Energy efficiency plays a major role in addressing contemporary energy-related challenges. This book consists of ten chapters that explain the scientific understanding of thermodynamics and energy conversion methods to enhance energy efficiency. Chapter 1 provides a brief overview of the topic and introduces related issues covered by the book. Chapter 2 discusses torrefaction processes of empty fruit bunches under various temperature conditions. Chapter 3 presents how to reduce the ash content of torrefied pulverized palm kernel shells. Chapter 4 discusses the characteristics of thermo-chemical processes of oleochemical wastes into useful energy. Chapter 5 discusses the effects of natural gas constituent on its flame behaviour for efficient combustion processes. Chapter 6 examines the cooling effects created from a thermoelectric module (TEM). Chapter 7 examines the performance of a thermoelectric module in generating power. Chapter 8 presents friction force behaviours of a wood-based Prony Brake dynamometer. Chapter 9 presents an energy efficiency analysis for a combined cycle power plant. Chapter 10 presents basic energy economic concepts that are required in an energy investment decision in order to be sustainable. Readers with basic understanding of thermodynamics will recognize problems associated with several energy conversion processes described in this book and their roles to improve energy efficiency potentials.