1. Introduction
Statistics prove that the performance of an individual human being is more effective in a conditioned space compared to that in an untreated environment, as a result of which the thermal comfort requirement becomes essential because people spend most of the time in confined environments. Human comfort conditions are achieved when temperature and moisture are held within a certain narrow range. The acceptable ranges of temperature and relative humidity for human comfort as given by the ASHRAE standards 55 are 20–26 °C and 30–60%, respectively. This increases the demand of air conditioning, both in the residential and commercial sectors. The use of conventional cooling system consumes large amount of energy to fulfill thermal comfort conditions. Furthermore, this technology is not energy efficient for climatic conditions where latent loads are dominant [
1].
Some alternative technology is required in order to overcome the drawbacks and rising demands of conventional cooling systems for residential as well as commercial buildings [
2,
3]. The regional demand of heating, ventilation, and air conditioning (HVAC) equipment is presented in
Figure 1 [
4]. It can be observed that the HVAC equipment demand is growing at a rapid rate for all regions. Secondly, the consumption of primary energy resources has increased significantly in recent years because of growing demand. An effective alternative is desiccant cooling technology. Due to the affinity to absorb/adsorb moisture, desiccants like silica gel, SiO
2, and zeolite attract moisture without any change in their chemical and physical composition. The desiccant material is regenerated using hot air so that the cycle can be repeated. After the dehumidification of the process air, the evaporative cooler is used to lower the temperature according to the desired comfort conditions [
5]. Desiccant cooling system is an effective and cost-efficient way to provide comfort in hot and humid environments. This technology has many potential advantages over other cooling techniques such as, better supply conditions, energy efficient, effective utilization of low grade energy sources, etc. [
6].
The desiccant cooling technology can be categorized based on the type of desiccant material used. Solid- and liquid-based desiccant cooling are two major types of this thermally driven technology. The solid desiccant cooling technology is more mature as compared to liquid desiccant. Liquid desiccants have advantage over the solid desiccants in that these require lower temperature heat sources (60–85 °C) for regeneration. This makes the usage of renewable energy resources like solar, biomass, etc. more feasible and effective [
7]. However, most of the studies carried out on liquid desiccant dehumidification systems are direct contact type in which there is a direct contact between liquid desiccant and the air streams. Direct contact type systems have the drawback that the supply air can carry some liquid desiccant droplets with it and can cause corrosion in the ducting and other equipment. This can increase the maintenance costs and decrease the equipment life. Secondly, desiccant carryover can affect adversely the quality of air and human health. Direct contact type systems also require high fan and pumping power because of the higher pressure drop and continuous supply of desiccant solution.
Different configurations are employed to overcome the problem of desiccant carryover and issues related to packed bed liquid desiccant dehumidifiers [
8]. The use of internally cooled/heated [
9,
10] and membrane type [
11] dehumidifiers are few of the solutions to the problem of desiccant carryover [
9,
10]. The rotary type liquid desiccant cooling system can also be designed to overcome drawbacks of packed bed liquid desiccant cooling systems. These wheels have large surface area and less pressure drops as compared to packed bed dehumidifiers. Some general advantages of using rotary desiccant wheel as a dehumidification unit are:
These systems are more convenient to install.
The process of dehumidification and regeneration are synchronized in these systems and operation is continuous.
For same contact surface area, these systems occupy a smaller space compared to packed bed dehumidifiers.
Dehumidification/regeneration capacity of the dehumidifier can be efficiently controlled by changing the rotational speed of the desiccant wheel.
In the past, most of the investigations were carried out for the solid desiccant rotary dehumidifiers and no in-depth work is available for energy analysis of rotary liquid desiccant dehumidifiers. The purpose of this research is to develop rotary type dehumidifier using a liquid desiccant material instead of solid desiccant to lower the required input heat for the operation of the system. In this paper, an experimental unit is designed and built to study the performance of the rotary liquid desiccant dehumidifier operating in conjunction with two stage evaporative cooler considering different parameters. The combination of direct and indirect evaporative cooler is used as a cooling unit. The experimental results show that better supply air conditions could be obtained to achieve human comfort in the hot and humid climate with effectiveness of the system largely dependent on air flow rate, regeneration temperature and humidity ratio of the process air. The developed rotary liquid desiccant cooling system is expected to overcome the disadvantages of liquid desiccants.
2. Two Stage Evaporative Cooling
In a desiccant-based cooling system, the latent load is controlled by the desiccant dehumidifier whereas for sensible load evaporative coolers can be utilized. Different configurations of evaporative cooler can be used in conjunction with desiccant dehumidifier depending upon the climatic conditions. The two basic evaporative cooler configurations are the direct and indirect type. Two stage evaporative coolers are an advanced technique mostly used for hot and humid climatic conditions. In hot and humid conditions, it is difficult to cool the air using only an indirect evaporative cooler and the use of larger size direct evaporative coolers will not be an economical option. Two stage indirect-direct evaporative coolers can cool the air to lower temperatures as compared to one stage evaporative coolers [
12]. This cooling option is much more energy efficient than cooling with refrigerants. According to ASHRAE, 60–75% savings on electricity can be achieved by replacing conventional vapor compression cooling systems with advanced two stage evaporative coolers.
Many researchers have investigated the use of the two stage evaporative cooler configuration as a standalone unit and in conjunction with desiccant dehumidifiers. Farahani et al. [
13] investigated a two stage evaporative cooling system for the climatic conditions of Tehran (Iran). The results demonstrated that the use of two stage evaporative cooling can fulfill the human comfort demands efficiently. El-Dessouky [
14] developed and tested a modified two stage evaporative cooler as an alternative to single stage evaporative cooling. The results showed that efficiency of two stage evaporative cooling system is better than a single stage evaporation system. Al-Sulaiman et al. [
15] also utilized two stage evaporative cooling system with liquid desiccant dehumidifier. Tashtoush et al. [
16] obtained 20% COP with two stage evaporative cooler than that achieved when employing either direct or indirect evaporative cooler systems alone. Different researchers have utilized multistage evaporative cooling systems to achieve desired supply conditions for thermal comfort [
17,
18]. Rafique et al. [
1] mentioned that the effectiveness of direct, indirect and indirect-direct evaporative coolers is 80–90%, 85%, and 110%, respectively. Furthermore, the energy saving potential of two stage evaporative coolers is better compared to single stage evaporative coolers.
8. Performance Indicators
A number of parameters are used in order to evaluate the performance of desiccant based cooling system. This section describes the performance indices used in the present system. These parameters are used for performance of a liquid desiccant enhanced cooling system to have lower regeneration temperature, no carryover of desiccant solution, and better supply air conditions. The relationships for dehumidification coefficient of performance (DCOP), cooling capacity, coefficient of performance (COP), electric coefficient of performance (ECOP), thermal coefficient of performance (TCOP) and sensible energy ratio (SER) are presented in Equations (5)–(10), respectively [
19,
20,
21]:
where, the value of equivalent conversion coefficient
) is taken as 0.3 for the present study [
12]:
The additional cooling required along with the dehumidification system in order to achieve comfort conditions in a conditioned room is defined by sensible energy ratio (SER) [
21]:
For better performance of the system, the achieved values of DCOP, CC, COP, ECOP, and TCOP should be higher whereas value of SER should be lower. The lower value of SER means less cooling is required after the desiccant dehumidifier.
9. Results and Discussion
The performance of the desiccant cooling system is tested to have lower regeneration temperature and better supply air conditions. Experiments were carried out using the ranges of operating parameters listed in
Table 3. Generally, high values of the determination coefficient or
R-squared (
R2) were obtained in all the cases with a linear fitted regression line. The high value of
R2 indicates that data is uniformly distributed along the regssion line.
The effect of the process air inlet humidity on system’s performance was investigated and the results are shown in
Figure 6. The electrical, thermal, and overall performance increased with the increase in inlet air humidity ratio. The mass transfer potential enhanced with increased humidity ratio of ambient air which in turn improved the capacity and performance of the system. In fact, the driving force for mass transfer is increased with the rise of humidity ratio of inlet air. Although, with the increase in ambient air humidity the required input heat for desorption of moisture also increased. However, this increase is not as high as capacity of the system to remove latent load. Thus, the higher the ambient air humidity, the better the performance (COP, TCOP, and ECOP, DCOP) of the system will be.
The COP, TCOP, ECOP, and DCOP represent the overall, thermal, electrical and dehumidification coefficient of performance, respectively. Similar to the moisture removal rate, a higher DCOP was achieved at higher inlet humidity ratio due to an increase in the moisture absorption capacity of desiccant dehumidifier as shown in
Figure 6. For instance, the DCOP increased by 62% when the humidity ratio is changed from 0.01 to 0.025 kg/kg. With regard to sensible energy ratio, the temperature at the exit of dehumidifier strongly increased with humidity ratio of inlet air because of larger quantity of water vapor absorbed by the desiccant. Therefore, temperature at state 2 increased due to the increase in the released heat of absorption. Thus, increase in humidity ratio of the inlet air increases the sensible energy ratio of the system at fixed regeneration value and supply air temperature.
Figure 7 illustrates the dependence of different performance parameters on air temperature. As regards to DCOP, an increase in the process air inlet temperature causes a slight reduction of moisture removal capacity but an increase in enthalpy of inlet air increases DCOP. In case of SER, the effect of inlet air temperature is insignificant due to change in temperature of inlet air because of small variations in absorption heat and air temperature at the exit of the dehumidifier.
An increase in the mass flux of the air leads to an improvement in the system performance coefficients COP, ECOP, and TCOP, (
Figure 8). This is due to the fact that the cooling capacity of the system increases while the input energy is kept constant. It is to be noted that the mass flux of regeneration air is kept constant and so the required thermal load remains the same. Due to respective high and low mass flow rates, the absorbed moisture may not be efficiently removed from the dehumidifier. Similarly, at low flow rate of process air and high flow rate of regeneration air, the required heat input will increase and may cause a decrease in system performance. The moisture absorbed by the desiccant dehumidifier may increase with the process air mass flux due to enhanced driving force for moisture transfer. However, the total moisture removal capacity decreases due to less residence time at higher flow rates. Thus, DCOP decreases with mass flux of process air. Furthermore, increasing the inlet flow rate of process air leads to an increase in both the supply air humidity ratio and temperature inside the dehumidifier. Thus, keeping the regeneration temperature constant, sensible energy ratio increases with inlet process air mass flux as shown in
Figure 8.
The effect of regeneration air flow rate on performance of the system is presented in
Figure 9. The COP and TCOP decreased by 25 and 60%, respectively with the regeneration air flow rate changed from 1.5 to 4.5 kg/m
2·s. An increase in regeneration flow rate decreases DCOP, as shown in
Figure 9. This is due to a proportional rise in the energy requirement for regeneration. With regards to SER, an increase in regeneration air flow rate resulted in the rise of absorbed moisture and heat of absorption. The regeneration temperature inversely affects the overall COP, TCOP, and DCOP of the system as it can be observed from
Figure 10. The variation of regeneration temperature from 55 to 85 °C, the values of COP, TCOP, and DCOP of the system decreased from 0.91 to 0.58, 3.4 to 1, and 0.6 to 0.4, respectively. The sensible energy ratio is decreased with the increase in regeneration temperature (
Figure 10). This decrease of SER is due to the increase in the difference between the regeneration and the process air inlet temperatures. As described above, the increase in ratio of flow rates causes a decrease in DCOP for desiccant dehumidifier due to an increase in regeneration heat. Also, the increase in this ratio will cause an increase in temperature at the exit of the dehumidifier due to increase in released heat of absorption. This increase in exit temperature in turn decreases the SER. Thus, with the increase in ratio of air flow rates and regeneration temperature, a reduction in SER occurs.
The developed system showed better performance compared to the results reported in the literature using other configurations of this technology. The results obtained at a regeneration temperature of 55 °C for the present system are; COP = 0.8, TCOP = 2.4 whereas the results obtained by Abdel-Salam et al. [
20] for the membrane liquid desiccant dehumidifier under similar operating conditions were: COP = 0.63 and TCOP = 1.48. Furthermore, the value of COP obtained by Bourdoukan et al. [
22] for the effect of regeneration temperature (55 °C), ambient temperature (35 °C), and inlet humidity ratio (0.014 kg/kg) were reported as 0.42, 0.29, and 0.35, respectively, whereas, under similar operating conditions, the present system give COP values of 0.8, 0.5, and 0.44 for the effect of regeneration temperature, ambient temperature and inlet humidity ratio, respectively. In comparison to a solid desiccant rotary cooling system, the achieved performance of the proposed system is much better. The dehumidification coefficient of performance achieved by Ge et al. [
19] for a solid desiccant cooling system at a regeneration temperature of 60 °C was 0.38 whereas in the present case it was 0.59. The performance of liquid desiccant assisted cooling system is expected to be higher compared to solid desiccant cooling system due to lower regeneration heat requirement. The required regeneration temperature in case of liquid desiccant (calcium chloride) is 60–85 °C [
7] whereas for solid desiccant (silica gel) it is 60–120 °C [
23]. More fieldwork is required in order to compete with other technologies available in the market. A summary of potential advantages which can be achieved by the implementation of proposed system is provided in
Figure 11.
10. Conclusions
Solar thermal cooling is not a new concept, nevertheless, it has been gaining relevance to provide efficient cooling at low costs without generating CO2 emissions. Throughout history, energy has been used in different forms such as mechanical, electrical, chemical, heat, etc. The use of alternative sources of energy can result in a more energy efficient and cost effective systems. Many efforts have been made to make the effective use of renewable energy sources due to escalating oil prices and the cost of other primary energy resources in recent years.
In this paper, the performance of a desiccant enhanced evaporative cooling system is investigated experimentally. The effects of different performance parameters on the system performance have been studied. With the increase in regeneration temperature, DCOP decreased due to increase in input energy at higher regeneration temperatures. It was concluded that the efficient control of input and output parameters can provide effective dehumidification capacity with the tested system. The total moisture removal capacity decreases at high process air flow rates due to the reduced residence time. The electrical, thermal, and overall performance increase with the increase in inlet air humidity ratio. The values of COP, ECOP, and TCOP increased from 0.41 to 0.59, 2.30 to 3.6, and 0.74 to 1.48, respectively, when the ambient air humidity ratio was changed from 0.01 to 0.025 kg/kg. A higher DCOP was achieved at higher inlet humidity ratio due to an increase in the moisture absorption capacity of the desiccant dehumidifier. For an increase in humidity ratio from 0.01 to 0.025 kg/kg the DCOP increased by 62%. The variations of regeneration temperature greatly affect the values of COP and TCOP. The increase in regeneration temperature from 55 to 85 °C lowered the COP and TCOP of the system by 32% and 53%, respectively. The desiccant-based technology is beneficial from both an economic as well as an environmental point of view. The system could be improved by developing composite desiccant materials and further lowering the required regeneration temperature. In this way better performance of the system can be achieved.