Experimental analysis of partial and fully charged thermal stratified hot water storage tanks

1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 62-69© IAEME
62
EXPERIMENTAL ANALYSIS OF PARTIAL AND FULLY
CHARGED THERMAL STRATIFIED HOT WATER
STORAGE TANKS
Dr V.Krishna Reddya
, Mr.M.Mastanaiahb
, Dr.S.Rama Krishna Reddyc
a
Professor, Department of Mechanical Engineering, KCIT, Markapur,
B
department of Mechanical Engineering, SGIT, Markapur,
c
department of Chemistry, SGIT, Markapur
ABSTRACT
Developing competent and economical energy storage devices such as thermal energy storage
have great importance as it reduces the gap between demand and supply of energy Presently, the
most common storage devices utilize phase change materials (commonly known as eutectic salts),
rock beds and hot water storage. In most cases stratified storage tanks with various capacities are
employed for storing energy which was most economical. Some analytical simulations and
experimental investigations of the effect of thermal stratification in storage systems were carried out
by a number of investigators who showed that stratification improves the performance of solar
systems. By considering the phenomenon of mixing and other degradation mechanisms will provide
an higher limit to the performance of a stratified thermocline hot water storage tank as they
influenced a lot and provides closure picture to the practical situation. An attempt was made to study
Thermal stratification in energy storage tanks systems using hot water experimentally as the works
available so far are very few. The experiments were carried out in static mode with hot water inlet at
the centre of the tank. This paper presents temperature stratification i.e. degradation of heat in
stratified thermocline storage was studied experimentally. Emphasis was given in this study for the
effects of aspect ratio, mix number, exergy efficiency and initial temperature difference. Data were
taken on fully, partially charged storage tanks and the temperature changes in axial and radial
directions were studied. The results showed that heat conduction to atmosphere through the
insulation contribute to greater loss than conduction across the thermocline in small diameter tubes
and is not happened in large diameter tanks with thick insulation.
Key words: Thermal storage, Thermocline, Stratification, Conduction Heat loss, Convection;
Mixing parameter.
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND
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ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 6, Issue 2, February (2015), pp. 62-69
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2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 62-69© IAEME
63
INTRODUCTION
Thermal energy storage in the form of sensible heat of water in stratified hot-water tanks is a
widely accepted practice. Energy storage schemes are necessary in solar energy systems because of
the intermittent nature of solar radiation. Presently, the most common storage devices utilize phase
change materials (commonly known as eutectic salts), rock beds and hot water storage. Water is
nontoxic, has a very high specific heat, is inexpensive and its vapor-liquid phase equilibrium is
suitable for the temperature range required for space and water heating.
Both for thermosyphon and forced-circulation types of solar domestic hot-water (SDHW)
systems, stratified storage tanks with capacities varying from tens to thousands of liters are
employed. These storage systems can act as buffer medium between the collection system and the
point of application. The hot water from the storage tank may then be used on demand at various
flow rates.
In stratified storage systems, we can store both the hot and cold fluids in a single tank. The
principle is based on the gravity (dense) separation of fluids of different densities. This intermediate
zone between the hot and cold water in the storage tank, where a considerable temperature gradient
is obtained, is called the thermo cline, and its thickness should be as little as possible. The increase in
thermo cline thickness with time is the degree of stratification decay in the Hot-water. These kinds of
systems are being suggested in the heating of buildings, as they can readily be retrofitted into
existing Hot-water systems [8].
Number of works on stratification has done through experiments and numerical studies for
storage tanks having different geometries, inlet and outlet positions, different tank wall materials,
and with various flow rates. The experimental observations of the degradation in a static thermocline
on static stratification, charging and discharging cycles are made by the different researchers.
LITERATURE SURVEY
Lavan and Thomson (1) done through investigation with storage tanks having different L/D
ratios and observed that 3.0 L/D ratio is optimum for better performance. These studies indicate that
the extraction efficiency decreases with flow rate, and the degree of stratification depends upon the
flow distributors used in the tank. Increasing the length/diameter ratio also increases the degree of
stratification, but increases heat losses through the walls. A drawback in their study is that the L/D
ratio was varied by changing the level of the exit pipe in the same tank because the storage volume
also varies with L/D.
A one-dimensional model of a thermally-stratified tank was presented by Cole and Bellinger
(2), who used their experimental results to estimate the empirical constants in a correlation for a
mixing parameter expressed in terms of Fourier and Richardson numbers. They identified a critical
Richardson number of about 0.25, below which mixing occurs, and concluded that the best
stratification occurs with tall tanks, low inlet velocities and large top-to-bottom temperature
differences.
Shyu et al. (3) studied experimentally the influence of interior lining on the stratification.
They confirmed that the outside insulation can improve the tank wall axial conduction, which leads
to decrease the degree of stratification. All these works are mainly focused on static stratification,
tank charging and discharging cycles. Dobbin (4) has studied stratification experimentally in water
tanks for closed-loop thermosyphon solar collectors. To achieve improved stratification, he
recommended the use of side-arm thermosyphon heat exchangers in preference to immersed coil
types. He also observed that the degree of stratification is highly dependent on the pressure drop in
the collector which, in turn, determines the flow rate. Fanney and Klein (5) have observed that

3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 62-69© IAEME
64
stratification exists at all times within the storage tank for the lower collector flow rates (=0.0033
kg/set m2 collector area); they also presented temperature profiles at different flow rates.
Ghaddar and Al-Marafie [6] published their numerical and experimental works on the effect
of finite wall thickness on stratification while charging of solar thermal storage tanks. In their work
they used a 2-dimensional spectral element model. The temperatures observed from their model were
compared with experimental data and with another one dimensional plug flow model. They
confirmed that thermocline degradation is because of the conducting wall. They also noticed that the
radial temperature distribution is almost uniform at low flow rates, but varies considerably at high
flow rates.
Wildin and Truman [7] conducted experiments to identify the factors influencing the
performance of stratified storage tanks, using natural stratified, diaphragm type and multi-tank
systems. Performance of the naturally stratified storage systems are observed to be the best out of all
above systems. While these studies provide a qualitative description of the phenomena of stratified
thermocline storage, it was felt that a systematic study of thermocline properties under controlled
conditions in static and dynamic modes to get the possibility of stratification closure to the practical
prevailing conditions.
OBJECTIVES OF THE WORK
Even though much work has been published on stratified heat storages but systematic
parametric study has not been done in the case of hot water storages. The degradation of
thermoclines and loss of available heating or cooling energy in the static mode of operation are
mainly because of mixing taking place in the tank between the incoming water and the water in the
tank. The level of mixing is a function of inlet stream velocity, temperature difference between the
water available in the tank and incoming water, and the thermo physical properties of the water.
Quantification of mixing is done in several cases by using some empirical relationships to compare
the experimental temperature profiles. The objective of this study is to experimentally determine the
mixing parameter which is measured as a function Reynolds number and Richardson number, and
the effect of several geometric and dynamic parameters such as aspect ratio on thermal stratification.
RESULTS AND DISCUSSIONS
Static Temperature Profiles
Temperature profile for an uninsulated fully charged hot water storage tank in static case with
varying time instant is shown in Fig.4.1. Initial temperature profile could not be maintained uniform
due to heat loss to ambient because of the large temperature differences between the stored hot water
and ambient. The top and bottom portions of the storage tank consist of radial diffusers made of low
thermal conductive material, so the heat transfer from the top and bottom surfaces can be considered
to be negligible. In addition, the top and bottom plates are insulated using 15mm thick thermo resin
material. It is therefore stated in the figures that convection takes place only through sidewalls. The
temperature profile in Fig.4.1 shows the top surface being high and bottom being lower. As time
increases this temperature decreases due to convective heat losses to the ambient from the bulk fluid
resulting in the formation of temperature gradients out the bulk fluid.
Mix Number
(a) Effect of aspect Ratio
The variation of Mix number with aspect ratio for increasing time in the case of fully charged
hot water storage rank is seen in Fig.4.2.

4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 62-69© IAEME
65
Fig 4.1
Fig 4.2
It is evident that more mixing (less stratification) takes place for small aspect ratio tanks. As
the aspect ratio increases, the Mix Number decreases due to increased thermal stratification inside
the storage tank.
(b) Effect of initial temperature
Figure 4.3 shows the variation of Mix Number with aspect ratio for different initial
temperatures. It is noted in this case that with increase in aspect ratio mixing decreases. The result
also shows mixing decreases with increase in the initial temperature differences.

5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 62-69© IAEME
66
Exergy efficiency
The exergy efficiency for a typical case of hot water storage tank at larger aspect ratio is seen
to be more compared to lower aspect ratio as seen in Fig.4.4 with initial temperature difference. The
reason is convective heat loss from the tank wall to the ambient by natural
Fig 4.3
Convection leads to development of temperature gradient, as a result the stored energy inside
the tank is decreased, leading to decrease in the value of exergy efficiency. The development of
temperature gradients is attributed due to mixing inside the storage tank. At higher initial
temperature differences heat leak is more resulting convective mixing hence showing lesser value in
the exergy efficiency.
Fig 4.4

6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 62-69© IAEME
67
Fig 4.5
Partially charged storage tank
A case with convective heat loss at top region and convective heat gain at the bottom region
is taken for presenting the results
Fig 4.6
Static Temperature Profiles
A clear variation of temperature for different levels of initial charge (½) is shown in Figs. 4.5
& figure 4.6 shows the temperature profile considering three levels of initial charge for an aspect
ratio of 3 with varying time.
Figure 4.6 also shows temperature variation for partially charged tank wherein the top region
is at a temperature higher than the ambient(heat loss condition).In partially charged storage tank for
different levels of initial charge, degradation of themocline is to be more in case of ¼ and ¾
charged tank as compared to ½ charged tank.
Thermocline is a zone between the warm fluid layers and cold fluid layers in which there is a
large temperature and density gradients.
The results show less dense warm water tries to induce convective mixing at the top of the
storage tank. Similar mixing effect can be explained for ½ and ¾ tank because of more mixing due
to buoyancy effects.
Temperature profile for partially charged storage tank with
different levels of initial charge
290
300
310
320
330
340
350
360
370
0 0.045 0.08 0.12 0.2 0.28 0.38 0.42 0.46 0.52 0.58 0.68 0.75 0.83 0.94
Dimensionless height
TemperatureK
1/4 CHARGE 0 MINUTES
1/4 CHARGE 30 MINUTES
1/4 CHARGE 60 MINUTES
1/4 CHARGE 120 MINUTES
1/2 CHARGE 0 MINUTES
1/2 CHARGE 30 MINUTES
1/2 CHARGE 60 MINUTES
1/2 CHARGE 120 MINUTES
3/4 CHARGE 0 MINUTES
3/4 CHARGE 30 MINUTES
3/4 CHARGE 60MINUTES
3/4 CHARGE 120 MINUTES

7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 62-69© IAEME
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Mix Number
The Mix Number variations with time for varying aspect ratios at different levels of initial
charges are seen in Figs. 4.7when the initial charge is 348K. The results shows
Fig 4.7
Degradation of thermocline in all the cases. Figure shows the quantitative degradation in
terms of Mix Number.
The Mix Number values increases with time for a given condition. The degradation is seen to
be more ¼ and ¾ charged tank compared to ½ charged storage tank. Providing insulation reduces
mixing as indicated by the dotted lines in Figure. It is also seen that varying trend of mixing for the
three different cases in which top and bottom region acts as either convective heat loss or heat gain.
The effect of insulating the storage tank considerably reduces mixing which is shown clearly in all
the cases of partial charging.
Fig 4.8
Exergy efficiency
Figure 4.8 shows the variation of exergy efficiency for different levels of initial charge in
case of partially charged storage tank. At ¼ charged storage tank, thermal degradation and
convective mixing leads to greater loss of energy whereas in case of ¾ charged tank, significant loss
is attributed due to axial conductive effects and thermal degradation. The storage tank with ½
charged accounts for lesser mixing and hence more exergy efficiency compared to ¾ and ¼ charged

8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 62-69© IAEME
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storage tank. It is to be noted that exergy efficiency calculation is not done for the case of dynamic
operating conditions as mixed convective effects leads to lesser accurate prediction of exergy
efficiency.
CONCLUSIONS
Numerical and experimental heat transfer analysis which can be described well by all major
factors relating to thermal degradation and the convective mixing is required in thermal stratification.
Majority of the present works focused on static or dynamic mode in a single mode of operation. Very
limited works are available in dynamic mode with inlet mixing and partially charged containers. In
the present work various studies are carried out for convection and stratification in both partially and
fully charged in both static and dynamic situations which resemble the practical situation. From this
work it is observed that the parameters like initial charge level, temperature difference, insulation
and aspect ratio between hot and cold water have substantial effect on the decay of stratification in
tanks.
Mix number increases with time for all aspect ratios and more mixing at lower aspect ratios
in fully charged tanks. In partially charged tanks also Mix number increases with time up to
particular values of aspect ratio and initial charge. With time mix number increases with aspect ratio
and elevated aspect ratios guide to more thermal degradation because of axial conduction and heat
gain through convection. One fourth charged tank gives higher mix number than in ½ and ¾
charged tanks and more stratification can be established with the raise in temperature gradients. In
case of dynamic charging and discharging storage efficiency was much influenced by non
dimensional numbers like peclet, Fourier number, tank wall material, insulation and inlet mixing.
Three dimensional geometry study can give better results and visualization of contour diagrams can
be done with numerical studies.
REFERENCES
1. Lavan Z, Thompson J.,“Experimental study of thermally stratified hot water storage tanks”,
Solar Energy 1977, 19, 519–524.
2. R.L. Cole and F.0. Bellinger, ASHARE, Trans. 88, 1005 (1982).
3. Shyu, R.J., Hsieh, C.K., Unsteady natural convection in enclosure with stratified medium.
Journal of Solar Energy Engineering 1987.109, 127–133.
4. W. Dobbin, Proc. ENERGEX &4, p. 501, F.A.Curtised. Saskatchewan, Canada (May 1984).
5. A. H. Fanney and S. A. Klein, Sol. Energy 40, 1 (1988).
6. Ghaddar N.K,Al-Marafie A.M. “Study of charging of stratified storage tanks with finite wall
thickness”. Int J Energy Res 1997; 21:411–417.
7. Wildin M.W, Truman C.R. “Performance of stratified vertical cylindrical thermal storage
tanks” part I ASHRAE, Transcactions, 1989, 95(Part1):1086–1095.
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