15 Mar 2016
A new paradigm is needed for electricity storage in the wind energy market. Today, what passes for “grid storage” are technologies providing, at most, eight to 12 hours of discharge time. If electricity generation is to be successfully weaned from fossil fuels, storage must have more than 1,000 hours of discharge time to back up renewables when they are intermittent. Sometimes the wind does not blow and the sun does not shine for long periods of time. Long discharge time provides reliability.
The good news is the low-cost storage technologies required to accomplish this transformation are being developed.
Strong need for storage
It’s clear that renewable energy from wind farms, as well as solar energy installations, remains intermittent and frustratingly dependent on fossil fuels. Because of the variable nature of the energy output, a generation plant must be available to be turned up or down. In general, natural gas-fired turbine generator plants accomplish this task for wind farms.
The renewable energy industry faces a dirty dilemma: behind every wind farm sits a fossil fuel plant to provide backup power when the skies are still. More wind farms mean more fossil fuel plants with more carbon emissions. Building more wind farms will not lead to full energy generation from wind.
To integrate this intermittent energy source, dispatchable plants of the same output must be ready to take over whenever wind production falls, as it does some 80 percent of the time, to keep the amount of generation constant. Some of the energy harvested by spinning wind turbines must be stored so it can be used when the wind isn’t blowing. That way, a wind farm becomes a dispatchable plant controlled just like a fossil fuel plant.
State of storage technology
To understand where we need to go, it’s important to look at how far we’ve come. What types of storage technologies are available or under consideration today and what are some of their attributes?
Lead acid is a tried and true technology, and there have been developments to improve the cycle life. Lithium-ion, a newer technology, provides higher power density and the cycle life is longer compared to lead acid. These technologies would need a cost reduction of 98 percent to economically store 1,000 hours of output.
Flow batteries have a different construction. An electrolyte liquid flows from tanks through a casing containing the electrodes. Because the electrolyte is separate from the electrode case, flow batteries can be significantly cheaper in cost. The cost still must be reduced by 95 percent to economically store 1,000 hours.
Metal air batteries involve the oxidation of metals with the oxygen from air. Using inexpensive metals, they can be cheaper as well but have limitations in their cycle life. The cost must come down 80 percent to economically store 1,000 hours.
Some other storage technologies are in the mix. First, flywheels are very quick to respond and are good for frequency and voltage control. Although high in power, they are low in energy and not suitable for long duration discharge.
Pumped storage is the only significant storage technology in use today. The idea is to pump water uphill when electricity is inexpensive and to harness gravitational energy by releasing the water back down through a hydroelectric turbine when power is needed. New pumped storage facilities are expensive to build and difficult to permit.
Another technology, Compressed Air Energy Storage, or CAES, consists of a compressor that pumps air into a pressure chamber. When electricity is needed, the air is heated and expanded via an air turbine generator. The lowest-cost CAES utilizes underground caverns. However, good caverns are in short supply.
Hydrogen on the horizon
In addition to the aforementioned storage solutions, a patented slurry technology is emerging as a contender to prevent wind farms from having a carbon footprint of their own. The projected costs make this technology economically viable.
The process uses electricity from a wind farm to split water into hydrogen and oxygen, through electrolysis. The hydrogen is stored in a magnesium slurry that can be safely stored on site or transported to where it is needed. The slurry is heated to release the hydrogen to fuel gas turbines, which drive generators. Discharged slurry is “recharged” at wind farms.
This technology has significant cost advantages over current battery storage technologies, as well as the potential to transform a wind farm into a dispatchable plant without need for a fossil fuel backup. When the wind blows hard, part of the energy streams directly to the grid, while the rest goes to storage. When the wind dies down, energy flows from the storage system to the grid.
Breakthrough energy storage
Last fall, technology titan Bill Gates gave the clean energy industry a shot in the arm, or perhaps a breath of fresh air, when he led a group of billionaires in launching the Breakthrough Energy Coalition.
“We will look for novel technologies as well as ways to make existing technologies dramatically cheaper, more efficient, or more scalable,” Gates stated in his five principals guiding the coalition’s effort. He also singled out storage as a key factor “on the path to a zero-carbon future,” while lamenting renewables’ intermittency, persistent dependence on fossil fuels, and the high cost of storage.
Aggressive innovation and investment is necessary to making meaningful advances in storage technology which will help clean energy reach its true potential.
Ken Brown is the CEO and Managing Partner of Safe Hydrogen, LLC, a storage technology company based in Massachusetts.