The Renewable Energy “Duck Curve”

The California grid operator (CAISO) has a high of share of  renewable energy.  CAISO has adopted a mascot that sums up the challenge: the duck curve.

The chart at left presents the duck curve.  It shows net power load during the day.  CAISO defines net power as the difference between forecast load and expected power production.  In certain times, net load curves have a “belly” in the middle of the afternoon given a surge in solar power production.  The chart then shows a rapid ramp-up in net load – as much as 13 to 15 GWh – as solar production declines and demand accelerates in the early evening.

Energy Goals Drive Create the Duck Curve

Energy policy in California is driving electric grid changes. Key initiatives
  • 50% percent of retail electricity from renewable power by 2030;
  • Greenhouse gas emissions reduction goal to 1990 levels;
  • Regulations in the next 4-9 years requiring power plants that use coastal water for cooling to either repower, retrofit or retire;
  • Policies to increase distributed generation; and
  • An executive order for 1.5 million zero emission vehicles by 2025.

New Operating Risks

The new operational risks include:
  • Short, steep ramps – when the ISO must bring on (or shut down) generation resources to match demand quickly, over a short period of time;
  • Oversupply risk – when more electricity is supplied by solar and hydro than is needed to satisfy real-time power requirements; and
  • Decreased frequency response – when less resources are operating to adjust electricity production to maintain grid frequency and reliability.

Teaching the Duck to Fly1

The new challenges force some utilities to question the long-term use of variable energy resources.  Flattening out the curve – so there is less ramping for traditional generation – will be the key.  The list below details how to avoid steep ramps using existing tools:

  1. Target energy efficiency in the hours with sharp ramps. Lighting is a significant portion of the load from 4 p.m. to 7 p.m. A switch to LEDs — in both homes and businesses — could have a notable impact on the curve.
  2. Orient solar panels to the west. Solar panels are often fixed on south-facing rooftops to maximize total energy production.  If the goal is matching the demand curve, solar panels might need to be tilted westward. Design engineering shows that west-facing solar panels produced 49% more electricity during peak demand compared to south-facing panels.
  3. Substitute solar PV production with some solar thermal generation. Simple really.  If solar PV causes the duck curve, then reduce and shift it.  Swapping a fraction of the solar PV to solar thermal would allow energy to be stored and used later.
  4. Implement new specs to better manage water heating loads. Water heaters have been used for decades as simple load control devices by many utilities. Supercharging water heater specs could offer as much as 500 megawatts of storage in any hour.
  5. Require air conditioners with thermal storage capacity and grid operator control. Air conditioning load is responsible for most of the peak use during the summer, and any solution should tackle commercial and residential AC units. AC thermal storage, such as the technology from Ice Energy, offers as much as 2 hours of thermal storage combined with automated demand response.

More Solutions, More Innovation

The grids themselves must also innovate to manage the duck curve:

  1. Apply utility demand charges in the “ramping hours”. Time-of-use pricing and demand charges are increasingly being levied on commercial customers. But instead of just charging customers more when demand is highest, rates should also match the periods when the ramp risks are highest.
  2. Retire inflexible generating plants. Older coal and nuclear plants just don’t play as well with solar PV as do newer gas-fired plants. Older plants also have higher baseload requirements in off-peak hours.  Low gas prices and new regulations are already doing this, but the limited ramping capability is also a reason.
  3. Deploy electrical energy storage in targeted locations. Flywheels, batteries, compressed air storage, pumped hydro and even electric vehicles could be deployed in areas with a lot of solar to take advantage of surplus generation.
  4. Implement aggressive demand-response programs. The U.S. is already a world leader in demand response, both to curb peak and to provide ancillary services like grid balancing. But load management capabilities can increase in many markets.
  5. Use interregional power exchanges. The power profile of the hydro-rich Northwest is vastly different from sunny Southern California. Regional planners have already started to take advantage of on-peak power exchanges, but there could be a whole lot more activity.  In the West, there are more than 8,000 megawatts of interregional transfer capability using the existing transmission system, and new capability is being developed.

Show 1 footnote

  1. Lazar, Jim, “Teaching the Duck to Fly” Regulatory Assistance Project, 2014
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