I dunno about this. Why did my professor claim that they’re way more efficient than gasoline or small diesel engines? Are they only efficient if the enrergy conversion is chemical (fuel) to kinetic (fast-flowing outlet air)? Turboprops and turbofans demonstrate that gas turbines can be used to produce shaft power, and the most efficient aircraft engines out there today are these combined kinetic/shaft producers.
Further, the increasing use of stationary gas turbines to produce electricity (especially within cogeneration plants) makes me think they can be pretty efficient… some of these cogen plants (like GE’s ‘H system’) have isentropic efficiencies above 60%.
I can see your point being valid if gas turbines can’t be made efficient in the mode of producing ONLY shaft power… but I don’t know if this is true.
With the Middle East already controlling the oil market, it would be unwise to just give them control over another form of power. They would be able to control the turban market just as easily…
Cogen plants use the heat from the gas turbine exhaust to convert water into steam, which then runs through a steam turbine. Can’t do that in a car - steam turbines are massive.
Gas turbines are indeed more efficient than piston engines[1]. That’s why we use them for power generation - otherwise, you would see many more peaking units[2] with Diesel engines[3].
sorry about the footnotes[5]
Dorfl
[1]Their efficiency, IIRC, is in the 30-35% order; Cogen are in the 60-70% order; gasoline/diesel piston engines are around 20-25%
[2] peaking units are power generation units that are not running 24/365, but are designed to kick in when the morning and afternoon peaks in electricity demand come. While sometimes large diesel engines are used, these are typically stationary gas turbines in the 75-110MW range that directly drive a generator. The exhaust gases are NOT used to drive stream turbines [4], these turbines are optimized for torque on the drive shaft, using all practically available heat from the combustion.
[3] Diesel engines have better eficciency that gasoline engines because their compression ratio is higher. For the thermodynamic cycle used by these engines, efficiency goes up with the pressure ratio, i.e. the ratio between the pressure when the piston is up vs. the pressure of the air outside [5]
[4] because the steam units take a long time to heat up and cool down, which is against the idea of a peaking unit that takes only about 10-15 seconds to kick in.
[5] a turbocharger increases this ratio by increasing the pressure before the air goes into the piston chamber
[6] If you know where I got the screenname from, you know where I learned to love footnotes
The thermodynamic efficiency of engines has at it’s heart the carnot cycle. An ideal engine cycle has 2 adiabatic cycles and 2 isothermal cycles. The maximum efficiency (no friction, etc) is based on the combustion temperatures and the ambient temperature. T(hot)-T(cold)/T(hot)
case 1. combustion at 400 degC, ambient 25 degC
((400+273)-(25+273))/(400+273)=55.44% efficiency
case2. combustion at 1000 degC, ambient 25 degC
((1000+273)-(25+273))/(1000+273)=76.44% efficiency
Based on this, an engine becomes MORE efficient (thermodynamically) as the combustion temperature increases.
The problem becomes a materials science program because there aren’t that many materials that can be used at high temperature. Most turbine engines use some type of metal turbine blades – generally these are single crystal/directionally solidified metals/superalloys. The holy grail for a heat engine has been a ceramic that has strength at temperature (>1000 degC) and zero thermal expansion (tolerances with turbines are a real bitch – most materials expand when they heat. Ceramic tend to crack when there is a temperature distribution trough them).
I am sceptical as to when this magic vehicle will be created. They started talking about beta spodumene (http://digitalfire.ab.ca/cermat/material/1287.html) as a magic material back when I started school 20 years ago.
Gas, diesel and gas turbine engines are all heat engines, and the Carnot cycle gives the maximum efficiency of a heat engine based on its inlet and outlet temperatures. You can’t beat the Carnot cycle - it would be equivalent to reversing entropy in a closed system.
Neither gas, diesel or gas turbine engines actually operate via the Carnot cycle - they all have their own little extra inefficencies. Incidently, the diesel cycle is actually slightly less efficient than the gas (Otto) cycle, but it makes up for it because diesel engines can have a higher compression ratio, giving them an effectively higher inlet temperature.
Modern gas turbines built to develop shaft power are more efficient than gas or diesel engines. They have much better power-to-weight, and they are much more expensive. To maintain their high inlet temperatures and power densities, they use exotic nikel-based alloys for turbine blades and nozzle guides. The blades and nozzles are cast with ceramic inserts that are then leached out to leave intricate internal cooling channels. The more expensive blades are directionally solidified or single crystal. They have multi-stage heat treatments, and they have oxidation resistant coatings such as platinum aluminide. They are not cheap. I’m currently investigating a nozzle guide vane and a couple of turbine blades from a power-generating gas turbine. Those three components alone cost more than a new car.
Ceramics hold the promise of cheaper gas turbine engines, but as xiao_wenti says, their problems haven’t been ironed out and possibly never will be.
I suppose it would be possible to build a simple low-performance gas turbine with e.g. solid inconel blades, but you’d have to lower the operating temperature and lose a lot of the advantages. wolfstu’s description of a heat reclaimation system might make up for this - that was news to me and an interesting idea.
You’ve made an error in assuming that “peaking” means “off to full power”. Peaking units are units which are put into service and which can have the power ramp up and down fairly quickly, without undue stress to the components, when needed. Many peaking GTs coast along at half power or less until needed, and having an HRSG (steam side) doesn’t really change their “peaking” status. The vast majority of the time a demand from a reliability operator is not like a “quick Scotty, we need power now!” situation but one where there is a generation/demand plan laid out the previous day. In fact, barring T&D issues, most likely an emergency demand for power is satisfied by ramping a large coal unit running at 60-75% MCR up to MCR, not by throwing in a GT. There are numerous exceptions, and I’ve been at a GT site where the phone rang and the main office said, verbatim, “power’s at $5000 a MWhr - GET THAT COCKSUCKER ONLINE NOW!”
Regardless, I would be very interested in any cite about a large GT coming online in “10 to 15 seconds”. You don’t even ramp up an operating one that fast because you want to watch temperatures and allow the components not to stress too quickly. These are devices that cost tens of millions of dollars after all…my experience has been more like fifteen minutes to half an hour from a cold start to MCR.
I recently was working at a 2x400 coal unit that was a “peaking” unit. It ran generally at about 40% of MCR until the word would come, then over an hour it would get up to 90-100% MCR. If the unit is in good condition and the waterwall and suspended tubes in good shape, you can leverage this.
Basically, each apex has a seal mounted on a spring bar. Originally the seal had two pieces, but in 1986 they split the contact piece along it’s length. The seal is only 2-3mm thick.
It’s made of pyrographite, a high-strength carbon material, and specially processed aluminum sintering, which provides some self-lubrication as it rubs against the nitrided iron surface of the side housing.
You live and learn. My practical knowledge was from working on several projects with a manufacturer of gas turbines[1], and we tracked reliability data etc. Turns out there were quite a few units in the 50MW range that ran about 20 hours a year - backup units for peaking plants etc. And yes, ramping up quickly poses a reliability problem - that’s why maintenance cycles are defines in terms of both start ups and time of operations.
However, the units that were derived from a/c turbines (IIRC, Rolls Royce used the Trent as a basis for a stationary gas turbine in the 35-50 MW range, and GE does the same) have similar characteristics as the a/c units - which means cold to full power in about 5 minutes max. For instance those from the new Volvo/GE partnership claim less than 10 minutes
Yes, but isn’t that because a coal unit runs a steam cycle, not a gas turbine cycle? It takes that long to {simplify} turn that much water into steam {/simplify}. Single Cycle Gas turbines do not have that problem.
cool links:
RR description of a turbine: http://www.rolls-royce.com/education/schools/gasturbine/gasturbines.pdf
I believe powdered coal can be used to fuel a gas turbine, however, molten coal ash (clinker?) often builds up in the combustion chamber and turbine blades, requiring periodic dissassembly and cleaning.
You are entirely correct in that the steam side does require additional time to come up to temperature or massflow, and thus power output. Thus, a combined cycle GT will come up in power slower than a simple cycle GT. What I was saying (or trying to, forgive me if I was unclear) was that as far as being considered a peaking unit, either can work because although there is a relative time difference in coming online or to full power, the absolute time overall is not so large.
I think that it needs to be reinforced that the main advantage to turbines over piston engines is the increased HP per weight of the gas turbines. The second advantage is increased reliability. (Far fewer moving parts in a gas turbine.) The third advantage is increased efficiency. And this increased efficiency is a fairly new aspect of gas turbines. The first turbines were much less efficient than piston engines.
And gas turbines currently ARE used in vehicles. Vehicles like the M1 Tank. http://people.howstuffworks.com/m1-tank2.htm And some busses (http://www.microturbine.com/applications/hybrid.asp somewhere on this site is a picture of a microturbine in a bus. Too tired to find it.) Things that need BIG hp. They just haven’t made a turbine small enough yet for a normal car, but they are working on it!
One cannot define reliability simply by having less moving parts. Some of the key moving parts in a GT are under much higher thermal and force stresses than in a piston-engined car. Plus, piston-engine technology is such that after more than a hundred years of evolution, failures of the main engine conponents (pistons, cylinders, main bearings, etc.) are very rare under 10 years of age. All the rest of the vehicle would have the same number of components.
Since there are no real GT cars out there, it might be best to compare with stationary power systems. And I am unaware of any studies which show, for stationary power production, that GTs are more reliable than piston-engined generators. Do you have some studies that show this?