During the life cycle of a PV plant, the efficiency of the modules and the performance of the electrical components will gradually decrease, and the power generation will then decrease year by year. In addition to these natural aging factors, there are also quality problems of the modules and inverters, line layout, dust, series and parallel losses, cable losses, and many other factors.
In the general financial model of PV power plant, the system power generation decreases by about 5% in three years, and the power generation decreases to 80% after 20 years.
How to calculate the power generation of PV power plant?
Theoretical annual power generation = total annual average solar radiation * total cell area * photoelectric conversion efficiency. Due to various reasons, the actual power generation of PV power plant is: actual annual power generation = theoretical annual power generation * actual power generation efficiency.
Ten main factors affecting the amount of photovoltaic power generated:
01 Amount of solar radiation
Solar cell module is a device that converts solar energy into electrical energy, and the intensity of light radiation directly affects the power generation capacity.
In the case of a certain conversion efficiency of solar cell modules, the power generation of photovoltaic systems is determined by the intensity of solar radiation. The efficiency of photovoltaic system for solar radiation energy utilization is only about 10% (solar cell efficiency, module combination loss, dust loss, control inverter loss, line loss, battery efficiency) The power generation of photovoltaic power plant is directly related to the amount of solar radiation, the intensity of solar radiation, spectral characteristics are changed with the meteorological conditions. The solar radiation data of each region can be obtained through meteorological information search websites or with the help of PV design software.
02 Tilt angle of photovoltaic modules
The information obtained from the weather station, generally the amount of solar radiation on the horizontal plane, is converted into the radiation on the tilted surface of the PV array in order to perform the calculation of the power generation of the PV system. The optimal tilt angle is related to the latitude of the project location. The approximate empirical values are as follows:
A, latitude 0 ° ~ 25 °, tilt angle equal to latitude
B, latitude 26° to 40°, tilt angle equal to latitude plus 5° to 10°
C, latitude 41 ° ~ 55 °, tilt angle equal to latitude plus 10 ° ~ 15 °
03 PV module efficiency
The mainstream material of solar photovoltaic cells is silicon. Therefore, the conversion rate of silicon has been an important factor limiting the further development of the entire industry.
The classical theoretical limit of the conversion rate of silicon is 29%. And the record set in the laboratory is 25%, and is putting this technology into industry. The laboratory has been able to refine high purity silicon directly from silica without having to convert it to silicon metal and then refine silicon from it. This reduces the number of intermediate steps and increases efficiency.
04 Connection method
Where series connections result in current losses due to differences in the currents of the components;
Where parallel connections result in voltage losses due to differences in the voltage of the components;
The combined loss can reach more than 8%, and the standard of China Engineering Construction Standardization Association stipulates less than 10%.
Measures:
1.To avoid combination loss, components with consistent current need to be strictly selected for series connection before power plant installation.
2.The attenuation of the components is kept consistent as much as possible. According to the national standard GB/T--9535, the maximum output power of solar cell modules shall not be attenuated by more than 8% when tested under specified conditions.
3.Isolation diodes are sometimes necessary.
05 Dust loss
Dust is a key factor affecting the overall power generation of a PV plant. The dust loss of a power plant may reach 6%!
Dust affects power generation and heat dissipation by blocking the light reaching the module, thus affecting conversion efficiency; dust with acidic and alkaline properties is deposited on the module surface for a long time, eroding the plate surface and causing it to be rough, expanding the dust to further accumulate and increasing the diffuse reflection of sunlight. Therefore, the modules need to be wiped and cleaned from time to time.
06 Temperature Characteristics
When the temperature rises by 1°C, the maximum output power of crystalline silicon solar cell decreases by 0.04%, the open circuit voltage decreases by 0.04% (-2mv/°C) and the short circuit current increases by 0.04%. To avoid the effect of temperature on power generation, it is necessary to keep the modules well ventilated.
07 Shading, snow shading
In distributed photovoltaic power plant, it is necessary to avoid the surrounding tall buildings. According to the circuit principle, when the components are connected in series, the current is determined by the least piece, if there is a piece of shadow, it will affect the power generation efficiency of this way components, similarly, when there is snow on the components, it will also affect the power generation, so it should be cleared in time
08 Line and transformer losses
The line loss of DC and AC circuits of the system should be controlled within 5%. In the design, it is necessary to use conductors with good conductivity and sufficient diameter, and pay special attention to the plug-in and terminal blocks during maintenance to ensure that they are secure.
09Controller, inverter efficiency
Inverters operate by having inductors, transformers and power devices such as IGBTs and MOSFETs, thus generating losses. The general string inverter efficiency is 97-98%, the centralized inverter efficiency is 98%, and the transformer efficiency is 99%. The charging and discharging circuit voltage drop of the controller shall not exceed 5% of the system voltage. The efficiency of grid-connected inverters is currently greater than 95%.
10 battery efficiency (stand-alone system)
Independent photovoltaic system requires the use of batteries, battery charging and discharging efficiency directly affects the efficiency of the system, that is, the impact of the independent system power generation, lead-acid battery efficiency of 80%; lithium iron phosphate battery efficiency of more than 90%.
The same capacity of photovoltaic power plant, power generation varies. There are actually many ways to effectively increase the power generation of a PV system and thus increase the revenue, depending on the factors that affect the power generation.
01 Reduce module loss
PV module is the most core factor affecting power generation, the higher the conversion rate of PV module, the better the power generation effect. Theoretically, the installation angle is generally the local latitude plus 5 degrees, and the installation angle is generally a little west of due south, but the actual situation needs to be calculated. PV modules generally have 3 temperature coefficients: open circuit voltage, peak power, and short circuit current. When the temperature rises, the output power of the PV module will drop. The peak temperature coefficient of the PV module is about -0.35%/°C, that is, the temperature rises, the lower the power generation of the PV module, theoretically, for every degree of temperature rise, the power generation decreases in about 0.35%.
(1) Design to avoid as much as possible. During the design of the power plant, care should be taken to avoid areas that may produce light shading. When the components are connected in series, the current is determined by the least piece. If one component has shading, it will affect the power generation of this way of components.
(2) Ensure component ventilation. Usually, in photovoltaic power station in the design of the time usually will raise the bracket, to ensure that the components before and after the left and right have enough space to ensure the circulation of air to achieve the purpose of cooling, in addition to the metal frame around the components also have a certain role in heat dissipation.
(3) Timely clean up debris. Be sure to ensure that the front of the photovoltaic module without dirt, such as the accumulation of debris, timely cleaning.
(4) regular inspection and cleaning. Surface ash and other conditions, will cause light blocking, reduce the output efficiency of the module, directly affect the power generation. At the same time, it may also cause the "hot spot" effect of the module, resulting in module damage.
02 Pay attention to the installation environment of inverter
Although the inverter itself has heat sink and fan to assist in heat dissipation, attention should be paid to ventilation during installation to avoid running the inverter at high temperature. An inverter working at room temperature will not only greatly improve its service life, but also help improve the conversion efficiency and power generation of the inverter, on the contrary, the long-term high temperature will accelerate the aging of electronic components.
(1) The installation location of the inverter should avoid direct sunlight as much as possible. If it is installed outdoors, it is best to choose to install it under the eaves on the back side of the sun or under the solar module, with the eaves or module blocking the top of the inverter. If it can only be installed in an open area, it is recommended to add a sunshade and rain shelter above the inverter.
(2) Both single inverter installation and multiple inverter installation should follow the installation space size given in the installation manual to ensure that the inverter has sufficient space for ventilation and heat dissipation and operation space for later operation and maintenance.
(3) During the operation and maintenance of the power plant, make sure that the cooling fans are running well, the fan blades are not damaged, there is no dust accumulation, and the bearings are well lubricated.
03 String design should be reasonable
The conversion efficiency of the inverter determines the ability of inverting the DC power of the module to the AC grid power. Theoretically, the closer the actual working voltage of the inverter is to the rated working voltage, the higher the conversion efficiency and the better the power generation yield.
The relationship between the rated operating voltage of the inverter and the type of AC grid it is connected to:
V amount ≈ 1.414 × Vac + 25V 230V grid-connected, the rated voltage is generally 360V; 400V grid-connected, the rated voltage is generally 580V; 480V grid-connected, the rated voltage is generally 700V.
04 Reduce system losses
(1) component series working voltage as close as possible to the rated working voltage of the inverter, the highest efficiency, inverter losses are small;
(2) The DC terminal configured on the inverter does not have to be connected to the full, and the number of strings connected is reduced as much as possible under the condition that the series working voltage of the components is suitable, so as to reduce the DC loss;
(3) Reasonable selection of AC and DC cable wire diameter, the inverter installation location should be moderate.
(4) consider the impact of temperature on the performance of the cable. The higher the temperature, the greater the resistivity of the wire, the loss will become larger, the cable should be installed in the ventilation and heat dissipation as far as possible.
05 Routine maintenance of photovoltaic power plant
After the normal operation of the photovoltaic power plant, we must regularly check the amount of power generation. There is also the solar panel is the most important part of the photovoltaic power plant, the solar panel operating environment affects the amount of electricity generated by the photovoltaic system, so to diligently maintain the photovoltaic components. The main thing to do on a daily basis is: find dirt on the solar panel to clean up in time; find solar panel failure to replace in time; prevent shadows on the solar panel, etc. From time to time real-time monitoring, on time maintenance, in order to ensure the maximum power generation of photovoltaic power plant.
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