Lightweight reactor design by additive manufacturing for preheating applications using metal hydrides

Thermal energy storage systems based on metal hydrides can be a solution for preheating fuel cells (FCs). They can provide thermal energy at temperatures below −20 °C during startup, while heat at 50 °C during operation is sufficient for regeneration. The challenge of such a system in mobile applications is the final weight specific thermal power. In this study, a reactor design based on additive manufacturing techniques for ~300 g of metal hydride is presented. Here, a reactor (passive) to hydride (active) mass ratio of 0.97 is realized, still reaching high weight specific thermal power of up to 2.1 kW/kg_MH at −20 °C and 8 bar (LmNi_4.91Sn_0.15). Considering the example of preheating a FC from −20 °C in ~120 s, the performance of LaNi₅ and LmNi_4.91Sn_0.15 is studied. While LaNi₅ requires higher regeneration temperatures than LmNi_4.91Sn_0.15 (>40 °C compared to >20 °C), its performance is less sensitive to operative variations due to its nearly ideal thermodynamic characteristic.     

Process simulation of hydrogen production by steam reforming of diluted bioethanol solutions: Effect of operating parameters on electrical and thermal cogeneration by using fuel cells

The possibility to exploit diluted bioethanol streams is discussed for hydrogen production by steam reforming. An integrated unit constituted by a steam reformer, a hydrogen purification section with high- and low-temperature water gas shift, a methanator reactor and a fuel cell were simulated to achieve residential size cogeneration of 5 kW electrical power + 5 kW thermal power as target output.Process simulation allowed to investigate the effect of the reformer temperature, of bioethanol concentration and of catalyst loading on the temperature and concentration profiles in the steam reformer. The net power output was also calculated on the basis of 27 different operating conditions.P_electrical output ranging from 3.3 to 6.0 kW were obtained, whereas the total heat output P_{thermal, total} ranged from 3.9 to 7.2 kW. The highest overall energy output corresponded to P_electrical = 4.8 kW, P_{Thermal, FC} = 3.1 kW, P_{heat recovery} = 4.1 kW, for a total 12 kW energy output. This was achieved by feeding a mixture with water/ethanol ratio = 11 (mol/mol), irrespectively of the catalyst mass, and setting the ref split temperature so to have an average temperature of 635 °C in the ESR reactor.     

Thermodynamic analysis and performance optimization of organic rankine cycles for the conversion of low‐to‐moderate grade geothermal heat

The present study considers a thermodynamic analysis and performance optimization of geothermal power cycles. The proposed binary‐cycles operate with moderately low temperature and liquid‐dominated geothermal resources in the range of 110°C to 160°C, and cooling air at ambient conditions of 25°C and 101.3 kPa reference temperature and atmospheric pressure, respectively. A thermodynamic optimization process and an irreversibility analysis were performed to maximize the power output while minimizing the overall exergy destruction and improving the First‐law and Second‐law efficiencies of the cycle. Maximum net power output was observed to increase exponentially with the geothermal resource temperature to yield 16–49 kW per unit mass flow rate of the geothermal fluid for the non‐regenerative organic Rankine cycles (ORCs), as compared with 8–34 kW for the regenerative cycles. The cycle First‐law efficiency was determined in the range of 8–15% for the investigated geothermal binary power cycles. Maximum Second‐law efficiency of approximately 56% was achieved by the ORC with an internal heat exchanger. In addition, a performance analysis of selected pure organic fluids such as R123, R152a, isobutane and n‐pentane, with boiling points in the range of ?24°C to 36°C, was conducted under saturation temperature and subcritical pressure operating conditions of the turbine. Organic fluids with higher boiling point temperature, such as n‐pentane, were recommended for non‐regenerative cycles. The regenerative ORCs, however, require organic fluids with lower vapour specific heat capacity (i.e. isobutane) for an optimal operation of the binary‐cycle. Copyright © 2015 John Wiley & Sons, Ltd.     

Solar assisted multi-generation system using nanofluids: A comparative analysis

In this comparative study, a parabolic trough solar collector and a parabolic dish solar collector integrated separately with a Rankine cycle and an electrolyzer are analyzed for power as well as hydrogen production. The absorption fluids used in the solar collectors are Al₂O₃ and Fe₂O₃ based nanofluids and molten salts of LiCl–RbCl and NaNO₃–KNO₃. The ambient temperature, inlet temperature, solar irradiance and percentage of nanoparticles are varied to investigate their effects on heat rate and net power produced, the outlet temperature of the solar receiver, overall energy and exergy efficiencies and the rate of hydrogen produced. The results obtained show that the net power produced by the parabolic dish assisted thermal power plant is higher (2.48 kW–8.17 kW) in comparison to parabolic trough (1 kW–6.23 kW). It is observed that both aluminum oxide (Al₂O₃) and ferric oxide (Fe₂O₃) based nanofluids have better overall performance and generate higher net power as compared to the molten salts. An increase in inlet temperature is observed to decrease the hydrogen production rate. The rate of hydrogen production is found to be higher using nanofluids as solar absorbers. The hydrogen production rate for parabolic dish thermal power plant and parabolic trough thermal power plant varies from 0.0098 g/s to 0.0322 g/s and from 0.00395 g/s to 0.02454 g/s, respectively.     

Small-capacity valve-regulated lead/acid battery with long life at high ambient temperature

Valve-regulated lead/acid (VRLA) batteries are widely used as back-up power sources for telecommunications and UPS. These applications require high-reliability under severe environmental conditions. To meet this demand, the authors' company have developed small capacity (12 V, 15–65 A h at C₂₀/20 rate), long-life VRLA batteries which can endure high ambient temperature. These batteries make use of a new alloy and grid design which has improved resistance to corrosion at the positive plate, while at the same time reduce float current at high temperature. As a result, these batteries have a life expectancy of 13 years at 25°C, and inhibited thermal runaway even under ambient temperatures up to 75°C. The batteries can be installed in outdoor and underground environments.     

A prototype truck powered by hydrogen from organic liquid hydrides

A prototype truck (17 tons, 6 cylinders, 150 kW(mech), ‘SAURER’) has been constructed to use a hydrogen-burning engine with H₂ injection under 10 bar pressure. The efficiency of the engine is ca 32%, the exhaust gas temperature ca 700°C and the power ca 150 kW(mech). The hydrogen (10 bar) is produced continuously by means of catalytic splitting of methylcyclohexane on board the truck.The reaction occurs under the following conditions: 10 bar pressure, 400°C, catalyst 0.25% Pt, 0.25% Re on alumina with an efficiency of ca 0.80 and a lifetime of several hundred hours, without hydrogen recycling. The approximate economics of the system, assuming ca 2 US¢ kWh^?1 (electric), results in ca 35 US¢ per litre of gasoline equivalent.     

Modeling of a thermal energy storage system based on coupled metal hydrides (magnesium iron – sodium alanate) for concentrating solar power plants

Concentrating solar power plants represent low cost and efficient solutions for renewable electricity production only if adequate thermal energy storage systems are included. Metal hydride thermal energy storage systems have demonstrated the potential to achieve very high volumetric energy densities, high exergetic efficiencies, and low costs. The current work analyzes the technical feasibility and the performance of a storage system based on the high temperature Mg₂FeH₆ hydride coupled with the low temperature Na₃AlH₆ hydride. To accomplish this, a detailed transport model has been set up and the coupled metal hydride system has been simulated based on a laboratory scale experimental configuration. Proper kinetics expressions have been developed and included in the model to replicate the absorption and desorption process in the high temperature and low temperature hydride materials. The system showed adequate hydrogen transfer between the two metal hydrides, with almost complete charging and discharging, during both thermal energy storage and thermal energy release. The system operating temperatures varied from 450 °C to 500 °C, with hydrogen pressures between 30 bar and 70 bar. This makes the thermal energy storage system a suitable candidate for pairing with a solar driven steam power plant. The model results, obtained for the selected experimental configuration, showed an actual thermal energy storage system volumetric energy density of about 132 kWh/m³, which is more than 5 times the U.S. Department of Energy SunShot target (25 kWh/m³).     

Performance analysis of two combined cycle power plants with different steam injection system design

A thermal analysis of two combined cycle power plants is performed. The steam injection system in the combustion chamber constitutes the main difference between the two designs. For the first power plant (design 1) the injected steam is generated in the HRSG. While for second power plant (design 2) this steam is provided using a heat recovery system installed at the compressor outlet. The steam turbine cycle engenders two pressure extraction levels connected to open feed-water heaters. The steam injection in the combustion chamber improves the overall combined cycle efficiency if this steam is generated outside the HRSG.The increase of the ambient temperature affects the overall cycle efficiency.The optimum thermal efficiency, for any temperature value during the year, may be obtained for suitable margin of steam injection ratio. The second design presents better overall efficiency then the first one. In winter season (T_am = 15 °C), the overall cycle efficiency is about 54.45% for a range of steam injection ratio within 11.8 and 14%. While in summer season (T_am = 35 °C) and for the same cycle efficiency, the required range of steam injection ratio is between 18.5 and 18.8%.     

Thermodynamic study on the integrated supercritical water gasification with reforming process for hydrogen production: Effects of operating parameters

In this paper, a conceptual process design of the integrated supercritical water gasification (SCWG) and reforming process for enhancing H₂ production has been developed. The influence of several operating parameters including SCWG temperature, SCWG pressure, reforming temperature, reforming pressure and feed concentration on the syngas composition and process efficiency was investigated. In addition, the thermodynamic equilibrium calculations have been carried out based on Gibbs free energy minimization by using Aspen Plus. The results showed that the higher H₂ production could be obtained at higher SCWG temperature, the H₂ concentration increased from 5.40% at 400 °C to 38.95% at 600 °C. The lower feed concentration was found to be favorable for achieving hydrogen-rich gas. However, pressure of SCWG had insignificant effect on the syngas composition. The addition of reformer to the SCWG system enhanced H₂ yield by converting high methane content in the syngas into H₂. The modified SCWG enhanced the productivity of syngas to 151.12 kg/100kg_feed compared to 120.61 kg/100kg_feed of the conventional SCWG system. Furthermore, H₂ yield and system efficiency increased significantly from 1.81 kg/100kg_feed and 9.18% to 8.91 kg/100kg_feed, and 45.09%, respectively, after the modification.     

New text comparison between CO2 and other supercritical working fluids (ethane,Xe, CH4 and N2) in line- focusing solar power plants coupled to supercritical Brayton power cycles

This study is focused on comparing four supercritical fluids: Ethane, Xenon, Methane and Nitrogen, as possible alternative to supercritical Carbon Dioxide (s-CO₂) in Brayton power cycles coupled to line- focusing solar power plants with Solar Salt (60% NaNO₃; 40% KNO₃) as heat transfer fluid. The Simple Brayton cycle with heat recuperation and reheating is the configuration selected in this paper, providing a balance of plant design with reduced number of equipment and cost. The gross plant efficiency is calculated fixing the recuperator conductance (UA) for different Turbine Inlet Temperatures (TIT), confirming the maximum plant gross efficiency is related with the minimum allowable recuperator pinch point temperature. The reheating pressure and compressor inlet temperature are optimized with the mathematical algorithms SUBPLEX, UOBYQA and NEWOUA. According to the REFPROP database ranges of applicability, the maximum TIT limits are established for the supercritical fluids (N₂ TIT = 550 °C, CO₂ TIT = 550 °C, C₂H₆ TIT = 400 °C, Xe TIT = 450 °C and CH₄ TIT = 350 °C). The reference scenario considered for calculating the thermosolar plant energy balances and simulations is the wet-cooling system with a Compressor Inlet Temperature (CIT = 32 °C). The gross efficiency results with the wet-cooling system are: N₂ (45.8%), CO₂ (44.37%), C₂H₆ (40.74%), Xe (39.88%), CH₄ (32.15%). The plant efficiency is also translated into solar field effective aperture area and estimated cost, for a fixed power output. For optimizing the solar collector aperture area and cost, the Primary Heat Exchanger (PHX) and the ReHeating Heat Exchanger (RHX) capacity ratio (C_R) are fixed (C_R = 1). The dry-cooling system scenario (CIT = 47 °C) is alto estimated: N₂ (43.34%), CO₂ (42.42%), C₂H₆ (37.34%), Xe (37.26%), CH₄ (29.53%).For predicting the recuperator heat exchanger dimensions for a fixed conductance (UA), the heat transfer coefficient (HTC) is calculated with the Dittus–Boelter correlation and compared with the CO₂ as reference. The C₂H₆, and CH₄ have relative higher HTC in relation with CO₂. Also is calculated the recuperator pressure drop. The C₂H₆, CH₄ and N₂ pressure drop is lower in comparison with the CO₂ for the same operating conditions.The energy efficiency in solar power station coupled to Brayton cycle is very constrained by the ambient temperature variation, impacting directly in the dry-cooling system performance. For this reason a Compressor Inlet Temperature (CIT) sensing analysis is carried out ranging from 32 °C to 57 °C, and also varying TIT from 400 °C to 550 °C. A sensing analysis is also developed varying the Turbine Inlet Pressure (TIP) from 200 bar to 375 bar. The CO₂ improves the plant efficiency when increasing the TIP from 250 bar to 350 bar, however the rest of fluids (Ethane, Methane, Nitrogen and Xenon) nearly not suffered any impact in the plant efficiency when increasing the TIP.     

Electrochemical investigations of cobalt-free perovskite cathode material for intermediate temperature solid oxide fuel cell

A novel cobalt-free perovskite zinc-doped lanthanum strontium iron oxide (La_0.8Sr_0.2Zn_xFe_1?xO_3?δ, LSZF, x = 0.1–0.3) is synthesized and evaluated as cathode material for intermediate temperature solid oxide fuel cell (IT-SOFC) with samarium doped ceria (SDC) electrolyte. LSZF cathode at x = 0.2 composition demonstrates the remarkable electrochemical activity at intermediate temperature (550 °C): such as, high electrical conductivity (13.63 S cm^?1), excellent thermal stability with SDC electrolyte (12.10 μK^?1), high surface area (4.52 m² g^?1), extremely reduced area specific resistance (0.69 Ω cm^?2) and low activation energy (0.117 eV). Furthermore, single fuel cells are fabricated using LSZF as a cathode, which exhibits the excellent performance by achieving the high power density of 409 mW cm^?2 under natural gas as a fuel and ambient air as an oxidant at 550 °C with good stability over 10 h. All experimental results indicate that the LSZF is a promising cathode material for natural gas based intermediate temperature fuel cell applications.     

Effect of YSZ addition on the performance of glass-ceramic seals for intermediate temperature solid oxide fuel cell application

The suitability of glass-based seal is evaluated for application in intermediate temperature solid oxide fuel cell (SOFC). Several glass-YSZ composite seals are investigated in temperature range from 650 °C to 800 °C. The leakage rates are obviously reduced with temperature increased. The seal containing 20 wt% YSZ exhibits excellent gas tightness and thermal cycle stability, obtaining the leakage rate of 0.005 sccm cm^?1 under input gas pressure of 6.8 kPa at 750 °C. Stable leakage rates can be maintained after ten thermal cycles, implying that YSZ addition suppresses crack propagation of the seals. It is also explained by both interfacial bondage and chemical compatibility examination. When large-area-cell is operated during five thermal cycling tests, its performance is found to be constant at 750 °C, all of cell tests achieving OCV of 1.2V and power density of 520 mW/cm². The above results demonstrate the possibility of using the H1-20 seals for SOFC application.     

Design,development, construction and operation of a novel metal hydride compressor

Metal Hydride Compressors (MHC) is a promising technology for thermal compression of hydrogen. Besides the absence of a necessity for significant mechanical or electrical energy input, this type of compressor has the advantage that no moving parts are involved. A brief review on the reported experimental set ups of metal hydride compressors is carried out and compared to the metal hydride compressor developed and constructed by HYSTORE Technologies Ltd in Cyprus. The compressor built by HYSTORE consists of 6 stages using AB₂ and AB₅ – type metal hydride alloys. The MHC is operated between 10 C and 80 °C, which is a temperature range that can be supplied by solar thermal collectors. Furthermore, the experimental results showed, that even lower temperatures of 17 C are sufficient thus reducing the demand for cooling capacity. During the operation, the compressor achieved stable compression of hydrogen from 7 bar more than 220 bar. The specific productivity of the compressor achieved values up to 67.2 l_H2 kg^?1 h^?1.     

Modeling of a direct solar receiver reactor for decomposition of sulfuric acid in thermochemical hydrogen production cycles

Hydrogen production thermochemical cycles, based on the recirculation of sulfur-based compounds, are among the best suited processes to produce hydrogen using concentrated solar power. The sulfuric acid decomposition section is common to each sulfur-based cycle and represents one of the fundamental steps. A novel direct solar receiver-reactor concept is conceived, conceptually designed and simulated. A detailed transport phenomena model, including mass, energy and momentum balance expressions as well as suitable decomposition kinetics, is described adopting a finite volume approach. A single unit reactor is simulated with an inlet flow rate of 0.28 kg/s (corresponding to a production of approximately 11 kg_H2/h in a Hybrid Sulfur process) and a direct solar irradiation at a constant power of 143 kW/m². Results, obtained for the high temperature catalytic decomposition of SO₃ into SO₂ and O₂, demonstrate the effectiveness of the proposed concept, operating at pressures of 14 bar. A maximum temperature of 879 °C is achieved in the reactor body, with a corresponding average SO₂ mass fraction of 27.8%. The overall pressure drop value is 1.7 bar. The reactor allows the SO₃ decomposition into SO₂ and O₂ to be realized effectively, requiring an external high temperature solar power input of 123.6 kJ/molSO2 (i.e. 123.6 kJ/mol_H2).     

Characterization of metal hydrides for thermal applications in vehicles below 0 °C

Metal hydrides promise great potential for thermal applications in vehicles due to their fast reaction rates even at low temperature. However, almost no detailed data is known in literature about thermochemical equilibria and reaction rates of metal hydrides below 0 °C, which, though, is crucial for the low working temperature levels in vehicle applications.Therefore, this work presents a precise experimental set-up to measure characteristics of metal hydrides in the temperature range of −30 to 200 °C and a pressure range of 0.1 mbar–100 bar. LaNi_4.85Al_0.15 and Hydralloy C5 were characterized. The first pressure concentration-isotherms for both materials below 0 °C are published. LaNi_4.85Al_0.15 shows an equilibrium pressure down to 55 mbar for desorption and 120 mbar for absorption at mid-plateau and −20 °C. C5 reacts between 580 mbar for desorption and 1.6 bar for absorption at −30 °C at mid-plateau.For LaNi_4.85Al_0.15, additionally reaction rate coefficients down to −20 °C were measured and compared to values of LaNi₅ for the effect of Al-substitution. The reaction rate coefficient of LaNi_4.85Al_0.15 at −20 °C is 0.0018 s⁻¹. The obtained data is discussed against the background of preheating applications in fuel cell and conventional vehicles.     

Parametric investigations on LCC1 based hydrogen storage system intended for fuel cell applications

Considering the necessity for compact hydrogen storage system for fuel cell stacks, 41 embedded cooling tube (ECT) reactor with an outer cooling jacket (OCJ) is designed, fabricated and tested with 3.75 kg of LCC1® alloy. To analyse the sensitivity of the system performance at various operating conditions and the applicability of this prototype as storage system, an extensive parametric investigation was carried out at varying supply pressure (P_s), absorption temperature (T_a) and desorption temperature (T_d). LCC1® alloy achieved maximum hydrogen storage capacity (HSC) of 1.6 wt% within 420 s at P_s of 25 bar, T_a of 25 °C and heat transfer fluid (HTF) flow rate (HTF_a) of 6 LPM. Supply pressure is found to have greater influence than absorption temperature over absorption performance and heating output. With T_a of 25 °C, HTF_a of 6 LPM, HSC of 1.58 wt% and 1.6 wt% were achieved at P_s of 20 bar and 30 bar, respectively, resulting in corresponding specific heating power (SHP) of 497.7 W/kg and 544.9 W/kg. Varying T_a from 25 °C to 35 °C at P_s of 20 bar and HTF_a of 6 LPM resulted in 3% reduction in HSC. During desorption, desorption temperature of above 20 °C is found to be favourable with more than 95% of stored hydrogen being desorbed. It is further observed that the dehydriding rate of LCC1® was nearly steady which is potentially suitable for fuel cell applications, as the average dehydriding rate is estimated to be about 15.75 NLPM and 22.91 NLPM at T_d of 20 °C and 25 °C, respectively, with 6 LPM of HTF flow rate. The analysed module is proposed as a potential hydrogen supply unit for a 1 kW fuel cell reported in literature.     

Thermodynamic evaluations of solar cooling cogeneration cycle using NaSCN–NH<Subscript>3</Subscript> mixture

The commercial refrigeration and air conditioning consumes more electric power for its operation. The solar vapor absorption refrigeration helps to minimize the electric power usage and it is renewable. Large size of solar collector area is required for producing the standalone power as well as cooling cycle. The integration of power and cooling cycle minimizes the number of components such as heat exchanger, separator and collector area. The main objective of the work is to integrate power and cooling for two outputs with single cycle using NaSCN–NH₃ as working fluid. The advantages of NaSCN–NH₃ are having high pressure and pure ammonia vapor at the exit of the generator. The integrated cycle is made by providing the turbine at the exit of the generator along with superheater. It has three pressures of generator, condensing and sink pressure, which is depending on separator and ambient temperature. At the separator temperature of 150°C with weak solution concentration of 0.30, it produces the cogeneration output of 284.80 kW with cycle and plant thermal efficiency of 0.49 and 0.20 respectively.     

Thermodynamic analysis of rhombic-driven and crank-driven beta-type Stirling engines

This work aims to compare beta-type Stirling engine performance (GPU-3 [ground power unit]) driven by rhombic and crank mechanisms. A modified non-ideal adiabatic model accounting for different frictional and thermal losses was adopted in this study. After validating the current model with engine experimental data, different scenarios of operating conditions including heater temperature, cooler temperature, charge pressure and engine speed were investigated. The results revealed that rhombic drive mechanism generates 32% more power and provides 20% more efficiency than crank mechanism at normal operating conditions. However, at low hot end temperature (300°C) and high charge pressure (50 bar) crank drive mechanism tends to slightly generate power more than rhombic drive mechanism at lower engine speeds. At low hot end temperature (300°C) and charge pressure (10 bar) both mechanisms cannot deliver any positive power. Higher power loss is recognized in crank drive mechanism at higher speeds due to increased pumping and gas spring hysteresis losses. This study highlights a wide analysis opportunity for designers and researchers of GPU-3 Stirling engine for further optimization.     

New PEEK-WC and PLA membranes for H2 separation

The potentialities of PEEK-WC (thermally treated at 120 °C) and PLA polymers have been studied in the field of membrane technology applied to H₂ separation/purification. In particular, for low/medium temperature operation (80 °C), PEEK-WC membranes (66 μm thick) showed good results in terms of H₂/CH₄ separation, showing an ideal selectivity value higher than 40. Meanwhile, we observed interesting selectivity also for H₂/N₂ and H₂/CO₂ separation, reaching values of 24 and 20, respectively. As expected, for PEEK-WC thermally treated membranes, the H₂ permeating flux increased from 25 to 80 °C and by increasing the transmembrane pressure. Furthermore, H₂ permeability at 80 °C was around 20 barrer. Concerning PLA membranes (26 μm thick), it is worth of noting that this polymer was pioneeristically used in this work as membrane application, showing great results in terms of H₂/CO₂ separation. Indeed, we overcame the Robeson's upper-bound (2008), achieving an ideal selectivity H₂/CO₂ around 25 with an H₂ permeability of 25 barrer. Further advantage due to the utilization of PLA membranes was related to the temperature operations set at ambient conditions, constituting a valuable and cost-effective solution for H₂/CO₂ separation processes via polymeric membrane technology.