One of the biggest technical challenges to a smart grid and cleaner electricity system is affordable storage. With energy storage, grid operators can avoid using natural gas power plants to maintain a steady frequency or avoid putting generators online to meet peak-power demands. Ultimately, the hope is that bulk storage will allow solar and wind farm operators to provide electricity to the grid--regardless of wind and sun conditions--making the electricity they provide more valuable.
That's why many companies, researchers, and the Department of Energy are investing in new energy storage technologies. Here is a battery system from Xtreme Power, which is supplying batteries to smooth out the flow of electricity to the grid from a wind farm in Hawaii and other places. The system in Hawaii will be capable of delivering 30 megawatts of power and have 10 megawatt-hours of storage--enough to supply power to thousands of homes.
Ohio-based utility AEP is one of the most advanced in using storage to buffer energy on the grid. Here, a sulfur-sodium battery from NGK Insulators is installed at a substation. It provides back-up power and holds enough energy to supply the local grid during peak times. Each megawatt battery can hold about 7.2 megawatt-hours.
Local storage is being tried out in a number of DOE-funded community storage projects. Like storage at a substation, these local units are available to supply electricity in the case of an outage, maintain voltage, and level the load at the substation. The idea is to link many of these small, relatively inexpensive units so that they can provide megawatts' worth of energy for a few hours. Here are AEP's "backyard storage" units being tested in Ohio.
Inside an Altairnano grid storage unit, which fits inside a truck trailer. Each tractor-size storage system provides 1 megawatt's worth of power for about 15 minutes. The company, like competitor A123 Systems, uses lithium ion batteries, which are well suited for delivering or receiving bursts of power quickly.
Another technology now being used on the grid is flywheels, which store electric energy from the grid as kinetic energy in spinning discs. Flywheels cannot store a lot of energy, but they can quickly deliver bursts on and off the grid to maintain a steady frequency and they don't take up a lot of space. In another DOE-backed project, this technology is being used in the New York grid for 20 megawatts' worth of power using 200 flywheels. Here is a picture of the flywheels being delivered before installation.
Another battery technology is called a flow battery, where liquid electrolytes from one chamber flow to another to cause a chemical reaction for either storing or discharging electricity. ZBB Energy is using its zinc-bromide flow batteries, pictured here, to reduce congestion at a substation. The system will be able to supply 2 megawatts of power and store 2 megawatt-hours of energy. The idea is to have a mobile unit so that it can be moved to where peak loads are happening.
When it comes to bulk storage, the method to beat on price is pumped hydro. A pumped hydro station moves water from a reservoir uphill during off-peak times and then releases the water through a generator during peak times. By generating electricity at peak times, the electricity is more valuable. The obvious challenge with pumped hydro stations, like this one operated by Pacific Gas & Electric, is finding suitable locations.
Another storage method is compressed air energy storage (CAES), which can store large amounts of energy in underground caverns, overcoming one of the biggest shortcomings of today's batteries. There are a handful of projects to couple compressed air storage with wind turbines. By coupling renewable energy with storage, renewable systems become "dispatchable," which means they can supply power whenever needed. Many people think that bulk storage is needed for renewable energy to become a large contributor to power generation in the U.S.
Ice is a form of thermal storage, which can help lower the load on the grid during peak times. This is an air conditioner from Ice Energy targeted at commercial and utility customers. At night, it freezes water to make ice and, during the day, it draws on ice to help power the air conditioner. Southern California Public Power Authority signed up to offset enough peak-time electricity to power 10,000 homes.
One of the most promising methods for storing many hours of solar energy is molten salt storage. Here is a diagram of a system developed by SolarReserve, which plans to deploy a 100-megawatt solar thermal plant in Nevada.
One novel technology being pursued is compressed air storage in tanks. SustainX, a company spun out of Dartmouth College, received a DOE grant to test out compressed air storage for utility-scale storage, which it is now doing with energy developer AES. The ultimate goal is to use compressed air storage to displace "peaker plants," which are natural gas plants that go online to meet electricity demand during peak times.