Steve Askins

Effects of Temperature on Hybrid Lens Performance

In hybrid Silicone‐on‐glass Fresnel lenses, an optical silicone is molded onto a glass substrate and forms the Fresnel structure. These lenses offer a cost effective solution as a primary optical element in point‐focus concentrator photovoltaic modules, as well as performance advantages. However, these lenses have a high performance variation with temperature due both to the change in index of refraction of silicone as well as to deformations in the facets caused by coefficient of thermal expansion (CTE) mismatch. In this study we perform measurements of the light flux at the focal plane of a family of SOG lenses, varying temperature and lens‐to‐receiver distances. The effect of varying silicone cure temperature and the depth of the silicone between the lens and the glass substrate on temperature dependence was investigated. A preliminary computer model of this behavior is presented.

Two-dimensional angular transmission characterization of CPV modules

This paper proposes a fast method to characterize the two-dimensional angular transmission function of a concentrator photovoltaic (CPV) system. The so-called inverse method, which has been used in the past for the characterization of small optical components, has been adapted to large-area CPV modules. In the inverse method, the receiver cell is forward biased to produce a Lambertian light emission, which reveals the reverse optical path of the optics. Using a large-area collimator mirror, the light beam exiting the optics is projected on a Lambertian screen to create a spatially resolved image of the angular transmission function. An image is then obtained using a CCD camera. To validate this method, the angular transmission functions of a real CPV module have been measured by both direct illumination (flash CPV simulator and sunlight) and the inverse method, and the comparison shows good agreement.

Luminescence inverse method For CPV optical characterization.

The luminescence inverse method may be used to optically characterize a concentrator photovoltaic module. With this method, the module angular transmission is obtained by evaluating the light emission of a forward biased module. The influence of the emission of the cell when measuring the angular transmission is evaluated, and the process of building a global angular transmission from the set of individual optics-cell unit functions is explained. A case study of a module composed by several optics-cell units is presented. In order to validate the proposed measurement, results for five different CPV technologies are compared for both direct methods (i.e., solar simulator) and indirect methods (i.e., Luminescence inverse method).

Characterization of CPV arrays based on differences on their thermal resistances

Thermal characterization of Concentrating Photovoltaics (CPV) modules and arrays is needed to determine their performance and modelling of energy forecast. Module-ambient thermal resistance is easily obtained from its definition but the cell-module thermal resistant needs to be estimated from indirect procedures, two of them are presented in this paper. In addition, an equivalent parameter is defined, the Concentrator Nominal Operating Module/Cell Temperature (CNOMT/CNOCT), the temperature at Concentrator Standard Operating Conditions (CSOC). Definitions and expression to relate (CNOMT/CNOCT) to thermal resistances are presented, plus several examples of estimations from real operating arrays.

Characterization Capabilities of Solar Simulators for Concentrator Photovoltaic Modules

Solar simulators for characterization of concentrator photovoltaic modules (CPV) are now commercially available, providing an alternative to outdoor performance characterization with clear advantages in terms of repeatability, availability, control of operating conditions, and cost. Nevertheless, optical concentration implies particular characteristics that introduce the need of new performance figures and characterization methods. CPV solar simulators have demonstrated characterization capabilities beyond simple current–voltage (I–V) curve acquisition. This paper summarizes other indoor performance tests developed at the Instituto de Energia Solar (IES-UPM) that have proven to be useful for the optical, mechanical, and spectral characterization and optimization of a CPV module.

Probing the effects of non-uniform light beams and chromatic aberration on the performance of concentrators using multijunction cells

A method is presented for estimating the photocurrent of the subcells of a multijunction cell within a concentrator, through the measurement of the cell’s short-circuit current under a large sweep of different spectral conditions. The spectrum is monitored by means of a set of component cells matched to the subcells of the concentrator cell. The method allows the calculation of the current-matching or J-ratio of the concentrator cell under any spectrum, and also the identification of losses due to the non-uniformity of the spectrum throughout the solar cell area. For illustration purposes, the method is applied to a concentrator composed of a Fresnel lens that concentrates light over a triple-junction cell 300 times smaller.

A review of the promises and challenges of micro-concentrator photovoltaics

Micro concentrator photovoltaics (micro-CPV) is an unconventional approach for developing high-efficiency low-cost PV systems. The micrifying of cells and optics brings about an increase of efficiency with respect to classical CPV, at the expense of some fundamental challenges at mass production. The large costs linked to miniaturization under conventional serial-assembly processes raise the need for the development of parallel manufacturing technologies. In return, the tiny sizes involved allows exploring unconventional optical architectures or revisiting conventional concepts that were typically discarded because of large material consumption or high bulk absorption at classical CPV sizes.

Spectral network based on component cells under the SOPHIA European project

In the frame of the European project SOPHIA, a spectral network based on component (also called isotypes) cells has been created. Among the members of this project, several spectral sensors based on component cells and collimating tubes, so-called spectroheliometers, were installed in the last years, allowing the collection of minute-resolution spectral data useful for CPV systems characterization across Europe. The use of spectroheliometers has been proved useful to establish the necessary spectral conditions to perform power rating of CPV modules and systems. If enough data in a given period of time is collected, ideally a year, it is possible to characterize spectrally the place where measurements are taken, in the same way that hours of annual irradiation can be estimated using a pyrheliometer.

Atmospheric parameters, spectral indexes and their relation to CPV spectral performance

Air Mass and atmosphere components (basically aerosol (AOD) and precipitable water (PW)) define the absorption of the sunlight that arrive to Earth. Radiative models such as SMARTS or MODTRAN use these parameters to generate an equivalent spectrum. However, complex and expensive instruments (as AERONET network devices) are needed to obtain AOD and PW. On the other hand, the use of isotype cells is a convenient way to characterize spectrally a place for CPV considering that they provide the photocurrent of the different internal subcells individually. Crossing data from AERONET station and a Tri-band Spectroheliometer, a model that correlates Spectral Mismatch Ratios and atmospheric parameters is proposed. Considering the amount of stations of AERONET network, this model may be used to estimate the spectral influence on energy performance of CPV systems close to all the stations worldwide.

Indoor characterization at production scale: 200 kWp of CPV solar simulator measurements

In order to complement ISFOC’s characterization capabilities, a Helios 3198 CPV Solar Simulator was installed in summer 2010. This Solar Simulator, based on a parabolic mirror and a high-intensity, small area Xenon flash lamp was developed by the Instituto de Energia Solar in Madrid [1] and is manufactured and distributed by Soldaduras Avanzadas [2]. This simulator is used not only for R&D purposes, but as a quality control tool for incoming modules that are to be installed in ISFOC’s CPV plants. In this paper we will discuss the results of recent measurements of close to 5000 modules, the entire production of modules corresponding to a small CPV power plant (200 kWp). We scrutinize the resultant data for signs of drift in the measurements, and analyze the light quality before and after, to check for changes in spectrum or spatial uniformity.