Abstract
A review on Liquid Desiccant based cooling technologies
with special focus on advances in liquid desiccant materials and configurations
of heat and mass exchangers have been discussed. Performance comparison and
energy saving potential of a hybrid liquid desiccant cooling system based on
vapour compression based sensible cooling and liquid desiccant based
dehumidification in comparison to conventional vapour compression system has
been reviewed. Hybrid liquid desiccant cooling system has enormous energy and
cost saving potential especially in hot and humid regions like India. The
ability of Liquid Desiccant cooling technology to be energized by Solar thermal
makes it an attractive alternative to high electrical energy intensive
conventional vapour compression based cooling for residential and commercial
HVAC applications.
Keywords: Liquid Desiccant, Dehumidification, Vapour
Compression, Solar Thermal.
1. Introduction
India is a tropical country and more
than 80% of Indian Sub continental area falls under Warm humid or Composite
Climatic zone [1]. These climatic zones are characterized by high annual
average temperatures and high humidity. With rapid urbanization and
industrialization in India, there is sharp rise in air conditioning load in
Industrial, commercial as well as residential buildings.
The Air conditioning load could be broadly
classified as sensible load and latent load. Conventional vapour compression
Air conditioners (VAC) meet the total air conditioning load by cooling the air
below the dew point temperature and thus condensing the moisture. These systems
require evaporator temperatures to be maintained much lower than required to
achieve sensible cooling alone. The dew point temperature is much below the set
temperature level and hence process air requires further heating to bring its
temperature to set temperature level. This requirement increases the capacity
rating of the compressors and requires high electricity and consequently
operates at reduced coefficient of performance (COP) [2].
There is a necessity to separate
the latent cooling load and sensible cooling load and handle them separately so
as to improve the COP of the air conditioners. The desiccant cycles can be used
to reduce the moisture content of air by partially converting latent cooling
load to sensible cooling load and then meeting the load by VAC’s. These systems
with vapour compression cycle for meeting sensible cooling load and liquid –
desiccant cycle for latent cooling load are called hybrid systems.
2. Liquid desiccant
dehumidification and air conditioning
A desiccant material has a strong attraction for water
vapour. Desiccants are commonly used in industrial applications where low
dew-point air is needed. The strength of a desiccant can be measured by its
equilibrium vapour pressure (i.e., pressure of water vapour that is in
equilibrium with the desiccant). This equilibrium vapour pressure increases
roughly exponentially with the temperature of the desiccant/water system. It
also increases as the desiccant absorbs water (a dilute liquid desiccant will
have a higher equilibrium vapour pressure than a concentrated liquid
desiccant). When the absolute humidity of air that has come into equilibrium
with a liquid desiccant of fixed concentration is plotted on a psychometric
chart, the equilibrium line closely follows a line of constant relative
humidity and the Fig1. illustrates this behaviour for solutions of lithium
chloride.
As
shown in the Fig 2., the brine-bulb temperature for a 43% solution of lithium
chloride and air at 30.0/25.6°C dry-bulb/wet-bulb will be 47.8°C. With an
ambient wet-bulb temperature of 25.6°C, a typical cooling tower might supply
water at29.4°C. It’s impractical to cool the ambient air using this cooling
water in a conventional heat exchanger, because the cooling water is only one
degree below the air temperature. However, a strong cooling effect could be
achieved by wetting the surfaces of the heat exchanger with the 43% lithium
chloride. Of course, one does not get this enhanced cooling for free. If the
cooling process is to be continuous, energy must be expended to regenerate the
desiccant back to its original concentration. If ambient air from the preceding
example is brought into equilibrium with 43% lithium chloride at 85°F (29.4°C),
the air will have a dew point of 33.5°F (0.8°C), a wet-bulb of 57.8°F (14.3°C),
and its enthalpy will be reduced from 41.5 Btu/lb (96.3 kJ/kg) to 24.9 Btu/lb
(57.8 kJ/kg). This large cooling effect, both in terms of latent cooling and
total cooling, and low dew point—both of which are achieved without a compressor—demonstrate
the potential for liquid desiccants to become an important part of HVAC
systems. Liquid desiccants have been successfully used to produce dry air for a
surprisingly long time. Dr. Russell Bichowsky, working for the Frigidaire
Division of General Motors, first used solutions of lithium chloride to dry air
in the 1930s. Also in the 1930s, the Niagara Blower Company introduced a liquid
desiccant technology that used glycol solutions to prevent frost from forming
on low-temperature evaporators. Both lithium chloride and glycol continue to be
used today in liquid-desiccant dehumidifiers, but their use is limited
primarily to industrial applications[3].
3.
Hybrid configuration: desiccant de-humidification and vapour compression based
cooling
An example of desiccant cooling application is
represented in fig. 3 [4].
Figure 3.Schematic of
Hybrid Liquid Desiccant aided Vapour compression air conditioning
Here,
the cool strong desiccant solution is sprayed onto the top of the dehumidifier through
spraying nozzles. By gravitation, it trickles through the structure of the
dehumidifier where it gets contact with the process air stream blown
perpendicularly to its trickling flow direction. Since, the cool and strong
desiccant solution vapour pressure is less than that of the air vapour
pressure, water vapour migrates from the air stream to the desiccant solution
and condenses therein. Consequently, the heat of condensation and mixing are
liberated causing an increase in the solution’s temperature. The process air
stream is slightly cooled down due to its contact with the cold desiccant
solution. The dehumidified and rather warm process air stream then passes
successively through the evaporative cooler and the evaporator of the
traditional refrigerant vapour compression air conditioner, before being
delivered into the conditioned space. The diluted desiccant solution, exited
from dehumidifier, is circulated through the regenerator where it is heated and
the moisture absorbed in the dehumidifier is now lost to the scavenger air
stream. In order for the system to keep functioning continuously and
effectively, an equal amount of water vapour absorbed from the humid air and
condensed to the desiccant solution in dehumidifier must be evaporated from the
desiccant solution in the regenerator. The hot and strong desiccant solution is
thereafter cooled down in the pre-cooler and then cooled further in the heat
exchanger (HX) before being ready again to dehumidify the incoming process air.
The
lowest limit temperature attainable by the evaporative cooler is the process
air wet bulb temperature which decreases with the decrease of the relative
humidity and increases with the elevation of the dry bulb temperature. The
essential role of the desiccant solution in this example is to lower the
relative humidity of the incoming air stream in order to enable the evaporative
cooler to function more effectively.
Here,
the desiccant assisted evaporative cooling is associated with the traditional
vapour compression air conditioning to reduce its size and enhance its
coefficient of performance. Because the latent load is handled independently by
the desiccant dehumidifier, the need of cooling the ventilation air below its
dew point is obviated. The temperature of evaporation can thus be lifted up to
15 °C from its generally practiced level of 5 °C for the traditional vapour
compression system. The increase in evaporation temperature will entail the
increase of the system’s coefficient of performance (COP).
This assemblage
can be useful in humid climates where the wet bulb temperature is fairly high.
In such climates, a significantly downsized vapour compression air conditioner
can be supplemented with a desiccant assisted evaporative cooler in order to
reach the desired indoor temperature, thus enabling costs and energy savings
and improving the indoor air quality.
4.
Liquid desiccant materials
Liquid
desiccants such as Glycols and solutions of halide salts are routinely used in
industrial de-humidifiers. Commonly used liquid desiccant materials include
lithium chloride, lithium bromide, calcium chloride, triethylene glycol and
mixture of salts etc. The choice of desiccant will have a profound effect on
the design of desiccant de-humidifiers.
The
desirable properties of liquid desiccants include large saturation absorption
capacity, low regeneration temperature, Low Viscosity, Good heat transfer, non
volatile, non – corrosive, odourless, non toxic, non flammable, stable and
inexpensive. Surface Tension of liquid desiccants is an important parameter of
liquid desiccants as it plays important role in static hold up and wetting of
the surface of heat and mass exchanger of Liquid desiccant system.
Halide
salts such as lithium chloride and lithium bromide are very strong desiccants.
A saturated solution of lithium bromide can dry air to 6% relative humidity and
lithium chloride to 15% but halide salts are corrosive in nature. Lithium
Chloride has good desiccant characteristics and does not vapourize at ambient
conditions but droplet filters are necessary to prevent any mixing of the
liquid droplets with process air. Cost of halide salts are relatively high
except calcium chloride whose cost is comparatively low compared to LiCl, LiBr
and TEG. Another advantage of Calcium chloride is its low viscosity which
reduces the pumping power. But the CaCl2 salt is highly corrosive in nature and
can be used in non metallic systems only [5].
The
least expensive alternative to lithium chloride is calcium chloride.
Unfortunately, calcium chloride is a relatively weak desiccant. A 42% solution,
which is about as strong as can be used without encountering crystallization,
will dry air to about 35% rh. (For comparison, a 43% lithium chloride solution
can dry air to a 15% rh.).
Glycols
are the second class of liquid desiccants now used in industrial equipment.
Both triethylene and propylene glycol have low toxicity, and their
compatibility with most metals has led several researchers to use them in LDACs
designed for HVAC applications. However, all glycols have one undesirable
characteristic that they are volatile and any evaporation into the supply air
makes it unacceptable for air conditioning for occupied buildings [6].
Salts
of weak organic acids, such as potassium or sodium formate and acetate, have
been explored as less corrosive alternatives to halide salts that are also not
volatile. Although it is a significantly weaker desiccant than lithium bromide
or lithium chloride, the ability to dry air below 30% relative humidity could
make potassium formate a good alternative desiccant in some applications.
Another less expensive alternative is potassium acetate. While potassium
acetate could dry air to about 25%, its viscosity becomes very high. At 70%
concentration and 27°C, a potassium acetate solution has a viscosity of about
28 cp. This is almost twice has high as a 43% lithium chloride solution at the
same temperature. Water at 27°C has a viscosity of close to 1.0. [3].
Studies
were also conducted on mixtures of calcium chloride and lithium chloride
solutions to take the advantage of good desiccant properties of LiCl and low
cost CaCl2 [7].
4.1
Advantages of using liquid desiccants include
1.
Lower
air pressure drop in process air stream.
2.
Liquid
desiccants are capable of providing equivalent dehumidification as solid
desiccant systems with lower regeneration temperature(mostly 70 - 80°C) due to
the internal cooling provided by cooling tower water or chilled water and
allowing utilization of solar heat or waste heat.
3.
Pumping
of liquid desiccants is possible makes it possible to connect several small
desiccant dehumidifiers to a larger regeneration unit which is especially
beneficial for large multi zonal commercial buildings.
4.
Liquid
desiccants have high COP’s as highly efficient liquid-liquid exchangers could
be employed.
5.
Simultaneous
air dehumidification and desiccant regeneration is not necessary as it is
possible to store dilute saturate liquid until regeneration heat is available.
6.
Liquid
desiccants are highly beneficial for their ability to filter microbial
contamination, bacteria, viruses, and moulds from process air stream.
5. Heat and mass exchangers for Liquid desiccant
de-humidification
The
Heat and mass exchanger of a desiccant dehumidification unit is where the
liquid desiccant comes in direct contact with the process air.
The
desirable characteristics for heat and Mass exchanger for high-performance
liquid desiccant dehumidifiers
1.
High
heat and mass transfer rates
2.
Low
pressure drop in process air flow
3.
Small
liquid-side resistance to moisture diffusion
4.
Large
contact transfer surface area per unit volume
5.
Compatible
desiccant/contact materials (non corrodible with high wetting coefficient)
6.
Zero
carryover of liquid desiccant droplets into process air
7.
Use
of common materials and inexpensive manufacturing techniques
8.
Classified
various thermally activated Desiccant cooling technologies as shown in fig. 4
Figure 4. Heat and mass exchanger configurations for
various desiccant cooling technologies
The
packed-bed conditioner has been the focus of many R&D projects on Desiccant
De-humidifiers. More recent R&D on packed-bed heat and mass exchangers
includes the work of [9] in which the performance of packed-bed heat and mass
exchangers flooded with lithium chloride solutions were experimentally
measured. The researchers first implemented their conditioner and regenerator
as internally cooled units using either copper tubes or polypropylene tubes as
the contact surface. However, the copper tubes were too easily corroded by the desiccant,
and the polypropylene tubes were too difficult for wetting.
[10] modelled
and experimentally measured the performance of packed-bed, lithium chloride
heat, and mass exchangers that used a random, polypropylene packing with a
volumetric surface area of 210 m2 per m3. They reported
that the lithium chloride solution did not uniformly wet the packing because of
its high surface
tension.
|
High
flooding rates are necessary to keep the desiccant cool and complete wetting.
But High flooding rates may cause carryover of liquid desiccant droplets into
air stream and also is responsible for pressure drop in air flow. Conditioners
that are internally cooled do not have to operate at the high flooding rates of
packed-bed units as the desiccant temperature is maintained close to coolant
temperature [11]. A cross flow heat exchanger is shown inthe Fig. 5 which is internally cooled by coolant where the process air flow and desiccant flow
contact at right angles. A coolant liquid provided from a cooling tower or
chilled water enters through the pipe section throughout the heat exchanger and
hence internally cooling desiccant.
At
low liquid desiccant temperature the vapour pressure also remains low and thus
allowing more moisture absorption into the liquid desiccant.
6. Solar Hybrid
Desiccant Cooling System
[12]
has investigated the solar hybrid desiccant cooling system (SHDCS) shown in
Fig. 7 for its applicability and performance in commercial premises with high
latent cooling load in subtropical Hong Kong. Vapour compression chiller was
used to provide chilled water to a supply air cooling coil. Desiccant wheel was
adopted and its regenerating heat primarily came from the solar thermal gain of
the evacuated tubes. The desiccant wheel dehumidified the fresh air to the
required level and the supply air coil provided the sensible cooling. For
commercial premises with high latent cooling load (60% RH) It is observed that
SHDCS had more superior cooling and energy performances than the conventional
centralized air-conditioning (AC) system in the subtropical Hong Kong. The
annual primary energy consumption saving could be around 49.5% in comparison to
conventional vapour compression systems.
Figure 7. Solar Hybrid Desiccant Cooling
system with solar heating for desiccant regeneration, Desiccant
Dehumidification and compression based cooling
[13]
simulated a hybrid desiccant cooling system comprising the conventional vapour
compression air conditioning system coupled with a liquid desiccant
dehumidifier which was regenerated by solar energy. The study suggested that,
when the latent load constitutes 90% of the total cooling load, the system can
generate up to 80% of energy savings. [14] conducted a comparative study of a
standalone VAC, the desiccant-associated VAC, and the desiccant and evaporative
cooling associated VAC as shown in following figure. The authors found an
increase of cold production by 38.8–76% and that of COP by 20–30%. [15] have
studied the performance of three possible hybrid system configurations in
supermarket applications and have compared their performance with traditional
VAC system. As reported, a total air conditioning saving ranging from 56.5% to
66% could be achieved for specified design conditions (ambient conditions: 30
°C, 16 g/kg.da; indoor conditions: 24 °C, 10.4 g/kg.da; room sensible heat
ratio: 0.35). [16] have modelled the performance of a desiccant integrated
hybrid VAC system. The waste heat rejected from a VAC cycle is utilized to
activate a solid desiccant dehumidification cycle directly. The performance
sensitivity of a first generation prototype hybrid VAC system to variable
outdoor conditions has been studied and compared to the performance of
conventional VAC systems. Results showed that the performance improvement over VAC
systems could be 60% at the same level of dehumidification under ARI summer
conditions. [17] have simulated the transient performance of a hybrid desiccant
VAC system for the ambient conditions of Beirut. The annual energy consumption
of the hybrid system in comparison with the conventional VAC system has been
studied for the entire cooling season. A payback period less than five years
was achieved.
[18]
has reviewed various thermally activated cooling technologies and has tabulated
a summary of the main features of up-to-date thermally activated cooling
technologies which is shown in table 1. It is observed that of all the
technologies liquid desiccant cooling technologies has lowest regeneration
temperatures and has better COP value compared to other technologies.
7. Conclusion
Liquid
Desiccant dehumidification systems although limited to industrial applications,
but could exhibit huge potential energy and economic savings for HVAC industry
by
·
Reducing
the peak electricity demand created by compressor – based AC’s
·
Improving
the indoor air quality and reduce the indoor humidity levels that could be
difficult to be controlled by conventional air conditioners.
Hybrid
liquid desiccant cooling technology has demonstrated its superior performance
for hot and humid climatic conditions and could save more that 50% operational
energy saving compared to conventional vapour compression cooling technology. One
of most important advantages of desiccant cooling systems undoubtedly lies in
the possibility of using solar energy which can be effectively utilized to regenerate
saturated liquid desiccants by using relatively low-cost solar thermal
collectors. LDAC’s have low regeneration temperatures (60 - 90°C) and have high
COP values (0.5 – 1.2) compared to other thermally activated cooling
technologies.
Future research should
include development of non corrosive desiccant materials having lesser
regeneration temperatures, developing control strategies to prevent mixing of
liquid desiccant droplets in the process air stream and design of small and
compact systems for application in residential buildings.
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