In this new blog series, we ask Energy Department researchers about their life as scientists working with energy efficiency and renewable energy technologies. Our aim is to inform readers about how scientific research is performed, learn from the people who produce our technological marvels, and to increase awareness of how this work impacts our nation’s energy needs.

Our first interview is with Dr. Sarah Kurtz, research fellow, National Renewable Energy Laboratory (NREL), Golden, Colorado; professor, University of California-Merced. Charlie Gay, director of the Solar Energy Technologies Office, sums her up in one word — passionate.

“For over 30 years, I’ve had the pleasure of working with Sarah Kurtz in various roles including the Director at NREL.  In fulfilling her responsibilities in varying roles, Sarah has developed technologies, mobilized diverse stakeholder groups and crafted information in form and substance suitable for a wide range of audiences. All of Sarah’s efforts are unified by a motivation to deliver unwavering support for globally affordable and reliable solar power.  In her dual role as an NREL Research Fellow and as Professor at UC-Merced, Sarah continues to inspire all of us and will lead future generations to even higher achievement.  Thank you, Sarah.”

Kurtz began working at the Solar Energy Research Institute—now the National Renewable Energy Laboratory (NREL)—in 1985. Educated as a chemical physicist at Harvard University, she is best known for her work at NREL in III-V multi-junction cells and for her efforts to improve the reliability and quality of solar energy systems.

Dr. Sarah Kurtz has worked in solar for more than 30 years

National Renewable Labortory

How did you decide on a career in science?

I always did well in math and enjoyed it. But it wasn’t clear what practical value math would provide by itself, so I thought I could apply my math skills to science. I was fortunate to have the opportunity to work with those who were looking for ways to solve the energy crisis in the late 1970s. 

Like most people, I am very pleased when I can do a little something to make the world a better place. Enabling solar energy in the United States appeared to be a wonderful opportunity.

What are the biggest challenges you've encountered as a scientist?

The two biggest challenges I have undertaken are creating high-efficiency multi-junction III-V cells and improving photovoltaic (PV) reliability.

With high-efficiency multi-junction solar cells, the challenge is doing everything right at the same time. It’s a little like being an Olympic gymnast—even a small hesitation could prevent getting the perfect score. There were many challenges along the way.

Growth of a multi-junction cell may include more than 50 steps—sometimes more than 100 steps—and each of these steps requires specifying between one and two-dozen control values like gas flow rates, gas flow direction, temperatures of baths and temperature of reactor. Merely designing the “recipe” that will fabricate the cell requires substantial review for typographical errors.

 If you’d like to contribute to scientific research, consider what position you’d like to “play.”

Dr. Sarah Kurtz

National Labs Improving Photovoltaic Technology

How do you improve PV Reliability?

To improve PV reliability, predicting the long-term durability of PV modules is like trying to hatch an egg in about six hours. Because PV modules can last more than 20 years, we need to simulate these conditions in the lab in a condensed time period to understand their durability and reliability. The solution, historically, has been to overdesign the modules so that they will last a long time, while also trying to improve our ability to understand failures in a more quantitative way. 

Is there any advice you'd most like to give a young student who's thinking about becoming a scientist?

Think about how you’d like to fit in.There are scientists that must market the project to a sponsor; otherwise, there won’t be funding for the work. Some scientists don’t like the marketing part of the job. There are also scientists that might prefer to get the work done in the lab. Many scientists are introverts and would prefer not to spend a lot of time in meetings. Some like programming; others like doing things with their hands.

Getting the job done requires a team. Just as a football team benefits from having a mixture of skills, a research team requires a broad set of skills and contributions. If you’d like to contribute to scientific research, consider what position you’d like to “play.”

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.

Generating your own electricity with solar photovoltaic (PV) panels works anywhere in the U.S. year-round. There’s plenty of sunshine for PV, even in winter, although at a slightly lower production level. Germany leads the world in solar power output and it doesn’t have a sunny climate. Its solar radiation is about the same as Alaska’s. So, cloudy days will come and go, but on average, it’s not going to affect the return on investment of solar panels significantly.

#1 Solar’s never been cheaper
Over the past ten years, PV panel prices have dropped more than 50 percent. Concurrently, solar leases and power purchase agreements have made getting solar easier—with little or no upfront costs. As for money, there are many new options for long-term, low-interest loans for clean energy and many states, utilities and others offer a variety of tax credits, rebates and incentives.

#2 Solar saves on utility bills
Solar gives immediate utility bill savings, but long-term savings are becoming less predictable. In Hawaii, California and other states, utilities are changing to time-of-use rate programs for homes with solar that charge more for electricity consumed during peak use periods (evenings) and less during off-peak periods (midday). Still, having solar will mean paying less than not having it, but maximizing savings may require adding energy storage to shift your electricity load.

#3 The federal investment tax credit for solar is still on
Congress renewed the 30 percent credit with no reduction until the end of 2019. On a $20,000 system that’s a $6,000 savings. Increasingly, states and local utilities are offering additional tax incentives and direct rebates.

#4 Net energy metering is now available in most states
Net energy metering means utilities compensate you for excess solar generation. You get credit to use utility-supplied power when your system isn’t producing enough solar power, like at night or on very overcast days, or your utility may offer cash for your contributions to the grid. Again, future utility rate structures, such as time-of-use billing, may reduce the value of credits.

#5 A good return on investment
You’re likely to produce 75-90 percent of the electricity you need and save $100-$200 per month, which means your system could pay for itself in 7-10 years. Because PV systems operate effectively for 20-30 years or longer, it adds up to secure long-term savings and increased home value.

#6 Solar increases your home’s value
Depending on where you live, solar PV panels can add up to $15,000 to the value of your home, according to a study by the U.S. Department of Energy’s Lawrence Berkley Laboratory.

#7 It’s good for the environment
A typical residential system will eliminate around 4 tons of atmospheric carbon emissions each year. That’s equal to about the annual amount of greenhouse gas emissions from one gas-powered car.

Bottom line on solar
Although the long-term value proposition for solar PV can shift with future changes in utility billing structures and rates, there really aren’t any substantial reasons not to go solar now.

Ready to get serious about solar?
Learn about installation costs, available incentives and more at CSE’s Solar Energy Center.
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