Renewables, such as wind and solar, generate electricity intermittently, where it’s impossible to predict when the wind will stop blowing or the sun will be clouded over.
The need for costly back-up power is well documented.
Not so obvious is the impact that intermittency has on existing coal-fired and natural gas combined cycle (NGCC) power plants.
NGCC and coal-fired power plants are designed to run at a steady load, except for planned maintenance periods where they are carefully shut down at the beginning of the maintenance period and slowly brought back on line when maintenance is completed.
The advent of renewables has resulted in these plants being cycled, up and down, to accommodate the variables caused by intermittent wind and solar power plants.
This cycling has resulted in less efficient use of the power plants, as well as costly damage to the plants.
This means higher costs and higher prices to the consumer for electricity.
The first casualty of cycling these plants is lower capacity factor. This means that these units are producing less electricity than they could. In other words, efficiently generated electricity is being replaced by inefficiently generated electricity from renewables.
The next casualty is damage to components from thermal cycles. In other words, components that are designed to operate at more or less stable temperatures, after being slowly brought up to operating conditions, are whip sawed between lower, then higher temperatures. These unplanned thermal cycles damage the boilers and turbines.
This damage causes increased spending on preventative and corrective maintenance, which results in higher costs to consumers.
The next casualty is to the pollution control systems. The changes in load result in changes to flu-gas temperatures and pressures that result in less efficient operation of these systems. In addition, the cycling of temperatures also damages this equipment.
Coal-fired power plants receive the most wear and tear from this thermal cycling.
Some examples.
- Headers in boilers are thick, expensive and hard to reach components that have been cracked as the result of thermal cycling.
- The boiler structure that supports firewalls and water-tubes expands and contracts at different rates than the firewalls, etc. This uneven expansion and contraction causes severe damage to the firewalls and tubing.
- Different expansion rates cause damage to the super heater tubes and their supporting structures.
- Boiler water chemistry is altered when boilers are operated under variable temperatures and pressures. Boiler water chemistry is crucial to preventing chemical attack of boiler tubes.
- Steam turbine rotors consist of huge, solid forgings. Gas turbines have buckets mounted on diaphragms keyed to forgings. These rotors, unless designed for thermal cycling, must be brought up to speed slowly so as not to be deformed by changing temperature. The coefficients of expansion between components are also different, which requires steady operating temperatures under load. Clearances are tight and deformation could cause rubbing and damage.
These are merely a few examples of the problems caused by operating coal-fired and NGCC power plants in anything other than steady conditions.
As an aside, I have worked inside the fire-box of marine boilers to point-up the brickwork, and to clean the tubes using a handheld steam lance. For a long while, I had the scar on my left arm from a burn caused by the high temperature of the steam hose.
The public isn’t aware of these problems and higher costs. All they hear about are the so-called “Green” benefits of wind and solar.
In the real world, renewables cause problems, and increase the price of electricity to consumers.
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