The basic structure of solar photovoltaic water pump system

Basic composition of solar photovoltaic water pump system

The photovoltaic water pump system is roughly composed of four parts: photovoltaic array, controller, motor and water pump.

1.1 Photovoltaic array

The photovoltaic array is composed of a large number of solar cells connected in series and parallel, and its function is to directly convert solar energy into DC electrical energy. 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 the solar cell 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 the single solar cell and the connection resistance between the components are ignored, it has an ideal consistency. The volt-ampere characteristic curve of the photovoltaic array can be regarded as the scale of the coordinates of the volt-ampere characteristic curve of the single solar cell enlarged in series and parallel.

1.2 Controller

The output characteristic curve of the photovoltaic array has a strong linear relationship and is closely related to the solar irradiance, ambient temperature, cloudy, sunny, rainy, foggy and other meteorological conditions. Its output changes with the sunshine. The DC power, and the photovoltaic water pump as the load of the photovoltaic array, its driving motor is sometimes a DC motor, sometimes an AC motor or even other new motors, which also have nonlinear properties. In this case, to make the photovoltaic pump system work in a relatively ideal working condition, and to maximize the output power potential of the photovoltaic array under any sunshine, an adapter is required to achieve a harmonious, efficient and stable working state between the load and the photovoltaic array. The content of the adapter is mainly the maximum power point tracker, inverter and some protection facilities.

1.2.1 Maximum Power Point Tracker (MPPT)

It can be seen from the volt-ampere characteristic curve of the photovoltaic array that the maximum power point position of the photovoltaic array under different solar irradiances is not fixed, and when the ambient temperature changes, the maximum power point position corresponding to the same irradiance will also change. In order to achieve maximum power point tracking to obtain the most energy under the current sunshine, MPPT is usually made into two forms, which are introduced below. • Constant voltage maximum power point tracker (CVT type MPPT).

Look carefully at the black dots in the figure that represent the maximum power output, the position of the maximum power point. They are all located near the straight line of Umax=const, especially when the sunlight is strong, they are closer to Umax=const. At the same time, considering that the temperature of the photovoltaic array is excellent, its open circuit voltage UOC will decrease under the same sunlight conditions, and the short-circuit current Isc will increase slightly. Considering the characteristics that the ambient temperature is generally higher when the insolation is high, and the ambient temperature is generally lower when the insolation is low, combined with the temperature characteristics of the solar cell, they are just conducive to making the trajectory of the maximum power point in a day closer to a vertical line Umax=const. That is to say, in engineering, people are allowed to approximate the trajectory of the maximum power point as a vertical line Umax=const, which constitutes the theoretical basis of TcvT MPPT.

• Real MPPT

The T-type MPPT has its shortcomings, mainly because the open circuit voltage Uoc and the maximum power point voltage U of the photovoltaic array are greatly affected by temperature. Once Um is set, there will be a large deviation in winter and summer, which will inadvertently lose a considerable amount of energy. Therefore, with the continuous improvement of the performance/price of microcomputer chips and their real-time performance, many systems have begun to adopt the "real MPPT" technology.

In the "real MPPT" technology, people adopt the concept of self-optimization, measure the output power of the photovoltaic array in real time, and automatically find the maximum power point after comparison. Constantly searching, constantly adjusting, and constantly searching again..., and so on, the system is always in micro-adjustment. This "real MPPT" can automatically adapt to the large temperature difference between winter and summer without manual intervention, which is very helpful to improve the system's efficiency throughout the year.

1.2.2 Variable frequency inverter

The output of the photovoltaic array after passing through the maximum power point tracker is a DC voltage. If the driving motor used by the water pump is a DC motor, of course, it can be directly connected when the voltage values of the two are adapted. The motor will drive the water pump to rotate and pump water, such as the quiet products of Solarjack Company in the United States in the early years. Since the cost of DC motors is generally high, it is also necessary to regularly maintain or replace its brushes. In recent years, due to the progress of new speed control theory and power electronic devices and technologies, AC speed control technology has made great progress. Its efficiency has gradually caught up with DC motors, while its convenience and firmness far exceed DC motors. Therefore, the drive mode of brushed DC motors is gradually being eliminated, and it is mainly replaced by high-efficiency three-phase asynchronous motors and DC brushless motors, and occasionally permanent magnet synchronous motors or reluctance motors are used. The latter types of motors must be driven by dedicated frequency conversion devices or corresponding power electronic drive circuits. ：Here, the basic principle of the three-phase asynchronous motor drive is explained as an example.

AC drive is usually divided into two categories: square wave drive (including step wave drive) and sine wave drive. Generally speaking, photovoltaic water pump systems with lower power (below 300W) mostly use square wave drive, while sine wave drive is often used to limit harmonic loss when the power is higher. Regardless of the type of drive used, its basic circuit structure can be divided into the following four parts, namely:

(1) Switching power supply part: Its function is to provide power for the controller. The controller often requires a control power supply such as ±5V or +12V, and the output voltage of the solar cell array cannot be used directly for this purpose in most cases. Therefore, a DC/DC conversion device is required to convert the DC voltage of the array into the required DC voltage, which is the switching power supply.

(2) Main circuit and its drive circuit

The main components of the three-phase inverter circuit as the main circuit are power electronic devices, which constitute a full-bridge inverter circuit. The large-capacity electrolytic capacitor is directly connected across the DC side as an energy storage element. When the inverter circuit is turned off, the solar cell array charges the capacitor. When the inverter circuit is turned on, the capacitor and the solar cell array together supply power to the load.

The design and production of the drive circuit should be carefully carried out. When using power MOSFETs, the gate drive circuit should have good performance.

3) Control circuit

At present, the control circuits of many photovoltaic water pumps have adopted advanced single-chip microcomputer technology. After the development process of MsC-51 series and MCS96 series, the more satisfactory 8XCI96 series has been launched recently, including the 80C196MC series specially used for motor speed regulation. In addition to many common features of the 196 series, it also has the characteristics of being particularly suitable for motor drive. Through assembly language programming, the following functions are mainly completed in this system.

• Complete the protection functions required by the system, such as overcurrent, undervoltage, low speed, and dry protection, and display the fault status;

• Detect the current and voltage on the DC side of the main circuit, calculate the output power of the solar cell array, and complete the tracking of the maximum power point of the array output during the variable frequency speed regulation process;

• According to the magnetic flux tracking or other corresponding variable frequency speed regulation principles, send SPWM signals.

(4) Protection circuit

For the sake of safe operation of the system, it is necessary to set up such as overcurrent, overvoltage, overload, underload, undervoltage, well water drying, automatic restart under various conditions after shutdown... Many protection links should be set in the circuit according to the selected control devices and control circuits. Since photovoltaic water pumps are "starting at sunrise and stopping at sunset" and working fully automatically in most cases, very reliable protection measures must be adopted.

1.3 Motor and water pump

All measures of the photovoltaic water pump system are to ensure stable and reliable water output, or in other words, they must be implemented in the work of the motor and water pump. 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 motor and water pump combination. Since the power level and voltage level of the motor are largely restricted by the voltage level and power level of the solar array, when the requirements for the pump head and flow are reflected on the motor, it is often necessary to design it specifically under the condition of taking into account the array structure. Due to the different requirements of different users, the driving 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, and high-power systems are still mainly based on high-efficiency three-phase asynchronous motors. When designing the motor, the specific operating conditions of the photovoltaic water pump should be fully considered, mainly: variable frequency operation, large changes in load rate in the morning and evening, etc. In this case, we should strive to make the total average efficiency of the motor the highest throughout the day and the whole year. It is not like ordinary motors that can be considered to be always driven by a power supply with a constant voltage.

The selection and design of water pumps in photovoltaic water pump systems are 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 require a large flow but a low head should generally use a self-priming water pump.

2. Efficiency and characteristics of photovoltaic water pump systems

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 efficiencies 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.