23 April 2012
The term “smart grid” means many things to many people, but for solar and utility engineers, the effort around perfecting solar smart grid tools has focused on refining the grid-capable properties of inverters and storage components. Communications protocols are being drafted and harmonized, and component manufacturers and utilities are entering a new phase of testing these protocols with the grid's next-generation hardware.
“We've gone as far as we can on paper, as far as we can in a committee room,” says Brian Seal, technical executive at the Electric Power Research Institute (EPRI). “Now we need field results, tangible results from field demonstrations that would shed light on how well these paper efforts actually work.”
The new smart communications standards efforts include updating IEEE 1547 (Standard for Interconnecting Distributed Resources with Electric Power Systems) to modify stipulations mandating solar systems automatically shut down when they sense a grid failure, and adding voltage regulation capabilities. Solar working groups are also mapping International Electrotechnical Commission (IEC) 61850 requirements for distributed energy resources to domain-specific protocols, such as Distributed Network Protocol (DNP)3 for distribution system networks, and Smart Energy Profile 2.0 for home-system nodes.
Seal, who served as project manager for the EPRI-led smart inverter/grid communication specification effort, expects the earliest results from field tests of the new specifications to come from large commercial installations, with utility-owned projects taking the lead.
Seal's sentiments are echoed by Michael Mills-Price, control solutions engineering manager at Advanced Energy Industries' solar division. Mills-Price, who is active in the effort to update the IEEE 1547 standard (he expects the new version, 1547.8, will reach a vote sometime in 2013) says the group is focusing on enhancing the voltage regulation and communications capabilities of solar installations of 30 kilowatts and larger, “and leaving the real small systems to more or less just inject watts,” though he believes smaller systems' capabilities will be addressed “as we migrate thru the myriad of conditions of the smart grid.”
Four Advanced Energy 260kW’s inverters were installed at this 1 MW ground mount PV Project in Knoxville, Tennessee. Photo courtesy Advanced Energy Industries
Combined inverter/storage smart grid research is bound to advance through deployments of many sizes.
Among the larger testbeds online is the Prosperity solar generation and storage project run by Public Service Co. of New Mexico (US), near Albuquerque. The 4.9-acre site, which can produce 500 kilowatts of power, includes 2,158 solar panels and 1,280 storage batteries capable of storing 1 MW/h of electricity. The project, which cost about $6 million, received about $2 million of American Recovery and Reinvestment Act funds from the Department of Energy. Prosperity, which launched in September, 2011, is the first fully grid-tied solar storage facility in the US.
Steve Willard, the project's principal investigator, says PNM engineers discovered the specifics of smoothing the PV array's output was quite complex.
“We really have to think hard about optimization,” Willard says. “How much battery is really required to smooth, and when have you done enough smoothing? We realize now that we have to have constraints in mind, and have to understand where and how smoothing PV is benefiting the system.”
Mills-Price says combined inverter/storage smart grid research is bound to advance through deployments of many sizes.
“The PV system or battery, when you're feeding it into an inverter, almost looks the same,” he says. “It's just a matter of setting up operational bounds from the utility's standpoint to say, 'We need at least this minimum amount of output from you over this time window,' so you design the battery system to fit that.”
A technician checks a battery bank at PNM's Prosperity Energy Storage project in New Mexico. The project's 1280 batteries can store up to 1 megawatt-hour of PV-generated energy. Photo courtesy PNM
He expects that either the IEEE or the IEC will address the solar storage/grid relationship at some point not too distant future, and within five to eight years after that, will combine storage and inverter properties together into a common standard.
Harvest potential next target
As inverters' reliability and conversion efficiency have climbed and plateaued, research into how to maximize inverter output — and how to best harmonize output potential with the architecture of a given installation — is likely to emerge as a key focus.
For example, Greg Madianos, product line director for Solarbridge Technologies' microinverter line, contends that, particularly in instances of partial shading, individually inverted modules will provide more optimized power than a centrally-inverted integrated array that suffers from shading issues might: Mills-Price agrees that such issues are important in smaller deployments, but that “as we go to large commercial and utility-type applications that's less of an issue because you're not going to put a 5MW array in the shade.”
Jon Hawkins, PNM's manager of advanced technology and strategy, says the company's experience has borne Mills-Price's observations out. Hawkins says averaging out irradiance sensors on an array introduces a host of data variables that may skew system performance.
“We're doing tests on irradiance sensors and finding they have to be averaged,” Hawkins says. “If they are not averaged, a cloud going over a corner of the array may cause our battery system to react whereas most the site is producing well.”
About the Author
Greg Goth is a writer who covers technology trends and policy.