ANALYSISMANUFACTURING COST


Figure 4: Using a NIC as a secondary


concentrator allows the system to be less sensitive to


misalignment by being a larger target for the primary optics without need for a increasing the cell size. Total internal reflection channels light from the wider receiving surface to the cell attached to the narrower base


Careful system-wide design needs to be followed to ensure best decisions based on the specific factors of the system in question. Once again, the modelling approach can help here by modelling failure rates associated with different options.


42


Module design for cost effective efficiency Economies of scale are essential to deliver the $/kWh figures needed for grid parity. Feed in tariffs are being used in many countries to help remove the ‘chicken and egg problem’ of not being able to mass produce solar collectors until sales are achieved that rely on the low costs of mass production in the first place. To maximise the savings from mass production, systems and sub components needs to be designed for manufacture and assembly. For example, modules for CPV systems are cheaper when arrays are designed to hold large numbers of identical units. But module size is inevitably a trade-off: too small and there is too much extra material adding cost and weight; too big and tolerances become an issue. Keeping the modules smaller prevents thermal expansion in the module lens from affecting optical alignment. This results in very large numbers of modules required for utility scale systems, demanding similar production techniques to those needed in the automotive or high volume consumer electronics sectors.


Heat dissipation is also a major issue for module efficiency, as temperature increases adversely effect cell efficiency, yet cells are obviously subjected to intense solar radiation! Being able to wick this heat away in a cheap, efficient manner can lead to real efficiency improvements.


Optical alignment for CPV systems Low cost production is essential but precise optical alignment is also critical as the light needs to be focussed perfectly on the small cell to achieve high efficiency. Deep drawn aluminium was found to offer significant benefits as a module housing, as it


is cheap, thin, thermally conductive and lightweight, but also stiff enough to preserve optical alignment. Focussing can also be affected by tracker accuracy, wind flexure in the array structure, mounting alignment of the lens, cell and module, and the accuracy of the lens optics. The accumulated tolerance error from all the sources of misalignment cannot allow concentrated light to miss the cell, or the efficiency will rapidly deteriorate.


Equally, the system cannot be overly expensive. Bigger cells are easier targets but they add extra cost. Modelling tolerance stack and module costs shows that using a secondary, non-imaging concentrator (NIC) allows the perceived target size for the concentrated light to be larger without significantly increasing costs. The NIC works by using a large receiving surface to collect light from the lens with a relatively small acceptance angle, then by total internal reflection similar to that seen in fibre optics, the light is conveyed to the narrower end of the NIC, where it shines onto the cell.


Conclusion


This article has discussed several important technical considerations for reducing the cost of large CPV solar farms, and how modelling can be used to optimize the balance between cost and efficiency.


A full system approach is the most balanced and efficient way to ensure cost effectiveness, not just in solar but indeed in any complex system. To help with this, companies with specific expertise or technologies are increasingly bringing in consultancies to add value in other areas, or even leading a system-wide cost optimisation programme to improve competitiveness.


Solar PV system designers need not only consider the cost hierarchy of the cell, the module and the system, but also the top level of the installation to include all aspects of the life cost. Taking a total system view to apportion technical development effort along each link in the value chain makes it easier to identify the big wins within the system, with the aim of making the system more cost effective as a whole. This holistic approach is best to address efficiency, reliability, serviceability and cost reduction of manufacture, assembly and installation; all leading to a lower cost per kWh and therefore a more competitive system, getting solar ever closer to grid parity and a better clean energy option to meet future demand.


© 2011 Angel Business Communications. Permission required.


www.solar-pv-management.com Issue VI 2011


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