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Dish-based high concentration PV system with Köhler optics

Blake M. Coughenour, Thomas Stalcup, Brian Wheelwright, Andrew Geary, Kimberly Hammer, and Roger Angel

Open Access Open Access

Abstract

We present work at the Steward Observatory Solar Lab on a high concentration photovoltaic system in which sunlight focused by a single large paraboloidal mirror powers many small triple-junction cells. The optical system is of the XRX-Köhler type, comprising the primary reflector (X) and a ball lens (R) at the focus that reimages the primary reflector onto an array of small reflectors (X) that apportion the light to the cells. We present a design methodology that provides generous tolerance to mis-pointing, uniform illumination across individual cells, minimal optical loss and even distribution between cells, for efficient series connection. An operational prototype has been constructed with a 3.3m x 3.3m square primary reflector of 2m focal length powering 36 actively cooled triple-junction cells at 1200x concentration (geometric). The measured end-to-end system conversion efficiency is 28%, including the parasitic loss of the active cooling system. Efficiency ~32% is projected for the next system.

© 2014 Optical Society of America

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Figures (9)
Fig. 1
Fig. 1 Paraboloidal dish concentrator with a central flat receiver.

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Fig. 2
Fig. 2 XRX-Köhler advantages. Shown is the imaging property of the projection lens (a) along with parallel ray concentration through the lens for on-axis (b) and off-axis (c) rays.

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Fig. 3
Fig. 3 XRX-Köhler concentrator XTP design. On-axis rays reflect off the XTP to evenly and symmetrically illuminate the solar cell for both on-axis (a) and off-axis (b) illumination.

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Fig. 4
Fig. 4 XRX-Köhler system design.

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Fig. 5
Fig. 5 XRX-Köhler XTP Design

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Fig. 6
Fig. 6 Univ. of Arizona XRX-Köhler Prototype (a). Dish reflector with ball lens at focus (b). Glass slumped segment of dish reflector (c), ball lens (d), XTP array (e), and single MJ cell (f).

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Fig. 7
Fig. 7 XTP Array (a) and diagonal cross-section (b) detailing segmented cell array concentration balance from decreasing XTP concentration matching as angle β increases.

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Fig. 8
Fig. 8 Gen 2 system measured acceptance angle (a), example daily power output (b), and cell operating temperature compared to ambient temperature throughout the day (c).

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Fig. 9
Fig. 9 Gen 3 handheld PCU (a), and renderings of two 6.4kW power generator units with 8 PCU’s and mirrors (b) and PCU active cooling pathways (c)

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Tables (1)

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Table 1 Comparison of Component Part Reduction of 3rd Generation PCU

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Equations (18)

Equations on this page are rendered with MathJax. Learn more.

ξ= n 2 ∬ pupil cosθd A S dΩ  ,
C= A A'  .
C= A A' = n ′ 2 n 2 sin 2 θ ′ sin 2 θ  .
C≤ C max = n ′ 2 sin 2 θ  .
CAP= C sin θ A  ,
CA P * = C sin θ A *  ,
θ A = a 2f  ,
ef l ball−lens = nR 2(n−1) = 1.458a 2(0.458) =1.59a .
b=1.546a .
s= f 2 [ 1+2 tan 2 ( β 2 )+ tan 4 ( β 2 ) ]  .
C 1 = A A ′ =[ s 2 / b 2 cos( β/2 ) ]cos( β 2 )= ( f b ) 2 [ 1+2 tan 2 ( β 2 )+ tan 4 ( β 2 ) ] .
C 1edge C 1center =1+ 1 8 ( F # ) 2 + 1 256 ( F # ) 4  .
C 1 ≈ 0.1 θ A 2  .
C 2 = [ 1− 2t αb tan(ω) ] −2 .
1 2 b⋅sinα=t⋅tan(2ω+α/2 ) ,
t/ αb= 1 2 tan(2ω+α/2 ) .
C 2 = [ 1− tan(ω) / tan(2ω+α/ 2) ] −2 .
C=η C 1 C 2  .
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