Regenerative braking and motion recovery: Science and thermodynamics
Why braking energy converts back so efficiently
What this tab covers The thermodynamics: the grade (temperature or pressure) of the energy being recovered, energy versus exergy efficiency, and how harsh the waste stream is on equipment.Braking and lowering energy is mechanical, the highest grade there is, so it converts back to electricity efficiently. The limit is whether something nearby can use or store it at the moment it appears.
Grade of energy
FavourableKinetic energy of a moving vehicle and potential energy of a lowered load are pure work, not heat. The 100 C rule does not apply, and a motor run as a generator can convert most of it back to electricity. Without recovery, the energy goes into friction brakes or into braking resistors as heat. Hoisting is the richest source in cranes. On a port rubber-tyred gantry (RTG) crane, about 84 to 89% of hoist energy was recoverable on typical days, against only about 3 to 5% for gantry travel.5
RTG hoist energy recoverable 83.8-89.0% (typical days)5 RTG gantry travel recoverable about 2.6-5%5 RTG peak hoist power about 400 kW5Exergy versus energy efficiency
FavourableEnergy and exergy are nearly the same here because the recovered product is electricity. What matters is the share of braking energy actually captured and reused. In EVs, regenerative braking returns a net 22% of energy on the US EPA combined city and highway cycle. This helps push overall EV efficiency to 87 to 91%, against about 30% for a petrol car. Alstom's reversible metro substations capture 99% of recoverable braking energy. Otis says its ReGen lift drives cut energy use by up to 75%. Its own worked example shows about 45% (calculated, 3,640 against 6,573 kWh a year).134
EV net regen, EPA combined cycle about 22%1 EV overall efficiency 87-91% (petrol car about 30%)1 Hesop capture of recoverable energy 99%3 Otis ReGen example saving about 45% (calculated); claim up to 75%4Media compatibility and corrosiveness
FavourableThere is no dirty process stream. The constraints are electrical and mechanical. Regenerated power must be clean enough for the local grid. Otis quotes total harmonic distortion (waveform distortion, THD) of 5% or less for ReGen drives, against more than 80% for older non-regenerative drives. Flywheels spin at up to 36,000 rpm in the GKN Gyrodrive bus system, so they need vacuum housings and containment. Brake dust falls as friction brakes do less work.46
ReGen drive THD 5% or less (non-regen over 80%)4 Gyrodrive flywheel speed up to 36,000 rpm6Variants compared134567
| Variant | Where energy goes | Reported saving | Note |
|---|---|---|---|
| EV regenerative braking | Back to the battery | About 22% net on EPA cycle | Standard on all EVs |
| Rail regen with reversible substations | Back to the grid | 99% of recoverable energy captured; 1,042 MWh/y at Hamburg | 136 Hesop units sold |
| Regenerative lifts | Building supply | Up to 75% (example about 45%) | Best in tall, busy buildings |
| Regenerative cranes (RTG) | Storage or grid | About 50-57% of energy recoverable | Diesel RTGs need on-board storage |
| Flywheel KERS (bus) | Flywheel, then wheels | 20-25% fuel | Mechanical store, no battery |
Sources
- FOTW #1360: A typical EV is 87%-91% efficient compared to 30% for a conventional gasoline vehicle US Department of Energy, Vehicle Technologies Office, 2024
- Capturing wasted energy in our cities (Hesop reversible substations) (company press release) Alstom, 2025
- ReGen drive (product brochure, UK) (vendor claims) Otis
- Analysis of energy usage for RTG cranes (Papaioannou et al.), Energy 125, accepted manuscript Elsevier (repository copy, Manchester Metropolitan University), 2017
- From fast lane to bus lane (GKN Gyrodrive order by Go-Ahead) Institution of Mechanical Engineers, 2014
- Gyrodrive hybrid technology first to make economic sense to bus operators (company claims as reported) Transport Engineer