Two sets of single line diagrams were created for this project, one for the system providing additional solar energy to the HQ and one for the system providing backup power to the LLC.
System 1: HQ Solar Power
String Sizing & DC Optimizers
The first step in this process involved us determining the maximum number of panels that could be placed on each string of the inverters. From initial calculations using information from the SunPower E20-327-COM solar panels and local weather data, we found the maximum number of modules per string to be 6 modules for both inverters, indicating we would only be able to use up to 18 panels of the 31 available to us.
However, as we continued research on the available equipment, we discovered that this limitation could be overcome through the use of the SolarEdge P400 DC power optimizers in our system. The power optimizers overcome the traditional string limit by using a proprietary DC-to-DC converter which decouples the PV module input voltage and current from the converter’s output voltage and current. This decoupling allows the associated inverter to operate at a constant DC voltage regardless of the number of power optimizers connected in series or ambient temperature since the optimizers use independent control loops to regulate the inverter input voltage at 350Vdc.
With the optimizers factored into the design, the number of modules per string for the SE7600A inverter was found to support 31 panels total (15 panels per string if divided evenly among two strings), and the SE3000H can support 14 panels total (7 panels per string if divided evenly among two strings). As a result, this non-traditional SolarEdge PV system will allow us to make use of the 30 modules we wish to use.
SolarEdge DC Optimizer Integration
The SolarEdge DC Optimizers were a critical component for our systems viability and installation. Click on the buttons below to learn more about how they work!
When designing the new solar array system to provide renewable power to the HQ, we discussed three possible scenarios for this design with EFS:
- AC Design: place the inverters next to the new solar arrays at the LLC and run an AC line to the HQ for interconnection with the grid.
- DC Design: place the inverters in the basement of the HQ and run DC lines from the solar modules at the LLC to the HQ.
- Field Design: place the inverters in the basement of the HQ and run DC lines from the solar modules in the field adjacent to the HQ to the HQ basement.
Designs 1 and 2 were our original candidates entering the project because they involved us placing most of the panels on the roof of the LLC, a flat roof that would be ideal for the the Dynoraxx racking system we were using. However, after analyzing the costs of installation and obstacles associated with running a conductor between the LLC and HQ, we found Design 3 to be ideal. This analysis is summarized in the table below.
|System||Wire Length||Wire Size||Total Wire Cost|
|AC Design||300ft x 2 wires||2 AWG||$775|
|DC Design||300ft x 4 wires||8 AWG||$800|
|Field Design||100ft x 4 wires||10 AWG||$180|
The AC and DC designs would also involve cutting through a road twice and routing the long conduit run around existing utility infrastructure that exists between the LLC and HQ, including water lines, phone lines, electrical lines, and a geothermal well. On the other hand, the Field design avoids existing utility infrastructure and only involves cutting through a parking lot once. The field design was made available once we determined that the Dynoraxx racking system can be used for ground mount systems as well as roof mount systems, allowing us to locate all of the panels together in a ground mount array in the field adjacent to the HQ.
System 2: LLC Backup Power
The integration of the Powerwall into the current LLC infrastructure involved close collaboration with Tyson’s Facilities Management to ensure that the system cooperated with the existing solar array system and was seamlessly added to the building without major renovation.
After reviewing the LLC simulation results, we decided to incorporate the Powerwall into the LLC by placing it on a subpanel that is feeds off the main the main panel and consists of the Powerwall, solar inverters, and the building’s critical loads. Since the simulation demonstrated the Powerwall’s ability to sustain the additional list of optional loads during prolonged outages under most conditions, we will also add these loads to the subpanel.
The Tesla Backup Gateway will be positioned in between the subpanel and building main panel. This device will monitor the main panel for utility grid outages and disconnect the subpanel from the main panel in the event of an outage, thus isolating the LLC subpanel as a small microgrid consisting of the solar energy, critical loads, and Powerwall. The Backup Gateway then signals the Powerwall to shift into backup mode, in which it will supply up to 5kW of power to loads and inverters, allowing the solar array to continue operation during the outage and supply energy to the building. This will continue until either the 13.5 kWh of energy is drained from the Powerwall or the Backup Gateway senses a restored grid connection, at which point it will switch back to normal grid-powered operation.
Visual Overview of Backup Gateway Installation. Source: Tesla
With respect to space requirements, a closet within the LLC has been located where the Powerwall can be stored. Tyson plans to redesign this space as a learning exhibit for students to learn about sustainable energy development and energy storage when they visit the building.