30The main problem with either wind or solar is that they generate electricity erratically, depending on the wind or sunshine. In contrast, a fossil-fuel plant can generate electricity predictably upon request. Blackouts are very expensive for society, so grid operators and designers go to a lot of trouble to make sure that blackouts are rare. The electrical grid should have spare capacity sufficient to meet the largest demand peaks even when some plants are out of commission. Plants in spinning reserve status stand by ready to take over if a plant trips (breaks down). Injecting erratic electricity into the grid means that other plants have to seesaw output to balance the ups and downs of wind or solar.
Adding wind or solar to a grid does not mean that existing fossil fuel plants can be retired. Often, neither wind nor solar is working and at those times a full complement of fossil fuel plants, or sometimes nuclear or hydro plants, must be available. Both wind and solar have pronounced seasonality. During low output times, as for summer wind, the fossil-fuel plants are carrying more of the load. Of course, solar stops working as the sun sets.
Wind behaves erratically hour to hour. Even though the Texas 18,000-megawatt system has thousands of turbines spread over a wide area, the net output is erratic changing by thousands of megawatts in a single hour. These shifts must be balanced by fossil-fuel plants slewing their output up and down to compensate and keep load matched to generation.
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Even very sunny southwest cities have 50 or more cloudy days per year, stopping or reducing solar generation. Wind turbines are very sensitive to wind speed. A 10% change in wind speed will change power output by 30%, amplifying the erratic nature of wind.
The big picture is that when wind or solar is added to a grid it is supplemental power. No coal or gas plants are eliminated. Those plants have to stay in place to handle periods when wind and solar are not producing electricity. This does not stop claims that wind or solar is replacing fossil fuel, but it is fuel that is replaced, not fossil-fuel plants. When wind or solar is producing, the fossil fuel plants are throttled back and they use less fuel. If, for example, a coal plant was closed when wind was added to the grid, the safety margin would be compromised.
