Photo‐Rechargeable Electric Energy Storage Systems

Abstract

use of solar energy, e.g., solar collectors, and concentrated solar thermal systems, as well as solar cells. Due to increasing manufacturing capacities and lower costs, the installed capacities of solar cells has grown massively in recent years. In 2012, the capacity of installed solar cells rose over the level of 100 GW worldwide. [ 10 ] The major drawback of solar power is that electricity generation is directly coupled to the availability of the sun (e.g., day and night, weather). This dependence of solar energy does not comply with the actual energy demand and requires a solution. The key technology is the combination of solar cells and effective energy storage systems, e.g., batteries or supercapacitors, to create independent electrical energy sources. [ 8,[11][12][13][14][15] Usually, the generated photovoltaic energy is stored by external batteries (e.g., Li ion or nickel/metal hydride batteries), which are directly connected to the solar cells by wires. [ 13,16,17 ] As a consequence, the relatively long distance between both parts lowers the energy storage effi ciency. [ 13 ] One approach to avoid this problem is the integration of the photo conversion system and the energy storage part within one device, for an effective storage of the excess energy. [ 6,18 ] The stored energy can be released and used throughout the day, and at different places, even if the sun is not shining. [ 18,19 ] Different possibilities for storing large amounts of energy are available, e.g., pumped hydroelectric energy storage or compressed air energy storage as large-scale, centralized systems. [ 19 ] Moreover, photo generated electricity can be stored as chemical energy. The best known system for conversion and storage of solar energy as chemical energy is the photosynthesis of plants, algae and bacteria, in which organic material and oxygen are generated by water and carbon dioxide under sunlight illumination. [ 20 ] Water splitting represents an artifi cial way, [ 19,21 ] where hydrogen and oxygen are produced by utilization of photo catalysts [ 22 ] or semiconducting electrodes. [ 23 ] Photo-electrochemical energy can be effi ciently stored by using two vanadium based redox couples. [ 24 ] Other possibilities for storage of solar energy are molecular energy storage systems, where new chemical bonds are formed; phase change materials, where the energy is stored as thermal energy; as well as electrochemical storage systems. [ 20,25 ] Many types of electrochemical storage applications such as rechargeable batteries (e.g., lithium ion batteries, hightemperature sodium batteries, or lead acid batteries), organic radical batteries, redox fl ow batteries, as well as electrochemical supercapacitors are available. [ 4,19,26 ] A solar rechargeable battery uses the solar light to generate electric energy, which is directly stored by an integrated storage The global energy demand is increasing at the same time as fossil fuel resources are dwindling. Consequently, the search for alternative energy sources is a major topic worldwide. Sol...

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