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Solar water desalination using an air bubble column humidifier
Desalination 372 (2015) 7–16
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Solar water desalination using an air bubble column humidifier A. Khalil, S.A. El-Agouz ⁎, Y.A.F. El-Samadony, Ahmed Abdo Mech. Power Eng. Department, Faculty of Eng., Tanta University, Egypt
H I G H L I G H T S • • • • •
Desalination using an air bubble column humidifier is investigated. Effect of water temperature and height, air flow rate and hole diameter is studied. At inlet water is 62 °C, productivity, efficiency and GOR are 21 kg, 63%, and 0.53. Air bubble column achieves higher performance than that conventional humidifier. Temperature difference along air column is less than 2.5 °C for all measurements.
a r t i c l e
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Article history: Received 18 April 2015 Received in revised form 6 June 2015 Accepted 13 June 2015 Available online xxxx Keywords: Humidification–dehumidification Air bubbles Solar desalination, sieve plate
a b s t r a c t An experimental study of a solar water desalination using an air bubble column humidifier is investigated. The characteristics of the generated bubbles are modified by using a different sieve plate with different hole size. The effect of water temperature, air flow rate, water height, and sieve's hole diameter on desalination performance is studied. The results showed that the daily productivity, efficiency and gain output ratio are 21 kg, 63%, and 0.53 respectively; at inlet water temperature is 62 °C. The change in the temperature difference along the column is less than 2.5 °C for all measurements. The best performance is observed from sieve with 1 mm hole diameter at which the outlet air from the bubble columns is always saturated. The air bubble column achieves higher performance than that for the conventional humidifier. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Desalination can be achieved by many methods. Thermal method is considered to be the simplest one. In this method, saline water is heated in an evaporator and generates water vapor free of salts. The generated vapor is condensed in a condenser then fresh water is collected. Water, air, or both can be heated by conventional energy source or by renewable energy source such as solar energy. The latter desalination process can be called solar air humidification– dehumidification desalination process. The humidification– dehumidification (HD) desalination process is one of the secondary desalination processes. This system is very useful for places which have low freshwater demand. The main advantages of this system are; consume a small amount of energy and simplicity in both plant layout and management. Bourouni et al. [1] presented the technique of air humidification– dehumidification (HD) process. The principle, technique and state of the art of the HD process were presented. Gahin et al. [2] presented a ⁎ Corresponding author. E-mail addresses: [email protected] (A. Khalil), [email protected] (S.A. El-Agouz), [email protected] (Y.A.F. El-Samadony), [email protected] (A. Abdo).
http://dx.doi.org/10.1016/j.desal.2015.06.010 0011-9164/© 2015 Elsevier B.V. All rights reserved.
preliminary design study of a solar collector humidification–dehumidification desalination unit. They studied different parameters affecting the global performance of the unit. Also, they studied the performance of the two most important components of the loop which are humidifying and dehumidifying columns or stacks. Farid and Al-Hajaj [3] designed and studied experimentally the performance of multi-effect solar air humidification desalination unit. The unit had two different loops; air closed loop and water open loop. The results showed that the multi-effect humidification–dehumidification process, with forced air circulation, was found suitable for water solar desalination. The unit achieved a daily productivity of 12 l/m2/d, which was over three times that for single-basin conventional solar still. Al-Hallaj et al. [4] studied experimentally the indoor and outdoor performance of two desalination units based on air humidification desalination. The results showed that water productivity was increased as the feeding water flow rate was increased to an optimum value. Moreover, they concluded that forced air circulation was effective in the unit performance at low operating water temperatures. Ben-Bachaa et al. [5] studied experimentally a solar multiple condensation evaporation cycle (SMCEC) desalination technique. The results showed that the pilot units produced as much as 60% of daily water needed for irrigation. Dai et al. [6] conducted experimentally a solar humidification and dehumidification desalination unit. The performance of the unit was strongly dependent on the
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temperature and mass flow rate of the inlet salt water to the humidifier and the process air mass flow rate, which was forced by a fan. The unit thermal efficiency was above 80%. Garg et al. [7] studied experimentally and theoretically multi-effect humidification/dehumidification solar desalination system when air was circulated by natural convection. The differences between the values of the experimental and theoretical results were increased as the water temperature was increased due to the energy losses in the humidifier. Nafey et al. [8,9] investigated theoretically and experimentally a solar humidification dehumidification desalination unit under different environmental and operating conditions. The productivity of the unit was strongly affected by the cooling water flow rate, airflow rate, and total solar energy incident through the daytime. Amer et al. [10] investigated experimentally and theoretically the effect of operating parameters on the characteristics of a humidification–dehumidification desalination system. The system was based on a closed cycle for the airstream and an open cycle for water. The results showed that the maximum daily productivity was 5.8 l/h at 2.8 kg/min water flow rate and 85 °C inlet water temperature. Yanniotis and Xerodemas [11] studied experimentally the performance of tubular spray humidifier and a pad humidifier in seawater desalination plants. The evaporation rate of the spray humidifier was approximately the same as 100 mm thickness pad humidifier. Also, the pad humidifier with 300 mm thickness gave the highest evaporation rate at high air to water flow rate ratios. Zhani [12] studied theoretically and experimentally solar humidification–dehumidification desalination unit. The results showed that the maximum GOR was obtained at 0.4 kg/s inlet hot water flow rate. Vlachogiannis et al. [13] studied experimentally and theoretically a novel desalination concept, combining between the principles of air humidification–dehumidification and mechanical vapor compression. Air was injected into the evaporation chamber through a porous bottom wall and dispersed as small diameter bubbles. Inaba et al. [14] studied experimentally heat and mass transfer of air bubbles in a hot water layer. The results showed that the mean diameter of generating air bubbles increased with an increase in the superficial air flow velocity.
EL-Agouz and Abugderah [15] studied experimentally the humidification process by air passing through seawater. The air was fed to the evaporator chamber from 32 holes of 10 mm diameter located on the surface area of a PVC pipe, which was submersed in the water of the evaporator chamber. They found that the maximum vapor content difference of the air was about 222 gmw/kga at 75 °C and exit relative humidity of air was reached to 95%. Kabeel [16] studied experimentally the performance of the liquid desiccant system during a dehumidification–humidification process using an injected air through the liquid desiccant solution (calcium chloride). The air was injected through a series of homogeneous distribution. Holes in a vertical pipe El-Agouz et al. [17] studied experimentally a single air bubbling humidification desalination unit. They studied the influence of the electrically heated water temperature, water level, and airflow rate on the desalination performance. The productivity was slightly affected by water level and the maximum productivity of the system reached to 8.22 kgw/h at 86 °C for water temperature and 14 kg/h for air flow rate. Zhang et al. [18] studied experimentally operating factors that affects bubbling humidification by using a single sieve plate. The result showed that air relative humidity reached to 100%. Moreover, humidification capacity was increased by about 80% when water temperature was increased by 10 °C. Zhang et al. [19] studied experimentally the influence of working design parameters of solar bubbling humidification desalination unit on its gain output ratio, electric power consumption, and fresh water production cost. They found that gain output ratio was increased as the humidification temperature was increased, while electrical power consumption decreased. Emily and John [20] experimentally studied the heat flux and effectiveness of a bubble column dehumidifier. It was found that the effectiveness was decreased while the heat flux was increased with decreasing coil area and increasing the temperature and air flow rate. Ghazal et al. [21] examined air bubble regeneration on the performance of a solar still using humidification–dehumidification desalination (HDD) process. The results showed that the exit air from the humidification process was fully saturated. The effectiveness of the solar
HUM
Humidifier
C.C
Cooling coils
PCV
Pressure control valve
SWC
Solar water collector
SP
Sieve plate
P
Pump
CV
Control valve
DEH
Dehumidifier
PG
Pressure gauge
FM
Flow meter
TST
Thermal storage tank
GL
Graduate level
RH
Relative humidity sensor
TC
Thermocouple
FWT
Fresh water tank
Fig. 1. Experimental set-up schematic diagram.
A. Khalil et al. / Desalination 372 (2015) 7–16
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(a) Photograph and dimensional of the humidifier
(b) Photograph of the experimental set-up Fig. 2. Photographs of the humidifier and the experimental set-up.
humidifier and the mass and heat transfer were upgraded by air bubbles regeneration. Sharqawy and Liu [22] studied experimentally the effect of the air pressure in bubble column dehumidifier dehumidification process, for absolute pressures in the range of 1–2 bar, superficial velocity range of 2–18 cm/s, and column height range of 3–7 cm. It was found that the total heat transfer rate was increased with the superficial velocity and pressure while effectiveness was increased with the superficial velocity and was decreased with pressure. The column height was slightly affected on the heat transfer and the effectiveness. Air bubble columns are suitable for humidification and dehumidification. Its great interface area, allows a great value of heat and mass transfer rate consequently improves productivity and efficiency. Therefore, the present work presents an experimental investigation of the use air bubble column humidifier for the solar water desalination system. The saline water, which is working in a closed loop, is heated by an
evacuated tube solar collector as an energy source in order to save electrical energy. Air working in an open loop is injected into the evaporation chamber from the bottom through sieve plates with 1200 holes and 1, 3, and 5 mm diameter. The characteristics of generating bubbles are changed by using the different hole diameter of sieve plates. The study includes the effect of water temperature and level, air flow rate, and orifice diameter of the sieve plate on the unit performance. 2. Experimental setup and instrumentation 2.1. Experimental setup Fig. 1 presented a schematic diagram of the experiment setup. The unit main components are HUM (humidifier), DEH (dehumidifier), and SWS (solar water collector). Fig. 2 shows photographs of the humidifier and the experimental set-up. The system consists of two
Table 1 Accuracies, ranges, and errors of measuring instruments. Property
Used instrument
Accuracy
Range
Error
Temperature Relative humidity Pressure Solar intensity Air velocity Fresh water volume Air flow rate
Digital thermometer, K-Type
±0.1 °C ±0.1% (RH) ±0.02 bar ±1 W/m2 ±0.1 m/s ±10 ml ±1 SCFM
0–100 °C 0–100% 0–2 bar 0–5000 W/m2 0–25 m/s 0–1000 ml 4–25 SCFM
0.25% 3% 2% 0.2% 2% 2% 2%
Pressure gauge (PG) MS-802 Pyranometer Meter TM-401 Graduate lab vessel Flow meter
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Turn on air compressor and then Open air control valve at (7) and water control valve at (2 and 3)
Turn on the pumps (2 and 4) and then Fill the evaporator to the design level by adjusting the control valves at (1 and 2)
Adjust the pressure and flow rate of air
Record 1. Water temperatures, air temperatures, and humidity of air every five minutes. 2. Record solar radiation every one hour
Fig. 3. A flow diagram of system operation.
loops, a closed water loop and open-air loop. In the closed water loop, the hot water at (1) is pumped to the HUM inlet (2) through CV (control value) and FM (flow meter) that heats in the evacuated tube solar
80
collector. At HUM exit (3) the hot water is pumped through CV and FM to TST (thermal storage tank) at (5). The hot salt water is circulated in a closed loop between HUM and TST. High salt concentration saline
1000
80
1000 Solar radiation
Twi Two
50
600
Tai Tao
40
400 17/6/2013 ma = 11 kg/hr H = 15 cm
30
200
20
0 8
10
12 14 16 Local Time, hr
18
800
60 50
Twi Two
40
Tai Tao
20 8
10
(a) 1000
600
Twi Two
400
Tai Tao
13/8/2013 ma = 15.4 kg/hr H = 15 cm
200
20
0 10
12 14 16 Local Time, hr
(c)
20
1000
18
20
70 Temperature, oC
60
Solar radiation, W/m2
Temperature, oC
800
8
0 18
Solar radiation
70
30
12 14 16 Local Time, hr
80
Solar radiation
40
200
(b)
80
50
400 21/7/2013 ma = 13.2 kg/hr H = 15 cm
30
20
600
800
60 50
Twi Two
40
Tai Tao
600 400 200
6/9/2013 ma = 17.6 kg/hr H = 15 cm
30 20 8
10
12 14 16 Local Time, hr
0 18
20
(d)
Fig. 4. Solar radiation, inlet, outlet water and air temperatures at humidifier at air flow rate: (a) 11 kg/h (b) 13.2 kg/h (c) 15.4 kg/h (d) 17.7 kg/h.
Solar radiation, W/m2
60
70 Temperature, oC
800
Solar radiation, W/m2
Temperature, oC
70
Solar radiation, W/m2
Solar radiation
A. Khalil et al. / Desalination 372 (2015) 7–16
11
Fig. 5. Inlet air pressure and exit air relative humidity of the humidifier.
water is rejected from CV at (6). HUM is made of Acrylic plastic sheet of 10 mm thickness with 580 mm × 580 mm cross section, and 900 mm height as shown in Fig. 2a. In the open air loop, air at (7) is supplied from the reservoir of a compressor containing 220 l at a pressure of 10 bar. The desired pressure and flow rate of air at (8) could be adjusted by a PCV (pressure control valve) and CV (control valve) then is passed through the FM and PG (pressure gauge) that indicate the pressure before the HUM. The air at (9) enters to the HUM through the sieve plate which is located at the bottom of the HUM. Three different sieves are used, 1200 holes each, with 1, 3, and 5 mm hole diameter. The air flow is humidified by passing through the water level in HUM and carries vapor water to DEH. Then, the produce water through a tube is desalinated to FWT (fresh water tank). Photographs of air flow through the water and the HUM dimensions are shown in Fig. 2a. DEH is a shell and tube heat exchanger. The shell is made of 0.7 mm thick steel with dimensions of 400 × 400 mm, and 900 mm height. Inside the shell, the cooling water tube is fixed. The type of SWC is an evacuated tube solar collector as shown in Fig. 2b. It is consisted of 25 evacuated glass tubes with 58 mm diameter and 1.8 m length. The capacity of TST is 250 l. The tilt angle of solar collector is 30° for all day.
to measure temperature and relative humidity of air at (9, 10). A pressure gauge (PG) is used to measure air pressure measurement at (9). A MS-802 Pyranometer sensor is used to measure solar radiation and wind speed is measured by air velocity meter TM-401. The air flow rate is measured at (9) by a flow meter, where the water flow rate is measured at (1 and 3) by a flow meter. All sensors are calibrated before using to determine the probes sensibility. The accuracy, range and errors that occurred in measuring instruments are shown in Table 1. The errors are calculated for thermocouples, solar meter, anemometer and collection tank. The minimum error that occurred in any instrument is equal to be the ratio between its last count and minimum value of the output measured. To estimate the uncertainties in the results presented in this work, the approach described by Barford [23] is applied. The uncertainty in the measurements is defined as the root sum square of the fixed error of the instrumentation and the random error observed during different measurements. Accordingly, the resulting errors of the calculated amount of extracting water, daily productivity, daily efficiency, and Gained Output Ratio of the desalination system are ± 0.24%, ± 0.65%, ± 1.52%, and ± 1.28%, respectively. 2.3. System operation
2.2. Instrumentation
The following steps describe the testing procedure in details and Fig. 3 presents a flow diagram of system operation.
As shown in Fig. 1, a K-Type thermocouple is used to sense water temperature at the point (2, 3). Digital sensor with data logger is used
1. Prior to testing, the system is checked. 2. Turn on air compressor to fill its tank with a compressed air.
Fig. 6. Water and air temperature difference of humidifier.
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A. Khalil et al. / Desalination 372 (2015) 7–16
Fig. 7. Effect of inlet water temperature on the amount of extracting water for different water height at air flow rate: (a) 11 kg/h (b) 13.2 kg/h (c) 15.4 kg/h (d) 17.6 kg/h.
3. Open air control valve at (7) to eject air through HUM. 4. Open water control valve at (2, 3), turn on the pumps (2, 4) and let the water flow to the evaporator to fill it, then adjust the flow rate and the level of water in evaporator by adjusting the control valves before and after the evaporator at (1, 2) manually. 5. After the water level became stable adjust the pressure and flow rate of air by using the pressure control valve to the desired pressure and adjusting flow control valve to the wanted volume flow rate. 6. Record water temperatures, air temperatures, and humidity of air, every five minutes. 7. Record solar radiation every one hour.
2.4. Efficiency and Gained Output Ratio of system It is important to calculate the hourly and daily efficiency of the desalination unit to know the performance achieved in real systems is calculated as:
ηh ¼ 100
ðMw LH wav Þ=3600 ðASWS IS þ PP þ Wc Þ
Fig. 8. Variations of air temperature and humidity ratio at dehumidifier inlet and exit at different air flow rate.
ð1Þ
A. Khalil et al. / Desalination 372 (2015) 7–16
where LHw is the latent heat of vaporization at the ambient condition, and [(m × h)w,i − (m × h)w,o] is the total enthalpy difference between entering and leaving the humidifier for hot water. The hot water flow rate, leaving the system is calculated as:
Table 2 The inlet and exit of average cooling water temperature in dehumidifier. ma, kg/h
Tdeh,wi, °C
Tdeh,wo, °C
11 13.2 15.4 17.6
22.3 20.3 19.6 20.0
26.1 23.0 23.4 27.0
13
mw;o ¼ mw;i −Mw :
ð5Þ
3. Results and discussion n 1X η n 1 h
ð2Þ
LHw:av ¼ 103 2501:9−2:40706 Tw þ 1:192217 10−3 T 2w −1:5863 10−5 T 3w
ð3Þ where ηh and ηd are the hourly and daily efficiency, Mw is the hourly productivity, LHw.av is the average of the latent heat of vaporization of water, El-Dessouky and Ettouney [24], ASWS is the solar water collector area, IS is the incident solar radiation on the collector surface, Pp = I × V is the power of pumps that are measured with different flow rate of cold and hot water, Wc is a power of compressor and n is the duration time of solar radiation. The Gained Output Ratio (GOR) is calculated as McGovern et al. [25]: M w LHw ðm hÞw;i −ðm hÞw;o
i
ð4Þ
3.1. Effect of 1 mm hole diameter Depending upon weather conditions which are different during the daytime, the wind speed, atmospheric temperature, and solar intensity. Fig. 4 shows the general trend of solar radiation, inlet and outlet water temperature and air temperature at humidifier inlet and out for 11, 13.2, 15.4, and 17.7 kg/h air mass flow rate. It can be seen that the solar radiation and air temperature at humidifier inlet increases in the morning hours, reaching its maximum values around midday, and then decreases in the afternoon. In case of water, water temperature started from high
25
80
20
60
15 Air flow rater, ma 11 kg/hr 13.2 kg/hr 15.4 kg/hr 17.6 kg/hr
10
5
40 Air flow rate, ma 11 kg/hr 13 kg/hr 15.4 kg/hr 17.6 kg/hr
20
0 56
58 60 62 Average hot water temperature, oC
64
56
58 60 62 o Average hott water temperature, C
(a)
(b)
0.55 0.50 0.45 GOR
Daily productivity, kgw/day
GOR ¼ h
Test rig external ambient conditions are very important. Average relative humidity of the ambient air is 15–40%. Wind speed during the experimental is 1.5–3 m/s in that period of time. The inlet water flow rate to HUM is 10 l/min while, the exit water flow rate is measured by FM and checked by using Eq. (5). Four different values of air flow rates are used 11, 13.2, 15.4, and 17.7 kg/h each value has been used during a whole working day. In addition, four different values of water height in humidifier are used 5, 10, 15, and 20 cm.
Daily effeciency, %
ηd ¼
Water Flow rate, ma 11 kg/hr 13.2 kg/hr 15.4 kg/hr 17.6 kg/hr
0.40 0.35 0.30 0.25 56
58 60 62 Average hot water temperature, 0C
64
(c) Fig 9. Effect of average hot water temperature on: (a) Daily productivity (b) Daily efficiency (c) GOR.
64
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A. Khalil et al. / Desalination 372 (2015) 7–16
Fig. 10. Effect of diameter holes of sieve plate on productivity, efficiency, and GOR.
temperature and then decreases in the morning hours, nearly remains constant around midday, and then decreases in the afternoon. The high temperature of the water at the beginning is because the water in the solar collector tank heating from sunrise to the desalination unit starts at 9.5 AM. In the morning hours, the water temperature decreases because the energy lost from water in the humidifier is larger than that the energy gain by solar radiation to the solar collector. In the midday, the water temperature nearly constant. This is because the energy lost from water in the humidifier is nearly equal the energy gain by solar radiation to the solar collector in this period. Fig. 5 shows inlet air pressure and exit air relative humidity of the humidifier. The inlet air pressure gauge at the humidifier change from 9 to 23 kPa, while the outlet air pressure is atmospheric pressure. The pressure drop (Δp) of the air bubble can be modeled as an exponential function of the air flow rate (ma), Δp ≈ (ma)2 [26]. Therefore, the increase of air flow rate increases the pressure drop. Also, the increase of air flow rate increases the bubble rise velocity and bubble size. In addition, the increase of the number of air bubbles increases the rate of interface area, which increase the productivity. Relative humidity reached to 100%, almost the time of the process (therefore, there is any reasons to use hole diameter less than 1 mm and pay more air side pressure drop), similar to Ghazal et al. [21] and higher than that El-Agouz and Abugderah [15] due to the decrease difference between the air drybulb/wet-bulb temperatures. Fig. 6 presents temperature difference of water and air at different water heights and air flow rates. As can be seen in Fig. 6, the water outlet temperature is slightly affected by both air flow rate and water height. It can be seen also that a temperature difference of 2.5 °C on water inlet temperature influences also the outlet air temperatures of 22.5 °C. Because this water has a high specific heat compared with low air specific heat (4.18 against 1 kJ/kg K), which causes a large amount of air to enhance water temperature. The air flow rate has no significant influence on the parameter and the difference value is constant for all the conditions considered, for a given inlet temperature. This could mean that with all air flow rate, the system is able to exchange all the available energy. It could be mean that the efficiency of the heat transfer is very high, because the air exit temperature is very close to the water inlet one as shown in Fig. 4. Fig. 7 shows the relation between inlet water temperature and amount of extracting water at different water height and air mass flow rate. It can be seen that as the inlet water temperature increases the amount of extracting water highly increases. Therefore, it has a great effect on the amount of extracting water. The amount of extracting water increases by 51.4, 76, 73, and 60.7% as inlet hot water temperature increases from 60 to 70 °C, at 11, 13.2, 15.4, and 17.7 kg/h, respectively, for air mass flow rate and the H = 20 cm. The amount of extracting water slightly increases with the increase of the air mass
flow rate due to increase of the air bubble number that increases the rate of interface area. Also, the amount of extracting water slightly increases as the water height increases due to the increase time contact of air bubbles with water and consequently heat and mass transfer increases. Fig. 8 presents air temperature and humidity ratio of dehumidifier inlet and exit at different air flow rate. The air temperature and humidity ratio of dehumidifier exit are slightly affected by both air flow rate and hot water temperature of humidifier. It can be seen also that the air humidity ratio of dehumidifier exit is about 25 gmw/kga. Also, Table 2 shows the inlet and exit of average cooling water temperature in dehumidifier. It can be seen also that a temperature difference from 3 to 7 °C on inlet cooling water temperature influences. Fig. 9 shows the effect of average hot water temperature on daily productivity, daily efficiency and GOR at different inlet hot water flow rate. It indicates that both of average daily water temperature and water flow rate promote the daily productivity, daily efficiency and GOR increases. The results indicate that the water flow rate and temperature play an important role in the system performance.
Fig. 11. The enhancement of humidity ratio difference at the humidifier for the present and previous results.
A. Khalil et al. / Desalination 372 (2015) 7–16
3.2. Effect of 1, 3, and 5 holes diameter
Pp T
Fig. 10 illustrates the effect of sieve's hole diameter on the amount of water extracted, daily efficiency and GOR. The amount of extracting water decreases as sieve holes diameter increases and 5 mm orifice diameter gave the lowest productivity. It is observed that at 5 mm sieve's hole diameter the flow started to transform to be jet flow. The extracting water for 1 mm hole diameter is higher than that 3 and 5 mm hole diameter by 9.1%, and 14.6%, respectively. The best performance is observed from sieve with 1 mm hole diameter. Because the diameter of the air bubbles decreases and the number of air bubbles increases as the sieve holes diameter decreases. Consequently, surface contact area between bubbles and hot water increases hence the heat and mass transfer. 3.3. Comparison with literatures Fig. 11 compares between enhancement of humidity ratio difference at the humidifier for the present and previous results. The percentage improvement in either system is given by: Enhancement ¼ 100
ΔωPresent −ΔωPrevious : ΔωPrevious
ð6Þ
It is inferred from Fig. 11, when the inlet water temperature has increased from 45 to 70 °C, the enhancement of presenting results is approximately 4–58% of Amer et al. [10], 22–206% from Zhani [12], and 30–270% of Garg et al. [7]. It can be noted that the present work achieved higher productivity than that listed for previous systems.
15
pump power, W temperature, °C
Greek letters η Efficiency Subscripts a air av average c compressor d daily h hourly i inlet o outlet s solar w water Abbreviations CC cooling coil CV control value DEH dehumidifier FM flow meter FWT fresh water tank GOR gain output ratio HUM humidifier PCV pressure control valve SWC solar water collector TC thermocouples TES Thermal Energy Storage
4. Conclusion References The experimentally a small-scale solar air bubbling humidification– dehumidification unit is studied. The effects of the water temperature, water height, airflow rate and holes diameter on the performances of the system are observed during the present experiments and the following points are concluded. • Air bubbling humidification is an efficient technique which can be used in HD systems and the outlet air from the bubble columns is always saturated. • The productivity, efficiency and GOR are affected strongly by inlet water temperature and air flow rate to the humidifier. • The value of the amount of the extracting water is increased slightly with the increasing water height. • The temperature difference of water is constant for all the water height and air flow rate conditions. The change in the temperature difference along the column is less than 2.5 °C for all measurements. • The present system achieved daily productivity, daily efficiency and GOR 21 kg, 63%, and 0.53, respectively, at inlet water temperature is 62 °C. • The best performance is observed from sieve with 1 mm hole diameter. • The air bubble column achieves higher performance than that for the conventional humidifier.
Nomenclature A area, m2 h enthalpy, kJ/kg H water height, cm LH latent heat of water, J/kg incident solar radiation, W/m2 IS m flow rate, kg/h M productivity, kg/h n duration time of solar radiation, h
[1] K. Bourouni, M.T. Chaibi, M.T. Tadrist, Water desalination by humidification and dehumidification of air: state of the art, Desalination 137 (2001) 167–176. [2] S. Gahin, M.A. Gazi, Engineering analysis for solar operated humidification– dehumidification plan, Desalination 2 (1980) 1097–1104. [3] M. Farid, A.W. Al-Hajaj, Solar desalination with a humidification–dehumidification cycle, Desalination 106 (1996) 427–429. [4] S. Al-Hallaj, M.M. Farid, A.R. Tamimi, Solar desalination with a humidification– dehumidification cycle: performance of the unit, Desalination 120 (1998) 273–280. [5] H. Ben Bacha, A.Y. Maalej, H. Ben Dhia, I. Ulber, H. Uchtmann, M. Engelhardt, J. Krelle, Perspectives of solar-powered desalination with the “SMCEC” technique, Desalination 122 (1990) 177–183. [6] Y.J. Dai, H.F. Zhang, Experimental investigation of a solar desalination unit with humidification and dehumidification, Desalination 130 (2000) 169–175. [7] H.P. Garg, R.S. Adhikari, R. Kumar, Experimental design and computer simulation of multi-effect Humidification (MEH) — dehumidification solar distillation, Desalination 153 (2002) 81–86. [8] A.S. Nafey, H.E.S. Fath, S.O. El-Helaby, A.M. Soliman, Solar desalination using humidification dehumidification processes Part I. A numerical investigation, Energy Convers. Manag. 45 (2004) 1243–1261. [9] A.S. Nafey, H.E.S. Fath, S.O. El-Helaby, A.M. Soliman, Solar desalination using humidification dehumidification processes. Part II. An experimental investigation, Energy Convers. Manag. 45 (2004) 1263–1277. [10] E.H. Amer, H. Kotb, G.H. Mostafa, A.R. El-Ghalban, Theoretical and experimental investigation of humidification–dehumidification desalination unit, Desalination 249 (2009) 949–959. [11] S. Yanniotis, K. Xerodemas, Air humidification for seawater desalination, Desalination 158 (2003) 313–319. [12] K. Zhani, Solar desalination based on multiple effect humidification process: thermal performance and experimental validation, Renew. Sust. Energ. Rev. 24 (2013) 406–417. [13] M. Vlachogiannis, V. Bontozoglou, C. Georgalas, G. Litinas, Desalination by mechanical compression of humid air, Desalination 122 (1999) 35-12. [14] H. Inaba, S. Aoyama, N. Haruki, A. Horibe, K. Nagayoshi, Heat and mass transfer characteristics of air bubbles and hot water by direct contact, Heat Mass Transf. 38 (2002) 449–457. [15] S.A. El-Agouz, M. Abugderah, Experimental analysis of humidification process by air passing through seawater, Energy Convers. Manag. 49 (2008) 3698–3703. [16] A.E. Kabeel, Dehumidification and humidification process of desiccant solution by air injection, Energy 35 (2010) 5192–5201. [17] S.A. El-Agouz, Desalination based on humidification–dehumidification by air bubbles passing through brackish water, Chem. Eng. J. 165 (2010) 413–419. [18] L. Zhang, S. Gao, H. Zhang, Experimental researches of factors affecting bubbling humidification, in: D. Jin, S. Lin (Eds.),CSISE AISC, 106 2011, pp. 359–364.
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[19] L. Zhang, S. Gao, W. Chen, Performance comparison of different solar desalination processes by bubbling humidification, Adv. Mater. Res. 282–283 (2011) 584–588. [20] W.T. Emily, H.L.V. John, Experiments and modeling of bubble column dehumidifier performance, Int. J. Therm. Sci. 80 (2014) 65–75. [21] M.T. Ghazal, U. Atikol, F. Egelioglu, An experimental study of a solar humidifier for HDD systems, Energy Convers. Manag. 82 (2014) 250–258. [22] M.H. Sharqawy, H. Liu, The effect of pressure on the performance of bubble column dehumidifier, Int. J. Heat Mass Transf. 87 (2015) 212–221. [23] N.C. Barford, Experimental measurements: precision error and truth, John Wiley & Sons, New York, 1990.
[24] H.T. El-Dessouky, H.M. Ettouney, Fundamentals of Salt Water Desalination, Elsevier Science BV, 2002. [25] R.K. McGovern, G.P. Thiel, G.P. Narayan, S.M. Zubair, J.H. Lienhard, Performance limits of zero and single extraction humidification–dehumidification desalination systems, Appl. Energy 102 (2013) 1081–1090. [26] T. Loimer, G. Machu, U. Schaflinger, Inviscid bubble formation on porous plates and sieve plates, Chem. Eng. Sci. 59 (2004) 809–818.
Contents lists available at ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
Solar water desalination using an air bubble column humidifier A. Khalil, S.A. El-Agouz ⁎, Y.A.F. El-Samadony, Ahmed Abdo Mech. Power Eng. Department, Faculty of Eng., Tanta University, Egypt
H I G H L I G H T S • • • • •
Desalination using an air bubble column humidifier is investigated. Effect of water temperature and height, air flow rate and hole diameter is studied. At inlet water is 62 °C, productivity, efficiency and GOR are 21 kg, 63%, and 0.53. Air bubble column achieves higher performance than that conventional humidifier. Temperature difference along air column is less than 2.5 °C for all measurements.
a r t i c l e
i n f o
Article history: Received 18 April 2015 Received in revised form 6 June 2015 Accepted 13 June 2015 Available online xxxx Keywords: Humidification–dehumidification Air bubbles Solar desalination, sieve plate
a b s t r a c t An experimental study of a solar water desalination using an air bubble column humidifier is investigated. The characteristics of the generated bubbles are modified by using a different sieve plate with different hole size. The effect of water temperature, air flow rate, water height, and sieve's hole diameter on desalination performance is studied. The results showed that the daily productivity, efficiency and gain output ratio are 21 kg, 63%, and 0.53 respectively; at inlet water temperature is 62 °C. The change in the temperature difference along the column is less than 2.5 °C for all measurements. The best performance is observed from sieve with 1 mm hole diameter at which the outlet air from the bubble columns is always saturated. The air bubble column achieves higher performance than that for the conventional humidifier. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Desalination can be achieved by many methods. Thermal method is considered to be the simplest one. In this method, saline water is heated in an evaporator and generates water vapor free of salts. The generated vapor is condensed in a condenser then fresh water is collected. Water, air, or both can be heated by conventional energy source or by renewable energy source such as solar energy. The latter desalination process can be called solar air humidification– dehumidification desalination process. The humidification– dehumidification (HD) desalination process is one of the secondary desalination processes. This system is very useful for places which have low freshwater demand. The main advantages of this system are; consume a small amount of energy and simplicity in both plant layout and management. Bourouni et al. [1] presented the technique of air humidification– dehumidification (HD) process. The principle, technique and state of the art of the HD process were presented. Gahin et al. [2] presented a ⁎ Corresponding author. E-mail addresses: [email protected] (A. Khalil), [email protected] (S.A. El-Agouz), [email protected] (Y.A.F. El-Samadony), [email protected] (A. Abdo).
http://dx.doi.org/10.1016/j.desal.2015.06.010 0011-9164/© 2015 Elsevier B.V. All rights reserved.
preliminary design study of a solar collector humidification–dehumidification desalination unit. They studied different parameters affecting the global performance of the unit. Also, they studied the performance of the two most important components of the loop which are humidifying and dehumidifying columns or stacks. Farid and Al-Hajaj [3] designed and studied experimentally the performance of multi-effect solar air humidification desalination unit. The unit had two different loops; air closed loop and water open loop. The results showed that the multi-effect humidification–dehumidification process, with forced air circulation, was found suitable for water solar desalination. The unit achieved a daily productivity of 12 l/m2/d, which was over three times that for single-basin conventional solar still. Al-Hallaj et al. [4] studied experimentally the indoor and outdoor performance of two desalination units based on air humidification desalination. The results showed that water productivity was increased as the feeding water flow rate was increased to an optimum value. Moreover, they concluded that forced air circulation was effective in the unit performance at low operating water temperatures. Ben-Bachaa et al. [5] studied experimentally a solar multiple condensation evaporation cycle (SMCEC) desalination technique. The results showed that the pilot units produced as much as 60% of daily water needed for irrigation. Dai et al. [6] conducted experimentally a solar humidification and dehumidification desalination unit. The performance of the unit was strongly dependent on the
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temperature and mass flow rate of the inlet salt water to the humidifier and the process air mass flow rate, which was forced by a fan. The unit thermal efficiency was above 80%. Garg et al. [7] studied experimentally and theoretically multi-effect humidification/dehumidification solar desalination system when air was circulated by natural convection. The differences between the values of the experimental and theoretical results were increased as the water temperature was increased due to the energy losses in the humidifier. Nafey et al. [8,9] investigated theoretically and experimentally a solar humidification dehumidification desalination unit under different environmental and operating conditions. The productivity of the unit was strongly affected by the cooling water flow rate, airflow rate, and total solar energy incident through the daytime. Amer et al. [10] investigated experimentally and theoretically the effect of operating parameters on the characteristics of a humidification–dehumidification desalination system. The system was based on a closed cycle for the airstream and an open cycle for water. The results showed that the maximum daily productivity was 5.8 l/h at 2.8 kg/min water flow rate and 85 °C inlet water temperature. Yanniotis and Xerodemas [11] studied experimentally the performance of tubular spray humidifier and a pad humidifier in seawater desalination plants. The evaporation rate of the spray humidifier was approximately the same as 100 mm thickness pad humidifier. Also, the pad humidifier with 300 mm thickness gave the highest evaporation rate at high air to water flow rate ratios. Zhani [12] studied theoretically and experimentally solar humidification–dehumidification desalination unit. The results showed that the maximum GOR was obtained at 0.4 kg/s inlet hot water flow rate. Vlachogiannis et al. [13] studied experimentally and theoretically a novel desalination concept, combining between the principles of air humidification–dehumidification and mechanical vapor compression. Air was injected into the evaporation chamber through a porous bottom wall and dispersed as small diameter bubbles. Inaba et al. [14] studied experimentally heat and mass transfer of air bubbles in a hot water layer. The results showed that the mean diameter of generating air bubbles increased with an increase in the superficial air flow velocity.
EL-Agouz and Abugderah [15] studied experimentally the humidification process by air passing through seawater. The air was fed to the evaporator chamber from 32 holes of 10 mm diameter located on the surface area of a PVC pipe, which was submersed in the water of the evaporator chamber. They found that the maximum vapor content difference of the air was about 222 gmw/kga at 75 °C and exit relative humidity of air was reached to 95%. Kabeel [16] studied experimentally the performance of the liquid desiccant system during a dehumidification–humidification process using an injected air through the liquid desiccant solution (calcium chloride). The air was injected through a series of homogeneous distribution. Holes in a vertical pipe El-Agouz et al. [17] studied experimentally a single air bubbling humidification desalination unit. They studied the influence of the electrically heated water temperature, water level, and airflow rate on the desalination performance. The productivity was slightly affected by water level and the maximum productivity of the system reached to 8.22 kgw/h at 86 °C for water temperature and 14 kg/h for air flow rate. Zhang et al. [18] studied experimentally operating factors that affects bubbling humidification by using a single sieve plate. The result showed that air relative humidity reached to 100%. Moreover, humidification capacity was increased by about 80% when water temperature was increased by 10 °C. Zhang et al. [19] studied experimentally the influence of working design parameters of solar bubbling humidification desalination unit on its gain output ratio, electric power consumption, and fresh water production cost. They found that gain output ratio was increased as the humidification temperature was increased, while electrical power consumption decreased. Emily and John [20] experimentally studied the heat flux and effectiveness of a bubble column dehumidifier. It was found that the effectiveness was decreased while the heat flux was increased with decreasing coil area and increasing the temperature and air flow rate. Ghazal et al. [21] examined air bubble regeneration on the performance of a solar still using humidification–dehumidification desalination (HDD) process. The results showed that the exit air from the humidification process was fully saturated. The effectiveness of the solar
HUM
Humidifier
C.C
Cooling coils
PCV
Pressure control valve
SWC
Solar water collector
SP
Sieve plate
P
Pump
CV
Control valve
DEH
Dehumidifier
PG
Pressure gauge
FM
Flow meter
TST
Thermal storage tank
GL
Graduate level
RH
Relative humidity sensor
TC
Thermocouple
FWT
Fresh water tank
Fig. 1. Experimental set-up schematic diagram.
A. Khalil et al. / Desalination 372 (2015) 7–16
9
(a) Photograph and dimensional of the humidifier
(b) Photograph of the experimental set-up Fig. 2. Photographs of the humidifier and the experimental set-up.
humidifier and the mass and heat transfer were upgraded by air bubbles regeneration. Sharqawy and Liu [22] studied experimentally the effect of the air pressure in bubble column dehumidifier dehumidification process, for absolute pressures in the range of 1–2 bar, superficial velocity range of 2–18 cm/s, and column height range of 3–7 cm. It was found that the total heat transfer rate was increased with the superficial velocity and pressure while effectiveness was increased with the superficial velocity and was decreased with pressure. The column height was slightly affected on the heat transfer and the effectiveness. Air bubble columns are suitable for humidification and dehumidification. Its great interface area, allows a great value of heat and mass transfer rate consequently improves productivity and efficiency. Therefore, the present work presents an experimental investigation of the use air bubble column humidifier for the solar water desalination system. The saline water, which is working in a closed loop, is heated by an
evacuated tube solar collector as an energy source in order to save electrical energy. Air working in an open loop is injected into the evaporation chamber from the bottom through sieve plates with 1200 holes and 1, 3, and 5 mm diameter. The characteristics of generating bubbles are changed by using the different hole diameter of sieve plates. The study includes the effect of water temperature and level, air flow rate, and orifice diameter of the sieve plate on the unit performance. 2. Experimental setup and instrumentation 2.1. Experimental setup Fig. 1 presented a schematic diagram of the experiment setup. The unit main components are HUM (humidifier), DEH (dehumidifier), and SWS (solar water collector). Fig. 2 shows photographs of the humidifier and the experimental set-up. The system consists of two
Table 1 Accuracies, ranges, and errors of measuring instruments. Property
Used instrument
Accuracy
Range
Error
Temperature Relative humidity Pressure Solar intensity Air velocity Fresh water volume Air flow rate
Digital thermometer, K-Type
±0.1 °C ±0.1% (RH) ±0.02 bar ±1 W/m2 ±0.1 m/s ±10 ml ±1 SCFM
0–100 °C 0–100% 0–2 bar 0–5000 W/m2 0–25 m/s 0–1000 ml 4–25 SCFM
0.25% 3% 2% 0.2% 2% 2% 2%
Pressure gauge (PG) MS-802 Pyranometer Meter TM-401 Graduate lab vessel Flow meter
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Turn on air compressor and then Open air control valve at (7) and water control valve at (2 and 3)
Turn on the pumps (2 and 4) and then Fill the evaporator to the design level by adjusting the control valves at (1 and 2)
Adjust the pressure and flow rate of air
Record 1. Water temperatures, air temperatures, and humidity of air every five minutes. 2. Record solar radiation every one hour
Fig. 3. A flow diagram of system operation.
loops, a closed water loop and open-air loop. In the closed water loop, the hot water at (1) is pumped to the HUM inlet (2) through CV (control value) and FM (flow meter) that heats in the evacuated tube solar
80
collector. At HUM exit (3) the hot water is pumped through CV and FM to TST (thermal storage tank) at (5). The hot salt water is circulated in a closed loop between HUM and TST. High salt concentration saline
1000
80
1000 Solar radiation
Twi Two
50
600
Tai Tao
40
400 17/6/2013 ma = 11 kg/hr H = 15 cm
30
200
20
0 8
10
12 14 16 Local Time, hr
18
800
60 50
Twi Two
40
Tai Tao
20 8
10
(a) 1000
600
Twi Two
400
Tai Tao
13/8/2013 ma = 15.4 kg/hr H = 15 cm
200
20
0 10
12 14 16 Local Time, hr
(c)
20
1000
18
20
70 Temperature, oC
60
Solar radiation, W/m2
Temperature, oC
800
8
0 18
Solar radiation
70
30
12 14 16 Local Time, hr
80
Solar radiation
40
200
(b)
80
50
400 21/7/2013 ma = 13.2 kg/hr H = 15 cm
30
20
600
800
60 50
Twi Two
40
Tai Tao
600 400 200
6/9/2013 ma = 17.6 kg/hr H = 15 cm
30 20 8
10
12 14 16 Local Time, hr
0 18
20
(d)
Fig. 4. Solar radiation, inlet, outlet water and air temperatures at humidifier at air flow rate: (a) 11 kg/h (b) 13.2 kg/h (c) 15.4 kg/h (d) 17.7 kg/h.
Solar radiation, W/m2
60
70 Temperature, oC
800
Solar radiation, W/m2
Temperature, oC
70
Solar radiation, W/m2
Solar radiation
A. Khalil et al. / Desalination 372 (2015) 7–16
11
Fig. 5. Inlet air pressure and exit air relative humidity of the humidifier.
water is rejected from CV at (6). HUM is made of Acrylic plastic sheet of 10 mm thickness with 580 mm × 580 mm cross section, and 900 mm height as shown in Fig. 2a. In the open air loop, air at (7) is supplied from the reservoir of a compressor containing 220 l at a pressure of 10 bar. The desired pressure and flow rate of air at (8) could be adjusted by a PCV (pressure control valve) and CV (control valve) then is passed through the FM and PG (pressure gauge) that indicate the pressure before the HUM. The air at (9) enters to the HUM through the sieve plate which is located at the bottom of the HUM. Three different sieves are used, 1200 holes each, with 1, 3, and 5 mm hole diameter. The air flow is humidified by passing through the water level in HUM and carries vapor water to DEH. Then, the produce water through a tube is desalinated to FWT (fresh water tank). Photographs of air flow through the water and the HUM dimensions are shown in Fig. 2a. DEH is a shell and tube heat exchanger. The shell is made of 0.7 mm thick steel with dimensions of 400 × 400 mm, and 900 mm height. Inside the shell, the cooling water tube is fixed. The type of SWC is an evacuated tube solar collector as shown in Fig. 2b. It is consisted of 25 evacuated glass tubes with 58 mm diameter and 1.8 m length. The capacity of TST is 250 l. The tilt angle of solar collector is 30° for all day.
to measure temperature and relative humidity of air at (9, 10). A pressure gauge (PG) is used to measure air pressure measurement at (9). A MS-802 Pyranometer sensor is used to measure solar radiation and wind speed is measured by air velocity meter TM-401. The air flow rate is measured at (9) by a flow meter, where the water flow rate is measured at (1 and 3) by a flow meter. All sensors are calibrated before using to determine the probes sensibility. The accuracy, range and errors that occurred in measuring instruments are shown in Table 1. The errors are calculated for thermocouples, solar meter, anemometer and collection tank. The minimum error that occurred in any instrument is equal to be the ratio between its last count and minimum value of the output measured. To estimate the uncertainties in the results presented in this work, the approach described by Barford [23] is applied. The uncertainty in the measurements is defined as the root sum square of the fixed error of the instrumentation and the random error observed during different measurements. Accordingly, the resulting errors of the calculated amount of extracting water, daily productivity, daily efficiency, and Gained Output Ratio of the desalination system are ± 0.24%, ± 0.65%, ± 1.52%, and ± 1.28%, respectively. 2.3. System operation
2.2. Instrumentation
The following steps describe the testing procedure in details and Fig. 3 presents a flow diagram of system operation.
As shown in Fig. 1, a K-Type thermocouple is used to sense water temperature at the point (2, 3). Digital sensor with data logger is used
1. Prior to testing, the system is checked. 2. Turn on air compressor to fill its tank with a compressed air.
Fig. 6. Water and air temperature difference of humidifier.
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Fig. 7. Effect of inlet water temperature on the amount of extracting water for different water height at air flow rate: (a) 11 kg/h (b) 13.2 kg/h (c) 15.4 kg/h (d) 17.6 kg/h.
3. Open air control valve at (7) to eject air through HUM. 4. Open water control valve at (2, 3), turn on the pumps (2, 4) and let the water flow to the evaporator to fill it, then adjust the flow rate and the level of water in evaporator by adjusting the control valves before and after the evaporator at (1, 2) manually. 5. After the water level became stable adjust the pressure and flow rate of air by using the pressure control valve to the desired pressure and adjusting flow control valve to the wanted volume flow rate. 6. Record water temperatures, air temperatures, and humidity of air, every five minutes. 7. Record solar radiation every one hour.
2.4. Efficiency and Gained Output Ratio of system It is important to calculate the hourly and daily efficiency of the desalination unit to know the performance achieved in real systems is calculated as:
ηh ¼ 100
ðMw LH wav Þ=3600 ðASWS IS þ PP þ Wc Þ
Fig. 8. Variations of air temperature and humidity ratio at dehumidifier inlet and exit at different air flow rate.
ð1Þ
A. Khalil et al. / Desalination 372 (2015) 7–16
where LHw is the latent heat of vaporization at the ambient condition, and [(m × h)w,i − (m × h)w,o] is the total enthalpy difference between entering and leaving the humidifier for hot water. The hot water flow rate, leaving the system is calculated as:
Table 2 The inlet and exit of average cooling water temperature in dehumidifier. ma, kg/h
Tdeh,wi, °C
Tdeh,wo, °C
11 13.2 15.4 17.6
22.3 20.3 19.6 20.0
26.1 23.0 23.4 27.0
13
mw;o ¼ mw;i −Mw :
ð5Þ
3. Results and discussion n 1X η n 1 h
ð2Þ
LHw:av ¼ 103 2501:9−2:40706 Tw þ 1:192217 10−3 T 2w −1:5863 10−5 T 3w
ð3Þ where ηh and ηd are the hourly and daily efficiency, Mw is the hourly productivity, LHw.av is the average of the latent heat of vaporization of water, El-Dessouky and Ettouney [24], ASWS is the solar water collector area, IS is the incident solar radiation on the collector surface, Pp = I × V is the power of pumps that are measured with different flow rate of cold and hot water, Wc is a power of compressor and n is the duration time of solar radiation. The Gained Output Ratio (GOR) is calculated as McGovern et al. [25]: M w LHw ðm hÞw;i −ðm hÞw;o
i
ð4Þ
3.1. Effect of 1 mm hole diameter Depending upon weather conditions which are different during the daytime, the wind speed, atmospheric temperature, and solar intensity. Fig. 4 shows the general trend of solar radiation, inlet and outlet water temperature and air temperature at humidifier inlet and out for 11, 13.2, 15.4, and 17.7 kg/h air mass flow rate. It can be seen that the solar radiation and air temperature at humidifier inlet increases in the morning hours, reaching its maximum values around midday, and then decreases in the afternoon. In case of water, water temperature started from high
25
80
20
60
15 Air flow rater, ma 11 kg/hr 13.2 kg/hr 15.4 kg/hr 17.6 kg/hr
10
5
40 Air flow rate, ma 11 kg/hr 13 kg/hr 15.4 kg/hr 17.6 kg/hr
20
0 56
58 60 62 Average hot water temperature, oC
64
56
58 60 62 o Average hott water temperature, C
(a)
(b)
0.55 0.50 0.45 GOR
Daily productivity, kgw/day
GOR ¼ h
Test rig external ambient conditions are very important. Average relative humidity of the ambient air is 15–40%. Wind speed during the experimental is 1.5–3 m/s in that period of time. The inlet water flow rate to HUM is 10 l/min while, the exit water flow rate is measured by FM and checked by using Eq. (5). Four different values of air flow rates are used 11, 13.2, 15.4, and 17.7 kg/h each value has been used during a whole working day. In addition, four different values of water height in humidifier are used 5, 10, 15, and 20 cm.
Daily effeciency, %
ηd ¼
Water Flow rate, ma 11 kg/hr 13.2 kg/hr 15.4 kg/hr 17.6 kg/hr
0.40 0.35 0.30 0.25 56
58 60 62 Average hot water temperature, 0C
64
(c) Fig 9. Effect of average hot water temperature on: (a) Daily productivity (b) Daily efficiency (c) GOR.
64
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A. Khalil et al. / Desalination 372 (2015) 7–16
Fig. 10. Effect of diameter holes of sieve plate on productivity, efficiency, and GOR.
temperature and then decreases in the morning hours, nearly remains constant around midday, and then decreases in the afternoon. The high temperature of the water at the beginning is because the water in the solar collector tank heating from sunrise to the desalination unit starts at 9.5 AM. In the morning hours, the water temperature decreases because the energy lost from water in the humidifier is larger than that the energy gain by solar radiation to the solar collector. In the midday, the water temperature nearly constant. This is because the energy lost from water in the humidifier is nearly equal the energy gain by solar radiation to the solar collector in this period. Fig. 5 shows inlet air pressure and exit air relative humidity of the humidifier. The inlet air pressure gauge at the humidifier change from 9 to 23 kPa, while the outlet air pressure is atmospheric pressure. The pressure drop (Δp) of the air bubble can be modeled as an exponential function of the air flow rate (ma), Δp ≈ (ma)2 [26]. Therefore, the increase of air flow rate increases the pressure drop. Also, the increase of air flow rate increases the bubble rise velocity and bubble size. In addition, the increase of the number of air bubbles increases the rate of interface area, which increase the productivity. Relative humidity reached to 100%, almost the time of the process (therefore, there is any reasons to use hole diameter less than 1 mm and pay more air side pressure drop), similar to Ghazal et al. [21] and higher than that El-Agouz and Abugderah [15] due to the decrease difference between the air drybulb/wet-bulb temperatures. Fig. 6 presents temperature difference of water and air at different water heights and air flow rates. As can be seen in Fig. 6, the water outlet temperature is slightly affected by both air flow rate and water height. It can be seen also that a temperature difference of 2.5 °C on water inlet temperature influences also the outlet air temperatures of 22.5 °C. Because this water has a high specific heat compared with low air specific heat (4.18 against 1 kJ/kg K), which causes a large amount of air to enhance water temperature. The air flow rate has no significant influence on the parameter and the difference value is constant for all the conditions considered, for a given inlet temperature. This could mean that with all air flow rate, the system is able to exchange all the available energy. It could be mean that the efficiency of the heat transfer is very high, because the air exit temperature is very close to the water inlet one as shown in Fig. 4. Fig. 7 shows the relation between inlet water temperature and amount of extracting water at different water height and air mass flow rate. It can be seen that as the inlet water temperature increases the amount of extracting water highly increases. Therefore, it has a great effect on the amount of extracting water. The amount of extracting water increases by 51.4, 76, 73, and 60.7% as inlet hot water temperature increases from 60 to 70 °C, at 11, 13.2, 15.4, and 17.7 kg/h, respectively, for air mass flow rate and the H = 20 cm. The amount of extracting water slightly increases with the increase of the air mass
flow rate due to increase of the air bubble number that increases the rate of interface area. Also, the amount of extracting water slightly increases as the water height increases due to the increase time contact of air bubbles with water and consequently heat and mass transfer increases. Fig. 8 presents air temperature and humidity ratio of dehumidifier inlet and exit at different air flow rate. The air temperature and humidity ratio of dehumidifier exit are slightly affected by both air flow rate and hot water temperature of humidifier. It can be seen also that the air humidity ratio of dehumidifier exit is about 25 gmw/kga. Also, Table 2 shows the inlet and exit of average cooling water temperature in dehumidifier. It can be seen also that a temperature difference from 3 to 7 °C on inlet cooling water temperature influences. Fig. 9 shows the effect of average hot water temperature on daily productivity, daily efficiency and GOR at different inlet hot water flow rate. It indicates that both of average daily water temperature and water flow rate promote the daily productivity, daily efficiency and GOR increases. The results indicate that the water flow rate and temperature play an important role in the system performance.
Fig. 11. The enhancement of humidity ratio difference at the humidifier for the present and previous results.
A. Khalil et al. / Desalination 372 (2015) 7–16
3.2. Effect of 1, 3, and 5 holes diameter
Pp T
Fig. 10 illustrates the effect of sieve's hole diameter on the amount of water extracted, daily efficiency and GOR. The amount of extracting water decreases as sieve holes diameter increases and 5 mm orifice diameter gave the lowest productivity. It is observed that at 5 mm sieve's hole diameter the flow started to transform to be jet flow. The extracting water for 1 mm hole diameter is higher than that 3 and 5 mm hole diameter by 9.1%, and 14.6%, respectively. The best performance is observed from sieve with 1 mm hole diameter. Because the diameter of the air bubbles decreases and the number of air bubbles increases as the sieve holes diameter decreases. Consequently, surface contact area between bubbles and hot water increases hence the heat and mass transfer. 3.3. Comparison with literatures Fig. 11 compares between enhancement of humidity ratio difference at the humidifier for the present and previous results. The percentage improvement in either system is given by: Enhancement ¼ 100
ΔωPresent −ΔωPrevious : ΔωPrevious
ð6Þ
It is inferred from Fig. 11, when the inlet water temperature has increased from 45 to 70 °C, the enhancement of presenting results is approximately 4–58% of Amer et al. [10], 22–206% from Zhani [12], and 30–270% of Garg et al. [7]. It can be noted that the present work achieved higher productivity than that listed for previous systems.
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pump power, W temperature, °C
Greek letters η Efficiency Subscripts a air av average c compressor d daily h hourly i inlet o outlet s solar w water Abbreviations CC cooling coil CV control value DEH dehumidifier FM flow meter FWT fresh water tank GOR gain output ratio HUM humidifier PCV pressure control valve SWC solar water collector TC thermocouples TES Thermal Energy Storage
4. Conclusion References The experimentally a small-scale solar air bubbling humidification– dehumidification unit is studied. The effects of the water temperature, water height, airflow rate and holes diameter on the performances of the system are observed during the present experiments and the following points are concluded. • Air bubbling humidification is an efficient technique which can be used in HD systems and the outlet air from the bubble columns is always saturated. • The productivity, efficiency and GOR are affected strongly by inlet water temperature and air flow rate to the humidifier. • The value of the amount of the extracting water is increased slightly with the increasing water height. • The temperature difference of water is constant for all the water height and air flow rate conditions. The change in the temperature difference along the column is less than 2.5 °C for all measurements. • The present system achieved daily productivity, daily efficiency and GOR 21 kg, 63%, and 0.53, respectively, at inlet water temperature is 62 °C. • The best performance is observed from sieve with 1 mm hole diameter. • The air bubble column achieves higher performance than that for the conventional humidifier.
Nomenclature A area, m2 h enthalpy, kJ/kg H water height, cm LH latent heat of water, J/kg incident solar radiation, W/m2 IS m flow rate, kg/h M productivity, kg/h n duration time of solar radiation, h
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