FIELD OF THE INVENTION The present invention relates to cooling exhaust gas of a power generation system, a method of producing fresh water, a power generation system and a system for producing fresh water.

BACKGROUND OF THE INVENTION

Heat energy from exhaust gases of power generation systems is generally lost. Utilisation of this energy can lead to improvements in overall efficiency of the system.

For example, combined cycle power plants having both gas and steam turbines are recognised as being efficient systems for generating electricity and useful heat from a fuel source, however a significant amount of energy, estimated to be in the order of 30%, is lost through heat in the form of steam exiting the steam turbine. Conventionally, this steam has been cooled through evaporation in a cooling tower where the evaporating steam is returned to the atmosphere. In such systems, a significant amount of the waste heat is not utilised and the evaporated steam is not available to be condensed and returned to the process as water for use in the steam cycle. Accordingly, previous combined cycle power plants consume both fuel and water and require readily available sources of both fuel and water for operation, thereby restricting the number of potential sites available for use. It has been found that the efficiency of a steam turbine reduces if the change in condensing pressure exiting the turbine increases. As such, it is frequently not possible to limit the condensing pressures to optimise the efficiency of the steam cycle as the cooling water temperature determines the pressure of the steam exiting the steam turbine to reduce water consumption. Some locations, such as those in the Middle East for example, have access to abundant fuel and sea water, which requires desalination for use, but limited fresh water. As significant energy is required for desalination processes, the overall efficiency of a power plant using desalinated water for cooling is significantly reduced. Also, as desalination plants draw water from oceans, marine life can be affected if careful consideration is not given to the location of a water intake and higher temperature discharge.

Examples of the invention seek to solve, or at least ameliorate, one or more disadvantages of previous power generation systems.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a method of cooling exhaust gas of a power generation system, including the steps of:

removing heat from the exhaust gas by evaporating a refrigerant of a vapour- compression refrigeration cycle system in a condenser or heat exchanger;

passing the refrigerant through the vapour-compression refrigeration cycle system to remove heat from the refrigerant; and

returning the cooled refrigerant to the condenser or heat exchanger,

to cool the exhaust gas continuously.

According to a preferred embodiment, the method being a method of cooling and condensing said exhaust gas, wherein the exhaust gas is cooled and condensed in said condenser to form a liquid.

Preferably, the exhaust gas is steam. The power generation system can be a combined- cycle gas/steam power generation system.

According to a preferred embodiment, the refrigerant comprises ammonia, the steam is condensed at a temperature in the range of 38°C to 48°C and the ammonia evaporates at approximately 75°C. The method can include the step of returning feed water, produced by condensing the steam, to a heat exchanger in communication with exhaust gases from a gas turbine, to convert the feed water to steam for use in a steam generator.

Preferably, the refrigerant comprises ammonia.

The method can further include the step of cooling the refrigerant with ambient air using a fan. The fan can be driven using electricity generated from said system,

Preferably, the refrigerant is compressed using electricity generated from the power generation system.

According to one aspect of the present invention, there is provided a method of producing fresh water, including the steps of:

cooling exhaust gas of a power generation system via a method according to any one of the preceding claims; and

heating salt water with the heat removed from the refrigerant to desalinate the water.

According to preferred embodiments, the salt water is sea water. Preferably, the salt water is subjected to a multiple effect distillation process using heat removed from the refrigerant. The heat removed from the refrigerant can be used to heat an intermediate fluid which is then used to heat the salt water.

Preferably the power generation system includes a condenser or heat exchanger for condensing or cooling exhaust gas of the system, the condenser or heat exchanger being configured to remove heat from the gas to evaporate a refrigerant of a vapour-compression refrigeration cycle system. The system can be configured such that the cooled refrigerant is returned to the condenser or heat exchanger, whereby the exhaust gas is cooled continuously. The power generation system can further include:

a gas turbine;

a steam turbine driven by steam heated by exhaust gases of the gas turbine; and said condenser, being a steam condenser for condensing exhaust steam, of which said exhaust gas of the system is comprised,

the condenser being configured to remove heat from the gas to evaporate the refrigerant.

The power generation system can include said vapour-compression refrigeration cycle system.

Preferably, a refrigerant compressor of the refrigeration cycle system is powered by electricity generated by the gas turbine and/or the steam turbine.

Preferably, the power generation system further includes a water return system to return water exiting the condenser for heating by exhaust gases of the gas turbine.

Preferably, said refrigeration cycle system is in communication with the condenser or heat exchanger and has an air-cooled refrigerant condenser for expelling process waste heat to the atmosphere.

According to preferred embodiments, the refrigerant used in the refrigeration cycle system is ammonia. Preferably, the said condenser is air-cooled.

The power generation system can further include, in parallel with said condenser, a secondary refrigerant condenser configured to heat district heating water. BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will be further described, by way of non-limiting example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of the energy distribution in a conventional combined cycle power plant;

Figure 2 is a schematic diagram of a conventional combined cycle power plant; Figure 3 is a schematic diagram of a power generation system of one embodiment of the invention; and

Figure 4 is a schematic diagram of a power generation system of another embodiment of the invention.

DETAILED DESCRIPTION

A power generation system 10 according to a one embodiment of the present invention is shown schematically in Figure 3. The power generation system 10 includes a gas turbine 12, a steam turbine 14 driven by steam heated by exhaust gases of the gas turbine 12, and a steam condenser 16 for condensing the exhaust steam from the steam turbine 14, The condenser 16 is configured to act as a heat pump and remove heat from the steam to evaporate a refrigerant of a vapour compression refrigeration cycle system 18.

The condenser 16 operates as a heat exchanger and evaporates the refrigerant directly so that the process is simplified and only one step of heat exchange is required. As the cooling water loop of previous power plants is removed, the process can be more efficient as high power condenser water circulating pumps are no longer required. The condenser 16 may be constructed as a conventional heat exchanger in which steam is received on one side and exits as water and on another side a refrigerant enters as either a liquid or gas, or a partial mixture thereof, passing through coils in contact with the steam so as to draw heat from the steam and causing the steam to condense and the refrigerant to evaporate, thereby providing a gaseous refrigerant from which heat can be expelled to the atmosphere and water for return to the steam cycle turbine.

In the described embodiment, the power generation system 10 includes a number of components common to conventional combined cycle gas/steam power generation systems, such as an air intake compressor 20, a combustor 22 in which fuel is combusted with air from the compressor 20, a generator 24 coupled to and driven by the gas turbine 12 for generating electricity, a heat recovery steam generator 26 for extracting heat from exhaust gases of the gas turbine 12 and generating steam for use in the steam turbine 14, and a generator 28 coupled to and driven by the steam turbine 14 for generating electricity.

The power generation system 10 further includes a vapour compression refrigeration cycle system 18 which is in communication with the condenser 16. The vapour compression refrigeration cycle system 18 operates in the same manner as a conventional mechanical refrigeration system and has a refrigerant compressor 30 for compressing the refrigerant, which has been evaporated in the condenser, to a superheated state. The vapour compression refrigeration cycle system 18 also has an air cooled refrigerant condenser 32 for expelling process waste heat to the atmosphere. In preferred embodiments, the refrigerant is selected so as to vaporise at ambient temperatures, thereby allowing air to be used to remove heat from the process without requiring the use of water.

The vapour compression refrigeration cycle system 18 operates in a closed state so that the refrigerant used is evaporated in the steam condenser 16, then superheated as it passes through the compressor 30, condenses in the air cooled condenser 32 to expel heat from the refrigerant to the atmosphere, passes through an expansion valve 34 to reduce pressure of the refrigerant, and then returns to the steam condenser 16 to remove further heat from the exhaust steam. As the described process is continuous, it will be appreciated that heat can constantly be removed from exhaust steam without evaporating the steam to the atmosphere. Once the steam has been condensed and cooled it can be re-used in the heat recovery steam generator 26. The power generation system 10 includes a water return system 36 to return water exiting the steam condenser 1 to the heat recovery steam generator 26 for heating by exhaust gases of the gas turbine 12. Although the exhaust steam will be condensed and returned to the process as liquid, a make-up water source 38 is provided to supplement any water consumed or lost in the process.

Conventional combined cycle gas/steam power plants, such as that shown schematically in Figure 2, have previously used a cooling tower 1 1 to remove waste heat from steam exiting the steam turbine to condense the steam. As the cooling tower evaporates steam into the atmosphere, water is consumed and not available for return to the heat recovery steam generator 26. Previous combined cycle power plants have used 1,800 litres/hour per megawatt of generation capacity or 43,200 litres per megawatt for a 24 hour day. For a 285 megawatt combined cycle power plant, the water consumption would be 12.36 million litres per day. Furthermore, cooling towers can be large and visually unpleasant and can be sources for legionella bacteria to grow.

Combined cycle gas/steam power plants used in locations having limited access to fresh water have previously used desalination processes to provide fresh water for the heat recovery steam generator 26. It is estimated that 0,7% of the plant generating capacity (based on an energy intensity of 4kWh per kl of fresh water) would be used by the desalination processes, thereby reducing the overall efficiency of the power plant. Other environmental issues also arise out of the use of cooling water.

It will be appreciated that energy is required to operate the refrigerant compressor 30 and a fan 35 of the air cooled refrigerant condenser 32. This energy is taken from the output of the generators 24, 28. It is estimated that 4.5% of the output of the plant would be used to drive the refrigerant compressor 30 and 1.5% of the output would be used to drive the fan 35. Although as much as 6% of the power generated is used to operate the vapour compression refrigeration cycle system 18, the power generation system 10 does not require a source of cooling water to operate.

In operating the power generation system 10, a method of cooling and condensing exhaust steam is used, the method including the steps of: condensing the exhaust steam in a condenser 16 to form a liquid, the condenser 16 being configured to remove heat from the steam and use the removed heat to evaporating a refrigerant of a vapour-compression refrigeration cycle system 18; passing the refrigerant through the vapour-compression refrigeration cycle system 18 to remove heat from the refrigerant; and returning the cooled refrigerant to the condenser 16 to continuously cool exhaust steam.

A power generation system 1 10 according to another preferred embodiment of the present invention is shown schematically in Figure 4. The poweT generation system 1 10 shares a number of common components with the power generation system 10 and these components are similarly numbered, though incremented by 1 0.

Although a number of components are common, the power generation system 1 10 does not include an air cooled refrigerant condenser 32 but instead has a heat exchanger or condenser 1 0 which is configured to provide useful heat for the production of fresh water or desalination of sea water, which will be explained further below.

The power generation system 110 also includes a first heat exchanger 142 in communication with exhaust gases exiting the heat recovery steam generator 126 for heating domestic and sanitary hot water. The power generation system 1 10 also includes additional lines G to supply liquid refrigerant to parts of the process. The liquid refrigerant can also be used in air inlet cooler 146 to cool air entering the gas turbine. The power generation system 1 10 also includes a second heat excha ger 144 in communication with refrigerant line G to collect heat from exhaust gases.

Via refrigerant line F, heated refrigerant is provided to the condenser/heat exchanger 140 to heat water for the production of fresh water or for desalination. Condenser/heat exchanger 140 may directly heat water which is to be subjected to treatment or an intermediate fluid may be utilised,

The production of fresh water/desalination process is carried out using a multi effect or multi stage distillation process in which the water to be treated is progressively distilled. Preferably, the process has between 5 and 7 stages.

Condenser 1 16 acts as a heat pump and it has been found that by condensing steam in the condenser 1 16, preferably by condensing steam on a surface of the condenser, within a temperature range of approximately 38 to approximately 48 degrees Celsius, and preferably at 43 degrees, ammonia can be evaporated to a temperature of around 70 degrees Celsius. Once compressed, the temperature of the ammonia rises to approximately 75 and by condensing ammonia at 75 degrees Celsius, it is expected that water available for the fresh water production/desalination will be approximately 70 degrees Celsius. It has been found that operating the multi effect distillation process at 70 degrees Celsius requires electric power 1.5 kWh/m ³ and thermal power of 8O.0 kWh/m ³ (290 kJ/kg),

Previous methods of desalination have taken steam directly from the steam turbine and used the steam for high temperature desalination processes. However, taking steam directly from the steam turbine reduces the amount of power that the process can generate.

It has previously been considered that high temperature steam was necessary for efficient desalination processes. As the efficiency of a heat pump greatly reduces with high temperatures, use of a heat pump in connection with utilising waste process heat for desalination has been considered counterintuitive. Furthermore, to maximise efficiency of the steam turbine it is desirable to maintain a large temperature differential over the condensing process, bearing in mind that condensing the steam too far can lead to damage to the turbine.

By utilising a heat pump, a greater temperature differential may be used over the steam turbine and the heat energy from the low temperature waste steam can still be efficiently harnessed for use in tire production of fresh water or a desalination process. It will be appreciated that further system improvements are possible. For example, a heat exchanger for recovery of heat from waste brine may be provided. Similarly, heat may be recovered from cooling oil used in either turbine. Furthermore, heat may be extracted from the intake air for the gas turbine to maximise its efficiency, particularly in hot climates.

It will be appreciated that the described invention has great potential in connection with harnessing heat energy that would be otherwise wasted. For example, a combined cycle can produce 0.67kWh of heat per kWh of electrical energy and a conventional power plant can produce 1.50kWh of heat per kWh of electrical energy.

In view of the above principles, it has been estimated that a for a combined gas/steam cycle power generation system, 65M1 of water per MW. of electricity produced per annum can be produced and 6 Ml of cooling water saved per MW per annum. Similarly, for a conventional power plant 145 Ml of water per MW of electricity produced per annum can be produced and 13 Ml of cooling water saved per MW per annum.

It has also been estimated that in a 760 MW power generation plant, approximately 500 MW of heat energy will be wasted, assuming approximately a 65% recovery and assuming a that around 80% of this wasted energy can be recovered, approximately 488 Gl of fresh water could be produced.

Although the illustrated embodiments have been described in relation to utilising steam to heat the refrigerant, it will be appreciated that other exhaust gases could be used, in which case the illustrated condenser could be substituted for a heat exchanger. For example, exhaust gases from a gas . turbine system may be used, or exhaust gases from the combustion of other combustible materials such as diesel.

Furthermore, it is envisaged that many different refrigerants may be used for the vapour- compression refrigeration cycle system 18, 1 18, such as carbon dioxide, ammonia or hydrocarbon based, for example. Preferably, the refrigerant used is ammonia. Preferred embodiments of the invention may provide the following advantages:

for hot climates, maximise combined cycle plant cycle to up to 70% efficiency with water cooled systems with air cooled refrigerant condensing, saving water and eliminating cooling tower vapour plumes;

for temperate climates, maximise cooling and heating with water cooled systems for hot dry climates, use refrigerant cooled condensing to save water associated infrastructures and water treatment chemicals. Further advantages of preferred embodiments of the invention will be apparent when consideration is given to the following tables.

Table 1 ■ Summary of parasitic energy use of tc

percentages of primary energy input into the system.

(1) Large consumplion of cooling Waler for refrigerant condensing in both compression and absorption cooling systems.

(2) Water is produced with RO procedure @ about 4kWh/kl

(3) Energy consumed by refrigerafioh compressors and fans of air cooled refrigerant condensers.

(4) Energy consumed by refrigeration compressor

NB. The parasitic loads are identified as those outside the district served by the district energy cent/e.

Table 2 - Evaluation of Total Systems

Table 3 - Evaluation of Total Systems (continued)

Chan 1 - Ammonia Compressor heat pump COPs

Π SWRO: Spiral wound reverse osmosis

Chart 2 - Percentage change in load & heat rate due to change in steam condensing pressure

Change in ondoncor BacK Pressure (in. g.)

The embodiments have been described by way of example only and modifications are possible within the scope of the invention disclosed.