Concentration photovoltaic optical system irradiance distribution measurements and its effect on multi‐junction solar cells

ABSTRACT This paper proposes an indoor procedure based on charge‐coupled device camera measurements to characterize thenon‐uniform light patterns produced by optical systems used in concentration photovoltaic (CPV) systems. Theseirradiance patterns are reproduced on CPV solar cells for their characterization at concentrated irradiances by usinga concentrator cell tester and placing high‐resolution masks over the cells. Measured losses based on the masksmethod are compared with losses in concentrator optical systems measured by using the Helios 3198 solar simulator forCPV modules. Copyright © 2011 John Wiley & Sons, Ltd. KEYWORDSconcentration photovoltaic systems; instrumentation, measurement, and metrology; CCD; multi‐junction cell; irradiance profile*CorrespondenceRebeca Herrero, Instituto de Energia Solar, Universidad Politecnica de Madrid, Madrid, Spain.E‐mail: rebeca.herrero@ies‐def.upm.esReceived 9 March 2011; Revised 29 March 2011 1. INTRODUCTION In concentration photovoltaic (CPV) systems, low‐cost op-tical elements concentrate light onto high‐efficiency solarcells, with the goal of producing a cost‐effective solutionfor solar energy production. These optical systems mustbe evaluated in terms of optical efficiency, angular trans-mission, and irradiance distribution on the solar cell,among other things [1]. An important concern in opticaldesigns for CPV systems is the non‐uniformity of the irra-diance distribution on the cell, which can cause loss of ef-ficiency due to an increase in the effective series resistance.Tunnel diodes within the structure of multi‐junction (MJ)cells commonly used in CPV also impose a limitation onmaximum local irradiance over the cell [2]. In addition,depending on spectral differences on light profile distribu-tions caused by refractive optics over top, middle, and bot-tom sub‐cells, the MJ cell efficiency could decrease due tocurrent mismatch between junctions [3]. As a result of thisconcern, many CPV optical systems are designed to pro-duce uniform irradiance distribution on the cell by meansof using a kaleidoscopic stage or Kohler integration [4].For a given concentration ratio and acceptance angle spec-ification, the uniform flux condition increases the numberand complexity of the optical surfaces and could increasecost.This paper seeks to characterize the electrical losscaused by non‐uniform irradiance profiles in order to re-veal the impact of the cell performance degradation on op-eration, compared with uniform light pattern on the cell.This information should allow CPV system designers tochoose an optimum solution in the trade‐off between cost,complexity, and illumination uniformity.In the past, the effects of having non‐uniform light dis-tribution on CPV systems have been evaluated by simula-tions based on distributed circuit models [5]. Experimentalresults have been obtained by measuring the I–V curve ofan MJ solar cell under localized high flux intensity(10000 times the standard solar irradiance) applied tosmall size MJ cells in which contribution of contact metal-lization to series resistance is negligible [6] and also withthe objective of determining the tunnel diode limit [7].

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