Toward More Efficient Large-Scale Green Hydrogen Systems via Waste Heat Recovery and ORC

Abstract

This research models a 20 MW PEM hydrogen plant. PEM units operate in the 60 to 80 °C range based on their location and size. This study aims to recover the waste heat from PEM modules to enhance the efficiency of the plant. In order to recover the heat, two systems are implemented: (a) recovering the waste heat from each PEM module; (b) recovering the heat from hot water to produce electricity utilizing an organic refrigerant cycle (ORC). The model is made by ASPEN^® V14. After modeling the plant and utilizing the ORC, the module is optimized using Python to maximize the electricity produced by the turbine, therefore enhancing the efficiency. The system is a closed-loop cycle operating at 25 °C and ambient pressure. 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 equivalents. The ORC is maximized by increasing the electricity output from the turbine and reducing the pump work while maintaining energy conservation and mass balance. The results show that the electricity power output reaches 555.88 kW, and the pump power reaches 23.47 kW.

1. Introduction

Hydrogen is a fundamental atom in many chemical compounds and processes. It is an intermediate molecule for many processes and a stand-alone energy carrier. In the last two decades, producing reasonably priced hydrogen without emitting tons of emissions has attracted attention among scientists and industry.

Hydrogen production pathways can be illustrated in many ways. This work illustrates two main pathways to reform feedstocks to hydrogen: (a) using thermal energy and heating processes; (b) using electricity to reform the feedstock. Basically, reforming the feedstock by burning and generating heat for refining purposes or using electricity to reform the feedstock. Electrified processes can be categorized as thermal and non-thermal. Steam methane reforming is the most convenient technology for producing hydrogen, utilizing 800 °C to crack natural gas, a fossil-based thermal-joule heating process. Results from research have shown that if electricity is used to reform the feedstock into hydrogen, there is a good opportunity to reduce emissions and be cost-competitive at scale production [1,2,3]. Figure 1 illustrates various hydrogen production pathways.

Finding the fittest to-purpose-cost beneficial with minimum environmental impact for producing hydrogen depends on the geographical location and resources. The proper system can be deployed based on the availability of resources such as water or natural gas. Technologies such as plasma [3,4], electric heating [5], and electrolyzers [6] utilize electricity to reform the feedstock to hydrogen. This research uses proton exchange membrane (PEM) electrolyzers [7] to reform water into hydrogen. A heat recovery system coupled with an organic refrigerant cycle (ORC) [8] is integrated into the system to enhance overall efficiency by converting wasted heat from PEM modules into electricity.

In current systems, the heat generated by modules is released into the air. In fact, if a heat recovery system is utilized, the generated heat can be returned to the system to produce electricity. There is a scarcity of literature and studies on heat recovery from PEM for large-scale hydrogen production. Norbet and Assma have conducted thermo-economic studies of waste heat recovery in PEMs using the Rankine cycle utilizing ethane and R1233zd(E) [9]. Els van and his collaborators looked at utilizing heat at a 2.5 MW PEM plant. They recommended expanding the model to a larger scale using industrial data, which this study tried to use [10].

1.1. Water Electrolyzer

There are various electrolyzer technologies for producing hydrogen, including alkaline electrolyzers (AWE) [11], polymer electrolyte membranes (PEMs) [11], solid oxide cells (SCOECs) [12], anion exchange membranes (AEMs) [11], supercritical water electrolysis (SWE), high-pressure electrolysis (HPE), and high-temperature electrolysis (HTE) [11].

Table 1. Electrolyzer types.

Technology	Electrolyte Type	Operating Temperature	Efficiency	Advantages	Challenges
Alkaline electrolyzer (AWE)	Liquid alkaline solution (KOH)	60–90 °C	70–80%	Mature technology, lower capital costs, uses non-precious metal catalysts	Larger physical footprint, slower response to load changes, lower current density
Proton exchange membrane (PEM) electrolyzer	Solid polymer membrane	50–80 °C	60–70%	Compact design, rapid response to power fluctuations, higher current density	Higher capital costs due to the use of precious metal catalysts, sensitive to water impurities
Solid oxide electrolyzer cells (SOECs)	Solid ceramic material	700–1000 °C	Up to 100% (theoretical)	High efficiency due to elevated temperatures, potential for waste heat utilization	High operating temperatures lead to material degradation, currently in developmental stages
Anion exchange membrane (AEM) electrolyzer	Anion-exchange membrane	<100 °C	Emerging technology	Potential for lower costs using non-precious metal catalysts combines benefits of AWE and PEM	Still under research, durability and performance need further validation
Supercritical water electrolysis (SWE)	Supercritical water	>374 °C and >22.1 MPa	High	High reaction rates, direct production of high-pressure hydrogen	Requires materials that can withstand extreme conditions, currently experimental
High-pressure electrolysis	Varies	Similar to AWE or PEM	Similar to base technology	Produces compressed hydrogen directly, reducing the need for external compression	Requires robust system design to ensure safety and durability under high pressure
High-temperature electrolysis (HTE)	Steam (solid oxide cells)	500–850 °C	Higher than AWE and PEM	Improved efficiency by utilizing thermal energy, suitable for integration with heat sources	High operating temperatures necessitate durable materials, system complexity

briefly compares these technologies.

1.2. Proton Exchange Membrane Electrolyzer

Proton exchange membrane electrolyzers are a type of equipment that utilizes a solid polymer electrolyte to produce hydrogen through the electrolysis of water. The main advantages of using PEM electrolysis are its compact design, high efficiency, high rate of hydrogen production, rapid response to power fractionation, and high hydrogen purity. The main challenges associated with PEM are material costs, water purity sensitivity, and the durability of the membrane and catalysts. PEM electrolysis occurs in three key steps:

(a)

Water splits at the anode:

2H₂O → 4H⁺ + 4e⁻ + O₂,

(1)

(b)

Protons migrate through the PEM toward the cathode.

(c)

The protons (H⁺) recombine with electrons supplied from the external circuit to form molecular hydrogen gas:

4H⁺ + 4e → 2H₂,

(2)

The overall reaction:

2H₂O → 2H₂ + O₂,

(3)

The schematic of PEM electrolysis process is shown in Figure 2.

1.3. Organic Refrigerant Cycle

Organic refrigerant cycles (ORCs) are thermodynamic processes that use organic working fluids to transfer heat. These cycles are particularly advantageous when dealing with low-to-moderate-temperature heat sources [14]. ORCs operate similarly to the traditional Rankine cycle but uses organic fluids with lower boiling points, such as hydrocarbons or refrigerants, instead of water. ORCs can be used in various applications such as waste heat recovery, geothermal power generation, and solar thermal power.

Choosing the appropriate working fluid depends on the application, thermodynamic properties, environmental impacts, safety, and economic concerns. Fluids like R123, R245fa, and R245ca are commonly selected based on their favorable characteristics.

Mojtaba Aghajani and his colleagues have surveyed R245fa and R134fa for the heat recovery of PEM units. For both working fluids (R−134a and R−245fa), they noticed that increasing the thermal load decreases the net power output while the efficiency of the hybrid system decreases [15]. Mohammad Ali Abdelkareem and his colleagues have reviewed the recovery of waste heat from PEM and fuel cells and discussed various applications for the system [16]. Huy and his colleagues discussed PEM fuel cell thermal management solutions [17]. Jie and his colleagues looked at PEM system waste heat recovery using an ORC utilizing R123, R245fa, R134a, water, and ethanol. They found that the maximum efficiency of the recovery system is 4.73% [18].

The efficiency of a typical with no heat recovery modules, PEM hydrogen production is in the range of 60 to 70% when considering the lower heating value of hydrogen [19]. In this research, a 20 MW plant is modeled to evaluate its efficiency after adopting ORC and waste-to-heat modules.

As the efficiency of the PEM module has increased, less attention has been focused on the desired heat from the modules. In typical PEM systems, the heat dissipates to the surroundings. In large-scale applications, the amount of wasted heat becomes more viable and cost-effective for recovery and use. In this design, the extra wasted energy is recovered through water flow and then passed through the ORC to enhance the efficiency of the system by generating electricity via a turbine. Moreover, the water returns to PEM units at ambient temperature, so the water for cooling the system is within a closed loop.

Figure 3 illustrates the organic refrigerant cycle module. The module includes a pump, an evaporator, a turbine, and a condenser. An organic working fluid is heated in a boiler until it is converted to vapor. Then, the vapor enters the turbine, where it is expanded and produces work. The working fluid from the turbine passes through a pump to be condensed. At this stage, the fluid’s pressure rises, making it ready to enter the evaporator again.

1.4. Aim and Novelty

As the PEM system is utilized to produce hydrogen, an excessive amount of heat will be released through the modules. This study aims to capture that heat via the ORC and use it to enhance the system’s efficiency. The novelty of this study lies in modeling a 20 MW electrolysis system with ORC. The ORC module is capable of producing electricity from waste heat. Additionally, optimization enhances system efficiency and the total efficiency of the ORC system by reducing pump power while increasing turbine power in the refrigerant unit.

2. Methods

The software Aspen Plus^® V14 is used to model and simulate 20 MW electrolysis processes with a cooling system for heat recovery modules, and the efficiency is compared with conventional systems. PENG−ROB library is hired as the property method.

The total system efficiency ($\eta$) is calculated by Equation (4), defined by the total heat transfer from the system ($Q_{out, sys}$), the total required electricity for hydrogen production ($W_{H2}$), the heat loss from the electrolyzer stack for cooling the hydrogen and oxygen ($Q_{loss}$), the electricity requirements for the electrical pump ($W_{water-in pump}$ and $W_{Ref. pump}$), and the electricity input to the total system ($W_{in}$). The total efficiency of the ORC system can be calculated by the difference between the turbine work and pump work, divided by the transferred heat into the system, as shown in Equation (5).

$$\eta ={\displaystyle \frac{Q_{out, sys}+W_{H2}-Q_{loss}}{W_{in}+W_{water-in pump}+W_{Ref. pump}}},$$

(4)

$${\eta }_{ORC}={\displaystyle \frac{W_{turbine}-W_{pump}}{Q_{in}}},$$

(5)

2.1. Technical Analysis

The electrolyzer model is built to analyze a 20 MW process utilizing ORC heat recovery modules with conventional systems. Technical analyses aim to investigate the quality and quantity of recoverable waste heat available from the electrolysis process and compare it with previously built systems. In each scenario, the amount of heat waste and, therefore, the efficiency of the system are calculated.

2.2. 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 °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 (O2HEX) and recovers heat (HOTW2). All recovered heat water streams, HOTW, HOTW2, and HOTW3, are mixed and enter the ORC module.

The Peng Robinson method is used to model the plant via ASPEN V.14^®. Hydrogen, water, oxygen, and R245fa are the components of the system. The ambient temperature and pressure are assumed to be 25 °C and 1.01325 bar, respectively. Model’s statistics are represented in Appendix A 

Table A1. Model statistics.

	Statistic
Number of variables	6270
Number of incident variables	6263
Number of fixed variables	563
Number of free variables	5707
Number of equations	5707
Number of excluded equations	0
Number of non-zeros	20,563
Number of incidents non-zeros	19,457
Number of incomplete connections	0

.

Based on experimental results, the operation temperature of the PEM is 70 °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. 2 MW PEM electrolyzer operation parameters.

Unit	Parameter	Value
PEM stack	Water flow	324 kg/hr
	Water temperature	25 °C
	Water pressure	1 bar
	Average energy	18.015 MW
Waste heat recovery	Water flow	7482 kg/hr
	Water temperature	25 °C
	Water pressure	1 bar
H2 cooling system	Water flow	688 kg/hr
	Water temperature	25 °C
	Water pressure	1 bar
O2 cooling system	Water flow	430 kg/hr
	Water temperature	25 °C
	Water pressure	1 bar

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 R245fa working fluid is utilized to recover heat, which is appropriate for recovering low-temperature heat [22,23]. Another advantage of R245fa 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. ORC operation condition.

Unit	Parameter	Value
Evaporator	Water inflow	86,000 kg/hr
Turbine	Discharge pressure	1.3 bar
	Efficiency	70%
Pump	Discharge pressure	2.7 bar
	Pump	70%
Condenser	Water inflow	2 × 106 kg/hr
	Water temperature	20 °C
	Pressure	1 bar
R245fa coolant	Flow rate	65,000 kg/hr
	Pressure	1.3 bar

.

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 °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. 20 MW operation parameters summary.

Parameter	Value
Working fluid	86,000 kg/hr
Water in temperature	25 °C
Water-in pressure	1 bar
Total flow-in water	324 kg/hr
Power input	20,000 kW
Water-in through pumps	8 kW

.

2.3. Maximizing ORC Efficiency

In order to increase the efficiency of the process, the ORC module is optimized to maximize the produced electricity and enhance the cycle’s efficiency. Therefore, the objective function is defined as

$$f\left(x\right)=-{\dot{\mathrm{W}}}_{net}+{Penalty}_{quality},$$

(6)

where the quality penalty is defined as

$${Penalty}_{quality}=\left\{\begin{array}{c}{10}^6\times \left(0.90-X_{quality}\right) if{X}_{quality}<0.90\\ 0 otherwise\end{array}\right.,$$

(7)

And

$${\dot{\mathrm{W}}}_{net}={\dot{\mathrm{W}}}_{turbine}-{\dot{\mathrm{W}}}_{pump},$$

(8)

${\dot{\mathrm{W}}}_{net}$ represents the cycle’s work, ${\dot{\mathrm{W}}}_{turbine}$ calculates the turbine’s work, and ${\dot{\mathrm{W}}}_{pump}$ measures the pump’s work.

$${\dot{\mathrm{W}}}_{turbine}={\dot{\mathrm{m}}}_{ref}\times \left(h_{turbine-in}-h_{turbine,out}\right)\times {\eta }_{turbine},$$

(9)

$${\dot{\mathrm{W}}}_{pump}={\dot{\mathrm{m}}}_{ref}\times \left(h_{pump,out}-h_{pump,in}\right)/{\eta }_{pump},$$

(10)

For limiting feasible solutions and meeting actual physical properties, the constraints below have been used to avoid any system failures during the process:

Superheat constraint:

$$T_{superheatet}\ge 5 °\mathrm{C},$$

(11)

Subcooling constraint:

$$T_{Subcooling}\ge 3 °\mathrm{C},$$

(12)

Pressure constraints:

$$1 bar \le P_{cycle}\le 40 \mathrm{b}\mathrm{a}\mathrm{r},$$

(13)

Net positive suction head (NPSH) constraint:

$${NPSH}_{available}\ge {NPSH}_{required}=3\mathrm{m},$$

(14)

NPSH is calculated using the equations below:

$$NPS{H}_{available}=\left(h_{subcooled}-h_{sat}\right)/g,$$

(15)

To make sure these calculations are correct, the overall energy balance for ORC is formulated as below:

$${\dot{Q}}_{hot water}={\dot{\mathrm{W}}}_{net}+{\dot{Q}}_{cooling water, out}+{\dot{Q}}_{loses},$$

(16)

$${\dot{Q}}_{hot water}=\dot{{\dot{m}}_{hotwater}}(h_{water,out}-h_{water,in}),$$

(17)

$${\dot{Q}}_{cooling water}=\dot{{\dot{m}}_{coolwater}}(h_{water,out}-h_{water,in}),$$

(18)

The evaporator heat transfer, condenser heat transfer, and recuperator heat transfer using the effectiveness-NTU method are calculated as follows:

$${\dot{Q}}_{evap}={\dot{\mathrm{m}}}_{ref}\times \left(h_{evap, out}-h_{evap,in}\right)\times {\eta }_{HX},$$

(19)

$${\dot{Q}}_{cond}={\dot{\mathrm{m}}}_{ref}\times \left(h_{cond,in}-h_{cond, out}\right)\times {\eta }_{HX},$$

(20)

$${\dot{Q}}_{recup}={\epsilon }_{recup}\times {\dot{\mathrm{m}}}_{ref}\times C_{p,min,in}\times \left(T_{hot,out}-T_{cold,in}\right),$$

(21)

The effectiveness-NTU is expressed as below:

$${\epsilon }_{recup}=1-e^{-NTU(1-C_{ratio})},$$

(22)

$$NTU={\displaystyle \frac{UA}{C_{min}}},$$

(23)

$$\mathrm{UA}={\displaystyle \frac{\dot{Q}}{LMTD}},$$

(24)

LMTD represents the lean mean temperature difference (LMTD) method for heat exchangers:

$$LMTD={\displaystyle \frac{\left[\left(T_{houtin}-T_{coldout}\right)-\left(T_{hotout}-T_{coldin}\right)\right]}{ln\left(\left(T_{hot,in}-T_{coldout}\right)/\left(T_{hot,out}-T_{cold,in}\right)\right)}},$$

(25)

In the end, the thermal efficiency of the cycle is calculated using Equation (26):

$${\eta }_{thermal}={\dot{\mathrm{W}}}_{net}/{\dot{Q}}_{evap},$$

(26)

3. Results

3.1. Hydrogen Production and Heat Recovery at a 20 MW PEM Electrolyzer Plant

ASPEN V14 is used to model a 20 MW hydrogen production plant that utilizes waste heat recovery and an ORC to produce electricity. The system is a closed-loop cycle operating at 25 °C and ambient pressure. The 20 MW PEM electrolyzer model produces 363 kg/hr of hydrogen and 2877 kg/hr of oxygen. Based on the higher heating value (HHV) of hydrogen, the plant produces 14,302.2 kWh hydrogen energy equivalents.

Table 5. Results for each 2 MW PEM electrolyzer.

Streams	Water Inflow to PEM Stack	H2 Stream	O2 Stream
Temperature (°C)	25	30.47	32.03
Pressure (bar)	1	29.97	29.98
Mass density (kg/m3)	993.96	2.36	38.69
Average energy MW	18.02	2.02	31.99
Mass flows (kg/hr)	324	36.25	287.75
Enthalpy flow (W)	−1.43 × 106	879.98	−147.58

represents the results for producing hydrogen and streams for each 2 MW stack.

As a result, each PEM stack rejects 546.36 kW of thermal energy. Therefore, the heat recovery captured from ten 2 MW stacks equals 5463.65 kW. This means that from the electricity provided to separate hydrogen and oxygen, 5463.65 kW of heat is lost, which is 27.32% of the power input and is available for recovery. The cooling water runs through the plant so as to (a) capture rejected heat from PEM units, (b) cool down the produced hydrogen, and (c) cool down the produced oxygen.

Table 6. Results for the cooling system of the 2 MW electrolyzer.

Stream	Water-In *	HOTW *	HOTW2 *	HOTW3 *	Rec-Wast *
Temperature (°C)	25	33.32	31.84	82.97	76.47
Pressure (bar)	1	0.98	0.99	1	0.98
Mass density (kg/m3)	939.96	985.88	987.32	963.11	942.80
Average energy (MW)	18.02	18.02	18.02	18.02	18.02
Mass flow (kg/hr)	8600	688	430	7482	8600
Enthalpy flow (W)	−3.81 × 107	−3.04	−1.9 × 106	−3.26 × 107	−3.75 × 107

* Refer to Figure 5.

represents the results for cooling water for each 2 MW PEM electrolyzer.

The cumulative amount of required electricity for producing hydrogen, recovered waste heat from stacks, and heat loss while producing oxygen and hydrogen are calculated.

Table 7. Energy portfolio of the units.

Energy-for Units	Value (kW)
Total required electricity for H2 production	14,536.44
Potential recovered waste heat from stacks	5463.65
Heat loss for cooling O2	37.01
Heat loss for cooling H2	72.01

provides the summary results.

Figure 6 presents the energy flow within the PEM stacks and demonstrates the energy flow for producing hydrogen. As it is clear, the majority of energy within the system is present in the form of hydrogen. The total electrical system conversion efficiency is 71.41%. By capturing waste heat, the thermal efficiency of the system will increase, and therefore, the system efficiency will be further enhanced. An ORC module is added to the plant to maintain the 25 °C cooling water recirculating through the plant and producing some electricity from the recovered heat, which is 27.32% of total input energy. In the following sections, the electricity produced and optimizing ORC is explained.

3.2. Organic Refrigerant Cycle

After collecting the waste heat via water from PEM stacks, the ambient temperature of the water reaches 76.48 °C, which is the operation temperature of the typical PEM fuel cells. The water enters the ORC module to return to ambient temperature, 25 °C. The water passes through an evaporator to exchange heat with the R245fa refrigerant. The refrigerant working fluid passes through the turbine and produces electricity, as Figure 3 shows. The water exits the evaporator and cools down to 25 °C to recirculate through the plant.

Table 8. ORC module stream results.

Stream	Stream 2 *	Rec Water *	R245fd-IN *	R245fa-OUT *
Temperature (°C)	76.47	24.8	21.44	30.97
Pressure (bar)	0.98	0.88	1.3	1.3
Mass density (kg/m3)	942.80	994.12	1350.42	7.17
Average energy MW	18.02	18.02	134.04	134.04
Mass flows (kg/hr)	86,000	86,000	65,000	65,000
Enthalpy flow (W)	−3.75 × 108	−3.81 × 108	−1.62 × 108	−1.58 × 108

* Refer to Figure 3.

presents the results for the ORC module.

As the ORC cools down the water to ambient temperature, the wasted heat is used to produce electricity from phase changes in the working fluid. A summary of results for the ORC is presented in

Table 9. ORC results for 86,000 kg/hr water at 76 °C.

Variable	Values
Electricity from ORC	169.97 kW
Exit water temperature from condenser	22 °C
Total waste heat recovered	27.32%
Required electricity	2.7 kW
Exit water for plant cooling	25 °C

.

3.3. Maximizing ORC Efficiency

The ORC is being maximized by increasing the electricity output from the turbine and reducing pump work while maintaining energy conservation and mass balance. Solving Equation (6) resulted in the electricity power output reaching 555.88 kW and the pump power reaching 23.47 kW. Therefore, the electricity power output increased by 555.88 kW from 169.97 kW. In order to achieve maximum efficiency, the mass flow rate of working fluid, cooling water, and hot water are also optimized. Based on the optimized value, the working fluid and cooling water mass flow rates are 61,200 kg/hr and 1,033,092 kg/hr, respectively. Additionally, this optimization improves the total system efficiency (η), calculated using Equation (4), from 72.95 to 74.80, and increases the ORC system’s total efficiency from 3.06 to 10.13 based on Equation (5).

Table 10. ORC variables before and after optimization.

Variable	Before	After
Working fluid R245fa mass flow rate kg/hr	65,000	61,200
Cooling water flow rate kg/hr	2,000,002	1,033,092
Power output kW	169.97	555.88
Net power kW	165.96	532.41

compares the results before and after optimizing the ORC.

Figure 7 illustrates the pressure enthalpy diagram for the ORC. In this optimization, the efficiency of the turbine and pump is assumed to be 70%, which is a conservative assumption.

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, R245fa, 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 °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 °C to 25 °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.

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.