Low-cost, grid-scale energy storage is often described as the holy grail of the renewable energy industry.
Wind and solar resources have come down dramatically in price in recent years, and installations have increased significantly, but the intermittency of the two energy resources remains a challenge for grid operators. So Utilities are ramping up testing storage technologies that can help balance and capture wind and solar energy when it’s not needed and save it for times of high demand. In 2013, California passed a law calling for 1.3 gigawatts of storage capacity from its largest utilities by 2020.
But not all storage and renewable pairings make sense when evaluating the energetic cost of the full lifecycle of the technologies, according to new study from Stanford University.
"Whenever you build a new technology, you have to invest a large amount of energy up front,” Michael Dale, a research associate at Stanford and lead author of the study, said in a statement. "Studies show that wind turbines and solar photovoltaic installations now produce more energy than they consume. The question is, how much additional grid-scale storage can the wind and solar industries afford and still remain net energy providers to the electrical grid?"
The researchers found that given full lifetime energy costs, which include the manufacturing of each technology, wind produces more energy than it consumes and could be paired with grid-scale energy storage in the form of batteries, pumped hydro, or compressed air capable of holding the energy for up to three days.
The study appeared in the 19 March online edition of Energy & Environmental Science.
"We found that onshore wind backed by three days of geologic storage can support annual growth rates of 100 percent—in other words, double in size each year—and still maintain an energy surplus," Dale said.
Solar, on the other hand, did not fare as well in terms of its lifetime energetic costs when paired with different forms of three-day grid storage. Given the lifetime energy needs of solar photovoltaic (PV) technology, there is only an energy surplus for up to 24 hours of storage in the form of a mix of batteries and pumped hydro, the researchers said. As the solar PV industry continues to reduce its energy intensity, and the energy intensity of storage technologies improves, the picture could change and become more similar to wind's.
But, the question of how best to pair renewable energy sources and and storage technology depends on the frame of the inquiry. Another study from Stanford last year found that for wind power, there is no battery technology that is good enough to deliver an energy return on investment that’s better than just curtailing wind energy when it's not needed. The 2013 study found that for solar power, the energy return on investment is always better if the energy is stored using any technology compared to curtailing it.
Some of the factors in evaluating the value of storage for different grid requirements are the length of storage and the development of new technologies. Pumped hydro and compressed air are often the best combination in theory because they are less than a tenth as energy intensive as battery technologies, according to researchers.
The two technologies, however, are limited by their geology; energy resources must be close to the water source or caverns for air storage. But batteries can be placed anywhere. (The study only examined lithium-ion, sodium-sulfur, zinc-bromine, and vanadium-redox batteries and did not look at other emerging technologies such as lithium air or liquid air.)
Also, a combination of storage technologies will be needed if large amounts of renewables are brought on the grid. Many utilities are both examining fast-responding storage, such as batteries and flywheels that can balance the grid, and looking at more long-term storage solutions to hang on to energy to for later use.
The most recent research from Stanford did not calculate the energy cost of replacing batteries every few years or the cost of installing grid-scale storage systems.
The study also did not take into account the comparison of pairing storage with renewables versus building new fossil-fuel peaking plants. And it did not investigate other grid services that storage can provide, such as frequency regulation, which could allow fossil generators to focus on operating more efficiently.
"People often ask, is storage a good or bad solution for intermittent renewable energy?" study co-author Sally Benson, a professor of energy resources engineering and director of the Global Climate and Energy Project at Stanford. "That question turns out to be way too simplistic.”