Saving Electricity For Later
Renewable energy. Electric cars. Wind power. Solar power.
You don’t have to think very hard about how those ripped-from-the-headlines phrases will affect our lives before you wonder: wouldn’t it be handy to be able to collect electricity to use at another time and place? Like when the wind isn’t blowing or the sun isn’t shining?
But stockpiling electrical energy is a lot harder than storing other energy.
When you go to the gas station to buy liquid fuel for your car, you put the fuel in a hollow tank. You can use some of the energy in the fuel to drive home, then park your car for a day or a week. The hollow tank is a very simple storage device, holding the fuel in its original form until you need it. The next time you drive your car, whether in 10 minutes or 10 hours, the fuel is ready to use immediately.
Electricity is different
If you want to store electricity to use 10 minutes from now, or tonight, or next week, you must convert its energy into something else. Then, when you want to use it, you have to change it back into electric energy.
Small rechargeable batteries, the kind we use in our cell phones and digital cameras, do this easily.
These small batteries are known as reversible electrochemical batteries. Electricity from the utility power grid enters the battery during the charging process. That incoming electricity causes temporary changes within the chemicals and other materials inside the battery. Later, when you use your cell phone or other battery-powered device, reactions among the chemicals inside the battery generate electricity for your device. As you use your device, the battery discharges, and eventually you have to add more electricity during the recharging process. These kinds of batteries can be charged, used, and charged again frequently, and can last a few years.
These small energy storage devices work just fine for the tiny amounts of electricity needed to operate a cell phone or a radio.
But what if you want to store enough electric power for all the houses in a subdivision, or for all the electric customers in an entire county?
Electric utility companies need to do that a lot to help manage the supply of electricity available to their customers. But instead of tiny batteries, electric utilities use a variety of much larger energy storage systems. Some involve batteries, and some use other methods.
Seeking a longer storage life
“Energy storage devices have always been useful as a way to deal with technical issues within certain parts of the transmission and distribution system,” says Bob Gibson, a senior program manager at the Cooperative Research Network, part of the National Rural Electric Cooperative Association. These kinds of short-term storage uses can be used to manage the flow of power for seconds or minutes while adjustments are made. Being able to quickly obtain electricity from a short-term storage system can help prevent brownouts (a voltage drop that can dim the lights or, worse, damage equipment). Energy storage systems also play a role in preventing many short-term outages that could black out power to an entire area. Energy from a storage system may provide enough electricity for a few minutes or a quarter of an hour while problems at normal generating sources are corrected.
Storing energy and then withdrawing it during a longer portion of a 24-hour time period has also been important for many years in certain areas.
Gibson says, “Many electric co-ops will soon need to use energy storage devices on a regular basis to support a section of the grid that is close to being overloaded.” As the number of customers in a geographic area increases, the amount of electricity available locally may not be enough to supply everyone with the power they need at all times. In more and more sections of the utility grid, the demand for electricity is greater than the amount of electricity that nearby generating plants can provide at certain times.
Installing an energy storage system could help provide extra electricity where and when it’s needed. Energy storage systems may be easier and cheaper to build and install than increasing the output at a generating plant, or building an entirely new one.
And in many parts of America, we’re adding generating plants that don’t perform in the same steady way that traditional generating methods, such as coal or natural gas, do. Energy from the sun isn’t available during the night, leaving a gap of many hours when electricity is not generated. Energy from wind often occurs during the nighttime hours, but not during the day when more people need electricity. That means utilities will need more energy storage systems to manage the quality and supply of power from these renewable but unsteady resources.
Many different kinds of energy storage systems can be used in the electric utility grid. Each system has advantages and disadvantages.
Utility company managers ask five basic questions when they consider the benefits of an energy storage device:
• How much energy can it store?
• How long can it store the energy?
• How quickly can the energy go in and come out (charge and discharge)?
• How many times can this cycle be repeated?
• Does the size of the device match the space available?
The answer to that last question is often a deal-breaker. Two good examples involve using large amounts of water or air.
Pumped hydro storage systems, which have been used since the 1930s, take advantage of gravity. Electric pumps move water to a high location in a reservoir at night, when demand for electricity is lower. During the day, when demand for electricity increases, the water is released. The force of the falling water is used to spin turbines that generate electricity.
About 150 pumped hydro systems scattered throughout 19 states consistently help generate electricity.
The Tennessee Valley Authority, which supplies electricity to several Kentucky counties, completed a pumped hydro storage facility at Raccoon Mountain in southeast Tennessee in 1978. When water in the upper reservoir is released, it can generate 1,600 megawatts of electricity for 22 hours.
Compressing electricity underground
But pumped hydro has a big disadvantage—it takes a lot of physical space to store the water both at the top and at the bottom of the system. The reservoir at TVA’s Raccoon Mountain covers 528 acres. Also, the top and bottom must be far enough apart for the falling water to have enough force to spin the turbines. This kind of system isn’t practical in flat or crowded areas.
The other option that requires a lot of space is a compressed air energy storage system (CAES). In this case, electric pumps force air into a confined space (such as an underground cavern) so the air is under pressure. When the pressurized air is released and heated, it can be used in turbine systems to generate electricity.
Although the idea for CAES was first discussed seriously during the 1960s and 1970s, so far there are only two in use in the world, one in Germany and the other in McIntosh, Alabama. The Alabama Electric Cooperative completed its CAES system in 1991. Using low-cost electricity to operate pumps during the nighttime hours and over weekends, air is compressed and stored in a 19 million cubic foot underground cavern. During the peak demand times for electricity, the compressed air is released, heated, and used in a turbine that generates electricity. The Alabama storage system can produce 110 megawatts of electricity and supply power to the grid for up to 26 hours.
Today, a different CAES system is being designed for use in the Des Moines, Iowa, area. Researchers and engineers at Sandia National Laboratory in Albuquerque, New Mexico, who are participating in this new project hope the Iowa system will be operating by 2012. Demonstration projects like this are important, but finding suitable underground storage areas will limit the places where CAES systems could be installed.
Storage options
A flywheel energy system is another way to store electricity. Electricity from the grid coming into the device makes a flywheel spin at very high speeds. When power stops coming into the flywheel mechanism, the flywheel continues to spin; this rotation can then be used to generate electricity that can go back into the grid. The advantages of flywheel systems include the ability to cycle on and off very quickly, operate a very long time without needing much maintenance, and last for years.
Flywheel energy systems have been used in the New York subway system for many years to help regulate the flow of electricity to the trains. And they can be a valuable tool in small sections of the utility power grid.
However, an individual flywheel can’t store very much energy, limiting their use so far.
Beacon Power, an independent engineering, assembly, testing, and integration company based in Massachusetts, has an established history with individual flywheels. Beacon engineers have developed improved flywheels and launched a new commercial pilot program within the New England region of the nation’s power grid. Many high-tech flywheels will be linked together into one system. Engineers expect that the new flywheel system can respond to a request to send power to the grid within seconds and help even out power fluctuations on a daily basis.
The Electric Power Research Institute (EPRI), a utility industry group with 700 members, constantly evaluates new choices for electric energy storage. Dr. Robert B. Schainker, senior technical executive at EPRI’s Palo Alto, California, office, says, “When you look at energy storage systems, there are engineering trade-offs in each case. There is no one ‘right’ answer because each option has its pros and cons. You need to find the right storage method for each application.”
Some energy storage systems will work best in only a few situations. The energy storage systems that are most likely to become common additions to the nation’s electric grid are variations on old familiar devices such as batteries and capacitors.
A QUICK GUIDE TO ELECTRICITY STORAGE TECHNIQUES
Reversible electrochemical batteries, like those in cell phones, use chemical reactions to take electricity from the utility power supply then release it for use as necessary.
Pumped hydro storage moves water to a reservoir high above the ground during those times of day when electricity use is at a minimum, then uses gravity during peak electric use to release water and turn an electric generating turbine.
Compressed air storage systems use electric pumps to force air into a confined space, such as an underground cavern, so the air is under pressure. When the pressurized air is released and heated, it can be used in turbines to generate electricity.
Flywheel energy systems use electricity from the utility grid to spin a flywheel. When power stops, the flywheel continues to spin, and that rotation can be used to generate electricity.
Next month: Energy storage use and research in Kentucky.
