Operational Excellence
September 20, 2023

Energy 101: How energy storage makes renewable energy more reliable

Xcel Energy is fortunate to operate in states with some of the best wind and solar resources in the country for producing electricity. Increasingly, we’re putting those resources to work for our customers—with wind farms and solar arrays sited in the Midwest, Colorado and the Texas Panhandle that can deliver energy more efficiently and at a lower cost. Renewable sources generated 40% of our electricity in 2022 and have set system records, supplying about 80% of our power for entire days. 
While wind and solar are important and growing parts of our energy supply, they bring operational challenges too. There are periods when the sun isn’t shining, or wind doesn’t blow, or times when those resources produce more electricity than our customers can use. For example, wind energy is often produced late at night when electricity use is low. Solar energy is strongest during the day and significantly drops off as the sun goes down — right when energy demand hits its peak in the late afternoon and evening as people return home from work and begin cooling or heating their homes and turning on lights and appliances.  
One solution to this challenge? Reliable large-scale energy storage. 
Large-Scale Storage Is a Viable Clean Energy Solution 
Technologies that store energy for use at a later time can improve the reliability and resiliency of our power grid by backing up wind and solar energy. They also support a clean energy future. To achieve our company-wide goal of delivering 100% carbon-free electricity by 2050, we’re exploring advanced 24/7 dispatchable clean energy solutions, including storage, that can be paired with our existing system. 
“Our company is focused new clean energy technologies that are furthest along in development, where the engineering, economics and public policy are moving in a positive direction,” said Steve Christensen, manager, Planning and Technical Assessment for Xcel Energy. “Energy storage technologies are building their supply chains and manufacturing capabilities for wider-scale adoption, and as a national leader in renewable energy, energy storage fills an important need for our system.” 
Researchers and technology developers are exploring all types of storage possibilities, from long-duration batteries to underground compressed air and even rail cars that use gravity when they store and generate electricity. At Xcel Energy, Steve Christensen and a team of others from around the company evaluate new technologies to modernize the grid and reduce carbon emissions reliably and affordably for our customers. To build our energy storage capabilities, Xcel Energy has committed to demonstrating two promising battery technologies and is expanding the capacity of an existing pumped hydroelectric plant in Colorado.   
Long-duration batteries 
Since the 1800s, batteries have converted chemical energy into electrical energy. Today’s standard AA battery uses zinc, manganese, potassium and graphite to create a chemical reaction to run your flashlight or TV remote. Those lightweight lithium-ion batteries inside your phone, laptop or electric vehicle hold significantly more energy and can both discharge energy for powering your devices and recharge to store energy for later use. 
The batteries used today on the grid often discharge over two to four hours to provide a variety of services to manage peak load, control frequency and store renewable energy. Long-duration energy storage refers to cases where the energy can be dispatched for 10, 20 or even 100 hours. Imagine a battery that the sun charges during the day and dispatches at night to provide a continuous energy supply like we currently get from coal or natural gas generation. Ideally, those batteries are made from materials that are readily available and sustainable, long lasting, efficient, safe and economical.  
“Long-duration battery systems are relatively simple technologies compared to other emerging dispatchable energy technologies such as hydrogen, carbon capture for natural gas or nuclear,” said Christensen. “You can site the systems next to renewable energy sources or elsewhere on the power grid to help smooth the peaks and valleys associated with operating intermittent wind and solar generation or to alleviate congestion on overcrowded transmission lines. We can add them any place where there’s space and a need to store and quickly dispatch energy.” 
The cost for long-duration storage technologies is steadily improving and becoming more competitive with the cost of other new dispatchable generation resources. New tax credits available under the federal Inflation Reduction Act are making projects more affordable and grant funding under the Infrastructure Investment & Jobs Act will help to build the market, which can further lower costs.  
Xcel Energy is partnering with Form Energy to build two 10-megawatt, 100-hour battery arrays near our retiring coal plants in Becker, Minnesota, and Pueblo, Colorado, as soon as 2025. Form Energy’s iron-air batteries can deliver enough energy to power 2,000 homes for up to five days. The batteries use a reversible and repeatable rusting technology to create electrochemical reactions between abundant, inexpensive materials — water, air and iron — to charge and discharge. Both batteries will tap into our existing transmission system. 
We’ve received a grant of up to $70 million from the U.S. Department of Energy to partially fund the project. This award, combined with a $20 million grant from Breakthrough Energy Catalyst, will help lower the cost for our customers.
In addition, we are working with Ambri to test its 300-kilowatt hour Liquid Metal battery system at the Solar Technology Acceleration Center in Aurora, Colorado — a 74-acre testing center for validating technologies in a real-world, grid-connected environment. 
Ambri’s system uses safe, readily available materials — calcium, antimony and calcium-chloride salt. Using the GridNXT Microgrid Platform at SolarTAC, the battery system will help integrate multiple energy sources, such as solar and wind, along with inverters, load banks and distribution connections. We expect to test the system over 12 months once it’s up and running in 2024. 
Pumped energy storage 
Hydroelectric pumped storage systems are the original large-scale storage technology, dating back to the late 1800s. Our Cabin Creek hydroelectric plant was the highest altitude pumped storage plant in the world when it was built in 1967 above Georgetown, Colorado, in the Rocky Mountains. 
Water is released from Cabin Creek’s upper reservoir to turn the plant’s two turbine generators and produce electricity. After, the water is returned to a lower reservoir where it is pumped and stored in an upper reservoir to start the generation process all over again when needed. Cabin Creek can be brought online within 10 minutes to respond to increased customer demand, quicker than any other plant on our system. 
Rather than retire the plant, we’ve renewed Cabin Creek’s operating license, extending its service life by 40 years, and we’re completing a project to increase the plant’s generating capacity and storage efficiency. When finished at the end of the year, the operating extension and expansion is expected to save more than $300 million compared to retiring and replacing the plant. Plus, Cabin Creek will continue as an important companion to our growing fleet of wind farms by pumping water when wind energy is plentiful and quickly responding to fill the gap when the wind stops blowing. 
Energy Storage in our Clean Energy Plans 
Beyond these projects, storage is moving forward in our energy plans on a smaller scale. In Pueblo, Colorado, the Neptune and Thunderwolf Energy Center — two cost-effective large-scale solar projects each combined with four-hour battery systems — began delivering energy to the grid in summer 2023. We’ve proposed 1,170 megawatts of new storage, both standalone systems connected to the grid and systems paired with renewable resources, in our Colorado Clean Energy Plan. In Texas, we’ve proposed installing three company-owned solar projects at existing power plant sites and are evaluating battery storage as part of those plans. 
The future of long-duration storage that provides days’ worth of power for our grid is promising. Read more about other technologies that are poised to deliver a clean energy future for our customers as part of our continuing Energy 101 series, including clean fuels and smart meters.