Renewable energy machines convert natural energy sources like sunlight, wind, and moving water into useful electrical or mechanical energy. Efficiency tells us what fraction of the incoming energy becomes the useful output we want. This matters because two machines can receive the same amount of energy but deliver different amounts of usable electricity.
Understanding efficiency helps engineers compare designs and reduce wasted energy.
Key Facts
- Efficiency = useful output energy / input energy
- Percent efficiency = (useful output energy / input energy) x 100%
- Energy is conserved, so input energy = useful output energy + wasted energy
- Power efficiency can be found using efficiency = useful output power / input power
- A Sankey diagram uses arrow width to show the relative amount of energy in each flow
- No real renewable energy machine is 100% efficient because some energy becomes heat, sound, vibration, or other unusable forms
Vocabulary
- Efficiency
- Efficiency is the fraction of input energy or power that is converted into useful output energy or power.
- Useful output energy
- Useful output energy is the energy transferred into the form and purpose the machine is designed to produce.
- Energy loss
- Energy loss is energy that is transferred into unwanted forms such as heat, sound, or vibration.
- Sankey diagram
- A Sankey diagram is a flow diagram where arrow widths represent the amounts of energy being transferred.
- Generator
- A generator is a device that converts mechanical energy into electrical energy using electromagnetic induction.
Common Mistakes to Avoid
- Treating wasted energy as destroyed is wrong because energy is conserved and only changes into less useful forms.
- Using output divided by losses for efficiency is wrong because efficiency compares useful output to total input.
- Forgetting to convert percent efficiency to a decimal in calculations is wrong because 40% must be used as 0.40 when multiplying energy or power.
- Assuming renewable means perfectly efficient is wrong because renewable machines still have friction, electrical resistance, drag, and other unavoidable losses.
Practice Questions
- 1 A wind turbine receives 8000 J of kinetic energy from moving air and produces 2800 J of electrical energy. What is its percent efficiency?
- 2 A solar panel has an efficiency of 18%. If 5000 J of sunlight reaches the panel, how much electrical energy is produced and how much energy is lost?
- 3 A Sankey diagram for a hydroelectric generator shows a thick input arrow, a medium useful electrical output arrow, and a smaller heat and sound loss arrow. Explain what the relative arrow widths tell you about the machine's energy conversion.