Open-Cycle OTEC

The open cycle consists of the following steps:  (i) flash evaporation of a fraction of the warm seawater by reduction of pressure below the saturation value corresponding to its temperature (ii) expansion of the vapor through a turbine to generate power; (iii) heat transfer to the cold seawater thermal sink resulting in condensation of the working fluid; and (iv) compression of the non-condensable gases (air released from the seawater streams at the low operating pressure) to pressures required to discharge them from the system. 

This process being iso-enthalpic,

h₂ = h₁ = h_f + x₂h_fg

Here, x₂ is the fraction of water by mass that has vaporized. The warm water mass flow rate per unit turbine mass flow rate is 1/x₂.

The low pressure in the evaporator is maintained by a vacuum pump that also removes the dissolved non condensable gases from the evaporator. The evaporator now contains a mixture of water and steam of very low quality. The steam is separated from the water as saturated vapour. The remaining water is saturated and is discharged back to the ocean in the open cycle. The steam we have extracted in the process is a very low pressure, very high specific volume working fluid. It expands in a special low pressure turbine.

h₃ = h_g

Here, h_g corresponds to T₂. For an ideal adiabatic reversible turbine,

s_5,s = s₃ = s_f + x_5,ss_fg

The above equation corresponds to the temperature at the exhaust of the turbine, T₅. x_5,s is the mass fraction of vapour at point 5.

The enthalpy at T₅ is,

h_5,s = h_f + x_5,sh_fg

This enthalpy is lower. The adiabatic reversible turbine work = h₃-h_5,s.

Actual turbine work w_T = (h₃-h_5,s) × polytropic efficiency

                                                                                                                                                

The condenser temperature and pressure are lower. Since the turbine exhaust will be discharged back into the ocean anyway, a direct contact condenser is used. Thus the exhaust is mixed with cold water from the deep cold water pipe which results in a near saturated water.That water is now discharged back to the ocean.

h₆=h_f, at T₅. T₇ is the temperature of the exhaust mixed with cold sea water, as the vapour content now is negligible,     

There are the temperature differences between stages. One between warm surface water and working steam, one between exhaust steam and cooling water and one between cooling water reaching the condenser and deep water. These represent external irreversibilities that reduce the overall temperature difference.

The cold water flow rate per unit turbine mass flow rate,   

Turbine mass flow rate,   

Warm water mass flow rate,        

Cold water mass flow rate     

Hybrid OTEC System

Another option is to combine the two processes together into an open-cycle/closed-cycle hybrid, which might produce both electricity and desalinated water more efficiently.  In a hybrid OTEC system, warm seawater might enter a vacuum where it would be flash-evaporated into steam, in a similar fashion to the open-cycle evaporation process. 

The steam or the warm water might then pass through an evaporator to vaporize the working fluid of a closed-cycle loop.  The vaporized fluid would then drive a turbine to produce electricity, while the steam would be condensed within the condenser to produced desalinated water.

Aplications

Ocean thermal energy conversion (OTEC) systems have many applications or uses. OTEC can be used to generate electricity, desalinate water, support deep-water Mari culture, and provide refrigeration and air-conditioning as well as aid in crop growth and mineral extraction. These complementary products make OTEC systems attractive to industry and island communities even if the price of oil remains low.
Benefits of OTEC

We can measure the value of an ocean thermal energy conversion (OTEC) plant and continued OTEC development by both its economic and no economic benefits.