• Thermodynamic Principle of Concentrating Solar Power (CSP):
• Power Cycle: Concentrating solar thermal power plants (parabolic troughs, solar power towers, linear Fresnel systems) concentrate sunlight onto a receiver to heat a thermal fluid (synthetic oil, molten salt) up to $400 - 600^\circ\text{C}$.
• Electricity Generation: The high-temperature thermal fluid generates superheated steam in a heat exchanger, driving a conventional steam turbine coupled to an electric generator based on the thermodynamic Rankine cycle.
• Photovoltaic Effect: Directly converts light into electricity in semiconductor PV cells without thermodynamic fluid cycles.
• Conclusion: Rankine cycle (Option 3).

Correct option: (3) Rankine cycle

• Thermal Storage Characteristics of Salt-Gradient Solar Ponds:
• Salinity Structure: A salt-gradient solar pond is divided into three distinct water layers:

• Upper Convective Zone (UCZ): Low-salinity surface layer.
• Non-Convective Zone (NCZ / Halocline): Increasing salt concentration (and density) with depth, suppressing upward natural buoyancy convection.
• Lower Convective Zone (LCZ / Storage Zone): Saturated brine layer at the bottom.
  • Unique Heat Storage: Because trapped heat cannot rise through the dense halocline, bottom water temperatures reach $80 - 90^\circ\text{C}$ and store thermal energy for extended periods (weeks to several months, providing day-night and seasonal storage).

• Conclusion: They can store heat for long periods (Option 1).