ISSN 0003-701X, Applied Solar Energy, 2018, Vol. 54, No. 2, pp. 119–125. © Allerton Press, Inc., 2018. Original Russian Text © J.S. Akhatov, K.A. Samiev, M.S. Mirzaev, A.E. Ibraimov, 2018, published in Geliotekhnika, 2018, No. 2, pp. 49–58.
SOLAR POWER PLANTS AND THEIR APPLICATION
Study of the Thermal Technical Characteristics of a Combined Solar Desalination and Drying Plant J. S. Akhatova, *, K. A. Samievb, M. S. Mirzaevb, and A. E. Ibraimovc aPhysical–Technical
Institute, Research and Production Association Physics–Sun, Academy of Sciences of the Republic of Uzbekistan, Tashkent, Uzbekistan b Bukhara State University, Bukhara, Uzbekistan cTashkent State Technical University, Tashkent, Uzbekistan *e-mail:
[email protected] Received February 2, 2018
Abstract—The paper presents the results of calculated research on determining the thermal technical indicators of a combined solar desalinization and drying plant. The structure of the plant is developed and proposed. A mathematical model is developed that describes the thermal processes occurring in the plant based on heat-balance equations solved using the Laplace method. Keywords: solar energy, heat transfer, drying, desalinization unit DOI: 10.3103/S0003701X18020032
Methods for drying plant cultivation products can be divided into natural, solar, and artificial. The natural method is the most common in Uzbekistan. The high potential of a dryer (high temperature, low relative humidity of atmospheric air), and coincidence of the ripening period of fruits and vegetables with the period of most intense solar radiation create favorable conditions for natural drying. It is considered that the use of solar energy in the natural method is quite effective and contributes to products suitable for consumption. However, the analysis carried out by the authors revealed some significant drawbacks of natural drying. This method envisages the use of various tools and the simplest types of structures, which complicates the drying process, makes it more expensive, requires significant costs for manual labor, larger land areas, dust pollution of the products, contamination from the insects and gnawing animals, and the impact of dew and precipitation that worsen the quality of the dried products. In addition, the duration of the drying process prevents the structures from being used a second time [1–5]. Researchers [6–14] have used solar drying plants to obviate the main drawbacks related to the quality of dried products. The products dried in solar drying plants (SDPs) are not exposed to dust pollution, contamination from insects, the impact from the dew and precipitations, and the substances determining their nutritional and biological value (sugar, vitamins, etc.) are better retained in them. As well, drying of products in solar drying plants has certain economic advantages due to the use of a cheap energy source.
At present, scientific research works on the development of the various types of solar drying plants are being carried out in Uzbekistan and abroad. All solar drying plants can be divided into chamber (convective), solar radiation, and combined in terms of the structural characteristics and operational principles. Chamber solar drying plants [15–19] consist of a solar thermal converter and drying chamber, where the heat transfer medium is either removed outside during drying or it circulates along a closed circuit. In the second variant, discharge of the drying agent is possible when a certain concentration of water vapor is achieved in the chamber. Combined solar drying plants [20, 21] consist of a radiation part and drying chamber. During drying, the product is moved to the radiation part and drying chamber of the plant. The atmospheric area is heated in the radiation part of the drying plant and is supplied to the drying chamber to accelerate the process. Solar drying plants for drying various fruits and vegetables are also created in the United States, Austria, Greece, Turkey, Bulgaria, India and other countries [22–25]. It follows from the analysis of the results in the above-mentioned works that solar drying plants have some advantages over natural drying. However, due to the dependence on weather factors, they cannot handle large-scale processing of vegetable products, which requires transfer of the drying process to a commercial basis using an artificial method. The results of studies on optimizing the characteristics of a solar tunnel dryer are presented in [26]. The operation mode of the dryer is designed in VisualBa-
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Solar drying plants
Active (forced circulation)
Passive (free circulation)
Direct
Combined
Directly
Black-box-type dryers
Building-type dryers
Tunnel dryers
Fig. 1. Classification of solar drying plants.
sicScript based on the simulation mode used to optimize the dryer characteristics. The optimized dryer is tested to compare its characteristics with the existing tunnel dryer. The results of the analysis showed an increase in the temperature of the water drain duct by 18–113%. In addition, bilateral dispersion analysis showed the presence of a quite sufficient difference between the pressure chamber temperatures for both dryers. Regression analysis and the t-test also show that the productivity of an optimized dryer is greater than in the existing dryer. In the solar tunnel dryer optimized using the simulation model, a decrease in depth of air heating of the chamber with a coefficient of 4.91 is achieved. Based on analysis of performed scientific research [27], the various types of the solar drying plants shown in Fig. 1 are classified.
MATHEMATICAL MODEL Figures 2 and 3 show, respectively, a circuit diagram of the proposed structure of a combined solar desalinization and drying plant and a circuit of the heat flows in the units of this plant. The heat-balance equations are based on the circuit of heat flows in the units of the considered plant for each of its elements [28–34]. The heat-balance equation for the relevant components of the units of the desalination part of the plant is for a translucent coating
aFtqfall + (hev + hcon + hrad )Ft (Т w − Т t ) = (hrt + hct )Ft (Т t − Т e );
(1)
for water in a desalinization unit D1
cw mw L3
L3
H2
dTw = ( τα )eff1 Fw qfall + hbw Fw (Tb − Tw ) dt + ( hrtw + hcon + hrw ) Fw (Tt − Tw ) ;
(2)
for the bottom part of the desalinization unit
(τα)eff 2 Fbqfall = hbw Fb (Т b − Т w ) + hwi Fb(Т b − Т wa ),
H3
L1
d h1 H1
h2
D2 L2
Fig. 2. Circuit diagram of combined solar desalination and drying plant.
(3)
where Tw is the water temperature in the desalinization unit; Tt is the temperature of the translucent coating; Tb is the temperature of the bottom part of the desalinization unit; Te is the ambient temperature; hev is the coefficient of heat transfer by evaporation; hcon is the coefficient of heat transfer by convection; hrad is the coefficient of heat transfer by radiation; Ft is the area of the translucent coating of the desalinization unit; Fb is the area of the desalinization base; Fw is water evaporation surface area. APPLIED SOLAR ENERGY
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The heat-balance equation for the drying chamber for moist air inside the drying chamber
dTwa = ( hev + hc2p )(Т p − Т wa ) Fp dt hww Fi (Т wa − Т wi ) − Сd Fout 2 g ΔH ΔP
сwa mwa +
∑
(4)
where
ΔP = [P (Twa ) − g wa P (Te )]; ΔH = ΔP , ρa g
(5), (6)
dTp = hc1pFp (Т e − Т p ) dt + ( hev + hc2p ) Fp (Т wa − Т p )
cp mp
(7)
is for the product;
α w Fwiqfalli = hwn Fwi (Т wi − Т e ) + hww Fwi (Т wi − Т wa )
(8)
is for the dryer wall,
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where Тp is the temperature of the product in the drying chamber; Тwi is the temperature of the drying chamber wall; Тwa is the temperature of moist air inside the drying chamber; hev is the coefficient of heat transfer by evaporation from the surface of the product; hc1p is the coefficient of convection heat transfer between the product and incoming air; hc2p is the coefficient of convection heat transfer between the product and moist air; hww is the coefficient of convection heat transfer between the moist air and drying chamber walls; Fp is the product surface area; Fwi is the drying chamber wall surface. Equation (8) is also described for each of the side walls and top and bottom surfaces of the drying chamber.
SELECTION OF THE HEAT TRANSFER COEFFICIENTS The coefficient of convective heat transfer between the water in the desalinization unit and the translucent enclosure is
⎡ Р (Tw ) − Р (Tt ) ⎤ = 0.884 ⎢(Tw − Tt ) + (Tw + 273) ⎥ , 3 268.9 × 10 − Р (Tw )⎦ ⎣ 13
hcon where
(
)
(10) Р (Т ) = ехр 25.317 − 5144 . Т + 273 The coefficient of heat transfer by radiation between the water in the desalinization unit and the translucent enclosure is hrad = εeff σ[(Т w + 273)
2
+ (Т t + 273) ] (Т w + Т t + 546 ) , 2
(11)
hrt =
(9)
(
ε tσ Tt4 − Tn4 Tt − Tn
),
(15)
where Тn = Тe[0.711+0.0056Tdp + 0.000073Tdp2 + 0.013cos(15t)]1/4. The coefficient of convection heat transfer between the translucent enclosure and the environment is determined as hct = 2.8 + 3.0V. (16)
where qfall
−1
⎛ ⎞ (12) εeff = ⎜ 1 + 1 − 1⎟ . ⎝ εt εw ⎠ The coefficient of heat transfer by evaporation between the water in the desalinization unit and the translucent enclosure is
hev = 16.273 × 10−3 hcon
Р (Т w ) − Р (Т t ) . Тw − Тt
qrtс
qcon
(13)
The hourly dynamics of condensate formation in the desalinization unit is determined as qev Т − Тt (14) × 3600 = hev w × 3600. L L The coefficient of heat transfer by radiation between the translucent enclosure and the environment is М ev =
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qct
qtw
qwe
Tw qte
qcp
qwt Tw1
qev Tp
qout Tw
Fig. 3. Diagram of heat flows in junctions of this plant.
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B C D E F
Solar radiation density, W/m2
1000 800 600 400
hev = 16.273 × 10−3 hc
200 0
To determine the coefficient of convection heat transfer between the moist air and drying chamber wall, the following expression is used for the Nusselt number: Nu = 0.036Re4/5Pr1/3. (19.2) The coefficient of heat losses from the product surface by evaporation is determined by the following expression:
6
8
10 12 14 Time of day, h
16
18
Fig. 4. Calculated values of solar radiation flux density incident on relevant surfaces of considered combined solar desalination and drying plant during daylight (B, C, D, E, F are respectively, surfaces of horizontal, east, north, and west walls, as well as frontal surface of translucent enclosure of desalinization unit).
The coefficient of convective heat transfer between the product and incoming air (hc1p), moist air (hc2p), as well as between the moist air and wall (hww) of the drying chamber
hww = hc1p = hc2p = hc = Nu Re =
U ol . ν
ko , D
(17) (18)
The air motion inside the drying chamber is turbulent, which is why the Nusselt number is calculated as follows: Nu = 0.023Re0.8Pr0.4.
(19.1)
Р (Т p ) − γ wa Р (Т wa ) , Т p − Т wa
(20)
where hrt is the coefficient of heat transfer by radiation between the translucent enclosure and the environment; hct is the coefficient of convection heat transfer between the translucent enclosure and the environment; V is the wind velocity; Uo is the velocity of the air flow inside the drying chamber; Re is the Reynolds number; Nu is the Nusselt number; Pr is the Prandtl number; γhi is the humidity inside the drying chamber; P(T) is the pressure at temperature Т; Тdp is the dew point; Мev is the mass of the formed condensate; L is the specific heat of evaporation; ko is the air leakage coefficient. Calculations were performed to determine the values of the solar radiation flux density falling on the relevant surfaces of the considered combined solar desalination and drying plant in daylight; the results are presented in Fig. 4. EXPERIMENTAL STUDIES FOR A TEST SAMPLE OF THE PLANT Based on the calculated studies, as well as the developed structure given in Fig. 2, a test sample of the plant was created to carry out experimental studies. Figure 5 shows a general view of the experimental
Fig. 5. General view of experimental sample of combined solar desalination and drying plant. APPLIED SOLAR ENERGY
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Table 1 Geometrical indicators of plant junction components (unit)
Value
Desalinization unit, L1, m
1.05
Desalinization unit height, h2, cm
6
Height of desalinization unit step, h1, cm
4.5
Distance between desalinization unit steps, d, cm
5.5
Thickness of the bottom part of desalinization unit, δ, cm
1.5
Width of desalinization unit, L4, m
0.5
Number of steps
16
Number of outlet tubes
17
Length of drying chamber base, L2, m
1.2
Width of holes for incoming air to drying chamber, D2, cm
2
Height of drying chamber, H1, m
0.85
Height of draft for outlet air, H2, m
1
Diameters of draft for outlet air, D1, cm
8
Length of top junction of dryer, 2L3 + D1, m
0.4
Distance between racks for products, H3, cm
15
Thickness of translucent coating (glass), δ1, mm
3
Thickness of drying chamber wall, δ2, mm
0.33
Thickness of outlet air draft, δ3, mm
0.33
Table 2 Optical and thermotechnical characteristics of the materials used when manufacturing the components of the plant (unit)
Value
Reflection coefficient of translucent coating (glass), ρ
0.04
Absorption coefficient of translucent coating (glass), α
0.08
Transmission coefficient of translucent coating (glass), τ
0.88
Reflection coefficient of drying chamber walls, ρ1 Absorption coefficient of drying chamber walls, α1
0.91
Heat conductivity coefficient of desalinization unit base, k, W/(m deg)
45.4
Heat conductivity coefficient of the desalinization unit walls, k1, W/(m deg)
45.4
Heat conductivity coefficient of the draft wall for the outlet air, k2, W/(m deg)
204
Heat conductivity coefficient of the translucent coating, k3, W/(m deg)
sample of the combined solar desalination and drying plant created at the Sun test area of Bukhara State University. Table 1 gives the geometric indicators of the main components of the test sample junctions of the considered plant. Table 2 gives the optical and thermotechnical characteristics of the materials used in manufacturing the components of the plant. APPLIED SOLAR ENERGY
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0.745
RESULTS Figure 6 shows the daily variation rate of the solar radiation flux density and ambient temperature on the day of the experiment. Figure 7 shows the temperatures of relevant junctions (3, 5, 6 is glass and water inside and in the outlet part) of the desalinization unit. As shown by the results of the measurements in Fig. 7 carried out during the experiment on June 21,
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600 500
32 400
1 2
300
30 8
200 9 10 11 12 13 14 15 16 17 18 Time of day, h
75 70 65 60 55 50 45 40 35 30 25 20
60 50 3 5 6
40 30 20 8
Fig. 6. Daily variation rate of solar radiation flux density and ambient temperature.
9
10
11
12 13 14 15 16 Time of day, h
17
18
Fig. 7. Temperature of relevant junctions (3, 5, 6 is glass and water inside and in outlet part) of desalinization unit.
55 50 Temperatura, °C
Temperatura, °C
70 Temperatura, °C
34
Total solar radiation on horizontal surface, W/m2
700 Temperatura, °C
80
800
36
7 8 9 10 11 12 8
9
10
11
12 13 14 Time of day, h
15
45 40 35 30
13 14 15
25 20 16
17
8
9
10
11
12 13 14 Time of day, h
15
16
17
Fig. 8. Numerals 7–12 are temperature of north wall of drying chamber, bottom part of desalinization unit, base, top, and east and west walls of drying chamber, respectively.
Fig. 9. Numerals 13–15 are temperature inside drying chamber, of air flowing out of it and of product, respectively.
2017, the maximum temperature values of the translucent coating of the desalinization unit, the water inside the desalinization unit, and outflowing water are, respectively 67, 75, and 57°С.
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Figure 9 shows the results of temperature measurements, respectively, inside the drying chamber, of the air flowing out of it, and of the product. From the analysis of the obtained calculated and experimental data, it can be concluded that such combined solar desalination and drying plants are efficient from the technical and economic viewpoints to improve the quality of domestic water supply to the populations of remote regions, particularly, Bukhara oblast, as well as to simultaneously improve the quality of drying and storage of agricultural products.
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Translated by Yu. Bezlepkina
