Aerial photo of a concentrating solar power plant

Every day our power needs fluctuate causing grid operators to make quick decisions to balance the grid. This can happen on hot summer days when people are turning on their air conditioners or in the middle of winter when they crank up the heat. Either way, grid operators must find a way to meet rapid spikes in energy demands.

Most concentrating solar thermal power (CSP) systems today are equipped with energy storage, which serves as a battery within the plant and allows utilities to use solar-generated power whenever it is needed. When grid operators have the choice to determine the best way to power the grid, this creates grid flexibility, making CSP a valuable asset as our grid demands evolve.

Despite steady developments in CSP technology, further innovation is needed to use high-tech components in holistically-designed systems that can rapidly and flexibly respond to consumer energy demands at low costs. With support from the Solar Energy Technologies Office’s (SETO) CSP research program, developments in these areas could improve grid flexibility by unlocking new choices for using CSP to better meet grid operator needs.

Concentrating Solar Power 101

Graphic explaining the concept of concentrating solar power.

CSP systems harness thermal energy from the sun and use this energy to create electricity or heat. State-of-the-art CSP systems use fields of mirrors called heliostats to reflect and concentrate sunlight onto a receiver that sits atop a tall tower. This receiver contains a heat transfer fluid that’s heated to around 565 degrees Celsius and then circulated throughout the system to drive a power cycle that generates electricity. This thermal energy can be easily and efficiently stored in tanks so it can be used whenever the energy is needed to meet demand, not just when the sun is shining. This enables CSP plants to operate independently and without backup fuel sources much like a conventional power plant.  

Size Matters: Flexible Plant Arrangements Can Meet a Variety of Needs

As electricity demands change, CSP’s flexibility as an on-demand resource can be used to the country’s advantage. Smaller systems, with lower up-front costs could be deployed to provide peak power while larger systems with many hours of storage can provide baseload power.

CSP systems are built from similar building blocks: mirrors to collect and concentrate sunlight, receivers to capture it and transfer it to a heat transfer fluid, thermal energy storage tanks, and a power block to convert the heat into electricity. For example, one 50-megawatt (MW) CSP plant can be configured as a type of peaker plant with less than six hours’ worth of energy storage. This plant can be used to supplement baseload generation when there’s a sudden, high spike in energy demand. That same plant can also be used with more than 12 hours of storage and a much larger mirror field to generate baseload power—allowing the plant to provide solar electricity throughout the day and night.

A graphic explaining how concentrating solar power can be customizable as a peaker, intermediate and baseload power.

While CSP plants can be designed in different sizes for different markets, the Energy Department’s solar office is looking ahead to the technology and research needed to ensure that the technology will be cost-competitive. Its 2030 cost targets for CSP peaker and baseload plants will help the solar industry stay on pace as competitive funding opportunities focus on rapid development. Solar Dynamics, for example, is already investigating the feasibility of a modular, molten-salt tower peaker plant that can be easily replicated and rapidly deployed in 24 months or less.

A graphic that looks at the cost targets for concentrating solar power.

Spinning and Non-Spinning Reserves Provide Grid Stability

CSP also provides essential grid stabilization features due to the use of a conventional, spinning turbine that adds inertia to the grid. Utilities and independent system operators (ISOs) are charged with meeting customer energy demands, and when there are rapid swings in energy needs, utilities need to ensure the grid remains stable.

For these needs, utilities and ISOs manage frequency and voltage regulation, short-circuit power, and spinning reserves, which is energy that’s already online and synchronized to the grid’s frequency. This makes it easier to maintain system frequency and quickly dispatch more energy. CSP can be a source of spinning reserves for immediate needs and non-spinning reserves for near-term needs, giving grid operators greater flexibility and control for ensuring reliability.

Putting a More Accurate Price Tag on Reliability Benefits

One of the biggest advantages of CSP is its reliability as an energy source and predictable costs. Unlike conventional fuels, there’s nothing to mine, ship, burn, or store as waste; there’s an abundant, unending supply of sunshine. Because the “fuel” is free, costs are predictable over the lifetime of a plant operation and its maintenance costs. In addition, more than 60% of the cost to operate a CSP power plant happens in the first year, enabling investors to have a better long-term understanding of costs and the return on their investment. 

Graphic explaining the no cost uncertainty for concentrating solar.

To help make the remaining cost of a CSP plant more transparent for project developers and investors, SETO is funding an open source modeling and simulation tool that optimizes CSP plant design and operations. This project accounts for maintenance, field exposure, and even solar generation uncertainties, helping project developers maximize the performance of a plant that that will last for more than 30 years.

This new vision for CSP technology can help grid operators better balance the grid, maximize their available energy resources, and better plan for future energy needs. This increased flexibility empowers grid operators to make the best decisions possible, ensuring the grid remains resilient and secure.

As the country’s energy demands evolve daily, so does CSP technology. While further innovations are needed to create these low-cost integrated systems, the research foundations SETO is laying now—like the Generation 3 CSP Systems funding opportunity—will enable CSP technologies to reach new heights. Its flexibility and predictability will make it a strong contender for meeting our changing energy needs today, tomorrow, and in a 100 years.

Learn more about SETO’s concentrating solar thermal power research. 

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. 

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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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