Energy conversion efficiency facts for kids

Useful output energy is always lower than input energy.

Efficiency of power plants, world total, 2008

Energy conversion efficiency (η) is a number that shows how well a machine changes energy from one form to another. It compares the useful energy coming out of a machine to the energy put into it.

The input and output can be different types of energy, such as chemical, electric power, mechanical work, light, or heat. The resulting value is usually between 0 and 1.

What is Energy Efficiency?

When we use machines, we often change energy from one type to another. For example, a car engine changes chemical energy from fuel into mechanical energy to move the car. A light bulb changes electrical energy into light.

Efficiency is often written as a percentage between 0% and 100%.

• If a machine has an efficiency of 0.8, it is 80% efficient.
• This means 80% of the energy is doing useful work, and 20% is lost, usually as waste heat.

The Limit of Efficiency

It is impossible for a machine to be more than 100% efficient. This would mean it creates energy out of nothing, which breaks the laws of physics. A machine that runs forever without adding energy is called a perpetual motion machine, and it cannot exist.

However, some devices like heat pumps and refrigerators can have a number higher than 1. This is because they move heat from one place to another instead of creating it. For these machines, we use a measure called the Coefficient of performance (COP). A higher COP means the machine is more efficient and costs less to run.

Types of Efficiency

There are different ways to measure efficiency depending on the machine:

• Electrical efficiency: How much useful electricity comes out compared to the power put in.
• Mechanical efficiency: How well a machine converts energy into movement (work).
• Thermal efficiency: How well a heat engine turns fuel into work.
• Luminous efficiency: How much visible light is produced from a light source.

Chemical Energy

Chemical reactions involve changes in energy. Batteries and fuel cells turn chemical energy into electricity.

• A fuel cell combines hydrogen and oxygen to make water and electricity.
• Electrolysis is the reverse process. It uses electricity to split water into hydrogen and oxygen.

Scientists use specific rules to calculate the energy in fuels. These are called Heating Values. The Lower heating value (LHV) assumes water stays as a gas (vapor) after burning. The Higher heating value (HHV) assumes the water turns back into liquid, which releases more heat. This distinction is important for engineers designing power plants and boilers.

Light and Vision

Different gases produce different colors of light when used in a flash tube.

The human eye sees green light best. It cannot see infrared or ultraviolet light.

Energy is lost at many steps when turning electricity into light in a fluorescent lamp.

When we talk about light bulbs and lasers, efficiency can be tricky. This is because our eyes only see certain colors of light.

Wall-Plug Efficiency

This measures the total energy change. It looks at how much light energy (radiation) comes out compared to the electricity from the wall plug. It does not care if our eyes can see the light or not. This is often used for lasers and industrial lights.

Luminous Efficiency

This measures how well a light source works for human eyes. Our eyes are very good at seeing green light (around 555 nanometers) but not as good at seeing red or blue. We cannot see infrared or ultraviolet light at all.

• A green laser pointer looks much brighter than a red one, even if they use the same amount of power.
• Luminous efficacy is measured in lumens per watt.

Because our eyes are picky, a lamp might produce a lot of invisible energy (like heat or infrared light). An incandescent light bulb gets very hot because most of its energy turns into heat, not light. This makes it very inefficient for lighting a room. Fluorescent lamps and LEDs are much better because they turn more electricity into visible light.

Examples of Efficiency

Here are some common examples of how efficient different things are.

See also