Asked by k
Why do energy sources not have 100% effciency. Why do you think some have lower efficiencys
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Answered by
GPT-5 mini
AI
Short answer: because converting energy from one form to another always produces unavoidable losses (mostly heat) due to the second law of thermodynamics and practical engineering limits. No real device can convert all input energy into useful work or the exact desired form.
Why that happens (main causes)
- Second law of thermodynamics / entropy: energy has a “quality.” Some forms (high-temperature heat, chemical energy) can be turned into work more readily than low-temperature heat. The maximum possible efficiency of a heat engine is limited by the Carnot efficiency, 1 − Tc/Th (temperatures in kelvin). That’s a fundamental limit — you can’t beat it.
- Irreversible processes: real processes involve friction, turbulence, viscous dissipation, electrical resistance, non‑ideal chemical reactions, mixing, and heat transfer across finite temperature differences. These generate entropy and turn usable energy into waste heat.
- Multiple conversion steps: each time you convert energy (chemical → heat → mechanical → electrical), you lose some fraction. Longer chains mean larger cumulative losses.
- Material and design limits: catalyst kinetics, semiconductor physics, mechanical tolerances, leakage, and degradation all reduce practical efficiency.
- Energy quality mismatch: sometimes you need a different form of energy (e.g., chemical to electricity) and the conversion itself is lossy; even “lossless” forms (electricity) incur losses in storage, transmission and conversion.
Why some sources are less efficient (with typical examples)
- Heat engines (coal/NG steam turbines): limited by Carnot. Practical large combined‑cycle gas plants reach ~50–62% LHV; older steam plants ~33–40%. Losses are exhaust heat and cooling losses.
- Internal combustion engines (cars): ~20–30% because of combustion losses, heat lost to coolant and exhaust, pumping and friction losses.
- Solar photovoltaics: limited by semiconductor physics (Shockley–Queisser limit ~33% for a single-junction cell); most commercial panels are ~15–22% due to nonideal absorption, reflection, resistance, and temperature effects.
- Wind turbines: limited by aerodynamics (Betz limit ~59% theoretical). Real turbines capture ~30–50% of wind’s kinetic energy because of mechanical/electrical losses and nonideal flow.
- Batteries: round‑trip (store → retrieve) efficiency ~70–95% depending on chemistry; losses come from internal resistance, self‑discharge, and conversion electronics.
- Hydroelectric and some electric motors/generators: relatively high (hydro ~85–95%, modern motors/gens ~90%+), because there are fewer thermodynamic barriers and lower temperature-driven losses.
Practical trade-offs
- Cost, weight, reliability, and safety often force choices that reduce peak efficiency (e.g., lighter vehicles use designs that sacrifice some efficiency).
- Sometimes you accept lower conversion efficiency for easier storage, portability, or controllability (batteries vs direct grid power, liquid fuels vs electricity).
- Environmental and resource constraints influence design choices (e.g., emissions controls add losses to engines and power plants).
How to improve efficiency
- Reduce irreversibilities: better insulation, better catalysts, lower friction, improved aerodynamics, higher operating temperatures (within material limits).
- Use combined systems: cogeneration (use waste heat), combined‑cycle plants, or hybrid systems to capture more of the original energy’s value.
- Shorten conversion chains: avoid unnecessary conversions and use higher‑quality energy forms when possible.
Bottom line: energy is conserved, but usable (high‑quality) energy is degraded in real processes. Some technologies are closer to the fundamental limits and have fewer practical losses, so they appear more efficient; others are further from those limits or involve more conversion steps, so they are less efficient.
Why that happens (main causes)
- Second law of thermodynamics / entropy: energy has a “quality.” Some forms (high-temperature heat, chemical energy) can be turned into work more readily than low-temperature heat. The maximum possible efficiency of a heat engine is limited by the Carnot efficiency, 1 − Tc/Th (temperatures in kelvin). That’s a fundamental limit — you can’t beat it.
- Irreversible processes: real processes involve friction, turbulence, viscous dissipation, electrical resistance, non‑ideal chemical reactions, mixing, and heat transfer across finite temperature differences. These generate entropy and turn usable energy into waste heat.
- Multiple conversion steps: each time you convert energy (chemical → heat → mechanical → electrical), you lose some fraction. Longer chains mean larger cumulative losses.
- Material and design limits: catalyst kinetics, semiconductor physics, mechanical tolerances, leakage, and degradation all reduce practical efficiency.
- Energy quality mismatch: sometimes you need a different form of energy (e.g., chemical to electricity) and the conversion itself is lossy; even “lossless” forms (electricity) incur losses in storage, transmission and conversion.
Why some sources are less efficient (with typical examples)
- Heat engines (coal/NG steam turbines): limited by Carnot. Practical large combined‑cycle gas plants reach ~50–62% LHV; older steam plants ~33–40%. Losses are exhaust heat and cooling losses.
- Internal combustion engines (cars): ~20–30% because of combustion losses, heat lost to coolant and exhaust, pumping and friction losses.
- Solar photovoltaics: limited by semiconductor physics (Shockley–Queisser limit ~33% for a single-junction cell); most commercial panels are ~15–22% due to nonideal absorption, reflection, resistance, and temperature effects.
- Wind turbines: limited by aerodynamics (Betz limit ~59% theoretical). Real turbines capture ~30–50% of wind’s kinetic energy because of mechanical/electrical losses and nonideal flow.
- Batteries: round‑trip (store → retrieve) efficiency ~70–95% depending on chemistry; losses come from internal resistance, self‑discharge, and conversion electronics.
- Hydroelectric and some electric motors/generators: relatively high (hydro ~85–95%, modern motors/gens ~90%+), because there are fewer thermodynamic barriers and lower temperature-driven losses.
Practical trade-offs
- Cost, weight, reliability, and safety often force choices that reduce peak efficiency (e.g., lighter vehicles use designs that sacrifice some efficiency).
- Sometimes you accept lower conversion efficiency for easier storage, portability, or controllability (batteries vs direct grid power, liquid fuels vs electricity).
- Environmental and resource constraints influence design choices (e.g., emissions controls add losses to engines and power plants).
How to improve efficiency
- Reduce irreversibilities: better insulation, better catalysts, lower friction, improved aerodynamics, higher operating temperatures (within material limits).
- Use combined systems: cogeneration (use waste heat), combined‑cycle plants, or hybrid systems to capture more of the original energy’s value.
- Shorten conversion chains: avoid unnecessary conversions and use higher‑quality energy forms when possible.
Bottom line: energy is conserved, but usable (high‑quality) energy is degraded in real processes. Some technologies are closer to the fundamental limits and have fewer practical losses, so they appear more efficient; others are further from those limits or involve more conversion steps, so they are less efficient.
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