Thermo-Economic Analysis of Organic Rankine Cycle with Different Working Fluids for Waste Heat Recovery from Coal-based Power Plant

The utilization of waste heat from power plants, which is generally lost to the atmosphere, can reduce energy waste significantly. Heat recovery systems can be integrated with power plants to utilize the waste heat, improving plant energy efficiency and reducing fossil fuel consumption and carbon emissions. The current study is focused on harnessing waste heat through the Organic Rankine Cycle (ORC) from 500 MWe supercritical power plant with CO2 capture. The simulation flow sheet program “Cycle-Tempo” models and simulates different plant layouts. This study considered five different working fluids for ORC, such as R245fa, Benzene, Methanol, Ethanol, and Acetone. The ORC generates additional electricity of 9.91 MWe for R245fa, 14.11 MWe for Benzene, 13.71 MWe for Methanol, 14.04 MWe for Ethanol and 13.97 MWe for Acetone. The thermodynamic study concludes that ORC based on benzene is the best, and the economic analysis discloses that ORC based on ethanol is the best among all working fluids with a payback period of 0.869 years and cost of electricity of Rupees 1.101 per kWh. This study also concludes that the novel technique used in the present study is economically viable, contributing to a more energy-efficient and environmentally friendly power generation system.


Introduction
India is a large country with high electricity demand, and waste heat utilization from various power generation units can help to meet some of that demand.The total energy generation by each sector from different resources in India is shown in figure 1.The total installed capacity in India as of June 2023, according to the Central Electricity Authority (CEA), was 421901.63MW [1].The chart depicts India's primary energy generation source as coal, comprising 48.8% of the total installed capacity.Power plants emit a substantial quantity of waste heat at extremely low temperatures, making their efficient conversion into useful work through conventional means challenging.Consequently, most of this heat is discharged into the atmosphere.
In thermal power plants, major energy losses take place in the condenser, followed by flue gas exhaust; as we know that technological development is moving forward as well as the energy consumption rate is also increasing.So the utilization of waste heat helps in the reduction of fossil fuel consumption rate and environmental pollution.Low-grade energy conversion cycles can utilize the waste heat available at different parts of the plant.Kalina Cycle, ORC, etc. are the most commonly used low-grade energy conversion cycles.The ORC is a promising method for obtaining electricity 1285 (2024) 012008 IOP Publishing doi:10.1088/1755-1315/1285/1/012008 2 from heat sources with low temperatures.It operates on the same thermodynamic principle as the Rankine cycle, but employs organic fluids instead of water [2].
Wei et al. did the thermodynamic analysis of ORC and discovered that maximizing waste heat utilization was an excellent way to increase the system's power output [3].Kong et al. investigated ORC thermodynamically with different heat sources at the evaporator [4].Bin et al. recovered the residual heat of a hybrid passenger vehicle using ORC and obtained an ORC efficiency of 5.4% [5].Kheiri et al. did a comparative study with different ORC configurations employing distinct working fluids.Their results show that efficiency of the proposed cycles increased with respect to simple ORC using R245fa refrigerant [6].Qiang Liu recovered waste heat from the power plants using ORC, and their findings show that incorporating ORC increases the plant's power output by up to 0.4% [7].Zhang et al. conducted a thermodynamic study on combining an ORC with an absorption heat pump.The outcomes of their research showed a 9.38% improvement in energy efficiency and a 1.71% increase in exergy efficiency [8].Campos et al. analyzed the ORC system thermo-economically and recovered waste heat from a micro gas turbine.The result reveals a 14.1kW increase in electrical power generation [9].Sherwani did a thermodynamic investigation on a solar-powered ORC-vapor compression system, and the study's findings demonstrate that the system achieves a maximum COP of 1.81 and an efficiency of 5.8% [10].Asim et al. did a thermodynamic investigation of an ORC that was integrated with a vapor compression cycle.They used the waste heat from the condenser outlet to generate an additional 1.41 kW of power [11].
From the literature, it is clear that the ORC is employed extensively in numerous environments where waste heat energy or low-quality heat is accessed.Still, very few have done the thermoeconomic study of the ORC integrated into the thermal power plant.l Figure 1.Total power production in India

Objective of the work
This study integrates the ORC with a 500 MWe supercritical plant with a CO2 capture (standalone plant).The ORC enhances the plant's overall efficiency by utilizing the waste heat available at compression processes of CO2, monoethanolamine (MEA) regeneration, and exhaust flue gas stream.ORC converts the low-grade thermal energy from the waste heat into useful work, increasing the plant's power output.This additional power also helps in reducing the exergy destruction, resulting in an improvement in efficiency.This integration helps to optimize the utilization of available energy resources and increase the plant's overall performance.The following are the goals of the current study:  Make a thermodynamic comparison by considering the same parameters of the standalone and proposed plants. Make a thorough analysis of ORC from an economic and thermodynamic perspective. Parametric analysis to investigate the changes in ORC efficiencies.

Base plant configurations
A 500 MWe supercritical plant equipped with monoethanolamine (MEA) based CO2 capture unit is considered a standalone plant [12], which utilizes coal as its fuel source, as shown in figure 2. It operates with a steam pressure of 242.2 bar and a temperature of 537°C, featuring a reheating process reaching 565°C and a feedwater temperature set at 280°C [12].The plant features two low-pressure turbines, an intermediate-pressure turbine, one high-pressure turbine, and seven feedwater heaters.To lower the plant's carbon emissions, it uses a CO2 capture unit that extracts CO2 from flue gas using MEA solvent.

ORC Configuration
ORC is a thermodynamic cycle comprising an evaporator, condenser, pump and turbine, shown in figure 3, which generates electricity through waste heat.It uses a fluid for operation, usually an organic substance with a lower boiling point.This study examines the performance of ORC with a stream parameter of 10 bar pressure using five distinct organic fluids as working fluids.Initially, the working fluid undergoes heating within the evaporator and is then transferred to the superheater heat exchanger, resulting in a rise in the fluid's enthalpy.After that, the fluid enters the turbine, generating power output through expansion.It enters the condenser after exiting the turbine, where water is used to cool it.The fluid is then pumped back into the evaporator to complete the cycle.Figure 4 depicts a schematic of the integration of ORC with the 500MWe power plant (Proposed plant).The various sources from which waste heat is extracted have been emphasized.

Coal Characteristics
Indian coal has been used as a fuel for the standalone plant, and Table 1 lists its characteristics.Indian coal is known for its low sulfur content and high mineral matter.However, despite being low in sulfur, it is considered low-grade because it contains high ash.

Characteristics of Working fluid used in ORC
The organic fluid used in an ORC system is an essential factor affecting the system's performance.The characteristics of the organic fluid can impact its thermal efficiency.Table 2 provides comprehensive details about the organic fluid, including essential parameters like critical temperature (C.T.), critical pressure (C.P.), and boiling point (B.P.).These parameters provide important information about how well the fluid performs and whether it is appropriate for use with ORC.The use of working fluids like R245fa and other hydrofluorocarbons in these applications poses a concern due to their high global warming and ozone depletion potential.Considering the environmental implications, there might be a potential phase-down of these working fluids in the future [13].Therefore, exploring alternative working fluids that share similar thermophysical properties while causing minimal environmental harm emerges as a promising avenue for sustainable implementation.

Assumptions and Input data
Based on the following listed assumptions, the thermodynamic analysis has been done:  For ORC 10 bar of evaporator pressure is considered. Five different working fluids are used in ORC. 7.5% of total power is considered as total auxiliary power consumption. For turbine and pump, the isentropic efficiencies are 90% & 85% [12].

Methodology
The thermodynamic study is carried out using a commercially available simulation tool called 'Cycle-Tempo' [14].The component modelling process begins with the power plant flow diagram.It then specifies various operational parameters for each component, such as pressure, temperature, flow rate at the intake (i) and exit (e), compressor, pump, and motor efficiency.A series of algebraic equations can be used to depict the complete power cycle that follows:

Performance Parameter
The following are the parameters that are used to calculate the results [12]: Energy Efficiency (η) = Net electrical power output m ̇ of the ̇ coal × Higher heating value of coal (5) Exergy Efficiency (ε) = Net electrical power output m ̇ of the coal× coal specific exergy (6)

Economic Assessment of ORC
Comparing only the thermodynamics of the proposed systems is an inadequate metric.Therefore, it is essential to check the system's performance in terms of thermodynamics and economics for more realistic results.Table 4 shows the purchase equipment cost (PEC) equation of each component of the ORC system [15][16][17].After calculating each component cost, the total PEC is calculated by adding all equipment costs.The heat transfer coefficient (U) utilized in the ORC system for both the evaporator and superheater is assumed to be 0.6 kW/m² [18].This economic study considered some economic limitation such as yearly operational time (n) of 8000 hour, interest (i) of 0.2, service factor (Φ) of 1.06 and lifespan (N) of 20 years [15,18].After calculating the PEC, capital recovery factor (CRF) is determined using [16,19,20]: The cost of the electricity (Celec) obtained through ORC is determined using [21]: Lastly, system's payback period (PB) is calculated using [22]: Here the Cpric is the regional cost of electricity (West Bengal) [23] and is accepted as 7.32 Rupees ≈ 0.08922$, where 1$ = 82.04 as on 26 th June 2023.

ORC Model Validation
This study employs the same parameters as the literature [24] to validate the mathematical model and calculation outcome of the ORC model obtained using the Cycle Tempo software.Table 5 compares the results of the calculations based on efficiency and demonstrates that outcomes of the present study are in good concordance with the calculation results in the literature.

Results and Discussion
Table 6 compares the efficiency between standalone plant and proposed plant with selected working fluids.The analysis reveals an increase in both energy and exergy efficiencies when comparing the proposed setups to the standalone unit.The table also describes the coal consumption rate of different plant configurations, and the results show that the proposed plant reduces the fuel consumption rate and associated carbon emissions.Table 7 indicates the additional power generation by utilizing waste heat using ORC.It is clear from the data that among the fluids, benzene exhibits the highest power output (14.11MWe).This is due to the thermo-physical properties of benzene, such as its critical pressure and critical temperature, which allow it to perform efficiently at high temperatures.These results suggest that benzene is a viable working fluid for ORC systems, especially for applications involving high-temperature waste heat sources.13.97

Energetic and Exergetic Comparison of the Proposed plant
The energy balance of the proposed plant is shown in Table 8.It provides the detail of efficiency and losses in each component.The losses are estimated by dividing the energy losses through the component by the heat input.This information can help identify the system's areas that require improvement to increase its energy efficiency and overall performance.The energy balance calculation in Table 8 shows that the maximum losses occur in the condenser, with 21.98% of the energy losses, followed by the losses in the steam cooling process for MEA regeneration, which account for 29.95% of the losses.These findings indicate that the condenser and the steam cooling process are the two primary sources of energy loss in the proposed plant.
The exergy balance calculation provides information on the quality of energy losses at various components in the proposed plant.The results are presented in Table 9.The exergy balance calculation considers the availability of energy and the quality of the energy, and it assesses the inefficiencies in the energy conversion processes.According to the exergy balance calculation, the combustor suffers significant exergy losses (33.57%).This is due to the irreversibility of the heat transfer and combustion processes, which reduces the quality of the energy produced.The exergy balance calculation highlights the areas of the plant where the energy conversion processes are the least efficient.5 depicts how the performance of the ORC, including power output and efficiency with the chosen working fluids, is influenced by evaporator pressure.The evaporator pressure holds a crucial role as an operational parameter within the ORC system, affecting its efficiency and power output.Adjustments are made to the evaporator pressures to comprehend the fluctuations in the ORC's performance.The figure demonstrates that increasing evaporator pressure enhances the system's output and efficiency.This is because an increase in evaporator pressure causes the temperature of the working fluid to rise, consequently leading to higher enthalpy.This intensifies the heat transfer, allowing it to expand more in the turbine, resulting in greater work output and thus increasing the system's efficiency.The same behaviour can also be seen in the literature [25].The study highlights the potential advantages of harnessing waste heat using ORC systems with existing power plants, offering valuable insights for both the energy sector and environmental sustainability initiatives.It also illustrates that the adopted technology is economically viable while enhancing the efficiency and make the energy generation processes eco-friendly

Conclusions
Harnessing waste heat generated by power plants, currently dissipated into the atmosphere due to low temperatures, holds significant potential for minimizing energy wastage.Heat recovery systems are essential for power plants, as they enhance efficiency while simultaneously curbing fossil fuel consumption and carbon emissions.The aim of this study is to use ORC to utilize the plant's waste heat.Various working fluids for ORC operation are considered, and the thermodynamic and economic

Figure 3 .
Figure 3. Organic Rankine Cycle2.3.Proposed plantFigure4depicts a schematic of the integration of ORC with the standalone plant (proposed plant).In the proposed plant configuration, ORC is integrated to use the waste heat available at various parts of plants.The various sources from which waste heat is extracted have been emphasized.

Figure 4 .
Figure 4. Layout of Proposed Plant

Figure 5 .
Figure 5.Effect of evaporator pressure on ORC Performance (a) Power output (b) Energy efficiency performance are assessed.Following are the key findings based on techno-economic analysis of the proposed model:  ORC produces an additional 9.91 MWe of electricity for R245fa, 14.

Table 2 .
Characteristics of refrigerants used

Table 3 .
Input data of ORC

Table 5 .
Model Validation

Table 6 .
Efficiencies and coal requirements of the plants

Table 7 .
Additional power generation using ORC

Table 8 .
Energy balance proposed plant

Table 9 .
Exergy balance proposed plant Economic results of ORC Table10indicates the results of ORC based on thermodynamic and economic analysis.The thermodynamic analysis reveals that benzene-based ORC shows high efficiency compared to others.In contrast, the economic analysis reveals that the ethanol-based ORC shows the best results among others because it has the lowest payback period of 0.869 years with 1.101 rupees of electricity generation cost per kWh.

Table 10 .
Thermodynamic and economic results of the ORC 11 MWe for benzene, 13.71 MWe for methanol, 14.04 MWe for ethanol, and 13.97 MWe for acetone. Compared to the standalone plant, the ORC improves energy efficiency by 1.61 %, 2.56 %, 2.48 %, 2.52 %, 2.52 %, and exergy efficiency by 1.64 %, 2.56 %, 2.44 %, 2.52 %, 2.48 %, with the use of R245fa, benzene, ethanol and acetone respectively. Other than the typical working fluid R245fa, the ORC performed better with uncommon working fluids, such as benzene, acetone, ethanol, and methanol. The thermodynamic study concludes that benzene-based ORC shows the best result among all working fluids.In contrast, the economic analysis concludes that the ethanol-based ORC is best because of the low payback period and low cost of electricity generation.