# Techno-Economic Analysis of Waste Heat Recovery in Automotive Manufacturing Plants
## 4 Results and Discussion
### Thermodynamic Analysis of the Proposed Carnot Battery  
Figure 2 presents the simulation results for the Carnot battery in the temperature of air and the temperature of waste heat at the outlet. Figure 2a shows the COP of the heat pump ranging from 2.55 to 2.87, which varies based on the temperatures of the waste heat and the air. When the temperature of the waste heat at the outlet is 60 \({}^{\circ}\)C and the air temperature is 25 \({}^{\circ}\)C, the COP is 2.55. If the air temperature increases to 35 \({}^{\circ}\)C while maintaining the same waste heat outlet temperature of 60 \({}^{\circ}\)C, the COP improves to 2.87. In addition, when the waste heat outlet temperature is lowered to 50 \({}^{\circ}\)C with an air temperature of 25 \({}^{\circ}\)C, the COP remains at 2.55. However, at a waste heat outlet temperature of 60 \({}^{\circ}\)C and an air temperature of 35 \({}^{\circ}\)C, the COP rises to 2.87. These COP values are based on the assumption that the heat pump consumes 100 kW of power. Figure 2b,c display the efficiency and net output power of the ORC, respectively. The efficiency of the ORC ranges from 0.125 to 0.155, while the net output power varies between 444.5 kW and 581.3 kW. At a waste heat outlet temperature of 60 \({}^{\circ}\)C and an air temperature of 25 \({}^{\circ}\)C, the ORC achieves its highest efficiency of 0.155 and the greatest net output power of 581.3 kW. However, when the air temperature rises to 35 \({}^{\circ}\)C while maintaining a waste heat outlet temperature of 60 \({}^{\circ}\)C, the efficiency drops to 0.139, and the net output power decreases to 509.9 kW. In the case where the waste heat outlet temperature is reduced to 50 \({}^{\circ}\)C and the air temperature remains at 25 \({}^{\circ}\)C, the ORC efficiency is 0.142, with a net output power of 514.5 kW. If the air temperature increases to 35 \({}^{\circ}\)C with a waste heat outlet temperature of 50 \({}^{\circ}\)C, the efficiency falls further to 0.125, and the net output power decreases to 444.5 kW. To achieve the desired results, the mass flow of the ethylene glycol from TES to ORC varies between 37.2 and 62.3 kg/s. The ORC system operated with a mass flow rate ranging between 7.4 and 7.7 kg/s. The primary energy source for charging the TES was the waste heat recovered from the exhausted air, which played a dominant role in the system's overall energy efficiency. Moreover, the heat pump contributed approximately 9.5% to 10.6% of the total energy required to charge the TES. This approach highlights the integration of both waste heat recovery and additional heat pump, optimising the energy  
storage process for better sustainability. Therefore, the outlet temperature of the waste heat can be maintained between 50 \({}^{\circ}\)C and 60 \({}^{\circ}\)C.  
Figure 2d illustrates the P2P of the Carnot battery ranges between 0.36 and 0.40 under various operating conditions. When the waste heat temperature at the outlet is 60 \({}^{\circ}\)C and the ambient air temperature is 25 \({}^{\circ}\)C, the P2P is 0.40, representing the higher end of the efficiency range. This suggests that a larger temperature difference between the waste heat and ambient air contributes to greater P2P. In another scenario, when the waste heat outlet temperature remains at 60 \({}^{\circ}\)C, but the ambient air temperature increases to 35 \({}^{\circ}\)C, the P2P remains at 0.40, indicating that the impact of ambient temperature may plateau under certain conditions. However, when the waste heat outlet temperature drops to 50 \({}^{\circ}\)C while the air temperature stays at 25 \({}^{\circ}\)C, the P2P decreases to 0.36. This reduction in temperature difference results in lower P2P, as expected in thermodynamic processes. Furthermore, when the waste heat outlet temperature remains at 60 \({}^{\circ}\)C and the ambient air temperature is increased to 35 \({}^{\circ}\)C, the P2P again drops to 0.36.