Renewables’ success seems to depend on whether the energy produced can be stored for periods when renewables are unavailable. While there are many candidate techniques for this I hear of no research into it as a topic, and your excellent column doesn’t mention anything about them. — Ian P., Bristol, EnglandIan, you raised one of the most important issues relating to energy efficiency and renewables — storage. We have a portfolio of storage technologies, some commercialized, some emerging, and others as lofty visions. My top picks are below. Pumped Storage — According to the California Energy Commission, “Conventional hydropower offers the potential for low-cost baseload electricity, but their output is dependent on the time of year as well as annual precipitation. By contrast, pumped storage methods are typically used to provide power during peak demand periods on very short notice and are not dependent solely on runoff. In a pumped storage facility, water is pumped during off-peak demand periods from a reservoir at a lower elevation for storage in a reservoir at a higher elevation. Electricity is then generated during peak demand periods by releasing the pumped water from the higher reservoir and allowing it to flow downhill through the hydraulic turbine(s) connected to generators. During the off-peak pumping cycle, the pumped storage facility is a consumer of electricity: in fact, the amount of electricity required to pump the water uphill is greater than the amount of electricity that is generated when the water is released during peak demand periods. Pumped storage facilities, however, are economical because they consume low-cost off-peak electricity, but generate high-value on-peak electricity.” http://www.energy.ca.gov/electricity/hydro.html Advanced Batteries — According to Advanced Battery and Technology News (September 2006) noted, “demand for advanced battery materials have reached $3.4 billion per year.” Cellular, computers, and electric vehicles are driving the domestic markets, while NASA, Defense and Homeland Security are driving the government markets. Cobysis in Ohio has built a new advanced battery plant and GridPoint in Washington, DC, is producing a 3.6 kW battery appliance for solar and wind systems, the first such device in the market. http://7ms.com/abt/index.shtml Compressed Air — According to Argonne National Labs, “Compressed Air Energy Storage (CAES) in Salt Caverns or mines have been used to store air under high pressure. Compressors use off-peak electricity to fill the cavern with compressed air. For peaking demand, the compressed air is withdrawn from the cavern, blended with natural gas, and used to drive a gas turbine to generate electricity. CAES Plants of 110 – 290 megawatts (MW) exist today. http://web.ead.anl.gov/saltcaverns/uses/compair/index.htm Thermal Salts — According to Sandia National Labs, solar thermal energy can be stored in molten salt. The molten salt is a mixture of 60 percent sodium nitrate and 40 percent potassium-nitrate, commonly called saltpeter. The salt melts at 430 degrees F and is kept liquid at 550 degrees F in an insulated cold storage tank. The salt is then pumped to the top of the tower, where concentrated sunlight heats it in a receiver to 1050 degrees F. The receiver is a series of thin-walled stainless steel tubes. The heated salt then flows back down to a second insulated hot storage tank. The size of this tank depends on the requirements of the utility; tanks can be designed with enough capacity to power a turbine from two to twelve hours. When electricity is needed from the plant, the hot salt is pumped to a conventional steam-generating system to produce superheated steam for a turbine/generator. The uniqueness of this solar system is in de-coupling the collection of solar energy from producing power; electricity can be generated in periods of inclement weather or even at night using the stored thermal energy in the hot salt tank. The tanks are well insulated and can store energy for up to a week. As an example of their size, tanks that provide enough thermal storage to power a 100-megawatt turbine for four hours would be about 30 feet tall and 80 feet in diameter. Studies show that the two-tank storage system could have an annual efficiency of about 99 percent, and could also be integrated with more commercial solar trough systems. http://www.sandia.gov/Renewable_Energy/solarthermal/NSTTF/salt.htm Fly Wheels — According to the Electricity Storage Association, “Most modern flywheel energy storage systems consist of a massive rotating cylinder (comprised of a rim attached to a shaft), which is substantially supported on a stator by magnetically levitated bearings that eliminate bearing wear and increase system life. To maintain efficiency, the flywheel system is operated in a low-vacuum environment to reduce drag. The flywheel is connected to a motor/generator mounted onto the stator that, through some power electronics, interact with the utility grid. Some of the key features of flywheels are little maintenance, long life (20 years or tens of thousands of deep cycles) and environmentally inert material. Flywheels can bridge the gap between short term ride-through and long-term storage with excellent cyclic and load following characteristics.” On June 29 , 2006, the California Energy Commission began Final-Stage Field Trial Testing of Beacon Power Flywheel Demonstration System. http://www.beaconpower.com Hydrogen – U.S. DOE states, “In the mid- to long-term, the hydrogen production technologies currently under development — renewables via electrolysis, direct renewables (photobiological, photoelectrochemical, etc.) — will become more cost effective and contribute to a diversification of domestic hydrogen production. One of hydrogen’s great strengths is its ability to be produced from a wide variety of resources. Each region may use a different combination of resources to produce hydrogen. Ovonics of Michigan stores hydrogen in solid nickel metal hydrides that can be used as electric storage as batteries, or the hydrogen can be disengaged to be used directly. The National Hydrogen Association (NHA) is the hydrogen trade organization led by over 100 companies dedicated to supporting the transition to hydrogen since 1989. http://www.HydrogenAssociation.org “Smart Controls” or “The Smart Grid” — According to Columbia University, “By 2050, it will take between 15 and 20 Terawatts (TW) of electric power to supply the North American economy. A little under 7 TW is currently used, with most of that consumed in the United States. The “Smart Electric Grid of the Future” must be able to efficiently and securely deliver this two- to three-fold-increase in power to all corners of the continent, in addition to being invulnerable to security breaches, attacks, natural disasters, and mechanical failures. The country can ill afford more blackouts like August 14, 2003.” In the immediate future, vast new renewable energy sources from wind, solar, and geothermal power generation must be added to gas, coal, hydroelectric and nuclear sources of the present. The new “Smart Electric Grid” must improve efficiency by 50% or more in order for this power technology revolution to be affordable. In addition, it must be far more sophisticated from a computerized control standpoint in order to deal with unpredictable and time-varying green power sources such as giant wind and solar farms located thousands of miles from metropolitan users. Distributed generation and local power storage at consumer and manufacturing sites must be designed and tested to further fortify grid stability and safety from terrorism, as well as better defend it from the usual weather and mechanical outages. And innovations such as ‘smart’ electric meters that can talk to storage and distributed generation devices, even appliances and industrial motors, are all in the offing. The electric grid can become a kind of virtual storage unit ‘in and of’ itself, as it now does so, albeit crudely, today. http://www.electricitystorage.org and http://www.eia.doe.gov/kids I could go on and on about energy storage… — Scott Sklar Scott Sklar is President of The Stella Group in Washington, DC, a distributed energy marketing and policy firm. Scott, co-author of “A Consumer Guide to Solar Energy,” uses solar technologies for heating and power at his home in Virginia. Have a question? Please contact Scott regarding new products, technologies or experiences for future Q&A columns.