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Future Carbon-Free Thermal Energy and Ship Propulsion

Published Apr 25, 2021 10:40 PM by Harry Valentine

The maritime sector seeks to reduce carbon emissions using ammonia, methanol, hydrogen and battery power and even liquefied air as the basis for propulsion. While solar, wind, ocean current and wave power contribute to the production of several forms of carbon-free propulsion energy, advances in lower cost nuclear power also have potential to make a very significant contribution.

Introduction

Concerns over the perceived long-term effects of carbon emissions and climate change have prompted initiatives in alternative zero-carbon and low-carbon forms of vehicle propulsion. Historically, wind-powered vessels carried passengers and freight over a period of centuries. On land, railways and municipal public transportation systems operated zero-emissions electrically powered trains, trams and buses in and between cities. While advances have occurred in wind-powered vessel propulsion to increase sailing speed and total vessel tonnage, the reliability of wind restricts the operation of wind powered vessels to a few routes such as across the North Atlantic.

The maritime sector has responded to the challenge of reducing carbon emissions by adapting unconventional fuels such as hydrogen, ammonia and methanol to large-vessel, extended-distance propulsion. Other forms of short-distance zero carbon vessel propulsion includes battery power, liquefied air and stored thermal energy. While there is great public opposition to nuclear power, recent advances in nuclear power development promise to increase the safety of future nuclear conversion, re-use spent nuclear fuel rods from an earlier era, reduce the cost of future nuclear power and resolve many of the earlier problems related to nuclear power conversion.

Molten Salt Reactor

While research scientists have developed different variations of the molten salt reactor, all variations operate free from the high steam pressure of earlier generation water-cooled reactors that typically converted less than one percent of the energy content of nuclear material to useable heat. A reactor operating free from high pressure steam offers greater safety at considerably lower cost, making modern molt salt reactor technology cost competitive against earlier nuclear technology as well as against some renewable energy conversion technologies. Japan’s Fukushima nuclear power station provides a lesson into the location and design of future coastal power stations.

Building a nuclear power station near a maritime port provides access to abundant liquid cooling of the condensers while providing the maritime port with access to large amounts of electric power and thermal energy. Ready access to such energy sources will provide the maritime sector with various forms of carbon-free propulsion engine. At such locations, the power station will provide for the combination of surrounding population, the local economy and adjacent energy-based installations that provide for the transportation sector. Power stations based on molten salt reactors have potential to sustain the production of alternative carbon-free transportation fuel.

Carbon-Free Thermal Energy

Concentrated solar thermal energy, geothermal energy and nuclear-fission energy represent the main forms of available carbon-free thermal energy capable of indirectly producing propulsion energy. Much research is still being applied to developing some form of workable radiation-free atomic fusion energy that in the future, would represent an additional form of carbon-free thermal energy capable of indirectly producing propulsion energy. The production of methanol combines thermal energy, water and methane (natural gas) that results in a liquid fuel capable of sustaining the operation of piston engines, turbine engines as well as high temperature solid-oxide fuel cells (SOFC’s) that generate electricity.

While the production of ammonia from natural gas requires thermal energy, the process also produces massive volumes of carbon dioxide. The solar thermal power industry has developed thermal storage material based on heat capacity that is capable of producing several thousand megawatt-hours of electric power per operational cycle. The same thermal storage and power conversion technology may be applied to short-distance ship propulsion and be cost-competitive over a period of decades against ammonia, methanol and hydrogen propulsion.

Conversion Efficiency

Molten salt nuclear conversion promises to be safer and more cost competitive against traditional nuclear power that converts less than one percent of the energy potential of nuclear fuel to thermal energy and to electric power. Advances in large-scale thermal storage technology applied to modern high-temperature nuclear conversion and to concentrated solar thermal energy involve direct transfer of thermal energy from the source of the thermal energy into thermal energy storage. That thermal energy storage technology may also be installed into a ship that would recharge at a coastal nuclear power plant.

A modern large-capacity positive-displacement steam engine installed aboard ship would likely convert thermal energy to propulsion at 15 percent to 24 percent efficiency. The conversion of thermal energy to electric power at 50 percent, followed by hydrogen-from-water production at 70 percent, fuel cell conversion to electric power at 50 percent and electric motor efficiency at 90 percent leaves an overall conversion efficiency of 12 percent to 17 percent. The production of ammonia using thermal energy is rated at 60 percent efficiency and methanol at 50 percent to 65 percent, with internal combustion engines using these fuels operating at 35 percent to 45 percent efficiency.

Cost Factors

The technology to produce large volumes of any of hydrogen, ammonia or methanol from thermal energy requires considerable investment and with long-term operating and maintenance expenses. By comparison, the investment is thermal energy storage technology is comparatively low at a concentrated solar thermal power plant or at a modern high-temperature nuclear power station, with the storage technology offering a useable service life extending into decades. The long-term operating and maintenance expenses of a thermal energy storage and power conversion installation would be lower than the production facilities for methanol and/or ammonia.

Short-Sea Sailing

A segment of the shipping industry saves fuel by sailing at 12 knots and requiring one-eigth the propulsive power of a ship sailing at 24 knots and requiring 32,000 horsepower. The combination of an alternative propulsion technology such as whale-based fluke propulsion or twin counter-rotating vertical-axis rotors offers the possibility of greater propulsive efficiency than traditional propellers, at lower sailing speeds. A ship of 30,000 tons deadweight carrying insulated tanks of super-cooled liquefied air and insulated tanks of molten salt to superheat the air prior to expansion in an engine could theoretically sail a distance of 800 nautical miles.

If the problems of rapidly removing heat from insulated tanks of high-temperature molten heat-of-fusion thermal storage material can be resolved, then such material would be able to provide the basis of future ship propulsion. Thermo-acoustic engines are still subject to research and development, with potential to convert heat to electricity at 40 percent to 60 percent efficiency for units of 100kW to 200kW electrical output. A bank of 20 such engines could provide the basis for short-sea sailing, using a mixture of 20 percent lithium fluoride and 80 percent lithium hydroxide that can hold over 1100KJ/Kg of thermal energy at 427 degrees C.

Conclusions

While solar thermal power stations will operate at some mainly arid locations internationally, molten salt nuclear reactors would operate at locations where solar thermal conversion would be impractical. Evolving nuclear energy conversion would indirectly produce methanol liquid fuel and ammonia required for future long-distance ship propulsion, with liquefied air and either heat-of-fusion or heat-capacity thermal storage technologies being applied to short-sea commercial sailing.

The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.