What is the difference between solar battery and lithium battery?

To be precise, these two are not the same thing. The solar "battery" is not a battery, but a photoelectric conversion semiconductor, and an additional battery pack is required for energy storage. Lithium battery is a chemical energy conversion electric energy device.

Solar cell is a special material of photovoltaic effect. It can not store electric energy. It can only convert solar radiation energy into electric energy instantly. When there is no sunlight, it can not produce current. Lithium battery is a kind of battery and can be used as solar cell. The electricity is stored for easy use without the sun or at night. In solar photovoltaic applications, the two are often used together.

It must be solar energy plus lithium battery. Adding battery, your solar panel is useless. Ordinary battery can't be charged. When the battery is used, there is no sun, you can't use it. If it is a lithium battery, you can use it. For a long time, although it was expensive at the beginning, it is practical, and a small lithium battery will not exceed 10. So it’s still good for lithium batteries.

The solar cell is a battery of photon energy conversion is to absorb the light energy and converted to electricity. The lithium battery is a rechargeable battery, can be repeated the electric charge in it.

Why not use lithium batteries to store solar cells after they are converted into electricity? Of course it can, but the voltage of the general lithium battery is higher, and the charging control circuit requires higher, and the charging current of the general solar battery is very small, so it is enough to use a general rechargeable battery.

Energy issues are an eternal topic in today's world, and they have led to the development of electronic devices, new energy vehicles, and smart grids. As a clean and sustainable energy source, solar energy can make up for the shortage of batteries, and the battery can make up for the intermittent problem of solar energy. How to combine solar cells and energy storage batteries organically? Recently, Professor QiquanQiao (communication author) from South Dakota State University in the United States summarized, discussed and looked forward to the problems encountered in the design of the "solar battery - energy storage battery" integrated system. Among them, three important parameters in the "solar battery - energy storage battery" integrated system: energy density, efficiency and stability are interpreted one by one.

1 The necessity of integrated solar cells - energy storage battery

Today's mass consumers rely heavily on energy technology and its development. The three key technologies related to current energy are smart electronic products, electric vehicles and smart grids. Smart electronics rely on batteries with limited capacity and require frequent charging of electronic devices using wired connections. Solar or Photovoltaics(PV) provide the potential for battery charging because solar energy can reach 100mWcm-2in outdoor sunlight. Another thriving market is the electric vehicle industry. Although electric vehicles do not produce carbon emissions, most of the electricity used by automobiles comes from fossil fuel-driven power grids. The sustainability of electric vehicles is of little use unless the vehicle uses electricity from renewable sources. 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 prospective application is the 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 intermittent nature leads to power fluctuation output, which is a key issue for grid applications. As a result, power companies limit the power of integrating photovoltaic power into the grid. This has not fully utilized the potential of photovoltaic power generation. Energy storage batteries can solve these problems. The battery can be charged during the day and discharged at night, which provides the possibility to connect photovoltaic power to the grid.

2 Traditional and advanced "solar cells - energy storage battery" system

The traditional method of charging a battery using a solar cell is to design two systems independently. It involves the solar cells and energy storage battery as two independent unit through the wires. Such systems tend to be expensive and cumbersome and inflexible, also need more space, besides the external wires can lead to loss of electric power.

The organic integration of capacity and energy storage into one unit for 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 costs and increases the practicality of the photovoltaic system. Although there are many advantages, it still has great challenges in terms of efficiency, capacity and stability. The current 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 system can be done by two different types of configuration: three electrodes (figure 1 b and 1 c) and double electrode (FIG. 1 d).One in the design of three electrodes, the electrode was used as a common electrode as a photovoltaic device and battery between cathode and anode. In the double electrode configuration, the positive and negative at the same time perform the function of light conversion function and energy storage.

3 Binary separate "solar cells - energy storage battery" design

This part summarizes the work of the previous separation of "solar battery - 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. Four series of perovskite solar cells charge lithium-ion batteries with an efficiency of 7.36%. The communication author Qiao Qiquan team used a transformer and maximum power point tracking to charge a lithium-ion battery using a single-cell perovskite solar cell, and its efficiency reached 9.36%. The research results were published on Advance Energy Materials.

4 design of a one-piece integrated "solar battery - energy storage battery"

Most about one-piece integrated "solar battery - energy storage battery" design work has focused on combining solar cells and capacitive energy storage rather than with the battery. Integrated system design can be divided into three types: (1) direct integration, (2) light auxiliary integration and integration (3) oxidation also influent flow batteries. Direct integration including solar cells and batteries stacked together (not including oxidation also concentrate flow battery).Light auxiliary integration using solar energy for the battery only provide part of the energy. Oxidation also influent flow integration involves the use of a solar charging oxidation also influent flow battery. Articles respectively on the three forms of predecessors' work has carried on the detailed summarization, figure 3, 4, and 5 are typical representative of them respectively.

5.1 The energy density

Conventional lithium-ion batteries are often packaged in a coiled package in order to increase their energy density, but are not feasible for a "solar cell-energy storage battery" integrated system. Because the way lithium-ion batteries are packaged affects the area that accepts solar energy. The number and power of solar cells need to match the energy storage portion to address the available PV surface area, the number of possible stacked cells, and the power matching needs. Using a high specific capacity material for the electrode can increase the overall energy density of the system. For example, a silicon-NMC battery has an energy density of 400 KW/kg, and silicon is a photovoltaic material. If the silicon is used as an integrated lithium ion electrode in an integrated system, it can also be used as a photovoltaic electrode and will be an ideal design. Silicon solar cells require high crystallinity, and lithium intercalation reduces the crystallinity of silicon, which requires finding an optimal balance point. Research on lithium metal batteries has also made it possible to increase the overall energy density of the system. In addition, according to reports in the literature of light conversion perovskite materials has been demonstrated the ability to embed lithium ion, and doped lithium ion in the perovskite have positive effects on the photovoltaic performance, which makes the perovskite could also be integrated photovoltaic cell system high capacity of dual functional materials. For high volume ratio of energy applications, will be more appropriate.

Idealized the overall efficiency of integration system is the product of the solar energy conversion efficiency and the energy storage system, integrated system can achieve the maximum efficiency is limited by the solar energy conversion efficiency, in the reality in the design of the efficiency of the integrated system to consider the various losses. Silicon solar cells and perovskite cells provide more efficient photoelectric conversion and provide better overall efficiency in integrated systems. If you want to make solar cells provide greater efficiency, another factor to consider is the most powerful tracking (MPPT), which makes the solar cell can provide maximum power. Energy storage battery, need to select the best match is negative to maximize the coulomb efficiency.

5.2 Stability

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 in the preliminary research stage. If you choose perovskite as the photovoltaic part of the integrated system, you need to have a bigger research on perovskite. Break through. The use of liquid electrolytes is also detrimental to the stability of the system. Solid electrolytes can be chosen to improve the safety and stability of the overall system. Because the solar cell part generates heat, it is also considered to have high temperature resistance while selecting the energy storage battery electrode material.

6. Future development direction and prospects

The integrated "solar cell - energy storage battery" system is still in the early stages of research and development. The literature reports so far have focused on the feasibility of innovative materials development and new equipment design, and 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 integrated system can use the following strategies, such as the use of energy conversion and storage dual functional materials, use of large-capacity energy storage materials, maximum power tracking, integrated lithium ion capacitor, using solid electrolyte, improve the compatibility between the electrochemical electrodes and the electrolyte. Integrated system can make use of the simulation or modeling method, to better predict the system performance, to provide better design solutions. In addition, future efforts should be referred to the "solar cells - energy storage battery" integrated system with such as sensor networks, wearable equipment and electronic equipment such as combination of practical application. Although the "solar cells - energy storage battery" the commercialization of the integrated system still has a long way to go, its development will greatly benefit from the rapid progress in the field of photovoltaic (PV) and battery. Its future direction will also evolve from the initial application to low-power, compact applications to large-scale energy applications.

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