Solar

Concentrator photovoltaics: a mature technology for solar power plants

Issue 3 and Volume 2.

Several of the first market-ready CPV products have been installed in power plants in 2008 and 2009; the technology is now ready for the next wave, with multi megawatts (MW) to be installed in 2010.

For the world’s future energy supply, the highly efficient conversion of solar energy to electrical energy using photovoltaic (PV) cells is one of the key elements. Researchers in the field of photovoltaics are faced with the problem how to increase the efficiency of this conversion process while reducing the cost significantly. The solution with the highest cost reduction potential is concentrator photovoltaics (CPV), where the cost reduction is incurred by replacing expensive PV cell material with lower cost optical systems covering the receiver aperture. In recent years, however, only expensive multijunction III-V concentrator solar cells with efficiencies >40% under concentration became available. This situation led to an intense development work on CPV system technology with the result that several of the first market-ready CPV products have been installed in power plants in 2008. These plants have been able to verify the maturity of this technology with very satisfying field data over the years [1,2].

Thanks to the data noted above, CPV is now ready for the next wave, with multi megawatts (MW) to be installed in 2010. The first example of a large scale project is the 1MW project in Questa, New Mexico – installed by Chevron – which demonstrates the confidence of major companies in CPV technology. Other examples will follow, leading to the scaling of this technology to tens, and then hundreds of MWs in the market. Thanks to the use of earth-abundant materials and small semiconductor content, scalability of this technology is virtually unlimited.

Benefits of CPV systems

CPV offers a number of benefits to utilities:

CPV is perfectly suited for high ambient temperatures. Due to the very low temperature coefficient of the III-V multi-junction concentrator solar cell, the performance of CPV systems is much less affected by temperature than any other PV technology, i.e., the loss of efficiency is approximately one third that of crystalline silicon modules. This attribute is extremely important for the best solar sites in the world, which are generally also located in hot climates. Because of the low temperature coefficient, the efficiency and the electricity production of CPV systems are only slightly affected by high ambient temperatures in comparison to other PV technologies. As can be seen in Fig. 1, the efficiency drop for a temperature difference of 40K (e.g., from 25°C at standard testing conditions to typical operating cell temperatures of 65°C) is by far the smallest for CPV systems.

 

Figure 1. Efficiency loss for different PV technologies due to a rise of temperature of 40K

CPV provides the power at the right time. CPV systems using a high concentration are always tracked using two axes. Two-axis tracking allows for a homogeneous electricity production profile over the day because the panels are always oriented perpendicularly to the incident irradiation from the sun. The most important effect is that the power production is at high levels when the power demand peaks in the afternoon. Afternoon peaking of electrical power consumption is very typical, particularly in sunny countries where the load is strongly influenced by air conditioning. In Fig. 2, the data for California are shown together with the production profiles of a fixed-PV installation and a CPV system at high concentration. The superior daily production profile together with the high efficiency enables CPV systems to achieve highest energy production per used area and highest temperature-corrected capacity factors of up to 34% on sites with a very good solar resource [3].

Figure 2. Electricity production profiles of fixed and two axes tracked PV installations together with the power demand (data for California).

CPV has the highest efficiency and energy output, i.e., the lowest LCOE (levelized cost of electricity). Until now, most of the industry has been focusing on PV module cost. As a very new and immature industry, a great deal of the affects of scaling, productivity, and the cost reduction learning curve had to be achieved. Now, GW-level production has been achieved by top PV module suppliers, meaning that most of this learning has been integrated. So cost reduction and efficiency improvement moves now into a continuous improvement mode, with a few percent maximum gain per year. Module cost reduction will be limited by raw materials cost contribution (glass, metals, semiconductor, etc.).

CPV has two fundamental cost advantages here: First of all, it minimizes the semiconductor content in the module thanks to high concentration. Any semiconductor material is far more expensive that any other material used in a PV solar plant. In addition, the higher the efficiency (CPV has 2X higher efficiency than multi-crystalline silicon technology), the better the usage of all the other material (glass, metal frame, etc.). That’s why, for similar volumes, a CPV module will be cheaper than any other PV technology.

From a mid- to long-term perspective, PV electricity costs in general will also be dominated by fixed costs associated with a PV plant: project development, installation, and O&M costs. In order to drive LCOE down to a very low level, energy output (kWh) has to be as high as possible to compensate for this fixed-cost structure. There are two ways to increase energy output: one is module efficiency, the second is sun tracking. Knowing that sun tracking only makes economic sense if module efficiency is high enough (because the tracker cost, which is measured in $/m2, has to be compensated for by the energy output “boost”) everything comes back again to the importance of efficiency.

So far, there is no alternative to CPV with respect to the highest efficiencies, making it the technology of the future in sunny regions.

A strong collaboration for next-generation CPV

Soitec, Concentrix Solar, Fraunhofer ISE and CEA-Leti started a collaboration on the development of the next generation of very high efficiency CPV solar cells based on Soitec’s proprietary technologies (Smart Cut, an enabling technology to transfer layers using ion implantation, and Smart Stacking, a wafer-level 3D integration technology) to boost CPV system performance. The newly developed cell is code named “Smart Cell,” and it will be integrated into the FLATCON CPV system. The modularity of the CPV approach, i.e., the independence of the module optimization from the cell optimization, allows for a very rapid and focused development.

Figure 3. Photo of a FLATCON CX-P6 system with modules CX-75 on a FLATCON power plant.

A complete system consists of the modules, the tracker, the inverter, the auxiliaries, and the control and monitoring hardware and software. In 2008, trackers were installed with 120 Gen I modules that had 150 lenses resulting in a total module aperture of 28.8m²/tracker. At that time, the nominal power of these trackers at a direct normal irradiation (DNI) of 850W/m² was 5.4kW. Installations were made at the ISFOC site (Instituto de Sistemas Fotovoltaicos de Concentration) at Puertollano, Spain, and at a site close to Seville, Spain. The tracker has a tracking accuracy determined to be better than 0.1° with a proprietary control system.

Figure 4. Solar-to-grid efficiencies of Gen I and Gen II FLATCON CPV systems.

Each of the trackers has its own inverter, which also serves as the control system of the mechanical tracking and as the communication port with its own IP address. This inverter for CPV systems was developed by Fraunhofer ISE in cooperation with Concentrix Solar and currently has an efficiency of 96%. For control and monitoring, each installation includes two DNI sensors, two GNI sensors and two wind sensors. A detailed analysis of the system and power plant performance can be made based on data that was taken at one-minute intervals.

The first system with Gen I modules was grid connected in Spain at the end of April 2008. Since then, the system demonstrated an AC energy efficiency – which means solar-to-grid efficiency – over the whole period of more than 20% with daily maxima above 23%. The daily system AC energy efficiency drops only in case of a low daily DNI (direct normal irradiation) energy, which is the case mainly in wintertime, whereas during summertime, when most of the electrical energy is gained, the efficiency is very high. Apart from the mentioned Gen I demonstrator system, two power plants of 100kW were grid connected at the end of September 2008; and one power plant of 100kW was grid connected at the end of October 2009. In the year 2009, all produced more than 400MWh of electrical energy and outperformed the predicted yield by 5%.

Figure 5. Photo of a module CX-75 (right) and view inside (left).

In 2009, several trackers of the same type, but with 90 of the CX-75 modules were installed, which resulted in the same aperture area per tracker of 28.8m² (Fig. 3). For this type of tracker, a DC system efficiency of 26% was measured, which is an indication for the homogeneity of the modules. The AC system efficiency and the respective AC power were determined to be 25% and 6kW at a DNI of 850W/m². Such a system was installed, for example, at the campus of the University College of San Diego. So far, the system AC energy efficiency during the monitoring period, which mainly included the winter, has been 22% (Fig. 4). The Gen II system efficiency exceeds the Gen I efficiency by 2%.

Design principles

The design of the module dates back to the late 1990s. The cover and bottom plate made out of glass, and a relatively small aperture of each of the primary lenses, which are assembled in an array, were developed at Fraunhofer ISE in Freiburg, Germany. The cells are mounted on heat spreaders which serve at the same time as contact pads for the internal electrical connection of the module.

The major reasons for using two glass panes are high durability, low cost, and the low coefficient of thermal expansion (CTE) that ensures that the foci remain on the cell in at varying operating temperatures. As the CTE of glass is 3 times lower than that of aluminum for example, it is possible to keep the focal position on the cell within 100µm at all operating temperatures. The bottom plate does not need to be thermally conductive as the heat spreading is already efficiently done by the heat spreader, for which highly thermally conductive materials are used. The glass also serves as a scratch-resistant cover plate.

The Fresnel lens array is replicated in one piece into a silicone rubber on glass (SOG), allowing for extremely UV stable materials in a cost effective mass fabrication. There are also good reasons for the relatively small lens aperture: thermal management and low module depth. In the case of this primary lens, a simple heat spreader made out of a metal with a satisfying thermal conductance is sufficient for the thermal management. It was shown that the cell temperature in a CPV module does not exceed 40K above ambient temperature on average [4]. Furthermore, a small lens allows also for small cells, which are advantageous with respect to obtaining the highest efficiency because of the low resistance losses in the cell.

The design is kept as simple as possible to enable robustness and a low manufacturing cost. A “unit” element of a FLATCON module consists of a primary lens and a solar cell plus bypass diode mounted on a small planar heat spreader. This planar design can be manufactured by using standard semiconductor assembly and printed circuit board machines (Fig. 5). On the top of Fig. 5, on the right side, one can see the SOG lens array, which is manufactured in one piece; on the bottom, the solar cell assembly array is interconnected by wire bonds. The lens and bottom plate are mounted by using proven standard technologies from the architectural glazing industry. Today, the FLATCON modules CX-75 have an average efficiency of 27%.

Conclusion

CPV is a field-proven technology ideally suited for sunny and hot environments. The AC system efficiencies of up to 25% today are the key factor for obtaining low system cost and low LCOE. First power plant installations in Spain using the FLATCON technology outperformed the predicted yield by 5%. The first MW power plant using III-V multi-junction solar cell CPV technology will be installed in the U.S. in 2010.

Acknowledgment

FLATCON is a registered trademark of Concentrix Solar GmbH, a division of the Soitec Group. Smart Cut and Smart Stacking are trademarks of Soitec.

References

  1.  M. Martínez, D. Sánchez, J. Perea, F. Rubio, P. Banda, “ISFOC Demonstration Plants: Rating and Production Data Analysis,” Proc. of the 24th European Photovoltaic Solar Energy Conf. and Exhibition, Hamburg, Germany, 21.-25.09.2007 (2009).
  2.  A. Gombert, A. Hakenjos, I. Heile, J. Wüllner, T. Gerstmaier, S. van Riesen, “FLATCON CPV Systems – Field Data and New Developments.” Proc. of the 24th European Photovoltaic Solar Energy Conf. and Exhibition, Hamburg, Germany, 21.-25.09.2007 (2009).
  3.  S. Kurtz, “Opportunities and Challenges for Development of a Mature Concentrating Photovoltaic Power Industry,” Technical Report NREL/TP-520-43208 Revised Nov. 2009.
  4.  G. Siefer, A. W. Bett, “Calibration of III-V Concentrator Solar Cells and Modules,” Proc. of the 4th World Conference on Photovoltaic Energy Conversion, Waikoloa, Hawaii, USA, 2006, pp. 745-748.

Andreas Gombert received his degree in photoengineering and photonic systems from the U. of Applied Sciences Cologne, Germany, and from the National School of Physics Strasbourg, France, and his PhD from the U. Louis Pasteur, Strasbourg. He is the CTO of Concentrix Solar GmbH (a division of the Soitec Group), Freiburg, Germany; email [email protected]

Christophe Desrumaux received his university diploma in engineering and materials physics from INSA Lyon, France, and is the business development manager at Soitec S.A., Parc Technologique des Fontaines, 38190 Bernin, France.