When Fueling Your Back-up Power System, Do You Choose Diesel or Gas?

by Lithium Technologies on ‎03-16-2011 09:51 AM


Moderated by: Jennie Tylec (jen-erator)
Featuring: Nick Kelsch (KelscNM) & Chad Dozier (diesel_dude)

Are you planning a facility expansion or looking to replace your existing standby power system? Diesel fuel has been the fuel of choice for standby backup power systems, but is it the best choice?

Our diesel guy, Chad Dozier will tell you, "There are many advantages to diesel engines, when used in standby applications." For example:

  • Start time and load acceptance: Diesel is your best bet to comply with NFPA 110 standards for 10 second power restoration and 100% rated load acceptance in a single load step. This is a requirement for life safety systems in the U.S. Further, the diesel generator set provides better transient response than an equivalent gas generator set.
  • Local fuel supply: Legally required standby systems require the generator set to operate for extended periods of time without being re-fueled. And often times a facility owner/operator will require that their generator sets can run for extended periods without the need for an external fuel supply. Both scenarios drive the need for on-site fuel storage. With diesel engines, on-site fuel storage is easier to accomplish.
  • Installed cost: Diesel gensets typically cost 40%-50% less to procure and install than an equivalently sized industrial grade gaseous fuel genset, although commercial grade automotive derivate engines less than 200 kW can be on par with diesels on installed cost.
  • Power density: Diesel generator sets provide 20%-40% more power per square foot than an equivalent gas generator set, resulting in smaller engine rooms when using multiple generator sets.

However, Gas Man Nick Kelsch will remind you, "Gas generator sets should not be ruled out." Reasons include:

  • Fuel maintenance: Diesel fuel that is left unmaintained and in storage tanks for extended periods can foster bacterial and microorganism growth that will plug filters and cause fuel system failure. This necessitates installation of fuel polishing equipment or is resolved by using pipeline gaseous fuel.
  • Run time in a natural disaster: During a hurricane, natural gas continues to flow whereas diesel fuel transportation trucks can not always reach hospitals, pumping stations, and hotels. Major storm events are much less of a threat to a natural gas pipeline than they are to road transit.
  • Environmental benefits: Lean burn gas gensets generally emit 90% less nitrous oxides than diesel as well as 60% less carbon monoxide (with oxidation catalyst). Visible soot emissions are nearly eliminated.
  • Non-emergency flexibility: Operating a genset during peak electrical demand in parallel with or isolated from a local electric utility is one way that genset owners can turn an emergency use asset into one that creates economic benefits. Given the strict use limitations related to diesel emission regulations today, gas gensets offer a good alternative. Further, as natural gas prices have declined dramatically in the US in recent years, gas gensets are more economical to own and operate at 500+ hours per year.

What are your thoughts on this?

  • Can gas generator sets be a viable option for standby installations?
  • Do you have experience with standby gas gensets applications?
  • How does it compare to a diesel in your experience?
  • What are some other considerations that should be factored into the decision between gas and diesel generator sets?
Jump into the debate with your thoughts below.

by Super Contributor
on ‎03-16-2011 12:57 PM

This is actually a pretty old debate for me, since I've been doing this for over 30 years and not just with CAT but with a number of other manufacturers units as well in a differing number of applications.  So the initial answer is, "it depends".


First, how critical is you requirement for backup power?  If you have a absolutely must get up and run as fast as possible in nearly every possible scenario, then in my opinion a diesel powered unit is still the best choice.  So why?  First off a spark ignited engine just doesn't have the same durability and tolerance to abuse that a compression ignition engine does.  The years of on the ground, hands on experience showed me that SI engines had more issues with fuel and ignition systems, failures to start, rough running and misfire, and inability to carry the connected load by a large factor over CI engines.  While owner maintenance plays a large part in this, in dealing with engines under service agreements, the service experience showed that the SI engines had more problems to deal with.  Engines without service agreements showed an even larger number of issues between SI and CI engines.


Where do you live and what kind of problems are you likely to see?  Natural gas may be a more reliable fuel source if tornados or hurricanes are an issue.  But I live in an area where our most likely natural threat is an earthquake.  After the Northridge quake we were able to load up 55 gallon drums of fuel and still get the most critical facilities up and going. While the storage and transport of NG is improving in the US, it's still a lot of effort to get NG fuel from a non-conventional source.  Propane may be another solution depending on where you are and what will likely happen in an emergency. And remember that in many emergency situations, while diesel fuel may become limited, aviation fuels like JP-4 and JP-5 can become available from emergency services and used in most CI engines if needed for a critical applications like a hospital or command center.


How well are you going to take care of it, lets face it, many standby systems today are installed based on cost, "how much cheaper can I make this, because it will hardly ever run anyway". In your initial evaluation of your need for a standby system, will you plan for maintenance, especially if you are in a strict emissions regulation environment.  One of the primary issues is how will you regularly test it?  If you have a CI engine you can get away with a much larger number of no load run hours.  You may get wet stacking and slobber, but the engines ability to start and run will not be affected for a fairly large number of hours on average. An SI engine will not tolerate nearly as well no load run conditions, and after a couple of fouled spark plugs, if it does start it likely won't perform as required. So back to the question, how are you going to test and maintain it?  Are you going to have loads you can apply in your facility that you can connect too regularly?  How about a dummy load, like a resistive load bank.  I had one customer who had a large electric water heater, a fairly simple manual transfer arrangement let him have a load and not waste the energy, can you do something like that in your facility?


Can you use your standby for demand response or seasonal peaking use?  If you have the ability to run your standby system in other than emergency conditions, then the choice for an SI engine based system may seem more attractive, however the primary questions still need to be asked.  How critical is my load?  What is my time allowable to up and on load?  What types of loads does my facility have with what kinds of transient response requirements?  If you can meet you primary goal of having a reliable backup power source, only then should you consider other uses for that system.  Decisions best made by engineers and plant operations people rather than accountants and MBA's.


Diesel fuel maintenance is a ever growing concern.  How much fuel should I have on hand and how to take care of it?  Many customers struggle with this, fuel testing and polishing can provide some help, but other alternatives may be available, especially now since most diesel fuel, at leas in the US, is low sulfur.  Maybe go ahead and buy on road fuel for your generator, and roll into your delivery fleet or make a deal with you fuel supplier to sell it to a volume user and refill with fresh fuel on a regular basis.  While the initial cost may go up, the cost of cleaning a dirty tank, or worse, having a fuel related failure during and outage can be significantly worse.  Try coming up with solutions outside the "normal" thinking for standby systems.  Starting batteries (and MV switchgear batteries) can have the same problems, most users let them run to failure.  How about rolling them into your fleet or make a deal with a large fleet or user to take your gently used batteries, and replace your batteries more often, before they fail?  I have a number of customers doing this now, and there are many other of these types of things that can be done to help improve system reliability and manage the cost of maintaining these systems.


In general I'm opposed to using SI engines in critical applications, way too many times standing in front of an end user, emergency response person and governmental agency person explaining why the unit didn't operate as expected.  Not that it hasn't happened with CI engine based units as well, but for the number of SI vs. CI engines in the field, the amount of problems with SI engines makes them a poor choice in my experience.  And it's not because I'm only a "diesel guy" I have the same number of years with SI engines in prime and cogeneration service, so my opinions are based on a pretty balanced set of experiences.


My two cents worth, Mike L.

by Visitor nirokaran
on ‎03-16-2011 08:56 PM

very good

i like

by New member littlerocker
on ‎03-22-2011 09:30 AM

Great information Mike L.Thanks for taking the time and sharing your experience with everyone.


We spec many generators in the 100 kW - 2000 kW range and 95% are diesel. From a pure cost standpoint we still see diesel as the clear choice for units 200 kW and up, and typically for smaller units too due to the costs of providing lp service when none exists. However, as the EPA and several states steadily restrict the allowable emissions from both moblie and fixed-location gensets this will likely decrease the cost advantage of diesel in relation to gaseous fuel units.


Also, we've seen instances where lp gensets were previously upgraded and the new units placed a larger fuel demand on the existing lp fuel supply, resulting in a situation where the new unit was starved for fuel under high loading. So a tip for anyone upgrading their natural gas genset to remember is always have the existing supply tested to be sure it is sized properly for the genset's full load demand.


Diesel gensets can fail as you said from wet-stacking from the cumulative effect of exercising with no load on the unit. This can be offset through establishing a proper exercise routine, typically one that includes a yearly test under 100% load using a load bank to blow out any crud buildup.


And last, no matter what the fuel choice end up being, it always pays off to have a monitoring unit installed on the genset that can remotely notify pertinent personnel of any system failure or impending failure that may prevent the unit from assuming load when called in an emergency situation.



by Regular Contributor
on ‎03-23-2011 03:44 PM

It sounds like you folks are focusing on the industrial customer and in that case imho diesel is the way to go for the reasons mentioned. ( I do have a LP burner at home though) However if you are paying attention the the Utility Industry you will note the trend for utility scale back up, standby, peak shaveing and now WIND FOLLOWING equipment is moveing away from single unit simple or combined cycle CT's to Spark fired NG. The problems with permitting a large oil fired plant are becomeing critical that along with fuel cost makes it cost prohibitive. While the problems with spark fire units still remain (ing. systems, etc) the industry is definetly moveing that way.       

by New member mporter
on ‎03-23-2011 05:42 PM

I would buy the diesel genset and Bi-fuel it with natural gas. That way you would have the best of both worlds.

Lower cost diesel package to install. With the option to run up to 70% on low cost natural gas. Check out Altronic.com for more on Bi-fuel systems. 

by New member jmsultan
on ‎03-23-2011 05:53 PM

Emergency and Legally Required cannot depend on gas unless it is stored on site. 

NEC  700.12(B) (3) and 701.11(B)(3).  The principle is the Owner must provide everything needed for the backup system to function.

On-site sotrage of large quantitieis of diesel fuel is a growing problem.


 (3) . . . Prime movers shall not be solely dependent on a public utility gas system for their fuel supply or on a municipal water supply for their cooling systems.

by Contributor Kengine7
on ‎03-23-2011 06:33 PM
Mikel, great job. The comment is loaded with useful information that does not just fall from the sky. I appreiciate the facts and figures. That quantitative comment, the number that carries the most weight in your analysis, 30 years, says it all. It seems appropriate in this particular era to listen-up techs, this BTO has been there, done that, and he has very obviously learned and earned a master technician/engineer designation by STICKING TO TASK. The comment by littlerocker about taking care to load test a gaseous unit rings true. Let me add that many NG suppliers are electric utilities. Do not expect that an ample supply of fuel is available and don't expect that the supply can be increased because the new b/u or s/b genset requires more fuel. Most gas lines are running higher pressure than design to accomodate growth. Add a 5,000,000 btu demand at the nursing facility at the end of beautiful, tree-lined, 1970's era Elm Street you may put 100 furnaces out of business during an outage.
by New member WallaceB
on ‎03-24-2011 05:36 AM

Could anybody share experiance of what % of rated power a gas genset can accept as a block load?  My area of interest is for 150-200kVA load.

by Contributor KelscNM
on ‎03-24-2011 07:47 AM

Hi WallaceB,


Gas genset transient response capability is less than that of a diesel's.  For example, if a typical 1000kW rated diesel genset is able to accept a 100% block load with a 35% instantaneous voltage dip and 10% instantaneous frequency dip and recover to nominal voltage and speed within 10 seconds, an equivalently sized gas genset may only be able to accept a 15%-20% block load without exceeding the same voltage and speed dip and take 15-20 seconds to recover to nominal speed and voltage.  The difference in block load capability is inherent to the design of a gaseous fuel system versus the diesel.


There are a large number of variables that can play into the gas gensets' transient response capability  Such variables include the type of fuel used, fuel pressure, fuel pressure stability, ambient conditions, cooling system sizing and aftercooler temperature, voltage regulator settings, speed governor settings, time between engine start and load accpetance (ie, operating jacket water temperature), and others.  Furter the type of loads being powered can also impact the how the gas genset will be sized.  Manufacturers like Caterpillar can make recommendations on the appropriate sizing of a genset to deliver the transient response that is required by a customer's specific set of loads and sequence of application. 

by Visitor stevenyoung
on ‎03-24-2011 11:52 AM

You are talking about Natural Gas verses Diesel? I've seen D399 and 3512 and now the 3600 series take shock and block loads equal to the diesel generators. Some of the factors that determin reaction and response are the choice of components, also the settings and applications of them. I'm sure youve seem generator sets that use a signal generator instead of a mag pickup to ensure the governors response time is as quickl as possible.


The settings and selection of a voltatge regulator is a major factor in voltage droop and recovery time. You can have a unit drooping and still produce the proper voltage an be within the proper hertz. I like the nateral gas units for there cleaner burning of fuel and the cost savings they have compared to diesel as it pertains to BTU per cubic meter (once the nateral gas unit gets hot).



by New member kellihej
on ‎03-24-2011 02:58 PM

I have been responsible for maintaining my firm's standby generators at for some 15 years and I my experience diesel is the superior choice for heavy industrial loads. In fact, in my tenure I have removed and replaced a few gas fired generators in the 100-400KW range with diesel engines.  Most of the time the units were replaced due to chronic failures.  The gas units just do not last as long against heavy loads when compared to their diesel counterparts.  When it comes to heavy industrial loads such as pumps, air compressors and refrigeration systems that are often block loaded you just can't beat the grunt and torque characteristics of the diesel engine. The gas engines have many advantages over diesel engines as described above when used in the right application.  My application is not one of them.


Like any job you must know when to use the right tool. This is no exception.



by Visitor JRoth
on ‎03-24-2011 11:14 PM

Besides all of the previous comments listed  by others , I want to add a summary of a  project I helped develop in New York City several years ago . The Diesel Generators needed major upgrade , so the idea was created to build a Multi- Genset plant  in individual soundproof, weatherproof enclosures on the roof of a major bank building . Financially, the Gas Gensets could be used for a Peak  removal scheme, OR STANDBY , and pay them selves off rather  than become a operating expense.  Much Preliminary  Engineering  was done , and the spiffy proposal was sent to the City for review .


The response from the City was immediate:  " Gents , one of FIRST things the the FIRE Dept . does when responding to a Hi- Rise  fire call is, to  TURN OFF ALL THE GAS " !!!!!!!!!


So, the project was somewhat downsized  for Emergency Loads only , and considerable money was spent upgrading the on-site  diesel fuel supply  to deliver additional fuel to the roof in an extended emergency.


For Industrial sites where low buildings can be supplied  via a nearby  power plant of Gas Gensets  primarily designed for a Peaking Solution , or a Cogen Plant,  then  sufficient  fire- safety margin should exist . Also , any managed plant will have immediate access to site personnel who can mitigate any start-up problems with Gas Gensets. I have seen many Cogen plants where the "Genset of last resort "  is  a DIESEL , designed  to be able to cover all the critical priority loads.





on ‎04-06-2011 11:03 PM

I believe JRoth is spot on.


It is my view that the genset should be capable of operating isolated from any utility, water and gas included. Unless directed otherwise.

on ‎04-22-2011 12:08 PM

Excellent discussion - some perspective from someone who's been working on then applying/selling systems for over 20 years:


For true emergency standby, you need to imagine your kid is on that operating table and his/her chest is cracked open.  Where are your priorities then?  Cost, etc go right out the window.  What becomes critical is reliability.  And what I've seen over the years that invariably compromises reliability is system complexity.  The more systems and moving parts in a standby system, the more likely it is to break.  When true life safety is on the line, I want a diesel generator set with robust, reliable, microprocessor controls that starts every time and can accept whatever load is thrown at it when the ATS transfers.  Maintenance is another discussion, so I'll leave it alone other than to say if I don't maintain that generator to a "t", I've forgotten about my kid on the operating table.  The good news is simplicity helps drive cost effective and straightforward maintenance.  I want a base fuel tank - no pumps or controls to fail when I need fuel the most.  I want lots of clean and unimpeded cooling air so the unit doesn't overheat.  I want a unit mounted radiator so I don't have to worry about valves, pumps, and electric fans for my cooling system to operate.  KISS.

Now sometimes you can't have all those things given your real estate.  However, the better job we do with out engineering and architechtural community the more of these things I can get.


For other standby applicaitions, there's a place for gas.  The cleaner emissions may allow for peak shaving and recovering some of your cost, so that's a plus.  They're generally well-built and reliable.  No good for block loads, but if you're powering a data center, how can you beat a theoretically unlimited fuel supply?  Your UPS will help with progressive loading so you're not slamming them with block loads.  Somebody mentioned that the first thing first responders do is kill the gas.  In many cases that may be true, but in many cases they also shunt all feeder breakers, including the generators, into the building at which point you're in the dark anyway.  I'm not sure that's a showstopper for me.


Someone else mentioned dual fuel as is diesel for pilot igntion and most of your energy coming from natural gas.  While these systems do work, there are some things to consider.  Many newer engines are charge air aftercooled.  That means the compressed air from the turbocharger leaves the engine, goes out to a cooling core integrated into the radiator for cooling, and is then returned to the engine.  Do we really want to add natural gas into that air mixture?  It's unlikely the external piping nor the cooling core are rated for natural gas use.  Will it work?  Probably.  But could there be building code issues - heck yes.


An added note on block loading of gaseous fueled units.  Many units below 200kW utilize automotive based engines with turbochargers to make sufficient power.  These have almost no block load capability and must be progressively loaded until some boost is built.  However, some manufacturers use gear boxes to allow the engines to turn at a higher rpm to make that same power without a turbocharger.  The rpm is reduced through the gear box to continue turning the generator at 1,800rpm.  The benefit?  These units will block load like crazy because there's no turbo lag and the higher rpm puts them higher in their torque curve.  I've had enigneers express concern about service life on such machines.  When they do, we take a ride in their car and watch the tachometer.  It's absolutely no big deal to operate an automotive engine at 3,000 or even 3,600rpm.  The gear boxes last the life of the engines too as opposed to a turbocharger which spins at 15,000 or 20,000rpm's.  Simply a more reliable way of skinning a cat.  I would caution the reader at this point, however, not all gas units below 200kW are high rpm.  Check with your supplier on that.


For smaller load applications where gas has many advantages but a source of onsite gas is needed, a different dual fuel system is available.  The unit can be set up to run normally on natural gas but switch to a local propane supply tank if the natural gas supply is interrupted.  Very simple system that works great.  Typically 2 hours of onsite fuel is required.  One or two tanks like what you'd have on your RV will provide that even for units as large as 100kW. 


Lastly someone also mentioned about gas supply.  I can tell you this is the #1 cause of early failure on these units.  They require a pretty large amount of gas.  The gas company should be consulted during design to be sure adequate supply is even available from them.  If it is, then the mechanical engineer should be provided with the gas consumption and required pressure (typically 8-13 inches of water column below 200kW) to be sure the piping to the genset is appropriately sized and a regulator is installed at the machine to step the pressure down into that range.  Careful note must be taken relative to codes for allowable pressures in the building if the genset is located indoors.


Both types have their place - the application and availability of fuels will dictate the best choice.

by New member sammathew
on ‎04-13-2012 02:48 AM - last edited on ‎04-13-2012 08:41 AM by Trusted Contributor

As far as fuel is concerned for backup power equipment, I have been using diesel as the power fuel for years now, but now I feel like switching to gas as a fuel. Diesel as a fuel has only one advantage that it is easily available on the contrary it effects the environment badly.

by Regular Contributor
on ‎04-21-2012 03:42 AM

way not boot fuel in same time, as dual fuel ?????, the tecnology of to day permet a lot of choise with low cost, so  i not understan way still this different, way  thing  in twoo via about use in genset aplication,first thing,  diesel to have respons from emergensy use wen star is fullblack  smoke, question is need all this quicly response, or is time to thing different aplication from engineering, need to have trasient responce  to near zero, this is mi idea

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