EROI Energy Return – AP Environmental Science

🌊

Hydroelectric Power

Massive initial energy investment to build the dam, but nearly zero energy to run it for a century. Incredible returns.

~80

🛢️

Conventional Oil (1950s)

Early gushers required merely sticking a pipe in the ground. Massive surplus energy powered the modern industrial boom.

~40

☀️

Solar Photovoltaic (PV)

Energy required to mine silicon and manufacture panels is high, but they silently produce energy for 25+ years.

~15

⛏️

Tar Sands / Shale Oil (Modern)

Extremely un-economical. Requires injecting steam underground to melt bitumen, or fracking rock. Huge energy investment just to get the oil out.

🌽

Corn Ethanol (Biofuel)

Tractors, massive fertilizers, harvesting, and distillation take almost as much energy as the fuel provides. Very poor net yield.

1.5

The Mathematics of Energy

EROI = Energy GatheredEnergy Invested

Why it matters: If you find an oil deposit that yields 100 barrels, but it takes the energy equivalent of 90 barrels to drill, pump, refine, and transport it, your EROI is barely > 1. Society only benefits from the 10 net surplus barrels.

AP Exam Tip

You will often be asked to evaluate the economic sustainability of a fuel type. Remember that Hydrogen Fuel Cells technically have a negative EROI (they take more electricity to split water than the H2 provides), meaning they are an energy carrier, not a source!