Concentrated PV

Module performance from week-to-week and month-to-month is impacted by factors, such as the available direct normal irradiance (DNI), prevailing spectral conditions, soiling, tracking accuracy, ambient temperature, and other factors

The performance of the CPV modules depends on Direct Normal Irradiation (DNI). As soon as DNI is at a high level, the spectral losses are not significant any more. For high performance, tracking has to be very precise, due to the high concentration of the sunlight. The focal point has to be directly on the solar cells.

CPV specialists like Soitec believe that they have racking under control. For its part, Soitec uses dual-axis tracking to find the optimal position of the CPV modules towards the sun from the morning to the evening. In a Soitec CPV system, a proprietary application and algorithm position the tracker. Astronomical positioning is used and DC power output is monitored to calculate the next optimum position for maximum power generation.

Impact

Module performance from week-to-week and month-to-month is impacted by factors, such as the available DNI, prevailing spectral conditions, soiling, tracking accuracy, ambient temperature, and other factors.

“CPV modules as a whole perform very well with conversion efficiencies as high as 30-35% but it is again critical to be able to predict how the efficiency will change with the prevailing outdoor conditions.  Interest rates provided by banks are reduced the more accurately performance can be predicted for a given installation and this is another means of lowering the LCOE for CPV,” says Matthew Muller, CPV performance testing and IEC CPV standards development, Engineer IV NREL, National Center for Photovoltaics.

In the past few months, the industry has witnessed steady progress.

In August, Amonix achieved a National Renewable Energy Laboratory (NREL) efficiency rating of 35.9% under recently adopted CPV IEC test conditions of 1000 W/m² and 25°C cell temperature.

A month later, Soitec launched its newest CPV module featuring a record power-generating efficiency of 31.8%. According to the company, its new module, already in industrial volume production, has the highest efficiency of any commercial product available for multi-MW installations.

“I have seen many members in the CPV industry make rapid improvements in module efficiency over the past few years,” says Muller. He adds that it is exciting to see the progress in performance “but NREL does not distinguish or compare one module manufacturer to another”.

Further Improvement

“The module power and performance is mainly driven by the cell,” says Hansjörg Lerchenmüller, Senior VP Product Strategy, Solar Energy Business Unit, Soitec.

“Soitec’s CPV modules today achieve a module efficiency of 31.8%. For these modules, advanced multi-junction solar cells with 42% efficiency are used,” Lerchenmüller says.

According to him, the potential for further efficiency increases is huge. Lerchenmüller explains: “We target a cell efficiency of 50% (recently achieved a new world record of 44.7% measured at a concentration of 297 suns). Soitec’s expertise in engineered substrates provides the right environment for the development of multi-junction solar cells with record efficiency.”

The company believes its epitaxy expertise can be used to grow rare semiconductor materials. And its Smart Cut technology can help to transfer ultra-thin layers of rare semiconductor materials plus the Smart Stacking technology can be used to stack multiple junctions.

“Apart from the solar cell efficiency, an optimised module design for the harsh environmental conditions in hot and sunny regions is vital. Soitec’s CPV modules are very durable. We have a planar glass-glass design and use Fresnel lenses made of Silicone on glass. Optics is also very important and we’ll also have improvements on the optics,” says Lerchenmüller.

Monitoring Conditions

Module performance is of course dependent on irradiance and temperature (as with all PV) but also can be significantly impacted by spectral conditions and tracking performance.

The current approach IEC WG7 is exploring in regards to spectral performance is to use component reference cells to monitor the spectral conditions.

“By specifically measuring spectral conditions is this way it is feasible to have a CPV performance rating standard that is repeatable and minimises uncertainty,” says Muller.

He clarifies that this is an issue associated with creating accurate models or appropriate standards but changing spectral conditions should not be equated with poor performance.

“Consider a module that has an absolute conversion efficiency of 33% in one month but 31% in another month due to seasonal spectral changes.  This module is performing much better in both months compared to other technologies.  At the same time is important the we can accurately predict/model performance regardless of the conditions."

Installation Costs

The industry is exploring the potential of bringing down installation costs because of the high efficiency of modules.

Less land, less steel, fewer foundations, and many other reductions come with higher efficiency modules.

Tne component of the balance of system costs is the cost of installation.  Pre-assembled systems have the potential to bring down system installation costs.

As Edward Cahill, Research Associate, Lux Research shared recently with PV Insider, pre-assembling modules will help with reliability and installation costs. Modules need to be perfectly aligned, which is difficult and expensive to do in the field. Pre-assembling arrays in tight, easy to ship configurations is key for CPV. Also key is light-weight, as this reduces shipping costs of such assemblies. If modules are big and cumbersome, they become uneconomical to ship and require onsite assembly and local manufacturing.

Certainty

CPV performance has improved dramatically on various counts over the past 4 to 5 years meaning that performance improvements have not really been the biggest hurdle for the industry in recent years.  A much bigger hurdle has been finding enough capital to ride out all the problems associated with the world financial crisis of recent years.

“Multiple years of performance data is needed to satisfy the banks and convincing venture capitalists to be patient for the results to come is a difficult proposition,” says Muller.

Companies like Soitec are working on plans to facilitate financing of solar projects featuring CPV modules.

Only recently, the company signed a performance-warranty insurance contract with Munich Re to cover all its CPV modules. This would in turn improve chances of securing financing for projects based on CPV technology.