ABSTRACT: This work describes possibilities and methods of application of smaller wind turbines in production of electric energy for specific operating conditions on the Croatian shore and islands. The results of simulation of possible annual production of electric energy in two smaller wind turbines for the characteristic wind potential are presented.

Smaller wind turbines could be used to support those consumers that are not connected to the power grid. Also, fossil fuel could be saved in autonomous electric energy systems with diesel generator units, which would contribute to the energy balance in mixed generator plants. Smaller wind turbines are clean energy sources suitable for meeting a part of energy demand in tourist facilities. Combined with photovoltaic devices, smaller wind turbines provide an alternative method for supplying households with electrical energy.

1. INTRODUCTION

The research conducted so far into possibilities for production of electric energy in the Republic of Croatia, [ 1] and [ 2] , and meteorological basis obtained for this research [ 3] , has shown that there exists a sufficient wind potential on the Croatian shore and islands for possible production of electric energy by means of up-to-date wind turbines of medium and higher power ratings (>100 kW). The results of these studies were obtained through simulation using the CRO-EOL expert system. On selected macrolocations with favorable wind potential for wind energy farms connected to the electric network, possible annual electric energy production amounts to 375.9 GWh with 250 kW generator units, or 605.6 GWh with 500 kW generator units. The total calculated cost of electric energy produced in this way on some macrolocations would be lower than the purchase price, so that potential investors could make a profit.

Beside macrolocations selected by the multicriterial analysis there is a number of smaller sites that have not been included in the studies mentioned and that have sufficient wind potential for conversion by means of wind turbines. These are, e.g. capes or hills near smaller settlements. It is not possible to install a large number of wind turbines of medium and higher power ratings on these sites, but it is possible to install one or more wind turbines of smaller power rating (<100 kW).

If smaller wind turbines were used, the technical wind potential, i.e. that part of the natural wind potential that can be converted into other forms of energy, could be used to a greater degree, [ 2] , so that “small” wind energy systems could play a significant role in power supply in Croatia.

Some theories state that small systems using renewable energy sources (wind, sun, water) will have the greatest effect on the world energy system in the twenty-first century, [ 4] .

A systematic approach to these issues in Croatia would greatly contribute to preservation of natural beauty of this part of Europe.

2. CLASSIFICATION OF SMALLER WIND TURBINES AND FARMS

To provide for better understanding of the text and for analysis of possibilities of application of smaller wind turbines, Table I proposes their classification. Each category is defined by generator power ratings in kW as intervals of the number 10 powers, [ 5] .

The class column describes wind turbines according to their dimensions, installation method, and energy yield.

The medium speed interval represents the mean annual speed at the wind turbine rotor axis height below which it is not advisable to install a certain type wind turbine.

The activity at the site column indicates the principle of calculation: it is not necessary to calculate the wind potential to select a site for a micro wind turbine, it is necessary to obtain an expert (meteorologist) opinion and have data obtained by observation near the place a wind turbine is to be sited (the Beaufort scale, the wind rose), [ 6] . To select a site for a mini wind turbine, a calculation of wind potential should be carried out based on the existing data.

Table I. Proposal of wind turbine classification, [5].

In the case of a small wind turbine, a study should be made, additional data on wind potential at or near the site should be gathered, and operation of a number of wind turbines suitable for use on that site should be simulated (e.g. using the CRO-EOL expert system, [ 7] ), while in the case of medium and large wind turbines, not only a study or a project is required, but the wind potential at the site chosen should be measured for at least a year, at the height of the rotor axis of possible wind turbines.

Table II provides a classification of wind farms according to the installed power criterion, using the same methodology as for the turbines.

Table II. Proposal of wind farm classification, [5].

Columns indicating category, wind farm class description, and installed power in MW as the interval of the number 10 powers, are followed by the column indicating the category of the wind turbine to be installed in the wind farm and the column indicating the need for a test or pilot plant for examination of the wind farm behavior before the wind farm is built.

Tables I and II contribute to standardization of terms in wind energy systems.

3. USE OF SMALLER WIND TURBINES

Wind turbines are appropriate for supplying consumers such as pump systems on farms, for navigation, telecommunications and signal devices on ships, light houses and remote stations, electrical devices in weekend houses, distant settlements etc.

It is known that medium and large wind turbines convert wind energy into electric energy in a more effective and economical way than do smaller wind turbines. The reasons for using smaller wind turbines in production of electric energy are, [ 8] : poor electric grid in the area as in the case of settlements with a small population and small electric power consumption; fluctuations of electric current in the grid are smaller if the wind farm consists of a number of smaller wind turbines; costs of installation of smaller wind turbines are smaller because it is not necessary to use large cranes, which presents a huge problem on complex ground; delivery of electric power from a wind farm is less affected in the case of failure in one wind turbine due to unforeseeable circumstances; sometimes the esthetic aspect of the site is less impaired by a large number of smaller wind turbines than by several larger ones. In areas with sufficient sun and wind energy, as is the case of the Croatian shore and islands, renewable energy sources can be combined. Thus it is possible to use wind turbines in combination with photovoltaic devices. Such hybrid systems reduce seasonal variations of the total wind and sun energy, [ 9] .

4. SELECTION OF WIND POTENTIAL FOR SMALLER WIND TURBINES

Manufacturers’ specifications and research indicate that wind potentials expressed by the mean annual speed indicated in Table I can be used for smaller wind turbines. Wind potentials expressed by the mean annual wind speed up to 7 m/s at the wind turbine rotor axis height are sufficient for effective operation of smaller wind turbine units (<100 kW); wind potentials of up to 7 m/s at a height of 25 m above ground are often encountered in Croatia, [ 1] and [ 3] , Figure 1., according to [ 10].

When choosing sites for smaller wind turbines, one should take into consideration the effect of the ground on the wind potential, especially the morphology, shapes, and obstacles, [ 11] . It is also necessary to know the wind rose and characteristics of wind turbulence at the potential site.

One of the fundamental features of the Croatian shore and islands is complexity of the ground, which can increase or decrease the wind energy potential. As the rotor axes of smaller wind turbines are placed relatively low above ground, from several m to approx. 30 m above ground, possible obstacles should be paid attention to, especially those lying in the direction of dominant winds that might obstruct the wind flow and significantly affect the local wind potential. This especially applies to micro and mini wind turbines, which require lower mean speed for

operation but are more susceptible to turbulence caused by obstacles in the wind flow.

5. POSSIBILITIES FOR APPLICATION OF SMALLER WIND TURBINES IN PRODUCTION OF ELECTRIC ENERGY

Production of electric energy in smaller wind turbines (micro, mini, and small) is less economic due to poorer efficiency of wind energy conversion in comparison to larger wind turbines (medium and large). However, the primary task of smaller wind turbines is to provide electric energy for previously defined devices so that efficiency takes the second place.

5.1 Production of electric energy in micro and mini wind turbines

This is the most frequent method of wind energy conversion in our country. There is a problem of relatively poor efficiency of wind energy conversion in smaller wind turbines. Also, there is a problem of reaction to turbulence in the wind flow, which greatly affects the efficiency of wind energy conversion.

Several dominant winds alternate along the Croatian shore with varying degrees of turbulence. There is a theory that given a proper choice of existing air profiles or development of special air profiles, adapted to varying degrees of turbulence in the wind flow, the same generator unit in smaller wind turbines would achieve higher wind energy conversion efficiency. For the purpose of these studies, research equipment, with a micro wind turbine for its basis, is being installed on the roof of the Faculty building, Figure 2.

Figure 2: A photo of a micro wind turbine (P=0.12 kW; D=1.2 m) used to examine the effect of turbulence and air profiles on wind energy conversion efficiency.

A small hybrid system consisting of a micro or mini wind turbine and a photovoltaic device and an electric energy accumulation and transformation system represents an almost ideal ecologically acceptable electric energy source for special consumers on the Croatian shore and islands. The quantities of electric energy produced are not large, but their contribution to preservation of the environment is. If solar systems for water heating are added to this hybrid system, a greater part of energy demand of an average household in areas rich with sun and wind energy (e.g. Mediterranean) can be met.

5.2 Production of electric energy in small wind turbines

According to data from 1991, [ 12] , a population of 120,000 inhabited 303 insular settlements. The average number of inhabitants per household being 3.1 for Croatia, this would mean that there were 38,700 households on the islands. If, incentives provided, every hundredth household invested in a wind turbine of e.g. 10 or 15 kW rated power, Figure 3, for complete or partial meeting of their needs, and accumulation or selling of the surplus electric energy produced, this would mean 387 units with 3.87 and 5.8 MW installed power respectively.

Figure 3: Presentation of operating features of two wind turbines of 10 and 15 kW rated power.

It is estimated that the installed power mentioned would not affect the stability of the electric energy network, [ 13] . As the sites for possible installation of small wind turbines have not been determined, so that there exist no data about the wind potential either measured or calculated, one should use the representative wind potential to simulate production of electric energy in a small wind turbine. The wind potential chosen has been calculated by WASP for a site with winds, orography and morphology similar to most sites suitable for siting small wind turbines, [ 3] . The basic features of the wind potential described are: the mean speed of 6 m/s at 25 m a.g.l., the Weibull distribution parameters A=6.5 and k=1.31, Figure 4.

Figure 4: Distribution of annual duration of individual wind speeds with parameters A=6.5 and k=1.31 with the mean annual speed of 6 m/s at a height of 25 m.

Figure 5. shows simulated annual production of electric energy for small wind turbines of 10 and 15 kW. These wind turbines could produce 24.7 (10 kW) i.e. 32.8 (15 kW) MWh/year per unit, for the mean annual wind speed of 6 m/s. With a total of 387 units it would be possible to produce 9.56 (10 kW) i.e. 12.7 (15 kW) GWh of electric energy annually.

Figure 5: Possible cumulative annual production of electric energy for two smaller wind turbines of rated power of 10 and 15 kW respectively for the wind potential in Figure 4.

Analyzing the results of the studies (1. chapter) and simulated production of e.g. 500 kW units of 605.6 GWh annually on the shore and the islands, it is evident that it would be possible to additionally obtain 2.1 % of that energy on islands only, if 15 kW units were used. Naturally, if the interest of investors into smaller wind turbines were higher, the percent of the electric energy produced could be higher. The insular electric network is connected to the mainland one, but it is not stable, especially in the period of peak consumption during tourist season. If a part of household demand for electric energy were met by operation of smaller wind turbines, the unstable network would be less burdened. Smaller wind turbines can be used in a hybrid system that includes a Diesel generator unit. Thus, a Diesel generator unit would be a stable source of electric energy in the unstable branches of electric network, and the operation of wind turbines would serve to save fuel.

6. CONCLUSIONS

- relatively high initial investment, poorer efficiency of energy conversion and wind instability, present an obstacle to investment into smaller wind turbines for production of electric energy. Provided some incentives by the local and state authorities, a systematic research and adaptation of wind turbine features to particularities of the wind potential in order to increase the wind energy conversion efficiency, smaller wind turbine systems would become more acceptable economically.

- for commercial production of electric energy in smaller wind turbines it is necessary to define in detail the technical requirements for connection to the electric energy system of the Croatian Utility, [ 13] . Possibilities for connection of smaller wind turbines as well as appropriate incentives have been discussed in a part of the National energy program - ENWIND, [ 10] .

- before smaller wind turbines are installed or smaller wind farms are built, it is necessary to gather empirical data on wind potentials from people living in the area of installation/building or in its vicinity as an aspect of expert knowledge taken from the empirical database. This knowledge should be gathered by polls and meteorologists should be consulted. A need for as extensive a base of measured data on wind potential as possible is indicated, as one of the most relevant factors affecting the choice and use of wind turbines.

- possible investors in smaller wind turbines should have the wind potential for the selected sites determined as precisely as possible. The number of possible sites being very large, wind potentials should be calculated at several heights above ground (e.g. at 10, 15, 20 m) on the territory of the whole country, and that information should be made available to possible investors into smaller wind turbines free of charge, as a contribution to the first paragraph of these conclusions.

REFERENCES

[ 4] Reid, B.: Modern small-scale wind power: case study, The World Directory of Renewable energy - Suppliers and Services 1996, James & James, 1996, 222-226.

[ 6] Klimatološki atlas Jadranskog mora, Hidrografski Institut RM, 1979.

[ 8] Hehenberger, G.: Utility-grade wind energy for remote areas: case study, The World Directory of Renewable energy-Suppliers and Services 96, James&James, 1996, 236-238.

[ 9] LMW System guide, LMW Renevables BV, Groningen, 1996.

[ 11] Troen I., Petersen E.L.: European wind atlas, Risř National Lab., Roskilde, 1989.

[13] Tehnički uvjeti za priključak malih elektrana na elektroenergetski sustav, HEP, Zagreb, 1995.