Modelling of an Air Turbine for a Hybrid System for Sea Wave

Energy Utilisation

Martin Pushkarov

, Rositsa Velichkova

2,3,*

Radostina A. Angelova

2,3

, Detelin Markov

2,3

Peter Stankov

, Ivan Denev

and Iskra Simova

2,3

Department of Energy and Mechanical Engineering, College of Energy and Electronics, Technical University of Sofia,

1000 Sofia, Bulgaria

Centre for Research and Design in Human Comfort, Energy and Environment (CERDECEN), Technical University of

Sofia, 1000 Sofia, Bulgaria

Department of Hydroaerodynamics and Hydraulic Machines, Technical University of Sofia, 1000 Sofia, Bulgaria

Department of Mechanics, Mechanical Engineering and Heat Engineering, Technical University of Sofia, Branch Sliven,

Bulgaria

ivan_denev.eng@abv.bg, iskrasimova@gmail.com

Keywords: Air Turbine, Energy Utilization, Hydbrid System.

Abstract: In connection with global warming and the depletion of energy production resources, new clean ways of

obtaining energy are increasingly being sought. A source of renewable energy harvesting is the energy of sea

wind waves. The present paper reports the design and modelling of an air turbine, which will be used in a

hybrid system for utilisation of the energy of wind waves.

1 INTRODUCTION

Wind energy is a type of solar energy, as it results

from solar energy. Solar energy is absorbed by both

the land and water surfaces (Mafimidiwo et all,2017),

but due to the water's high heat capacity, the

efficiency of the accumulated solar energy in the

water basins is higher than that in the land. The

statistics on the wave energy market show that wave

energy production and utilisation reached $43.8

million in 2019, with the expectations of increasing

its value up to $141.1 million by 2027 (Dixit et

all,2020).

The only water basin in Bulgaria where the sea

waves could be considered an energy source is the

Black Sea. To assess the resources of sea waves, the

main characteristics and features of the respective

water basin are used (Kalogeri et all,2017) Among the

meteorological stations located on the Black Sea

coast, Kaliakra station gives the best information

about wind resources on the high seas. The reason is

that Cape Kaliakra is the most protruding point of the

land in the sea. Usually, the wind speed during a

storm is not constant - it increases and decreases after

reaching a peak, with fluctuations in time and

direction (Rusu et all, 2018).

Determining is, in the end, the power of the sea

waves in the deep-water zones, normalised to one

linear meter of the wave format. The value of this

indicator in the USA and Japan's coastal areas is about

40 kW/m, on the west coast of England – up to 60

kW/m, and for the Black Sea coast (Bulgaria) – up to

12-15 kW/m (Rusu et all,2018),(Valchev et all,2012),

(Markov et all,2017), (Lehmann et all,2017). An

estimation of the theoretical energy resource of the

waves in the Bulgarian territorial sea waters was

made in 20 points, almost evenly distributed along the

coastline, based on the results of numerical

simulations of the wave climate in the western Black

Sea (Markov et all,2018).

The Wells turbine for capturing wave energy was

invented in the 1980s (Shehata et all, 2017). The

Wells turbine is mainly used in power plants to absorb

wave energy, but some drawbacks make the

technology difficult to implement. The efficiency is

very low, and in case of low airflow, the turbine

switches off. The turbine blades have a large spread

but are closely spaced, which requires them to be

permanently used.

Pushkarov, M., Velichkova, R., Angelova, R., Markov, D., Stankov, P., Denev, I. and Simova, I.

Modelling of an Air Turbine for a Hybrid System for Sea Wave Energy Utilisation.

DOI: 10.5220/0011358400003355

In Proceedings of the 1st International Joint Conference on Energy and Environmental Engineering (CoEEE 2021), pages 82-86

ISBN: 978-989-758-599-9

The aim of our paper is to present results from the

modelling and design of an air turbine for a hybrid

system for wave energy utilisation. The hybrid

system experimental stand consists of two turbines:

an air turbine, which is presented in this paper and a

water turbine with oscillating blades, which has

already been presented in (Velichkova et all, 2018).

The designed air turbine is a modification of the

Wells turbine, and the designed calculations are

presented.

2 DESIGN CALCULATIONS

2.1 Design Principle of the Wells

Turbine

The Wales turbine is an axial reaction flow turbine

used to extract wave energy by changing the airflow.

It is connected to the electric generator and works

with or without guide vanes (Raghunathan et all

1982). The turbine is made of symmetrical wing

profile type blades, located around a central hub and

rotating in one direction, regardless of airlow

direction. It works based on the general aerodynamic

theory of the aerodynamic wing (Gato et all, 1988),

(Raghunathan et all,1995).

The blades are horizontal that allows them to

rotate in the same direction no matter where the

working fluid comes from – Figure 1. The air's

absolute speed strikes the blades in the axial direction,

and the tangential speed of the blade acts in a

direction parallel to the plane of rotation. The relative

speed W acting at an angle α (angle of attack) relative

to the turbine blade causes a lifting force L

perpendicular to W and a force of frontal resistance D

in the W direction. In this case, the lifting force can

be divided into axial and tangential forces, the

direction of the tangential one always being in the

same direction or the direction of turbine's rotation.

Figure 1. Wing profile of a Wells turbine blade.

Where F

tan

is draft force, F

is lift force and W,D

and L represent the reduced of drag force.

2.2 Design and Calculations

The turbine we designed and built for our

experimental stand is a modification of a Wells

turbine and should work without a guide device. It

should be made of a whole aluminium block and to

have 5 blades with a wing area of F = 0.0147 m

, or

for all blades, the turbine area is F

total

= 0.0735 m

The diameter of the hub is Ф = 200 mm, and the entire

diameter of the turbine with blade span is Ф = 500

mm. Figure 2 presents a drawing of the turbine wheel

and its projections. The section of the turbine blade is

presented in figure 2c and gives clarity about the

construction of the wing profile of the blade itself and

a cross-section of the turbine blade.

)

view of the turbine wheel

) right view of the turbine view

Modelling of an Air Turbine for a Hybrid System for Sea Wave Energy Utilisation

)

cross-section throu

h the turbine wheel blade

Figure 2. Drawing of the turbine wheel.

The following data and assumptions are used to

calculate the turbine:

• NACA 0015 wing profile is selected. It has

proven its potential in scientific developments

among different wing profiles for air turbines,

which have been tested several times. The wing

profile allows the turbine to rotate in the same

direction no matter where the airflow comes

from.

• The air density is assumed to be ρ = 1.29 kg/m

• The pressure difference is H = 3 m.

• The turbine efficiency is assumed to be η = 0.9.

• The air velocity is v = 1.8 m/s;

• The diameter of the pipe, in which the air

turbine will be installed is D

tube

= 0.53 m.

• The area of the tube of the experimental stand

is S = 0.217 m

• The flow rate is determinate by:

Q=V.S=0,39m /

(1)

• The power of the turbine is calculated as:

283

1000

ρη

gQH

(2)

• The velocity of rotation is:

()

0.586

0.364

21, 432

67.762 min

−

nQgH

(3)

• The angular speed of the turbine is determined

using the equation:

7.096

−

(4)

• The specific velocity of rotation is calculated

as:

1,25

9,128==

(5)

3 MODELLING AND DESIGN OF

THE AIR TURBINE

Before the design of the real air turbine, it was

modelled using Solid Works 2019. The basic

drawings of the model, showed in figure 2, were

considered. Figure 3 presents the 3D model of the

experimental stand with the Wells type air turbine.

a.) cross view from the top

) view of air turbine and butterfly valve

CoEEE 2021 - International Joint Conference on Energy and Environmental Engineering

c) view form front

Figure 3. 3D model of the experimental stand.

The turbine (figure 3a) is housed in a specially

designed tower (figure 3b). The tower is made of

sheet steel with a circular cross-section through which

the airflow passes and transfers its energy to the

turbine blades. The cylinder, in which the air turbine

is housed, has an inner diameter of 0.526 m.

Several segments and details are used in the

modelled stand to build the turbine tower (figure 3c).

In the upper and lower part, segments with an inner

diameter of 0.526 m are mounted, after which the

transition segments are attached using flange

connections, which change the inner diameter from

0.526 m to 0.346 m. This transition has a double

function in the stand:

• to allow the assembly of a butterfly valve in the

lower part again by means of a flange

connection;

• to allow the increment of the fluid velocity

directed to the turbine blades.

The butterfly valve is installed to stop the air flow

to the turbine; certainly, a blockage is provided in the

generator against unintentional rotation of the turbine.

In the model stand, shown in Fig. 3, the turbine is

mounted in two places, and the bearing bodies are

reinforced to the respective segments.

The principle of operation of the experimental

stand (figure 3c) is the following: airflow is supplied

from the underside of the turbine, which in turn

rotates the turbine and transfers its energy to the

windings of the generator. Then an airflow is supplied

from the upper side, and again the turbine produces

electricity through a generator coupled to it.

In this way, the action of the waves, for which the

installation is intended, is simulated. The airflow

action is expressed in the wave’s amplitude: the

airflow climbs the tower when it is in the wave's

maximum amplitude and gives energy to the turbine.

In the other case, when the airflow is in the lowest

part of the wave, it is sucked from the upper part of

the tower and energy is again given to the turbine

blades.

As a result, the system produces energy all the

time. When the butterfly valve is closed, it separates

the lower airflow from the system hermetically and

thus, neither air can be forced in nor sucked in. This

valve stops the operation of the system.

Figure 4 shows the designed air turbine based on

the performed calculations and computer modelling.

)

view

)

lade

rofile

Figure. 4. The manufactured air turbine

4 CONCLUSIONS

The design of an air turbine that is a part of an

experimental stand for investigation of the harvesting

of the energy of wind waves is shown. The main

calculations are shown. The design of a modification

of the Wells turbine is presented.

Further investigations in the project include

different tests related to the performance of the hybrid

system and its parameters, as well as the effect of

environmental factors.

Modelling of an Air Turbine for a Hybrid System for Sea Wave Energy Utilisation

ACKNOWLEDGEMENTS

This study is part of the project „Utilisation of wave

energy by hybrid system”

, KP06-H37/30, funded by

the Bulgarian Science Fund of the Ministry of

Education and Science.

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CoEEE 2021 - International Joint Conference on Energy and Environmental Engineering