The Solar Trifecta: A Path to Smart Utility-Scale Solar

What can smart solar do for the grid? Higher penetrations of utility-scale solar are coming. Dive into this article to learn more about the potential for solar to provide more than just energy.

“Traditional” utility-scale solar creates operational challenges at high penetrations

  • Traditional utility-scale solar is designed and operated to generate and deliver the maximum amount of electricity in real time. At high penetrations, these systems create significant operational challenges for the electric grid operators (e.g., California duck curve)
  • A common response to this challenge is to simply pair traditional utility-scale solar with flexible natural gas generation. However, this is not the only option as utility-scale solar has the potential to become smarter and provide significant value to the electric grid

In the near term, “controllable” utility-scale solar may emerge as a near-term resource to support electric grid operations

  • Controllable utility-scale solar system uses existing technology to provide additional value and flexibility through targeted curtailments, smoother output, and expanded ancillary services

Over the long term, “smart” utility-scale solar could emerge as a flexible and dynamic grid asset

  • Smart utility-scale solar is capable of cost competitively offering operational attributes that are comparable to conventional generation assets. By leveraging smart inverters and battery storage, smart utility-scale solar can be optimized for the grid, not just maximum output

Smart utility-scale solar could become a reality and may behave more like a dispatchable resource with the convergence of three important market requirements, which we call the “solar trifecta.” The three pillars of the solar trifecta are:

  • Value from Good Grid Citizenship – With smart inverters, utility-scale solar PV systems provide smoother, more predictable output; a broad suite of ancillary services; and targeted curtailment
  • Energy When You Need It
    – Utility-scale solar PV plus storage (PV+S) allows a system to dispatch energy and capacity to meet evening and nighttime load
  • Cost-Competitive Resource
    – Advances in operating intelligence coupled with declines in the cost of utility-scale PV and batteries bring promise for cost performance competitive with other options

Traditional Utility-Scale Solar: Growing Penetrations, Growing Challenges

Controllable Utility-Scale Solar: A Major Step Forward

The past is not always indicative of the future. Rather than the unabated deployment of traditional utility-scale solar, “controllable” utility-scale solar may emerge as a near-term resource to support electric grid operations

  • ScottMadden defines controllable utility-scale solar as a utility-scale solar system that trades some energy output to cost competitively offer operational attributes comparable to conventional generation. Key features of controllable utility-scale solar include the following:
    • Consists of utility-scale solar PV
    • Leverages smart inverters and operating intelligence to provide:
      • Targeted curtailments to better manage evening ramping of conventional generation
      • Smoother, more predictable output by withholding a portion of output ramping blocks of inverters up and down to achieve desired output
      • Suite of grid reliability services, such as regulation (both up and down) and frequency response
    • Competes on cost with new natural gas generation and other conventional generation

Smart Utility-Scale Solar: An Even Better Way

Controllable utility-scale solar becomes “smart” utility-scale solar with the addition of battery storage. Smart utility-scale solar is a dynamic grid resource that could become a significant asset on the grid over the long term

  • ScottMadden defines smart utility-scale solar as a utility-scale solar system capable of cost competitively offering comparable operational attributes to conventional generation. Key features of smart utility-scale solar include the following:
    • Consists of utility-scale solar PV plus battery storage (PV+S)
    • Provides controllability through targeted curtailments, smoother output, and expanded ancillary services
    • Leverages battery storage to provide grid reliability services and offer dispatchable energy around the clock
    • Competes on cost with new natural gas generation and other conventional generation
    • Becomes a dynamic and desired grid asset, rather than a growing challenge like traditional utility-scale solar – optimized for the grid, not just maximum output

The Solar Trifecta: The Path to Get There

There are three market requirements that, when delivered, give a generation asset a competitive advantage:

In combination, these requirements address the constraints of traditional utility-scale solar and advance the deployment of smart utility-scale solar. We call the convergence of these three pillars and emergence of smart utility-scale solar the “solar trifecta.”

Value from Good Grid Citizenship: Grid-Friendly Capabilities

A common view is traditional utility-scale solar has not been a good grid steward because the resource is variable and the generation asset does not provide a broad suite of ancillary services. As a result, other resources are required to respond to solar-related grid integration challenges

  • Passing cloud cover can create significant variation in the net output of a traditional utility-scale solar system
  • In addition, unlike conventional generation which has rotating machines synchronized to the electric grid that can naturally provide reactive power and voltage control, utility-scale solar plants are connected to the grid through an inverter which converts DC to AC energy (non-synchronous), and traditional inverter setting cannot provide a suite of ancillary services
  • So traditional utility-scale solar is dependent upon the grid to provide electrical support, flexibility, and reliability. Increasing penetration of traditional utility-scale solar generation increases the need for additional support from the grid—potentially limiting future utility-scale solar deployments
  • The support provided to the grid by typical inverters and controls in today’s traditional utility-scale solar systems is rudimentary (e.g., disconnecting solar systems during grid disturbances) and does not provide a net positive effect to the grid—not good “grid citizenship”

With smart inverters and operational intelligence, utility-scale solar can become much more grid friendly by performing new functions and providing new grid services which improve stability and reliability rather than reducing it—potentially setting the stage for future large-scale integration of utility-scale solar into the electric grid

  • Solar PV is capable of providing nearly constant output. This is done by designing in additional capacity and controlling the system to meet the interconnection limit (e.g., operating below maximum capacity). When this is done, blocks of inverters can be ramped up to mitigate passing cloud cover, resulting in nearly constant net output. In a sense, this makes solar dispatchable within a range of operation. Targeted curtailments can also allow grid operators to better manage ramps in net load
  • Demonstrations have also shown the capability to provide a suite of ancillary services by trading energy output in favor of regulation (both up and down) and frequency response. In many instances, utility-scale solar is already providing dynamic voltage and power factor regulation based on bulk power system interconnection requirements

Value from Good Grid Citizenship: Demonstrating Potential

Three recent demonstrations have shown the potential capabilities and provide tangible evidence that smart utility-scale solar is capable of delivering multiple grid services, thereby making smart utility-scale solar a better grid steward

  • The first two demonstrations were included in a study designed to prove the ability of utility-scale solar plants to provide various types of active and reactive power control with the implementation of new controls in two different wholesale markets with varying characteristics

Energy When You Need It: Extending Net Output

  • By exporting generation only in real time, traditional utility-scale solar is often unavailable during evening load periods
    • As a result, conventional generation must follow changes in solar output and be available to serve evening load
    • The most extreme example of this dynamic is the California duck curve
      • Conventional generation is limited mid-day during high solar production, but must ramp exceedingly fast to meet evening load
  • Smart utility-scale solar addresses these operational challenges by using PV+S to provide energy and capacity during evening and nighttime hours
    • A PV+S system would be capable of serving evening load by using excess generation to charge battery storage during the day (see chart)
    • As solar production declines in the evening, the battery storage would switch from charging to discharging at a constant output level
    • The result is less strain on the system during noon and more solar power for the system to serve evening load
    • In wholesale markets, battery storage would allow system owners to benefit from higher prices often available during evening hours
    • Depending on market needs, a PV+S system could be designed to meet a variety of operational specifications by altering solar capacity, battery capacity, battery discharge, and system configuration

Declining technology costs—coupled with increasing PV+S system size and the federal investment tax credit—have resulted in a rapid decline in power purchase agreement (PPA) prices for PV+S systems. In September 2015, a PV+S system in Hawaii signed a PPA for 13.9 cents per kWh.

The solar trifecta provides a clear pathway for smart utility-scale solar to offer tremendous benefits to consumers, the environment, and grid management. However, advancements within all three pillars of the trifecta will be needed to ensure scalable deployment of smart utility-scale solar:

  • The opportunity and potential for smart utility-scale solar would be dampened if advancements in any one of the pillars of the trifecta fail to materialize
    • Utility-scale solar must begin providing a broad suite of ancillary services in regular operations, not just in demonstration settings
    • PV+S systems must become cost competitive and in broad use. The National Renewable Energy Laboratory recently estimated the PV+S systems would become cost competitive with standalone PV by 2020
    • Utility-scale solar and battery storage costs must continue to decline in order to compete with conventional generation

However, a critical intermediate step will be the emergence of controllable solar

  • Controllable solar will be achievable by increasing the value from being a good grid citizen (i.e., smoother output, robust ancillary services, and targeted curtailment)
  • The addition of cost-effective storage will allow the transition from controllable to smart utility-scale solar

Several market developments and regulatory decisions could either hamper or accelerate advancement of the solar trifecta and ultimately the deployment of controllable and smart utility-scale solar. Key signposts to watch include:

  • Recognition of good grid citizenship – Most utilities do not compensate or set minimum requirements for solar assets to provide broad ancillary services. As a result, traditional utility-scale solar systems do not leverage these potential capabilities. New and innovative PPA structures and market rules could accelerate the learning curve and encourage future utility-scale solar systems to be model grid citizens
  • Success of early PV+S systems – For evening and nighttime dispatchable solar to gain broad industry acceptance, early PV+S systems must prove their ability to reliably and consistently deliver energy and capacity during evening load periods
  • Continued learning-curve impacts – With increasing installed capacity, both solar PV and battery storage costs have benefited from learning curves as installed capacity grows. Continued declines in technology will be important if smart solar is to become cost competitive

Are we there yet? So what are the key milestone that will mark the dawn of smart-utility solar and a new era for the electric grid?

  • An initial milestone will be controllable utility-scale solar becoming commonplace, supplanting the deployment of traditional utility-scale solar
    • For example, solar output could be curtailed in the late afternoon to mitigate steep ramps for conventional generation
    • In many markets, leveraging the full potential of existing technology may require new PPA constructs, as well as regulatory changes
  • Longer term, a critical milestone will be a PV+S system outcompeting a new natural gas peaking plant and successfully providing energy, capacity, and a broad suite of ancillary services


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Contributing Authors

Paul Quinlan Clean Tech Manager

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