Abstract

Solar energy is one of the most abundant, clean, and widespread energy in the world, which has the potential to address the issues of environmental pollution, global warming, and energy crisis, while the intermittent distribution of solar energy in time and space limits its utilization. Among various approaches of solar energy utilization, converting solar energy into chemical fuel (e.g., hydrogen) by thermochemical approach could maintain the steady and high-efficient energy supply and can make use of the full-spectrum solar energy. The research about solar thermochemical fuel generation lasts more than 40 years, and lots of reaction system and reactors have been proposed. This chapter reviews the state-of-the-art progress of solar thermochemical fuel generation, and the characteristics of different systems have been compared and discussed, which may give systematical insight into the development and improvement of solar fuel generation by thermochemical approach in the future.

Keywords: solar energy, solar fuel, solar thermochemistry, thermochemical fuel, solar membrane reactor

## 1. Introduction

### 1.1 Energy and environmental problems and solutions

Due to population growth and rapid industrial development, world energy demand has increased significantly. Compared with the previous generation, the world's population has increased rapidly by 2 billion [1], and this mainly comes from the population growth of developing countries, and rapid population growth poses more severe challenges to increasingly scarce energy and resource supplies. The importance of energy for social development is self-evident. In order to ensure the supply of energy, a large amount of fossil energy is used, which at the same time has a serious impact on the environment, leading to increasingly serious problems of atmospheric pollution and the greenhouse effect. Therefore, improving energy efficiency and using more clean energy and exploring a sustainable development path compatible with energy use and the environment have become one of the important topics in energy science research.

The efficient use of renewable energy is of great significance. Among the many renewable energy sources, solar energy has become one of the best choices for future energy sources with its unique advantages, which is the most abundant renewable energy source and widely distributed.
