Table of Contents
Introduction
Energy conversion is the process of changing energy from one form into another useful form. Modern societies depend on long chains of such conversions, from sunlight to electricity, from fuel to motion, and from electricity to light or heat. This chapter introduces the core ideas that describe and limit these conversions. The focus here is on physical principles, not on detailed descriptions of particular technologies, which are handled elsewhere in the course.
Forms of Energy and Conversion Pathways
Energy appears in many forms. For energy conversion, some of the most relevant forms are chemical energy in fuels and batteries, thermal energy in hot substances, mechanical energy in moving or elevated objects, radiant energy such as sunlight, and electrical energy in electric currents and fields.
An energy system usually converts energy through several stages. For example, in a coal power plant, chemical energy in coal becomes thermal energy in steam, then mechanical energy in a turbine, and finally electrical energy in the generator. In a solar photovoltaic system, radiant energy from the sun is converted directly into electrical energy in the solar cells, and then may be converted again into mechanical energy in an electric motor.
Each conversion step is governed by physical laws that determine how much energy is available, how much is lost, and in what form the energy appears at the end.
Conservation of Energy
The most fundamental principle for all energy conversions is conservation of energy. In a closed system, energy cannot be created or destroyed. It can only change form or move from one place to another.
This principle can be written as:
$$E_{\text{in}} = E_{\text{out}} + E_{\text{stored}}$$
where $E_{\text{in}}$ is the total energy entering a system, $E_{\text{out}}$ is the total energy leaving the system, and $E_{\text{stored}}$ is the change in energy stored inside the system.
In practical devices, what people often call “energy losses” are not losses in the sense of energy disappearing. Instead, useful energy is converted into less useful forms, such as waste heat spread out in the environment. The total energy is still conserved, but its ability to do useful work is reduced.
Energy cannot be created from nothing or destroyed. Every energy conversion must obey conservation of energy:
$$E_{\text{in}} = E_{\text{out}} + E_{\text{stored}}$$
This rule means that any device that claims to output more energy than it receives is not physically possible.
Work, Heat, and Useful Output
When we speak about the usefulness of energy, we often distinguish between work and heat. In simple terms, work is associated with ordered motion, such as turning a shaft or moving an object, while heat is associated with random motion of particles in a substance.
Energy conversion devices are usually designed to produce a certain useful output. In a power plant, the useful output is electrical work delivered to the grid. In a car engine, the useful output is mechanical work on the wheels. Heat can be useful as well, for example in heating buildings or in industrial processes, but low temperature heat that is spread out in the environment is usually difficult to use again.
The idea that not all energy is equally useful leads to the distinction between total energy and the portion of that energy that can actually be converted into work under given conditions. This portion is often called “available energy” or “exergy,” and it is closely linked to the rules described by the second law of thermodynamics.
The Second Law and Irreversibility
While conservation of energy tells us that total energy is preserved, it does not tell us anything about the direction in which processes occur or how much useful work can be extracted. The second law of thermodynamics fills this gap. It states that in natural processes, there is a tendency for energy to spread out and for systems to move toward more disordered states.
In the context of energy conversion, this means that every real process is irreversible and produces some waste heat. That waste heat represents useful energy that could not be turned into work. Even if there were no friction, no electrical resistance, and no other obvious “losses,” it would still be impossible to convert all of the input energy into useful work.
A typical example is a heat engine that converts heat into work. It operates between a hot source and a cold sink, for example between high temperature steam and the cooler environment. Even in an ideal case, the second law states that only a fraction of the heat from the hot source can be turned into work. The rest must be rejected to the cold sink.
No heat engine can convert all input heat into work. There must always be some heat rejected to a colder sink. This is a direct consequence of the second law of thermodynamics.
The difference between ideal and real processes is captured by the idea of irreversibility. A reversible process is a theoretical limit in which no entropy is produced, and every step can be exactly undone. Real devices always have irreversibilities, such as friction, turbulence, electrical resistance, and heat conduction across temperature differences, which reduce the maximum possible useful output.
Maximum Theoretical Efficiency of Heat Engines
For devices that convert heat into work, such as conventional thermal power plants or some renewable systems that rely on temperature differences, there is a theoretical upper limit to efficiency. This limit is described by the Carnot efficiency, named after the French engineer Sadi Carnot.
If a heat engine operates between a hot reservoir at absolute temperature $T_H$ and a cold reservoir at absolute temperature $T_C$, the maximum possible thermal efficiency is
$$\eta_{\text{Carnot}} = 1 - \frac{T_C}{T_H}$$
where temperatures are measured in kelvins (K), not in degrees Celsius.
For example, if the hot steam in a power plant is at $T_H = 873 \,\text{K}$ (which is 600 °C) and the environment is at $T_C = 298 \,\text{K}$ (about 25 °C), the maximum theoretical efficiency is
$$\eta_{\text{Carnot}} = 1 - \frac{298}{873} \approx 0.66$$
So even in perfect conditions, no more than about 66 percent of the heat input could be converted into work. Real power plants operate below this limit because of irreversibilities in turbines, generators, and other components.
For any heat engine operating between a hot temperature $T_H$ and a cold temperature $T_C$ (in kelvins), the maximum theoretical efficiency is:
$$\eta_{\text{max}} = 1 - \frac{T_C}{T_H}$$
No real device can exceed this limit.
This result has major implications for all thermal energy systems, including many fossil plants and some renewable technologies. It also shows why higher temperature sources and lower temperature sinks can improve the theoretical efficiency of such systems.
Conversion Chains and Cumulative Losses
In practical applications, energy usually passes through several conversions before providing the desired service. Each step has its own efficiency, which is defined more fully in another chapter. Here it is important to understand that the overall effectiveness of a conversion chain is the result of multiplying the efficiencies of each step.
Consider a simple chain where chemical energy in a fuel is converted to electricity in a power plant, and then to mechanical work in an electric motor. If the power plant has efficiency $\eta_1$ and the motor has efficiency $\eta_2$, the overall efficiency is
$$\eta_{\text{overall}} = \eta_1 \times \eta_2$$
If $\eta_1 = 0.40$ and $\eta_2 = 0.90$, then
$$\eta_{\text{overall}} = 0.40 \times 0.90 = 0.36$$
which means only 36 percent of the original chemical energy appears as mechanical work in the final device.
In a chain of conversions, overall efficiency equals the product of the efficiencies of each step:
$$\eta_{\text{overall}} = \eta_1 \times \eta_2 \times \eta_3 \times \dots$$
Small inefficiencies at each stage multiply to create large overall losses.
This idea helps explain why some energy pathways are more favorable than others. Shorter chains with fewer conversion steps, or chains that avoid inherently limited conversions from heat to work, can result in higher overall effectiveness.
Direct vs Indirect Conversion
Some technologies convert one form of energy directly into another, while others use several intermediate forms. Direct conversion often has fewer losses, but it may be limited by other physical or material constraints.
For example, solar photovoltaic cells convert radiant energy from the sun directly into electricity without an intermediate heat stage. In contrast, concentrated solar power plants first convert sunlight into heat, then heat into mechanical energy in a turbine, and finally mechanical energy into electricity in a generator.
Direct mechanical to electrical conversion, as in a wind turbine, avoids a thermal stage and therefore is not limited by the Carnot efficiency. However, other limits exist, such as the maximum fraction of wind energy that can be extracted, described by aerodynamic principles rather than thermal ones.
Understanding whether a technology relies on thermal conversion or on direct nonthermal conversion is essential to identify which physical limits apply. Thermal conversions are constrained by temperature differences and the Carnot limit, while nonthermal conversions have their own specific governing principles, such as electromagnetic or quantum effects.
Matching Energy Quality to Use
The concept of energy quality refers to how easily a form of energy can be converted into other forms, especially work. Electricity and mechanical energy are considered high quality because they can be converted into many other forms with high efficiency. Low temperature heat is generally considered low quality because it is difficult to use to produce work.
An important principle in energy conversion is to match the quality of the energy source to the quality required by the end use. Using high quality energy for low quality needs often wastes potential.
For instance, using electricity to generate low temperature heat for simple space heating can be very inefficient from a systems perspective if the electricity came from a thermal power plant, although technologies like heat pumps can change this picture. In contrast, using low temperature solar heat directly for water heating can make better use of the available energy, because the quality of the source roughly matches the quality demanded by the service.
In renewable energy planning, this idea encourages the direct use of renewable resources in forms that fit the final application whenever possible, in order to minimize unnecessary conversions and losses.
Storage as a Conversion Process
Energy storage, which is discussed in depth in another part of the course, is itself an energy conversion process. When energy is stored, it is converted from some input form into a stored form, and later converted back into a usable form.
For example, in a battery system, electrical energy is converted into chemical energy during charging, then back into electrical energy during discharging. In pumped hydro storage, electrical energy is converted into gravitational potential energy by pumping water uphill, then back into electrical energy when the water flows down through turbines.
Each of these steps has its own efficiency. The product of charging and discharging efficiency gives the round trip efficiency of the storage system.
Energy storage always involves one or more conversion steps. The round trip efficiency is less than 100 percent, so you always get back less useful energy than you put in.
Recognizing storage as a conversion process helps explain why it is important to minimize unnecessary storing and releasing of energy if direct use is possible, and why technological improvements that reduce conversion losses are valuable.
Energy Conversion and the Shift to Renewables
Although the basic physical principles apply equally to fossil and renewable energy systems, the way energy is converted in renewable systems can be quite different. Many renewable technologies harness natural energy flows more directly, for example sunlight to electricity or wind to mechanical energy. This can reduce the number of conversion steps and avoid some of the fundamental limits that apply to thermal systems.
At the same time, the variable nature of many renewable resources means that conversion often needs to be combined with storage, power electronics, and control systems. Each of these involves additional energy conversions and potential losses, so understanding conversion principles becomes even more important when designing efficient and reliable renewable energy systems.
In summary, energy conversion principles provide the common physical framework behind all energy technologies. They explain why some pathways are more efficient than others, why losses are unavoidable, and how the structure of conversion chains influences the overall performance of energy systems.