Global warming,environmental and sustainability aspects of a geothermal energy based biodigester integrated SOFC system

In this study, global warming, environmental and sustainability aspects of a geothermal energy based biodigester integrated SOFC system are parametrically analyzed. In this regard, a system is designed, consisting of three main subsystems such as Solid Oxide Fuel Cell, Anaerobic Digester, and a Heat Recovery Steam Generator. In order to investigate the global warming, environmental and sustainability aspects of the system, the energy and exergy analyses are performed, and the following indicators are taken into consideration, which are i) unit CO₂ emission, ii) environmental effect factor, iii) waste exergy ratio, iv) exergy destruction ratio, v) exergy recovery ratio, vi) exergetic sustainability index. Accordingly, the maximum exergetic sustainability index and exergy efficiency of the integrated system are calculated to be 0.486 and 0.367, respectively, in case the SOFC inlet temperature is equal to 633.3 °C while electric current density is 5500 A/m². On the other hand, the minimum exergy destruction ratio and the minimum environmental effect factor are obtained to be 0.74 and 2.33 while SOFC inlet temperature is 633.3 °C and SOFC current density is 8000 A/m². The minimum unit CO₂ emission of the whole system is determined to be 368.4 kg/MWh at 5500 A/m² of SOFC current density and 727 °C of SOFC inlet temperature while determined as 258.3 kg/MWh at 8000 A/m² of SOFC current density and 680 °C of SOFC inlet temperature. Thus, it can be said that such a system may be applied for reducing the CO₂ based global warming effects and improving the environmental sustainability.     

Thermo-electrochemical modeling of ammonia-fueled solid oxide fuel cells considering ammonia thermal decomposition in the anode

Ammonia (NH₃) is a promising hydrogen carrier and a possible fuel for use in Solid Oxide Fuel Cells (SOFCs). In this study, a 2D thermo-electrochemical model is developed to investigate the heat/mass transfer, chemical (ammonia thermal decomposition) and electrochemical reactions in a planar SOFC running on ammonia. The model integrates three sub-models: (1) an electrochemical model relating the current density-voltage characteristics; (2) a chemical model calculating the rate of ammonia thermal decomposition reaction; (3) a 2D computational fluid dynamics (CFD) model that simulates the heat and mass transfer phenomena. Simulations are conducted to study the complicated physical-chemical processes in NH₃-fueled SOFCs. It is found that increasing the inlet temperature of NH₃-fueled SOFC is favorable for a higher electric output, but the temperature gradient in the SOFC is considerably higher, particularly near the inlet of the SOFC. The effects of operating potential and inlet gas velocity on NH₃-fueled SOFC performance are investigated. It is found that an increase in inlet gas velocity from 1 m s⁻¹ to 10 m s⁻¹ slightly decreases the SOFC performance and does not affect the temperature field significantly. For comparison, decreasing the gas velocity to 0.2 m s⁻¹ is more effective to reduce the temperature gradient in SOFC.     

Effects of dual-inlet on premixed hydrogen/air combustion characteristics and thermal performance for micro thermophotovoltaic application

Premixed H₂/air combustion in the micro-combustor with dual-inlet is investigated. Effects of the second-inlet settings on the combustion characteristics, thermal performance and radiation efficiency are discussed and compared. The results indicate that the second-inlet can efficiently modify burning characteristics and heat transmission, improving the burner thermal performance. With the increase of the distance between second-inlet and inlet, the mean outer wall temperature T_m is first risen and then down with a fixed reactants ratio of second-inlet Q_in. It also affects the flow field and radiation temperature, and combustor C3 achieves the highest T_m 1180.9 K and radiation efficiency 28.231% at the total flow rate m_f = 3.376 × 10^?5 kg/s and Q_in = 10%. For an augment of m_f, the radiation temperature is improved while radiation efficiency is declined, and the combustor C2 obtains the highest radiation power 42.3 W at m_f = 5.626 × 10^?5 kg/s and Q_in = 20%.     

Experimental performance investigation of modified cavity receiver with fuzzy focal solar dish concentrator

In this paper, thermal performance analysis of 20 m² prototype fuzzy focal solar dish collector is presented. The focal image characteristics of the solar dish are determined to propose the suitable design of absorber/receiver. First, theoretical thermal performance analysis of the fuzzy focal solar parabolic dish concentrator with modified cavity receiver is carried out for different operating conditions. Based on the theoretical performance analysis, the total heat loss (conduction, convection and radiation heat losses) from the modified cavity receiver is estimated. It is observed that the maximum theoretical efficiencies of solar dish collector are found to be as 79.2% for no wind conditions and 78.2% and 77.8% for side-on and head-on winds speed of 5 m/s respectively. Latter, real time analysis of parabolic dish collector with modified cavity receiver is carried out in terms of stagnation test, time constant test and daily performance test. From stagnation test, the overall heat loss coefficient is found to be 356 W/m² K. The time constant test is carried out to determine the influence of sudden change in solar radiation at steady state conditions. The daily performance tests are conducted for different flow rates. It is found that the efficiency of the collector increases with the increase of volume flow rates. The average thermal efficiencies of the parabolic dish collector for the volume flow rate of 100 L/h and 250 L/h are found to be 69% and 74% for the average beam radiation (I_bn) of 532 W/m² and 641 W/m² respectively.     

Grape drying by using hybrid photovoltaic-thermal (PV/T) greenhouse dryer: An experimental study

A hybrid photovoltaic-thermal (PV/T) greenhouse dryer of 100 kg capacity has been designed and constructed at Solar Energy Park, Indian Institute of Technology, New Delhi (28°35′N, 77°12′E, 216 m above MSL), India. The developed dryer has been used to dry the Thompson seedless grapes (Mutant: Sonaka) when DC fan was in operation for forced mode convection. The drying of grapes was also performed in open as well as shade for comparison. Experiments were conducted for drying of grapes in the month of April, 2007. Various hourly experimental data namely moisture evaporated, grape surface temperatures, ambient air temperature and humidity, greenhouse air temperature and humidity, etc. were recorded to evaluate heat and mass transfer for the proposed system. It has been found that the value of the convective heat transfer coefficient for grapes (GR-I) lies between 0.26 and 0.31 W/m² K for greenhouse and 0.34–0.40 W/m² K for open conditions, respectively and that for grapes (GR-II) lies between 0.45–1.21 W/m² K for greenhouse and 0.46–0.97 W/m² K for open conditions, respectively.     

Heat loss of a trapezoidal cavity absorber for a linear Fresnel reflecting solar concentrator

The present paper studies the heat loss of a linear absorber with a trapezoidal cavity and a set of pipes used for a linear Fresnel reflecting solar concentrator. The study includes the measurements on a 1.4 m long prototype installed in a laboratory, and its thermal simulation in steady-state using EnergyPlus software. Results of the measured vertical temperature variation inside the cavity, the surface interior and exterior wall and window temperatures, the global heat loss at steady-state and the heat loss coefficients, are presented for six different temperatures of the pipes. Measurements revealed a stable thermal gradient in the upper portion of the cavity and a convective zone below it. Around 91% of the heat transferred to outdoors occurs at the bottom transparent window, for a pipe temperature of 200 °C. The heat loss coefficient per area of absorbing pipes ranged from 3.39 W/m²K to 6.35 W/m²K (for 110 °C < T_pipe < 285 °C), and it increased with the increase of T_pipe. Simpler and less time-consuming available free software originally designed for heat transfer in buildings was tested to be a possible replacement of the highly complex CFD software commonly used to simulate the steady-state heat loss of the absorber. The experimental and predicted data sets were found to be in good agreement.     

In situ sinterable cathode with nanocrystalline La0.6Sr0.4Co0.2Fe0.8O3−δ for solid oxide fuel cells

A nanocrystalline powder with a lanthanum based iron- and cobalt-containing perovskite, La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF), is investigated for solid oxide fuel cell (SOFC) applications at a relatively low operating temperature (600-800 °C). A LSCF powder with a high surface area of 88 m² g⁻¹, which is synthesized via a complex method with using inorganic nano dispersants, is printed onto an anode supported cell as a cathode electrode. A LSCF cathode without a sintering process (in situ sintered cathode) is characterized and compared with that of a sintering process at 780 °C (ex situ sintered cathode). The in situ sintered SOFC shows 0.51 A cm⁻² at 0.9 V and 730 °C, which is comparable with that of the ex situ sintered SOFC. The conventional process for SOFCs, the ex situ sintered SOFC, including a heat treatment process after printing the cathodes, is time consuming and costly. The in situ sinterable nanocrystalline LSCF cathode may be effective for making the process simple and cost effective.     

Permeability and thermal conductivity of compact chemical and physical adsorbents with expanded natural graphite as host matrix

Permeability and thermal conductivity test units were designed to study the heat and mass transfer performance of compact chemical and physical adsorbents, i.e. compact CaCl₂ and activated carbon (AC) using expanded natural graphite (ENG) as host matrix. The thermal conductivity was studied by steady-state heat source method and the permeability was tested with nitrogen as a gas source. For the compact CaCl₂ adsorbents, results show that permeability and thermal conductivity vary with the ratio of ENG and the density of compact adsorbents. The value of permeability is 10^-13 ～ 10^-11 m² when the density of compact sample change from 400 kg/m³ to 550 kg/m³, and it keeps increasing linearly with the less ratio of ENG. The value of thermal conductivity is 1.08 W/(m·K), which is increased by 5 times compared with granular CaCl₂ when the density is 550 kg/m³ and the ratio of ENG is the minimum value of 16.7%. The compact physical adsorbent of AC with ENG as matrix has the highest thermal conductivity of 2.61 W/(m·K) when the ratio between ENG–AC is 1.5:1. Similarly, thermal conductivity drops down with the ratio of ENG decreasing. When the ratio of ENG reaches the minimum ratio of 28.6%, the thermal conductivity is 2.08 W/(m·K), which is increased by 5.8 times if compared with the result of granular AC, and corresponding permeability is 5.16 × 10^-11 m². The thermal conductivity and permeability of compact physical adsorbent of AC are all better than the values for the compact chemical adsorbent of CaCl₂.     

Parametric study on the thermoelectric conversion performance of a concentrated solar‐driven thermionic‐thermoelectric hybrid generator

A concentrated solar‐driven thermionic‐thermoelectric hybrid generator composed of solar heat collector, thermionic generator (TIG), thermoelectric generator (TEG), and radiator is introduced in this paper. A theoretical model of thermoelectric conversion performance for the hybrid generator is built up based on the heat source of the concentrated solar radiation rather than isothermal heat source. Based on the model, the impacts of related parameters on the internal temperature distributions, output power, and efficiency have been discussed. Moreover, the optimal operating conditions of the TIG‐TEG hybrid device at its maximum output power and efficiency have been determined. Results show that when cascading the TEG with the TIG, there is very little change of the TIG cathode temperature in most conditions, namely, T_C ≈ T_C′. Meanwhile, the anode temperature becomes higher, and the TEG cold end temperature T₂ is close to the anode temperature T_A′ for the single TIG system, ie, T_A > T_A′ ≈ T₂. In theory, the optimal concentrated solar radiation I₀ for the maximum output power P_max and the maximum efficiency η_max differs, which are I_0,P = 2.5 × 10⁶ W/m² and I_0,η = 2 × 10⁶ W/m², respectively, whereas the output power and efficiency of the TIG‐TEG hybrid system simultaneously reach their maximum values when the optimal TIG anode temperature T_A,opt = 1025 K, the optimal TIG output voltage V_opt = 2 V, and the optimal ratio of load resistance to internal resistance (R₂/R)_opt = 2. However, in practice, the parameter values of I₀, Φ_A, and T_A should be strictly controlled under 1.8 × 10⁶ W/m², 1.4 eV, and 660 K, respectively. Generally, the maximum output power and efficiency of the hybrid TIG‐TEG system are, respectively, 35% and 4% higher than that of the single TIG.     

Assessing the influence of four bonding methods on the thermal contact resistance of open-cell aluminum foam

A thermal application of open-cell aluminum foam typically requires it to be bonded on a substrate. The resulting thermal contact resistance is investigated for four bonding methods. This is done by minimizing the difference between the calculated heat transfer via a zeroth order model and experimental data. The bonded metal foam, used to obtain the experimental data, are manufactured in-house. This allows varying pore size, porosity, aluminum alloy, foam height, air mass flow rate, air inlet temperature and bonding method. The latter is found to have an overwhelming impact. The resulting four thermal contact resistances are: 0.7 × 10^?3 m²K/W for brazing, 0.88 × 10^?3 m²K/W for co-casting, 1.25 × 10^?3 m²K/W for a single-epoxy bonding and 1.88 × 10^?3 m²K/W for a press-fit bonding. The uncertainty on these values is 11%.     

Novel hybrid two-stage alkali metal thermoelectric converter and thermally regenerative electrochemical cycle to exploit waste heat of solid oxide fuel cell: Performance assessment and optimization

An innovative combination of a two-stage alkali metal thermoelectric converter (TAMTEC), and thermally regenerative electrical cycle (TREC) is employed to utilize the high-quality heat dissipated from solid oxide fuel cell (SOFC) for further electricity production. The superiority and effectiveness of the SOFC-TAMTEC-TREC system are verified compared to existing SOFC-based hybrid systems and sole SOFC. The performance of the system based on energy, exergy, and economic indicators is evaluated by varying the main design parameters. Parametric assessment demonstrates that the SOFC-TAMTEC-TREC system can reach the maximum power density of 12126 W m^?2 with energy and exergy efficiencies of 47.13% and 50.46% as TAMTEC proportional constant increases to 10^7.5 m² and rising SOFC pore and gain diameters to 3.77 × 10^?6 m and 2.5 × 10^?6 m, respectively reduce the cost rate density of system by 3.55 $ h^?1 m^?2. Furthermore, to achieve the maximum power density and exergy efficiency, and minimum cost rate density, NSGA-III multi-criteria optimization, and decision-making techniques are conducted. Outcomes indicate that Shannon entropy leads to the maximum power density of 8597.2 W m^?2 with a 35.94% enhancement relative to a single SOFC and 1 $ h^?1 m^?2 increment in cost rate density of the hybrid system, while LINMAP and TOPSIS ascertain the minimum increase in the cost rate density by 0.6 $ h^?1 m^?2 with 31.04% improvement in power density relative to single SOFC.     

Drying saturated zeolite pellets to assess fluidised bed performance

This work studies the drying rates of zeolite pellets (Type 1 [D.M. Ruthven, Principles of Adsorption and Adsorption Processes, John Wiley & Sons, New York, 1984]) in spouted beds and in conventional fan assisted ovens. Three temperatures were studied: 48, 85 and 133 °C for drying times up to 1 h. Heat transfer is deduced from measured mass transfer rates and literature correlations. Maximum heat transfer coefficients were found to be 115.6 W/m² K for the fluidised bed and 31.8 W/m² K for the oven using correlations from literature. Higher figures were found by experimentation and calculation, typically about 200 W/m² K. A finite element model was used to estimate the temperature profile within pellets, the predictions agree well with experimental temperature measurements.     

Analysis of adsorption equilibrium of hydrogen on graphene sheets

The adsorption equilibrium of hydrogen on graphene sheets (GS) was studied based on a sample of GS with S_BET = 300 m²/g at the temperatures of 77.15 K–293.15 K and the pressures of 0 MPa–6 MPa. In the meantime, the adsorptions (Excess adsorption measurements) of hydrogen on granular coconut shell SAC-02 activated carbon (S_BET = 2074 m²/g) and carbon nanofiber (CNFs, S_BET = 205 m²/g) were investigated at the pressures of 0–8 MPa and the temperature of 77.15 K. The outcomes from experiments were used to determine the parameters in Toth equation by way of Non-linear fit. The absolute adsorption amounts of hydrogen on the GS, which were calculated from the equation, were used to calculate the isosteric heat of hydrogen adsorption by use of adsorption isosteres.     

Evaluation of heating performances and associated variability of domestic cooking appliances (oven-baking and pan-frying)

By comparison with those in industry, domestic cooking processes are rarely studied with respect to heat transfer although they can drastically modify the quality of heated products. In order to compensate for this lack of information in the literature, the aim of this work was to propose test procedures to evaluate the variability of domestic appliance heating performances in the case of oven-baking and pan-frying. The measurements included the continuous recording of pan temperature during pan-frying using different types of hobs (electric, halogen, gas, induction) and pans; the continuous recording of air temperature and measurement of the convective heat transfer coefficient and equivalent radiative temperature during oven-baking using different types of ovens. The results revealed broad variations in heating conditions depending on the type of appliance used and on consumer behaviour. For pan-frying, it was shown that pan temperature varied constantly during heating. From an initial value of 200 °C and a given product load, it could fall to 150 °C or rise to 330 °C at medium or high heat, respectively. For oven-baking, heating was sometimes performed at an actual air temperature that differed considerably from the air temperature set on the oven. These measurements also showed relatively low convective heat transfer coefficient values, ranging from 6 W/m² K under free convection to 16 W/m² K under forced convection.     

NUMERICAL MODEL AND PARAMETRIC ANALYSES OF AN INFLATABLE SOLAR HEATER

This paper presents a new solar heater model which is shaped as an inflatable hemisphere. It is called SOLARBALL©. This solar heater is above all distinguished by its small weight and volume. The final form is blown up by the user. The objective of this work was to develop a mathematical and a numerical model to analyse solar radiation and heat transfer in such a heater. The numerical model was verified by a series of experiments. After successful verification, the numerical model was used for the parametric analyses to establish the time required to heat water for different meteorological data, size of reflector and optical properties of the transparent cover, reflector and absorber surface. It was found out that typical optical efficiency and overall heat transfer coefficient of the hemispherical solar heater are between 0.45 to 0.5 and 0.6 to 1.6 W/m² K respectively. By standardised conditions (G_cov=700 W/m² and temperature difference T_w−T_a equal 50°C) the required heating time for heating 0.33 l of water varies between 15 and 25 min according to the construction parameters.     

Importance of pressure gradient in solid oxide fuel cell electrodes for modeling study

The pressure gradients in the electrodes of a solid oxide fuel cell (SOFC) are frequently neglected without any justification in calculating the concentration overpotentials of the SOFC electrodes in modeling studies. In this short communication, a comparative study has been conducted to study the effect of pressure gradients on mass transfer and the resulting concentration overpotentials of an SOFC running on methane (CH₄) fuel. It is found that the pressure gradients in both anode and cathode are significant in the fuel cell electrochemical activities. Neglecting the anode pressure gradient in the calculation can lead to underestimation of the concentration overpotential by about 20% at a typical current density of 5000 A m⁻² and at a temperature of 1073 K. The deviation can be even larger at a higher temperature. At the cathode, neglecting the pressure gradient can result in overestimation of the concentration overpotential by about 10% under typical working conditions.     

Results from an industrial size biogas-fed SOFC plant (the DEMOSOFC project)

The EU-funded DEMOSOFC project aims to demonstrate the technical and economic feasibility of operating a 174 kW_e Solid Oxide Fuel Cell (SOFC) in a wastewater treatment plant. The fuel for the three SOFC modules (3 × 58 kW_e) is biogas, which is available on-site from the anaerobic digestion of sludge collected from treated wastewater. The integrated biogas-SOFC plant includes three main units: 1) the biogas cleaning and compression section, 2) the three SOFC power modules, and 3) the heat recovery loop. Main advantages of the proposed layout are the net electric efficiency of the SOFC, which is in the range 50–55%, and the near-zero emissions. A specific focus of the demonstration project is the deep and reliable removal of harmful biogas contaminants. The presented work is related to the design of the SOFC system integrated into the wastewater treatment plant, followed by the analysis of the first results from the plant operation. We analyzed the biogas yearly profile to determine the optimal SOFC capacity to install that is 3 SOFC modules. The rational is to maintain high the capacity factor while minimizing the number of shutdown per year (due to biogas unavailability). First results from plant operation are also presented. The first SOFC module was activated in October 2017 and the second in October 2018. The measured SOFC efficiency from compressed biogas to AC power has always been higher than 50–52%, with peaks of 56%. Dedicated emissions measurements have been performed onsite during December 2017. Results on real biogas operation show NO_x < 20 mg/m³, SO₂ < 8 mg/m³ (detection limits for the instrument) and PM lower than ambient air values.     

Thermal Modeling and Management of Solid Oxide Fuel Cells Operating with Internally Reformed Methane

A detailed three-dimensional mechanistic model of a large-scale solid oxide fuel cell (SOFC) unit running on partially pre-reformed methane is developed. The model considers the coupling effects of chemical and electrochemical reactions, mass transport, momentum and heat transfer in the SOFC unit. After model validation, parametric simulations are conducted to investigate how the methane pre-reforming ratio affects the transport and electrochemistry of the SOFC unit. It is found that the methane steam reforming reaction has a “smoothing effect”, which can achieve more uniform distributions of gas compositions, current density and temperature among the cell plane. In the case of 1500 W/m² power density output, adding 20% methane absorbs 50% of internal heat production inside the cell, reduces the maximum temperature difference inside the cell from 70 K to 22 K and reduces the cathode air supply by 75%, compared to the condition of completely pre-reforming of methane. Under specific operating conditions, the pre-reforming ratio of methane has an optimal range for obtaining a good temperature distribution and good cell performance.     

Energy saving analysis and thermal performance evaluation of a hydrogen-enriched natural gas-fired condensing boiler

Hydrogen-enriched natural gas (HENG) has attracted widespread attention due to its lower pollutant emissions and industrial decarbonization in the past decades. HENG combustion boosts the water content in the flue gas, which is highly favorable for condensing boilers to recover additional latent heat. The energy saving and thermal performance of a condensing boiler burning HENG were evaluated at a constant heat load of 2.8 MW in this study. The variations in combustion products and boiler efficiency were investigated based on the material balance and energy conservation. The heat transfer calculations were employed to evaluate the thermal performance of boiler heating surfaces. The energy recovery performance of the condenser was assessed via a thermal design method. Results show that H₂ enrichment enhances the radiation intensity of the flame due to the incremental triatomic gases with higher emissivity in the furnace. The heat absorption ratio increases with H₂ enrichment in the radiative heating surface, while it shows a reverse tendency in the convective heating surface. The condensing boiler efficiency based on lower heating value increases from 101.83% to 110.60%, the total heat transfer rate of the condenser increases from 2.77 × 10⁵ W to 4.61 × 10⁵ W, and the total area required decreases from 46.45 m² to 42.16 m², as the H₂ enriches from 0 to 100% under the exhaust flue gas temperature of 318 K. Although the amount of recoverable heat in the exhaust flue gas increases considerably after H₂ blending, the original condenser with natural gas as the designed fuel could meet the requirements of the heat recovery for HENG without increasing the extra heating surface. When the H₂ fraction is enriched from 0 to 100%, CO₂ emission intensity drops from 6.05 × 10⁻⁸ kg J⁻¹ to 0. This work may offer some theoretical references for the application and generalization of HENG condensing boilers.     

Modeling and performance analysis of an absorption chiller with a microchannel membrane‐based absorber using LiBr‐H2O,LiCl‐H2O,and LiNO3‐NH3

In order to develop compact absorption refrigeration cycles driven by low heat sources, the simulated performance of a microchannel absorber of 5‐cm length and 9.5 cm³ in volume provided with a porous membrane is presented for 3 different solution‐refrigerant pairs: LiBr‐H₂O, LiCl‐H₂O, and LiNO₃‐NH₃. The high absorption rates calculated for the 3 solutions lead to large cooling effect to absorber volume ratios: 625 kW/m³ for the LiNO₃‐NH₃, 552 kW/m³ for the LiBr‐H₂O, and 318 kW/m³ for the LiCl‐H₂O solutions given the studied geometry. The performance of a complete absorption system is also analyzed varying the solution concentration, condensation temperature, and desorption temperature. The LiNO₃‐NH₃ and the LiBr‐H₂O solutions provide the largest cooling effects. The LiNO₃‐NH₃ can work at a lower temperature of the heating source, in comparison with the one needed in a LiBr‐H₂O system. The lowest cooling effect and coefficient of performance are found for the LiCl‐H₂O solution, but this mixture allows the use of lower temperature heating sources (below 70°C). These results can be used for the selection of the most suitable solution for a given cooling duty, depending on the available heat source and condensation temperature.