Performance of plastic solar air heating collectors with a porous absorber

The thermal performance of solar air heaters consisting of a porous textile absorber between two PVC foils has been investigated. The efficiency of the heaters depends strongly on the characteristics of the textile forming the absorber and on the back insulation. For an incident solar radiation of 687 W/m² at the collector's surface, a temperature rise of 16-6°C in the air flowing through the solar collector at a rate of 800 m³/h, was achieved, thus yielding an efficiency of nearly 71 percent. Further it was found that the linear approximation for the Hottel-Bliss equation leads to erroneous estimations for the collector's parameters when the absorber is porous; for the same type of collector with a denser textile as absorber, however, such an approximation yields, as usual, correct numerical values for the characteristic parameters of the collector.     

Thermal performances comparisons of the glass evacuated tube solar collectors with shapes of absorber tube

The thermal performance of a glass evacuated tube solar collector is numerically and experimentally investigated. The solar collector considered in this paper consists of a two-layered glass tube and an absorber tube. Air is used as the working fluid. The length and diameter of this glass tube are 1200 and 37 mm, respectively. Four different shapes of absorber tubes are considered, and the performances of the solar collectors are studied to find the best shape of the absorber tube for the solar collector. Beam irradiation, diffuse irradiation, and shade due to adjacent tubes are taken into account for a collector model to obtain a realistic estimation. In addition, a single collector tube with only beam irradiation is studied as a simplified model, and the results of the simplified model are compared to those of the collector model to identify the difference between these two models. The performance of a solar collector is affected by the shape of the absorber, incidence angle of solar irradiation, and arrangement of collector tubes. The results obtained from the simplified model are very different from those from the collector model, which considered not only beam and diffuse irradiation but also shade due to adjacent tubes.     

Heat loss characteristics study of a trapezoidal cavity absorber with and without plate for a linear Fresnel reflector solar concentrator system

The numerical and experimental studies are conducted to analyze the heat loss in the cavity absorbers of linear Fresnel reflecting solar concentrator (LFRSC). The cavity is trapezoidal shape in cross section, which is placed at focus of the concentrator, has multiple tubes and water is used as the working fluid. The upper surface of the cavity has two models; with copper plate, above which absorber tubes are placed together and without copper plate i.e. absorber tubes alone without copper plate underneath. In both the models, the heat loss coefficient of projected absorber surfaces is analyzed with and without black chrome coating. For the numerical simulation of the trapezoidal cavity absorber, ANSYS FLUENT 12.0 version is used to develop the two dimensional model with non-Boussinesq numerical approximation. For the experimental study, two cavity absorbers are designed for operating in conjunction with a LFRSC experimental set up for the area of 4.0 m². The overall heat loss coefficients are also estimated analytically by cavity correlations. The trend of variation of estimated heat loss coefficient by both methods is similar to experimental values. Also, estimated values by numerical study are very close to analytical and experimental values and the numerical model can be used for further analysis.     

Experimental study of the performances of a modified diesel engine operating in homogeneous charge compression ignition (HCCI) combustion mode versus the original diesel combustion mode

Homogeneous charge compression ignition (HCCI) combustion mode provides very low NO_x and soot emissions; however, it has some challenges associated with hydrocarbon (HC) emissions, fuel consumption, difficult control of start of ignition and bad behaviour to high loads. Cooled exhaust gas recirculation (EGR) is a common way to control in-cylinder NO_x production in diesel and HCCI combustion mode. However EGR has different effects on combustion and emissions, which are difficult to distinguish. This work is intended to characterize an engine that has been modified from the base diesel engine (FL1 906 DEUTZ-DITER) to work in HCCI combustion mode. It shows the experimental results for the modified diesel engine in HCCI combustion mode fueled with commercial diesel fuel compared to the diesel engine mode. An experimental installation, in conjunction with systematic tests to determine the optimum crank angle of fuel injection, has been used to measure the evolution of the cylinder pressure and to get an estimate of the heat release rate from a single-zone numerical model. From these the angle of start of combustion has been obtained. The performances and emissions of HC, CO and the huge reduction of NO_x and smoke emissions of the engine are presented. These results have allowed a deeper analysis of the effects of external EGR on the HCCI operation mode, on some engine design parameters and also on NO_x emission reduction.     

Performance of a residential heat pump operating in the cooling mode with single faults imposed

The system behavior of a R410A residential unitary split heat pump operating in the cooling mode was investigated. Seven artificial faults were implemented: compressor/reversing valve leakage, improper outdoor air flow, improper indoor air flow, liquid line restriction, refrigerant undercharge, refrigerant overcharge, and presence of non-condensable gas in the refrigerant. This study monitored eight fault detection features and identified the most sensitive features for each fault. The effect of the various fault levels on energy efficiency ratio (EER) was also estimated. Since the studied system employed a thermostatic expansion valve (TXV) as an expansion device, it could adapt to some faults making the fault less detectable. The distinctiveness of the fault depended on the TXV status (fully open or not).     

Computational study of heat transfer in solar collectors with different radiative flux models

Two‐dimensional steady incompressible laminar Newtonian viscous convection‐radiative heat transfer in a rectangular solar collector geometry is considered. The ANSYS FLUENT finite volume code (version 17.2) is used to simulate the thermo‐fluid characteristics. Extensive details of computational methodology are given to provide engineers with a framework for simulating radiative‐convection in enclosures. Mesh‐independence tests and validation are conducted. The influence of aspect ratio, Prandtl number ( Pr), Rayleigh number ( Ra) and radiative flux model on temperature, isotherms, velocity, and pressure is evaluated and visualized in colour plots. In addition, local convective heat flux is computed, and solutions are compared with the MAC solver for various buoyancy effects achieving excellent agreement. The P1 model is shown to better predict the actual influence of solar radiative flux on thermal fluid behaviour compared with the limited Rosseland model. With increasing Ra, the hot zone emanating from the base of the collector is found to penetrate deeper into the collector and rises symmetrically dividing into two vortex regions with very high buoyancy effect. With increasing Pr there is a progressive incursion of the hot zone at the solar collector base higher into the solar collector space and simultaneously a greater asymmetric behaviour of the dual isothermal zones.     

Numerically study on a new hybrid photovoltaic thermal (PVT) collectors with natural circulation

Traditional design of Hybrid photovoltaic thermal (PV/T) system has the solar cells fixed on the top of the absorber. A new PV/T system in which the solar cells are pasted on the bottom of the glass cover is suggested with the aim of realizing higher electricity output considering the lower temperature of glass cover compared with that of absorber. A numerical analysis model is set up to compare the performances of the traditional PV/T and the new PV/T in this study. It is found that compared to the traditional PV/T, the new PV/T shows higher daily electric efficiency. But this superiority is not as apparent as expected. The key point to increase the daily electric efficiency of a PV/T lies in increasing the solar energy transfer efficiency of the solar cell. The total energy gain of the new PV/T is about 7% lower than that of the traditional PV/T because of smaller mass rate and more energy loss from glass cover with higher temperature.     

Theoretical and experimental study of a two channel air heater with perforated first absorber

The performance of a two channel solar air collector with perforated first absorber plate is discussed. The transient equations, written by considering the energy balance for every individual component of the air collector, are solved explicitly. The effect of some of the design parameters are discussed. The mentioned design is investigated experimentally. It was found that there is a strong relationship between collector efficiency and first absorber plate thickness, holes density and dimensions.     

The study of the performance and efficiency of flat linear Fresnel lens collector with sun tracking system in Iraq

Two flat linear Fresnel lenses and two absorbers connected in series. Tracking system is constructed so that it tracks the sun in two directions. Thermal and optical losses are introduced. The thermal efficiency of the first lens is higher than the second lens and reaches 0.65. The FLFL all-day collector efficiency reaches 0.58 and it varies depending on weather condition.     

Corrosion protection of Sunselect, a spectrally selective solar absorber coating, by (3-mercaptopropyl)trimethoxysilane

The formation of a protective layer from (3-mercaptopropyl)trimethoxysilane (MPTMS) on commercial Sunselect, cermet-based spectrally selective coating (Alanod, DE), was studied by non-electrochemical (infrared reflection–absorption—IR RA, X-ray photoelectron spectroscopy—XPS), electrochemical (cyclic voltammetry (CV) in the presence of a redox probe (Cd⁺²), and potentiodynamic (PD)) techniques. By simple immersion and subsequent dip coating of the Sunselect substrate in the MPTMS sol, the hydrolyzed MPTMS precursor was adsorbed on the substrate, forming a protective layer imparting corrosion stability to Sunselect in a salt spray chamber for at least 20 days, outperforming any other sol–gel coating used so far for the corrosion protection of Sunselect. This was attributed to the penetration of MPTMS into the porous cermet structure through the upper antireflective Sn-oxide layer, as shown from XPS depth profile. Detailed analysis of the hydrolysis and condensation reactions of the MPTMS precursor by ²⁹Si NMR and IR spectroscopic techniques is reported.The most important finding was the observation that the applied MPTMS layer did not affect the spectral selectivity, as inferred from the solar absorptance increase of 1% and thermal emittance increase of not more than 2%.     

Two 3D thermomechanical numerical models of friction stir welding processes with a trigonal pin

ABSTRACT

During the friction stir welding (FSW) process, the behavior of the material is at the interface between solid mechanics and fluid mechanics. This article deals with a comparison of two 3D numerical models of FSW processes with a trigonal pin. The first model is based on a solid formulation and the second one is based on a fluid formulation. Both models use a Norton–Hoff constitutive model with the high temperature sensitivity of the parameters’ value and advanced numerical techniques such as the Arbitrary Lagrangian Eulerian (ALE) formalism. It can be concluded that, basically, these two formulations lead to the same results.     

Theoretical analysis of the thermal performance of all-glass evacuated tube solar collectors with absorber coating on the outside or inside of the inner tube

Theoretical efficiencies (η) and thermal behaviour of all-glass Evacuated Tube solar Collectors with an Internal Absorber Film (ETCIAF), i.e. the absorber film deposited in the inner surface of the inner tube, are compared and contrasted with the traditional design of all-glass Evacuated Tube solar Collectors with an External Absorber Film (ETCEAF), using the absorber film on the external surface of the inner tube. The values of η of the ETCIAF are unacceptably lower than that of ETCEAF for any particular value of the heat transfer coefficient (h_b) for the annular space, except in the case of a highly leaky ETCEAF, with h_b > 2.6 W/m² K. However, it is shown that the use of a transparent conductive coating with moderately low emittance 0.1−0.25 on the outside of the absorber tube of ETCIAF can offer efficiences 0.75−0.63, respectively, for f = 0.1 °C m²/K, competing well (η = 0.76) with the ETCEAF design operating under best conditions (α = 0.91, = 0.05, and h_b = 0.026 W/m² K).     

3D thermomechanical behaviour of solid oxide fuel cells operating in different environments

Hermetic sealing and long-term structural reliability of fuel cell stacks depend strongly on the thermomechanically induced stress–strain behaviour. These are especially affected by the environment; the fuel cell is operating in. Most of the research and development studies, as well as laboratory studies are conducted within electrically heated furnaces rather than operating in an insulated system environment. The thermomechanical comparison of them is not fully understood, yet. The present study utilises a previously developed full scale three dimensional planar type 6-cell SOFC short stack model to shed light on the thermomechanical response of high temperature fuel cells operating in system and furnace environments. The physically resolved coupled computational fluid dynamics and computational structural mechanics model has been improved, accounting for the rate dependent creep strain, as well as including the furnace domain and thermal radiation to fully describe the thermal and deformation behaviour of the stack. The non-linear elastoplastic behaviour of the metal components as a function of temperature is considered. The results are validated using creep strain data from the literature and in-house post-mortem images. The study gives an insight about the critical regions prone to failure due to creep strain operating in different environments and the long-term fuel cell behaviour. Moreover, the critical locations appear to be prone to high creep strain after 1000 h operation time.     

Results of a 3D seismic survey at the Travale (Italy) test site

In the last 15 years geothermal exploration in Tuscany, Italy, has addressed deep reservoirs (depth ≥ 3000 m), hosted within complex geological systems, such as metamorphic formations and/or intrusive bodies. Reservoir productivity is linked to fractured and permeable zones that are rather confined and not uniformly distributed. In this context, the seismic methods represent one of the most reliable geophysical techniques for locating potential drilling targets. A 3D seismic survey has been acquired at the Travale test site, and its results have been used to develop a geological and structural model of the site, and to identify and characterize fractured zones inside the deep geothermal reservoir. A correlation between a high-amplitude reflector (H marker) and fractured contact-metamorphic rocks has been highlighted. More than 70% of the total geothermal fluid production at the Travale area comes from this seismic marker.     

The use of moderate vacuum environments as a means of increasing the collection efficiencies and operating temperatures of flat-plate solar collectors

It is shown that evacuating a flat-plate solar collector to a pressure 1–25 torr results in elimination of the natural convection heat loss from the absorber for absorber-to-cover spacings up to 15 cm. This mode of heat transfer then reduces to pure conduction through the air space between the absorber and the cover. The effect of this reduction on the total upward heat loss from the collector is considered for a variety of collector operating conditions and is shown to be especially pronounced for collectors employing wavelength-selective surfaces (high absorptance for solar radiation, but low emittance for the energy re-radiated by the absorber). Computer simulations of collector performance for the Dallas, Texas area indicate that the combination of a moderate vacuum and a selective surface (α = 0·90, = 0·15) can increase daily energy collection as much as 278 per cent over that obtained with a non-vacuum collector using a flat-black (α = = 0·95) surface and can make it possible to operate at a temperature of 150°C with a daily energy collection efficiency of more than 40 per cent. The theoretical predictions are supported by the results of twelve experiments with a no-load solar tester. At an absorber-to-cover spacing of 7·5 cm, the steady-state temperature of a moderately selective absorber (α = 0·75, = 0·3) was increased from 115°C at atmospheric pressure to 179°C at a pressure of 25 torr.     

燃煤电厂湿法烟气脱硫吸收塔镍基合金材料国产化可行性分析

目前国内燃煤电厂湿法烟气脱硫吸收塔的镍基合金材料多采用进口C276,由于其价格昂贵,供货周期长,建议用国产NS334代替进口C276;对两者进行了分析比较,提出了使用国产NS334材料的可行性。     

Optimization of the photovoltaic thermal (PV/T) collector absorber

In an effort to reduce the cost of conventional fin and tube photovoltaic thermal (PV/T) collectors a novel mathematical analysis was developed which determines the optimum absorber plate configuration having the least material content and thus cost, whilst maintaining high collection efficiency.The analysis was based on the “low-flow” concept whose advantages include: improved system performance, smaller pump (less expensive with lower power consumption), smaller diameter tubes requiring lower thickness and thus cost of insulation, less construction power and time for the optimum absorber configuration.From the optimization methodology developed it was found that very thin fins (typically 50 μm) and small tubes (of 1.65 mm inside diameter for the risers, in the header and riser arrangement and 4.83 mm for the serpentine arrangement), with a tube spacing of 62 mm and 64 mm (both corresponding to 97% fin efficiency) and a mass of 1.185 kg/m² and 2.140 kg/m², respectively, can be used. This optimum serpentine absorber plate contains 40.50% less material content and mass, as compared to the serpentine prototype proposed by others. In one such design a mass of 3.596 kg/m² was used (with 10 mm diameter tubes, 95 mm tube spacing and 200 μm thick absorber).To predict the performance of the determined optimum configurations, a steady-state model (using the EES code) was developed. To validate the steady-state model two prototypes, one in Header and Riser and the other in Serpentine configuration, were built and tested. It was found from the experiments that there is a good agreement between the computational and the experimental results. Moreover, it was found that optimum PV/T configurations do indeed have thermal and electrical performance comparable to non-optimum ones of greater mass and cost.     

An experimental study on water transport through the membrane of a PEFC operating in the dead-end mode

Water transport through the membrane of a polymer electrolyte fuel cell (PEFC) was investigated by not only measuring the voltage variation but also visualizing the accumulation of water at the anode for various values of operating parameters, such as the humidity, current density, stoichiometry, location of humidification, and membrane properties. The PEFC was operated in the dead-end mode to prevent the discharge of water from the anode. The water transport in the PEFC was characterized by the elapsed time for the voltage to reach its limit. Anode visualization showed water transport under various conditions. In addition, the mass balance of water at the anode of the PEFC was considered. The variations of water diffusion and electro-osmotic drag were analyzed based on the experimental results.     

A thermal comparison performance of CPC with modified (duel-cavity) and non-modified absorber

The main objective of this work is the design and development of non-imaging solar collector systems for heating purposes. The aim is to increase the efficiency of the Compound Parabolic Concentrator CPC, by proposing a novel modification to the absorber. A modifications have been undertaken in the absorber design by introducing two cavities in the most appropriate location. The new design has been tested and compared with conventional designs to prove that the novel design of CPC with a dual cavities is ameliorate, as it was able to increase the efficiency considerably and at the same time reduces the heat losses. In this paper a comparative experimental results for CPC with two different absorber. The experiment was done under a real condition of sun. The comparative results was for the efficiency of each collector.