Q7Power Generation Sources

(a) OTEC (Ocean Thermal Energy Conversion) Cycles

OTEC harnesses the natural temperature gradient that exists in tropical oceans between warm surface water (around 25-28°C) and cold deep water (around 4-6°C at depths of 800-1000 m) to run a heat-engine (Rankine) cycle and generate electricity. Although the temperature difference (typically 20-24°C) is small, giving low thermodynamic (Carnot) efficiency (a few percent), the enormous volume of stored solar thermal energy in tropical oceans makes OTEC an attractive long-term renewable resource in equatorial coastal/island regions.

There are three main OTEC cycle configurations: (1) Open (Claude) cycle — warm surface seawater is admitted into a low-pressure flash evaporator chamber, where a small fraction flashes into low-pressure steam (since boiling point drops sharply at reduced pressure); this steam drives a low-pressure turbine-generator directly, after which it is condensed using cold deep water (often producing desalinated fresh water as a valuable by-product), and the non-condensable gases are removed by a vacuum pump. (2) Closed (Anderson) cycle — warm seawater is passed through a heat exchanger (evaporator) to vaporize a separate, low-boiling-point working fluid (such as ammonia or a refrigerant) in a fully closed loop; this vapor drives the turbine, and is then condensed in a second heat exchanger cooled by cold deep water, before being pumped back to the evaporator — analogous to the closed-cycle gas turbine concept but using a liquid-vapor working fluid instead of a permanent gas. (3) Hybrid cycle — combines features of both, typically flashing warm water as in the open cycle to produce steam, which is then used to vaporize a secondary working fluid (as in the closed cycle) that actually drives the turbine, aiming to capture the desalination by-product of the open cycle along with the more compact turbine machinery of the closed cycle.

OTEC plants require a large volume of both warm and cold water flow (the cold water typically drawn up through a long, large-diameter deep-sea intake pipe) and are generally sited offshore or on tropical island coastlines with a steep continental shelf providing quick access to deep cold water close to shore.

(b) Solar Pond

A solar pond is a naturally or artificially constructed body of saline water that functions simultaneously as a solar collector and a long-term thermal energy storage device. It exploits a stable salinity (density) gradient to suppress the natural convection that would otherwise occur in a heated body of water, allowing solar heat absorbed at the bottom to remain trapped there rather than rising and being lost to the atmosphere by convection and evaporation at the surface.

A typical (salt-gradient) solar pond has three distinct zones: (i) Upper Convective Zone (UCZ) — a thin surface layer of relatively fresh, low-salinity water at near-ambient temperature, exposed to the atmosphere; (ii) Non-Convective Zone (NCZ) — a middle layer with a gradually increasing salt concentration with depth, which acts as a transparent insulating blanket, since the increasing density with depth prevents convective mixing even as the water is heated (the salinity gradient overcomes the natural buoyancy-driven convection that a uniform-density heated fluid would exhibit); (iii) Lower Convective Zone (LCZ) or storage zone — a dense, highly concentrated (near-saturated) brine layer at the bottom, blackened or dark-colored to maximize solar absorption, where solar radiation that has penetrated through the upper layers is absorbed and the resulting heat is effectively trapped by the insulating NCZ above it, allowing this bottom layer to reach temperatures of 70-90°C even though the top surface remains near ambient temperature.

Hot brine is withdrawn from the lower convective zone through a heat exchanger to extract useful heat — either for direct process/space heating applications or to vaporize a low-boiling-point organic working fluid in an Organic Rankine Cycle turbine-generator to produce electricity — after which the cooled brine is returned to the pond. Solar ponds provide an inherent, low-cost thermal energy storage capability (since the large thermal mass of brine in the LCZ can store heat for days to months), making them valuable for continuous, non-intermittent low-grade heat supply despite solar energy's inherently intermittent input, and have been demonstrated for applications including desalination, industrial process heat, and small-scale power generation in high-solar-insolation regions such as Israel, India (e.g., Bhuj) and Australia.