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International Journal of Advances in Science and Technology,
Vol. 1, No. 5, 2010
Effect of Operating Parameters on Thermal Stratification in Sensible Heat Storage Systems
- A Simulation Study
Dr.K.Hema Chandra Reddy1, Dr.G.Naga Malleshwara Rao2*
1. Director Academic & Planning, JNTUA, Anantapur,A.P.
2. Professor (Mech. Engg.), Shri Shirdi Sai Institute of Science and Engineering, Anantapur,A.P.
Email: nmgujjar@gmail.com, mallesh@teachers.org
* Author for correspondence
Abstract
Renewable energy supplies are steadily gaining increasing importance in all the countries. In particular, solar energy
being non-polluting, clean and inexhaustible has received wide attention among scientists and engineers. Though there are
many advantages, an important factor is that solar energy is time dependent energy source with an intermittent character.
Hence some form of thermal energy storage is necessary for the most effective utilization of this energy source. Most of the
thermal energy storage systems in use rely on the specific heat or sensible heat of the storage material, such as water, oil and
rock beds and they are known as sensible heat storage systems.
Thermal performance of sensible heat storage systems mainly depends on the thermal stratification. Thermal
stratification in solar tanks is essential for a better performance of energy systems where these tanks are integrated.
The objective of this paper is to study the thermal stratification of energy storage tanks, by means of experiments and
computer simulations. The current study is proposed in order to quantify the thermal stratification inside the storage. A special
attention is given to the validation of the model considered in this study by using a commercial CFD tool ANSYS
WORKBENCH-CFX. Good thermal stratification is vital in order to achieve high thermal performance on solar thermal
storages where the tank design is one of the most important elements to accomplish it. As it is known, the best tank design
depends on several factors such as the system typology and storage size. In this paper an attempt is made to quantify thermal
stratification by considering three such influencing parameters with the help of experiments and computer simulations. With
this work, it is possible to conclude what are the important design parameters to build up and best preserve thermal
stratification in a solar energy storage tank.
Keywords: Thermal energy storage, Sensible heat storage, thermal stratification, CFD, ANSYS-CFX, Influencing factors, Tank
design.
1.Introduction
There are many good methods and sources used to store warm thermal energy. These include solar heaters, solar ponds,
geothermal storage methods, and many others. The advantage of warm thermal energy storage is that usually, the warm thermal
energy storage (TES) is obtained from an abundant and ecologically friendly source, such as the sun. As a result, heat storage is
usually very cost friendly and good for the environment.
A mantle tank is a cylindrical storage tank surrounded by an annulus through which hot liquid from the collector flows
thereby transferring energy to the tank contents. The separating wall is the heat exchange surface. A possible advantage of this
flow configuration is reduced internal tank mixing and, as a consequence, improved temperature stratification resulting in higher
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solar collector efficiency. Mantle heat exchangers are limited to small scale hot water systems because the mantle heat transfer
area to volume ratio decreases with increasing storage capacity.
Figure 1 shows a solar domestic hot water system with a vertical mantle heat exchanger. The mantle fluid is slowly
pumped down through the mantle in order to exchange heat between the mantle fluid and the water in the tank.
A schematic diagram of a solar domestic water heater with a mantle heat exchanger as shown in fig1.
Figure1. Solar water heater with mantle heat exchanger
1.1. Salient Features of the Mantle Heat Exchanger System
A). Thermal Stratification: The mantle tank system is one of the simplest ways of producing high heat exchanger
effectiveness while promoting thermal stratification. The mantle configuration provides a large heat transfer area and effective
distribution of the collector loop flow over the wall of the tank. Most of the incoming mantle fluid seeks the thermal
equilibrium level in the mantle, and thermal stratification in the mantle and the inner tank is not disturbed.
B). Cost: The cost of tank is less compared to the increased performance initiated by the fine thermal stratification.
C). Size limitation: The mantle tank design is not suitable for large low flow SDHW systems, as the heat transfer area
gets too small for tanks with volumes over 800-1000 lts.
2. Literature Review
Zurigat [1], Shyu, [2] investigated the factors influencing the thermal stratification degradation in solar energy tanks are
forced convection flow through the tank, heat loss to the surrounding environment, thermal mixing at inlet, natural convection
flow induced by conduction within the tank walls and heat diffusion inside the tank due to the vertical temperature gradient within
the tank.
Lavan and Thompson [3] performed an experimental study on a thermally stratified, vertical hot water storage tank.
Experimental measurements were made on various height/diameter ratios, inlet port location and geometry, inlet-outlet
temperature difference and different mass-flow rate. Their study showed that better thermal stratification can be obtained by
increasing the ratio of the tank height to its diameter, increasing the diameter of the inlet port, or increasing the difference between
the inlet and outlet water temperature
Cole and Bellinger [4] concluded that maximum thermal stratification could be achieved inside the storage tank with a
height to diameter ratio of four. The studies of Ismail et al. [5] and Hahne and Chen [6] confirmed Cole and Bllinger’s conclusion
while Nelson et al. [7] suggested aspect ratio of three for the best thermal stratification.
Lightstone et al. [8] showed that in the dynamic mode, the effect of wall thermal conductance on the thermal
stratification degradation is negligible. Mixing flow in the inlet opening is considered one of the most important parameters in the
degradation of thermal stratification. Therefore, in order to reduce the inlet mixing, different inlet configurations were considered
and studied.
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3. Thermal Stratification Phenomenon in Storage Systems
Thermal stratification as the name suggests, denotes formation of horizontal layers of fluid of varying temperature with
the warmer layers of fluid placed above the cooler ones. When a heat source is placed vertically in a pool of water, the fluid
adjacent to the source gets heated up. In the process, its density reduces and by virtue of the buoyancy force, the fluid in this
region moves up. At the top free surface, the heated water gets reflected downwards. Since, density of this heated water is low, it
does not flow downward. Instead, it rises up and moves towards the wall of the pool along the surface. Over a period of time, the
greater part of the pool gets thermally stratified except the regions close to the heat source where there can be horizontal
temperature gradients as well. In the stratified region the fluid is nearly static with negligible movement. Owing to the occurrence
of thermal stratification, the heat transfer from the source to the pool under thermally stratified conditions becomes significantly
low.
3.1. Natural stratification in storage systems
The principle of operation of stratified storage tanks is based on the natural process of stratification. When a hot water tank
without external flow is subjected to the ambient temperature, a thermal stratification of the water is formed in the course of the
cooling process. Thus, the cold water accumulates at bottom while hot water inside the tank is at uniform temperature. It is
originated from the fact that, prior to releasing heat to the ambient, the tank wall cools a thin vertical layer of water along the tank
wall. Part of this heat is then transferred towards the core of the tank by diffusion. The water of the vertical layer becomes denser
than its surrounding and then slips towards the bottom of the tank creating the stratification. [3].
3.2. Artificial stratification in storage systems
In solar thermal tanks, the cold fluid which is to be heated at the heat source is withdrawn from the bottom, e.g. solar
collectors, and returned to the top of the tank at a relatively higher temperature. As a result, a temperature difference between the
top and the bottom parts of the tank arises with the consequent variation of the density in the medium. At the inlet zone appears a
mixing fluid region that is gradually pushed down as more fluid enters the tank. As a consequence, a region with a steep
temperature gradient is formed which is known as the thermocline region. Once the thermocline region is developed, it travels
down as the charging process continues, limiting the mixing between the cold and the hot regions. In fact, the thickness of the
themocline has been used as a means to qualify how well a stratified tank has been designed. The higher the mixing at the inlet,
the thicker is the thermocline zone. [9].
4. Influencing Factors of Thermal Stratification in Thermal Storage Tanks
As it is known that cold water is heavier than warmer water, which ascends to a layer with the same density. This is the
natural process behind thermal stratification a critical factor on the design of effective water heating systems with high thermal
performances. In a well stratified storage tank, high temperatures in the top and low temperatures in the bottom lead to better
operation conditions of the solar system. Good stratification guarantees that the warmer water remains on the top of tank
producing less starts and stops in the auxiliary energy mechanism. Further, the inlet water to the solar collector is removed from
the lower and the coldest part of the storage tank improving the operation conditions of the collector. Thus, the solar collector can
be in use for longer periods without the use of auxiliary energy supply.
5. Evaluation of Thermal Stratification
The thermal stratification can be measured by the vertical temperature profile in the tank. A mixing layer is defined as
the zone between hot and cold regions in the tank. This layer can be described by the share with a temperature range with respect
to minimum and maximum temperature such as Tmin + 10% (Tmax – Tmin) < T < Tmin + 90% (Tmax – Tmin). The degree of
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Vol. 1, No. 5, 2010
stratification can also be expressed in terms of dimensionless temperature ratio, which assumes a value between zero and one
(0<θ<1) where θ is the degree of stratification and is calculated mathematically by the following equation [10].
T T min
---------- (1)
T max T min
Where T is temperature of mixing layer in the tank
Tmin is cold fluid inlet temperature
Tmax is mantle fluid inlet temperature
θ is the degree of stratification
The objective of this part of the study involves the estimation of the effect of operating parameters on thermal
stratification in mantle heat exchanger, by means of experiments and computer simulations. The current study is proposed in order
to quantify the thermal stratification inside the storage tank. Good thermal stratification is vital in order to achieve high thermal
performance on solar thermal storages where the tank design is one of the most important elements to accomplish it. An attempt is
made in this part of the study to quantify thermal stratification by considering five such influencing parameters with the help of
experiments and computer simulations.
6. Experimental Set-up
In this paper, the convective heat transfer at the mantle wall and at the tank wall is investigated under operation
conditions. In this study, the convective heat transfer in the mantle and in the inner tank is analysed by means of dimensionless
heat transfer theory in order to obtain heat flux and heat transfer coefficients for vertical mantle heat exchangers. It is not enough
only to be able to predict the heat transfer at the mantle wall and at the tank wall to model the thermal stratification in the inner
tank, it is also necessary to know the heat transfer at different levels due to the heat flows inside the inner tank. An effort is also
made to model heat transfer.
An experimental arrangement of a vertical mantle heat exchanger for the solar water heater as illustrated in fig 2. The
purpose of this experimentation is to evaluate the overall heat transfer characteristics of the mantle heat exchanger over a range of
mantle flow rates and thermal boundary conditions. These experiments are carried out at heat transfer laboratory of JNTU College
of Engineering, Anantapur, Andhra Pradesh, India.
Figure 2. Photographic view of Experimental set-up.
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The mantle is constructed with an annular spacing of 30 mm wrapped around the bottom half of a stainless steel (SS 304
grade) tank. The tank is insulated with glass wool. In the experiments hot water is supplied to the hot side of the heat exchanger
from a conventional hot water tank as the heat source and the cold water is supplied from the bottom side of the storage tank.
Eight thermocouples (copper-constantan) are fitted at different levels inside the core of the tank and four thermocouples
(copper-constantan) are mounted on the mantle tank wall side of the heat exchange surface, to measure tank temperatures at
different locations to know the stratification. The inlet and outlet temperatures and flow rate of the hot water (mantle fluid)
1
through the heat exchanger are also measured. A3 digit digital display unit is used to display the temperatures.
2
7. Thermal Experiments in the Laboratory
In this study, tests are carried out with a mantle inlet size of 12 mm for two different initial conditions (starting modes) of the
temperatures inside the tank. The initial conditions are that the tank is either mixed at around 26 0C or preheated with temperatures
at 250C in the bottom and 750C in the top. The initial stratification is created in the following way. All the water in the inner tank
is heated to 750C and then approximately half of the volume of the inner tank is changed with cold water. The cold water is
entering at the bottom with a flow rate of 1.0 lit/min (0.0167 kg/sec) and the hot water is leaving from the top with the same flow
rate.
The tests are also performed for three different mantle fluid entry patterns, regular, spray, nozzle injections into the mantle
heat exchanger. The first one is regular entry, in which the fluid is just allowed into the mantle heat exchanger at a fixed flow rate.
In the second pattern, the mantle fluid is sprayed onto the mantle wall. For spray injection, small orifices are made at the exit of
tin sheet tube to sprinkle mantle fluid over the mantle inner wall. The third pattern of fluid entry is nozzle injection in which the
mantle fluid is sent through a nozzle of 6 mm diameter into the mantle tank.
All these experiments are performed at two inlet temperatures of mantle fluid i.e., 50°C and 70°C respectively.
8. Measurements during the Experiments
In the heat storage test facility it is possible to control the flow rate in the mantle and mantle inlet temperature. During
the experiments the following parameters are measured.
a) The mantle flow rate
b) The mantle inlet temperature
c) The mantle outlet temperature
d) The temperature of the domestic water in the inner tank at eight points inside the tank
e) The temperature at four points on the outside of the mantle wall
f) The ambient temperature.
8.1. Specifications of measuring devices
The specifications of the equipment used I these experiments are as follows.
Tank with mantle heat exchanger: Stainless Steel of grade SS 304.
Thermocouples: copper –constantan with the specifications of P-200°C (PT100)
Digital temperature indicator panel: 216Y0811DTI
Measuring jar: 1000 ml made up of glass to measure mantel flow rate.
8.2. Uncertainty in the experiments
The errors associated with various measurements and in calculations of performance parameters are computed in this
section. The maximum possible errors in various measured parameters namely temperature, pressure, exhaust gas emissions time
and speed estimated from the minimum values of output and accuracy of the instrument is calculated using the method proposed
by Moffat [11]. This method is based on careful specification of the uncertainties in the various experimental measurements.
The maximum possible error in the case of temperature measurement is calculated from the minimum values of the
temperature measured and accuracy of the. The errors in the temperature measurement are 0.004714 or 0.4714 %.
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9. CFD Modeling
The experiments are first simulated by taking into account of the steady state natural convective flow on the tank side. A
3-D CFD model of a mantle heat exchanger coupled with a storage tank is developed with the same dimensions as the prototype
unit. The CFD modeling of this type of heat exchanger would lead to very time consuming CFD simulations. In order to make the
computational solution viable, the computational domain is simplified by modeling the mantle tank system into three regions
which made the problem simpler. Although, the model appears symmetric, grid restrictions required a full three dimensional
analysis of the model. The purpose of the configuration is to investigate the influence of the flow on the stratification in the tank,
the heat transfer inside the tank and the natural convection in the tank loop. The buoyancy for an incompressible fluid with
constant properties except density and viscosity is modeled by using the Boussinesq approximation in ANSYS CFX 11.0. No slip
and adiabatic boundary conditions are applied to the walls of the tank. The Boussinesq approximation uses the thermal expansion
coefficient (β) to capture natural convection. Thus, if β is presumed to be constant, a linear dependency of the density of water on
temperature is used.
The computational domain is simplified by modeling the mantle tank system into three regions which made the problem
simpler. The computational domain is simplified by modeling the top and bottom of the tank as flat walls, and the mantle-tank
volume as same as that of the prototype unit. High concentration mesh is used in the high temperature gradient regions near the
heat transfer wall between the mantle and the storage tank. A total of 537077 grid points are used in the computational domain
within the mantle gap and the inner tank.
For each simulation, the inlet temperature, flow rate of mantle fluid and flow rate of cold fluid from the experiments are
specified as inputs to the CFD model. The typical running time for simulation is approximately 3 days. On the tank side, the
temperature profile along the tank height is initialized based on the measured data. Heat loss from the tank is modeled using a
constant convective heat transfer coefficient on the outer surfaces of the tank. All physical properties of water and glycol/water
mixture are assumed to be constant except the density and viscosity in the buoyancy term in order to obtain faster numerical
convergence. In order to have sufficient number of grid points for solving the flow and heat transfer in mantle (steady state model)
particularly near the inlet jet impingement region and the high temperature gradient near the heat transfer wall, a mesh sensitivity
check is undertaken by comparing the numerical results for different mesh sizes. Figure 3 shows the mesh model of the work
respectively.
Figure.3: 3-D mesh model of solar water tank with mantle heat exchanger.
10. Results and Discussions
Investigating the process of formation of thermal stratification inside the storage tank for various governing parameters is
the main objective of the present study. In this paper, the effect of following five parameters on thermal stratification in energy
storage systems are considered in order to study and quantify the thermal stratification in energy storage tanks.
1. Inlet position of mantle heat exchanger.
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2. Mass flow rate of mantle fluid.
3. Type of heat transfer fluid.
10.1. Effect of mantle inlet position on thermal stratification
In this trial, tests are carried out with two different locations of the mantle inlet ports at two different mantle inlet
temperatures of 50°C and 70°C. Throughout these experiments, the mass flow rate is maintained constant at 0.5 lit/min. Figure 4
shows the degree of stratification against the height of tank for the first and second inlets for the mantle inlet temperature of 50°C.
Figure. 4. Figure.5.
Figure 4.Thermal Stratification for two inlets and at 50°C
Figure 5.Thermal Stratification for two inlets at 70°C
Better thermal stratification can be promoted in the heat storage when high energy is supplied to the top part than that of
the bottom part of the storage tank. From the figure 7.1, it can be noticed that high energy is transferred for lower mantle inlet
temperature when the mantle fluid is allowed through the second inlet and lower stratification is noticed when the mantle fluid is
allowed to pass through the first inlet.
Figure 5 shows the degree of stratification against the height of tank for the first and second inlets for the mantle inlet
temperature of 70°C. Better stratification is observed when the mantle fluid is passed through the first inlet at higher mantle inlet
temperatures. It is therefore, concluded that for high inlet temperature (70°C), the use of the first inlet position gives the high
degree of stratification and for lower mantle inlet temperature (50oC), the second inlet position gives high degree of stratification.
10.2. Effect of mass flow rate of mantle fluid on thermal stratification
In this set of experiments, an attempt is made to study the effect of mass flow rates of mantle fluid on the energy supplied
to the heat storage. Two different mass flow rates of 0.5 lit/min (0.00833kg/sec) and 0.75 lit/min (0.0125 kg/sec) at two different
mantle inlet temperatures of 50°C and 70°C are considered in the investigation. In both the cases top inlet is chosen.
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Figure 6 Figure 7
Figure 6. Thermal Stratification for two mass flow rates at 50°C
Figure 7. Thermal Stratification for two mass flow rates at 70°C
Figures 6 and 7 show the degree of stratification obtained with both mantle flow rates at two mantle inlet temperatures
respectively. It is evident from the figures 6 & 7 that high degree of stratification is obtained with high mass flow rate. It is
observed from this study that the time taken to reach higher degrees of stratification is less when mass flow rate is high when
compared to lower mass flow rate.
10.3. Effect of type of heat transfer fluid on thermal stratification
To estimate the effect of type of mantle fluid on thermal stratification, two types of mantle fluids viz., water and glycol-
water mixture are allowed to pass through the first inlet for two mantle inlet temperatures of 50°C and 70°C. Throughout these
experiments, the mass flow rates of the mantle fluid are maintained constant at 0.5 lit/min.
From the figure 8, the stratification with glycol-water mixture as the mantle fluid is noticed to be higher than that of the
water for a mantle inlet temperature of 50°C. It is observed from this study that, in case of glycol-water mixture the stratification
is noticed to be low in the beginning of the experiments and attains higher values as the experiment progresses. This can be
attributed to the lower viscosity at elevated temperatures and high heat capacity of glycol-water mixture. Hence, it can be
concluded from this study that at higher mantle inlet temperatures, glycol-water mixture perform better than water and at lower
temperatures both the fluids act similar.
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Figure 8 Figure 9
Figure 8. Thermal Stratification for two fluids at 50°C
Figure 9. Thermal Stratification for two fluids at 70°C
11. Conclusions
The study of stratification is important for determining the weighted average temperature, which is the sole parameter to
evaluate the total heat, gained by the water inside the tank. Through experiments and CFD analysis, it is ascertained that
stratification inside the storage tank of solar domestic water heater with a mantle heat exchanger is influenced by several factors.
These include mantle fluid inlet position, mass flow rates of mantle fluid, heat transfer fluids, startup methods, mantle fluid
injection and initial degree of stratification of the water inside the tank.
The study of this work reveals that, for high inlet temperature (70°C), the use of a top inlet position gives the high degree
of stratification and for lower mantle inlet temperature (50oC), the bottom inlet position gives high degree of stratification. It is
evident that, there should be more contact between the hot mantle fluid and tank outer wall to impart heat quickly to the tank
contents. This is possible with high mass flow rates and direct impingement of mantle fluid. The properties of heat transfer fluids
which have great impact on stratification are the coefficient of thermal expansion, viscosity and specific heat. This is observed by
replacing water with propylene glycol-water mixture. Better stratification characteristics are noticed with 30% propylene glycol
and 70% water mixture at higher mantle inlet temperature when compared to water alone.
12. References
[1] Zurigat, Y.H., “A comparison study of one-dimensional models for stratified thermal storage tanks”. ASME J. Solar Energy
Eng. 111, 205-210.
[2] Shyu, R.J., J.Y. Lin, and L.J. Fang, “Thermal analysis of stratified storage tanks”. ASME J. Solar Energy Eng. 111, 54-61.
[3] Lavan, Z. and Y. Thompson, “Experimental study of thermally stratified hot water storage tanks”. Solar Energy 19, 519-524.
[4] Cole R.L. and Bellinger F.O., “Thermally stratified tanks”, ASHRAE Transactions 88, 1005-1017.
[5] Ismail, K.A.R., Leal, J.F.B., Zanardi, M.A., “Models of Liquid Storage Tanks”, International Journal of Energy Research 22,
805-815.
[6] Hahne, E. and Y. Chen, “Numerical study of flow and heat transfer characteristics in hot water stores”, Solar Energy 64, 9-18.
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[7] Nelson, J.E.B., Balakrishnan, A.R. and Murthy, “Experiments on Stratified Chilled Water Tanks”, International Journal of
Refrigeration, Vol. 22, 216-234.
[8] Lightstone, M.F., Raithby G.D., and Hollands K.G.T., “Numerical simulation of the charging of liquid storage tanks:
comparison with experiment”, Journal of Solar Energy Engineering 111, 225-231.
[9] Shah, L.J. and Furbo, S, “Entrance Effects in Solar Storage Tanks”. Solar Energy, Vol. 75, 337-348.
[10] Robert Huhn, “Correlation of Design, Material, “Flow conditions and the thermodynamic losses in hot water storage tanks”,
10th International Symposium on District Heating and Cooling.
[11] Moffat R.J., (1988), “Describing the uncertainties in experimental results”,Experimental Thermal and Fluid Science, pp:3-17.
Authors Profile
Dr K. Hema Chandra Reddy received MTech and PhD. from Jawaharlal Nehru Technological University (JNTU),
Hyderabad, India. He is RUNNER-BOLT awardees from Indian Air-Force. He is a Professor in the Department of
Mechanical Engineering, JNTU College of Engineering Ananthapur. He also worked as Training and Placement Officer at
JNTU College of Engineering Ananthapur and during this tenure many of their students are globally placed. Currently he
is the Director of Academics and Planning at Jawaharlal Nehru Technological University, Ananthapur. He is a Fellow of
IE (India), and life member of various professional bodies. Many research scholars are pursuing their PhDs under his
esteemed guidance and some of them have been awarded.
Dr. G. Naga Malleshwara Rao is a Professor of Mechanical Engineering at Shri Shirdi Sai Institute of Science and
Engineering, Ananthapur of Andhra Pradesh State, India. His area of research includes Heat transfer and Renewable
energy. He has presented many articles in national and international conferences. He also worked as Training and
Placement Officer at Intel Engineering College between 2006 and 2009. He is a life member of IE (India). He is having
a total experience of 20 years of which 6 years is Industrial and 14 years academic.
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