Energy and exergy analyses of hydrogen production via solar-boosted ocean thermal energy conversion and PEM electrolysis

Energy and exergy analyses are reported of hydrogen production via an ocean thermal energy conversion (OTEC) system coupled with a solar-enhanced proton exchange membrane (PEM) electrolyzer. This system is composed of a turbine, an evaporator, a condenser, a pump, a solar collector and a PEM electrolyzer. Electricity is generated in the turbine, which is used by the PEM electrolyzer to produce hydrogen. A simulation program using Matlab software is developed to model the PEM electrolyzer and OTEC system. The simulation model for the PEM electrolyzer used in this study is validated with experimental data from the literature. The amount of hydrogen produced, the exergy destruction of each component and the overall system, and the exergy efficiency of the system are calculated. To better understand the effect of various parameters on system performance, a parametric analysis is carried out. The energy and exergy efficiencies of the integrated OTEC system are 3.6% and 22.7% respectively, and the exergy efficiency of the PEM electrolyzer is about 56.5% while the amount of hydrogen produced by it is 1.2 kg/h.     

Thermodynamic assessment of geothermal energy use in hydrogen production

In this study, geothermal-based hydrogen production methods, and their technologies and application possibilities are discussed in detail. A high-temperature electrolysis (HTE) process coupled with and powered by a geothermal source is considered for a case study, and its thermodynamic analysis through energy and exergy is conducted for performance evaluation purposes. In this regard, overall energy and exergy efficiencies of the geothermal-based hydrogen production process for this HTE are found to be 87% and 86%, respectively.     

Life cycle assessment of high temperature electrolysis for hydrogen production via nuclear energy

A life cycle assessment (LCA) of one proposed method of hydrogen production—the high temperature electrolysis of water vapor—is presented in this paper. High temperature electrolysis offers an advantage of higher energy efficiency over the conventional low-temperature alkaline electrolysis due to reduced cell potential and consequent electrical energy requirements. The primary energy source for the electrolysis will be advanced nuclear reactors operating at temperatures corresponding to those required for the high temperature electrolysis. The LCA examines the environmental impact of the combined advanced nuclear-high temperature electrolysis plant, focusing upon quantifying the emissions of carbon dioxide, sulfur dioxide, and nitrogen oxides per kilogram of hydrogen produced. The results are presented in terms of the global warming potential (GWP) and the acidification potential (AP) of the system. The GWP for the system is 2000 g carbon dioxide equivalent and the AP, 0.15 g equivalents of hydrogen ion equivalent per kilogram of hydrogen produced. The GWP and AP of this process are one-sixth and one-third, respectively, of those for the hydrogen production by steam reforming of natural gas, and are comparable to producing hydrogen from wind- or hydro-electricity powered conventional electrolysis.     

A specific exergy costing assessment of the integrated copper-chlorine cycle for hydrogen production

In this study the specific exergy costing (SPECO) approach is employed on a four-step integrated thermochemical copper-chlorine (Cu Cl) cycle for hydrogen production for a second-law based assessment purposes. The Cu–Cl cycle is considered as one of the most environmentally benign and sustainable options of producing hydrogen and is thus investigated in this study due to its potential of ensuring zero greenhouse gas (GHG) emissions. Several conceptual Cu–Cl cycles have been exergoeconomically examined previously, however this study aims at investigating the four-step integrated Cu–Cl cycle developed at the Clean Energy Research Laboratory (CERL) at the Ontario Tech University thereby contributing to the thermo/exergoeconomic assessments of the thermochemical hydrogen production. In this study, the cycle is first thermodynamically modeled and simulated in a process simulation software (Aspen Plus) through exergy and energy approaches. The basic principles of the SPECO methodology are applied to the system and exergetic cost balances are performed for each cycle component. The exergetic costing of each cycle stream is then performed based on the cost balance equations. The purchased equipment cost and the hourly levelized capital cost rates for each cycle component is also obtained. The exergoeconomic factor, relative cost difference and exergy destruction cost rate for various cycle components are also evaluated. Moreover, the effect of several parameters on the total and hourly levelized capital cost rates is analyzed by performing a comprehensive sensitivity analysis. Based on the analysis, the exergy cost, the unit or specific exergy cost, and the unit costs of hydrogen are evaluated to be 6407.55 $/h, 0.042 $/MJ, and 4.94 $/kg respectively.     

Thermodynamic assessment of a lab-scale experimental copper-chlorine cycle for sustainable hydrogen production

An integrated lab-scale copper-chlorine (Cu-Cl) thermochemical cycle for hydrogen production at the University of Ontario Institute of Technology (UOIT) is presented and analyzed in this paper. In a practical operation of the Cu-Cl cycle, besides the main steps of hydrolysis, thermolysis, electrolysis and drying, the oxidized anolyte (consumed anolyte at the electrolyzer cell) needs to be recycled to be concentrated sufficiently for the electro-chemical process. Recycling of the oxidized anolyte through the separation processes is achieved by distillation of anolyte, drying unit, separation cell, pressure swing distillation and CuCl₂ concentrator. This study examines the thermodynamic performance of all unit operations in the lab-scale Cu-Cl cycle. A process simulation model with Aspen Plus is used to assess the system by energy and exergy analyses. For the specific system design characteristics, the cycle is capable of producing 100 L/h of hydrogen. From the simulation results, the overall energy and exergy efficiencies of the lab-scale Cu-Cl cycle are determined to be 11.6% and 34.9%, respectively. Furthermore, after the thermolysis and hydrolysis reactors, the quench cell and CuCl₂ concentrator have the highest exergy losses with thermal energy transferred through CuCl solidification and water vaporization phase-change processes at relatively high temperature. Additional results of the processes are presented and discussed.     

Energy and exergy analyses of electrolytic hydrogen production with molybdenum-oxo catalysts

This paper analyzes a new low-temperature electrolysis hydrogen production system using molybdenum-oxo catalysts in the cathode and a platinum based anode. A thermodynamic model is developed for the electrolysis process in order to predict and analyze the energy and exergy efficiencies. The new electrolysis system with molybdenum-oxo catalysts consists of two half cells of PEM (proton exchange membrane) and alkaline electrolysis. The effects of temperature and membrane thickness are reported at varying current densities. The results are presented and compared with previous studies to demonstrate the promising performance of the system.     

A comprehensive comparative energy and exergy analysis in solar based hydrogen production systems

In the presented paper, energy and exergy analysis is performed for thermochemical hydrogen (H₂) production facility based on solar power. Thermal power used in thermochemical cycles and electricity production is obtained from concentrated solar power systems. In order to investigate the effect of thermochemical cycles on hydrogen production, three different cycles which are low temperature Mg–Cl, H₂SO₄ and UT-3 cycles are compared. Reheat-regenerative Rankine and recompression S–CO₂ Brayton power cycles are considered to supply electricity needed in the Mg–Cl and H₂SO₄ thermochemical cycles. Furthermore, the effects of instant solar radiation and concentration ratio on the system performance are investigated. The integration of S–CO₂ Brayton power cycle instead of reheat-regenerative Rankine enhances the system performance. The maximum exergy efficiency which is obtained in the system with Mg–Cl thermochemical and recompression S–CO₂ Brayton power cycles is 27%. Although the energy and exergy efficiencies decrease with the increase of the solar radiation, they increase with the increase of the concentration ratio. The highest exergy destruction occurred in the solar energy unit.     

Design and assessment of a solar energy based integrated system with hydrogen production and storage for sustainable buildings

The current study investigates a holistically developed solar energy system combined with a ground-sourced heat pump system for stand-alone usage to produce power, heat, and cooling along with domestic hot water for residential buildings. An integrated system is proposed where three types of building-integrated photovoltaic plant orientation are considered and integrated with a vertical-oriented ground-sourced heat pump system as well as an anion exchange membrane electrolyser for hydrogen-based energy storage along with proton exchange membrane fuel cells. The ground-sourced heat pump system covers the heating requirements and exploits the available thermal energy under the ground. Hydrogen subsystem enables the integrated system to be used anytime by compensating the peak periods with stored hydrogen via fuel cell and exploiting the excess energy to produce hydrogen via electrolyser. The photovoltaic plant orientations are extensively designed by considering geometries of three different applications, namely, rooftop photovoltaic, building-integrated photovoltaic façade and photovoltaic canopy. The shading and geometrical losses of photovoltaic applications are extensively identified and considered. In addition, the openly available high-rise building load profiles are obtained from the OpenEI network and are modified accordingly to utilize in the current study. The building requirements are considered for 8760 h annually with meteorological data and energy usage characteristics of the selected regions. The integrated system is assessed via thermodynamic-based approach from energy and exergy points of views. In order to increase generality, the proposed building energy system is analyzed for five different cities around the globe. The obtained results show that a 20-floor building with approximately 62,680 m² residential area needs between 550 kWp and 1550 kWp of a photovoltaic plant in five different cities. For Ottawa, Canada, the overall energy and exergy efficiencies are found as 18.76% and 10.49%, respectively, in a typical meteorological year. For the city of Istanbul in Turkey, a 20-floor building is found to be self-sufficient by only using the building's surface area with a 495 kWp BIPV façade and a 90 kWp rooftop PV.     

Energy and exergy analyses of integrated hybrid sulfur isobutane system for hydrogen production

This paper analyzes an integrated HyS cycle (hybrid sulfur cycle), isobutane cycle and electrolyzer for hydrogen production. The operating parameters such as concentration, pressure and temperature are varied to investigate their effects on the energy and exergy efficiencies of the system with/without heat recovery and integration, as well as the decomposer and rate of hydrogen produced. A new heat exchanger network is also developed to recover heat within the HyS cycle in the most efficient manner. The exergy destruction rate in each component is analyzed and discussed. From the results, increasing the pressure is beneficial up to 3222 kPa, after which the performance remains constant. The exergy efficiency varies more significantly with operating parameters than the energy efficiency. The maximum exergy destruction occurs in the heat exchanger so this component should be the focus to enhance the overall performance of the system.     

Model development and analysis of a novel high-temperature electrolyser for gas phase electrolysis of hydrogen chloride for hydrogen production

In this study, a high temperature electrolyser for the gas phase electrolysis of hydrogen chloride for hydrogen production is proposed and assessed. A detailed electrochemical model is developed to study the J-E characteristics for the proposed electrolyser (a solid oxide electrolyser based on a proton conducting electrolyte). The developed model accounts for all major losses, namely activation, concentration and ohmic. Energy and exergy analyses are carried out, and the energy and exergy efficiencies of the proposed electrolyser are determined to be 41.1% and 39.0%, respectively. The simulation results show that at T = 1073 K, P = 100.325 kPa and J = 1000 A/m², 1.6 V is required to produce 1 mol of hydrogen. This is approximately 0.3 V less than the voltage required by a high temperature steam electrolyser (based on a proton conducting electrolyte) operating at same condition (T = 1073 K, P = 101.325 kPa and J = 1000 A/m²), suggesting that the proposed electrolyser offers a new option for high temperature electrolysis for hydrogen production, potentially with a low electrical energy requirement. The proposed electrolyser may be incorporated into thermochemical cycles for hydrogen production, like CuCl or chlorine cycles.     

Energy and exergy performance assessment of a novel solar-based integrated system with hydrogen production

In this study, a new solar power assisted multigeneration system designed and thermodynamically analyzed. In this system, it is designed to perform heating, cooling, drying, hydrogen and power generation with a single energy input. The proposed study consists of seven sub-parts which are namely parabolic dish solar collector, Rankine cycle, organic Rankine cycle, PEM-electrolyzer, double effect absorption cooling, dryer and heat pump. The effects of varying reference temperature, solar irradiation, input and output pressure of high-pressure turbine and pinch point temperature heat recovery steam generator are investigated on the energetic and exergetic performance of integration system. Thermodynamic analysis result outputs show that the energy and exergy performance of overall study are computed as 48.19% and 43.57%, respectively. Moreover, the highest rate of irreversibility has the parabolic dish collector with 24,750 kW, while the lowest rate of irreversibility is calculated as 5745 kW in dryer. In addition, the main contribution of this study is that the solar-assisted multi-generation systems have good potential in terms of energy and exergy efficiency.     

Energy and exergy analyses and optimization study of an integrated solar heliostat field system for hydrogen production

In this paper, we propose an integrated system, consisting of a heliostat field, a steam cycle, an organic Rankine cycle (ORC) and an electrolyzer for hydrogen production. Some parameters, such as the heliostat field area and the solar flux are varied to investigate their effect on the power output, the rate of hydrogen produced, and energy and exergy efficiencies of the individual systems and the overall system. An optimization study using direct search method is also carried out to obtain the highest energy and exergy efficiencies and rate of hydrogen produced by choosing several independent variables. The results show that the power and rate of hydrogen produced increase with increase in the heliostat field area and the solar flux. The rate of hydrogen produced increases from 0.006 kg/s to 0.063 kg/s with increase in the heliostat field area from 8000 m² to 50,000 m². Moreover, when the solar flux is increased from 400 W/m² to 1200 W/m², the rate of hydrogen produced increases from 0.005 kg/s to 0.018 kg/s. The optimization study yields maximum energy and exergy efficiencies and the rate of hydrogen produced of 18.74%, 39.55% and 1571 L/s, respectively.     

Energy and exergy analyses of a solar based hydrogen production and compression system

In this study, a solar thermal based integrated system with a supercritical-CO₂ (sCO₂) gas turbine (GT) cycle, a four-step Mg–Cl cycle and a five-stage hydrogen compression plant is developed, proposed for applications and analyzed thermodynamically. The solar data for the considered solar plant are taken for Greater Toronto Area (GTA) by considering both daily and yearly data. A molten salt storage is considered for the system in order to work without interruption when the sun is out. The power and heat from the solar and sCO₂-GT subsystems are introduced to the Mg–Cl cycle to produce hydrogen at four consecutive steps. After the internal heat recovery is accomplished, the heating process at required temperature level is supplied by the heat exchanger of the solar plant. The hydrogen produced from the Mg–Cl cycle is compressed up to 700 bar by using a five-stage compression with intercooling and required compression power is compensated by the sCO₂-GT cycle. The total energy and exergy inputs to the integrated system are found to be 1535 MW and 1454 MW, respectively, for a 1 kmol/s hydrogen producing plant. Both energy and exergy efficiencies of the overall system are calculated as 16.31% and 17.6%, respectively. When the energy and exergy loads of the receiver are taken into account as the main inputs, energy and exergy efficiencies become 25.1%, and 39.8%, respectively. The total exergy destruction within the system is found to be 1265 MW where the solar field contains almost 64% of the total irreversibility with a value of ～811 MW.     

Thermodynamic and electrochemical analyses of a solid oxide electrolyzer for hydrogen production

In this paper, a modeling of the Solid Oxide Electrolysis Cell (SOEC), through energetic, exergetic and electrochemical modeling approaches, is conducted, and its performance, particularly through exergy efficiency, is analyzed under various operating conditions and state properties for optimum hydrogen production. In a comprehensively performed parametric study, at a single electrolysis cell scale, the effects of varying some operating conditions, such as temperature, pressure, steam molar fraction and the current density on the cell potential and hence the performance are investigated. In addition, at the electrolyzer system scale, the overall electrolyzer performance is investigated through energy and exergy efficiencies, in addition to the system's power density consumption, hydrogen production rate, heat exchange rates and exergy destruction parameters. The present results show that the overall solid oxide electrolyzer energy efficiency is 53%, while the exergy efficiency is 60%. The exergy destruction at a reduced operating temperature increases significantly. This may be overcome by the integration of this system with a source of steam production.     

Green methods for hydrogen production

This paper discusses environmentally benign and sustainable, as green, methods for hydrogen production and categorizes them based on the driving sources and applications. Some potential sources are electrical, thermal, biochemical, photonic, electro-thermal, photo-thermal, photo-electric, photo-biochemical, and thermal-biochemical. Such forms of energy can be derived from renewable sources, nuclear energy and from energy recovery processes for hydrogen production purposes. These processes are analyzed and assessed for comparison purposes. Various case studies are presented to highlight the importance of green hydrogen production methods and systems for practical applications.     

Energy and exergy analyses of magnesium-chlorine (Mg-Cl) thermochemical cycle

In this paper, we conduct energy and exergy analyses of the magnesium-chlorine (Mg-Cl) thermochemical cycle for hydrogen production and examine the respective cycle energy and exergy efficiencies. We also undertake a parametric study to investigate how the overall cycle performance is affected by changing the reference environment temperature and operating conditions. The results show that Mg-Cl cycle offers a good potential due to its high energy and exergy efficiencies as 63.63% and 34.86%, respectively, based upon the conditions and parameters considered. In this regard, Mg-Cl cycle appears to be a promising low temperature thermochemical cycle. It may, therefore, compete with other low temperature thermochemical and hybrid cycles such as the copper–chlorine cycle.     

Steam and air fed biomass gasification: Comparisons based on energy and exergy

Results are reported of thermodynamic analyses of a biomass gasification unit in which sawdust is the biomass feed and the gasifying medium is either air or steam. Energy and exergy analyses are performed for the system and each of its components. A parametric study reveals the effect of design and operating parameters on the system's performance and energy and exergy efficiencies. The results show that the adiabatic temperature of biomass gasification significantly changes with the type of the gasifying medium. In addition, the exergy and energy efficiencies are observed to be higher when air is the gasifying medium rather than steam, while the system performance and exergy efficiencies are dependent on the moisture content of the feed biomass. The results are significant because they quantify the strong dependence of biomass gasification, which can be used for syngas or hydrogen production, on moisture content, and gasifying medium.     

Exergetic life cycle assessment of a hydrogen production process

Exergetic life cycle assessment (ExLCA) is applied with life cycle assessment (LCA) to a hydrogen production process. This comparative environmental study examines a nuclear-based hydrogen production via thermochemical water splitting using a copper–chlorine cycle. LCA, which is an analytical tool to identify, quantify and decrease the overall environmental impact of a system or a product, is extended to ExLCA. Exergy efficiencies and air pollution emissions are evaluated for all process steps, including the uranium processing, nuclear and hydrogen production plants. LCA results are presented in four categories: acidification potential, eutrophication potential, global warming potential and ozone depletion potential. A parametric study is performed for various plant lifetimes. The ExLCA results indicate that the greatest irreversibility is caused by uranium processing. The primary contributor of the life cycle irreversibility of the nuclear-based hydrogen production process is fuel (uranium) processing, for which the exergy efficiency is 26.7% and the exergy destruction is 2916.3 MJ. The lowest global warming potential per megajoule exergy of hydrogen is 5.65 g CO₂-eq achieved a plant capacity of 125,000 kg H₂/day. The corresponding value for a plant capacity of 62,500 kg H₂/day is 5.75 g CO₂-eq.     

Comparative assessment of various gasification fuels with waste tires for hydrogen production

In this paper, waste tires are comparatively studied and assessed as a feedstock relative to coal and coconut char. An Integrated Gasification Combined Cycle (IGCC) is developed by using the Aspen Plus to assess the suggested gasification feedstocks based on their carbon dioxide emissions and hydrogen production to feed rate ratios. Note that many tires are disposed of every year in North America and are stockpiled in the masses in landfills, which cause various environmental implications. In the present study, it is found that waste tires as a feedstock for gasification are a viable solution to this ever-rising problem. The hydrogen production to feed rate ratio is found to be 0.158 which is very competitive with high-quality coals and coconut char. The net power production from the combined cycle when tires are used as the feedstock for the gasifier is found to be 11.1kW. The optimal hydrogen production to feed rate ratio is also achieved at the maximum net power production rate. The energy and exergy efficiencies of the overall system are found to be 55.01% and 52.31% when the waste tires are used as a feedstock.     

Thermodynamic performance assessment of boron based thermochemical water splitting cycle for renewable hydrogen production

The present study is related with the thermodynamic performance assessment of renewable hydrogen production through Boron thermochemical water splitting cycle. Therefore, all step efficiencies and overall cycle efficiency are calculated based on complete reaction. Additionally, a parametric study is conducted to determine the effect of the reference environment temperature on the overall cycle efficiency. In this regard, exergy efficiencies, exergy destruction rates and also inlet and outlet exergy rates of the cycle are calculated and presented for various reference temperatures. The exergy efficiency of the cycle is calculated as 0.4393 based on complete reaction and occurs at 298 K. This study has shown that Boron thermochemical water splitting cycle has a great potential due to cycle performance. As a result, Boron based thermochemical water splitting cycle can help achieve better environment and sustainability due to high exergetic efficiency. By the way, economic and technical issues of the storage and transportation of the hydrogen can find a proper solution if the hydrogen production reaction of the Boron thermochemical water splitting cycle takes place on-board of a vehicle.