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 conditioningA 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 wetbulb
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 basedcooling
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.
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Air cooled heat exchangers are only one of various kinds of heat exchanger. Heat exchangers are gadgets which are specially designed for aiding the transfer of heat from one fluid towards the other. For us to better understand, typical samples of devices that use heat exchangers include the air conditioners, refrigerators as well as space heaters.
Besides learning the actual functions of ACHEs, we should have a view upon how it operates inside and its elements. Usually, air coolers for process application are composed of a finned-tube bundle with rectangular container headers on both ends of the tubes. Headers are boxers that dispense the fluid from the piping to the tubes. The other parts include one or more fans that give cooling air. The fans are generally powered by electric motors and are also accompanied by speed reduction devices. The fan drive assemblage is sustained by the metal mechanical drive support system. Generally, the air flows up-wards all the way through a horizontal tube bundle. The fans could be either forced or induced draft, dependant upon whether the air is pushed or pulled through the tube bundle. Forced draft unit enables a simple access for maintenance for the fans and the bundles. Furthermore, the fans stay in the cold ambient air. Whilst the induced draft unit provides a stable as well as strong thermal performance because of the protection of the finned surface against wind, rain, hail and snow by the plenum chamber. The induced draft also guarantees an improved air distribution, much less hot air recirculation, much less air-side fouling and lower noise levels at grade. Space in between the fans and the tube bundle is encircled by a plenum chamber, that directs the air. The full assembly is usually installed on legs or perhaps a pipe rack.
Understanding and knowing the inside and out of ACHEs is fairly difficult, especially for those who have no background in mechanical matters. And for this reason, let us once again move our attention on another subject matter, which is the "why" question. With regards to economy, air coolers are preferred due to the lower maintenance expenses. Some other benefits offered by air coolers are as follows: lack of the need for cooling water or other cooling media, no issue shall arise with regards to the thermal as well as chemical pollution of cooling fluids, and it quite fast to install by bolted assembly.
