A newer approach to optimizing the efficiency and reliability of solar systems is to use a micro-inverter connected to each solar panel. Equipping each solar panel with a separate micro-inverter allows the system to adapt to changing loads and weather conditions, providing optimum conversion efficiency for a single panel and the entire system.
The micro-inverter architecture also simplifies wiring, which means lower installation costs. By making the consumer's solar power system more efficient, the time required for the system to "recover" the initial investment in solar technology will be reduced.
The power inverter is the key electronic component of the solar power system. In commercial applications, these components connect photovoltaic (PV) panels, batteries that store electrical energy, and local power distribution systems or utility grids. A typical solar inverter converts the extremely low DC voltage from the PV array output into several voltages such as battery DC voltage, AC line voltage, and distribution network voltage.
In a typical solar energy harvesting system, multiple solar panels are connected in parallel to an inverter that converts the variable DC output from multiple photovoltaic cells into a clean 50 Hz or 60 Hz sine wave inverter power supply.
In addition, it should also be pointed out that the microcontroller (MCU) module TMS320C2000 or MSP430 usually contains key on-chip peripherals such as pulse width modulation (PWM) modules and A/D converters.
The main goal of the design is to increase the conversion efficiency as much as possible. This is a complex and repetitive process involving a maximum power point tracking algorithm (MPPT) and a real-time controller that executes the relevant algorithm.
Maximize power conversion efficiency
An inverter that does not use the MPPT algorithm simply connects the photovoltaic module directly to the battery, forcing the photovoltaic module to operate at the battery voltage. Almost without exception, the battery voltage is not the ideal value for collecting the most available solar energy.
Describes the traditional current/voltage characteristics of a typical 75W photovoltaic module at a battery temperature of 25°C. The dashed line represents the ratio of voltage (PV VOLTS) to power (PV WATTS).
The solid line shows the ratio of voltage and current (PV AMPS). As shown in Figure 2, at 12V, the output power is approximately 53W. In other words, by forcing the photovoltaic module to work 12V, the output power is limited to about 53W.
But with the MPPT algorithm, the situation has changed fundamentally. In this example, the voltage at which the module can achieve maximum output power is 17V. Therefore, the MPPT algorithm's responsibility is to make the module work at 17V, so that regardless of the battery voltage, all 75W of power can be obtained from the module.
The high-efficiency DC/DC power converter converts the 17V at the controller input to the battery voltage at the output. Since the DC/DC converter reduces the voltage from 17V to 12V, in this example, the battery charge current in the system that supports the MPPT function is: (VMODULE/VBATTERY) × IMODULE, or (17V/12V) × 4.45A = 6.30A.
Assuming that the DC/DC converter's conversion efficiency is 100%, the charge current will increase by 1.85A (or 42%).
Although this example assumes that the inverter processes energy from a single solar panel, the conventional system is usually an inverter connecting multiple panels. Depending on the application, this topology has both advantages and disadvantages.
MPPT algorithm
There are three main types of MPPT algorithms: perturbation-observation, conductance increment, and constant voltage. The first two methods are often referred to as "climbing" because they are based on the fact that on the left side of the MPP, the curve is on the rise (dP/dV>0), while on the right side of the MPP, the curve is down (dP/dV <0 ).
Perturbation-observation (P&O) methods are the most commonly used. The algorithm perturbs the operating voltage in a given direction and samples dP/dV. If dP/dV is positive, the algorithm "understands" that it was just adjusting the voltage toward MPP. It will then adjust the voltage in this direction until dP/dV becomes negative.
The P&O algorithm is easy to implement, but in steady-state operation, they sometimes oscillate around the MPP. Moreover, their response speed is also slow, and even in the rapidly changing weather conditions, it is possible to reverse the direction.
The incremental conductance (INC) method uses the conductivity increment dI/dV of the PV array to calculate the positive and negative dP/dV. INC can track rapidly changing light irradiation conditions more accurately than P&O. But like P&O, it may also oscillate and be deceived by rapidly changing atmospheric conditions. Another disadvantage is that the increased complexity increases the calculation time and reduces the sampling frequency.
The third method “constant pressure method†is based on the fact that, in general, VMPP/VOC≈0.76. The problem with this method is that it requires instantaneous adjustment of the PV array current to zero to measure the open circuit voltage of the array. Then, set the operating voltage of the array to 76% of the measured value. However, the available energy is wasted during the array disconnection. It has also been found that although 76% of the open circuit voltage is a good approximation, it is not always consistent with MPP.
Since none of the MPPT algorithms can successfully meet all common usage environment requirements, many design engineers will let the system first evaluate the environmental conditions and then select the algorithm that best suits the prevailing environmental conditions. In fact, there are many MPPT algorithms available, and it is not uncommon for solar panel manufacturers to provide their own algorithms.
For cheap controllers, in addition to the normal control functions of MCUs, it is not easy to implement MPPT algorithms, which require these controllers to have superior computing capabilities. Advanced 32-bit real-time microcontrollers such as the Texas Instruments C2000 platform series are suitable for a variety of solar applications.
Power inverter
There are many benefits to using a single inverter, the most prominent of which is simplicity and low cost. The use of MPPT algorithms and other techniques has improved the efficiency of single-inverter systems, but this is only to a certain extent. Depending on the application, the disadvantages of a single inverter topology can be significant. The most prominent is the reliability problem: as long as the inverter fails, all the energy generated by the panel is wasted before the inverter is repaired or replaced.
Even if the inverter works properly, the single inverter topology may have a negative impact on system efficiency. In most cases, each solar panel has different control requirements for maximum efficiency. The factors that determine the efficiency of each panel are: differences in the manufacturing of photovoltaic modules contained within the panel, different ambient temperatures, shades, and azimuth resulting in different light intensities (received raw solar energy).
Compared to using an inverter in the entire system, a micro-inverter for each solar panel in the system will once again improve the conversion efficiency of the entire system. The main benefit of the micro-inverter topology is that even if one of the inverters fails, energy conversion can still be performed.
Other benefits of using micro-inverters include the ability to use high-resolution PWM to adjust the conversion parameters of each solar panel. Since clouds, shadows, and shades change the output of each panel, equip each panel with a unique micro-inverter that allows the system to adapt to changing load conditions. This provides the best conversion efficiency for each panel and the entire system.
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