Solar Panels Guide

The Principle of Solar Cells

Solar cell modules are also called solar panels and photovoltaic modules. They are the core part of the solar power generation system and the most important part of the solar power generation system. Its function is to convert solar energy into electrical energy. The energy converter of solar photovoltaic power generation is a solar cell, also known as a photovoltaic cell. The principle of solar cell power generation is the photovoltaic effect. When sunlight shines on the solar cell, the cell absorbs light energy and generates photogenerated electron-hole pairs. Under the action of the built-in electric field of the battery, the photogenerated electrons and holes are separated, and the accumulation of opposite-signal charges occurs at both ends of the storm.

A “photogenerated voltage” is generated, the “photovoltaic effect.” If the electrodes are drawn on both sides of the built-in electric field, and the load is connected, the load will have a “photo-generated current” flowing through, thereby obtaining power output. In this way, the sun’s light energy is directly converted into electricity that can be used. At the same temperature, the effect of light intensity on the solar panel: the greater the light intensity, the greater the open-circuit voltage and short-circuit current of the solar panel, and the greater the maximum output power. At the same time, it can be seen that the open-circuit voltage changes with the irradiation intensity. Not as obvious as the change of short-circuit current with irradiation intensity. Under the same light intensity, the effect of temperature on the panel: when the temperature of the solar cell increases, the output open-circuit voltage decreases significantly with the temperature, and the short-circuit current gains slightly, and the general trend is that the maximum output power decreases.

Features of Solar Cells

The solar cell module has high photoelectric conversion efficiency and high reliability; advanced diffusion technology ensures the uniformity of conversion efficiency throughout the chip; good electrical conductivity, reliable adhesion, and good electrode solderability; high precision. The silk-screen printed graphics and high flatness make the battery easy for automatic welding and laser cutting.

Application of Solar Photovoltaic Modules

Small Solar Power System

1：The small solar panel power supply system ranges from 10 to 100W. It is used for the daily electricity consumption of military and civilians in remote areas without electricity, such as plateaus, islands, pastoral areas, and border posts. 2：3-5KW home roof off-grid grid-connected power generation system. 3：Photovoltaic water pump: solve the drinking and irrigation of deep wells in areas without electricity.

Transportation

Beacon lights, traffic/railway signal lights, traffic warning/signal lights, Yuxiang street lights, high-altitude obstruction lights, highway/railway wireless phone booths, unattended road class power supply, etc.

Communication

Solar unattended microwave relay station, optical cable maintenance station, broadcasting/communication/paging power supply system, rural carrier telephone photovoltaic system, small communication machine, GPS power supply for soldiers, etc.

Oil, Ocean, Weather

Oil pipeline and reservoir gate cathodic protection solar power system, life and emergency power supply of oil drilling platform, marine detection equipment, meteorological/hydrological observation equipment, etc.

Home Lighting Power Supply

Garden lights, street lights, portable lights, camping lights, mountaineering lights, fishing lights, black lights, tapping lights, energy-saving lamps, etc.

Photovoltaic power station

10KW-50MW independent photovoltaic power station, wind-solar (diesel) complementary power station, various large parking plant charging stations, etc.

Solar Building

Combining solar power generation with building materials will enable large buildings to achieve self-sufficiency in electricity, which is a major development direction in the future.

Other

Matching with cars: solar cars/electric cars, battery charging equipment, car air conditioners, ventilation fans, cold drink boxes, etc.; Renewable power generation system of solar hydrogen production and fuel cell; Power supply for desalination equipment; Satellites, spacecraft, space solar power plants, etc.

Classification and Characteristics of Solar Cells

According to the different materials used, solar cells can be divided into silicon solar cells, multi-compound thin-film solar cells, polymer multilayer modified electrode solar cells, nanocrystalline solar cells, organic solar cells, plastic solar cells, among which silicon solar cells are the most mature and dominate the application.

Silicon Solar Cells

Silicon solar cells are divided into monocrystalline silicon solar cells, polycrystalline silicon solar cells， and amorphous silicon thin-film solar cells.

Monocrystalline silicon solar cells have the highest conversion efficiency and the most mature technology. The highest conversion efficiency in the laboratory is 26%, and the efficiency in large-scale production is 20% (22% by 2022). It still occupies a dominant position in large-scale applications and industrial production, but due to the high cost of monocrystalline silicon, it isn’t easy to greatly reduce its cost. Polycrystalline silicon and amorphous thin films have been developed as monocrystalline solar cells to save silicon materials. Replacement product. Compared with monocrystalline silicon, polycrystalline silicon solar cells have lower costs and higher efficiency than amorphous silicon cells. The highest conversion efficiency in the laboratory is 20%, and the conversion efficiency in industrial-scale production is 15% (as of 2022, it is 18%). Therefore, polycrystalline silicon cells will soon dominate the solar cell market.

Multi-compound Thin-film Solar Cells

Polycrystalline thin-film batteries cadmium sulfide and cadmium telluride polycrystalline thin-film batteries are more efficient than amorphous silicon thin-film solar cells. Their cost is lower than that of single-crystal silicon batteries, and they are also easy to mass-produce. The environment causes serious pollution. Therefore, it is not the most ideal substitute for crystalline silicon solar cells.

Polymer Multilayer Modified Electrode Solar Cells

Nanocrystalline chemical energy solar cells are newly developed, and their advantages lie in their cheap cost, simple process, and stable performance. Its photoelectric efficiency is stable above 10%, and the production cost is only 1/5 to 1/10 of that of silicon solar cells. The lifespan can reach more than 20 years. The research and development of such batteries have just started and will gradually enter the market in the near future.

Nanocrystalline Solar Cells

Organic thin-film solar cells are solar cells whose core parts are made of organic materials. It is reasonable that everyone is unfamiliar with organic solar cells. More than 95% of solar cells in mass production are silicon-based, while less than 5% are made of other inorganic materials.

Organic Solar Cells

In dye-sensitized solar cells, a pigment is attached to TiO2 particles and soaked in an electrolyte. Free electrons and holes are generated when the pigment is irradiated with light. The free electrons are absorbed by TiO2, flow out from the electrode into the external circuit, pass through the electrical appliance, flow into the electrolyte, and finally return to the pigment. The low manufacturing cost of dye-sensitized solar cells makes them highly competitive. Its energy conversion efficiency is around 12%.

Plastic Solar Cells

Plastic solar cells use recyclable plastic films as raw materials and can be mass-produced through “roll-to-roll printing” technology, which is low-cost and environmentally friendly. However, plastic solar cells are not yet mature. In the next 5 to 10 years, solar cell manufacturing technology based on organic materials such as plastics will mature and be used on a large scale.

Introduction to The Classification of Silicon Solar Cells

Silicon solar cells refer to solar cells with silicon as the base material. The earliest silicon solar cells arose from an interest in using silicon for point-contact rectifiers. The rectifying properties of sharp metal contacts for various crystals were discovered as early as 1874. In the early days of radio technology, such crystal rectifiers were widely used as detectors in radio reception equipment. However, with the development of thermionic tubes, this crystal rectifier has been replaced by thermionic tubes, except that it is still used in the ultra-high frequency field. The most typical example of such a rectifier is the point contact of tungsten on the silicon surface. This technique has improved silicon purity and hopes for a better understanding of silicon’s properties. Silicon solar cells have special design and material requirements compared to most other silicon electronic devices.

To obtain high energy conversion efficiency, silicon solar cells require almost ideal passivation of the silicon surface, and the bulk material properties must be of uniformly high quality. This is because some wavelengths of light must travel hundreds of micrometers in silicon to be absorbed, and the resulting carriers must still be able to be collected by the cell. Classification of silicon solar cells are solar cells based on silicon. According to the thickness of the silicon wafer, it can be divided into crystalline silicon solar cells and thin-film silicon solar cells. According to the crystalline form of the material, crystalline silicon solar cells are divided into two types: monocrystalline silicon (c-Si) and polycrystalline silicon (p-Si) solar cells; thin-film silicon solar cells are divided into amorphous silicon (a-Si) thin-film solar cells, microcrystalline silicon solar cells There are three types of crystalline silicon (c-Si) solar cells and polycrystalline silicon (p-Si) thin-film solar cells.

Monocrystalline Silicon Solar Cells

Monocrystalline silicon solar cells have the highest conversion efficiency and the most mature technology. The highest conversion efficiency in the laboratory is 26%, and the efficiency in large-scale production is 20% (22% by 2022). It still occupies a dominant position in large-scale applications and industrial production, but due to the high cost of monocrystalline silicon, it isn’t easy to reduce its cost greatly. Polycrystalline silicon and amorphous silicon thin films have been developed as monocrystalline silicon solar cells to save silicon materials. Replacement product.

Polycrystalline Silicon Solar Cells

Compared with monocrystalline silicon, polycrystalline silicon solar cells have lower costs and higher efficiency than amorphous silicon cells. The highest conversion efficiency in the laboratory is 20%, and the conversion efficiency in industrial-scale production is 15% (as of 2022, it is 18%). Therefore, polycrystalline silicon cells will soon dominate the solar cell market.

Amorphous Silicon Thin-film Solar Cells

Amorphous silicon thin-film solar cells are low-cost, lightweight, easy to mass-produce, and have great potential. Amorphous silicon, its atomic structure is not as regularly arranged as crystalline silicon, but a semiconductor with an amorphous crystal structure. Amorphous silicon belongs to the direct band system material, which has a high absorption coefficient of sunlight. Only a one μm thick film can absorb 80% of sunlight. Amorphous silicon thin-film solar cells came out in 1976 due to the shortage of raw materials and rising prices, which promoted the technology of using silicon efficiently and the development of amorphous silicon thin-film solar cells.

The low cost of amorphous silicon thin-film cells makes up for their lack of photoelectric conversion efficiency. However, due to many defects in amorphous silicon, the efficiency of the prepared solar cell is relatively low. Due to the photoelectric efficiency decline effect caused by its material, the stability is not high, which directly affects its practical application. Microcrystalline silicon (μc-Si) thin-film solar cells are also unstable due to the photoelectric efficiency decay effect. Development is limited. In recent years, polycrystalline silicon thin-film solar cells have been a hot spot in solar cell research. Although polysilicon is an indirect bandgap material and is not an ideal thin-film solar cell material, with the continuous development of light trapping technology, passivation technology, and carrier confinement technology, it is possible to prepare high-efficiency and inexpensive polysilicon thin-film solar cells.

Detailed Explanation of The Composition

Monocrystalline silicon solar cells are solar cells that use high-purity monocrystalline silicon rods as raw materials and are currently the fastest-developed solar cells. The structure and production process of monocrystalline silicon solar cells have been finalized, and the products have been widely used in space and ground.

1. Tempered glass: tempered glass protects the main body of power generation (such as a battery), and the selection of tempered glass is required. First, the light transmittance must be high (generally more than 91%); second, ultra-white tempering treatment.
2. EVA: EVA is used to bond and fix the tempered glass and the main power generation body (such as solar cells). The quality of the transparent EVA material directly affects the module’s life. The EVA exposed to the air is easy to age and turns yellow, thus affecting the light transmittance of the module. In addition to the quality of EVA itself, the lamination process of module manufacturers is also very influential. For example, the adhesion of EVA is not up to standard, and the bonding strength of EVA to tempered glass and backplane is not enough, which will cause EVA to age prematurely. , affecting component life.
3. Solar cell: The main function of solar cells is to generate electricity. The mainstream in the power generation market is crystalline silicon solar cells and thin-film solar cells, both of which have advantages and disadvantages. Crystalline silicon solar cells have relatively low equipment costs, high consumption, and cell costs, and high photoelectric conversion efficiency; thin-film solar cells are suitable for power generation in outdoor sunlight, with relatively high equipment costs, low consumption, battery costs, the photoelectric conversion efficiency is more than half that of crystalline silicon cells. Still, the low-light effect is perfect, and it can also generate electricity under ordinary light, such as solar cells on calculators.
4. Backboard: The function of the backplane is to seal, insulate, and be waterproof (generally, TPT, TPE, and other materials must be resistant to aging). Component manufacturers have a 25-year warranty. Tempered glass and aluminum alloy are generally no problem. The key is whether the backplane and silicone can meet the requirements.
5. Aluminum Frame: The aluminum frame plays a certain role in sealing and supporting.
6. Junction box: The junction box protects the power generation system and is a current transfer station. The junction box automatically disconnects the short-circuit battery string if the module is short-circuited. The selection of the most critical diode in the junction box prevents the entire system from burning out. According to the cells in the module, Different types of diodes correspond to different diodes.
7. Silica gel: Silicone acts as a seal to seal the junction between the component and the aluminum alloy frame, the component， and the junction box. Some companies use double-sided tape and foam instead of silicone. Silica gel is widely used in China, the process is simple, convenient, and easy to operate, and the cost is meager.

Solar Module Electrical Parameters

Taking a certain line of brand polycrystalline components as an example, the electrical parameters are as follows: 

Standard Test Conditions[STC]: irradiance 1,000 W/m2 ; AM 1,5; module temperature 25℃. Measuring uncertainty of power is within士3%. Tolerance of Pmpp: 0-+3%. Certified in accordance with IEC61215, IEC61730-1/2.

Maximum Power Pmax

Pm=Im*Vm, corresponding to the vertex of the power parabola in the figure below. The parabola is the power curve, and the other is the UI curve. Component parameter nominal, generally based on “Standard Test Conditions STC”. With changes in environmental conditions such as temperature and irradiance, the corresponding parameters of the components will change. In addition, the power characteristic curve of the module is a “quasi-parabola”, which has the highest point and the working point that the inverter MPPT “Maximum Power Point Tracking” needs to find.

Power Tolerance

“0~+5” means positive tolerance. Such as 265W solar modules, the power range between 265W to 270W is qualified.

Maximum Power Point Operating Voltage VM

It corresponds to the abscissa corresponding to the vertex of the power parabola in the above figure, which represents the working voltage of the component at the maximum power.

Maximum PowerPoint Operating Current IM

Mechanical Parameters of Solar Modules

Solar Cell

The Solar cell is the smallest unit of photoelectric conversion, and the commonly used size is generally 125mm*125mm or 156mm*156mm. By 2022, the size of the largest solar cell will be 210*210mm, and the maximum power solar panel can reach 600W. The working voltage of the solar cell is about 0.5V and generally cannot be used alone. After the solar cells are packaged in series and parallel, they become photovoltaic modules.

Number of Solar Cells

For example, a 300W monocrystalline solar panel comprises 60 156*156mm solar cells.

Solar Module Size

The size of the 340W to 390W solar panel in the above picture can be 1956*992*40mm and 1980*1002*40mm. Usually, the size of the solar panels can also be customized if the quantity is sufficient.

Solar Module Weight

The weight per square meter of polycrystalline silicon modules commonly found on the market is about 12 kilograms (about 19 kilograms for a single solar module with 60 solar cells).

Backboard

Performance requirements for crystalline silicon cell module backboards typically include the following:

• It has good weather resistance
• No change in the lamination temperature
• Bonds firmly to viscous materials
• Must have low thermal resistance and prevent the ingress of water or water vapor

Do you know why the back panel is usually white? The white color is conducive to the reflection of the light in the gap between the cells to the front surface, and part of the light will be reflected by the solar cell, which increases the utilization of light energy by the solar cell and is conducive to the improvement of the photoelectric conversion efficiency.

Frame

Flat-panel components must have a frame to protect their components and facilitate the connection and fixation of the components. The main materials of the frame are stainless steel, aluminum alloy, rubber, and reinforced plastic. Conventional photovoltaic frames are generally made of aluminum materials. The frame structure should have no protrusions to avoid accumulating water, dust, or other objects. The thickness of its surface oxide layer is greater than 10μm, which can ensure that it will not be corroded in the outdoor environment for more than 30 years, and it is firm and durable.

Junction Box

The positive and negative poles of the components are connected with the designed cables in the junction box and associated with the external circuits. The junction box has the following main characteristics:

• The shell has strong anti-aging and UV resistance
• Meet the requirements of use in harsh outdoor environmental conditions

Temperature Rating Parameters

This group of parameters mainly reflects the influence of temperature on some parameters of solar modules. 

Rated Solar Cell Temperature

Nominal solar cell temperature is achieved when the solar module or cell is in an open circuit state under the following conditions. This value is generally 45±2℃ (module operating temperature). Since the power temperature coefficient of photovoltaic modules is negative, the lower the rated battery temperature, the better.

Power Temperature Coefficient

Change of power with temperature, unit %/K, negative value.

Voltage Temperature Coefficient

Variation of voltage with temperature, unit %/K, negative value.

Current Temperature Coefficient

Change of current with temperature, unit %/K, positive value. Usually, when the module temperature decreases, the output current of the cell will fall, and the voltage will increase accordingly. Therefore, when designing a photovoltaic system string-matching inverter, it is necessary to consider the local shallow temperature to prevent the string voltage from exceeding the nominal range of the inverter’s voltage.

Limit Parameters

Working Temperature

The operating temperature of the solar module usually refers to the external ambient temperature range in which the solar module can work. Generally, the working temperature of photovoltaic modules can meet the ambient temperature. But one day, you need to build solar power plants in Antarctica, the North Pole, and even outer space, so please don’t forget this parameter.

Maximum System Voltage

System voltage refers to a solar power generation system with several solar panels. The maximum DC voltage of this power generation system can be up to 1500V. Assuming that the voltage temperature coefficient of the open-circuit voltage Voc of the photovoltaic module is -0.32%, the Voc under STC=44.7V, and the Voc=44.7*(1+0.32%*(25+40))=54V under the extreme operating low temperature of -40℃, the number of designed series components in each string is generally required to be ≤1000/54=18.

Maximum Fuse Rated Current

This value will be greater than the maximum operating point operating current to protect solar modules and cables. Taking a typical centralized power station as an example, in the current centralized power station, the number of strings connected in parallel is as high as 100 strings. When a short-circuit fault occurs in one string, the current of all strings will be reversed to the faulty string, and the reversed current may be reversed. More than 800A, far beyond the safety requirements of cables and components, is prone to fire accidents, so fuses must be used to cut off fault currents and protect cables and components.

What is The Hot Spot Effect of Solar Modules？

A shaded solar cell module in a series branch will be used as a load to consume the energy generated by other illuminated solar cell modules. The shaded solar cell module will heat up at this time, which is the hot spot effect. This effect can seriously damage solar cells. A shaded cell may consume part of the energy produced by a lit solar cell. Just a piece of bird droppings may cause the hot spot effect. To prevent the solar cell from being damaged due to the hot spot effect, it is better to connect a bypass diode in parallel between the positive and negative poles of the solar cell module to prevent the energy generated by the light module from being consumed by the shaded module. When the hot spot effect is severe, the bypass diode may be broken down, causing the component to burn, as shown in the figure below.

What is The PID Effect of Solar Modules？

Potential Induced Degradation (PID, Potential Induced Degradation) is a long-term high voltage action of solar cell modules, which causes leakage current between glass and packaging materials, and a large number of charges are shot on the surface of the solar cell, which makes the passivation effect of the surface of the solar cell deteriorated. As a result, the performance of the solar module is lower than the design standard. When the PID phenomenon is serious, it will cause a solar module power attenuation of more than 50%, affecting the power output of the entire solar string. PID phenomenon is most likely to occur in coastal areas with high temperatures, high humidity， and high salinity.

The main reasons for the PID phenomenon of solar modules are as follows:

1) System design reasons: The lightning protection grounding of the photovoltaic power station is realized by grounding the component frame at the edge of the square array, which causes a bias voltage between a single component and the frame. The higher the bias voltage of the component, the more PID phenomenon occurs. More serious. For P-type crystalline silicon modules, grounding the negative pole of the inverter with a transformer can effectively prevent the occurrence of PID by eliminating the forward bias of the module frame relative to the cell. Still, grounding the negative pole of the inverter will increase the corresponding system construction, cost.

2) Photovoltaic module reasons: The high temperature and high humidity external environment cause leakage current between the solar cell and the ground frame, and a leakage current channel is formed between the packaging material, the backplane, the glass， and the frame. One of the ways to achieve anti-PID for solar modules is to change the insulating film by using ethylene vinyl acetate (EVA). Under the conditions of using different EVA packaging films, the anti-PID performance of solar modules will be different. In addition, the glass in photovoltaic modules is mainly calcium soda glass, and the influence of glass on the PID phenomenon of photovoltaic modules is still unclear.

3) Solar cell reason: The uniformity of the sheet resistance of the solar cell, the thickness of the anti-reflection layer, and the refractive index have different effects on the PID performance.

What is EL Testing of Solar Modules？

When there is a problem with the photovoltaic module, the local resistance increases and the temperature in that area increases. The EL tester is like the X-ray machine in our physical examination, which can perform the physical analysis on photovoltaic modules – through infrared images, the images show different colors according to different temperatures, so it is effortless to find many problems of photovoltaic modules: cracks, hot spot, PID effect, etc. In the process of transportation, handling, and installation, photovoltaic modules are easily trampled and hit, resulting in invisible cracks in the solar modules, which greatly affect the output power of the modules. EL can detect it.

The picture below is an infrared photo of the PID effect. The solar cell with severe PID effect is black.