Solar-cell researchers have made great strides in recent years increasing the efficiency of photovoltaic (PV) conversion—and pushing the boundaries on the material systems that can be used to produce energy from the sun.
Although amorphous silicon remains the linchpin of the solar-cell industry, it has been joined by an array of new technologies, ranging from thin-film processes based on cadmium telluride to concentrator-based designs that deploy multiple layers of PV material to harness the energy from as wide a spectrum of visible light as possible.
However, the headline conversion figures for solar cells based on standards such as the AM1.5D spectrum specified by the American Society for Testing and Materials (ASTM) only tell half the story when you consider the amount of energy that an integrated solar panel will produce once installed.
What counts is how many kilowatt-hours a panel will deliver in real-world conditions. The electronics tucked away underneath each panel play an increasingly crucial role in that equation.
Shade Plays A Role
Only rarely will the light hitting a panel resemble closely the AM1.5D spectrum used to test individual cell performance. Factors such as shading dramatically affect the efficiency of a PV panel.
Shade falling on less than 3% of the area of a solar installation can reduce its output efficiency by more than 15%, according to tests carried out by the U.S. National Renewable Energy Laboratory. This is because shade falling on one part of a panel can have a knock-on effect on other cells within the panel if they are on the same circuit.
Each PV module has a characteristic current-voltage (IV) curve that depends on temperature and incident light. A typical silicon-based module might generate a high voltage but very low current on a cold, drab winter day.
As illumination rises, the voltage will drop slightly but current will increase dramatically. Beyond that, current might rise slightly further but voltage can drop dramatically. As a result, the knee of the curve presents the optimum operating point for the module.
Temperature also has a dramatic effect on the peak output efficiency of a module. High temperatures cause the output voltage of the module to drop. As a result even during periods of intense sunlight, when they should be at peak efficiency, PV panels can suffer from drops in conversion efficiency if the electronic circuitry does not compensate.
Keys To Efficiency
There are two keys to efficiency. One is a design that keeps the module as cool as possible, which implies the use of electronic devices designed to conduct excess heat away from the cells. This is why power transistors and diodes developed for the PV market focus on heat-transfer efficiency.
For example, the SPx family developed by Microsemi comprises the only low-profile power transistor modules that incorporate copper baseplates to conduct heat away from the module as efficiently as possible.
Similarly, the LX2400 solar bypass device uses Microsemi’s CoolRUN technology to cut the resistance that causes forward voltage drop, minimizing heat generation. Bypass devices are important for the efficient operation of modules, as they make it possible to temporarily switch out shaded cells that are in danger of making the overall module far less productive.
Conduction losses are also important in the power conversion stages. The MOS 8 family of insulated-gate bipolar transistors (IGBTs) from Microsemi can minimize these losses.
The other key to efficiency in PV module design is a flexible conversion topology that can react to changes in the operating point. This is the key concept behind maximum power point tracking (MPPT).
An inverter that uses MPPT can search for the optimum combination of voltage and current and supply a load with the necessary resistance to allow efficient energy harvesting.
Traditionally, architectures have deployed a single MPPT engine in a common inverter. This has the benefit of simplicity, but the conditions won’t be optimal for all panels in an array.
Depending on shading conditions or dirt buildup on individual panels, each will have its own characteristic maximum power point at any point in time that will not be reflected by the conditions set by the common MPPT engine.
Architectures that deploy an inverter within each module make it possible to have a finer level of granularity in terms of MPPT. This will increase the amount of circuitry within each panel but the architecture can ensure a faster payback, particularly for domestic and industrial installations where shading from nearby buildings is an issue.
By offering much finer control over power conversion, it becomes more practical to install modules on surfaces that might experience some level of shading during the day because these modules will not affect the optimum power conversion of those that rarely suffer from shading.
The Heart Of The Inverter
Typically, the heart of the solar inverter is a programmable microcontroller running a number of control loops that implement the switching algorithms and MPPT functions. The programmability of the microcontroller makes it possible for PV system manufacturers to differentiate their offerings.
Changes to the MPPT algorithm can deliver efficiency improvements that look small on paper. But over the operational life of the module, they can make a huge difference to the payback equation.
Inverter designers can choose from a wide variety of algorithms, and each has advantages and drawbacks. With the “perturb and observe” technique, the voltage or current is changed and the change in power output is noted. This technique is susceptible to oscillations if the lighting conditions change quickly or if the steps are too large, particularly at low levels of irradiance.
Incremental conductance also uses small changes to feed data to a control loop that calculates the direction in which the power point should be moved. It generally provides a better guide on the direction in which the maximum power point is moving. If implemented in software on a microcontroller, though, the delay in processing can be high, which can lead to errors in fast-changing conditions and unwanted oscillation.
Field-programmable gate array (FPGA) technology provides a way to give the microcontroller assistance for the more complex MPPT algorithms and improve response times. The FPGA can be configured to act as a coprocessor for the core microcontroller, not just offloading many of the repetitive calculations but also accelerating them dramatically, reducing the time between power-point updates.
Microsemi’s Igloo family of FPGAs offers very low power consumption, contributing minimally to the heating of a PV module or inverter. For modules that include their own inverters to support fine-granularity MPPT, the SmartFusion family offers the high level of integration needed to minimise the space taken up by the control electronics.
Each SmartFusion device contains, alongside the programmable logic used to accelerate MPPT algorithms, a 32-bit ARM Cortex-M (see the figure). This microcontroller core provides the performance necessary to run not just the core control loops but also the diagnostic and communications functions needed in modern solar arrays.
However, performance can be further optimised through the use of the analogue compute engine (ACE). The ACE combines a sample-sequencing engine (SSE) with a post-processing engine (PPE) to offload the job of reading analogue inputs from the Cortex-M3.
The SSE captures data from the analogue inputs, passing it to the PPE, which can perform functions such as low-pass filtering to remove noise and transform the data into a format convenient for the processor. Devolving these functions into hardware ensures that vital data samples if the software is performing diagnostic or communications functions, improving MPPT reliability.
Microsemi’s system-oriented FPGAs and solar-optimised power devices provide a combination designed to improve the power harvesting of PV modules and their overall profitability, ensuring that the high efficiency of modern solar cells does not go to waste.