This invention relates generally to power generation and
simultaneous desalinization of seawater. Specifically, the invention relates to
improving thermal efficiency by using the heat of exhaust gases of a gas turbine
to heat water used in the distillation of seawater.
Commercially available desalinization techniques can be
classified generally into two categories. The first includes distillation processes
that require mainly heat plus some electricity for ancillary equipment. The second
includes reverse osmosis processes. In the distillation processes, vapor is produced
by heating the seawater close to its boiling temperature and passing it through
a series of stages under successively decreasing pressures to induce flashing. The
vapor produced is then condensed and cooled as distillate.
In reverse osmosis processes, pure water is forced to pass
under pressure through special semi-permeable membranes, while salt is rejected.
The pressure differential must be high enough to overcome the natural tendency of
water to move from the low salt concentration side of a membrane to the high concentration
side, as determined by osmotic pressure.
DE 2 520 936
discloses a process for evaporation and heating using exhaust gas.
This invention is particularly adapted for distillation
processes that typically use low pressure steam to heat the seawater as a first
step in the distillation process.
When desalinization plants are integrated into gas turbine
power plants, they are invariably incorporated as combined cycle power plants that
utilize both gas and steam turbines. In combined cycle plants, electricity is produced
with high-pressure steam, generated by heat exchange with gas turbine exhaust gases,
to run turbines that in turn power electric generators. In a typical case, boilers
produce high-pressure steam at about 540°C (1,000°F). As this steam expands
in the turbine, its temperature and energy level is reduced. Distillation plants
need steam having a temperature of about 120°C (248°F) or below, and this
steam can be obtained by extracting lower temperature steam at the low pressure
end of the turbine after much of its energy has been used to generate electricity.
This low pressure steam is then run through the distillation plant's brine heater,
thereby increasing the temperature of the incoming seawater. The condensate from
the extracted steam is then returned to the boiler to be reheated.
This invention uses the calories of the gas turbine exhaust
gases without having to resort to a complex and expensive installation as in the
case of a combined cycle power generation plant. More specifically, a thermal transfer
fluid (such as demineralized water) is heated by the gas turbine exhaust gases and
used in place of low pressure steam to heat the seawater in the initial step of
the distillation process.
In the exemplary embodiment, a boiler is placed in the
gas turbine exhaust gas flowpath. A damper upstream of an open cycle exhaust stack
permits diversion of the gas turbine exhaust gases into the boiler in a cogeneration
mode. The heat of the exhaust gases is transferred via a heat exchanger in the boiler
to the thermal transfer liquid (demineralized water) and this liquid is supplied
to the desalinization plant where it is passed in heat exchange relationship with
the seawater to heat the latter in an otherwise conventional distillation process.
After heat exchange, the exhaust gases are released to atmosphere.
The advantages of using hot water as the thermal transfer
fluid for the desalinization by distillation include:
- a) Simplicity of the boiler and its installation;
- b) Low cost compared to an installation that uses steam to heat the seawater;
- c) Low pressure of the circulating pump; and
- d) No consumption of the thermal transfer fluid.
One disadvantage of using hot water as the thermal transfer
fluid for desalinization by distillation is the significant flow of water required
in the circuit (for the same transfer of calories), in comparison with the amount
of low pressure steam required. However, this disadvantage is outweighed by the
advantages noted above.
In its broader aspects, therefore, the invention relates
to a method of generating electrical power while simultaneously converting salt
water to fresh water comprising: a) supplying exhaust gases from a gas turbine used
to generate electrical power to a boiler located downstream of the gas turbine and
upstream of a gas turbine exhaust gas stack; b) employing a closed thermal transfer
fluid circuit between the boiler and a desalinization plant for recirculating the
thermal transfer fluid between a first heat exchange in the boiler and a second
heat exchanger in the desalinization plant where the thermal transfer fluid passes
in heat exchange relationship with the seawater to thereby heat the seawater as
a first step in a distillation process.
In another aspect, the invention relates to a combined
gas turbine power generating plant and seawater desalinization plant comprising
a gas turbine for generating electrical power and producing exhaust gases to be
released to atmosphere; a desalinization plant for removing salt from water; and
a closed thermal fluid circuit arranged between an exhaust gas duct of the gas turbine
and the desalinization plant, the circuit including a first heat exchanger arranged
to heat a thermal transfer fluid with heat from the gas turbine exhaust gases and
to supply heated thermal transfer fluid to the desalinization plant for heat exchange
with the seawater.
The invention will now be described in connection with
the accompanying drawing, which is a schematic flow diagram of a combined gas turbine
power generation/desalinization plant in accordance with the exemplary embodiment
of the invention.
With reference to the sole Figure, the plant 10 includes
a gas turbine 12 of conventional construction, used to produce electrical power.
The gas turbine is provided with an exhaust duct 14 leading to a first exhaust stack
16 used when the turbine is operated in an open cycle mode. A damper 18, operated
by control 20, is arranged to open or close the inlet to the stack 16. When stack
16 is closed by the damper, the gas turbine will operate in a cogeneration mode,
and the exhaust gases will flow into a boiler 22 provided with a heat exchanger
24. The exhaust gases will flow across the heat exchanger and be released to atmosphere
via a second exhaust stack 26. Heat from the exhaust gases is transferred to a thermal
transfer fluid flowing in the heat exchanger 24, and the heated thermal transfer
fluid is then supplied to a desalinization plant 28 via stream 30. One satisfactory
thermal transfer fluid is demineralized water, but other suitable liquids may be
employed. An accumulator 31 is employed in the stream 30 upstream of the desalinization
plant 28 to compensate for the dilatation of the thermal transfer fluid. The plant
28 is an otherwise conventional unit that desalts seawater by a commercially available
distillation process. Stream 30 feeds the thermal transfer fluid to heat exchanger
34 in the plant 28. Seawater flows into the desalinization plant 28 via inlet 36
and flows across the heat exchanger 34 to thereby increase the temperature of the
seawater to the level required for distillation. After the distillation process
is complete, brine is returned to the sea and removed via outlet 32, and fresh water
produced by the distillation process is taken from the plant 28 via outlet stream
The thermal transfer fluid, cooled via heat exchange with
the seawater, is returned to the boiler 22 via stream 40 and a circulation pump
42. Thus, a closed circuit thermal transfer fluid circuit is formed that includes
insulated piping (for streams 30, 40), heat exchangers 24 and 34, the accumulator
31 and pump 42.
The simple and effective installation makes efficient use
of the gas turbine exhaust gases for heating seawater in a distillation process.