# Toward More Efficient Large-Scale Green Hydrogen Systems via Waste Heat Recovery and ORC
## 2 Methods
### Model Development  
As shown in Figure 4, the 20 MW plant includes waste heat capture, and an ORC module is modeled. The plant includes ten 2 MW PEM units, water waste heat recovery, and the ORC module. Each of the 2 MW modules are illustrated in Figure 5. As shown in each 2 MW module, each module includes a PEM unit, three heat exchangers, and two separators. The PEM stack is fed with two streams: (a) 2 MW of electricity; (b) water at ambient temperature and pressure conditions (20 \({}^{\circ}\)C and 1 bar). The water flows through the PEM units and is divided into oxygen and hydrogen. Hydrogen is separated from oxygen at a separation column (H2-SEP). The generated heat is absorbed by the water, which is heat water recovery (Waste-HE). The hydrogen passes through a heat exchanger (B1) to recover the heat from the produced hydrogen. The separated oxygen is mixed with some water. The water and oxygen separate at the O2\(-\)SEP, and the water returns to the PEM unit. The oxygen passes through a heat exchanger (O2 HEX) and recovers heat (HOTW2). All recovered heat water streams, HOTW, HOTW2, and HOTW3, are mixed and enter the ORC module. Figure 4: 20 MW PEM hydrogen production with heat recovery and ORC module. Purple, blue, green, and orange indicate electricity, water, hydrogen, and oxygen flows, respectively. The Peng Robinson method is used to model the plant via ASPEN V.14(r). Hydrogen, water, oxygen, and R245 fa are the components of the system. The ambient temperature and pressure are assumed to be 25 \({}^{\circ}\)C and 1.01325 bar, respectively. Model's statistics are represented in Appendix A Table 1.  
Based on experimental results, the operation temperature of the PEM is 70 \({}^{\circ}\)C at a pressure of 30 bar [20; 21]. Inside the PEM stack, the model simulates the electrolysis reactions shown in Equations (1)-(3), where the water splits into hydrogen and oxygen. The fraction reaction conversion in ASPEN is assumed to be 0.75. The maximum number of iterations to convert equations is 30, with an error tolerance of 0.0001. Currently, manufacturers such as FEST GmbH in Germany, ITM Power in United Kingdom, IMI VIVO in Italy, Nel Hydrogen in Norway, Siemens in Germany, Hele Titanium in China, Plug Power in USA and many more offer 2 MW PEM electrolyzers. Table 2 summarizes the operating parameters of the 2 MW PEM electrolyzer and the heating capture system. In this operation, all water runs at ambient pressure and temperature conditions to optimize the energy consumption of cooling systems. The hydrogen production mass flow rate and output pressure are compared to evaluate the performance of the modeled 2 MW PEM electrolyzer and the available PEM electrolyzer benchmarked technologies. In order to further increase the operation efficiency, water captures heat from each stack and flows out of each module. Then, the total water enters the ORC. The R245 fa working fluid is utilized to recover heat, which is appropriate for recovering low-temperature heat [22; 23]. Another advantage of R245 fa is the coolant's low viscosity, which reduces the pumping power required in the ORC system. The input parameters for ORC are summarized in Table 3.  
Figure 5: 2 MW PEM module. The results show that the work performed by the turbine is sufficient to provide the energy needed for cooling hydrogen or oxygen water. After the heat exchange, the water leaves the evaporator at 41.73 \({}^{\circ}\)C and passes through a turbine. At this stage, electricity will be produced from the turbine coupled with a generator. In the next step, the water will pass through a condenser. The cold water will be pumped back into the evaporator for the next cycle. The total efficiency of the ORC system can be calculated by the difference between the turbine and pump works, divided by the transferred heat into the system, as shown in Equation (5). The operation condition for the 20 MW plant is summarized in Table 4.