Potential for Propulsive Thermal Energy Storage in a Modern Steam Powered Ship

 

by Harry Valentine

Steam powered ships and marine vessels have sailed the oceans and waterways of the world for well over a century. Many navies still boil water to produce steam in their ships and submarines to drive steam turbines that in turn drive electric generating equipment. The propellers on many navy and commercial ships are electrically driven.

New developments in thermal energy storage technology provides opportunity to re-introduce steam power to ships and vessels that sail relatively short distances, in contrast to extended oceanic voyages. Some thermal storage systems involve groups of well-insulated accumulators capable of holding saturated water at high-pressure, even within the super-critical range. Other systems store thermal energy in the latent heat of fusion of mixtures of molten salts.

Thermal Energy Storage:

The solar thermal power industry has found it necessary to develop some form of grid-scale energy storage that can allow solar thermal power stations to continue to provide electric power after sunset, or during short periods of cloud cover. Several companies are developing thermal storage systems based on the heat-of-fusion of mixtures of cost-competitive salt mixtures. Salts such as sodium nitrate, potassium carbonate along with related rock salts occur naturally and are commercially available in large quantities at low cost.

The molten salt thermal energy storage installation in Nevada USA is rated at 280MW and stores thermal energy in a molten mixture of naturally occurring sodium and potassium salts. The stored heat is able to generate superheated steam that sustains the operation of steam turbines over periods of several hours, driving electrical generators. There may be scope to scale down the thermal energy storage technology for use in short-distance marine transportation.

Future development of heat-of-fusion thermal energy storage systems may include eutectic mixtures of the oxides and hydroxides or oxides and fluorides of the same naturally occurring metals. Several bi-metallic oxides such as lithium aluminum oxide (Li-O-Al=O, LiAlO2) occur naturally in the earth. Mixtures of closely related bi-metallic oxides offer the potential for increased thermal energy storage in the same size of package. Such metallic-oxides mixtures may extend the voyage distances of thermal rechargeable ships, the optimal mixture being the fluoride and oxide of thorium.

Marine Application:

The sheer size of a ship provides scope to undertake such research and the ocean provides a natural heat sink to sustain the operation of a marine steam condenser. Steam turbines have been used in ships and electrically driven propellers are long proven in commercial marine transportation. Oil tankers are the largest ships afloat at some 1600-ft length, 200-ft width and weighing in at over 500,000-tonnes deadweight, with engines rated at 35,000kW (35MW) output.

A solar thermal energy storage system of 28,000-tonnes of molten salt mixture can develop 50MW for 7.5-hours. The system flows the molten mixture between the storage tanks and the boilers. Some 20,000-tonnes of that low-cost mixture plus some 1,000-tonnes of engine equipment would constitute less than 5% of the fully laden deadweight of the ship and provide some 7.5-hours of service. Modern steam ships that use thermal storage technology may operate short-distance routes carrying freight or as a bulk carrier. Thermal power stations and/or thermal storage installations may be located close to the marine terminal.

The thermal-battery ships would be recharged during layovers at port, using interconnecting insulated steam pipes. It is also possible to combust gaseous fuels such as producer gas on board the ship during layovers at port, to recharge the thermal storage system. The ship could recharge on thermal sources and fuels that may otherwise present a hazard if carried onboard ship, especially services that include passenger transportation. Stored onboard thermal energy may offer a low-cost marine propulsive option.

Higher Performance Thermal Storage:

Heat-of-fusion thermal storage systems offer cost advantages over chemical battery energy storage technologies, as well as many times the service duration. While molten mixtures of naturally occurring nitrates of sodium and potassium are now being used for thermal energy storage, many other molten mixtures involving naturally occurring metallic oxides are possible. It is also possible to manufacture any of several bi-metallic oxides to increase thermal storage capacity per unit weight, at slightly higher temperatures.

Reacting recycled molten aluminum with caustic soda (NaOH) will produce Na3AlO3 that can be mixed with naturally occurring cryolite (Na3AlF6) to produce a useable melting temperature, with higher thermal storage capacity. Reacting aluminum with potassium hydroxide will produce K3AlO3 that can be mixed with both cryolite and Na3AlO3. It will melt at a temperature closer to that of the molten nitrates of sodium and potassium, with higher thermal storage capacity (KJ/Kg or BTU/lb).

There is the option of mixing naturally occurring minerals such as cryolite (Na3AlF6) plus aluminum-oxide-hydroxide [AlO (OH)] also know by trade names Diaspore and Bhoemite] to raise thermal storage capacity at competitive cost. The objective would be either to raise power output or extend operating range. The molten mixture of sodium and potassium nitrates will still flow through piping systems to carry heat between the thermal storage tank(s) and the boilers.

Direct Drive Option:

While a downsized version of a power station size thermal storage system could sustain the operation of steam turbines in a large ship, there is the option of using a direct drive propulsion system. Many of the large marine diesel engines directly drive the propeller that rotates at 75 to 100RPM. These engines are built to rotate in both clockwise and counter-clockwise directions.

Several designs of positive-displacement rotary engines are compact and able to operate on steam. There may be scope to adapt several of these engines to bi-directional rotation capability so as to directly drive large-scale marine propellers. This may be accomplished by installing piston-type control valves and bypass valves in the pipes connect to the engine ports, to allow the same pair of inlet/outlet ports to operate in the reversed order.

Adapting positive-displacement rotary engines for bi-directional rotation using piston-valves can also introduce variable inlet timing to the engine. It is a method by which to efficiently adjust power output. The engines may also operate as a 3-stage steam expansion system involving high-pressure, intermediate-pressure and low-pressure sections of the engine to ensure optimal thermal efficiency. It would be possible to use double-jointed cardan drive shafts and closely spaced rod drive mechanisms housed inside a casing to connect the rotary engine system to a propeller installed on an azipod.

Ship Routes:

Thermal energy storage ships using salt mixtures may see service on many short-distance routes around the world. These routes would include:

- Florida and Nassau, Bahamas or may provide service between some of the Caribbean islands.
- Buenos Aires and Montevideo
- Barcelona (Spain) and the Balearic Islands
- Rome, Italy and the islands of Sardinia and/or Corsica
- Havana and Miami and/or Tampa
- Liverpool and Dublin, Liverpool and Belfast
- Port Sudan and Jeddah
- Nagasaki (Japan) and Pusan (South Korea).
- Vancouver and Nanaimo, Sydney (NS) and Port-aux-Basque
- Melbourne and Tasmania

Extended Ship Routes:

Thermal energy storage ships using eutectic mixtures metallic-oxide or bi-metallic oxides for thermal storage may sail extended routes that may include:

- Wellington and Christchurch
- Melbourne and Hobart
- Shanghai and Taipei, Nagasaki and/or Pusan
- Hong Kong and Taiwan
- Helsinki and Stockholm
- Tunis and Rome
- Barcelona and Algiers
- London and Rotterdam

Conclusion:

The earth’s crust offers a range of materials that may form the basis of grid-scale and marine-scale thermal energy storage batteries. Such technology can allow power stations to generate electric power and allow large ships to undertake short-distance voyages at locations where marine transportation has logistical advantages over land-based transportation. There is scope do undertake further research into higher performance thermal energy storage mixtures, suitable for short-distance marine propulsive applications. A steam powered ship engine powered by a bi-directional rotary engine offers the option for direct-drive operation.

Harry Valentine can be contact via email at  [email protected] or [email protected].