Rooftop PV and behind-the-meter batteries can team up to reduce customer energy bills, shave off expensive peaks in building energy consumption, and store midday solar-generated electrons for evening discharge to help smooth out their impact on the grid. But more complicated functions, like balancing voltage on distribution grid circuits, or disconnecting and reconnecting to the grid for emergency backup power, require a third technology: smart inverters.
On Thursday, the SunSpec Alliance, the University of California-San Diego, and SolarCity launched a project meant to test this emerging standard for advanced inverter functionality. It’s the first real-world test of a technology set to be mandated for all new solar and battery projects in California in 2016, a fact that’s drawn some of the world’s biggest inverter makers into the project.
“We have an ensemble cast of partners here,” SunSpec Alliance chairman Tom Tansy said. Funded by a $2 million California Energy Commission EPIC grant, the $4 million project will run interoperability tests featuring inverters from seven different global manufacturers -- ABB, SMA, KACO, Outback, SolarEdge, Enphase and Ideal Power.
Starting late this year, each company will submit their smart inverters, along with chosen battery and solar integration partners, to testing by UCSD. The university’s state-of-the-art microgrid, which includes pretty much every form of distributed energy known to humanity, will provide a useful control and renewable power resource for the testing, Tansy said.
“That’s where we’ll prove out our communications interoperability, via the SunSpec standard,” he said. SunSpec and partners have built a set of standards around linking inverters with components like batteries, solar panels, and energy management systems. “They’ll be doing things like curtailment, voltage regulation, frequency regulation, both on power from the solar array and from storage.”
Almost all of today’s solar and battery inverters come with advanced features that fit this description of “smart.” But very few companies are turning them on, let alone communicating with utilities and grid operators about what they’re doing, and what they’re capable of.
That’s largely because most utility and grid regulatory frameworks haven’t kept up with distributed energy’s growth. There isn’t even an Underwriters Laboratories specification for smart inverters yet, although a California-led group is working with UL and inverter makers to fill that gap by next year.
UCSD is also providing its digital models of its distribution circuits -- the campus is like a grid in miniature, and submetered to an unusual degree for public buildings. OSIsoft, the biggest provider of data management software for utility SCADA deployments and other resources of circuit-level data, is participating in that part of the project as well, he said. Project partners will also pull data from the distribution circuit maps newly unveiled by the state’s big three utilities, including UCSD’s utility, San Diego Gas & Electric.
The broader goal is “to see how deeply a circuit can be penetrated,” he said. At most utilities, “there’s an artificial cap of about 15 percent of the total demand capacity that can be offset with renewable energy” and other distributed energy resources (DERs), he said, in terms of how much a distribution grid circuit can bear before causing potential problems.
But tests at Department of Energy labs, and analyses of real-world circuit data in DER-rich grid locales in Hawaii and California, indicate that many circuits can bear a much higher portion of distributed energy -- and even benefit from it -- as long as it’s planned and managed well. CEC’s grant request form (PDF) describes the project’s goals: “To develop a complete smart inverter data communication standardization and go-to-market solution to enable photovoltaic (PV) penetration beyond the 15% Institute of Electrical and Electronics Engineers (IEEE) guideline, incorporate energy storage as a standard building block of PV systems, and evaluate the market-expansion potential of a standardized communication interface.”
SolarCity’s energy-smart neighborhood, plus virtual power plant
This brings us to SolarCity’s part in the project, which is distinct from the UCSD work. The aggregator of hundreds of thousands of solar systems across California will seek out about 50 customers on a specific test circuit of Southern California Edison’s sprawling distribution grid, and equip each with a lithium-ion battery system, capable of providing roughly 7 to 10 kilowatt-hours of storage, Tansy said.
That just happens to also be the range of specs for Tesla’s new Powerwall home energy storage systems, by the way. Ryan Hanley, senior director of grid operations for SolarCity, wouldn’t say which battery and inverter partners the company planned to work with on the project, which is set to start some time early next year. But SolarCity is “cost-sharing more than we’re getting from the grant -- we’re putting up more in R&D and program support than we’re receiving.”
“We are going down to one circuit and finding 50 residential customers, and deploying 50 smart energy homes on that circuit,” he said. “In each home, we’ve got solar PV, a smart inverter, a residential battery, and a smart thermostat.” That last control point allows access to air conditioning, a key ingredient of household electricity load that could provide more flexibility in absorbing and redirecting solar power. Think of precooling a home with plentiful solar energy, and “storing” that cool to let the AC idle through the late afternoons and early evenings, when large swaths of Southern California circuits reach their peak, for example.
On the inverter-grid interconnection front, SolarCity plans to provide three main services with its aggregated 350 to 500 kilowatt-hours of storage. “The first one we’ll do is support voltage needs on the feeder,” he said -- something that requires advanced inverter functions to operate in concert with each other and utility-facing grid sensors and controls. Second, 50 homes will also support local capacity needs for the substation serving the circuit, much as SCE’s local capacity resource procurements are doing with storage from Stem, Ice Energy/NRG and Advanced Microgrid Solutions.
“The third one, which is my favorite, is we’re aggregating all 50 of those systems and providing wholesale grid support,” Hanley said, through the Proxy Demand Resource demand-response program run by grid operator CAISO. “What’s of note here is that it’s a heterogeneous portfolio. It’s the first time we’ve aggregated different technologies and bid them into CAISO. The rules were just recently changed to allow this,” he said, with a big demand-response auction set for later this year, and rules for how distributed assets can play in DR markets still under development for rollout over the next few years.
SolarCity’s control platform, which manages its small but growing fleet of Tesla battery-backed solar homes and businesses, will also control this 50-home fleet as a virtual power plant, capable of responding to utility signals and, in some instances, turning themselves over to utility control, he said. The combination of customer and utility benefits from this arrangement are complex, and “part of the goal of the project is to quantify that,” he said.
SolarCity isn’t the first to bid behind-the-meter battery flexibility into California’s grid markets -- behind-the-meter startup Stem has done that in pilot projects in the past two years. Nor is it the first to test solar-battery grid support and load shifting capabilities. The Sacramento Municipal Utility District has a big residential solar-battery test underway. Southern California Edison has the stimulus-grant-funded Irvine smart grid demonstration test bed, and California’s big three IOUs want to do many more pilots over the next few years as part of their distribution resource plans.
But the project SolarCity is part of is the first to use the standard smart inverter specification so soon to become a mandatory part of California’s new solar fleets. UCSD’s inverter tests will serve as a blueprint for SolarCity to interconnect its various distributed energy assets, Hanley said, although it’s not planning to test every new inverter in its 50-home pilot -- “We’ll probably be using one inverter, maybe two,” although that could expand over time.
More broadly speaking, “we believe the industry is better off the sooner smart inverters are widely deployed, and we want to do everything we can to accelerate that,” he said. California is in the midst of reworking its utility regulations to bring DERs into play with utility grid operations and planning, and stand in for part of utilities' multi-billion-dollar investment plans, to reward what they do for the state's renewable energy and carbon reduction goals. The sooner advanced inverter functionality is part of that DER market, the faster new rules and markets will evolve to express that value in terms of kWh and kVAR, and eventually, dollars and cents.