In our previous TerraBlog post on microgrids, Technical Challenges & Solutions for Facility Microgrids, we share some technical considerations in analyzing a microgrid. This post is part four of our microgrid blog post series and presents a methodology for sizing and modeling a system for resiliency.

TerraVerde Energy has developed two tools to assist in microgrid sizing. The first, TerraGrid, utilizes a Monte Carlo simulation to determine the ideal battery power and duration for a statistical analysis on duration of backup power availability. The second, MegaCharge, simulates daily battery operations (charges & discharges) to determine the strategy that provides optimal financial benefits. Depending on the objective of the study these tools can be utilized in either order.

Sizing a Battery Primarily for Resiliency Purposes

For this scenario, TerraGrid will be utilized first to determine the required battery size for the desired duration of back-up power needs. Then, MegaCharge will simulate the financial benefits associated with that battery.

Given the necessary inputs (facility energy usage profile and solar PV production profile), TerraGrid first determines the worst day of the year based on the usage profile, meaning the day the facility uses the most energy. Alternate days can also be used as inputs into the model. Once the study day(s) are selected, and the associated solar production available, TerraGrid calculates the battery size needed to provide backup power for the desired duration.

With this battery size determined, a Monte Carlo simulation is conducted, randomizing the following:

• The likelihood that the solar is producing as expected
• The starting state of charge of the battery
• The day in which the outage occurs
• The time at which the outage occurs

TerraGrid then provides the probability for that battery supporting the selected loads over various grid outage durations, as depicted in the following graph.

Now that the battery size has been determined, MegaCharge will simulate the financial benefits associated with the battery. Utilizing the same inputs as TerraGrid, as well as rate schedule information, and solar & storage tariffs (e.g., NEM2), MegaCharge optimizes a battery cycling strategy to maximize demand reduction and arbitrage savings (for details on these value streams, see our recent article on Evaluating the Potential of a Microgrid). The optimization is performed by first discharging the battery to reduce demand, then utilizing the remaining battery capacity to store power during periods of low electricity cost, and discharge at higher electricity costs.  The key rule to follow under current regulations is that the battery must do only one of the following:

• Charge from the grid, but not discharge to the grid
• Discharge to the grid, but not charge from the grid, meaning the battery must solely charge from solar production.

It is important that this simulation accurately captures what a real-world battery could achieve given the applicable rules and regulations, actual facility usage patterns (which can be challengeing to predict), and in some cases variable real-time-market energy prices. For this reason, it is important to buffer the demand reduction targets, and to avoid fully discharging the simulated battery.

Sizing a Battery Primarily for Savings and Understanding the Associated Resiliency

For this scenario, MegaCharge will analyze the various battery strategies, as well as different battery sizes to determine the optimal combination to maximize financial benefits. With the determined battery size, TerraGrid will then conduct a Monte Carlo simulation as described above to determine the probability of the battery lasting various grid outage durations.

For our next TerraBlog post, we will share a case study utilizing these tools to create a cash flow projection.