Solar energy systems have witnessed significant advancements in terms of efficiency, reliability, and productivity for generating electricity and charging batteries, surpassing the capabilities of older solar systems. Notably, the efficiency of solar cells has garnered substantial attention and improvement within the industry. Additionally, maximum power point tracking has emerged as a significant breakthrough, benefiting both grid-tied arrays and solar systems with battery storage.
While solar photovoltaic (PV) panels and batteries form a successful duo, neither component possesses inherent intelligence. When solar PV panels do not operate at the optimal voltage for the batteries, they receive less current, leading to decreased efficiency. Without maximum power point tracking (MPPT) capabilities, batteries frequently fail to harness the full power potential of solar panels, resulting in energy wastage.
What is the Maximum Power Point Tracking (MPPT)?
Maximum power point tracking (MPPT), occasionally referred to as power point tracking (PPT), is a technique to extract maximum power from a PV module, especially when conditions vary.
PV solar systems exhibit varying relationships to external grids, batteries, inverters, and electrical loads. The primary challenge tackled by MPPT revolves around the efficiency of power transfer from the PV systems, which is influenced by factors such as sunlight availability, shading, solar panel temperature, and the electrical characteristics of the load. As these conditions fluctuate, the load characteristic (impedance) that enables the most efficient power transfer changes. System optimization occurs when the load characteristic adapts to ensure that power transfer remains at the highest efficiency. This state of optimal load characteristic is commonly referred to as the maximum power point (MPP).
How Does MPPT Work?
Understanding Current-Voltage and Power-Voltage Curves
Within the datasheet of a solar panel, an array of information is provided, enabling individuals to comprehend the fundamental parameters of the device and create mathematical models to represent its behavior within an electrical circuit.
As a standard practice, the datasheet generally incorporates graphical representations of the solar panel's characteristics, such as the "current-voltage curve" (referred to as IV curve) and the "power-voltage curve." These graphs provide valuable insights into the panel's performance.
Examining the power-voltage curve, makes it possible to identify the specific point or points where the solar panel achieves its maximum power output. The IV curve typically highlights two values, namely "Vmp" and "Imp,” which represent the voltage and current levels at which the solar panel's power output is maximized under standard test conditions (STC). It is important to note that the solar panel is not constrained to operate solely at maximum power. Any point along the IV curve is considered a valid operating point.
When employing string inverters in system designs, the inverters determine the operating point. The capability of the inverters to identify the specific operating point of a solar array where the output power is maximized is commonly known as maximum power point tracking (MPPT).
When a solar array consists of uniform solar panels operating under identical irradiance and temperature conditions, resulting in each module having the same IV curve and maximum power point, the collective IV curve of the entire array (which incorporates the IV curves of each module) will exhibit a shape resembling the red curve illustrated in Figure 1 below.
The blue curve represents the relationship between the output power of the array and the output voltage. It is essential to observe that there is a single peak in power situated at the "knee" of the IV curve. The inverter aims to identify this one specific point where the array’s power is maximized.
The Role of Bypass Diodes
The IV curve becomes significantly more complex when certain array sections are shaded.
Shaded modules exhibit distinct IV curves compared to unshaded modules, particularly regarding the output current of the shaded modules. When a module receives low irradiance, it can lead to a decline in the power of the entire string connected to that module. This reduction is attributed to the fact that the current flowing through the string is constrained by the current through the most shaded module.
Manufacturers incorporate bypass diodes into their modules to alleviate these impacts. Functioning like an on/off switch, a bypass diode conducts current in the "on" state and prevents current flow when in the "off" state. When the bypass diode is activated, it effectively bypasses the shaded module by redirecting the string current through the diode.
By incorporating bypass diodes, the inverter can effectively bypass shaded panels instead of operating at their reduced current levels. As a result, the IV curve of a partially shaded array differs from that of an unshaded array.
The resulting IV curve in this scenario may resemble the red curve depicted in Figure 2, the blue line represents the corresponding power- voltage (P-V) curve. It is worth noting that there are two distinct operating points where power is maximized. The first is a global maximum, characterized by higher current and lower voltage, while the second is a local maximum, characterized by lower current and higher voltage.
The global maximum is achieved when the shaded modules are bypassed, allowing the system to operate at its highest power output. Conversely, the local maximum is attained when the shaded modules are not bypassed, resulting in a slightly lower power output.
Maximum Power Point (MPP)
A photovoltaic cell behaves as a constant current source for most of its useful curve. However, within the maximum power point (MPP) region, the cell's curve demonstrates an approximately inverse exponential relationship between voltage and current. According to fundamental circuit theory, the power delivered to a device is maximized where the derivative ( dI/dV) of the cell's I-V curve is equal and opposite to the I/V ratio. This optimal point, known as the "knee" of the curve, ensures maximum power transfer.
Thus, the load with a resistance denoted by R=V/I, the reciprocal of I/V, extracts the maximum power from the device. This resistance is often referred to as the "characteristic resistance" of the cell. It is a dynamic quantity influenced by factors such as illumination level, temperature, and cell condition. Deviating from this optimal resistance value, either higher or lower, reduces power output. Maximum power point trackers employ control circuits or logic to identify and maintain this point.
Maximum Power Point Tracking (MPPT) Implementation
When a load is directly connected to a solar cell, it is rare for the panel to operate at its peak power point. The operating point of the panel is determined by the impedance it faces. By properly setting the impedance, peak power can be attained. As solar panels operate on DC, DC-DC converters are used to transform the impedance from the source circuit to the load circuit. Adjusting the duty ratio of the DC-DC converter alters the impedance (duty ratio) perceived by the solar cell.
MPPT algorithms consistently measure the currents and voltages of solar panels and subsequently adjust the duty ratio based on the measurements. These algorithms are typically implemented using microcontrollers. In modern implementations, advanced computing systems are often employed for enhanced analytics and load forecasting purposes.
Controllers have the flexibility to employ various strategies to optimize power output. MPPTs can dynamically switch between multiple algorithms based on prevailing conditions.
Perturb and Observe
The controller employs a method that adjusts the voltage from the array by a small increment and measures the resulting power. If the power increases, further adjustments in that direction are made until no further power increase is observed. This method, commonly known as perturb and observe (P&O), is widely used, although it may introduce power output oscillations. It is commonly known as a hill-climbing technique, as it relies on tracking the rise and fall of the power-voltage curve to find the maximum power point.
The perturb and observe method is popular due to its simplicity of implementation. When a proper predictive and adaptive hill climbing strategy is implemented, the perturb and observe method can achieve high efficiency.
In this approach, the controller analyzes incremental changes in current and voltage to anticipate the impact of a voltage adjustment. This method involves more computational requirements for the controller but offers a faster adaptation to changing conditions compared to P&O. It ensures stable power output without oscillations.
The method leverages the PV array’s incremental conductance (dI/dV)to determine the direction of power change relative to voltage (dP/dV). This method calculates the MPP by comparing the array conductance (I/V) to the incremental conductance (dI/dV). When these two values are equal, the output voltage corresponds to the MPP voltage. The controller ensures the system operates at this voltage until there is a change in irradiation conditions, prompting the process to be repeated.
This method involves applying a sweep waveform to the array current, allowing the I-V characteristic to be achieved and updated at regular intervals. The maximum power point (MPP) voltage can be determined by analyzing the characteristic curve at these intervals.
Constant voltage methods encompass two approaches: one regulates the output voltage to a constant value regardless of conditions, while the other adjusts the output voltage based on a fixed ratio to the measured open circuit voltage (VOC). The latter technique is also known as the "open voltage" method.
Although the constant voltage approach does not explicitly track the MPP, it serves as a supplementary technique when MPP tracking encounters difficulties. The open voltage method briefly interrupts the power flow to measure the open-circuit voltage (VOC) at zero current. Subsequently, the controller resumes operation by maintaining the voltage at a fixed ratio, typically around 0.76, of the VOC. This predetermined value, either empirically or model-based, represents the expected MPP for the particular operating conditions. By regulating the array voltage and aligning it with the fixed reference voltage (Vreference = kVOC), the array's operating point remains close to the MPP.
The specific value of Vreference can be chosen to optimize performance considering other factors alongside the MPP; however, the fundamental concept revolves around determining Vreference as a ratio to VOC. It's worth noting that this method entails inherent approximations, as the ratio between the MPP voltage and VOC is only approximately constant, allowing for further potential optimization.
Comparison of Methods
Both the perturb and observe (P&O) method and the incremental conductance method are considered "hill climbing" techniques as they aim to locate the local maximum of the power curve specific to the operating condition of the array, thereby ensuring accurate Maximum Power Point (MPP) tracking. The perturb and observe (P&O) method tends to cause power output oscillations around the maximum power point, even in scenarios of steady-state irradiance.
The incremental conductance method effectively identifies the maximum power point without oscillations and is more accurate than P&O, particularly under rapidly changing irradiation and daylight conditions. However, this method may still exhibit oscillations and unpredictable behavior when encountering rapid variations in atmospheric conditions. The algorithm’s complexity leads to a decreased sampling frequency compared to P&O.
The constant voltage ratio method may result in energy loss when the current is set to zero. The approximation of 76% as the ratio (VMPP/VOC) may not always be precise. While this approach is simple and cost-effective to implement, intermittent interruptions can decrease the efficiency of the array and do not guarantee the precise identification of the actual maximum power point (MPP). However, certain systems may achieve efficiencies exceeding 95%.
MPPT in Battery Operation
During the nighttime, when a PV system depends on batteries to supply power to loads, the voltage of the fully charged battery pack may be close to the MPP voltage of the PV panel. However, this alignment is unlikely to occur at sunrise when the battery is partially discharged. Charging can initiate at a voltage significantly lower than the MPP voltage of the PV panel, and an MPPT system can rectify this mismatch.
Once the batteries reach full charge and the PV production surpasses the local loads, the MPPT encounters limitations in operating the panel at its MPP due to the absence of a load to utilize the surplus power. Consequently, the MPPT must adjust the operating point of the PV panel, deviating from the peak power point, until the production aligns with the demand.
All generated power by solar modules is fed into the grid in a system connected to the grid. Consequently, the MPPT in such a system consistently strives to operate at the MPP (maximum power point).
Key Takeaways of MPPT
Solar PV is recognized as a promising energy source in renewable energy generation systems, primarily due to the abundant availability of sunlight. However, it has limitations, including weather inconsistencies, lower efficiency levels, and a significant initial investment. To overcome these challenges, MPPT serves as a crucial power electronics interface that enables the extraction of maximum power output from PV systems, both in uniform and non-uniform shading scenarios. Extensive research has been undertaken to enhance the efficiency of PV systems under diverse weather conditions through MPPT techniques. Nevertheless, selecting the most suitable MPPT for specific PV system configurations and conditions has remained challenging. Hence, this article comprehensively reviews various MPPT techniques, thoroughly examining their advantages and disadvantages.