Evaluation of a hybrid photovoltaic-wind system with hydrogen storage performance using exergy analysis

This article presents and discusses the results of an exergy analysis conducted during the operation of a test-bed hybrid wind/solar generator with hydrogen support, designed and constructed at the Industrial Engineering School of the University of Extremadura, Badajoz (Spain). An exergy analysis is made of the different components of the system, calculating their exergy efficiencies and exergy losses, and proposing future improvements to increase the efficiency of the use of the surplus energy produced by the wind/solar generator. The results show the electrolyzer to have an acceptable efficiency (η_ex = 68.75%), but the photovoltaic modules a low exergy efficiency (η_ex = 8.39%) as also is the case, though to a lesser extent, for the fuel cell (η_ex = 35.9%).     

Performance assessment of an electrochemical hydrogen production and storage system for solar hydrogen refueling station

This paper investigates the performance of a hydrogen refueling system that consists of a polymer electrolyte membrane electrolyzer integrated with photovoltaic arrays, and an electrochemical compressor to increase the hydrogen pressure. The energetic and exergetic performance of the hydrogen refueling station is analyzed at different working conditions. The exergy cost of hydrogen production is studied in three different case scenarios; that consist of i) off-grid station with the photovoltaic system and a battery bank to supply the required electric power, ii) on-grid station but the required power is supplied by the electric grid only when solar energy is not available and iii) on-grid station without energy storage. The efficiency of the station significantly increases when the electric grid empowers the system. The maximum energy and exergy efficiencies of the photovoltaic system at solar irradiation of 850 W m-2 are 13.57% and 14.51%, respectively. The exergy cost of hydrogen production in the on-grid station with energy storage is almost 30% higher than the off-grid station. Moreover, the exergy cost of hydrogen in the on-grid station without energy storage is almost 4 times higher than the off-grid station and the energy and exergy efficiencies are considerably higher.     

Energy and exergy analysis of hydrogen production from ammonia decomposition systems using non-thermal plasma

In the current study, the energy and exergy efficiencies of three hydrogen production systems from ammonia decomposition using dielectric barrier discharge plasma (DBD) were comparatively evaluated. The hydrogen gas was separated in a cylindrical plasma membrane reactor (PMR) using the Pd–Cu40% membrane with a thickness of 20 μm. The pre-catalytic reactor (CR) is added to the second system (CR-PMR), additionally, the CR is filled with the catalytic material type of 2%Ru/Al₂O₃ and the CR temperature is raised to 450 °C. Furthermore, the zeolite material type of SA-600 A was added to the PMR in the third H₂ production system (PMR) to enhance the hydrogen permeation through the Pd–Cu membrane. The hydrogen production rate was enhanced by combining the plasma and zeolite material in the third system (CR-CPMR). Moreover, the maximum obtained hydrogen production rates were 2.66, 81.6, and 96.6% in PMR, CR-PMR, and CR-CPMR or catalytic PMR, respectively. Also, it was observed that the energy efficiency increased by adding the CR to the system, while, the exergy efficiency values of all ammonia decomposition systems were still low due to the effect of system irreversibility. Additionally, the maximum energy efficiencies values were 0.8, 16.1, 44.1%, while the maximum exergy efficiencies values were 0.156, 4.91, and 6.344% for PMR, CR-PMR, and CR-CPMR, respectively. The exergy destruction rate of all NH₃ decomposition systems was still high although using the modified systems. The depletion factor is enhanced with the feeding ammonia flow rate increased while the sustainability index decreased at the same flow rates. Moreover, it was seen that the depletion factor results of PMR only were higher than other systems due to the exergy destruction rate was high.     

A review on biomass‐based hydrogen production and potential applications

In this paper, a detailed review is presented to discuss biomass‐based hydrogen production systems and their applications. Some optimum hydrogen production and operating conditions are studied through a comprehensive sensitivity analysis on the hydrogen yield from steam biomass gasification. In addition, a hybrid system, which combines a biomass‐based hydrogen production system and a solid oxide fuel cell unit is considered for performance assessment. A comparative thermodynamic study also is undertaken to investigate various operational aspects through energy and exergy efficiencies. The results of this study show that there are various key parameters affecting the hydrogen production process and system performance. They also indicate that it is possible to increase the hydrogen yield from 70 to 107 g H₂ per kg of sawdust wood. By studying the energy and exergy efficiencies, the performance assessment shows the potential to produce hydrogen from steam biomass gasification. The study further reveals a strong potential of this system as it utilizes steam biomass gasification for hydrogen production. To evaluate the system performance, the efficiencies are calculated at particular pressures, temperatures, current densities, and fuel utilization factors. It is found that there is a strong potential in the gasification temperature range 1023–1423 K to increase energy efficiency with a hydrogen yield from 45 to 55% and the exergy efficiency with hydrogen yield from 22 to 32%, respectively, whereas the exergy efficiency of electricity production decreases from 56 to 49.4%. Hydrogen production by steam sawdust gasification appears to be an ultimate option for hydrogen production based on the parametric studies and performance assessments that were carried out through energy and exergy efficiencies. Finally, the system integration is an attractive option for better performance. Copyright © 2012 John Wiley & Sons, Ltd.     

Performance and thermo-economic assessments of geothermal district heating system: A case study in Afyon,Turkey

In this study energy, exergy and exergoeconomic analysis of the Afyon geothermal district heating system (AGDHS) in Afyon, Turkey is performed through thermodynamic performances and thermo-economic assessments. In the analysis, actual system data are used to assess the district heating system performance, energy and exergy efficiencies, exergy losses and loss cost rates. Energy and exergy losses throughout the AGDHS are quantified and illustrated in the flow diagram. The energy and exergy efficiencies of the overall system are found to be 37.59% and 47.54%, respectively. The largest exergy loss occurs in the heat exchangers with 14.59% and then in the reinjection wells with 14.09%. Besides, thermo-economic evaluations of the AGDHS are given in table. Energy and exergy loss rates for the AGDHS are estimated to be 5.36 kW/$ and 0.2  kW/$, respectively.     

Evaluation of sludge liquors from acidogenic fermentation and thermal hydrolysis process as feedstock for microbial electrolysis cells

In this study, mesophilic acidogenic fermentation, thermophilic acidogenic fermentation, and thermal hydrolysis process (THP) were compared to generate sludge liquors for bioenergy recovery with microbial electrolysis cells (MECs). The results showed that THP at 170 °C was the most effective for hydrolysis of particulate organics in sewage sludge, while fermentation under thermophilic temperature led to the highest accumulation of volatile fatty acids (VFAs) in sludge liquor. However, THP yielded the highest percentage of acetate in VFAs, which resulted in superior MEC performance compared to fermented sludge liquors in terms of current density (2.7 vs. ~1.3 A/m²), coulombic efficiency (50% vs. 31–34%), bio-H₂ potential (1114 vs. 839–881 mL), and H₂ production rate (50.3 mL/d vs. 28–32 mL/d). The utilization sequence of the VFAs was found to be acetate > butyrate > propionate. Overall, our results show that generating sludge liquors through THP could provide a feasible solution to produce bio-H₂ from sewage sludge; however, coulombic efficiencies should be further improved before practical application.     

Hydrogen production through biomass gasification in supercritical water: A review from exergy aspect

Hydrogen production through supercritical water gasification (SWG) of biomass has been widely studied. This study reviews the main factors from exergy aspect, and these include feedstock characteristics, biomass concentration, gasification temperature, residence time, reaction catalyst, and reactor pressure. The results show that the exergy efficiencies of hydrogen production are mainly in the range of 0.04–42.05%. Biomass feedstock may affect hydrogen production by changing the H₂ yield and the heating value of biomass. Increases in biomass concentrations decrease the exergy efficiencies, increases in gasification temperatures generally increase the exergy efficiencies, and increases in residence times may initially increase and finally decrease the exergy efficiencies. Reaction catalysts also have positive effects on the exergy efficiencies, and the reviewed results show that the effects are followed KOH > K₂CO₃ > NaOH > Na₂CO₃. Reactor pressure may have positive, negative or negligible effects on the exergy efficiencies.     

A renewable energy based waste-to-energy system with hydrogen options

A pressurized gasification combined system is studied in a novel integration with geothermal energy to produce hydrogen-enriched syngas. This system utilizes dewatered sludge, which leaves the biological wastewater treatment facility during the wastewater treatment process and is used as a feedstock to produce hydrogen as a useful output. The hydrogen produced is transformed in a proton-exchange fuel cell to electricity for community use. This system also incorporates a wind farm with a hydrogen storage system to meet societies’ energy need when the energy demand fluctuates. The integrated system is then analyzed with thermodynamic-based energy and exergy approaches. The Greater Toronto area is chosen as the case study location and comprehensive thermodynamic analysis and simulation are completed on the Aspen Plus and Engineering Equation Solver softwares while the annual wind speed data are obtained from the RETScreen software. The daily total energy delivered to the community from this proposed system is recorded to be 2.1 GWh. In addition, the hydrogen production ratio at the gasification system is observed to be 0.12 through the sludge utilization where the energy and exergy efficiencies of the integrated gasification combined cycle were calculated to be 24% and 28%, in this order. The highest energy and exergy efficiency with 38.6% and 42.2%, respectively, are observed in January where the wind farm operated at a capacity of 41.7% and the average wind speed was 6.3 m/s for Greater Toronto Area. The overall energy and exergy efficiencies of this waste-to-energy system are calculated as 32.7% and 36.6%, respectively.     

Thermal modeling of a packed bed thermal energy storage system during charging

The process of charging of an encapsulated ice thermal energy storage device (ITES) is thermally modeled here through heat transfer and thermodynamic analyses. In heat transfer analysis, two different temperature profile cases, with negligible radial and/or stream-wise conduction are investigated for comparison, and the temperature profiles for each case are analyzed in an illustrative example. After obtaining temperature profiles through heat transfer analysis, a comprehensive thermodynamic study of the system is conducted. In this regard, energy, thermal exergy and flow exergy efficiencies, internal and external irreversibilities corresponding to flow exergy, as well as charging times are investigated. The energy efficiencies are found to be more than 99%, whereas the thermal exergy efficiencies are found to vary between 40% and 93% for viable charging times. The flow exergy efficiency varies between 48% and 88% for the flows and inlet temperatures selected. For a flow rate of 0.00164 m³/s, the maximum flow exergy efficiency occurs with an inlet temperature of 269.7 K, corresponding to an efficiency of 84.3%. For the case where the flow rate is 0.0033 m³/s, the maximum flow exergy efficiency becomes 87.9% at an inlet temperature of 270.7 K. The results confirm the fact that energy analyses, and even thermal exergy analyses, may lead to some unrealistic efficiency values. This could prove troublesome for designers wishing to optimize performance. For this reason, the flow exergy model provides the most useful information for those wishing to improve performance and reduce losses in such ITES systems.     

Exergoeconomic and environmental impact analyses of a renewable energy based hydrogen production system

In this study, exergoeconomic and environmental impact analyses, through energy, exergy, and sustainability assessment methods, are performed to investigate a hybrid version renewable energy (including wind and solar) based hydrogen and electricity production system. The dead state temperatures considered here are 10 °C, 20 °C and 30 °C to undertake a parametric study. An electrolyzer and a metal hydride tank are used for hydrogen production and hydrogen storage, respectively. Also, the Proton Exchange Membrane Fuel Cell (PEMFC) and battery options are utilized for electricity generation and storage, respectively. As a result, the energy and exergy efficiencies and the sustainability index for the wind turbine are found to be higher than the ones for solar photovoltaic (PV) system. Also, the overall exergy efficiency of the system is found to be higher than the corresponding overall energy efficiency. Furthermore, for this system, it can be concluded that wind turbine with 60 gCO₂/month is more environmentally-benign than the solar PV system with 75 gCO₂/month. Finally, the total exergoeconomic parameter is found to be 0.26 W/$, when the energy loss is considered, while it is 0.41 W/$, when the total of exergy loss and destruction rates are taken into account.     

Energy and exergy analysis of a new hydrogen-fueled power plant based on calcium looping process

A novel hydrogen-fueled power plant with inherent CO₂ capture based on calcium looping process is proposed in this paper. The analyzed system has been evaluated from the energy and exergy points of view, it enables determination of the contribution of main component to the total exergy loss. The results show that energy and exergy efficiencies of the system are 42.7% and 42.25% respectively, combustion chamber and regenerator are responsible for large exergy destructions, mainly due to irreversibilities associated with the combustion reactions, they have great potential for system efficiencies improvements. The effects of various air pressure ratios and gas turbine inlet temperatures on the system thermodynamic performance are also presented. The thermodynamic efficiencies increase with the increase in air pressure ratios and gas turbine inlet temperatures.     

Performance analysis of a CCHP system based on SOFC/GT/CO2 cycle and ORC with LNG cold energy utilization

An innovative CCHP system based on SOFC/GT/CO₂ cycle and the organic Rankine cycle (ORC) with LNG cold energy utilization is proposed to achieve cascade energy utilization and carbon dioxide capture. The mathematical models are developed and the system performance is analyzed using the energy and exergy methods. The results illustrate that the comprehensive energy utilization, the net power generation and the overall exergy efficiencies of the system can reach about 79.48%, 79.81% and 62.29%, respectively, while the power generation efficiency of the SOFC is 50.96% and the CO₂ capture rate of the proposed CCHP system is 79.2 kg/h under the given conditions. It shows that the proposed CCHP system can reach a high energy utilization efficiency with near zero emissions. The influence of some key parameters, such as the fuel utilization factor, the air-fuel ratio, the oxygen concentration in the cathode feed and the compression ratio of the SCO₂ turbine on the performance of the entire system is studied.     

Exergy analysis of industrial pasta drying process

In this study we present an energy and exergy modelling of industrial final macaroni (pasta) drying process for its system analysis, performance evaluation and optimization. Using actual system data, a performance assessment of the industrial macaroni drying process through energy and exergy efficiencies and system exergy destructions is conducted. The heat losses to the surroundings and exergy destructions in the overall system are quantified and illustrated using energy and exergy flow diagrams. The total energy rate input to system is 316.25 kW. The evaporation rate is 72 kg h^?1 (0.02 kg s^?1) and energy consumption rate is found as 4.38 kW for 1 kg water evaporation from product. Humidity product rate is 792 kg h^?1 (0.22 kg s^?1) and energy consumption rate is found about 0.4 kW for 1 kg short cut pasta product. The energy efficiencies of the pasta drying process and the overall system are found to be as 7.55–77.09% and 68.63%. The exergy efficiency of pasta drying process is obtained to be as 72.98–82.15%. For the actual system that is presented the system exergy efficiency vary between 41.90 and 70.94%. Copyright © 2006 John Wiley & Sons, Ltd.     

Energy and exergy analyses of a combined molten carbonate fuel cell - Gas turbine system

This study deals with the thermodynamic analysis of molten carbonate fuel cell combined with a gas turbine, based on the first- and second-law of thermodynamics. The mass, energy, entropy and exergy balance equations are written and applied to the system and its components. Some parametric studies are performed to investigate the change of system performance through energy and exergy efficiencies with the change of operating conditions. The irreversibilities occuring in different devices of the integrated system are also investigated through the exergy destruction analysis in these devices. The maximum output work of the MCFC is estimated to be 314.3 kW for an operating temperature of 650 °C. The overall energy and exergy efficiencies achieved for this system are 42.89% and 37.75%, respectively.     

Optimization and comparison of two improved very high temperature gas-cooled reactor-based hydrogen and electricity cogeneration systems using iodine-sulfur cycle

To enrich the existing research methods and content, two improved very high temperature gas-cooled reactor and iodine-sulfur (I–S) cycle-based nuclear hydrogen and steam and helium gas turbines electricity cogeneration systems, including the series connection system (SCS) and the parallel connection system (PCS), are proposed and studied. The energy and exergy analysis methods are used to model these two systems, and Aspen Plus is adopted to build the I–S hydrogen production system. The energy consumption and thermal efficiency of the I–S system are analyzed in detail, and the parametric optimization of two improved systems is performed using particle-swarm optimization (PSO) algorithm. Lastly, the performance comparison of the two systems under different operating conditions is conducted. The simulation results show that more than 99% of the energy consumption of the I–S system is occupied by H₂SO₄ section and HIx section, and the system's thermal efficiency is estimated to be in the range of 17.7%–43.3%. After using an internal heat exchange network, a conservative thermal efficiency of 23.7% is achieved. The optimization results show that under zero hydrogen production load, the proposed PCS and SCS can respectively achieve the net electrical power outputs of 172.8 MW and 125.7 MW, the global thermal efficiencies of 49.36% and 35.91%, and the global exergy efficiencies of 51.94% and 37.79%. With the increase of hydrogen production load, the global efficiencies of both systems decrease significantly, but the decreasing rate of PCS is faster than that of SCS. In addition, the performance comparison results indicate that when the hydrogen production load is small or the intermediate heat exchanger's primary side helium outlet temperature is close to the reactor inlet temperature, the PCS would be a better option than the SCS.     

Enhanced generation of hydrogen,power, and heat with a novel integrated photoelectrochemical system

Hydrogen is an essential component of power-to-gas technologies that are needed for a complete transition to renewable energy systems. Although hydrogen has zero GHG emissions at the end-use point, its production could become an issue if non-renewable, and pollutant energy and material resources are used in this step. Therefore, a crucial step for the fully developed hydrogen economy is to find alternative hydrogen production methods that are clean, efficient, affordable, and reliable. With this motivation, in this study, an integrated and continuous type of hydrogen production system is designed, developed, and investigated. This system has three components. There is a solar spectral splitting device (Unit I), which splits the incoming solar energy into two parts. Photons with longer wavelength is sent to the photovoltaic thermal hybrid solar collector, PV/T, (Unit II) and used for combined heat and power generation. Then the remaining part is transferred to the novel hybrid photoelectrochemical-chloralkali reactor (Unit III) for simultaneous H₂, Cl₂, and NaOH production. This system has only one energy input, which is the solar irradiation and five outputs, namely H₂, Cl₂, NaOH, heat, and electricity. Unlike most of the studies in the literature, this system does not use only PV or only a photoelectrochemical reactor. With this approach, solar energy utilization is maximized, and the wasted portion is minimized. By selecting PV/T rather than PV, the performance of the panels is maximized because recovering the by-product heat as a system output in addition to electricity, and the PV/T has less waste and higher efficiency. The present reactor does not use any additional electron donors, so the wastewater discharge is only depleted NaCl solution, which makes the system significantly cleaner than the ones available in the literature. The specific aim of this study is to demonstrate the optimum operating parameters to reach the maximum achievable production rates and efficiencies while keeping the exergy destruction as little as possible. In this study, there are four case studies, and in each case study, one decision variable is optimized to get the desired performance results. Within the selected operating parameter range, all performance criteria (except exergy destruction) are normalized and ranked for proper comparison. The maximum production rates and efficiencies with the least possible exergy destruction are observed at the operating temperature of 30 °C. At 30 °C, 4.18 g/h H₂, 127.55 g/h Cl₂, 151 W electricity, and 716 W heat are produced with an exergy destruction rate of 95.74 W and 78% and 30% energy and exergy efficiencies, respectively.     

Enhanced methane fermentation of municipal sewage sludge by microbial electrochemical systems integrated with anaerobic digestion

Sewage sludge from a municipal wastewater treatment plant was fed into a microbial electrochemical system, combined with an anaerobic digester (MES-AD), for enhanced methane production and sludge stabilization. The effect of thermally pretreating the sewage sludge on MES-AD performance was investigated. These results were compared to those obtained from control operations, in which the sludge was not pretreated or MES integration was absent. The soluble chemical oxygen demand (SCOD) in the raw sewage sludge after pretreatment was 31% higher than the SCOD in untreated sludge (5804.85 mg/L vs. 4441.46 mg/mL). The methane yield and proportion of methane in biogas generated by the MES-AD were higher than those of the control systems, regardless of the pretreatment process. The maximum methane yield (0.28 L CH₄/g COD) and methane production (1139 mL) were obtained with the MES inoculated with pretreated sewage sludge. Methane yield and production with this system using pretreated sewage were 47% and 56% higher, respectively, than those of the control (0.19 L CH₄/g COD, 730 mL). Additionally, the maximum SCOD removal (89%) and current generation were obtained with the MES inoculated with a pretreated substrate. These results suggested that sewage sludge could be efficiently stabilized with enhanced methane production by synergistic combination of MES-AD system with pretreatment process.     

Analysis and assessment of a geothermal based cogeneration system and lithium extraction

This paper presents the thermodynamic analyses for a double flash-binary based integrated geothermal power plant which consists of two steam turbines and one expander in the organic Rankine cycle that uses ammonia as the working fluid and a lithium extraction sub system. The main useful outputs of the plant are electricity, heat for floor heating and lithium carbonate (Li₂CO₃). The aim of this study is to assess the overall system performance energetically and exergetically. Based on the results obtained from this study, the overall energy and exergy efficiencies are 58.41% and 66.63%, respectively. The present results also show that the Li₂CO₃ is produced at the rate of 9.52 × 10⁻³ kg/s. In addition, the effects of changing several important operating parameters and ambient conditions on the energy and exergy efficiencies and the performance of the subsystems are investigated.     

Investigation of a unique integrated photoelectrochemical system for multigeneration purposes

This study develops, investigates and analyze a continuous type hybrid photoelectrochemical-chloralkali H₂ production reactor that converts the by–products into useful industrial commodities (i.e., Cl₂ and NaOH). The proposed system maximizes solar spectrum use by taking advantage of photocatalysis and PV/T. Furthermore, by using electrodes as electron donors to support the photochemical reaction, the potential risk of pollutant emissions is minimized. The final products of this novel integrated system can be listed as H₂, Cl₂, NaOH, heat, and electricity. In this study, the effects of operating temperature and inlet mass flow rates on H₂, Cl₂, heat, and electricity production, energy and exergy efficiencies, and exergy destruction rates are presented. The results of this investigation show that the proposed system is capable of producing hydrogen up to 70 L/h, chlorine up to 60 L/h, heat up to 800 W, electricity up to 160 W, with energy and exergy efficiencies up to 80% and 30%, respectively.     

Thermodynamic analysis of a Proton Exchange Membrane fuel cell

In this study, energy and exergy analyses of a 1 kW Horizon H-1000 XP Proton Exchange Membrane (PEM) Fuel Cell has been investigated. A testing apparatus has been established to analyze the system efficiencies based on the first and second laws of thermodynamics. In this mechanism pure hydrogen has been directly used as a fuel in compressed gas formation. Purity of hydrogen was above 99.99%. The system performance was investigated through experimental studies on energy and parametric studies on exergy by changing the operating pressure and operation temperature. The results showed that the energy efficiency of PEM fuel cell is 45.58% for experimental study and 41.27% for parametric study at full load. Also, 2.25% and 4.2% performance improvements were obtained by changing the operating temperature ratio (T/T₀) from 1 to 1.2 and operating pressure ratio (P/P₀) from 1 to 2, respectively.