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Experimental study on combined low temperature regeneration of liquid desiccant and evaporative cooling by ultrasonic atomization
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Experimental study on combined low temperature regeneration of liquid desiccant and evaporative cooling by ultrasonic atomization B.S. Arun , V. Mariappan , Valeriy S. Maisotsenko PII: DOI: Reference:
S0140-7007(19)30499-2 https://doi.org/10.1016/j.ijrefrig.2019.11.023 JIJR 4590
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International Journal of Refrigeration
Received date: Revised date: Accepted date:
7 August 2019 6 November 2019 17 November 2019
Please cite this article as: B.S. Arun , V. Mariappan , Valeriy S. Maisotsenko , Experimental study on combined low temperature regeneration of liquid desiccant and evaporative cooling by ultrasonic atomization, International Journal of Refrigeration (2019), doi: https://doi.org/10.1016/j.ijrefrig.2019.11.023
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Highlights
Fabricated M-cycle based evaporative cooler with liquid desiccant regeneration.
Liquid desiccant regeneration using ultrasonic atomization was studied.
Simultaneous evaporative mist cooling was also analyzed.
Low temperature (35-50 oC) regeneration of CaCl2 solution was accomplished.
Wet-bulb effectiveness of unity was achieved with simple geometry.
Experimental study on combined low temperature regeneration of liquid desiccant and evaporative cooling by ultrasonic atomization. B. S. Arun a, V. Mariappan, b Valeriy S. Maisotsenko c a, b
Department of Mechanical Engineering, National Institute of Technology, Tiruchirappalli 620015, Tamil Nadu, India. c
Chief Scientist and Founder of M-Cycle Corporation, Centennial, CO 80015, USA
Authors: First author: B. S. Arun Address: Research Scholar, Department of Mechanical, National Institute of Technology, Tiruchirappalli 620015, Tamil Nadu, India. Mobile phone: +919495239176 Email: [email protected], [email protected]. Corresponding author: V. Mariappan Address: Associate Professor, Department of Mechanical, National Institute of Technology, Tiruchirappalli 620015, Tamil Nadu, India. Mobile phone: +919894471094 Email: [email protected] Co-author: Dr. Professor Valeriy S. Maisotsenko Address: Chief Scientist and Founder of M-Cycle Corporation, 5628 South Idalia Street, Centennial, CO 80015, USA Phone: 720-220 9314; Fax: 303-375-1693; Skype: valeriy3940 E-mail: [email protected] Abstract
Liquid desiccant cooling is a great alternative to vapour compression cooling for air-conditioning system in buildings with immense energy saving potential. After dehumidification of fresh air, the dehumidified water can be used further for evaporative cooling. However, the recovery of water from regenerator exhaust is a slow process and impractical to be utilised as feed water for evaporative cooling. To overcome this problem, low temperature regeneration of diluted desiccant solution with evaporative mist cooling can be achieved by ultrasonic technology. In this work, a novel regenerative evaporative cooler through the Maisotsenko Cycle (M-Cycle) integrated with an ultrasonic atomizer for liquid desiccant regeneration was fabricated. Moisture removal and effectiveness of the system were studied under various air and desiccant temperature, air and desiccant flow rate, humidity ratio and desiccant concentration. Low temperature (35-50 oC) regeneration of diluted desiccant was accomplished by 0.208 to 0.619 g.s-1 of moisture removal rate. Room cooling supply air temperature below the wet-bulb temperature of intake air up to a minimum temperature of 19 oC was effortlessly achieved by evaporative cooling of atomized water mist from the regenerator. Keyword: liquid desiccant, evaporative cooling, regeneration, M-Cycle, ultrasonic Nomenclature T
temperature (oC)
Q
heat capacity (W)
r
ratio
I
enthalpy of air (J.kg-1)
cp
specific heat of air (J.(kg.oC)-1)
Lfg
latent heat of vaporization of water (J.kg-1)
m
mass flow rate (kg.s-1)
w
work done
P
pressure
A
surface area
ultra
ultrasonic
VCR
vapour compression refrigeration
REC
regenerative evaporative cooler
MRR
moisture removal rate (g.s-1)
DCi
desiccant concentration increase
RH
relative humidity (%)
Greek symbols ω
humidity ratio (kgv.kga-1)
ε
effectiveness
ν
velocity of air (m.s-1)
Subscripts hum
humidity
enth
enthalpy
p
primary air
s
secondary air
sol
solution
i
inlet air
o
outlet air
wb
wet-bulb
v
vapour
atm
atmospheric
ex
extraction
w
water mist
eq
equivalent
1. Introduction Vapour compression air-conditioning systems consume a large amount of energy. An alternative to VCR systems, desiccant-evaporative cooling systems are currently adopted because of less energy consumption and free from ozone depletion substance. Also, waste exhaust heat and solar thermal energy could be utilised (Chua et al., 2013). Liquid desiccant dehumidification unit removes moisture from air directly using desiccant substances like LiBr, LiCl and CaCl2. This dehumidified air is then introduced into an evaporative cooler for room air-conditioning (Mujahid Rafique et al., 2015)(Rafique et al., 2016). A regenerative evaporative cooler (REC) has the potential to cool intake air below its wet-bulb temperature. The working of REC is based on thermodynamic cycle known as Maisotsenko cycle (Mcycle) (Maisotsenko and Treyger, 2011). The energy and exergy analyses and sustainability assessment of M-cycle evaporative air cooling system which cools the intake air to its dew point temperature was presented by Caliskan et al. (Caliskan et al., 2011). There is a transient investigation on the performance of a solar desiccant cooling system integrated with a counter flow M-cycle heat and mass exchanger as an air-conditioning system for the buildings in hot and humid areas. (Maryam Karami et al., 2019). M-cycle coolers were first produced by Coolerado Corporation for residential and commercial purpose. (Sadighi Dizaji et al., 2018). It consists of alternative dry and wet channels similar to an indirect evaporative cooler. A part of the intake air from the dry channel is extracted to the wet channel, where it undergoes evaporative cooling due to the wet surface. As a result, heat transfer takes place between primary air and secondary air leading to advance sensible cooling of the primary air. Heat and mass transfer of REC has been studied numerically and experimentally. Numerical results showed that the velocity of air, channel size and extraction ratio of air have significant influence on the effectiveness (Zhao et al., 2008)(Heidarinejad and Moshari, 2015)(Fakhrabadi and Kowsary, 2016)(B Riangvilaikul and Kumar, 2010). Riangvilaikul and Kumar (B. Riangvilaikul and Kumar, 2010), Bruno (Bruno, 2011) have carried out experimental studies for different climatic conditions. They found that the wet-bulb effectiveness ranged between 0.92-1.29. Duan et al. (Duan et al., 2016) experimentally investigated the
intake and exit parameters like flow rate, temperature and humidity ratio of the air for a evaporative cooler. The results showed that the wet-bulb effectiveness and energy efficiency ratio varied from 0.55 to 1.06 and from 2.8 to 15.5 respectively. Lin et al. (Lin et al., 2017) also experimentally investigated on a cross-flow dew-point cooler for the temperature of 25 – 37 oC and humidity ratio of 12 – 13 g.kg-1. They observed that the wet-bulb effectiveness and dew-point effectiveness of the cooler reached 1.25 and 0.85 respectively. An integrated liquid desiccant-indirect evaporative cooler was developed by Gao et al (Gao et al., 2015) and Cuce (Cuce, 2017). Moisture was removed in the desiccant dehumidifier, and sensible heat was removed in the indirect evaporative cooler. Experimental results showed that the variation of dehumidification capacity directly affected the cooling capacity. An average reduction of 5.3 oC for supply air temperature and 63.7 % dehumidification effectiveness was achieved. The Desiccant regenerator is the main energy consuming component of a desiccant dehumidification system where the diluted desiccant after dehumidification is regenerated and then recirculated for next cycle. Fumo et al. (Fumo and Goswami, 2002), Elsarrag (Elsarrag, 2006) and Bassuoni (Bassuoni, 2011) have experimentally investigated on the performance of a packed tower regenerator using liquid desiccant solution. The moisture removal rate and humidity effectiveness of the regeneration process were evaluated under the effects of variables such as air and desiccant flow rates, air and desiccant temperature, humidity ratio and desiccant concentration. Significant increase in moisture removal rate for regenerator was observed when the air and desiccant flow rate was increased. Yin et al. (Yin et al., 2008) carried out an experimental study on internally heated regenerator. They suggested that the regeneration efficiency of the internally heated regeneration was more than that of the adiabatic regeneration. Liu et al. (Liu et al., 2007) conducted an experimental investigation on a cross-flow regenerator using LiBr desiccant solution. Moisture removal rate and humidity effectiveness were taken as the performance parameters. They found that the moisture removal rate increased with the increase of air and desiccant flow rate, and desiccant temperature. It decreased with air humidity and desiccant concentration. Also, the humidity effectiveness
increased with desiccant flow rate and desiccant concentration. It decreased with air flow rate and desiccant temperature. Dong et al. (Dong et al., 2019) carried out an experimental study to determine the optimal regeneration temperature of LiCl desiccant solution in a cross-flow regenerator. The solution temperature was varied from 50 to 90 oC, and an optimum solution temperature of 65 oC was found from a validated theoretical model. Ahmed et al. (Ahmed et al., 2017) and Audah et al. (Audah et al., 2011) have investigated on a conventional liquid desiccant cooling system for cooling, waste energy recovery and condensed freshwater from regenerator exhaust air. Using LiCl and CaCl 2 desiccant solution, the system produced 84.6 kg and 15 kg of freshwater per hour respectively. Additionally, the optimal regeneration temperature required was around 50-52 oC. From the literature, it is found that high temperature was necessary for desiccant regeneration and water vapour from scavenging air has to be condensed for freshwater needs. On the other hand, the desiccant can be regenerated using ultrasonic technology under low temperature (Yao, 2016). Zhang et al. (Zhang et al., 2010) and Yang et al. (Yang et al., 2012) have conducted experimental investigations on the regeneration of solid desiccant (silica gel) using power ultrasound under different ultrasound frequencies, humidity and regeneration temperature (35-70 oC). The results showed that lower frequency and temperature was beneficial for application of power ultrasound on silica gel regeneration. Yang et al. (Yang et al., 2016) developed a novel ultrasonic atomization liquid desiccant regeneration system in which LiCl desiccant solution was atomized into tiny droplets of 50 μm. They found that ultrasonic atomisation enabled lower temperature regeneration of liquid desiccant. In sum from the literature, it is noted that regenerative evaporative cooler can achieve temperature below wet-bulb temperature with simple geometric construction based on M-cycle and also, it requires additional water circulation throughout the cooling pad. Regenerative evaporative cooler integrated with liquid desiccant dehumidifier works effectively and have great energy saving potential. Moreover, the dehumidified water can be recovered from regenerator exhaust by condensation of scavenging air. As the condensed water will be at high temperature and collected at a slower rate, it is unsuitable to be utilised as
feed water for evaporative cooling. The influence of feed water temperature in regenerative evaporative cooling was discussed by the authors in previous paper (Arun and Mariappan, 2019). To address the above problem, low temperature regeneration of liquid desiccant can be carried out using ultrasonic technology and the water mist generated could be used for evaporative cooling in the same apparatus. Based on U.S. patent 6497107B2 (Maisotsenko et al., 2002) in which the M-cycle combines heat exchange and evaporative cooling in a single apparatus. In this apparatus, the intake ambient air is dehumidified by strong liquid desiccant flowing through the dry channel as a moving fluid film. The wet channel is wetted by weak liquid desiccant which is pumped after the adsorption process from the dry channel. The redirected dehumidified air in wet channel undergoes evaporative cooling and simultaneously cools the intake air by heat transfer through the heat exchanger walls. Hereafter, the cold dehumidified air can be used to cool the room supply air directly or indirectly. The regenerated strong liquid desiccant from the wet channel is collected in a tray and recycled. The pressure difference between channels is created by a pressure reduction baffle, a forced draft fan for dry channel and an induced draft fan for wet channel. In this study, a first-generation M-cycle is adopted from above mentioned concept, and a novel regenerative evaporative cooler integrated with an ultrasonic atomizer for liquid desiccant regeneration in wet channel side was fabricated. Under different conditions of air and desiccant, experiments were conducted to investigate the moisture removal rate and effectiveness of the system. This prototype could be further developed to a fully-fledged waterless desiccant cooling system. 2. Experimental setup The system consists of mainly two components: the indirect evaporative cooling unit and liquid desiccant regeneration chamber. Regenerative type indirect evaporative cooling unit is used, which includes alternative dry and wet channels. The desiccant regeneration chamber consists of an ultrasonic atomizer (piezoelectric transducer) of acoustic frequency ranging from 800 kHz to 1.6 MHz which produces water droplets of size ranging from 15 to 70 microns. Table 1 shows the construction specification of the system. CaCl2 solution was selected as the liquid desiccant (Conde, 2004). Fig. 1 and
Fig. 2 demonstrates the schematic layout and photographic view of the test rig respectively. As shown in Fig. 1, the solution tank is filled with a prepared weak solution of the required mass fraction, and it flows to the regeneration chamber through a control valve and heater. A pump is used for recirculation of the solution. In the desiccant regeneration chamber, the solution undergoes ultrasonic dehydration, and small droplets of water (mist) is produced. This mist is carried over to wet channels of the REC where dehumidified secondary air undergoes evaporative cooling and also, a small quantity of salt carryover is witnessed. Secondary air is extracted from primary air flowing through the dry channels, which is redirected through a conduit. The secondary humidified air exits through the top of the unit. Three AC motor fans are used to create required airflow and are controlled by an electrical fan regulator. It is tested in the laboratory by using dehumidified air from Bry-Air FFB 300 desiccant wheel dehumidifier under a humidity ratio range of 9 – 17 gv.kga-1. The temperature is maintained by heating/cooling coils. The value ranges of dynamic parameters are shown in Table 2. To measure the temperature, relative humidity and airflow velocity, a K-type thermocouple with a 10k ohm thermistor, a HC2 hygrometer (rotronic) and a hot wire anemometer are used respectively. The measured intake and exit parameters of air are recorded by keysight 34972A LXI data logger. 2.1. Ultrasonic regeneration and heat-mass transfer process Fig. 3 shows the airflow direction and heat transfer for the dry and wet channels. The working of the regenerative evaporative cooler is similar to an indirect evaporative cooler. In this unit, a weak solution enters into the ultrasonic desiccant regeneration chamber where the ultrasonic transducers atomize the solution with a high-intensity ultrasonic wave which vibrates the thin layer of the solution resulting in excitation of small water droplets (mist) from the surface of the solution. This process is known as cavitation which is induced under certain power intensity and frequency. The water mist which is in contact with the dehumidified secondary air undergoes evaporative cooling in the wet channels. Heat transfer between dry and wet channels takes place through heat exchanger wall, which results in precooling of the intake primary air. Consequently, primary air temperature decreases below its wet-bulb
temperature. The humid secondary air is then exhausted, and the humidity of primary air remains the same as intake air. 2.2. Test procedure To obtain the experimental conditions as shown in Table 2, the procedures taken for the test are as follows. Before starting the experiment, the distilled water was mixed with CaCl2 salt and a weak solution of require mass fraction was prepared. The weak solution is then stored in a solution tank. The dehumidifier along with heating/cooling coil was switched on to make inlet conditions of air to values as shown in Table 2. The airflow rate was varied by controlling the fan regulator. The diluted liquid solution from the tank was circulated through the regeneration chamber using the pump and its temperature and flow rate was controlled by heating element and valve respectively. Hereafter, ultrasonic atomizer was turned on to atomize the desiccant solution and mist were produced which are carried by secondary air. . The experiments are carried out for a steady state condition where relative humidity and temperature variation are within 3% and 0.3oC respectively, and are logged with above mentioned instruments. The increased desiccant concentration was measured from calibration curve plotted with respect to density. After the completion of regeneration, the solution and air operating conditions were set to default values. Different experimental runs are carried out by varying one operating parameter and keeping others as fixed.
3. Performance indicators 3.1. Effectiveness The regeneration performance is commonly measured by humidity and temperature effectiveness, which have been applied in various studies (Gandhidasan, 2005)(Taylor et al., 2011). The humidity effectiveness is defined as the ratio of actual change to the maximum possible change of humidity ratio and given as εhum. (1)
Here, ωs,i and ωs,o are the humidity ratios of secondary air at the inlet and exhaust of wet channel respectively. Secondary air inlet humidity remains same as primary air intake humidity ratio. ωeq is the equilibrium humidity ratio of air, which is the maximum theoretical humidity level to which air can be humidified. It is given by, (2)
where pv is the partial vapour pressure (Pascal) on the surface of CaCl2 desiccant solution. Using the Gad correlation (Gad et al., 2001)(Bassuoni, 2011) pv is calculated in mm of Hg as follows, (3)
Value for the constants of the Eq. (3) are a0 =10.0624, a1=4.4674, b0=739.828, b1=1450.96 and C=111.96. The enthalpy effectiveness of the regeneration based on change in temperature of air is given by, (4)
where, Is,i and Is,o are the enthalpies of secondary air at the inlet and exhaust of wet channel respectively. The enthalpy of air can be found from Eq. (5) in terms of humidity ratio ωs and temperature Ts as follows, (
)
(5)
Similarly, the enthalpy of solution may be found from Eq. (6) as given below: (6) where, Cp,sol is the specific heat capacity of solution at constant pressure which can be calculated in terms of desiccant solution temperature Tsol and concentration ξsol. In Eq. (4), Ieq is the enthalpy of air when the temperature of air is in thermal equilibrium (T s=Tsol) with desiccant solution at the common boundary. For evaluating regenerative evaporative cooler which is similar to indirect evaporative cooler the wetbulb effectiveness εwb is used as given below:
(7)
Here, Tp,i and Tp,o are the temperatures of primary air at the inlet and outlet of horizontal duct (dry channel) respectively as shown in Fig. 1, and Tp,wb is the wet-bulb temperature of inlet fresh air. 3.2. Moisture removal rate (MRR) Moisture removal rate is used in few studies (Yang et al., 2015)(Yang et al., 2016) to evaluate the rate of moisture removed by air the from the weak desiccant solution. It is defined as, (8) where, ωs,i and ωs,o are the humidity ratios of secondary air at the inlet and exhaust of wet channel respectively, and ms is the secondary air mass flow rate which can be determined in terms of extraction ratio and mass flow rate of primary air (ms = rex mp). 3.3. Desiccant concentration increase (DCi) During the regeneration process the weak solution is converted into strong solution, which is normally recirculated through the dehumidification unit. Similar to MRR, desiccant concentration increase measures the performance based on concentration of the desiccant solution before and after regeneration process. It is defined as, (9) where, ξsol,i and ξsol,o are the CaCl2 solution concentrations before and after the ultrasonic regeneration process measured from the regeneration chamber. 4. Uncertainty Propagation The accuracy of all measuring sensors is given in Table 3. From the measuring variables the performance indicators were calculated. The uncertainty of the performance indicators can be obtained by following Eq. 10 √∑
(10)
Where, U represents the uncertainty of the variable. It is caused by errors in the measuring sensors. Uncertainties of the measured variables propagate into the value of the calculated quantity y. The results of uncertainty propagation for certain performance indicators are given in Table 4. 5. Result and discussion of experimental results The validity of experimental results is checked by calculating the energy balance of the regenerative evaporative cooler. The steady performance was analysed by varying operating parameters of the desiccant regeneration evaporative cooler. 5.1. Conservation of energy The change in enthalpy of primary air must be equal to change in enthalpy of secondary air based on the theory of energy conservation. Energy changes on secondary and primary air are expressed as: (
) (11)
(
)
The energy changes between air streams shown in Fig. 4. For all the experimental results the discrepancies are within ±15 %. 5.2. Effect of operating conditions on the system The exit parameters of the air are examined by varying the operating conditions. Wet-bulb effectiveness is used as the evaluation scale for rating evaporating cooling. Humidity and enthalpy effectiveness, MRR and DCi are used for evaluation of desiccant regeneration. Different experimental runs are carried out by varying one operating parameter and keeping others as fixed. 5.2.1. Effect of air velocity The airflow rate is increased by increasing the velocity of air using the primary fan. Fig. 5 presents the effect of air velocity on the regeneration and cooling performance. As the airflow rate increases the enthalpy, humidity and wet-bulb effectiveness decrease from 75.8 to 67 %, 65.1 to 54.9 % and 1.1 to 0.6 respectively. The reduction in effectiveness is due to less time of contact with the plate
surface and an increase in the quantity of primary air. However, the MRR increases from 0.39 to 0.58 g.s-1 and DCi increases from 0.8 to 0.99 % with the increase in airflow rate for regeneration as shown in Fig. 5. For a fixed extraction ratio, primary air and secondary air velocity would increase simultaneously. As the quantity of air increases, more moisture is removed from the desiccant solution because of low humidity and vapour pressure. Here, more amount of air will be in contact with the unit mass flow rate of solution. Accordingly, moisture will be removed from the weak desiccant solution increasing the MRR and DCi. 5.2.2. Effect of extraction ratio Fig. 6 presents the effect of extraction ratio on the effectiveness and moisture removal rate. In Fig. 6, it can be seen that by increasing the extraction ratio from 0.3 to 0.5, the enthalpy and humidity effectiveness decreases because of the increase in secondary air mass flow rate as discussed in previous section 5.2.1. However, the wet-bulb effectiveness, MRR and DCi increase from 0.90 to 1.05, 0.385 to 0.445 g.s-1 and 0.794 to 0.921 % respectively. This can be explained as follows. The more amount of secondary air in wet channel acts as a heat sink absorbing heat from the low velocity primary air through the plate wall, thereby improving sensible heat transfer. Moreover, MRR and DCi increase as more moisture is removed. 5.2.3. Effect of inlet humidity ratio The effect of humidity ratio within low humidity condition where the air humidity ratio is maintained between 0.009 - 0.017 kg.kg-1 is presented in Fig. 7. The effectiveness remains almost constant by increasing inlet air humidity ratio as shown in Fig. 7. It can be seen that only the enthalpy effectiveness slightly varies from 70.3 to 61.5 % as the air becomes more humid. Even though the wetbulb effectiveness is unchanged, with the increase in humidity, the wet-bulb temperature of inlet air also increases simultaneously which results in less primary air temperature drop at the outlet of the cooler. In this case, both the MRR and DCi decrease as shown in Fig. 7. It can be seen that by increasing humidity the moisture removal rate reduces because the high humid air gets saturated quickly during evaporative cooling. The humidity effectiveness remains constant as both the outlet and inlet humidity increase
together. Moreover, all the generated water droplets are not carried over to exhaust because of low capacity fans, and it is settled within the regeneration chamber. 5.2.4. Effect of inlet air temperature Fig. 8 presents the effect of inlet air temperature between 28-36 oC on regeneration and cooling performance. It can be seen that the MRR increases 0.357 to 0.619 g.s-1 and DCi increases from 0.735 to 1.10 % with increase in the inlet air temperature. This can be described as follows, Even though the humidity ratio determines the vapour pressure of air, the relative humidity will decrease with increase in temperature which could improve the evaporative potential. Also, the humidity and enthalpy effectiveness increase from 48 to 83.16 % and 63.1 to 81.2 % respectively. This increase in regenerator performance is due to heat transfer through the plate surface, which results in the rise of secondary air temperature and thereby more moisture is carried by hot air to the exhaust. Here, the intake air temperature is selected below the desiccant solution temperature (35 oC). As a result, no moisture removal by adiabatic heating of desiccant solution by air is witnessed. Same can be concluded for wet-bulb effectiveness which remains constant as shown in Fig. 8, where the wet-bulb temperature will also increase with inlet air temperature. However, wet-bulb effectiveness might remain unchanged for ambient conditions up to 40 oC. At a very high temperature of inlet air, a considerable amount of sensible heating will takes place which cannot be compensated by evaporative cooling in this apparatus. 5.2.5. Effect of desiccant solution flow rate Fig. 9 presents the effect of the desiccant solution flow rate on regeneration and cooling. It can be seen that by increasing the desiccant solution flow rate, the MRR remains unchanged. The atomizer used in the present study runs at its full capacity. As a result, the ultrasonic atomization load on the system is increased significantly. With increases in solution flow rate power output of the atomizer should also be enhanced proportionally to achieve more moisture removal. Additionally, the outlet air humidity is being saturated, and the airflow rate should also be increased. The conventional desiccant regeneration is mainly heat and mass transfer process. However, in ultrasonic atomization the weak desiccant solution is dehydrated without thermal energy. The vapour pressure between solution and air would increase with the
addition of more warm solution as the gross temperature of solution increases, resulting in more moisture removal still the desiccant remains as a weak solution due to more inflow. The DCi reduces from 0.869 to 0.389 % with the increase in solution flow rate as shown in Fig. 9. In this case, the effectivenesses of the evaporative cooler are unchanged as the solution flow rate has a negligible impact on evaporative cooling. 5.2.6. Effect of inlet solution concentration. The effect of inlet desiccant solution concentration between 28.4-35.6 % on moisture removal and effectiveness is presented in Fig. 10. It can be seen that MRR decreases from 0.461 to 0.281 g.s-1 and DCi decreases from 1.02 to 0.78 % with the increase in inlet solution concentration. It is due to the reduction in surface vapour pressure of the desiccant solution as the ultrasonic power remains the same. Consequently, the vapour pressure difference between the solution and secondary air reduces. The humidity effectiveness increases with the increase in inlet solution concentration as shown in Fig. 10. It can be explained as, by increasing the solution concentration the water mist generated from the strong solution to secondary air reduces, which results in lower humidity of outlet air. Moreover, the equivalent humidity ratio also decreases. The outlet air humidity ratio and equivalent humidity ratio compensate with each other, and the humidity effectiveness increases. The wet-bulb effectiveness decreases as the amount of water mist from the solution reduces resulting in less evaporative cooling as shown in Fig. 10. 5.2.7. Effect of desiccant solution temperature. Fig. 11 presents the effect of desiccant solution temperature on moisture removal and effectiveness. It can be seen that by increasing desiccant solution temperature from 35 to 50 oC, the MRR increases from 0.39 to 0.58 g.s-1 and DCi increases 0.8 to 0.99 %. This is due to the increase in surface vapour pressure of the solution, which aids in easy atomisation of water mist from the solution which consequently enhances the moisture removal rate. However, the humidity effectiveness and wet-bulb effectiveness decreases from 56 to 30 % and 1 to 0.73 respectively as shown in Fig. 11. It can be explained as follows. When the desiccant solution temperature is increased, the equivalent humidity ratio also increased simultaneously. Although the secondary air outlet humidity ratio increases, the equivalent humidity ratio would increase drastically due to more mass transfer potential of desiccant solution at the
high temperature, which reduces the humidity effectiveness. In this case, the decrease in wet-bulb effectiveness is negligible up to 40 oC as water mist does not carry sensible heat from the solution. Above 40 oC up to 50 oC, decrease in wet-bulb effectiveness is observed. This is due to considerable addition of sensible heat from the desiccant solution which dominates the evaporative cooling (latent heat) of the secondary air. Table 5 shows the comparison of wet-bulb effectiveness and regeneration temperature with respect to the regeneration load from previous studies. The proposed system was operated at a relative low regeneration load. However, low temperature regeneration of desiccant was achieved effortlessly. 6. Conclusions A combined ultrasonic desiccant regenerator and the indirect evaporative cooler through the MCycle was fabricated, and experimental investigation was conducted. For this study, the conclusions are as follows: 1. Wet-bulb effectiveness of unity with a minimum temperature of 19 oC is achieved with basic constructions. It is improved by increasing extraction ratio and remain unchanged by increasing humidity ratio, inlet air temperature and solution flow rate. When the inlet air velocity, desiccant concentration and desiccant temperature are increased, wet-bulb effectiveness reduces. 2. Humidity effectiveness also known as regeneration effectiveness improves by increasing the inlet air temperature as the regeneration potential increases and remains unchanged with the increase in inlet air humidity and desiccant flow rate. Although effectiveness enhances with the increase in desiccant concentration, it is due to the negative effect of drop in equivalent humidity ratio. Regeneration effectiveness reduces with increase in inlet air velocity, extraction ratio and solution temperature. 3. Low temperature desiccant regeneration (35-50 oC) is accomplished with 0.208 to 0.619 g.s-1 of moisture removal rate. MRR improves by increasing air and desiccant temperature, inlet air velocity and extraction ratio, and reduces by increasing inlet air humidity and desiccant concentration.
4. Low temperature ultrasonic regeneration of desiccant solution can be developed to the waterless desiccant-evaporative cooling system through the M-Cycle where dehumidified water can be reused for evaporative cooling.
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List of Figures Fig. 1. Schematic layout of test rig. Fig. 2. Photographic view of test rig. Fig. 3. Temperature distribution along channel length in dry and wet channel. Fig. 4. Energy balance between air streams. Fig. 5. Effect of inlet air velocity on regeneration performance. Fig. 6. Effect of extraction ratio on regeneration performance. Fig. 7. Effect of inlet air humidity on regeneration performance. Fig. 8. Effect of inlet air temperature on regeneration performance. Fig. 9. Effect of desiccant flow on regeneration performance. Fig. 10. Effect of desiccant concentration on regeneration performance. Fig. 11. Effect of desiccant temperature on regeneration performance.
Fig. 1. Schematic layout of test rig.
Fig. 2. Photographic view of test rig.
Fig. 3. Temperature distribution along channel length in dry and wet channel.
Fig. 4. Energy balance between air streams.
Fig. 5. Effect of inlet air velocity on regeneration and cooling performance.
Fig. 6. Effect of extraction ratio on regeneration and cooling performance.
Fig. 7. Effect of inlet air humidity on regeneration and cooling performance.
Fig. 8. Effect of inlet air temperature on regeneration and cooling performance.
Fig. 9. Effect of desiccant flow on regeneration and cooling performance.
Fig. 10. Effect of desiccant concentration on regeneration and cooling performance.
Fig. 11. Effect of desiccant temperature on regeneration and cooling performance.
List of tables Table 1. Geometric properties Table 2. Ranges of dynamic parameters Table 3. Accuracy of measuring sensors Table 4. Results of uncertainty propagation for certain performance indicators Table 5. Comparison of regeneration load and temperature with previous studies
Table 1. Geometric properties Parameters
System
Cooler length
0.5 m
Cooler height
0.3 m
Primary channel width
3 mm
Secondary channel width
3 mm
Number of plates
40
Plate thickness
0.2 mm
Ultrasonic atomiser
6 L.h-1, 300 W
Table 2. Ranges of dynamic parameters Parameters
Unit
Range
Fixed value
Inlet air temperature (Tin)
o
28-36
30
Inlet air humidity ratio (ωin)
kgv.kga-1
0.009-0.017
0.011
Inlet air velocity (νin)
m.s-1
1.5-4
2.2
Desiccant solution temperature (Tsol)
o
35-50
35
Desiccant flow rate (msol)
kg.s-1
0.015-0.033
0.015
Desiccant concentration (ξsol)
%
28.4-35.6
30.1
Extraction ratio (rex)
-
0.30-0.50
0.40
C
C
Table 3. Accuracy of measuring sensors Parameters
Apparatus
Measurement range
Accuracy
Relative humidity
Hygroclip HC2 Humidity sensor
0-90% RH
±1% RH
Air velocity
Hotwire anemometer
0-20 m.s-1
±0.03 m.s-1 or 5%
Pressure drop
Digital manometer
0–200 kPa
±2% FS
Temperature
Thermocouple K-type
0–100 oC
±0.2 oC
Thermistor 10K
-20-100 oC
±0.25 oC
Air flow rate
Turbine flowmeter
0–10 CMM
±1% FS
Desiccant flow rate
Glass flow meter
0-10 LPM
±2 % FS
Desiccant density
SG. Hydrometer
1000-1500 kg.(m3)-1
±1 kg.(m3)-1
Table 4. Results of uncertainty propagation for certain performance indicators
Symbol
Performance indicators
Value ±uncertainty
Units
mp,i
Mass flow rate of air
0.128 ±0.00109
kg.s-1
ωp,i
Humidity ratio
0.011 ±0.000170
kg.kg-1
εwb
Wet-bulb effectiveness
0.997 ±0.02768
-
εhum
Humidity effectiveness
0.5398 ±0.02153
-
εenth
Enthalpy effectiveness
0.6786 ±0.01348
-
MRR
Moisture removal rate
0.4023 ±0.01815
g.s-1
DCi
Desiccant concentration increase
0.875 ±0.01414
%
Cooling capacity
1263 ±38.93
W
Qp
Table 5. Comparison of temperature, regeneration load and wet-bulb effectiveness with previous studies Study
Desiccant/
Solution
Inlet air
Inlet humidity
Desiccant
MRR
Wet-bulb
Cooler
temperature
temperature
ratio (g.kg-1)
mass fraction
(g.s-1)
effectiveness
(oC)
(oC)
(%)
Liu et al.,2007
LiBr
50-62
29-35
12-22
40-52
4-8
Fumo et al.,
LiCl
60-70
30-40
14-21
32.8-35
1.4-2.4
TEG
72-80
-
18-24
90.4-95
1.7-2.36
LiCl (ultra)
45-64
29-36
14-22
24-32
0.07-0.18
M-cycle
-
25-45
7.0-26.0
-
-
0.92-1.14
M-cycle
-
22.7-38.9
9.3-9.4
-
-
0.55-1.06
M-cycle
-
22.5-40.3
10.7-26.7
-
-
0.93-1.06
CaCl2/M-
35-50
28-36
9-17
28.4-35.6
0.39-0.58
0.4-1.05
2002 Elsarrag., 2006 Yang et al., 2016 Riangvilakul et al., 2010 Z. Duan et al., 2016 F. Bruno et al., 2011 Present
cycle (ultra)
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Experimental study on combined low temperature regeneration of liquid desiccant and evaporative cooling by ultrasonic atomization B.S. Arun , V. Mariappan , Valeriy S. Maisotsenko PII: DOI: Reference:
S0140-7007(19)30499-2 https://doi.org/10.1016/j.ijrefrig.2019.11.023 JIJR 4590
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
7 August 2019 6 November 2019 17 November 2019
Please cite this article as: B.S. Arun , V. Mariappan , Valeriy S. Maisotsenko , Experimental study on combined low temperature regeneration of liquid desiccant and evaporative cooling by ultrasonic atomization, International Journal of Refrigeration (2019), doi: https://doi.org/10.1016/j.ijrefrig.2019.11.023
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Highlights
Fabricated M-cycle based evaporative cooler with liquid desiccant regeneration.
Liquid desiccant regeneration using ultrasonic atomization was studied.
Simultaneous evaporative mist cooling was also analyzed.
Low temperature (35-50 oC) regeneration of CaCl2 solution was accomplished.
Wet-bulb effectiveness of unity was achieved with simple geometry.
Experimental study on combined low temperature regeneration of liquid desiccant and evaporative cooling by ultrasonic atomization. B. S. Arun a, V. Mariappan, b Valeriy S. Maisotsenko c a, b
Department of Mechanical Engineering, National Institute of Technology, Tiruchirappalli 620015, Tamil Nadu, India. c
Chief Scientist and Founder of M-Cycle Corporation, Centennial, CO 80015, USA
Authors: First author: B. S. Arun Address: Research Scholar, Department of Mechanical, National Institute of Technology, Tiruchirappalli 620015, Tamil Nadu, India. Mobile phone: +919495239176 Email: [email protected], [email protected]. Corresponding author: V. Mariappan Address: Associate Professor, Department of Mechanical, National Institute of Technology, Tiruchirappalli 620015, Tamil Nadu, India. Mobile phone: +919894471094 Email: [email protected] Co-author: Dr. Professor Valeriy S. Maisotsenko Address: Chief Scientist and Founder of M-Cycle Corporation, 5628 South Idalia Street, Centennial, CO 80015, USA Phone: 720-220 9314; Fax: 303-375-1693; Skype: valeriy3940 E-mail: [email protected] Abstract
Liquid desiccant cooling is a great alternative to vapour compression cooling for air-conditioning system in buildings with immense energy saving potential. After dehumidification of fresh air, the dehumidified water can be used further for evaporative cooling. However, the recovery of water from regenerator exhaust is a slow process and impractical to be utilised as feed water for evaporative cooling. To overcome this problem, low temperature regeneration of diluted desiccant solution with evaporative mist cooling can be achieved by ultrasonic technology. In this work, a novel regenerative evaporative cooler through the Maisotsenko Cycle (M-Cycle) integrated with an ultrasonic atomizer for liquid desiccant regeneration was fabricated. Moisture removal and effectiveness of the system were studied under various air and desiccant temperature, air and desiccant flow rate, humidity ratio and desiccant concentration. Low temperature (35-50 oC) regeneration of diluted desiccant was accomplished by 0.208 to 0.619 g.s-1 of moisture removal rate. Room cooling supply air temperature below the wet-bulb temperature of intake air up to a minimum temperature of 19 oC was effortlessly achieved by evaporative cooling of atomized water mist from the regenerator. Keyword: liquid desiccant, evaporative cooling, regeneration, M-Cycle, ultrasonic Nomenclature T
temperature (oC)
Q
heat capacity (W)
r
ratio
I
enthalpy of air (J.kg-1)
cp
specific heat of air (J.(kg.oC)-1)
Lfg
latent heat of vaporization of water (J.kg-1)
m
mass flow rate (kg.s-1)
w
work done
P
pressure
A
surface area
ultra
ultrasonic
VCR
vapour compression refrigeration
REC
regenerative evaporative cooler
MRR
moisture removal rate (g.s-1)
DCi
desiccant concentration increase
RH
relative humidity (%)
Greek symbols ω
humidity ratio (kgv.kga-1)
ε
effectiveness
ν
velocity of air (m.s-1)
Subscripts hum
humidity
enth
enthalpy
p
primary air
s
secondary air
sol
solution
i
inlet air
o
outlet air
wb
wet-bulb
v
vapour
atm
atmospheric
ex
extraction
w
water mist
eq
equivalent
1. Introduction Vapour compression air-conditioning systems consume a large amount of energy. An alternative to VCR systems, desiccant-evaporative cooling systems are currently adopted because of less energy consumption and free from ozone depletion substance. Also, waste exhaust heat and solar thermal energy could be utilised (Chua et al., 2013). Liquid desiccant dehumidification unit removes moisture from air directly using desiccant substances like LiBr, LiCl and CaCl2. This dehumidified air is then introduced into an evaporative cooler for room air-conditioning (Mujahid Rafique et al., 2015)(Rafique et al., 2016). A regenerative evaporative cooler (REC) has the potential to cool intake air below its wet-bulb temperature. The working of REC is based on thermodynamic cycle known as Maisotsenko cycle (Mcycle) (Maisotsenko and Treyger, 2011). The energy and exergy analyses and sustainability assessment of M-cycle evaporative air cooling system which cools the intake air to its dew point temperature was presented by Caliskan et al. (Caliskan et al., 2011). There is a transient investigation on the performance of a solar desiccant cooling system integrated with a counter flow M-cycle heat and mass exchanger as an air-conditioning system for the buildings in hot and humid areas. (Maryam Karami et al., 2019). M-cycle coolers were first produced by Coolerado Corporation for residential and commercial purpose. (Sadighi Dizaji et al., 2018). It consists of alternative dry and wet channels similar to an indirect evaporative cooler. A part of the intake air from the dry channel is extracted to the wet channel, where it undergoes evaporative cooling due to the wet surface. As a result, heat transfer takes place between primary air and secondary air leading to advance sensible cooling of the primary air. Heat and mass transfer of REC has been studied numerically and experimentally. Numerical results showed that the velocity of air, channel size and extraction ratio of air have significant influence on the effectiveness (Zhao et al., 2008)(Heidarinejad and Moshari, 2015)(Fakhrabadi and Kowsary, 2016)(B Riangvilaikul and Kumar, 2010). Riangvilaikul and Kumar (B. Riangvilaikul and Kumar, 2010), Bruno (Bruno, 2011) have carried out experimental studies for different climatic conditions. They found that the wet-bulb effectiveness ranged between 0.92-1.29. Duan et al. (Duan et al., 2016) experimentally investigated the
intake and exit parameters like flow rate, temperature and humidity ratio of the air for a evaporative cooler. The results showed that the wet-bulb effectiveness and energy efficiency ratio varied from 0.55 to 1.06 and from 2.8 to 15.5 respectively. Lin et al. (Lin et al., 2017) also experimentally investigated on a cross-flow dew-point cooler for the temperature of 25 – 37 oC and humidity ratio of 12 – 13 g.kg-1. They observed that the wet-bulb effectiveness and dew-point effectiveness of the cooler reached 1.25 and 0.85 respectively. An integrated liquid desiccant-indirect evaporative cooler was developed by Gao et al (Gao et al., 2015) and Cuce (Cuce, 2017). Moisture was removed in the desiccant dehumidifier, and sensible heat was removed in the indirect evaporative cooler. Experimental results showed that the variation of dehumidification capacity directly affected the cooling capacity. An average reduction of 5.3 oC for supply air temperature and 63.7 % dehumidification effectiveness was achieved. The Desiccant regenerator is the main energy consuming component of a desiccant dehumidification system where the diluted desiccant after dehumidification is regenerated and then recirculated for next cycle. Fumo et al. (Fumo and Goswami, 2002), Elsarrag (Elsarrag, 2006) and Bassuoni (Bassuoni, 2011) have experimentally investigated on the performance of a packed tower regenerator using liquid desiccant solution. The moisture removal rate and humidity effectiveness of the regeneration process were evaluated under the effects of variables such as air and desiccant flow rates, air and desiccant temperature, humidity ratio and desiccant concentration. Significant increase in moisture removal rate for regenerator was observed when the air and desiccant flow rate was increased. Yin et al. (Yin et al., 2008) carried out an experimental study on internally heated regenerator. They suggested that the regeneration efficiency of the internally heated regeneration was more than that of the adiabatic regeneration. Liu et al. (Liu et al., 2007) conducted an experimental investigation on a cross-flow regenerator using LiBr desiccant solution. Moisture removal rate and humidity effectiveness were taken as the performance parameters. They found that the moisture removal rate increased with the increase of air and desiccant flow rate, and desiccant temperature. It decreased with air humidity and desiccant concentration. Also, the humidity effectiveness
increased with desiccant flow rate and desiccant concentration. It decreased with air flow rate and desiccant temperature. Dong et al. (Dong et al., 2019) carried out an experimental study to determine the optimal regeneration temperature of LiCl desiccant solution in a cross-flow regenerator. The solution temperature was varied from 50 to 90 oC, and an optimum solution temperature of 65 oC was found from a validated theoretical model. Ahmed et al. (Ahmed et al., 2017) and Audah et al. (Audah et al., 2011) have investigated on a conventional liquid desiccant cooling system for cooling, waste energy recovery and condensed freshwater from regenerator exhaust air. Using LiCl and CaCl 2 desiccant solution, the system produced 84.6 kg and 15 kg of freshwater per hour respectively. Additionally, the optimal regeneration temperature required was around 50-52 oC. From the literature, it is found that high temperature was necessary for desiccant regeneration and water vapour from scavenging air has to be condensed for freshwater needs. On the other hand, the desiccant can be regenerated using ultrasonic technology under low temperature (Yao, 2016). Zhang et al. (Zhang et al., 2010) and Yang et al. (Yang et al., 2012) have conducted experimental investigations on the regeneration of solid desiccant (silica gel) using power ultrasound under different ultrasound frequencies, humidity and regeneration temperature (35-70 oC). The results showed that lower frequency and temperature was beneficial for application of power ultrasound on silica gel regeneration. Yang et al. (Yang et al., 2016) developed a novel ultrasonic atomization liquid desiccant regeneration system in which LiCl desiccant solution was atomized into tiny droplets of 50 μm. They found that ultrasonic atomisation enabled lower temperature regeneration of liquid desiccant. In sum from the literature, it is noted that regenerative evaporative cooler can achieve temperature below wet-bulb temperature with simple geometric construction based on M-cycle and also, it requires additional water circulation throughout the cooling pad. Regenerative evaporative cooler integrated with liquid desiccant dehumidifier works effectively and have great energy saving potential. Moreover, the dehumidified water can be recovered from regenerator exhaust by condensation of scavenging air. As the condensed water will be at high temperature and collected at a slower rate, it is unsuitable to be utilised as
feed water for evaporative cooling. The influence of feed water temperature in regenerative evaporative cooling was discussed by the authors in previous paper (Arun and Mariappan, 2019). To address the above problem, low temperature regeneration of liquid desiccant can be carried out using ultrasonic technology and the water mist generated could be used for evaporative cooling in the same apparatus. Based on U.S. patent 6497107B2 (Maisotsenko et al., 2002) in which the M-cycle combines heat exchange and evaporative cooling in a single apparatus. In this apparatus, the intake ambient air is dehumidified by strong liquid desiccant flowing through the dry channel as a moving fluid film. The wet channel is wetted by weak liquid desiccant which is pumped after the adsorption process from the dry channel. The redirected dehumidified air in wet channel undergoes evaporative cooling and simultaneously cools the intake air by heat transfer through the heat exchanger walls. Hereafter, the cold dehumidified air can be used to cool the room supply air directly or indirectly. The regenerated strong liquid desiccant from the wet channel is collected in a tray and recycled. The pressure difference between channels is created by a pressure reduction baffle, a forced draft fan for dry channel and an induced draft fan for wet channel. In this study, a first-generation M-cycle is adopted from above mentioned concept, and a novel regenerative evaporative cooler integrated with an ultrasonic atomizer for liquid desiccant regeneration in wet channel side was fabricated. Under different conditions of air and desiccant, experiments were conducted to investigate the moisture removal rate and effectiveness of the system. This prototype could be further developed to a fully-fledged waterless desiccant cooling system. 2. Experimental setup The system consists of mainly two components: the indirect evaporative cooling unit and liquid desiccant regeneration chamber. Regenerative type indirect evaporative cooling unit is used, which includes alternative dry and wet channels. The desiccant regeneration chamber consists of an ultrasonic atomizer (piezoelectric transducer) of acoustic frequency ranging from 800 kHz to 1.6 MHz which produces water droplets of size ranging from 15 to 70 microns. Table 1 shows the construction specification of the system. CaCl2 solution was selected as the liquid desiccant (Conde, 2004). Fig. 1 and
Fig. 2 demonstrates the schematic layout and photographic view of the test rig respectively. As shown in Fig. 1, the solution tank is filled with a prepared weak solution of the required mass fraction, and it flows to the regeneration chamber through a control valve and heater. A pump is used for recirculation of the solution. In the desiccant regeneration chamber, the solution undergoes ultrasonic dehydration, and small droplets of water (mist) is produced. This mist is carried over to wet channels of the REC where dehumidified secondary air undergoes evaporative cooling and also, a small quantity of salt carryover is witnessed. Secondary air is extracted from primary air flowing through the dry channels, which is redirected through a conduit. The secondary humidified air exits through the top of the unit. Three AC motor fans are used to create required airflow and are controlled by an electrical fan regulator. It is tested in the laboratory by using dehumidified air from Bry-Air FFB 300 desiccant wheel dehumidifier under a humidity ratio range of 9 – 17 gv.kga-1. The temperature is maintained by heating/cooling coils. The value ranges of dynamic parameters are shown in Table 2. To measure the temperature, relative humidity and airflow velocity, a K-type thermocouple with a 10k ohm thermistor, a HC2 hygrometer (rotronic) and a hot wire anemometer are used respectively. The measured intake and exit parameters of air are recorded by keysight 34972A LXI data logger. 2.1. Ultrasonic regeneration and heat-mass transfer process Fig. 3 shows the airflow direction and heat transfer for the dry and wet channels. The working of the regenerative evaporative cooler is similar to an indirect evaporative cooler. In this unit, a weak solution enters into the ultrasonic desiccant regeneration chamber where the ultrasonic transducers atomize the solution with a high-intensity ultrasonic wave which vibrates the thin layer of the solution resulting in excitation of small water droplets (mist) from the surface of the solution. This process is known as cavitation which is induced under certain power intensity and frequency. The water mist which is in contact with the dehumidified secondary air undergoes evaporative cooling in the wet channels. Heat transfer between dry and wet channels takes place through heat exchanger wall, which results in precooling of the intake primary air. Consequently, primary air temperature decreases below its wet-bulb
temperature. The humid secondary air is then exhausted, and the humidity of primary air remains the same as intake air. 2.2. Test procedure To obtain the experimental conditions as shown in Table 2, the procedures taken for the test are as follows. Before starting the experiment, the distilled water was mixed with CaCl2 salt and a weak solution of require mass fraction was prepared. The weak solution is then stored in a solution tank. The dehumidifier along with heating/cooling coil was switched on to make inlet conditions of air to values as shown in Table 2. The airflow rate was varied by controlling the fan regulator. The diluted liquid solution from the tank was circulated through the regeneration chamber using the pump and its temperature and flow rate was controlled by heating element and valve respectively. Hereafter, ultrasonic atomizer was turned on to atomize the desiccant solution and mist were produced which are carried by secondary air. . The experiments are carried out for a steady state condition where relative humidity and temperature variation are within 3% and 0.3oC respectively, and are logged with above mentioned instruments. The increased desiccant concentration was measured from calibration curve plotted with respect to density. After the completion of regeneration, the solution and air operating conditions were set to default values. Different experimental runs are carried out by varying one operating parameter and keeping others as fixed.
3. Performance indicators 3.1. Effectiveness The regeneration performance is commonly measured by humidity and temperature effectiveness, which have been applied in various studies (Gandhidasan, 2005)(Taylor et al., 2011). The humidity effectiveness is defined as the ratio of actual change to the maximum possible change of humidity ratio and given as εhum. (1)
Here, ωs,i and ωs,o are the humidity ratios of secondary air at the inlet and exhaust of wet channel respectively. Secondary air inlet humidity remains same as primary air intake humidity ratio. ωeq is the equilibrium humidity ratio of air, which is the maximum theoretical humidity level to which air can be humidified. It is given by, (2)
where pv is the partial vapour pressure (Pascal) on the surface of CaCl2 desiccant solution. Using the Gad correlation (Gad et al., 2001)(Bassuoni, 2011) pv is calculated in mm of Hg as follows, (3)
Value for the constants of the Eq. (3) are a0 =10.0624, a1=4.4674, b0=739.828, b1=1450.96 and C=111.96. The enthalpy effectiveness of the regeneration based on change in temperature of air is given by, (4)
where, Is,i and Is,o are the enthalpies of secondary air at the inlet and exhaust of wet channel respectively. The enthalpy of air can be found from Eq. (5) in terms of humidity ratio ωs and temperature Ts as follows, (
)
(5)
Similarly, the enthalpy of solution may be found from Eq. (6) as given below: (6) where, Cp,sol is the specific heat capacity of solution at constant pressure which can be calculated in terms of desiccant solution temperature Tsol and concentration ξsol. In Eq. (4), Ieq is the enthalpy of air when the temperature of air is in thermal equilibrium (T s=Tsol) with desiccant solution at the common boundary. For evaluating regenerative evaporative cooler which is similar to indirect evaporative cooler the wetbulb effectiveness εwb is used as given below:
(7)
Here, Tp,i and Tp,o are the temperatures of primary air at the inlet and outlet of horizontal duct (dry channel) respectively as shown in Fig. 1, and Tp,wb is the wet-bulb temperature of inlet fresh air. 3.2. Moisture removal rate (MRR) Moisture removal rate is used in few studies (Yang et al., 2015)(Yang et al., 2016) to evaluate the rate of moisture removed by air the from the weak desiccant solution. It is defined as, (8) where, ωs,i and ωs,o are the humidity ratios of secondary air at the inlet and exhaust of wet channel respectively, and ms is the secondary air mass flow rate which can be determined in terms of extraction ratio and mass flow rate of primary air (ms = rex mp). 3.3. Desiccant concentration increase (DCi) During the regeneration process the weak solution is converted into strong solution, which is normally recirculated through the dehumidification unit. Similar to MRR, desiccant concentration increase measures the performance based on concentration of the desiccant solution before and after regeneration process. It is defined as, (9) where, ξsol,i and ξsol,o are the CaCl2 solution concentrations before and after the ultrasonic regeneration process measured from the regeneration chamber. 4. Uncertainty Propagation The accuracy of all measuring sensors is given in Table 3. From the measuring variables the performance indicators were calculated. The uncertainty of the performance indicators can be obtained by following Eq. 10 √∑
(10)
Where, U represents the uncertainty of the variable. It is caused by errors in the measuring sensors. Uncertainties of the measured variables propagate into the value of the calculated quantity y. The results of uncertainty propagation for certain performance indicators are given in Table 4. 5. Result and discussion of experimental results The validity of experimental results is checked by calculating the energy balance of the regenerative evaporative cooler. The steady performance was analysed by varying operating parameters of the desiccant regeneration evaporative cooler. 5.1. Conservation of energy The change in enthalpy of primary air must be equal to change in enthalpy of secondary air based on the theory of energy conservation. Energy changes on secondary and primary air are expressed as: (
) (11)
(
)
The energy changes between air streams shown in Fig. 4. For all the experimental results the discrepancies are within ±15 %. 5.2. Effect of operating conditions on the system The exit parameters of the air are examined by varying the operating conditions. Wet-bulb effectiveness is used as the evaluation scale for rating evaporating cooling. Humidity and enthalpy effectiveness, MRR and DCi are used for evaluation of desiccant regeneration. Different experimental runs are carried out by varying one operating parameter and keeping others as fixed. 5.2.1. Effect of air velocity The airflow rate is increased by increasing the velocity of air using the primary fan. Fig. 5 presents the effect of air velocity on the regeneration and cooling performance. As the airflow rate increases the enthalpy, humidity and wet-bulb effectiveness decrease from 75.8 to 67 %, 65.1 to 54.9 % and 1.1 to 0.6 respectively. The reduction in effectiveness is due to less time of contact with the plate
surface and an increase in the quantity of primary air. However, the MRR increases from 0.39 to 0.58 g.s-1 and DCi increases from 0.8 to 0.99 % with the increase in airflow rate for regeneration as shown in Fig. 5. For a fixed extraction ratio, primary air and secondary air velocity would increase simultaneously. As the quantity of air increases, more moisture is removed from the desiccant solution because of low humidity and vapour pressure. Here, more amount of air will be in contact with the unit mass flow rate of solution. Accordingly, moisture will be removed from the weak desiccant solution increasing the MRR and DCi. 5.2.2. Effect of extraction ratio Fig. 6 presents the effect of extraction ratio on the effectiveness and moisture removal rate. In Fig. 6, it can be seen that by increasing the extraction ratio from 0.3 to 0.5, the enthalpy and humidity effectiveness decreases because of the increase in secondary air mass flow rate as discussed in previous section 5.2.1. However, the wet-bulb effectiveness, MRR and DCi increase from 0.90 to 1.05, 0.385 to 0.445 g.s-1 and 0.794 to 0.921 % respectively. This can be explained as follows. The more amount of secondary air in wet channel acts as a heat sink absorbing heat from the low velocity primary air through the plate wall, thereby improving sensible heat transfer. Moreover, MRR and DCi increase as more moisture is removed. 5.2.3. Effect of inlet humidity ratio The effect of humidity ratio within low humidity condition where the air humidity ratio is maintained between 0.009 - 0.017 kg.kg-1 is presented in Fig. 7. The effectiveness remains almost constant by increasing inlet air humidity ratio as shown in Fig. 7. It can be seen that only the enthalpy effectiveness slightly varies from 70.3 to 61.5 % as the air becomes more humid. Even though the wetbulb effectiveness is unchanged, with the increase in humidity, the wet-bulb temperature of inlet air also increases simultaneously which results in less primary air temperature drop at the outlet of the cooler. In this case, both the MRR and DCi decrease as shown in Fig. 7. It can be seen that by increasing humidity the moisture removal rate reduces because the high humid air gets saturated quickly during evaporative cooling. The humidity effectiveness remains constant as both the outlet and inlet humidity increase
together. Moreover, all the generated water droplets are not carried over to exhaust because of low capacity fans, and it is settled within the regeneration chamber. 5.2.4. Effect of inlet air temperature Fig. 8 presents the effect of inlet air temperature between 28-36 oC on regeneration and cooling performance. It can be seen that the MRR increases 0.357 to 0.619 g.s-1 and DCi increases from 0.735 to 1.10 % with increase in the inlet air temperature. This can be described as follows, Even though the humidity ratio determines the vapour pressure of air, the relative humidity will decrease with increase in temperature which could improve the evaporative potential. Also, the humidity and enthalpy effectiveness increase from 48 to 83.16 % and 63.1 to 81.2 % respectively. This increase in regenerator performance is due to heat transfer through the plate surface, which results in the rise of secondary air temperature and thereby more moisture is carried by hot air to the exhaust. Here, the intake air temperature is selected below the desiccant solution temperature (35 oC). As a result, no moisture removal by adiabatic heating of desiccant solution by air is witnessed. Same can be concluded for wet-bulb effectiveness which remains constant as shown in Fig. 8, where the wet-bulb temperature will also increase with inlet air temperature. However, wet-bulb effectiveness might remain unchanged for ambient conditions up to 40 oC. At a very high temperature of inlet air, a considerable amount of sensible heating will takes place which cannot be compensated by evaporative cooling in this apparatus. 5.2.5. Effect of desiccant solution flow rate Fig. 9 presents the effect of the desiccant solution flow rate on regeneration and cooling. It can be seen that by increasing the desiccant solution flow rate, the MRR remains unchanged. The atomizer used in the present study runs at its full capacity. As a result, the ultrasonic atomization load on the system is increased significantly. With increases in solution flow rate power output of the atomizer should also be enhanced proportionally to achieve more moisture removal. Additionally, the outlet air humidity is being saturated, and the airflow rate should also be increased. The conventional desiccant regeneration is mainly heat and mass transfer process. However, in ultrasonic atomization the weak desiccant solution is dehydrated without thermal energy. The vapour pressure between solution and air would increase with the
addition of more warm solution as the gross temperature of solution increases, resulting in more moisture removal still the desiccant remains as a weak solution due to more inflow. The DCi reduces from 0.869 to 0.389 % with the increase in solution flow rate as shown in Fig. 9. In this case, the effectivenesses of the evaporative cooler are unchanged as the solution flow rate has a negligible impact on evaporative cooling. 5.2.6. Effect of inlet solution concentration. The effect of inlet desiccant solution concentration between 28.4-35.6 % on moisture removal and effectiveness is presented in Fig. 10. It can be seen that MRR decreases from 0.461 to 0.281 g.s-1 and DCi decreases from 1.02 to 0.78 % with the increase in inlet solution concentration. It is due to the reduction in surface vapour pressure of the desiccant solution as the ultrasonic power remains the same. Consequently, the vapour pressure difference between the solution and secondary air reduces. The humidity effectiveness increases with the increase in inlet solution concentration as shown in Fig. 10. It can be explained as, by increasing the solution concentration the water mist generated from the strong solution to secondary air reduces, which results in lower humidity of outlet air. Moreover, the equivalent humidity ratio also decreases. The outlet air humidity ratio and equivalent humidity ratio compensate with each other, and the humidity effectiveness increases. The wet-bulb effectiveness decreases as the amount of water mist from the solution reduces resulting in less evaporative cooling as shown in Fig. 10. 5.2.7. Effect of desiccant solution temperature. Fig. 11 presents the effect of desiccant solution temperature on moisture removal and effectiveness. It can be seen that by increasing desiccant solution temperature from 35 to 50 oC, the MRR increases from 0.39 to 0.58 g.s-1 and DCi increases 0.8 to 0.99 %. This is due to the increase in surface vapour pressure of the solution, which aids in easy atomisation of water mist from the solution which consequently enhances the moisture removal rate. However, the humidity effectiveness and wet-bulb effectiveness decreases from 56 to 30 % and 1 to 0.73 respectively as shown in Fig. 11. It can be explained as follows. When the desiccant solution temperature is increased, the equivalent humidity ratio also increased simultaneously. Although the secondary air outlet humidity ratio increases, the equivalent humidity ratio would increase drastically due to more mass transfer potential of desiccant solution at the
high temperature, which reduces the humidity effectiveness. In this case, the decrease in wet-bulb effectiveness is negligible up to 40 oC as water mist does not carry sensible heat from the solution. Above 40 oC up to 50 oC, decrease in wet-bulb effectiveness is observed. This is due to considerable addition of sensible heat from the desiccant solution which dominates the evaporative cooling (latent heat) of the secondary air. Table 5 shows the comparison of wet-bulb effectiveness and regeneration temperature with respect to the regeneration load from previous studies. The proposed system was operated at a relative low regeneration load. However, low temperature regeneration of desiccant was achieved effortlessly. 6. Conclusions A combined ultrasonic desiccant regenerator and the indirect evaporative cooler through the MCycle was fabricated, and experimental investigation was conducted. For this study, the conclusions are as follows: 1. Wet-bulb effectiveness of unity with a minimum temperature of 19 oC is achieved with basic constructions. It is improved by increasing extraction ratio and remain unchanged by increasing humidity ratio, inlet air temperature and solution flow rate. When the inlet air velocity, desiccant concentration and desiccant temperature are increased, wet-bulb effectiveness reduces. 2. Humidity effectiveness also known as regeneration effectiveness improves by increasing the inlet air temperature as the regeneration potential increases and remains unchanged with the increase in inlet air humidity and desiccant flow rate. Although effectiveness enhances with the increase in desiccant concentration, it is due to the negative effect of drop in equivalent humidity ratio. Regeneration effectiveness reduces with increase in inlet air velocity, extraction ratio and solution temperature. 3. Low temperature desiccant regeneration (35-50 oC) is accomplished with 0.208 to 0.619 g.s-1 of moisture removal rate. MRR improves by increasing air and desiccant temperature, inlet air velocity and extraction ratio, and reduces by increasing inlet air humidity and desiccant concentration.
4. Low temperature ultrasonic regeneration of desiccant solution can be developed to the waterless desiccant-evaporative cooling system through the M-Cycle where dehumidified water can be reused for evaporative cooling.
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List of Figures Fig. 1. Schematic layout of test rig. Fig. 2. Photographic view of test rig. Fig. 3. Temperature distribution along channel length in dry and wet channel. Fig. 4. Energy balance between air streams. Fig. 5. Effect of inlet air velocity on regeneration performance. Fig. 6. Effect of extraction ratio on regeneration performance. Fig. 7. Effect of inlet air humidity on regeneration performance. Fig. 8. Effect of inlet air temperature on regeneration performance. Fig. 9. Effect of desiccant flow on regeneration performance. Fig. 10. Effect of desiccant concentration on regeneration performance. Fig. 11. Effect of desiccant temperature on regeneration performance.
Fig. 1. Schematic layout of test rig.
Fig. 2. Photographic view of test rig.
Fig. 3. Temperature distribution along channel length in dry and wet channel.
Fig. 4. Energy balance between air streams.
Fig. 5. Effect of inlet air velocity on regeneration and cooling performance.
Fig. 6. Effect of extraction ratio on regeneration and cooling performance.
Fig. 7. Effect of inlet air humidity on regeneration and cooling performance.
Fig. 8. Effect of inlet air temperature on regeneration and cooling performance.
Fig. 9. Effect of desiccant flow on regeneration and cooling performance.
Fig. 10. Effect of desiccant concentration on regeneration and cooling performance.
Fig. 11. Effect of desiccant temperature on regeneration and cooling performance.
List of tables Table 1. Geometric properties Table 2. Ranges of dynamic parameters Table 3. Accuracy of measuring sensors Table 4. Results of uncertainty propagation for certain performance indicators Table 5. Comparison of regeneration load and temperature with previous studies
Table 1. Geometric properties Parameters
System
Cooler length
0.5 m
Cooler height
0.3 m
Primary channel width
3 mm
Secondary channel width
3 mm
Number of plates
40
Plate thickness
0.2 mm
Ultrasonic atomiser
6 L.h-1, 300 W
Table 2. Ranges of dynamic parameters Parameters
Unit
Range
Fixed value
Inlet air temperature (Tin)
o
28-36
30
Inlet air humidity ratio (ωin)
kgv.kga-1
0.009-0.017
0.011
Inlet air velocity (νin)
m.s-1
1.5-4
2.2
Desiccant solution temperature (Tsol)
o
35-50
35
Desiccant flow rate (msol)
kg.s-1
0.015-0.033
0.015
Desiccant concentration (ξsol)
%
28.4-35.6
30.1
Extraction ratio (rex)
-
0.30-0.50
0.40
C
C
Table 3. Accuracy of measuring sensors Parameters
Apparatus
Measurement range
Accuracy
Relative humidity
Hygroclip HC2 Humidity sensor
0-90% RH
±1% RH
Air velocity
Hotwire anemometer
0-20 m.s-1
±0.03 m.s-1 or 5%
Pressure drop
Digital manometer
0–200 kPa
±2% FS
Temperature
Thermocouple K-type
0–100 oC
±0.2 oC
Thermistor 10K
-20-100 oC
±0.25 oC
Air flow rate
Turbine flowmeter
0–10 CMM
±1% FS
Desiccant flow rate
Glass flow meter
0-10 LPM
±2 % FS
Desiccant density
SG. Hydrometer
1000-1500 kg.(m3)-1
±1 kg.(m3)-1
Table 4. Results of uncertainty propagation for certain performance indicators
Symbol
Performance indicators
Value ±uncertainty
Units
mp,i
Mass flow rate of air
0.128 ±0.00109
kg.s-1
ωp,i
Humidity ratio
0.011 ±0.000170
kg.kg-1
εwb
Wet-bulb effectiveness
0.997 ±0.02768
-
εhum
Humidity effectiveness
0.5398 ±0.02153
-
εenth
Enthalpy effectiveness
0.6786 ±0.01348
-
MRR
Moisture removal rate
0.4023 ±0.01815
g.s-1
DCi
Desiccant concentration increase
0.875 ±0.01414
%
Cooling capacity
1263 ±38.93
W
Qp
Table 5. Comparison of temperature, regeneration load and wet-bulb effectiveness with previous studies Study
Desiccant/
Solution
Inlet air
Inlet humidity
Desiccant
MRR
Wet-bulb
Cooler
temperature
temperature
ratio (g.kg-1)
mass fraction
(g.s-1)
effectiveness
(oC)
(oC)
(%)
Liu et al.,2007
LiBr
50-62
29-35
12-22
40-52
4-8
Fumo et al.,
LiCl
60-70
30-40
14-21
32.8-35
1.4-2.4
TEG
72-80
-
18-24
90.4-95
1.7-2.36
LiCl (ultra)
45-64
29-36
14-22
24-32
0.07-0.18
M-cycle
-
25-45
7.0-26.0
-
-
0.92-1.14
M-cycle
-
22.7-38.9
9.3-9.4
-
-
0.55-1.06
M-cycle
-
22.5-40.3
10.7-26.7
-
-
0.93-1.06
CaCl2/M-
35-50
28-36
9-17
28.4-35.6
0.39-0.58
0.4-1.05
2002 Elsarrag., 2006 Yang et al., 2016 Riangvilakul et al., 2010 Z. Duan et al., 2016 F. Bruno et al., 2011 Present
cycle (ultra)
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: