Advanced Materials Research
ISSN: 1662-8985, Vols. 608-609, pp 1045-1048
doi:10.4028/www.scientific.net/AMR.608-609.1045
© 2013 Trans Tech Publications, Switzerland
Online: 2012-12-13
Experimental Study on Heat Storage and Release of the Phase Change
Thermal Energy Storage Unit With Bushings Between Double Flow
Feng Xu1, a, Yong Sun1, b, Haichuan Tian1, c and Yongjiang Shi1, d
1
Hebei Institute of Architecture & Civil Engineering, Zhangjiakou 075024, Hebei Province, China
a
b
c
d
[email protected], [email protected], [email protected], [email protected]
Keywords: PCM, Thermal storage unit, Bushings, Double flow, Characteristic of heat storage and
release.
Abstract. The phase change thermal storage unit With Bushings Between Double Flow was proposed
through adopting a kind of new phase change material (PCM), JDJN-60. In addition, the
corresponding heating experimental device was built. The study with regard to the heat storage
characteristic of JDJN-60 was performed. The results show that the thermal energy storage process
has three stages and there is a certain undercooling of the PCM. The effects of natural convection on
the heat storage and release process are different. The natural convection accelerates the liquefaction
process and alleviates the solidification process.
Introduction
In recent years, with the development of global industrialization, the problem of energy attracts more
and more attention. Heat storage technology has been widely applied as an important means for
alleviating the energy crisis [1]. Phase Change Thermal Storage is the most potential and important
technology because of several advantages, such as high thermal energy storage capacity, the compact
structure, high thermal efficiency, constant temperature of heat storage and heat release and so on
[2,3,4].
However, most of phase change materials are usually exhibited the poor transfer capacity. In order
to strengthen the capacity of their heat storage and release, the structure of thermal energy storage
device should be optimized. Thereby, this paper presents a new phase change thermal energy storage
unit with bushings between double flow. The corresponding heating experimental device is built and
the study on characteristics of heat storage and release is given.
Phase Change Experimental Device
The Structure of Thermal Energy Storage Unit. The unit is a multi-tube heat exchanger with a
double-way flow. The main part is three tubes, the inner and outer layers are the channels of heat
transfer fluid and the phase change material is between the two layers. The heat transfer fluid flows
into the inter tube and it will bend to the outer tube after reaching to bottom. The heat exchange
between phase change materials and heat transfer fluid goes on along the way. This structure is
equivalent to extend the heat transfer fluid channels. So, the heat exchange area is greatly increased
and the effect of heat exchange is significantly improved.
Physical Properties of the Phase Change Material. The phase change material adopted is
JDJN-60. Its melting and freezing point is 58~60oC and the latent heat of fusion is 262kJ/L. The
specific heat is 1.42 kJ/(kg·k) in solid and 2.08 kJ/(kg·k) in liquid, separately. The thermal
conductivity is 1.22W/(m·K) in solid and 0.58 W/(m·K) in liquid, respectively.
Experimental Device. The experimental device is designed and built for mastering the
characteristics of heat storage and release of the unit. The main equipments include electric heater,
temperature controller, the heat storage unit, radiator, circulating water pump and water-replenishing
tank etc. The system maintains the constant pressure through expansion tank. The principle of the
experimental device is shown in Fig. 1.
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Progress in Renewable and Sustainable Energy
1- heat storage unit 2- radiator 3- electric heater
4- flowmeter 5- circulating water pump 6- expansion tank
Fig. 1 Schematic diagram of the experimental device
Test Point Arrangement. There are seven measuring points in the whole experimental system. T1,
T2 and T3 are arranged longitudinally in the phase change material. In the center position there are T4,
T5 and T6 around diameter. Moreover, two measuring point are set in the import and export to monitor
the temperature of water entering and leaving the unit, respectively. The arrangement diagram is
shown in the Fig 2.
Fig. 2 Schematic diagram of the measuring point arrangement
Experiments and Result Analysis
The Phase Transformation Law during Heat Storage. The heat storage experiment is performed
under the conditions that the temperature of water is 70 oC and the system flow is 200L/h. The
temperature of each measuring point is shown in the Fig. 3 and Fig. 4.
Fig. 3 Temperature change for longitudinal
measuring points
Fig. 4 Temperature change for radial measuring
points
The temperature change of all the measuring points is basically identical. The phase change
processes include roughly three stages. These are solid-phase sensible thermal storage, phase change
latent thermal storage and Liquid-phase sensible thermal storage.
Advanced Materials Research Vols. 608-609
1047
At the stage of solid-phase sensible thermal storage, the material absorbs heat from the heat
transfer fluid and the temperature increases. The heat is stored with the form of sensible thermal. It is
worth noting that there are special features in this stage, such as higher temperature, the shorter
duration and the little heat storage.
When the temperature of each point rises to near 55 oC, the temperature change becomes slow. The
fact indicates that the material begins phase change. The material temperature changes slowly due to
the material absorbing a large amount of latent thermal in. Liquid layer forms natural convection
which appears between the tube wall and solid phases with the material melting. Heat transfer process
is enhanced. So, the temperature everywhere has not been suspends in the phase change temperature.
It shows an increasing tendency but the amplitude is less. The features of phase change stage are the
longer duration and the more heat storage.
Then, the temperature of measuring points jumps in different degree after phase change. The fact
explains that the stage of liquid-phase sensible thermal storage begins. By the comparison with the
phase change stage, this stage is rather short and the heat storage is the least.
The Fig. 3 shows that there is significantly different among the temperature change of T1, T2 and T3.
The liquid-phase material rises and the solid-phase material falls under buoyancy force because of
natural convection existing in liquid layer. This makes the temperature of T1 located in the top steep
rise. T2 located in the middle significantly lags behind T1. However, T3 located in the bottom is higher
than T2. The probable reason is that the bottom of material and the heat transfer fluid is direct contact,
which constrains material stratified. But natural convection strengthens along with the increase of
molten material, the temperature of T2 speeds up. The temperature difference between T1 and T2 is
diminishing.
As shown in Fig. 4, there is also significantly different among the temperature change of the
measuring points around diameter. The probable reason is that the heat absorbed by solid-liquid
interface is equal to the latent heat which it absorbs plus the heat conducted to the outer along the
radial direction. The difference among their quantities leads to the difference of the temperature
change around diameter. The material around T4 reaches to phase change stage after about 15 minutes.
The temperature of T5 rises also more quickly, because the temperature of outboard fluid in outer tube
is lower than the inner, the material around T5 begin phase change after around 30 minutes. It lags
behind the T4. The temperature of T2 is the lower than T4 and T5. But it keeps slow ascent in the phase
change stage. This is different from T4 and T5 which exist more obvious thermostatic process.
Because the T2 locates in the center of heat transfer, the materials on either side conduct heat to it
simultaneously. The heat transforms into its latent thermal almost completely but no transfer with the
outer. The experiment of Sari and Kaygusuz also shows the same results [5,6,7].
The Phase Transformation Law during Heat Release. The heat release experiment was
performed under the conditions that the temperature of water is 40 oC and the system flow is 200L/h.
The temperature of each measuring point is shown in the Fig. 5 and Fig. 6.
Fig. 5 Temperature change for longitudinal
measuring points
Fig. 6 Temperature change for radial measuring
points
The Fig5 and Fig 6 show that the exothermic process is similar to the phase change processes,
which also include roughly three stages. But the effect of natural convection on is different from the
heat storage process, it slows the material heat release. Along with the solidification, the liquid layer
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Progress in Renewable and Sustainable Energy
gradually thinned, natural convection relents. But the inner natural convection make the liquid
material circulation flow, which make the temperature of the whole material uniformity, the
temperature gradient decreases continuously and the heat transfer is weakened. The results show that
the temperature difference among longitudinal measuring points is lower than the heat storage process.
The time of heat release is about one hour longer than the former. There is a certain undercooling of
the phase change material JNDN-60, the solidification temperature is lower than the melting
temperature.
Conclusions
The experimental study of heat storage and release of the phase change thermal energy storage unit
With Bushings Between Double Flow was performed in this paper. The characteristic of PCM is
obtained by analyzing the experimental data.
(1) The thermal energy storage process has three stages: Solid-phase sensible thermal storage,
phase change latent thermal storage and Liquid-phase sensible thermal storage. There is a certain
undercooling of the phase change material whose phase change temperature is in the range of 55 oC to
65 oC.
(2) The temperature change of all the measuring point is basically identical in the phase change
process. But the temperature of material near the heat fluid is higher than that in the center.
(3) The effect of natural convection is different between the heat storage and release process. It
enhances the liquefaction process but alleviates the solidification process. So, the latter time is longer
than the former.
References
[1] Jun Hu, Hua Dong: J. Acta Energiae Solaris Sinica. Vol. 27 (2006), p. 399 (in Chinese)
[2] Haiting Cui, Zhenhui Wang: J. Acta Energiae Solaris Sinica. Vol. 30 (2009), p. 1368 (in Chinese)
[3] Yin Chen, Zequan Sun: J. Journal of Guangdong Mechanical Institute. Vol. 12 (1994), p. 80 (in
Chinese)
[4] Hansain S.M: J. Energy Conversion and Management. Vol. 39 (1998), p. 1127
[5] Sari A, Kaygusuz K: J. Energy Conversion Manage. Vol. 43 (2002), p. 863
[6] Sari A, Kaygusuz K: J. Energy Conversion Manage. Vol. 43 (2002), p. 2493
[7] Sari A, Kaygusuz K: J. Renewable Energy. Vol. 24 (2001), p. 303
Progress in Renewable and Sustainable Energy
10.4028/www.scientific.net/AMR.608-609
Experimental Study on Heat Storage and Release of the Phase Change Thermal Energy Storage Unit
with Bushings between Double Flow
10.4028/www.scientific.net/AMR.608-609.1045
DOI References
[4] Hansain S. M: J. Energy Conversion and Management. Vol. 39 (1998), p.1127.
10.1016/S0196-8904(98)00025-9
[5] Sari A, Kaygusuz K: J. Energy Conversion Manage. Vol. 43 (2002), p.863.
10.1016/S0196-8904(01)00071-1
[6] Sari A, Kaygusuz K: J. Energy Conversion Manage. Vol. 43 (2002), p.2493.
10.1016/S0196-8904(01)00187-X
[7] Sari A, Kaygusuz K: J. Renewable Energy. Vol. 24 (2001), p.303.
10.1016/S0960-1481(00)00167-1