MANUFACTURINGOUTLOOK
T
he manufacturing of Si solar systems, which today consists in processing the individual Si solar cells and then stringing them together into modules, is expected to undergo drastic modifications. This is driven not only by the trend towards thinner back-contacted solar cells but also by the recent efforts to increase the energy yield of the PV modules. The latter implies the integration at module level of so-called smart components: future PV systems will contain distributed power converters, active bypass diodes, low-loss switches, energy storage components... At the same time, the industry will be further progressing towards fundamental understanding and modelling the different failure modes of modules. Both trends will eventually enable PV manufacturers to go for a customized module and system design suiting specific operating conditions for PV plants across the globe.
Over the last decades, the photovoltaic (PV) industry has succeeded in continuously lowering the cost per installed Watt-peak (cost/Wp), with learning ratios in the order of 20%. For crystalline- Si (c-Si) PV, the workhorse of the PV industry, the production costs of the solar cells are constantly lowered by improved manufacturing practices, increase of areal throughput of equipment and up- scaling of fabs [1, 2]. Add to this the tremendous efforts worldwide to increase the efficiency of solar cells, a measure that has just as much a large leverage effect on cost.
This progress in terms of cost/Wp has been the main focus of the PV industry so far, as an obvious means to make PV energy economically viable. Without any doubt, the PV community will pursue these efforts in the years to come. Recently, a new trend has emerged: the dynamic increase of the energy yield of the PV modules. This can be done by either extending their operational lifetime, or by improving their yearly production (expressed as Wh/year for each installed Wp).
Extending the lifetime of the modules beyond the typical 20 years requires reliable ageing models that can predict the modules degradation rate. Improving the yearly production can be achieved by adding more functionality to the PV system – at the module or even within the module – and by moving to smarter control strategies. This would eventually allow the realization of reconfigurable smart PV modules that would link up with the smart electricity grid of the future [3].
Figure 1: Today, Si solar cells are processed individually and then stringed together into modules. The figure shows a PV module consisting of 3 substrings connected in series. A bypass diode is placed across each substring
In view of this vision, we discuss a number of building blocks that will most likely constitute the smart PV module of the future: an integrated converter, active bypass devices, ultra-low-loss switches, local energy storage devices, and monitor and control functions. Besides these functionalities, we need an appropriate technology that allows embedding the different components in the PV modules.
This development will also be influenced by another trend in Si PV manufacturing, namely the steady decrease in Si wafer thickness. We will also stress the importance of outdoor testing and the availability of ageing models to test any new PV module concept. Finally, we discuss the impact of this new emerging market segment on the overall PV market.
Making PV modules more intelligent Today, a PV module can be defined as a series connection of individual solar cells integrated into a package that is transparent to sunlight but likewise protects the cells from all other environmental factors. The modules are connected in series to constitute a larger PV system. The power produced by the modules is converted via a central dc-ac inverter to ac power that is used on- site or pumped into the grid. Current conventional PV system designs are unintelligent solutions that do not allow for maximum power output in different conditions and at different times of the day.
Tracking of the maximum power point is performed by the central inverter with the inherent drawback that, if for any reason the current of one module drops significantly (e.g. by shadowing, or by local temperature increase), the overall power output of the system is jeopardized. An alternative approach is to connect every module to its own converter stage which operates at its own maximum power
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www.solar-pv-management.com Issue IV 2011
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