**Preface to "Challenge and Research Trends of Solar Concentrators"**

Challenge and research trends of primary and secondary solar concentrators are key issues for advanced solar energy research. The main purpose of the book is therefore to present some of the most recent developments related to solar concentrators. This book is designed to provide the readers a better understanding of solar concentrators and their impacts on solar-pumped lasers. The chapter order follows a logical flow of ideas from novel primary solar concentrators to novel secondary concentrators, concluding finally with the design details of novel solar-pumped lasers.

1. Novel primary solar concentrators for high solar flux applications

A novel three-dimensional elliptical-shaped Fresnel lens (ESFL) analytical model was presented to evaluate and maximize the solar energy concentration of Fresnel-lens-based solar concentrators.

2. Novel compound primary solar concentrator for pumping a solar laser

Significant numerical improvement in end-side-pumped solar laser collection efficiency and beam brightness was presented by combining a Fresnel lens and a modified parabolic mirror.

3. Novel fixed solar concentrator for illumination

A fixed fiber light guide system using concave outlet concentrators as its receiving unit was proposed. The absence of a tracking structure highlights this research.

4. Novel luminescent concentrator

The authors show that by shaping the Luminescent Solar Concentrators (LSCs) in the form of an elliptic array, its emission losses can be drastically reduced.

5. Conical secondary concentrator for thermal applications

In this study, thermal performance of a conical solar collector was assessed with a new design of concentric tube absorber and the results were compared to the existing circular tube absorber.

6. New progress in solar-pumped lasers by NOVA University of Lisbon

6.1. This study focuses on the influence of two secondary concentrators: an aspherical lens and a rectangular light guide. A Ce:Nd:YAG solar laser pumped through the rectangular fused silica light guide was experimentally investigated, attaining 40 W solar laser output power.

6.2. Most efficient solar laser emission from a single Ce:Nd:YAG rod

The utilization of a small diameter Ce:Nd:YAG rod was essential to significantly enhance solar laser efficiency, attaining 4.5% record solar-to-laser power conversion efficiency.

6.3. Novel solar concentrator design for the production of doughnut-shaped and top-hat solar laser beams. The first numerical simulations of doughnut-shaped and top-hat solar laser beam profiles were accomplished through both ZEMAX® and LASCAD® analysis.

6.4. Novel solar concentrator design for the emission of 5 kW-class TEM00 mode solar laser beams from one megawatt solar furnace

A novel multi-rod solar laser pumping concept was proposed to significantly improve the TEM00 mode solar laser power level and its beam brightness by novel zigzag beam merging technique.

In summary, this book may empower the readers with up-to-date approaches for designing and implementing the next generation solar concentrators, as well as solar-pumped lasers.

The book would not be possible without the valuable contributions of all authors. Sustained helps from MDPI editorial teams are highly appreciated.

#### **Dawei Liang and Changming Zhao** *Editors*
