Thermal efficiency

Efficiency diagram by Zureks

Carnot heat engine 2

Thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a steam turbine, or a steam engine, a boiler, a furnace, or a refrigerator for example. Specifically, it is the ratio of the work output to the heat input to the system, expressed as a percentage. This concept is significant in engineering and physics, particularly in the fields of mechanical engineering, thermal engineering, and energy engineering.

Definition

The thermal efficiency (\(\eta\)) of a system or process is defined as the ratio of the output work or useful energy (\(W\)) to the input heat energy (\(Q_{in}\)). Mathematically, it is expressed as:

\[\eta = \frac{W}{Q_{in}}\]

where:

• \(\eta\) is the thermal efficiency,
• \(W\) is the work output or useful energy output,
• \(Q_{in}\) is the heat input.

It is important to note that due to the Second Law of Thermodynamics, no real-world engine can convert 100% of its heat input into work, leading to the inevitable conclusion that all real engines have a thermal efficiency of less than 100%.

Factors Affecting Thermal Efficiency

Several factors can affect the thermal efficiency of a system, including:

• Carnot Efficiency: The theoretical maximum efficiency that a heat engine can achieve, which depends only on the temperatures of the hot and cold reservoirs.
• Heat Losses: In practical applications, heat losses to the surroundings always occur, reducing the actual efficiency.
• Friction and mechanical losses within the system also reduce efficiency.
• The quality of the working fluid and the working conditions, such as pressure and temperature, can also influence the efficiency.

Applications

Thermal efficiency is a critical parameter in the design and operation of various systems:

• In internal combustion engines, improving thermal efficiency can lead to better fuel economy and lower emissions.
• In power plants, higher thermal efficiency means more electricity can be generated from a given amount of fuel.
• In heating, ventilation, and air conditioning (HVAC) systems, higher efficiency can reduce energy consumption and operating costs.

Improving Thermal Efficiency

Improvements in thermal efficiency can be achieved through various means, such as:

• Using advanced materials that can withstand higher temperatures and reduce heat losses.
• Implementing regenerative braking in vehicles, which recovers kinetic energy as electrical energy, which can then be used to improve fuel efficiency.
• Employing combined cycle power plants that utilize both gas and steam turbines to generate electricity, thereby increasing the overall efficiency of the power plant.

See Also

• Energy conversion efficiency
• Heat engine
• Carnot cycle
• Entropy
• Second Law of Thermodynamics

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