Stephen Askins

Design and modeling of a cost-effective achromatic Fresnel lens for concentrating photovoltaics

This paper presents a novel Fresnel lens capable of significantly reducing chromatic aberration in solar applications. The optical performance of this achromatic lens has been analyzed through ray-tracing simulations, showing a concentration factor three times higher than that attained by a classic silicone on glass (SOG) Fresnel lens while maintaining the same acceptance angle. This should avoid the need for a secondary optical element, reducing the cost associated with its manufacturing and assembly and increasing the module reliability. The achromatic lens is made of inexpensive plastic and elastomer which allows a highly scalable and cost-competitive manufacturing process similar to the one currently used for the fabrication of SOG Fresnel lenses.

Understanding causes and effects of non-uniform light distributions on multi-junction solar cells: Procedures for estimating efficiency losses

This paper presents the mechanisms of efficiency losses that have to do with the non-uniformity of the irradiance over the multi-junction solar cells and different measurement techniques used to investigate them. To show the capabilities of the presented techniques, three different concentrators (that consist of an acrylic Fresnel lens, different SOEs and a lattice matched multi-junction cell) are evaluated. By employing these techniques is possible to answer some critical questions when designing concentrators as for example which degree of non-uniformity the cell can withstand, how critical the influence of series resistance is, or what kind of non-uniformity (spatial or spectral) causes more losses.

Module optical analyzer: Identification of defects on the production line

The usefulness of the module optical analyzer when identifying module defects on production line is presented in this paper. Two different case studies performed with two different kind of CPV modules are presented to show the use of MOA both in IES-UPM and Daido Steel facilities.

Evaluation of misalignments within a concentrator photovoltaic module by the module optical analyzer: A case of study concerning temperature effects on the module performance

Instituto de Energia Solar, Universidad Politecnica de Madrid (IES-UPM) has developed a method [referred to as the luminescence inverse (LI) method] and equipment [called module optical analyzer (MOA)] to fast measure the optical-angular properties of a CPV module without illumination system nor module movement. This paper presents how the MOA can investigate the optical performance of concentrator photovoltaic (CPV) modules optical-angular performance (in particular, misalignments between the optical components comprising the module) at different temperature conditions.

Spectral Impact on Multijunction Solar Cells Obtained by Means of Component Cells of a Different Technology

Spectral indices can be defined to characterize how the solar spectrum affects photovoltaic devices. The use of such indices is particularly important in technologies which are highly dependent on spectral variations, such as tandem, or multijunction (MJ), solar cells. In this paper, we analyze the capability of three effective irradiance values, given by the photocurrents of the components of triple-junction solar cells, and covering different spectral bands, to define the spectrum condition. Even though these parameters will be based on a particular type of MJ solar cell, they provide enough information to accurately assess the spectral impact on other types of MJ solar cell technologies with significantly different spectral responses. Consequently, specific component cells for each MJ solar cell technology may not be necessarily needed and a reference technology could be used for all of them, providing a way for the future standardization of these devices as a mean of defining the solar spectral condition.

Hybrid dome with total internal reflector as a secondary optical element for CPV

Secondary optical elements (SOEs) are used in Concentrator Photovoltaic (CPV) modules to allow the concentration ratio to exceed those typically achievable by Fresnel lenses, reducing cell costs, without sacrificing tolerance to tracking errors. One option is a “dome” SOE: a simple, single surface refractive optic that images the primary lens onto the cell while immersing it. In this article, we explore the limits of this type of SOE and propose an evolved version, which we dub the Hybrid Dome Reflector (HDR), which offers advantages especially for high concentration modules with large cells, where reflective secondaries do not offer sufficient acceptance angle, but other dielectric secondaries, such as the Dielectric Totally Internally Reflecting Concentrator DTIRC, may be too large for economical manufacture. We discuss aspects of HDR design and share selected ray-tracing simulations and experimental results. We show that the new HDR design improves acceptance angle and tolerances to manufacturing error...

Experimental characterization of achromatic doublet on glass (ADG) Fresnel lenses

In this paper we present a comprehensive experimental characterization of Achromatic Doublet on Glass (ADG) Fresnel lenses. When compared to a Silicone on Glass (SoG) Fresnel lens, the ADG Fresnel lens shows higher tolerance to displacements of the lens with respect to the optimal lens-to-cell distance. Furthermore, lower sensitivity of the ADG Fresnel lens to temperature variations has been experimentally proven.

Rating of CPV modules: Results of module round robins

The results of three CPV module round robins are presented. Ten test labs around the world participated to the round robins in total. Each round robin used a different CPV module technology (Daido Steel, Soitec, Suncore). The data gathered at the test labs was used to test CSOC power rating procedures as basis for the IEC draft standard 62670-3. The deviation between the minimum and the maximum power output rated at the test labs was in average 4.4 % with a standard deviation of 1.8 %abs. This underlines that power ratings or CPV modules are reliable and reproducible.

A novel achromatic Fresnel lens for high concentrating photovoltaic systems

In this paper we present a novel manufacturing method to produce achromatic Fresnel lenses for photovoltaic application. These achromatic lenses are capable of reaching a concentration factor three times higher than that attained by a conventional Silicone-on-Glass (SOG) Fresnel lens. The manufacturing method presented to fabricate the achromatic lens, which we refer to as Achromatic Doublet on Glass (ADG) Fresnel lens, is simple, cost-effective and highly scalable. A comprehensive ray-tracing analysis and its comparison with experimental results is presented in this work.

Temperature effects on two-stage optics made of silicone

This paper presents the experimental characterization of the efficiency thermal sensitivity of a system whose optics is composed of a silicone on glass Fresnel lens and a secondary element also made of silicone. In addition, some indications are provided on the adequate chemical formulation of the silicone that withstands longtime outdoor exposure under concentrated light without showing appreciable degradation.