Researched, Designed and built
by the
Department of Industrial Electrical Power
Conversion
at the
Faculty of Engineering, University of Malta
Introduction
The
exploitation of natural resources is one of the main elements of economic
growth and development; however this has serious negative effects on the
environment. During the 20th century the consumption of energy has
clearly increased, and this is largely sustained by the extraction of non-
renewable fossil fuels. In the last few years, depletion of natural
resources, pollution caused by burning
of the said resources to create energy together with the on-going rise in oil
prices have become great concerns to mankind.
In
order to reduce the exploitation of natural resources and pollution, mankind
decided to opt for other energies classified under Renewable Alternative Energy
which are more sustainable. These alternative energies include solar, wind,
hydro, tidal, geothermal and biomass. This project explores the viability of
building a solar powered catamaran which is environmentally friendly but at the
same time offering the same or better functionality.
The
catamaran is driven using
an electrical propulsion drive whose energy is supplied from battery banks
on-board the catamaran. These battery banks are charged by means of solar
energy and a fuel cell. For the
design of the catamaran a large amount of research was carried out and all
components of the catamaran were calculated and simulated before the actual
implementation.
Project goal
Before designing
the catamaran a target goal was set. This goal was that the catamaran had to be
able to do a successful trip around Malta. The best route was selected assuming
that the field test is done on a sunny day in August in calm seas. Thus the
distance that had to be covered by the catamaran was calculated.
The first thing
to calculate was the hull speed which theoretically is limited by the wave that
the hull creates. The theoretical maximum hull speed of a displacement boat is
when the wave length is equal to the waterline length. Therefore the length of
the wave is relative to the length of the boat. Thus the equation for a displacement boat is:
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| Figure 1: Map showing the route around Malta along with the distance involved. |
The length of the
catamaran is 4.88m and therefore will reach a velocity of 2.76m/s which is equal
to 5 knots. The propulsion motor when reaching a speed of 5 knots would need 1.2kW of power. Since
a field trip around Malta will cover approximately 76km of water, thus it will take around 9 hours to complete the
trip with a cruising speed of 5 knots.
Hence for this trip the total energy consumption of the motors is 10.8kWh. This energy has to be gathered
from the solar panels assisted by
the fuel cell.
Catamaran hull design
In order to
design the hull a number of tests were first carried out on a canoe shape hull
which was available and the results were recorded.
This enabled the design team to come up with the ideal hull that will offer the
minimum drag while having the right bouyancy to be able to handle rough seas.
This resulted into a knife edged shape for one third of its length and the rest
as a rounded hull with a light curved banana shaped keel. The bow was also
enlarged from the top part so as to ride over waves easily. Before the actual
hull construction a small scale model was built and a number of tests to
determine the drag were carried out in a large water tank which is available in
our laboratories.
System
Modelling
A
sum of equations relating thrust and drag were used to simulate the effect of
both forces acting on the boat. Figure 9 shows the model that was used to
represent the Torqeedo outboard motor, where in this case the motor was being
simulated for no load.
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| Figure 2: Motor Model |
The
Saw tooth generator is used to simulate the throttle’s varying input of the
outboard motor, where the PWM varies according the constant which represent the
throttle position. The constant will be fed into a subsystem block where the
rotor angular position is directly fed back to achieve commutation. For load
conditions the model was amended to take into consideration the torque
developed by the drag of the propeller blades using equation equ 1
And the drag developed by the boat during run
time using equation equ 2
These equations helped determine the losses during
sailing. These results are very important since they show the efficiency at
which our power is operating, how much power is actually being used to propel
the catamaran.
The last simulation done was that of the system
under load conditions. Using the drag coefficient found in previous experiments
and using motor and sea water parameters the whole system could be simulated.
The system was simulated running at different amounts of power.
 |
Figure 3
- Simulation of motor under load conditions
with different input power
|
Using the simulation different plots of the
motor speed were plotted. From the graph it can be seen that the motor maximum
speed is approximately 8.7km/hr which is equal to around 4.7 knots.
System Design and
Implementation
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Figure 4 - Block diagram showing how all
components work together.
|
The PV system consists
of 6 panels with a maximum power of 245
W, nominal voltage of 29.6V and nominal current of 7.78A. The power generated
by the PV system can either be used to charge the batteries or if the batteries
are already charged, can be supplied to the grid using an inverter. When the
solar charger is used,
the PV panels are connected in such a way that there are two sets of three PV
panels in series. These two sets are placed in parallel to each other and are
fed to the solar charger. The solar charger has an integrated Maximum power
point tracking system; this allows the charger to monitor the output of the
panels and compares it to the battery bank voltage to figure out what is the
best power that the solar panels can put to charge the batteries. The DC rated
output power of the solar charger is 1200W, which is approximately equal to
the maximum DC input power of 1250W. This shows that the charger is capturing
the maximum power from the panels and reducing the voltage to the battery rated
voltage of 24V, and in the process converting all the remaining voltage into
current, so as to obtain a maximum DC power at the output. It is estimated that
during the trip around Malta the PV system is calculated to produce an overall
energy of 7.34kWh. Since the maximum efficiency of the solar charger is 97.3%,
therefore from the 7.34Kwh produce by the solar panels, 7.137Kwh can be stored
in the battery bank.
Research on different
types of fuel cells was done to see what would be best suited for the
catamaran. In this case the direct methanol fuel cell was preferred from other
types since it has a low operating temperature and has a high efficiency. The
fuel cell monitors the voltage of the battery bank and switches on when the
voltage reaches 24.6V. This is done so that the batteries don’t get damaged due
to over-discharging. The fuel cell also switches OFF automatically once the
battery is fully charged at 28.4V. The fuel cell has a nominal current of 3.75A
and if during the field trip it is left continuously ON charging either one of
the battery banks it will produce an overall energy of 810Wh. The fuel cell
consumes 0.9litres of methanol for every 1kWh produced, hence for a whole trip
we require only 0.729 litres of methanol.
The two battery banks
that were used on board the catamaran consist of four batteries each. Each 6V
battery is rated at 180Ah and ideally its state of charge is not depleted less
than 20% to avoid deep discharge. Thus the total usable stored energy is:
Adding all the input
energy from the PV panels, fuel cell and batteries it can be seen that a
surplus energy of 3.259kWh will be supplied by the system in the ideal scenario
of this field trip. In reality this extra 21% of the energy supplied might be
utilized due to adverse conditions such as sea currents, wind resistance and
over cast.
Battery Bank testing
For
preliminary testing, each subsystem to be implemented on the catamaran was
tested using separate test rigs set up in the laboratory. For the charging of
the battery bank three circuits were set up, one using a battery charger, the other by means of a fuel
cell, and one using solar power. The battery bank was discharged through a 40A
resistor. A battery management unit (BMU) was installed to measure the charging and
discharging parameters.
The
chargers starts charging with a constant current were the voltage increases up
to a point where it reaches the maximum voltage, 2.4V/cell. On reaching the
maximum voltage, the charger will decrease
the current to maintain this maximum voltage. All chargers switch off
automatically when the battery has reached its maximum capacity.
 |
Figure 5 – Charging Profiles Shore Charger
and Solar Charger
|
The shore charger
supplies the battery bank with a constant current of 25A whereas the solar
charger supplied
the battery bank with a constant current of 17A. Hence the solar charger took
longer than the shore charger to charge the battery bank to full capacity.
If
the battery is sufficiently charged the fuel cell will remain in standby mode.
Charging mode is only possible when battery voltage is below 26.4V. The fuel
cell will be used as a backup charge since it has a very slow charging process,
hence testing was started from a voltage just below 26.4V (2.4V/cell). As can
be seen, the device goes through a cold start phase which takes about 20minutes
before reaching the full rated current.
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Figure 6 – Charging Profile Fuel Cell and
Discharge Profile through a 40A Resistor
|
The
battery bank was then discharged through a 40A resistive load. To maximize the
service life of the battery bank, total discharge should be avoided; hence the
battery bank was discharged until the current drawn from the battery is equal
to 80% of the nominal capacity. The limit factor when discharging batteries is
the voltage, the lower voltage limit of a lead acid battery should not go under
1.83V/cell. A continuous load of 40A for 3hrs will give a capacity rating of
120Ah, leaving the battery with an efficiency of less than 67%.
 |
Figure 7 – Layout of catamaran and all
implemented components.
|
Also a battery management unit had to be used so
that the state of charge of the batteries was monitored to see if there is any
degradation and also to show the captain how much energy he has left. After designing all
the vessel circuirty and simulating various components the catamaran started being built. The
different components described were fitted in the catamarans hull. During
fitting it was made sure that a lot of attention was placed on what components
were used and how they were used. This is because the catamaran had to always
comply with all marine safety regulations. It was made sure that all cables
were selected with the correct current capacity. The cables were double insulated to have twice the
protection and to resist moisture. Furthermore the cables were made from
fire-retardant material in case of any fire. All
components were installed in water proof enclosures (IP 65) and the cables were
properly glanded to eliminate any ingress of moisture which can lower the
electrical insulation and hence increase the probability of faults and
maulfunction. The control console is the main operating unit that
will control, monitor and display all the commands for all the catamaran
movements and manouvering operations. From the control panel the captain has the start key input, the switches that select which of the battery feeds the power to the
propulsion motors, the charging of the batteries from
either the fuel cell or PV and much more. This was done so that the Captain has
full control of the vessel. The control panel is equipped
with a mimic display that includes the state of charge status of the batteries.
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Vessel Safety
Another important marine standard had to be kept in mind
when designing the Catamaran was the safety equipment requirements. The vessel was therefore equipped in accordance to marine and classification
regulations and standards. These included
various components like navigation lights, an all-around white light, horn and
also two float switches along with two bilge pumps. Also for safety to prevent
the boat from capsizing or lifted up with the wind, the centre of gravity had
to be low and centre as much as possible. One of the methods to obtain a low
centre of gravity is to add a lot of weight. This was easily obtained by the positioning of the
heavy system components
such as the lead acid batteries. Two battery compartments are installed in each hull.
Although the calculations show that one battery bank of 24V 180Ahr is enough
for the scope of the trip there is the possibility to double this capacity in
each hull. This means that the weight will counteract the force coming from the
solar panel roof, making the boat stable
even in high winds. The battery compartment was
designed such that the batteries are held
in an upright sturdy position and protected from
exposure to outside elements.
Since the batteries were placed inside the closed hulls, ventilation was
compulsary to ensure that any hydrogen (which is extremetly flammable)
generated from either the fuel cell or the lead acid batteries is immediately
extracted.
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Figure 9 - Hull ventilation system
|
When
designing the electrical circuitry standards were followed and safety
measurements were taken to prevent any accidents. The
electrical circuitry designs are based on the International Standard ISO
10133:2012 which specifies the requirements for the design, construction and
installation of extra-low voltage direct current electrical systems. The designed
circuits for the boat had to satisfy the following safety criteria:
·
Interlock system -
Propulsion should be switched ‘OFF’ when the shore charger and the grid
connection cables are plugged in
·
If ignition switch is
‘ON’ or control system is ‘ON’ a fire switch and ventilation system should be
switched ‘ON’
·
An alarm will sound in
case of smoke or fire or other hazard such a hydrogen leak
·
To charge the two
battery bank, one has to switch ‘On’ the battery isolation switches in each
hull and the battery bank selection switch together with the source charging switches should be ‘On’ as
well (from the selected charging source to the respective battery bank). The Key switch is only used for propulsion
control.
·
Ignition Key should be
inserted in order to operate one of each propulsion drive or the two drives
together. Thus making boat safe at its
mooring especially when it will be unattended.
·
Navigation lights
should be switched ‘ON’ even if batteries are charging
·
BMU is connected directly
to battery banks irrespective of Ignition Switch position, so as to ensure
continuous data logging.
·
Control of contactors
and relays should be switched ‘ON’ in accordance to the operation and energy
conservation of the catamaran.
One
must note that the design of the catamaran had the main junction boxes kept on
the starboard side of the catamaran, this helps minimize the number of cables
crossing between the hulls, reducing the overall resistance of the cables. The
power propulsion box however was put in the centre in order to have cables in
equal proportion for equal power distribution.
Conclusion
Todays
Catamaran presentation is the result of 3 years ongoing research, design,
manufacturing and construction. The catamaran is now ready to undergo the
necessary pre-comissioning tests, followed by load tests and eventually a 6
month field test. During the past 3 years, the building of this
catamaran has served as an excellent educational tool for a number of students
who carried out their undergraduate and postgraduate studies. The catamaran will be tested to see if all calcultions
and test were correct and if the vessel can truly do the complete trip around Malta. Further
improvements on the catmaran would be the add an auotmated rudder system along
with an autopilot. Furthermore the control panel could be replaced by an
automated system which monitors the inputs and controls accordingly. . The
solar powered catamaran has several environmental advantages over fossil fuel
boats and is a great opportunity to create a cleaner sea.
“The
catamaran is intended to serve as a renewable operation platform for students
to train and educate themselve on the potential of Renewable energy and energy
storage. One can say it is a innovative design that will serve numereos
purposes, but mainly a floating renewable laboratory which can be a very
interesting way for students to enjoy the results of their research work. I
sincerely thank all those, especially Charles Azzopardi, who have helped me
realise this dream.” says Professor Joseph
Cilia
Research Project Team
Mr Charles Azzopardi (Project manager who is a
QA/QC Marine Surveyor)
Professor Charles Pule
Ing Daniel Zammit
Mr Donald Cassar
Mr John Camilleri
Students who did their thesis work on the projects
Ing Neville Azzopardi
Ing David Grixti
Ing Ian Busutil
Ing Sarah Baldwin
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