An interview with Xcel Energy’s Carlos Hill: How electrical storage works with renewable energy

Energy utilities face incredible challenges in the coming years due to the electrification of houses, businesses, and automobiles. Even as efficiency improves, policies mandating increased use of renewable energy encourage utilities to invest in energy storage methods that can work within their electrical grids.

We spoke with Hill to get his insight on the state of utility-scale energy storage, residential energy storage, how renewable energy generation impacts the existing grid, and the new technologies he sees influencing the electricity industry in the coming years.

Utility-scale storage for the transition to clean energy

Arrow Electronics: What are the existing options for storing electricity?

Carlos Hill: This is the question everyone is trying to answer right now. You can store oil inside barrels or propane inside tanks, but electricity is something that generally must be consumed immediately. There are four primary storage options in my opinion: pumped storage hydropower (PSH), battery storage, flywheel energy storage (FES), and thermal storage.

Pumped hydropower involves pumping water upward to an elevated location, typically during the night when there’s a lot of free wind generation powering the grid. The water is stored at the higher elevation until peak hours in the late afternoon, when more people are consuming electricity. The Potential Energy, which equals mass x gravity x height, of the Water is released to flow downstream to a turbine, generating electricity thanks to the transfer to kinetic energy. The big advantage of pumped hydropower is that potential energy is stored at a higher elevation until the utility needs to convert it to kinetic energy for the grid.

Regarding battery storage, most people default to lithium-ion batteries, but Form Energy’s iron-air is another popular battery chemistry that can store power for days. Teams at Xcel Energy are working with Form Energy to eventually provide 10 MW capacity batteries that can hold energy for nearly 100 hours.

Xcel Energy also executed a Battery Connect pilot where residential customers in Colorado with SolarEdge or Tesla batteries in their garages participated in a solar shifting and peak shaving pilot. The utility charged the residential batteries in the mornings during periods of high irradiance (sunshine) and then discharged the batteries in the early evening during peak-hours to shave load off the customer’s electrical demands.

Very few people talk about FES, but it’s an effective method for energy storage. Think about times when you spin a round object, the object continues spinning because you transferred energy into the object as angular momentum. Utilities can leverage periods when electricity is cheaper to start spinning giant cylinders to transfer energy as angular momentum. Utilities can then extract the energy back to the grid during peak hours.

Finally, there’s thermal energy storage. One example is heating a water tank when there’s abundant energy available on the grid. One needs several units of energy to increase large volumes of water by one degree in temperature, but water has a high heat capacity such that the liquid can hold the energy in the appropriate conditions. One could then dispatch this energy to the grid at peak hours. It’s less common in the U.S. right now as far as I know.

Arrow Electronics: How do each of these work within the infrastructure that currently exists?

Carlos Hill: Pump hydro requires a big capital expenditure. These systems are hefty to build and work best in hilly or mountainous areas. Open-loop systems exist where the water continues downstream past a dam, but there are also closed-loop systems where the water cycles between an upper and lower reservoir.

Flywheel energy storage we can do anywhere—a park, a desert, a neighborhood—basically, anywhere there’s a flat patch of land.

Batteries can also be put anywhere in commercial and/or residential settings. They are the most-common option today that the average American knows about. Small battery systems, like 5 kilowatts of capacity, can fit in a garage. If you want something larger, like 50kw, you’d want a warehouse.

Arrow Electronics: What are the challenges associated with storing renewable energy using existing infrastructure?

Carlos Hill: Renewable energy is variable. We can burn coal and natural gas whenever we want, but wind and solar are intermittent. There’s also the complication of overproducing renewable energy when it’s abundant and unfortunately having to turn generators off if there’s not capacity to store it.

If you’re generating electricity from solar or wind, you could use batteries to store your excessive generation. You should always have an electrical engineer involved who understands the voltage, amps, inverters, capacity, etc.

Entering the next phase of the sustainable energy transition

Arrow Electronics: Do you have any examples of grid-level renewable energy storage in practice today?

Carlos Hill: Xcel has the Cabin Creek pumped hydropower facility in Colorado. We’ve also partnered with Form Energy to develop multi-day energy storage. Their iron-air batteries are a gamechanger with impressive utility-scale storage that holds 10 MW of power for up to four days. That’s 1,000 MW hours of energy the iron-air battery can potentially store. These potential 1,000 MWh iron-air battery systems with long-duration storage are critical to creating a clean grid. These multi-day energy storage projects are designed to maximize the use of renewable energy while maintaining the reliability of the Xcel Energy grid. The involved parties will construct the batteries at the sites of retiring coal plants like the Sherburne Station in Minnesota and the Comanche Station in Colorado. These utility-scale batteries will allow Xcel Energy to store renewable energy and then distribute said energy during periods of lower energy production. There are a few of these utility-scale batteries around the country.

Arrow Electronics: Are there any other new energy storage technologies on the horizon?

Carlos Hill: One energy storage technology that's becoming more popular is green hydrogen. There are different colors to represent different production methods: gray hydrogen, brown hydrogen, blue hydrogen, green hydrogen, and pink hydrogen. Green hydrogen means using green energy to run an electrolyser, which is an impressive machine that does electrolysis, or splitting water apart. You can split water into hydrogen atoms and oxygen atoms, and then store the hydrogen to later power a fuel cell during high-demand periods.

The idea is to use surplus renewable electricity to run the electrolyser to produce the hydrogen, and then let the hydrogen sit in a storage tank, similar to how oil sits in barrels. Unfortunately, the economics of this process are still challenging.

The consumer impact on renewable energy adoption

Arrow Electronics: What trends are you seeing that impact progress on renewable energy storage?

Carlos Hill: The adoption of electrical vehicles is putting new loads on the grid that never existed before. Houses consumed electricity based on the refrigerator, the lights, the air conditioner, the washer and dryer, and other basic appliances. Now there’s this sudden demand spike where the house consumes massive amounts of electricity while an EV charges. There’s a lot of discussion going on about how to manage that extra load. Some strategies around this are called static/passive charging and dynamic/active charging. Under these managed charging strategies, EVs will charge at certain times less stressful for the grid.

Kitchen rebuilds that forgo gas for electrification are also putting stress on the grid. Utilities need to ensure that the feeders, transformers, and substations are sufficient to compensate for this growing electrical demand.

The views expressed are those of the interviewee and do not reflect the opinions of Arrow Electronics Inc. or its affiliates.

Are you on the list?

Sign up to receive exclusive offers, product announcements, and the latest industry news.