IntroductIon
Traditionally, the choice between diesel-fueled and gaseous-fueled generators has
been relatively straightforward. Power density as well as capital cost advantages in
large-kilowatt applications typically favored diesel for standby power of 150 kilowatts
of electricity (kWE) or more—large commercial and industrial standby applications.
However, technological innovations are making gaseous-fueled generators both more
powerful and more cost effective. Additionally, issues of fuel storage and reliability in
diesel-fueled generators are becoming a bigger challenge. Finally, as more and more
companies seek to reduce their carbon footprints, they are more open to options that
are more environmentally friendly.
As a result, standby generator system designers, electrical contractors and electrical
engineers have significantly more gaseous-fueled choices than they had before.
GASEouS FuEL
GEnSEt
SoLutIonS
BEcoMInG
MorE
AFFordABLE,
GrEEnEr
EM
Er
G
In
G
t
r
En
d
S
white paper
GASEouS VS. dIESEL FuEL GEnSEtS
E
M
E
r
G
E
n
c
Y
P
o
W
E
r
2
dIESEL VS. GASEouS-FuELEd GEnSEtS:
A trAdItIonAL PErSPEctIVE
Diesel and gaseous-fueled generators each offer advantages
to consider when designing a standby power solution. The
most noticeable advantage of a gaseous-fueled generator is the
extended run time provided by a continuous supply of natural gas.
The natural gas infrastructure has shown itself to be extremely
reliable in situations that cause power outages; through four
Florida hurricanes in 2004 and the Northeast grid failure of 2003,
the natural gas supply was unaffected.
By comparison, diesel-fueled generators provide access to
backup power in remote areas that do not have a gaseous-fuel
infrastructure. When applied to standby power applications of 150
kWE or more, a diesel-fueled generator delivers a lower capital cost
per kilowatt of electricity than a gaseous-fueled generator. Attempts
to lessen this disparity, such as converting industrial diesel engines
to gaseous fuel, only add engineering costs to the project. As a
result, diesel-fueled generators have a capital cost advantage over
their spark-ignited counterparts in larger standby applications,
making them the traditional market norm.
Additionally, a significant part of the market—especially those
segments with mission-critical applications like hospitals and
911 call centers—uses diesel-fueled generators because of code
requirements for on-site fuel. While gaseous-fueled generators
using on-site LP fuel can often meet such code requirements,
as can systems designed to run in a dual-fuel configuration with
natural gas as a primary fuel and LP as the secondary fuel, the
capital cost advantage of diesel-fueled generators typically makes
them the preferred solution when on-site fuel is a must.
Both diesel and gaseous-fueled units share applications below 150
kWE. Gaseous-fueled generators are often chosen for residential
and small commercial standby applications. The automotive-style
engines used in these units are readily available in high volumes,
making them extremely cost effective. The ready supply of LP and
natural gas in these applications also makes them ideal.
dIESEL chALLEnGES
In spite of their widespread use for standby power in large-kilowatt
applications, diesel-fueled generators can have some significant
drawbacks that are often overlooked. Fuel storage and reliability
considerations are foremost, as are environmental issues.
Fuel Storage
There is no question that the ability to store diesel fuel on site fills a
critical need for backup power in remote areas without a gaseous-
fuel infrastructure. However, because it is typically stored for long
periods of time, contamination and breakdown are real concerns.
According to Exxon Mobil, diesel fuel can be stored for up to one
year without a reduction in quality if it is kept clean, cool, and dry.
Longer storage periods require periodic filtrations and the addition
of fuel stabilizers and biocides1. In the case of a diesel generator
set, however, it could easily take a diesel-fueled generator with a
tank sized for 72 hours of full-load operation about 20 years to
turn a single tank of fuel2. Without proper maintenance, the fuel will
become contaminated with water and biomass.
Water enters the tank as humidity through the normal vent and
condenses. Moisture binders in the fuel capture and contain the
moisture, but as these binders become overloaded, water will drop
to the bottom of the tank and begin accumulating. If pulled into
the engine, it could result in loss of power, loss of lubrication, and
corrosion. Water also creates an environment that will support
biomass at the water/fuel interface. When these microbes are
pulled into the engine, they clog the fuel filter, resulting in power
loss and shutdown. To minimize these effects, fuel tanks require
a well-defined low point where the water can collect, and monthly
maintenance to drain the water. Periodic fuel polishing (filtering
and water removal) may be required, as well.
In addition to contamination, fuel breakdown seems to be more
prevalent with today’s low sulfur fuels. The additional refining
processes necessary to remove the sulfur may also be removing
some of the fuel’s stability elements. As diesel fuel gets older, a
fine sediment and gum forms in it, brought about by the reaction of
diesel components with oxygen from the air. Additives are helpful in
treating common fuel breakdown issues when integrated into a fuel
filtering preventive maintenance program. However, at some point
the fuel may simply need to be replaced.
diesel-fueled generators have
a capital cost advantage over
their spark-ignited counterparts
in larger standby applications, making
them the traditional market norm.
1 “Diesel FAQ,” last modified July, 2010, accessed September 8, 2010,
http://www.exxon.com/USA-English/GFM/fuels_quality_diesel_faq.aspx
2 Assuming a 60% typical load level, weekly no-load exercising, and average power
outages of only four hours per year.
GASEouS VS. dIESEL FuEL GEnSEtS
E
M
E
r
G
E
n
c
Y
P
o
W
E
r
3
By contrast, natural gas is continuously supplied via the local
municipal infrastructure, so storage is not an issue. As for LP, it
can be stored on site for several years—more affordably and more
safely than diesel fuel—providing an additional redundant backup
to the already stable gaseous-fueled pipeline infrastructure.
reliability of Fuel delivery
According to the Edison Electric Institute, severe weather events
account for 62% of unexpected power outages in the United States.
These events can close roads and cripple municipal infrastructures,
making it difficult or impossible to refuel the diesel generators
used in so many standby applications. Designers who size fuel
tanks more modestly to address the aforementioned issues of
fuel contamination or breakdown run the risk of implementing a
solution that will run out of fuel in an emergency.
Environmental concerns
More and more companies are considering how their overall
environmental footprint affects the world. This trend holds true in
virtually all areas of business, and automatic standby generators
are no exception. In this regard, diesel-fueled generators face
significant challenges. Not only do diesel engines emit more nitrogen
oxides and particulate matter than comparable spark-ignited units,
but diesel engines are also being ever more scrutinized to minimize
their environmental impact.
For example, diesel engines have been subject to intense emission
level regulations, and have seen aggressive Environmental
Protection Agency (EPA) tier changes. This additional oversight
has increased the total cost of both diesel engines and fuel. Future
governmental cap and trade regulations for emissions trading may
cause diesel engines to be taxed at a higher rate due to higher CO2
emissions.
Fuel containment and the environmental concerns surrounding
large quantities of fuel stored on site are considerable issues, as
well. Because quantities of fuel are typically kept in a main storage
tank and then transferred to a smaller day tank at the generator for
usage, fail-safe controls must be designed into the system to avoid
spillage. Additionally, no single point of failure in the fuel delivery
system should result in fuel spillage. Many localities have their own
code requirements covering fuel containment and delivery—some
of which include concrete-walled secondary containment, double-
walled piping, fire-rated tanks, special fill and spill requirements,
special permitting, etc. In fact, permitting costs for diesel storage
also continue to rise as local governments attempt to control the
environmental impact of accidental spills, odors and other factors.
Thus, while diesel-fueled generators above 150 kWE do offer
significant capital cost advantages when compared to similar
gaseous-fueled units, managing the fuel storage, reliability, and
environmental concerns tends to flatten this cost differential.
tEchnoLoGY ShIFtS In GASEouS-
FuELEd GEnErAtorS
Thanks to advances in technology, gaseous-fueled generators are
growing in popularity for larger applications. Key advancements
include the optimization of engine speed (RPM), integrated
approaches to generator paralleling, and bi-fuel (combined diesel
and gaseous-fuel) operation. These technologies are reducing the
historical cost advantage of diesel-fueled generators.
optimizing Engine rPM
The AC frequency of the generator electrical output is a function of
engine speed and alternator design. To achieve 60 Hz, the alternator
rotor must revolve at a specific speed for a given alternator pole
configuration. Fifty years ago, most generator engines operated at
speeds below 900 RPM. Within the last 30 years, however, engine
outputs have increased. As such, the diesel standby generator
market has moved from 1200 to 1800 RPM.
This trend has affected gaseous-fueled generators in applications
up to 150 kWE, as well. Historically operating at 1800 RPM,
current technology is optimizing these automotive-style engines
for operation at 2300, 3000, and 3600 RPM. Some manufacturers
utilize a simple gear reduction device between the engine and a
four-pole alternator to achieve the optimal amount of mechanical
power at a 60 Hz electrical output.
Increasing the operating speed of spark-ignited automotive-style
engines offers many advantages, including improved transient
performance, less stress on engine bearings, and increased power
densities. The graph below illustrates the transient performance
increase associated with optimizing the RPM of these engines.
70
60
50
40
30
0 2 4 6 8 10
Hz
Seconds
6.8L Nat Gas (optimized 3000 rpm)
8.1L Nat Gas (Traditional 1800 rpm)
Typical Diesel
125 kW Block Load, Transient Performance Data
GASEouS VS. dIESEL FuEL GEnSEtS
E
M
E
r
G
E
n
c
Y
P
o
W
E
r
4
Most importantly, though, this trend means more powerful engines
and reduced capital costs.
Integrated Generator Paralleling
Using these speed-optimized, spark-ignited engines as building
blocks, manufacturers are connecting smaller gaseous-fueled
generators together and combining their output in an integrated
approach to generator paralleling. In this way, they are able to provide
a cost-effective alternative to single, large diesel-fueled units.
Parallel power solutions have always offered significant advantages,
including application flexibility, scalability, and redundancy.
However, the implementation of such solutions had been limited
to mission-critical applications and large kilowatt projects largely
because of panel board constraints including cost, space, and
issues of single source responsibility. Traditional parallel power
solutions were also extremely complex. Each generator in the
system normally required four to six analog and digital micro-
controllers from various manufacturers, all hardwired together.
A typical two-generator system would have between nine and
fourteen controllers (including the master control section) to
manage the speed governor, load-share controller, synchronizer,
voltage regulator, generator controller, and protective relay.
Today, manufacturers have mitigated cost and complexity by
using one digital controller per generator to control all functions,
significantly enhancing system performance and reliability. Some
have also integrated the paralleling switch into the generator
connection box, eliminating the cost and space requirements
of external panel boards. Today, three 300 kWE gaseous-fueled
generators operating in parallel could replace a single large 1000
kWE diesel-fueled generator at a more feasible cost—but with the
added advantage of built-in redundancy. With a single 1000 kWE
unit, generator failure means the facility will be without backup
power. In a parallel solution, however, if one of the 300 kWE units
doesn’t run, the most critical loads will be distributed among the
remaining two generators.
Bi-fuel Generators
Bi-fuel generators combine the power density and capital cost
benefits of diesel engines with the extended run time of natural
gas. Using mass-produced diesel engines as prime movers, bi-
fuel generators start up on diesel fuel in a normal manner. As
loads are added, natural gas is introduced to the combustion air
while diesel fuel is reduced. Under typical load conditions, bi-fuel
generators will operate on a ratio of 25% diesel and 75% natural
gas, with no reduction in power. An example fuel mix for a 600 kWE
generator is illustrated below.
Only slightly more expensive than diesel-only designs, bi-fuel
generators offer several important advantages. For one, the lower
capital cost of a compression-ignited engine is retained while
capitalizing on many of the advantages of gaseous fuel, such as an
improved emissions profile.
Run times are significantly extended, as well, due to the reduced
consumption of diesel fuel. This can be very important, since
refueling may be difficult during emergencies that cripple municipal
infrastructure. It can also allow for smaller diesel tanks, because
natural gas is the predominant fuel. With smaller fuel tanks, the
risk of fuel contamination and the cost of fuel maintenance is
significantly reduced.
Finally, fuel redundancy is built into the system. If the natural gas
supply is interrupted for any reason, or if there is a fault in the
bi-fuel delivery system, the controls automatically revert to 100%
diesel without interruption.
Bi-Fuel % Diesel & Natural Gas
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 100 200 300 400 500 600
kW
Pe
rc
en
t F
ue
l Diesel
Natural Gas
GASEouS VS. dIESEL FuEL GEnSEtS
E
M
E
r
G
E
n
c
Y
P
o
W
E
r
5
EFFIcIEncY And co2 EMISSIonS
To evaluate the bigger environmental picture, it is important to compare the source-to-site cost of the fuel used in diesel and gaseous-fueled
generators. This is the total cost associated with using the fuel. It includes such items as extraction and transportation costs, the current
market price of the fuel, and the cost per kilowatt hour. Consideration of total CO2 emissions is critical, as well, in determining environmental
impact. It is possible to quantify all of these items and bring them into the cost-benefit analysis regarding generator fuel choice.
The following chart shows efficiency and CO2 comparisons of gaseous-fueled and diesel generating sets.
Combining fuel cost with environmental impact provides companies with a broader view to the true bottom line, and overall environmental
impact, of their generator choice. For example, while a 100 kWE diesel engine burns fuel 23% more efficiently than a similar-sized
gaseous-fueled engine (column J), its fuel cost per kilowatt hour is almost double (column I) and its total CO2 emissions are more than 16% greater
(column O). Additionally, gaseous-fueled generators have historically cost less per installed kilowatt than their diesel counterparts in the smaller sizes
(column K).
Given the new technologies associated with gaseous-fueled generators, and their advantages in terms of fuel reliability and cost, customers
might be more willing to consider a gaseous-fueled solution where historically they would have purchased a diesel-fueled unit.
A B C D E F G H I J K L M N O
Unit
kWE
Fuel
Type
Engine
Volume
(liters)
Engine
RPM
Power
(kWE)
Nat
Gas
Fuel
Flow
(ft3/hr)
Diesel
Fuel
Flow
(gal/hr)
Cost/Hr1 Cost/kW-hr1 Btu/kWE Capital
Cost/kWE
CO2
Emissions
From Engine
Exhaust Only
(lbs/hr)
Energy
use
(mBtu/hr)
Total CO2
Emissions
from
Engine and
Processing
(lb/hr)2
% CO2
Increase
vs
Natural
Gas
100 Gas 6.8 2300 100 1380 — $13.11 $0.131 13,800 $205 168.4 1.380 328 —
100 Diesel 6.7 1800 100 — 7.9 $20.15 $0.201 10,490 $220 175.4 1.116 383 16.7%
200 Gas 13.3 1800 200 2750 — $26.13 $0.131 13,750 $270 335.5 2.750 654 —
200 Diesel 8.7 1800 200 — 15.5 $39.53 $0.198 10,291 $180 344.1 2.189 751 14.9%
1 Based upon fuel usage and overall current price of fuel, 100% rated load. Natural gas: $0.95/therm. Diesel: $2.55/gal.
2 GREET Transportation Fuel Cycle Analysis Model, developed by Argonne National Laboratory, September 5, 2008.
diesel vs. Gaseous-fueled Generators: total cost and total co2 Emissions
SuMMArY
Gaseous-fueled generators are becoming more attractive for
standby applications above 150 kWE. The capital cost and
thermal efficiency advantages of diesel over gaseous fuels have
been moderated by the maintenance and reliability challenges of
storing diesel fuel over long periods of time. Additionally, a number
of recent technological innovations in implementing high-kWE
gaseous-fueled solutions are allowing such generators to compete
with their diesel-fueled counterparts on cost and power.
Additionally, the fact that spark-ignited engines offer improved
emission profiles over compression-ignited engines—and that
gaseous fuels do not pose the kinds of environmental risks inherent
in diesel fuel storage—is making gaseous-fueled generators more
attractive to the ever-growing number of companies looking to
reduce their overall carbon footprint.
Diesel-fueled generators will most assuredly continue to serve
the standby power market; no single solution is ideal for every
application. Gaseous-fueled generators, however, are becoming
more cost-effective and environmentally-friendly options for
applications above 150 kWE.
GASEouS VS. dIESEL FuEL GEnSEtS
E
M
E
r
G
E
n
c
Y
P
o
W
E
r
6
GEnErAc InduStrIAL PoWEr: dIESEL And GASEouS-FuELEd SoLutIonS
Generac Industrial Power offers diesel and gaseous-fueled generators for every standby power solution.
• Diesel up to 600 kWE (1 MWE with Gemini®)
• Gaseous up to 300 kWE
• Bi-Fuel™ 600 kWE
• Parallel power solutions up to 9000 kWE using Generac’s
Modular Power Systems (MPS)
Modular Power Systems (MPS)
Generac’s Modular Power System (MPS) is a combination of the industry’s most reliable generators and state-of-
the-art integrated paralleling technology. Powered by either diesel or gaseous fuel, MPS is appropriate for numerous
types of businesses, including hospitals, airports, office buildings, manufacturing plants, data centers, and retail
superstores. Not only does MPS boast a reliability rate of up to 99.9999%, it is more cost effective and flexible than
single generator sets with the same load capacity, making expensive stand-alone switchgear obsolete.
Gemini®
Generac’s Gemini Twin Pack houses two generators within a single enclosure, providing the same amount of power in
a footprint that is 20% smaller than many single engine units. Gemini provides built-in redundancy for superior system
reliability and scalability along with load shedding capabilities. Parallel up to seven diesel or gaseous-fueled Gemini
systems without additional switchgear.
Bi-Fuel™
Perhaps the solution that most effectively combines the power of diesel with the environmental friendliness of
natural gas, Generac’s Bi-Fuel system generators start on diesel fuel and add natural gas as load is applied. During an
outage, each Bi-Fuel unit can optimize at a fuel mixture of 75% natural gas and 25% diesel. If the natural gas supply is
interrupted, the generator automatically switches to 100% diesel without any power drop during the transition. At varying
loads, the advanced fuel system maximizes the use of natural gas while closely monitoring the system for safe operation.
For more information about the information contained in this white paper, or to learn more about the full line of Generac Industrial Power
generators, contact Generac at 1-888-GENERAC (1-888-436-3722) or visit us at www.generac.com to find your local dealer.
GASEouS VS. dIESEL FuEL GEnSEtS
E
M
E
r
G
E
n
c
Y
P
o
W
E
r
7
notES
GASEouS VS. dIESEL FuEL GEnSEtS
E
M
E
r
G
E
n
c
Y
P
o
W
E
r
8
Bulletin 0187520SBY Printed in USA 11.10
©2010 Generac Power Systems, Inc. All rights reserved.
Generac Power Systems, Inc.
S45 W29290 Hwy. 59
Waukesha, WI 53189
1-888-GENERAC (1-888-436-3722)
gene rac .com