Advances in Thermal Energy Storage and Boat Propulsion

Historical records indicate that the first powered submarines of the late 19^th century used steam-thermal energy storage to power a piston engine and drive the propeller. For several decades between the late 19^th to mid/late 20^th centuries, several short line railways operated shunting locomotives that stored saturated water in high-pressure, insulated tanks. During that period, several efforts aimed at extending the operating range of such locomotives involved using heat-of-fusion thermal energy storage technology, using molten caustic soda that melted at about 320ºC (600ºF).

Ongoing developments in the concentrated-solar thermal power industry led to developments in grid-scale thermal energy storage technology that could storage enough heat to generate superheated steam that would drive steam turbines and electrical generators. During the early to mid 20^th century, the premium steam powered passenger ships such as the Queen Mary and Queen Elizabeth used steam turbines to generate electric power to drive multiple propellers. Boats and ships that sailed short voyages burned coal and/or wood fuel to raise steam that drove triple-expansion steam piston engines.

The rising price of oil has encouraged various segments of the maritime industry to explore possibilities of using alternative, lower-costing fuel energy for propulsion. One sector uses trains of kites that fly in the powerful trade winds that blow at elevations of between 1,000 and 3,000-feet, to pull maritime craft that sail parallel to the prevailing winds. The higher price of fuel has also sparked renewed interest in steam power and the development of boilers capable of generating steam in excess of 600ºC (1100ºF) and steam piston engines that use steam injectors able to deliver steam at over 4,000-psi (275-atmospheres).

The combination of new developments in thermal storage technology for solar-steam power and renewed interest in steam power by the maritime sector has prompted interest in adapting thermal storage technology for short-distance maritime propulsion.  Once branch of the solar-thermal power sector uses a mixture of sodium nitrate and potassium nitrate that both occur naturally and melts at some 500ºC (950ºF) to store some 80-KJ/Kg (36-BTU/lb) of thermal energy. Ongoing research has identified other materials that can store many times that amount of thermal energy on a per-unit-weight basis.

Heat-of-Fusion Storage:

A mixture of 80% lithium hydroxide and 20% lithium fluoride (by molecular weight) has been tested as absorbing over 1100-KJ/Kg (500-BTU/lb) as it melts at some 465ºC (870ºF). Both compounds are stable individually and in the mixture and corrosion-resistant steam lines may pass through the molten mixture to deliver and extract thermal energy. The compound lithium hydride is a temperamental and needs to be stored inside a container that contains an inert gas instead of atmospheric air, to prevent a chemical reaction. It melts between 680ºC and 690ºC (1255ºF to 1275ºF) and absorbs some 4700-KJ/Kg (2300-BTU/lb) of thermal energy.

Corrosion-resistant pipes carrying high-pressure helium would deliver heat to an insulated container of lithium hydride and extract heat from the container and transferring the heat to a boiler. At the present time, China’s high-temperature nuclear power industry is developing thermal technology that can operate at elevated temperatures and be adapted to maritime thermal energy storage applications. A shore-based, high-temperature micro-reactor could generate heat to thermally recharge a small fleet of thermal energy tug vessels that may push and navigate sea-based and inland waterway barge vessels that may sail voyages of 100 to 500-miles (160 to 800-kms).

Thermo-Chemical Storage:

The thermal power industry has and still is examining prospects for thermo-chemical energy storage, by heating a compound such as magnesium hydride or calcium carbonate to cause chemical-molecular separation. At high temperature, hydrogen will dissociate from magnesium in magnesium hydride, which calcium carbonate (CaCO3) will break down into carbon dioxide (CO2) and calcium oxide (CaO). A mixture of carbon monoxide (CO) and hydrogen (3H2) can react thermally to form the combination of methane (CH4) and steam (H2O). Many other mixtures and compounds either separate or change at elevated temperature, then release heat when they recombine.

There may be scope to adapt some power industry thermo-chemical energy storage developments for future application in maritime propulsion, especially as future oil prices rise. Given the space that thermal energy storage systems may occupy aboard a ship, tugs would be the most likely vessels to operate on stored thermal energy, moving ships around harbors and/or pushing and navigating barges on short coastal voyages or along inland waterways. Such tugs may spend limited time in port under thermal recharge, as spent material may be transferred off and reprocessed material transferred on to the vessel during short layovers.

Comparative Efficiency:

Electro-chemical batteries produce and dissipate heat during the recharging cycle and have lower useable life expectancy than heat-of-fusion and some thermo-chemical thermal storage technologies. If a thermal power station that operates at 50% efficiency produces electric power, a battery may dissipate up to 20% of the power as heat during the recharge cycle to return 35% to 40% overall efficiency. Electrolysis of water produces hydrogen at 70%-efficiency while a fuel cell may operate at up to 50% efficiency, yielding at overall efficiency of 0.7 x 0.5 x 0.5 = 17.5% efficiency for electricity from a thermal power station.

A well insulated thermal storage system could dissipate about 5% of thermal input and operate at 40% to 50% thermal efficiency using super-critical steam, with up to 65% overall thermal efficiency using a bottom-cycle engine that runs of engine exhaust heat. A conventional steam power system may operate at 25% to 32% thermal efficiency, with prospects for 40% to 50% using combined bottom cycle technology. A tug powered by a thermal storage system could offer higher energy efficiency, lower operating cost and greatly extended service life than electro-chemical battery storage systems that receive electricity from thermal power stations.

Trans-Oceanic Vessels:

Ongoing developments in small-scale, high-temperature thorium-nuclear technology would include reprocessing the partially spent thorium for continual use generating heat and producing steam. A relatively small amount of the same thorium could provide trans-oceanic maritime propulsion for several decades, while another small amount of it may recharge thermal storage vessels. The future availability and future cost of oil would determine the development of alternate maritime propulsion technologies, with steam power being a possible option.

Conclusions:

Thermal energy storage technology can be competitive in terms of long-term cost and overall efficiency against electro-chemical battery storage. In short-distance maritime service, thermal energy storage holds the promise of long service life and competitive levels of energy efficiency. Such technology would be applicable at coastal locations where a low-cost source of high-grade thermal energy is available.

Harry Valentine frequently contributes to the MarEx newsletter. He can be reached for questions/comments at [email protected].