Boil-off gas reliquefaction plant
The onboard refrigeration plant that returns boil-off gas to the tank as liquid, its cycle options, its power demand, and where it earns its capital cost.
A reliquefaction plant returns boil-off gas to the cargo or fuel tank as liquid instead of burning or venting it. It compresses the vapour, rejects heat to seawater or to a refrigerant cycle, and expands the condensed liquid back into the tank, so the inventory is preserved at the cost of the electrical power the cycle consumes.
Whether that trade pays depends on what the gas is worth and on how much of it there is. On a gas carrier the boil-off is cargo, and preserving it is straightforward economics. On a gas-fuelled ship the inventory is far smaller and the plant competes against pressure accumulation, engine consumption and a gas combustion unit, so it appears mainly where port dwell is long and the hotel load is large.
The full article will cover the cycle options and their coefficients of performance, compressor and expander arrangements, the power demand and its effect on the ship energy balance, partial against full reliquefaction, and the comparison against the other boil-off disposal routes. See boil-off gas management and LNG fuel system .