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Energies 2020, 13, 2447 14 of 18 varied temperature differences between CO2 and the exhaust gases at the inlet and outlet of the heat exchanger, ranging from 3 to 20 K. The relationship between the LMTD and cycle efficiency is presented in Figure 8. Figure 8 shows that the internal efficiency of the cycle increases if the difference in temperature of CO2 at the entry to and exit from the recuperator (between hot and cold side) is smaller. The smaller temperature difference between CO2 streams flowing countercurrent means that more heat of returning CO2 is transferred to CO2 after compression. Otherwise, this energy would have to be “taken” from the system at the cooler so the energy would be “wasted”. The increase in CE with the decrease in the LMTD results from the fact that the smaller the LMTD, the more energy is transferred from the hot side of recuperator to the cold side of recuperator. By this way, more internal energy in the system is re-used and thus the efficiency of the cycle is increasing. The relationship between the LMTD and percent of waste heat transferred to CO2 is presented in Figure 9. In Figure 9 a very clear relationship is presented, where, with the increase in LMTD in recuperator, the percent of waste heat transferred to CO2 also increases. This results from the fact that with larger values of LMTD in recuperator, more energy is “wasted” and “taken” from the system at the cooler. To compensate this, a correspondingly greater amount of waste heat from the flue gases must be supplied to the system. In Figure 10 the relationship between the LMTD and waste heat utilization rate is presented. Waste heat utilization rate is an energy efficiency indicator, which combines CE and PWHT. The value of WHUR increases both with the growth in CE and PWHT. The data presented in Figure 10 shows that the WHUR value increases slightly with the decrease of LMTD in recuperator. Results presented in Figures 8–10, show that for the adopted operating conditions, the increase in internal efficiency has a greater impact on the WHUR than decrease in percent of waste heat transferred to CO2 (PWHT). The increase of WHUR is much more significant when the temperature difference between CO2 and flue gases in the heat exchanger decreases. This is due to the fact that both CE and PWHT grow with the temperature difference drop. By overlapping these growths, the increase in WHUR is evident. 5. Results and Discussion As mentioned in the introduction, no other publications were found that presented an analysis of S-CO2 cycle application for the waste heat recovery at a natural gas compressor station. Detailed comparison of cycle efficiency indicators values between its applications in various industries is pointless, because these are different operating parameters (power of devices, waste heat temperature, etc.). For example, Kim et al. [6] studied the supercritical CO2 power cycle for landfill gas fired gas turbine bottoming cycle. They concluded that for the simple recuperated cycle the net produced work is 2.18 MWe with 29.98% cycle net efficiency. For comparison, in this paper maximum net power system was 47 kW with cycle efficiency 14%. However, it should be noticed that Kim et al. assumed exhaust temperature 519.69 ◦C and mass flow rate of the flue gases of 21.3 kg/s. In this work it was, respectively, 343.23 ◦C and 3.55 kg/s. Similarly, the maximum system operating pressure was taken as 27.6 MPa (22.5 MPa in this paper). Taking this into account, it was decided to compare the values of selected S-CO2 efficiency indicators with ORC efficiency indicators, analyzed for the same gas compressor station. Kowalski et al. [2] analyzed the application of ORC system at Jarosław II natural gas compression station. The ORC system was proposed for the production of eclectic energy. They conclude that the average net electric power of about 32 kWe could be obtained when ORC system was fed with waste heat from a compressor engine. The obtained exergy efficiency of the cycle was about 7.06%. In another paper, Kostowski et al. [3] analyzed the methods of waste heat recovery at gas compressor station. They performed a case study for the Jarosław II gas compressor station and compared different technologies including ORC. The ORC output power was estimated in a simplified black-box approach based onPDF Image | Supercritical CO2-Brayton Cycle Nat Gas Compression Station
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