Black Metals Beat A New Path To Solar Cell Efficiency

It looks like black is the new black when it comes to solar cell efficiency. A team of researchers at Lawrence Livermore National Laboratory (LLNL) has come up with a way to increase solar cell efficiency, by creating “black  metals” etched with nanoscale structures that harvest energy from wavelengths all along the solar spectrum. The idea is to take advantage of the plasmonic effect, and if the research develops apace it could enable the prime affordability target of the solar market to expand from high-sunlight locations to just about anywhere.

Along similar lines, we’ve been following “black silicon” research that uses nanoscale etching to increase solar absorption, so let’s take a closer took at both and see what’s going on.

The LLNL solar cell efficiency project, loosely speaking, involves “roughening up” metals at the nanoscale level, which is where the concept crosses paths with its silicon-based cousin. The random, nanoscale irregularities increase the number of reflections, trapping more light.

The result is a black surface that has lower reflectivity and higher absorption than the original material, covering both the visible and infrared sections of the spectrum.

According to writer Kenneth K Ma at LLNL, existing work in gold and silver black metals has come up against a fabrication obstacle, in which it has been difficult to replicate the full solar absorption rate.

The research team worked on that angle and developed a “nanopillar” structure that can be manipulated with more predictability, enabling the team to create metals “as  black as they want.”

LLNL has some competition out there, by the way. The research made the cover of Applied Physics Letters back in May, but a team from China also published the results of its work on light harvesting nanopillar structures in the publication’s online edition in April.

Also involved in the nanopillar horserace is UC-Santa Barbara, which has been developing a high efficiency solar cell based on a “forest of gold nanorods.”

That leads us into the next question, which is how metals can generate an electrical charge from sunlight. Basically, it’s the same idea as semiconductors such as silicon, in which sunlight causes electrons to shift positions, leaving positively charged “holes.”

In metals, this shift creates free electrons and electromagnetic pulses similar to sound waves, called plasmons.

The nanorod approach provides one way to control and manipulate the plasmonic effect, but there are others. Over at Stanford University, for example, a team is working on a “waffle iron” plasmonic concept that involves creating nanoscale dimples in a layer of the semi-porous metal titania.

Meanwhile, a team at the University of Buffalo is testing out a low-efficiency but low-cost solar cell that incorporates the plasmonic effect into thin film organic solar cells. The idea is to create an affordable solar “paint” that could be applied to building surfaces.

On the meta-level, Duke University has been hot on the trail of an atomic-level explanation of the plasmonic effect, by studying the optical scattering that occurs when gold nanoparticles interact with a thin film of gold.

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Obama-California Energy Efficiency Mandates Coming to a Town Near You

President Obama’s recent “climate action” speech includes an expansion of subsidies and mandates requiring additional energy efficiency. The president calls these policies a “great deal” for consumers, but they are anything but—energy efficiency mandates impose lawmakers’ and bureaucrats’ preferences on the American people. These mandates assume that most Americans do not make wise choices and therefore Washington must require more energy-efficient products.

Energy efficiency mandates are truly a lousy deal for the American people. But that does not stop bureaucrats from imposing new energy efficiency requirements. California, for example, began regulating energy efficiency in 1974 with the creation of the California Energy Commission. Since then, the state claims it has been “at the forefront of American energy policy, working to protect the stability of California’s energy resources and the security of its energy consumers.” As alleged proof, the government boasts that its regulatory policies have kept per-capita electricity consumption flat since the 1970s.

While California’s per capita energy use has stopped increasing over the last three decades, a recent study commissioned by the National Bureau of Economic Research (NBER) finds that the state’s energy efficiency standards are not the cause. The study points to other factors which account for 90 percent of California’s residential electricity savings since the mid-1970s, concluding that the state’s policies provide “no lessons for other states or countries considering adopting or tightening their energy efficiency standards.”

The last part is important. President Obama’s climate action plan calls for reducing carbon dioxide emission by at least 3 billion metric tons by 2030, which the administration says is equivalent to half of all CO2 emissions from the U.S. energy sector in a given year. To that end, the administration will impose a host of new subsidies and regulations through unilateral executive action without input from Congress or the American people.

The Department of Agriculture (USDA), for example, will provide up to $250 million through its Energy Efficiency and Conservation Loan Program to encourage rural utilities to adopt energy efficiency upgrades. The USDA will also “streamline” its Rural Energy for America subsidy, which has already doled out nearly $24 million in grants and loan guarantees for agricultural producers since 2009. This is in addition to the almost $14 billion the Obama administration has spent on tax subsidies for energy conservation programs since 2009, according to IER’s new Federal Energy Spending Tracker.

If California’s history is any indication, the Obama administration’s latest efforts to engineer energy savings will fall woefully short. But it should come as no surprise, since many economists have long questioned the merits of energy efficiency mandates. An IER summary of recent research into energy efficiency shows how “rebounds” explain why government mandates often fail to live up to expectations.

As Professor Robert Michaels explains, rebound effects often negate the purported benefits of policies designed to reduce energy consumption. In general, rebounds occur because consumers who use products that are more energy efficient tend to use more of those products. The economics literature reveals, for example, that household behavior before and after installation of energy-efficient appliances produces rebounds between 10 and 60 percent. In some cases, efficiency mandates even increase net energy use, known as “backfires.” Even the environmentalistBreakthrough Institute recognizes the failure of energy efficiency mandates.

A 2008 study from Stanford University supports the conclusion that energy efficiency policies, at least in the case of California, fall short. The study finds that only 23 percent of California’s energy savings since the 1970s can be attributed to energy conservation measures. In other words, it is unlikely that exporting California’s energy efficiency schemes to the rest of the country would significantly reduce energy use.

Indeed, decades of energy efficiency mandates have left California “at the forefront” of high energy prices. California’s retail electricity rates are seventh highest in the nation, while the state’s gasoline prices are nearly 40 cents a gallon higher than the national average. California’s electricity rates are also higher than its neighbors: Arizona, Nevada, and Oregon.

The reality is that while energy efficiency is important, it is just one of many factors for consumers to consider. For example, if the choice is between two cars that cost the same, have equally powerful engines, and have the same amenities, but one gets better fuel economy, everyone will choose the more fuel-efficient vehicle. People are not stupid about energy efficiency.

But unlike the hypothetical car example, in real life there are tradeoffs. Cars with high fuel economy are usually smaller and more expensive than comparable cars. Energy efficiency is just one of many factors that matter when buying an automobile. But when the government mandates higher fuel efficiency, automakers are more likely to scrimp on other attributes such as size, comfort, safety, and price to meet the mandate.

Energy efficiency mandates are based on the premise that Americans need government bureaucrats to make wise choices about energy. The problem is that bureaucrats could never know what is in the best interest of all 114 million American households. Instead of imposing more costly and dubious mandates on the American people, the president should let the public decide for itself how to best use and conserve energy resources.

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UCLA scientists double efficiency of novel solar cell

UCLA researchers have developed photovoltaic cells with twice the energy harvesting capacity of cells developed in 2012. The cells, which can be processed to be transparent or in shades ranging from light green to brown, could be used to make building windows, smartphone screens, car sunroofs and other surfaces into sources of sustainable energy.

Nearly doubling the efficiency of a breakthrough photovoltaic cell they created last year, UCLA researchers have developed a two-layer, see-through solar film that could be placed on windows, sunroofs, smartphone displays and other surfaces to harvest energy from the sun.

The new device is composed of two thin polymer solar cells that collect sunlight and convert it to power. It's more efficient than previous devices, the researchers say, because its two cells absorb more light than single-layer solar devices, because it uses light from a wider portion of the solar spectrum, and because it incorporates a layer of novel materials between the two cells to reduce energy loss.

While a tandem-structure transparent organic photovoltaic (TOPV) device developed at UCLA in 2012 converts about 4 percent of the energy it receives from the sun into electric power (its "conversion rate"), the new tandem device—which uses a combination of transparent and semi-transparent cells—achieves a conversion rate of 7.3 percent.

Researchers led by Yang Yang, the Carol and Lawrence E. Tannas, Jr., Professor of Engineering at the UCLA Henry Samueli School of Engineering and Applied Science, said the new cells could serve as a power-generating layer on windows and smartphone displays without compromising users' ability to see through the surface. The cells can be produced so that they appear light gray, green or brown, and so can blend with the color and design features of buildings and surfaces.

The research was published online July 26 by Energy & Environmental Science, a Royal Society of Chemistry journal, and it will appear later in a published edition of the journal.

"Using two solar cells with the new interfacial materials in between produces close to two times the energy we originally observed," said Yang, who is also director of the Nano Renewable Energy Center at the California NanoSystems Institute at UCLA. "We anticipate this device will offer new directions for solar cells, including the creation of solar windows on homes and office buildings."

The tandem polymer solar cells are made of a photoactive plastic. A single-cell device absorbs only about 40 percent of the infrared light that passes through. The tandem device—which includes a cell composed of a new infrared-sensitive polymer developed by UCLA researchers—absorbs up to 80 percent of infrared light plus a small amount of visible light.

Chun-Chao Chen, a graduate student in the UCLA materials science and engineering department who is the paper's primary author, said using transparent and semi-transparent cells together increases the device's efficiency, and that the materials were processed at low temperatures, making them relatively easy to manufacture.

Explore further: Transparent solar cells for windows that generate electricity

More information: xlink.rsc.org/?doi=10.1039/c3ee40860d

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