Performance of CPV modules based on vertical multi-junction cells under non-uniform illumination

Abstract

High-voltage silicon Vertical Multi-Junction (VMJ) cells can accept high concentration with peak efficiency potentially reaching close to 30%. Dense-array modules with high-voltage cells should allow a parallel connection with voltage matching rather than series connection with current matching, leading to lower mismatch losses under non-uniform illumination. The performance of a dense array with VMJ cells connected in parallel under non-uniform illumination is investigated, in comparison to a module with conventional cells connected in series. The incident flux distribution corresponds to that of a parabolic dish with and without a homogenizer. The number of junctions in each VMJ cell is a free parameter for optimization. The results show a clear advantage of the VMJ module over the conventional module under non-uniform illumination, allowing reduction or even elimination of homogenizing secondary optics, and easing the tracking accuracy requirements.

Introduction

Non-uniform or unequal illumination is a well-known problem in photovoltaic systems (Louis and Bucciarelli, 1979). When a cell array is subject to non-uniform illumination, series connection of cells in the array will result in current mismatch among the cells, leading to severe degradation in system performance, as well as danger of cell damage due to reverse-bias operation and overheating (Kovach and Schmid, 1996). In standard PV panels, cells may receive different illumination due to mutual shading by neighboring panels, or due to shading by adjacent objects (Woyte et al., 2003). In dense-array concentrating photovoltaic (CPV) systems, the non-uniformity in incident flux on the cells may result from the inherent optical behavior of the concentrator, and due to tracking inaccuracies. This is less of an issue in single optic/single cell CPV systems, as long as all optical units are reasonably well aligned and produce the same incident power for all cells. In dense array CPV systems, a known solution is to use an optical flux homogenizer (Ries et al., 1997). Although such a device can improve the flux homogeneity on the cell array, the introduction of an additional optical device will inflict additional losses, which can reach 10% or more for typical designs (Kreske, 2002), as well as additional cost and complexity.

A common method to protect cell arrays from reverse bias damage under non-homogeneous illumination is to install bypass diodes parallel to each cell or a string of cells. This measure protects cells against damage but does not fully recover the power lost due to current mismatch. Non-standard arrangements and connections of the cells were proposed in an attempt to match the expected flux distribution, for example a radial receiver with custom-shaped cells that divide the incident flux evenly between the cells (Vivar et al., 2010); however, such designs are vulnerable to optical misalignment and tracking errors. Fitting each cell in the array with an individual DC–DC converter (Salemi et al., 2011) can theoretically allow optimal operation of each cell independent of its neighbors, but obviously this leads to excessive cost and complexity.

The source for the series mismatch problem is the fact that PV cells typically offer low voltage (around 0.6 V for silicon cells, and around 3 V for III–V multi-junction cells), and therefore need to be connected in series to produce an overall high voltage of the module. If individual cells could provide high voltage and low current, then they could be connected in parallel rather than in series and still provide a reasonably high module output voltage. This latter arrangement leads to voltage matching rather than current matching within the module. Since cell voltage is less sensitive to illumination, voltage matching should produce lower performance degradation under non-uniform illumination, compared to the series connection used in conventional dense array modules. Two architectures have been proposed for high voltage cells: the Vertical Multi-Junction (VMJ) cell, and the Monolithic Inline Module (MIM) cell.

Silicon VMJ cells have been proposed since the 1970s (Sater et al., 1973, Gover and Stella, 1974). Such cells can be produced in a process of stacking multiple wafers, followed by orthogonal cutting (Sater and Sater, 2002). The number of wafers in the stack, which is identical to the number of junctions connected in series in the final cell, determines the cell voltage. The high cell voltage corresponds to low cell current, allowing the VMJ cell to operate under high concentration, without the severe degradation due to series resistance that is experienced by conventional cells under concentration. Silicon VMJ cells have shown capability to operate under high concentration of up to 2500 suns, with peak efficiency of about 20% (Sater and Sater, 2002). An alternative approach for producing the VMJ structure is by monolithic fabrication on a single wafer (Pozner et al., 2012), where the junction geometry can be optimized with greater freedom compared to the wafer stacking approach. The efficiency of geometrically optimized and monolithically produced VMJ cells was predicted to reach close to 30% under concentration of about 1000. While analyses of vertical junctions and VMJ cells exist, the performance of a module composed of VMJ cells under non-uniform illumination, and in particular the impact of series vs. parallel connections, has not been addressed.

The MIM is a high-voltage design that relies on segmenting a conventional horizontal junction with trenches, and adding a series connection using top-to-bottom electrical links across the trenches (Keller et al., 2000, Ortega et al., 2008). Although high output voltage was achieved in MIMs, reported conversion efficiencies were in the range of 7–12% in silicon (Keller et al., 2000, Ortega et al., 2008). MIMs based on GaAs and dual junction tandem cells have shown efficiencies up to 22% and 26% under high concentration (Loeckenhoff et al., 2008, Helmers et al., 2010). The mismatch losses in a MIM-like series connected GaAs PV cells illuminated by laser were evaluated (Peña and Algora, 2003), but the main focus of that work is power transmission application in fiber optics systems, where the illumination input is a uniform disc that is misaligned with a circular receiver. Hence, these results cannot be adapted to CPV applications where the flux non-uniformities are of a different nature.

In this work we model the performance of a CPV module made of monolithic silicon VMJ cells (Pozner et al., 2012), electrically connected in parallel under non-uniform concentrated illumination produced by a parabolic dish concentrator with a reflective homogenizer. The performance of this module, and in particular the loss attributed to electrical mismatch between the cells, is compared to the performance of a module made from conventional cells connected in series under the same incident radiation distribution. The effect of tracking errors on a module made of VMJ cells is also presented.