8/19/2019 Underwater Solar Cells

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JULY-AUGUST 2012 | VOLUME 15 | NUMBER 7-8 3

Solar cells that work nine metres under the sea have

been developed by US scientists. These could be used

to power autonomous electronic sensing systems.

Electricity is currently supplied to under-water

sensors by on-shore sources, batteries or solar cells on

platforms above the water. However, the development

of stand-alone devices is desirable. “The use of

autonomous systems to provide situational awareness

and long-term environment monitoring underwater

is increasing,” says Phillip Jenkins, an expert in solar

cell R&D at the Naval Research Laboratory where this

research was carried out.

Earlier attempts used silicon-based solar cells and

researchers struggled with their performance due to the

type of light available at this depth. “Solar cells based ongallium indium phosphide were shown to produce 2 – 3

times more power than conventional silicon solar cells

when used underwater,” explains Jenkins. At a depth of

9.1 m, the GaInP cells were able to produce 7 watts of

electricity per square metre: enough power to operate

electronic sensor systems.

“Although water absorbs sunlight, the technical

challenge was to develop a solar cell that can efficiently

convert these under-water photons to electricity,”

 Jenkins says . “The

spectral response

of GaInP solar cells

is well matched to

sunlight transmitted

underwater and

converts this light

more efficiently

than conventional

silicon solar cells,”

he adds.

The sun is filtered by

the water and more

of the blue/green

part of spectrum getsthrough. “Virtually

all photons two

metres and below the surface, have an energy greater

than 1.8 electron volts (eV). The most efficient way to

convert this radiation is with a solar cell having a band

gap energy close to this cut-off energy. GaInP has a band

gap close to 1.8e V - much higher than silicon’s band gap

of 1.1 eV,” Jenkins explains. “Additionally, high quality

GaInP solar cells have very low parasitic losses and thus

operate better at low intensities (found underwater)

compared to silicon.”

Next, the team are planning an in situ study: “We

intend to field an underwater power supply to

understand issues of lifetime and reliability of long

term deployment,” Jenkins told Materials Today .

Nina Notman

Underwater solar cellsENERGY

Super powered shock absorbersMECHANICAL PROPERTIES

A prototype shock absorber capable of significantly

reducing vibrations, such as those experienced

while driving, has been developed by German

researchers. The device can also convert vibrations

into energy, meaning it has the potential to power

inaccessible sensors.

Shock absorbers are devices that dampen

unwanted vibrations. Most are passive in nature

and made of materials called elastomers that are

 yielding and malleable. An alternative approach

is to use an active shock absorber that works to

counteract the vibrations: hence improving the

dampening effect.

Researchers at the Fraunhofer Institute for

Structural Durability and System Reliability

have made an active shock absorber containing

an electroactive elastomer. “These are elastic

substances that change their form when exposed

to an electrical field,” team member William Kaal

explains.

An alternating current is applied to the electroactive

elastomer, causing it to vibrate. If the elastomer

vibrations are tuned to complement those of the

external vibrations they can effectively cancel

each other out. “By sensing the vibrations and

feeding the signal back to the electronics an active

absorber can virtually be tuned in its stiffness and

damping properties,” Kaal told Materials Today.

The team built a prototype dielectric stack actuator

consisting of 40 alternating layers of thin films of

natural rubber (the electroactive elastomer) and

nickel electrodes. The key part of this work is the

design of the electrode layers. “The challenge was

the design of the electrodes with which we apply

the electric field to the elastomer layers,” explains

Kaal‘s colleague Jan Hansmann.

As metals are rigid they hinder the vibration of

the elastomer layers. To overcome this “we put

microscopic-sized holes in the electrodes”, says

Hansmann. “If an electric voltage deforms the

elastomer, then the elastomer can disperse into

these holes.”

To test the prototype, a vibrating oscillator

was attached. After a short period of time the

vibrations stopped, as the frequency of the stack

actuator’s vibrations adjusted to counteract those

coming from the oscillator.

One potential use for this is in passenger vehicles.

“The vibrations [of a car’s engine] are channelled

through the chassis into the car‘s interior, where

the passengers start to feel them,” says Kaal.

“Active elastomers may help further reduce

vibrations in the car.”

The team also demonstrated that it is possible

to change the function of the stack actuator, so

it can produce energy through the absorption of

external vibrations. When an electromagnetic

oscillator was attached to their prototype,

they demonstrated that the vibrations were

converted into power. “That would be of interest

if you wanted to monitor inaccessible sites where

there are vibrations but no power connections,”

says Hansmann. One example given is for the

temperature and vibration sensors that monitor

the condition of bridges.

Nina Notman