Ship Boil-Off Gas Re-Liquefaction Using Self-Refrigeration
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Solution Overview
Problem
Existing partial re-liquefaction systems for boil-off gas in liquefied gas storage tanks are inefficient in re-liquefying boil-off gas and do not effectively utilize the boil-off gas as a refrigerant, leading to energy loss and increased storage tank pressure.
Innovation Solution
A system that includes a boil-off gas heat exchanger, a compressor, an extra compressor, a refrigerant heat exchanger, and a decompressing device to compress and cool boil-off gas, allowing for additional cooling and re-liquefaction without a separate refrigerant, and utilizing the boil-off gas as both a fuel and refrigerant in a closed loop.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a separate refrigerant is used for re-liquefying boil-off gas, then the re-liquefaction process can be maintained, but the system complexity and energy consumption increase
Solution Approach 1:
The system uses the boil-off gas itself as the refrigerant for re-liquefaction, eliminating the need for separate refrigerants. The boil-off gas is compressed, heat-exchanged, and expanded to create a refrigeration cycle that re-liquefies additional boil-off gas, making the system self-sufficient and reducing complexity
Solution Approach 2:
The boil-off gas serves multiple functions: it is used as fuel for the engine, as a refrigerant for re-liquefaction, and as a working fluid in the heat exchangers. This multi-functionality eliminates the need for separate refrigerant systems and reduces overall system complexity
2Quantity of substance
If boil-off gas is accumulated in the storage tank, then the re-liquefaction amount increases, but the internal pressure of the tank rises excessively
Solution Approach 1:
The system extracts boil-off gas from the storage tank through a dedicated discharge line and processes it externally through compression, heat exchange, and expansion. This prevents pressure accumulation in the storage tank while enabling continuous re-liquefaction of the extracted gas
Solution Approach 2:
The system introduces intermediary components (compressor, heat exchangers, expansion valve) between the storage tank and the re-liquefaction process. These intermediaries facilitate the controlled extraction and processing of boil-off gas, maintaining storage tank pressure within safe limits while enabling continuous re-liquefaction
3Use of energy by moving object
If the boil-off gas is used as fuel for the engine, then the energy utilization improves, but the amount of gas available for re-liquefaction decreases
Solution Approach 1:
The system segments the boil-off gas flow into multiple pathways: one stream is directed to the engine as fuel, while another stream is directed to the re-liquefaction system. The re-liquefied gas is then returned to the storage tank, effectively increasing the overall utilization of the boil-off gas by serving dual purposes
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This system increases re-liquefaction efficiency, reduces energy loss, and allows for flexible control of refrigerant flow, enhancing the economical efficiency and space utilization on ships by using existing compressors and heat exchangers.
Implementation Method 1
a boil-off gas heat exchanger which is installed on a downstream of a storage tank and heat-exchanges a compressed boil-off gas (hereafter referred to as 'a first fluid') by a boil-off gas discharged from the storage tank as a refrigerant to cool the boil-off gas
Implementation Method 2
the refrigerant heat exchanger heat-exchanges and cools both the first fluid and second fluid by the boil-off gas, which passes through the refrigerant decompressing device, as a refrigerant
Implementation Method 3
a first decompressing device which expands the first fluid that has been cooled by the boil-off gas heat exchanger and the refrigerant heat exchanger
Data Source
AI summary
A ship includes: a boil-off gas heat exchanger which is installed on a downstream of a storage tank and heat-exchanges a compressed boil-off gas (“a first fluid”) by a boil-off gas discharged from the storage tank as a refrigerant to cool the boil-off gas; a compressor installed on a downstream of the boil-off gas heat exchanger and compresses a part of the boil-off gas from the storage tank; an extra compressor which is installed on a downstream of the boil-off gas heat exchanger and in parallel with the compressor and compresses the other part of the boil-off gas from the storage tank; a refrigerant heat exchanger which additionally cools the first fluid; and a refrigerant decompressing device which expands a second fluid, which is sent to the refrigerant heat exchanger, and then sends the second fluid back to the refrigerant heat exchanger.