Use of adsorbents for thermal energy storage of solar or excess heat- improvement of energy density, Sci ...

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INTERNATIONAL JOURNAL OF ENERGY RESEARCH
Int. J. Energy Res. (2012)
Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.2913
Use of adsorbents for thermal energy storage of solar or
excess heat: improvement of energy density
Dan Dicaire and F. Handan Tezel
*
,
†
Department of Chemical and Biological Engineering, University of Ottawa, Ottawa, Ontario, Canada
SUMMARY
The current paper describes the design of a prototype system to explore the feasibility of the adsorption thermal energy storage.
Water was chosen as the adsorbate, and three different adsorbents were tested. Zeolite 13X, NaLSX zeolite, and an activated
alumina (AA)/zeolite 13X composite adsorbent were used as adsorbents. Experiments were performed at varying
ow
rates and different relative humidities to determine the optimal operating conditions for the system. The regeneration of the
adsorbents also was explored by performing repeated runs on the same adsorbent sample. The results indicate that complete
regeneration was achieved. A maximum energy density of 160 kWh/m
3
has been achieved with the AA/13X adsorbent, and
this adsorbent was chosen for further studies. After this adsorbent screening, the system was modi
ed to improve the data
recording and system performance. Tests were performed on AA/13X, and a maximum energy density of 200 kWh/m
3
was
achieved, which was much higher than the maximum energy density reported in the literature for adsorption thermal energy
storage systems (165 kWh/m
3
). Copyright © 2012 John Wiley & Sons, Ltd.
KEY WORDS
Thermal Energy Storage; Adsorption Heat Storage; Zeolite 13X; Water Adsorption; NaLSX Zeolite; Activated Alumina/13X zeolite
hybrid adsorbent
Correspondence
*F. Handan Tezel, Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur, Ottawa, Ontario K1N
6N5, Canada.
†
E-mail: Handan.tezel@uottawa.ca
Received 28 March 2010; Revised 31 January 2012; Accepted 7 February 2012
1. INTRODUCTION AND
LITERATURE SEARCH
In sensible heat storage systems, a material, like rock or
water, is heated inside an insulated container that slows
thermal leaking. The performance of this type of storage is
measured by the temperature difference between the material
and the ambient temperature. Examples of this type of
storage systems are as follows: Aquifer Thermal Energy
Storage [4], Borehole Thermal Energy Storage [5], or water
tank storage [6]. This method of storing energy is extremely
cheap and is currently the dominant form of thermal energy
storage. However, sensible heat storage systems have low
energy densities, in the range of 40
–
60 kWh/m
3
[7], which
requires large volumes to store sufcient energy for heating
applications. The thermal energy is also sensibly stored and
is constantly diffusing to the environment. As a result, large
amounts of insulation are required to slow the heat loss to
the surroundings. Most systems keep the energy for less
than a week, and even the state-of-the-art systems do not
last more than a few months, even with volumes above
30 000m
3
[8]. These sensible heat storage systems have
become widespread as short-term thermal energy storage
with small volumes. The systems can provide a limited
amount of energy depending on the climate, which means
Conservation and sustainability are integral parts of our
society. It drives us to explore new sources of energy and
nd value in what used to be considered waste. As resources
become depleted and prices for standard commodities, like
oil, keep rising, clean sustainable technologies are getting
more attention and are economically feasible. The
‘
Clean
Tech
’
movement has arrived.
Thermal energy storage is one of these resulting technol-
ogies. In Canada, energy is a basic requirement of all
industrial, commercial, and residential operations. Instead
of obtaining thermal energy from conventional sources, like
coal or oil, it can prove more protable to collect it from
unconventional sources like solar heat, when it is abundant
and store it until it is required. There is a great demand for
utilization of low-mid grade waste heat and thermal energy
storage system to provide space heating.
There are three groups of thermal storage systems:
sensible heat storage, latent heat storage and thermochemical
heat storage [1
–
3].
Copyright © 2012 John Wiley & Sons, Ltd.
D. Dicaire and F. H. Tezel
Improvement of energy density for adsorption thermal energy storage
that auxiliary heating systems are usually required, espe-
cially in northern regions.
Latent heat storage systems rely on the energy released or
absorbed during the phase change of a material to store
energy, which is why they are typically referred to as phase
change materials. These materials are either free owing or
encapsulated for easy handling and placed in large heat
transfer containers, or they are infused into building
materials like dry wall. These materials are mainly designed
to maintain a constant temperature around the fusion
temperature and are subject to continuous heat loss to the
environment. Therefore, these systems also require a great
deal of insulation and cannot be used for long-term thermal
energy storage. Typical phase change materials like parafn
waxes have energy densities around 55 kWh/m
3
[9] and are
very good for cooling applications. Some phase change
materials, like molten salts, have been developed for high
temperature (300
C and up) applications and can have
energy densities as high as 300 kWh/m
3
[10]. The latter
materials are best suited for steam production and would
not be feasible in residential settings. Several prototypes of
latent heat storage systems are being developed, and the
parafn waxes are starting to be commercialized, but these
systems are not widespread.
Thermo-chemical heat storage utilizes reversible exo-
thermic/endothermic reactions or processes to store heat.
Excess heat is used to perform the endothermic reaction,
which usually separates a product into reactants. Once
the reactants are separated, the energy is stored as chemical
potential. When the energy needs to be released, the reac-
tants are brought together for the exothermic synthesis
reaction, and as the product is made, the energy is released.
Because these systems store energy as chemical potential,
they do not require insulation, and the stored energy does
not degrade with time. An example of this kind of thermal
energy storage is NH
3
dissociation into H
2
and N
2
being
developed by the Australian National University [11].
However, most reactions require very high temperatures
(400
C and up) for both endothermic and exothermic reac-
tions, which makes them well suited for steam production
but not for typical heating applications [11]. Only a few
experimental systems exist worldwide, and the technology
is still being developed.
As can be seen from the previous paragraphs, although
thermal energy storage is an old concept, it is still young in
terms of technology. There are some solutions that can cater
to niche markets, but widespread thermal applications, like
residential heating applications, are out of range for currently
available technology. The biggest hurdle is the lack of
permanent long-term thermal energy storage with charging
temperatures below 250
C. The second hurdle is the need
for more compact thermal energy storage systems that will
not take up large volumes and could be retrotted into
existing homes. The commercialization target set by the
international thermal energy storage community is a thermal
storage system that has 8 to 10 times the energy density of
water, around 480 kWh/m
3
.
Energy storage through adsorption is a physical process
and is a possible solution as the heat quality does not degrade
with time and stays locked away until the two interacting
components are brought together. Adsorption is an
exothermic process, which releases thermal energy as the
adsorbent adsorbs the gas it is being exposed to, into its
crystalline structure [7]. The reverse of adsorption is a
desorption process, which is endothermic. After an adsorbent
bed is saturated with gas, it needs to be regenerated to be
used again as an energy source. Using a heat source (like
solar radiation), the necessary energy for the endothermic
desorption process is provided, which releases the gas from
the adsorbent and restores the adsorbent to its original state,
ready to dispense more energy. Both of these processes are
explained in Figure 1. The temperature and amount of
thermal energy used in the regeneration determines the
amount of gas that can be released. Although, typically,
temperatures higher than 200
C are necessary to desorb all
the gas and attain maximum available energy [12], it is
sometimes more practical to operate the system between
the 110 and 150
C, sacricing some potential adsorption
capacity for practicability.
The heat released from adsorption is not constant and
varies as a function of the amount of gas already adsorbed
into the system as illustrated by Gopaletal. [12]. The
amount of gas, which can be adsorbed, and therefore the
amount of energy that can be released, also is a function of
the gas partial pressure [7] because it acts as the driving force
for adsorption. It is good to have a high heat of adsorption as
more energy is available to the system, but it should be
noted that a high heat of adsorption requires a high drying
temperature for regeneration and therefore can be a
disadvantage [13]. Many factors must be considered to
Figure 1. Description of adsorption and desorption.
Int. J. Energy Res.
(2012) © 2012 John Wiley & Sons, Ltd.
DOI: 10.1002/er
Improvement of energy density for adsorption thermal energy storage
D. Dicaire and F. H. Tezel
obtain the maximum performance from an adsorbent thermal
energy storage system. Because the energy stored suffers no
degradation with time, they are ideal candidates for thermal
energy storage. A basic design, which outlines a possible
residential thermal energy storage system, can be seen in
Figure 2. Both desorption and adsorption cycles are shown
in this gure.
When humid air is passed through the adsorbent bed
(shown as
‘
Water Vapour In
’
in Figure 2), water vapour gets
adsorbed by the adsorbent, which releases heat (because
adsorption is an exothermic process). This increases the
temperature of the air leaving, going into the house and used
as space heating (shown as
‘
Energy Out
’
in Figure 2).
Because the adsorbent bed will get saturated eventually, it
will need to be regenerated. This regeneration will require
heat, which will be provided by the solar panel (shown
as
‘
Energy In
’
in Figure 2). This heat causes the water
vapour to be desorbed from the adsorbent bed (shown as
‘
Water Vapour Out
’
leased, and the consumer would only be paying for the
energy used.
Working adsorbent systems have been reported in the
literature. Gantenbein et al. [13] produced a working zeo-
lite system, which had an energy density of 106 kWh/m
3
.
The Institute for Sustainable Technologies in Austria
investigated thermo-chemical energy storage using 200 kg
of silica gel as the adsorbent [15]. Hauer [16] reported a
successful full scale 7000-kg zeolite 13X storage system,
which heated a school and was charged by district heating
over night to offset the peak energy demands and performed
at 124 kWh/m
3
of energy density. Jaenchen et al.[17]
studied a zeolite system which could measure energy densities
of 160 kWh/m
3
. Dawoud et al. [18] reported their working
zeolite 13X system performing at 165 kWh/m
3
.Evennatural
zeolite, which has a lack in performance when compared
with its synthetic counter parts, has been identied as a
potential candidate for use in an adsorption thermal energy
storage system [19].
In this study, an adsorbent thermal energy storage system
has been designed and built. The prototype presented here is
the rst step in the design process and is a useful tool for
determining the maximum performance of an adsorbent
system and for screening different types of adsorbents for
thermal energy storage applications.
in Figure 2), preparing it to be ready
to adsorb more.
The ultimate goal of this technology would be to store
heat from solar panels or any other source of heat (like waste
heat from aluminum plants or nuclear reactors) when it is in
excess and release it later when it is necessary. Adsorbent
beds are ideally suited for these applications because the
physical storage of the energy does not degrade with time.
This means that with an on-site system, excess energy stored
during the summer can be used during the winter. Energy
stored during the day can be used at night or that left over
heat from a process can be recycled to be used at a later time.
Adsorbent bed systems also could be mobile, allowing
distribution of the energy away from its generation similar
to gas distribution through propane tanks. Dedicated solar
farms or aluminum reneries could act as regeneration
stations. The excess thermal energy would be stored and then
be distributed in mobile adsorbent beds to surrounding
commercial or residential sectors, which may not have the
physical or nancial resources to install solar systems [14].
Spent adsorbent beds would be picked up, and charged
beds would be dropped off;
2. MATERIALS AND METHODS
A literature search provided ample suggestions for
adsorbates (NH
3
, methane, water, ethane and CO
2
) for a
variety of different adsorbents (silica gel, ZSM-5, activated
carbon, Mordenite and Na-A) [20
–
22].
Suitable adsorbent and adsorbate pairs were identied
based on cost, availability and non-toxicity. Water was
identied as the most feasible adsorbate and could be
coupled with a number of adsorbents. Zeolite 13X, Na
LSX and a hybrid of activated alumina and zeolite 13X
(AA/13X) were retained. Details of these adsorbents are
given in Table I.
the equipment would be
Figure 2. Basic thermal energy storage system.
Int. J. Energy Res.
(2012) © 2012 John Wiley & Sons, Ltd.
DOI: 10.1002/er
D. Dicaire and F. H. Tezel
Improvement of energy density for adsorption thermal energy storage
Table I. Details of the adsorbents used in this study.
Adsorbent
Company
Chemical name
Mesh
Zeolite 13X
Ceca, France
Ceca G5CO2
8
12
NaLSX
Air Products, Allentown PA USA
UOP APG III
8
12
AA/13X
Axens (Formerly RioTinto Alcan), Brockville, ON Canada
ACTIGUARD 650PCAP
8
14
The primary goal of these experiments was to validate
adsorbent beds as thermal energy storage systems by
measuring how much energy could be released from such
a system. More specic tests also were performed to
understand how the operating conditions, mainly ow rate
and relative humidity, affect the release of energy from the
bed. Finally, the system was used to screen different types
of adsorbents and test the regeneration of the adsorbents
by repeating the experiments with the same sample of
adsorbent. Figure 3 shows the schematic diagram of the
prototype thermal energy storage system that was built.
The column is composed of 10 cm of ½ inch stainless
steel tube, with a volume of 9.04ml and weight of 100 g. It
was loaded with adsorbent by blocking one end with quartz
wool, loading the adsorbent pellets and shaking the column
to ensure that the packing was tight and uniform. The
other end was then blocked with quartz wool as well to
prevent the adsorbent particles to leave the column during
experiments. All the runs presented in this study were done
with fresh adsorbent (adsorbent, which had not been through
an adsorption cycle yet), except those which were tested for
the regeneration of the adsorbents. The column is insulated
in a cylinder of breglass insulation 1
”
think and has two
valves at either ends to seal it from the rest of the system,
which is connected with 1/4
”
Stainless Steel tubing. The
heater is a 200-W electric heater insulated and attached to a
voltage controller. The air comes from a compressed air
supply and goes through the wet and dry rotameters. If the
‘
dry
’
rotameter is used, the air is fed directly into the system;
if the
‘
wet
’
rotameter is used, the air is fed through a bubbler
where it is saturated with water vapour and then fed into the
system.
Once the column was packed, it was installed into
the system and regenerated by blowing hot dry air at
250
C through it for 8 hours to remove any impurities
it may have accumulated during storage or loading. After
8 hours, the column was sealed by closing the inlet and
outlet valves, and the column was allowed to cool to room
temperature.
The experimental phase had two parts. The rst was to set
the relative humidity of the system by using the by-pass
line that avoids the column and goes to the hygrometer.
During this time, the inlet relative humidity was adjusted
to the desired value (between 30% and 100%) using the
hygrometer measurements at the end of the system as well
as controlling the ows through the wet and dry rotameters.
The wet rotameter produces air with 100% RH, whereas the
dry air produces air with 0% RH. By combining different
ows from both, the desired relative humidity can be
reached. The total ow rate was measured with a precision
ow meter. After the relative humidity was set, the data
acquisition system was started, the inlet and outlet valves
to the column were opened and the humid air was passed
through the column. The thermocouples monitored the
temperature of the air entering and leaving the column, the
temperature of the outside of the column and the temperature
of the stream when it reached the hygrometer. The RH at the
exit of the column was measured with the hygrometer. The
column was considered to have reached its saturation when
the outlet temperature of the column had returned to that of
the inlet and the outlet relative humidity was equal to that
of the inlet. At this point, the system was shut down.
When the column is being regenerated, dry air is fed into
the system, and the heater is activated. The heater heats the
air to the desired temperature (between 130
Cand
250
C), which is measured by a thermocouple at the exit
of the heater. The hot air is fed into the column, where it
regenerates the adsorbent, releasing the water and drying
the zeolite. The now warm wet air exits the column and
passes through the hygrometer where its relative humidity
is measured to monitor the water leaving the column.
The end of the regeneration cycle was marked when no
Figure 3. Schematic diagram of the experimental setup.
Int. J. Energy Res.
(2012) © 2012 John Wiley & Sons, Ltd.
DOI: 10.1002/er
 Improvement of energy density for adsorption thermal energy storage
D. Dicaire and F. H. Tezel
moisture was detected at the outlet of the column. This
indicated that
2.2. Analysis
the adsorbent bed was ready to adsorb
The experimental system used in this study was designed to
determine the potential of different adsorbents to be used for
energy storage applications. With its strategically placed
thermocouple, it can measure heat released and, therefore,
the energy stored by any adsorbent and therefore determine
its potential as an energy source. The hygrometer can be used
to determine how much water is entering and leaving the
system, which indicates how much water the system has
adsorbed. The system can accommodate a wide range of
different
moisture again.
To start the adsorption cycle (to start releasing the stored
energy), humidied air (with the use of the bubbler) is fed
into the column at room temperature (the heater is kept
off). The water vapour is adsorbed as the humid air
passes through the adsorbent bed. Because adsorption is an
exothermic process, water adsorption releases heat, and
the energy released heats up the air leaving the column.
The temperature of this warm air is measured by a thermo-
couple as it leaves the column. The air stream then passes
through the hygrometer to monitor the water leaving the
column.
Before the adsorbent was discarded, a second regenera-
tion run was performed with it to measure the amount of
water it had adsorbed.
Controlled variables:
ow rates and relative humidity and can be used
to determine the optimal operating conditions as well as the
optimal performance of any adsorbent.
The total amount of water adsorbed was calculated
using Equations (1)
–
(4):
TotalH
2
OAdsorbed
¼
X
N
dm
H
2
OAdsorbed
ð
Þ
(1)
n
¼
1
Relative humidity at the inlet of the column
where
Flow-rate
g
air
dm
a
Regeneration temperature
RH
inlet
100
dm
H
2
OAdsorbed
ð
Þ¼
dm
a
(2)
Amount of adsorbent
RH
outlet
100
g
air
Dependant variables:
is the amount of water adsorbed between two time steps
(t
n
and t
n-1
) and
g
air
¼
:
0044e
0
:
0607T
Temperature at the outlet of the column
Relative humidity at the outlet of the column
;
Tin
C
(3)
Measured and assumed to be constant:
is the maximum amount of water the air can hold at a given
temperature at 100% relative humidity. Equation (3) was
extrapolated from the Perry
’
s Chemical Engineering
Handbook [23], by tting an exponential curve to the
psychometric chart for temperatures between 15
Cand
35
C. The mass of air that has passed through the column
between two time stamps (i.e., between t
n
and t
n-1
), which
is denoted by dFlow, is calculated as follows:
Inlet temperature, it is measured at the beginning and
then assumed to be constant for the rest of the
experiment
Flow-rate and RH,
Ambient temperature
Pressure of the system
1m
3
1000l
r
air
2.1. Assumptions
dm
a
¼
t
n
t
n
1
ð
Þ ðÞ
(4)
Following assumptions were made to simplify calculations
or simplify complex concepts:
The total energy released from the system is calculated
using Equations (4)
–
(7):
The relations for heat capacity, water capacity and
density of humid air, which are presented in the analysis
section, are true in the temperature range used for the
experiments.
d EnergyReleased
ð
Þ ¼
dm
a
ð
T
n
T
n
1
Þ
C
p
;
a
¼
dE
(5)
is the energy released between two time stamps, t
n
and t
n-1
.
To calculate the total energy released from the start until
the end of the experiment, these d(Energyreleased) values
need to be added up as follows:
The inlet relative humidity remains constant throughout
the experiment. The air coming out of the air source and
into the bubbler is very dry, which means that there is
some evaporative cooling occurring in the bubbler,
which gradually lowers the temperature of the water,
which lowers the temperature of the air and its water
capacity. It is assumed that because the experiments
are relatively short, the air entering the system is at its
initial measured value and does not change as the
experiment progresses.
X
N
n
TotalEnergyReleased
¼
1
d EnergyReleased
ð
Þ
n
(6)
¼
C
p
;
a
¼
1
:
005
þ
1
:
88
H
(7)
where H=RH/100. Equation (7) was taken from Perry
’
s
Chemical Engineering Handbook [23], and it corrects the
Int. J. Energy Res.
(2012) © 2012 John Wiley & Sons, Ltd.
DOI: 10.1002/er
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