Sustainable energy is a high priority for the Napa (California) Sanitation District. That leads the operators of the district’s 15.4 mgd wastewater treatment plant to some innovative projects.
One of those projects is the installation of batteries designed and manufactured by Tesla. The batteries help the district make more efficient use of electric power from the wastewater treatment plant’s cogeneration system and its array of solar panels.

“We use a fairly large amount of electricity,” says Jeff Tucker, chief financial officer. “At any given time, we use 500 kW to 1.6 MW. Electricity is expensive, so we try to reduce the energy we consume and produce our own energy. Part of that package and way of thinking is the batteries. They allow us to take some of the energy we create when electricity is less expensive and use it when electricity is more expensive.”

Substantial savings

Founded in 1945, the Napa Sanitation District collects and treats wastewater for some 82,000 residents in its city and surrounding unincorporated areas. The district’s energy demand is relatively high because it recycles about 2,000 acre-feet of water each summer and pumps it to farms, vineyards, parks, cemeteries and other sites.

The batteries are expected to save about $110,000 per year in electric utility charges, to be split between the district and Tesla. “So far, it’s looking very positive,” Tucker says.
Napa Mayor Jill Techel, who also chairs the sanitation district, considers the project a great success. Long-range planning helped put the district in position to capture the opportunity. “It’s just great to be producing green energy and using it at peak times,” she says.

The five white metal cabinets containing lithium-ion batteries stand on a concrete slab. Each cabinet is 8 feet tall, 8 feet wide and 6 feet deep. The batteries reduce energy charges by discharging at times of day when utility rates are highest. They can also reduce the district’s peak demand on the utility grid and thus limit demand charges.

“The battery system is either charging, discharging or waiting,” says Tucker. “If we have a high load, it knows not to charge. It’s constantly sensing our meter and determining what to do.” The battery pack is rated at 1 MW/2 MWh, which means the batteries could discharge 1 MW for two hours or a lesser amount for a longer time.

Operating partnership

The battery project cost $3.2 million, offset by a $1.9 million Self-Generation Incentive Program grant from Pacific Gas & Electric. Tesla paid the rest and handles operations and maintenance. “It’s fairly passive and automated,” says Tucker. “Tesla’s programmers and monitors are all remote. They have telemetry directly into our system. If anything is going on with the batteries, they can tell right away. If it’s something small, they ask us to fix it. If it’s something larger, then they come out. We see them only if there is a problem, and there haven’t been many problems.”

Most electricity generated at the district comes from biogas produced in its 1.2-million-gallon, egg-shaped anaerobic digester, which rises 80 feet above ground and extends 40 feet below ground. At first, the biogas provided about 25 percent of the district’s energy. That increased to 40 percent with the addition of fats, oils and grease from restaurants. The 1 MW, 4-acre solar energy system (SunPower) installed in 2016 boosted the total to 60 percent.

The battery system helps the district manage the self-generated electricity. Typically, it’s charged using electricity from a 415 kW engine-generator, which runs around the clock. “But it could be charging from the solar as well,” says Tucker. “It doesn’t see that distinction.”

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More opportunities

As part of its application for a grant to fund the Tesla battery project, the Napa Sanitation District and Pacific Gas & Electric performed an energy audit that revealed some other potential energy- and cost-saving projects. They include the following opportunities:

• Better dissolved oxygen control. The audit found the district was putting more air than necessary into its aeration process.
• Increased variable-frequency drives. The audit recommended adding VFDs to water pumps and air-exchange pumps. The VFDs are expected to pay for themselves in about eight years.
• Chemically enhanced primary treatment. The audit revealed ferric chloride could be added at the headworks to enhance solids settling. This would capture more solids for conversion to biogas and reduce the secondary treatment load. The district already uses ferric chloride, but would add it at a different process stage.