Download thermal conductivy enhancement of thermochemical energy storage

Proceedings of the Asian Conference on Thermal Sciences 2017, 1st ACTS
March 26-30, 2017, Jeju Island, Korea
ACTS-P00569
THERMAL CONDUCTIVY ENHANCEMENT OF THERMOCHEMICAL
ENERGY STORAGE MATERIALS FOR EFFICIENT WASTE HEAT
UTILIZATION
Yukitaka Kato1*, Massimiliano Zamengo2, Keiko Fujioka3
1
Tokyo Institute of Technology, 2-12-1-N1-22, Ookayama, Meguro-ku, Tokyo 152-8550, Japan
Tokyo Institute of Technology, 2-12-1-S8-29, Ookayama, Meguro-ku, Tokyo 152-8550, Japan
3
Functional Fluids Ltd., 5th Fl., Chiyoda Bldg. Annex, 1-4-5, Utsubohonmachi, Nishi-Ku, Osaka, 550-0004, Japan
2
*
Presenting and Corresponding Author: [email protected]
ABSTRACT
High-temperature waste heats over 200°C have not been utilized well in some industrial processes. Efficient
utilization of the high-temperature waste heat is one of important ways for an improvement of energy efficiency of
high-temperature processes. For solar thermal energy system, cogeneration engine and industrial heat processes in
practical use, a mismatch between heat output from heat source and heat demand generates plenty amount of waste
heat. Then, waste heat storage function for high-temperature heat becomes important for an efficient operation of
these processes. Thermochemical heat storage makes it possible to store high-temperature heats. Magnesium
oxide/water/magnesium hydroxide (MgO/H2O/Mg(OH)2) thermochemical energy storage is one of candidates.
∆H° = -81.0 kJ mol-1
(1)
MgO(s) + H2O(g) ↔ Mg(OH)2(s),
Although the MgO pellet has high reactivity, thermal conductivity enhancement of packed bed materials is
important for efficient heat exchange and thermal performance of the heat storage system because of its low
effective thermal conductivity. Expanded graphite (EG) is a good candidate for thermal conductivity enhancement.
Mg(OH)2 composite material mixed with EG, named as EM, was developed. EM has higher mold-ability, which
means capability of obtaining a shape by compression, in comparison with pure-Mg(OH)2. EM materials had
higher thermal conductivity than pure-Mg(OH)2. EM packed bed reactor experiments indicated that heat could be
transported more rapidly through to the center of the bed. The EM material resulted to be a more practical material
for a packed bed reactor in thermochemical energy storage systems, because of its higher thermal conductivity and
mold-ability than Mg(OH)2 pellets
KEYWORDS: Themochemical energy storage, Magnesium oxide, Water, Thermal conductivity enhancement,
Waste heat recovery
1. INTRODUCTION
High-temperature waste heats are lost from solar thermal energy system, internal combustion engine, cogeneration and high-temperature processes. In Japan [1], waste heat at over 200°C is 1.3×1018 J/y, corresponding
to 40 % of total Industrial heat demand of 2.9×1018 J/y. Amount of exhaust gas emission is quite large, and needs
to be utilized well for energy efficiency improvement. High-temperature heats over 200°C have not been utilized
well. Efficient utilization of the high-temperature heat would be one of important way for an improvement of
energy efficiency of high-temperature processes. For heat process in practical use, an influence of instable thermal
operations on a reduction of total energy efficiency is not negligible. For solar thermal energy system,
cogeneration engine and so forth in practical use, a mismatch between heat output from heat source and heat
demand generates plenty amount of waste heat. Then, waste heat storage function for high-temperature heat
becomes important for an efficient operation of high-temperature processes. Thermochemical heat storage (TCES)
1
has possibility to store high-temperature heats.
Magnesium oxide/water/magnesium
(MgO/H2O/Mg(OH)2) thermochemical heat storage is one of candidates [2].
MgO(s) + H2O(g) ↔ Mg(OH)2(s), ∆H° = -81.0 kJ mol-1
hydroxide
(1)
Although the Mg(OH)2 pellet has high reactivity, thermal conductivity enhancement of material is important for
efficient heat exchange and thermal performance of the heat storage system because of low effective thermal
conductivity of it. Expanded graphite (EG) is a good candidate for thermal conductivity enhancement. Mg(OH)2
composite material mixed with EG, named as EM, was developed. EM has higher mold-ability, which means
capability of forming easily a prescribed figure by compaction, in comparison with pure-Mg(OH)2. EM is
expected to have higher thermal performance, and to contribute on re-utilization of high temperature heats.
2. THERMAL CONDUCTIVITY ENHANCEMENT FOR TCES MATERIAL
Fig. 1 Developed Mg(OH)2 pellets (MH-V05G, φ
1.9 × 5∼10 mm) having high durability to
repetitive reaction.
Fig. 2 An example of a packed bed reactor
for thermochemical energy storage based on
a fin-tube type heat exchanger.
Fig. 1 shows thermochemical heat storage material pellet using a pure-Mg(OH)2 (MH-V05G) [3]. Fig. 2 shows
an example of a packed bed reactor for TCES based on fin-tube type heat exchanger. In practical use of TCES
system, the TCES material will be charged between heating fins in the reactor. Fig. 3(a) shows heat exchange
models of TCES for Mg(OH)2 pellet as shown in Fig. 1. Mg(OH)2 pellet has low-thermal conductivity (around
High thermal
Composite conductivity
material of EG
Low contactand Mg(OH)2
thermal
resistance
Low thermal
conductivity
High contactMg(OH)2 pellets
thermal
resistance
Heat exchanging plate
(a)
(b)
Fig. 3 Concept of thermal conductivity enhancement of a packed bed reactor for thermochemical
energy storage performance: (a) Conventional heat exchange model between Mg(OH)2 pellet and
heat exchanging plate, (b) Improved heat exchange model between composite material of EG and
Mg(OH)2, and heat exchanging plate.
Heat exchanging plate
2
0.2 W m-1 K-1), and because of its cylindrical form the pellet has weak contact with the heat exchanging plate
surface. Then, the pellet is thought to have relatively lower thermal performance induced by the high thermal
contact resistance at the surface. Thermal conductivity enhancement of material and mold-ability for tight contact
with heating fin are important for efficient heat exchange and thermal performance of the heat storage system.
Expanded graphite (EG) as shown in Fig. 4(a) is good candidate for thermal conductivity enhancement. EG has
high thermal conductivity, chemical stability and large void fraction. Mg(OH)2 composite material mixed with EG,
named as EM, was developed as shown in Fig. 4(b) which is tablet figure. It was demonstrated that the EM
composite had higher effective thermal conductivity and reactivity than pure-Mg(OH)2 pellet [4]. EM has higher
mold-ability, which means capability of forming easily a prescribed figure by compaction, in comparison with
pure-Mg(OH)2. Mold-ability is an important property for the thermochemical energy storage material, because it
helps to achieve tight contact between material and heat exchanging surface and enhances heat conduction between
heat exchanging plate and TCES material, resulting effective for practical reactor for energy storage. Fig. 3(b)
shows a packed bed reactor of EM. EM has low-thermal conduct resistance between plate and material because
EM can form a flat surface, and have a larger contact surface with the plate surface, and high thermal
conductivity of EM itself: then, the EM reactor is expected to have higher thermal performance than pellet bed.
Thermal performance advantage of EM bed in comparison with Mg(OH)2 pellet was discussed experimentally.
The ideal form of EM would be a slab which fits well with a space between heating fins. Upper limit of the
thickness of slab would be given by the diffusivity of vapor in the material. In this study, tablet figure was used
as an elemental part of the slab.
100 μm
(a)
(b)
(c)
Fig. 4 Composite material of expanded graphite and Mg(OH)2: (a) SEM image of expanded graphite
(EG), (b) tablet form (φ 7 mm × h 3.5 mm) of the composite material (EM8), (c) Packed bed reactor
(φ 48 mm × h 48 mm ) charged with the tablets.
3. PACKED BED EXPERIMENT FOR EM
3.1 EM TABLET PREPARATION
Pure-Mg(OH)2 pellets (φ1.9 mm × length 5 - 10 mm) in Fig. 1 were used as the precursors for preparing the EM
composite. EG was obtained from graphite flakes after thermal treatment (700°C for 10 min) in an electric
muffle furnace under atmospheric conditions. In this study, EM material which has the mass ratio Mg(OH)2 :
EG is α : 1, hereafter will be named as EMα. EM8 in Fig. 4(b) was prepared by the following method:
(1) The Mg(OH)2 powder and EG in mixing mass ratio of 8:1 were placed in a glass dish; then, purified water
was added (approx. 30 ml) to the powder mixture, and the resulting slurry was mixed gently using a spatula.
(2) After obtaining a homogeneous mixture, the glass dish was placed in a furnace for approximately 15 min at
120°C under atmospheric conditions. A homogeneous mixture of Mg(OH)2 and EG (EM) was obtained by remixing it with a spatula before the complete evaporation of water. The mixture was kept in the furnace for 12
hours until it resulted completely dry.
3
(3) After removal from the furnace, the EM composite material was compressed into tablets using a stainless
steel mold set. The EM tablets had a diameter of 7 mm and a thickness of 3.5 mm.
3.2 EXPERIMENTAL PACKED BED REACTOR
Pure-Mg(OH)2 pellet and EM tablets were examined by an experimental apparatus of a packed bed reactor
(PBR) [5]. A PBR for the experiment is shown in Fig. 5. A cylindrical reactor made of stainless-steel had a
diameter of 48 mm and depth of 48 mm and steel thickness of 2 mm. The reactants were charged into from the
top, and water vapor was allowed to move from the top surface of the bed. K-type thermocouples were used for
temperature measurement of the locations in the Figure. The reactor is stored in a reaction chamber, which is
put on an electric balance. The whole reaction conversion of charged material during reaction was calculated
from a mass change of the reaction chamber measured by the balance.
Thermo couple
Tcenter
Tr-wall
Tmiddle
(4)
Tr-bottom
(5)
48
(3)
Twall
12 12 12 12
(1)
∅ 48
Ttop
(2)
Units: [mm]
(6)
Fig. 5 Experimental packed bed reactor: (1)
reaction chamber, (2) packed bed reactor, (3)
reactant bed, (4) Sheath heater, (5) Insulator,
(6) reactor stem
4. RESULTS AND DISCUSSION
4.1 THERMAL CONDUCTIVITY ENHANCEMENT
Apparent thermal conductivity of EM4, EM8, EM16 and pure Mg(OH)2 pellet were measured using a quick
thermal conductivity meter (QTM500, Kyoto Electronics). The pellets were arranged so that they completely
covered the hot wire sensor of the meter. The thickness of the sample bed was 2 cm, which was the same as the
thickness of the samples used for calibrating the instrument and the size recommended by the instrument
manufacturer. The density of the samples was the same as that in the PBR experiment. Because the contact
between the pellets and the hot wire sensor was random and not optimal, a single measurement would not give
an accurate result. After the completion of each measurement, the hot wire sensor was lifted from the sample,
and the pellets were rearranged to change the contact condition for the following measurement. Measurements
were repeated 30 times, and the average of the 30 values was calculated. The averaged apparent thermal
conductivities λbed were measured. λbed increased gradually with EG mixing ratio increase. Compared with the
original Mg(OH)2 pellets of 0.20 W m-1 K-1, the λbed of EM4 of 0.40 W m-1 K-1 was two times greater. It was
demonstrated that EG mixing was beneficial for enhancing thermal conductivity of EG composite.4.2 HEAT
STORAGE PERFORMANCE OF THE REACTOR BED
From packed bed reactor experiments, pure Mg(OH)2 pellet needed over 90 min for completion of dehydration.
On the other hand, EMs had shorter periods for dehydration. EM8 showed the shortest period. Greater content
of EG into EM4 shows higher thermal conductivity and shorter dehydration period, however, surplus content of
EG inhibits vapor diffusion in EM4, then, EM8 resulted the composite characterized by the optimum content of
4
EG and finished dehydration by 40 min. Heat-storage capacity per unit mass of Mg(OH)2 in materials charged
in the packed bed reactor (Td = 400°C, Pcond = 2.3 kPa, Tcond = 20°C) was evaluated from the experiments.
EM8 showed the highest-heat storage capacity over 800 kJ kgMg(OH)2-1 in the initial 40 min. EM8 resulted again
the optimum material. It was demonstrated that the improvement in the heat-transfer properties and heat
diffusion for EM8 allowed an overall increase of dehydration rate and shortening of dehydration completion
time than pure Mg(OH)2 pellet. It was demonstrated that EM has higher reactivity than pure Mg(OH)2 pellet by
its enhanced thermal conductivity.
5. CONCLUSIONS
EM materials had higher thermal conductivity than pure-Mg(OH)2 material. EM packed bed reactor indicated that
heat could be transported rapidly through to the center of the bed. The temperature of the inner part of the bed rose
faster because of the higher effective thermal conductivity of the EM bed. EM8 was optimum material under this
operation condition. It was demonstrated that the EM material was more practical material for a packed bed reactor
showing better heat exchanger functions for thermochemical energy storage than Mg(OH)2 pellets because of its
high thermal conductivity and mold-ability. The development of heat transfer enhanced material for
thermochemical energy storage was a key strategy for efficient utilization of surplus thermal energy at high
temperature.
ACKNOWLEDGMENT
This study was executed with the financial support of the Grant-in-Aid for Scientific Research (B) #24360404
from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
REFERENCES
[1] Energy Conservation Center of Japan (ECCJ), Nation wide waste gas heat at industrial sectors at
temperature ranges in Japan, 2004, (2006) http://www.eccj.or.jp/diffusion/04/diff_07_09.html
[2] Kato, Y., et al., "Kinetic study of the hydration of magnesium oxide for a chemical heat pump", Appl. Therm.
Eng., 16 (11): 853-862 (1996).
[3] Kato, Y., Saito, T., Soga, T., Ryu, J., Yoshizawa, Y., “Durable Reaction Material Development for
Magnesium Oxide/Water Chemical Heat Pump”, J. Chem. Eng. Japan, 40(13): 1264-1269 (2007).
[4] Zamengo, M., Ryu, J., Kato, Y., “Magnesium hydroxide – expanded graphite composite pellets for a
packed bed reactor chemical heat pump”, Applied Thermal Engineering, 61(2): 853-858, (2013).
[5] Zamengo, M., Ryu, J., Kato, Y., “Thermochemical performance of magnesium hydroxide–expanded
graphite pellets for chemical heat pump”, Applied Thermal Engineering, 64(1–2): 339-347 (2014).
5