We’ve seen a lot of combinations of solar power and energy storage lately, with battery-PV combos promising to help mitigate the issues that arise from connecting solar to the grid. This week saw the launch of a new project aimed at integrating ultracapacitors -- the “flash” storage of the grid -- into the mix.
The project, funded by a $1.39 million California Energy Commission grant, is meant to test the costs and benefits of combining ultracapacitors from Maxwell Technologies and concentrating photovoltaic (CPV) solar systems from Soitec. It’s taking place at the University of California San Diego’s cutting-edge microgrid, a combination of solar power, energy storage, on-site generation and energy management controls that provide 90 percent of the campus’ power needs during normal times.
UCSD’s microgrid has also helped save utility San Diego Gas & Electric from blackouts on more than one occasion, and is working with SDG&E on its broader smart grid plans as well. Because nearly half of the microgrid's standing energy comes form solar power, however, it’s also interested in using building energy management systems, backup generators, batteries and other technologies to help balance that intermittent, yet vital resource.
Where Solar and Ultracapacitors Fit
The Maxwell/Soitec project is aimed at similar goals, but at a different scale, Shaw Lynds, senior systems engineer at Maxwell, told me in an interview this week. In simple terms, “We’re trying to make the solar panels play nicely with the rest of the grid,” he said.
Ultracapacitors, like their less powerful cousins, capacitors, differ from batteries in that they store energy in an electric field, rather than via chemical reaction. That means they can essentially charge and discharge instantaneously, whereas batteries have a maximum power input/output. But it also means they’re cost-prohibitive for more than a handful of minutes' worth of energy storage, compared to batteries.
Lynds put the sweet spot at less than six to seven minutes of storage capacity. In the case of the Soitec project, that means a 2.5 kilowatt-hour ultracapacitor array, about the size of two refrigerators side by side, next to a 22-kilowatt CPV system, which adds up to roughly five minutes of nameplate production capacity, he said.
For any application that requires more than seven or eight minutes of capacity, batteries are a better bet, he said. That means that long-term load shifting, backup power and other such applications aren’t for ultracapacitors. But for shorter-term applications, “With a relatively small amount of storage, you can do a lot of good,” he said.
“That’s what we’re going after with this project. We hope to demonstrate that most of the big things that make solar hairy for the grid, we can solve, with a kind of storage that doesn’t require a large amount of maintenance.”
The Problem With Fluffy Clouds, and Other Solar Smoothing Issues
Solar panels, whether they’re scattered across thousands of rooftops or aligned in massive desert arrays, react to different weather in different ways. Obviously, a storm front that blocks out the sun can shift an entire region’s solar output from full nameplate to nearly nothing. Luckily, these kinds of weather conditions are easy to predict, giving grid operators time to ramp up other resources to cover the difference -- and that’s not one of the things Maxwell and Soitec are looking at handling, Lynds noted.
But even partially cloudy skies can mess with solar power output, in both quantity and quality, he said. Indeed, highly variable patterns of cloud and shade, which can cause unpredictable fluctuations across distribution circuits with lots of rooftop solar, can be harder to deal with than really nasty weather that you can see coming, he said.
The two also want to use stored energy to “ramp” their output to the grid in a way that helps grid operators manage it, he said. Ramping solar output is tricky for systems that have no storage, because the only way to do it is to “burn off” energy, slowly decreasing output for several minutes in advance of an oncoming cloud front -- something that requires some ability to forecast when that shade is going to hit, as well as wasting some energy.
Of course, ultracapacitors and batteries alike need sophisticated inverters and power electronics to connect with the grid. Maxwell is used to this large-scale integration work through its uninterruptible power supply (UPS) business, Lynds said, and the Soitec/UCSD integration is using all off-the-shelf components typical for solar systems of its size.
Energy Storage and the Long-Term Challenge of Distributed PV
While we haven’t seen enough solar power deployed yet to find out how big of a problem these kinds intermittency and power quality issues may become, it’s certainly something that utilities and the smart grid industry are focused on. Germany, which is the world’s solar leader, is considering a subsidy for PV-connected energy storage to deal with distributed problems like these.
But to date, it’s been hard to pin down just how serious of a problem these issues are for today’s grid operators, utilities and solar project developers, Lynds noted. “Where that starts to become apparent to the public is when you get your hands on a PPA between the solar installer and the power company,” he said. “That’s where they start to nail down details on what the penalties are for violating ramp rates and for non-delivery of power that’s been promised in a forward-looking 15-minute interval.”
As states like California, Hawaii and others start adding more and more solar power to their grids, we’re bound to see more details on the challenge emerge. In the meantime, we’ve got a host of projects around the world testing the ability of technologies from next-generation metal and air-based batteries, to lots and lots of smart thermostats, air conditioners, refrigerators and water heaters, to balance intermittent wind and solar power.
Maxwell and Soitec will be looking at market issues as well as technical issues in their two-year project, Lynds noted, with plans to produce an economic model for how these smoothing and ramping capabilities could alter the cost-benefit equation of distributed solar on a broader scale. That means that, by 2015 or so, “while this conversation is still a major point of debate, we will have years of historical data” on hand to prove out the concept, he said.