SUN ON THE WIRES

Will Solar Crash the Smart Grid? Utilities don’t have much time to revamp their grids to accommodate the growing popularity of rooftop solar. Here are some technologies that could help.
Jeff St. John, November 2, 2009 (Greentech Media)

"…The proliferation of solar panels will effectively transform commercial districts and neighborhoods into small, localized power plants. While that will allow utilities to cut back on coal, the unpredictable, varying nature of solar power will force grid operators to dispatch or throttle power rapidly. Solar-balancing smart grid systems now confined to pilot projects will need to become common features pretty soon…

"…[W]hat happens when 20 percent or more of the homes in a neighborhood go solar and a cloud passes overhead? That changes a neighborhood of solar power producers to utility power customers in a matter of minutes – and grids built to deliver power one way at constant voltages and frequencies have trouble accommodating that two-way, intermittent flow…Too much solar power, and local grid voltage could rise, causing potential problems for motors, lights and other equipment. Too little, and voltage can sag. That may only flicker light bulbs at home, but it can lead to million-dollar work stoppages for customers like semiconductor manufacturers and server farms that need clean power at a near-to-constant voltage and frequency…"

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"...[Nobody knows for sure] the maximum amount that the neighborhood distribution grids of today can take…Solving that problem is the focus of smart grid projects for utilities across the country… Some California utilities have already pushed back against efforts in the state legislature to expand the 2.5 percent cap on how much customer-owned solar power they're obliged to credit to customers…But Pacific Gas & Electric has asked the California Public Utilities Commission for permission to expand its cap to 3.5 percent…Right now, PG&E is seeing "some localized issues" with grid instability in neighborhoods where rooftop solar penetration has grown to around 5 percent…But both the Federal Energy Regulatory Commission and the Institute of Electrical and Electronics Engineers, a standards-setting body, have put limits on distributed power sources like solar panels making up more than 15 percent of a distribution substation's load…

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"PG&E and other California utilities might have to face a much larger percentage of their power coming from such sources in the near future…[One solution] is lots of energy storage….A lot of that would be big storage…That could include pumped hydro and compressed air energy storage, which…will remain the most economical form of energy storage, as well as large-scale batteries like the one Southern California Edison wants A123 Systems to build to manage wind turbines…Other batteries could fit into neighborhoods, where they could balance out rooftop solar sags and surges…[including] household batteries linked to solar panels…General Electric, a project partner, has said it will bring "net-zero energy homes" to market by 2015…[M]uch of today's rooftop PV comes with small batteries to cushion household wiring from the panels' ups and downs in power delivery…

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"Utilities…are looking at ways to control existing PV systems…[M]ost systems out there today weren't made to be controlled by, say, a smart meter calling for a battery to discharge power to meet a utility's peak demand…More smart grid-ready systems involving new batteries and inverter technologies would be useful…But they could add too much to the already daunting costs of a home solar system for most customers…Some utilities are looking at cargo container-sized flow batteries or sodium-sulfur batteries at substations…Other utilities are considering a similar, small-scale distributed approach known as community energy storage…Inverters, which convert solar panel DC output into grid-ready AC, could help by delivering short bursts of energy…That could give them a role in niche utility power needs such as frequency regulation…"

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"There's another form of balancing that can be done by subtracting, rather than adding, power to the grid – demand response, or the ability for utilities to turn down customer's power use when they're facing peak loads on the system…Demand response tends to be a centralized affair at present, with utility dispatchers paying agreeable customers and giving them ample advance warning to turn down their big power loads – sometimes via radio signals, text messages, emails or phone calls – when needed…But automated systems running over smart meter communications networks could open up new ways to tap the demand response potential in the home – imagine turning down the air conditioner when the solar panels detect a cloud passing overhead…

"Of course, utilities that can predict when solar panels are about to fade could fire up peaker plants or dip into demand response capacity to match it. That's where microclimate forecasting comes in…Every utility forecasts the weather as a part of their day-to-day business. It's hard to miss a heat wave's effect on air conditioning loads. Pinpointing local weather conditions in real time – and then adjusting the local grid in response – is a more complicated matter…Given the complications, projects to integrate solar panels, storage and smart grid systems are more easily contemplated in smaller units…"