# Toward More Efficient Large-Scale Green Hydrogen Systems via Waste Heat Recovery and ORC
## 4 Discussion  
This study aimed to enhance the efficiency of the PEM hydrogen plant by incorporating heat recovery modules into the system. Heat is utilized throughout the system: (a) the heat recovered from each PEM module; (b) the heat recovered from the refrigerant cycle to generate electricity and enhance the system's efficiency. The 20 MW PEM electrolyzer plant produces 363 kg/hr of hydrogen and 2877 kg/hr of oxygen. Based on the higher heating value of hydrogen, the plant produces 14,302.2 kWh of hydrogen energy equivalent. The ORC is being maximized by increasing the electricity output from the turbine and reducing pump work while maintaining energy conservation and the mass balance. The results prove that 5463.65 kW of energy is available to recover from ten 2 MW PEM modules. This heat is recovered through a flow of water. The heated water passes through the ORC module and a turbine unit to heat the working fluid, R245 fa, producing electricity. The heat recovery system and ORC units make the water cooling system for the plant a closed loop since the exit water from the ORC unit returns to the initial point of the plant at the same temperature, which is 25 \({}^{\circ}\)C.  
The results show that the work performed by the turbine is sufficient to provide the energy needed for cooling hydrogen or oxygen water. As the B1 unit requires 7.2 kW, the O2 Hex unit requires 3.7 kW, and there are 10 PEM modules, the total duty work required by them is equal to 109 kW, which is less than the total electricity produced from the turbine. Comparing these results with previous works utilizing ORCs [24] shows this system operates at ambient temperature, using cooling fluid water. Moreover, a lower water mass flow rate is required to cool down the system. The ORC module effectively utilizes waste heat from the PEM stacks, reducing the water temperature from 76.47 \({}^{\circ}\)C to 25 \({}^{\circ}\)C, generating 169.97 kW of electricity in its initial configuration. Optimization efforts further enhanced this performance, increasing the ORC power output to 555.88 kW and improving the total system efficiency to 74.80%. By comparing the achieved efficiency and the previous work summarized in Table 1, it can be concluded that efficiency has improved by 4.8%. These improvements stem from optimizing the mass flow rates of both the working fluid and cooling water while maintaining energy conservation principles. Figure 7: Pressure-enthalpy diagram for the optimized organic Rankine cycle (ORC). The waste heat recovery system enhances efficiency and supports sustainable energy utilization by reducing the cooling load and repurposing otherwise lost thermal energy. This integration is particularly beneficial for large-scale hydrogen production facilities, where efficiency gains translate. Optimizing the ORC not only reduced the volume of required water for cooling but also increased the net efficiency of the refrigeration cycle. Performing techno-economic analyses would be a great next step in evaluating the use of ORC waste heat recovery for producing hydrogen. S.S.N. : Conceptualization (lead); methodology (equal); software (lead); validation (lead); formal analysis (lead); investigation (lead). ; resources (lead); data curation (lead); writing--original draft preparation (lead); writing--review and editing (equal); visualization (equal); supervision (lead); project administration (lead). F.M. : Conceptualization (supporting); methodology (equal); software (supporting); validation (supporting); formal analysis (supporting); (writing--original draft preparation (supporting); writing--review and editing (equal); visualization (equal). E.P. : Review and editing. All authors have read and agreed to the published version of the manuscript. * [PERSON] et al. (2003) [PERSON]; [PERSON]; [PERSON]; [PERSON] A parametric study of PEM fuel cell performances. _Int. J. Hydrogen Energy_**2003**, _28_, 1263-1272. [CrossRef]
* [PERSON] et al.