Optimizing photovoltaic systems

Solar energy continues to grow as a renewable energy source, and continued interest in it has driven down the price of solar panels and increased efficiency. At the same time, balance of system (BOS) components such as inverters, chargers, and energy optimizers have made significant progress. This article will introduce new architectures and components that affect the performance of solar BOS.

Transformerless DC/AC inverters are widely used in Europe, but in the United States, they have only recently been used in some areas. There are many transformerless inverter topologies, and the HERIC topology developed by Fraunhofer Institute shows high efficiency. The structure of a traditional full-bridge inverter is shown in Figure 1, and the HERIC topology is shown in Figure 2, which also shows two new switch/ diode pairs. This topology uses a unique freewheeling path to reduce switching and conduction losses, increasing efficiency to more than 98%.

Figure 2 HERIC topology used in transformerless inverter

Advantages of Transformerless Inverters

Transformerless inverters have several advantages. The transformer stage of a conventional inverter, which provides galvanic isolation, is heavy, expensive, and has high losses. Even high-frequency inverters with very small transformers have significant energy losses, up to 1% to 2%. In the continued process of reducing the cost of installing PV systems, every little bit of energy is critical. Therefore, the transition to transformerless inverters will continue.

Disadvantages of Transformerless Inverters

Transformerless inverters do have some disadvantages. As mentioned above, they do not include the galvanic isolation provided by a transformer, which is a significant safety concern. However, the integration of comprehensive safety mechanisms, such as isolation resistance testing and residual current detection, makes transformerless inverters as safe as transformers. In addition, there is evidence that grounding problems in these inverters can cause permanent damage to thin-film panels, especially some CIGS solar panels.

The switches in an H-bridge are common in inverter topologies. As mentioned above, inverter design is moving towards reducing the size and cost of inductors/ capacitors and transformers at higher and higher powers. High voltage/high frequency switching is necessary in solar inverters. However, running MOSFETs at high voltage/high frequency results in severe conduction losses. IGBTs are often used because they have lower conduction losses than MOSFETs. However, they produce tail currents during turn-off – increasing switching losses.

ESBT

ST's emitter-switched bipolar diode (ESBT) provides a good solution. As shown in Figure 3, the ESBT's common-base amplifier structure contains a high-voltage BJT and a power MOSFET, and the entire device has a very low on-state voltage drop.

Figure 3 ESBT with MOSFET driver

When an ESBT is paired with an external MOSFET and a diode/ resistor , the entire circuit looks like a 3-terminal device that can be driven to operate like an IGBT or power MOSFET. The turn-off energy of an ESBT is much lower than that of an IGBT, enabling high-efficiency design and is very suitable for high-frequency, high-voltage inverter design.

The traditional rooftop solar system installation process is also reducing BOS costs and improving performance. In this structure, solar panels are connected together in a series / parallel array, which is very sensitive to shadows and mismatches. For example, if the performance of a panel in a series array is affected by shadows or dust, the output of the entire string will be severely affected. One solution to this problem is to add a DC/DC converter and a large power point tracker at the panel or series level.

optimization

Energy optimization at the panel level is a very important energy conversion and control task. These functions optimize the energy collected by the solar panel and then convert it into a continuous voltage or current, while sending the operating status to the central controller. This requires a microcontroller or state machine, analog sensing circuits, DC/DC current conversion, and wired or wireless communication.

These specific functions are well understood and suitable for integration into a single module. Doing so provides cost, reliability and performance advantages. Optimized MPPT output increases system performance and leads to increased efficiency, helping to reduce system costs.

A typical MPPT integrated solution is ST's SPV1020. It includes an integrated boost converter, an MPPT wired state machine, analog sensing circuits and a PLM. The converter uses a high-frequency interleaved structure that can accommodate smaller inductors and capacitors . This highly integrated solution will be launched later in 2010.

Solar energy is suitable for most industrial applications, such as off-grid solar powered street lights, signs, collision warning lights, safety systems, data acquisition and remote communications. Typically, solar energy is used in places where the grid is not accessible. However, in these places, the use of solar energy is limited by cost factors. However, like rooftop solar, off-grid industrial solar power systems will increase in use as cost and efficiency improvements are made.

Off-grid power generation systems require large energy harvesters, especially batteries. These circuits need to be charged safely and efficiently to continuously improve completeness and integration. For example, Cypress Semiconductor has launched an integrated solar charger reference design using the PowerPSoC processor. It uses a 12V solar panel to slowly charge a 12V lead-acid battery. This reference design includes MPPT optimization and a lead-acid battery charger.

The architecture of this product uses a current controlled buck rectifier for MPPT and battery charging (see Figure 4). The MPPT and battery charger embedded in the PowerPSoC uses voltage and current feedback to operate the panel at peak power by controlling the switching of the buck controller to operate the panel at peak power.

Figure 4 MPPT/charger controller structure diagram

In another example, STMicroelectronics has developed a highly integrated HBLED solar MPPT charger/driver. This fully integrated solution comes with 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.