Research Paper
Thermoeconomic and environmental analysis and optimization of the supercritical CO2 cycle integration in a simple cycle power plant

https://doi.org/10.1016/j.applthermaleng.2019.02.052Get rights and content

Highlights

  • Thermoeconomic and thermo-environmental study has been implemented.

  • A comprehensive parametric study has been done on thermoeconomic performance.

  • Optimization is performed considering thermoeconomic and thermo-environmental impacts.

  • Minimum total cost and total product unit cost are obtained.

Abstract

In the present work, a thermoeconomic and thermo-environmental analysis is carried out in Santa Rosa simple cycle thermal power plant (air Brayton cycle), located in Peru. Subsequently, the same analysis is applied to a proposed integrated thermal power plant, formed by the integration of a supercritical CO2 partial heating Brayton cycle in the mentioned simple cycle thermal power plant. Also, an exhaustive parametric study is carried out to investigate the effects of some decision variables on the thermoeconomic performance of the proposed system. Finally, the integrated thermal power plant is optimized from the thermoeconomic and environmental point of view, the objective function considered is the total cost, which is the sum of the capital investment cost, exergy destruction cost and environmental cost. The results show that the total product unit cost (electricity) is reduced by 10.37% respect to the initial condition. In addition, the specific investment cost and net efficiency are lower by 25.25% and 0.45% respectively, compared to a conventional combined cycle thermal power plant.

Introduction

Currently, energy is one of the most important elements for the economic development of a certain place because of its wide applicability, such as heat and electricity, in daily and industrial processes. As a consequence, combined cycle thermal power plants have greater interest in the field of power generation, due to their greater efficiency and lower environmental impact compared to single cycle and Rankine cycle power plants independently [1].

However, recently supercritical carbon dioxide (S-CO2) Brayton cycles have been investigated, which compared to other cycles, have greater cost advantage because of the use of more compact equipment, which is a consequence of the high density and work pressure of S-CO2. In addition, these cycles operate at lower temperatures, favoring the structural materials of the equipments [1], [2].

Due to the high costs of energy and the reduction of fossil fuels, energy conversion systems are being evaluated and optimized from the exergetic and thermoeconomic (exergoeconomic) points of view. The exergy analysis based on the first and second law of thermodynamics has become a key tool to quantify the inefficiencies in the processes and distinguish if the energy consumption is optimal [3]. The exergoeconomic is the field of the engineering that combines the exergética analysis with economic restrictions to provide crucial information for the design and profitable operation of a system [4].

Recent researches have been carried out about the thermoeconomic and environmental optimization of conventional thermal power plants, cogeneration systems and alternative thermodynamic cycles such as S-CO2 cycles, with the objective of increasing performance and reducing the total product unit cost. Ganjeh Kaviri et al. [5] conducted the modeling and optimization of a combined-cycle thermal power plant, whose results showed that the exergy destruction cost is greater in the combustion chamber and was reduced by increasing the turbine inlet gases temperature. In addition, with the pinch point increase, the exergy efficiency was reduced and the total cost increased. According to Selçuk Mert et al. [6] performed an exergoeconomic analysis in a cogeneration plant. The results showed that the highest amount of exergy destruction and potential for improvement occurs in the combustion chamber, while the lowest values occur in the gas turbine. Ganjeh Kaviri et al. [7] studied the effect of inlet gas temperature to heat recovery steam generator (HRSG) on cycle efficiency and exergy destruction. As a result, it was found that the greatest exergy destruction occurs in the high pressure evaporator. In addition, the inlet gas temperature to HRSG up to 650 °C increased the energetic and exergetic efficiency, after 650 °C, these efficiencies began to decrease. Luo et al. [8] studied a reconditioned natural gas cogeneration system (RNGCS) and an original natural gas cogeneration system (ONGCS). The results showed that the total energy and exergy efficiency of the RNGCS were 1357% and 1325% higher than those of the ONGCS.

Vandani et al. [9] studied the effect of the use of diesel instead of natural gas in a combined cycle thermal power plant. For natural gas, exergy efficiency and annual cost may improve by 2.34% and 4.99% respectively, while for diesel by 2.36% and 1.97%.

Respect to S-CO2 cycle configurations, multiple investigations are shown in the literature. Ahn et al. [10] presented a comparison of the different S-CO2 cycle configurations in terms of performance. These cycles can achieve higher performance with temperature of fluids between 450 and 600 °C. Crespi et al. [11] categorized and compared these configurations in terms of their performance. As a result, it was found that the average efficiency of the autonomous cycles was 40% and between 50% and 60% in the combined cycles. Persichilli et al. [12] conducted a commercial study of an S-CO2 cycle and a Rankine steam cycle. The authors reported that the levelized electricity cost (LCOE) of the S-CO2 cycle ranges from 0.019 to 0.031 $/kWh in base load, while between 0.047 and 0.075 $/kWh in cyclic operation. In the Rankine steam cycle, the LCOE ranges from 0.036 to 0.043 $/kWh in base load and between 0.105 and 0.126 $/kWh in cyclic operation. Nami et al. [13] analyzed and optimized in an exergoeconomic and exergoambiental way a new cogeneration system, integrating an S-CO2 cycle. The authors achieved a reduction of 0.56 $/GJ in the average product unit cost of the system, which represents 2.8% with respect to the base condition. Kim et al. [14] studied 9 S-CO2 cycles in order to obtain the cycle with the highest thermodynamic performance. The authors determined that the cycle of double heating and flow division generates the maximum net power of 3.23 MW, however, the partial heating cycle can generate 2.75 MW with a lower number of equipment and less complexity in the operation. Akbari et al. [15] performed a thermoeconomic analysis and optimization of a S-CO2 recompression Brayton cycle (SCRBC) and a combined cycle, consisting of an SCRBC and an Organic Rankine cycle (SCRB/ORC). The authors reported that the exergy efficiency of the SCRB/ORC is 11.7% greater than that of the SCRB while the total product unit cost of the SCRB/ORC is 5.7% lower than that of the SCRB.

Some research works have been published in literature regarding other heat recovery cycles. According to Yari et al. [16] studied from the thermodynamic and thermoeconomic viewpoints a trilateral Rankine cycle (TLC), an organic Rankine cycle (ORC) and a Kalina cycle (KCS11). Using n-butane and considering the isentropic efficiency of the TLC expander of 0.85, the lowest product cost equal to $16.01/GJ (57.63 $/MWh) was obtained. Khaljani et al. [17] integrated an organic Rankine cycle (ORC) in a cogeneration plant. As a result, they obtained an additional power of 1.93% and an exergoeconomic factor of 10.59%. Iglesias et al carried out a review of the most relevant thermodynamic cycles used for low-grade heat sources in two main sections; Rankine organic cycles (ORC) and trilateral cycles (TC). Most of the revised ORCs have first law efficiencies between 5% and 10%. These values are low compared to those obtained by the reviewed TCs, which are between 36% and 51% [18].

With the present study it is proposed to obtain a new configuration of a thermal power plant, considering the integration of an S-CO2 cycle in a single cycle thermal power plant, whose characteristics are low total product unit cost (electricity), high energy efficiency and lower specific investment cost that the combined cycle thermal power plants.

The main objective of this article is to reduce the total product unit cost of the integration of a S-CO2 partial heating cycle in a single cycle thermal power plant. For the optimization, the total cost of the proposed system, which is formed by exergy destruction cost, environmental cost and capital investment cost, is considered as objective function.

Considering variables such as compression ratio of compressor 1, the used air by required air ratio, the compression ratio of compressor 2, the isentropic efficiency of compressor 2, the isentropic efficiency of gas turbine 2, the temperature difference between the hot fluid at the inlet and the cold fluid at the outlet of the heat recovery unit and the outlet gases temperature of the heat exchanger 1, a parametric study is developed to see the effects on the total product unit cost and the total net power of the integrated cycle. Finally, thermoeconomic and environmental optimization of the integrated cycle is carried out, using the Direct search method in EES software.

Section snippets

System description and assumptions

The generation unit TG8, which forms Santa Rosa single-cycle thermal power plant in Peru, is evaluated in a thermoeconomic and exergoenvironmental way. Subsequently, the integration of a partial heating cycle S-CO2 in the single cycle thermal power plant, is analyzed and optimized in a thermoeconomic and environmental way. Table 1 [19] shows the technical specifications of the SGT6 5000F model turbine, which corresponds to the generating unit TG8 of the Santa Rosa thermal power plant. Table 2

Thermoeconomic analysis

Thermoeconomic analysis allows the combination of thermodynamic analysis with economic principles at the component level in a thermal system. Also, provides information that is not possible to obtain through an independent analysis of energy, exergy and economic. The main objective of the thermoeconomic analysis is to determine the unit cost of the products, in different components of the thermal system. In the present work, each component of the integrated thermal system is considered as a

Thermo-environmental analysis

The total cost of the proposed thermal power plant is proportional to the costs associated with the exergy destruction, the environmental impact and the investment, operation and maintenance cost. The reduction of environmental impact and destroyed exergy implies a higher investment cost. The thermo-environmental analysis provides the cost function related to environmental damage. In the present study, the effects of carbon monoxide (CO) and nitrogen oxide (NOx) on the cost of total

Thermoeconomic optimization

The main objective of this article is to reduce the total product unit cost of the integrated thermal power plant, through thermoeconomic and environmental optimization of the total cost rate. The total cost rate, which is the objective function, considers the costs associated with exergy destruction, environmental impact and capital investment as well as operating and maintenance. The three mentioned costs are in conflict, because the reduction of the sum of the first two costs, would imply an

Exergetic and thermoeconomic analysis.

For the exergy and thermoeconomic analysis carried out in the present study, the current conditions of the Santa Rosa simple cycle thermal power plant are considered.

The results presented in Table 8 indicate that for the simple cycle, the variation between the output variables presented by Chiok [27] and those obtained in the present work are less than 5%.

Also, the results of the S-CO2 cycle presented in Table 9 indicate that the variation between the results obtained by Kim et al. [14] and the

Conclusion

By means of the thermoeconomic and environmental analysis applied to the simple cycle thermal power plant, the total cost and total product unit cost are determined and have a value of 2.38 $/s and 47.19 $/MWh, respectively.

The integration of a S-CO2 partial heating cycle in the simple cycle thermal power plant to form the integrated thermal power plant, is also analyzed in a thermoeconomic and environmental manner. The results obtained show an exergoeconomic factor of 62.36%, in addition,

Glossary

Acronyms

C1
Compressor 1
CC
Combustion chamber
TG1
Gas turbine 1
G1
Generator 2
IC1
Heat exchanger 1
IC2
Heat exchanger 2
HR
Heat recovery
IC3
Heat exchanger 3
C2
Compressor 2
TG2
Gas turbine 2
G2
Generator 2

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