Prospects for Gas/Air Turbine Powered Thermal Rechargeable Ship Propulsion
By Harry Valentine
The history of ship propulsion began with the development of steam piston engines that were displaced by steam turbines engines on the largest commercial ships. Over time the steam engines gave way to diesel engines and occasionally gas turbines that provide commercial maritime propulsive power. While navy services may favor gas turbine engines due to high output from compact dimensions, such engines typically operate at optimal efficiency while delivering peak output. They generally operate at lower thermal efficiency than do diesel engines and especially so while operating at reduced output.
New developments in the power industry has created potential for a thermally efficient, externally heated gas/air turbine engine to be adapted for commercial marine propulsion. The nuclear power industry is developing high-temperature technology of up to 1000?C (1832?F) that can activate high-powered, closed-cycle, externally heated turbine engines that flow either helium or carbon dioxide gas at extreme pressure. The low pressure upstream of the low-pressure compressor may be 7-bar to 10-bar (100-psi to 150-psi) and peak system pressure may reach 45-bar to 65-bar (650-psi to 950-psi) immediately upstream of the high-pressure power turbine.
Possible Marine Turbine:
Using highly pressurized gas within the turbine system gas allows an extremely compact engine to deliver very high power output. The ability to vary the pressure in the turbine system allows for variable engine output dropping to as low as 25% of maximum output, at very high efficiency. The outer dimensions of a closed-cycle, externally heated air/gas turbine engine of 20Mw to 50Mw output and capable of propelling a ship may theoretically fit inside the box of a delivery van. For marine service, the engine may operate using highly pressurized atmospheric air.
Operating on the ocean allows for water-cooling of certain engine operations, such as the intercooler that cools the air between the low-pressure and high-pressure turbo compressors. There may be additional need to cool the pressurized air within the turbine system immediately upstream of the low-pressure compressor. A recuperative air-to-air heat exchanger would transfer exhaust heat leaving the low-pressure power turbine to preheat compressed air flowing from the high-pressure compressor. That compressed air would be further heated to a much higher temperature in a high performance heat exchanger that sources heat from a thermal storage system.
After passing through the high-pressure power turbine, the partially cooled air would be reheated in a second high performance heat exchanger connected to thermal storage, before passing through the low-pressure power turbine. Hot exhaust air from the low-pressure turbine would flow through the recuperative heat exchanger to preheat compressed high-pressure air, prior to being further cooled by seawater and recycled through the turbine system. The overall thermal efficiency of a variable pressure, closed-cycle, and externally heated 2-stage turbine engine system would exceed 40% while driving electrical generating equipment to energize electrically driven propellers.
Thermal Storage:
Researchers at several technical universities in Japan have been testing the performance of several thermal rechargeable energy storage systems. One system involves the decomposition of naturally occurring calcium carbonate into calcium oxide and carbon dioxide, when heated at 810?C (1490?F). When carbon dioxide encounters calcium oxide in a pressure chamber, the ensuing reaction generates some 375-BTU/lb of heat at some 1000?C (1832?F) that happens to coincide with the operating temperatures of some new generation nuclear power reactors.
Barium carbonate also occurs naturally and decomposes into barium oxide and carbon dioxide at 1300?C (2372?F) at atmospheric pressure. It appears possible for the decomposition to occur at lower temperature in a vacuum chamber. The reaction between barium oxide and carbon dioxide can occur at near atmospheric pressure and generate heat around 1000?C (1832?F) that could energize an externally heated, closed-cycle turbine engine.
When onboard ship, the metallic oxide would be carried in a storage-chamber and use an auger screw mechanism to push it to a reaction chamber. A separate tank would carry pressurized carbon dioxide. A third storage chamber would contain the spent fuel in the form of the metallic carbonate that would be transferred off the ship and on to railway cars when at port. Processing of the metallic carbonate into the oxide and carbon dioxide would occur away from port, perhaps at a facility that operates a micro nuclear reactor or a radiation-free fusion power system. The carbon dioxide tanks would be refilled during layovers at power.
Depending on the size of the ship and the size of the onboard storage areas for the metallic oxide, the carbon dioxide and the metallic carbonate, it may be possible to develop a thermal rechargeable air turbine ship with an operating range of some 1000-miles (1600Km). Such ships may operate in regions where oil resources are either scarce or prohibitively expensive and where nuclear operations exist. Shanghai may become the likely center of operations for such ships that may likely carry freight to and from such ports as Seoul, Tokyo, Taipei, Manila and possibly Hong Kong.
Alternative Energy Storage:
The reversible reaction between carbon dioxide and certain metallic oxides represents one form of thermo-chemical energy storage. Steam will react with several metals that include barium and calcium to produce heat and a metallic hydroxide that may be decomposed at a higher temperature. Powdered metal may be carried in one tank while a battery of insulated pressure vessels may carry saturated water. Some of the exhaust heat may serve to maintain temperature in the pressure vessels that may otherwise begin to cool. A third tank may carry the metallic hydroxide that may be transferred off ship while at port, when a fresh load of powdered metal in loaded onboard, along with saturated water.
Hydrogen is known to react with certain metals to generate heat and produce a metallic hydride. However, the storage of hydrogen aboard ship may be problematic due to the tendency of the gas to permeate the chemical structure of several metals and reduce its structural strength. While heat of fusion thermal storage technology may be quite possible, such technology may best involve lower temperature applications that result in the production of superheated steam. Reversible reaction thermo-chemical technology may be well suited for high temperature thermal storage applications and also be compatible with evolving high temperature nuclear fission and radiation free nuclear fusion.
Conclusions:
As world oil prices rise over the long term, researchers will explore alternative transportation propulsion technologies. There is scope to adapt several developments from the grid electrical energy sector to commercial marine application. An externally heated, closed-cycle air turbine engine that operates at variable high pressure and sources heat from a rechargeable chemical thermal energy storage system, is one possible future marine propulsion option. Recharging of reversible reaction, thermal storage systems would be undertaken at specialized facilities using any of nuclear fission, radiation-free nuclear fusion or possibly concentrated solar thermal energy.
Harry Valentine can be reached for comment at [email protected] or
[email protected].
MarEx does not necessarily endore the opinions herein.