Solar technology is a sector that is really buzzing and evolving rapidly. One innovation follows another. Recently, scientists at the Massachusetts Institute of Technology (MIT) found a solution to store solar energy during the day and release it at a later time as heat energy. A technique like this opens a wide range of opportunities. Car windshields, for example, can melt away a layer of ice. In this case, energy intensive auxiliary heating could become a thing of the past. The material used, a transparent polymer film, can be applied to different surfaces, not only windows or glas. There is no separate battery storage needed to store the absorbed energy of the sunlight.
“This work presents an exciting avenue for simultaneous energy harvesting and storage within a single material,” says Ted Sargent, university professor at the University of Toronto, who was not involved in this research.
The stable molecular configuration retains the absorbed energy of the sunlight indefinitely
The finding, by MIT professor Jeffrey Grossman, postdoc David Zhitomirsky, and graduate student Eugene Cho, is described in a paper in the journal Advanced Energy Materials. The energy is not stored in form of heat or electricity. A chemical reaction of the polymer film enables the long-term and stable storage. As in the case of the thermal storage system which has been recently brought into service by researchers at the German Aerospace Center the chemical energy is storable for a very long period of time until it is needed. The stable molecular configuration retains the absorbed energy of the sunlight indefinitely. Heat energy is not storable for an unlimited period because it inevitably dissipates over time. Even the most professional insulation is not able to prevent heat losses. The energy release can be triggered by a small jolt of heat, light or electricity.
Fundamental to the whole process are molecules with two different and stable configurations. Solar power moves the molecules into their charged configuration where they stay a very long period. If triggered by a specific temperature, the molecules change to their original shape while spending heat energy – 10 degrees Celsius above the surrounding temperature.
This solar thermal fuels (STF) have been already developed in former times. But in this case, the chemically-based storage material is not using a liquid solution. It is made of a durable solid-state film. According to MIT, it is also based on inexpensive materials and widespread manufacturing technology. In the beginning, the team used so-called azobenzenes. This material is able to store a useful amount of heat. To improve its energy density, ability to form smooth, uniform layers, and its responsiveness to the activating heat pulse the researchers modified the chemistry of the material. Another advantage of the material is the transparency. For this reason, it is possible to embed it in car windshields. No thin heating wires or auxiliary heaters are needed anymore. “We did tests to show you could get enough heat to drop ice off a windshield.” Grossman says.
The team still tries to improve the film’s properties. Because of a slight yellowish tinge, the researchers are looking for more transparency. They also try to boost the released temperature from 10 degrees Celsius above the surrounding temperature to 20 degrees. “The approach is innovative and distinctive,” says Sargent, from the University of Toronto. “The research is a major advance towards the practical application of solid-state energy-storage/heat-release materials from both a scientific and engineering point of view.”