Renewable energy machines such as wind turbines, solar panels, batteries, and hydro turbines make electricity with little or no fuel burned during operation. However, they still require energy before they ever produce a kilowatt-hour. Energy is used to mine raw materials, refine metals, manufacture parts, transport equipment, install systems, maintain them, and recycle or dispose of them later.
This hidden energy cost is called embodied energy, and it matters when comparing clean-energy technologies.
Embodied energy is usually studied with life cycle analysis, which follows a machine from raw materials to end of life. A renewable machine becomes environmentally beneficial when it produces more useful energy than was required to build and support it. This point is described by energy payback time, and the overall benefit is often measured using energy return on energy invested.
Technologies with long lifetimes, high efficiency, good recycling, and low-energy manufacturing usually have better energy performance.
Key Facts
- Embodied energy is the total energy used to make, transport, install, maintain, and retire a device.
- Energy payback time = embodied energy ÷ annual energy output.
- EROEI = lifetime energy output ÷ lifetime energy input.
- Net energy = lifetime energy output - lifetime energy input.
- A shorter energy payback time usually means a renewable machine repays its energy cost faster.
- Manufacturing, mining, and materials such as steel, concrete, silicon, copper, lithium, and rare earth elements can strongly affect embodied energy.
Vocabulary
- Embodied energy
- The total energy required to produce, transport, install, maintain, and eventually recycle or dispose of a product.
- Energy payback time
- The time a machine must operate before it generates the same amount of energy that was used to create and support it.
- Life cycle analysis
- A method for studying the total environmental and energy impacts of a product from raw material extraction through end of life.
- Energy return on energy invested
- A ratio comparing the useful energy a system produces over its lifetime to the energy required to build and operate it.
- Net energy
- The amount of useful energy left after subtracting the energy needed to create and run the energy system.
Common Mistakes to Avoid
- Ignoring manufacturing energy, which is wrong because renewable machines still require mining, refining, factories, transport, and installation before they generate electricity.
- Confusing power with energy, which is wrong because power is the rate of energy use or production while embodied energy is a total amount of energy.
- Assuming zero emissions during operation means zero total impact, which is wrong because life cycle impacts include materials, construction, maintenance, and end-of-life processing.
- Comparing devices without considering lifetime output, which is wrong because a machine with higher embodied energy can still be better if it produces much more energy over many years.
Practice Questions
- 1 A solar panel system has an embodied energy of 18,000 kWh and produces 6,000 kWh per year. What is its energy payback time?
- 2 A wind turbine requires 2,500,000 kWh of lifetime energy input and produces 75,000,000 kWh over its lifetime. What is its EROEI?
- 3 Two batteries store the same amount of electricity, but Battery A uses more energy to manufacture and lasts twice as many charge cycles as Battery B. Explain what information you would need to decide which battery has the lower embodied energy per kWh delivered.