Study of Adsorbent Energy Density and Regeneration
for Long Term Thermal Energy Storage
Of Solar and Waste Heat
Dan Dicaire (dandicaire@hotmail.com) and F. Handan Tezel (*)
Department of Chemical and Biological Engineering
University of Ottawa, 161 Louis-Pasteur, Ottawa, Ontario K1N 6N5 CANADA
Abstract
A lab scale system has been developed in this study to demonstrate long term thermal
energy storage for space heating using adsorbent beds. The reversible exothermic
adsorption process releases heat when exposed to humidity and can be regenerated using
low grade heat obtained from solar or waste heat. The energy does not degrade with time
and the system can be recharged repeatedly without significant loss of performance. The
level of regeneration/energy storage achieved from various regeneration conditions has
been studied. The practical feasibility of the system is verified by applying the resulting
regeneration parameters, efficiency, and energy densities to typical residential space
heating needs. The experiments were performed on a new hybrid material as well as a
natural zeolite in order to confirm that the trends are reproducible in different adsorbents.
3 different scenarios have been considered for this seasonal thermal storage and
corresponding adsorbent beds have been sized.
Introduction
Solar energy is the most abundant and accessible source of energy on earth. However, it
is not constant through the seasons producing an excess during the summer and a
deficiency during the winter. The development of more environmentally responsible
energy practices is pushing to increase efficiencies and find value in previously unusable
waste. The key to utilizing wasted solar heat is storage; to accumulate energy when it is
in excess and use it when it is lacking. Although there are many different types of
Thermal Energy Storage (TES) systems, the focus of this paper is on “Long Term
Adsorption Thermal Energy Storage” and its many advantages over typical sensible
thermal storage system like aquifers or water tanks.
Adsorption is an exothermic reversible process. When a bed of adsorbent is exposed to
humid air, adsorption of water takes place and the exothermic process releases heat.
Once the adsorbent is saturated, the system can be regenerated by introducing heat
(provided by solar energy or waste heat) to release the adsorbed water and regenerate the
adsorbent so that it is ready to adsorb water again. The stored energy does not degrade
with time as long as the adsorbent is kept dry. They can also be recharged repeatedly
with limited loss of performance [9].
_______________________________________________________
(*)
Author to whom correspondence should be sent to: handan.tezel@uottawa.ca
1
Long Term Adsorption TES have been developed in the past for a variety of applications.
The technology has been proven in large scale systems using electric resistance during
off peak times to regenerate the adsorbent. Systems typically report an energy density
between 125-150 kWh/m3 [3] but theoretically could reach up to 200-250kWh/m3. A
prototype system has been developed to study the process, optimize the energy density,
and characterize the regeneration parameters.
The application considered in this study is space heating since 60% of residential energy
use and 52% of commercial energy use in Canada goes towards space heating [6]. The
heat for the storage system can come from a variety of sources and can have varying
thermal quality. The cases considered in this study include a residential household, a 10storey building and a retail/office space, all fitted with a concentrating solar panel array
and an adsorption thermal energy storage system to fulfill the complete annual space
heating needs.
The panels supply heat to the building all year round. They have an excess of heat in the
summer and can not supply the complete demand during the winter. The idea is to use
the storage system to store the excess heat from the summer in the adsorption TES
system and release it during the winter when space heating is required. The goal of this
paper is to see if such a system is practically feasible.
Prototype System
An in depth description of the prototype system, adsorbent screening process and
experimental procedure can be found in Dicaire 2009 [2].
The chosen adsorbent is a new hybrid adsorbent material produced by Rio Tinto Alcan in
Brockville, Ontario, Canada. The adsorbent was placed in a stainless steel column and
insulated. The adsorbent was exposed to humid air at varying flow rates and the energy
released was determined with the use of thermocouples. A hygrometer was used to
measure water input/outputs and to determine when the adsorbent was saturated. For
regeneration, the adsorbent was exposed to hot air at varying temperatures until all
possible humidity was removed. The energy required for the regeneration was measured.
Adsorption Results & Regeneration Characterization
The usefulness of an adsorption TES system depends on the energy released during the
adsorption phase and the level of regeneration possible using the waste or solar heat. The
focus of the research has been to determine the parameters that maximize the energy
density during the adsorption phase and to characterize the level of regeneration based on
the temperature of the available heat source. The system reaches temperatures as high as
70oC and has a maximum energy density of 200kWh/m3. It was found that if the system
is well insulated, energy densities are fairly constant as a function of flow rate. It is
possible to control the rate of energy released by manipulating the flow rate and relative
humidity of the system. Lower flow rates and lower relative humidity cause slower
release of energy. In these cases, the temperature of the outlet also diminishes and heat
losses are greater, reducing the useful energy density of the system. This is why it is
better to operate at 100% relative humidity and higher flow rates.
2
Figure 1 summarizes the regeneration characterization for the hybrid material. It shows
the energy density achieved from different regeneration temperatures. The adsorbent bed
was regenerated with hot air at various temperatures to remove the moisture and then put
through a 100% relative humidity adsorption cycle to measure the amount of energy
stored. Regeneration was performed at various temperatures (80oC to 250oC) and
superficial velocities (0.15 m/s, 0.3 m/s and 0.45m/s). The regeneration temperature has
a non linear relationship (NLR) with the energy density which is also displayed in the
figure. Experiments were performed on clinoptilolite as well to confirm the regeneration
trend seen in the hybrid adsorbent. Clearly, although the energy densities are not as high
for this natural zeolite, the trend is identical and the model still applies. Repeat
experiments were performed to obtain a standard deviation of the energy density which is
represented on the graph by error bars. The regeneration flow rate was not found to have
a significant impact on the energy density.
250
0.3 m/s
0.45 m/s
0.15 m/s
NLR
3
Energy Density (kWh/m )
200
Hybrid adsorbent
150
100
Clinoptilolite
50
NRL =
ax
b+ x
0
0
50
100
150
200
250
300
Regeneration Temperature (°C)
Figure 1: Regeneration characterization of the adsorbents studied.
The final piece of the regeneration characterization is the efficiency of the thermal
storage. The energy released during the adsorption cycle is divided by the amount of
energy used during the regeneration process to determine the percentage of energy stored,
or efficiency of the thermal storage, which is displayed in Figure 2 as a function of the
regeneration temperature for different superficial velocities. The efficiency varies
linearly with regeneration temperature between 30% and 50%. The efficiency decreases
as temperature increases, likely because of the additional heat losses from operating at a
higher temperature. Experiments were performed at various superficial velocities and all
3
show similar trends. Therefore, between 30% and 50% of the thermal energy inputted to
the system will produce useful heating.
0.7
0.6
0.3 m/s
0.45 m/s
0.2 m/s
0.15 m/s
Storage Efficiency
0.5
0.4
0.3
0.2
0.1
0
0
50
100
150
200
250
300
o
Regeneration temperature ( C)
Figure 2: Efficiency of hybrid adsorbent thermal energy storage system as a function of regeneration
temperature for different superficial velocities.
Feasibility Analysis
The characterization of the adsorption TES from the previous section will be combined
with a solar heat source and typical space heating requirement to complete the long term
TES system. The analysis will explore three different scenarios; a residential household
(250m3), a 10-storey building (34 000m3) and a retail/office space (14 000m3). The solar
heat source for the scenarios considered in this study are concentrating solar panels
designed for thermal energy collection produced by Menova Energy Inc (Ottawa,
Ontario, Canada) [10]: the residential PS-35 (21 kWthermal, span of 6.5m x 6.5m and a
collector area of 35m2) and the industrial PS-140 (85 kWthermal, span of 13m x 13m and a
collector area of 140m2). The space heating load has been modeled by HOT2000 V10.3
and EE4E, which are residential and commercial energy simulation software developed
by the CANMET Energy Technology Center (Ottawa, Ontario, Canada) [5].
The residential household is combined with one PS-35 and the 10 Storey building and
retail/office space are matched with several PS-140s. Figure 3 displays the monthly
supply of energy by the PS-35 panel and demand for the household. The solid curve
represents the space heating requirements of a typical Canadian household located in
Montreal, Quebec. The other curve represents the average thermal output from the PS-35
including a 3% heat loss for piping. Similar trends are observed with the other two
scenarios considered, although the magnitude of the curves is much greater. The
4
concentrating solar panels operate all year and can provide approximately 70% of the
building space heating needs. They can supply the space heating needs alone for
approximately 8 months of the year but require a complementary system for the
remaining 4 months. The goal of the adsorption TES is to store the excess heat from the
summer and use it for heating during the winter months to make up the missing 30%.
3,500
PS-35 TH Thermal
Output
3,000
Energy (kWh / Month)
Typical Space
Heating Requirement
2,500
2,000
1,500
1,000
500
0
1
2
3
4
5
6
7
8
9
10
11
12
Months
Figure 3: Monthly Energy Supply and Demand for a typical Canadian household located in
Montreal, Quebec.
The months where the solar panel outputs exceed the space heating needs shall be
referred to as “summer excess” and the remaining months which require supplemental
heat to fulfill the space heating needs shall be referred to as “winter needs”. The
concentrators produce heat at approx 100°C for regeneration which means the TES has
an energy density of 150kWh/m3 and an efficiency of 50% (see Figure 1 and Figure 2).
Table 1 summarizes the parameters of the complete adsorption TES for on-site use of
excess thermal energy. When the energy required for regeneration of the TES (to store
enough energy for the winter needs not supplied by the PS-35) is compared to the excess
of energy from the summer, it is clear that there is enough energy.
Table 1: Summary of Parameters for Adsorption TES
Heat Source
Excess Thermal Energy from
Summer (kWh)
Total Winter Needs not supplied
by PS-35 (kWh)
Energy Required for TES
Regeneration (kWh)
3
Bed Size (m )
House
10 Storey Building
Retail / Office
PS-35 Thermal
5.5xPS-140 Thermal
5xPS-140 Thermal
16 515
310 249
278 806
1 884
152 616
132 019
3 768
305 232
264 038
12.56=(2.32m)
3
1034.16 = (10.11m)
3
929.35 = (9.76m)
3
5
For the residential scenario, there is about 4 times more excess energy than required,
which means that one PS-35 could supply more than one household with an Adsorption
TES for winter space heating needs or even be used to also supply the Domestic Hot
Water (DHW) needs. If we were to include DHW needs for the entire year, the PS-35
would still be able to deliver, but the adsorbent bed required would be 25 m3. For the 10
storey building and the retail/office building, the excess summer heat is just enough to
supply the regeneration of the TES and store enough energy for the winter needs. These
large buildings are very likely to have enough area on their roof or somewhere else on
their property to put the concentrating solar panels as well as the adsorbent bed
necessary.
It is possible to minimize the size of the adsorbent bed size by upgrading the regeneration
heat from 100°C to 250°C by supplementary heating such as off-peak electricity or
petroleum fuels. By increasing the regeneration temperature, the energy density is also
increased which decreases the size of the adsorbent bed required to store the same
amount of energy. However, this adds an operating cost for electricity or fuel usage. At
250°C, the energy density of the adsorbent is 200kWh/m3 and has an efficiency of 30%
(from Figure 1 and Figure 2). Upgrading with supplementary heat can reduce the
adsorbent bed size by 25%. Although the size requirements for adsorption TES are still
larger than conventional heating, it is a considerable improvement over the competing
methods of thermal energy storage as can be seen in
Figure 4 for the residential scenario.
50
45
3
Storage Volume (m )
40
Adsorption(100°C)
TES
Hybrid
Sensible TES
Water
Conventional Heating
Wood
35
30
25
20
15
10
5
0
Hybrid
(100°C)
Hybrid
(250°C)
Water
Rock
<100°C
PCM
> 300°C
PCM
Wood
Figure 4: TES size comparison for the proposed household being studied: Water 60kWh/m3 [8], Rock
40 kWh/m3 [8], <100°C PCM 56kWh/m3 [1], >300°C PCM 300kWh/m3 [4], Wood 15-19 MJ/kg [7]
Sensible storage systems, such as water, rock, or Phase Change Materials (PCMs) have
very low energy densities and as a result require large volumes to achieve the same level
of energy storage. High temperature PCMs have much higher energy density but require
temperatures above 300°C, which is not useful for residential solar panels. It must also
be recognized that energy storage through sensible TES degrades with time and would
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not be useful for long term seasonal energy storage unless much larger volumes and
extensive insulation were used. Also included on
Figure 4 are the storage requirements for a year’s worth of space heating with wood with
a conventional wood furnace. The adsorption TES is not only much smaller and ideally
suited for long term thermal storage compared with other TES, but its storage size is
comparable to a conventional wood burning stove for space heating.
Conclusions and Recommendations
Adsorption TES systems are feasible for long term thermal energy storage and space
heating applications. The prototype has proven that thermal energy can be stored and
released from the adsorbent. Models for storage efficiency and energy density as a
function of regeneration temperature have been developed through experimentation and
are applied to determine the parameters of an adsorption TES for real life space heating
applications. Although the size required for the sufficient energy storage is large
compared to natural gas or electric heating, it is comparable to heating with a wood
furnace and it is renewable and sustainable. Adsorption TES is much more practical than
other competing forms of thermal energy storage. These types of systems could be
applied to any residential, commercial or retail building. Since this TES system could
also be portable, it can be applied for use of waste heat in commercial manufacturing and
refineries or large solar farms and then transported to building which do not have the
space for on-site solar energy collection. Using this approach, thermal energy could be
delivered similarly to propane for space heating.
Future experiments for this project will focus on improving the energy density of the
adsorption TES in order to reduce the size of the storage bed necessary as well as the
feasibility of upgrading the energy obtained from the power spars to achieve a higher
level of regeneration. Future work will also include performing an economic analysis of
the complete system to verify if implementation of a solar panel with adsorption TES is
feasible and if it can compete with current heating applications. Possible uses for the
remaining excess thermal energy produced from the power spars, such as supplying
domestic hot water needs or space heating for a second home, will also be investigated.
Acknowledgements
The authors wish to thank the industrial partners that have made this publication and
research possible: Menova Energy Inc, Rio Tinto Alcan and Natural Resources Canada.
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