Discussion/Comments

Summary

There have been a lot of new developments on the renewable-energy front, and yet solar energy is not a good candidate for boat propulsion. AAmpere (amp), SI unit of electrical current few years ago powering a boat with solar energy would require towing a solar-barge larger than the boat, and in 2016 not much has changed.

But still the headlines persist about the miracle of renewable wind and solar energy:

• Sol Voltaics in April 2016 announced a photovoltaic (PV) module with efficiencies of 27%percent or more.
• Holographic foil that is twice as efficient as typical photovoltaic (PV) solar cells, using light selection, deflection, and concentration. The Dresden-based company Apollon GmbH & Co claims this has 28% efficiency compared to a typical 17%.
• Modern silicon and indium-tin-oxide-based solar cells are approaching the theoretical limit of 33.7% efficiency but a research team at Princeton has used nanotechnology to create a mesh that increases efficiency over organic solar cells nearly threefold.
• Houses all over the world are being powered by roof-top solar panels, and selling excess energy back to the grid.
• PlanetSolar is the biggest solar ship in the world.
• Solar Impulse HB-S1A airplane left on a transcontinental flight acorss the USA on May 4 2013.
• New solar cells adjust sensitivity for latitude.

There are only two possible renewals for powered boats:

• Solar
• Hydrogen

Wind is a non-starter for propulsive power because, well, if you want to use wind power, get a sailboat! Right?

Wind energy, like solar is useful for supplementary power; or, for supporting a minimal lifestyle on a sailboat.

Top

Solar Facts

Here'sSecond the short version on solar energy.

According to NASA, the earth's surface receives a maximum of 137 Watts per square meter (WWatt/m²) at noon at the equator when the sun is perpendicular. At other latitudes and sun angles the energy is less.

In practice, for an 8-hour summer day, 40 degree latitude, the sun delivers an average 600 W/m². Currently the best commercial solar panels are ~16% efficient. At an efficiency of ~16% on a perfect sun day this is a yield of 96 W/m² averaged over 8 hours (0.768 kWhKiloWatt hour).

UPDATE: In 2015 solar panel company SolarCity announced commercial cells that are 22.5% efficient.

Now, metric horsepower, widely used in the auto industry, is defined as 0.73549875 kilowatt (kWKiloWatt, 1000 watts). Assuming we need 200 horsepower (hphorsepower) to drive a 70-ftFoot boat, this is ~147kW. At 96 W/m² this equals 1,531 m² (16,472 ft2) of panel acreage [1 m² = 10.76 ft²]. And this is just for propulsion during daylight hours. There is no extra for storage in a battery for night operations.

The highest density commercial solar cell has a capacity of 174W/m² or ~16W/ft². Assume this is feasible output, even though we only receive 137 W/m² at the equator and 96 W/m2 at 40 degrees latitude. For our 147-kW motor we would need 845 m² (9,092 ft²) of panels. At most a 70-ft yacht would have less than 1,000 ft² of surface area, so you can see why most boats using solar have hybrid diesel-electric propulsion systems.

Top

Solar at a Cost

The huge carbon-fibre catamaran PlanetSolar MSMotor Ship Tûranor has 537 m² (5,780 ft²) of 38,000 photovoltaic panels with an 18.8% yield. These are mounted on deck and on large fold-out wings [1]. The panels feed six blocks of lithium-ion batteries, like those used in the Boeing Dreamliner. The reported maximum daily yield during one 24-hour period was 661 kWh. According to the logs, recharging each day typically took until noon. Build cost was of the order of USD $16 million.

The Solar Impulse HB-S1A airplane can carry only the pilot. Using high-technology materials it weighs 3,527 lbs and has a gigantic wing span of 208 ft -- as much as an Airbus A340 or a Boeing 747. It has four 10-hp electric motors driving propellers. These are powered by 11,628 monocrystallane solar cells spread across every available surface. "With 200m² of photovoltaic cells and a 12% total efficiency of the propulsion chain, the plane’s motors achieve an average power of 8 HP or 6kW," according to the design team. So if we had 100% efficiency the plane would have 67 hp average available. Build cost is reported to be 90 million Euros (USD $118 million).

People reading this stuff are seized by the vision and write things like, "This is awesome. Let’s get electric passenger airplanes as soon as possible." Or boats. Or cars.

Top

Worked Example

Let’s cut to the chase with a real worked example. Assume a boat moored in Montreal’s Old Port at 45.5 degrees NNewton - Unit of force and 73.35 degrees W. The sun’s azimuth angle varies between 20 and 68 degrees from winter to summer. Assume 365 perfectly sunny days and solar panels that are horizontal on the boat deck and do not track the sun’s azimuth and east-to-west travel. The average available energy over a year is 3.40 kW/m²/day. At 16% solar-panel conversion efficiency, this is only 0.56 kW/m²/day.

Assume a solar field 10x3 mMetre, SI unit of length (30 m²). This would yield an average of 16.8 kW/day. In comparison 200 hp is equivalen to 150 kW.

The reality (do the math yourself) is that even with solar panels at 100% solar-conversion efficiency it isn't feasible -- for practical purposes -- to fully power a trawler only with a solar platform that is sized within the hull's boundary. It doesn't compute. Solar is best as a supplemental charging system for the boat's batteries.

Links

Solar Sunflower

Top