Using Salt, Thermal Energy for Propulsion

The history of maritime propulsion includes many centuries of boats and ships being propelled by renewable, naturally occurring energy. 

The switch from flat-bottom vessels to vessels built with a lengthwise vertical central plank, the keel, greatly improved directional control. The combination of a keel and a triangular movable sail allowed forward propulsion from both coastal winds that often blew perpendicular to the sailing direction and winds that blew toward the vessel at angles of 45 degrees.

Wind Power

For many centuries, wind powered ships carried trade between distant nations, sometimes sailing parallel to and in the direction of prevailing trade winds as was the case for galleons, barque ships and clipper ships. An alternate design of wind-powered ship evolved from the trading dhow, the schooner that could sail into prevailing winds at angles of 30 degrees and performing zig-zag maneuvers known as ‘tacking’ into the wind. In some parts of the Asian world, kites made with bamboo frames and fabric covering were used to pull small boats in the general wind direction, over short distances.

While wind power may appear to be a maritime propulsion technology from a bygone era, there has been ongoing research and development in wind powered maritime technology. The innovations include new frameless wind-inflatable kites for towing large vessels, airplane-type wing-airfoils being used in place of sails and developing wind-powered vessels that ride on small-water area multiple hulls, water-skis and/or hydrofoils to reduce hydrodynamic drag. A wind-powered vessel has achieved a speed of 65 knots or 1.7 times the wind speed while kite-towed vessels are carrying tourists and other kite-assisted vessels sail parallel to trade winds carrying freight.

Wave Power

The classical form of wave-powered transportation is the surfboard. British explorer Captain James Cook and his crew first encountered surfboards in 1778 during a stop at Hawaii. Hydrofoil surfboards that ride above the water are the latest development in the technology and allow surfers to ride a series of waves. 

Patents for wave powered boats were filed in the U.S. prior to 1900, with several private inventors building wave-powered boats prior to 2000. During 2008, a wave-powered boat dubbed Suntory Mermaid II took 110 days to sail from Hawaii to Japan.

Calm seas are the nemesis of wave powered vessels, hence the extended duration of the voyage of Suntory Mermaid II from Hawaii to Japan. Depending on location and ocean depth, different measurements of wavelengths and amplitudes for ocean waves occur in different locations. 

While wave-powered boats have sailed at speeds of six knots, a sailing speed of 10 knots is believed possible. Perhaps future innovative research and design could develop a wave-powered vessel capable of competitive sailing speeds in regions where waves are frequent. This may restrict wave-powered vessels to coastal service instead of trans-oceanic sailing.

Tropical Thermal Energy

In tropical waters, the surface temperature may exceed 25oC while at a depth of 1,000 meters, water temperature of 4oC may be typical. Ocean thermal energy conversion technology can actually generate electric power from the difference in temperature, using organic Rankin cycle engines. The technology has been tested and demonstrated off the coast of Hawaii. 

There are several variations that could be adapted to low-speed maritime propulsion in tropical regions. One option could involve a deep tube extending to a depth of 1,000 meters (3,200 feet) to gain access to water at near the freezing point. However, this would be impractical due to the combination of the weight of the tube and the hydrodynamic drag it would cause. 

A possible option instead would include carrying a heat sink onboard the vessel, perhaps using air tanks built into the hull to provide additional buoyancy to compensate for the weight. The heat sink would need to absorb massive volumes of heat, meaning that it would require a measure of latent heat of fusion for it to absorb heat over a very narrow range of temperature.

Adding Ice Energy

It may be possible to combine ocean thermal energy conversion with onboard stored ice energy. During the overnight hours, a heat pump connected by tube to the 1,000-meter depth could cool a tank of onboard water to produce ice. Pure water ice would require 144 BTU per pound of thermal energy to convert from solid into liquid water and serve as a heat sink for an organic Rankin cycle engine that would provide propulsion. Mixing sodium chloride salt into the water would reduce its melting temperature and increase the latent heat of fusion thermal storage capacity aboard a boat.

Sodium chloride salt is soluble in water and usually melts in pure form at over 800oC (1,470 degrees Fahrenheit) with a heat of fusion of over 200 BTU per pound. An insulated heat sink tank filled with extreme saline water would be installed under a boat that would include air ballast tanks for additional buoyancy. 

Depending on the size of the boat and size of the saline tank, the organic Rankin cycle engine could generate several hundred kilowatts of power using the difference in temperature of surface tropical ocean water and the heat sink tank.

Storage Tanks

The onboard heat sink tanks of salt water would need to be lined with plastic in order to reduce salt induced corrosion of metal, perhaps with some carbon dioxide gas inside the tank to prevent oxidation of metal. Insulation technology would need to surround the tanks, perhaps using vacuum technology. Heat exchange pipes that pass through the storage tanks would require a corrosion resistant coating or be made from a plastic that offers a high level of thermal conductivity. The sodium chloride salt to be contained inside the tanks is commercially available at very low cost.

Unlike electrochemical batteries that offer limited life expectancy, a water-salt mixture would offer many decades of useable service. In this case, the thermal battery heat sink technology would be more rugged, more durable and more economical to use than electrochemical battery technology. The low cost of the sodium chloride salt allows for a very large vessel with massive heat sink capacity to allow a vessel to sail at a low speed between tropical islands. Recharging of the heat sink tanks could be achieved using off-peak energy from any of the power grid or various renewable energy technologies.

Tank Recharging

Mega-scale size port-based heat sinks could compliment the vessel onboard heat sink tanks. During a layover at port, heavily insulated pipes would be connected between port and vessel heat sinks to pull massive volumes of the heat from the vessel tanks. The ability to rapidly cool the contents of the vessel tanks would translate to additional time in service for the vessels, allowing them to sail additional short-distance voyages in the form of inter-island ferry services in tropical locations in the Caribbean region, the Philippines as well as at other Asian locations.

Power and Speed

Assume that a vessel of 200 meters (650 feet) in length requires 27,000kW engine power to sail at a speed of 21 knots. Power varies according to the cube of the velocity, with power level resulting from force multiplied by velocity. The force required to balance the water drag varies to the square of the velocity. Sailing the vessel at one-third the speed or at seven knots would reduce the power requirement by a factor of (3 x 3 x 3 = 27) 1/27 or 1,000kW. Ocean thermal energy conversion technology for stationery applications has been developed to levels of power output of just over 1,000kW.

The distance that the vessel will travel would depend on the thermal storage capacity of the heat sink, with larger vessels offering increased storage capacity and possibly increased operating range. Closed-cycle organic Rankin cycle engines operating over a very narrow range of temperature typically operate at a thermal efficiency or about two percent. The easy availability of the needed temperature range in tropical locations makes the operation of engines of such low efficiency both viable and practical. Despite the low efficiency, there may be market application for the services of an organic Rankin cycle powered boat that sails over short distances.

Two-Tank Storage

A two-tank system of warm and cold insulated thermal energy storage tanks would offer higher engine efficiency. A low-tech warm tank would involve using heat pumps to transfer heat from tropical surface seawater into a thermal storage tank held at a temperature of over 60oC (140 degrees Fahrenheit). A heat pump could transfer up to four units of thermal energy for every unit of energy to operate the heat pump. A higher concentration of low-cost salt in seawater could store large amounts of thermal energy at constant temperature and remain at constant temperature for several hours during a ferry voyage.
 
While sodium chloride salt would work well to help store energy in a cold tank, storing thermal energy at higher temperature allows for the addition of other compounds such as small amounts of sodium hydroxide and sodium fluoride into the chemical mixture to increase thermal storage capacity.  A small tank at higher temperature could serve as the vessel starting tank to accelerate the vessel to cruising speed when thermal energy from seawater would blend in to sustain organic Rankin cycle engine operation. The advantage of maritime dimensional scale would allow for the application of ocean thermal energy conversion technology for short-distance vessel propulsion.

Conclusions

Wind and wave powered vessels depend on favorable winds to provide the source of propulsion, waves being a manifestation of wind energy which in turn is a manifestation of solar thermal energy. Applying ocean thermal energy conversion technology to maritime propulsion bypasses the problem of insufficient wind to provide maritime propulsion. Ocean thermal energy conversion technology involves low cost and a very extensive useful service life. However, it would be restricted to low-speed, short-distance sailing in tropical waters.