Mechanical and thermal energy storage

Energy storage stores surplus energy when generation is more than demand and returns it when demand is high. It supports variable renewables (solar, wind), peak shaving, frequency regulation and reliability.

Mechanical energy storage

1. Pumped storage hydropower (PSH)

• Two reservoirs at different levels. Off-peak power pumps water up; at peak, water flows down through the turbine.
• Energy stored: $E = \rho g V H$.
• Round-trip efficiency about 70–80%; large capacity (hundreds of MW, many hours); long life.
• Needs suitable topography; high capital cost. Nepal has good sites in the hills.

 Upper reservoir
      |   ^
 gen  v   | pump  (reversible pump-turbine)
 Lower reservoir

2. Compressed air energy storage (CAES)

• Off-peak power compresses air into underground caverns; at peak, air is heated and expanded in a turbine.
• Efficiency about 40–70%; large scale; needs suitable caverns.

3. Flywheel energy storage

• Energy stored in a rotating mass: $E = \frac{1}{2}J\omega^2$.
• Fast response (milliseconds), high power, many cycles; short duration (seconds to minutes); self-discharge by friction.
• Used for UPS, power quality and frequency regulation.

4. Gravity storage – lifting heavy blocks and lowering them to generate power (emerging).

Thermal energy storage

1. Sensible heat storage – heat stored by raising temperature of water, rock, sand, concrete or molten salt: $Q = mc\Delta T$. Example: hot water tanks, molten salt in concentrated solar power plants (allows generation after sunset).

2. Latent heat storage – uses phase change materials (PCM) such as paraffin wax or salt hydrates; stores large heat at nearly constant temperature: $Q = mL$.

3. Thermochemical storage – heat stored in reversible chemical reactions; highest energy density, still developing.

4. Cold storage – ice or chilled water made at night for daytime air conditioning, shifting load to off-peak.

Comparison