Boats powered by sunlight represent one of the most successful and attractive applications of PV in the field of sustainable transport. Less well-
known to the public than the solar car races that have achieved international fame in Australia and the USA, solar boating has recently made headlines with a growing number of international events and a transatlantic crossing. Solar circumnavigation of the globe is a definite prospect. Unlike road vehicles, boats do not have to climb hills or travel at high speed and they require surprisingly little power for propulsion in calm conditions. This makes solar-powered boating on lakes, rivers, and canals relatively inexpensive and opens up a new market for PV in an important leisure industry.
The low power levels needed to propel boats at modest speeds in calm water can be nicely illustrated with a historical example. Two hundred years ago Britain was in the middle of a canal-building frenzy. The heavy materials of the early industrial revolution, including coal and iron, needed to be transported over considerable distances for which the road network was totally inadequate. So the English narrow canals, with locks just over 2 m wide and 22 m long, were carved through the countryside by gangs of ‘navvies’ (derived from the word navigation) using picks, shovels, wheelbarrows and human muscle power. This extraordinary feat of civil engineering revolutionised inland transport and allowed cargos up to about 30 tonnes to be carried in individual barges, the so-called narrowboats, that just squeezed into the locks. And how did the boats move in those early days? They were towed, often two at a time, by a single horse! Admittedly at slow speed, typically 2-3 km/h (kph), but it was a vast improvement on existing methods of transport by land.
This example suggests that a single ‘horsepower’, nowadays taken as equivalent to 746W, is enough to shift many tonnes of boat at modest but useful speeds. And if careful attention is paid to design by making hull, motor, and propeller as efficient as possible, we now know that one or two horsepower (HP) can propel a modern leisure craft with several passengers at realistic speeds – say up to 10kph (6.4mph) in calm water. The quest for efficiency mirrors that of solar car design with its emphasis on streamlined bodywork and high-performance motors, transmission, and tyres. But in the case of boats the power levels, and therefore costs, tend to be much lower.
Electric boats are a novelty to many people. For the last hundred years most motorboats have used petrol or diesel engines for propulsion, helping to deplete the Earth’s valuable fossil fuels, making a lot of noise and polluting the waterways. But it was not always so. In the period from the 1880s up to the start of the First World War in 1914 there were plenty of battery – powered electric boats on the lakes and rivers of Europe, including some that could carry over 50 passengers. The river Thames in England boasted a scheduled passenger service, with electric charging stations along the bank. However the advent of internal combustion engines proved nearly fatal and by 1930 electric boating was in severe decline. Half a century later it began to emerge again, largely due to increasing environmental awareness, and today represents a small, but flourishing sector of the leisure boating industry. The essential components – batteries, control circuits, electric motors and propellers – are constantly being developed and refined, giving wonderfully silent cruising with minimal disturbance to wildlife and riverbank.
Solar electric boats are even more of a novelty. We are not talking about the many boats that use a PV panel or two to power their electronic equipment and cabin lights, but true electric boats that use PV for propulsion. These exciting craft literally ‘ cruise on sunlight ’ . Today there are many examples on the inland waterways of Europe, North America, and Australia, and the number rises year by year. The combination of a virtually silent, nonpolluting electric drive and solar energy is extremely attractive.
As already noted, quite a lot can be achieved with a propulsive power of 1HP, equivalent to 746 W. In fact the range 200 W to 3kW covers most modern electric leisure boats at normal cruising speed, and there are a few larger craft, including passenger ferries, that require considerably more. We are referring to the mechanical power needed to propel the boat forward; more electrical power is required because of combined motor and propeller losses, typically amounting to 40%.
We now describe three recent boats with different design criteria, specifications, and passenger accommodation. The first, 6.2 m catamaran Solar Flair III, cruises on inland waterways in England. Designed as an experimental boat to test various combinations of PV modules, motors, and propellers, she also appears at boat shows and rallies, helping to promote PV and solar boating and convince the public of its viability, even in the British climate. She carries six 75 W. monocrystalline silicon PV modules in front of a small cabin, plus two more behind (not visible in the photo), giving a total of 600 Wp to charge batteries that power an electric outboard motor. A smaller additional motor, mounted below the front module, acts as a bow thruster to aid sharp turning on narrow canals and rivers. The main motor takes about 450W of input power to attain a cruising speed of 8 kph in calm conditions. Average summer sunshine produces enough PV electricity to move Solar Flair III about 32km (20 miles) per day at this speed. The design aims at technical performance and a streamlined appearance rather than passenger accommodation.
Figure 5.29 Solar-powered catamaran Solar Flair III (Paul A. Lynn).
Our second example, the 6.7 m (22 ft) pontoon boat Loon, has been designed and developed in Ontario, Canada, as a spacious canal and river cruiser able to accommodate up to eight passengers in comfort. Raising the 1 kWp of PV modules on a canopy greatly increases passenger space and gives protection against rain – and maybe also sun! The input motor power to achieve 8 kph is about 1 kW and the PV provides enough electricity, in the Canadian summer months, to travel an average of about 24 km (15 miles) per day at this speed. On long cruises the boat’s batteries may be fully recharged by plugging into shore power electricity.
The third example, 14 m Swiss catamaran Sun21, made history in 2007 by completing the first Atlantic crossing entirely on solar power. She carries 10kWp of crystalline silicon PV modules on a canopy, and needs about 3.8 kW of motor power to cruise at 8 kph in calm water. On the Atlantic voyage the PV provided up to about 45 kWh/day and since the boat was travelling day and night the motor input power had to be kept down to an average of around 1.5kW, giving a speed of about 5kph (3 knots) in sea conditions. Sun21 is an impressive catamaran with accommodation for five crew members. Before the Atlantic voyage very few people believed that
Figure 5.30 The pontoon boat Loon (Tamarack Lake Electric Boat Company).
a motorboat, travelling entirely on sunlight, could achieve such a feat and she received a rapturous welcome on reaching New York.
The catamaran or pontoon form of hull is very popular for solar-powered boats, with sleek twin floats providing a good stable platform for PV, especially when raised on a canopy. However there is nothing to stop designers from using conventional monohulls; the main criterion is an efficient low-drag hull that creates minimal wash and uses the precious PV energy to best advantage.
Finally, we consider the question ‘What exactly makes a boat solar- powered?’ Exaggerated claims are sometimes made; it is easy to stick a PV module or two on a boat, and claim that it is powered by the sun. But it does PV no good to overstate its performance and capabilities, leading to disappointment and scepticism. One answer is to use a simple measure known as the solar boat index (SBI) to quantify performance and allow sensible comparison of a wide variety of boats carrying different amounts of PV10
Figure 5.31 Sun 21 arrives in New York (Dylan Cross Photographer).
The SBI is based on the peak sun hours concept introduced in Section 3.3.2. We have also used it to size PV arrays for water pumping in the previous section. It involves compressing the daily radiation received by an array into an equivalent number of hours of standard ‘bright sunshine’ (1 kW/m2). In this case the most relevant radiation data is that for a horizontal surface (most PV modules on boats are mounted horizontally) during the summer months of the boating season. An array rated at peak power PPV watts and receiving an average Sp peak sun hours per day is expected to yield about Sp PPV watt hours per day. If the boat needs an input motor power PM watts to cruise at a standard speed (normally taken as 8 kph) in calm conditions, then the SBI is defined as:
SBI = nSp Ppv/Pm (5.11)
where n is a system efficiency that accounts for the PV generally operating away from its maximum power point (MPP), and for battery storage losses. Using typical figures of 80% (0.8) for the PV and 75% (0.75) for the batteries, the system efficiency n = 0.8 x 0.75 = 0.6. If we now assume Sp = 5
(typical daily peak sun hours for midsummer in Western Europe), Equation (5.11) becomes:
SBI = 3 Ppv/ Pm (5.12)
This is easy to remember and is in fact used in the UK to quantify the performance of solar-powered boats.10
The SBI has a simple interpretation. It represents the approximate number of hours per day, in average summer weather, that a boat can travel at standard speed on its PV electricity. For example if a boat’s SBI is unity, this means it can travel about 1 hour a day, or 7 hours a week at 8 kph to give a range of 56 km. Most inland leisure boats are weekend boats, for which this amount of cruising is fairly typical. Therefore it seems reasonable to describe leisure boats with SBI values of 1.0 or above as ‘solar – powered ’ in the West European and similar climates; otherwise they are ‘solar-assisted’. Although the SBI is only approximate, it does provide a simple quantitative measure of a boat’s cruising range on sunlight, and allows the solar performance of different boats to be compared. The SBIs for the three examples are:
Solar Flair III: 4.0 Loon: 3.0 Sun21: 7.9
Clearly, these values need sensible interpretation because the patterns of use of the three boats are different and so are the solar climates in which they operate. What we can say is that, if the three boats met together on a European lake, their SBIs should give a good indication of relative solar performance.
Worldwide, there are a number of competitions for solar – powered boats that act as good catalysts for new ideas and designs, encouraging young people to get involved. A good example is the Frisian Solar Challenge,11 held biannually on canals and lakes in the Netherlands. Such events do an excellent job of bringing to public attention the exciting future of solar – powered boats with their silence, lack of pollution, and minimal environmental impact.