NSD Partners With Tesla on Battery Storage Project

July  29, 2016

In 2016 the Napa Sanitation District (NSD) implemented a Battery Storage Project, in partnership with Tesla and Pacific Gas & Electric (PG&E). The five Tesla batteries can capture and store 1MW/2MWh of electricity and will help reduce NSD’s electricity costs.
The project is partly paid for through a $1.9 million Self-Generation Incentive Program (SGIP) grant. Tesla is covering the remaining $1.3 million of the total $3.2 million cost. They will also pay for operations and maintenance for the term of the project.
The district and Tesla will split the electricity bill savings currently estimated to be $110,000 per year. The project was conceived of in 2014, with engineering and design taking place in 2015, and permission to operate the batteries received from PG&E in March 2016.
We spoke with NSD’s Robin Gamble Holley, Engineer & Project Manager, Stephanie Turnipseed, Pollution Prevention & Outreach Coordinator and Jeff Tucker, Chief Financial Officer.
BIOSOLIDS & RENEWABLE ENERGYWhat is the history of the NSD wastewater treatment plant?
The treatment plant on the current site was built in 1975. In the late 1990s, NSD became a tertiary treatment facility, and in 2001, the anaerobic digester was added. The capacity of the plant is 15.4 million gallons per day (MGD).
NSD uses a combination of physical, chemical, and biological treatment processes. To physically treat the water, the District utilizes screens at our influent pump station, bar screens at the headworks, then primary and secondary clarifiers. At the primary clarifiers ferric chloride is added. Additional chemical treatment occurs in the flocculating clarifier, the Dissolved Air Flotation (DAF) clarifier and the chlorine contact basins. Biological treatment occurs in the aeration basins, the anaerobic digester and four treatment ponds.
When the digester was installed in 2001, a co-gen system was also installed. The typical daily load of material into the plant and digester only provides the gas to run the co-gen system at 50% of its capacity, although we do generate additional power when FOG (Fats, Oils, Grease) are added directly to the digester.
How did the battery project come about?
One of NSD’s strategic goals is Sustainable Energy. Initially we were looking at solar arrays as a source of renewable energy. Our Renewable Energy Consultant, Gopal Shanker of  Récolte Energy,  brought the Tesla project forward.
We applied for a SGIP grant. Once we were notified that we qualified for the grant we moved forward quickly with design and planning. Tesla and NSD had several planning and field meetings to coordinate the location of the batteries.
Finding a suitable location for the batteries wasn’t an easy task. Areas that appear to be open because there isn’t anything on the surface may have an underground maze of piping, which need to be accessible. The batteries are placed on a concrete slab, which would limit accessibility. Ultimately a location that met both Tesla and NSD’s needs was found.
As a requirement of the SGIP grant, NSD performed an energy audit with PG&E. Several energy saving projects were identified in the audit, so in the process of doing the battery project we learned about other energy and cost saving projects we could implement. This was an unexpected bonus of the battery project.
How does using batteries save you money?
Our electricty bill is based on two components 1) peak electricity demand (or the maximum amount we consume in a 15-minute interval) and 2) electricity usage over time.
Prior to the installation of the batteries, the power generated from the co-gen was used on an ‘as produced’ basis, and lowered our peak demand, but it could not be stored during low rate periods for later use in peak demand periods.
Approximately 90% of the savings from the batteries is based on reducing the demand during the Peak Demand Period and approximately 10% is based on reducing consumption. NSD is currently on PG&E’s E20 rate tariff. The tariff did not change when the batteries went online but will change to E20 Option R once the solar array is interconnected.
The co-gen system is run continuously at varying capacities based on flow into the plant and how much FOG is received (at the grease receiving station) and pumped into the digester. Typically the co-gen system is run at high capacity during the District’s highest energy use periods, which can change throughout the year. For example, at times when irrigation of District lands is running during the day, large amounts of energy must go to powering the recycled water pumps, so the co-gen would be run at high capacity when that irrigation needs to occur.
The amount of FOG delivered to the plant is roughly 8,000 gallons per day. These FOG deliveries have increased methane production in the digester by roughly 40%. This extra methane production provides the fuel to run the co-gen system at approximately 70% of its capacity. This results in more energy produced by the district and substantial energy cost savings for NSD.
The battery storage units don’t have an effect on the gas or energy produced, but the system does allow us to further reduce peak demand charges by charging the battery during off-peak hours and discharging during peak hours.
How smoothly did the project go?
The project went very smoothly. The Tesla team was easy to work with and communicated regularly, although there are still some operational kinks to work out. The short term benefit of the batteries is that there will be a reduction in energy costs for NSD. Once the solar array is interconnected in September 2016, there will be another source of alternative energy which will further improve NSD’s savings profile.
What recommendations would you make to other utilities looking at similar projects?
It is important to have a really good understanding of how energy billing works, and to know the facility’s demand profile. This affects the programming of the system. We learned a lot about the plant’s energy demands that we didn’t know before going into this project, and the more you know in advance, the easier it is to get maximum energy savings.