The current worldwide electric generation capacity is estimated to be about 20 terawatt hours. Approximately 68% of today’s electrical energy is supplied from fossil fuels: coal (42%), natural gas (21%), oil (5%), nuclear (14%), hydro (15%), and the remaining 3% from renewable energy technologies. Even with aggressive conservation and development of new, efficient technologies, the worldwide electricity demand is predicted to double by the middle of the century and triple by the end of the century. Electricity is the dominant form of energy used (e.g., 40% of all energy consumption in the United States by 2002), and the demand for electricity is increasing at a faster pace than overall energy consumption. At the same time, oil and natural gas production is predicted to peak over the next few decades. Coal has been the dominant source of electricity generation in the world; abundant coal reserves may maintain current consumption levels longer than oil and gas. However, every kWh of electricity generated by burning coal coproduces an average 1000 g lifecycle CO2 emission, a greenhouse gas that is widely considered as the primary contributor to global warming.3,4 In the United States alone, coal power plants emit 1.5 billion tons of CO2 per year, and emissions from developing countries are accelerating.

To reduce greenhouse gas emissions, many countries are adopting emission regulations (i.e., cap-and-trade or variants) and carbon “trading,” which benefits industries with a small “carbon footprint” and requires those producing higher emissions to purchase carbon “allowances.” The environmental concerns over the use of fossil fuels and their resource constraints, combined with energy security concerns, have spurred great interest in generating electric energy from renewable sources. Solar and wind energy are among the most abundant and potentially readily available.3,5,6 The solar radiation energy the Earth receives in 1 h is enough to meet worldwide energy requirements for a year.

Capturing a small percentage of potential wind energy could also contribute significantly to meeting the world’s electrical energy requirements. While advances in technology are still needed to harvest renewable energy economically, solar and wind power technologies have grown quickly. Globally, the total electricity from installed wind power reached 74.3 gigawatts (GW) in 2006 and 94 GW in 2007.7 The World Energy Council estimates that new wind capacity worldwide will total up to 474 GW by 2020. The output from photovoltaic (PV) module installations is currently growing at 40% per year worldwide.5 The United States targets 100 GW solar power by 2020.

However, solar and wind are not constant and reliable sources of power. The variable nature of these renewable sources causes significant challenges for the electric grid operators because other power plants (usually fossil fueled power plants) need to compensate for the variability. For example, as shown in Figure 1a, wind power profiles in Tehachapi, California, vary over minutes, hours, and days while peaking at night when demand is low. During the day, wind power can be a fewGWat some moments and only a few megawatts (MW) and even zero at others. Similarly, in Figure 1b, solar power is generated only during the daytime and varies when clouds pass by.

A further concern is the fact that the renewable resources are localized and are often away from load centers. In the United States, wind sources are concentrated in the midwest regions, and solar sources in southwest regions. To smooth out the intermittency of renewable energy production, low-cost electrical energy storage (EES) will become necessary. EES has been considered as a key enabler of the smart grid or future grid, which is expected to integrate a significant amount of renewable energy resources while providing fuel (i.e., electricity) to hybrid and electrical vehicles,8 although the cost of implementing EES is of great concern.

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