Modelling and performance study of a continuous adsorption refrigeration system driven by parabolic trough solar collector

This article suggests a numerical study of a continuous adsorption refrigeration system consisting of two adsorbent beds and powered by parabolic trough solar collector (PTC). Activated carbon as adsorbent and ammonia as refrigerant are selected. A predictive model accounting for heat balance in the solar collector components and instantaneous heat and mass transfer in adsorbent bed is presented. The validity of the theoretical model has been tested by comparison with experimental data of the temperature evolution within the adsorber during isosteric heating phase. A good agreement is obtained. The system performance is assessed in terms of specific cooling power (SCP), refrigeration cycle COP (COP_cycle) and solar coefficient of performance (COP_s), which were evaluated by a cycle simulation computer program. The temperature, pressure and adsorbed mass profiles in the two adsorbers have been shown. The influences of some important operating and design parameters on the system performance have been analyzed.The study has put in evidence the ability of such a system to achieve a promising performance and to overcome the intermittence of the adsorption refrigeration systems driven by solar energy. Under the climatic conditions of daily solar radiation being about 14 MJ per 0.8 m² (17.5 MJ/m²) and operating conditions of evaporating temperature, T_ev = 0 °C, condensing temperature, T_con = 30 °C and heat source temperature of 100 °C, the results indicate that the system could achieve a SCP of the order of 104 W/kg, a refrigeration cycle COP of 0.43, and it could produce a daily useful cooling of 2515 kJ per 0.8 m² of collector area, while its gross solar COP could reach 0.18.     

Exploring the effects of symmetrical and asymmetrical relative humidity on the performance of H2/air PEM fuel cell at different temperatures

This article is dedicated to study the interlinked effects of symmetric relative humidity (RH), and asymmetric RH on the performance of H₂/air PEM fuel cell at different temperatures. The symmetric and asymmetric RH were achieved by setting the cathode relative humidity (RHC) and anode relative humidity (RHA) as equal and unequal values, respectively. The cell performance was evaluated by collecting polarization curves of the cell at different RH, RHC and RHA and at different cell temperatures (T_cell). The polarization curves along with the measured internal cell resistance (membrane resistance) were discussed in the light of the present fuel cell theory. The results showed that symmetric relative humidity has different impacts depending on the cell temperature. While at RH of 35% the cell can show considerable performance at T_cell = 70 °C, it is not so at T_cell = 90 °C. At T_cell = 70 °C, the cell potential increases with RH at lower and medium current densities but decreases with RH at higher currents. This was attributed to the different controlling processes at higher and lower current densities. This trend at 70 °C is completely destroyed at 90 °C. Operating our PEM fuel cell at dry H₂ gas conditions (RHA = 0%) is not detrimental as operating the cell at dry Air (O₂) conditions (RHC = 0%). At RHA = 0% and humidified air, water transport by back diffusion from the cathode to the anode at the employed experimental conditions can support reasonable rehydration of the membrane and catalysts. At RHA = 0, a possible minimum RHC for considerable cell operation is temperature dependent. At RHC = 0 conditions, the cell can operate only at RHA = 100% with a loss that depends on T_cell. It was found that the internal cell resistance depends on RH, RHA, RHC and T_cell and it is consistent with the observed cell performance.     

Optical, electrical and structural properties of indium-doped cadmium oxide films obtained by the sol-gel technique

Indium-doped cadmium oxide films were obtained by mixing cadmium oxide and indium oxide precursor solutions by the sol-gel technique. The indium atomic concentrations in solution (x) studied were 0, 2, 5 and 10 at%. The films were sintered at two different sintering temperatures (T_s) 350 and 450 °C, and after that annealed in a 96:4 N₂/H₂ gas mixture atmosphere at 350 °C. X-ray diffraction patterns showed that all films sintered at T_s=350 °C only consisted of cadmium oxide crystals. The films sintered at T_s=450 °C consisted of cadmium oxide crystals also; however, for the highest indium atomic concentration (10 at%) the formation of cadmium indate oxide crystals was evident. All films show high optical transmission (>85%) and an increase of the direct band gap value from 2.4 to 3.1 eV, as the indium atomic concentration in solution increases. The minimum resistivity value obtained was 6.3×10⁻⁴ Ω cm for the films with x=5 at%, T_s=450 °C and annealed at 350 °C.     

A combined power and ejector refrigeration cycle for low temperature heat sources

A combined power and ejector refrigeration cycle for low temperature heat sources is under investigation in this paper. The proposed cycle combines the organic Rankine cycle and the ejector refrigeration cycle. The ejector is driven by the exhausts from the turbine to produce power and refrigeration simultaneously. A simulation was carried out to analyze the cycle performance using R245fa as the working fluid. A thermal efficiency of 34.1%, an effective efficiency of 18.7% and an exergy efficiency of 56.8% can be obtained at a generating temperature of 395 K, a condensing temperature of 298 K and an evaporating temperature of 280 K. Simulation results show that the proposed cycle has a big potential to produce refrigeration and most exergy losses take place in the ejector.     

Fiber-based flexible thermoelectric power generator

Flexible thermoelectric power generators fabricated by evaporating thin films on flexible fiber substrates are demonstrated to be feasible candidates for waste heat recovery. An open circuit voltage of 19.6 μV K per thermocouple junction is measured for Ni–Ag thin films, and a maximum power of 2 nW for 7 couples at ΔT = 6.6 K is measured. Heat transfer analysis is used to project performance for several other material systems, with a predicted power output of 1 μW per couple for Bi₂Te₃/Sb₂Te₃-based fiber coatings with a hot junction temperature of 100 °C. Considering the performance of woven thermoelectric cloths or fiber composites, relevant properties and dimensions of individual thermoelectric fibers are optimized.     

Optimization of bio-ethanol autothermal reforming and carbon monoxide removal processes

Experimental investigation of bio-ethanol autothermal reforming (ATR) and water-gas shift (WGS) processes for hydrogen production and regression analysis of the data is performed in the study. The main goal was to obtain regression relations between the most critical dependent variables such as hydrogen, carbon monoxide and methane content in the reformate gas and independent factors such as air-to-fuel ratio (λ), steam-to-carbon ratio (S/C), inlet temperature of reactants into reforming process (T_ATRin), pressure (p) and temperature (T_ATR) in the ATR reactor from the experimental data. Purpose of the regression models is to provide optimum values of the process factors that give the maximum amount of hydrogen. The experimental ATR system consisted of an evaporator, an ATR reactor and a one-stage WGS reactor. Empirical relations between hydrogen, carbon monoxide, methane content and the controlling parameters downstream of the ATR reactor are shown in the work. The optimization results show that within the considered range of the process factors the maximum hydrogen concentration of 42 dry vol. % and yield of 3.8 mol mol⁻¹ of ethanol downstream of the ATR reactor can be achieved at S/C = 2.5, λ = 0.20-0.23, p = 0.4 bar, T_ATRin = 230 °C, T_ATR = 640 °C.     

Design of a thermally integrated bioethanol-fueled solid oxide fuel cell system integrated with a distillation column

Solid oxide fuel cell systems integrated with a distillation column (SOFC-DIS) have been investigated in this study. The MER (maximum energy recovery) network for SOFC-DIS system under the base conditions (C_EtOH = 25%, EtOH recovery = 80%, V = 0.7 V, fuel utilization = 80%, T_SOFC = 1200 K) yields Q_Cmin = 73.4 and Q_Hmin = 0 kW. To enhance the performance of SOFC-DIS, utilization of internal useful heat sources from within the system (e.g. condenser duty and hot water from the bottom of the distillation column) and a cathode recirculation have been considered in this study. The utilization of condenser duty for preheating the incoming bioethanol and cathode recirculation for SOFC-DIS system were chosen and implemented to the SOFC-DIS (CondBio-CathRec). Different MER designs were investigated. The obtained MER network of CondBio-CathRec configuration shows the lower minimum cold utility (Q_Cmin) of 55.9 kW and total cost index than that of the base case. A heat exchanger loop and utility path were also investigated. It was found that eliminate the high temperature distillate heat exchanger can lower the total cost index. The recommended network is that the hot effluent gas is heat exchanged with the anode heat exchanger, the external reformer, the air heat exchanger, the distillate heat exchanger and the reboiler, respectively. The corresponding performances of this design are 40.8%, 54.3%, 0.221 W cm⁻² for overall electrical efficiency, Combine Heat and Power (CHP) efficiency and power density, respectively. The effect of operating conditions on composite curves on the design of heat exchanger network was investigated. The obtained composite curves can be divided into two groups: the threshold case and the pinch case. It was found that the pinch case which T_SOFC = 1173 K yields higher total cost index than the CondBio-CathRec at the base conditions. It was also found that the pinch case can become a threshold case by adjusting split fraction or operating at lower fuel utilization. The total cost index of the threshold cases is lower than that of the pinch case. Moreover, it was found that some conditions can give lower total cost index than that of the CondBio-CathRec at the base conditions.     

Performance of hybrid refrigeration system using ammonia

This paper investigates the performance of a hybrid refrigeration system that combines sorption–conventional vapour compression refrigeration machine driven by dual source (heat and/or electricity). The dual source makes the system highly flexible and energy efficient. The ammonia refrigerant (R717) is used in both adsorption and associated conventional refrigeration cycles. The model of thermal compressor corresponds to a multiple pair of compact adsorption generators operating out of phase with both heat and mass recovery for continuous cooling production and better efficiency. Each generator is based on a plate heat exchanger concept using the activated carbon–ammonia pair. The model of conventional vapour compressor is a reciprocating compressor from Frigopol. The hybrid refrigeration performances are presented mainly for ice making and air conditioning applications (T_C = 40 °C, −5 °C < T_E < 20 °C). The exhaust temperature of the compressor (driving temperature for thermal compressor) varies from 90 °C to 250 °C. The results show a cooling production ranging from 4 kW to 12 kW with back-up mode (both cycles not operating simultaneously) and from 8 kW to 24 kW with complementary mode (both cycles operating simultaneously). The effective overall COP based on the total equivalent heat rate input varies from 0.24 to 0.76.     

Proposal and analysis of a novel ammonia–water cycle for power and refrigeration cogeneration

Cogeneration has improved sustainability as it can improve the energy utilization efficiency significantly. In this paper, a novel ammonia-water cycle is proposed for the cogeneration of power and refrigeration. In order to meet the different concentration requirements in the cycle heat addition process and the condensation process, a splitting /absorption unit is introduced and integrated with an ammonia–water Rankine cycle and an ammonia refrigeration cycle. This system can be driven by industrial waste heat or a gas turbine flue gas. The cycle performance was evaluated by the exergy efficiency, which is 58% for the base case system (with the turbine inlet parameters of 450 °C/11.1 MPa and the refrigeration temperature below −15 °C). It is found that there are certain split fractions which maximize the exergy efficiency for given basic working fluid concentration. Compared with the conventional separate generation system of power and refrigeration, the cogeneration system has an 18.2% reduction in energy consumption.     

High performance polymer electrolytes based on main and side chain pyridine aromatic polyethers for high and medium temperature proton exchange membrane fuel cells

Novel aromatic polyether type copolymers bearing side chain polar pyridine rings as well as combination of main and side chain pyridine units have been evaluated as potential polymer electrolytes for proton exchange membrane fuel cells (PEMFCs). The advanced chemical and physicochemical properties of these new polymers with their high oxidative stability, mechanical integrity and high glass transition temperatures (T_g's up to 270 °C) and decomposition temperatures (T_d's up to 480 °C) make them promising candidates for high and medium temperature proton exchange membranes in fuel cells. These copolymers exhibit adequate proton conductivities up to 0.08 S cm⁻¹ even at moderate phosphoric acid doping levels. An optimized terpolymer chemical structure has been developed, which has been effectively tested as high temperature phosphoric acid imbibed polymer electrolyte. MEA prepared out of the novel terpolymer chemical structure is approaching state of the art fuel cell operating performance (135 mW cm⁻² with electrical efficiency 45%) at high temperatures (150-180 °C) despite the low phosphoric acid content (<200 wt%) and the low platinum loading (ca. 0.7 mg cm⁻²). Durability tests were performed affording stable performance for more than 1000 h.     

Two-phase flow in high-heat-flux micro-channel heat sink for refrigeration cooling applications: Part I--pressure drop characteristics

Two-phase pressure drop was measured across a micro-channel heat sink that served as an evaporator in a refrigeration cycle. The micro-channels were formed by machining 231 μm wide × 713 μm deep grooves into the surface of a copper block. Experiments were performed with refrigerant R134a that spanned the following conditions: inlet pressure of P_in = 1.44-6.60 bar, mass velocity of G = 127-654 kg/m² s, inlet quality of x_e,in = 0.001-0.25, outlet quality of x_e,out = 0.49-superheat, and heat flux of q″ = 31.6-93.8 W/cm². Predictions of the homogeneous equilibrium flow model and prior separated flow models and correlations yielded relatively poor predictions of pressure drop. A new correlation scheme is suggested that incorporates the effect of liquid viscosity and surface tension in the separated flow model’s two-phase pressure drop multiplier. This scheme shows excellent agreement with the R134a data as well as previous micro-channel water data. An important practical finding from this study is that the throttling valve in a refrigeration cycle offers significant stiffening to the system, suppressing the large pressure oscillations common to micro-channel heat sinks.     

Combustion behavior in a dual-staging vortex rice husk combustor with snail entry

The combustion characteristics of rice husk fuel in a dual-staging vortex-combustor (DSVC) are experimentally investigated. In the present work, the vortex flow is created by using a snail entrance mounted at the bottom of the combustor. The temperature distributions at selected locations inside the combustor, the flue gas emissions (CO, CO₂, O₂, NO_x), and the combustion/thermal efficiency are monitored. Measurements are made at a constant rice husk feed rate of 0.25 kg/min with various excess airs (37%, 56%, 74% and 92%) and different secondary air injection fractions (λ = 0.0, 0.15 and 0.2), respectively. The combustion chamber is 1800 mm high and 300 mm in diameter (D) with a centered exhausted pipe while the middle chamber of the combustor is set to 0.5D. The smaller section at the middle chamber is introduced to split the chamber to be dual-staging chamber where a large central toroidal recirculation zone induced by swirl flow through the small section is generated in the top chamber. The experimental results reveal that the highest temperature inside the combustor is about 1000 °C whereas both the thermal and the combustion efficiency are 41.6% and 99.8% for 74% excess air without the secondary air injection (λ = 0.0). In addition, the emissions are CO₂ = 8.1%, O₂ = 9.3%, CO = 352 ppm, NO_x = 294 ppm and small amount of fly ash. Therefore, the DSVC shows an excellent performance, low emissions, high stabilization and ease of operation in firing the rice husk.     

Optical and structural properties of the sol-gel-prepared ZnO thin films and their effect on the photocatalytic activity

ZnO thin films were obtained by the sol-gel method, using the dip-coating procedure. Glass slides were used as substrates. The sintering temperature (T_s) was varied in the range of 200-600 °C in intervals of 50 °C, in an open atmosphere. Films with 1 and 5 coatings were prepared for each T_s. An increase of the grain size from 10 to 34 nm as the T_s increased was observed from X-ray diffraction measurements. The thickness of the films prepared starting from five coatings, decreased by 36% when T_s increased, and denser films were obtained. This result was corroborated with the refractive index values, calculated from the UV-Vis transmission spectra. The films were tested as a photocatalyst by the photobleaching of methylene blue in an aqueous solution under UV light exposure during 5 h. The photocatalytic activity (PA) increased with T_s, around 72% for the films with one coating and 66% for those with five coatings. The samples with one coating and a T_s=500 °C showed the best PA. However, the glass substrate had a negative effect on the PA for T_s>500 °C, even when the surface morphology of the samples showed an increase in roughness when T_s increased. The observed negative effect can be due to the presence of an amorphous compound formed by Si, Zn and O at the glass-ZnO interface.     

Reciprocating air flow for Li-ion battery thermal management to improve temperature uniformity

The thermal management of traction battery systems for electrical-drive vehicles directly affects vehicle dynamic performance, long-term durability and cost of the battery systems. In this paper, a new battery thermal management method using a reciprocating air flow for cylindrical Li-ion (LiMn₂O₄/C) cells was numerically analyzed using (i) a two-dimensional computational fluid dynamics (CFD) model and (ii) a lumped-capacitance thermal model for battery cells and a flow network model. The battery heat generation was approximated by uniform volumetric joule and reversible (entropic) losses. The results of the CFD model were validated with the experimental results of in-line tube-bank systems which approximates the battery cell arrangement considered for this study. The numerical results showed that the reciprocating flow can reduce the cell temperature difference of the battery system by about 4 °C (72% reduction) and the maximum cell temperature by 1.5 °C for a reciprocation period of τ = 120 s as compared with the uni-directional flow case (τ = ∞). Such temperature improvement attributes to the heat redistribution and disturbance of the boundary layers on the formed on the cells due to the periodic flow reversal.     

Turbulent convection in round tube equipped with propeller type swirl generators

This article is aimed at studying the heat transfer, friction loss and enhancement efficiency behaviors in a heat exchanger tube equipped with propeller type swirl generators at several pitch ratios (PR). The investigation is performed for the Reynolds number ranging from 4000 to 21,000 under a uniform heat flux condition. The experiments are also undertaken for several blade numbers of the propeller (N = 4, 6, and 8 blades) and for different blade angles (θ = 30°, 45°, and 60°). The influences of using the propeller rotating freely, on heat transfer enhancement, pressure loss, and enhancement efficiency, are reported. In the experiments, the swirl generator is used to create a decaying swirl in the tube flow. Average Nusselt numbers are determined and also compared with those obtained from other similar cases. The experimental results indicate that the tube with the propeller inserts provides considerable improvement of the heat transfer rate over the plain tube around 2.07 to 2.18 times for PR = 5, blade angles θ = 60° and N = 8. The use of the propeller leads to maximum enhancement efficiency up to 1.2. Thus, because of strong swirl or rotating flow, the propellers and their blade numbers become influential upon the heat transfer enhancement. The increase in friction factor from using the propeller is found to be 3–18 times over the plain tube. Correlations for Nusselt number (Nu) and friction factor (f) for the inserted tube are provided and the performance evaluation criterion to access the real benefits in using the swirl generators of the enhanced tube is also determined.     

Interrelationships between bioreactor volume,effluent recycle rate,temperature, pH, %H2, hydrogen productivity and hydrogen yield with undefined bacterial cultures

Simultaneous achievement of high volumetric productivities (HP = 231.3 mmol H₂/L/h) and high hydrogen yields (HY = 3.55 mol H₂/mol glucose) was obtained by increasing the temperature to 70 °C and by reducing the total bioreactor system volume (V) to 5.74 L and increasing the degassed effluent recycle rate (F_er) to 3.2 L/min, giving a V/F_er value of 1.8 min. The bioprocess involved the recycling of degassed effluent at a high flow rate through a quasi-stationary fluidized granular bed. In this process the rate of physical removal of H₂ trapped in the bulk liquid phase surrounding the fluidized granules reduced the thermodynamic constraints preventing the simultaneous achievement of high HPs and high HYs in the anaerobic fluidized granular bed bioreactor. Energy balance analysis showed that with heat recycling the bioreactor system could achieve a net positive volumetric energy output of 11.76 W/L at an energy efficiency of 49.3%.     

SrCo1−xSbxO3−δ perovskite oxides as cathode materials in solid oxide fuel cells

The SrCo_1−xSb_xO_3−δ (x = 0.05, 0.1, 0.15 and 0.2) system was tested as possible cathode for solid oxide fuel cells (SOFCs). X-ray diffraction results show the stabilization of a tetragonal P4/mmm structure with Sb contents between x = 0.05 and x = 0.15. At x = 0.2 a phase transition takes place and the material is defined in the cubic Pm-3m space group. In comparison with the undoped hexagonal SrCoO₃ phase, the obtained compounds present high thermal stability without abrupt changes in the expansion coefficient. In addition, a great enhancement of the electrical conductivity was observed at low and intermediate temperatures (T ≤ 800 °C). The sample with x = 0.05 displays the highest conductivity value that reaches 500 S cm⁻¹ at 400 °C and is over 160 S cm⁻¹ in the usual working conditions of a cathode in SOFC (650-900 °C). Moreover, the impedance spectra of the SrCo_1−xSb_xO_3−δ/Ce_0.8Nd_0.2O_2−δ/SrCo_1−xSb_xO_3−δ (x ≥ 0.05) symmetrical cells reveal polarization resistances below 0.09 Ω cm² at 750 °C which are much smaller than that displayed by the pristine SrCoO_3−δ sample. The composition with x = 0.05 shows the lowest ASR values ranging from 0.009 to 0.23 Ω cm² in the 900-600 °C temperature interval with an activation energy of 0.82 eV.     

Experimental and numerical study of the natural convection from a heated horizontal cylinder wrapped with a layer of textile material

Natural convection heat transfer enhancement from a horizontal cylinder with a textile coating is studied experimentally and numerically. The coating layer may be used for two purposes. According to the thickness of the coating it may be used as an insulating material or for surface augmentation. In the experimental study, two cylinders having different diameters of 4.8 mm and 9.45 mm are used. The bare cylinders having a radius smaller than a certain critical size were wrapped with a textile material. Wrapped cylinder diameters were increased to 9 and 12.8 mm respectively after coating and constant heat flux was applied to all bare and wrapped cylinders. Experimental study was carried out at different ambient temperatures in a conditioned room which can be maintained at a stable required value and inside a sufficiently designed test cabin. The ambient and cylinder surface temperatures (T_∞ and T_w) varied between 10 °C – 40 °C and 20 °C – 60 °C respectively. Heat transfer rates from bare and wrapped horizontal cylinders were compared and heat transfer enhancement was observed. On the basis of the experimental data average Nusselt numbers were calculated and compared with the well known correlations on natural convection heat transfer from a horizontal cylinder in the specified range of Rayleigh number, and it is seen that the results are in good agreement. The problem is also investigated numerically. Experimental and the numerical results fall in ± 30% band.     

Thermal analysis of solar parabolic trough with porous disc receiver

In this paper, 3-D numerical analysis of the porous disc line receiver for solar parabolic trough collector is presented. The influence of thermic fluid properties, receiver design and solar radiation concentration on overall heat collection is investigated. The analysis is carried out based on renormalization-group (RNG) k–ε turbulent model by using Therminol-VP1 as working fluid. The thermal analysis of the receiver is carried out for various geometrical parameters such as angle (θ), orientation, height of the disc (H) and distance between the discs (w) and for different heat flux conditions. The receiver showed better heat transfer characteristics; the top porous disc configuration having w = d_i, H = 0.5d_i and θ = 30°. The heat transfer characteristic enhances about 64.3% in terms of Nusselt number with a pressure drop of 457 Pa against the tubular receiver. The use of porous medium in tubular solar receiver enhances the system performance significantly.     

Effects of fin geometry on boiling heat transfer from silicon chips with micro-pin-fins immersed in FC-72

Experiments were performed to study the effects of the height and thickness of square micro-pin-fin on boiling heat transfer from silicon chips immersed in a pool of degassed or gas-dissolved FC-72. Six kinds of micro-pin-fins with the dimensions of 30 × 60, 30 × 120, 30 × 200, 50 × 60, 50 × 200 and 50 × 270 μm² (thickness, t × height, h) were fabricated on the surface of a square silicon chip with the dimensions of 10 × 10 × 0.5 mm³ by using the dry etching technique. The fin pitch was twice the fin thickness. The experiments were conducted at the liquid subcooling, ΔT_sub, of 0, 3, 25 and 45 K under the atmospheric condition. The results were compared with previous results for a smooth chip and three chips with enhanced heat transfer surfaces. The micro-pin-finned chips showed a considerable heat transfer enhancement in the nucleate boiling region and increase in the critical heat flux, q_CHF, as compared to the smooth chip. The wall temperature at the CHF point was always less than the maximum allowable temperature for LSI chips (=85 °C). For a fixed value of t, q_CHF increased monotonically with increasing h. The increase was more significant for larger t. The q_CHF increased almost linearly with increasing ΔT_sub. The maximum value of allowable heat flux (=84.5 W/cm²), 4.2 times as large as that for the smooth chip, was obtained by the chip with h=270 μm and t=50 μm at ΔT_sub=45 K.