Insulated solar storage tanks

This paper presents the theoretical and experimental investigation of an insulated parallelepiped, outdoor solar, water-filled storage tank of size 1 m × 0.5 m × 0.3 m, that is made from galvanized iron. The absorption coefficient of the insulating material has been determined. The effects of plastic covers and insulation thickness on the water temperature and the energy gained or lost by water are investigated. Moreover, the effects of insulation thickness on the temperature profiles of the insulating material are discussed. The results show that the absorption coefficient decreases as the insulation thickness increases. Also, it is found that the glass wool insulation of 2.5 cm thickness has the best results compared with the other thicknesses (5 cm, 7.5 cm, and 10 cm) as far as the water temperature and the energy gained by water are concerned.     

Mixed convective flow and thermal stratification in hot water storage tanks during discharging mode

An analysis of transient, two dimensional, mixed convection and thermal stratification in cylindrical hot water storage tanks is presented. The governing equations together with inflow and outflow boundary conditions are written for laminar mixed convection flow using a finite volume based computational code in the dynamic discharging mode based on Boussinesq approximations and conjugate heat transfer. The equations are solved numerically and the results are obtained for aspect ratios of the tanks ranging from 1 to 4 in the Richardson number range of 10⁵ to 10⁸ using a finite volume based computational code. The dynamic discharging mode is considered using a conjugate heat transfer model. The transient temperature profiles in the bulk fluid reveal reduced mixing at higher Richardson numbers during discharging process. The system performance in the dynamic mode of operation is defined by a Mix Number and discharging efficiency parameter. Mixing at the bottom of the tank due to inflow of low temperature water from the load is found to have significant influence on the storage efficiency. The discharging efficiency decreases with Fourier number due to increased thermal degradation with time.     

Heat-transfer considerations for large liquefied-natural-gas storage tanks

Natural gas is used worldwide as a practical energy source. In order to have a concentrated form of energy, natural gas is liquefied and stored under a pressure slightly above atmospheric and at a corresponding temperature just above its normal boiling point (112 K). This investigation presents a general steady-state study of the heat transfers into such a storage tank. Two mathematical models are proposed in order to help improve the thermal design process for such a tank. It is concluded that the aspect ratio (i.e. height-to-radius) of such a tank for the minimum rate of heat gains should be approximately unity. This value differs from the economically most favourable aspect ratio, i.e. that leading to the least total financial expenditure over the lifetime of the tank. However, the two models allow a complete simulation of the thermal costs. As the unit cost of fuel rises relative to other costs (e.g. for the construction of the tank), the overall most economic tank design approaches that of the optimal thermal design of tank as deduced in this investigation.     

Entrance effects in solar storage tanks

A theoretical and experimental analysis of water jets entering a solar storage tank is performed. CFD calculations of three inlet designs with different inlet flow rates were carried out to illustrate the varying behaviour of the thermal conditions in a solar store. The results showed the impact of the inlet design on the flow patterns in the tank and thus how the energy quality in a hot water tank is reduced with a poor inlet design. The numerical investigations were followed by experiments. A test solar store, similar to the store investigated by numerical modelling was constructed with cylindrical transparent walls so that the flow structures due to the inlet jets could be visualized. With the three inlets, nine draw-off tests with different inlet flow rates were carried out and the temperature stratification in the tank was measured during the draw-offs. The experimental results were used in an analysis using the first and second law of thermodynamics. The results showed how the entropy changes and the exergy changes in the storage during the draw-offs influenced by the Richardson number, the volume draw-off and the initial tank conditions.     

Phase change characteristic study of spherical PCMs in solar energy storage

This paper investigates the phase change behavior of 65 mol% capric acid and 35 mol% lauric acid, calcium chloride hexahydrate, n-octadecane, n-hexadecane, and n-eicosane inside spherical enclosures to identify a suitable heat storage material. Analytical models are developed for solidification and melting of sphere with conduction, natural convection, and heat generation. Both the models are validated with previous experimental studies. Good agreement was found between the analytical predictions and experimental study and the deviations were lesser than 20%. Heat flux release at the wall, cumulative energy release to the external fluid, are revealed for the best PCM. The influence of the size of encapsulation, initial temperature of the PCM, the external fluid temperature on solidified and molten mass fraction, and the total phase change time are also investigated.     

Optimization of hydrogen storage in metal-hydride tanks

The storage time of hydrogen in metal-hydride tanks (MHTs for short) is strongly influenced by the rate at which heat can be removed from the reaction bed. In the present work a two-dimensional mathematical model is developed and validated against experimental results. This model is used, first, to evaluate the impact of the tank wall thermal mass on the hydriding process. Walls in steel and in brass are tested and the obtained results show that there is no significant effect on hydrogen storage time. Then, the established model is used to study the dynamic behaviour inside various designs of MHTs: i) a cylindrical tank, ii) a cylindrical tank with external fins, iii) a cylindrical tank with a concentric tube filled with flowing cooling fluid and iv) a cylindrical tank with a concentric tube equipped with fins. Optimization results indicate that almost 80% improvement of the storage time can be achieved over the case where the tank is not optimized.     

Thermal stratification in ribbed liquid hydrogen storage tanks

A significant decrease in the degree of thermal stratification is demonstrated by improvising transverse wall ribs on the inner cylindrical surfaces of large liquid hydrogen storage tanks. The ribbed surfaces are modeled as fins and a conjugate transient heat transfer problem is formulated for predicting flow currents and heat transfer. Turbulent Rayleigh numbers between 1.2×10¹²$1.2\times {10}^{12}$ and 6×10¹⁶$6\times {10}^{16}$ are considered. A stratification parameter based on the moment of energy is defined to quantify the degree of stratification and this parameter is seen to be about 30% lower for the ribbed tanks. The degree of stratification is not sensitive to changes in the ratio of the rib height to the spacing between the ribs. The transient free convection is shown to be characterized by the parameter Fo×Ra^0.15$\mathrm{Fo}\times {\mathrm{Ra}}^{0.15}$. The process of stratification takes place more slowly in the ribbed tanks than in smooth-walled tanks. The free convective heat transfer coefficient for tanks having ribbed surface is also seen to be significantly lower. Incorporation of ribs over the inner surface of the insulated tanks is demonstrated to offer a simple means of reducing the stratification and boil-off losses.     

New computation method for stratification pipes of solar storage tanks

The efficiency of low-flow solar systems is strongly influenced by the quality of the thermal stratification in the storage tank. The better a thermal stratification can be generated and maintained, the higher can be the yield of the solar system. Fluid mechanical charge systems are often used for this purpose, which cause, however, undesirable sucking effects. Therefore, the knowledge of the appearing fluid flows as well as the knowledge of the consequences of constructive changes are very important for the design of such charge systems. However, simulations with CFD (Computational Fluid Dynamics) are very costly and time-consuming. In this article a new and much simpler computation method is introduced making the determination of the individual fluid flows and the estimation of the effects of constructive changes possible. The computations can be carried out within short time. The comparison with CFD gives a qualitatively good agreement for a simple charge system. The results of a constructive modification of the charge system reducing the sucking effect are discussed. The remaining quantitative differences result from the discrepancies between the non-ideal behaviour of the real fluid and the model assumptions and point out improvement potentials.     

An analysis of bottom entry nozzles for atmospheric storage tanks

In this paper, we describe the analysis of soil settlement in the vicinity of a bottom entry nozzle of an atmospheric storage tank for several practical loadings. These are the pressure of the liquid stored in the tank, the body force of the rock and soil foundation, and an external moment applied to the nozzle where it emerges from the foundation. The foundation is modelled as a three-dimensional elastic medium which can support no tensile stress and is composed of two materials (rock and soil) with an elastic cylindrical shell embedded in the rock phase. The solution is accomplished with a general-purpose finite element program (ICES—STRUDL II). The overall conclusion reached is that bottom entry nozzles are acceptable for tankage installed on reasonably good soil.     

Design and performance of hot-oil storage tanks

Because the ratio of surface area to capacity decreases with increasing volume for a particular shaped hot-oil storage tank, there is a trend towards larger tanks in order to incur only relatively small heat losses per unit capacity. The present investigation suggests, for conditions encountered in the UK, that the aspect ratio (i.e. height-to-radius) for least heat losses from bare or fully insulated hot-oil tanks should be approximately 0·4. If the cylindrical walls of the tank are to be insulated, the roof remaining bare, then the optimal aspect ratio should be appropriately greater. However, when designing a tank, the running cost is only one of several considerations—although an increasingly important one as fuel costs inflate—the capital investment in the tank and site also radically affect the choice. The economically most favourable aspect ratio, i.e. that leading to the least total financial expenditure over the lifetime of the tank, is considerably in excess of the optimal aspect ratio corresponding to minimum rate of energy loss. This maximum energy thrift or minimum financial expenditure dichotomy is now serious when neither sufficient, cheap fuel nor adequate industrial investment is available.     

Temperature distributions inside electrical hot-water storage tanks

The presented mathematical model is solved analytically to determine: (i) the effect of different design parameters on the thermal stratification within the tank; (ii) the time required by the tank to supply water with a specified outlet temperature.     

Performance study of one-dimensional models for stratified thermal storage tanks

Two basic approaches are used to model the temperature distribution in thermal storage tanks for solar domestic hot water (SDHW) systems. In the multinode approach, the tank is divided into N nodes, with an energy balance written for each node. This approach results in a set of N differential equations that can be solved for the temperatures of the nodes as a function of time. In the plug flow approach, segments of liquid of different temperatures and sizes are assumed to move through the tank in a plug flow manner. The sizes of the fluid elements are determined mainly by the simulation time step and the flow rates. Whenever the incoming fluid from the heat source is colder than the fluid at the top of the tank, “plume entrainment” occurs. A model describing plume entrainment has been incorporated into both the multinode and the plug flow models in the TRNSYS program[1]. A performance study of the TRNSYS tank models has been carried out with experimental data from two different sources. Three performance numbers have been defined for quantifying the accuracy of the models compared with experimental data. Recommendations are given as to which tank model should be used under which conditions.     

Integral approximate solution for the charging process in stratified thermal storage tanks

This paper presents the approximate integral solutions to the one-dimensional model describing the charging process of stratified thermal storage tanks with fluid mixing at the inlet. The temperature is assumed to be a form of the Fermi–Dirac distribution function, which can be separated into two sets of cubic polynomials for the hot and cold sides of the thermal boundary layers. The proposed approximate integral solutions are compared with previous works on approximate analytic solutions and show reasonable agreement. This approach, however, benefits from reduced mathematical complexity as compared with the complicated solution form and unstable convergence of the series solution found in the previous analytic solutions. For the ideal case of no fluid mixing at the inlet, the thermocline thickness is proportional to the square root of time and reversely proportional to the flow rate. However, if the fluid is mixed perfectly in the region near inlet, the thermocline thickness could be thicker as the flow rate increases because of the increased mixing region caused by promoted flow mixing in this region. Thus the optimal flow rate depends on the relationship between the flow rate and the size of the mixing region.     

Comparison between models for the simulation of hot water storage tanks

Numerical and experimental analyses of velocity and temperature fields inside a tank submitted to internal natural and mixed convection are presented in this paper. The numerical analyses were performed with two approaches: one using a two-dimensional model in cylindrical coordinates through the finite volume method and another using a one-dimensional model. A turbulence model for low Reynolds numbers was added to the two-dimensional model in mixed convection regime. The two-dimensional model was experimentally validated and then adopted as reference. Its results were compared to those obtained with one-dimensional models (combined with certain computational artifices described in this work) with a good agreement.     

Theoretical model of the charging process for stratified thermal storage tanks

In order to provide an upper limit of the performance for stratified thermal storage tanks, a theoretical model of the charging process is studied. First, by introduction of reasonable assumptions in addition to the perfect piston flow, an idealized model is developed. Governing equations derived from the model appear to be characterized by the only parameter, Peclet number. Application of the Laplace transform technique to the equations results in a simple closed-from solution for the transient temperature distribution. The model is validated by examining a distinction from a simpler one which is heat conduction between two semi-infinite regions in contact with a moving interface. Temperature profiles for representative cases as well as the effect of the Peclet number on them are illustrated and discussed. Also, the storage efficiency is analytically expressed in terms of the Peclet number. The efficiency by the present model presents similar trends, but is smaller in value in comparison to that by the semi-infinite case. Consequently, the feasible range of the storage efficiency by the present model, which is the difference between the upper and lower limits of the efficiency, becomes more specific. For the convenience of the usage, a simple correlation of the efficiency is proposed as a function of the Peclet number.     

Large DHW solar systems with distributed storage tanks

The thermal behaviour of a central DHW solar system, the design of which is based on a new Central Collection-Separate Storing (CCSS) approach, has been investigated theoretically. The common practice for large DHW solar systems, of employing a central storage and delivery facility, has been shown in the past to exhibit a rather poor performance and considerable heat losses. This is due to the extensive lengths of pipework required for both the transfer of solar energy and the delivery of hot water. The CCSS solar system presented can overcome the above problems by employing separate storage tanks for each family, thus being best suited for multistory buildings. The simulation analysis has revealed a number of interesting features for the system performance: (i) the collected energy is distributed to all users in a fair manner, irrespective of their distance from the collector field and the daily hot water consumption profiles; (ii) an energy saving behaviour is most likely to evolve by most users, since the auxiliary energy consumptions are charged individually (unlike in large DHW solar systems with central water storage and delivery); and (iii) high values of solar fractions, comparable with those attained by thermosiphon systems, have been derived.     

Design of steel storage tanks with spherical bottoms

A new approach to the design of a vertical cylindrical storage vessel with a spherical bottom, based on bending theory instead of membrane analysis of shells, is proposed. The design of cylindrical shells employs the analysis of a cylindrical shell subjected to a wind load. Axial membrane stresses along the height caused by wind loading are evaluated from the prepared charts for various R/h ratios. These stress values are multiplied by a multiplication factor to obtain the meaningful bending stresses. The design of the spherical bottom involves the analysis of a shallow spherical shell on a Pasternak foundation. The distribution of membrane and bending stresses in the spherical bottom is plotted against the radial distance. To facilitate the work of the designer charts have been provided for rapid determination of various terms involved in the calculation of net stresses in the bottom.     

Solar ICS systems with two cylindrical storage tanks

Two types of ICS solar water heaters designed, constructed and tested. The systems consist of two cylindrical storage tanks, which are connected in series and are horizontally incorporated in a stationary asymmetric CPC type mirror. The efficient operation of the system is due to the thermal losses suppression of the two inverted cylindrical surfaces and the effective use of the two tanks during sunshine period. Low cost and durable materials are used to construct the systems. The mean daily efficiency and the thermal performance of the hot water storage during night are calculated from outdoor experimental data. The results show that the proposed ICS systems are efficient and suitable for practical use as DHW systems.     

固液相变蓄热技术的研究进展

综述了相变蓄热材料、相变传热问题求解方法、典型相变传热过程以及相变潜热蓄热系统(LHTES)优化设计及强化传热等诸多固液相变蓄热技术相关问题的研究进展情况     

Analytical and experimental investigation of thermal stratification in storage tanks

An analytical and experimental investigation of transient turbulent two-dimensional charging and discharging of a sensible heat storage tank has been conducted. Parametric studies showed that the turbulent mixing factor due to hydrodynamic disturbances at the inlet ports is the most significant item in the performance of thermal stratification storage tanks. Furthermore, the effect of the aspect ratio and convection at the walls in promoting stratification have been studied. Comparison with experimental data showed the capability of the present analytic approach to accommodate, with a satisfactory degree of accuracy, such problems.