Questions & explanations
1. How does a metal hydride store hydrogen? What are the pros and cons for a ship?
A metal hydride stores hydrogen by chemically bonding it to a metal alloy, forming a solid. When the metal is cooled under hydrogen pressure, it absorbs hydrogen. To release the hydrogen, the metal is heated. The advantage is that hydrogen is stored at low pressure and room temperature, making it safer than compressed or liquid hydrogen. Also, the volumetric density can be higher than compressed gas. However, the metal hydride is heavy, adding significant weight to the ship. It also requires heat to release hydrogen, which uses energy. The charging process is slow, and the metal can degrade over time. For ships, it may be suitable for small vessels where safety is paramount and weight is less critical.
2. What are the three main ways to store hydrogen on a ship? Compare their energy density.
The three main methods are compressed hydrogen gas (CH2), liquid hydrogen (LH2), and metal hydrides. Compressed hydrogen is stored at high pressure (350-700 bar) in strong tanks. It has low energy per volume, so tanks are large. Liquid hydrogen is stored at -253°C in cryogenic tanks, giving higher energy per volume than compressed, but it requires heavy insulation and some hydrogen boils off. Metal hydrides store hydrogen in a solid material, offering high volumetric density but adding weight. For ships, liquid hydrogen has the best energy per volume, but boil-off and insulation are challenges. Compressed is simpler but bulky. Metal hydrides are heavy and need heat to release hydrogen.
3. Why is storing hydrogen as a liquid challenging for marine use? What safety issues arise?
Liquid hydrogen must be kept at -253°C, which requires very good insulation to prevent boil-off. Even with insulation, some hydrogen evaporates, creating pressure buildup. This boil-off must be managed, either by reliquefying or using it as fuel. The extreme cold makes materials brittle, so special alloys are needed. Also, liquid hydrogen is less dense than water, so a spill could float and spread. Hydrogen is highly flammable and can leak through tiny gaps because its molecules are very small. Leaks can form explosive mixtures with air. Ventilation and gas detection are critical. Despite these challenges, liquid hydrogen offers higher energy density than compressed gas.
4. Give an example of how a ship could use a PEM fuel cell for propulsion. What would be the main challenge?
A small ferry could use a PEM fuel cell to power an electric motor for propulsion. The fuel cell would take compressed hydrogen stored in tanks and convert it to electricity, which drives the motor. The main challenge is storing enough hydrogen on board because hydrogen has low energy per volume. Even compressed to 700 bar, hydrogen takes up much more space than diesel for the same energy. This limits the ship's range unless very large tanks are used. Also, PEM fuel cells need very pure hydrogen, so any impurities in the fuel can damage them. Despite these challenges, such a system produces zero emissions at the point of use, which is good for coastal or inland waters.
5. Compare the fuel efficiency of a two-stroke and a four-stroke marine diesel engine. Which one is generally more efficient and why?
Four-stroke marine diesel engines are generally more fuel-efficient than two-stroke engines. In a four-stroke, the separate intake and exhaust strokes allow better removal of exhaust gases and more complete combustion. Two-stroke engines lose some fresh air out the exhaust during scavenging, which wastes fuel. However, two-stroke engines have less friction because they have fewer moving parts, and they produce more power per revolution. For large ships that run at constant low speed, two-stroke engines are often used despite lower efficiency because they are simpler and cheaper to build. Modern two-stroke engines with advanced turbocharging can be quite efficient too.
6. What is a fuel cell and how does it differ from a battery?
A fuel cell is a device that converts the chemical energy of a fuel (like hydrogen) directly into electricity through an electrochemical reaction. Unlike a battery, which stores energy inside and must be recharged, a fuel cell generates electricity as long as fuel is supplied. In a fuel cell, hydrogen and oxygen from air react to produce electricity, water, and heat. Batteries store energy in chemical form inside and release it when needed, but they have a limited capacity and need to be recharged. Fuel cells can run continuously with refueling, making them good for long voyages. However, fuel cells are generally less responsive to sudden power changes than batteries.
7. What are the main differences between PEM and SOFC fuel cells for marine use?
PEM (Proton Exchange Membrane) fuel cells work at low temperature (about 80°C) and start quickly, making them good for auxiliary power on ships. SOFC (Solid Oxide Fuel Cell) runs very hot (600-1000°C) and can use various fuels like natural gas directly, but takes time to warm up. PEM needs pure hydrogen to avoid poisoning its catalyst, while SOFC is more tolerant of impurities. For marine applications, SOFC offers higher overall efficiency (up to 60%) and can use LNG without external reforming. However, PEM's quick response suits dynamic loads like ship maneuvering. Both produce electricity with very low emissions, but SOFC's heat can be used for ship heating.
8. What safety measures are needed for compressed hydrogen storage on a ship?
Compressed hydrogen tanks must be strong to withstand high pressure (350-700 bar). They are usually made of carbon fiber composite to be lightweight. The tank area must have gas detectors to warn of leaks because hydrogen is odorless and invisible. Ventilation is needed to prevent hydrogen accumulation, as it can explode at concentrations from 4% to 75% in air. Tanks should be placed on deck or in well-ventilated spaces, not in enclosed compartments. Fire protection includes water spray to cool tanks and prevent pressure rise. Also, pressure relief valves are required to release gas safely if pressure gets too high. Regular inspection for damage is essential.
9. Compare the use of velocity potential versus stream function for wave-structure interaction.
Both velocity potential and stream function describe flow, but they are used for different flow types. Velocity potential works for irrotational flow (no spin) and is easier for three-dimensional problems. Stream function works for two-dimensional flows and automatically satisfies continuity. For wave-structure interaction, potential is more common because waves are nearly irrotational. Stream function is sometimes used for steady currents or nonlinear waves. The potential gives pressure directly via Bernoulli's equation, while stream function gives velocities but not pressure directly. In most offshore engineering, potential theory is the standard choice.
10. How does methanol injection work in a marine diesel engine? What changes are needed?
Methanol can be injected into a diesel engine either by mixing it with air before combustion (port injection) or by direct injection into the cylinder. Because methanol has a high octane number but low cetane number, it does not auto-ignite easily like diesel. So a small amount of diesel or a glow plug is used to ignite the methanol. The engine needs modified fuel injectors and a separate methanol supply system. Methanol's lower energy density means more fuel must be injected to get the same power. Also, the injection timing and pressure must be adjusted for proper combustion. These changes allow the engine to run on methanol with lower emissions.
11. Compare the efficiency of a SOFC system with a conventional diesel engine for a ship. Which one is better and why?
A SOFC system can achieve electrical efficiency of 50-60% when using natural gas, while a modern diesel engine typically has a brake thermal efficiency around 45-50%. However, the SOFC can also use its waste heat for cogeneration, raising total efficiency to 80% or more. Diesel engines lose a lot of heat through exhaust and cooling water. SOFC also produces very low NOx and SOx emissions, unlike diesel which requires exhaust treatment. But SOFC is more expensive and less proven at large scale. For a ship that runs mostly at steady load, like a cruise ship, SOFC could be more efficient overall. For dynamic operation, diesel might still be better.
12. Compare the safety challenges of using ammonia versus LNG as a marine fuel.
Ammonia is toxic, while LNG is primarily a flammability hazard. Ammonia leaks can poison people even without ignition, whereas LNG leaks create a fire or explosion risk but are not toxic. Ammonia is also corrosive to some metals, requiring special materials. LNG is cryogenic (-162°C) and can cause cold burns, but ammonia is stored at -33°C or under pressure. Both require gas detection and ventilation, but ammonia detectors must be set to very low ppm levels. Firefighting for ammonia is different: water can be used to dilute ammonia, but for LNG, dry powder or foam is used. Both need careful handling, but ammonia's toxicity adds a layer of risk.