1.1 Evaluation index of photovoltaic power generation system
1.1.1 Calculation of peak sunshine hours and power generation
18.104.22.168 Peak sunshine hours and resource division
First of all, we must distinguish the difference between the concept of sunshine time and sunshine time.
Sunshine time refers to the exposure time of sunlight from sunrise to sunset in a day. The sunshine hours refers to the time when the sunlight reaches a certain irradiance (usually 120W/m2 measured by the meteorological station) in a day until the time is less than this value. Obviously, the sunshine hours are less than the sunshine time.
The peak sunshine hours is the number of hours under the standard test conditions (irradiance G=1000W/m2) converted from the local solar radiation. That is, HG, the ratio of the total solar radiation H to the standard irradiance G on the ground level within a period of time (month by month or year), sometimes referred to as equivalent utilization hours. Internationally, solar energy resources are usually divided into three types of regions:
①High irradiance areas (2200kW·h/m2·a), such as the southwestern United States and the Sahara Desert in Africa
②Medium irradiance areas (1700kW, h/m2·a), such as most areas of the United States and southern Europe, etc.
③ Low irradiance areas (1100kW·h/m2·a), such as Central Europe, Germany, etc.
22.214.171.124 Estimation of power generation
Before the photovoltaic power generation system (power station) is formally established, the power generation of the photovoltaic array must be predicted. This is indispensable in the pre-feasibility or feasibility study report. Because investors are eager to know the amount of electricity generated when they consider investing in a photovoltaic power generation project in a certain place, so as to understand the economic and social benefits such as the expected profit level and the potential for reducing CO2 emissions after the project is implemented. Of course, the factors that affect the cost of power generation or the on-grid price include a series of factors such as local solar resources, photovoltaic system efficiency and power generation, system reliability, and energy grid quality, initial investment scale, operation and maintenance costs, loan ratios, and interest rate taxes. It is one of the most important indicators. If a period of time is set as a year, the annual power generation estimate formula of the photovoltaic power generation system (power station) is:
In the formula, EAC—— annual power generation of photovoltaic system (AC), kW·h;
P0——Rated DC power of photovoltaic array;
Yr——The annual peak sunshine hours of solar energy;
K——comprehensive system efficiency.
Regarding the K value, there are many influencing factors, including the comprehensive effects of photovoltaic module series, parallel matching loss, temperature rise loss, module performance degradation, line loss, shading, and dust and stain loss. The best K value is 0.84. In engineering design, the grasp of K value is related to actual experience. At present, for China’s estimated photovoltaic system power generation, the K value is 60%-70% for stand-alone photovoltaic power stations; 80% to 85% for centralized grid-connected photovoltaic power stations; 75% to 80% for distributed grid-connected photovoltaic systems .
1.2 Performance ratio of photovoltaic power generation system
In evaluating the performance of photovoltaic power plant systems, performance ratio (PR) is one of the main indicators. It is defined as the ratio of photovoltaic power generation to solar energy resources, namely:
Performance ratio = full power generation hours / peak sunshine hours = actual AC power generation / ideal state DC power generation
Expressed by the formula:
PR=EAC / P0Yr
Compared with the previous calculation formula of power generation, the performance ratio PR is equivalent to the comprehensive system efficiency K. Because the reflected factors include: the electrical efficiency of the system (series and parallel loss of components, inverter efficiency, transformer efficiency and other equipment efficiency, temperature rise loss, line loss, etc.), component degradation, shielding conditions, light reflection loss, MPPT error, Measurement errors, failure conditions, and operation and maintenance levels are undoubtedly the most important evaluation indicators for photovoltaic systems. This indicator excludes regional and solar resource differences, and more objectively reflects the performance and quality of the photovoltaic system itself.
In addition, after performing temperature and spectral corrections, and excluding the differences between these two aspects, it can be called the standard performance ratio. Divide the performance ratio by the floor area to get the performance ratio per unit area, that is, not only the quality is good, but the area design is also excellent.