Solar energy continues to evolve as a renewable resource, driving down costs and boosting efficiency in solar panel technology. Advances in Balance of System (BOS) equipment like inverters, chargers, and energy optimizers have also been remarkable. This article explores new architectures and components impacting the performance of solar BOS.
Transformerless DC/AC inverters are widely adopted in Europe but are relatively new in the U.S., particularly in specific regions. Various transformerless inverter topologies exist, with the HERIC topology developed by the Fraunhofer Institute standing out for its high efficiency. A traditional full-bridge inverter's structure is depicted in Figure 1, while the HERIC topology is illustrated in Figure 2. This topology introduces two new switch/diode pairs, utilizing a unique freewheeling path to cut down on switching and conduction losses, pushing efficiency beyond 98%.
Figure 1 Full H-bridge for transformerless inverters
Figure 2 HERIC topology for transformerless inverters
Advantages of Transformerless Inverters Transformerless inverters offer several benefits. Traditional inverters rely on a transformer stage for galvanic isolation, which adds weight, expense, and substantial losses. Even high-frequency inverters with tiny transformers incur significant energy loss, up to 1-2%. Every bit of saved energy is crucial in reducing the overall cost of PV installations. Thus, the shift towards transformerless inverters remains a strong trend.
Disadvantages of Transformerless Inverters Despite their advantages, transformerless inverters come with drawbacks. Without the galvanic isolation provided by a transformer, they pose a significant safety risk. However, incorporating comprehensive safety mechanisms, such as isolation resistance testing and residual current sensing, ensures that transformerless inverters can be as safe as those with transformers. Additionally, there’s evidence suggesting grounding issues with these inverters could cause irreversible damage to thin-film panels, especially CIGS Solar Panels.
Common inverter topologies feature switches within the H-bridge. As mentioned, inverter designs aim to minimize the size and cost of inductors, capacitors, and transformers while maximizing power. High voltage and high-frequency switching are essential in solar inverters. However, operating MOSFETs under these conditions can lead to severe conduction losses. While IGBTs have lower conduction losses compared to MOSFETs, they generate tail currents during turn-off, increasing switching losses.
ESBT
ST’s Emitter-Switched Bipolar Diode (ESBT) offers an excellent solution. As shown in Figure 3, the ESBT’s common base amplifier structure combines a high-voltage BJT and a power MOSFET, resulting in a very low turn-on voltage drop.
Figure 3 ESBT with MOSFET driver
When paired with an external MOSFET and a diode/resistor, the entire circuit resembles a 3-terminal device that can be driven similarly to an IGBT or power MOSFET. ESBT’s turn-off energy is significantly lower than that of IGBTs, enabling more efficient designs and making it ideal for high-frequency, high-voltage inverter applications.
Traditional rooftop solar system installation processes are also reducing BOS costs and enhancing performance. In this setup, solar panels are connected in series/parallel arrays, which are highly sensitive to shadows and mismatches. For instance, if a panel in a series array is shaded or dusty, the entire output is severely impacted. One solution is to integrate a DC/DC converter and a powerful Maximum Power Point Tracker (MPPT) at the panel or series level.
Optimizing panel-level energy optimization is a key energy conversion and control task. These functions maximize the energy harvested by the solar panel, converting it into a stable voltage or current while reporting operational status to the central controller. This requires a microcontroller or state machine, analog sensing circuitry, DC/DC current conversion, and wired or wireless communication.
These specific features are straightforward to implement and ideal for integration into a single module. This approach brings cost, reliability, and performance benefits. Optimized MPPT output boosts system performance and improves efficiency, contributing to reduced system costs.
A typical MPPT integration solution is ST’s SPV1020. It integrates an integrated boost converter, an MPPT wired state machine, analog sensing circuitry, and a PLM. The converter employs a high-frequency interleaving structure, allowing the use of smaller inductors and capacitors. This highly integrated solution will launch later in 2010.
Solar energy is versatile and suitable for various industrial applications, including off-grid solar-powered lighting, signage, collision warning lights, security systems, data acquisition, and telecommunications. Solar energy is typically deployed in areas where grid access is unavailable. However, cost remains a limiting factor in these locations. Similar to rooftop solar systems, off-grid industrial solar power systems will see improvements in both cost and efficiency.
Off-grid power generation systems require robust energy harvesters, particularly batteries. These circuits need safe and efficient charging to enhance performance and integration. For example, Cypress Semiconductor has introduced an integrated solar charger reference design using the PowerPSoC processor. Powered by a 12V solar panel, it slowly charges a 12V lead-acid battery. This reference design includes MPPT optimization and a lead-acid battery charger.
The product’s architecture uses a current-controlled buck rectifier for MPPT and battery charging (see Figure 4). The MPPT and battery charger embedded in the PowerPSoC utilize voltage and current feedback to operate the panel at peak power, controlling the buck controller’s switches to maintain peak power operation.
Figure 4 Block diagram of the MPPT/charger controller
In another example, ST Microelectronics developed a highly integrated HBLED solar MPPT charger/driver. This fully integrated solution features an MPPT-optimized battery charger and an integrated HBLED driver. This product will be released in late 2010 and is ideal for HBLED street lighting applications.
JIANGSU BEST ENERGY CO.,LTD , https://www.bestenergy-group.com