Renewable Energy and Energy Efficiency
Renewable energy systems convert natural resources into electricity. For solar PV, understand the I-V curve, maximum power point (MPP), and the role of MPPT controllers. Know that panel output falls with temperature (a negative temperature coefficient) even as irradiance rises, and that series strings add voltage while parallel strings add current. Inverters convert DC to AC; grid-tied systems synchronise to the grid, while off-grid systems need storage and charge control.
For wind, know that power in the wind is proportional to the cube of wind speed (P ∝ v³) and to the swept area, and that the Betz limit caps theoretical extraction at about 59%. Understand cut-in, rated, and cut-out speeds.
Energy efficiency covers load management, power factor correction, high-efficiency motors, LED lighting, and building envelope measures. Power factor correction reduces reactive current and line losses; know that capacitors are added in parallel to improve pf.
Worked example: A load draws 100 kW at pf 0.8. Apparent power S = 100/0.8 = 125 kVA; reactive Q = √(125² − 100²) = 75 kvar. Correcting to pf 0.95 needs Q_new = 100 × tan(cos⁻¹0.95) ≈ 32.9 kvar, so add about 42 kvar of capacitance.
Traps: assuming doubling wind speed doubles power (it multiplies it by eight); ignoring temperature derating of PV; and confusing kW with kVA.