MANUFACTURINGOUTLOOK
point. Besides an enhanced energy yield, other advantages of this approach are improved safety, more monitoring capabilities, and improved modularity and flexible system design. We can distinguish here two different categories: module integrated inverters or module integrated dc-dc converters.
A module integrated inverter converts the power so it can be applied directly to the utility grid. Alternatively, module integrated dc-dc converters convert the variable current from the PV module to match the common output dc-voltage shared with other modules in a series string connection. The combined string output is used by a larger string or central inverter to deliver ac power to the grid. First attempts in this direction are presently appearing on the market (SolarMagic [4], SolarEdge [5], Enphase Energy [6], Enecsys [7]).
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But are we ready for this evolution from a technological point of view? The power efficiency of PV converters has increased through the years and in the lab, 99% efficiency has already been demonstrated. At the same time, we see a trend to increase the switching frequency in order to reduce the size of the converter passives (and thereby the whole converter), paving the way to micro-converters for PV modules. But module integrated converters must not only be compact: they must be capable of sustaining larger
temperatures, be highly efficient and cheap and, being an integral part of the module, have a lifetime of more than 20 years. In this perspective, wide bandgap materials, especially GaN based devices, are very attractive. GaN has a high mobility, high power density, high breakdown voltage and can be grown on large-diameter Si substrates. At imec, first convertor circuits based on GaN-on-Si were successfully designed, built and tested [8]. The converter uses AlGaN/GaN/AlGaN double heterostructure field- effect transistors (DHFETs) and is designed for high switching frequencies from 205kHz up to 1MHz. High conversion efficiencies of around 96% were reached at 400kHz.
Active bypass diodes and low-loss switches Other obvious components to integrate inside the module volume are bypass diodes. Their function is to limit the amount of power dissipated in the cells that are reverse biased by e.g. partial shadows on the module. It is expected that future PV modules will incorporate more functionalities than just bypass diodes, such as active bypass diodes with lower losses and thus less heat dissipation than regular diodes.
An example of a state-of-the-art active bypass switch is the LX2400 from Microsemi [9]. A further step forward would be the incorporation of ultralow-loss switches that would allow a dynamic reconfiguration of cells into modules in order to act upon non-uniformities. Moreover, integrated on-off switches controlling the module output would ensure safety during maintenance and anti-theft protection by offering the possibility to ‘lock-up’ the panels.
Local energy storage
Figure 2: (a) GaN DHFET mounted on AlN carrier and experimental set-up of a high-frequency convertor; (b) efficiency at high frequency operation
It is also believed that modules may one day include local energy storage components, i.e. passives such as standard capacitors or dedicated storage devices such as batteries or supercapacitors. Indeed, the penetration of PV systems in the smart grid will require some sort of storage. The storage function may be centralized or distributed, and possibly micro-storage will even be introduced at the module level – in particular in close conjunction to module-level converters.
The reason to include energy storage is twofold. A first function is to locally ‘micro-balance’ the output power. Although local mismatches in power output can be dealt with instantaneously by the dc-dc converters, the temporary power output difference over a certain operation cycle can additionally be balanced by locally storing and recovering energy. Secondly, they help to balance the electricity grid
www.solar-pv-management.com Issue IV 2011
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