04-08-2010 05:20 PM
The question of the appropriate fuel for one or more standby generators for data center use is touched on in several places, but I wanted to focus the topic in one place.
As I understand it, here are some issues in choosing between diesel and natural gas/propane/low energy gas for standby generator fuel:
- start time for diesel faster
- diesel has more emissions issues than gas (including permitting constraints from air quality, health authorities)
- if other nearby standby generators are on NG and take on load in a disaster, will pressure drop causing an outage?
- Can't plan on switching NG fueled generator to propane because of differing BTU of fuel leading to timing problems
- natural gas (at least) has less power than diesel for comparable physical generator size
- diesel engines respond much faster to load transients than gas engines
- availability of natural gas supply at all after a seismic event
- availability of diesel from tanker after a seismic event (mitigation: fuel from 55 gallon drums?)
- greater failure rate for gas powered.
04-22-2010 10:47 PM
There are many different variables that are going to determine the fuel best suited for your application (mostly the size of the generator, you didn't specify how big you're looking to get and if you're looking to parallel multiple gensets), but I'll try to advise from a general application standpoint.
1. Start times are much faster with diesel engines. We never recommend a natural gas engine if the customer requests a 10 second start up time (meaning from when the lights go out to when the lights come back on in an outage situation).
2. In the next 3-4 years, emissions restrictions are getting MUCH worse for off-road diesel applications. But yes, generally diesels do not burn as cleanly as natural gas.
3. There would have to be A LOT of natural gas generators in the area for the gas pressure to drop enough to not allow your generator to run. Contact your natural gas supplier if this is a concern.
4. We have units that switch over from Natural Gas to propane with minimal drop in frequency, the only problem that we occasionally see with changeover NG/LP is if the customer doesn't exercise the microswitches and solenoid valves in the two systems. No timing problems though, propane is used as a backup to the natural gas.
5. Yes, the engine is going to be larger in your natural gas than in a diesel set of the same kW rating.
6. Diesel engines will handle load much better and respond to various types of load better.
7 & 8. You're rolling the dice with seismic events, it's impossible to say whether natural gas lines or roads will be broken, or both. Again, depending on the size of the unit... I would say it would be better to have a large underground tank, base tank, or day tank. 55 gallon drums?? 2 megawatt generators burn upwards of around 100 gph at full load, since I don't know what kind of loads you're running, I can't say any better.
9. Failure rates will depend on the amount of hours the generator is run and how well the maintenance is kept up. For a standby application, it will be able the same type of failure rates. Obviously natural gas has a separate ignition system that will require maintenance, where a diesel wouldn't require this extra maintanance.
If you could provide more information such as generator size required, located near a major city or a remote site, what time requirements you need in an outage, where do you want to put the generator (outside, rooftop, indoors, basement?).
Hope this helps!
04-24-2010 05:58 PM
Thanks for your helpful reply.
My concern is not so much with a specific case as with general best practices in data center design. For years I've been specifying diesel as fuel for data center backup power, without any additional thought, but recently I was in a workshop with someone who had some credible sounding arguments for natural gas as a fuel for data center generators, so I thought I'd ask here.
The comment on 55 gallon drums was based on a comment elsewhere that mentioned that, in a regional disaster where roads were impaired, fuel could be delivered in 55 gallon drums. Admittedly this wouldn't be anyone's choice.
Just to give some general sizing, this might most typically be for a multi generator site for a total 2MW load. I'd expect a multiple UPS with either battery or flywheel carry through, so 10 second startup wouldn't absolutely be required. Life safety functions would not be on these generators. The typical data center would generally be within reasonable driving distance of a major airport, thus a major city, but close to cheap, clean power is a major consideration.
Unfortunately I can't retrieve any case studies of how generators performed, by fuel type, in the aftermath of recent major disasters, such as the Chilean earthquake.
So, to summarize, it's an issue of what are general best practices for data centers in the 1MW+ range, located globally, but probably in North America.
09-01-2011 05:28 PM
Dual fuel is an excellent alternative for the best of both worlds. I am President of a Water Utility in South Texas. After Hurrican Ike, we could not get fuel delivered for 6-7 days, so ordered our engineers to evaluate how long our genset would run and the answer was about 1-2 days on 1000 gallons of diesel. I did not feel this would be sufficient, so we had a Natural Gas line installed and also installed a dual fuel control system. This system injects natural gas into the intake manifold of the diesel engine, therefore subsituting natural gas for diesel. After installation and testing, my engineer estimates we can run for 2-3 weeks. In addition, the NOx emissions are greatly reduced as are other harmful by-products.
Unfortunately, for emission regulations it is still considered a diesel engine, but I am working with a supplier that is working with regulatory agencies to get a system certified as a dual-fuel engine.
These dual fuel systems are gaining popularity on Oil & Gas drilling rigs which are prime power applications and are one of the toughest transient load applications known. They typically use the CAT 3512 diesel engine and have seen no negative ramifications using the dual-fuel system as long as it is calibrated correctly. As a practical matter, I would highly recommend looking at this type of system. As a regulatory matter, it still is not accepted as an alternative to a diesel engine.
If you would like to discuss further off-line, my email is firstname.lastname@example.org.
09-01-2011 07:24 PM
Thanks for this informative reply.
Do you have any data on cost differential for CAPEX and OPEX, other than having the natural gas line installed?
Who is the regulator that you're trying to educate? Do you still test with the pure diesel source?
(I'm replying here because the discussion may have wider interest.)
09-02-2011 04:48 PM
I will have to get back with you on this. We are a few months from releasing our product and much of what we are doing is highly confidential right now. There are existing systems on the market right now for $30-50k plus installation, but they have lower substitution rates than I believe are attainable, which directly reflects on emissions, so I want something better. In addition, some areas may require oxidizing catalyst as well.
Keep my email and contact me in another 60-90 days for more info.
09-04-2011 10:24 PM
For a set of "general guidelines for data centers 1 MW+", really hard to make a broad set of recommendations that will work for a market segment because of the differing regulatory requirements, types of environmental drivers to possible outages, availability of different fuel sources, and the critical nature of the affected facility in all the different regions.
I've spent a lot of time lately with a new facilities manager for a group of local COLO data centers, he's a smart guy and wants to make things right, but certainly is finding that working in California much different than Iowa. Maybe instead of a general set of recommendations, it would be refreshing to find someone in this business who knew how to properly survey a sites needs, taking into account many of the things discussed in this thread and in other threads in the forum. Biggest problem I see is that most people making the recommendations have something to sell, and tend to drive the recommendations based on what they have to offer. I see it with UPS systems, generating systems, switchgear and BMS systems. And in not many places do I see well planned comprehensive power systems that can meet nearly every possible need of a critical facility. Sometimes it's really good hardware and poor installation and workmanship, sometimes it's a choice to use a new and improved proprietary control system with the only source of support 2500 miles away, and sometimes it's just the wrong piece of hardware for the job.
Fuel choice is always going to be a major concern and a moving target, as noted above, emissions regulations will continue to get stricter, and new systems will need to meet very tight requirements, and older systems will need replacement or retrofit on a likely increasing basis. The ability of gas fueled engines to operate in island mode and provide some reasonable level of transient response is improving, but will never likely meet the abilities of a modern high speed diesel unit with a proper turbo match. One of the main driving factors to fuel choice really is the number of run hours you expect in a year. In sunny southern CA with not much at all in the way of bad weather the average total run time for most standby sets is 40-50 hours a year. Outages over a few minutes are rare, and the possible sources of long term outages are pretty small compared to the same kind of facility built in the Southeast. Maybe you have a need to operate longer, take advantage of demand response or do some peaking, in any case the power system design would need to either accommodate those desired operating modes initially, or be built to be economically retrofitted at a later date.
Emissions, this is an area that is tough for most data center folks (and many other standby customers) to keep up with. Buy a current tier unit, dump it on the ground and away you go,right? How about if it needs a catalyst, additional AF controls, fuel and NOx reporting system? Let's broach one of these main misconceptions, is a modern tier diesel engine really "dirtier" that a natural gas fueled engine? I'm talking engine stack, non-federated engines here. NOx is likely lower in the certified diesel compared to a stoich (standard) gas engine, yep, makes more particulate (at least PM10, the whole PM2 thing is yet to come), and in many cases the CO is actually lower from the diesel. And I've got two 4" binders of emissions data sheets, 90% CAT the rest competitive engines I've been able to collect over the years to support that in IC engines between 200 and 5000 bhp. So the real answer is "it depends", what are you comparing to, what are the non-attainment constituents in the air quality area you're dealing with, what are your load factors and what additional environmental conditions exist at the site that affect engine emissions output? Is a gas fueled engine easier (generally) to reduce and produce lower stack emissions than a diesel unit? A qualified yes, again depends on what you have to deal with where you're at and what you have installed.
How will you operate your engine to perform regular testing and maintenance? Run no load, run regular light load runs on non-critical equipment, run regular on load test runs, or have an on site or portable load bank at regular intervals? How well you plan to take care of your engine has an impact on what you can get away with.
In general, diesel units, especially fully packaged (unit radiators, most everything bolted to the skid) have the lowest installed cost per kW, take a lot of abuse (although newer engines meeting higher tiers are a bit more fussy) and serviceability and local support is usually very good. You can almost always figure out a way to get fuel to them in most situations, not always easy, but at least in my experience it can be done, just not always a pretty solution.
SI engine driven units, meaning natural gas and propane, are getting better, but I have shared a number of opinions regarding their suitability for standby service in other threads and my opinion is still the same, I would not recommend an SI engine in a critical standby application, but there may be a data center application or a combination system where they may make sense. And if you desire to do peak shaving or demand response then it may improve their overall desirability. In my own experience engines that operate for a reasonable number of hours in a year, say at least 250 and up to 1000 hours per year, appear to have better operational histories as to responding to emergency run demand than do SI engines that are only in standby service and only run very short times for testing and maintenance. I have worked on a number of SI engine driven systems that used natural gas as the primary fuel and propane as a backup fuel. they work, I generally wouldn't stake my life on them, and they have a number of things you need to keep on top of, but with a good in house operator and local service support they can meet some needs in data center applications.
Biofuel in a standby unit, we did a dual fuel conversion for one of our major sewage treatment plants on their only standby, everyone loved it, the plant operators touted being "green", the rate payers thought it was a great idea, the local air board loved the decision. We worked as part of the team that did the conversion, noted my concerns, and 14 months later when the unit wouldn't carry full load on straight diesel fuel because the aftercooler was nearly plugged solid, I really struggled with not yelling "I told you so". Operating an SI engine on biofuel can provide a reliable and efficient means to produce electricity from a waste stream, however the negative impact the fuel has on the engine service life and reliability is significant, and in my opinion is not a good option to use in a system that you need to rely on starting and carrying load everytime you ask it to.
Dual fuel conversions, the dealer I used to work for for many years was way ahead of the curve on that, we did a pretty good number of units, worked with some other companies who were trying to come up with commercially viable retrofit systems, and struggling to get a buy in from CAT (and othe manufacturers). We learned a lot, and again there are a few threads in this forum where I've made my experience and opinions pretty clear. I like the idea of dual fuel conversions in the right applications, like oil and gas service, where you have experienced mechanics and technicians who understand both gas and diesel engine operation and maintenance. There is currently a lot of people looking at it again, some seem on the right track, some seem to be selling snake oil. My former employer has received a number of requests for information regarding those systems and lately it seems they have been passing my name to a number of those customers. I think it can be done effectively and provide and good balance between meeting your critical needs and offering you longer operating times if fuel is available and the air quality people will buy off on it. Currently there are two major issues I'm seeing as I discuss these issues with people looking to move forward with dual fuel, first is how the engine manufacturer and local dealer will feel about you you making a major modification to what is likely an EPA/CARB certified engine and its impact on warranty issues. Next is who is going to work on it? The local dealers are struggling to just keep technicians up to date on what they're selling (not just a problem with CAT, I see it with all the suppliers in the areas I work in), so if it doesn't work who referees the pissing match that happens when it doesn't work as expected, the end user wants to hang it on someone and the folks who did the conversion and the engine dealer are standing there pointing fingers at each other? A third issue is the impact of the local air quality regulations, frankly most people staffing Air Quality boards ( at least in the US) aren't looking to be helpful or innovative, they just want to follow poor drafted and in some cases unattainable rules, and let you figure it out while paying your way the whole time.
Well, good luck in your quest, hope you found some of this helpful. Mike L.
09-20-2011 08:16 PM
You know, the whole purpose of a standby set is based on the need to have power in the event of a utility failure. Relying on town gas is reliance on just another utility, another stress raiser and another connection to grid (indirectly). If I had a choice, probably look at what I need as an Island.