Leading Solar Expertise—A Launch Pad to the Future

Leading Solar Expertise—A Launch Pad to the Future

NREL is speeding solar devices from the lab to utility-scale operation.

Before a rocket blasts off into the atmosphere, the stratosphere, and eventually to orbit and payload stage, it must first be earthbound, supported within a strong framework, and perched on a rock-solid foundation. When it comes to solar research, NREL's trajectory has had the benefit of a solid foundation in fundamental science and has followed a successful "flight plan"—a well-tuned strategy. Consequently, the lab has entered its own "payload stage" in commercialization and deployment of solar technologies.

NREL's 30-year history and unique leadership make it especially suited to lead the effort in speeding solar devices from the lab to utility-scale operation.

Since its inception in 1977, NREL has focused on increasing solar efficiency, reducing the costs of producing those technologies, and helping to bring them to market. NREL's successes in the solar realm are legion—from the dozens of times it has broken the record for solar efficiencies, to a host of patents and licenses on solar devices, to hundreds of U.S. companies that have collaborated with the lab and adopted the resulting technological breakthroughs.

Another stellar achievement is the number of R&D 100 Awards that the NREL solar research teams have garnered—21 since 1984 (see sidebar). The awards are given out by R&D Magazine and identify each technology as one of the top 100 technological innovations of the award year.

These accomplishments are the direct results of a strategy that NREL is using to bring about the vision of creating a national, sustainable energy system by 2050 that is carbon-neutral, highly efficient, affordable, reliable, and that supports high-value domestic jobs. At the utility-scale level, the laboratory has made great progress in two solar technology areas: photovoltaics (PV) and concentrating solar power (CSP).

Photovoltaics

The first viable PV cells were developed in 1954 by Bell Laboratories. In the 1970s the global oil crisis demonstrated the need for alternative energy. It was then that the Solar Energy Research Institute (which later became NREL) began managing many research subcontracts involving crystalline silicon materials. The contracts were for R&D that successfully reduced the amount of silicon required in PV devices, and hence their cost. These activities played an instrumental role in helping the solar silicon industry evolve and led to NREL's leadership in solar cell research.

Since 1996, PV research has been performed at the National Center for Photovoltaics (NCPV), which is based at NREL and funded by the U.S. Department of Energy (DOE). The NCPV is charged with accelerating PV as a viable energy option in the United States. It focuses on innovations in PV technology that drive industry growth in U.S. PV manufacturing. DOE has directed the center to use the resources and capabilities of the national labs and universities to serve the U.S. PV industry. The NCPV enhances communication and catalyzes strategic partnerships between these entities and also functions as a source for knowledge and research facilities within the DOE system.

Bob Hawsey, NREL Associate Lab Director for Renewable Electricity and End Use Systems, recognizes NREL's unique leadership contributions, "The NCPV is truly without peer when it comes to PV research and development and is currently the envy of the wider international PV community," he says. "We are most recognized for tremendous basic science and technology research that is enabling photovoltaic systems to make a real difference in the everyday lives of citizens of our nation, and around the world."

Within the PV research sphere, NREL has concentrated on two areas: thin-film and high-efficiency solar cells.

Thin-Film Solar Cells

Photo of concentrating PV system. Enlarge image

Before the turn of the century, NREL worked on some of the world's first solar power towers—Solar One and Solar Two, shown here.

Thin-film solar cells use less than 1% of the raw material of silicon wafer-based solar cells, leading to significant cost advantages. These cells can also be applied to flexible materials such as metal foil or even plastic film, expanding their use.

Thin-film research at NREL gained notice in 1980 by scientists worldwide when efficiencies passed 10%. NREL collaborated with Boeing in 1984 for the first solar cell to pass 10% efficiency, using films thinner than a human hair. The cell was made from copper indium diselenide (CIS). In the early 1990s, the group worked with Golden Photon to create the first large-area device made from cadmium telluride (CdTe).

NREL made rapid progress in 1994 by surpassing 15% efficiency and then reaching 17.7% in 1996 for copper indium gallium diselenide (CIGS). One of the more popular thin-film solar cells to be developed with NREL participation in the last 30 years is the Uni-Solar triple-junction amorphous-silicon solar module, which resembles a traditional roof shingle.

In 2003, NREL co-developed, with First Solar, a new method for producing CdTe modules. In 2004, NREL joined with Global Solar to develop a new lightweight, flexible, CIGS module. For 16 years, NREL held the world record for conversion efficiency. Today, CIGS cell efficiencies at NREL are at 20%.

High-Efficiency Solar Cells

In the late 1980s, NREL experimented with a new type of solar cell made by using multiple solar cell junctions of differing materials: gallium indium phosphide and gallium arsenide. The resulting "tandem" solar cell led to record efficiencies. The technology was licensed for space applications in the early 1990s to Spectrolab and EMCORE corporations and has since become the industry standard for powering Earth-orbiting satellites.

The technology came back to earth quickly, however, when NREL stimulated interest in high-efficiency solar cells. In 2002, the lab organized the first international conference on solar concentrators using high-efficiency solar cells. Solar concentrators use lenses or mirrors to concentrate the sun's energy several hundred times, increasing the electricity generated by super-efficient solar cells by the same factor.

In May 2005, NREL announced that it had confirmed a new solar cell record efficiency of 37.9%. A month later, Spectrolab, Inc., the lab's industry research partner, announced an even higher record with a 39%-efficient cell.

NREL built on that success in a partnership with Amonix to develop the Amonix 7700 Solar Power Generator, which uses Fresnel lenses to focus the sun's rays onto ultrahigh-efficiency solar cells. This bulk power generator produces 40% more energy than conventional fixed PV panels and is well-matched to utility-scale solar energy projects, especially in dry, sunny climates. In 2010, NREL and Amonix received an R&D 100 Award for this technology.

According to the Solar Energy Industries Association, as of January 2011 a total of 2.1 GW of PV capacity was installed in the United States with more than 16 GW under construction or in the development stage.

The DOE Solar Program recently announced the SunShot Initiative (see sidebar), which is dedicated to expanding the market for solar technologies by helping solar energy reach cost parity with other baseload electricity generation sources across the United States

Concentrating Solar Power

Photo of an angled concentrated PV tracking system Enlarge image

The Amonix 7700 Solar Power Generator is an example of a concentrating PV system that is well-matched to utility-scale projects. Thirteen of these generators have been installed at the Solar Technology Acceleration Center in Aurora, Colorado.

CSP systems produce electricity by using mirrors to concentrate the sun's energy to heat a working fluid to drive conventional turbines that convert heat to electricity. By using thermal storage, such as molten salt, or by supplementing the solar plant with natural gas, CSP systems can deliver electricity when utilities need it most, which is typically at times of high demand in late afternoon or early evening. This "dispatchability" adds significantly to the value of power delivered by utility-scale solar power plants.

NREL's roots in CSP go back to the 1970s with the development of the High-Flux Solar Furnace. Since the early 1990s, NREL has added many more CSP research capabilities, including laboratories dedicated to optical materials and thermal storage. These capabilities have allowed NREL to make solid gains in developing CSP technology for utility-scale use, including scientific advances in materials and processes used in parabolic trough systems and power towers.

Parabolic Trough Systems

A parabolic trough system consists of arrays of parabolic mirrors that collect heat from the sun and focus it on receiver tubes that contain a heat-transfer fluid. The hot fluid is sent through a series of heat exchangers, which release the heat to generate high pressure steam. That steam is then fed to steam turbines that generate electricity.

In the mid-1980s to early 1990s, nine commercial parabolic trough power plants were constructed at three locations in the Mojave Desert in California for a combined capacity of 354 MW—and they are still operating today. In an effort to make parabolic trough plants cost-effective in southwest markets and without incentives, NREL established the USA Trough Initiative, a partnership with U.S. industry. The effort helped expand U.S. industry involvement and competitiveness in worldwide trough development activities and helped advance U.S. knowledge of this technology.

Today, NREL uses state-of-the-art facilities to characterize collectors and receivers. The laboratory's work in this area falls primarily within the following: determining optical efficiency, measuring heat loss, developing and testing concentrators, and advancing optical characterization. NREL also enables the CSP industry by developing and testing advanced mirrors and receiver tube coatings.

One such effort involved collaboration with SkyFuel, Inc., a small company headquartered in Arvada, Colorado, and earned an R&D 100 Award for the SkyTrough parabolic trough. The result of more than a dozen years of collaboration, SkyTrough is unique in that its mirrors are made of lightweight aluminum sheets covered with ReflecTech mirror film, the component that NREL helped develop. Lighter and less expensive to manufacture than glass mirrors, ReflecTech is also much easier to transport and install, and less prone to break.

These types of scientific advances at NREL have helped bring about reductions in operation, maintenance, and system costs, which in turn have led to a significant decrease in the cost of parabolic trough-generated electricity. The costs have fallen from more than $ 0.28 per kilowatt-hour (kWh) in the 1980s (in 2009 dollars) to costs approaching $ 0.18/kWh today, which is approaching current costs in intermediate load markets.

Power Towers

R&D 100 Awards for NREL Solar Program Technologies

1984 - CIS Solar Cell
1991 - Tandem Solar Cell
1991 - CdTe Solar Cell
1992 - Solar Detoxification of Contaminated Groundwater
1993 - Silicon Defect Mapping System
1994 - Transpired Solar Collector
1997 - PV Optics Software
1998 - Solar Roof Shingle
1999 - CIS Solar Module
2001 - Triple-Junction Solar Cell
2002 - PowerView™ Module Smart Glass
2003 - High-Rate Module Process
2004 - Flexible CIGS Module
2005 - Sinton Silicon Evaluation System
2007 - HEMM Concentrator Solar Cell
2008 - IMM Solar Cell
Hybrid CIGS
2009 - SkyTrough™ Parabolic Trough
Ultra-Accelerated Weathering System
2010 - Black Silicon Wet-Chemical Etch
Amonix 7700 Solar Power Generator

See a complete list of NREL's R&D 100 Awards.

A power tower system uses a large field of flat, sun-tracking mirrors called heliostats to focus and concentrate sunlight onto a receiver on the top of a tower. The receiver contains heat-transfer fluid that becomes hot enough to convert water into steam, which is then used in a conventional turbine generator to produce electricity.

In the 1980s and early 1990s, NREL worked with Sandia National Laboratories on some of the world's first solar power towers—Solar One and Solar Two. Solar One used water/steam as the heat-transfer fluid. The plant was later converted into Solar Two, which used molten nitrate salt because of its superior heat-transfer and energy-storage capabilities.

Today, NREL continues to improve power tower technology by supporting the U.S. industry with the development of advanced system performance models, conducting research on advanced thermal energy storage materials and design concepts, and investigating advanced high-temperature thermodynamic cycles.

NREL is also researching ways to improve the thermal characteristics of currently available storage materials and developing and characterizing advanced nanofluids and phase-change materials for future thermal storage applications.

A Proven, Working Technology

According to the Solar Energy Industries Association, as of February 2011, 508 MW of CSP utility-scale projects were operating in the United States, 399 MW were under construction, and 9,146 MW were under development, for a total of more than 10 GW.

For more information on this technology, view the CSP 101 video on the DOE website.

Rock-Solid Launch Pad + Well-Aimed Flight Plan = Direct Hit

Using tools such as cooperative research and development agreements, licensing, and technology partnerships, NREL has helped stimulate the market for solar technologies and assisted the growth of solar start-ups, such as Abound Solar, Amonix, First Solar, Global Solar, and SkyFuel.

Companies such as these exist across the country and create jobs as they grow. The National Solar Jobs Census was compiled in August 2010 by The Solar Foundation (a nonprofit organization), Green LMI Consulting, Cornell University, and others. The census identified 93,502 solar workers in the United States, roughly double the number estimated for 2009. In addition, employers from all of the studied subsectors expected significant employment growth well into 2011.

A 30-year history of excellence in solar R&D and a well-honed strategy have combined to make NREL the pre-eminent laboratory to lead solar technologies to the utility-scale level. In this way, NREL is enabling solar energy to help protect the environment, achieve U.S. energy security, reduce petroleum dependence, create new jobs, and boost the nation's economy.

Learn more about solar energy research at NREL.

Credit, top and bottom photo: Dennis Schroeder. Credit, slideshow: PIX 16464, credit Pat Corkery; 16604, photo courtesy SkyFuel, Inc.; 13975, illustration credit Susannah Pedigo; 13419, credit Global Solar Energy; 15942 and 17841, credit Dennis Schroeder; 17841. Credit, middle photo: Warren Gretz

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