The increasing need for building energy efficiency, the advancement of green technology, and the development of thermal energy-efficient systemsThermal energy-efficient systems are crucial in world today. These systems optimize the use of renewable energy and provide an effective solution for the common mismatch between building energy supply and demand. This chapter focuses on the design, development, validation, and demonstration of low-cost, modular thermal energy-efficient systemsThermal energy-efficient systems for various building applications, including heating, cooling, and domestic hot water generation, utilizing technologies such as solar collectors and highly efficient heat pumps. The primary goal for researchers and engineers is to develop energy-efficient thermal systems and processes with optimized designs to address rising energy costs and stringent greenhouse gas emissionGreenhouse gas emissions targets. Improving energy efficiency is essential for businesses to stay competitive, leading to substantial efforts in enhancing both overall systems and individual components. These improvements ultimately reduce energy consumption, costs, and environmental impact. This chapter provides a comprehensive discussion on thermal energy-efficient systemsThermal energy-efficient systems, detailing their description, working mechanisms, and applications.