Modelling of an Air Turbine for a Hybrid System for Sea Wave
Energy Utilisation
Martin Pushkarov
1
, Rositsa Velichkova
2,3,*
, Radostina A. Angelova
2,3
, Detelin Markov
2,3
,
Peter Stankov
2
, Ivan Denev
4
and Iskra Simova
2,3
1
Department of Energy and Mechanical Engineering, College of Energy and Electronics, Technical University of Sofia,
1000 Sofia, Bulgaria
2
Centre for Research and Design in Human Comfort, Energy and Environment (CERDECEN), Technical University of
Sofia, 1000 Sofia, Bulgaria
3
Department of Hydroaerodynamics and Hydraulic Machines, Technical University of Sofia, 1000 Sofia, Bulgaria
4
Department of Mechanics, Mechanical Engineering and Heat Engineering, Technical University of Sofia, Branch Sliven,
Bulgaria
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.
82
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
Copyright
c
 2022 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved

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
x
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
2
, or
for all blades, the turbine area is F
total
= 0.0735 m
2
.
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.
a
)
to
p
view of the turbine wheel
b
) right view of the turbine view
Modelling of an Air Turbine for a Hybrid System for Sea Wave Energy Utilisation
83

c
)
cross-section throu
g
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
3
.
• 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
2
.
• The flow rate is determinate by:
3
Q=V.S=0,39m /
s
(1)
• The power of the turbine is calculated as:
283
1000
ρη
==
gQH
PW
(2)
• The velocity of rotation is:
()
0.586
0.364
1
21, 432
67.762 min
−
−
==
=
nQgH
(3)
• The angular speed of the turbine is determined
using the equation:
1
7.096
30
π
ω
−
==
n
s
(4)
• The specific velocity of rotation is calculated
as:
1,25
9,128==
s
nP
n
H
(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
b
) view of air turbine and butterfly valve
CoEEE 2021 - International Joint Conference on Energy and Environmental Engineering
84

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.
a
)
to
p
view
b
)
b
lade
p
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
85

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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