Re-Examining Compressed Air Ferry Propulsion

Onboard a ferry, an engine powered by compressed air could drive the propeller and bow/stern thrusters. There are precedents for such a technology in other industries.

Several years ago, a story circulated in the automotive transportation media about plans in Spain to develop a car powered by compressed air. The story shifted to India where Tata Motors was allegedly evaluating such a concept. 

During the early to mid-20th century, mining companies did operate their own short-line railways to haul ore out of mines, using locomotives converted from operating on steam to operating over short distances on compressed air. 

After the resources exploration sector discovered massive caverns deep in the earth’s bedrock, energy companies began to store compressed natural gas in such caverns. Energy companies also explored the option of storing highly compressed air in such caverns, for grid-scale energy storage applications. 

More recently, the energy sector has explored securing balloons to the seafloor and filling them with compressed air during the overnight periods. During peak electrical usage periods, the compressed air is released through engines that drive electrical generators to supply the electrical power market. 

It appears possible in a deep and narrow channel, to connect a flexible tube between a ferry vessel and submerged balloons filled with compressed air to provide a daytime ferry service.

At the present time, hobbyists use compressed air to launch low-cost rockets made from large plastic beverage bottles, skyward. The combination of compressed air over water yields spectacular flight results. With air pressure at 100-psia or 700-kPa and at room temperature, water would have a density of 125-times that of compressed air. Jet propulsion is based on the mass (related to weight) of the fluid being ejected at high velocity, while engine power also depends on mass flowrate through an engine.

The Maritime Advantage

The maritime sector offers greater prospects for short-distance compressed air propulsion than short-line railway or roadway transportation primarily because maritime offers the advantage of scale. 

Maritime can use the compressed air over water propulsion of a bottle rocket, on a greatly magnified scale. Even with air compressed to 1000-psia (7-MPa) at room temperature, seawater will still offer more than 12.5-times the density and greatly increasing the mass flow rate through an engine or through a jet pipe. Boats that float on water can also be designed to use water in a compressed air over water propulsion system.

Venturi Water Pump

Venturi water pumps use a high-speed jet of water from a small diameter tube installed inside a large diameter tube to drive a large volume of water at a much lower speed. Such pumps are widely used to pump water to higher elevation. 

The most efficient boat propellers move a large volume of water at slightly higher speed and in the opposite direction than the sailing speed of a boat. It is technically possible to adapt a venturi water pump to propel a boat using compressed air over water energy storage and frequently recharged at quayside. 

A compound propulsion venturi pump would involve a small diameter discharging into a submerged venturi tube that in turn discharges into an even larger venturi. At extremely low sailing speeds, propellers operate at reduced efficiency and make venturi propulsion competitive in terms of low-speed sailing efficiency. 

A compressed air over water powered vessel using venturi propulsion could operate short-haul ferry service or operate as a tug or push vessel, helping larger vessels sail to and from ports that enforce strict exhaust emissions standards. The absence of rotating or sliding machinery assures reliable service.

Operation in Service

Maritime vessels carry spherical, high-pressure air tanks built to extraordinary dimensions and capable of holding air at extraordinary pressure levels. Air would be released through check valves into a secondary air tank that would remain at constant internal pressure, supplying air into cylindrical high pressure water tanks. The water tanks would in turn connect to a battery of small jet pipes of different diameters, each with a control valve that is either open or closed. Maximum possible speed of different mass flow rates of water jet would blast into the venturi, assuring optimal efficiency during sailing.

The system may be set to hold 1,000-psia or 7.0MPa maximum air pressure and 435-psia or 3.0MPa minimal operating pressure before propulsion water is exhausted. 

Whether operating ferry service or tug boat service, the energy storage system would regularly be recharged during layovers by pumping in water under high pressure to pressurize the air. System top-off would involve pumping in of air to extreme pressure inside the main air storage tank, with an optional thermal storage tank absorbing heat of compression. During sailing, air would be preheated before exerting pressure inside the water tanks.

Sequential Pumping of Water

A boat with multiple high-pressure water tanks can refill non-pressurized water tanks while sailing as pressurized tanks provide propulsion. As a pressurized tank becomes exhausted, inlet valves close and another set of inlet valves open to admit high-pressure into a filled water tank. A multiple-tank system reduces the amount of water that the vessel would carry and increase payload. At port, a multiple tank system can alternately fill and empty with water under high pressure, to recharge the compressed air storage system that could function with zero to minimal loss of air.

Competitive Technologies

During a layover at a terminal, a compressed air over water energy storage system lends itself to rapid recharge from the electrical power grid, while also offering the prospect of greatly extended service life. 

By comparison, most electro-chemical storage battery technologies require extended recharge durations, with a useable service of life of 500 to 3,000-deep cycle recharges. A system that recharges eight times per day to accomplish four return trips would require new batteries after three years of service. Thermal energy storage systems would require extended recharge durations and may be better suited to operate extended journeys.

Maritime vessels that operate on compressed air over hydraulic energy storage would incur low propulsion system maintenance costs. Water storage tanks would need corrosion resistant interior surfaces and corrosion resistant hydraulic valves and pipes. 

These vessels would be competitive in operation at locations where: 

Electric power is cheaper than competing energy sources
Electric power is readily available at several ports or terminals
Vessel will provide short-haul ferry service (e.g. across Strait of Gibraltar)
Vessel may operate as a large tug pushing vessels over short distances
Stringent ship emissions standards are enforced at the dock area

Conclusions

Maritime technology offers the advantage of magnitude of scale that far exceeds the capabilities of railway or road transportation in terms of energy storage capacity and travel distance. 

A boat using such technology could operate viable commercial service. By comparison, a short-distance coastal railway could operate as a tourist attraction, using compressed air over water storage technology and transfer exhaust water into a trough or drain installed along the track.