design to maintain high vacuum and large volume and
weight relative to traditional systems.
Approaches undertaken to overcome the drawbacks
include enhancement of heat and mass transfer of the
adsorber, improving adsorption properties of adsorbents, design changes, adoption of different kinds of
cycles, improvement of regenerative heat and mass
transfer between beds etc. (Wang et al., 2018). Heat
transfer problems in adsorption cycles have been intensively investigated by Cacciola et al. (1993). The
main factors affecting heat transfer between the thermal fluid and the adsorbent bed include the physical
contact at the metal–adsorbent interface and the effective conductivity of the adsorbent bed. The thickness
of the adsorbent bed affects temperature distribution
between the surface and bottom layers in a flat plate
bed arrangement leading to uneven adsorption and
desorption. Evacuated tube type of solar collectors can
be used to improve heating efficiency in such systems.
The adsorbents in use in most solar adsorption systems have low thermal conductivity and poor porosity
characteristics. The effect is a bulky collector component and thus, excessive heating capacity and consequently a low COP. Heat dissipation to the environment
limits solar heating efficiency.
To improve performance of adsorbents, utilization
of composite adsorbents has been studied by Patel et al.
(2016) from which recommendations for adsorbentadsorbate working pairs with high cyclic adsorption
capacity and thermal conductivity, low resistance to
adsorbate flow as well as better heat management
during the adsorption cycle have been proposed.
5 IMPROVEMENT STRATEGIES
Adsorption systems must have their size and cost
reduced to become more commercially attractive. The
most promising alternatives to achieve these goals
include the improvement of heat management to
increase the COP and enhancement of internal and
external heat transfer of the adsorber to increase the
SCP. The main technologies to enhance the external
heat transfer in the adsorber are related to the increase
of the heat exchange area, the use of coated adsorbers
and utilization of heat pipe technology. To improve the
internal heat transfer, the most suitable option is the
deployment of consolidated adsorbents.
5.1 Optimization of adsorption bed design
Some of the methods used to expedite improved efficiency of the adsorber bed include improving the heat
transfer structure of the adsorption bed, enlarged mass
transfer channels, thermal insulation and automation.
Xu et al. (2014) developed an enhanced heat and mass
transfer finned tube casing in which it was found that
in comparison with the metal casing of same dimension, finned tube has 51.4 % more heat transfer per
unit length.
In his study of heat pipes, Zhang (2002) contends
that selection of proper adsorption pairs, heat and mass
transfer enhancement in the adsorption bed, invention
of advanced refrigeration cycles are always a focus of
research interests.
Dusane & Ghuge (2016) conducted studies to analyse performance of a glass tube and reflector system
using domestic charcoal-methanol pair and concluded
that system performance depended on adsorption pairs
and process parameters in agreement with Khattab
(2004), whose study on use of domestic charcoal
adsorbent achieved higher COP than that of activated
charcoal. They recommended further studies for new
low cost adsorption material and high performance
adsorbent bed.
5.1.1 Extended surfaces
Several types of extended surfaces can be considered,
such as finned tubes, plate heat exchangers and plate–
fin heat exchangers. The drawback of this technology
is that it increases the thermal mass of the adsorber;
and therefore requires efficient heat management to
produce reasonable COPs. Furthermore, this solution
should be avoided if the operation pressure is very low
and the Knudsen effect can occur (Meunier, 1998).
5.1.2 Advanced cycles
Research on advanced cycles involve cycles with heat
and mass recovery. A continuous multiple bed adsorption regenerative system with 32 solar modules has
been proposed by Critoph (2000) in which the effect
of thermal capacity ratio, number of modules, air
temperature from solar heater, generator heat transfer coefficient and evaporator air inlet temperature on
COP and CSP were reported. Cycles with heat management have achieved energy recovery of about 35%
of the total energy transmitted to the adsorber. Besides
utilization of heat management cycles, it is possible to
employ refrigerant mass recovery between two adsorbent beds to effectively enhance both the cooling power
and the COP. Szarzynski (1997) analysed cycles with
refrigerant mass recovery and concluded that the SCP
could be increased by about 20%. Wang (2001) compared the COP of adsorption systems with and without
mass recovery and found that the former could produce
a COP from 10% to 100% higher than the latter. The
difference between the COPs is higher at lower generation temperatures. Although advanced cycles can
increase the performance of the adsorption systems,
the complexity of the system also increases. Therefore,
among the studied advanced cycles, the mass recovery
cycle seems to be one of the most cost effective ways
to improve both COP and SCP.
5.2 Improvement in adsorption properties of
working pairs
Koyama et al. (2014) investigated three pairs of
adsorbent/adsorbate according to Malaysia climate
conditions. The selected pairs were activated carbonmethanol; activated carbon fibre –ethanol and silica
gel-water. Adsorption was highest in activated carbon
fibre –ethanol and lowest in silica gel/water. In a simulation experiment, activated carbon fibre –ethanol
269
weight relative to traditional systems.
Approaches undertaken to overcome the drawbacks
include enhancement of heat and mass transfer of the
adsorber, improving adsorption properties of adsorbents, design changes, adoption of different kinds of
cycles, improvement of regenerative heat and mass
transfer between beds etc. (Wang et al., 2018). Heat
transfer problems in adsorption cycles have been intensively investigated by Cacciola et al. (1993). The
main factors affecting heat transfer between the thermal fluid and the adsorbent bed include the physical
contact at the metal–adsorbent interface and the effective conductivity of the adsorbent bed. The thickness
of the adsorbent bed affects temperature distribution
between the surface and bottom layers in a flat plate
bed arrangement leading to uneven adsorption and
desorption. Evacuated tube type of solar collectors can
be used to improve heating efficiency in such systems.
The adsorbents in use in most solar adsorption systems have low thermal conductivity and poor porosity
characteristics. The effect is a bulky collector component and thus, excessive heating capacity and consequently a low COP. Heat dissipation to the environment
limits solar heating efficiency.
To improve performance of adsorbents, utilization
of composite adsorbents has been studied by Patel et al.
(2016) from which recommendations for adsorbentadsorbate working pairs with high cyclic adsorption
capacity and thermal conductivity, low resistance to
adsorbate flow as well as better heat management
during the adsorption cycle have been proposed.
5 IMPROVEMENT STRATEGIES
Adsorption systems must have their size and cost
reduced to become more commercially attractive. The
most promising alternatives to achieve these goals
include the improvement of heat management to
increase the COP and enhancement of internal and
external heat transfer of the adsorber to increase the
SCP. The main technologies to enhance the external
heat transfer in the adsorber are related to the increase
of the heat exchange area, the use of coated adsorbers
and utilization of heat pipe technology. To improve the
internal heat transfer, the most suitable option is the
deployment of consolidated adsorbents.
5.1 Optimization of adsorption bed design
Some of the methods used to expedite improved efficiency of the adsorber bed include improving the heat
transfer structure of the adsorption bed, enlarged mass
transfer channels, thermal insulation and automation.
Xu et al. (2014) developed an enhanced heat and mass
transfer finned tube casing in which it was found that
in comparison with the metal casing of same dimension, finned tube has 51.4 % more heat transfer per
unit length.
In his study of heat pipes, Zhang (2002) contends
that selection of proper adsorption pairs, heat and mass
transfer enhancement in the adsorption bed, invention
of advanced refrigeration cycles are always a focus of
research interests.
Dusane & Ghuge (2016) conducted studies to analyse performance of a glass tube and reflector system
using domestic charcoal-methanol pair and concluded
that system performance depended on adsorption pairs
and process parameters in agreement with Khattab
(2004), whose study on use of domestic charcoal
adsorbent achieved higher COP than that of activated
charcoal. They recommended further studies for new
low cost adsorption material and high performance
adsorbent bed.
5.1.1 Extended surfaces
Several types of extended surfaces can be considered,
such as finned tubes, plate heat exchangers and plate–
fin heat exchangers. The drawback of this technology
is that it increases the thermal mass of the adsorber;
and therefore requires efficient heat management to
produce reasonable COPs. Furthermore, this solution
should be avoided if the operation pressure is very low
and the Knudsen effect can occur (Meunier, 1998).
5.1.2 Advanced cycles
Research on advanced cycles involve cycles with heat
and mass recovery. A continuous multiple bed adsorption regenerative system with 32 solar modules has
been proposed by Critoph (2000) in which the effect
of thermal capacity ratio, number of modules, air
temperature from solar heater, generator heat transfer coefficient and evaporator air inlet temperature on
COP and CSP were reported. Cycles with heat management have achieved energy recovery of about 35%
of the total energy transmitted to the adsorber. Besides
utilization of heat management cycles, it is possible to
employ refrigerant mass recovery between two adsorbent beds to effectively enhance both the cooling power
and the COP. Szarzynski (1997) analysed cycles with
refrigerant mass recovery and concluded that the SCP
could be increased by about 20%. Wang (2001) compared the COP of adsorption systems with and without
mass recovery and found that the former could produce
a COP from 10% to 100% higher than the latter. The
difference between the COPs is higher at lower generation temperatures. Although advanced cycles can
increase the performance of the adsorption systems,
the complexity of the system also increases. Therefore,
among the studied advanced cycles, the mass recovery
cycle seems to be one of the most cost effective ways
to improve both COP and SCP.
5.2 Improvement in adsorption properties of
working pairs
Koyama et al. (2014) investigated three pairs of
adsorbent/adsorbate according to Malaysia climate
conditions. The selected pairs were activated carbonmethanol; activated carbon fibre –ethanol and silica
gel-water. Adsorption was highest in activated carbon
fibre –ethanol and lowest in silica gel/water. In a simulation experiment, activated carbon fibre –ethanol
269