Soaking up the rays: The MS Turanor Planetsolar uses solar panels
When the solar-powered catamaran MS Turanor Planetsolar sailed down the Thames last month, it prompted a sudden interest in alternative methods of ship propulsion.
The huge vessel, said to be the largest solar ship in the world, employed 512m² photovoltaic cells made with 809 solar panels to power its two 60kW electric motors that drive the standard propeller. On a sunny day, the solar panels can generate 480kWh, it is claimed.
As with all industries, the shipping sector needs to reduce greenhouse gas emissions because of concerns about climate change. But as things stand, achieving such a reduction would entail paying higher prices for low-sulphur fuels. So the industry needs to investigate alternative, more economic, ship-propulsion systems. Accordingly, much research is under way into the greater use of liquefied natural gas, batteries, even nuclear-powered ships.
But what about renewable technologies? MS Turanor Planetsolar proved that the sun can power ocean-bound vessels, so why is there seemingly so little activity in the area of renewable propulsion?
Some timely answers are provided in a report by the Royal Academy of Engineering. Future Ship Powering Options: Exploring alternative methods of ship propulsion, put together by a working party of engineering experts led by John Carlton, professor of marine engineering at City University London, surveyed current and potential future marine propulsion systems, measuring them against the objectives of energy efficiency and environmental sustainability.
The report suggests solar- and wind-powered ships have potential, but that significant limitations have to be overcome before seagoing vessels such as MS Turanor Planetsolar are regarded as anything other than a technological oddity.
The main problem for solar-powered ships is that photovoltaic methods offer only limited amounts of power generation, more suitable for auxiliary power requirements. The maximum contribution is small when compared with the power required to drive the ship.
The average raw power of sunshine depends on the latitude and the angle at which the photovoltaic cell is positioned relative to the sun. In the UK, the average value over the year is about 100W/m2 on a horizontally mounted surface. Throughout the world, the variation in power availability under average cloud cover is typically between 87W/m2 in Alaska, and 273W/m2 in Mauritania.
However, in terms of the energy that can be derived, the effect of cloud cover is important. Consequently, weather conditions and position on the planet are significant influencing factors in developing the potential of solar power, says the report.
That said, there is design potential to adopt a range of rigid and flexible technologies onboard a ship. However, the principal constraint is the ability to find a large deck surface area on the ship that does not interfere with cargo handling or other purposes for which the ship was designed. In this context, car transporters are an obvious candidate for the application of solar technology in a commercial setting.
Even if efficiency could be improved to 100%, solar technology inherently suffers from low generation capability. This limitation, coupled with a maximum attainable specific power from the sun at given global locations and the generally limited available deck area, suggest that the power attainable would suffice only to augment the auxiliary power demands, the report finds.
One proposal is to increase the available area for energy capture from the sun with solar panels on mast-like structures along the deck, sometimes in combination with wind augmentation. Again, the number of masts that can be accommodated depend on the type of ship and its duty, as well on the angle of the panels in relation to the sun to maximise the panels’ effectiveness. So the report remains cautious, suggesting that these arrangements will be effective only in augmenting auxiliary power requirements.
What about wind energy? A variety of techniques use wind to provide energy to navigate ships. Typically, these include Flettner rotors, kites or spinnakers, soft sails, wing sails and wind turbines.
Soft sails are the oldest of these techniques. While some remarkable sailing passages have been made, particularly by the tea clippers in the 19th and early 20th centuries, soft sail-derived power depended on the availability of the wind, and relied on the skill of seamen to make the best use of the available weather.
However, mimicking these skills today lends itself, to some extent, to automated control systems. The Flettner rotor, which uses the Magnus effect of fluid mechanics – whereby if wind passes across a rotating cylinder a lift force is produced – made its appearance in the 1920s on a ship called the Baden-Baden, formerly the Buckau.
This force has a linear relationship with wind speed and, unlike with conventional sails, a true cross-wind relative to the ship will produce a useful forward thrust at any ship speed, even when this is greater than the wind speed. For a large ship, Flettner rotors can provide a small but significant proportion of the total propulsive power. However, the vorticity produced by a rotor is complex, and understanding of the mechanisms is still evolving, principally through computational fluid dynamics, the Royal Academy report suggests.
If more than one rotor is fitted to a ship, the vorticity in the wake of a rotor raises the issue of vortex interaction. This requires exploration for a particular design, to look at any interference with superstructure or high freeboard under certain wind conditions.
On the Baden-Baden, two rotors, 18m high and 2.7m in diameter, were fitted in place of the previously fitted three masts. It was found that the ship could sail much closer to the wind than when previously under sail, and in 1926 the ship successfully crossed the Atlantic.
Subsequently, another 3,000-tonne cargo-passenger, the Barbara, was ordered. This ship sailed between Hamburg and Italy for six years. In this case, there were three rotors, 17m high and 4m in diameter, rotating at 150rpm.
More recently, the E-Ship 1, a 10,500dwt vessel, was built in 2010. As well as being fitted with two 3.5MW diesel engines, E-Ship 1 has four Flettner rotors: two aft, port and starboard, and two forward behind the bridge and accommodation structure. With this arrangement, the ship is capable of a service speed of 17.5 knots.
The principal use of sail power today, apart from leisure craft, is in the luxury cruise market or with sail training ships. However, wing sails have been used, and several trials have been undertaken in recent years. An example of wing sail application is the MV Ashington, for which sea trials have shown potential benefits in augmenting propulsive power. However, the loadings derived from the sails fluctuate. While there is a broad linearity of resultant load in the context of wind speed, there can be significant scatter in the results, which has to be taken into account in the control system design. Such fluctuations require attention in the fatigue and structural design of the installation.
Meanwhile soft sails, along with kites, have also been explored experimentally on modern merchant shipping. Their contribution in the ahead and leeway directions is a function of the relative magnitude and direction of the ship and wind speed.
Similar considerations apply to wind turbines mounted on ships to generate electric power, says the report, in that they require an adequate differential wind speed over the turbine rotor. For small ships and leisure boats, gyroscopic couples from a wind turbine also need to be taken into account to prevent stability issues in a seaway.
Power provided by wind sources will tend to alter the design basis of the propeller and lead to an off-design performance in some operating conditions. So to optimise performance, some allowance of the average power to be derived from the wind must be taken into account in the propeller design.
Hydrogen: storage volume and safety worries to sort out
The Royal Academy of Engineering’s report also looked at hydrogen as a potential alternative fuel for ship propulsion.
Ivo Veldhuis, from the faculty of engineering, science and mathematics at the University of Southampton, assessed the application of liquid H2 to a concept propulsion study of a high-speed container vessel designed for high-value, time-sensitive goods. Liquid H2 benefits from a much higher specific heat per unit weight than conventional fuels, but needs a much greater storage volume.
If stored at 700 bar (70MPa) pressure, the storage tanks would be at least six times bigger than for conventional fuels. New ship designs would require increased above-water structures to accommodate this storage capacity – this might create difficulties in retrofitting ships to use liquid H2 fuel.
An advantage of liquid H2 fuel is that it generates no carbon dioxide or sulphur oxides emissions to the atmosphere. Nitrogen oxides emissions can be managed as for any other fuel but where H2 is burned in a fuel cell, there are no NOX emissions.
However, there are ship safety design concerns. These centre on the flammability of the fuel when stored and the pressure vessels and cryogenic systems that would be required. These issues are similar to, but more extreme than, the challenges that have been solved with liquefied natural gas (LNG) or liquid petroleum gas (LPG) ships. Similarly to LNG fuelling, for liquid H2 to become a realistic possibility for deepsea ship propulsion, a liquid H2 supply infrastructure would need to be developed.
H2 fuels might also be used in conjunction with fuel cells, which offer potential for ship propulsion in the medium to long term. Encouraging experience is being gained through auxiliary, hybrid and low-power propulsion machinery.
For marine propulsion, the fuel cells that show the most promise are the high-temperature solid oxide and molten carbonate types, says the report. For lower powers, the low-temperature proton exchange membrane fuel cells are better suited.
The downside is that the use of more conventional marine fuels in fuel cells would present problems and necessitate complex onboard preprocessing. They would, in this case, be a more expensive way of generating electricity than conventional methods.
Fuel cells also produce DC electrical output, so they are not as suited to ships with mechanical transmission systems. In addition, fuel cells have lower specific powers and power densities than diesel engines.