Routes to a higher efficiency

1. Efficiency is the fraction of the input energy that is transferred usefully, so increasing it means making a greater fraction of the input reach the useful store.
2. Every method starts by finding the process that dissipates the energy, such as friction, air resistance, electrical resistance or unwanted heating, and then tackling that process.
3. Reducing wasted transfers: if less energy is dissipated, more of the same input reaches the useful store.
4. Recovering wasted energy: if energy that would have been dissipated is captured and put to use, it becomes part of the useful output.
5. Choosing a better device: if a different design produces the same useful output from a smaller input, the fraction that is useful goes up.
6. None of these methods creates energy, because the total is still conserved, and they only change how much of it ends up in a useful store.
7. No method can raise the efficiency to $100\%$, because some energy is always dissipated in any real transfer.

Reducing friction and air resistance

1. Wherever parts of a machine rub together, work is done against friction, and energy is transferred to the thermal energy stores of the parts instead of to the useful store.
2. Bearings and wheels: ball bearings let parts roll over one another instead of sliding, and rolling produces far less friction than sliding.
   1. This is why ball bearings are fitted in bicycle wheel hubs, skateboard wheels, electric fans and car gearboxes.
3. Maintenance: worn, rusty or badly aligned parts rub harder against each other, so servicing a machine and replacing worn parts keeps friction low.
4. Streamlining: a smooth, tapered shape lets air flow around a vehicle with less disturbance, so the vehicle does less work against air resistance at the same speed.
   1. Less energy is dissipated to the kinetic and thermal energy stores of the air, so a greater fraction of the energy from the fuel or battery keeps the vehicle moving.
   2. Examples include the pointed nose of a high-speed train, the curved deflector on the cab of a lorry and a cyclist crouching low over the handlebars.
5. Tyre pressure: a tyre inflated to the correct pressure squashes less as it rolls, so less energy is dissipated in the rubber and the car needs less fuel to cover the same distance.

Reducing electrical waste

1. Whenever there is a current in a wire, its resistance transfers part of the energy to the thermal energy store of the wire.
2. Low-resistance cables: thick copper cables have a lower resistance than thin ones, so less energy is dissipated in heating them when a current flows.
3. Better motors: motors built with thicker, low-resistance coils and well-made bearings waste less energy, so more of their input reaches the kinetic energy store of the moving part.
4. Better lamps: an LED lamp gives the same amount of light as a $60W$ filament lamp while taking in only about $10W$, because far less of its input is transferred by heating.
5. Replacing an old appliance with a more efficient model is often the simplest way to increase efficiency, because the improvement is built into its design.

Recovering wasted energy

1. In an ordinary car, braking dissipates the kinetic energy store of the car through friction in the brakes.
2. In an electric or hybrid car, regenerative braking uses the motor as a generator while the car slows down, so energy is transferred from the kinetic energy store of the car to the chemical energy store of its battery.
3. That stored energy is used again when the car speeds up, so less input energy is needed for the whole journey.
4. A combined heat and power station pipes the hot water left over from generating electricity to heat nearby homes and buildings, so energy that would have been dissipated into the air does a useful job.
5. A condensing boiler passes its hot exhaust gases over the cold water returning from the radiators, so energy that would have escaped through the flue warms the water instead.