Power/Performance Bits: Jan. 12

Incandescent bulbs might not be dead yet

Can incandescent bulbs be as efficient – or even more so – than LEDs? More than 95 percent of the energy that goes into incandescents is wasted, most of it as heat, so researchers at MIT and Purdue University struck out to see if that could be changed.

A conventional heated metal filament, with all its attendant losses, served as the basis. But instead of allowing the waste heat to dissipate in the form of infrared radiation, secondary structures built of photonic crystals surrounded the filament to capture the radiation and reflect it back to the filament to be re-absorbed and re-emitted as visible light. Incandescents built on the new system could reach efficiencies as high as 40 percent, the team says.

For this to work, they had to redesign the incandescent filament from scratch. “In a regular light bulb, the filament is a long and curly piece of tungsten wire. Here, the filament is laser-machined out of a flat sheet of tungsten: it is completely planar,” said Peter Bermel, a Purdue physics professor. A planar filament has a large area, and is therefore very efficient in re-absorbing the light that was reflected by the filter. In describing how the new device differs from previously suggested concepts, Marin Soljačić, professor of physics at MIT, emphasizes that “it is the combination of the exceptional properties of the filter and the shape of the filament that enabled substantial recycling of unwanted radiated light.”

A proof-of-concept device built by MIT researchers demonstrates the principle of a two-stage process to make incandescent bulbs more efficient. (Source: Ognjen Ilic/MIT)

The first proof-of-concept units made by the team do not yet reach that level, achieving about 6.6 percent efficiency. But even that preliminary result matches the efficiency of some of today’s CFLs and LEDs, they point out. And it is already a threefold improvement over the efficiency of today’s incandescents.

The technology involved has potential for many other applications besides light bulbs, Soljačić says. The same approach could “have dramatic implications” for the performance of energy-conversion schemes such as thermo-photovoltaics. In a thermo-photovoltaic device, heat from an external source (chemical, solar, etc.) makes a material glow, causing it to emit light that is converted into electricity by a photovoltaic absorber.

“LEDs are great things, and people should be buying them,” Soljačić says. “But understanding these basic properties” about the way light, heat, and matter interact and how the light’s energy can be more efficiently harnessed “is very important to a wide variety of things.”

He added that “the ability to control thermal emissions is very important. That’s the real contribution of this work.” As for exactly which other practical applications are most likely to make use of this basic new technology, he says, “it’s too early to say.”

Solar record

Scientists at the Energy Department’s National Renewable Energy Laboratory (NREL) and the Swiss Center for Electronics and Microtechnology (CSEM) set a new world record for converting non-concentrated sunlight into electricity using a dual-junction III-V/Si solar cell.

The newly certified record conversion efficiency of 29.8 percent was set using a top cell made of gallium indium phosphide developed by NREL, and a bottom cell made of crystalline silicon developed by CSEM using silicon heterojunction technology. The two cells were made separately and then stacked by NREL.

“The performance of the dual-junction device exceeded the theoretical limit of 29.4 percent for crystalline silicon solar cells,” said David Young, a senior researcher at NREL.

Safer batteries

Aiming to prevent the kind of fires that have prompted recalls and bans on a wide range of battery-powered devices, Stanford researchers developed the first lithium-ion battery that shuts down before overheating, then restarts immediately when the temperature cools.

A typical lithium-ion battery consists of two electrodes and a liquid or gel electrolyte that carries charged particles between them. Puncturing, shorting or overcharging the battery generates heat. If the temperature reaches about 300 degrees Fahrenheit (150 degrees Celsius), the electrolyte could catch fire and trigger an explosion.

Several techniques have been used to prevent battery fires, such as adding flame retardants to the electrolyte. In 2014, Stanford engineer Yi Cui created a ‘smart’ battery that provides ample warning before it gets too hot.

“Unfortunately, these techniques are irreversible, so the battery is no longer functional after it overheats,” said Cui, an associate professor of materials science and engineering and of photon science. “Clearly, in spite of the many efforts made thus far, battery safety remains an important concern and requires a new approach.”

For the battery experiment, the researchers coated tiny particles of nickel with nanoscale spikes protruding from their surface with graphene and embedded the particles in a thin film of elastic polyethylene.

Bunched together, as shown here, nanoparticles of graphene-coated nickel conduct electricity. When the battery overheats, the particles separate and electric current stops flowing. During cooling, the particles reunite and the battery starts producing electricity again. (Source: Stanford)

“We attached the polyethylene film to one of the battery electrodes so that an electric current could flow through it,” said Zheng Chen, lead author of the study. “To conduct electricity, the spiky particles have to physically touch one another. But during thermal expansion, polyethylene stretches. That causes the particles to spread apart, making the film nonconductive so that electricity can no longer flow through the battery.”

When the researchers heated the battery above 160 F (70 C), the polyethylene film quickly expanded like a balloon, causing the spiky particles to separate and the battery to shut down. But when the temperature dropped back down to 160 F (70 C), the polyethylene shrunk, the particles came back into contact, and the battery started generating electricity again.