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Improve efficiency of photovoltaic systems

Posted: 30 Nov 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Photovoltaic? inverter system? customisable system-on-chip? FPGA?

Among the various renewable energy sources we employ nowadays, solar energy is by far the most prevalent. To harness this source, arrays or solar panels are formed by linking solar modules that contain semiconducting material which absorbs photons of sunlight. The photons energise electrons in the semiconducting material, freeing them from their atoms. This, in turn, creates direct current (DC) that must be converted to alternating current (AC). Photovoltaic (PV) technology is the mechanism used to implement this solar-to-electrical conversion.

Unfortunately, it generally can only achieve efficiency of roughly 19 per cent. The only way to maximise the use of harvested solar energy while minimising module and system size is to achieve efficiency of greater than 95 per cent.

There are two types of PV systems. The first type is configured as an off-grid or stand-alone system that operates independently of the electric utility grid. The second type of PV system can be integrated with the utility grid, which enables energy to be shared between the PV system and the grid. One benefit of this approach is that surplus power can be sold back to the utility.

Regardless of which approach is taken, each PV system uses similar components, including PV modules, a cooling system, an energy storage system or battery bank, the load, a utility grid interface, and a PV inverter system (figure 1). While these components vary depending on functional and operational requirements, the PV inverter system is the heart of any implementation. It performs all DC-to-AC conversion, power protection, monitoring, and control functions.

Figure 1: Typical PV energy system.

There are a number of decisions to make in the design of PV inverters, including power system interconnection regulations and international standards. Specifications such as IEEE 1547 and EN50160 impose constraints including the necessity for galvanic isolation, as well as the maximum harmonic distortion of the current injected at the point of common coupling (PCC), and the maximum permitted DC current injection.

Designing PV inverter system
The two primary sub-components in a PV inverter systems typically are the controller used to implement system management tasks and control algorithms, and the AC-to-DC conversion circuit.

The controller is used for tasks including grid and system monitoring, system synchronisation with utility power for grid-connected systems, and output power quality monitoring. The controller also performs protective functions for safety and compliance with various standards and regulations. Other key functions include data logging, firmware updates, and communications with the system operator, as well as battery charging control for stand-alone systems, and smart metering used for grid-connected PV systems. One other important controller responsibility is the execution of control and energy management algorithms. In addition to being very computationally demanding, these tasks can also impact power efficiency.

The DC-to-AC conversion circuit also plays an important role, handling all the tasks related to converting raw DC power from the panels into clean AC power that is consistent with the utility grid's voltage and power quality requirements. To accomplish this, the circuit uses a set of switching power devices such as metal oxide semiconductor field effect transistors (MOSFETs) or insulated gate bipolar transistors (IGBTs). The inverter circuit also includes active filtering circuitry to reduce the distortion caused by harmonics resulting from high-frequency switching.

The right configuration
Designers have a choice of several possible PV conversion circuit configurations. The choice depends on a number of factors, including the number of power processing stages, the type of power decoupling, the types of intra-stage interconnections, and the type of grid interface.

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