Solar-Reports:

Concentrated photovoltaics: the path to high efficiency

by Francisca Rubio & Pedro Banda,
ISFOC, Puertollano, Spain
2009-04-24

The costs of a photovoltaic installation are driving the market and the need for subsidized schemes, such as feed-in tariffs. Concentrated photovoltaics (CPV) is leading the development of future lowcost renewable energy sources in two ways: on one hand offering high efficiency systems, and on the other, being most capable of reducing manufacturing costs. The idea to decrease the cost of the photovoltaic system using optical elements to focus the radiation into the cell to reduce the size of the cells has been in the mind of the scientists since the 1970s. But, apart from a reduced market, there were several issues that did not allow CPV success at that time. This paper puts forth the proposition that the key is to replace the area of active material, which is the most expensive, with optic elements, which are well known and cheaper. This Solar Report is an abridged/edited version of a paper that was originally published in "Photovoltaics International" (2nd issue 2008.)
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Tiny solar cells in large power plants: CPV systems by Concentric Solar in Spain.
Tiny solar cells in large power plants: CPV systems by Concentric Solar in Spain.
Picture courtesy: Concentrix Solar GmbH

1980s cell technology did not permit very high concentration levels

In order to optimize the performance under concentrated light, the cells should be very efficient – more efficient than the 20% record lab efficiency reached in the initial days of research. Cell technology did not permit very high concentration levels; the maximum concentration ratio was 40% and therefore the optical elements were very simple. Cell technology based on Si and its low efficiency did not allow a large reduction of the cell size and therefore, the temperature budget was low, creating degradation problems in the system. For example, in the SOLERAS project in Saudi Arabia, installed by Sandia Labs in the 1980s, the systems degraded by up to 20% in six years, mainly due to delamination issues at high temperature. A further issue resulted from the use of outdated tracker technology, creating problems with the algorithms and control systems. Despite these hurdles, this project continued in operation for 18 years.

CPV technology relies particularly on these three areas: high efficiency cells, optics and sun tracking. In each of these areas there have been significant developments in the last five years, leading to current commercial systems already competitive in the current market.

High efficiency cells for CPV applications

Cell technology has been greatly improved and the efficiency consequently increased. Si-based solar cells with back point contact reached an efficiency record of 27.6% and some manufacturers like Amonix or Guascor Foton use this type of cell under more than 400X concentration. However, the best improvement in cell technology lies in the use of compound III-V semiconductors. Ternary alloys of III-V elements like GaAs together with In, Al and P allow a better temperature behaviour of the cell. GaAs’ thermal coefficient is 1.76*10-3 ºC, compared to 3.21*10-3 ºC for Si.

Also, III-V technology allowed the development of multijunction solar cells. Two or more p-n junctions are monolithically integrated into a single device. The use of these cells allows a better use of the solar spectrum as each of the junctions is optimized to capture the radiation of a different part of the spectrum. Based on the spectral response, the theoretical limit efficiency of silicon cells is 40%, whereas multijunction cells could potentially reach 86%. Current records for laboratory multijunction cell are Spectrolab’s 40.7% record, 40.8% from NREL and Fraunhofer ISE reached 41,1 % at the beginning of 2009.

These very high efficiencies would allow the reduction of the cell size down to 1cm2 or even 1mm2, like in Isofotón or Concentrix systems. This decreasing area also allows the improvement of the thermal behavior and the reduction of the serial resistance. Currently, almost all systems using multijunction cells have concentration ratios of more than 350X, while the majority feature more than 450X (Concentrix, Solfocus, Arima…), with some using ratios of up to even 1000X (Isofotón). The way is paved to proceed to higher concentrations.

Solar spectrum in Si and multi junction cells.
Figure 1. Solar spectrum in Si and multi junction cells (courtesy of A. Luque, CONSOLIDER project).

Thermal management

Thermal management continues to be a challenge for CPV systems. Most of them use some type of heatsink or thermal dissipater as passive cooling. In most of the designs an alumina plate is used to mount the cell die as it is a good electrical insulator and a good thermal conductor. In order to improve the thermal behavior, there are designs which use a very simple and small Cu or Al plate; others link directly to the metal housing of the module, whereas others use intermediate thermal systems. In large parabolic mirror dish systems, such as those from Solar Systems (Australia), an active cooling system is needed due to the fact that all the cells are together and the thermal dissipation area is lower.


Cell featuring alumina and thermal simulation
Figure 2. Cell featuring alumina and thermal simulation (Courtesy of Renovalia CPV).

Optics

Optical designs that meet the strict criteria for high concentration systems are key to new CPV concepts. Traditionally, flat Fresnel lenses or mirrors have been used, such as those in Sandia, Ramon Areces, and Euclides systems. In current systems, the Fresnel lens is still the optical element of choice for companies such as Concentrix, Amonix, Guascor, Emcore, Arima ECO, Renovalia CPV, Sol 3G and Entech, among others.

Ease of design and simulation, as well as reduced cost, are the contributing factors for this being considered a lead component. Nevertheless, its manufacture meets a number of challenges under the requirements for high concentration CPV. There are various technological approaches to the manufacture of Fresnel lenses, from injection or compression molding, with very difficult tuning, to silicone films, which do not always allow good prices. In order to improve the acceptance angle of the system, a secondary element can be used. This element is normally made of a solid glass or hollow metal prism, both of which allow a better light distribution on the cell and improve the collection of the light; optical systems without a secondary element can also provide very high efficiencies.

Reflective optics are an alternative method, capable of being used in several ways, such as in a large parabolic mirror dish, using a parquet of cells, like Solar Systems’ technology, or as compact mirror systems, with one parabolic mirror per cell, which can incorporate additional reflection steps like Solfocus’ system.

Additionally, other non-imaging optical concepts like the Total Internal Reflexion (TIR) system are used by companies such as Isofotón. In recent years, new optical concepts such as XR or RXIhave been developed, concepts that combine reflection and refraction and have the potential to achieve very high concentration with a wide acceptance angle.

Different optics layouts: Fresnel without secondary, Fresnel with secondary, mirror, TIR, XR optic and RXI optic
Figure 3. Different optics layouts: Fresnel without secondary, Fresnel with secondary, mirror, TIR, XR optic and RXI optic

Tracking systems

Depending of the acceptance angle of the optics, tracking systems should be more or less accurate and should be adapted for every module technology. Up to a concentration ratio of 2.5X, there is no need for sun tracking. For mid concentration (up to 40X), the tracker can be lineal, like the Euclides system, with elevation movement in a polar axis or east-west movement in a north-south axis. However, for high concentration systems, two-axis sun tracking with a very high accuracy is mandatory.

The control of the tracking system can be carried out in open loops (sun movement equations) or closed loops (light sensor). Use of a single control method can pinpoint tracking errors, while tracking accuracy can be reduced by using an open loop control on a system that has not been precisely installed and positioned. A closed loop system will not work properly if there are clouds shadowing the light sensor. Current systems use hybrid control, which include both sun movement algorithms and radiation or power output sensors.

Euclides system.

Two-axis tracker.

Figure 4. Euclides system. Figure 5. Two-axis tracker.

Institute for Concentration Photovoltaics Systems – ISFOC

Technology developments and a buoyant PV market have made it possible for CPV to be deployed. High efficiency cell technology is already available for terrestrial applications; new optical concepts or processes have also been developed and, finally, tracker systems and controls are being developed and validated. With proven technology and manufacturing processes available, manufacturing lines and utility-sized CPV power plants are ready for deployment.

Armed with the objective of making this a reality, ISFOC, formed in September 2006, is the result of the R&D plan promoted by the Spanish Department of Education and Science from the Castilla La Mancha Government and the IES-UPM (Institute of Solar Energy from the Universidad Politécnica de Madrid). The project is financed by the Spanish Ministry of Education and Science.

In order to generate key knowledge on this technology, ISFOC is executing a number of power plants (up to 3MW in total) incorporating different concentrator technologies. Companies featuring in the first phase include Isofotón (ES), Solfocus (US) and Concentrix (DE). The second phase includes Emcore (US), Sol 3G (ES), Arima Eco (TW) and Renovalia CPV (ES).

Currently, there are 1.1MW already installed and connected to the grid from the first phase. The objective of these pilot plants is to assist the industries in the setting up of pilot fabrication lines. There will also be valuable information obtained from the process such as reliability, suitability and production from each technology.

ISFOC HCPV power plants in Puertollano.
Figure 6. ISFOC HCPV power plants in Puertollano. Picture courtesy: Denis Lenardic; pvresources.com

Standards

In December 2007, the first standard for CPV (IEC 62108) was approved. This standard establishes the reliability tests for the CPV modules. The tests to be carried out are, electrical tests, climatical test, mechanical tests and ambient condition tests. There are a number of laboratories that are now in the process gaining of accreditation for performing these tests. All companies under ISFOC’s project must pass the most important tests of this standard before installation. During performance of the standard procedures, designs were improved and modifications were added to comply with the IEC tests. The isolation test and the damp heat test are among the most difficult trials, as the modules need to show they are watertight while maintaining electrical isolation. Other standards under development include CPV modules power rating, CPV system energy rating (with ISFOC participation) as well as tracker requirements and safety.

Conclusion

With the available technology, approved standards, installation of the first demonstration plants and the availability of their results, the CPV industry is experiencing extraordinary progress. The number of CPV and cell manufacturers has increased in the last two years and the number of CPV installations has become considerable.

CPV is in the early developmental stages of deployment, and holds great potential for rapid growth. With a global PV market driven by government subsidies, CPV will rely on special measures that will allow its growth, independent of other renewable energy sources. We expect that early deployment will be sufficient to show the capabilities of this technology in fulfilling the needs of cost and productivity to reach grid parity in the short term.

ABOUT THE AUTHORS

Francisca Rubio graduated as an electronic engineer from Granada University in Spain. She worked for ten years in the fields of electronics and LEDs and was responsible for the R&D Department of Valeo Lighting in Martos, where she developed LED-based headlamps for the automotive industry. She joined ISFOC in September 2006 as head of the R&D Department.

Pedro Banda has a Ph.D. in electronic engineering from RMIT in Australia and graduated as a telecommunications engineer from UPM in Spain. His background is in the semiconductor industry, where he held various technology, development and management positions at Lucent Technologies, BPSolar (Spain) and IMEC (Belgium). He currently holds the role of ISFOC Director General.

MANUFACTURERS Company Today on sun (MW; Nov. 2008) Plant location Manufacturing capacity (MW/year)
Abengoa Solar Spain
1.2
Spain
Guascor foton Spain
10.275
Spain
15
Isofotón Spain
0.4
Spain
5
Renovalia CPV CS La Mancha Spain
0
Spain
11
Sol 3G Spain
1.4
Spain
12
Concentrix Germany
0.3
Spain
25
Solfocus USA
0.5
Spain
50
EMCORE USA
0.35
Spain
10
AMONIX USA
0.61
USA
Arima ECO Taiwan
0.05
Spain
7.5
Solar Systems Australia
1.2
Australia
5
TOTAL
16,285
140.5
Table 1. Main CPV manufacturers showing total installed power versus actual manufacturing capacity.
Other Companies Company
location
Boeing USA
Concentrating Technologies USA
Cool earth USA
Energy Innovations USA
Enfocus Engineering USA
Entech USA
Green volts USA
JX Crystal USA
Opel International USA
Pyron Solar USA
Semprius USA
Skyline USA
Solartech USA
Soliant Energy USA
Menova Canada
Morgan Solar Canada
ENEA Italy
Magpower Portugal
Solar Tec AG Germany
Whitfield Solar UK
Daido Steel Japan
Sharp Solar Japan
Everphoton Energy Taiwan
Green and Gold Energy Australia
Table 2. Other CPV manufacturers
ENQUIRIES

ISFOC
Juan Bravo 22
13500 – Puertollano
Spain
Tel: +34 926 441 673
Fax: +34 926 429 142
Email: isfoc@isfoc.com

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