Introductory Chapter: Energy Consumption, Conversion, Storage, and Efficiency

1. Introduction

Energy has emerged as the unspoken life force that sustains our world in the modern era. From powering our homes, propelling our vehicles, and fueling the engines of industry, energy underpins nearly every aspect of our lives. Energy influences our economies, geopolitics, and environment, and the importance of energy cannot be overstated. The global growth of energy demand is accompanied by rising prosperity and better life quality. Energy consumption, storage, conversion, and efficiency are interconnected components of the world energy system, each playing an important role in shaping our energy landscape. This chapter presents an introductory review of energy consumption, storage, conversion, and efficiency, inviting us on a journey into the intricate interplay of energy within our society.

2. Energy consumption

Energy consumption encompasses the utilization of energy in various forms for meeting human needs and fulfilling societal demands. This includes the utilization of electricity, natural gas, coal, oil, biomass and renewable solar, and wind, hydro, and geothermal energy as well. Energy consumption is directly linked to the prosperity of the world and is at the heart of our modern civilization. However, energy consumption is closely related to environmental problems. For example, carbon dioxide, the by-product of burning of fossil fuels, is the major cause of global warming or climate change [1].

The United States Department of Energy (DOE) publishes the nation’s energy consumption and energy sources every year [2]. The US consumes 97.3 quadrillion BTU of energy in 2021, about 5% increase from 2020. A remarkable finding is that the renewable energy portion, including solar, wind, and biomass energy, increases significantly by more than 10%. This increase is consistent with the increasing public awareness of energy consumption and its environmental problems. Sustaining the growing energy consumption in a safe and environmentally responsible way remains a key challenge. Addressing this challenge requires a multifaceted approach that prioritizes the conversion of renewable energy into electricity, efficient energy storage, and sustainable and efficient energy consumption.

3. Energy conversion

Energy conversion involves transforming one form of energy into another to meet specific energy needs and applications. According to the first law of thermodynamics, energy can be transformed into different forms, but it cannot be created or destroyed [3]. Mechanical, chemical, thermal, optical, and electrical energy conversion processes take place around us all the time. Car engines transform chemical energy into mechanical energy. Solar panels convert optical energy directly into electricity. Space heaters convert electricity into thermal energy. LED bulbs convert electricity into optical or light energy.

Among these energy conversions, the conversion of various forms of energy into electricity is particularly interesting because electricity is as important as water and food to people in the modern era. Electricity is used for heating, cooling, and lighting and for operating cell phones and appliances in our daily lives. There are many different types of electric generators. The vast majority of electrical generators are based on Faraday’s law of induction discovered in 1831 [4]. Other types of electrical generators include photovoltaic cells, fuel cells, and thermoelectric generators. Photovoltaic cells convert solar energy directly into electricity through the photovoltaic effect. Fuel cells produce electricity through an electrochemical process wherein a hydrocarbon fuel reacts with oxygen. A thermoelectric generator is composed of p- and n-type semiconductor legs that convert thermal energy directly into electricity through the Seebeck effect.

Renewable energy such as solar, wind, and geothermal energy has become one of the fastest growing sources of electricity generation around the world. Enhancing the efficiency and sustainability of energy conversion processes is critical for reducing greenhouse gas emissions, minimizing resource depletion, and mitigating environmental impacts.

4. Energy storage

The surplus energy is stored in different forms and can be used later as needed. There are many different types of energy storage, including batteries and thermal and mechanical systems. The first electrochemical battery, storing and releasing a charge through a chemical reaction, was invented in 1800 by Italian chemist Alessandro Volta [5]. The modern high-energy density lithium-ion batteries were commercialized in the 1970s [6]. Now, the batteries have been widely deployed in applications ranging from electric vehicles and consumer electronics to grid-scale energy storage projects.

Thermal energy storage systems store heat or cold. There are three main types of thermal energy systems: sensible heat, latent heat, and thermo-chemical heat storage. A water tank is often used to store sensible heat, while phase change materials are for latent heat storage. Thermo-chemical energy storage systems store heat or cold through reversible chemical reactions. The main types of mechanical energy storage systems are pumped hydro, flywheel, and compressed air. The pumped hydro storage uses two reservoirs with one at a higher elevation than the other and currently is the most widely used large-scale mechanical energy storage system [7].

Energy storage plays a pivotal role in bridging the gap between energy supply and demand, enabling the efficient utilization of various forms of energy. Energy storage technologies find applications across diverse sectors, including transportation, residential, commercial, industrial, and utility-scale energy systems. Many challenges to wide deployment of energy storage systems remain to be addressed. These include cost competitiveness, technological limitations, regulatory barriers, grid integration issues, and environmental considerations.

5. Energy efficiency

Energy efficiency refers to the use of less energy to achieve maximum output, thereby reducing costs, producing less waste, and mitigating environmental impacts. Technological advancements in energy-efficient appliances and industrial processes drive improvements in energy efficiency. For example, LED (light emitting diode) lighting consumes significantly less electricity compared to traditional incandescent lighting while providing the same illumination. Adding thermal insulation material in walls and roofs can minimize heat loss in buildings and then maintain the desired temperature more efficiently. Utilizing fuel-efficient vehicles, such as hybrid and electric vehicles, reduces fuel consumption and emissions. Combined heat and power systems generate both electricity and useful heat that may maximize energy efficiency compared to separate generation of heat and power.

The efficiency of electric generation systems is particularly interesting as electricity serves as the backbone of industries, infrastructure, transportation, and communication in the world. The second law of thermodynamics puts an upper limit on achievable efficiencies of heat engines and then heat engine-based electric generators such as steam turbine generators, gas turbine generators, and internal combustion engine generators [3]. The energy efficiency of electric generators depends on various factors including the type of generators, operating conditions, design, and technology. Continuous efforts to improve efficiency through advancements in technology and operational practices are crucial for reducing energy consumption and minimizing environmental impact.