CHEMICAL ENGINEERING TRANSACTIONS
VOL. 88, 2021
A publication of
Guest Editors: Petar S. Varbanov, Yee Van Fan, Jiří J. Klemeš
Copyright © 2021, AIDIC Servizi S.r.l.
ISBN 978-88-95608-86-0; ISSN 2283-9216
Feasibility Study: Effect of Tidal Turbines Cut-in Speed for
Power Generation in New Zealand
Navid Majdi Nasab*, Jeff Kilby
School of Engineering, Computing and Mathematical Sciences, Auckland University of Technology, Auckland, New
Zealand
navid.nasab@aut.ac.nz
This paper proposes and evaluates an optimized system of tidal turbines as a renewable energy generating
unit in New Zealand. A comprehensive simulation model has been set up, using several available commercial
software packages to test the performance, capacity and efficiency of the proposed system. Available tidal
records have been used to find four regions that are suitable for tidal energy generation and conduct
simulation model runs at one of them, Foveaux, to provide the electricity for the isolated area of Stewart
Island. Generation of electricity with tidal turbines depends on the water currents and cut-in speed of the tidal
turbines. In other words, to reach maximum efficiency from tidal power, the first step is to analyse the water
currents of the site where turbine will be located and then estimate the generated power from that turbine. To
do it, it is necessary to find the range of water currents and the tidal directions with demonstrating tidal rose
and also find how to get maximum power by choosing a proper tidal turbine. Based on tidal flows of the
Foveaux straight, two tidal turbines (Schottel 54 kW and 70 kW) with different cut-in speeds are selected. The
results indicate that choosing Schottel 54 kW with 0.2 m/s less cut-in speed in comparison to Schottel 70 kW
can generate 4.7 times more power.
1. Introduction
Globally electricity demand is provided from different sources such as fossil fuels, i.e., coal, oil, and natural
gas. The issues with fossil fuels are that they have limited resources and cause pollution. Other sources of
electrical energy which are economically viable are renewable energies from solar, wind and hydro, and can
be used for commercial purposes to solve the vast demand for electrical energy (Mathew 2006).
The use of offshore energy commenced in the 1990s, and its potential has been estimated as greater than
onshore (Esteban et al., 2011). But no offshore energy installations have been installed in New Zealand yet.
However, to ensure their better operation in an offshore environment, some technical challenges remain to be
resolved (Nasab et al., 2020). The speed at which tidal turbines needs to rotate and produce sufficient torque
to generate electrical power is called cut-in speed. The cut-in speed is an essential parameter in designing or
selecting a tidal turbine system to provide any electricity demand. Generally, a tidal current less than 1 ms-1 in
most turbines cannot generate electricity. So, the selection of the turbine needs to be compatible with the
strength of tidal power. In this paper, the Foveaux Strait between Steward Island and the mainland of the
South Island in New Zealand is selected for providing electricity demand of Stewart Island. Currently, the cost
of electricity in Stewart Island is higher than the available grid-connected network in New Zealand and
electricity for 456 (as recorded in 2017) customers is provided by a central diesel power station consisting five
diesel gensets by the Stewart Island Electrical Supply Authority (SIESA). Proposing a solution of electricity
generation with lower cost is a priority for Stewart Islanders (P. Botha, G. M., 2018). What is novel about the
project described in this paper is that it investigates the technical feasibility of Stewart Island's renewable
power output by tidal turbine installation. The results will be optimized by decreasing the cut-in speed
switching design from Schottel 70 kW to Schottel 54 kW.
13
DOI: 10.3303/CET2188002
Paper Received: 11 May 2021; Revised: 29 July 2021; Accepted: 12 October 2021
Please cite this article as: Nasab N.M., Kilby J., 2021, Feasibility Study: Effect of Tidal Turbines Cut-in Speed for Power Generation in New
Zealand, Chemical Engineering Transactions, 88, 13-18 DOI:10.3303/CET2188002
The Italian Association
of Chemical Engineering
Online at www.cetjournal.it
2. Methods
The MetOcean atmospheric model of tidal currents, as shown in Figure 1, indicates that most areas have very
low mean current speeds (<1 m/s), and only four areas have mean current speeds that exceed this value
(MetOcean Solutions is a science-based consultancy providing specialist numerical modeling and analytical
services in meteorology and oceanography); From north to south, Cape Reinga, Cook Strait, Foveaux Strait,
and south of Stewart Island (Huckerby, et al., 2008).
Figure 1: Goring model of tidal flow speeds around New Zealand
Cape Reinga and the south of Stewart Island were not considered further, since both areas are significant
distances from existing electricity infrastructure and potential markets, both of which would considerably
increase the cost of potential projects in these areas (Huckerby, et al., 2008).So, the rest areas which can be
considered are: The first one is in Cook Strait in south of northern island and second one is in the Foveaux
strait which is in south of southern island. Among these points, Foveaux is selected in terms of closer distance
to provide power for Stewart Island. The geographical coordinates of Foveaux are summarised in Table 1.
Table 1:The geographical coordinates of the site
Location Latitude (deg) Longitude (deg) Water Depth (m) Annual Water Speed (ms
-1)
Foveaux −46.6325 °S 168.2025 °E 30 0.52
The speed at which turbines start to turn while there is insufficient torque is called the cut-in speed. Cut-in
speed is an important parameter in design of a system to provide the electricity demand of any site. Tidal
flows less than 1 ms-1 in most of the turbines can not generate electricity. Also, the selection of turbine needs
to be compatible to strength of tidal power. This means that to select a turbine, it is necessary to look at tidal
currents of site at first and then according to power curves of tidal turbines, select a suitable turbine. Based on
tidal flows of Foveaux, the results will show how the cut-in speed can affect power generation. According to
characteristics of Foveaux, two tidal turbines (Schottel 70 kW and Schottel 54 kW) are compared in this
section to show how a turbine with lower cut-in speed can make a big difference in power generation. The
pertinent details for the tidal turbines are set out in Table 2. As can be seen, the cut-in speed of Schottel 54
kW and 70 kW is 0.7 m/s and 0.9 m/s, respectively.
Table 2: Tidal turbines details("Homer Pro,").
Parameter Schottel [70 kW] Schottel [54 kW]
Rated Capacity (kW) 70 54
Cut-in Speed (m×s-1) 0.9 0.7
Cut-out Speed (m×s-1) 6.75 4.6
Rated Power at (m×s-1) 3.8 2.6
Swept Area (m2) 7.06 7.06
Rotor Diameter (m) 3 3
14
An example of the ocean tide data from the TPXO9 (2021) model over the period of year 2020 (with 1-hour
data sampling interval) is shown in Figure 2.
Figure 2: The TPXO9 Results for Foveaux in year,2020 (1-hour data sampling interval).
For plotting tidal roses, the flow speeds in x directions (u) and y directions (V) of National Institute of Water
and Atmospheric Research (NIWA) used. Figure 3 shows that this site has a single distinct ebb and flood
direction separated almost exactly by 180°.
Figure 3: Tidal flow components in Foveaux
The tidal rose of Figure 4, which is plotted with WRPLOT (2021), indicates an NW-SE direction for tidal
currents. It is categorized into three tidal classes: Current speeds of 0-0.7 m/s when electricity can not be
generated in both tidal turbines, 0.7-0.9 m/s when electricity is produced in only Schottel 54 kW and, more
than 0.9 m/s when electricity produces in Schottel 70 kW and as well Schottel 54 kW.
Figure 5 shows the tidal distribution for different speeds in Foveaux. It indicates that 35.4% of tidal currents
are more than 0.7 m/s when a Schottel 54 kW can generate electricity, while using Schottel 70 kW makes
electricity generation possible for only 10.6% of tidal currents.
Based on NIWA ‘s flow currents, the yearly hours tidal flows from different directions in Foveaux are shown in
Figure 6. NW and SE with 3,776 and 3,772 h/y are prominent directions.
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
-1 -0.5 0 0.5 1
Cu
rr
en
t V
el
oc
it
y
V
(m
/s
)
Current Velocity u (m/s)
15
Figure 4: Tidal Rose of Foveaux
Figure 5: Tidal distribution for different speeds in Foveaux using Schottel (a) 70 kW, (b) 54 kW
16
Figure 6: The yearly hours tidal flows from different directions in Foveaux
3. Results
According to characteristics of Foveaux, two tidal turbines (Schottel 70 kW and Schottel 54 kW) are compared
in this section to show how a turbine with lower cut-in speed can make a big difference in power generation.
Figure 7 shows the annual tide run at instantaneous speed, within specified direction range. From this Figure,
it can be seen that SE and NW directions with 7,128 and 7,120.8 km/y tidal run are superior using 70 kW
turbine. Changing turbine to 54 kW increases these values to 10,924 and 10,936 km/y.
Figure 7: Annual tide run obtained when tide flows, within specified direction range in Foveaux using Schottel
(a) 70 kW, (b) 54 kW
0
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
h
/y
Tidal Direction
1.01-1.5 m/s
0.7-1.00 m/s
0-0.69 m/s
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
N
N
N
E
N
E
E
N
E E
E
S
E
S
E
S
S
E S
S
S
W
S
W
W
S
W W
W
N
W
N
W
N
N
W
A
n
n
u
a
l
ti
d
a
l
ru
n
(
k
m
/y
)
Tidal Direction
(a)
0
2,000
4,000
6,000
8,000
10,000
12,000
A
n
n
u
a
l
ti
d
a
l
ru
n
r
(
k
m
/y
)
Tidal Direction
(b)
17
Figure 8 shows the annual power output obtained while turbine generates, within specified direction range. It
indicates that power generation in SE and NW directions for 70 kW turbine is only 940 and 900 kWh/y,
respectively. Changing turbine to 54 kW increases these values to 4,341 and 4,362 kWh/y.
Figure 8: The annual power output obtained while tidal turbine generates, within specified direction range in
Foveaux using Schottel (a) 70 kW, (b) 54 kW
4. Conclusions
The cut-in speed of Schottel 54 kW tidal is 0.7 m/s which is 0.2 m/s less than 70 kW turbine. This means in the
range of 0-0.69 m/s, power generation is zero. Using this turbine increases electricity generation 4.7 times
more in comparison of Schottel 70 kW tidal turbine. Considering this point is important that rated capacity of
70 or 54 kW is achievable only after rated speed which according to Table 2 is 2.6 and 3.8 m/s for Schottel 54
and 70 kW respectively and before these speeds turbine can not generate its nominal capacity. Definitely, the
rated water speed of 2.6 m/s rarely occurs during a year. That is why it is important to consider a turbine at a
lower cut-in speed.
References
Esteban M.D., Diez J.J., Lopz J.S., Negro V., 2011, Why offshore wind energy?, Renewable Energy, 36, 444 - 450.
Homer Pro, 2020. Make Efficient, Informed Decisions About Distributed Generation and Distributed Energy
Resources, , accessed 02.05.2021.
Huckerby J., Johnson D., Nobs Line N.P., 2008. New Zealand’s wave and tidal energy resources and their timetable
for development. International Conference on Ocean Energy, October 1, Brest, France, 15-17.
Mathew, S., 2006. Wind energy: fundamentals, resource analysis and economics (Voi. 1): Springer.
Nasab, N. M., Kilby, J., Bakhtiaryfard, L., 2020 (2020). Wind Power Potential Assessment for Three Locations in
New Zealand. Chemical Engineering Transactions, 81, 73-78.
P.Botha, G. M., 2018. Stewart Island Wind Investigation Retrieved from , accessed 08.08.2021.
TPX09, 2021. Tidal Predictions Using TPX09, , accessed 02.03.2021.
WRPLOT, 2021. WRPLOT View™ - Freeware, , accessed
02.01.2021.
0
100
200
300
400
500
600
700
800
900
1,000
N
N
N
E
N
E
E
N
E E
E
S
E
S
E
S
S
E S
S
S
W
S
W
W
S
W W
W
N
W
N
W
N
N
W
A
n
n
u
a
l
p
o
w
e
r
o
u
tp
u
t
(k
W
h
/y
)
Tidal Direction
(a)
0
1,000
2,000
3,000
4,000
5,000
N
N
N
E
N
E
E
N
E E
E
S
E
S
E
S
S
E S
S
S
W
S
W
W
S
W W
W
N
W
N
W
N
N
W
A
n
n
u
a
l
p
o
w
e
r
o
u
tp
u
t
r
(k
W
h
/y
)
Tidal Direction
(b)
18