Monitoring is the key to unlocking the energy production of the solar cell. It is easy to lose efficiency through the use of circuit architectures that assume constant energy production when the solar environment is constantly changing.
The change in current-voltage properties as a solar module heats up or receives more light can be an important source of efficiency losses in solar arrays. If the inverter that generates grid-compatible electricity is not tuned to the output voltage and current conditions, it will waste more of the electricity than it should. In response, electronics companies have produced ICs that perform the maximum power-point tracking (MPPT) needed to optimize energy conversion as well as bypass electronics to prevent temporarily unproductive modules from disrupting the output of active cells.
Close to the equator on a clear day, energy from the Sun means that a single square meter of Earth’s surface receives more than 1 kW of power. Today’s solar cells make it possible to convert a significant portion of this energy into electricity. Silicon-based solar cells tested under laboratory conditions have achieved conversion efficiencies approaching 25 percent. However, these cells have not made it into production photovoltaic (PV) panels, and even if they did there are other issues that reduce the efficiency of the total PV system.
The electronic controls behind the solar cells provide the key to leveraging the raw efficiency of the solar cell. The control systems need to be able to react to changing weather conditions to ensure that the PV cells and modules operate as close to their peak as possible. Each solar cell has a characteristic current-voltage (IV) curve that reflects its response to both temperature and incident-light levels.
The cells packed into a photovoltaic module might generate a high voltage but very-low current on a drab winter day. As light levels rise the voltage will drop slightly, but current will increase dramatically until it approaches its peak level. As the module heats up, the output voltage of the module will fall, reducing its overall energy output. As a result, even during periods of intense sunlight when they should be at peak efficiency, PV panels can suffer from significant drops in conversion efficiency if the electronic circuitry does not compensate for this.
Part of the secret to high efficiency lies in the use of components that minimize heat generation, so that output voltage is maintained at a high level. More important is an architecture that takes account of changing conditions to ensure that the cells and modules are operated at peak potential for the current conditions. Local shading will dramatically reduce the output of the shaded cells, with knock-on effects on modules that contain them and potentially complete PV installations or fields if not managed effectively. Shade falling on less than 3 percent of the area of a solar installation can reduce its output efficiency by more than 15 percent, according to tests carried out by the US National Renewable Energy Laboratory. If a cell is not producing energy, its resistance will rise and it will start to consume the current fed to it by other cells, as they are normally wired in series. The result is a build-up of heat. This hot-spot generation will damage the cells over time, as well as reduce the output energy of the entire solar array.
In a typical installation, groups of PV panels are connected in series to make a PV string. Each string is connected in parallel to the others and then connected to the inverter, which adapts the produced power to the characteristics of the public electrical grid. Within each string, bypass diodes protect each panel by providing an alternative path for the current flow generated by other panels. These diodes guarantee both the panel protection and the whole system functionality in case of damaged or shaded panels. Blocking or cut-off diodes protect each string from current reflow from other strings due to a lower voltage on the string, typically caused by shading on a part of the string.
Figure 1: The architecture of a basic solar installation using a single string.
A bypass diode should have a low forward-bias voltage drop to allow current from upstream panels to flow easily on its way to the inverter that will convert the energy to grid-compatible electricity. To prevent heat buildup and energy loss, a very-low forward voltage drop is crucial. The LX2400 solar bypass device uses Microsemi’s CoolRUN technology to cut the resistance that causes forward voltage drop, and therefore minimize heat generation. The voltage drop of the LX2400 is just 50 mW at 10 A and the temperature rise for such a high current is only 10°C, preventing efficiency-sapping heat increases near productive modules.
STMicroelectronics also has a solar bypass diode in the form of the SPV1001. It is a system-in-package solution designed for cool operation through the use of a power MOSFET working in combination with a capacitor to provide bypass rectification similar to that of a Schottky diode, but with lower voltage drop and reverse leakage current. The MOSFET charges a capacitor during its off-time and is then driven during its on-time through the charge stored in the capacitor. The on-time and off-time are scaled to reduce the average voltage drop across the drain and source terminals, which reduces power dissipation.
The remaining key component of the solar array or field is the inverter section, which generates grid-compatible electricity. Inverters are complex systems that normally comprise three functions: DC/DC conversion, DC/AC conversion, and Anti-Island control.
Anti-island control is a safety-control that forces system disconnection from the grid during a grid-level power failure, preventing the inverters from continuing to feed power into small sections, or ‘islands’, of the grid. If islands are powered, workers attempting to repair the system are put at risk. A further issue is that without a grid signal with which to synchronize, the power output of the inverters may drift out of the range that customer equipment is meant to operate within the island.
The inverter needs to be tuned to the conditions experienced by the solar array, This is generally achieved using maximum power-point tracking (MPPT). Using MPPT, the inverter circuitry can use the optimum combination of voltage and current and supply a load with the necessary resistance to allow efficient energy harvesting.
Figure 2: By using independent DC/DC conversion and MPPT, a solar installation can react more appropriately to changing environmental conditions.
The simplest architecture is where one MPPT engine is used to control a single common inverter. This approach is simple but has the problem of applying a single power point to modules in the array that may be experiencing different operating conditions. If a module is partially shaded as clouds pass across the sun from its perspective, it will have a different power point to others that are fully illuminated. Despite being shaded, it may still be productive to not warrant bypassing. Dirt buildup on individual modules will lead to changes in power point over time that also cannot be reflected by a common-MPPT controller. One alternative is to split the array by employing a number of strings in parallel, each feeding a different inverter front-end. A more flexible approach is to associate a DC/DC converter and MPPT controller with each PV module, each feeding DC power at a common voltage to the inverter section.
There are a number of ways to determine the optimum power point at any given time, each with its own advantages and disadvantages. A commonly-used technique is perturb-and-observe. Using this technique, the voltage or current is changed and the change in power output noted. If the output increases, the perturbation moved in the correct direction. If not, the next perturbation will be in the opposite direction. Typically, the operating voltage employed by the DC/DC converter will oscillate around the MPP. The SPV1020 from STMicroelectronics employs a perturb-and-observe algorithm.
An alternative is incremental conductance, which relies on the observation that the power curve derivative versus voltage at the MPP is zero and will be positive on the left and negative on the right-hand side. Implementations compare the incremental conductance, the change in current versus voltage (∆I/∆V), to the instantaneous conductance (I/V). At the MPP ∆I/∆V = -I/V. To the left of the MPP, ∆I/∆V is greater than -I/V and less than -I/V to the right of the MPP.
Figure 3: Graph of current and power versus voltage for a typical solar cell, showing the maximum power point and the cell’s current curve.
Incremental conductance generally provides a better guide on the direction in which the maximum power point is moving and results in less oscillation than perturb-and-observe, but is more computationally intensive. However, a microcontroller such as the Microchip Technology PIC16F1503 is easily able to perform the necessary calculation for a PV module using incremental conductance, and includes a numerically controlled oscillator (NCO) peripheral unit that can provide the timing signals for high-resolution pulse-width modulation (PWM) needed by high-efficiency DC/DC conversion.
Figure 4: The perturb-and-disturb algorithm moves the operating point in order to bracket the maximum power point.
As the voltage output of a PV module can vary significantly with environmental conditions, any DC/DC converter attached to the module needs to be flexible enough to cope. One possibility is to use a boost converter topology with a high intermediate DC voltage being supplied to the grid-tied inverter. The SPV1020 employs a four-phase interleaved boost topology that enables it to operate in either continuous or discontinuous mode. The interleaving dramatically reduces output voltage ripple. Phases are shut down during burst mode when low current output is needed. Burst mode is generally used during startup to avoid voltage oscillations and each phase is activated gradually during the startup process.
The alternative to a boost topology is a buck-boost DC/DC converter topology so that the voltage of the module can move around the intermediate voltage; this can be leveraged to support high-efficiency operation when the module is converting energy at near-optimal levels.
The Texas Instruments Solar Magic SM72442 MPPT and DC/DC converter controller, which is supported by an evaluation board, provides the option of direct connection for almost lossless operation when the panel input voltage and the desired output voltage are within ±2 percent of each other. This allows the DC/DC converter to shut down temporarily. On either side of this range, the controller engages buck or boost modes.
The SM72442 uses PWM dithering techniques to smooth the transition between buck, boost, and the direct-connection panel mode. The MPPT algorithm used by the device typically provides convergence within one hundredth of a second, monitoring both input current and voltage.
As the solar industry continues to expand, more solutions can be expected to arrive that provide MPPT and power control, particularly as attention shifts to more sophisticated architectures that provide per-module efficiency tuning. Those solutions will include both specific devices and more general-purpose products that allow the implementer to tune their MPPT and power-conversion algorithms.