One of the glaring flaws (far more than a mere foible) of "renewables" (wind and PV) is that they are unreliable. SO unreliable, as a matter of fact, that they force the adoption of much dirtier fossil-fired generators to accommodate their output swings.
Naive greenies think that "RE" can just be thrown onto the grid, but in fact an RE-heavy grid requires different generating technologies than one with little or none. You can generally follow the normal load curve using a CCGT plant, which can be up to 64% efficient (LHV). Following the bumpiness of "renewables" mostly requires simple-cycle gas turbines (the CCGT steam systems don't like rapid power variations); the best open-cycle I've read about gets only 46% efficiency, and I recall that the single-shaft industrial models often get something like 38%. IOW, you're burning a lot more fuel for the same electric output. This puts you way behind emissions-wise.
Let's use a real-world example: the Mitsubishi-Hitachi M501JAC gas turbine,
which is available in both simple-cycle and combined-cycle versions. This allows a head-to-head comparison. The single-unit combined-cycle version of the M501JAC is rated at 614 MW and 64.0% LHV efficiency. It doesn't even HAVE a specified turndown ratio, minimum rated output, rated ramp rate or startup time. One can conclude from this that it really isn't suitable for trying to follow the ups and downs of "renewables", though it can probably handle normal load curves because other steam-turbine plants have been doing it for the last century.
The simple-cycle heat rate of this unit is 7775 kJ/kWh (LHV). Since a kilowatt-hour is 3600 kJ, we just divide that by 7775 to get 0.463, or 46.3%. The rated output is 425 MW and the rated ramp rate is 42 MW/minute, or about 10% per minute; it can be turned down to 50% of rated output, so it can go from minimum to full output in 5 minutes. This can track things like surges and sags from passing clouds and weather fronts pretty well. Its startup time is specified as 30 minutes.
What you pay for this flexibility is efficiency. Going from 64.0% down to 46.3% means burning 38% more fuel. Put another way, you need to get 27.6% of your juice from emissions-free sources just to break even on the increased emissions from going from combined-cycle to simple-cycle... and that assumes that you maintain the 46.3% efficiency at lower output power, which you won't. GE makes this data very hard to find, but the efficiency of the LMS100 gas turbine drops from 44.3% at rated power down to under 40% at half rated power (the minimum). This means even MORE fuel required.
Typical capacity factors for wind are 30-40%; PV is much lower. If you're getting 30% of your juice from "renewables", and you're burning at least 38% more fuel per kWh to get the rest, you're saving less than 3.3% from the CCGT emissions figure. At low enough capacity factors, you can actually burn more
fuel with the addition of "renewables" than what you could do with all-fossil.
Is it worth spending so much money for such paltry gains? Even if your wallet can stand it, can the planet?
Now, don't let it be said that there aren't ways around this. With enough excess RE capacity you can just brute-force the issue by dumping excess power to resistance heaters in a CCGT's gas turbines, substituting electricity for fossil fuel and managing the rapid power swings on the demand side. But this is going to hit the economics, and nobody even seems to be thinking that far out of the box.
Labels: renewable energy