Storing Solar Energy

Credit: actu.epfl.ch

The Nobel Prize in Physics 1921 was awarded to the great scientists of this century Albert Einstein" for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect." The law which became the basic tool to convert light energy into electricity. Solar energy can be used as a direct source of heat or the heat can be converted into electricity with the help of solar cells based on the application of photoelectric effect. With time, wind and sun, two unpredictable resources, are becoming ever more important as non-polluting and renewable sources of energy all over the world. Many consider the sun the energy source of the future.

This means that we face growing need for solar energy storage facilities, because if solar energy cannot be used immediately when it is generated, it needs to be stored either in the form of heat or electricity until it is needed. At one hand, researchers are coming forward with developments in battery having large storage capacity at a faster rate with safety measures and cost effective whereas on other hand they are also looking for different ways to store solar energy as a heat directly to make its use at desired time and place. This would also prove invaluable for applications such as heating systems in off-grid systems or remote locations, or as an environmentally-friendly supplement to conventional heating in houses and offices. It could potentially also be produced as a thin coating and applied to the surface of buildings, or used on the windscreens of cars where the stored heat could be used to de-ice the glass in freezing winter mornings.

Converting into hydrogen

Solar energy can be stored by converting it into hydrogen. But current methods are too expensive and don’t last long. Using commercially available solar cells and none of the usual rare metals, researchers have now designed a device that outperforms in stability, efficiency and cost. To store solar energy for period when the sun doesn’t shine, one solution is to convert it into hydrogen through water electrolysis. The idea is to use the electrical current produced by a solar panel to ‘split’ water molecules into hydrogen and oxygen. Clean hydrogen can then be stored away for future use to produce electricity on demand, or even as a fuel. Even though different hydrogen-production technologies have given promising results in the lab but they are still too unstable or expensive and need to be further developed to use on a commercial and large scale.

Liquid storage of solar energy

Researchers have demonstrated efficient solar energy storage in a chemical liquid. The stored energy can be transported and then released as heat whenever needed. A research team has shown that it is possible to convert the solar energy directly into energy stored in the bonds of a chemical fluid -- a so-called molecular solar thermal system. The liquid chemical makes it possible to store and transport the stored solar energy and release it on demand, with full recovery of the storage medium. The process is based on the organic compound norbornadiene that upon exposure to light converts into quadricyclane. Combining the chemical energy storage with water heating solar panels enables a conversion of more than 80 percent of the incoming sunlight. The research project was initiated more than six years ago and at that time, the solar energy conversion efficiency was 0.01 percent and the expensive element ruthenium played a major role in the compound. Now, the system stores 1.1 percent of the incoming sunlight as latent chemical energy -- an improvement of a factor of 100. Also, ruthenium has been replaced by much cheaper carbon-based elements which prove an opportunity to develop molecules that make the process much more efficient.

Solar energy storing materials

Researchers studying a crystalline material have discovered it has properties that allow it to capture energy from the sun. The energy can be stored for several months at room temperature, and it can be released on demand in the form of heat. With further development, these kinds of materials could offer exciting potential as a way of capturing solar energy during the summer months, and storing it for use in winter - where less solar energy is available. Study shows promising material can store solar energy for months or years. The material is based on a type of 'metal-organic framework' (MOF). These consist of a network of metal ions linked by carbon-based molecules to form 3-D structures. A key property of MOFs is that they are porous, meaning that they can form composite materials by hosting other small molecules within their structures. The MOF pores were loaded with molecules of azobenzene a compound that strongly absorbs light. These molecules act as photos witches, which are a type of 'molecular machine' that can change shape when an external stimulus, such as light or heat, is applied.

In tests, the researchers exposed the material to UV light, which causes the azobenzene molecules to change shape to a strained configuration inside the MOF pores. This process stores the energy in a similar way to the potential energy of a bent spring. Importantly, the narrow MOF pores trap the azobenzene molecules in their strained shape, meaning that the potential energy can be stored for long periods of time at room temperature. The energy is released again when external heat is applied as a trigger to 'switch' its state, and this release can be very quick -- a bit like a spring snapping back straight. This provides a heat boost which could be used to warm other materials of devices. Further tests showed the material was able to store the energy for at least four months. This is an exciting aspect of the discovery as many light-responsive materials switch back within hours or a few days. The long duration of the stored energy opens up possibilities for cross-seasonal storage. These proof-of-concept findings open up new avenues of research to see what other porous materials might have good energy storage properties using the concept of confined photoswitches. It also has no moving or electronic parts and so there are no losses involved in the storage and release of the solar energy. Researchers hope that with further development they will be able to make other materials which store even more energy.

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