Growing demand for energy and limited human resources along with increase in world energy prices in recent years leads us to the fact that taking steps towards optimizing energy consumption is inevitable. Nowadays, solar energy has greater potential for replacing fossil fuels. Solar energy is an environmentally friendly source and can be useful to improve and decontaminate the environment.
Desiccant dehumidifier system is a method of cooling that answers maximization of efficiency, economy and environment friendly concerns; it also meets the standards of indoor air quality.
Combination of conventional air conditioning systems and absorbent systems results in less sensible and latent heat in hybrid systems which could be an answer for controlling temperature and humidity within thermal comfort limit. The air conditioning system with a hybrid desiccant system has the potential to reduce energy consumption thanks to its less latent cooling load is.
In summary, the use of these systems has the following benefits:
||Reducing energy consumption
||Reducing bacteria and fungi
||Improving indoor air quality
||No greenhouse gases by dehumidifier
Liquid desiccant cooling cycle studies are followed in theory and practice. Here some of the earlier studies and the obtained results obtained are reviewed.
The hybrid liquid desiccant cooling systems were made by Mago and Goswami,
University of Florida. Comparison between vapor compression refrigeration system
and liquid desiccant hybrid system showed that the rate of condensation increases
in the former with air flow rate and temperature (Mago
and Goswami, 2003).
An experimental study on dehumidifier and regenerator of liquid desiccant cooling
air condition was performed by Yin et al. (2007a).
They showed the relationships of regenerator mass transfer coefficient as a
function of heating temperature and desiccant concentration (Yin
et al., 2007a).
An ideal liquid desiccant dehumidification system was featured by Wang
et al. (2010) with idealization of practical liquid desiccant dehumidification
system, regarding its exergy performance and thermodynamic properties.
A solar energy input for regenerating the lithium chloride liquid desiccant
was modeled and created in TRNSYS for Toronto, Chicago and Miami; and results
indicated that solar input could provide half of the required thermal input
(Andrusiak et al., 2010).
Artificial neural network analysis with a reasonable degree of accuracy was
used for modeling liquid desiccant dehumidification system (Gandhidasan
and Mohandes, 2010).
The open cycle solar collectors/regenerators was used to regenerate the desiccant
solution. The effect of ambient condition variables and operating condition
were also experimentally investigated (Yutong and Yang,
A simplified theoretical model was used by Chebbah (2002)
to investigate the complex phenomena of simultaneous heat and mass transfer
and the results showed that the cooling of air dehumidification process enhanced
the mass transfer and reduced the strong desiccant solution requirement.
Mechanism of proposed cooling cycle: The following overview shows design
of the cooling needs of the building under consideration. In Fig.
1, ambient hot and humid air in point1 enters to the dehumidifier tower
and the humidity of the air is reduced by passing through the liquid desiccant
solution and then enters compression cooling system; consequently thermal comfort
In order to regenerate the sorbent in the desiccant cycle, regenerator liquid
was used. By crossing in the regenerator the hot oil output of the collector
provides the thermal energy needed for regenerating desiccant liquid solution.
|| Schematic diagram of the buildings cooling cycle
Absorbing air moisture by absorbing the solution can be considered as a mass
transfer from gas-phase into liquid. In the dehumidifier, humid air and liquid
desiccant solution enter from the bottom and the top of the tower, respectively;
thus heat and mass transfer between the gas and liquid phases occur simultaneously.
This process produces a steady and continuous stream.
At a specific height of the tower, due to transferring of the solute (water vapor) from the gas phase into the liquid there must be a gradient density in mass transfer direction. In a liquid system, the liquid absorbent separates moist in direct contact with in the air. In this study, lithium bromide was utilized for dehumidifying of air. The most important parts of any desiccant system are absorbent and dehumidifier as described in the following sections.
Regenerator and dehumidifier: Desiccant solution absorbs air moist based on partial pressure difference between the desiccant surface and partial pressure of the vapor. The moist air loses humidity by passing over liquid desiccant which result to fine absorbent. After saturation desiccant solutions loses its efficiency and cannot absorb more moisture from the air. Thus, to continue the process of dehumidification it should be regenerated. Regeneration of this material usually is done by utilizing the hot air with lower vapor partial pressure than that is used in desiccant. This difference causes transferring moisture from saturated absorbent to hot air and finally provoked regenerated desiccant.
In the regenerator, the absorbent is reheated and revived. Therefore, a heat source is needed which here it is solar energy. It is notable that the capacity of regenerator is affected by the heat source and concentration of dryer.
As mentioned; mass and heat transfer are significant factors affecting performance
of the system. Therefore, there is a particular interest in this area in all
researches conducted. In what follows, the thermodynamic equations of each component
of desiccant and regenerator are given (Yin et al.,
2007b), (Yin and Zhang, 2010). In the Table
1 summary of results is indicated:
Compression refrigeration cycle: The purpose of hybridize desiccant
cycle with compression refrigeration cycle is to reduce temperature of the air
entering the designed space. Liquid desiccant cycle is not capable of providing
comfort conditions, so compression refrigeration cycle is used. In this investigation,
solar energy is used for supplying electrical power and reducing fossil fuels
needed in compression refrigeration cycle.
|| Summary of results and comparison of predecessors in the
field of liquid desiccant
|↑: Increasing trend, ↓: Decreasing trend
Compression cooling systems are conventional systems used in various areas
which are based on compressing a fluid and changing its phases from liquid to
Organic Rankine cycle: For low temperature working fluid (100-400°C) organic fluids are the best for the Rankine cycle. For selection of working fluid in Rankine cycle the flowing parameters should be considered:
||Fluid cost should be low
||Have high density in vapor phase
Using organic fluid is more efficient for producing heat from low temperature
heat. By reviewing the literature and considering that thermal energy supplied
by solar energy, n-pentane was selected in this investigation; it has been also
used in organic Rankine cycle with solar energy supply (Quoilin
et al., 2012). The governing equations in organic Rankine cycle are:
Summary of the results is listed in Table 2.
Simulation of cycle: All thermodynamics equations governing the organic Rankine cycle, compression refrigeration cycle, regenerative desiccant dehumidifier in the system was written by EES. Simulation of energy in the sample building under Bushehr's weather was done in TRNSYS. The thermal energy produced by the solar system is used for two purposes, one to set up an organic Rankine system and the other to provide the required heat in reducing the liquid desiccant system.
Solar design by using TRNSYS software is pictured below. As can be observed from Fig. 2 the defined cycle includes solar collector, pump and auxiliary heater.
According to the Fig. 2 solar heating system which is simulated
in TRNSYS is constituted of different components. In this system, solar collector
concentrators (Type 74) are used to provide 270°C. Temperature of working
fluid (oil) increases by passing through the solar collector and then leaves
the collector into the auxiliary heater (Type 6).
|| Summary of results and comparison with its predecessors in
the field of organic Rankine cycle
|| Solar heating system
When the collector outlet temperature is insufficient for the organic Rankine
cycle, the Auxiliary heater increases the fluid temperature up to design temperature.
Exhaust fluid (oil) then enters the regenerator of liquid desiccant cycle and
by losing heat it is pump back into the collector. If the pump (Type 3b) which
is controlled by controller is turned off according to scheduled plan for the
building (Type 14 h), the entire cooling system stops working. In addition,
the component (Type 24) is used for data collecting throughout the year, especially
when system is working. Furthermore, component (Type 109-TMY2) is utilized for
gathering weather properties for selected city and links the collector to the
next stage of process. At the end, component (Type 65c-2) demonstrates results
of analysis of cycle dynamically at specific time period.
Building cooling cycle optimization: For optimization of the cooling system, cycle is divided into two parts, one is liquid desiccant system with compression cooling cycle and organic Rankine cycle and the other is solar heating system. This is because of the fact that the system was modeled in two parts in two different software. As a result, optimization of each section is completed in its pertinent software. The mass rate of absorber fluid, air temperature and air flow can be defined using the basic equations of heat and mass transfer and the parameters that can influence the design of absorber tower and its efficiency.
Basic equations of heat mass transfer show that the main parameters in the design of absorbent tower are absorbent fluid rate, temperature of absorbent fluid, air temperature and also air flow rate. Furthermore, capacity and dimensions of the tower and rate of air mass to absorber fluid are the main design parameters, by these parameters; efficiency of dehumidifier can be defined. By increasing rate of liquid desiccant in HVAC systems which use liquid desiccant solution, more air volume in dehumidification can be dried, therefore more moisture can be removed. On the other hand, greater amount of liquid desiccant solution leads to more energy consumption and extra costs for instance in regenerator section. In other words, although more mass rate of fluid can absorb moister but system efficiency does not necessarily leads to a good performance. By using optimization methods, the optimal ratio between air mass flow rate and fluid flow rate can be obtained for increasing the performance coefficient of the tower.
Effect of regeneration temperature on the efficiency of regenerator: As shown in the Fig. 3, by increasing of regeneration temperature, efficiency of regenerator increases. The parameters are moist air flow of 1.5 kg sec-1, hot fluid (oil) flow rate of 0.4 kg sec-1, ambient humidity of 0.6 and ambient temperature of 40°C.
Effect of room temperature on energy consumption in the compressor: By increasing room temperature (comfort design) energy consumption in compressor is reduced. Parameters here are air flow rate of 1.5 kg sec-1 and relative humidity of 0.4.
As can be seen from Fig. 4, by changing room temperature from 20-28°C the amount of energy consumption in the compressor is decreased. Therefore, in accordance with national regulations regarding building energy consumption it is better to insulate the walls and the doors, use double layer windows and, set room temperature at 28°C in summer.
Effect of moist air mass flow rate on the compressor consumption: As seen in Fig. 5, by rising moist air flow rate inflow to dehumidifier, energy consumption in compressor is increased. The oil flow rate 0.4 kg sec-1, relative humidity equal to 0.6 and ambient temperature 38°C were used.
Effect of increasing turbine inlet pressure (pressure ratio) on the performance of organic Rankine cycle: As illustrated in Fig. 6, by increasing pressure of inlet fluid working at turbine in an organic Rankine cycle, efficiency of the cycle is increased considerably.
|| Effect of regeneration temperature on the efficiency of regenerator
|| Effect of room temperature on energy consumption in the compressor
|| Effect of moist air mass flow rate on the compressor consumption
By increasing the fluid inlet pressure to the turbine, work output of the turbine increases, due to the pressure difference and causes increases in the efficiency of the organic Rankine.
Effect of dehumidification besides cooling in hot and humid weather condition: Considering that the most of the cooling of buildings is done by compression refrigeration cycle, in this study, the advantage of using desiccant cycle besides compression refrigeration cycle is reduction of the cooling load of the building (Fig. 7). Using the dehumidifier besides the vapour compression refrigeration cycle led to reduction of building cooling load by about 13%.
||Effect of increasing turbine inlet pressure (pressure ratio)
on the performance of organic Rankine cycle
|| Using desiccant cycle besides vapour compression refrigeration
Structural optimization of solar heating system: Optimization of the structure solar system, such as the area and acceptance angle of the collector was studied. Oil with fixed flow rate was considered as working fluid and given that the fluid rate is used to supply energy for Rankine cycle and regeneration heat, it was remained fixed as with its variation, other parameters change considerably. Solar system structure and its changes are discussed below.
Solar fraction index was defined at the outset which is defined as the share
of energy consumed by cooling systems which can be supplied by the collector.
Optimization was based on the amount of solar fraction. When the auxiliary heater
has the lowest energy consumption or SF has the highest value, the model is
optimal. Different studies have proposed different definitions of solar fraction.
In this study, the following definitions were under consideration:
Optimal collector acceptance angle: Collector acceptance angle based on the heat absorbed by the collector during the cooling period is shown in Fig. 8. The picture shows the climatic conditions in Bushehr with latitude of about 30 degrees and oil as the fluid used in the collector with the working hours of 7 am to 18 pm.
As pictured in Fig. 8 with increasing acceptance angle up to 90°, the amount of useful energy gain increases. However, there are few changes up to 70°. Therefore, 70° angle is considered as the optimum angle. Schematic diagram of CPC collector is depicted in Fig. 9. As can be seen the red angle indicates acceptance angle of the CPC collector.
Optimal collector area: To optimize collector area, change of SF was examined by varying the values of collector area. The variations of SF based on changes of the collector area are pictured below.
As shown in Fig. 10, with increase of the solar collector area, contribution of solar energy to meet the increased cooling requirements of the building increases and the share of auxiliary heater decreases. By taking the cooling load of building approximately 50 tons of refrigeration for hottest months as a reference of designing solar collector system, variation of collector area was input in the software and an area of approximately 350 m2 was considered as the appropriate choice. By this way the optimum collector area is obtained.
Solar system simulated results: In this section, components of the solar
heating system during the performance of an office unit in Bushehr and area
in 1000 square meters for 48 h and cooling capacity of 50 ton in August was
|| Effect of increasing acceptance angle of collector on useful
|| Schematic diagram of CPC collector
|| Effect of increasing collector area on solar fraction
The optimum operating conditions of the cycle, on the basis of solar fraction
are summarized in Table 3 as follows.
The graph in Fig. 11 compares the amount of heat transfer
rate which is supplied by solar energy, with the amount of heat supplied by
auxiliary heater in the first days of August. As an overall trend, it is clear
that at the end of working hours the system automatically switches off and restarts
the next working day. Clearly, in some days the collector can provide even more
than what is needed to the system setup and the oil temperature is higher than
set point temperature.
|| Solar simulated results
|| Dynamic performance of the solar system
The purple line indicates ambient temperature in Bushehr at this time of year.
It was demonstrated that it is feasible to use solar liquid desiccant cooling cycle in hybrid mode with vapour compression cycle to meet cooling load of the building in hot and humid climate. Merits of using suggested cycle in comparison with vapour compression cycle are: considerable decrease of cooling load of the building by dehumidifying supply air; and decrease in fossil fuel consumption by using solar energy.
Because of load latency in the traditional vapour compression systems, the air needs to be cooled below the dew point. High latent needs low temperatures to evaporate at the cooling cycle. There are limitations to the low temperature and the temperature cannot be less than the freezing temperature range. However, the cold air cannot enter the space. Thus in traditional methods when the latent heat is high, reheating should be used. Therefore, efficiency is not promising by any mean. By combining vapour compression cycle and absorbent system, higher efficiency is obtained. By comparison, solar liquid desiccant cooling cycle in hybrid mode with vapour compression cycle and vapour compression cycle resulted in 13% reducing of cooling load of case study building.
||Area of CPC collector (m2)
||Heat transfer (kJ)
||Concentrate of liquid (kg kg-1)
||Enthalpy (kJ kg-1)
||Flow rate (kg sec-1)
||Humidity ratio (kg kg-1)
||Specific heat capacity (kJ kg-1K-1)
||Useful energy gain from solar collector (kJ h-1)