Three-dimensional analysis and investigation of the thermal and hydrodynamic behaviors of cylindrical storage tanks

A numerical analysis of the three-dimensional temperature and velocity fields in horizontal cylindrical storage tanks was performed. The phenomena of laminar natural convection and vertical stratification of temperature were considered. The developed three-dimensional transient computing code solves the equations of energy and momentum through the finite volume method. The simulation of fluid cooling process inside the tank showed the formation of stratified temperature profiles that matched those obtained experimentally. Based on several simulations, a correlation was proposed for determining the degree of thermal stratification inside the tank regarding thermal and geometrical parameters. From this correlation, an expression was proposed to predict the fluid temperature profiles along the time. This information is very important in many applications, such as in thermosiphon solar water heating systems, where the global efficiency of the system increases with the thermal stratification degree of the working fluid. Another case studied considered that the tank was connected to solar collectors, aiming at investigating the influence of the inlet jet position with and without a baffle plate on the preservation of the thermal stratification. Results showed that the baffle plate modified the velocity and temperature fields close to the inlet jet, allowing a better thermal stratification. Also the suitable choice of the inlet jet position allowed the formation of a more effective thermal stratification. Some other aspects of the internal dynamics of this kind of storage tank are presented and discussed. For the cases studied, the inlet jet next to the top led to a greater thermal stratification. However, it was verified that when the inlet jet temperature remains constant for a long period of time, and thus its temperature approaches the temperature of the water inside the tank, for the same height, the temperature profiles obtained become similar to the case of the inlet located at usual height of 2/3 of the diameter.     

Numerical Study of the Transient Cooling Process of Water Storage Tanks under Heat Losses to the Environment

The unsteady laminar heat transfer and fluid flow phenomena inside a water storage tank during its static mode of operation and submitted to heat losses to the environment has been investigated by means of CFD and heat transfer numerical simulations. A parametric study has been carried out considering several situations and varying the aspect ratio, insulation thickness, initial temperature, and tank volume. In order to characterize the cooling process, a nondimensional analysis has been performed. From the analysis of the results of several detailed numerical simulations, correlations for the Nusselt number at the lateral, top, and bottom walls have been obtained. The verification of the mathematical model assumed, together with the verification and validation of the results, have also been pointed out.     

冰球式蓄冷槽内冰球冻结和放热特性的数值模拟分析

在冰蓄冷系统中,冰球式蓄冰槽是一种常用的蓄冰设备;通过相变材料的潜热保存冷量。对冰球式蓄冰槽内冰球的冻结和解冻过程进行了数值模拟,通过对比冻结曲线和解冻曲线,探究了冰球不同的排布方式对冰球冻结速率和解冻速率的影响。结果显示:采用顺排方式的蓄冰槽冰球冻结时间为29 400 s,而采用错排方式的蓄冰槽冰球冻结时间为25 200 s,错排方式能够提高冰球在预冷阶段的冷却速率,从而降低冻结时间;在解冻过程中,错排方式融冰相变过程的时间明显小于顺排方式,而融冰后的显热阶段两种排列方式的差异不大。     

Cooling load reduction of building by seasonal nocturnal cooling water from thermosyphon heat pipe radiator

In this study, a concept of using thermosyphon heat pipe to extract heat from water in a storage tank to generate cooling water was proposed. Heat pipe condenser was attached with an aluminum plate and acted as a thermal radiator while its evaporator was dipped in the water storage tank. Cooling water in the tank could be produced during the nighttime and used to serve the cooling load in a room during the daytime. A heat transfer model to calculate the water temperature and the room temperature during both the nighttime and daytime was developed. The input data were ambient temperature, dew point temperature, area of the radiator, volume of cooling water and room cooling load. The experiment was setup to verify the heat transfer model. A 9.0 m² tested room with six cooling coils, each of 0.87 m² was installed at the ceiling, was constructed along with the 1.0 m³ water storage tank. A 500–2000 W adjustable heater was taken as an artificial load inside the room. A 6.36 m² radiator is installed on a 45° tilting roof of the tested room. The simulated results agreed very well with those of the experimental data. With the developed model, a simulation to find the sizing of the radiator area and the volume of cooling water for cooling water production during winter of Chiang Mai, Thailand was carried out. The cooling water was used for cooling during summer in an air‐conditioned room with different cooling loads. The parameters in terms of room temperature, radiator area, volume of cooling water, cooling load and UA of cooling coil were considered to carry out the percent of cooling load reduction. Copyright © 2009 John Wiley & Sons, Ltd.     

Numerical analyses of the impact of plates for thermal stratification inside a storage tank with upper and lower inlet flows

In this paper, numerical analyses have been carried out to describe the velocity and temperature fields inside a storage tank to be used in a solar system under various boundary conditions with upper and lower inlet flows. The aim of the study was to evaluate the effect of different plate sizes situated opposite the inlet in order to increase the thermal stratification. A numerical model was developed, and validated using experimental results. Two different initial temperature assumptions were taken into account along with a two-layer configuration. Two cases were analysed, the thermocline in the vicinity of the plate and the thermocline in the middle of the tank. In the latter case, the plate diameter had little impact, but moving the thermocline closer to the plates resulted in the diameter having a greater influence. It was also found that larger plates made it possible to preserve stratification with at larger inlet flow rates than the flow rates of the conventional low flow systems. Cold water inflow into the top of the tank was also studied. The influence of the plate diameter for the colder inflow was examined along with two temperature differences between the inlet and the tank. It was found that the diameter of the plate and the distance between the plate and the top of the tank have a significant effect on the temperature stratification within the tank when cold water enters at the top of the tank.     

冰蓄冷空调蓄冰罐特性的研究

本文通过对某大厦冰蓄冷空调系统的运行情况的测试,详细研究和分析了蓄冰罐在不同流量下的蓄冷和释冷特性,在融冰工况时,蓄冰罐进口温度出口温度和释冷量的影响。     

Numerical Study of Convection Melting Inside Ice Storage Systems by the Lattice Boltzmann Model

Abstract

The internal melt ice-on-coil tank with horizontal pipes is widely used in ice storage systems. The tank’s discharge process is greatly affected by the natural convection process that is caused by melting of the phase change material outside the pipes. To achieve an optimal arrangement of the pipes, a double-population lattice Boltzmann model was developed to simulate the transient solid-liquid phase change behavior in a section of an internal-melt ice-on-coil thermal storage tank with nine aligned built-in horizontal pipes. The evolutions in the phase change interface and melting rate was illustrated with different pipe shapes and pipe connections. Based on the melting rate, the whole melting process was divided into three stages: sharp decrease stage, continuous decrease stage, and snail-melting stage. The numerical results showed that a high melting rate was obtained by preferentially assigning the high-temperature pipes to the upper part of the tank, while a stable melting rate could be obtained when high-temperature pipes were preferentially assigned to the bottom part of the tank.     

1MW塔式太阳能电站蓄热系统模拟分析

以1MW塔式太阳能电站蓄热系统为研究对象,通过理论分析,建立了管壳式换热器、热罐、冷罐以及蒸汽蓄热器的动态数学模型,经验证模型具有合理性.在此基础上对蓄热系统的充、放热过程进行了模拟计算,分析研究了换热器A和C的动态特性、冷热罐内油温的动态变化以及蒸汽蓄热器的充、放热特性.模拟结果为双级蓄热系统的过程控制提供了理论依据.     

Effect of water inlet velocity on thermal stratification in a mantled hot water storage tank

Thermal stratification in a mantled hot water storage tank is analysed numerically for different water inlet velocities. The aim is to obtain higher thermal stratification and supply hot water for usage as long as possible. Twelve different water inlet velocities to the hot water storage tank are considered. The numerical method is validated by comparing its results against experimental and numerical results from the literature. It turned out that the results obtained from the numerical analysis have shown very good agreements with the results from previous works. As a result, the water temperature in the tank increases with the increase of the water inlet velocities to the mantle but this increment is not proportional. After a period of operation of 7.2 h, which corresponds to the average sunshine duration in Turkey, temperature increments of 6.5 and 35 K have been estimated for the hot water inlet velocities of 0.01 and 0.3 m s^?1, respectively, at a radial distance of 0.1 m and a height of 1 m inside the storage tank. Copyright © 2005 John Wiley & Sons, Ltd.     

Performance analysis of a new tank configuration applied to the natural gas storage systems by adsorption

This work presents a numerical study of a new tank configuration applied to natural gas storage systems by adsorption. The traditional tanks employed in natural gas storage by adsorption reveal serious limitations for use in fast charge systems because of their inefficiency in the dissipation of adsorption heat. In order to eliminate the detrimental effects of adsorption heat, and to make viable the fast charge of gas in automotive tanks, a vessel made up of several tubes, compacted with activated carbon, was proposed. In the charge process, the gas circulates through the tank and all non-adsorbed gasses pass through an external heat exchanger installed close to the gas source of the refueling station. The numerical results obtained in the present work showed that the charge time of the new system can vary from 50 to 200 s, depending on the applied mass flow rate. These time periods are considered satisfactory for fast charge conditions. Another advantage of this new system is that there will be no need to include the accessories employed in traditional tanks, such as: fins, perforated tube in the tank center and a cooling external jacket, which would increase the complexity of the vessel design.     

Parametric Analysis of Solar Heating and Cooling Systems for Residential Applications

Abstract

In this paper, a parametric analysis of two solar heating and cooling systems, one using an absorption heat pump and the other one using an adsorption heat pump, was performed. The systems under investigation were designed to satisfy the energy requirements of a residential building for space heating/cooling purposes and domestic hot water production. The system with the absorption heat pump was analyzed upon varying (i) the solar collectors’ area, (ii) the volume of the hot water storage, (iii) the volume of the cold water tank, and (iv) the climatic conditions. The system with the adsorption heat pump was evaluated upon varying (i) the inlet temperature of hot water supplied to the adsorption heat pump, (ii) the volume of the hot water storage, (iii) the volume of the cold water tank, and (iv) the climatic conditions. The analyses were performed using the dynamic simulation software TRNSYS in terms of primary energy consumption, global carbon dioxide equivalent emissions, and operating costs. The performance of the solar heating and cooling systems was compared with those associated with a conventional system from energy, environmental and economic points of views in order to evaluate the potential benefits.     

Analytical approach to ground heat losses for high temperature thermal storage systems

A new approach to estimate the heat loss from thermal energy storage tank foundations is presented. Results are presented through analytical correlations based on numerical solutions for the steady‐state heat conduction problem for thermal energy slab‐on‐grade tanks with uniform insulation. Model results were verified with other well‐established benchmark problems with similar boundary conditions and validated with experimental data with excellent agreement. In addition to the TES foundation heat loss, new correlations for the maximum temperature and for the radial evolution of the temperature underneath the insulation layer are also provided, giving important information related to the tank foundation design. The correlated variables are of primordial importance in the tank foundation design because, due to the typical high operating storage temperatures, an inappropriate tank foundation insulation would lead not only to a not desired loss of energy but also to an inadmissible increase of the temperatures underneath the insulation layer, affecting the structural stability of the tank. The proposed correlations provide a quick method for the estimation of total tank foundation heat losses and soil maximum temperature reached underneath the insulation layer, saving time, and cost on the engineering tank foundation design process. Finally, a comprehensive parametric analysis of the variables of interest is made and a set of cases covering a wide range of tank sizes, insulation levels, depths to water table, and storage temperatures are solved.     

Efficient heat utilization of a space cooling system over a range of atmospheric temperatures and solar radiation

Behavior of a space cooling system using the diurnal range of atmospheric temperatures in the summer is examined by numerical simulation. The system consists of radiators, a thermal energy storage tank, fan‐coil coolers, and an electric cooler. Conditions for the calculation were obtained from actual weather records and characteristics of appliances. Total consumption of electric power was used for evaluating the system performance. The larger the volume of the storage tank becomes, the smaller the consumption. The consumption is smaller when the radiator is operated for a period of some limited hours than when operating through the night. To decrease the consumption, there are optimum values of the flow rate of the pumps and in the on–off temperature of the fan‐coil cooler. Stratified tanks could reduce the consumption compared with mixed ones. This system has the possibility of reducing demand for electricity or of shifting the demand to off‐peak hours. © 2001 Scripta Technica, Heat Trans Asian Res, 30(2): 126–138, 2001     

Performance of solar-desiccant cooling system with Silica-Gel (SiO2) and Titanium Dioxide (TiO2) desiccant wheel applied in East Asian climates

This paper shows the numerical investigation of the developed solar-desiccant cooling system applied in the East Asian climatic conditions with two different desiccant wheel coating materials – the Silica-Gel (SiO₂) and the Titanium Dioxide (TiO₂). The developed and validated numerical model of the system is currently used in the present study incorporating the two new materials in the desiccant wheel. The system was applied in temperate climate (Beijing and Tokyo), subtropical climate (Taipei and Hong Kong) and tropical climate (Manila and Singapore). The study showed that the specification of the solar-desiccant cooling system varies depending on the climatic conditions. It showed that the required flat plate collector area was getting larger from the temperate climate to the tropical climate. The storage tank requirement was getting bigger in the tropical climate compared to the subtropical and temperate climate. The volumetric flow rate of air was getting higher from temperate climate to tropical climate. In the comparison of the two materials, it was found that the Titanium Dioxide (TiO₂) can support lower indoor temperature and humidity ratio than the Silica-Gel (SiO₂) with the same specification of the solar thermal system and desiccant cooling system. In general, the solar-desiccant cooling system can provide the required indoor temperature and humidity ratio. However, for the hot and humid climate such as in tropical, large size of the solar thermal system is needed. In addition, higher volumetric flow of air to support the high cooling load is required. With regard to the new material, Titanium Dioxide, it is proven to be a good alternative material since it can provide lower indoor temperature and humidity ratio with higher cooling performance than the Silica-Gel.     

Experimental results of a sensible heat storage system for residential and light commercial application

This paper presents the performance results for a sensible heat storage system. The system under study operates as an air source heat pump which stores the compressor heat of rejection as domestic hot water or hot water in a storage tank that can be used as a heat source for providing building heating. Although measurements were made to quantify space cooling, space heating, and domestic water heating, this paper emphasizes the space heating performance of the unit. The heat storage system was tested for different indoor and outdoor conditions to determine parameters such as heating charge rate, compressor power, and coefficient of performance (COP). The thermal storage tank was able to store a full charge of heat. The rate of increase of storage tank temperature increased with outdoor temperature. The heating rate during a charge test, best shown by the normalized rate plots, increased with evaporating temperature due to the increasing mass flow rate and refrigerant density. At higher indoor temperature during the discharge tests, the rate of decrease of storage tank temperature was slower. Also, the discharge heating rate decreased with time since the thermal storage tank temperature decreased as less thermal energy became available for use. Copyright © 1999 John Wiley & Sons, Ltd.     

Flash vapor solar powered desalination unit optimization study

In this work a flash vapor desalination unit was assembled which consisted of a solar water-cooled system of three flat plate collectors and a storage tank. A copper coil was immersed inside the tank, the saline water was pumped through the coil and delivered to the flash tank, the vapor then being flashed to the surroundings. The saline water level in the flash tank was taken as a measure of the quantity flashed. The temperature of both cooling and saline water at the inlet and outlet were measured; solar irradiation and wind speed were also recorded. The mass ratio (R) which is the ratio of the rate of the mass flashed to that of saline water circulated and the total unit efficiency were taken as dependent variables. Graphical and numerical optimization methods were conducted to find the values of the dependent variables. The optimization algorithm is described and the results are shown.     

Modelling of a thermosyphon solar water heating system and simple model validation

A thermosyphon solar water heater consisting of two flat plate collectors of total aperture area of 2.7 m² and 150 l storage tank is modelled using TRNSYS. Simple experiments were conducted in order to validate the model. During the experiments weather conditions were measured every 10 min and integrated over an hour. The temperature of the water in the storage tank was also measured at the beginning and at the end of the day. The storage tank temperature rise was used to validate the model by using the actual weather data as input to the program. Validation tests were performed for 25 days spread over 6 months and the mean deviation between the predicted and the actual values of water temperature rise is 4.7% which is very satisfactory. Subsequently, long term system performance is estimated by using TRNSYS model run with the weather values of TMY file for Nicosia, Cyprus. The annual solar fraction obtained was 79% and the system could cover all the hot water needs of a house of four people during the three summer months. The maximum auxiliary energy was needed during the months of December and January (about 280 MJ/month). In addition, an economic analysis of the system was carried out. The pay-back time of the system was found to be 8 years and the present worth of life cycle savings was found equal to C£ 161.     

Performance study of a partitioned thermally stratified storage tank in a solar powered absorption air conditioning system

Accurate modeling of solar heating or cooling with storage generally requires an accounting of the stratification within such storage tank, since overall system performance is significantly affected by the storage temperature distribution. In this study, a simple one-dimensional multi-node approach, taking into account of the axial heat conduction between nodes, has been used to theoretically analyze temperature stratification in the thermal storage tank. The results indicate that, for less collector area, the heat removal factor plays a major role in increasing the system performance, than the thermal stratification. Also, an optimum ratio of tank volume over collector area exists for a solar powered absorption air conditioning system. This paper also reviews the state of the art on different kinds of variable inlet design, and a simple new inlet design (partitioning the tank) has been introduced to effect better thermal stratification in storage tank.     

Analysis of natural circulation solar water heating systems

This paper presents an analysis of the performance of a solar water heating system with natural thermosyphon circulation between the collector and the storage tank. The analysis is based on the formulation by Ong except that provision for withdrawal of hot water from the tank (for domestic/ industrial use) has been made in the energy balance equation; further in contrast to the use of the finite difference method by Ong, explicit expressions have been obtained. The results of the present analysis (in the absence of withdrawal of hot water from the tank) are seen to be in better agreement with experiments than the corresponding results of Ong, obtained by use of the finite difference method.

Numerical results, corresponding to hot water retrieved from the storage tank, have been presented for two modes of hot water withdrawal viz. the constant flow rate and constant mean storage tank water temperature.     

Multi-objective optimization of cascade storage system in hydrogen refuelling station for minimum cooling energy and maximum state of charge

Compared with single-stage hydrogen storage refuelling, cascade storage refuelling has more advantages and significantly reduces cooling energy consumption. In the cascade system, the parameters of cascade storage tanks are critical, especially the initial pressure and volume. This article analyzes the thermodynamic processes in a cascade hydrogen refuelling station (HRS) and establishes the simulation model in Matlab/Simulink platform. The state of charge (SOC) of the onboard storage tank and the cooling energy consumption of the refuelling system are obtained from different initial pressures and volumes of the cascade storage tanks by using the simulation model. These data are introduced into the artificial neural networks in Matlab to generate a relationship between the decision variables and objective functions. The decision variables are optimized to minimize the cooling energy consumption and maximize the SOC through the genetic algorithm and Pareto optimization. So that optimal initial pressure and volume of the cascade storage tanks are determined. The research shows that when the ambient temperature is 293.15 K, and the SOC is 0.98–0.99, using the optimal initial pressure and volume of the cascade storage tanks can reduce the cooling energy consumption by up to 11.43%, compared with the baseline situation. Among the factors affecting cooling energy consumption and SOC, initial pressure is more sensitive than volume, so optimizing initial pressure, especially for the high-pressure cascade storage tank, seems more meaningful than volume. This research is instructive for the construction of the cascade HRS.