The principle, composition and development trend of solar lighting

1 Solar Lighting Overview

Solar energy is "inexhaustible, inexhaustible", non-polluting renewable energy, the amount of radiant energy sent to the Earth's surface every day is equivalent to about 250 million barrels of oil. For a long time, solar energy has slid away from people. With the rapid development of science and technology, solar energy has gradually been developed and utilized, and has become one of the most promising environmentally friendly energy sources.

At present, there are two main ways to use solar energy.

(1) Converting solar energy into electric energy through solar cells for power generation, that is, photovoltaic power generation, such as solar lighting, solar power stations, and the like.

(2) The solar energy is converted into heat energy by a heat collector, such as a solar water heater, a solar cooker, and the like.

Among them, solar lighting is one of the most important ways to use solar energy, and it is also our main concern and research.

In outdoor lighting, the development of solar lighting is strong. Solar street lights (non-main roads), solar garden lights, solar lawn lights, solar traffic lights, solar advertising lights, etc. are not uncommon, and have become the highlights of green lighting. We refer to solar lights, which are not only lamps, light sources, but also solar cells, batteries, controllers, inverters, light poles and other supporting devices.

The most notable features of solar lighting are energy saving, economy, and environmental protection. There is no need to obtain electric energy from the traditional public power system, which saves investment in power distribution equipment, cables, switches, etc., and basically has no operating expenses. Solar energy is converted into electrical energy, avoiding atmospheric and environmental pollution caused by coal, oil, nuclear and other power generation. Under today's technical conditions, solar lighting is particularly suitable for applications where the power supply is remote or difficult to supply.

2 Solar lighting principle

Solar panels convert solar energy into electrical energy and charge the battery through high-power diodes and control systems. When charging to a certain level, the self-protection system in the controller operates to cut off the charging power. In the evening, the solar panel acts as a photoelectric controller, activates the controller, the battery supplies power to the lighting, ignites the lighting; in the early morning, the solar panel acts as a photoelectric controller, activates the controller, cuts off the power of the lighting, and restarts the conversion. Solar energy works for electrical energy. When the solar light is on, it is also possible to dim according to the settings.

3 solar lighting composition

Solar lighting consists of solar panels, batteries, special controllers for solar lights, illuminants and poles. The following focuses on solar cells, batteries, inverters (for AC lighting bulbs), solar-powered controllers, and illuminators. Figure 1 is a schematic diagram of solar lighting.

3.1 Solar cells

3.1.1 Principle of solar cells

Solar cells are devices that use the principle of photoelectric conversion to convert solar radiation into electrical energy through semiconductor materials. This photoelectric conversion process is usually called "photovoltaic effect", and solar cells are also called "photovoltaic cells."

When sunlight is irradiated onto a P-N junction composed of two homogeneous conductive materials of different conductivity types, P, N, under certain conditions, solar radiation is absorbed by the semiconductor material to form a built-in electrostatic field. If the electrodes are taken from both sides of the built-in electrostatic field and connected to the appropriate load, voltage and current are formed. This is the basic principle of the solar cell, as shown in Figure 2.

A monolithic solar cell is a thin-film semiconductor P-N junction. The rated output voltage is 0.48V under standard lighting conditions. In order to obtain a higher output voltage and a larger capacity, a plurality of solar cells are often connected together.

The output power of the solar cells is random. The output power of the same solar cell is different at different times, different locations and different installation modes.

At present, the photoelectric conversion rate of solar cells is generally more than ten percent, and the photoelectric conversion rate of solar cells in some developed countries can reach about 30%.

3.1.2 Solar cell classification

(1) silicon solar cells;
(2) batteries using inorganic salts such as gallium arsenide III-V compounds, cadmium sulfide, copper, indium, selenium and the like as materials;
(3) a solar cell prepared from a functional polymer material;
(4) Nanocrystalline solar cells, etc.
Silicon solar cell technology is relatively mature. The forbidden band of semiconductor materials is not too wide, the photoelectric conversion rate is high, and the material itself does not cause pollution. Therefore, silicon is currently the most ideal solar cell material. Solar cells based on other materials are also being developed and developed.

3.1.3 Silicon solar cells (1) Monocrystalline silicon cells: At present, monocrystalline silicon solar cells have the highest photoelectric conversion rate (about 20%), and the technology is the most mature. However, due to the high price of single crystal silicon materials, the manufacturing process is cumbersome. The high cost of single crystal silicon has become a major obstacle to the development of monocrystalline silicon solar cells, and is now being replaced by polycrystalline silicon thin film solar cells and amorphous silicon thin film solar cells.
At present, monocrystalline silicon solar cells are widely used in outdoor lighting such as road lighting and traffic signals, and the photoelectric conversion rate is 11% to 24%, and the service life is long.
(2) Polycrystalline silicon battery: Since polycrystalline silicon thin film battery uses much less silicon than single crystal silicon, there is no problem such as efficiency degradation, and it is possible to prepare on an inexpensive substrate material. The cost of polycrystalline silicon thin film solar cells is much lower than that of single crystal silicon cells, and the photoelectric conversion rate is nearly 20%, which is higher than that of amorphous silicon thin film batteries. Therefore, polycrystalline silicon thin film batteries are expected to become the leading products of solar cells.
The application of polycrystalline silicon solar cells in outdoor lighting will become more and more extensive.
(3) Amorphous silicon battery: amorphous silicon thin film solar cell is simple to manufacture, low in cost, convenient for mass production, and has been widely recognized and rapidly developed. The photoelectric conversion rate is above 14.5%, but stability. Poor. Increasing conversion rate and stability is the development direction of amorphous silicon thin film solar cells. At present, amorphous silicon cells are widely used in low-power power systems.

3.1.4 Other solar cells under development

(1) Nanocrystalline chemical solar cells: Nanocrystalline chemical solar cells are new types of solar cells, which are still under development, among which nanocrystalline TiO2 solar cells have attracted much attention. The photoelectric efficiency of the nanocrystalline TiO2 solar cell is above 10%, and the fabrication cost is 1/5 to 1/10 of the silicon solar cell, and the lifetime can reach more than 20 years.
(2) Polymer multilayer modified electrode type solar cell: the raw material is organic material, the flexibility is good, the production is easy, the material source is wide, and the cost is low. Performance and longevity are far less than silicon batteries, but it is possible to provide cheap electricity. The study has just started.
(3) Multi-component thin film solar cells: Some metal compounds such as cadmium sulfide and cadmium telluride polycrystalline thin film cells are more efficient than amorphous silicon thin film solar cells, have lower cost than single crystal silicon cells, and are also easy to mass produce. However, since cadmium is highly toxic, it causes serious pollution and, therefore, cannot be developed.

3.1.5 Capacity selection of solar cells

The solar cell power must be more than 4 times higher than the load power for the system to work properly.

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