Diesel gensets and gas turbines.

Hello,

I've been reading about gas turbines and one specific point didn't became clear for me, specially after reading a Wärtsilä article comparing energy generation power plants, when they tell (short version) using their diesel engines are more efficient when there is no need to have the power plant working at (near) full power all the time.

When comparing the efficiency between equally powered diesel generator and a gas turbine with both working at (near) full power, which one would (usually) be more efficient (burning less fuel)?

Thank you.

 

So, if you need constant 70-80% output, GT's have better efficiency...if you float 30-70% output, then diesel has the advantage in efficiency. FWIW, modern ships with variable power needs are using multiple GTs or diesels in a ring powering system with engines brought up or dropped off as needed. It all depends on what is the major fuel tankage system.

 

Unless you are talking about gas turbines designed for base load power generation, which are equipped with recuperators and steam generation plant, the fuel consumption would be much higher than a diesel engined generator. Obviously the turbine is much more scaleable in power output, but the only time "ordinary" gas turbine generators are used is for peak clipping at times of maximum utility demand.

Usually a utility will charge whatever the maximum draw was for the entire month of service, Im talking rail services or cities. Therefore, 2x or 3x the fuel burn is peanuts compared to paying a utility for that power level for an entire month.

 

It's not just steady load, but operation fraction as well. As expensive as they are, they can never the less manage to burn several times their installation cost worth of fuel each year. So even a seemingly small increase in efficiency can justify large price differences between plants if they are expected to run for 30 years with out much down time. A 1500 kW genset might cost $1,000,000 but could burn nearly 900,000 gallons of diesel in a year. Turbines and diesels are also quite different in terms of maintenance and operation budget. Hopefully something this size actually has a knowledgeable operator in the vicinity all the time, but that isn't always the case. The cost of efficiency has to be justified by the present value of future savings, and depends more on operating hours and fuel prices than anything else. So the most important things to match in a comparison are duty fraction, design life, and fuel price models. The industry standard way of making these comparisons is to use the LCoE number. In the US, diesel fuel cost contributes about 70% to 90% of the LCoE of a diesel peaker plant. It would be at the high end for base loads. Diesel recip is a fair bit more costly than gas turbine (or gas recip).

There's a good table of diesel vs gas turbine vs renewables in this document from Lazard.
Levelized Cost of Energy 2017 https://www.lazard.com/perspective/levelized-cost-of-energy-2017/

 

A gas turbine with waste heat recovery (recuperated cycle) are generally about as efficient as a diesel engines at high load. If the recuperator has a high effectiveness you can have similar fuel consumption down to about 35% power. Below that it the diesel is better, but only if speed is varied. Most diesel fuel consumption curves are based on a propeller load and the diesel efficiency looks good a lower loads at low speed. But if you are running the diesel at low load and higher speed (as you would do in the case where you are operating a synchronous generator at a constant rpm no matter what the load is) then there is very little difference between the turbine with waste heat recovery and a diesel. Of course turbines beat the heck out of diesels in terms of maintenance requirements, with only occasional topping up of the oil tank, compared with oil changes and injector maintenance as well as much more frequent overhauls. The weight of the turbine will be an order of magnitude lower than a comparable diesel, but the first cost will be significantly higher than the diesel. That's pretty much the bottom line. If power density or long life with low maintenance are the basic requirements, then the turbine wins. If the system weight doesn't matter and first cost is the primary consideration then the diesel gets the nod.

 

I think you should also consider the cost of maintenance and repair. That includes the difference in salary between a diesel and a turbine mechanic. Further, the life expectancy has to be taken into consideration. In short, the economic analysis has to take every factor into consideration.
https://www.webpages.uidaho.edu/~ml...ering_Economics_Excerpt_from_FE_Reference.pdf
Engineering Economic Calculator https://web.njit.edu/~wolf/calculator.html

 

Hello,

First of all sorry about me being silent, I was expecting to receive some e-mail when here had an answer, like usually happens, but I didn't get any this time.

That seems to be one of those cases when everything depends on the concrete case. To give you some perspective I was thinking (hypothetically) about something like CODLAG[1], with the turbines used to archive higher speeds; however, with more and more vessels using natural gas, what about both (engines and turbines) using the same fuel?

Thank you.

[1] I think weight matters here.

 

Typically CODAG is the worst of both worlds. Let's consider boats with a length of 100 feet or less. Most of these applications are planing or semi-planing hulls. Most CODAG systems are configured with a fairly large diesel engine that is capable of producing moderate speeds and then a turbine is added for "dash" speed. Since the turbines aren't used for continuous operation they designers generally install a lower priced turbine that is not very efficient. So when you add in the turbine power it sucks fuel like crazy since the specific fuel consumption of the turbine is poor and you need a lot of power to get the requisite increase in speed. In order to produce moderate speeds the diesel engine is heavy and this adds a huge amount of mass that impacts the top speed when the turbine is being used. Also, to go much faster with a turbine you need to provide the diesel engines with a two speed gearbox so that you don't overspeed the diesel at higher speeds. The two speed gearbox adds weight and complexity, or you could declutch the diesels and operate just on the gas turbines, but then you're carrying the weight of the diesels as dead weight..

A far better solution is to use a smaller diesel that would allow for docking and operation up to hull speed, but then go with gas turbines for all operation above hull speed. This approach requires more expensive, but considerably higher efficiency turbines. In that case the turbine does burn more fuel than a diesel. But.. The boat is significantly lighter and requires about 10% less power for a given speed so while the turbine burns about 20% more fuel than the diesel the power required is 10% less and the overall fuel burn is only 10% more than a diesel powered system and it's much better than a CODAG system. In such a case you're looking at two 1500 kw turbines an a single 300 kw diesel ( or two 15o kW diesels), just to give you an idea of scale. And those turbines (without gearboxes) only weigh about 400 pounds each... Compare that with the weight of a pair of 1700 kw diesels (you need more power to produce the same top speed because of the heavier weight of the diesel system) and you get an idea of how it all trades. With the CODAG you would have a pair of 1000kW diesels and a 1500kW turbine, multispeed gearboxes for the diesels, and a bigger fuel tank...

So it can clearly be done, the only issue is cost. And that is where the turbine generally loses. Turbines are more expensive, but the maintenance cost are lower so there is a trade off there, but don't expect it to pay off unless you're using the boat every day, like a patrol boat or a commercial craft like a crew boat where time is money...

 

@Yellowjacket

This is a quite interesting real-world information since on the paper CODAG looks to be the exact opposite, at least to the eyes of someone like me, who don't know a thing.

But since your explanation was based on planing or semi-planing hulls, could you also bring some light to large displacement hulls? I mean, frigates apparently often use this or similar mix of engines and turbines, what usually would be the "right" setup for large (and heavy) hulls with mixed operational profiles, like those kind of military equipment?

Thank you.

 

Ships like frigates or destroyers generally employ much larger gas turbines that are pretty efficient. Not quite as efficient as large diesels, but within 5%. Engines like the LM2500 are not only efficient, but they are much more reliable and run forever. For these larger ships that are running at pretty high power levels all the time, gas turbines are the way to go. Part power specific fuel consumption of the turbine is worse than a diesel. If you look at ships like LCS you'll see a combined power system that relies on diesels for lower speed cruising (which is what they do most of the time) and go to turbines for dash speed. That works, but it would be better from a weight and overall performance standpoint of they could drive both props with just one turbine at lower speeds and run that engine at a higher percentage power. That would require a means of transferring power to both shafts so that you didn't have a dead prop. But again, it depends on the mission. If the idea is to cruise at lower speeds most of the time, then a CODAG is the way to go. If you want to cruise at planing speeds, then you really shouldn't bother with the diesel. The weight of the diesel just loads down the boat so much that it isn't worth it.

Just to give you an idea of the numbers, a typical diesel has a specific fuel consumption at full power of around .34 -.36 pounds per hp hr and fuel consumption on the prop curve is pretty flat. A poor gas turbine (like a PT6) has an SFC of .58 lb/hp-hr, and a good helicopter engine, like a T700 or T800 have SFC's of about .43.. A LM2500 is around .38. Again this is high power sfc, at part power the PT6 is around .70 or worse. So if you aren't running that inefficient turbine hard you're guzzling fuel.

Every designer makes choices and the configuration he ends up with is a product of those choices. There are lots of ways to skin the cat. Choices of what engine to use and the cost of the overall propulsion installation enter into it. Also there's a limited number of gas turbines that are available to the marine market so this really limits what the designer can specify. If there were more engines in different sizes available then it would a lot easier.

 

I got it.

Thank you.