Systems

The CPV system is composed of many parts which must cooperate efficiently; generally, the modules or assemblies must follow the Sun in its apparent motion, to ensure the collection of the direct irradiation from the cells, through the optics. The possibility of the concentrators to catch only the direct portion of the sunlight, with an additional circumsolar light dependent on the acceptance angle of the optics, is an important limitation for the CPV respect to standard photovoltaics. Diversely, the necessity to follow the Sun is not generally a limitation; indeed tracking installations are already in fields for standard, flat plate modules too. The tracking of the Sun gives a significant improvement in the energy collection, because of it allows for a constant maximal intercepted area of the modules for the sunrays. This fact permits to improve the energy production of 30-40% respect to fixed installations of same peak power, with this percentage depending on the latitudes of the installation. So, for an economical point of view, the additional costs introduced by the Sun- tracker have to be balanced by the gain in the energy production; this is the straightforward
evaluation in the case of standard modules; for CPV the trackers are fundamental parts of the systems, so, it’s an integral element and must be considered as an essential component as well as the inverters or the modules.

For these reasons, high efforts in the designing and production of cheap and reliable trackers are fundamental for the CPV establishment.

As previously described, CP Vs, depending on the technology employed for the modules and cells, can use single axis trackers and two-axis trackers. While for the HCPV the 2-axis tracker is compulsory, the low concentration systems can be found, depending on the technology, on 1-axis or 2-axis trackers. In fig. (14) a 2-axis system mounting 25x concentrating modules with high angular acceptance is shown; in this case, the high optical acceptance permits to use standard trackers generally used for flat plate modules (Antonini et al., 2009a).

The most common kind of CPV systems are constituted with panels of many modules. These CPV modules are treated similarly to standard flat plate modules on a tracking structure; in the CPV panels, the rigidity of the structures and the precision of mounting on the frames are more critical than for standard modules, as well as the pointing precision in the Sun tracking. These modules are made of many cell-optics units, electrically connected internally into a closed, water proofed box. Each cell-optics unit play the role of a single cell in a standard flat plate module.

An alternative approach uses a large concentrating optics collecting the light onto a dense array of cells. The most classic designs consist of big reflective dishes with paraboloid or similarly curved shapes and dense arrays positioned in the focuses of the concentrators or at the end of a secondary optical elements (Stefancich et al., 2007). In fig. (15) a dense array of silicon solar cells is shown. The main advantage of this approach is that there is a high technology core of small area, which can be assembled with standard equipments for electronics, while in the CPV modules the cells are distributed on all the module surface with consequent high area to be considered for the CPV receivers. However, the dense arrays have some important limitations too; first, an even light irradiation is required on the series connected string of cells. This is because the less illuminated limits the current of all the string. Second, it is necessary to reduce at the minimum the spaces between the cells and to reduces the bus-bars and interconnections areas; indeed, all these zones give optical losses for the photovoltaic concentrator. These two points are not in common with CPV modules, because the light irradiance on the optics-cell units is equal for all, and the connectors and bus bars of the cells are usually kept out of the illuminated region, using for these purposes the large area between the cells in the module receiver.

Fig. 15. 30cm x 30cm dense array of silicon solar cells (Courtesy of CPower srl)

The CPV systems have the advantage of a lower energy payback time (EPBT) respect to standard c-Si modules. The EPBT, an indicator for the energetic sustainability of a system, is the time a system for energy production needs to generated the input energy required during its whole life-cycle. The shorter EPBT for CPVs is because the material used for the concentrators are usually produced with low energy consumption. The high level of purification required for the silicon to achieve the electrical properties essential for the photovoltaic use needs a high energy utilization. To understand the order of magnitude, to produce about 100W of silicon for standard photovoltaic cells with efficiency of about 15%, about 300 kWh are necessary; considering an average annual production of 1400 kWh/kWp, more than 2 years are required to pay back the energy for the solar grade silicon alone. Adding to this energy consumption needed for the silicon purification the other fabrication steps to get a compete standard PV modules, the EPBT usually reported for the modules is in the order of 3-4 years (Stoppato, 2008). The CPVs technologies have only a small fraction of very purified materials, being mainly composed of plastics, glass and metallic frames. This fact leads to shorter energy payback time, in the order of 1 year (Peharz & Dimroth, 2005).

The localization for CPV installation is strongly dependent on the weather conditions; diversely than for standard flat plate modules, the fundamental irradiation data is not just the global irradiation, but it is the direct normal irradiation (DNI), i. e. the component of light collected by the concentrators. The humidity, the clouds, the dust and the pollution scatter the light coming from the Sun deflecting the rays; usually, the best conditions for CPV are in dry and highly sunny climates. The higher DNI/GNI ratios are typical of desert areas or elevated terrains. The evaluation of this parameter is fundamental, and the knowledge of the global irradiation is not sufficient to estimate the energy production of a CPV system; indeed, the yearly average DNI/GNI ratio can vary from 50% to 80% (NREL, 1994). Reliable solar maps for direct irradiance are not yet available for everywhere as for the GNI.

Sometimes, even the DNI is not enough to evaluate the energy production of a system; indeed, the light impinging the cells in a concentrator systems is not necessary the same read from the pyroheliometer, i. e. the instrument used to measure the direct irradiation; this instrument, basically a sensor of irradiation with a tube limiting the angle of incidence for the incoming rays, usually has a view angle of ±2.5° and a limit angle of about ±4°. Depending on the optical solution adopted for the photovoltaic concentrator, the acceptance angle of a CPV system can be higher or lower respect to the pyroheliometer, so the light seen by the cells can be higher or lower respect to the reference instrument. The effect of the soiling on the modules is similar to the scattering effect due to the atmospheric conditions; indeed, the particles deflect the sun rays and can contribute to significant losses. Generally, the higher the acceptance angle of the optics, the lower is the effect of the soiling on the performances; for low concentrator systems with high acceptance angle the losses seem to be comparable with that of standard modules (Antonini et al., 2009b).

The peak power for the CPV modules and assemblies is usually defined under a DNI of 850 W/m2. Although the conditions for the performances testing of CPVs are not yet defined in international standards, the main producers and research institutions recently refer to the 850 W/m2 of DNI and module temperature of 25°C; performances tests with the cell’s temperature of 60°C are often found too (Hakenjos et al., 2007). The temperature is a more thorny issue for testing respect to the irradiation, because of the temperature in field are usually significantly higher than in lab. The outdoor characterizations are fundamental to evaluate the performances losses due to the heating up of the cells.

The irradiance condition of 850 W/m2 of DNI has been selected because of a DNI/GNI ratio in the order of 85% is frequently observed in many locations around the world when the GNI is of 1000 W/m2, i. e. the standard irradiance condition for the test of flat plate modules. The energy productivity of a CPV installation can be evaluated, similarly than standard installations for flat plate modules, using the energy yield (Yf) and the Performance Ratio (PR) (Marion et al., 2005). The energy yield represents the energy production for installed peak power of a system; it is measured in kWh/kWp and strongly depends on the location because of it doesn’t take care of the incident radiation. It’s the first parameter for the comparison of different installations in the same site. Diversely, the Performance Ratio (PR), dimensionless and defined as in (14), normalizes the energy production to the incident irradiation, delivering a useful parameter for the comparison of installation under different irradiation conditions; it quantifies the losses due to temperature, AC/DC conversion, soiling, down-times, failures and mismatching.

Yf

PR = ^~ (14)

Y

r

The PR can be read as the equivalent time the system has delivered it’s nominal peak power (Yf) respect to the time of equivalent nominal irradiance conditions on the panel.

The Reference Irradiance (the irradiance for the DC peak power estimation) for CPVs is 850 W/m2 of DNI, instead of 1000 W/m2 of GNI as for the standard modules. This difference must be taken into account during the comparison of the CPV with other different PV technologies.