What is thermal efficiency?

Short Answer:

Thermal efficiency is the measure of how effectively a system converts heat energy (thermal energy) into useful work or output energy. It is the ratio of the useful work output to the total heat energy supplied. The higher the thermal efficiency, the better the system uses heat for doing work.

In practical systems like engines, turbines, or boilers, not all the supplied heat can be converted into useful work because some heat is lost to the surroundings. Thermal efficiency is usually expressed as a percentage, and it helps to compare the performance of different heat-based systems or machines.

Detailed Explanation :

Thermal Efficiency

Thermal efficiency is an important concept in thermodynamics and mechanical engineering. It describes how efficiently a device, engine, or system converts the input heat energy into useful work output. In all real processes, some energy is always lost due to heat dissipation, friction, or other inefficiencies, so the thermal efficiency of any real system is always less than 100%.

The formula for thermal efficiency is given by:

This efficiency helps engineers and designers understand how well an energy conversion device performs and how much of the supplied energy is wasted as losses.

Concept of Thermal Efficiency

When a system like a heat engine or turbine receives heat energy from a source (for example, burning fuel or steam), part of this energy is converted into mechanical work, and the rest is lost to the surroundings, usually as waste heat.

For example, in a steam power plant, heat from burning coal or oil is used to produce steam. The steam expands in a turbine to generate electricity. However, only a portion of the input heat is converted into useful work — the rest is lost through exhaust steam, cooling water, and friction.

Therefore, thermal efficiency measures how effectively the energy from the fuel or heat source is used to produce useful work.

Mathematical Representation

Where,

  • η = Thermal efficiency (%)
  • Wₒᵤₜ = Useful work or energy output (Joules or kJ)
  • Qᵢₙ = Total heat energy input (Joules or kJ)

In words, the formula states that thermal efficiency is the percentage of input heat energy that is converted into useful work.

For example:
If a heat engine receives 1000 kJ of heat energy and produces 300 kJ of useful work,

This means 30% of the input heat is converted into useful work, and the remaining 70% is lost.

Thermal Efficiency in Different Systems

  1. Heat Engines:
    • Convert thermal energy into mechanical work.
    • Examples: Internal combustion engines, steam engines, gas turbines.
    • Their efficiency depends on temperature difference between the heat source and the sink.
  2. Boilers:
    • In a boiler, efficiency indicates how effectively heat energy from fuel combustion is transferred to the steam.
    • Boiler efficiency typically ranges between 70% to 90%.
  3. Refrigeration and Heat Pumps:
    • For these systems, efficiency is expressed differently using Coefficient of Performance (COP) instead of thermal efficiency.
  4. Power Plants:
    • In thermal power plants, efficiency depends on the working cycle such as Rankine cycle or Brayton cycle.
    • Typical efficiency of steam power plants is around 35–40%, while modern combined cycle plants may reach 55–60%.

Factors Affecting Thermal Efficiency

  1. Temperature Difference:
    • Greater the difference between the source temperature and sink temperature, higher the possible efficiency.
    • This is the basis of the Carnot principle.
  2. Heat Losses:
    • Energy lost through exhaust gases, cooling water, or friction reduces efficiency.
  3. Design of the System:
    • Proper insulation, high-quality materials, and optimized design improve efficiency.
  4. Type of Working Fluid:
    • Fluids like steam or gas influence heat transfer and conversion efficiency.
  5. Operating Conditions:
    • Pressure, temperature, and load conditions affect overall system efficiency.

Carnot Efficiency (Theoretical Maximum)

The Carnot efficiency represents the maximum possible thermal efficiency that any heat engine can achieve operating between two temperature limits. It is given by:

Where,

  • Tₕₒₜ = Temperature of heat source (in Kelvin)
  • Tₗₒₗd = Temperature of heat sink (in Kelvin)

This formula shows that efficiency increases as the temperature difference between the heat source and sink increases. However, even the Carnot cycle cannot reach 100% efficiency, because some heat must always be rejected to the surroundings.

For example:
If a heat engine works between 1000 K and 300 K,

This means no real engine operating between these temperatures can exceed 70% efficiency.

Examples of Thermal Efficiency in Real Systems

  1. Steam Turbine Power Plant:
    • Efficiency ranges between 35% to 40%.
    • Losses occur due to exhaust steam and heat rejection to cooling water.
  2. Diesel Engine:
    • Efficiency about 35% to 45%.
    • Some energy lost through exhaust gases and friction.
  3. Petrol Engine (Otto Cycle):
    • Efficiency between 25% and 30%.
    • Limited by high exhaust heat and incomplete combustion.
  4. Gas Turbine (Brayton Cycle):
    • Efficiency about 30% to 40%, improved to 55% with combined cycles.
  5. Boiler System:
    • Efficiency depends on fuel type and design, ranging from 70% to 90%.

Improving Thermal Efficiency

  1. Increasing Source Temperature:
    • Using higher temperature fuels or advanced combustion methods.
  2. Reducing Heat Losses:
    • Proper insulation and minimizing exhaust losses.
  3. Combined Cycles:
    • Using waste heat from one cycle to power another (e.g., gas-steam combined plants).
  4. Better Material and Design:
    • Use of heat-resistant materials and advanced blade designs in turbines.
  5. Regular Maintenance:
    • Cleaning, lubrication, and monitoring of system components reduce friction and improve performance.
Conclusion :

Thermal efficiency is a key indicator of how well a heat-based system or engine converts heat energy into useful work. It helps determine the performance, economy, and environmental impact of machines such as engines, turbines, and boilers.

Although no real system can reach 100% efficiency due to unavoidable losses, engineers continually improve design, materials, and technology to approach the theoretical limit. Understanding and enhancing thermal efficiency is essential for achieving energy conservation, cost reduction, and sustainable engineering operations.