Concentrating photovoltaics (CPV) refer to photovoltaics used with optical concentration, in which mirrors and/or lenses capture sunlight and focus it onto a smaller area of photovoltaic materials, sometimes called receivers. Most CPV systems employ optical elements that actively track the sun as it moves across the sky, keeping sunlight in focus on the receivers. Most such CPV systems fall into two broad categories:
- Single-axis tracking systems in which the optical elements focus light into strips.
- Multiple-axis tracking systems in which the optical elements focus light into spots.
This relationship of optical and tracking geometries is true of both refractive and reflective optics.
A CPV system can be characterized by its design concentration ratio -- the ratio of its aperture area to its receiver area. In contrast to a CPV system, a flat-plate photovoltaic panel has a concentration ratio of one. A system's aperture is the area of the shape defined by the intersection of the sunlight captured by the system and a plane perpendicular to the sunlight. Thus, a CPV system having an aperture of one square meter and photovoltaic receivers whose total area 20 square centimeters has a design concentration ratio of 500.
Higher concentration ratios have the advantage of lessening the quantities of photovoltaic material required to convert a given flux of sunlight to electricity, while imposing design requirements on the system such as the ability to dissapate heat from the receivers.
Practical design concentration ratios of a CPV system depend greatly on the system's type, with multiple-axis spot-focusing types offering much higher ratios than single-axis strip-focusing types. Assuming perfect optics and tracking, the maximum effective concentration ratio of a strip-focusing system is about 100-to-one and that of a spot-focusing system is about 10,000-to-one. The limit in ratios is a consequence of the sun's angular size, which is about one-half degree of arc.
Multi-Junction PV Cell Configurations
PV cells used in CPV systems are generally multi-junction, where each of several junctions is tuned to a different frequencty of light. Two methods of configuring multi-junction cells are stacked -- in which the different junctions are stacked on top of each-other and light not captured by one layer is passed onto the layers beneath it, and split-spectrum -- where the different junctions are arranged side-by side such that each junction recevies light of its frequency.
In a stacked configuration, the junction having the highest bandgap energy is on the top of the stack, and each deeper layer has a lower bandgap energy. Most of the photons whose energies are below the bandbandgap energy of a given junction are passed to the junction beneath it. Typically the junctions are electrically connected in series via tunnel diodes.
In contrast using a stacked configuration, an the split-spetrum approach lays the different junctions out side-by side and uses spectrum-splitting optics to direct light to the different junctions.
Professor Martin Green and Anita Ho-Baillie from the University of New South Wales (UNSW) used five different cells and spectrum-splitting optics to reach an experimental efficiency of 43% - measured under the global ASTM G173-03 spectrum, a terrestrial standard. Earlier this year, a US team put together a similar system operating at 42.7% efficiency.