Man B&W S80ME-C7, Project Manual

Page: 87 / 353

MAN B&W 4.05
Page of 8
MAN Diesel
MAN B&W S80MC6, S80MC-C7/8, S80ME-C7/8/9,
K80MC-C6, K80ME-C6/9
198 57 99-0.2
Due to the increasing fuel prices seen from 2004
and onwards many shipowners have shown inter-
est in efficiency improvements of the power sys-
tems on board their ships. A modern two-stroke
diesel engine has one of the highest thermal effi-
ciencies of today’s power systems, but even this
high efficiency can be improved by combining the
diesel engine with other power systems.
One of the possibilities for improving the efficien-
cy is to install one or more systems utilising some
of the energy in the exhaust gas after the two-
stroke engine, which in MAN Diesel terms is des-
ignated as WHR (Waste Heat Recovery Systems).
WHR can be divided into different types of sub-
systems, depending on how the system utilises
the exhaust gas energy. Choosing the right sys-
tem for a specific project depends on the electric-
ity demand on board the ship and the acceptable
first cost for the complete installation. MAN Diesel
uses the following designations for the current
systems on the market:
• PTG (Power Turbine Generator):
An exhaust gas driven turbine connected to a
generator via a gearbox.
• STG (Steam Turbine Generator):
A steam driven turbine connected to a generator
via a gearbox. The steam is produced in a large
exhaust gas driven boiler installed on the main
engine exhaust gas piping system.
• Combined Turbines:
A combination of the two first systems. The ar-
rangement is often that the power turbine is
connected to the steam turbine via a gearbox
and the steam turbine is further connected to a
large generator, which absorbs the power from
both turbines.
The PTG system will produce power equivalent to
approx. 4% of the main engine SMCR, when the
engine is running at SMCR. For the STG system
this value is between 5 and 7% depending on
the system installed. When combining the two
systems, a power output equivalent to 0% of the
main engine’s SMCR is possible, when the engine
is running at SMCR.
As the electrical power produced by the system
needs to be used on board the ship, specifying
the correct size system for a specific project must
be considered carefully. In cases where the elec-
trical power consumption on board the ship is
low, a smaller system than possible for the engine
type may be considered. Another possibility is to
install a shaft generator/motor to absorb excess
power produced by the WHR. The main engine
will then be unloaded, or it will be possible to in-
crease the speed of the ship, without penalising
the fuelbill.
Because the energy from WHR is taken from
the exhaust gas of the main engine, this power
produced can be considered as ”free”. In reality,
the main engine SFOC will increase slightly, but
the gain in electricity production on board the
ship will far surpass this increase in SFOC. As an
example, the SFOC of the combined output of
both the engine and the system with power and
steam turbine can be calculated to be as low as
55 g/kWh (ref. LCV 42,700 kJ/kg).
Waste Heat Recovery Systems (WHR)