*2.4. LCA in the Brayton S-CO2-ORC System*

The LCA procedure was adopted to investigate the potential environmental impacts of the components and organic fluids of the *Brayton S-CO2-ORC* in each phase of the life cycle. This procedure is developed according to the ISO 140009 environmental management standards and is supplemented by some steps such as definition, inventory, life cycle impact assessment analysis, results, and interpretation [38].

This analysis was applied in the Barranquilla city (Colombia) in the year 2020. The suggested practical unit is 1 kWh produced by the Brayton S-CO2-ORC system, while the scope of the analysis considers the assembling procedures of materials and divisions of the cycle (construction phase). Also, the operation and maintenance phase was considered as a function of the energy ratio of the equipment. The decommissioning period of the system is likewise considered in the LCA of the thermal system, as shown in Figure 3.

**Figure 3.** Life cycle assessment system boundary diagram.

Some considerations were adopted to conduct the LCA, such as the Eco-indicator 99 approach, which was utilized in [38]. The environmental impact of the organic working fluid, the components, and the thermal oil are considered in the three phases of the process lifecycle, which are the construction, operation, and maintenance and decommissioning [39]. The normal loss of organic working fluid in an ORC ranges from 0 to 2% of the total filled fluid. So, for a period of 20 years, the working fluid loss considered was 0.5%. As needs are, the liquid loss in the operation phase is 10%, while the organic loss in the decommissioning stage is just 3%. Also, it was assumed that the composition of the thermal oil (Therminol) is 73.5% Diphenyl Oxide for the environmental impact assessment of the thermal oil. Concerning the toxicity of working fluids, in the selection of these, it has been recommended to use nontoxic and nonflammable organic fluids. Therefore, only fluids with classifications A1, B1, A2L, B2L were selected, under ASHRAE standard 34-2001 or the NFPA 704 standard [40,41].

By applying the energy balance in the heat exchangers (ITC1, ITC2, and ITC3), the heat transfer area is obtained with Equation (14)

$$\mathbf{A}\_{l} = \frac{\dot{\mathbf{Q}}}{\Delta \mathbf{t}} \cdot \frac{1}{\mathbf{U}} \tag{14}$$

where U is the heat transfer coefficient in kW/m2·K, and <sup>Δ</sup><sup>t</sup> is the true temperature difference, determined with the Equation (15)

$$
\Delta \mathbf{t} = \mathbf{C} \mathbf{F}\_{\mathbf{T}} \cdot \text{LTD} \tag{15}
$$

where CFT is the correction factor calculated with the Equation (16), and LTD is the logarithmic mean temperature differences calculated according to Equation (17) [42]

$$\text{CF}\_{\text{T}} = \frac{\frac{\sqrt{\mathbf{R}^2 + 1}}{\mathbf{R} - 1} \cdot [\ln(1 - S) - \ln(1 - RS)]}{\ln \frac{2 - S \cdot \left(R + 1 - \sqrt{R^2 + 1}\right)}{2 - S \cdot \left(R + 1 + \sqrt{R^2 + 1}\right)}}\tag{16}$$

$$\text{LTD} = \frac{\Delta \text{T}\_{10-1AT} - \Delta \text{T}\_{11-3AT}}{\ln \left( \frac{\Delta \text{T}\_{10-1AT}}{\Delta \text{T}\_{11-3AT}} \right)} \tag{17}$$

where *R* corresponds to the effectiveness coefficient, and *S* is the heat power ratio.

The modeling of the printed circuit heat exchangers is carried out according to the mathematical model presented in the literature [31]. This heat exchanger is the Recuperator (HTR) and the Reheater (RH) in the Brayton cycle, which are fabricated by such technologies as chemical etching and diffusion bonding, where flow channels are imprinted chemically on the metal plates and produce one block by diffusion adhering, as shown in Figure 4.

**Figure 4.** Printed circuit heat exchangers geometrical design.

The heat exchanger is calculated by dividing it into small sub-exchangers (Equation (18)) in which the properties of the fluid, temperature, and pressure are known, and an iterative process is followed until both the heat exchanged and the maximum pressure drop are satisfied. The correlations used for each of the *i* divisions were obtained from the literature [31]

$$N\_{III} = \begin{cases} 4.089 & \text{if } \qquad R\_{t\_i} < 2300\\ 4.089 + \frac{N\_{\parallel L\_{5000}} - 4.089}{5000 - 2300} \cdot \left( R\_{t\_i} - 2300 \right) & \text{if } \quad 2300 < R\_{t\_i} < 5000\\ \frac{\left(\frac{t\_i}{8}\right) \cdot \left( R\_{t\_i} - 1000\right) \cdot P\_{r\_i}}{1 + 12.7 \left( P\_{t\_i} \frac{2}{3} - 1 \right) \sqrt{\frac{t\_i}{8}}} & \text{if } \qquad R\_{t\_i} > 5000 \end{cases} \tag{18}$$

where *f* is the Darcy factor and *Pr* is the Prandtl number.

The heat transfer coefficient is then calculated using Equation (19)

$$H\_i = N\_{lI\_l} \cdot \left( k / D\_{hid} \right) \tag{19}$$

where *k* is the conductivity of the exchanger material [W/m·K].

Finally, the overall coefficient U of each element is calculated using Equation (20), and the length of each of the sub-exchangers with Equation (21).

$$\mathcal{U}\_i = \frac{1}{\frac{1}{H\_{h\_l}} + \frac{1}{H\_{c\_l}} + \frac{1}{k}} \tag{20}$$

$$L\_i = \frac{Q\_i}{P\_{i^\*} Q\_{i^\*} \left(T\_{h\_m} - T\_{c\_m}\right)}\tag{21}$$

where *t* is the thickness of the plate, and *Pi* is the wet parameter.

*Energies* **2020**, *13*, 2259

When calculating the hea◦t exchanger, you check that it meets the introduced pressure drop using Equation (22).

$$
\Delta P\_i = f\_i \cdot \left(\frac{L\_i}{D\_{hid\_i}}\right) \cdot \left(\frac{c\_1^2}{2}\right) \tag{22}
$$

The heat exchanger mass can be calculated using Equation (23)

$$M\_i = \rho \cdot A\_i \cdot \delta \tag{23}$$

where ρ is the density (steel is 7930 kg/m3, and copper is 8900 kg/m3), and δ is the material thickness with a value of 0.002 m [32].

The turbine and pump masses are determined using Equation (24)

$$M\_{\bar{i}} = \alpha \cdot \mathsf{W}\_{\bar{i}} \tag{24}$$

where *Wi* is the turbine power produced, or the pump power consumed α is the required quality of the material in kg/kW. For steel, the value for α is 14 kg/kW and 31.22 kg/kW for the pump and the turbine, respectively, while for copper, its value is 35.03 kg/kW and 15.71 kg/kW for the turbine and the pump, respectively [32].

According to Equation (25), the environmental impact of each equipment can be described

$$Y\_i = w\_i \cdot M\_i \tag{25}$$

where *wi* is the component coefficient Eco 99.

Therefore, the environmental impact of equipment (*YLCAk*) is obtained using Equation (26) [43]

$$Y^{LCA}{}\_i = Y^{co}{}\_i + Y^{om}{}\_i + Y^{dc}{}\_i \tag{26}$$

where *Ycoi*, *Yomi* and *Ydei* lead to the environmental impacts of the corresponding phases: construction, operation, and decommissioning.

Finally, the total environmental impacts of components are determined with Equation (27)

$$Y\_i = Y^{LCA}\_{\;\;i} + Y^{wf}\_{\;\;i} \tag{27}$$

where *Yw f <sup>i</sup>* is the effect of the organic fluid volume used in each component, and is a function of the component's exergy destruction, which is determined using Equation (28).

$$Y^{wf}\_{\,i} = \frac{Y^{wf} \cdot \dot{E} D\_{di}}{\dot{E} D\_{d-total}} \tag{28}$$

#### **3. Results and Discussion**

#### *3.1. System Thermodynamic and Exergy Performance*

Figure 5 shows a sensitivity analysis of the thermodynamic and exergy performance indicators of the thermal cycle, based on the inlet temperature of the main turbine (T1) in the Brayton S-CO2-ORC configuration. For the development of the sensitivity analysis, the following considerations were taken into account ORC pressure ratio was 30, evaporator pinch point 25 ◦C, main turbine inlet temperature (TIT) 750K, Brayton cycle high pressure 25 kPa, Brayton turbine efficiency 93%, ORC turbine 80%, compressor efficiency 89%, pump efficiency 75%, and HTR effectiveness 95%. Figure 5a shows the net power generated with the different organic working fluids simulated in the system, observing that as the temperature increases there is a 36% increase in the net power delivered by the system for a range between 550 ◦C and 800 ◦C, showing similar trends among the three organic working fluids studied.

**Figure 5.** System performance parameters with respect to TIT, (**a**) net power; (**b**) thermal efficiency Brayton-ORC; (**c**) absolute increase in thermal efficiency; (**d**) exergy efficiency Brayton-ORC; (**e**) brake-specific fuel consumption; (**f**) absolute decrease in brake-specific fuel consumption.

In the case of the thermal efficiency of the Brayton S-CO2-ORC integrated system, as shown in Figure 5b, acetone is the fluid with the best thermal performance in the operation of the configurations, this is because its thermo-physical properties benefit the best use of energy and the performance of the system components, reaching a maximum thermal efficiency of 50.44% at 800 ◦C, this being the best operating condition of this fluid. Therefore, each fluid presents its particular operating conditions that must be studied to optimize the energetic, exergetic, and environmental performance of each proposed alternative [33,34].

Figure 5d shows the total exergetic efficiency of the system studied, where the turbine T1 inlet temperature has a positive influence on all the organic working fluids studied. Toluene and cyclohexane exhibited similar behavior. However, acetone exceeds an exergetic efficiency of 71.4% at a turbine inlet temperature of 800 ◦C. Through the analysis of this parameter, it contributes to the objective of making more effective use of the nonrenewable energy resource used in the Brayton cycle by establishing the components, modes, and actual amounts of exergy destruction and loss in the integrated system. The values obtained for exergetic efficiency are of great importance in this study since they allow for the design of a more effective integrated Brayton S-CO2-ORC configuration, aimed at the reduction of inefficiencies in these systems. The acetone presented the best average behavior among the fluids in the temperature range studied, concerning toluene and cyclohexane, with a difference of 2.9% and 2.7%, respectively, which is a consequence of the thermal properties of this fluid. However, before implementing these results at the industrial level, advanced economic analyses and exertion of these systems must be carried out [35], in addition to thermo-economic studies to determine the technical and economic feasibility of this solution [36].

The high turbine pressure in the system is another relevant operational parameter that impacts on the energy and exergy efficiency of the integrated system; however, this variable has less impact than the main turbine inlet temperature (T1), producing less variability in the performance parameters, as can be seen in Figure 6. For the performance of the net power, it presents less efficiency in its behavior concerning the performance affected by the inlet temperature (Figure 5a), presenting an approximate increase of 36% beside 9% of the energy generated by the system.

**Figure 6.** System performance parameters with respect to PHIGH, (**a**) net power; (**b**) thermal efficiency Brayton-ORC; (**c**) absolute increase in thermal efficiency; (**d**) exergy efficiency Brayton-ORC; (**e**) brake-specific fuel consumption; (**f**) absolute decrease in brake-specific fuel consumption.

On the other hand, Figure 6e shows the decrease in the specific fuel consumption for the three fluids studied in the integrated configuration of the Brayton S-CO2-ORC system, with acetone showing the best performance of the three fluids due to its better specific consumption reduction. This result is closed near the other parameter calculated. This result implies a significant reduction in the system operating costs. It allows better use of resources, improving its performance between the energy input and the power produced at high temperatures.

The exergetic efficiency behavior for each component of the system under the three organic working fluids is shown in Figure 7. From the results. The lower exergy efficiency is presented in the thermal oil pump (P1), with a 10.98% (Toluene) and 11.06% (Cyclohexane), while the efficiency in the organic fluid pump (P2) was 77.6% for the three fluids. These results are because of the higher-pressure ratios required to pump the thermal oil, which implies higher irreversibilities for heat transfer in this component. Thus, a thermo-hydraulic design should be proposed for both the evaporator (ITC2) and the shell and tube heat exchanger (ITC1) with the lowest pressure drop, and highest heat transfer.

**Figure 7.** Exergetic efficiency of the Brayton S-CO2-ORC components.

For the operating conditions studied, the turbine 1 (T1) and the turbine (T2) are the components that present the best behavior, presenting exergetic efficiencies of 98% in the three fluids used to analyze this configuration. Therefore, they present minor irreversibilities and allow better use of the energy in the system.

#### *3.2. Exergy Destruction*

To develop an exergy destruction analysis of the system, it was necessary to apply the exergy balance for each of the components as shown in Figure 8, where the exergy destruction fraction of each component is evaluated at different inlet turbine temperature ranging from 550 ◦C to 800 ◦C. For this analysis, the operational considerations in Section 3.1 have been taken to determine exergy destruction. The component with the minimum exergy destroyed is the pump (P2) compared to the heat exchanger components that have the greatest exergy destruction in the system. Thus, the ITC2, which is the component with the greatest exergy destroyed with values ranging from 3.93 kW to 9.75 kW for the temperature range evaluated, showing a decrease between the value of exergy destroyed from the base condition of 11% and an increase in exergy destroyed at the temperature of 800 ◦C of 3%. Although technological improvements do not translate into significant improvements in exergetic efficiency, this result can be improved if ITC1 is designed with rational energy use and sustainability criteria, that leads to sensible improvements in how heat transfer is performed in this type of exchanger, with the aim to obtain more compact, economical and efficient equipment.

The thermal oil pump (P1), and the organic fluid pump (P2) were the components that presented minimum exergy destruction. An alternative to having equipment with less exergy destruction would be to propose pumping systems with higher exergy efficiency, which would imply an increase in the net power produced by the cycle because the pump will consume less energy. However, the pump power is less than that produced by the turbine, which is the reason for the lower contribution of the exergy destroyed and the isentropic efficiency on the overall thermal cycle performance, as the contribution of these is almost indistinguishable when compared with the exergy destroyed from the other components of the cycle at the different inlet turbine temperatures.

**Figure 8.** Components destroyed exergy with respect to turbine inlet temperature (TIT).

The influence of the PHIGH on the destroyed exergy by components is presented in Figure 9, where the ITC2 is the component with the highest irreversibility in the configuration studied. This trend of the exergy destruction continues as the high-pressure turbine decreases, obtaining approximately 8.67 kW of exergy destroyed in the system by this component when the pressure is 20 MPa, which is reflected in a significant energy loss in this component due to the large size of this heat exchanger. Therefore, the operating conditions in the Brayton cycle at the input of operation, although it makes the cycle deliver more power, at the time of coupling with ORC cycle, show important irreversibilities by heat transfer in the thermal circuit of coupling due to operational limitations to ensure thermal stability on the thermal oil and organic fluid. These results can be compared with the results made by the authors in the following references, where they use other working fluids and different models and heat recovery systems [44,45].

**Figure 9.** Components destroyed exergy with respect to PHIGH.

Similarly, the exergy destruction in the HTR used in the system increases as the PHIGH increases from 20 MPa to 28 MPa, being this the second component with more impact in the process, reaching a value of 5.10 kW in the exergy destruction at the maximum operating condition. These results are due to the presence of greater irreversibilities in the thermal source, as it is required that the organic working fluid reaches a higher pressure and temperature; therefore, this variable must be considered as an objective variable in energy and exergetic optimization to obtain competitive efficiencies for the system operating with the energy source under study.

#### *3.3. Life Cycle Assessment*

Based on the thermodynamic parameters of each state of the Brayton S-CO2-ORC integrated system, the energy and exergetic parameters of the organic components and organic fluids are shown in Table 2. In the proposed system, a lifetime of 20 years was considered [46], in which the components and working fluids will have 7446 working hours [47,48].

**Table 2.** Exergy efficiency, destroyed exergy, destroyed exergy ratio of each equipment, and heat exchanger area.


The results allow for identifying that the RHR is one of the components that presents greater environmental impacts due to their heat transfer area in comparison with the rest of the components of the system. Also, the RHR presents the greatest exergy destroyed, with values of 46% (cyclohexane), 32% (toluene), and 32% (acetone).

The masses in the different phases of the life cycle of the Brayton S-CO2-ORC system are presented in Table 3. The LCA methodology was developed to obtain the environmental impacts of each component of the system under the three phases of the lifetime, which are the construction, operation and decommissioning, as shown in Table 4, selecting steel as the material. Table A1 shows the results when copper is proposed as the construction material for the devices.


**Table 3.** Organic fluids and thermal oil masses in each life cycle phase.



For the three organic working fluids (cyclohexane, toluene, and acetone), the results of the assessment of the environmental impact of the system are presented in Figure 10, where the material of the components is the steel and copper. The components with the greatest environmental impacts are the main turbine (T1) and the secondary turbine T2 of the Brayton S-CO2 cycle, with values of 226,893 mPts and 321,518 mPts, respectively, when steel is selected as the material. With this, the T1 and T2 turbines obtain, respectively, percentage values of 18.19% and 25.78% (cyclohexane), 20.54% and 29.10% (toluene), 20.36% and 28.85% (acetone). In the ORC, the ITC1 is the component with the greatest environmental impact, with percentage values of 10.67% (cyclohexane), 12.05% (toluene), and 11.94% (acetone) concerning the environmental impacts of all components. When the material is copper, there is a decrease in the percentage values of the T1 and T2 turbines due to the increase in environmental impacts in the ITC1, which obtains values of 11.45% (cyclohexane), 12.85% (toluene) and 12.94% (acetone).

**Figure 10.** Environmental impact of each equipment with respect to the organic fluids and materials.

In general, the environmental impacts of each equipment when steel is selected as material steel are lower than those of the components with copper because in the methodology applied the Eco99 coefficient is higher than that of steel. Therefore, considering sustainability criteria complementary to the energy and exergetic aspects studied, the organic working fluid and material in which the greatest exergetic opportunities for improvement are found can be selected to obtain the least environmental impact.

Regarding the environmental impacts of working fluids and thermal oil, the results are presented in Figure 11 where the thermal oil has a very large environmental impact on the organic fluids, representing 83% of the environmental impacts compared to cyclohexane (4%), toluene (4%), and acetone (9%), respectively. Among the organic fluids, because it has a higher Eco99 coefficient, acetone has greater environmental impacts than cyclohexane and toluene, and the suggested material is the steel, which is a result close to that reported in the literature for the case of the ORC as heat recovery [37]. Therefore, with the ORC being built with this material and operating with this fluid, it can be widely used for heat recovery from natural gas generation engines [38].

The results show that the greatest differences in LCA are obtained in the environmental impacts in the construction phase, where acetone presents better results for this system, which coincides with the results obtained from the analysis of the energy and exergetic indicators.

**Figure 11.** Environmental impact of the working fluid.

#### **4. Conclusions**

The main contribution of this study was to analyze the energy, exergy, and environmental performance of an integrated Brayton S-CO2-ORC, where some performance indicators and the potential environmental impact were studied using Toluene, Acetone, and Cyclohexane as the organic working fluids. Therefore, a complementary assessment was developed based on the energy, exergetic, and environmental analysis to determine the best behavior of the components in this cycle, and the effect of the relevant operating conditions.

For identifying the best performance of the components that integrate this system, the study evaluates exergetic and energy parameters such as the exergetic and overall thermal efficiency under the different organic working fluids used in this study. In these parameters, it is possible to evaluate the power generated by the integrated system for each fluid operating in the Brayton S-CO2-ORC configuration, where the acetone presented the best behavior with respect to toluene and cyclohexane. The performance of the system for the operating conditions studied found that turbine 1 (T1) and the turbine (T2) are the components that present the best exergetic efficiencies with 97%. Therefore, the acetone offers minor irreversibilities and allows for better performance in the ORC as a bottoming cycle from the Brayton cycle.

Conversely, another relevant parameter studied is the exergy destroyed of each component, presenting the pump (P2) least performance compared to the heat exchanger components that have the most considerable exergy destruction in the system, thus observing that ITC1 is the component with the most exergy destroyed, with values ranging from 75 kW to 127 kW for the temperature range evaluated. So, this component presents greater opportunities for energy improvement, and therefore environmental improvement, since its potential environmental impacts are based on the heat transfer area of this equipment.

For the three working fluids (cyclohexane, toluene and acetone), the results of the potential environmental impacts of the system were studied using the steel and copper as construction materials for the components. The components with the greatest environmental impacts are the main turbine (T1), and the secondary turbine (T2) of the Brayton S-CO2 cycle, with values of 226 Pts and 321 Pts, respectively. The T1 and T2 turbines obtained a percentage value of 18.19% and 25.78% of the total environmental impact with cyclohexane, 20.54%, and 29.10% with toluene, 20.36% and 28.85% with acetone. Also, the ITC1 in the ORC cycle is the component with the greatest environmental impact, with a percentage value of 10.67% (cyclohexane), 12.05% (toluene) and 11.94% (acetone).

In general, the environmental impacts of the components with steel are lower than those of the components with copper, because the methodology applied suggests a higher Eco99 coefficient than steel. Therefore, through this methodology, the organic working fluid and the material in which the most significant opportunities for improvement are found can be selected to obtain the smallest environmental impact.

Among the organic fluids studied, acetone has lower potential environmental impacts than cyclohexane and toluene, which is a consequence of the Eco99 coefficient. However, some safety and health consideration such be considered to implement the ORC alternative industrially as the bottoming system from the Brayton cycle.

Finally, this study allows for evaluating the performance of the combined cycles for applying this technology in industries for generating energy and net power. Acetone is the fluid with the best thermodynamic and environmental performance results on this configuration because of their thermal properties, giving an option for other studies to proposed an eco-design of this and obtain better exergy and environmental results according to other performance parameters.

**Author Contributions:** Conceptualization: E.E.B.; Methodology: G.V.O. and J.D.F.; Software: E.E.B., G.V.O., and J.D.F.; Validation: E.E.B., and J.D.F.; Formal Analysis: E.E.B., G.V.O., and J.D.F.; Investigation: E.E.B., G.V.O., and J.D.F.; Resources: G.V.O. and J.D.F.; Writing—Original Draft Preparation: G.V.O.; Writing—Review and Editing: G.V.O. and J.D.F.; Funding Acquisition: G.V.O., and J.D.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Universidad del Atlántico, and Universidad Francisco de Paula Santander in Ocaña - Norte de Santander.

**Acknowledgments:** This research was supported by the Mechanical Engineering Program of Universidad del Atlántico. The Kai Research Group supports G. Valencia and J. Duarte.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Nomenclature**


## **Appendix A**

The LCA results in each component of the proposed configurations using copper as the material are presented in Table A1.


**Table A1.** Results of Life Cycle Assessment for each component selecting copper as the material.

## **References**


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