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 batterys? Recently, Prof. Qiquan Qiao (Corresponding author) from South Dakota State University in the United States has summarized, discussed, and looked forward to the problems encountered in the design of an integrated system for “solar cell-energy storage battery”. Among them, the three important parameters in the " solar cell-energy storage battery " integrated system: energy density, efficiency, and stability are each interpreted one by one.
2. Comparison of traditional and advanced " solar cell-energy storage battery " systems
The traditional method of using a solar cell to charge a battery is that the two systems are designed independently (FIG. 1A), which involves the solar cell and the energy storage battery 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. 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 1B and 1C) and two electrodes (Figure 1D). 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.
Figure 1 Independent design of traditional solar cell and energy storage battery (A), three-electrode design (B and C) and two-electrode design (D)
3. Design of binary separation type "solar cell-energy storage battery"
This section summarizes the work of predecessor separation “solar cell-energy storage battery” design. Silicon solar cells, perovskite solar cells, and dye-sensitized solar cells can be combined with lithium ion batteries in different forms. Among them, Figures 2A 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 2C and D).
Figure 2 separated photovoltaic cell system
(Figure A, Figure B) Recharging Li4Ti5O12/LiFePO4 Lithium-Ion Batteries Using Four Perovskite Solar Cells
(Figure C, Figure D) Recharge Li4Ti5O12/LiCoO2 Lithium-ion Battery with Single-cell Perovskite Solar Cell under assistance of DC-DC Converter.
4. Monolithically integrated "solar cell-energy storage battery" design
Most of the design work on the monolithically integrated “solar cell—energy storage battery” 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 3, 4 and 5 are their typical representatives.
Figure 3 Direct integration
A schematic diagram of a three-electrode (Figure A) design of a silicon solar cell filled with a Li4Ti5O12/LiCoO2 lithium ion battery and (Figure B) Photo-electricity charge/constant current discharge cycle performance. A schematic diagram of the two-electrode design (Figure C) and the charging process and (Figure D) charge/discharge voltage curves of the mixed dye and lithium iron phosphate as the positive electrode and the lithium metal as the negative electrode.
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.
Figure 5 Solar and Flow Battery Integration
5. Technical challenges and opportunities
5.1 Energy density
Conventional lithium-ion batteries often use a roll-type packaging method to increase their energy density, but it is not feasible for a "solar cell-energy storage battery" integrated system. Because the lithium-ion battery's packaging affects the area that receives solar energy. The number and power of solar cells need to be matched with the energy storage section to solve the available PV surface area, the number of possible stacked cells and the power matching requirements. The use of high specific-capacity materials as electrodes can increase the overall energy density of the system. For example, silicon-NMC batteries have an energy density of 400 kW/kg, and silicon is a photovoltaic material if silicon can be used as a lithium ion in an integrated system. The electrodes can also be used as photovoltaic electrodes, which will be an ideal design. Silicon solar cells require a high degree of crystallinity, and the insertion of lithium will reduce the crystallinity of silicon, which requires finding an optimal balance point. The study of lithium metal batteries also offers the possibility to increase the overall energy density of the system. In addition, it has been reported in the literature that the photoconversion material perovskite has been shown to have the ability to intercalate lithium ions, and that doping lithium ions in perovskites has a positive influence on its photovoltaic performance, which makes it possible for perovskites to become integrated photovoltaic cells. System high-capacity dual-function material. For applications requiring higher volumetric energy, it will be more appropriate.
The overall efficiency of the idealized integrated system is the product of the solar energy conversion efficiency and the energy storage system. The maximum efficiency that the integrated system can achieve is limited by the solar energy conversion efficiency. In reality, the efficiency of the integrated system in the design must also take into account various losses. Silicon solar cells and perovskite batteries can provide more efficient photoelectric conversion and provide better overall efficiency in integrated systems. Another factor to consider if the solar cell is to provide greater efficiency is Maximum Power Tracking (MPPT), which allows the solar cell to provide maximum power. For energy storage batteries, the most suitable positive and negative electrodes need to be selected to maximize Coulomb efficiency.
Stability needs to consider light stability, electrochemical stability, and environmental stability, which requires careful selection of electrode materials. Although people have made gratifying progress in the study of the stability of perovskite solar cells, they are still at a preliminary stage of research. If perovskite is selected as the photovoltaic part of the integrated system, there is a need for greater research on perovskites. Break through. The use of liquid electrolyte is also not conducive to the stability of the system, you can choose to use solid electrolyte to improve the overall system safety and stability. Because the solar cell part will generate heat, the high temperature performance of the energy storage battery electrode material must be considered at the same time.
6. Future Directions and Prospects
The integrated "solar cell-energy storage battery" system is still in the early stage of research and development. The literature reports so far have focused on the feasibility of innovative material development and new equipment designs. Future research should continue to develop in this direction. The novel design needs to be combined with high capacity, high efficiency and more stable materials. Optimization of the integrated system can use the following strategies, such as the use of energy conversion and storage of dual-function materials, the use of large-capacity energy storage materials, maximum power tracking, integrated lithium-ion capacitors, use of solid-state electrolytes, improved compatibility between electrochemical electrodes and electrolytes, etc., Integrated systems can use simulation or modeling methods to better predict system performance and provide better design solutions for integrated systems. In addition, future efforts should combine the integration of "solar cell-energy storage battery" systems with practical applications such as sensor networks, wearables, and electronic devices. Although the current commercialization of "solar cell-energy storage battery" integrated system still has a long way to go, its development will greatly benefit from the current rapid progress in the field of photovoltaic and battery. Its future development will also be directed toward low-power, compact applications, and then to large-scale energy applications.