Quote:
Originally Posted by jamesqf
If diesels are more efficient than gas turbines, then why are there many gas turbine-powered generating plants, while diesels are only a niche application, mostly for backup & emergency power? And why are there so few diesel-engined airplanes?
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For aircraft: Power to weight. A piston engine of the same output as a large turbine would be so heavy the aircraft won't fly (imagine a 747 with 10 piston engines, as one example, or look at the complexity of the engines that were used on the Bristol Brabazon).
For power stations: Exhaust temperature. Turbine exhaust is hot enough to drive steam plant (cogeneration) and the combined efficiency brings the relatively poor efficiency of the turbine past that of a similar diesel. Unlike other space-constrained applications it's perfectly feasible to run a 1+N solution so that turbines can be down for maintanance without affecting overall output.
(Open cycle turbine plants exist, they're expensive to run and generators tend to keep their older diesels maintained for this kind of peaking load as a result. These plants are getting more and more workouts due to renewables intermittency and this in turn is driving up power prices - it's one of the hidden costs of renewables that noone wants to talk about)
Diesel (or CNG) piston engines don't have hot enough exhaust temperatures to drive steam turbine generation efficiently, so you don't get DGCT plants.
For ships: Piston engines win on efficiency every the time. You usually only see turbines on military vessels where rapid response time and compact size are bigger advantages than fuel consumption. Theoretically on an electric-drive vessel one could run a CCGT generation plant but this doesn't happen
For rail locomotives: gas turbines were tried and failed. They were useful to haul loads up the west side of the Rocky Mountains but noise was a killer and heavy consumption meant the locos were deadweight when running down the plains of the midwest (diesels were used for the downhill runs). Hauling up the Midwest plains was viable but noise complaints were a major issue.
The american power grid systems (note plural) are not interconnected or robust enough to handle longhaul electric rail systems - which have a nasty tendency to go from drawing 4-5MW to generating 3-4MW as trains go over hills. That's bad enough that in some countries (New Zealand being one example), the power distribution companies PAY rail companies to not run electric locomotives. European rail systems are dense enough and interconnected enough that a train generating power in one segment can supply a train in another segment without stressing the intermediate power distribution systems (this is more or less the same issue that intermittent renewables plants cause on grids, only on a much faster timeframe)
For vehicles: Turbines have been proposed on a few high performance hybrids. These are NOT being pushed for fuel efficiency, but for power-to-weight and powerplant size reasons.
For the actual power requirements and fuel efficiency required on vehicles piston engines are the right choice if you need ICE. Truckmakers have decades of experience at "rightsizing" the engine to the load and whilst electric transmission is a good idea for boosting on hills or stop-start urban delivery work, a longhaul rig has more-or-less constant load which means that direct coupling an appropriately sized engine directly to the wheels is the most efficient overall design. It may be that a smaller donkey engine setup is the best way of boosting or providing electric drive for noise-sensitive end-point operation but (as with a hybrid setup and batteries) that extra mass cuts into carrying load plus overall efficiency and therefore it makes more sense to setup your distribution depots so that the longhaul work is point-to-point with (hybrid) delivery vehicles from there - funnily enough, that's exactly what tends to happen.
The thing about longhaul transport is that trucks have lifespans of several million miles and in many cases the engines are never allowed to cool down between servicing (not running == not earning). Providing "free fuel" for 150,000 miles isn't a big incentive over having an overall more-efficient solution - and that overall solution includes appropriate location of your endpoints.
Yes, the hybrid is cool - but unless it makes economic sense a haulier will never buy it. The technology is more likely to make economic sense in the 2-12 ton delivery vehicle market than in the 30-50 ton hauling one - and in such markets a turbine would be far too noisy to tolerate.
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