents for each kWh produced and sold to the net for the 15 years next to the start of the production.
After 15 years the incentive is no longer supplied, but just the basic tarif.
As an option, it is possible to submit the mechanism of the Green Certificate, that are titles which certify the production of a certain quantity of electric energy from renewable sources and they can be exchanged in a special market or through bilateral negotiations.
The recent law dispositions, in particular the last financial laws, on the contrary, have introduced rules and incentives for the increasing and development of such technologies. As for the wind plants, the mechanism of exchange on place or the option of the direct sale has been extended to all the installations up to a maximum of 200 kW (beyond this limit the direct sale involves the automatic acceptance of the Green Certificates).
The costs of a small wind system can change according to the size of the turbine and of the support devices used (batteries, inverters, charge regulators, hight of the supporting tower). During the last years the costs of the small wind generators have progressively reduced thanks to the technological development of systems more and more integrated and standardized.
On average, a complete system, all inclusive, has a price from 5.000 to 7.000 euro/kW power for machines up to about 5 kW, while it decreases to 2.000 – 3.000 euro for larger dimensions.
The repayment of such an investment is sustainable provided that the location is windy enough.