3 ms·

I've been wondering about this very same question a lot myself and accidentally stumbled across the answer just a few days ago: Efficiency of turbines smaller

by usrusr 3y ago

I've been wondering about this very same question a lot myself and accidentally stumbled across the answer just a few days ago: Efficiency of turbines smaller than grid scale is simply not anywhere close to what piston engines can do at e.g. car size. Even at naval scale, turbines only win in use cases where power density is more important than fuel efficiency. Helicopters are deep in the (specific) camp of power density beats efficiency because carrying a heavier but more efficient engine would easily eat the fuel savings. Fixed wing aircraft gain range by climbing high, but up there both efficiency and power density of piston engines decreases dramatically with decreasing air density, so they are also in the camp of power density over efficiency (turbines are also affected, but not quite as much). Note that despite all this, the Otto Aviation 500L that is all about fuel efficiency at high altitudes uses a piston engine (they probably put a lot of effort into their turbocharger, those can lessen the impact of thin air)

anjel 3y ago
Celebrity and Cunard built a few Cruise ships that use turbines to generate power to its electric motor propulsion.[1] They regret the endeavor owing to cost of operation.[2] [1]https://www.ge.com/gas-power/industries/cruise-lines https://www.ge.com/gas-power/industries/cruise-lines [2]https://www.airliners.net/forum/viewtopic.php?t=1202425 https://www.airliners.net/forum/viewtopic.php?t=1202425
KennyBlanken 3y ago
The bit about the naval use and efficiency isn't quite accurate. The issue isn't that they're not efficient. It's that they are only efficient at high power level, and the minimum power level they're efficient at (and even their minimum power level, period) - is quite high. To compare: Britain's current aircraft carrier has four diesels that total 40MW...combined those diesels equal one of its two 40MW turbines. This minimum power level is why jet airplanes have an APU, and often taxi with just one engine running, with the second started up with enough time to get up to operating temperature for takeoff. Example: https://en.wikipedia.org/wiki/Rolls-Royce_MT30 https://en.wikipedia.org/wiki/Rolls-Royce_MT30 ~40MW, minimum efficient power level 25MW. 25MW, even if it's very efficient in terms of turning kerosene into shaft power, means the ship is moving really fast, and thus there's enormous fuel consumption and drag.
- usrusr 3y ago
  I don't think that it's just inefficiency at certain power levels: The big table on https://en.wikipedia.org/wiki/Brake-specific_fuel_consumption https://en.wikipedia.org/wiki/Brake-specific_fuel_consumptio... has the Marine Trent rated less efficient than some car engines. And that table is peak efficiency, not efficiency at some arbitrary inconvenient power level. But you are right in so far as efficiency at the low end of the power range is very important in commercial shipping. Because the energy demand (at the drive shaft) of a given trip is highly dependent on the desired speed and if going show ruins your engine's efficiency any dip in demand will ruin you economically (because others on the market are much better at compensating the dip by "slow steaming")
- taneq 3y ago
  Then wouldn’t they just have a 5MW turbine and run it at its peak efficiency?
jabl 3y ago
Another reason many navies like turbines is that big diesels produce a lot of low frequency noise which travels very far underwater, so submarines can hear you coming from a very long distance. Although many navies do operate combinations of diesels and gas turbines (CODAG etc.), in such cases the diesels need to be installed on special shock absorbing mountings, in some cases even with a diesel-electric drive to avoid coupling the vibrations of the engine to the hull via the propeller shaft.

by usrusr 3y ago

anjel 3y ago

Celebrity and Cunard built a few Cruise ships that use turbines to generate power to its electric motor propulsion.[1] They regret the endeavor owing to cost of operation.[2] [1]https://www.ge.com/gas-power/industries/cruise-lines https://www.ge.com/gas-power/industries/cruise-lines [2]https://www.airliners.net/forum/viewtopic.php?t=1202425 https://www.airliners.net/forum/viewtopic.php?t=1202425

KennyBlanken 3y ago

The bit about the naval use and efficiency isn't quite accurate. The issue isn't that they're not efficient. It's that they are only efficient at high power level, and the minimum power level they're efficient at (and even their minimum power level, period) - is quite high. To compare: Britain's current aircraft carrier has four diesels that total 40MW...combined those diesels equal one of its two 40MW turbines. This minimum power level is why jet airplanes have an APU, and often taxi with just one engine running, with the second started up with enough time to get up to operating temperature for takeoff. Example: https://en.wikipedia.org/wiki/Rolls-Royce_MT30 https://en.wikipedia.org/wiki/Rolls-Royce_MT30 ~40MW, minimum efficient power level 25MW. 25MW, even if it's very efficient in terms of turning kerosene into shaft power, means the ship is moving really fast, and thus there's enormous fuel consumption and drag.

usrusr 3y ago
I don't think that it's just inefficiency at certain power levels: The big table on https://en.wikipedia.org/wiki/Brake-specific_fuel_consumption https://en.wikipedia.org/wiki/Brake-specific_fuel_consumptio... has the Marine Trent rated less efficient than some car engines. And that table is peak efficiency, not efficiency at some arbitrary inconvenient power level. But you are right in so far as efficiency at the low end of the power range is very important in commercial shipping. Because the energy demand (at the drive shaft) of a given trip is highly dependent on the desired speed and if going show ruins your engine's efficiency any dip in demand will ruin you economically (because others on the market are much better at compensating the dip by "slow steaming")
taneq 3y ago
Then wouldn’t they just have a 5MW turbine and run it at its peak efficiency?

jabl 3y ago

Another reason many navies like turbines is that big diesels produce a lot of low frequency noise which travels very far underwater, so submarines can hear you coming from a very long distance. Although many navies do operate combinations of diesels and gas turbines (CODAG etc.), in such cases the diesels need to be installed on special shock absorbing mountings, in some cases even with a diesel-electric drive to avoid coupling the vibrations of the engine to the hull via the propeller shaft.