Photovoltaic arrays in photovoltaic systems Motors Water pumps

Photovoltaic array

The photovoltaic array is composed of numerous solar cells connected in series and parallel, and its function is to directly convert solar energy into DC electricity. At present, most solar cells used in photovoltaic water pump systems are silicon solar cells, including single crystal silicon, polycrystalline silicon and amorphous silicon solar cells. The volt-ampere characteristic curve of solar cells is shown in the figure: It has strong nonlinearity.

The volt-ampere characteristic curve of the photovoltaic array has the same shape as that of a single solar cell. If the differences in the production process of single solar cells and the connection resistance between components are ignored, it has ideal consistency. The volt-ampere characteristic curve of the photovoltaic array can be regarded as only the scale of the volt-ampere characteristic curve of a single solar cell enlarged in series and parallel.

Motor and water pump

All measures of the photovoltaic water pump system are to be able to output more water stably and reliably, or in other words, they must be implemented in the work of the motor and water pump in the end. They often constitute an assembly, which requires maximum reliability and high efficiency. For photovoltaic water pumps, the combination of motors and water pumps is not as "casual" as the common combination of motors and water pumps. The power level and voltage level of the motor are largely restricted by the voltage level and power level of the solar array. Therefore, when the requirements for the pump head and flow rate are reflected on the motor, it is often necessary to design it specifically while taking into account the array structure. Due to the different requirements of different users, the drive motors for photovoltaic water pumps include: traditional DC motors of different voltage levels, DC brushless permanent magnet motors, three-phase asynchronous motors, permanent magnet synchronous motors, reluctance motors, etc. From the current usage, three-phase asynchronous motors and DC brushless motors are the most common, and high-power systems still mainly use high-efficiency three-phase asynchronous motors. When designing the motor, the specific operating conditions of the photovoltaic water pump must be fully considered, mainly: variable frequency operation, large changes in load rate from morning to evening, etc. In this case, we should strive to make the motor The total average efficiency of the machine throughout the day and the whole year is the highest. It is not like an ordinary motor that can be considered to be always driven by a power supply with a constant voltage.

The selection and design of the water pump in the photovoltaic water pump system is also very special. According to the different requirements of users for flow and head, the pump type can be selected according to the following principles based on economy and reliability:

Users who require small flow and high head should choose a positive displacement water pump; users who require a large flow and a high head should choose a submersible electric pump; users who need a large flow but a low head should generally use a self-priming water pump.

2. Efficiency and characteristics of photovoltaic water pump system

The system efficiency ηsys can be the instantaneous system efficiency, the hourly average system efficiency, the daily average coefficient efficiency, the monthly average system efficiency, the quarterly average system efficiency, the annual average System efficiency, etc. When calculating these efficiencies, it should be noted that in the above definition formula, the integral limits selected for the numerator and denominator are the same hour, day, month, quarter or year, and it is often easy to confuse such as considering the daily average efficiency as the arithmetic mean of the hourly average efficiency, or considering the monthly average efficiency as the average of the daily average efficiency of each day in a month.

The system efficiency of commercial photovoltaic water pump systems generally ranges from 1% to 6%. The quality of products varies. Moreover, since the efficiency of solar cells is included in the system efficiency, it is difficult to judge the efficiency of major components such as inverters and pump assemblies based on the system efficiency alone. Therefore, the efficiency of solar cells is usually excluded in the market, and a "pump system efficiency" is given. Here, the word "system" means that the efficiency of MPPT and variable frequency inverter has been taken into account. Judging from the current product situation in the world market, the efficiency of pump systems varies greatly, with the highest reaching about 60% and the lowest only about 10%. Of course, different power levels, pump types and motor types will have different efficiency ranges. Under the premise of ensuring its reliability, its performance/price ratio within a certain period of time should be carefully considered.

The daily water lifting capacity of the photovoltaic water pump system is greatly affected by the weather (mainly sunshine), and it is difficult to give a specific result. The corresponding system working condition can only be given according to the specific sunshine distribution on a specific day.

For example, for a photovoltaic water pump system, the solar irradiance at the beginning of water lifting (corresponding to a certain lift value) is about 420w/m2 (about 7:50 in the morning). People call this "420W/m2" the "water lifting threshold" of the system when the lift is equal to this certain value. The water lifting threshold is an important indicator to measure the working condition of a photovoltaic water pump system. It depends on the quality of the photovoltaic water pump system and the size of the solar cell array. For the same system with the same array capacity and lift, the smaller the threshold, the better, which means that the pump pumps water earlier and more.

In order to further develop and improve the design, manufacture and testing of photovoltaic water pump systems, my country has launched corresponding software packages such as system simulation, system CAD and optimized configuration.