Echogen claims the use of its supercritical CO2 technology in combined cycle systems can boost gas turbine prime mover efficiency by over 50 percent. (Image courtesy of Echogen)
US-based Echogen Power Systems has partnered with GE to design a closed-loop exhaust energy recovery system that uses supercritical CO2 (sCO2) rather than steam as the working fluid.
The high density of CO2 in its supercritical state allows for small, lightweight turbomachinery, and Echogen claims the combined cycle system can boost gas turbine prime mover efficiency by over 50 percent at a 35 percent lower footprint and weight than equivalent steam systems.
Efficiency will be on par with that of low speed diesels, says Echogen’s Chief Technical Officer Dr. Tim Held, and the combined power density increases arrangement flexibility for enhanced cargo carrying capabilities.
The system can use waste heat from engines to produce electricity for onboard services or to increase shaft power. It can also be incorporated into an integrated power system to function as an additional generator, with no fuel consumption or emissions.
By moving away from steam systems, operation and maintenance requirements are greatly reduced, says Held. CO2 is nonflammable and thermally stable, and the exhaust energy recovery system eliminates the need for the water chemistry balancing required for steam systems.
The system is designed for fully automatic operation and works efficiently with exhaust gas supplied from either gas turbines or diesel engines, with exhaust temperatures ranging from 240°C to 600°C (464°F to 1,112°F).
Echogen has been developing the technology for over 10 years. In 2011, the company was awarded US Navy funding to demonstrate its ability to improve efficiency and reduce specific fuel consumption for marine gas turbine prime movers and power generation modules.
The primary commercial application for the system is LNG carriers, cruise ships and other large applications where the narrowing fuel price differential between marine gas oil and very low sulphur fuel oil makes the operation of gas turbine engines favourable, says Held. However, he notes that while the GE gas turbine engine LM2500+ with the sCO2 solution offers advantages for vessel types such as LNG carriers, the adoption of gas turbines in some other commercial marine applications is still in its early stages.
Given the major impact of the propulsion system on ship design, Held expects the primary applications will be newbuild rather than retrofit. However the power density of the gas turbine and the sCO2 system do present opportunities to install the equipment above deck, making retrofits potentially more viable.
The company has already designed, built and tested an sCO2 power system for combined cycle applications with a land-based 20-25MW gas turbine. The design modifications for marine applications have been studied, but the company is waiting for a launch customer to begin design work.