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 A further issue resulted from the use of outdated tracker technology, creating problems with the algorithms and control systems.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.

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.

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.

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.

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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