What is the Duck Curve?

Learn about the duck curve and how solar can help balance hourly energy loads.

In 2013, the California Independent System Operator published a chart that is now commonplace in conversations about large-scale deployment of solar photovoltaic (PV) power. The duck curve—named after its resemblance to a duck—shows the difference in electricity demand and the amount of available solar energy throughout the day. When the sun is shining, solar floods the market and then drops off as electricity demand peaks in the evening. The duck curve is a snapshot of a 24-hour period in California during springtime—when this effect is most extreme because it’s sunny but temperatures remain cool, so demand for electricity is low since people aren’t using electricity for air conditioning or heating.

The duck curve represents a transition point for solar energy. It was, perhaps, the first major acknowledgement by a system operator that solar energy is no longer a niche technology and that utilities need to plan for increasing amounts of solar energy. This is especially true for places that already have high solar adoption, such as California, where one day this past March, solar contributed nearly 40% of electricity generation in the state for the first time ever.  

Utility Challenges

California Independent System Operator

High solar adoption creates a challenge for utilities to balance supply and demand on the grid. This is due to the increased need for electricity generators to quickly ramp up energy production when the sun sets and the contribution from PV falls. Another challenge with high solar adoption is the potential for PV to produce more energy than can be used at one time, called over-generation. This leads system operators to curtail PV generation, reducing its economic and environmental benefits. While curtailment does not have a major impact on the benefits of PV when it occurs occasionally throughout the year, it could have a potentially significant impact at greater PV penetration levels.

While the mainstream awareness of these challenges is relatively recent, the U.S. Department of Energy’s Solar Energy Technologies Office (SETO) has been at the forefront of examining strategies for years. Most of the projects funded under SETO’s systems integration subprogram are performing work to help grid operators manage the challenges of the duck curve.

Duck Curve Solutions

Using Storage to Improve Grid Resiliency

Learn about SETO's project with Austin Energy.

Solar coupled with storage technologies could alleviate, and possibly eliminate, the risk of over-generation. Curtailment isn’t necessary when excess energy can be stored for use during peak electricity demand. SETO launched several projects in 2016 that pair researchers with utilities to examine how storage could make it easier for utilities to rely on solar energy to meet customer needs around the clock. This research will enable even more solar energy to be integrated into the grid, while tackling the obstacles utilities face when incorporating solar.

In 2012, SETO also launched a research program that helped utilities, grid operators, and solar power plant owners to better predict when, where, and how much solar power will be produced. More accurate solar power predictions, known as forecasts, allow utilities and electric system operators to better understand generation patterns and maximize solar resources. One key success came from IBM, whose machine-learning technology enabled prediction accuracy to be improved by 30%. However, as the amount of solar energy generation connected to our electric grid continues to grow at a rapid rate, further improvements in predictive accuracy will be needed.

Bringing it Back to the Duck Curve

There are many potential solutions to the duck curve. The lessons learned from SETO’s projects will be critical to improving the flexibility of the grid and addressing over-generation risks as solar grows throughout the country. According to the Energy Information Administration, the installed amount of PV is expected to triple by 2030—potentially migrating the duck curve outside of California. New and improved technologies will allow PV to provide on-demand capacity and fulfill a greater fraction of total electricity demand.

Learn more about our work to improve grid integration.

A new solar collector is starting a trend when it comes to concentrating solar power (CSP) technology. For the first time ever, “ganged heliostats” could be a viable option for new CSP systems.

Skysun, a startup out of Bay Village, Ohio, developed the new design that could help cut the cost of a CSP system by more than 30%.

Ganged Heliostat Technology

CSP technologies use mirrors to reflect and concentrate sunlight onto receivers that collect solar energy and convert it to heat. The mirrors, also known as heliostats, typically require their own base, foundation, and motor.

Skysun’s solar collector groups together heliostats through shared motors and support structures, which has the potential to cut the total installed cost of CSP systems in half. While other ganged heliostat concepts have previously been proposed, none of them have shown to be cost competitive or viable—until now.

Ganged heliostat prototype installed at Sandia National Laboratories' National Solar Thermal Test Facility.

SkySun partnered with Sandia National Laboratories through a $275,000 Small Business Vouchers project funded by the U.S. Department of Energy (DOE) SunShot Initiative. Sandia reported that Skysun’s ganged heliostats can achieve an average price point around $80/m2. That’s 33% lower than the lowest average cost for today’s conventional heliostats ($120/m2) and close to the SunShot Initiative’s goal of lowering the cost of solar collectors to $75/m2.

Path to Market Adoption

Skysun’s biggest barrier was showing that the technology is not just comparable to current heliostats in terms of performance, but more affordable. They used a grant from Innovation Fund America to build their first lab-scale prototype, then worked with Sandia to model and optimize the system. Alongside Sandia, Skysun designed custom codes for mirror positioning to reduce shading from other mirrors within the system, making its peak efficiency comparable to those deployed today. So far, modeling on Skysun’s solar collectors show that its mirrors achieve CSP industry accuracy standards with winds up to 15-20 miles per hour.

Skysun founder Jim Clair believes he will be able to leverage the outcomes from Skysun’s collaboration with Sandia in his search for a strategic partnership to prepare this technology for market adoption. Describing Sandia as “the mecca for CSP,” Clair said Sandia’s support in demonstrating the ganged heliostat’s stability, performance, and cost will be instrumental in showing the technology’s viability to potential partners.

Learn more about the SunShot Initiative and Tech-to-Market program within DOE’s Office of Energy Efficiency and Renewable Energy. 

Read More

Learn more about Tech-to-Market’s Small Business Vouchers program, which opens the national labs to qualified small businesses by making the contracting process simple, lab practices transparent, and access to the labs' unique facilities practical. 

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Anurag Garg, Vice President – Solar Business, Schneider Electric India

1. Throw some light on the current size and the growth seen in the solar inverter market in India? What are some of the key market trends?

The global solar photovoltaic (PV) inverters market is anticipated to capture an aggregate market value of over $22 billion during the period 2018-2022. In terms of market share – China, US, and India were the top countries and collectively they held over 70% of the global market share in 2017 with respect to installed capacity.

China led the solar PV inverters market globally with 41 GW of installations in the year 2017 wherein it held over 49% of the global market share. The US emerged as second-largest market globally for solar PV inverters in 2017, registering 13.2 GW. It accounted for more than 15% of the global market during the year. India ranked third, next to the US, with a market volume of 8.07 GW in 2017, accounting for nearly 9.6% of the global market.

The PV inverter market in India and other emerging markets like South Korea and Brazil is expected to further gain momentum as a result of the growing power demand in these countries and their commitment to shift towards clean energy.

The solar inverter market is extremely competitive characterised by shrinking margins and new players. In this environment, it is important for the manufacturers to stay ahead of the competition curve through regular upgradation and by leveraging advancements in technology. Technology trends such as DC/AC ratio optimisation, increased voltages and neutral point clamped topology are a few of the means that global inverter manufacturers are adopting. This helps in keeping the costs down and increasing efficiency.

2. How do you look at the competition in this space? What are your plans and strategies to stand out of the crowd?

Solar inverter market is extremely competitive and to stand ahead of the competition across business verticals, some of our focus areas include:

Solution view: Our products are always part of a modular system: electrical, mechanical, digital. The sum of the parts always work better together from a customer application perspective.

Digital: We work with our entire ecosystem to ensure all Schneider Electric products are connected with open protocols and cybersecurity integral to the design. In this endeavor, we have developed an innovative platform – EcoStruxure™. The EcoStruxure architecture enables the ability to create an interoperable and open platform which is IoT enabled.

Innovative, Simple & Easy: Simplicity and ergonomic design are mandatory in all our offers that are intuitive and easy to design, commission, install, operate, and service

Services: We are partner for our customers and customer satisfaction is paramount to us. Deployment of our best-in-class solutions and products are followed by world class quality service across countries.

3. Tell us about your company’s product offering in PV inverter segment and your current manufacturing capacity? Also, brief us on some of the product’s salient features that will give it an edge in the Indian solar market?

At Schneider Electric, we offer inverters for different applications from utility-scale power plants, de-centralised solar farms, residential and battery backup.

The Conext SmartGen inverter is a 1,500 V cloud-connected power conversion platform for utility-scale renewable power and energy storage with unmatched reliability, lower total cost of ownership and faster return on investment. It combines the best in power conversion technology with Industrial Internet of Things (IIoT) to provide a better levelized cost of energy. It highlights 30+ year service life, with a predictive maintenance feature that reduces maintenance costs by making decisions based on actual operating conditions.

The Conext CL-60 string inverter is the ideal solution for decentralized power plants and large commercial buildings, offering a highly integrated configurations, easy installation, commissioning and services, and world-leading efficiency performance.

The Conext XW+ hybrid inverter/charger is a complete solution for grid-interactive and off-grid, residential and commercial, solar and backup power applications. The flexible architecture supports single or three-phase off-grid systems from 7.0 kW to 76.5 kW. It also supports charging of lithium ion battery packs.

We seek to provide our customers with tailored and optimised solutions for their needs. Our cloud-based monitoring software allows us, in combination with our other services, to deliver greater efficiency than our competitors. We are able to help our customers analyse their data and optimize their parameters for increased profitability.

4. In your view, what are some of the key challenges in the market?

Some of the key challenges that are faced by the solar inverter segment are:

Bankability – Bankability of Inverter supplier is extremely critical for solar plants as they need to last for 25-30 years. Therefore, Inverter is critical for the Plant operation & life, making it important to choose a bankable supplier.

Schneider Electric is a 181 Year-old company, global business operating in over 100 countries, having dedicated business for Solar & we have the scale to support installation anywhere.

Incorrect installation: Construction of Solar inverters is generally outsourced to EPC companies which in turn appoints installers for final installation. In this perspective, the installers’ expertise plays a pivotal role in ensuring correct installation of the inverters. A common problem that occurs in the installation process is the incorrect programming of the invertors.

At Schneider Electric, considering the criticality, we have a dedicated team who verifies all the parameters and make sure the EPC have correctly installed all the solution. We offer this service to our customers to ensure they have the insurance of a leading partner. 3

Overheating: Solar inverters made up of electronic components are sensitive to temperatures. High temperatures will lead to a significant reduction in production, and can even result in a production stop if the maximum operating temperature is reached. It is therefore, imperative to asses and install cooling technology in the production line to achieve maximum output.

At Schneider Electric, Our Smart gen range of inverters have double circuit cooling and high flow low pressure system, to address the cooling requirements of Inverters due to high power, in most stringent weather conditions around the globe and especially in India.

Isolation Fault: Isolation Fault is a result of a short-circuit between various parts of the circuit. The short-circuit is usually the result of a combination of moisture and damage to the sleeve on the cabling, faulty installation, poor connection of the DC cables to the panel. In the event of an isolation fault, the inverter will work at the minimum “required” isolation level or will stop working which leads to a loss in production. Special emphasis therefore should be laid on procuring high quality DC cables and ensuring their correct installation. Inverters should have a robust DC protection which is supported by DC fuses and DC motorized Air Circuit Breakers(ACB) to add to the safety, reliability and protection of the equipment.

Ingress Protection(IP) of the Inverters – In India, Solar plants are generally developed in areas with high radiation which generally have harsh external environments like deserts or coastal areas, and thus it is very important that the Inverters, especially, Outdoor type in Large Power Plants have the right level of IP Ratings for protection from any extreme weather condition, for safety, as well as ensure performance of plant and its life.

Schneider Outdoor Inverters are designed considering such arid and harsh external environments, and therefore are 1500V Solar Central Inverters (Smart Gen) are rated IP65/NEMA4X to suit all kinds of environments with Aluminum enclosure.

5. What are your future plans?

We have been working towards ensuring energy efficiency while honoring individual’s right to clean, safe and reliable, sustainable and un-interrupted power. Today we are adding our competencies in the IoT space through digitisation and advisor software for better anticipation.

Hence, digitisation will remain at the core of our strategy and we will continue addressing the energy dilemma of our country through our various products and solutions. Almost all energy equipment of the future will demand system-on-chip technologies; when we integrate energy, automation and software with connectivity, we drive greater energy and process optimisation.

Our focus thus will be on Innovation at every level to transform the places where we live, work and play riding on the wave of the Internet of Things, making everyone and everything more connected.

Finally, we define projects in the context of an access to energy program that allows people who do not have electricity to be able to obtain solar energy. Life is On.

The Paris Agreement calls for a “balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of the century.” This emphasis on “a balance” – or what is also referred to as “net-zero emissions” in the case of the energy system – is a critical development because it recognises that surface temperature warming is directly related to the cumulative total of carbon dioxide (CO2) emitted to the atmosphere.

As climate change is a key issue to deal with and it is a cumulative problem, there is a need for the energy system to get to net-zero emissions to bring a halt to the rising level of carbon dioxide in the atmosphere. However, the time period society has to achieve this is less than it will take to find and/or scale up alternatives for all energy services, goods and products for 7.5 billion people that are currently provided by fossil fuels. In other words, society needs to fill an important gap between when we need to get to net-zero emissions and when we can actually get to zero emissions. The former is around 2070 to meet the goal of the Paris Agreement, whereas the latter may not be until sometime in the 22nd century.

That gap will be filled by “sinks”, which includes;

  • Geological storage of CO2 with carbon capture and storage (CCS);
  • Increasing the carbon stock in products held by society (i.e. carbon capture and use or CCU);
  • Increasing the carbon uptake of the biosphere through land management practices such as reforestation.

CCS and CCU will include;

  • Direct capture and storage applied in large point source emitting facilities such as cement plants and smelters;
  • Energy facilities operating with a sustainably produced biomass feedstock resulting in net removal of CO2 from the atmosphere when capture and geological storage is applied;
  • The production of various products, such as plastics, from fossil fuels or biomass.

All of the above feature in the new Shell scenario, Sky – but sharing the understanding of how they fit into the energy system and what can be achieved was one of the communication challenges behind the Sky scenario publication that was recently released.

Sky reaches net-zero emissions in the energy system by 2070, resulting in stabilisation of atmospheric CO2 and a limit on warming of around 1.75°C in 2100, or well below 2°C as called for by the Paris Agreement. By the time 2070 is reached, the global power generation system is largely renewables and nuclear, but fossil fuel use in certain modes of transport and heavy industry remains significant, albeit declining. The remaining fossil fuel emissions must be dealt with to achieve the goal of net-zero emissions.

To help scenario readers understand the change and see how emissions are managed, we developed a chart that illustrated the carbon flows in Sky as time went by. The sequence starts in 2020 and includes visual elements to help the reader understand what is going on. For starters, we used strata to show;

  • The upper lithosphere where fossil fuels are found;
  • Surface based energy activities;
  • The atmosphere.

Legend for Balances

Balance 2020

In the 2020 illustration above, fossil fuel production results in 35.2 Gt of CO2 emissions, from a potential of 37.8 Gt based on actual extraction. The materials produced include bitumen for roads and petrochemicals that don’t end up as energy products (e.g. combustion of plastic waste). The biofuel energy system is also shown, which results in a loop as CO2 is absorbed by plants, then released again as the biofuel is manufactured and finally used for energy. At this stage there is no real interaction between the systems.

Balance 2050

By 2050, the scene is very different. CCS has emerged at scale, both directly in fossil fuel applications such as industrial facilities using natural gas in furnaces and indirectly in the bio-energy system, where geologically stored carbon amounts to permanent removal of CO2 from the atmosphere. By 2050, BECCS (bioenergy with CCS) has also emerged at scale – this is the use of biomass for energy service provision, although principally for the generation of electricity, but linked with CCS.

BECCS is illustrated in the diagram below and is a technology that exists in parts today but hardly as a whole. For example, biomass is used to make ethanol (corn and sugarcane) at large scale in both the USA and Brazil and is a major energy source (woodchips) for the power generation sector in Sweden. CCS also functions at some forty sites around the world, but the combination of bioenergy with CCS is very limited. One such example is the Arthur Daniels Midland ethanol plant in Illinois, USA which stores one million tonnes per year of CO2. As this CO2 is from the ethanol process itself, it is, in effect, drawdown of CO2 from the atmosphere.

By 2070 the energy system has reached net-zero emissions, primarily through a scale-up of the bioenergy with CCS sector when compared with 2050.

Balance 2070

Finally, by 2100, the energy system is net-negative, drawing 6.4 Gt of CO2 per year from the atmosphere. Fossil fuel use has declined further to 13.7 Gt equivalent of CO2, but this is all captured through one of the mechanisms discussed above.

Balance 2100

As BECCS does not exist at scale today, there are always questions about this pathway forward. However, bioenergy does exist at scale and CCS has been demonstrated many times at scale in early projects, so the combination of the two is entirely plausible. A competing future technology for drawdown of CO2 from the atmosphere is direct air capture linked with CCS (DACCS), but this doesn’t exist today other than at pilot plant size. Experience has shown that the time taken for energy and energy related technologies to move from pilot plant to meaningful commercial scale (i.e. >1% of system size, so in the case of DACCS, >200 big installations around the world) is measured in decades rather than years. For this reason, the drawdown pathway selected in Sky utilized BECCS rather than DACCS, but the latter should not be ruled out from future consideration.


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