A team of researchers from Lancaster University studying a crystalline material has discovered it can capture solar energy and store it for several months at room temperature and release it on demand as heat.

The team hopes to make further development with the material as there is ample potential to capture solar energy during summers and store it for use during winters.

The research, supported by the Leverhulme Trust, is outlined in the paper ‘Long-Term Solar Energy Storage under Ambient Conditions in a MOF-Based Solid-Solid Phase Change Material,’ by the journal Chemistry of Materials. The team comprises John Griffin, Kieran Griffiths, and Nathan Halcovitch, all from the Lancaster University’s Department of Chemistry.

According to the team, the discovery is vital for heating in off-grid systems or remote locations. The material can also function as an environmentally-friendly supplement to conventional heating in homes and offices.

The team claims that the material could be produced as a thin coating and applied to building surfaces or on car windscreens where the heat stored could de-ice the glass in the cold winters.

The crystalline material is based on a form of ‘metal-organic framework’ (MOF) consisting of a network of metal ions linked by carbon-based molecules to form 3D (three-dimensional) structures. MOFs are porous, vital to form composite materials by hosting other small molecules within their structures.

The research team at Lancaster tested a MOF composite, prepared by a Kyoto University-based research team in Japan known as ‘DMOF1’; the team wondered whether DMOF could be used to store energy not researched earlier.

The MOF pores were filled with azobenzene molecules, which absorb light and act as photoswitches- a type of ‘molecular machine’ that can change shape when an external stimulus, such as light or heat, is applied.

During the tests, the researchers exposed the material to UV light, causing the azobenzene molecules to change shape into a strained configuration inside the MOF pores. This process stores energy like a bent spring stores potential energy. The narrow MOF pores trap the azobenzene molecules in their strained shape, aiding in potential energy storage for long periods at room temperature.

The energy is released when external heat is applied as a trigger to switch its state, which the researchers claim is almost instantaneous. This provides a heat boost which could be used to warm other materials of devices.

Further tests demonstrate that the material can store the energy for at least four months. The extended storage capacity is an exciting aspect of the discovery. It opens up possibilities for cross-seasonal storage since many light-responsive materials switch back within hours or a few days.

The concept of storing solar energy in photoswitches has been studied before. However, previous examples required the photoswitches to be in a liquid. Since the MOF composite is a solid, not a liquid fuel, it’s chemically stable and quickly contained, making it easier to develop coatings or standalone devices.

Dr. John Griffin, senior lecturer in materials chemistry at Lancaster University and joint principal investigator of the study, said: “The material functions a bit like phase change materials, which are used to supply heat in hand warmers. However, while hand warmers need to be heated to recharge them, the nice thing about this material is that it captures ‘free’ energy directly from the sun. It also has no moving or electronic parts, so there are no losses involved in solar energy storage and release. We hope that with further development we will be able to make other materials which store even more energy.”

Although the results were promising for this material’s ability to store energy for long periods, its energy density was modest. The next steps are to research other MOF structures as well as alternative types of crystalline materials with more significant energy storage potential.

Researchers at the National Institute of Technology Kurukshetra recently claimed to have developed a new method to regulate direct current voltage for grid-connected solar photovoltaic plus battery energy storage systems. The researchers explained that power system engineers faced several technical challenges due to the high-penetration of renewable energy into the distributed generations. The voltage level increases due to the active power supply from solar to the distribution system. The issue becomes severe when solar power generation is at its peak during the day time.