Effects of a porous manifold on thermal stratification in a liquid storage tank

A numerical model for a porous manifold has been developed to study its effects on thermal stratification in a typical storage tank. The tank considered has a capacity of 300 l and a height-to-diameter ratio of 2. The numerical model employs a transient stream function-vorticity formulation to predict the development of flow and temperature fields in a charging process. The Reynolds number of the inlet flow is fixed at 200 and a wide range of the Richardson number (0.01≤Ri≤100) is considered. Other parameters considered include the Biot number (i.e. heat loss from the tank), the Darcy number (permeability of the porous manifold), the location of baffle, and the thickness of the porous tube. The results show that all these factors should be taken into account for a better design of a thermal storage tank. In addition, the results provide an envelope for these design parameters.     

Thermal characteristics of a volumetric solar absorption system

Solar volumetric heating is one of the alternative clean energy applications, and the improvement of thermal storage capacity of the system is necessary for the efficient applications. Consequently, volumetric solar absorption flow system incorporating the absorber plate in the channel was investigated for different Reynolds numbers and solar concentrations. The influence of the location of the absorbing plate on the heat transfer and hydrodynamic losses was also examined in the channel. In order to increase the thermal storage capacity of the working fluid, phase change materials of 7% concentration was incorporated in the analysis. Lauric acid was used as phase change materials, and water was considered as the carrier fluid in the channel. The performance and pump power loss parameters were introduced to assess the thermal performance of the volumetric solar absorption system. It is found that the performance parameter attained the highest value for the absorber plate location at the top of the channel, which was about 10% higher than those corresponding to the other locations. This was more pronounced with increasing Reynolds number and solar concentration. The pump power loss parameter was the highest for the absorber plate location at the mid‐height of the channel. Copyright © 2013 John Wiley & Sons, Ltd.     

自然分层型水蓄冷槽布水器的模拟

介绍了自然分层型水蓄冷技术的国内外研究现状，分析影响自然分层型水蓄冷槽蓄冷特性的主要因素。采用CFD商业模拟软件FLUENT6对三种不同类型的布水器建立模型，模拟其充冷过程中的温度分布和速度场，并进行比较分析，优化布水器的设计。     

多孔布水器蓄冷水槽性能影响因素分析

自然分层蓄冷水槽中的出流流动是多孔平行射流流动，决定蓄冷效率的因素是布水器的性能及蓄冷水槽的几何因素。首先对布水器的性能进行了分析，通过对单孔出流控制方程组进行数量级分析得到了无因次方程组；对无因次方程组的分析得到了影响布水器出流性能的决定因素是R8数和Fr数，并且指出Re数和Fr数中应选择单孔的几何尺寸作为定型尺寸。进一步对蓄冷水槽出流流动的影响因素进行了分析，指出蓄冷水槽的几何因素对蓄冷效率有影响。     

A comparison of experimental thermal stratification parameters for an oil/pebble-bed thermal energy storage (TES) system during charging

Six different experimental thermal stratification evaluation parameters during charging for an oil/pebble-bed TES system are presented. The six parameters are the temperature distribution along the height of the storage tank at different time intervals, the charging energy efficiency, the charging exergy efficiency, the stratification number, the Reynolds number and the Richardson number. These parameters are evaluated under six different experimental charging conditions. Temperature distribution along the height of the storage tank at different time intervals and the stratification number are parameters found to describe thermal stratification quantitatively adequately. On the other-hand, the charging exergy efficiency and the Reynolds number give important information about describing thermal stratification qualitatively and should be used with care. The charging energy efficiency and the Richardson number have no clear relationship with thermal stratification.     

Performance Characteristics of a Volumetric Solar Receiver: Presence of an Absorber Plate with a Selective Surface

Performance characteristics of a concentrated solar volumetric absorber are examined numerically. The thermal system considered consists of parabolic trough, glass tube, absorbing plate, and slurry containing 7% lauric acid as a phase change material and water as a carrier fluid. To assess the effect of the absorbing plate on performance characteristics, two locations of the absorbing plate on the glass tube surface are incorporated in the analysis. A selective surface is considered at the absorber plate surface for improved absorption of solar radiation and reduced thermal emission due to temperature increase at the surface. Temperature ratio, gain parameter, and pump power loss parameters are introduced to quantify the performance characteristics of the volumetric receiver. The study is extended to include the effect of Reynolds number on the receiver performance characteristics. A heating model incorporating radiation, convection, and conduction is adopted to simulate the thermal process. It is found that the gain parameter of the concentrated solar volumetric receiver improves by 15% when the absorber plate is located at the left face of the glass tube opposing the trough surface. The effect of Reynolds number on gain parameter is found to be inconsiderable.     

Effect of obstacles with different opening means on thermal stratification in hot water storage tanks

为研究具有内置隔板的太阳能蓄热水箱隔板开孔尺寸及位置对其内部热分层效果的影响,对9种隔板开孔位置的太阳能蓄热水箱内温度场进行了数值分析,结果显示：在相同的流动参数及开孔面积条件下,隔板中心开1个圆孔的水箱热分层效果最好。对于多开孔的水箱,开孔位置对水箱内热分层影响不大,但对蓄热量影响显著。对于隔板中心开1个圆孔的水箱,在不同流动参数条件下,冷、热水出口温差随着冷水入口流速的增大呈先增后减的趋势,当冷水入口流速大于0.9 m/s时,减弱了热分层的稳定性。     

辅助火电机组调峰系统的储热参数设计研究

目的  为适应原有火电机组对新能源电力消纳的需求,提高其调峰能力是关键因素之一。 方法  储热系统作为燃煤热电机组“热电解耦”的重要方式,评价其参数匹配性具有工程参考价值。文章采用一种耦合储热装置来增加燃煤机组深度调峰能力的方法,并结合热负荷以及电负荷的功率曲线,对储热装置参数的影响规律展开了系统研究。 结果  结果表明：以区域负荷曲线为典型案例,随着储热罐的储热容量和充放热速率参数的提高,储热系统对机组的热负荷调节能力先逐渐提升并在分别在112.75 MW和129 37 MW·min时达到上限,此时深度调峰参数为77 MW左右;此外,储热容量与充放热速率具有一定的匹配关联,二者中瓶颈因素将直接制约系统的深度调峰性能。 结论  通过对储热和放热边界情景的积分,精确的展示了储热罐在辅助调峰过程的中作用,同时结合其运行策略的优化设计能够进一步分析储热辅助调峰系统的参数匹配关系,为后续火电机组耦合储热系统的参数设计提供参考。     

Optimization of a smooth flat plate solar air heater using stochastic iterative perturbation technique

Due to low heat transfer capability, the thermal efficiency of solar collectors is very low and various techniques are implemented to increase the performance of solar air heaters. There is a need for optimization of design and operating parameters for maximizing the thermal gain from the solar air heating systems. In this paper a stochastic iterative perturbation technique (SIPT) is implemented to obtain the optimized set of different system and operating parameters i.e. the number of glass cover plate, emissivity of the plate, mean plate temperature, rise in temperature, tilt angle and solar radiation intensity for different Reynolds number. The results obtained have also been compared with the results obtained from genetic algorithm and random search global optimization technique for smooth flat plate solar air heater.     

Thermal Modelling of a Natural Convection Solar Water Heating System with a Thermal Trap Collector

A mathematical model has been developed to study the performance of a solar water heating system with a thermal trap flat plate collector and in which the flow of water between the collector and the storage tank is maintained by natural convection. An expression has also been developed for the mass flow rate in terms of known parameters. The model yields exact expressions for the temperature of water in the storage tank as a function of time in terms of collector's parameters and the solar insolation. Numerical calculations have been performed to compare the performance of the hot water heating system with a thermal trap collector with the one with an ordinary flat plate collector.     

Experimental study of CPC type ICS solar systems

Extensive experimental study on solar water heaters, which were developed in our laboratory, is presented. These solar devices are integrated collector storage (ICS) systems with single horizontal cylindrical storage tank properly placed in symmetric CPC type reflector trough. In this paper we study ICS solar systems, which differ in storage tank diameter and correlate their thermal performance and the ratios of the stored water volume per aperture area and also per total external surface area. Based on the results of this study and aiming to achieve improved ICS systems, we considered an effective tank diameter and we extracted by outdoor tests the performance of a number of experimental models differing in the absorbing surface, reflector and transparent cover. We calculated the mean daily efficiency and the thermal loss coefficient during night of each system combination. In addition, 24 h and four days operation diagrams of the variation of water temperature of the studied ICS systems are compared with the corresponding diagrams of two flat plate thermosiphonic units with mat black and selective absorbing surface, respectively. The experimental results show that ICS system with selective absorbing surface, high transmissivity of the transparent cover and high reflectance of its reflector surface performs efficiently enough, both during the day and night operation, approaching the thermal performance of the corresponding thermosiphonic unit of flat plate collector with selective absorber.     

Molten-salt thermal energy storage in thermoclines under different environmental boundary conditions

Operation during the charge and discharge cycles of molten-salt thermoclines used for solar thermal energy storage depends strongly on the environmental boundary conditions to which the tanks are exposed. A comprehensive model which accounts for thermal transport in the molten-salt heat transfer fluid and the filler material in the tank is developed for exploring the effects of boundary conditions on thermocline performance. Heat loss from the tank under non-adiabatic boundary conditions is found to distort the temperature and salt flow distributions relative to the uniform conditions found in adiabatic thermoclines; as a result, the outflow temperature drops more rapidly in the former case. Such effects of non-adiabatic boundaries become insignificant at large salt-flow Reynolds numbers. As the Reynolds number increases beyond 250, the discharge efficiency of non-adiabatic thermoclines approaches that of the adiabatic counterparts. In the case of significant heat loss at the walls, the discharge efficiency of thermoclines increases with increasing Reynolds number, a trend that is opposite to that in adiabatic thermoclines.     

Energy and exergy analyses of an ice-on-coil thermal energy storage system

In this study, energy and exergy analyses are carried out for the charging period of an ice-on-coil thermal energy storage system. The present model is developed using a thermal resistance network technique. First, the time-dependent variations of the predicted total stored energy, mass of ice, and outlet temperature of the heat transfer fluid from a storage tank are compared with the experimental data. Afterward, performance of an ice-on-coil type latent heat thermal energy storage system is investigated for several working and design parameters. The results of a comparative study are presented in terms of the variations of the heat transfer rate, total stored energy, dimensionless energetic/exergetic effectiveness and energy/exergy efficiency. The results indicate that working and design parameters of the ice-on-coil thermal storage tank should be determined by considering both energetic and exergetic behavior of the system. For the current parameters, storage capacity and energy efficiency of the system increases with decreasing the inlet temperature of the heat transfer fluid and increasing the length of the tube. Besides, the exergy efficiency increases with increasing the inlet temperature of the heat transfer fluid and increasing the length of the tube.     

Experimental characteristics of a storage tank on a harvest-type ice storage system

This paper is concerned with the development of a new method for making and separating ice and saving floated ice by installing an evaporation plate at underwater within a storage tank. In a conventional harvest-type ice storage system, a tank saves ice by separating a formed ice from an installed evaporation plate, which is located above an ice storage tank as an ice storage system. A new harvest-type method shows better heat transfer efficiency than a convectional method. It is because the evaporation panel is directly contacted with water in a storage tank. Also, at a conventional system a circulating pump, a circulating water distributor and a piping are installed, but these components are not necessary in a new method. In this study two kinds of ice storage systems are experimentally investigated to study the thermal characteristics of ice storage tanks. The results show the applicable possibility and performance enhancement of a new type.     

Modeling of the CSU heating/cooling system

This paper resents a thermal simulation of the Colorado State University solar house. A computer model of the solar energy system was developed and computer runs were made using one year of meteorological data to determine the important design features. The system consists of a flat plate solar collector, main storage tank, service hot water storage tank, auxiliary heater, absorption air conditioner with cooling tower and heat exchangers between the collector and storage, storage and service hot water tank and storage and residence. This system very closely models the CSU house in operating mode one.The results are in the form of monthly integrated values for the pertinent energy quantities. In addition, results are presented which show the effect on the system performance of the collector tilt, collector area and number of covers.     

Influence of homogeneous‐heterogeneous reactions and induced magnetic field on the nonlinear thermal convective radiative Jeffrey liquid flow with heat source/sink

An analysis has been carried out to investigate the effect of homogeneous‐heterogeneous reactions and induced magnetic field on the unsteady two‐dimensional incompressible nonlinear thermal convective velocity slip flow of a Jeffrey fluid in the presence of nonlinear thermal radiation and heat source/sink. We assumed that the flow is generated due to injection at the lower plate and suction at the upper plate. We obtained a numerical solution for the reduced nonlinear governing system of equations via the shooting technique with fourth‐order Runge‐Kutta integration. We plotted the graphs for various nondimensional parameters, like Deborah number, heat source/sink parameter, nonlinear convection parameter, nonlinear radiation parameter, magnetic Reynolds number, Strommer's number, velocity slip parameter, strengths of homogeneous, heterogeneous reaction parameters and skin friction over the nondimensional flow, temperature, concentration profiles and magnetic diffusivity fields. Also, we calculated the numerical values of boundary properties, such as the skin friction and heat transfer rate. We noticed that the temperature of the fluid is enhanced with the radiation parameter, whereas the concentration decreases with increase of the magnetic Reynolds number. The present results have good agreement with published work for the Newtonian case.     

Experimental and CFD investigation of an ICSSWH at various inclinations

The integrated collector storage (ICS) is the type of solar water heater that has retained its existance for well over a century. The flat absorber plate ICS collector type is a relatively recent addition. Being effective, low cost and simple to manufacture, their importance has been further enhanced by the recent upsurge in efforts to effectively tap renewable energy resources. Having different inclinations based on latitude, the design of flat plate heaters can benefit from extensive amount of research on the topic of natural convection in inclined cavities. More than half-century of exploration on inclined cavities has witnessed added activity particularly in the last three decades. Despite this consistent research, efforts to apply the outcomes to the flat plate collectors have been few and collectors reported in the literature appear to be deficient in embedding the knowledge into the design parameters. For an ICS type heater, natural convection studies gain even more weight as the apparatus is functionally an assembly of two natural convection cavities: an air cavity (space between the absorber and cover plates) and a water cavity (water storage tank). An extensive review of previous studies on inclined cavities relevant to flat plate collectors has been complied and discussed. Experimental tests of the ICS heater have been conducted for controlled heat flux up to 400 W. The thermal performance of the heater is recorded experimentally at angles 0–60° from horizontal, in 15° intervals. CFD analysis is also carried out for the same and is found to be in good agreement with previous studies. It was found that for any given constant value of heat flux, the performance of the heater is a strong function of the angle of inclination. The optimum configuration of the heater for Edinburgh conditions (latitude 55°55′N) is also evaluated. The present study also covers the convective behavior inside the water tank, which has been neglected in the past. A step-by-step build-up approach is adopted to resolve water tank behavior as its treatment as a simple natural convention cavity is invalid. This article would serve as a design guide for developing heaters tailored for a specific geographical location.     

Thermal performance optimization of a flat plate solar air heater using genetic algorithm

Thermal performance of solar air heater is low and different techniques are adopted to increase the performance of solar air heaters, such as: fins, artificial roughness etc. In this paper an attempt has been done to optimize the thermal performance of flat plate solar air heater by considering the different system and operating parameters to obtain maximum thermal performance. Thermal performance is obtained for different Reynolds number, emissivity of the plate, tilt angle and number of glass plates by using genetic algorithm.     

Performance Estimation of a Latent Heat Thermal Energy Storage System with Different Packing Module Arrangements,Boundary Conditions and Melting Models

In a latent heat thermal energy storage system, the shape of the container for encapsulating the phase change material (PCM) and the arrangement of the PCM vessels within the thermal storage tank have a high influence on the performance of the thermal storage tank. In the present study, a newly designed PCM container was used to investigate the effect of the arrangement of the packing module on the performance of the thermal storage tank. To reflect an actual situation, the system should be modeled using the unconstrained melting model, which includes a density difference between the solid and liquid PCM, and also the convective boundary condition with heat transfer fluid should be applied. The amount of deviation from a real situation was analyzed for simplified models of a constrained melting model and an isothermal boundary condition, which have been commonly used in most previous works. The horizontal arrangement of the packing module showed higher performance than the vertical arrangement. Compared to the unconstrained melting model, the constrained melting model underestimated melting by 50 min and 70 min for the horizontal and vertical arrangements, respectively. Compared to the convective boundary condition, the isothermal boundary condition overestimated melting by 115 min and 100 min for the horizontal and vertical arrangements, respectively.