Increasing the Energy Density and Power Ratio of a Staggered VAWT

Wind Farm by using The Rotor's Diameter as a Reference

BUDHI MULIAWAN SUYITNO

Department Mechanical Engineering

Universitas Pancasila

Srengseng Sawah, Jagakarsa, Jakarta 12640

INDONESIA

REZA ABDU RAHMAN

Department Mechanical Engineering

Universitas Pancasila

Srengseng Sawah, Jagakarsa, Jakarta 12640

INDONESIA

ISMAIL

Department Mechanical Engineering

Universitas Pancasila

Srengseng Sawah, Jagakarsa, Jakarta 12640

INDONESIA

ERLANDA AUGUPTA PANE

Department Mechanical Engineering

Universitas Pancasila

Srengseng Sawah, Jagakarsa, Jakarta 12640

INDONESIA

Abstract: - The development of wind energy systems has achieved a higher technology readiness level for

Horizontal Axis Wind Turbine (HAWT). Unfortunately, the HAWT is only suitable for high wind speed areas.

The Vertical Axis Wind Turbine (VAWT) is considered the ideal model to utilize wind energy in the low wind

speed region. However, VAWT has a lower power coefficient. Therefore, developing a VAWT wind farm can

improve the overall energy density for power generation in the low wind speed region. In this study, staggered

configuration for three turbine clusters is evaluated through numerical simulation and experimental tests. The

pitch distance is set by using the rotor's diameter as a reference for placing the 3rd rotor at the second row. The

turbulence intensity in the area wake superposition is highly affected by the position of the 3rd rotor. The flow

characteristic indicates that the 3D layout has a high concentration at the front area of the 3rd rotor. It leads to

higher achievement of power ratio for the clusters. The overall power ratio for 3D layout can achieve more than

0.9, whereas, at a speed 3 m/s, the highest power ratio is obtained at 1.0. The finding in this study can be set as

an essential reference for developing a VAWT wind farm with a specific arrangement and improving the

overall power density of the turbine clusters.

Key-Words: - Savonius rotor, Turbulence, VAWT, Wake superposition, Wind farm

Received: May 18, 2021. Revised: January 22, 2022. Accepted: February 19, 2022. Published: March 26, 2022.

1 Introduction

The energy crisis encourages researchers and

designers to improve renewable energy sources'

utilization continuously [1]. As a result, the growth

of renewable energy sources increases rapidly, with

a positive trend in the near future. The options for

storing renewable energy are enlarged by using

thermal or electrical energy storage [2]–[4]. Further,

advanced storage systems by indirect hydrogen

production and pumped hydro storage offer better

flexibility for utilizing renewable energy sources. In

WSEAS TRANSACTIONS on FLUID MECHANICS

DOI: 10.37394/232013.2022.17.6

Budhi Muliawan Suyitno,

Reza Abdu Rahman, Ismail, Erlanda Augupta Pane

E-ISSN: 2224-347X

Volume 17, 2022

recent years, wind energy development has been an

excellent example of sustainable development in

renewable energy [5].

High-efficiency wind turbine model Horizontal

Axis Wind Turbine (HAWT) is achieved owing to

massive research for improving the design of the

turbine [6]. However, the HAWT turbine is limited

by geographical requirement since it works ideally

for an area with high wind speed. In order to utilize

the wind energy for an area with low potential wind

speed, the Vertical Axis Wind Turbine (VAWT) is a

better option. The VAWT model is able to harvest

low wind speed and produce sufficient rotational

torque to produce mechanical work. The produced

work can be used for generating electricity for small

utilities. Therefore, the development of VAWT is

highly recommended for a particular application

such as lighting and pumping system [7], [8].

To improve the power output from the turbine,

several wind turbines are assembled under a

specified area called a wind farm. The average

output of windfarm is considerably high since it

employs more than one turbine and is able to

improve the produced energy density [9].

Nevertheless, the layout of the turbine should be

appropriately adjusted to reduce the area as well as

to minimize the wake superposition effect [10]. The

improper arrangement leads to low energy density

and increases the processing and production cost.

For example, wake superposition can be minimized

by increasing the distance between the first and

second-row turbine, but it increases the area's usage

and eventually decreases the energy density [11]. If

the turbine is set closer, the wake superposition

increases and the power generation on the second-

row turbine drop significantly. Besides, the layout

arrangement of the wind farm has to consider the

operational aspect, such as energy storage,

maintenance and power distribution [12].

Numerous studies have analyzed the ideal

configuration for a wind farm. Unfortunately, it

focuses on the HAWT windfarm [13]. The study

that addressed the VAWT wind farm layout is

limited since the VAWT is only suitable for low

wind speed areas with small energy production.

Also, the design of VAWT is relatively complex

and makes this turbine is less attractive. The

complexity for VAWT is aimed at the design of the

turbine and low power produced, which makes it

less feasible for large-scale application [14]–[16].

Despite that, one study reports that an optimal

configuration for the VAWT wind farm is able to

maximize the potential energy of wind energy [17].

The study describes that with a proper turbine

design and suitable layout for VAWT, low wind

speed can convert as usable energy when it is

addressed for specific applications instead of large-

scale electric production like HAWT. For example,

an inline configuration of two VAWT can improve

the power ratio up to 0.34 [18]. Another study

reported that staggered configuration for VAWT

wind farms with three turbine clusters could

increase the overall power ratio by 0.8 [19]. Further

research is still necessary for the VAWT wind farm

by considering the achievement for the VAWT wind

farm and the opportunity to improve its power ratio

[20].

The VAWT wind farm is highly recommended to be

improved by addressing suitable configuration for

staggered configuration. Increasing the power ratio

will improve the impact factor for windfarm VAWT

in the renewable energy system and broader

application. A previous study shows that by using

the Myring Equation for designing the Bach-type

blade for Savonius VAWT, a better power

coefficient can be achieved [21]. Thus, the present

study aims to find a suitable configuration for a

VAWT wind farm in a staggered layout. The three

turbine clusters are chosen and adjusted using the

rotor's diameter (D) as a specific base point. By

finding the ideal layout and the highest power ratio,

the energy density for the VAWT wind farm can be

increased significantly and make it suitable to be

used as an essential reference for developing a

large-scale VAWT wind farm.

2 Materials and Method

Bach-type Savonius rotor is used in this study. The

consideration for using the given model is a better

air distribution in any wind direction and relatively

easy to be manufactured [22]–[24]. The curvature of

the rotor was adjusted by using the Myring

Equation; thus, the model can be adapted precisely

[25].

Fig. 1: The geometry of the Savonius turbine

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Budhi Muliawan Suyitno,

Reza Abdu Rahman, Ismail, Erlanda Augupta Pane

E-ISSN: 2224-347X

Volume 17, 2022

The designed rotor was used the curve parameter

control (θ = 10°) with three blades. The height and

diameter of the turbine were 300 mm and 150 mm,

respectively (Fig. 1).

The staggered configuration was used three

turbine clusters. In order to study the effect of pitch

distance between the turbine in the first and second

row, the 3rd rotor was varied by taking the turbine's

diameter as reference. The diameter is taken as a

specific reference to refer to a specific arrangement

for the designing windfarm. Fig. 2 presents the basic

orientation for the layout. The measurement point at

x and y were chosen since this area has the highest

turbulence intensity according to the wind speed

direction. An experimental test for the turbine was

performed using an open-loop wind tunnel with

high feasibility and reduced the deviation for the

measurement [26].

Fig. 2: Basic layout configuration and measurement

at the test section

The 3rd rotor (at second row) is varied by pitch

distance at 1D – 4D. Numerical simulation using the

software was conducted first to evaluate the

turbulence value, power ratio, and flow

characteristic at the wake superposition area. The

simulation was intended to obtain the best

configuration of the turbine by understanding the

effect of wake superposition and turbulence on the

power ratio.

Fig. 3: a) Initial measurement, b) Experimental test

for best staggered configuration

The power ratio was taken as the main indicator to

determine the best configuration where it is taken

from the rotational speed and power generated of

the turbine at the first and second row. According to

the standard for low wind speed area, the wind

speed for measurement is set at 1–5 m/s. One

turbine prototype was used to check the readability

of the instrument (Fig. 3a). The wind speed in the

wind tunnel was measured using an anemometer

(hot wire), where a tachometer measured the

rotational speed of the turbine. The best staggered

configuration (Fig. 3b) was evaluated

experimentally and repeated ten times to ensure the

data acquisition and minimize the deviation of

measurement.

3 Results

The changes in turbulence value at the wake

superposition area are mainly affected by the

arrangement of the second-row turbine. Fig. 4

presents the turbulence value under different pitch

configurations given wind speed (1 – 5 m/s). The

turbulence value at the front area of the first-row

turbine (measurement point at 1st and 2nd, as seen

in Fig. 2) shows an identic value where the

turbulence tends to increase along with an increment

in the wind speed. However, the turbulence value at

the midpoint (x) and rear (y) change significantly as

the wind speed increases, related to the pitch

distance between the first and second-row turbine.

At pitch 1D and 2D, the turbulence value at the area

before the 3rd rotor (x) is always smaller than the

turbulence at the rear area (y). In contrast, at

positions 3D and 4D, the turbulence value changes

at x and shows a smaller value than turbulence at y.

In order to understand the flow pattern at the

designed configuration, Fig. 5 presents the

characteristic of wind speed at different

configurations. The pattern is plotted at wind speed

3 m/s since it is the median value of the tested wind

speed. The flow distribution at the first-row turbine

presents a stable flow associated with the turbulence

value in this area as the distance to the second-row

turbine (3rd rotor) changes, the flow characteristic at

the wake superposition area is also different. It can

be seen at 1D that the concentration of wind speed

at the front area of the 3rd rotor is high. When the 3rd

rotor is moved farther from the first-row turbine, the

flow characteristic at this area varies greatly. At

pitch 2D, the passing wind from the first row

collides, making the velocity concentration at the

front area of the 3rd rotor split. The passing wind

from the first-row turbine promotes a stable flow at

pitch distance 4D since the space between the first

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DOI: 10.37394/232013.2022.17.6

Budhi Muliawan Suyitno,

Reza Abdu Rahman, Ismail, Erlanda Augupta Pane

E-ISSN: 2224-347X

Volume 17, 2022

and second-row turbine is far enough to achieve a

laminar flow. The 3D configuration obtains the ideal

flow where the passing wind from the first-row

turbine is concentrated at the middle of the 3rd rotor.

The changes in the flow affect the power

generated by the turbine, particularly for the second-

row turbine (3rd rotor). The power generated by the

turbine gained from the simulation is analyzed to

obtain the power ratio. Fig. 6 presents the power

ratio comparison at different layout configurations

under specific wind speeds. The lowest power ratio

is found at pitch distances 1D and 4D, where the

Fig. 4: Turbulence value at different configuration

Fig. 5: Flow characteristic at different configurations with wind speed at 3 m/s

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Budhi Muliawan Suyitno,

Reza Abdu Rahman, Ismail, Erlanda Augupta Pane

E-ISSN: 2224-347X

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value is less than 1 at wind speed 1–5 m/s. It

emphasizes that the power ratio is affected by the

layout configuration of the turbine cluster. For pitch

distance 1D, the short distance between the first and

second-row turbine disrupts the flow after passing

the first-row turbine. It makes the power generation

for the second-row turbine decrease. On the

contrary, a wide distance at pitch 4D reduces the

flow concentration and leads to low power

generation on the second-row turbine. According to

the power ratio, pitch 2D and 3D show a suitable

result where the power ratio at wind speed 3 m/s is

more than 1. However, pitch 3D has a higher mean

power ratio than pitch 2D, where most of the power

ratio is more than 0.9.

According to the simulation result, pitch 3D has

the highest power ratio. This layout is chosen for the

experimental test to obtain the actual power ratio.

The measurement is taken through a wind tunnel

and is repeated 10 times to ensure the reliability of

the measurement. The power generated on the first-

row turbine is compared to the power generated on

the 3rd rotor. As shown in Fig. 7, the power ratio is

relatively high for the configuration 3D, with an

average value of more than 0.9. the power ratio at

wind speed 3 m/s is more than 1, indicating an

excellent indicator of the suitable layout for

staggered configuration with three turbine clusters.

Also, both experimental and simulation results show

a good agreement that the power ratio for 3D has the

highest value.

4 Discussion

The effect of pitch distance between the first and

second-row turbine is mainly on the change of

turbulence intensity. It is the main reason for finding

the optimal configuration to adjust the turbulence

intensity around the wind farm, particularly for the

second-row turbine. The increment in turbulence

intensity is primarily on the area wake superposition

as well as the rear area of the second-row turbine.

As observed in Fig. 4, the area wake superposition

and after 3rd rotor shows a high turbulence intensity.

The highest turbulence intensity is obtained by pitch

distance of 3D and 4D while it elevates as the wind

speed increases. The turbulence intensity before and

after area wake superposition also can be used to

determine the ideal flow distribution on the second-

row turbine. The turbulence intensity for 3D

configuration decreases significantly after passing

the 3rd rotor. It shows that the second-row turbine

can absorb a sufficient amount of wind energy. It

also can be observed by understanding the flow

characteristic in this area.

The flow characteristic is plotted at wind speed 3

m/s for each layout configuration. Fig. 5 presents

that pitch distance 3D indicates a high concentration

flow at the front area of the 3rd rotor. It is the major

Fig. 6: The change in power ratio at different configuration

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DOI: 10.37394/232013.2022.17.6

Budhi Muliawan Suyitno,

Reza Abdu Rahman, Ismail, Erlanda Augupta Pane

E-ISSN: 2224-347X

Volume 17, 2022

factor that leads to a high turbulence intensity at the

area wake superposition for 3D configuration. A

high concentration of airflow in the front of the

second-row turbine leads to a significant energy

conversion which can be addressed based on the

differences in turbulence intensity between the area

before and after 3rd rotor. The wind speed

distribution is spread equally with minor

degradation. It emphasizes that the turbulence

intensity for 3D configuration at the area before and

after the 3rd rotor slightly decreases. The finding is

also supported by the previous study, which

concluded that a suitable adjustment for the turbine

in the first row could lead to a coupled effect for the

area wake superposition and increase the power

characteristic for the second-row turbine [27]. It is

advantageous since the power generation for the

second-row turbine could be increased and raise the

power ratio of the layout configuration.

The highest power ratio is obtained for 2D and

3D layout configuration, as seen in Fig. 6. However,

the average power ratio for pitch distance 2D is

relatively smaller than pitch distance 3D. It is

mainly affected by the wind distribution at the area

wake superposition and turbulence intensity on the

2D configuration, which leads to low power

generation on the second-row turbine. It points out

that area wake superposition is important for wind

farms with more than one-row cluster and depends

adjusting the distance between first and second-row

turbines. A suitable pitch distance is able to improve

the power ratio of the clusters. Also, the use of

Bach-type blade for Savonius rotor promotes a

better wind direction after passing the rotor and

increases the wind concentration at the front of 3rd

rotor, leading to a higher power generation for the

second-row turbine [28]–[30]. The importance of

adjusting the pitch distance between first and

second-row turbines also can be proven by 1D and

4D layout configuration. Both configurations show

a low power ratio since the pitch distance is too

close (for 1D) or too wide (for 4D) and lead to a

significant drop in the power ratio (less than 0.9)

[31].

Experimental evaluation for 3D configuration

shows a high-power ratio for the all-ranged wind

speed and is relatively close to the simulation result

(Fig. 7). It proves that 3D configuration is able to

deliver a high-power density for the staggered

VAWT wind farm. Thus, the ideal layout for a

staggered VAWT wind farm can be adjusted

precisely, as seen in Fig. 8. The specific area of the

configuration is 3D × 6D. With a smaller area for

the windfarm and a high-power ratio, the area's

energy density could be improved significantly [32].

Thus, the proposed model using 3D configuration

can be used to design a staggered VAWT wind

farm.

Fig. 7: Experimental result for power ration at pitch 3D

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Budhi Muliawan Suyitno,

Reza Abdu Rahman, Ismail, Erlanda Augupta Pane

E-ISSN: 2224-347X

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5 Conclusion

A detailed study is done to find an ideal VAWT

wind farm with a staggered configuration. Though

VAWT is only suitable for low wind speed areas

and has a lower power coefficient, by adjusting a

proper layout, the overall power density of the wind

farm can be utilized effectively. The change in pitch

distance between the first and second-row turbine

shift the turbulence intensity, particularly for the

area before and after the second-row turbine. The

flow characteristic of each layout is observed and

emphasized the turbulence intensity for each model.

The pitch distance 3D shows a better turbulence

intensity with a fair distribution of wind speed in the

area wake superposition. It increases the power

generated for the second-row turbine (3rd rotor).

Thus, the power ratio obtained from the power

generation between the first and second-row turbine

shows a higher value for the 3D configuration. Both

results from the experiment and simulation indicate

a high-power ratio for 3D configuration. The

average power ratio is obtained more than 0.9 where

at wind speed 3 m/s, the power ratio is able to

achieve value by 1.02. The proposed layout in this

study can be referred for further improvement in the

VAWT wind farm and is expected to increase the

technology readiness level for wind power

generation with VAWT.

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Reza Abdu Rahman, Ismail, Erlanda Augupta Pane

E-ISSN: 2224-347X

Volume 17, 2022

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Contribution of Individual Authors to the

Creation of a Scientific Article (Ghostwriting

Policy)

Budhi Muliawan Suyitno: Conceptualization and

methodology

Reza Abdu Rahman: Writing and visualization

Ismail: Formal analysis and supervision

Erlanda Augupta Pane: Data acquisition and

investigation

Creative Commons Attribution License 4.0

(Attribution 4.0 International, CC BY 4.0)

This article is published under the terms of the

Creative Commons Attribution License 4.0

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DOI: 10.37394/232013.2022.17.6

Budhi Muliawan Suyitno,

Reza Abdu Rahman, Ismail, Erlanda Augupta Pane

E-ISSN: 2224-347X

Volume 17, 2022