Effect of air leak on long term performance of air heater

In this paper, the effect of an air leak on the air heater performance is investigated. “Air leaking in” and “air leaking out” systems are examined. The effect of various parameters like leakage rate, mass flow rate, solar insolation, plate length and ambient temperature has been studied. The possibility that air leakage can take place from any where along the length of the collector has been incorporated in the model. It is found that, for “air leaking in” systems, efficiency goes up, while for “air leaking out” systems, efficiency decreases from the no-leak situation, and it depends on the position of the leak.     

Thermal performance of an n-pass solar air heater

A mathematical model is developed for the prediction of the thermal performance for an n-pass solar air collector. The thermal efficiency and the temperature rise in teh collector are increased by preheating the air within the collector before it comes into contact with the absorber plate. The thermal losses to the ambient, through the bottom plate, are minimized due to the lower bottom plate temperature. The analysis describes how the governing equations are manipulated with the aid of the boundary conditions to obtain a set of equations which is solved numerically. In order to demonstrate the capability of the model, the analysis is applied to investigate the effect of increasing the number of air passes within the collector on the thermal efficiency and the temperature rise for a range of air mass flow rates. The results indicate that the thermal efficiency of the collector increases rapidly and approaches the optical efficiency of the collector as the mass flow rate is increased. In general, the model can be used to analyze the effect of various design parameters on the thermal performance.     

Simulation of small-scale releases from liquid hydrogen storage systems

Knowledge of the concentration field and flammability envelope from small-scale leaks is important for the safe use of hydrogen. These small-scale leaks may occur from leaky fittings or o-ring seals on liquid hydrogen-based systems. The present study focuses on steady-state leaks with large amounts of pressure drop along the leak path such that hydrogen enters the atmosphere at near atmospheric pressure (i.e. Very low Mach number). A three-stage buoyant turbulent entrainment model is developed to predict the properties (trajectory, hydrogen concentration and temperature) of a jet emanating from the leak. Atmospheric hydrogen properties (temperature and quality) at the leak plane depend on the storage pressure and whether the leak occurs from the saturated vapor space or saturated liquid space. In the first stage of the entrainment model ambient temperature air (295 K) mixes with the leaking hydrogen (20-30 K) over a short distance creating an ideal gas mixture at low temperature (∼65 K). During this process states of hydrogen and air are determined from equilibrium thermodynamics using models developed by NIST. In the second stage of the model (also relatively short in distance) the radial distribution of hydrogen concentration and velocity in the jet develops into a Gaussian profile characteristic of free jets. The third and by far the longest stage is the part of the jet trajectory where flow is fully developed. Results show that flammability envelopes for cold hydrogen jets are generally larger than those of ambient temperature jets. While trajectories for ambient temperature jets depend solely on the leak densimetric Froude number, results from the present study show that cold jet trajectories depend on the Froude number and the initial jet density ratio. Furthermore, the flammability envelope is influenced by the hydrogen concentration in the jet at the beginning of fully developed flow.     

Theoretical analysis of some configurations of double exposure solar air heaters

We present a theoretical analysis of three different configurations of double exposure solar air heaters. The heat-balance equations are written in each case and are solved explicitly. The periodic responses have been studied for different combinations of parameters. Measured values of solar radiation in the plane of the collector and the ambient temperature are used in the analysis. The effects of collector length, reflectance and flow rate have been studied.The outlet air temperature increases with an increase in plate length but the instantaneous and average efficiency decrease. The outlet air temperature is approximately the same, for a given model and combinations of parameters, when the ratio of the length of the air heater to the mass flow rate is constant. A double exposure solar air heater performs better than a single exposure air heater if the reflectance for solar radiation is greater than 10%.     

Analysis of heat transfer and frost layer formation on a cryogenic tank wall exposed to the humid atmospheric air

In this paper heat transfer characteristics and frost layer formation are investigated numerically on the surface of a cryogenic oxidizer tank for a liquid propulsion rocket, where a frost layer could be a significant factor in maintaining oxidizer temperature within a required range. Frost formation is modeled by considering mass diffusion of water vapor in the air into the frost layer and various heat transfer modes such as natural and forced convection, latent heat, solar radiation of short wavelength, and ambient radiation of long wavelength. Computational results are first compared with the available measurements and show favorable agreement on thickness and effective thermal conductivity of the frost layer. In the case of the cryogenic tank, a series of parametric studies is presented in order to examine the effects of important parameters such as temperature and wind speed of ambient air, air humidity, and tank wall temperature on the frost layer formation and the amount of heat transfer into the tank. It is found that the heat transfer by solar radiation is significant and also that heat transfer strongly depends on air humidity, ambient air temperature, and wind speed but not tank wall temperature.     

Flow distribution in unglazed transpired plate solar air heaters of large area

Unglazed transpired plate solar air heaters have proven to be effective in heating outside air on a once-through basis for ventilation and drying applications. Outside air is sucked through unglazed plates having uniformly distributed perforations. The air is drawn into a plenum behind the plate and then supplied to the application by fans. Large collectors have been built that cover the sides of sizeable buildings, and the problem of designing the system so that the air is sucked uniformly everywhere (or nearly so) has proven to be a challenging one. This article describes an analytical tool that has been developed to predict the flow distribution over the collector. It is based on modelling the flow-field in the plenum by means of a commercial CFD (computational fluid mechanics) code, incorporating a special set of boundary conditions to model the plate and the ambient air. This article presents the 2D version of the code, and applies it to the problem of predicting the flow distribution in still air (no wind) conditions, a situation well treated by a 2D code. Results are presented for a wide range of conditions, and design implications are discussed. An interesting finding of the study is that the heat transfer at the back of the plate can play an important role, and because of this heat transfer, the efficiency of a collector in non-uniform flow can can actually be greater than that of the same collector in uniform flow.     

Analytical and experimental investigation of small-scale unintended releases of hydrogen

Knowledge of the concentration field and flammability envelope from a small-scale hydrogen leak is an issue of importance for the safe use of hydrogen. A combined experimental and modeling program is being carried out by Sandia National Laboratories to characterize and predict the behavior of small-scale hydrogen releases. In contrast to the previous work performed by Sandia on large, momentum-dominated hydrogen leaks, these studies are focusing on small leaks in the Froude number range where both buoyant and inertial forces are important or, in the limit, where buoyancy dominates leak behavior. In the slow leak regime-buoyant forces affect the trajectory and rate of air entrainment of the hydrogen jet leak and significant curvature can occur in the jet trajectory. Slow leaks may occur from leaky fittings or o-ring seals on hydrogen vehicles or other hydrogen-based systems where large amounts of pressure drop occur across the leak path. Low-pressure electrolyzers or vents on buildings or storage facilities containing hydrogen are also potential sources for slow leaks.     

Performance of evaporator-collector and air collector in solar assisted heat pump dryer

A solar assisted heat pump dryer has been designed, fabricated and tested. This paper presents the performance of the evaporator-collector and the air collector when operated under the same meteorological conditions. ASHRAE standard procedure for collector testing has been followed. The evaporator-collector of the heat pump is acting directly as the solar collector, and the temperature of the refrigerant at the inlet to the evaporator-collector always remained below the ambient temperature. Because of the rejection of sensible and latent heats of air at the dehumidifier, the temperature at the inlet to the air collector is lower than that of the ambient air. Hence, the thermal efficiency of the air collector also increases due to a reduction of losses from the collector. The efficiencies of the evaporator-collector and the air collector were found to vary between 0.8–0.86 and 0.7–0.75, respectively, when operated under the meteorological conditions of Singapore.     

Performance analysis of a double-pass thermoelectric solar air collector

The thermoelectric (TE) solar air collector, sometimes known as the hybrid solar collector, generates both thermal and electrical energies simultaneously. A double-pass TE solar air collector has been developed and tested. The TE solar collector was composed of transparent glass, air gap, an absorber plate, thermoelectric modules and rectangular fin heat sink. The incident solar radiation heats up the absorber plate so that a temperature difference is created between the thermoelectric modules that generates a direct current. Only a small part of the absorbed solar radiation is converted to electricity, while the rest increases the temperature of the absorber plate. The ambient air flows through the heat sink located in the lower channel to gain heat. The heated air then flows to the upper channel where it receives additional heating from the absorber plate. Improvements to the thermal and overall efficiencies of the system can be achieved by the use of the double-pass collector system and TE technology. Results show that the thermal efficiency increases as the air flow rate increases. Meanwhile, the electrical power output and the conversion efficiency depend on the temperature difference between the hot and cold side of the TE modules. At a temperature difference of 22.8 °C, the unit achieved a power output of 2.13 W and the conversion efficiency of 6.17%. Therefore, the proposed TE solar collector concept is anticipated to contribute to wider applications of the TE hybrid systems due to the increased overall efficiency.     

Theoretical and experimental study of efficiency factor,heat transfer and thermal heat loss coefficients in solar air collectors with selective and nonselective absorbers

The thermal heat performance of a solar air collector depends strongly on the thermal heat loss and the efficiency factor. In order to increase these performances, it is necessary to use a solar air collector which is well insulated and where the fluid flow is fully developed turbulent flow. It needs a high heat transfer between the absorber plate and the fluid to decrease the absorber‐plate temperature and hence the heat loss by radiation from the absorber to the ambient. This increases the efficiency factor. In the present paper, the heat loss and efficiency factor are treated for solar air collectors with selective and nonselective absorber plate. It is shown that the selectivity of the absorber plate cannot play an important role in a well‐insulated solar collector with a fanned system which permits a fully developed turbulent flow and, in consequence, high heat transfer. Copyright © 1999 John Wiley & Sons, Ltd.     

Efficiency investigation of a new-design air solar plate collector used in a humidification–dehumidification desalination process

The aim of this research is to experimentally study the efficiency of a new-design plate collector used to heat air in a new desalination humidification–dehumidification process. In fact, in such processes, the air solar collectors work at unusual experimental parameters (forced convection, elevated air humidity, high solar irradiation…). At these stressed experimental conditions, few published works are available in literature. Furthermore, the comparison of the efficiency of collectors running with normal air humidity content (about 10–20 g kg⁻¹) and air of elevated humidity (20–50 g kg⁻¹) were not yet published as our knowledge. In the present investigation, a new air solar plate collector was designed and developed for its use in a desalination process. Moreover, a characterization of such collector was performed under different experimental conditions. The effect of different parameters, namely: the solar radiation, the wind velocity, the ambient temperature, the air mass flow rate, the inlet air humidity and temperature, on the collector efficiency was also investigated. The collector was optimized for its use in a new solar desalination process. In fact, the air solar collector was designed in order to lower its economic cost making them applicable for water desalination.     

Performance analysis of new-design solar air collectors for drying applications

This paper presents a performance analysis of four types of air heating flat plate solar collectors: a finned collector with an angle of 75°, a finned collector with an angle of 70°, a collector with tubes, and a base collector. In this study, the first and second laws of efficiencies were determined for the collectors and comparisons were made among them. The results showed that the efficiency depends on the solar radiation and the construction of the solar air collectors. The temperature rise varied almost linearly with the incident radiation. The first law of efficiency changed between 26% and 80% for collector-I, between 26% and 42% for collector-II, between 70% and 60% for collector-III, and between 26% and 64% for collector-IV. The values of second law efficiency varied from 0.27 to 0.64 for all collectors? The highest collector efficiency and air temperature rise were achieved by the finned collector with angle of 75°, whereas the lowest values were obtained for the base collector. The effectiveness order of the collectors was determined as the finned collector with angle of 75°, the finned collector with angle of 70°, the collector with tubes, and the base collector.     

Performance of jet impingement in unglazed air collectors

Jet impingement is effective at improving the heat transfer between air and a heated surface. Studies have shown that jet impingement can marginally improve the thermal efficiency of a glazed collector. However, little attention has been placed on applying jet impingement to an unglazed solar air collector. This paper presents a theoretical and experimental investigation identifying the performance characteristics of jet impingement. Overall, jet impingement was able to improve the thermal efficiency of the collector by 21%. An increase in the pressure loss was also measured but found to be small. The flow distribution of jets along the collector was the most significant factor in determining the efficiency. Increasing the hole spacing was found to improve the efficiency.     

Performance investigation of flat plate,v-corrugated and finned air collectors

In this study, flat plate, finned and v-corrugated air heaters were investigated both experimentally and theoretically in an effort to improve the performance of conventional air heaters. Collectors were also tested in double pass mode to investigate the extent of improvement in efficiency that could be achieved without increasing the collector size or cost. A series of experiments were conducted, based on the ASHRAE standard, under climatic conditions of Singapore. The performance of all three collectors was examined over a wide range of operating and design conditions. The v-corrugated collector was found to be the most efficient collector and the flat plate collector to be the least efficient. Results showed that the v-corrugated collector is 10–15 and 5–11% more efficient in single pass and double pass modes, respectively, compared to flat plate collectors. The double pass operation of the collector improved the efficiency of all three collectors. The improvement in efficiency for double pass mode was most significant in the flat plate collector and least in the v-groove collectors.     

Transient test of a solar air heater with a compound parabolic concentrator

A method for measuring the performance parameters of a solar thermal collector under non-steady conditions has been applied to an air heater with a truncated compound parabolic concentrator having an aperture area of 1.44 m² and a flat absorber with concentration ratio three. This type of collector was chosen in order to obtain high air temperatures in a tropical climate where the proportion of diffuse solar radiation is high. The parameters found were the optical efficiency, the linear and nonlinear heat loss coefficients, and the effective heat capacity. These parameters were determined individually in separate experiments, some with and some without solar radiation falling on the collector. Agreement within 2% was found between the optical efficiency measured in the experiments and the optical efficiency determined from direct measurements of the optical properties of the materials in the collector. The method can be performed all the year round in a tropical climate when steady conditions do not occur.     

An improved thermal and electrical model for a solar photovoltaic thermal (PV/T) air collector

In this paper, an attempt is made to investigate the thermal and electrical performance of a solar photovoltaic thermal (PV/T) air collector. A detailed thermal and electrical model is developed to calculate the thermal and electrical parameters of a typical PV/T air collector. The thermal and electrical parameters of a PV/T air collector include solar cell temperature, back surface temperature, outlet air temperature, open-circuit voltage, short-circuit current, maximum power point voltage, maximum power point current, etc. Some corrections are done on heat loss coefficients in order to improve the thermal model of a PV/T air collector. A better electrical model is used to increase the calculations precision of PV/T air collector electrical parameters. Unlike the conventional electrical models used in the previous literature, the electrical model presented in this paper can estimate the electrical parameters of a PV/T air collector such as open-circuit voltage, short-circuit current, maximum power point voltage, and maximum power point current. Further, an analytical expression for the overall energy efficiency of a PV/T air collector is derived in terms of thermal, electrical, design and climatic parameters. A computer simulation program is developed in order to calculate the thermal and electrical parameters of a PV/T air collector. The results of numerical simulation are in good agreement with the experimental measurements noted in the previous literature. Finally, parametric studies have been carried out. Since some corrections have been down on thermal and electrical models, it is observed that the thermal and electrical simulation results obtained in this paper is more precise than the one given by the previous literature. It is also found that the thermal efficiency, electrical efficiency and overall energy efficiency of PV/T air collector is about 17.18%, 10.01% and 45%, respectively, for a sample climatic, operating and design parameters.     

Field study of various air based photovoltaic/thermal hybrid solar collectors

In the present work a comparative study for thermal and electrical performance of different hybrid photovoltaic/thermal collectors designs for Iraq climate conditions have been carried out. Four different types of air based hybrid PV/T collectors have been manufactured and tested. Three collectors consist of four main parts namely, channel duct, glass cover, axial fan to circulate air and two PV panels in parallel connection. The measured parameters are, the temperature of the upper and the lower surfaces of the PV panels, air temperature along the collector, air flow rate, pressure drop, power produced by solar cell, and climate conditions such as wind speed, solar radiation and ambient temperature. The thermal and hydraulic performances of PV/T collector model IV have been analyzed theoretically based on energy balance. A Matlab computer program has been developed to solve the proposed mathematical model.The obtained results show that the combined efficiency of collector model III (double duct, single pass) is higher than that of model II (single duct double pass) and model IV (single duct single pass). Model IV has the better electrical efficiency. The pressure drop of model III is lower than that of models II and IV. The root mean square of percentage deviations for PV outlet temperature, and thermal efficiency of model IV are found to be 3.22%, and 18.04% respectively. The calculated linear coefficients of correlation (r) are 0.977, 0.965 respectively.     

A two-pass solar air heater

An air heater, in which the air first flows between metallic and cover plates and is then made to flow between two metallic plates in opposite direction, is discussed theoretically. The governing equations of the model are solved explicitly under suitable conditions. The measured values of the solar insolation and ambient temperature are represented by Fourier series. The effects of collector length and flow rate have been studied. The air heater in this mode of operation is more efficient than one in which air flows between two metallic plates in the same direction, provided the plate length is less than 5 m.     

水在玻璃管真空管太阳集热器热性能计算与分析

在水在玻璃管式全玻璃真空管集热器能量平衡分析的基础上，推导了集热器热损系数、效率因子等性能参数计算公式，理论计算结果与实验数据吻合良好。计算分析表明，真空管热损系数与吸热管温和环境温度之差是非线性关系，将两者的计算关系式按环境温度分段整理将使计算结果更接近实际；随着吸热管内对流换热系数的增大，其对效率因子和效率的影响逐渐减小，这说明对于水在玻璃管连接方式而言，为得到较大的管内对流换热系数而将流速过度提高是没有必要的；涂层发射率对集热器的热损系数和效率影响较大，降低涂层发射率是提高集热器效率的有效途径。     

非晶硅太阳能光伏/光热空气集热器性能对比实验研究

文章设计了新型非晶硅太阳能PV/T空气集热器,该空气集热器能够解决传统太阳能PV/T热水器在高温波动情况下,晶硅电池热应力大的问题,同时避免了冬季管道发生霜冻的现象。文章通过实验对比,分析了非晶硅太阳能PV/T空气集热器、单独非晶硅光伏电池和传统太阳能空气集热器的能量效率和[火用]效率的差异。分析结果表明:非晶硅太阳能PV/T空气集热器的平均热效率为45.70%,比传统太阳能空气集热器的平均热效率降低了约25.88%;当空气质量流量增大至0.048 kg/s时,非晶硅太阳能PV/T空气集热器中的非晶硅光伏电池的平均电效率高于单独非晶硅光伏电池,它们的平均电效率分别为4.70%,4.54%;非晶硅太阳能PV/T空气集热器的总[火用]效率高于传统太阳能空气集热器的热[火用]效率和单独非晶硅光伏电池的电[火用]效率,非晶硅太阳能PV/T空气集热器总[火用]效率最大值为7.14%。文章的分析结果为非晶硅太阳能PV/T空气集热器的推广提供了参考。