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Sun in a Box: The Liquid That Solar Energy for Two Decades

Solar power is considered one of the most promising alternatives to fossil fuel. However, in order to embrace this sustainable energy enti...

Solar power is considered one of the most promising alternatives to fossil fuel. However, in order to embrace this sustainable energy entirely, there are still challenges we need to overcome — one of which is the long-term storage of solar energy. Storage is vital to ensuring we have access to power even when the sun isn't shining.

A series of research papers offer hope though, as they outline a novel approach to storing the sun's energy.

Liquid acts like an efficient battery

In 2018, scientists in Sweden developed "solar thermal fuel," a specialized fluid that can reportedly store energy captured from the sun for up to 18 years

"A solar thermal fuel is like a rechargeable battery, but instead of electricity, you put sunlight in and get heat out, triggered on demand," Jeffrey Grossman, an engineer who works with these materials at MIT explained to NBC News.

The fluid has been in development for more than a year by scientists from Chalmers University of Technology in Sweden. 

The solar thermal collector named MOST (Molecular Solar Thermal Energy Storage System) works in a circular manner. A pump cycles the solar thermal fuel through transparent tubes. When sunlight makes contact with the fuel, the bonds between its atoms are rearranged and it transforms into an energy-rich isomer. The sun's energy is then captured between the isomers' strong chemical bonds.

Incredibly, the energy stays trapped there even when the molecule cools down to room temperature. To put the trapped energy to use, the liquid flows through a catalyst (also developed by the research team) creating a reaction that warms the liquid by 113 °F (63 °C). This returns the molecule to its original form, releasing energy in the form of heat.

"When we come to extract the energy and use it, we get a warmth increase which is greater than we dared hope for," the leader of the research team, Kasper Moth-Poulsen, Professor at the Department of Chemistry and Chemical Engineering said in the press release.

When an energy demand occurs, the energy-rich fluid can be used to power a building's water heater, dishwasher, clothes dryer, and much more. There could be industrial applications as well, including low-temperature heat used for cooking, sterilization, bleaching, and distillation. 

The liquid is then pumped back into the solar thermal collector to be reused. So far the researchers have put the fluid through this cycle more than 125 times without significant damage to its molecular structure. 

Moth-Poulsen has calculated that, at its peak, the fuel can store up to 250 watt-hours of energy per 2.2 lb (1 kg). Pound for pound, that's roughly twice the energy capacity of the Tesla Powerwall batteries.

Large-Scale Application

As of late 2020, an EU project led by a team at Chalmers will work to develop prototypes of the technology for large-scale applications. The project has been granted 4.3 million Euros from the EU and will last 3.5 years. 

“With this funding, the development we can now do in the MOST project may lead to new solar-driven and emissions-free solutions for heating in residential and industrial applications. This project is heading into a very important and exciting stage,” says Kasper Moth-Poulsen.

Along with this grant comes news of advances in the development of MOST. The researchers have used the technology in a window film to even out the temperature indoors on hot days. Application of the molecule in blinds and windows has already begun through a spin-off company called Solartes AB

Researchers believe the technology could be in commercial use within 10 years.

 

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