Cheapest long-duration storage for systems with high renewables
Researchers at the US Department of Energy's National Renewable Energy Laboratory (NREL) have assessed the cost and performance of most long-duration energy storage (LDES) technologies. They have also looked at flexible power plants to help electricity systems to deal with extremely high levels of renewable energy penetration and have found that, given current and future capital cost scenarios, that geologic hydrogen storage and natural gas combined-cycle (NGCC) plants with carbon capture storage (CCS) technologies offer the lowest levelized cost of energy (LCOE) for 120-hour discharge applications and that pumped hydro, compressed air, and batteries are the cheapest solutions for 12-hour discharge.
“Since energy storage technologies will compete with low-carbon power generation technologies such as NG-CC with CCS to provide the grid with electricity during times when wind and solar are not producing electricity, we compare them all together within this paper,” researcher Chad Hunter told pv magazine. “This allows for a quick comparison of technologies that have not all been looked at in the same analysis before our paper.”
The techno-economic analysis considered the LDES and flexible power generation technologies in the US Western Interconnection, which is a wide-area synchronous grid stretching from Western Canada to Baja California in Mexico, with an 85% share of renewable energy in the area's electricity mix.
“LDES requires large energy capacities so that a typical rate of charging or discharging can be sustained for days, weeks, or even longer,” the scientists explained. “In this study, flexible power plants and LDES system power generation equipment are sized at 100 MW, in the range of peaking and load-following plant sizes today.”
LDES systems are sized to supply rated power for durations from 12 hours to seven days and the LCOE is calculated for both current and future capital costs.
Through their analysis, the academics found that, for the maximum duration of seven days, NG-CC plants with CCS are the cheapest solution. For the minimum 12-hour threshold, the options with the lowest costs are compressed air storage (CAES), lithium-ion batteries, vanadium redox flow batteries, pumped hydropower storage (PHS), and pumped thermal energy storage (P-TES), which they said is mainly due to their moderate power-related capital costs and high round-trip efficiency.
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“Batteries will likely play a large role in grid energy storage moving forward, especially if battery prices continue their strong decline as we have seen over the past decade,” Hunter explained. “Shorter-duration battery storage will be complemented by low-cost, longer-duration storage technologies, such as geologic hydrogen storage.”
For more than four days of storage, the least-cost solutions are diabatic compressed air energy storage (D-CAES), NG-CC, NG-CC with CCS, natural gas combustion turbine (NG-CT), and hydrogen storage in salt caverns with re-electrification in heavy-duty vehicle proton exchange membrane (HDV-PEM) fuel cells. They also determined that pumped hydro storage and the HDV-PEM fuel cells with salt cavern storage offer the lowest LCOE for the 12-hour and 120-hour durations, respectively.
“Although hydrogen systems with geologic storage and natural gas with CCS are the least-cost technology options to support high variable renewable energy (VRE) grids at durations beyond 36 h, several challenges are associated with them,” the NREL research team said. “First, neither technology offers the lowest cost for short-duration storage (12 hours), which will likely dominate the storage market until high VRE penetrations are reached; thus, market adoption and learning must be driven by other sectors or use cases, such as using HDV-PEM fuel cells in heavy-duty trucking or deploying CCS for industrial applications.”
The NREL group said that minimizing storage capital is economically convenient at durations longer than approximately 48 hours and that the LCOE is more sensitive to storage energy capacity costs than storage power capacity costs. The team presented its findings in “Techno-economic analysis of long-duration energy storage and flexible power generation technologies to support high-variable renewable energy grids,” which was recently published in Joule.
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To cut U.S. greenhouse gas emissions in half within a decade, the Biden administration’s goal, the U.S. is going to need a lot more solar and wind power generation, and lots of cheap energy storage.
Wind and solar power vary over the course of a day, so energy storage is essential to provide a continuous flow of electricity. But today’s batteries are typically quite small and store enough energy for only a few hours of electricity. To rely more on wind and solar power, the U.S. will need more overnight and longer-term storage as well.
While battery innovations get a lot of attention, there’s a simple, proven long-term storage technique that’s been used in the U.S. since the 1920s.
It’s called pumped hydro energy storage. It involves pumping water uphill from one reservoir to another at a higher elevation for storage, then, when power is needed, releasing the water to flow downhill through turbines, generating electricity on its way to the lower reservoir.
Pumped hydro storage is often overlooked in the U.S. because of concern about hydropower’s impact on rivers. But what many people don’t realize is that most of the best hydro storage sites aren’t on rivers at all.
We created a world atlas of potential sites for closed-looped pumped hydro – systems that don’t include a river – and found 35,000 paired sites in the U.S. with good potential. While many of these sites, which we located by satellite, are in rugged terrain and may be unsuitable for geological, hydrological, economic, environmental or social reasons, we estimate that only a few hundred sites are needed to support a 100% renewable U.S. electricity system.
Why wind and solar need long-term storage
To function properly, power grids must be able to match the incoming electricity supply to electricity demand in real time or they risk shortages or overloads.
There are several techniques that grid managers can use to keep that balance with variable sources like wind and solar. These include sharing power across large regions via interstate high-voltage transmission lines, managing demand – and using energy storage.
Batteries deployed in homes, power stations and electric vehicles are preferred for energy storage times up to a few hours. They’re adept at managing the rise of solar power midday when the sun is overhead and releasing it when power demand peaks in the evenings.
Pumped hydro, on the other hand, allows for larger and longer storage than batteries, and that is essential in a wind- and solar-dominated electricity system. It is also cheaper for overnight and longer-term storage.
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An off-river pumped hydro system comprises a pair of reservoirs spaced several miles apart with an altitude difference of 200-800 meters (about 650-2,600 feet) and connected with pipes or tunnels. The reservoirs can be new or use old mining sites or existing lakes or reservoirs.
On sunny or windy days, water is pumped to the upper reservoir. At night, the water flows back down through the turbines to recover the stored energy.
A pair of 250-acre reservoirs with an altitude difference of 600 meters (1,969 feet) and 20-meter depth (65 feet) can store 24 gigawatt-hours of energy, meaning the system could supply 1 gigawatt of power for 24 hours, enough for a city of a million people.
The water can cycle between upper and lower reservoirs for a hundred years or more. Evaporation suppressors – small objects floating on the water to trap humid air – can help reduce water evaporation. In all, the amount of water needed to support a 100% renewable electricity system is about 3 liters per person per day, equivalent to 20 seconds of a morning shower. This is one-tenth of the water evaporated per person per day in the cooling systems of U.S. fossil fuel power stations.
Storage to support 100% renewables
Little pumped storage has been built in the U.S. in recent years because there hasn’t been much need, but that’s changing.
In 2020, about three-quarters of all new power capacity built was either solar photovoltaics or wind power. Their costs have been falling, making them cheaper to build in many areas than fossil fuels.