INNOVATIVE SOLUTIONS FOR SMALL SCALE VERTICAL AXIS WIND TURBINES USED IN HARBOURS AND SHORE AREAS

Headnote

Abstract: The paper aims to analyse the wind turbine solutions implemented in harbours and on shore areas. Also a thorough study of the blade design solutions for small power Vertical axis wind turbines (VAWTs) has been conducted, with their advantages and disadvantages, in order to find the best solution that minimises the loads and helps with the self-starting capabilities of the wind turbine. First are presented all the solutions, next are discussed several research results for each solution and, in the end, a combination of solutions is chosen for our new small power VA WT with a pre-dimensioning analysis.

Key words: Vertical axis wind turbines (VAWTs), wind turbines implemented in shore areas, blade design, aerodynamic profile.

INTRODUCTION

The research in the area of renewable energies is more and more focusing on bringing the energy sources closer to the place where it is actually needed and used. This is the reason why, implementing wind turbines in areas where a reduction of energy production from conventional sources is of a huge interest [1],

When it comes to sea or seashore wind energy, almost all the research is focused on large, offshore. Horizontal Axis Wind Turbines (HAWTs). Thus, overlooking the huge potential that the seashore has for small power wind turbines. Hence, in this article, the study is aiming to investigate the requirements, problems and solutions raised by integrating the small Vertical Axis Wind Turbines (VAWTs) into seashore building and in harbour areas (on shore and on ships).

Several researches have been conducted in order to define the requirements regarding what a building or ship implemented wind turbine should fulfil [1-3]. Among the requirements are: to cope with turbulence and gusty wind, to have the optimum dimensions with respect to the energy demand and to the building/ship, to be silent and without vibrations, to have the ability to produce electricity at small wind speeds (3-5 m/s), to be coated for marine enviromnent, and to not harm the people or the birds. These requirements have lead to a first, main solution: the Vertical Axis Wind Turbines (VAWTs), especially the lift-type Darrieus. These have the advantage of functioning in winds coming from any direction, have good efficiency due to their airfoil blade profile and can be light enough to be carried on the roof or deck for mounting.

In this paper will be analysed the different solutions for improving the performances and reducing the loads and vibrations, in small power VAWTs. [4, 5],

2. INTEGRATION OF WIND TURBINES

The integration of VAWTs takes into consideration the dimensions of the building / ship to be mounted on, the wind potential of the place, the energy demand or the amount of energy aimed to be covered.

Now, the majority of marine wind turbines are with horizontal axis and are used for small remote resort applications or for small ship or yacht implementation. Usually these are derived from an onshore destined design, and adapted for seashore or marine enviromnent. These wind turbines are small (400W - 1 kW), covering partially the electricity requirements.

The seaside comes with some advantages with respect to urban integration of wind turbines: the higher wind speeds, influence of the coastal landscape on accelerating the wind speed, influence of the buildings. This means: a certain amount of turbulence and a skewed flow [1], Hence, the small power wind turbines can be successfully used in applications such: energy demand for houses, resorts or for the illuminating system of the ships while staying in harbour.

There are several aspects that should be taken into consideration regarding the emplacement: to be placed away enough as to not disturb the pedestrians; to not be a danger to the crew, passengers or other ships or animals, to produce as less noise as possible, to not produce vibrations. Thus, the best solution for implementation should be on the mast and not be obstructed by elements that can block the free airflow from reaching the turbine [1.22],

The marine and seashore enviromnent gives relatively steady winds, with gradual changes in speed or direction, not sudden, as coimnon in other urban enviromnents [1],

In view of these considerations, next will be analysed several solutions for VAWTs.

3. ROTOR DESIGN SMALL POWER VAWTs

In order to improve the performances and reduce the loads and noise in a wind turbine, several solutions for the blades design have already been tested. Thus, there are: 1. Aerodynamics: blade shape [10], blade number [4], rotor aspect ratio [4], self-starting solutions [11, 12], blade aerodynamic profile [13] and variable cross-section blade [10], additional elements such as flaps [14, 15], vortex generators [15, 16], flexible blades [17] and slots [15]; 2. Structural materials [5]; 3. Other solutions: control [18] etc. Next will be presented each solution.

3.1 The blade shape

There are two distinct categories of VAWTs: the lift- type (Fig.3.b-d) and the drag-type (Fig.3.a). A mainly drag type VAWT is the Savonius design [4], which is a turbine made by two overlapping semi-cylinders with a gap between them. These machines are well known for their ability to self start. The only drawback is the poor efficiency (C'p «î 0,14 at 6 m/s [20]). The most coimnon shape is the classical Darrieus design (eggbeater) (Fig.3.b). The main advantage of this design is the ideal Troposkein shape of the blades that induces the minimum loads on the blades and on the structure [4, 5, 10], H-Darrieus design (Fig.3.c), offers maximum diameter along the whole height, the only drawback being at structural level [4, 30, 31], where at very high rotational speeds it can induce fatigue and shorten the life of the turbine. In order to reduce the loadings of the H- Darrieus design, many coimnercial wind turbines are presenting helical blades (Fig.3.d).

3.2. Blade number

The number of blades has an effect both on the maximum power coefficient as well as on the vibrations and pulsating torque. Hence, a higher number of blades decreases the maximum Cp [4], but in the same time diminishes the torque vibrations.

Increasing the number of blades it also increases the self-starting capabilities of the wind turbine (Fig.3).

3.3. Rotor aspect ratio

The rotor aspect ratio (Diameter/Height) lias an important role regarding the final output of the wind turbine. Depending on the design (Troposkein shape or H-Darrieus), for the same diameter, the height of the rotor can reach a lower or a higher power coefficient.

3.4. Self-starting solutions

One of the main problems of lift-type VAWTs is their poor ability to self-start at low wind speeds [21], The typical solution is to use the generator as a motor, until the turbine starts to rotate.

As a first recoimnendation for Darrieus design is the increased number of blades, from 2 to 3 for a more even distribution of the loads. Another solution, that augments the Re number is increasing the blade chord length, thus, the turbines with "thicker" blades have a better ability to self-start, as the Reynolds number depends on the chord length. Kirke is recoimnending in his thesis [21] the solution with cambered airfoils instead of symmetrical airfoils, for improving the self-starting capabilities. He also proposes increasing the blade aspect ratio (depends on the airfoil chord and blade length) which should be higher than 7.5 in order to facilitate the self starting. A too small ratio leads to big tip losses, and thus leads to poor self-starting.

As a solution for self-start there are the Hybrid- designs, which are using a small Savonius rotor, together with the Darrieus rotor, in order to help with the self- starting. Unfortunately, the additional Savonius rotor also decreases the maximum power output of the assembly.

3.5. Blade aerodynamic profile

The aerodynamic profile has a very important role in the performance of a wind turbine. It can influence both the final power output and self-starting capability.

The standard profile for a VAWT is a symmetrical one. It has been proven that, in case of VAWTs it is preferably to have a thicker airfoil, with a bigger nose radius [24], Hence, the NACA0018 [13, 24] or NACA0021 are recoimnended [25] and not the ones with thinner profile like NACA0012 [23], Several coimnercial H-Darrieus designs are now implementing cambered airfoils coupled with helical blades, due to their higher lift - and thus, ability to self-start the turbine, and also to their contribution at reducing the loads (eg. Improvements proposed by Claessens to the TU Delft Turby VAWT) [11, 13],

3.5. Additional elements

In [15], Pechlivanoglou makes an analysis of the flow control solutions applicable for wind turbine blades, dividing them in passive and active flow solutions. For small power VAWTs, in this paper, the authors have thus, selected as feasible solutions, the following flow control solutions:

3.5.1 Active flow control

These solutions are implying movable parts that are having a controlled behaviour according to flow conditions and performance requirements.

a. Flaps

Are the most coimnon flow-control devices. Taken from airplane wings (Fig.5) [28], have the purpose of changing the camber of the airfoil and, thus changing the lift coefficient of the profile. In vertical axis wind turbines the flaps are controlled aiming to maintain optimum performance, and thus reducing the loads and improving the performance. They can be coupled with a pitching device [15],

b. Slots and slats

Similar to flaps, are taken from airplane wings solutions (Fig.5). The slots are small airfoil-shaped parts placed near the leading edge of the blade. Slots have the advantage of delaying the stall [32] and increasing the lift. Also, they offer the advantage of increasing the functional angle of attack (AoA) interval. However, the influence of slots should be carefully analysed and requires further investigation for VAWT implementation. The difference between slots and slats is slightly perceptible, but mainly one is part of the airfoil blade, being movable, and the other is a passive fixed airfoil element.

3.5.2. Passive flaw control

There are many passive flow control solutions. Their main advantage is their self-adjusting behaviour and that they do not require any active control. For VAWT integration have been chosen the ones with the biggest impact on flow control and with minimum negative impact on Cl/CD, as well as the ones which are easier to predict and to manufacture (with minimum additional costs).

a. Vortex generators

The vortex generators are small thin plates, mounted normal to the blade surface and at an angle from the chord wise direction. Their aim is to create small vortices that are propagating downstream and are improving the mixing between the free flow and the airfoil boundary layer, thus helping the boundary layer to reattach to the airfoil. Similar to slats are delaying the stall but with no improvement in the airfoil lift.

3.6. Structural materials

The blades should be as light as possible in order to have a low moment of inertia. So, the manufacturing process and the chosen materials are very important. In [30] this respect, the small power VAWTs are now mainly manufactured by composite materials such as carbon fibre (CFRP) - being the lightest one - or fibre glass (GFRP) that are heavier but are offering a higher flexibility.

4. ANALYSIS AND FURTHER STEPS

The solutions that have been previously presented have the potential to increase the performance of VAWTs, either individually or in combination. The flaps are one of the best solutions for flow control. Besides the possibility to increase the lift they can control and reduce the loading of the blade. The feasibility of delaying the stall with the aid of fixed slots or vortex generators needs further investigation. The slots are a new idea in the area of small power wind turbines, as their usage is more found at big HAWTs in the area near the hub, for increasing the lift at very thick airfoils. In Fig. 8 it is shown a comparison of the Cl-a curve for different solutions: plain bladed or with flaps, with slots and with both.

An important aspect regarding the building or ship implementation of a wind turbine is the effect of the building on the turbine power curve. It is a known fact that the buildings, the turbulences and other factors are decreasing the performances of the wind turbine [32], Thus, when placing a wind turbine it is very important to consider the location wind potential and the energy demand of the application.

5. CONCLUSIONS

Considering the above studied variants, it follows that a thorough study of the combined variants applied to VAWTs lias to be done. Nevertheless, from a preliminary simplified analysis can be seen that by adding elements such as flaps (Fig.8), the power output of the wind turbine augments considerably.

When choosing the best solution have to be taken into consideration the vibrations reduction, turbulent and gusty wind flow working conditions, small wind turbine dimensions. Thus, the best solution offers the highest improvement with minimum separate parts and minimum added costs.

In conclusion, for seashore and marine applications the turbine should be silent and vibration-less, with good self-starting capabilities; requirements achieved by a relatively low tip speed ratio (the speed of the blade divided by speed of wind) and at least 3 blades.

ACKNOWLEDGEMENT

This paper is supported by the Sectorial Operational Programme Human Resources Development (SOP HRD), ID134378, respectively 137070, financed from the European Social Fund and by the Romanian Government.

References

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