MAN B&W 4.05
Page 1 of 9
MAN Diesel
198 57 97-7.5
MAN B&W 98-60 engines
Waste Heat Recovery Systems (WHRS)
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 & Turbo terms
is designated as WHRS (Waste Heat Recovery
Systems).
WHRS 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
& Turbo 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. 3.5% of the main engine SMCR, when
the engine is running at SMCR. For the STG sys-
tem this value is between 5 and 7% depending
on the system installed. When combining the two
systems, a power output equivalent to 10% of the
main engine’s SMCR is possible, when the engine
is running at SMCR.
The WHRS output depends on the main engine
rating and whether service steam consumption
must be deducted or not.
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 WHRS. The main engine will then
be unloaded, or it will be possible to increase the
speed of the ship, without penalising the fuel bill.
Because the energy from WHRS is taken from the
exhaust gas of the main engine, this power pro-
duced 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 en-
gine and the system with power and steam turbine
can be calculated to be as low as 152 g/kWh (ref.
LCV 42,700 kJ/kg).
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