For both LNG-fuelled and electric propulsion concepts, advanced cooling is emerging as a key efficiency enabler
New research on glycol freezing has driven developments in heat exchanger technology that are set to benefit users of LNG fuel. A fluid mix including glycol is often used as a medium for heat exchange, but the extreme temperatures at which gas is liquefied means that freezing the mix is a risk. This presents problems for the use of plate-and-shell heat exchangers used in the vaporisation of gas fuel
“With more and more ships relying on LNG as fuel, we are seeing greater demand for durable vaporiser technology that can dependably resist freezing and the fatigue caused by pressure and temperature,” says Alfa Laval head of marine heat transfer equipment Jonny Hult. “A glycol mix with a freezing point of around -50°C has the potential to freeze when it meets plate surfaces as cold as -90°C. Solving this problem with a higher glycol flow is a very expensive solution for our customers.”
Alfa Laval has worked with SINTEF Energy Research to find the optimal correlation of flows on both sides and thereby avoid freezing of fluid on the hot side. This will allow shipbuilders use a more compact design with smaller pumps and pipes, reducing cost.
“To maintain performance and extend the operating life of drivetrain components, temperatures must be kept as low as possible”
Mr Hult explains that the company’s own plate heat exchanger, DuroShell, has been optimised for use as a vaporiser in fuel gas applications. The stainless-steel design allows it to handle LNG entry temperatures that approach -170°C or lower. A patented ‘cutwing’ plate pattern also helps, providing high turbulence that improves heat transfer efficiency and reduces the risk of freezing and fouling.
There is also scope for rethinking the role of heat exchangers in cooling for hybrid or electric propulsion. Whilst electrical propulsion systems are generally more tolerant of temperature increases compared to traditional combustion engines, there is an important area where higher temperatures can have a detrimental effect on the performance of the components within a drivetrain. Tests undertaken by manufacturers show that in order to maintain performance and extend the operating life of drivetrain components, temperatures must be kept as low as possible.
According to Bowman marketing manager Phil Allman, a universal principle of electronics says that a 10⁰C lower temperature will double the life expectancy of electrical components and that is certainly true for electric marine propulsion systems. Whilst component lifecycle is significant, there is another issue to consider, he says: “In many electric propulsion systems, sophisticated sensor-based controls are used to monitor the health and performance. If the water temperature within the cooling circuit rises beyond specified levels, this will be identified by the sensors and power to the drive train reduced to protect the system components.”
For users, the implications of this switch into ‘limp mode’ could range from simply frustrating, to the downright dangerous, depending on water and weather conditions. Mr Allman explains how one company is overcoming this problem by designing the cooling circuit of its 100 kW drivetrain to operate at a maximum temperature of 60⁰C. This is based on a maximum cooling water intake temperature of 35⁰C and using Bowman heat exchangers to ensure consistent and accurate cooling of all components.
The rapid development and growth of higher powered (60 kW plus) drivetrains has also created a need for more efficient component cooling circuits to manage the heat loads generated, something which companies are now designing into their ranges. Primary cooling requirements for these systems include the battery pack and on-board charger (where fitted), AC-DC converter, DC-DC converter, plus the electric drive motor itself.