Making Renewable Energy Practical: Can We Reliably, Efficiently, and Economically Store Energy to Make Solar and Wind Power Viable Options to Replace Fossil-Fuel or Nuclear Plants?

Article excerpt

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Few things get our attention more quickly than a loss of electric power. Many of our activities cease the instant energy from a generator many miles away stops supplying the electricity to light our homes or businesses, run our computers, lift our elevators, operate the industrial machinery on which our food supply depends, and countless other laborsaving tasks. While just a slap in the face at first, after a few hours, as food begins to spoil, sewage begins to accumulate, and night closes in, the inconvenience moves toward the potentially dangerous. Why can't our utility companies do something to prevent outages?

Unlike our water supply, which is often stored in elevated tanks that can supply us for hours or days, or our food supply in the pantry, electricity is used at the moment it is generated. But why can't large amounts of electricity be stored in batteries or through other means? And why couldn't this stored energy be used to make practical the large-scale use of wind or solar power, which don't generate energy when the wind is not blowing or the sun is not shining?

Batteries

A power plant generates alternating current (AC) where the plus and minus poles change 60 times per second--the reason behind the 60 Hz you see on all appliances. Batteries are direct current (DC) devices that produce energy continuously between the positive and negative poles with no alternation. Providing DC to an AC motor--everything from your air conditioner to an electric razor--would provide lots of smoke, but no rotation. To store AC energy in batteries, it must first be "rectified" to DC. Then when AC is required, the DC is converted to AC using an "inverter" that chops up the electric current, polarizes it, and reconstructs it into the semblance of an AC sine waveform.

Second, the number of batteries needed to provide storage for the output of a typical 1,000 megawatt (MW) power plant--one million kilowatt hours (kWh) each hour--would be incredible. A typical 12-volt automotive battery is rated at 70 amp-hours, which means it should deliver 70 amps at 12 volts for an hour. Watt-wise, this is equal to 840 Wh or 0.84 kWh. A simple calculation shows it would take 1,200,000 automotive batteries to store the power required to replace an hour of electrical production by the 1,000 MW plant. Of course, to have a reasonable lead-acid battery life, the batteries should never be discharged over 50 percent. Then there are the conversion losses from rectifying AC power to DC power and back again. Taking this into account, storing an hour's production from a 1,000 MW plant would require about 3,750,000 batteries, containing 75,000,000 pounds of lead. On three-foot centers, this would require a field of batteries covering more than a square mile--about 774 acres. And remember, this is for one hour's worth of electrical energy from a typical 1,000 MW nuclear or coal-fired power plant.

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But why talk about storing power from conventional power plants to alleviate once-in-a-blue-moon power outages? Of more practical interest would be to determine the requirements to store energy from a wind or solar farm so that we could eliminate a conventional power plant altogether. Because wind and solar always need to be backed up against outages--owing to windless days of high pressure or clouds and precipitation--we assume that we need at least a two-day storage period. To use batteries to store enough wind power to generate something approaching reliable wind power equivalent to a 1,000 MW power plant would require 180,000,000 batteries covering about 58 square miles. This is in addition to approximately 2,000 well-sited wind turbines situated on something over 490 square miles. Keep in mind that these are not the windmills used by Farmer Jones to pump water for his cattle, but 1.5 MW behemoths with blades reaching up over 30 stories. The capacity factors are generously assumed to be 30-35 percent. …