Solar boats are better boats than conventional boats because they:
- do not pollute water
- do not release harmful emissions in air
- are very silent and comfortable for passengers
- have lower cost of ownership (high initial cost but low operating cost)
A solar boat design process is completely different from that of a conventional boat so much so that it can be termed as “solar boat design philosophy.” It starts with defining functional and performance needs.
Normally such functional and performance specifications are made to match conventional boats. Solar boats are very effective in passenger boats where the propulsion power needed is less (unlike high speed boats, tugs, trawlers, cargo vessels).
A successful solar boat requires two different but critical focus areas:
- Reduction in Propulsion power
- Optimisation of Energy management
It is challenging to design solar ferries that run over 6 hours daily and almost every day in a year, compared to solar cruise boats that run 3 hours daily and 200 days in a year.
Reduction in Propulsion power
The first critical step is to drastically reduce the propulsion power compared to a conventional boat. This can be achieved by five different process, all of which are necessary.
1) Almost always an adopted practice for solar passenger boat is to make the boat as a multi-hull, usually catamaran, which ensures a lower resistance for the same displacement and speed. This also has higher stability compared to a single hull vessel. Designing a solar boat as a catamaran has an added benefit of having larger deck area for providing solar panels.
2) Apart from this the propulsion power can be reduced by lowering the drag of the hull. This can be achieved by optimising the hull form using the latest techniques like computational fluid dynamics (CFD). The hull shape has to be optimised for the operational condition (displacement, draft, trim, speed) so that the resistance of the boat is minimum. Fore-body and aft-body shape is optimised to get best results.
3) Another approach to reduce the propulsion power is by reducing the weight of the boat. The greatest contributor to a boat weight is the hull. Solar boats are almost always built of composite (FRP, Carbon fibre) or Aluminium. The superstructure of the boat is also of such light material. Every single item that goes to the boat is checked to meet two conditions:
- Is it necessary. Can it be avoided?
- Can it be lighter?
4) In addition the propeller design is critical to ensure high efficiency so that maximum power delivered to propeller is converted to useful thrust. It usually leads to larger propeller diameter and optimum rotating speed (RPM), pitch and area ratio to give the highest efficient propeller, usually in the range of 55%.
5) Apart from the major factors outlined, other minor aspects that affect propulsion power demand need to be taken care – flow to propeller, clearances, shaft bearing, rudder design, etc.
Once the above steps are followed, the propulsion power needed for a solar passenger boat compared to a conventional boat is half in case of comparable composite boat and less than one-third of a steel boat.
Optimisation in Energy management
Once the propulsion power is finalised the motor power is decided. Motors have to withstand the rugged marine conditions and continuous usage. For larger systems, more than 6 kW, the motors may be 3 phase AC motors (synchronous or asynchronous). A suitable controller would vary the speed/power input and thereby vary the speed of the boat. The shafting also contain a thrust bearing to protect the motor and transfer the propeller thrust to the hull.
The next step is to decide the battery bank. The size of the boat drives the area available for solar panel and thereby decide the solar power available. Usually this is maximised since the cost of power from solar energy is cheaper than stored energy from the grid and far cheaper than from small generators (in that order).
The total energy available during the day from solar panel depends on the location and weather condition. Locations closer to equator receives more solar energy than those far away and bright sunny day receives more energy from sun than a cloudy day.
The battery bank is designed based on
- number of running hours of the boat
- when these running hours are
- design condition (available energy from sun)
- type of battery
The running hours of the boat will determine the propulsive power consumed. The charging from the sun (through solar panels) usually starts about 8:00 AM until about 5 PM in summer, in areas close to equator, like some place in Kerala. The rate of charging increases with the intensity of sun and peaks about 1-2 PM.
The design condition is also critical. The available energy from the sun varies with the months of year. It also varies between a sunny day and cloudy day. For example in Kerala the average for the month range between lowest of 4.68 in June to 6.83 standard sun in March. The average for the whole year is 5.72. For a cloudy day these might be close to one-third. The battery bank size increase if one were to design for a cloudy day compared to an average sunny day. This also changes between months of the year.
The depth of discharge allowed for longer life of battery depends on type of battery which will in turn drive the battery bank size. For example lithium batteries can be discharged by 80% compared to 50% for lead-acid batteries. At this depth of discharge lithium batteries have a life of 2500 cycles compared to 500 for lead acid batteries.
An additional step is required in design of power train of solar ferries. Unlike cruise boat that run for 3 to 4 hours a day, ferries need to run for 6 to 10 hours a day. This means that the batteries, motors and systems are of higher reliability and efficiency. Usually batteries are lithium based (iron magnesium phosphate being more safer) with battery management system.
Additional safety features usually include independent auto-bilge pump in all compartments.
Almost always a questions that comes to everyones mind is, “Is the solar boat worth the high price?” The resounding answer is “YES”.
Large solar ferries are usually four times costlier than a conventional single hull steel ferry. However if one raises the ergonomics and safety standards (built under classification society) of the conventional boat to that of solar ferries the ratio is close to three.
The consumable cost (fuel and lubes) is usually 20-25 lakhs for a large conventional ferry compared to zero for solar ferries. Diesel engines and steel hull structure have higher maintenance compared to electric motors and FRP hull. The total operating costs of a conventional ferry comes to about 25-30 lakhs per year.
In addition if we add the carbon credit, govt. subsidy, and then do an economic analysis, the break-even period is usually six years (see case study). If we were to add the cost of pollution for conventional ferries the break-even period is even lower.
Case Study of 75 Pax Solar Boat
Mission: To transport 75 passengers across a 1.1 nautical miles (2 km) backwater stretch of Kerala at 5.5 knots average speed in a solar ferry built under class. The energy storage size is to be designed for sunny condition to reduce the cost (reduce size of battery bank).
For providing comfortable seating for 75 passengers and 3 crew, a catamaran with deck area of 15 m length and 5.5 breadth is sufficient. However after multiple iteration it was found that about 19.5 kW (18 kW for propulsion) solar panel power is needed to provide good energy management. For this purpose the boat size was defined as 20 m length and 7 m breadth.
After multiple iterations the weight of the boat in full loaded condition was determined as 22T. A catamaran hull with was selected that provided the best performance in terms of lowest
resistance. This selection of hull was based on optimisation using computational fluid dynamics (CFD). This hull needs about 16 kW to propel at 5.5 knots and about 22 kW during manoeuvring for short period leading to average 18 kW consumption. Two motors of 20 kW power are selected ensuring 100% redundancy. At full power the boat will cruise at 7.5 knots.
In Kerala, the average solar energy production from 1 kW of solar panel is 5.72 kWh. Taking a efficiency of 70% for conversion, the energy produced by 18 kW solar panel is 72 kWh.
The total running hours for the boat is 6 hrs starting at 7 AM and ending at 7 PM, with focus during rush hour hours in morning and evening. This translates to total energy need of 108 kWh. On a sunny day the gap between energy needed and obtained from sun is 36 kWh. This has to provided by the battery. Since lithium batteries can be discharged by 80% the battery bank need to be at least 45 kWh. A 50 kWh battery bank of lithium iron magnesium phosphate is selected for easy of arrangement and to provide energy buffer.
The power train is kept as two independent system on each hull so that in the eventuality of failure in one, the other system would provide redundancy. In normal operation motors will operate at 50% of load.
There is a separate solar panel group (1.5 kW), separate battery bank (24V, 300 Ah) for utilities. These include indoor LED lights, navigation lights, fire pump, bilge pumps, electric horn, bulkhead lamps, search lights, wiper motor, battery management system which are divided between the two groups and also having a redundancy of being able to select from other battery bank for critical items.
The batteries can be charged from the normal single phase plug overnight so that they are full charged in the morning. The charging time is about six hours.
The above energy curve shows how the running hours of the ferry can be planned. During rush hours (8-10 AM and 5-7 PM) the embarkation time is reduced to 5 mts which otherwise is 15 mts.
Also there is a 2 hours noon break during which the ferry will not be running (only charging from sun). Grid charging during noon break is only done on cloudy or rainy days.
For various scenarios for the sunny conditions time schedule can be made to optimise the energy usage. On an average sunny day the boat can run for 6 hours and on a cloudy/rainy day it can run 5 hours (charging from grid during the noon break).
A conventional 75 passenger ferry that has to be build with similar ergonomics and safety standards under classification society approval would cost about 75 lakhs. With a 100 HP main engine and consumption of 12 litres diesel per hour on average, and total of 120 litres per day (engines are running all the time). At current price of diesel that amounts to ₹6600 per day and ₹20 lakhs per year (300 days running).
Lubricating oil can be taken as 10% of fuel cost, which is ₹2 lakhs per year. Maintenance cost for engines is another 10%, ₹2 lakhs per year. Steel hull maintenance is another ₹2 lakhs per year.
Total operating cost per year is ₹26 lakhs (excluding labour and other expenses that are same in both). These are expected to grow each year at 10%.
For solar ferry an operating cost that comes every five years is replacement of battery bank. This can be taken as ₹50 lakhs. A daily grid charging cost of ₹400 (@ ₹10/unit) and total of ₹ 1.2 lakhs per year expected to grow at 10% per year.
A 2.5 Crore boat, would get a central govt. subsidy of 30% of 100 lakhs (solar panels, charger, battery, battery management system) i.e., 30 lakhs, thereby a net price of 2.2 Crore.
The total cost of a conventional ferry and that of a solar ferry is listed in the two graphs.
If we were to compare the total cost of ownership between the two boats, see graph below.
The above graph shows that although solar boats have higher upfront costs, the total cost of ownerships is lwer. This boat has a break-even period of six years.
If one were to factor the cost of pollution and uncomfortable journey by conventional ferry then break-even period would be even lower.
AltEn (Alternative Energies), France is the world’s leading solar ferry builder from France. They have successfully designed and build 15 solar ferries from 30 to 75 passengers. These ferries run over 10 hours daily and 300 days in a year. AltEn, after extensive R&D over 20 years, has patented many core technologies needed for building a high quality solar ferry. AltEn was founded in 1997.
EVE Systems, France, is a firm specialised in electrical power management and control for use in electric vehicles including boats. EVE System was founded in 2001.
Navgathi is a marine design and construction firm with extensive experience in designing boats and ships. They have done new building project management of large oil tankers, container vessels, bulk carriers. Navgathi also have yards in three locations with experience of construction in steel, wood and FRP. Navgathi was founded in 2008.
NavAlt is a joint venture between the three firms, a private limited company registered in India, with ownership stake as Navgathi (50%), AltEn (40%) and EVE Systems (10%). The aim of the company is to build solar boats in India for the Asian market. NavAlt was founded in 2013.
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