Combined-cycle gas turbines

Fuel costs represent a large proportion of the operating costs of a ship and this has led to major efforts to reduce fuel consumption

Combined-cycle gas turbines were tried in two Russian ships in the 1970s but not to my knowledge repeated (Letters, PE December).

Fuel costs represent a large proportion of the operating costs of a ship and this has led to major efforts to reduce fuel consumption. In addition to development of the efficiency of the main engines, the course followed has been to optimise heat recovery from the main engines but utilising it to generate some of the electrical power for the ship rather than linking with the main engines as an addition to propulsive power, although this procedure can be followed if the electrical load cannot absorb all the potential recovery power.

Current large marine diesels have a standalone efficiency of 50% and the overall ship's fuel consumption is reduced by two main methods:

• The main engine exhaust gases supply a recovery boiler which feeds a steam-turbine generator.
  
• The efficiency of turbochargers, such as those used on the main engine, has been so improved that the engine air requirements can be supplied by using only a proportion of the available exhaust gases. Surplus gas can then be used to supply a so-called power turbine to drive a generator.
  

Both types of recovery can be used together, with the two turbines arranged to drive a generator from opposite ends. If electrical demand is insufficient to absorb the power produced, clutches can be used to enable this surplus to supplement the main engine, in which case at least some operation would be in combined-cycle!

Modern ships, particularly large container vessels, utilise well over 60% of the heat in the fuel, whilst burning low-grade residual oil, and are a great example of efficiency improvements by continual development, giving lower emissions together with cost savings.

D G Nicholas, Rugby

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