Thermal efficiency facts for kids

Thermal efficiency is a way to measure how well a device uses energy. It's like asking: "How much useful work or heat did we get out compared to the energy we put in?"

Imagine you have a machine, like a car engine or a furnace. You put energy into it, usually as heat from fuel. The machine then gives you something useful back, like making the car move (mechanical work) or heating your house (heat).

Thermal efficiency is a number without units, usually between 0 and 1. When you see it as a percentage, it's between 0% and 100%.

Here's the simple idea: Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \text{Efficiency} = \frac{\text{What you get out}}{\text{What you put in}}

Because of basic physics rules, you can't get more energy out than you put in. So, the efficiency can never be more than 100%. In real life, it's always less than 100%. This is because some energy is always lost, for example, due to friction or as waste heat.

For example, a typical car engine might only be about 25% efficient. This means only a quarter of the energy from the gasoline actually helps move the car. The rest becomes heat that goes out the exhaust or warms up the engine. Large power plants that burn coal are around 36% efficient. Some advanced power plants, called combined cycle plants, can reach nearly 60% efficiency!

How Heat Engines Use Energy

A heat engine is a device that turns heat into movement or work. Think of a car engine or a steam turbine. The thermal efficiency of a heat engine tells you what percentage of the heat energy is changed into useful work.

We can write it like this: Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \eta_{th} = \frac{\text{Work out}}{\text{Heat in}}

Or, thinking about the heat that's wasted: Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \eta_{th} = 1 - \frac{\text{Waste heat out}}{\text{Heat in}}

Let's say you put 1000 joules of heat energy into an engine. If it produces 300 joules of mechanical work, then 700 joules are lost as waste heat. In this case, the thermal efficiency is 30%. This means 30% of the energy was used for work, and 70% was wasted.

Efficiency in Heating Devices

For devices that convert energy to heat, like a boiler or a furnace, thermal efficiency measures how much of the input heat becomes useful output heat.

The formula is: Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \eta_{th} = \frac{\text{Useful heat out}}{\text{Heat in}}

For instance, if a boiler takes in 300 kW (kilowatts) of heat energy and produces 210 kW of useful heat, its efficiency is 210/300 = 0.70, or 70%. This means 30% of the energy is lost to the surroundings.

An electric resistance heater is very efficient, almost 100%. If you put in 1500 watts of electrical energy, it produces almost 1500 watts of heat. When comparing different heating systems, like an electric heater to a natural gas furnace, you need to look at both their efficiency and the cost of the energy they use.

Heat Pumps and Refrigerators: Moving Heat Around

Heat pumps, refrigerators, and air conditioners work differently. They don't convert energy; they move heat from one place to another. Because of this, we use different ways to measure their performance, like the "coefficient of performance" (COP).

Here are the formulas for how well they work:
For a Heat Pump (HP):

For a Refrigerator (R):

Where:

• Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): Q_H is the heat moved to the hot side.
• Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): Q_L is the heat moved from the cold side.
• is the work (energy) put in to make it run.

If the temperatures are steady and the process is ideal:

Here, Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): T_H is the higher temperature (hot side) and Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): T_L is the lower temperature (cold side).

Understanding Energy Efficiency

Sometimes, "thermal efficiency" is also called "energy efficiency." In places like the United States, for cooling devices and heat pumps, people often talk about the SEER (Seasonal Energy Efficiency Ratio). This is a common way to measure how energy efficient they are.

For heating devices, their highest efficiency is often stated, like "this furnace is 90% efficient." But a more detailed measure for how well they work over a whole season is called the Annual Fuel Utilization Efficiency (AFUE).

Related topics

• Thermodynamics

See also

In Spanish: Rendimiento térmico para niños