NewEnergyNews: TODAY’S STUDY: The Many Ways To Use Solar Plus Storage


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    Tuesday, November 06, 2018

    TODAY’S STUDY: The Many Ways To Use Solar Plus Storage

    Solar Plus Storage System – Configuration Impacts Operational Flexibility And Economic Value; Solar power coupled with energy storage, and its linkage with the grid, gains interest.

    October 30, 2018 (ScottMadden)

    National Renewable Energy Laboratory (NREL) Explores Tradeoffs among Different Configurations

    ∙ As states like California continue to aggressively pursue carbon-free power generation and solar power penetration increases, more attention is being paid to solar plus storage systems to help manage temporal variations in output

    ∙ In an August 2017 report, NREL explored a variety of photovoltaic (PV) plus storage system configurations

    ∙ NREL’s modeling exercise assumed a 50-MW, fixed-tilt PV system and 30 MWs/120 MWh of battery storage in Southern California. The PV system included an inverter loading ratio of 1.3, i.e., panel capacity exceeded inverter capacity. This common practice results in solar output being “clipped” or lost during peak production

    ∙ The report noted that traditional levelized cost-of-energy metrics will always be higher for solar plus storage systems because storage adds overall costs to the system; therefore, the study examined a benefit-cost ratio defined as the annualized benefits (energy revenue and capacity value) divided by the annualized cost (capital and operating expenses)

    ∙ Key factors driving the benefit-cost ratios of solar plus storage configurations include balance of system costs, operational flexibility, and the 30% federal investment tax credit (ITC)

    Independent PV and Storage Systems

    ∙ Systems operate at different locations and do not share hardware components

    ∙ Storage responds to overall grid conditions and stores energy from any grid source

    ∙ Configuration represents the vast majority of PV and storage systems currently operating

    AC-Coupled PV Plus Storage System

    ∙ Systems are co-located and share point of common coupling on the AC grid

    ∙ Reduces balance of system costs, including siting, permitting, engineering, and land costs

    ∙ With no common hardware components, storage system can store energy from any grid source and can act independently of PV system, but may not get ITC (see next page)

    DC-Coupled PV Plus Storage (Flexible Charging)

    ∙ PV and storage are coupled on the DC side of a shared bi-directional inverter

    ∙ Configuration allows storage system to charge from the grid and PV system

    ∙ Storage system can capture clipped solar output when panel capacity exceeds inverter capacity, but decreased operational flexibility due to single inverter (see next page)

    DC-Coupled PV Plus Storage (Tightly Coupled)

    ∙ PV and storage are coupled on the DC side of a shared DC to AC-only inverter

    ∙ Storage system can capture clipped solar output, but operational flexibility is further reduced as storage can only charge from the PV system

    ∙ Storage systems in tightly coupled configuration can receive full ITC

    Battery Storage Can Qualify for ITC

    ∙ Battery systems that are charged by a renewable energy system more than 75% of the time are eligible to leverage the federal ITC, per private letter rulings from the U.S. Internal Revenue Service

    - Battery systems must meet eligibility requirements (i.e., 75% renewable charging) on an annual basis for a period of five years

    - Eligible battery systems may claim an ITC value equal to the proportion charged from renewable energy (e.g., 80% renewable charging results in 80% of ITC), significantly improving battery project economics

    - If renewable charging drops below the benchmark of the first-year percentage, then the system may be subject to recapture provisions (i.e., system owner must pay back proportional amount of the tax credit claimed in earlier years)

    Coupling Limits Storage Utilization and Results in Non-Optimal System Dispatch

    ∙ The two NREL examples (see figures at right) illustrate how DC coupling of solar and battery storage can create times when storage cannot be fully utilized because of PV system operations

    ∙ In the independent configuration, the two inverters allow the solar and storage systems to operate separately, resulting in a combined output as high as 70 MWs

    ∙ Meanwhile, the DC-coupled system requires the solar and storage systems to share one inverter, thereby limiting total output to 50 MWs

    Despite Operational Limitations, Coupling Increased Value to System Owner ∙ NREL explored the benefit-cost ratio of solar plus storage in 2020 by examining two ITC scenarios (30% ITC and no ITC) and two different solar penetration scenarios (15% and 24% PV penetration)

    ∙ In the 30% ITC scenario (left chart above), tight DC coupling produced the greatest benefit-cost ratio as the storage system leverages full ITC value. In addition, the value of a PV only system collapses with 24% PV penetration, yet solar coupled with storage retained a favorable benefitcost ratio

    ∙ In the no ITC scenario (right chart above), all coupling scenarios show a higher benefit-cost ratio than the comparable PV-only system design

    ∙ Driven by technology cost declines, the modeling results show many cases where solar plus storage is more favorable than standalone PV in 2020


    System configuration will be an important consideration as solar plus storage garners increased attention as a dispatchable resource.

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