This July, India will witness its first pro-planet water transport system-a solar-powered ferry sailing through the waters of Kerala; which once operational, would be the world´s cheapest solar boat, after calculating the construction cost per passenger.
Designed from scratch, NavAlt Solar Ferry is an ode to Indian innovation and engineering prowess. A start-up in Kerala, NavAlt has taken the initiative to construct India´s largest solar-powered ferry (See detailed case study on page 57). The important part about which is, that it will be a contribution towards a non-water, non-air and non-noise polluting mode of transport. With no fuel requirement, the ferry´s operational cost too, would be negligible.
Considering the cost comparison between conventional boat and a solar boat, the latter would cost around Rs 2.5 crore, while the former, of same capacity (75 passengers), safety standards and features costs around Rs 1.5 crore. Explaining the cost comparison, Sandith Thandasherry, CEO, NavAlt, says, ´Solar ferries would need to spend Rs 30 lakh on battery replacement every six years (five years warranty) and a daily grid charging cost of Rs 1.4 lakh per year. This averages out to Rs 6.4 lakh per year against Rs 30 lakh annually for diesel ferries.´
´With this cost structure, the solar ferry has a break-even period of three years. If one were to factor the cost of pollution and uncomfortable journey by conventional ferry, then break-even period would be even lower. In addition, if we add the carbon credit and, additional government subsidy like sales tax exemption (provided to electric cars) the cost is even lower,´ Sandith points out.
It is expected that the solar ferry, which is under construction, will be operational by July 2016.
Understanding solar boat
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),´ explains Sandith.
It is challenging to design solar ferries that run over six hours daily and almost every day in a year, compared to solar cruise boats that run three hours daily and 200 days a year.
As is wont to happen in India, an innovation once shared with the government is either struck with bureaucratic procedures or put on the backburner; however the solar ferry has been an exception. The Kerala State Water Transport Department (KSWTD), following a feasibility study by CUSAT, undertook a bold initiative to implement a pilot project for solar powered ferry and decided to place an open tender, which was awarded to NavAlt, based on their experience and expertise. Sharing his experience, a visibly excited Sandith mentioned, ´The very fact that they (Kerala government) was willing to try out something new, and the first in the country is something great.´
Ergo, it is not only the state government that is positive about this innovation, but, the Ministry of New and Renewable Energy (MNRE) is also willing to provide special concessions which will be transferred to the client to bring down overall ownership costs.
Construction of such ferries takes close to nine months. Typically all these projects--whether it is conventional or solar-based--are customised, as they have to cater to the specific needs of the particular route or location etc. Thus, efforts are directed towards the designing of the ferry.
That said, Kerala government has announced as many as nine innovative ferries based on solar, electric and hybrid models. Meanwhile, NavAlt is also in talks with the governments of Maharashtra and West Bengal to replicate the Kerala model in their respective states.
It is India´s first solar-powered ferry.
World´s cheapest boat, according to construction cost per passenger.
India´s largest boat equipped with lithium battery storage.
Equipped with two 20 kW electric motors powered by 50 kWh lithium battery pack.
Maximum speed of 7.5 knots, with 6 hr runtime on cruise.
NavAlt was founded by three people - Sandith, Philippe and Frederic and backed by each of their firms - Navgathi, AltEn and EVE (latter two from France) with ownership of 50%, 40% and 10% each. Navgathi is a marine design and construction firm; AltEn is the leading solar ferry builder; and EVE is a leading electric vehicle controls and automation expert. NavAlt´s aim is to build solar, electric and hybrid boats in India for the Asian market.
The first critical step is to drastically reduce the propulsion power compared to a conventional boat. This can be achieved by five different processes, all of which are necessary.
1. The commonly adopted practice for solar passenger boat design is to make it 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. The propulsion power can be reduced by lowering the drag of the hull. This can be achieved by optimising the hull form using 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. The propulsion power is also reduced 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.
4. 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 per cent.
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.
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 three 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 contains 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 decides 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 receive more solar energy than those far away and bright sunny days receive 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 5:00 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 1:00-2:00 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 increases, 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 per cent compared to 50 per cent for lead-acid batteries. At this depth of discharge lithium batteries have a life of 2,500 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 runs for three to four hours a day, ferries need to run for six 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 safer) with battery management system.
Additional safety features usually include independent auto-bilge pump in all compartments.
India´s first solar-powered ferry
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 members, 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 required 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 22 T. 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 manoeuvering for short period leading to an average 18 kW consumption. Two motors of 20 kW power are selected ensuring 100 per cent 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 efficiency of 70 per cent for conversion, the energy produced by a 18 kW solar panel is 72 kWh. The total running hours for the boat is 6 hours starting at 7:00 am and ending at 7:00 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 per cent 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 per cent of load.
There is a separate solar panel group (1.5 kW), separate battery bank (24 V, 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 (8:00-10:00 am and 5:00-7:00 pm) the embarkation time is reduced to 5 mts which otherwise is 15 mts. Also there is a two hour noon break during which the ferry will not run (only sun charge). Grid charge 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 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.