The world's renewable power capacity—the amount of energy that can be produced from sources comprising hydropower, wind, solar, bio-energy, and geothermal—has doubled in the last decade. From 2008 to 2017, it went from 1,060 gigawatts to 2,179 gigawatts, according to research from the International Renewable Energy Agency (IRENA), an intergovernmental organization.
Over that period, world renewable energy production has also increased, from over 3.7 million gigawatt hours to almost 5.9 million gigawatt hours.
To be clear, capacity, according to the U.S. Department of Energy, is the "maximum output of electricity that a generator can produce under ideal conditions," whereas generation is what actually gets produced.
In both cases, much of the growth, according to data from IRENA, comes from wind and solar sources. This is good news for clean energy advocates who hope to see our world less reliant on polluting fossil fuels that contribute to climate change.
Renewable power sources provide "fuel" that is free and clean. This is highly attractive to businesses that generate electricity, and those that run on electricity. Yet sources like wind and solar are also intermittent; after all, the wind stops blowing at times, and the sun stops shining at night. In contrast, power generated from coal and natural gas is consistent, and able to be ramped up and down quickly to meet demand.
This presents a conundrum for clean energy advocates. How do you make renewable energy not only cheap enough, but reliable enough, to compete with fossil fuels?
The answer may lie in the types of energy storage used to capture the power generated from these renewable sources. Better batteries could make it possible for utility companies to leverage renewable energy sources at scale as primary power sources, rather than as clean backups to traditional dirty fuels.
The power grids that charge and recharge modern life are complicated beasts, comprised of many different companies and technologies that make it possible to generate power, then route that power to homes and businesses.
These grids operate on a basic principle: once energy is generated, it is sent out for consumption. If it is not being consumed immediately, it needs to be stored in some fashion so it can be released back into the grid when end users are ready to consume it.
It sounds easy in theory, but it quickly gets complicated in practice.
When businesses and homes need power, they typically need it immediately. This means demand for grid power can fluctuate a lot, and it can fluctuate quickly. Power supply must match demand as adequately as possible, on time and on budget. Failure to meet demand sufficiently and on time results in power failures or blackouts. Failure to meet demand on-budget means a power provider eventually goes out of business.
This is where the benefits of fossil fuels become evident. Fossil fuel-powered energy plants generate consistent power, since humans, not nature, control their fuel sources, and the different fossil fuel power generation technologies (such as coal, oil, and natural gas) can be scaled up or down to meet demand fluctuations. Despite being dirty (basically, the fuels are burned to heat water to turn turbines that generate electricity, releasing carbon dioxide and other contaminants into the atmosphere in the process), they are (at the moment) reliable.
"Incumbent technologies are still on a steep cost-down curve, which is challenging for new technologies to compete with," says Stanford University's William Chueh.
Renewable energy sources, on the other hand, are clean and their fuel is free and abundant. However, the power generated by water, wind, and solar sources must be stored somewhere after it is generated, since it is intermittent.
The top method utilized in the U.S. for renewable energy storage is pumped-storage hydroelectric, which provides 95% of grid-scale electricity storage in the country, according to the U.S. Department of Energy. Pumped-storage hydro utilizes multiple reservoirs to store and release electricity in a highly efficient and responsive manner, allowing power grids to react quickly to fluctuations in demand.
According to electric power holding company Duke Energy, "Pumped-storage hydro plants store and generate energy by moving water between two reservoirs at different elevations." When demand is low, "Excess energy is used to pump water to an upper reservoir." When there's high demand, the water is released from the upper reservoir to generate power.
However, building these storage systems requires massive time and resource investments, and the location of these storage systems is highly dependent on geography.
This is where battery technology comes in. On the grid, the right battery technology may be able to store and release energy at far less cost than pumped-storage hydro. Some battery developments are also enabling the transition to electric vehicles, further reducing reliance on non-renewable fossil fuels.
One of the main battery types currently used for energy storage is the lithium-ion battery. Lithium-ion batteries power the handheld tech gadgets we use every day, including smartphones. They also provide energy storage for electric cars, and extremely large lithium-ion batteries are increasingly being used in power grids to store energy from renewable sources.
Lithium-ion batteries are inexpensive and energy-dense compared to batteries made with other materials. They also degrade relatively slowly, losing just a fraction of their power after each use.
"Over the past decade, we have seen a tremendous cost reduction of lithium-ion battery technology, by approximately 10 times," says William Chueh, an assistant professor of Materials Science and Engineering at Stanford University working on renewable energy storage technologies. "This has been responsible for the boom in electric vehicles and for storing intermittent solar and wind electricity."
However, lithium-ion batteries have one big problem: they still are not priced competitively enough to be used at scale on grids to store energy from renewable sources.
"To realize a complete penetration of batteries for storing intermittent renewable electricity, the cost needs to decrease by another order of magnitude, and the scalability needs to be greatly improved," says Chueh.
Those developments are unlikely, says Ben Schiltz, head of Energy Storage Communications at the U.S. Department of Energy's Argonne National Laboratory, which has multiple energy storage research projects in progress.
"Today, the industry continues to make incremental improvements to lithium-ion batteries. However, we are reaching the theoretical limit of what can be done with these batteries," Schiltz says. "Developing safe new battery technologies with higher energy capacity, lower cost, and longer life, that can also be charged and discharged fast, is very challenging. Oftentimes you can improve one [factor] at the expense of others."
This situation has researchers hunting for alternative technologies to store energy from renewable sources.
There are many alternatives when it comes to grid energy storage, says Antonio Baclig, a renewable energy storage researcher at Stanford and a member of Chueh's team.
"Lithium-ion is now the front-runner for grid storage batteries, but lead-acid batteries are a low-cost alternative, and flow batteries are continuing to improve," he says. Lead-acid batteries are solid batteries used in automotive applications. Rechargable flow batteries, however, use liquids instead of solids to conduct electricity, which may give researchers more options to find chemical combinations that dramatically improve efficiencies and reduce costs.
Lead-acid batteries have the "largest market share for rechargeable batteries," according to research published in The Journal of Energy Storage by Geoffrey May, Alistair Davidson, and Boris Monahov. Much of the market for these batteries is in traditional motor vehicles—standard car batteries.
While lead-acid batteries are relatively inexpensive and widely available, they do have some drawbacks: for one, they are heavy, which makes them less than ideal for electric cars (Tesla's vehicles, for example, use lithium-ion batteries.)
The real advantage of lead-acid over lithium-ion has historically been cost, but that is changing. Research by Joe O'Connor, manager of application engineering at battery technology company Farasis Energy Inc., showed that while individual lead-acid batteries are cheaper to buy than lithium-ion batteries, the total lifecycle cost for off-grid lithium-ion batteries is reaching parity with that of lead-acid batteries.
Lithium-ion batteries require little maintenance and are more resilient to irregular discharging than lead-acid batteries, according to O'Connor.
While lead-acid and lithium-ion technologies duke it out over marginal cost reductions and efficiency gains, advances and improvements in flow batteries may just be getting started.
Chueh, Baclig, and a team of researchers have developed a new type of flow battery that could eventually be a low-cost, high-power alternative to existing energy storage methods.
According to Stanford, "The group found a suitable ceramic membrane made of potassium and aluminum oxide to keep the negative and positive materials separate while allowing current to flow." The membrane "doubled the maximum voltage of conventional flow batteries, and the prototype remained stable for thousands of hours of operation."
With those advances, says Chueh, "We aim to simultaneously achieve high energy density, lifetime, and reduced cost."
However, this new flow battery is still in the prototype phase. While promising, it will not be mass-produced any time soon.
Another new battery type that is commercially farther along in scaling the storage of renewable energy is the zinc-air battery, a metal-air battery powered by oxidizing zinc with oxygen from the atmosphere. These batteries have high energy densities, and are relatively inexpensive to produce. Late last year, energy storage technology company NantEnergy and its billionaire founder Patrick Soon-Shiong announced they had developed a battery that uses zinc and air to store renewable energy.
This zinc-air battery has been tested "in Africa and Asia, as well as cellphone towers in the United States for the last six years, without any backup from utilities or the electric grid," according to The New York Times. The company claims the new battery can store and release electricity at a cost of less than $100 per kilowatt-hour (kWh). In comparison, Elon Musk told shareholders that Tesla was working to get to that price point for its lithium-ion battery cells by the end of last year.
The $100/kWh mark is seen in the energy storage community as a tipping point for widescale adoption of electric cars, according to Bloomberg.
Despite these advances, commercially viable and scaleable grid-ready alternatives to lithium-ion batteries remain to be seen.
"Incumbent technologies are still on a steep cost-down curve, which is challenging for new technologies to compete with," says Chueh. However, Chueh is confident that, given a long-enough timeline, alternative grid batteries will be part of the answer to scaling renewable power storage.
"Batteries used at the grid scale will look more like a chemical plant, rather than the batteries used in electric vehicles, drones, and robots today," Chueh says.
Lead batteries for utility energy storage: A review, Journal of Energy Storage, February 2018, https://www.sciencedirect.com/science/article/pii/S2352152X17304437
Battery Showdown: Lead-Acid vs. Lithium-Ion, Medium, Jan. 23, 2017, https://medium.com/solar-microgrid/battery-showdown-lead-acid-vs-lithium-ion-1d37a1998287
Cheaper Battery Is Unveiled as a Step to a Carbon-Free Grid, The New York Times, Sept. 26, 2018 https://nyti.ms/2MiFwAj
Renewable Energy Statistics 2018, International Renewable Energy Agency, July 2018 http://www.irena.org/publications/2018/Jul/Renewable-Energy-Statistics-2018
2017 Hydropower Market Report, U.S. Department of Energy, April 2018 http://bit.ly/2TWJUaN
Pumped-Storage Hydro Plants, Duke Energy http://bit.ly/2MfvhN6
What's the Difference Between Installed Capacity and Electricity Generation?, U.S. Department of Energy, Aug. 7, 2017 http://bit.ly/2MhLrWv
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