What are the factors that affect the efficiency of photovoltaic power plants?

1. Temperature reduction

I think the natural factor that has the greatest impact on system efficiency is temperature. The temperature coefficient is a very important parameter for photovoltaic modules. In general, the temperature coefficient of crystalline silicon batteries is generally -0.35~-0.45%/℃, while the temperature coefficient of amorphous silicon batteries is generally around -0.2%/℃. The temperature of photovoltaic modules is not equal to the ambient temperature. The following figure shows the variation of output power of photovoltaic modules with module temperature.

Around 12:00 noon, the temperature of the photovoltaic module in the picture reaches around 60 degrees Celsius, and the output power of the photovoltaic module is only about 85%.

In addition to photovoltaic modules, when the temperature increases, the conversion efficiency of electrical equipment such as inverters will also decrease with the increase of temperature.

The reduction caused by temperature can be estimated based on the temperature coefficient of the photovoltaic module and the local temperature.

2. Unavailability of sunlight

The total radiation value we obtain is the sum of direct radiation, scattered radiation, and reflected radiation of various radiation intensities, but not all radiation can generate electricity. For example, inverters require irradiance greater than 50W/m2 to supply power to the grid, but the output power is extremely low when irradiance is below 100W/m2.

Even in the sunny western region, although this portion is included in the total radiation data, the unusable solar radiation can still reach 2-3%.

2、 The impact of equipment factors on system efficiency

I think the equipment factor is the most important factor affecting the efficiency of photovoltaic systems.

1. Matching degree of photovoltaic modules

The nominal deviation is also an important parameter of photovoltaic modules, which is generally acceptable within ± 3%. This indicates that although the nominal parameters of the components are the same, there are actually differences in the output characteristic curves, which leads to a decrease in efficiency caused by inconsistent current when multiple components are connected in series. At present, component manufacturers such as Tianhe and Yingli generally use positive deviation to reduce losses caused by power mismatch.

2. Efficiency of inverters and box transformers

Although the European efficiency in the inverter technical specifications considers the weighted conversion efficiency of different load rates, few inverters can achieve the current 98.5% commonly used efficiency in practical use. It should be good if the weighted efficiency of the inverter can reach 97.5% during the process of DC to AC conversion.

The MPPT tracking effect of different inverters also varies. When the maximum power point voltage changes with irradiance, the inverter needs to constantly change the voltage value to find the maximum power point voltage, which can also cause energy loss due to the hysteresis of tracking. In addition, a 500kW inverter needs to track approximately 100 MPPT strings, and the differences between the strings will affect the tracking accuracy. At present, some inverter manufacturers use multi-channel MPPT to reduce this loss.

Within the maximum DC input voltage range, as many series components as possible can increase voltage and reduce current, which can improve the conversion efficiency of the inverter and reduce line losses.

The box transformer will inevitably experience energy loss during the process of boosting voltage, which is determined based on the parameters of the box transformer, usually around 1.5%.

3. DC line loss, AC line loss

The area of a 1MW unit is approximately 3.5-4 hectares. To deliver the electricity generated by such a large area of photovoltaic modules to a single location, it requires a long DC line. There are two ways to reduce line loss: choose good cables and increase voltage. In general, DC line loss can be estimated at 2-3%.

The AC line is short and the line loss is relatively small, which can generally be estimated at 1%.

4. Equipment malfunction

Equipment failures and maintenance are an important cause of low system efficiency. The following figure shows the causes of failures in photovoltaic power plants, with half of them coming from equipment.

3、 The impact of human factors on system efficiency

1. Improper design

The most serious loss of power generation caused by improper design is "improper spacing design". Due to the current vertical layout of photovoltaic power plants, a small amount of obstruction along the lower edge often leads to a significant decrease in the output power of the entire string. According to statistics, in some power plants with a small distance between the front and back, the loss of power generation caused by front and rear obstruction can even reach 3%. In addition, in addition to considering the front and rear obstructions, mountain power stations also need to consider the obstructions caused by the east-west height difference. In power plants with larger slopes and smaller east-west distances, this reduction can reach 2%.

In addition to spacing, I often see tall buildings (structures) designed within the photovoltaic power plant area, which obstruct the surrounding photovoltaic arrays.

2. Inadequate cleaning

In the northwest region, a sandstorm may directly reduce power generation by more than 5%; In the east, during severe haze weather, the photovoltaic power plant has almost no output. The following figure shows the comparison of the output of the photovoltaic power plant before and after cleaning.

It can be seen that the higher the irradiance and the stronger the penetration of sunlight, the less damage caused by dust.

In addition to dust, if snow is not removed in a timely manner, it can also cause significant losses in power generation.