POWER October 2015 - 51

DISTRIBUTED GENERATION
Diesel Gensets Aim at the Future
Though newer distributed generation options like microturbines, solar, and
batteries have grabbed the headlines, diesel remains the go-to option because
of its established technology and reliability. But growing competition
and new regulations are threatening its traditional role. Here's what
diesel is doing to stay relevant.
Thomas W. Overton, JD
I
f you pay attention to nothing but the
headlines in the energy media (including
POWER), you can be forgiven for having
mostly forgotten about diesel. Staid and reliable,
diesel generation isn't terribly exciting-it's
just there when you need it.
Exact figures for how much diesel generation
is in service worldwide are difficult
to come by, in part because official statistics
from the U.S. Energy Information Administration
and the International Energy Agency
tend to lump diesel together with fuel oil, but
there's no question there's a lot of it. Diesel is
used extensively for backup generation in developed
countries and for primary generation
in developing countries where the national
grid may be unreliable or simply nonexistent,
as well as for island grids, where large power
plants are not cost-effective.
Diesel's advantages are in its simplicity,
reliability, fast response, and low cost. Compared
to other options such as gas engines,
microturbines, and renewable sources such as
wind and solar, diesel generators are typically
the least expensive in terms of capital outlay.
Diesel fuel also has substantially higher energy
density than natural gas and other options,
which can simplify fuel supply logistics.
With simple maintenance requirements and a
well-understood, 100-year-old technology, a
diesel genset doesn't need much attention to
provide many years of reliable service-an
ideal quality for remote areas.
But diesel comes with some drawbacks.
Diesel fuel can be expensive, especially
when it must be imported long distances,
such as to a remote island. Much of the impetus
toward renewable generation on islands
such as Hawaii and Puerto Rico, and other
areas such as the Caribbean and South Pacific,
has come from high electricity costs
driven by diesel generation using imported
fuel. Hawaii in particular has made the decision
to abandon diesel-and all other fossil
fuels-by 2045.
Diesel plants built without consideration
of this challenge can quickly turn into white
elephants, as was the case with the $335 million
Tarakhil power plant built by the U.S.
October 2015 | POWER
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51
Agency for International Development outside
Kabul, Afghanistan. Because importing
diesel into the country is so expensive
and dangerous, the plant has sat mostly idle,
managing around a 2% capacity factor since
it was completed in 2010, according to a government
report released in August.
Another concern that has grown in recent
years is emissions. Compared to gas engines
and microturbines, let alone renewables, diesel
engines have higher levels of particulate,
NOx
, and SOx
emissions. For most of its existence,
diesel flew under the Environmental
Protection Agency's (EPA's) radar, but that
came to an end in 2006 with the first national
emission regulations under the New Source
Performance Standards, which generally
required at least a 90% reduction in particulates
and NOx
for new engines. Those standards
have since been tightened even further,
and new regulations have also been issued for
existing engines.
Efficiency and Environmental
Gains
Staying competitive has meant a host of changes
for diesel generation over the past few years
to improve efficiency and reduce emissions.
Efficiency in a diesel engine is most directly
tied to combustion rate, the degree to
which the fuel is completely burned during
ignition. This is typically a function of how
finely and evenly dispersed the fuel is during
injection into the combustion chamber.
Turbocharging, which forces excess air into
the chamber, also improves combustion rate,
which is why two-stage turbocharging (with
intercooling between the stages) is now common
for diesel gensets.
Modern diesel engines use a method
known as high-pressure common-rail
(HPCR) fuel injection (Figure 1). This method
replaces traditional mechanical injection
with electronically controlled multiple highpressure
injections during each combustion
1. High pressure, high efficiency. Modern high-pressure common-rail diesel fuel systems,
such as the Cummins XPI (extreme pressure injection) system shown here, allow for
much higher fuel pressures and much more precise and flexible injection of fuel into the combustion
chamber. Courtesy: Cummins

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POWER October 2015

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