Recycool™, a cutting-edge recondenser system
As environmental regulations continue to evolve, improving energy efficiency and reducing emissions from LNG-fuelled ships have become critical challenges. High-pressure ME-GI engines, chosen for their low methane slip and high-power efficiency, are often selected as the main vessel propulsion. However, managing the boil-off gas (BOG) in such high-pressure engines remains complex, requiring a reliable, simple and cost-effective solution.
In LNG-fuelled ships powered by ME-GI engines, the fuel gas supply system requires a forced vaporisation of LNG, generating a large amount of cold energy. Recycool™, the Recondenser system developed by GTT, optimises cold energy recovery, offering several advantages compared to other solutions available on the market. The first successful applications on container ships have confirmed its efficiency and reliability in operation.
Regulatory Landscape and Market Context
The adoption of the Initial IMO Strategy on GHG emissions reduction is pushing ship operators toward a cleaner fuel choice. LNG, thanks to its low carbon-to-hydrogen ratio, is seen as the most viable transition towards low-emissions solutions. Additionally, LNG has become the industry's preferred alternative fuel in terms of the number of orders placed in 2024. The latest regulations encourage ship operators using LNG as fuel to adopt efficient engine technologies with low GHG emissions.
One of the preferred choices is the two-stroke high-pressure ME-GI engine, which requires a high gas supply pressure (around 300 bar). Compared to two-stroke low-pressure engines, ME-GI engines provide higher efficiency and a significantly lower methane slip.
However, maintaining high pressures at the engine intake requires large, energy-intensive and expensive high-pressure compressors, regardless of tank technology (whether atmospheric or pressurised). These compressors also help maintain tank pressure and avoid any overpressure, which could force gas to be burned unnecessarily.
An alternative approach is to integrate a recondenser solution within a fuel supply system equipped with a low-pressure compressor.
GTT’s Recycool™ Solution
The primary recondenser solution available on the market today (Direct Contact Type Recondenser) originates from the Floating Storage and Regasification Unit (FSRU) segment. This system, which allows direct contact between LNG and BOG streams, effectively promotes BOG condensation, but comes with an operating limit. The maximum separator pressure is constrained by the maximum pressure of the BOG compressors, which, in turn, limits the maximum temperature to which the LNG can be heated, reducing cold energy recovery. The need for a separator limits the compactness of the unit.
To overcome these limitations, GTT has developed Recycool™, which utilises an indirect heat exchange process between excess BOG and LNG. Indirect heat exchange decouples the inlet pressure of the high-pressure pump, and is independent from the BOG compressor pressure ratio, therefore eliminating the need for a separator.
Recycool™ offers the best reliquefaction performance. While A low BOG compressoroutlet pressure favours the Recycool performance and therefore gives a higherperformance than the Direct Contact Recondenser.
Assuming a high pressure pump inlet pressure for Recycool™ of 9 bar, when the low pressure compressor outlet pressure is at a typical operating value of 8 bar, the system performance is 13% better than the Direct Contact Recondenser performance. Moreover, this performance improvement could be considered even higher if the the additional heat input due to the presence of a buffer/separator is taken into account.
The Recycool design allows for:
- A more compact system (eliminating the need for a buffer/separator)
- Optimised efficiency and stable operation
- Increased BOG reliquefaction capacity
Recycool™ minimises the volume of BOG that would otherwise be directed to the boilers. This is achieved by cooling the compressed BOG (from the BOG compressor) by recovering cold energy from the fuel gas sent to the high-pressure engines. The condensates are then returned to the tank in liquid form.
The system operates when the ship's main engine is running on gas (i.e. the ship is in motion), with cold energy recovery occurring in two different locations:
- Downstream of the high-pressure pumps in the Precooler.
- Upstream of the high-pressure pumps in the Recondenser.
When integrated with a power take-off (PTO), Recycool™ further enhances the advantages of using the ME-GI engine for electricity generation, offering operational expense reduction and lower greenhouse gas emissions.
First Worldwide Application and Performance Validation
Recycool™ has been successfully installed on ten LNG-powered containerships built at Samsung Heavy Industries shipyard. All ten vessels are now in commercial operation. Another order to equip ten more LNG-powered containerships was announced on September 2024.
The Recycool™ skid is placed next to the high-pressure pump skid, allowing seamless integration.
In this first application, GTT combined the Precooler with the high-pressure vaporiser into a single heat exchanger, further enhancing system compactness.
Recycool™ performance, validated during gas trials and commercial operations, demonstrates stable operation enabling the crew optimal management of tank boil-off and pressure.With its proven performance and reliability, compact design, and optimised efficiency, Recycool™ stands out as a key solution for LNG-fuelled vessels powered by high-pressure two-stroke engines, contributing to both operational cost savings and emissions reduction.
