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Prof. Wong Ching-ping, Dean of Engineering at The Chinese University of Hong Kong (CUHK), has been leading a cross-institutional research team to conduct a five-year research project entitled ‘Smart Solar Energy Harvesting, Storage and Utilization’. The research team of CUHK has successfully invented the most efficient asymmetric supercapacitors and the flow batteries with the highest volumetric capacity reported to date. The inventions can strongly enhance the efficiency of renewable energy storage and transmission and increase the penetration of renewable electricity generation. The research findings on the asymmetric supercapacitors have been published in Nano Energy. The research findings on the new flow batteries have been published in Nature Communications and Advanced Energy Materials. 

With the fast-growing demand for energy consumption, the greenhouse gas emissions caused by fossil fuels accelerate climate change. The development of renewable energy becomes critical to the future development of society. According to the report of International Renewable Energy Agency (IRENA) published in June 2016, it is forecasted that the electricity generation from solar energy will increase from less than 2% today to 13% of the world by 2030. It is equivalent to an average annual capacity addition of more than double in the next 14 years. The report also illustrates that the electricity generation from solar energy in 2014 has grown by nearly 40% as compared to 2013. The electricity generation cost is forecasted to decrease by nearly 60% in the next ten years. These figures reflect the growing emphasis on and the promising potential for solar energy technology in future. 

Many governments are devoted to promoting the use of renewable energy, but its unstable supply and its inability to provide sufficient energy when electricity demand peaks, limit its penetration. The asymmetric supercapacitors and the new flow batteries invented by the CUHK research team are able to push beyond the limit of the current technology.

Asymmetric supercapacitors with record performance in efficiency 

Traditional batteries have high energy density, but low power density. They can store large amounts of energy, but the charging time is longer. Capacitors have high power density, but low energy density. The charging time is faster, but they can only store a small amount of energy. Traditional batteries and capacitors, therefore, cannot reach the best level of performance due to their drawbacks. 

Prof. Wong has been working with Prof. Zhao Ni, Assistant Professor of the Department of Electronic Engineering, CUHK, as well as their team of students and post-doctoral fellows, to develop nanostructured metal oxide-carbon composites for asymmetric supercapacitors. The energy density and power density of asymmetric supercapacitors can reach as high as 98.0 W h kg^-1 and 22,826 W kg^-1 respectively, making them the best performing supercapacitors. 

Prof. Wong Ching-ping said, “The supplies of renewable energy, such as solar energy and wind energy, are not stable. They cannot generate electricity at night or on windless days. Therefore, a more effective capacitor is needed to store more renewable energy on sunny or windy days, thereby supplying electricity stably. Asymmetric supercapacitors combine the advantages of batteries and capacitors and successfully improve the amount of energy stored and transmitted. This can address the unstable supplies of renewable energy.” 

Catholyte flow batteries with the highest volumetric capacity reported to date 

The research team led by Prof. Lu Yi-Chun, Assistant Professor of the Department of Mechanical and Automation Engineering, CUHK, has successfully developed catholyte flow batteries last year whose volumetric capacity reached  294 Ah L^-1. The team has made a breakthrough again this year. They have further increased the volumetric capacity to 550 Ah L^-1 by combining liquid phase, lithium iodide and solid phase sulphur flow cathodes. It is the highest catholyte volumetric capacity to date. Prof. Lu successfully obtained a patent on the technology and planned to apply it to electric cars.

Prof. Wong said, “With the popularity of electric cars, this technology’s future development is promising. In fact, the research findings are widely recognised and supported by the industry. Some companies have approached us for further collaboration. Meanwhile, it also signifies a step forward in the penetration of renewable energy and improvement of air quality.” 

The research team is now conducting a field demonstration at a student hostel at Lee Woo Sing College, CUHK. The team has installed rooftop solar panels, a smart power storage system and microgrids. Led by Prof. Chiu Dah-ming, Winston, Research Professor of the Department of Information Engineering, CUHK, the research team has been collecting data for analysis and assisting the College to deploy appropriate energy-saving initiatives. The data can also provide reference figures to smart cities.    

‘Smart Solar Energy Harvesting, Storage and Utilization’ Research Project

The five-year research project has been funded by the Theme-based Research Scheme (TRS) of the Research Grants Council (RGC) of Hong Kong Government (HK$ 60.33 million) since 2014, with another HK$ 13.8 million support from CUHK and HK$ 3 million from other partner universities. More than 30 scholars and experts from CUHK, The Hong Kong Polytechnic University, The Hong Kong University of Science and Technology and The University of Hong Kong has been working together to enhance the efficiency of solar power and the penetration of the technology. 

Project website: https://sse.erg.cuhk.edu.hk/sse/