Energy Efficiency, Cogeneration, and Where the Energy Goes
- Trace energy losses from primary source to end use
- Explain cogeneration and why it raises overall efficiency
- Evaluate efficiency measures against generation as a policy choice
Most primary energy never does useful work
Following energy from source to service is the most clarifying exercise in this unit. A coal plant converts roughly 35% of the fuel's chemical energy into electricity; the rest leaves as waste heat, because a heat engine is limited by thermodynamics rather than by engineering sloppiness. Transmission and distribution lose a further several percent. An incandescent bulb then converts about 5% of the electricity into light and the rest into heat — so the useful light delivered is on the order of 1–2% of the coal's original energy. Replacing that bulb with an LED at roughly 90% conversion multiplies the whole chain. The general lesson the exam wants: efficiency gains compound backward through the chain, so a unit saved at the point of use saves several units of primary fuel.
Cogeneration
Cogeneration, or combined heat and power, captures the waste heat from electricity generation and uses it — for building heat, industrial process heat, or hot water — instead of dumping it into a river or the air. Because the heat was going to be produced regardless, using it raises overall system efficiency from around 35% to as much as 80%. The constraint is geographic: heat cannot be transported far without large losses, so cogeneration requires a customer for the heat near the plant, which is why it is common at industrial sites, universities and district-heating systems in dense cities and rare at remote large power stations. That geographic limitation is exactly the kind of qualification a free-response answer should include.
Efficiency as a resource
The policy point worth arguing is that energy not used requires no generation, no fuel, no emissions and no land — which makes efficiency competitive with new supply, and usually cheaper. Concrete measures with mechanisms: building insulation and sealing reduce the heat that must be supplied; LED lighting cuts the conversion loss at the end of the chain; efficiency standards for appliances and vehicles move the whole stock over time as old units are replaced; public transit and denser development reduce vehicle-kilometers per person; smart grids shift demand to match supply. Two honest qualifications: efficiency gains can be partly offset by the rebound effect, where cheaper operation increases use, and efficiency alone cannot decarbonize a system that still burns fossil fuel — it reduces the quantity, not the kind.
A coal plant is 35% efficient, transmission delivers 92% of what is generated, and a household replaces incandescent bulbs (5% efficient) with LEDs (90% efficient). Compute the overall efficiency before and after.
- 1.Before: multiply the three steps. 0.35 × 0.92 × 0.05.
- 2.0.35 × 0.92 = 0.322. Then 0.322 × 0.05 = 0.0161, or about 1.6%.
- 3.After: 0.35 × 0.92 × 0.90.
- 4.0.322 × 0.90 = 0.2898, or about 29.0%.
- 5.The ratio is 0.90/0.05 = 18, so the useful light per unit of coal rises eighteenfold.
Multiply efficiencies through a chain rather than averaging them, and identify the weakest link. A chain's overall efficiency is dominated by its worst step, which is why replacing a 5% bulb matters more than a marginal improvement at a 35% power plant.
Cogeneration raises overall efficiency primarily by
The rebound effect refers to the phenomenon that
Answer the 2 checkpoints as you read.
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