Big ships, using 2 stroke slow speed marine diesels, use air to start them. Not saying they can’t use other means, but from what I’ve seen, its been air.
I probably mis-interpreted your cite request. If your looking for a land based system cite, then I have no idea. Belay my last.
Anthracite, are diesels still used in electric powerplants? Can you give us some typical sizes, speeds, descriptions, etc? I do not believe I have ever heard of stationary diesels that come anywhere the size of marine engines but I was also under the impression that they had all been pretty much replaced by turbines.
At any rate, for a very large diesel, compressed air is the simplest and easiest way to go as all you have to do is pump air into the cylinders and air compressors are pretty standard equipment one can find anywhere. If your own local compressor breaks down you can rent one around the corner. There is no danger of mechanical problems like you could have with other mechanical starters and air allows for a graduated speedup at any rate you choose. Electric motors are much more complex to adjust to variable speed.
IANAE by any stretch of the imagination but I was under the impression that electric motors are great for low RPM high torque situations akin to the deisel-electric arrangement on train locomotives.
The ones I deal with are only there for startup power, although, when power prices get outrageous ($1000/MWh), some plants have the ability to turn them on and supply additional power to the grid, or allow additional power to be sent to the grid by covering some station service. Assuming diesel fuel at $3.50 per MBtu, and a diesel with a net generating efficiency of 17,065 Btu/kWh (20% efficiency), that equates to a production cost of:
17.065 MBtu per MWh * $3.50 per MBtu = $59.72 per MWh
So that’s a good deal.
I don’t know sizes, manufacturer, etc. of these. I have some info at my office, but as I said, details on these generators is really stretching the limit of my knowledge and experience.
From HowStuffWorks.com, talking about diesel locomotives:
Typically diesels are only economic when the excess heat can be utilised.
The electricity is sometimes treated as a desireable by-product rather than being the main utility.
On former pit sites and trash dumps where there is a supply of methane available, many diesel genrators are dual fuel and make use of this otherwise wasted resource, a small amount of diesel is injected to provide ignition, but most of the cylinder charge is gas.
I’ve only seen fairly small diesels around 5MW(electricity + 5MW heat) or so, I have seen images of larger ones and these are almost always turbines, but the heat produced from them is even greater, so use of this is even more critical.
The heat is enough to drive steam turbines and can then be utilised to provide heating and chilling.
The main difficulty here seems to be getting the local planning permission, no-one wants a power station on their doorstep, and yet to make good economic and efficient use, you need to be close to a population centre to pipe the waste heat around.
Heep reading that article bud, the actual motors that push the train are electric (called traction motors). The main deisel engine drives a big generator feeding them. Clutching and shifting something with hundreds of tons of load is pretty much impossible.
In retrospect I could see how air driven systems would be better low maintenance options for systems that rarely shut down. Maintaining a starter system that might only be used a few times a year vs a couple valves and a compressed air tank that only needs to be charged when a startup is needed makes pretty good sense on some level. Guess its just a matter of how well things scale up.
Right, but the question was starting the engine: air start v. electric starting motor. The article seems to indicate that locos are started via huge electric starting motor, whereas diesels in the marine environment are air start.
What? Where do you get your information? Diesels are used for multitude of uses and most of the time the excess heat is just discarded except for ocassional use for heating (like in passenger vehicles, etc). If you want to heat all you need is a furnace which is much cheaper and simpler. Diesel or any other motor is used when you need mechanical energy as the main product. A large diesel can only yield in mechanical energy from 30 to 40 % of the energy contained in the fuel and the rest is lost as heat. Power plants have cooling towers or use rivers and lakes for cooling for a reason. Mechanical energy is much more desirable than heat which is considered low grade energy.
Nuclear power plants are not allowed to start up unless they have a source of offsite power. They’re not allowed to use their diesel generators to start up the plant.
Wow - there is so much that begs for clarification here I hardly know where to start.
First of all - Electrical Generation by Diesel Engines. The US Dept of Energy keeps track of all the generation facilities in the US. Here is the webpage:
There are a few things here that will surprise a lot of people. There are over 16,000 generating units in the US. Over 4,000 of these are Internal Combustion (virtually all diesel) units. You wouldn’t know this because most of these are older municipal or industrical cogeneration units. However, new diesel stations are still occasionally installed in the US.
The great majority of these units are in the 1 to 3 megawatt (about 1000 to 4000 horsepower) range. These could either be electrically or air started. However, of more interest are the larger units. In the US, I think the largest engines are about 20 MW (over 25,000 horsepower). However, internationally I believe stationary engines of 40 to 50 MW or more have been built. I haven’t got a reference for that right now, but here’s a link to a 108,000 hp marine diesel (5 million ft-lb torque):
I can remember seeing pictures of very old central station diesels that operated at about 50 rpm and they had pistons with about 10 feet diameter.
In some applications diesels are better choices than turbine plants. Diesel efficiency is not much lower than a simple cycle combustion turbine and installed cost is not much different. Lakeland Florida installed the 50MW Winston plant in 2002 with 4 engines of 12.5 MW each. Over the last 10 years, the University of Illinois has installed 4 natural gas diesels of 4 to 6 MW.
Nuclear plant emergency diesels range from about 3 to 7 MW. All the ones I have seen are air start and I’ve seen a lot of them.
KenGr, thanks for your post. Very interesting. We had seen that huge diesel before but it is always interesting to see it again. The crankshaft weighs 300 tons! And the thermal efficiency can exceed 50%. That is amazing.
Yeah, that’s what I thought. It is the easiest and most practical way.
Why not? What are the diesels there for if not to start up the plant?
Exactly my point, to make the power generated from this source economic you have to find a use for the heat.
I worked in a CHPS plant in Leeds General Infirmary where we would sell electricity to the network.
Using diesel only our electrical output was not economic unless the pool prices went very high, in other words we were competing against steam plants whose output price was lower than ours.
Our price was governed by the heating and chilling load required in the hospital main site, if that was using less energy then we would reduce heat output and consequently the electrical output. This in turn would increase our pool price due mainly to fixed costs.
There comes a point where it is uneconomic to produce your own electricity and you import it, and use gas boilers to provide the heat.
Running on gas (with injected diesel for ignition) puts the balance much further in our favour, often and especially at night it would be cheaper for grid to take our power despite it being more expensive rather than put online a large 500MW unit, just to cover say an extra 10MW demand.
Since I left, further capacity has been installed, inclucing a couple of gas turbines and steam turbines which run on the waste heat from the gas units.
The limit to total capacity has proven to be the ability to extend the number of local users, such as the University and the Polytechnic college which both use a lot of heat and also the feeders back to the substation.
I used to have a link to a website that gave the stats on this plant but I guess I must have deleted it, I think Anthracite will have it as I recall she was familiar with the site.
They are there only for operating the safety systems in the event of a loss of offsite power. There are very strict rules to minimize the cases where dedicated safety systems are used for normal plant operation or startup. The security diesels are separate from the safety diesels. The startup pumps are different from the safet shutdown pumps, etc.
Not necessarily relevant to the US, but in Ireland there are two or three stations on the system with “black-start” capability. In the event of a system-wide failure such as occurred in North America during the week, the system has the theoretical capability to split into three geographic regions, in the hope that power can be maintained in at least one region.
The black-start capability consists of a diesel engine rated to around 2 to 4 MW to provide excitation current to the main generator. The power to start the diesels comes from batteries. Other batteries provide emergency auxiliary power for control systems, etc.
When the unit with black-start capability has run up to speed, the breakers are closed to allow it to energise the transmission system in its region, allowing the other units in the region to be restarted. Once enough units are running, the distribution networks can be re-energised. The whole process is planned to take 7 to 9 hours, but we will never really know until it actually happens.
As has been stated earlier in the thread.
It should be noted, though, that “not allowed” is different than the theoretical ability to do so.
Anyway, this whole discussion only applies to land-based nuclear power plants. At sea (on a submarine), I have conducted fast-recovery startups numerous times using the battery (and motor-generator sets to convert DC to AC).
I conducted one complete startup at sea using the on-board diesel generator. (The battery was nearly depleted while some equipment was being replaced.)
Lastly, our onboard emergency diesel generator had to be able to be air-started. It wouldn’t be very useful as as emergency power source if it took electricity to start…
Wanted a cite on starting diesels on compressed air instead of battery?