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The advantage of solar rechargeable batteries
- Jun 22, 2018 -

The energy issue is an eternal topic in today's world. It has led to the development of electronic devices, new energy vehicles, and smart grids. Solar energy as a clean and sustainable energy source can make up for the deficiency of the battery, and the battery can make up for the intermittent problem of solar energy. How to organically integrate solar cells and energy storage cells? Recently, Prof. Qiquan Qiao from South Dakota State University and others have summarized, discussed, and looked forward to the problems encountered in the design of integrated systems for "solar battery-energy storage batteries." Among them, the three important parameters in the "solar battery-energy storage battery" integrated system: energy density, efficiency, and stability are each interpreted one by one.

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1. The need for integrated solar cells - energy storage cells

Today's mass consumers rely heavily on energy technologies and their development. The current three key energy-related technologies are smart electronics, electric vehicles and smart grids. Intelligent electronic products rely on batteries with limited capacity and require the use of wired connections to frequently charge electronic devices. Solar energy or photovoltaics offer the possibility of charging the battery because the energy density of solar energy can reach 100 mW/cm2 in outdoor sunlight. At present, another prosperous market is the electric vehicle industry. Although electric vehicles do not produce carbon emissions, most of the electricity used by the vehicles comes from the fossil fuel-driven power grid. Unless vehicles use electricity from renewable sources, the sustainability of electric vehicles is of little significance. In addition, the distribution of charging stations also limits their practical application. Distributed generation such as photovoltaic power generation is the most suitable charging method for electric vehicles. Another foreground application is the power grid. The application of renewable energy is steadily expanding. The biggest problem with using photovoltaic energy is the lack of sunlight at night or on cloudy days, resulting in intermittent power supply during use. This intermittency can result in power fluctuation output, which is a key issue for grid applications. Therefore, power companies limit the power of photovoltaic power into the grid. As a result, the potential of photovoltaic power generation is not fully utilized. The energy storage battery can solve these problems. The battery can be charged during the day and discharged at night, which provides the possibility of photovoltaic power generation access to the power grid.

2. Comparison of traditional and advanced "solar battery-energy storage battery" systems

The traditional method of using a solar cell to charge a battery is to design the two systems independently (FIG. 2A), which involves a solar cell and an energy storage cell connected as two separate units by wires. Such systems are often expensive, cumbersome, and inflexible. They also require a relatively large space. In addition, external wires can cause power loss.

Combining production capacity and energy storage into one unit to achieve an integrated design will effectively solve the energy density problem of solar cells and batteries. This design has the characteristics of miniaturization, which in turn reduces the cost and increases the practicality of the photovoltaic system. Although there are many advantages, there are still great challenges in terms of efficiency, capacity, and stability. At present, research in this area is still in its infancy, and the focus of research is mainly on the design of materials and devices.

Integrated photovoltaic cell systems can be implemented in two different configurations: three electrodes (Figures 2B and 2C) and two electrodes (Figure 2D). In the three-electrode design, one electrode is used as a common electrode as a cathode or anode between the photovoltaic device and the battery. In the two-electrode configuration, the positive and negative electrodes perform the light conversion function and the energy storage function simultaneously.

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 Fig. 2 Independent design of traditional solar cell and energy storage cell (A), three-electrode design (B and C) and two-electrode design (D)

3. Design of binary separation type "solar battery-energy storage battery"

This section summarizes the work of predecessor separation “solar cell-energy storage cell” design. Silicon solar cells, perovskite solar cells, and dye-sensitized solar cells can be combined with lithium ion batteries in different forms. Among them, FIGS. 3A and B show that four series-connected perovskite solar cells are used to charge lithium ion batteries with an efficiency of 7.36%. The Qiao Qiquan team of the paper used transformers and maximum power point tracking to realize the use of single-cell perovskite solar cells to charge lithium-ion batteries. The efficiency reached 9.36%. The results of the study were published on Advance Energy Materials (Figure 3C). And D).

 

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Figure 3 separated photovoltaic cell system

(A, B) Charging Li4Ti5O12/LiFePO4 Lithium-Ion Batteries Using Four Perovskite Solar Cells

(C, D) Charging Li4Ti5O12/LiCoO2 Lithium-Ion Batteries with the Help of a DC-DC Converter Using a Single Perovskite Solar Cell

4. One-piece integrated "solar battery-energy storage battery" design

Most of the design work on the monolithically integrated “solar cell—energy storage cell” focuses on the combination of solar cells and capacitive energy storage rather than batteries. The integrated system can be divided into three types of designs: (1) direct integration, (2) light-assisted integration, and (3) redox flow battery integration. Direct integration involves stacking solar cells and batteries together (excluding redox flow batteries). Light-assisted integration uses solar energy to charge the battery with only a portion of the energy. Redox flow integration involves the use of redox flow batteries with solar charging. The article gives a detailed summary of the work of these three forms of predecessors. Figures 4, 5 and 6 are their typical representatives.

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 Figure 4 Direct integration

A schematic diagram of the (A) design of a three-electrode silicon solar cell filled Li4Ti5O12/LiCoO2 lithium-ion battery and (B) photocharge/constant-current discharge cycle performance. The (C) schematic diagram of the two-electrode design with the mixed dye and lithium iron phosphate as the positive electrode and the lithium metal as the negative electrode and the charging process and the (D) charge/discharge voltage curve.

 

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Figure 5 Optical Assist Charge Integration

The (A) schematic of the dye-sensitized TiO2 photoelectrode and the oxygen electrode of the lithium-oxygen cell are integrated with the (B) charge curve. Dye sensitized solar cells integrated with Li/LiFePO4 lithium-ion battery (C) schematic and (D) light-assisted charge profiles.

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 Figure 6 Solar and Flow Battery Integration

A schematic diagram of a solar rechargeable redox flow battery (A) based on a Li2WO4/LiI two-phase electrolyte and (B) a photovoltaic charge and constant current discharge voltage curve. An integrated (C) schematic of a dual silicon photoelectrochemical cell and a bismuth/bromine redox flow battery, (D) constant current discharge curve and (E) overall efficiency.