However, such plants are only economical
for significant power outputs, typically
several MW, with high capital costs, fixed
installations and significant project duration.
Turbo-compounding is suitable for much
smaller engines, as demonstrated by its use
in Formula 1. Within power generation, the
focus is on gensets in the 150kW-2.5MW
range, where it is possible to improve
the efficiency of diesel and gas-powered
piston engines by 4-7%. “For modern
turbocharged engines, we can achieve a 4%
improvement in fuel efficiency. For older
engines, or low-energy gas applications,
we can achieve a 7% increase in efficiency.
It primarily depends on how much energy
there is in the exhaust, and the exhaust
temperature,” says Keith Douglas, head of
performance at Bowman Power.
Although the backpressure caused
by turbo-compounding reduces
engine efficiency, the turbine recovers
approximately 2.5 times more energy than
is lost, leading to a net efficiency gain. As
an example, consider a 1MWe engine with
a baseline of 38% electrical efficiency.
Applying the ETC system while keeping
the fuel flow constant would result in an
approximate loss in engine load due to
pumping losses of 40kWe, giving 36.5%
electrical efficiency for the engine. But
the ETC system would then generate
approximately 100kWe, leading to a net gain
of 60kWe and 40.3% electrical efficiency in
total.
THE COMPETITION
The main competing technology is Organic
Rankine Cycle (ORC), which are also able to
generate useful work from exhaust gas and
may be fitted to gensets. However, ORC is
significantly more complex and expensive,
while the organic fluids used may also be
highly damaging if they are released to the
atmosphere, such as CFC’s and HCFC’s.
ETC gains efficiency from a multiple-stage
expansion, with a system that is said to be
much simpler and easier to install.
Bowman Power’s ETC systems have
been fitted to gensets produced by many
major OEMs. “ETC is very much a proven
OIL & GAS – TURBO-COMPOUNDING
“Electric turbo-compounding has
been used for 22 million operating hours,”
Mike Essex, head of marketing, Bowman Power
technology, with 22 million operating hours
across 800 systems,” explains Mike Essex,
head of marketing at Bowman Power. “Our
third generation of ETC (2017) reduced
costs by 50% and is now being used for
landfill gas, wastewater treatment, the
rental market and others.”
For typical genset installations, the
addition of an ETC system does not add any
significant cost in terms of unit cost per kW.
The increase in power offsets the additional
cost, resulting in a more economical system
with payback in as little as 12 months.
“We are always careful to ensure that by
increasing power output, we don’t overload
the genset,” adds Douglas. “Older engines
can achieve efficiency and power gain, while
newer engines are operated at the same
power with only an efficiency gain. This is
done to ensure firing pressures are kept
within the engine’s limits. Our OEM engine
data allows us to increase power where
possible. The only significant maintenance
is a bearing change every 30,000 hours,
aligned with host engine service.”
REDUCING EMISSIONS
Improved fuel efficiency also results in
lower CO2 emissions – important to climate
change.
When it comes to local air quality, other
emissions are more significant, such as
particulate matter, oxides of nitrogen,
sulphur oxides, carbon monoxide and
unburned hydrocarbon (UHC). These
emissions can have serious public health
implications and, in the case of UHC, may
also act as greenhouse gases. Bowman
recently demonstrated a 32% reduction
in UHC emissions for a gas-powered
generator – achieved due to the increased
backpressure on cylinders, which reduces
fuel short-circuiting (methane slip).
Although other emissions are typically the
same per unit of fuel as without ETC, they
are lower relative to the energy produced.
Electric turbo-compounding enables an
improvement in engine efficiency with only
a modest cost/complexity increase. As with
turbo-charging, it looks set to become a
standard feature of modern engines.
Key components of turbo-compounding in Nomad aero engine
Contra rotating
propellers
Nomad 1
12 cylinders
(the half is opposed)
Extra combustion
chamber
Main turbine
Exhaust jet
Air
intake Axial
compressor
Air
intake
Single propeller Axial compressor
Hydraulic clutch
Turbine
Exhaust jet
Valve
Centrifugal Extra turbine
compressor
Intercooler
Wikimedia Commons/Tataroko
Nomad 2
May 2019 www.operationsengineer.org.uk 29
/www.operationsengineer.org.uk