What’s the difference between the installed capacity and electricity generation of energy sources?

It’s a good question and one that’s commonly misunderstood.

In the energy world, these two terms are often used to describe the growth of energy resources in the United States.

Take wind or solar, for example.

According to the EIA, around 1% of U.S. electricity generation came from solar energy in 2016.


There might be an article about wind making up 8% of all energy generation capacity. Or, that solar will make up 1% of electricity generation in a specific year.

So what’s the difference? Let’s break it down for you.

What is Capacity?

The U.S. Energy Information Administration (EIA) refers to capacity as the maximum output of electricity that a generator can produce under ideal conditions. Capacity levels are normally determined as a result of performance tests and allow utilities to project the maximum electricity load that a generator can support. Capacity is generally measured in megawatts or kilowatts.

Let’s look at an example.

According to EIA, wind turbines accounted for 8% of U.S. installed electricity generation “capacity,” as of December 2016. This means under ideal conditions, utilities would be able to supply 8% of the country’s electricity needs with wind power, but this won’t necessarily be the actual amount of electricity produced.

According to EIA, wind turbines accounted for 8% of U.S. installed electricity generation capacity as of December 2016.


What is Generation?

Electricity generation, on the other hand, refers to the amount of electricity that IS produced over a specific period of time. This is usually measured in kilowatt-hours, megawatt-hours, or terawatt-hours (1 terawatt equals 1 million megawatts). To understand the unit of megawatt-hours (MWh), consider a wind turbine with a capacity of 1.5 megawatts that is running at its maximum capacity for 2 hours. In this scenario, at the end of the second hour, the turbine would have generated 3 megawatt-hours of energy (i.e. 1.5 megawatts X 2 hours).

If the wind was not blowing strongly enough for the turbine to operate at its maximum capacity, and the same turbine was only producing 1 megawatt of power for 2 hours, the total energy generation would be 2 megawatt-hours (i.e. 1 megawatt X 2 hours). This simple thought exercise demonstrates how calculations of generation take into account the fact that not all generation sources are operating at their maximum capacity at all times, such as when the sun isn't shining or when the wind isn't blowing.

Where Can I Learn More?

The EIA has a roster of Frequently Asked Questions on electricity usage and every other energy topic under the sun.

Learn more about recent advancements in wind energy and solar energy.

Inkjet-printed perovskite solar cells with efficiency above 16%.

Dr. Maikel van Hest, National Renewable Energy Laboratory

Did you know there are alternatives to standard silicon solar panels? Or that someday soon, you might be able install a solar panel that is 50% more efficient than the average silicon photovoltaic (PV) solar panel?

That’s exactly what Iris Photovoltaics, Inc. (Iris PV) is aiming to produce. The Berkeley, California-based company is working to modernize how silicon solar panels are manufactured. In addition, they are attempting to increase the efficiency of PVs to a range of 25-30%.

The U.S. Department of Energy (DOE) Small Business Vouchers (SBV) program award recipient’s technology adds a crystalline metal-halide perovskite layer to coat standard silicon solar panels, which produces additional electricity from infrared light. This is then layered on top of traditional silicon solar cells to create a “tandem” solar panel. These “tandem” solar panels, composed of two materials instead of one, generate a greater amount of electricity per panel.

From Manufacturing Floor to Rooftops

Iris PV is receiving technical assistance from researchers at the National Renewable Energy Laboratory (NREL) through SBV as part of DOE’s Office of Energy Efficiency and Renewable Energy Technology-to-Market program. Iris PV cofounders Colin Bailie and Chris Eberspacher are working with NREL researchers to manufacture the technology at scale and accelerate the adoption of solar with their high-efficiency PV products.

“Through the SBV program, we are addressing critical manufacturing challenges so that production facilities can be built,” said Bailie.

“Commodity silicon solar cells are mired in the 18-22% efficiency range. The theoretical maximum for combining two solar cell materials is 46% efficiency, though we’re aiming to fly a little less close to the sun and hit 30-35% efficiency.”

Compared to today’s standard solar panel, Iris PV’s design minimizes costs for manufacturers and is compatible with most existing PV technologies. It could also save individual homeowners thousands of dollars in upfront costs and utility bills compared to current technology. Once this technology is commercially available, we hope to get an enthusiastic response from both solar installers and homeowners,” said Bailie.

Overcoming Manufacturing Challenges

Today’s metal-halide perovskite solar cells have manufacturing limitations. Specific manufacturing techniques, such as spin-coating, limit the size of individual glass panels. The spin-coating process deposits thin layers of solvents or coating materials, like silicon wafers, using centrifugal force. This process also requires additional patterning in solar cell production, adding to overhead costs.

With the technical assistance of NREL’s researchers, Iris PV is overcoming these limitations using inkjet printing. Inkjet printing can uniformly coat large areas and complete patterns by dispensing single drops with controlled print design. Because inkjet printers are more precise than spin-coaters, the production process is more efficient and uniform.

According to Iris PV, inkjet printing allows for rapid prototyping and low-cost custom products down the road, including the Iris PV form factor. To date, the project has printed single-junction perovskite cells with efficiencies of more than 16%, on par with devices made using other scalable technologies. And another benefit to Iris PV’s tandem panel design: Because it will be compatible with existing manufacturing tools and methods, costs for current solar manufacturers to switch technologies will be minimal.

Iris PV’s next step is to demonstrate the technology’s scalability. If they are able to print perovskite films on a 6” x 6” area, the demonstration will be considered a success.

As for Bailie and Eberspacher, their team especially valued the support of NREL researchers who helped through the SBV program--Maikel van Hest, Rosie Bramante, and James Whitaker.

“The development of inkjet-printed perovskite photovoltaics would not have been possible without the support provided by the Department of Energy’s Small Business Vouchers program,” said Bailie.

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. 




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