Date Published: May 17, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Bin Luo, Delai Ye, Lianzhou Wang.
Over the last few decades, there has been increasing interest in the design and construction of integrated energy conversion and storage systems (IECSSs) that can simultaneously capture and store various forms of energies from nature. A large number of IECSSs have been developed with different combination of energy conversion technologies such as solar cells, mechanical generators and thermoelectric generators and energy storage devices such as rechargeable batteries and supercapacitors. This review summarizes the recent advancements to date of IECSSs based on different energy sources including solar, mechanical, thermal as well as multiple types of energies, with a special focus on the system configuration and working mechanism. With the rapid development of new energy conversion and storage technologies, innovative high performance IECSSs are of high expectation to be realised for diverse practical applications in the near future.
Energy shortage and environmental deterioration resulting from insufficient fossil fuel supplies and increasing consumption has becoming two major global problems for human beings.1 Developing new technology to make full use of the abundant “green” energies in the forms of solar, mechanical, and thermal energies have been recognised as a promising and effective way for our long‐term energy needs and environmental sustainable development.2 Over the past few decades, a large number of energy conversion technologies such as solar cells,3 mechanical generators,4 and thermoelectric generators,5 have been developed to convert the “green” energies into electrical energy, which is the most widely used energy type in our current society.[[qv: 2b,6]] However, the major drawback of these “green” energies is that electricity generation is highly dependent on the availability of the energy sources (e.g., sunlight, wind, heat), which is always not in good alignment with the actual demand.
According to the different ways of energy conversion during the charging process, the solar energy based IECSSs can be divided into two groups as shown in Figure1. In the first group (Figure 1a), the energy conversion and storage units are normally separated and have independent electrochemical behaviour during the photo‐charging and discharging processes. There are no photo‐induced redox reactions during the photo‐charging process, which can be defined as photovoltaic charging system. For example, IECSSs based on most of the semiconductor solar cells such as silicon solar cells, organic solar cells and perovskite solar cells (PSCs), should fall into this group. The charging voltage on the energy storage part can be provided or partially provided by photovoltaic solar cells. In contrast, photo‐induced redox reactions will be involved during the energy storage (photo‐charging) process in a photocatalytic charging system. As illustrated in Figure 1b, during the photo‐charging process, the photoactive material on the photoelectrode will be excited to generate the electron hole pairs. The resultant electrons will then transfer to the energy storage electrode and the holes will be involved into the redox reactions and finally neutralized by the electrons from the counter electrode. During the discharging process, the charges will then come back to the counter electrode from the energy storage electrode, and the cations or anions in the electrolyte between these two electrodes will provide the counterbalance. In this section, recent progress and working mechanism of some typical representatives of the above two solar based IECSSs will be discussed. It should be noted that although the dye sensitized solar cells (DSSCs) have always been recognised as one kind of photovoltaic cell, the energy conversion in the DSSC is basically a photoelectrochemical process that involves photo‐induced redox reactions, which is different from the other semiconductor based solar cells. Therefore, here we place all the DSSC based IECSSs into the group of photocatalytic charging system.
Mechanical energy is another important energy source that can be converted into electrical energy to power electronics. Based on piezoelectric or triboelectric effects, various mechanical energies such as wind, waves, fluids etc. can be effectively converted into electricity that can be employed in powering nano‐devices.6, 57 In the past decade, many efforts have focused on the development of mechanical energy based IECSSs, and the progress on IECSSs related to piezoelectric effect will be reviewed as follows.
Thermal energy is another abundantly available energy source, and most of it especially the low‐grade heat from such sources as industrial wastes, geothermal activity, and solar heating, is often wasted. Thermal‐electric energy conversion and storage has been demonstrated as an attractive technology to utilize this vast energy. Investigations in this field have focused on the exploration of solid‐state devices based on semiconductor materials for the conversion of thermal energy into electricity.[[qv: 5a]] The main problems with this way include the lack of energy storage capacity and the high cost semiconducting materials. Liquid‐based thermoelectrochemical cells (also known as thermogalvanic cells or thermocells) become an alternative due to the potential low cost design and scalablility.64 In thermoelectrochemical cells, the electrochemical potentials of redox couples (e.g., [Fe(CN)6]4−/3− and Co2+/3+) are temperature dependent and can be used directly to generate a output voltage at different temperatures. The current research in this area are focusing on increasing the energy conversion efficiencies and output power and many strategies including selecting suitable redox couples[[qv: 60h,65]] and electrode materials,66 as well as the use of ionic liquids,[[qv: 65d,67]] have been proposed. However, only a few approaches were developed to endow thermoelectric system with energy storage capability.[[qv: 65c,68]]
In the above sections, we have introduced several types of IECSSs to collect the solar, mechanical, thermal energies, respectively. The development of more functional IECSSs that can scavenge and store multi‐mode energies from environment individually or simultaneously is more attractive but also highly challenging.69 Devices powered by this system can endure more complex environment by using whatever energy that might be available.
In summary, this review provides an overview of the recent advances of IECSSs based on solar energy, mechanical energy, thermal energy, or multiple energies, which has been recognised as a promising way to simultaneously capture and store energy from the environment. Some of the key aspects and comparisons of these IECSSs are summarized in Table1.
The authors declare no conflict of interest.