Updated: What Worked and What Didn’t at Six School Microgrids in California

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Salinas, California set out out to build six school microgrids two years ago as models for others. Today the project offers what schools do best — lessons — but not only about its successes.

The project developers are talking about their challenges, an all too uncommon, but valuable gift to a rapidly growing industry that needs to know what can go astray.

Located at schools in the Santa Rita Union School District, the microgrids demonstated that they could operate off-grid last year. But some hiccups have delayed their completion.

school microgrids

Solar on a school at the Santa Rita Union School District. Photo courtesy NantEnergy

For example, the microgrids can’t transition into island mode automatically, and school staff members need to be trained to do this. Troubles accessing school property during school hours — and, now, during the COVID-19 crisis — have made it hard to finish work. The project also ran into problems with its inverters and faced delays interconnecting to the utility, Pacific Gas & Electric (PG&E),

The project began about two years ago, when the school district installed solar plus storage at six sites to form microgrids that would provide power during outages. Financed by Generate Capital, the microgrids were designed to provide up to seven hours of power at each school during grid outages. In total, the systems include 1 MW of solar PV that is integrated with 1.1 MWh of behind-the-meter energy storage.

Solar cutting costs and carbon

Solar panels at each school are already offsetting about 80% of the school’s energy and demand usage, creating utility bill savings and environmental benefits, said Ted Flanigan, president and CEO of EcoMotion, which envisioned the project and provided the solar.

About 80% of the schools’ annual usage comes from solar, covering 100% of the bill under a net metering arrangement, he said. “This is a function of PG&E’s old A-6 rate in which summer generation was more valuable and thus provided the district with attractive bill credits.” 

Flanigan added, “When the sun is shining in summer, and there’s no one on campus, a lot of green electrons go on the grid,” and the school district receives net metering credits. That means that those credits can be used in the winter.

The 20% that the district purchases includes some carbon-free power, so the school’s arrays are about 93% carbon free, he said.  

The multi-campus systems will enable the schools to support the local Salinas community as Powered Emergency Response Centers in the event of disasters that cause prolonged outages.

School microgrids as teachers

Another goal is for students to learn from the project.

“The idea is for this to be a model for other schools. The model shows that low income school districts could go solar and get resilience at no marginal cost,” said Flanigan. “The project involves the community, the students and the operation of the schools, and it’s all financed through the private sector.”

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NantEnergy, which provides storage and other clean power solutions, is now tweaking the system to allow the school staff to manually island the microgrids.

AC or DC inverters?

“We can not conclude that one technology is better than the other” — Mario Barbaresso

In addition, NantEnergy concluded that AC inverters — as opposed to DC inverters — are a more viable solution.

The system installed at the six schools uses DC coupling, with solar DC and battery DC going into the same inverter, and then coming out as AC.

“The inverters are bi-directional, they charge and discharge the battery. When the grid is not there, the system is capable of islanding and establishing a microgrid,” said Carl Mansfield, vice president of system solutions at NantEnergy.

The company now only uses AC coupling for its other projects. With AC coupling, solar has its own inverters and the batteries have their own inverters.

“We can not conclude that one technology is better than the other,” responded Mario Barbaresso, president and CEO of CE+T Energy Solutions, Austin, Texas, which provided the inverters.

“It all depends on the way the control algorithm is designed. This requires a good understanding between the inverter manufacturer and the controller designer. Those two have to work together to define the behavior of the microgrid, understand the limitations of each component and provide an optimum solution to customers.”

Interconnection delays

Another challenge has been interconnecting with PG&E, Mansfield said.

Part of the problem has been the lack of standard utility interconnection procedures for solar plus storage systems that have a resilience component.

Utilities are more familiar with net metered solar, which is connected to the grid. But the solar shuts down if there’s an outage, he explained. Utilities are also more familiar with backup generation — diesel plants, for example. With backup generation, the building isolates from the grid and a generator is fired up.

This microgrid project doesn’t fit into either box, said Mansfield.

“We had to go through a lot of design choices to get over the finish line with PG&E. Part of the concern is safety,” he explained. “When you have a system that can generate electric power when the grid is down, the utility needs to make sure linemen are safe.”

Switching into island mode

When the COVID-19 crisis eases and NantEnergy can gain access to the school buildings, the company expects to complete the project and train personnel at each school to switch into island mode.

Switching into island mode is not automatic because the system doesn’t have enough capacity to power the entire buildings. In order to switch over to island mode, school personnel must make sure the ovens in the kitchens aren’t on and that the air conditioning is turned down, said Mansfield. Then the microgrids can be put in island mode.

EcoMotion, NantEnergy, Santa Rita show way forward

In recent years, NantEnergy has seen a huge demand for this type of resilience solution. With lithium ion costs dropping and public safety power shut offs increasing, “businesses are all looking for solutions to help them,” he said.

“We are seeing a huge shift. Most of our systems are becoming larger and longer duration,” Mansfield said. “In California, we’re working with organizations providing community choice aggregation (CCAs). People are looking for these kinds of programs to provide resiliency, including critical facilities and schools.”  As early pioneers in school microgrids, EcoMotion, NantEnergy and the Santa Rita schools are showing them the road forward — and its bumps to avoid.

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Comments

  1. Jay Handleson says:

    Why no word on the components used? If we’re having challenges in inverters meeting their specified performance goals, why not pass that along so the rest of us integrators don’t get sold a bill of goods as well?

    • The inverters were from CE+T Power
      Sharp Energy Systems & Services was the project integrator
      Black & Veatch did the engineering.

  2. “In addition, NantEnergy has made some design changes, concluding that AC inverters — as opposed to DC inverters — are a more viable solution. […] The company has had trouble with the reliabilty of the DC inverters.”

    I hate to nitpick, but for folks working in this industry, this statement is confusing. By definition, an inverter converts from DC to AC, so there’s no such thing as “AC inverters” and “DC inverters.” I’d recommend just referring to “AC-coupled” and “DC-coupled” systems.

    Also, like the commenter above, I think think most of us would love to know which DC-coupled product was used.

    • The inverters were from CE+T Power
      Sharp Energy Systems & Services was the project integrator
      Black & Veatch did the engineering.

  3. Emma Eriksson says:

    Please provide some details on the model number of the DC/AC inverters used. How were there DC bus configured ? What system reliabilities were faced with a DC coupled system and how was that overcome ? What were the efficiencies achieved in the AC coupled system as opposed to the DC system ? What sized batteries were used ? What models ? What was the load profile of the schools ? Why wasn’t automatic load shedding adopted where ovens and such were switched off automatically ?
    Many thanks,
    Emma

  4. There has been many confusing messages sent by this article. CE+T provided some of the inverters during the project but ours are neither DC or AC coupled. Our converter is a 3 ports DC-DC-AC using an internal magnetic core that mixes all energy sources and reroute them where needed. The converter has embedded intelligence that allows efficient grid forming and grid following capabilities along with features compliant to UL1741SA. It is a proven and reliable technology when applied with proper system control practices.

  5. George J Kamburoff says:

    We will see how they compare here. I have an older system with 18 255 Watt panels feeding a common inverter. We added six 370 Watt panels with their own inverters, producing AC, and they all go into the same bus. I was concerned about the differing waveshapes interacting in the bus, but it works.

  6. From the comments above, I can see that “inverter technology” is an unknown to many. There are well established “inverter” manufacturers out there that have been around for about 50 years. YASKAWA a well known VFD manufacturer (Variable Frequency Drive) seems to also be in the solar PV inverter marketplace. The VFD has become quite cheap in comparison to the solar inverter product. The (fact) that since Texas Instruments invented its DSP in the 1980’s, most drive manufacturers are using DSP to generate waveforms to control industrial motor speeds. A solar PV inverter is just a fixed sub-set of VFD controls. Instead of 0-60 Hertz to ramp up an electrical motor, one is looking at 59.5 to 60.5 Hertz switched A.C. waveform out onto the grid. A 224kW VFD costs about $10.5K, what does a 224kW solar PV inverter cost?

    Just sayin’ the average cost of a VFD is lower than a solar PV inverter. There are VFDs out there that are using the new SiC drive finals, just like the newest solar PV inverters. The Solar PV inverter is a (sub-set) of the capabilities of the VFD already on the market.

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