Prospects of Using Compressed Air Storage for Ferry Propulsion
By Harry Valentine
The prospect of higher future fuel prices is encouraging development of alternative sources and forms of transportation propulsive energy, including for marine transportation. Compressed air storage is one option that may be applied to short-distance marine services. It has successfully been used to power an early generation of mining locomotives and is being developed for grid-scale energy storage applications.
The natural gas industry has developed much of the technology that may be applied to a variety of compressed air energy storage (CAES) systems. Some of the large volume, cylindrical shaped natural gas storage tanks can hold a pressure of over 750-bar or over 10,000-psia. Many large marine vessels can accommodate the volume of such tanks.
Underground Air Storage:
Onboard tanks may store a small amount of compressed air to provide short bursts of additional power. The natural gas industry stores massive volumes of compressed natural gas in naturally occurring caverns in the earth’s bedrock that have been flushed of rock salt. Some of these caverns may measure up to 1-mile in diameter by up to almost 6-miles in vertical height. Depending on the depth of the cavern dome below ground surface, the caverns may hold pressure at anywhere between 1000-psi to over 3000-psi.
Electric utilities have introduced compressed air energy storage (CAES) into available large caverns in the south-central USA and as well as in Germany. During the overnight off-peak period for power stations, excess electric power drives banks of air compressors to pump air into the underground caverns. The heat of compression of the air may often by put to numerous productive uses, including thermal desalination of seawater and district heating during winter.
During summer, the heat of compression may be pumped into underground seasonal thermal storage systems, such as the system that is already operational in a community in Alberta, Canada. There is scope to develop such technology at other locations where local geology permits. The stored heat may contribute to wintertime district heating or it may be put to some other productive industrial use.
Compressed air powered ships may operate to/from terminals located near grid-scale, compressed air energy storage (CAES) systems. During layover at port, high-pressure lines would transfer compressed air from land-based storage into onboard tanks. There may be scope to pump the compressed air into the onboard storage tanks at higher pressure and transfer the heat of compression into onboard thermal energy storage system. Such systems may be based on either heat-of-fusion or heat-of-transformation technology.
A compressed air powered ship may carry a compliment of combustible fuel that would be used to superheat the compressed air immediately upstream of the engine inlet. During operation, compressed air would transfer from a high-pressure tank to a lower-pressure running tank that would remain at constant pressure for the duration of each voyage. Heat from the onboard thermal energy storage system would preheat the air as it flows from the constant-pressure tank.
The air would be further heated in the recuperative heat exchanger that would recover some of the exhaust engine heat, then superheated in a high-temperature heat exchanger prior to expansion in the first stage of the engine. Exhaust air from the high-pressure engine would be reheated prior to being expanded in the intermediate-pressure section of the engine. Exhaust air from that section would be reheated prior to being expanded in the low-pressure stage of the engine.
Low-pressure exhaust heat would flow through the recuperative heat exchanger to preheat the incoming air. Residual exhaust heat may be used to provide onboard heating to various sections of the ship. The 3-stages of engine expansion may involve either a continuous-flow engine or a positive-displacement engine technology, or a hybrid or combination of the engine technologies.
Some positive-displacement engines may be able to directly drive the propeller, while some continuous flow engines may drive electrical generation equipment to provide energy to power azipods. The size of the ship, the size and storage volume of the onboard storage tanks as well as available onboard thermal energy would determine the operating range of a ship propelled by compressed air.
Deep-water Compressed Air Storage:
A recent variation in compressed air energy storage involves pumping air into air cells secured to the floor of a deep body of water. The pressure in some 1000-ft depth of seawater is over 450-psia (over 31-bar). Deep-water storage of compressed air may be possible in some fiords in Norway, in the Strait of Gibraltar, between the islands of Corsica and Sardinia and near the Strait of Messina.
During overnight off-peak periods when electric power sells at a low price, electrically driven air compressors would pump air into the submerged air cells. The heat of compression may productively be used on shore. When the ferryboat is in operation, compressed air may be transferred at high pressure from the submerged air cells into the ferry ship through an interconnecting pressure pipe system.
Other Marine Operations:
A shore-based or underwater-based compressed air storage system could provide propulsive power for tugboats that assist ships at deep-sea ports. The pressure lines that carry compressed air to the tugs would be located below the draft-depth of the large ships, to prevent interference. Given that tugboats typically operate at low speed over very short distances, there may be scope for some tugboats to carry a large submerged tank of highly compressed air pumped to near the liquid state. The top of the tank would be located below the depth of the vertical-axis propeller.
Much of the technology needed to develop a marine compressed air propulsion system already exists and is well proven in related applications. The technology may be suitable for ships and tugs that operate short-distance voyages, such as ferry services or short-distance services at select ports. There is scope to further refine and develop compressed air propulsion for a variety of marine vessels.
MarEx does not necessarily endorse the opinions herein.
The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.