Energy and Environmental Comparison of Different Scenarios for Cogeneration of Power and Desalinated Water in Iran

Nomenclature

1. Introduction

1.1.   Literature review

Co-generation of desalinated water and electricity in southern areas of Iran which suffer from lack of enough freshwater sources is critical in attaining sustainable development targets. As a result, efforts have been made to utilize the desalination systems to produce fresh water from seawater. On the other hand, there are several gas turbine power plants in the southern region which produce large amounts of exhaust gases with relatively high temperature. So, there is a great opportunity for integrating the existing gas turbine power plants with thermal desalination systems for covering parts of the freshwater demands of southern cities of the country.  As in the wintertime, the total natural gas consumption of the country increases considerably, the total efficiency of these systems is of great importance [1].

Thermal desalination is one of the most useful applications of cogeneration. Two major types of thermal desalination systems include MED and MSF. Different combinations of desalination systems have been studied in the literature [2, 3]. Desalination systems, in general, can be operated using low-grade heat from fossil fuel plants or renewable energy systems [4-6]. Different renewable energy systems may be incorporated into thermal systems for desalinated water production, including biomass combustion [7], a combination of solar and geothermal [8], and biomass gasification [9]. In our previous work, an analysis of multi-generation system fed by combined natural gas and synthesis gas (produced in a biomass gasification reactor) was performed [10].

Table 1 summarizes the recent research works, regarding the combined production of electricity and desalinated water with the use of fossil fuels.

Table 1: research works regarding the combined generation of freshwater and electricity

1.2.  knowledge contribution and novelties

The major novelties of this paper are as follows:

-        Simultaneous thermodynamic, economic, and environmental analysis of three different configurations for co-production of electricity and freshwater

-        Introducing a novel evaluation methodology for comparing combined cycles of power, cooling, and water production based on the aggregate effects of economic and thermodynamic characteristics.

-        Determination of the impact of inlet air cooling on the CO₂ emission of different configurations of heat recovery steam generator linked to the gas turbine package and desalination system.

Using the proposed configuration, both electricity and water demands of an area could be covered with higher efficiency. So, the proposed system can help in reaching a more sustainable future.

2. Process description and system configuration

As it will be shown in the figures, all configurations are similar in having a gas turbine which is a Siemens V94.2 (with a nominal power output of 148.8 MW in 15°C inlet air temperature) and the desalination system which is chosen to be MSF (Multi-Stage Flash) system, but the type of steam turbine and HRSG and consequently the way the desalination system is fed differs from case to case.

Furthermore, each configuration is analyzed in two different conditions: with inlet air cooling and without inlet air cooling.

2.1. Scenario 1

In this scenario (Fig. 1), the cycle is primarily composed of a Siemens v94.2 gas turbine, a one pressure HRSG, a backpressure steam turbine, a Multi-Stage Flash (MSF) desalination system for producing desalinated water, and a chiller for providing chilled water for inlet air cooling (If required).

As the steam turbine is backpressure type, the exiting steam of the steam turbine with required pressure is directed to brine heater as motive steam to heat the seawater. The heated seawater then goes to the first stage of the MSF system. Part of the heated seawater is flashed due to the low pressure of this stage relative to the vapor pressure of the heated brine. The flashed water is collected after condensing on the outer surface of the tube bundle of that stage.

The inputs of this scenario in two conditions, with inlet air cooling and without inlet air cooling, are presented in the table2:

Fig. 1 The schematic diagram of the cycle of scenario 1

Table 2: input parameters of scenario 1 

Fig. 2 The schematic diagram of the cycle of scenario 2

Table 3: input parameters of scenario 2

2.2. Scenario 2

As can be seen in Fig. 2, there is no steam turbine in this scenario, and the steam produced in the one pressure HRSG is directly used in the brine heater of the MSF system. The motive steam has a pressure of 2bar and a temperature of 121^c. The steam leaves the brine heater with a temperature of 117.5^c and in a saturated condition. The produced condensate returns to HRSG through the feed pump. The input data of this scenario is shown in table 3.

2.3. Scenario 3

In this case, two pressured HRSG is utilized. All the produced low-pressure steam plus a percent of low-pressure steam which is extracted from low-pressure stages of steam turbine, are mixed and directed to the brine heater, to increase the brine temperature and act as motive steam. The superheated steam generated in a high-pressure section of HRSG is utilized for power generation. Fig. 3 shows the schematic diagram of this configuration. Extraction of a portion of steam will decrease the steam turbines’ power output; on the other hand, it increases the produced desalinated water. The presence of a condenser is indispensable in this configuration. The HRSG is comprised of three economizers, two evaporators, one deaerator, and one superheater with an industry acceptable configuration. Table 4 shows the major inputs of this configuration.

Fig. 3 The schematic diagram of the cycle of scenario 3

Table 4: input parameters of scenario 3

3. Simulation

3.1. Thermodynamic Simulation

Thermodynamic, economic, and environmental simulation of the three configurations are described in this section. The first step is to determine both the inputs and the outputs of the simulation.

The inputs include  changeable decision variables and system parameters which are fixed during the simulation. The outputs are calculated from inputs, which are the fundamental laws of thermodynamics. 

The major assumptions for simulation are shown in table 5.  Siemens V94.2 gas turbine is selected for all three configurations and the ambient air temperature is considered 35 °C.

Other required inputs that are necessary for simulation include HP and LP steam pressure and temperature as well as the pinch and approach point temperature of heat recovery steam generator.

Dependent variables that are the desired outputs of the configurations are calculated based on the decision variables and simulation parameters. Major dependent variables are:

1- Power produced by the gas turbine

2- The gross power generation

3- The net power generation

4- The natural gas (input fuel) flow rate

5- Freshwater production flow

6- Total capital investment

7- Period of return

8- The net annual profit

9- CO₂ emission

Table 5: Major assumptions in all the three configurations

3.1.1. Gas Turbine Simulation

The gas turbine simulation is conducted through the thermodynamics laws, namely the first and the second laws of thermodynamics. Isentropic efficiency as well as mechanical efficiency concepts are employed in the simulation. It is assumed that the streams' physical properties are calculated in mean temperature and both combustion products and ambient air act as an ideal gas. The gas turbine schematic diagram is displayed in Fig. 4. The complete formulation for gas turbine modeling can be found in [26].

Fig. 4 The schematic diagram of the gas turbine

After analyzing the gas turbine and finding the mass flow rate, the composition, and the temperature of the leaving flue gas, the next stage is to calculate the characteristics of the water streams (i.e. the enthalpy and the entropy). For this purpose, it is necessary to determine the pressure and temperature of the waterside.

3.1.2. HRSG simulation

Finding the generated steam flow rate and water and gas temperature of all involved streams is the goal of HRSG simulation. For this purpose, it is necessary to do the pressure analysis of the gas and waterside of the system.

According to the assumptions of pressure drop in different parts of HRSG and with the use of the known pressures of the cycle, the pressure of every single stream can be found. The waterside pressure drop of the economizer is considered 3% of inlet pressure. Also, the waterside pressure drop of the superheater is considered 3.5% of inlet pressure. No pressure drop is considered for the waterside of the evaporator as it works in a saturated condition. The gas side pressure drop is assumed 2 mbar for each part of the HRSG.

3.1.3. Power calculation procedure

To calculate the output power of the steam turbine or the input power of water pumps, it is necessary to know flow rates, the enthalpies, and the mechanical efficiencies of the steam turbine and water pumps. The flow rates and the enthalpies are calculated according to the procedure mentioned in 3.1.2. The mechanical efficiencies are considered fixed as was mentioned in table 5.

3.2. Economic modeling 

The major goal of the economic simulation is to determine the produced power and freshwater cost. It is also necessary to calculate the total annual profit of each configuration as one of the aims of the economic simulation. Power and freshwater costs are dependent on the net power and water output, O&M costs, fuel cost, etc. Since each configuration has its characteristics, its power and freshwater costs are different. Table 6 shows the major assumptions for economic simulation.

Table 6: Major economic data

3.2.1. Electrical power cost

The cost of produced electricity is composed of four terms, which stem from capital investment, fixed O&M cost, variable O&M cost, and fuel cost.

Capital investment includes the cost of all the major pieces of equipment of the system as well as all the minor costs. Furthermore, structural and civil works, piping work, equipment erection, wiring and electrical as well as the startup and engineering should be taken into account The electricity cost related to capital investment is determined through equation 1 below:

“CAP” is the total investment (10^6 $) which is extracted from Thermoflow software [27], “CRF” is the capital recovery factor which is calculated according to equation 2, “n” is operation years, “i” is the rate of interest, “P” is the net electricity output (MW), and “av” is the availability of the plant.

Fixed O&M cost ranges from 10 to 40 $/kW-year. for  the three above-mentioned configurations, 20 $/kW-year is considered [27].

Variable O&M cost which is usually expressed in $/kWh. For a co-generation plant, a reasonable value is 0.002 $/kWh as per the currently working plants of Iran.

Considering the price of the fuel which is assumed 0.003 $/(MJ LHV) and using equation 3, fuel cost can be calculated

HR is the Heat Rate (MJ/kWh) and is calculated by equation 4:

is the fuel flow rate (kg/s) and LHV is the fuel low heating value (kJ/kg).

3.2.2. Freshwater cost

The freshwater cost is composed of three parts which stem from capital investment, fixed O&M cost, and variable O&M cost ($/m³.)

Capital investment is composed of transfer pumps, brine heat exchanger, stages, and related electrical, civil and structural, and piping works.

Equation 5 is utilized to calculate freshwater costs.

The fixed and variable O&M costs of the desalination plant, are assumed 0.1159 $/m³ and 0.0783 $/m³ respectively. The total cost of freshwater production is the summation of the three above-mentioned terms.

3.3. Results of thermodynamic and economic modeling

The final results of thermodynamic and economic simulation of the first configuration are displayed in table 7.

As can be seen from Table 7, the total cost of produced power is 0.0286 $/kWh and the total cost of produced water is 0.7408 $/m³. Also, the total annual profit which comes from selling power and water to the market is 35.103 M$ and the CHP efficiency is 67.08.

In table 8, a precise comparison between the three configurations for the combined production of power and desalinated power mentioned above is shown. CHP efficiency, Total annual profit, and the total cost of produced power and water are among the comparison parameters. For every configuration, two conditions are presented. The first one is the condition in which no inlet air cooling is applied (i.e. the inlet air temperature is 35 °C) and the second one is the condition that incorporates inlet air cooling for power augmentation (i.e. the inlet air temperature is 15 °C).

Table 8 shows that configuration 1 with inlet air cooling, has the highest total annual profit whereas configuration 2 with inlet air cooling, in which the steam produced in the one pressure HRSG is directly used in the brine heater, brings the highest CHP efficiency. The cost of electricity generation is the highest in configuration 3 with inlet air cooling (which is 0.0317 $/kWh) while the corresponding cost is lowest in configuration 2 with inlet air cooling (which is 0.0257 $/kWh). The cost of produced water is almost independent of inlet air cooling. The highest price of freshwater generation occurs in configuration 3 and the lowest occurs in configuration 2.

Table 7: thermodynamic and economic results of the first configuration (15 °C inlet air temp.)

Table 8: the comparison between thermodynamic and economic performance of all three configurations

3.3.1. Impact of the GT inlet air temperature on CO₂ emission

The Impact of changing inlet air cooling on the value CO₂ emission per MW of generated electricity is displayed in Fig. 5. In configuration 2, the inlet air temperature increases will increase the CO₂ emission too. On the other hand, the rate of CO₂ increase is marginal in configuration 1 and almost negligible in configuration 3. In fact, in configuration 2, increasing the air inlet temperature from 5 to 30 will increase the CO₂ emission from 599.05 kg/MWh to 632.41 kg/MWh which is a 5.5% increase.

The reasoning is that in configuration and 3; the rate of decrease of electricity generation (as a result of inlet air temperature increase) is almost identical to the rate of decrease in natural gas consumption and consequently the exhaust flue gas mas flow rate.  But in configuration 2, the rate of decrease of electricity generation (as a result of inlet air temperature increase) is higher than the rate of decrease in the exhaust flue gas mass flow rate which is mainly due to the system arrangement.

Fig. 5 Carbon Dioxide emission as a function of GT inlet air temperature

3.3.2. Impact of the GT inlet air temperature on desalinated water production

The impact of inlet air temperature on total desalinated water generation can be seen in Fig. 6.

As can be observed, as the inlet air temperature increases, three different trends are observable depending on the configuration. For the first configuration, a marginal increase in freshwater generation (from 488.2 kg/s to 492.6 kg/s) occurs. For the second configuration, a decrease in freshwater generation from 855.2 kg/s to 829.1 kg/s occurs and in the third configuration, no change occurs due to the change in inlet air temperature.

Fig. 6 Total desalinated flow as a function of GT inlet air temperature

3.3.3. Impact of the GT inlet air temperature on gross electrical output

By changing the inlet air temperature from 5 ºC to 30 ºC the gross electric power decreases with the same trend in all three configurations. In configuration 3, the gross power decreases from 198.57 MW to 175.57 MW. Similarly, in configuration 2, the gross power decreases from 153.43 MW to 130.08 MW. Finally, in configuration 3, a decrease from 224.12 MW to 199.87 MW can be observed.

Fig. 7 Gross electric output as a function of GT inlet air temperature

3.3.4. Impact of the GT inlet air temperature on net electricity generation output

Similar to the trend of change of gross electricity generation due to the inlet air temperature, the net electricity generation decreases as the inlet air temperature increases from 5 ºC to 30 ºC. The course of change is shown in Fig. 8 below. Fig. 8 shows the rate of change of net electricity generation, due to the change of inlet air temperature is gentler than the rate of change of gross electricity generation. It is also observable that the rate of change of net power generation due to the inlet air temperature variation is not linear in all of the three configurations.

For example, in configuration 1, the maximum rate of power generation change occurs between 10 ºC and 25 ºC. Generally speaking, the minimum change rate occurs between 5 ºC and 10 ºC in all the configurations.

Fig. 8 Net electrical output as a function of GT inlet air temperature

3.3.5. Impact of the GT inlet air temperature on gross electrical efficiency

The impact of changing gas turbine inlet air temperature on gross electrical efficiency is demonstrated in Fig. 9 below. As it is displayed, the gross electric efficiency is independent of inlet air temperature in configuration 3. On the other hand, in configuration 2, the gross electric efficiency decreases as the inlet air temperature increases. Finally, in configuration 1, a marginal decrease in gross electric efficiency can be observed as the inlet air temperature increases, the efficiency decreases from 43.17% to 42.64%.

Fig. 9 Gross electrical efficiency as a function GT inlet air temperature

3.3.6 Impact of the GT inlet air temperature on net electrical efficiency

Fig. 10 demonstrates the effect of inlet air temperature on net electrical efficiency. Contrary to the trend of gross electrical efficiency which was discussed above, the net electrical efficiency increases as the inlet air temperature increases. It is observed that as a result of increasing inlet air temperature from 5 ºC to 30 ºC, the efficiency increases from 38.12% to 39.64%. It is also observed that the rate of increase is quite the same for configuration3. In configuration 2, a marginal increase from 26.57% to 26.88% occurs as the temperature increases from 5 ºC to 10 ºC but no further increase occurs as the temperature increases from 10 ºC to 30 ºC.

Fig. 10 Net electrical efficiency as a function GT inlet air temperature