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Making energy from the sun a more versatile resource by pairing it with storage technologies is gaining momentum as the grid evolves and prices fall.

Storage is “at the same point in the growth curve that PV was at circa 10 years ago,” Bernadette Del Chiaro, California Solar and Storage Association executive director, said. Of the approximately 1.3 million residences equipped with photovoltaic solar systems, about 15%--or 30,000, include battery storage. As long as the state and federal governments continue to remove barriers and create a level playing field for storage, its growth over next decade should continue to follow the trajectory of the PV industry, Del Chiaro added.

Owners of solar-plus-storage rooftop systems can take the excess amount of solar their systems generate during the day when electricity prices are cheap, store it and then discharge the battery during the 4 p.m. to 9 p.m. period when electricity costs much more per kilowatt-hour as demand rises. They also can provide onsite power during outages, be it caused by extreme cold, heat, or wildfires.

To do essentially that and more, Sunrun signed two contracts last year for residential solar-plus-storage capacity with Glendale Water & Power and East Bay Community Energy.

Glendale's 12.8 MW solar-storage deal

Glendale in Southern California is rolling out a $240 million scheme pairing distributed solar and storage through a power purchase agreement with Sunrun. Glendale will purchase the solar power, the associated renewable energy certificates representing the renewable attribute, and the storage from 3,000–4,000 customers and 30–40 multifamily apartment buildings within its service area.

The resulting virtual power plant will provide 28 MW of capacity and 25 MW of 2-hour storage to Glendale’s system over the course of 25 years. It will provide backup power to participating customers in the event of a grid outage. At no cost, participants will host solar and storage systems that will benefit the municipal utility while receiving on average $200 per year.

East Bay is working with Sunrun to line up 1,000 residential backup battery systems, with 200 contracts nearly finalized to date. Customers receive a one-time payment of $1,250. Sunrun manages the network of batteries, an anticipated 5 MW, calling on them when demand is high. When a power outage hits, the home will continue to receive power from the battery.

Community microgrids

Another promising approach is to develop semi-autonomous microgrids where solar generation, storage and consumption is aggregated and optimized within an energy community.

One example is a project financed by the San Francisco Bay Area Air Quality Management District where developer Gridscape Solutions is integrating solar PVs and lithium-ion battery storage systems to provide energy, emergency backup power and grid resilience by relying on EnergyScope, its secret sauce, a proprietary optimization software.

Based on the projected growth of Gridscape’s microgrids, the air district’s Climate Tech Finance team estimates the installations will produce between 1,440 MWh of AC power in 2021 to 21,457 MWh in 2025, Kristina Chu, the air district’s spokesperson, said.

Utility-scale projects

Utility-scale solar developers and thermal generators are co-investing in large-scale storage to enhance the value of the solar generation by stretching its output into the early evening hours after sunset to slim the California grid operator’s “duck curve.” That phenomenon highlights excess solar production in the middle of the day, representing the protruding belly of the duck.

In mid-December, for example, Vistra Energy completed the first phase of the Moss Landing Energy Storage Facility, currently the world’s largest utility-scale battery storage system.

The newly operating 300 MW/1,200 MWh operating lithium-ion battery at Vistra’s Moss Landing Power Plant in Monterey County, south of San Francisco, will add an additional 100 MW/400 MWh by August 2021. That will bring its total capacity to 400 MW/1,600 MWh by the summer peak demand generally occurs.

The battery, which captures excess electricity from the grid, mostly during high solar-output hours, can release it back to the grid when demand is at its peak–and prices are high–after the solar output has declined. Together the two systems allow the discharge of enough electricity to power roughly 250,000 California homes for four hours during evenings, heatwaves, and other times when energy demand outstrips supply, according to Vistra.

This project “revitalizes an existing power plant site and utilizes active transmission lines, enhances grid stability, fills the reliability gap created by intermittent renewables, provides emission-free electricity, supports California's sustainability goals and mandates, significantly benefits the local community, and ultimately provides affordable electricity to consumers,” Curt Morgan, chief executive officer of Vistra said.

Vistra’s investment will be recovered though a long-term resource adequacy contract with Pacific Gas & Electric.

Roofless options

But what about customers who don’t have a roof for solar PVs and/or a garage for storage – since building codes in the U.S. do not allow batteries to be installed within homes? The answer: When there is a need, someone will find a way.

The behind-the-meter space is rapidly evolving in other ways, allowing established companies such as OhmConnect to offer traditional demand response services.

OhmConnect has developed a network of 16,000 homes in Alameda County, a San Francisco suburb, managing pre-programmed smart devices such as thermostats, hot water heaters, lighting, and other connected devices. Participants get paid to sign up and receive bonus points for responding to signals to lower their demand. The average participant can expect $300-$500 a year for reducing load on the grid.

During the mid August 2020 heat wave in California, OhnConnect reduced the energy use of its customers by nearly 1 GWh while rewarding them $1 million. The company is now working to create 550 MW of flexible demand in time for the next summer’s peak demand in California.