In this paper we present a method of optical characterization of solar concentrators based on the use of a laser beam.
The method, even though constrained by lengthy measurements, gives nevertheless interesting information on local
mirror surface defects or manufacturing defects, like internal wall shape inaccuracies. It was applied to 3D-CPC-like
concentrators and the measurements were supported by optical simulations with commercial codes. The method,
simple to apply, requires just a laser to scan the CPC input aperture following a matrix-like path, at a controlled
orientation of the beam. Maps of optical efficiency as function of the laser beam incidence angle are obtained by
matching the CPC exit aperture with a photodetector with an efficient light trapping. The integration of each map gives
the CPC efficiency resolved in angle of incidence, so curves of optical transmission (efficiency) as function of
incidence angle can be drawn and the acceptance angle measured. The analysis of the single maps allows to obtain
interesting information on light collection by the different regions of CPC input area. It reveals, moreover, how the
efficiency of light collection depends on several factors like surface reflectivity, number of reflections of the single
beam, local angle of incidence, local surface defects, and so on. By comparing the theoretical analysis with the
experimental results, it is possible to emphasize the effects directly related to manufacturing defects.
The optical characterization of a CPC concentrator is typically performed by using a solar simulator producing a
collimated light beam impinging on the input aperture and characterized by a solar divergence (± 0.27°). The optical
efficiency is evaluated by measuring the flux collected at the exit aperture of the concentrator, as function of incidence
angle of the beam with respect to the optical axis, from which the acceptance angle can be derived.
In this paper we present an alternative approach, based on the inverse illumination of the concentrator. In
accordance with this method, a Lambertian light source replaces the receiver at the exit aperture, and the light
emerging backwards at the input aperture is analyzed in terms of radiant intensity as function of the angular
orientation. The method has been applied by using a laser to illuminate a Lambertian diffuser and a CCD to record the
irradiance map produced on a screen moved in front of the CPC.
Optical simulations show that, when the entire surface of the diffuser is illuminated, the "inverse" method allows to
derive, from a single irradiance map, the angle resolved efficiency curve, and the corresponding acceptance angle, at
any azimuthal angle. Experimental characterizations performed on CPC-like concentrators confirm these results. It is
also shown how the "inverse" method becomes a powerful tool of investigation of the optical properties of the
concentrator, when the Lambertian source is spatially modulated inside the exit aperture area.
The Perkin Elmer Lambda 900 spectrophotometer has been used to perform measurements of spectral reflectance in the NIR/Vis/UV range from 250 nm to 2200 nm at variable angle of the incident light from 8° to 78°. To perform this type of measurements, the Lambda 900 has been equipped with two integrating spheres, one for accommodating the sample and orienting it with respect to the incident sample beam, the other for measuring the intensity of radiation reflected by the sample. Measurements of angle-resolved spectral reflectance were then performed on a set of standards of diffuse reflectance from Labsphere.
The optical modeling of a Compound Parabolic Concentrator (CPC), as photovoltaic solar light concentrator, shows that the light distribution on the circular receiver is far from being uniform, as requested for this type of application. The solution lies in the adding of a prismatic optical mixer, which breaks the circular symmetry of the CPC, distributing light over a square surface. This is paid by a ~30% reduction of the original CPC concentration ratio. The CPC-mixer system has been optimized for medium levels of concentration (~100). The optical efficiency of the CPC-mixer concentrator has been evaluated for different values of the walls reflectance, R, and a total loss of ~6% has been found for a 98% reflecting silver coated wall.
A new method is presented for measurements of angle-resolved reflectance of a plane surface. The main innovation introduced by the method consists in allowing to carry out reflectance measurements from 0° to 90° of incidence angle. This was obtained by changing the technique of sample illumination: instead of illuminating a portion of the test surface, the entire test surface is illuminated by a collimated and uniform beam at any angle of incidence. To be applied, the method requires, besides measurement on the unknown sample, also measurements on two different standards of diffuse reflectance. The method is particularly suitable for the optical characterization of photovoltaic devices, generally exposed to the inclined light of the direct component of solar radiation.
High-concentration PV systems offer a viable alternative to conventional modules mainly because of the high energy density produced for unit surface of photovoltaic cells. Optimal performances can be obtained by employing simultaneously receivers sensitive to different wavelengths by a proper partitioning of the concentrated solar spectrum. At the same time secondary concentrators with the ability to homogenize the illumination field must be employed to maximize cells performances. The use of a dichroic mirror and compound parabolic concentrator (CPC) homogenizers allows to forecast overall system performances capable of justifying the development of systems for residential applications. Moreover the concentrating systems may open prospects for large scale energy applications.
A camera for recording the intensity of light backscattered from textured photovoltaic samples is described. It was realized by modifying a professional folding camera, for use on optical bench. The collimated light from a laser source crosses the camera and impinges on the test sample. The light backscattered by the sample exposes a punched photographic plate on the front side of the camera. With a proper choice of the laser light wavelength, the optical effects produced by surface or subsurface features of the sample can be in this way recorded. The analysis of the light diffusing characteristics of textured photovoltaic samples can be directed towards improving their light collection capabilities.
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