A 6-focus high-concentration photovoltaic-thermal dish system Max Schmitz a , Nicolay Wiik a , Gianluca Ambrosetti b , Andrea Pedretti b , Stephan Paredes c , Patrick Ruch c , Bruno Michel c , Aldo Steinfeld a,⇑ a ETH Zurich, Dept. of Mechanical and Process Engineering, 8092 Zurich, Switzerland b DSolar Ltd, Via Industria 10, 6710 Biasca, Switzerland c IBM Research – Zurich, 8803 Rüschlikon, Switzerland article info Article history: Received 10 April 2017 Accepted 29 May 2017 Keywords: Solar dish CPV Photovoltaic-thermal Optical design Characterization Performance modeling abstract We present the design, optical characterization and full-system modeling of a novel 6-focus, high-concentration photovoltaic-thermal solar polygeneration system, aiming at an energy-efficient and cost-effective utilization of the solar resource. Essential to this system is a compact, modular solar dish con- centrator design optimized for mass-production, structural rigidity, and scalability, with a high geometric concentration ratio of 1733 at each of its six receivers. Every receiver comprises 36 triple-junction CPV cells, interconnected in a unique hybrid parallel-serial scheme that mitigates mismatch losses caused by non-uniform irradiance distributions. Cogeneration is enabled by using high-performance microchannel heat exchangers, allowing the extraction of low grade heat for secondary thermal processes. The tested pro- totype achieves an average solar radiative flux of 1374 suns on each of the receivers. By optimizing several design parameters, the CPV-thermal system can deliver a solar-to-electricity conversion efficiency of 28.5% in PV-only mode and 26.6% in cogeneration mode while extracting heat at 89.8 °C, and a power of 12.1 kW el and 11.3 kW el /21.5 kW th respectively, matching the performance of state of the art CPV commercial systems, while striving towards a reduction of the investment costs. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction The fundamental principle behind concentrating photovoltaics (CPV) is the substitution of expensive cell area with inexpensive optics (Luque and Andreev, 2007; Pérez-Higueras and Fernández, 2015). As a consequence, high-efficiency multi-junction cells, with a recently demonstrated record photovoltaic efficiency of 46.0% (Green et al., 2016), can be employed. However, in order for an eco- nomic benefit to be upheld, several requirements need to be met. The cost per active area of the optical concentrator has to be sub- stantially lower than the cost per area of the cells to offset the additional system costs (Cooper et al., 2016; Ittner, 1980), mandat- ing the use of inexpensive materials, efficient fabrication and assembly, high concentrations, and suggesting the use of large con- centrator apertures to reduce the specific number of system com- ponents. While a high concentration – beyond several 100 suns – is a key driving factor for cost decrease due to the reduced cell area, it introduces the problem of thermal management of the cells (Algora, 2004; Ittner, 1980). To maintain safe operating conditions at high photovoltaic efficiencies, active cell cooling is required to evacuate the produced heat, which, even with the highest- efficiency cells, is typically more than 50% of the total incident solar radiation, from the dense-arrays. Conversely, if heat can be efficiently extracted at high enough temperatures using photovoltaic-thermal (PVT) receivers (Paredes et al., 2015; Zimmermann et al., 2015), this heat can then be used further in applications such as space heating, cooling, and water desalination, and increase the overall solar resource utilization (Mittelman et al., 2009; Mittelman et al., 2007; Ong et al., 2012). In this work, we address the aforementioned challenges in an effort to develop an energy-efficient and cost-effective viable solu- tion for solar energy utilization relying on polygeneration of elec- tricity and heat. We present the design concept of a novel modular, multi-focus solar dish for CPV applications. Characteriza- tion and modeling of its optical components is coupled to a CPV receiver model for forecasting the on-sun performance under var- ious operating conditions and for design optimization. 2. A modular 6-focus solar dish concentrator 2.1. Multi-module asymmetric solar dish Line focus (2D) designs can in theory reach a geometrical con- centration of C g,ideal,2D = 1/sinh i = 215  for a half-acceptance angle h i = 4.65 mrad (Winston et al., 2005). However, practical designs http://dx.doi.org/10.1016/j.solener.2017.05.087 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: aldo.steinfeld@ethz.ch (A. Steinfeld). Solar Energy 155 (2017) 445–463 Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener