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New solar cell keeps working at night

Developed by researchers at Stanford University, the device could eventually eliminate the need for batteries to supply electricity after sunset.

Researchers at Stanford University have built a solar cell capable of harvesting renewable energy both day and night.

According to research published Tuesday in the journal Applied Physics Letters, the device under development could eliminate the need for batteries that store energy for use at night after a typical solar cell can no longer convert energy from the sun’s rays into electricity. Instead, the device harvests energy from the heat drifting from Earth back into space as temperatures cool at night.

“The solar cell is a pretty good thermal emitter, meaning it likes to emit heat a bit at night,” said Sid Assawaworrarit, author of the study. Assawaworrarit explained that at night, the cell is cooler than the air around it. “That’s the property that in this work we used to generate electricity.”

The researchers “basically convert the difference in temperature into electricity” using a thermoelectric module, according to Assawaworrarit. When it has contact with both the colder solar cell and the warmer ambient environment, the module uses the temperature change between the cell and the air to generate voltage and current.

According to a statement accompanying the research, the thermoelectric module could be incorporated within existing solar cells and be particularly helpful in more remote areas where battery access and maintenance are less feasible.

The study indicated that the thermoelectric module also works during the day and can boost the power of the traditional solar cell before the sun sets.

Assawaworrarit said the research team is now exploring how to optimize the cell cooling ability without minimizing its solar energy capture during the day. They are also seeking to make improvements to the thermoelectric module itself.

“If we have the freedom to design and make our own thermoelectric generator, we would want to make it smaller to use less material” — and improve performance — said Assawaworrarit.

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A few watts of power are necessary for lighting at night. The device used in the research generated 50 milliwatts per square meter, meaning just over 215 square feet of solar cell area would be required for nighttime lighting.

However, the scientists noted that their research utilized components that were not specifically engineered for the new device’s purposes. If the components are engineered particularly for a solar cell with a thermoelectric generator, the researchers said they believe the performance of the device can be further enhanced.

Assawaworrarit said with their desired improvements, which “should be achievable,” the device could prove a cost-effective and attractive alternative to the batteries that solar power largely depends on.                        

“Because batteries tend to go bad every few years, you don’t want the cost associated with that maintenance, especially if you’re going to put it far away,” Assawaworrarit said. “If we can improve the performance by a little bit, it would bring down the cost comparison versus the battery to the point where it’s actually competitive and it will be cheaper to do this.”

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