When it came time for one large fishing trawler to replace its on-site refrigerating plant, there were two solutions based on natural refrigerants: an ammonia system or a CO₂ system. Global Seas opted for a CO₂ system from Seattle-based Highland Refrigeration for its 148’ long Northern Defender. 

CO₂ had been seldom used on ships, but Steve Schwitters of Highland Refrigeration thinks that might change. “CO₂ systems have one great advantage. The plant is very compact and can easily be made to fit during vessel conversions,” Schwitters said.

The compact dimensions also contribute to its cost-effectiveness. More capacity can be installed within the same dimensions, and the small disadvantage in efficiency a CO₂ system has compared to ammonia can easily be compensated for by size. On the Northern Defender, for example, there was only space to install a refrigerating capacity of 500 kW. The new CO₂ recirculated seawater (RSW) chiller generator fit perfectly, which is largely due to the compact compressors.

In addition, three semi-hermetic GEA Bock compressors operate in the refrigerating plant on the Northern Defender. They are installed in a row, and each compressor is equipped with a frequency converter for speed control. 

The Northern Defender normally operates in the Bering Sea off Alaska and returns to port with up to 340 tons of pollock. For the fish to stay fresh during the voyage of up to several days, it is chilled with sea water at 0 to -1°C.