Xcel Energy may not have solved the renewable energy variability issue, but an ongoing energy-storage project shows promise. The utility is partnering with S&C Electric, the National Renewable Energy Laboratory, the Great Plains Institute, GridPoint and others to conduct a wind-to-battery project using sodium sulfur (NaS) technology at an 11 MW wind farm in Minnesota.
The NaS battery shifts wind energy from off-peak to on-peak availability, reduces the need to compensate for wind's variability, strengthens the grid by providing voltage support and responds to real-time imbalances between generation and load, according to Xcel.
‘We were kind of surprised that it worked as well as it did,’ says Frank Novachek, director of corporate planning for Xcel. ‘It's actually very good technology. Most of the glitches that we ended up with were more communications-related.’
The NaS system was recently tested at the MinWind project in Luverne, Minn., for basic generation storage by scheduling it to discharge during on-peak periods and charge during off-peak times at a rate that was proportional to the wind farm's output. The system performed as expected, the utility noted in a report about the project.
The system was also tested using forward and spot energy prices in the Midwest Independent Transmission System Operator market and, again, performed as expected. Additional tests included frequency regulation, ramp-rate control and steady-output control.
The wind-to-battery project began in 2008 as part of Xcel's environmental strategy. ‘We're trying to use clean energy technologies to put ourselves in a better position to meet the future challenges in the electric utility market,’ says Novachek. ‘We view wind as being strategic to our portfolio in the future, so we're trying to come up with ways to add more and more wind to our system.’
After a selection process, Xcel chose NaS technology because, at the time, it was the only commercially available technology that allowed the utility to research bulk-energy and short-burst storage at the same time, according to Novachek.
‘We wanted to test the battery in a number of different ways to see how it might be able to provide value to the grid,’ he adds.
The utility purchased the battery from Japan-based NGK Insulators Ltd. This type of technology is in use in other parts of the world and in the U.S. Earlier this year, Electric Transmission Texas, a joint venture between subsidiaries of American Electric Power and MidAmerican Energy Holdings Co., energized a 4 MW NaS battery system in the small town of Presidio, Texas. The system is expected to provide transmission backup in the event of a transmission-line outage, which the town experiences frequently. The battery system also is designed to improve power quality and reduce voltage fluctuations.
Xcel's NaS system comprises 20 50 kW battery modules that, together, are about the size of two semi-trailers and weigh approximately 80 tons. The system can store about 7 MWh of electricity. When fully charged, the battery can power 500 homes for about seven hours, according to the utility.
Xcel's report also shows that the NaS project could work for solar power. ‘We weren't really sure how quickly the battery would respond,’ says Novachek. ‘As it turns out, it responds very quickly, so we think it can serve in that role as well.’
In fact, the utility is a founding partner of the Solar Technology Acceleration Center in Aurora, Colo., where a different type of storage technology – which has not been selected yet – will be tested.
Looking ahead, the University of Minnesota – another of Xcel's partners in the project – will examine the electrochemical and lifetime performance of the battery. The university's power engineering department will look at how placement of such technology can optimize its value to the grid.
Xcel will now work to determine whether the wind-to-battery system can handle larger penetrations of wind energy on the grid and assess the cost-effectiveness of the technology.
Even though the report shows promising results, the cost of this type of technology will need to come down in order for it to be widely deployed, Novachek explains. He adds that Xcel is also exploring other types of technology, including compressed-air storage and zinc-bromine and lithium-ion batteries.
‘There are other types of batteries out there that look like they have some promise for coming down in price,’ he says. ‘The sodium-sulfur battery works very well, but at this point in time, we don't really know what the price point is going to be for economic value. But we do know that the current price point is too high.’