Why we store energy
Many sources work only part of the time: the sun at night, wind on a calm day. Many users need energy all the time. A store takes energy in when it is plentiful and gives it back when it is needed. The path is: source → (converter) → store → (converter) → use.
Good stores are judged on: how much they hold (capacity), how fast they can give it back (power), how much is lost (efficiency), how long they keep it (leakage), cost, size and safety.
Mechanical storage
Energy is kept as movement or position.
- Height (gravity): a raised weight or water in a high tank. Energy = m × g × h. Pumped storage uses spare power to pump water uphill, then runs it down through a turbine.
- Spring (elastic): a squeezed or wound spring, as in a wind-up toy or a mechanical watch.
- Flywheel (movement): a heavy wheel spinning fast. It keeps smooth power for a short time.
- Compressed air: air squeezed into a tank, released to run a tool.
Good: lasts many cycles, fast release. Limits: bulky for small energy, friction and air losses.
Chemical storage
Energy is kept in the bonds of chemicals.
- Fuels: wood, coal, petrol, gas, hydrogen. Burning gives heat. Very high energy per kilogram.
- Batteries: a chemical reaction makes electricity. Rechargeable batteries (like lithium-ion) let the reaction run backward when charging.
Energy in a battery (watt-hours) = voltage (V) × capacity (Ah). A 12 V, 50 Ah battery holds 600 Wh. It can run a 60 W lamp for 600 ÷ 60 = 10 hours (in theory).
Good: compact and portable. Limits: wear out after many cycles, cost, heat and fire risk, safe disposal.
Thermal storage
Energy is kept as heat in a material.
Q = m × c × ΔT, where m is mass (kg), c is specific heat capacity (water: about 4200 J/kg·K) and ΔT is the rise in temperature. Water is a great store because c is large and it is cheap.
- Hot-water tanks, solar water heaters, night-storage heaters, hot stones or molten salt.
- Latent heat stores use melting and freezing (ice, wax): lots of energy at an almost steady temperature.
Good: cheap and simple. Limits: heat leaks out, so insulate the tank; it gives heat, not easy electricity.
Converters, modulators and adapters
These parts change the form of electrical energy so that source, store and load fit together.
- Converter: changes the type of current or voltage. A rectifier turns AC into DC; an inverter turns DC into AC; a DC–DC converter steps voltage up or down. Charging a phone uses an AC–DC converter.
- Modulator: controls how much power goes out by switching quickly on and off. In pulse-width modulation (PWM) a longer 'on' time gives more power (for example, a dimmer or a motor speed control).
- Adapter: makes two parts fit and work together: plug shape, voltage, signal. A phone charger is an adapter.
Each conversion wastes some energy as heat, so fewer conversions mean better efficiency.
Gearboxes, belts and couplings
These parts carry rotation from a motor to a machine and change speed and turning force (torque).
- Gearbox: toothed wheels. Ratio = teeth on driven gear ÷ teeth on driving gear. A ratio of 3 makes the output 3 times slower but with about 3 times more torque (ideal case).
- Belt and pulleys, or chain: pass motion across a gap. A belt is quiet and can slip to protect the machine; a chain does not slip.
- Coupling: joins two shafts end to end so they turn together, and may allow a small misalignment.
Speed × torque stays about the same (power), minus friction loss.
Guiding in rotation and translation
Moving parts must be guided so they move only the way we want, with little friction.
- Rotation (turning): shafts run in bearings. A plain bearing is a smooth sleeve; a ball or roller bearing uses rolling balls, so friction is much lower.
- Translation (sliding in a straight line): rails, slides and guide rods, for example a drawer runner or a printer head.
Oil or grease reduces wear. A well-guided part has less friction, so less energy is lost as heat.
Sealing
A seal stops liquid, gas or dust from passing where parts meet.
- Static seals (parts do not move): gaskets and O-rings between flanges and covers.
- Dynamic seals (one part moves): lip seals on rotating shafts, piston seals in cylinders.
Seals keep oil in, dirt out, and pressure up. They wear out, so they are checked and replaced. A leaking seal on a thermal tank or hydraulic store wastes stored energy.
Key formulas and definitions
- Mechanical (gravity): E = m × g × h (g ≈ 9.8 m/s²)
- Battery energy (Wh) = V × Ah; run time = energy ÷ power
- Thermal: Q = m × c × ΔT (water c ≈ 4200 J/kg·K)
- Efficiency = useful energy out ÷ energy in × 100%
- Gear ratio = teeth on driven gear ÷ teeth on driving gear
Worked examples
1. A 100 kg weight is raised by 10 m. How much energy is stored? (g = 9.8 m/s²)
E = m g h = 100 × 9.8 × 10 = 9800 J = 9.8 kJ.
2. A 12 V, 50 Ah battery runs a 60 W lamp. For how long, in theory?
Energy = 12 × 50 = 600 Wh. Time = 600 ÷ 60 = 10 hours.
3. A tank holds 200 kg of water. Solar heating raises it by 40 °C. How much heat is stored? (c = 4200 J/kg·K)
Q = m c ΔT = 200 × 4200 × 40 = 33 600 000 J = 33.6 MJ. In kWh: 33.6 ÷ 3.6 = 9.3 kWh.
4. A battery stores 600 Wh. Charging uses 700 Wh from the supply. What is the charging efficiency?
Efficiency = 600 ÷ 700 × 100 ≈ 85.7%. The other 100 Wh was lost as heat.
5. A 20-tooth gear drives a 60-tooth gear. The motor turns at 1200 rpm. What is the output speed and the ideal torque change?
Ratio = 60 ÷ 20 = 3. Output speed = 1200 ÷ 3 = 400 rpm. Torque becomes about 3 times larger (ideal, before friction).
Common mistakes
- Mixing up power and energy. Power is how fast (watts); energy is how much (joules or watt-hours).
- Forgetting losses. No store returns 100% of the energy that went in.
- Using grams instead of kilograms in Q = m c ΔT or E = mgh.
- Thinking a gearbox adds energy. It trades speed for torque; it never makes extra energy.