The whys of Combined Cycle power plants

Charles123

That you for your answers! Again very clear ones!
I have a question about your first reply, can you elaborate on the issue of downsizing in gas turbines and its associated decrease in efficiency?
About coal, I am familiar with those cleaner ways to use it, like the example you used of carbon sequestration. As well as with the fact the it holds huge interest for its price, availability and for its wide geographical distribution as a geological resource. My question was purely related to thermodynamic efficiency, how much electric energy can I generate with a x amount of fuel. Any ideas of what its being done or can be done in the near future?
Regards

Pkruse

If we could build steam engines to run at the same temperatures as gas turbines, then their efficiencies would be about the same. That would mean using an inert gas, or at least a gas that is not terribly reactive at these temperatures. Jim suggested Helium in one of the notes above. That would be ideal. But all proposals to use anything other than water and steam are a whole lot more expensive than water or steam. That is why the industry has chosen to operate as it does.

As for scaling down a GT, that is a complicated issue. One of the causes of inefficiency in GTs is leaks past the clearances between moving parts. So we keep the clearances as small as possible. This is complicated by the fact that the parts get bigger under stress or when they get hot, so we always have some clearance. We just try to make that clearance as small as possible so as to leak as little as possible through it. That clearance cannot be scaled down when you make smaller GTs, because it is always as small as we can make it. In big GTs, the effect of a leak is a small percentage of the total gas flow. In a small GT, the percentage is bigger and therefore has a bigger effect on the inefficiency of the machine.

Charles123

Thank you for the answers! This was very elucidating.
Regards

jim hardy

Take a look at this link. It lets you enter temperatures and calculate Carnot efficiency,
which is theoretical best any machine could possibly do.
http://hyperphysics.phy-astr.gsu.edu...mo/carnot.html

Steam turbine plants can attain perhaps 70% of Carnot efficiency limit.
Type in 1000F for Thot
and 120F for Tcold
observe 60% is best any machine could do
our Fossil plants made roughly 40%.

Type in 516F for Thot, same 120 for Tcold,
observe 40% is limit
our Nukes made ~30%.
In winter when condenser cooling water is colder all steam plants do better. Type in 90F for Tcold...


Probably Pkruse knows inlet and exhaust temperature for a gas turbine, and how close it can approach Carnot limit.
From that you could figure from 10,000 BTU's of coal (a pound of the good stuff)
how much energy you'd get out.
Our combined cycle units of early nineties could make about 55% overall efficiency burning oil or natural gas.


Have Fun !!!

Charles123

Thank you! I was familiar with the site, but not with this particular link.

AlephZero

For a modern gas turbine, try a turbine inlet temp of 1500C (note, C not F). These guys are claiming a "record" of 1600C. http://www.mhi.co.jp/en/news/story/1105261435.html

That should push the Carnot efficiency up to around 80%.

jim hardy

And a practical machine that achieves 70% of Carnot efficiency would be 56%.

Power plant operators closely track efficiency for obvious reasons.
They measure it as "Heat Rate",
which is BTU's in per Kilowatt Hour out.

Conversion from BTU to Kilowatt hour turns up a few different numbers all close to 3413 BTU/KWH. My company used 3412.7.

So a power plant that was 100% efficient would have a heat rate of 3412.7 BTU/KWH of course impossible.
It'd take a perfect Carnot engine rejecting heat to absolute zero to make that.


Divide heat rate into 3413 and you'lll be close
A nuke at 10,500 is 3413/10,500 = 32.5%
a fossil at 9,000 is 3413/9,000 = 38%

So look up heat rates and convert to efficiency if you wish...
A search on "power plant heat rate" will turn up plenty of hits. Here's one,
http://www.econsci.com/euar9801.html
http://wps.aw.com/wps/media/objects/...cs/topic07.pdf how'd that happen ?


That link tabulates some effficiencies from around industry.

gmax137

just as an aside, the reason you see various values for this conversion factor is because there is more than one "BTU." A BTU is the heat to raise one pound of water by one degree F. But the amount of heat to do that depends on the temperature -- so there is a "BTU" as 32F, and a slightly different one at 20 C (68 F) and yet another value at 60 F, and so on.

jim hardy

oops i got wrong link in above post, fixed now.
Sorry - probably i put heat rate link into electron thread, too..

ahh the joys of aging!

ehilge

A couple of minor observations on what I've read so far, most of what I can contribute has already been said so I'll keep it short.

- In nuke plants, I know the temperature of the steam is lower than coal plants because the steam is generally not superheated for safety reasons. To superheat the steam, you would need to expose the rods directly to the steam which can make it difficult to cool the reactor in an emergency. I can't comment if the materials can't take the higher temp as well.

- I once did some research into a coal-fired gas turbine. If I remember right, you could somehow gasify the coal first and then use it. But that is generally more work than its worth.

Pkruse

1600 C is certainly higher than most turbine entry temperatures, but hardly a record. I have no direct experience with the P&W F135 engine, which goes on the new F35. I would expect that the real number would be classified. But judging by the performance of the engine, it is certainly higher than we have seen in any other production engine. Mainline engineering and aerospace publications have said the number is 3600 degrees F, or 1982 C. So I would expect the real number to be at least that. More commonly, I'm used to seeing TITs of 1800-2400 degrees F. As for the exit temperatures, you can make them pretty much whatever you want, according to what your design objectives are. If you were planning a combined cycle plant, my expectation is that you would not mind if they were a little on the high side.

AlephZero

You have to factor requirements on component life and overhaul intervals into the equation. The military don't mind too much if their blade life is only a few hundred hours - that's still longer than the expected life of the plane in a real combat situation. Some military fast jets only fly a few hundred hours a year, and spend literally half their time on the ground being maintained and overhauled. Compare that with civilian long haul aircraft which clock up 5000 to 6000 flying hours a year.

On the other hand, power generators think 20,000 hours between major overhauls is unreasonably short...

Pkruse

Aleph nailed it. But DOD is demanding & getting much longer overhaul intervals.

boudzmawed

hi every1 ,
I'm trying to do the concept of a turbosteamer that the bmw is workking on but i'm facing few problems :
the flow rate of water will be 9 ml/s and of pressure 6 bars . based on htri xchanger program , the best design of my shell and tube heat xchanger will be of 22 cm diameter and 6.4 mm tube diameter where number of tubes will be 215 and the length will be around 0.5 meters.

my problem is how to allow this very low flow rater ( 9 ml/s) enter the 215 tubes equally ?

my 2nd problems is that i have no experience with microturbines , i need to know how to design that turbine, ihope you can help guys

thank you

Pkruse

This thread on gas turbines has pretty well run its course. I suggest you open a new thread on steam turbines, & maybe one on heat exchanges. Both of your questions are interesting.

boudzmawed

hehe we can just open it here :)

Charles123

This thread gained life of its own!