98 POWER PLANT ENGINEERINGEt = Er × Cr × FA
= 5.457 × 1018 × 0.5 × 0.7
= 1.9 × 1018 MJ/year
This corresponds to an average terrestrial incidence on the waters of the solar constant
S = 1353 W/m^2 × 0.5 = 676 W/m^2.
This energy is not totally absorbed by the water because some of it is reflected back to the sky.
A good estimate of the amount absorbed is obtained from the annual evaporation of water,
Annual evaporation (EV) = 1.20 m
Average water surface temperature (T) = 20°C
The latent heat of vaporization(C) = 2454 kJ/kg
Seawater density (Ω)= 1000 kg/m^3.
The annual energy absorbed (Eab) = EV × ρ × C
= 1.20 × 1000 × 2454
= 3 × 106kJ/m^2 = 95 W/m^2.
This amount is, of course, replenished by rainfall back on the water and by runoff from land.
Absorbed energy as % of incident energy = Eab/S = (95/676) × 100 = 14%2.21.1 Power Plants Based on Ocean Energy
Ocean thermal energy is used for many applications, including electricity generation. There are
three types of electricity conversion systems: closed-cycle, open-cycle, and hybrid.
Closed-cycle systems use the ocean’s warm surface water to vaporize a working fluid, which
has a low-boiling point, such as ammonia. The vapor expands and turns a turbine. The turbine then
activates a generator to produce electricity. Open-cycle systems actually boil the seawater by operating
at low pressures. This produces steam that passes through a turbine/generator. Hybrid systems combine
both closed-cycle and open-cycle systems.
Depending Upon these electricity conversion systems the Ocean power plant can be divided
mainly in to two groups.
The Open or Claude OTEC Cycle Power Plant. The Frenchman Georges Claude constructed
the first OTEC plant in 1929 on the Mantanzas Bay in Cuba.
The Claude plant used an open cycle in which seawater itself plays the multiple role of heat
source, working fluid, coolant, and heat sink.