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Thermal performance of a dual-purpose collector-cum-storage type air-water heater
Applied Thermal Engineering 171 (2020) 115094
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Thermal performance of a dual-purpose collector-cum-storage type airwater heater
T
⁎
Aneesh Somwanshi , Niladri Sarkar Department of Mechanical Engineering, MATS University, Raipur, Chhattisgarh, India
H I GH L IG H T S
novel design of a dual-purpose collector cum storage type air–water heater has been developed. • The proposed collector can deliver hot water and air simultaneously. • The maximum thermal efficiency reached during day time is 82.4% • The covering the collector by insulating cover during night about 19.9% of heat energy can be saved. • By • The maximum water and air temperature reached is 89.6 °C and 81.1 °C during summer.
A R T I C LE I N FO
A B S T R A C T
Keywords: Dual heater Collector-cum-storage device Water heating Air heating
The aim of this study is to design and develop a dual-purpose integrated solar collector-storage device that can simultaneously provide hot water and hot air. The device can be used as a single purpose air or water heater, based on the requirement. A Simple mathematical model of the proposed dual-purpose heater is presented and validated for the climate of Raipur, Chhattisgarh, India (21.25oN, 81.62oE). The performance of the collector is numerically computed for the composite climate of Delhi, India (28.70oN, 77.10oE). The results reveal that about 19.9% of the heat energy can be conserved by covering the collector with an insulating cover at night. The dual collector performs well when a low mass flow rate of air flows through the upper compartment, in comparison to high mass flow rate. The maximum thermal efficiency during day time is about 82.8% at a mass flow rate of 0.020 kg/s. Thermal efficiency of the proposed dual-purpose heater is higher than that of conventional single purpose water heater. The maximum water and air temperatures in December (winter) where 56 °C and 50 °C, respectively, whereas those for May (summer) where 89.6 °C and 81.1 °C, respectively.
1. Introduction A flat plate solar collector is a commonly used technology for heating water or air. This collector is a heat exchanger that transforms solar energy into heat. The solar energy that is absorbed into the absorber plate is converted into heat and then is transferred to the stream of liquid or air that comes in contact. The utilization of solar energy, which is freely available, is one of the major advantages of these systems. Flat plate collectors are mostly used to heat water for domestic or industrial use. However, these collectors are sometimes utilized as air collectors for space heating and drying purposes. Although the flat plate collector is a well-established technology, the researchers all over the globe are constantly working on improving the overall performance and efficiency of these collectors. One of the recent trends is to combine two known technologies, for ⁎
developing a device that works in combination, known as a hybrid or dual-purpose device. Few studies have been conducted to design and develop a cost-effective dual air–water collector that can generate both hot air and hot water. Ma et al. [1] designed a dual- function solar collector that can simultaneously provide hot water and hot air. They found that the thermal efficiency of the proposed collector attened a value of 50% in water heating water and varied from 41% to 55% in heating air depending on the ambient conditions and mass flow rate. Assari et al. [2] developed and experimentally validated a mathematical model based on the effectiveness method. They considered three different channels rectangular, triangular and no fins. The Simulation results revealed that the channels with rectangular fins perform better. Nematollahi et al. [3] experimentally investigated a dual-purpose solar heating system. They studied the effect of parameters such as temperature variations in an absorber plate, average temperature of storage
Corresponding author. E-mail address: aneeshsomwanshi@rediffmail.com (A. Somwanshi).
https://doi.org/10.1016/j.applthermaleng.2020.115094 Received 24 June 2019; Received in revised form 13 February 2020; Accepted 16 February 2020 Available online 17 February 2020 1359-4311/ © 2020 Elsevier Ltd. All rights reserved.
Applied Thermal Engineering 171 (2020) 115094
A. Somwanshi and N. Sarkar
Nomenclature
Ap ac ap
ca cw dd de hag hap hca hrpg hramb
hw
hb Ka K L1 L2 ṁ a
Mw S t
Ta
Area of the absorber plate [m2 ] Area of cross section of upper compartment (air heating channel) [m2 ] Perimeter of the upper compartment (air heating channel) [m ] Specific heat of air [ Jkg −1K−1] Specific heat of water [ Jkg −1K−1] Diameter of duct [m ] Equivalent diameter [m ] Convective heat transfer coefficient between air and glass cover [Wm−2K−1] Convective heat transfer coefficient between air and plate [Wm−2K−1] Convective heat transfer coefficient between glass cover and ambient [Wm−2K−1] Radiative heat transfer coefficient between plate and glass cover [Wm−2K−1] Radiative heat transfer coefficient between glass cover and ambient [Wm−2K−1] Convective heat transfer coefficient between plate and water [Wm−2K−1] Convective heat transfer coefficient between bottom and air [Wm−2K−1] Thermal conductivity of air [Wm−1K−1] Thermal conductivity of insulation [Wm−1K−1] Length of dual collector [m ] Width of dual collector [m ] Mass flow rate of air flowing through upper compartment of dual collector [kgs−1] Mass of water in collector [kg ] Solar radiation intensity [Wm−2 ] Thickness of insulation [m ]
Tw Tae Tai Tg Tp Tamb Tb
Ub Ut U ′t
va ρa ατ εg εeff αc σ ηa ηw ηdu Xpre Xexp Nu Re
Temperature of air flowing through upper compartment [°C] Temperature of water in lower compartment [°C] Temperature of exit air flowing through upper compartment [°C] Temperature of inlet air flowing through upper compartment [°C] Temperature of glass cover [°C] Temperature of absorber plate [°C] Ambient temperature [°C] Temperature of bottom of dual collector (base temperature) [°C] Overall heat transfer coefficient between base to ambient [Wm−2K−1] Overall heat transfer coefficient between glass cover to ambient [Wm−2K−1] Overall heat transfer coefficient between insulated cover to ambient [Wm−2K−1] Velocity of ambient air [ms−1] Density of air [Kgm−3 ] Effective transmittance-absorptance product Glass cover emittance Effective emissivity of plate glazing system Absorptance of glass cover Stefan’s constant [W / m2K 4 ] Efficiency of air heating Efficiency of water heating Efficiency of proposed dual heater Theoretical value Experimental value Nusselt number Reynold’s number
proposed collector was compared with other types of solar collectors (type 2 and type 3). The simulation results reveal that the heat loss coefficient of the type 1 collector is approximately 54.4% lower compared with type 2 collectors and is approximately 65.8% lower compared with the type 3 collectors. Luo et al. [10] proposed a dual-function solar collector with tile shaped Methyl Methacrylate (PMMA) covers to match the roof appearance. The test result reveals that the daily thermal efficiency of the water heating system with tile shaped collector system varied from 54% to 61.8% whereas the efficiency of the water heating system without PMMA covers varied between 35.5% and 67.4%. In present study, we proposed a novel design dual heater that combines the collector-storage type of solar water heater with an air collector. This proposed heater in known as a dual-purpose collectioncum-storage type air–water heater (DCS-AWH). The collector-storage type of solar water heater has been a well-known technology previously to heat water during winters. The performance of this type of heater has been investigated by researchers [11–16]. The heater comprises a rectangular tank in a suitable container. All the sides and the bottom of the tank are insulated with glass fiber, and the top surface is blackened and glazed. Thus, the top surface acts as a solar radiation absorber, whereas the tank filled with water acts as a storage. The proposed heater can simultaneously provide hot air and hot water. Depending on the requirement, the heater can be used as a single-purpose collector for heating water or air. Unlike the previous designs of dual collectors, the proposed design is very simple and does not include any grid of tubes that are bonded to the absorber plate. This reduces the conduction losses and eliminates the leakage problems through joints. Because the proposed design incorporates storage tank
tank and air velocity on the system performance. They concluded that the average efficiency of a single- purpose system is 67.8%. However, the efficiency for a dual-purpose system is between 71.6% and 72.5%, which is approximately 3% to 5% higher than the efficiency of a single system. Jafari et al. [4] conducted energy and exergy analysis of a dual air–water solar collector by using the number of transfer unit (NTU) method. They concluded that the optimal efficiency can be achieved if the temperature of the inlet water is 60 °C and the triangular type of air channel is used. More and Pote [5] designed a model of a dual-purpose solar collector (DPSC) with horizontal water tubes. They developed mathematical model based on the effectiveness method and obtained maximum thermal efficiency up to 72.4%. Venkatesh and Christraj [6] experimentally investigated a multipurpose solar collector system by combining a solar water collector and solar air heater. They found the maximum stagnant temperature of the multipurpose solar air heater to be 88 °C for the overall heat loss coefficient of 12.83 W/m2 K at no load condition. Moreover, they confirmed that the efficiency of the multipurpose system is higher than that of the conventional system. Shemelin and Matsuka [7] formed a mathematical model of a dual air–water solar collector (DWAC). They compared four different solar collector systems. Moreover, they found that the combining of a domestic hot water preparation system and recirculating air- heating system provides 30% higher solar energy yield then the conventional solar domestic hot-water system. Balotaki and Saidi [8] designed and analyzed a dual-purpose solar collector with rectangular fin. They found that the rectangular channel collector provides approximately 11% increment in the efficiency and outlet temperature than V corrugated and flat plate collectors. Hu et al. [9] designed a novel wavelike roof solar collector (type 1) to provide hot water and space heating. The 2
Applied Thermal Engineering 171 (2020) 115094
A. Somwanshi and N. Sarkar
insulated cover that is removed back during the day time (sunshine hours). One of the advantages of the proposed design of DCS-AWH is that the heat stored by the water in the lower tank is utilized to heat the air flowing though upper compartment during no sunshine hours. One of the additional features of the proposed design is that it contains an auxiliary water- heating element to provide hot water and air at a desired temperature or in bad weather conditions. A simple mathematical model of the proposed design has been developed and experimentally validated for the climate of Raipur, Chhattisgarh, India. The model will be helpful to predict the temperature of stored water and the exit air. The effect of covering the glass cover during night by an insulated cover has been investigated in the present study. The temperature of exit air (hot air) and the temperature of water in the tank is affected by the mass flow rate of air flowing through the upper compartment. The effect of the mass flow rate of air flowing through the upper compartment on water and air temperature was investigated. The performance of dual collector is numerically computed for a day in December (winter) and May (summer) for the composite climate of Delhi, India. The monthly hourly averages of the ambient temperature and solar radiation considered for computations where determine from a study by Tiwari [17]. The area and capacity of the proposed DCS-AWH where taken as 1.5 m2 and 105L, respectively, for computations. The dimensions and the other relevant parameters used for the computations are listed in Tables 1 and 2.
and a collector in a single unit, it is more cost-effective. The heater can simultaneously provide hot water for domestic purposes and hot air for heating rooms during winters. During summers, the hot water can be delivered to industries/hospitals, whereas the hot air can be used for drying crop. The overall annual usage of the proposed DSC-AWH is higher than that of the conventional single- purpose air -water heaters. 1.1. Introduction to the proposed heater (DSC-AWH) The proposed design is a modification of the existing design of solar collector-storage type of water heater. The device comprises two different compartments lower and upper. Both compartments are separated by a common absorber plate. The Lower compartment is used for heating water, and the upper is used for heating air. The lower compartment comprises an insulated rectangular tank containing water. The top portion of the tank is covered by an absorber plate that is painted black. The upper compartment comprises an insulated rectangular tank with a common absorber plate at the bottom and a glass cover at the top. The solar energy that is absorbed by the absorber plate is transferred to the water in the lower compartment and the air that flows through the upper compartment. A schematic of the proposed design is displayed in Fig. 1. The lower tank is connected to an overhead tank to supply water to the collector. There is a provision to collect hot water from the top of the lower tank. The inlet and outlet are provided in the upper compartment to allow the air to flow through the upper compartment. An exhaust fan is connected to the outside duct of the upper compartment to drive the airflow. To avoid the heat loss during night- time (no sunshine hours) the glass cover is covered by an
2. Mathematical analysis The following assumptions have been made in writing the energy
Fig. 1. Proposed dual collector. 3
Applied Thermal Engineering 171 (2020) 115094
A. Somwanshi and N. Sarkar
Nu = 0.0158Re0.8
Table 1 Design parameters of proposed collector. Overall size
Material Insulation Insulation thickness Transparent cover Absorber plate Area of absorber plate Frame Dimensions of air channel
Dimensions of water channel
Water tank capacity
Length Width Height Mild steel plate (0.3 cm) Glass-wool 5 cm* Glass (4 mm thick) M.S. Plate (Painted Black) 1.5 m2 Wooden Length Width Depth Air duct diameter Length Width Depth 105 L
hag = (
160 cm 110 cm 17 cm
de is the equivalent diameter and it is given by, 4ac ap
de =
ac and ap are area of cross section and wetted perimeter of upper compartment (air heating section) 150 cm 100 cm 5 cm 15 cm 150 cm 100 cm 7 cm
2.2. Air stream Considering small length ‘dx’ the steady state energy balance of air stream will be,
dTa dx = hag (Tg − Ta) L2 dx + hap (Tp − Ta) L2 dx dx
ṁ a ca
Table 2 Relevant parameters used for numerical computation.
Ta (L1) = A1 Tp + B1
Mw = 105 kg Ap = 1.5 m2
cw = 4200 J/kgK Ag = 1.5 m2
ca = 1005 J/kgK L1 = 1.5 m*
ατ = 0.85* σ = 5.67 × 10−8 W/ m2 K4*
αc = 0.05*
εeff = 0.85*
hw = 108.6W / m2K **
ρa = 1.2kg / m3
dd = 0.15m
t = 0.05m
K = 0.004W / moC
A1 =
B [1 − exp(−AL1)] A
(8)
B1 =
C [1 − exp(−AL1)] + Ta0 exp(−AL1) A
(9)
A=
hag hag L2 [hap + hag − ] ṁ a ca Ut + hrpg + h ag
(10)
hag hrpg L2 [ + hap ] ṁ a ca Ut + hrpg + h ag
(11)
L2 U T + αc S [hag ( t amb )] ṁ a ca Ut + hrpg + hag
(12)
balance equations 1. The temperatures of the glass cover, absorber and back plate vary only in the direction of air flow 2. The system is in quasi-steady state condition 3. The flow rate is assumed constant. 4. The side losses are neglected since collector area is larger than the thickness.
B=
C=
From Eqs. (1) and (7),
Tg = A2 + B2 Tp
The energy balance equations of glass cover, absorber plate, air, water and base are as follows: -
A2 = (1)
B2 =
Here Ut is the total overall heat transfer coefficient between the glass cover and ambient, hag is convective heat transfer coefficient between air and glass cover is the radiative and convective heat transfer coefficient between plate and glass cover. α c is the fraction of solar energy absorbed by glass cover, Ut will be replaced by U ′t when glass cover is covered by insulating cover during off-sunshine hours (night time)
hrpg =
− (Tg +
Tp − Tg
hrpg Ut + hrpg + h ag
Tp = A3 + B3 Tw
A3 =
εg σ [(Tg + 273) 4 − (Tamb + 267) 4]
εeff σ [(Tp +
+
(14)
(15)
(16)
From Eqs. (7), (13) and (16), we have
(3)
273) 4
hag A1 Ut + hrpg + h ag
(ατ ) S = hap (Tp − Ta) + hrpg (Tp − Tg ) + h w (Tp − Tw )
(2)
Tg − Tamb
hag B1 Ut Tamb + α c S + Ut + hrpg + hag Ut + hrpg + h ag
2.3. Absorber plate
The correlation to find hca, hramb, hrpg is given by Tiwari [17],
hramb =
(13)
Here,
2.1. Glass cover
hca = 2.8 + 3va
(7)
Here,
* Tiwari [17]. ** Garg [11].
Ut = hca + hramb
(6)
here hag and hap are convective heat transfer between air and glass cover and air and absorber plate here hag = hap . L2 is the width of flat plate collector. Eq. (6) can be written as,
* Sodha [12,13].
Ut (Tg − Tamb) = hag (Ta − Tg ) + hrpg (Tp − Tg ) + α c S
ka )0.0158Re0.8 de
(17)
(ατ ) S + hap B1 + hrpg A2 hap − A1 hap + h w + hrpg − B2 hrpg
(18)
hw hap − A1 hap + h w + hrpg − B2 hrpg
(19)
(4)
B3 =
(5)
2.4. Basin
273) 4]
The correlation used for calculating the convective heat transfer coefficient between air and glass cover is given by Kays [18],
h w (Tw − Tb) = Ub (Tb − Tamb) 4
(20)
Applied Thermal Engineering 171 (2020) 115094
A. Somwanshi and N. Sarkar
Tb = A 4 + B4 Tw
Ub Tamb Ub + h w
A4 =
(21)
A5 =
(22)
B5 =
B4 =
hw Ub + h w
(23)
1 −1 t ] + K hb
Ub = U ′t when the glass cover of the collector is covered by insulated cover of the same thickness and material. 2.5. Water
(24)
Eq. (24) can be written as,
dTw + A5 Tw = B5 dt Tw (t ) =
B5 [1 − exp(−A5 t )] + Tw0 exp(−A5 t ) A5
Ap A3 h w + Ap A 4 h w Mw c w
(28)
The design parameters of the proposed model are listed in Table 1. The schematic of the experimental set-up is displayed in Fig. 1, and the pictorial view is shown in Fig. 2. The solar radiation transmitted by a glass cover and absorbed by an absorber plate was subsequently transferred to the water and air that comes in contact. The entire unit is encased in a wooden box. Moreover, to minimize any heat transfer from the sides and the bottom, glass wool insulation layer of 5 cm thickness was placed between the walls of the tank and wooden box. The experiment was performed in January (12/01/2020) in Raipur, Chhattisgarh, India. The dual collector was placed facing south, the temperature of the inlet air, exit air, and water was recorded using calibrated resistance temperature detector (RTD). The water temperature was measured at three different depths, and the average temperature was considered. The initial temperatures (t = 0) of the absorber plate, glass cover and water were recorded before initiating the observations. The total solar radiation incident on the glass cover of the collector was measured by using a pyranometer (Kipps and Zenon). The pyranometer was placed over the glass cover in the same plane, and faced south. A fan (40 W) was connected in the exit duct to induce the air flow below the glass cover in the upper compartment. Velocity of exit air was measured using a hot wire anemometer (HTC) of an
hb = 2.8 + 3va
dTw = h w (Tp − Tw ) Ap − h w (Tw − Tb) Ap dt
(27)
3. Experimental set-up and validation
Here t and K are the thickness and thermal conductivity of insulation and hb is the convective heat transfer coefficient between bottom and air and it is given by Tiwari [17],
Mw c w
Mw c w
Eq. (26) can be solved considering the time step of one hour, Tw0 is the temperature of initial water at time t = 0. By putting the value of Tw (t ) in Eq. (7) the temperature of air exit Ta (L1) can be found out.
Ub in Eq. (20) is the bottom loss coefficient and it is given by Tiwari [17], Ub = [
Ap h w − Ap h w B3 + Ap h w − Ap B4 h w
(25)
(26)
Here,
Fig. 2. Photograph of experimental set-up. 5
Applied Thermal Engineering 171 (2020) 115094
A. Somwanshi and N. Sarkar
accuracy of ( ± 0.1 m/s). The mass flow rate of air flowing through the collector was computed by equation ṁ a = ρa vad , here ρa is air density (1.2 kg/m3) and ad is the duct area. A regulator was provided with fan to control the mass flow rate of air that flows through the collector. The accuracy and the percent uncertainty of instruments are listed in Table 3. The uncertainty in determining the mass flow rate was expressed by following relation,
Δma Δv Δd = +2 ma v d To validate the proposed mathematical model, we recorded the temperature of water and exit air at the time interval of 30 min and compared it to the theoretical values obtained from numerical computation by using our model. Before initiating the observations, the lower compartment of the collector was filled with fresh water by using an inlet pipe. Moreover, the initial water, glass and plate temperatures were recorded to be 24.8 °C, 14.1 °C and 14.4 °C respectively. The mass flow rate of air flowing through the upper compartment was measured by measuring the velocity of exit air. The mass flow rate of air was 0.0178 kg/s, and the collector was covered with an insulated cover (glass wool 5 cm thick) during no sunshine hours (5.30 pm–8 am). The ambient temperature and intensity of solar radiation measured by us during the experiment is presented in Fig. 3. The theoretical and experimental values of exit air and water temperature are shown in Figs. 4 and 5. The experimental values are reasonably close to the theoretical values. The closeness of the theoretical values to the experimental values can be expressed in terms of the root mean square of percent deviation (e) and is given in a study by Jain and Tiwari [19],
e=
ei = [
Fig. 3. Ambient temperature and solar radiation during experiment.
∑ (ei )2 n
Xpre (i) − Xexp t (i) Xpre (i)
] × 100
The relationship between the theoretical and experimental values is given by a coefficient known as the coefficient of correlation (r). The experimental and theoretical values are said to be in a strong correlation, if the value of r is close to 1. The coefficient of correlation can be evaluated by following expression
r=
N ∑ Xpre Xexp −( ∑ Xpre )( ∑ Xexp )
Fig. 4. Experimental and theoretical water temperature.
2 2 N ∑ Xexp − ( ∑ Xexp )2 N ∑ Xpre − ( ∑ Xpre )2
through the upper compartment of collector, numerical computations have been made for a day in winter December (winter) for the composite climate of New Delhi, India. The design and dimensions considered for computations are same as shown in Table 1. The area and capacity of the collector were considered to be 1.5 m2 and 105L, respectively. Hourly averages of the ambient temperature and solar radiation considered for the numerical computations are presented in Fig. 6. The initial glass cover and plate temperatures were assumed to be equal to the ambient temperature. The initial water temperature was assumed to be equal to the tap water temperature (20 °C).
Here N is the number of observations. The values of the percentage deviation (e) and coefficient of correlation (r) was determined and presented in Figs. 4 and 5. The value of “e” is in between 8.01 and 10.58 and the correlation coefficient “r” is in between 0.9862 and 0.9857 with the degree of freedom (number of observation points) as 48. The value is reasonably good for solar thermal experiments. 4. Numerical computations To analyze the effect of covering with a glass cover during no sunshine hours and the effect of the mass flow rate of air flowing Table 3 Details of instruments used during experiment. Instrument
Range
Accuracy
% Uncertainty
K-type temperature sensor(Constantan) RTD(Platinum) Hot wire anemometer(HTC) Pyranometer(Kipps and Zenon) Mass flow rate
0 °C to 150 °C −50 °C to 199.9 °C 0 m/s–25.0 m/s 0 W/m2–1500 W/m2
± 0.2 °C ± 0.1 °C ± 0.1 m/s 73 µV/W/m2(Sensitivity)
2%–0.4% (10 °C–50 °C) 1%–0.2% (10 °C–50 °C) 5% (2 m/s) 10% 6.3%
6
Applied Thermal Engineering 171 (2020) 115094
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drop was observed up to 38 °C. When the cover was used, the drop in the water temperature was less. The temperature dropped up to 45 °C. Similarly, the exit air temperature dropped from 52 °C to 26 °C when the cover was not used. The drop in the temperature of exit air was up to 37 °C when the cover was used. The losses in the air and water temperatures during the no sunshine hours were minimized by covering the collector with an insulated cover. The amount of heat delivered (MJ) to the water and air in a day (during winter) was computed for both the conditions with and without the insulation cover (Fig. 8). The amounts of heat delivered to the water and air when the insulation cover was used were 138 MJ and 6 MJ, respectively. The amounts of heat delivered to the water and air when the insulation cover was not used were 110 MJ and 5.4 MJ, respectively. The total heat delivered to the air and water with the insulated cover was 144 MJ and without the cover was 115.4 MJ. The total heat delivered to the air and water when a cover was used was approximately 19.9% higher than that delivered when a cover was not used.
4.2. Effect of the mass flow rate of air Fig. 5. Experimental and theoretical exit air temperature.
The mass flow rate of the air flowing below the cover plate
Fig. 6. Hourly average ambient temperature and solar radiation of December for the composite climate of Delhi (India).
4.1. Effect of cover insulation During the no sunshine hours, the heat stored by the absorber plate is lost from the top of the collector into the atmosphere. This heat loss affects the exit air and stored water temperatures. To minimize the heat loss from the top, an insulated cover with a thickness of 5 cm [9,10] was placed at the top of the glass cover during the no sunshine hours and was removed back during the sunshine hours. The effect of covering with the glass cover was investigated for a day in December in the climate of New Delhi (India). Glass cover was covered using an insulated cover from 4 pm onwards and removed early in the morning at 7am. Hourly exit air temperature and stored water temperature were numerically computed and presented in Fig. 7 (a, b). The mass flow rate of the air flow was considered to be 0.005 kg/s and the initial water temperature (t = 0) was considered to be equal to the tap water temperature (20 °C). The performance of the collector was affected during no sunshine hours (4 pm–7 am) when the collector was uncovered. In this case, considerable amount of heat was lost from the top of the collector. The performance highly improved when the collector was covered during no sunshine hours. The water temperature increased from 20 °C to a maximum value of 56 °C (4 pm). Then, a continuous
Fig. 7. Performance of dual collector with and without cover insulation. 7
Applied Thermal Engineering 171 (2020) 115094
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collectors conducted by Nematollahi et al. [3] and More and Pote [5]. The results are shown in Fig. 12. The average thermal efficiency in the present study is 76% which is 5.5% higher than that in the study by Nematollahi et al. [3] and about 24.6% higher than that in the study by More and Pote [5]. The maximum thermal efficiency attend is almost same as that in the study by Nematollahi et al. [3] and about 14.4% higher than that the study by More and Pote [5]. Moreover, the thermal efficiency of the dual heater is higher than that of the conventional single -purpose water heater. 6. Performance of the proposed DCS-AWH The performance of the DCS-AWH was computed for December (winter) and May (summer) for the composite climate of Delhi, India. The temperature of the stored water and exit air was computed for 24 h from 7am to the next morning. The area and capacity of the collector considered for the computations were taken as 1.5 m2 and 105L, respectively. The performance was computed at low (0.005 kg/s) and high (0.020 kg/s) mass flow rates of air flowing through the upper compartment (air-heating section). The degree of heating of air (Tae-Tai) was computed for no sunshine and sunshine hours. To minimize the heat loss during the no sunshine hours, the collector was covered with an insulating cover (4 pm–7 am). In December (winter), the ambient temperature varies between 8.0 °C − 33.4 °C, and the maximum solar radiation is 637 W/m2. Figs. 14 and 15 reveal that the maximum water temperature attained was 56 °C at a low mass flow rate (0.005 kg/s) and was 51 °C at a high mass flow rate (0.020 kg/s). The temperature of water available next morning at 7 am ranges between 44 °C and 34 °C. The maximum temperature of the exit air was between 50 °C at a low mass flow rate and 37 °C at a high mass flow rate. The average degree of heating of air during sunshine hours was 23.7 °C at a low mass flow rate and 12.5 °C at a high mass flow rate. For the no sunshine hours, the average degree of heating at a low mass flow rate was 13 °C and at a high mass flow rate it was 10.5 °C. In May (summer), the ambient temperature varied between 26.6 °C and 40.5 °C, and the maximum solar radiation was 1011 W/m2 [Fig. 13]. Figs. 16 and 17, reveal that the maximum water temperature attained was 89.6 °C at a mass flow rate of 0.005 kg/s and was 80.1 °C at a mass flow rate of 0.020 kg/s. The maximum temperature of the exit air was 81.1 °C at a mass flow rate of 0.005 kg/s and was 53.6 °C at a mass flow rate of 0.020 kg/s. The average degree of heating of air during the sunshine and no sunshine hours were 30.1 °C and 26.04 °C at
Fig. 8. Heat energy (MJ) given to air/water for a day in winter month (December) For Delhi (India).
influences the air and water temperatures. A Low mass flow rate of air increases the contact time with the heated absorber plate, thus enhances the heat transfer between the plate and air. To study the effect of the mass flow rate of air flowing, numerical computations have been done for a day. The mass flow rates considered were 0.005 kg/s, 0.075 kg/s, 0.010 kg/s, 0.015 kg/s and 0.020 kg/s. Other relevant parameters are the same as discussed earlier in the study. Figs. 9 and 10 reveal that the dual collector performs well at low mass flow rate of air. Although at a high mass flow rate, the Reynolds number increases and higher values of the convective coefficient are obtained. However, the intensity of interaction between the air and heated plate tends to decrease due to decrease in the contact time with the heated absorber plate. The maximum water temperature attained was 56 °C at a mass flow rate of 0.005 kg/s and was 51 °C at a high mass flow rate of 0.020 kg/s. The water temperature obtained next morning (7a.m) was 44 °C at low mass flow rate of 0.005 kg/s and 34 °C at a high mass flow rate of 0.020 kg/s. The exit air temperature reached a maximum value in the range of 50 °C to 37 °C for a mass flow rate between 0.005 kg/s and 0.020 kg/s. The degree of heating of air and water was higher at a lower mass flow rate of air. 5. Efficiency of the proposed DCS-AWH The instantaneous thermal efficiency of the proposed dual heater is given as follows,
ηdu =
ηw + ηa ∑ SAc dt
Here, ηw and ηa are the efficiencies of heating water and air, respectively. Ac is the collector area and dt is the time. The efficiency of heating the air and water is given as follows,
ηa =
ṁ a ca (Tae − Tai ) dt ∑ SAc dt
ηw =
Mw c w (Tw (n) − Tw (n − 1) ) ∑ SAc dt
Thermal efficiency of the dual collector was computed for the sunshine hours at different mass flow rates for a day in winter in Delhi, and the results are presented in Fig. 11. The maximum thermal efficiency of approximately 82.80% was obtained at 12 pm at a mass flow rate of 0.020 kg/s. The maximum thermal efficiency attended at a mass flow rate of 0.005 kg/s was about 82.49%. The maximum and daily average thermal efficiencies in the present study were compared with those obtained in earlier studies on dual
Fig. 9. Variation in water temperature with mass flow rate of air. 8
Applied Thermal Engineering 171 (2020) 115094
A. Somwanshi and N. Sarkar
Fig. 13. Hourly average ambient temperature and solar radiation of May for the composite climate of Delhi (India).
Fig. 10. Variation in exit air temperature with mass flow rate of air.
Fig. 14. Water temperature of winter for climate of Delhi. Fig. 11. Efficiency of dual-collector at different mass flow rate of air.
Fig. 15. Exit air temperature of winter month for Delhi.
Fig. 12. The comparison between present work and previous work.
9
Applied Thermal Engineering 171 (2020) 115094
A. Somwanshi and N. Sarkar
foggy days. This is a main limitation of the proposed design. However, the problem can be rectified by using an additional backup electric heating coil in the system.
8. Conclusions The proposed DCS-AWH can be used to heat air and water simultaneously or separately, depending on the requirement. The design is more cost effective than the previously proposed designs. A simple mathematical model was developed and validated by experimentation. The model is helpful to analyze the performance of the dual collector for various climatic conditions. Numerical computations have been made to analyze the effect of mass flow rate of air on the heater performance. The heater performs well when air flowing through the upper compartment at a low mass flow rate. The maximum temperature of water was between 56 °C and 51 °C during December (winter). The temperature of hot water available early in the morning during winter was between 44 °C and 32 °C. In May (summer), the maximum water temperature was between 89.1 °C and 80.1 °C. The maximum temperature of exit air for winter was between 50 °C and 36 °C. Covering the collector with an insulating cover was highly recommended to avoid heat loss during the night. Covering glass cover during no sunshine hours can conserve about 19.9% of heat energy. The maximum and average thermal efficiency was 82.81% and 76% respectively. The efficiency of a dual-purpose heater was higher than that of the single purpose water heater. In the present study, computations were conducted by considering the area and length of heater to be 1.5 m2 and 1.5 m respectively. Because the design is highly scalable, one can create a design depending on the requirement.
Fig. 16. Water temperature of summer for climate of Delhi.
Declaration of Competing Interest 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. Fig. 17. Exit air temperature of summer month for Delhi.
a lower mass flow rate of 0.005 kg/s. At a mass flow rate of 0.020 kg/s, the average degree of heating during the sunshine and no sunshine hours were 7.1 °C and 11.1 °C respectively.
Acknowledgement We are grateful to Prof. M.S Sodha for suggesting the problem and helpful suggestions during work and Chattisgarh Council of Science and Technology, Raipur (CGCOST) for financial assistance. We must thank editor and reviewers for their valuable comments which helped to improve the quality of our work.
7. Limitation of the proposed design The proposed system functions inefficiently on cloudy, rainy and Appendix Flow chart showing solution procedure
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Appendix B. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.applthermaleng.2020.115094.
[10] Luo Bingqing, Hu. Zhongting, Hong Xiaoqiang, He Wei, Experimental study of the water heating performance of a novel tile-shaped dual-function solar collector, International Conference on Solar Heating and Cooling for Buildings and Industry, SHC 2014, 2015, pp. 87–94. [11] H.P. Garg, Usha Rani, Theoretical and experimental studies oc collector/storage type solar water heater, Sol. Energy 29 (6) (1982) 467–478. [12] M.S. Sodha, J.K. Nayak, S.C. kaushik, S.P. Sabberwal, M.A.S. Malik, Performance of a collector/storage solar water heater, Energy Convers. Manage. 19 (1979) 41–47. [13] M.S. Sodha, P.K. Bansal, N.D. Kaushik, Performance of collector/storage solar water heater; arbitrary demand pattern, Energy Convers. Manage. 21 (1981) 229–238. [14] M. Smyth, P.C. Eames, B. Norton, Techno-economic appraisal of an integrated collector/storage solar water heater, Renewable Energy 29 (2004) 1503–1514. [15] C. Dharuman, J.H. Arakari, K. Srinivasan, Performance evaluation of an integrated solar water heater as an option for building energy conservation, Energy Build. 38 (2006) 214–219. [16] Tadahmun, A, Yaseen., Naseer, D, Mohklif., Muhammad, Asmail, Eleivi, Performance investigation of an integrated solar water heater with corrugated absorber surface for domestic use, Renewable Energy (2019) doi:10.1016/J.renene. 2019.01.114. [17] G.N. Tiwari, Solar Energy Fundamentals, Design Modeling and Applications, Narosa Publishing House, New Delhi, 2002. [18] W.M. Kays, Convective Heat and Mass Transfer, Mc Graw- Hill, NewYork, 1993. [19] D. Jain, G.N. Tiwari, Thermal aspects of open sun drying of various crops, Energy 28 (2003) 37–54.
References [1] Jinwei Ma, Wei Sun, Jie Ji, Yang Zhang, Aifeng Zhang, Wen Fan, Experimental and theoretical study of the efficiency of a dual-function solar collector, Appl. Therm. Eng. 31 (2011) 1751–1756. [2] M.R. Assari, H. Tabrizi Basirat, I. Jafari, Experimental and theoretical investigation of dual-purpose solar collector, Sol. Energy 85 (2011) 601–608. [3] Omid Nematollahi, Pourya Alamdari, Mohammad Reza Assari, Experimental investigation of a dual-purpose solar heating system, Energy Convers. Manage. 78 (2014) 359–366. [4] I. Jafari, A. Arshadi, E. Najafpour, N. Hedayat, Energy and exergy analysis of dualpurpose solar collector, Int. J. Mech. Mechatron. Eng. 5 (9) (2011) 1712–1714. [5] N.G. More, R.S. Pote, Numerical and experimental investigation of dual-purpose solar collector, Int. J. Eng. Res. Technol. (IJERT) 7 (2018) 81-88. (ISSN: 22780181). [6] R. Venkatesh, W. Christraj, Experimental Investigation of multipurpose solar heating system, J. Energy Eng. (2015) 141–143. [7] V. Shemelin, T. Matuska, Performance modelling of dual air/ water collector in solar water and space heating application, Int. J. Photoenergy (2009), https://doi. org/10.1155/2019/8560193. [8] H.K. Balotaki, M.H. Saidi, Experimental investigation of dual-purpose solar collector using rectangular channels, J. Therm. Eng. 3–1 (2017) 1052–1059. [9] Zhongting Hu, Birigqing Luo, Wei He, Dengyun Hu, Jie Ji, Jinwei Ma, Performance study of a dual function roof solar collector for Chinese traditional building application, Appl. Therm. Eng. 128 (2018) 179–188.
11
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Thermal performance of a dual-purpose collector-cum-storage type airwater heater
T
⁎
Aneesh Somwanshi , Niladri Sarkar Department of Mechanical Engineering, MATS University, Raipur, Chhattisgarh, India
H I GH L IG H T S
novel design of a dual-purpose collector cum storage type air–water heater has been developed. • The proposed collector can deliver hot water and air simultaneously. • The maximum thermal efficiency reached during day time is 82.4% • The covering the collector by insulating cover during night about 19.9% of heat energy can be saved. • By • The maximum water and air temperature reached is 89.6 °C and 81.1 °C during summer.
A R T I C LE I N FO
A B S T R A C T
Keywords: Dual heater Collector-cum-storage device Water heating Air heating
The aim of this study is to design and develop a dual-purpose integrated solar collector-storage device that can simultaneously provide hot water and hot air. The device can be used as a single purpose air or water heater, based on the requirement. A Simple mathematical model of the proposed dual-purpose heater is presented and validated for the climate of Raipur, Chhattisgarh, India (21.25oN, 81.62oE). The performance of the collector is numerically computed for the composite climate of Delhi, India (28.70oN, 77.10oE). The results reveal that about 19.9% of the heat energy can be conserved by covering the collector with an insulating cover at night. The dual collector performs well when a low mass flow rate of air flows through the upper compartment, in comparison to high mass flow rate. The maximum thermal efficiency during day time is about 82.8% at a mass flow rate of 0.020 kg/s. Thermal efficiency of the proposed dual-purpose heater is higher than that of conventional single purpose water heater. The maximum water and air temperatures in December (winter) where 56 °C and 50 °C, respectively, whereas those for May (summer) where 89.6 °C and 81.1 °C, respectively.
1. Introduction A flat plate solar collector is a commonly used technology for heating water or air. This collector is a heat exchanger that transforms solar energy into heat. The solar energy that is absorbed into the absorber plate is converted into heat and then is transferred to the stream of liquid or air that comes in contact. The utilization of solar energy, which is freely available, is one of the major advantages of these systems. Flat plate collectors are mostly used to heat water for domestic or industrial use. However, these collectors are sometimes utilized as air collectors for space heating and drying purposes. Although the flat plate collector is a well-established technology, the researchers all over the globe are constantly working on improving the overall performance and efficiency of these collectors. One of the recent trends is to combine two known technologies, for ⁎
developing a device that works in combination, known as a hybrid or dual-purpose device. Few studies have been conducted to design and develop a cost-effective dual air–water collector that can generate both hot air and hot water. Ma et al. [1] designed a dual- function solar collector that can simultaneously provide hot water and hot air. They found that the thermal efficiency of the proposed collector attened a value of 50% in water heating water and varied from 41% to 55% in heating air depending on the ambient conditions and mass flow rate. Assari et al. [2] developed and experimentally validated a mathematical model based on the effectiveness method. They considered three different channels rectangular, triangular and no fins. The Simulation results revealed that the channels with rectangular fins perform better. Nematollahi et al. [3] experimentally investigated a dual-purpose solar heating system. They studied the effect of parameters such as temperature variations in an absorber plate, average temperature of storage
Corresponding author. E-mail address: aneeshsomwanshi@rediffmail.com (A. Somwanshi).
https://doi.org/10.1016/j.applthermaleng.2020.115094 Received 24 June 2019; Received in revised form 13 February 2020; Accepted 16 February 2020 Available online 17 February 2020 1359-4311/ © 2020 Elsevier Ltd. All rights reserved.
Applied Thermal Engineering 171 (2020) 115094
A. Somwanshi and N. Sarkar
Nomenclature
Ap ac ap
ca cw dd de hag hap hca hrpg hramb
hw
hb Ka K L1 L2 ṁ a
Mw S t
Ta
Area of the absorber plate [m2 ] Area of cross section of upper compartment (air heating channel) [m2 ] Perimeter of the upper compartment (air heating channel) [m ] Specific heat of air [ Jkg −1K−1] Specific heat of water [ Jkg −1K−1] Diameter of duct [m ] Equivalent diameter [m ] Convective heat transfer coefficient between air and glass cover [Wm−2K−1] Convective heat transfer coefficient between air and plate [Wm−2K−1] Convective heat transfer coefficient between glass cover and ambient [Wm−2K−1] Radiative heat transfer coefficient between plate and glass cover [Wm−2K−1] Radiative heat transfer coefficient between glass cover and ambient [Wm−2K−1] Convective heat transfer coefficient between plate and water [Wm−2K−1] Convective heat transfer coefficient between bottom and air [Wm−2K−1] Thermal conductivity of air [Wm−1K−1] Thermal conductivity of insulation [Wm−1K−1] Length of dual collector [m ] Width of dual collector [m ] Mass flow rate of air flowing through upper compartment of dual collector [kgs−1] Mass of water in collector [kg ] Solar radiation intensity [Wm−2 ] Thickness of insulation [m ]
Tw Tae Tai Tg Tp Tamb Tb
Ub Ut U ′t
va ρa ατ εg εeff αc σ ηa ηw ηdu Xpre Xexp Nu Re
Temperature of air flowing through upper compartment [°C] Temperature of water in lower compartment [°C] Temperature of exit air flowing through upper compartment [°C] Temperature of inlet air flowing through upper compartment [°C] Temperature of glass cover [°C] Temperature of absorber plate [°C] Ambient temperature [°C] Temperature of bottom of dual collector (base temperature) [°C] Overall heat transfer coefficient between base to ambient [Wm−2K−1] Overall heat transfer coefficient between glass cover to ambient [Wm−2K−1] Overall heat transfer coefficient between insulated cover to ambient [Wm−2K−1] Velocity of ambient air [ms−1] Density of air [Kgm−3 ] Effective transmittance-absorptance product Glass cover emittance Effective emissivity of plate glazing system Absorptance of glass cover Stefan’s constant [W / m2K 4 ] Efficiency of air heating Efficiency of water heating Efficiency of proposed dual heater Theoretical value Experimental value Nusselt number Reynold’s number
proposed collector was compared with other types of solar collectors (type 2 and type 3). The simulation results reveal that the heat loss coefficient of the type 1 collector is approximately 54.4% lower compared with type 2 collectors and is approximately 65.8% lower compared with the type 3 collectors. Luo et al. [10] proposed a dual-function solar collector with tile shaped Methyl Methacrylate (PMMA) covers to match the roof appearance. The test result reveals that the daily thermal efficiency of the water heating system with tile shaped collector system varied from 54% to 61.8% whereas the efficiency of the water heating system without PMMA covers varied between 35.5% and 67.4%. In present study, we proposed a novel design dual heater that combines the collector-storage type of solar water heater with an air collector. This proposed heater in known as a dual-purpose collectioncum-storage type air–water heater (DCS-AWH). The collector-storage type of solar water heater has been a well-known technology previously to heat water during winters. The performance of this type of heater has been investigated by researchers [11–16]. The heater comprises a rectangular tank in a suitable container. All the sides and the bottom of the tank are insulated with glass fiber, and the top surface is blackened and glazed. Thus, the top surface acts as a solar radiation absorber, whereas the tank filled with water acts as a storage. The proposed heater can simultaneously provide hot air and hot water. Depending on the requirement, the heater can be used as a single-purpose collector for heating water or air. Unlike the previous designs of dual collectors, the proposed design is very simple and does not include any grid of tubes that are bonded to the absorber plate. This reduces the conduction losses and eliminates the leakage problems through joints. Because the proposed design incorporates storage tank
tank and air velocity on the system performance. They concluded that the average efficiency of a single- purpose system is 67.8%. However, the efficiency for a dual-purpose system is between 71.6% and 72.5%, which is approximately 3% to 5% higher than the efficiency of a single system. Jafari et al. [4] conducted energy and exergy analysis of a dual air–water solar collector by using the number of transfer unit (NTU) method. They concluded that the optimal efficiency can be achieved if the temperature of the inlet water is 60 °C and the triangular type of air channel is used. More and Pote [5] designed a model of a dual-purpose solar collector (DPSC) with horizontal water tubes. They developed mathematical model based on the effectiveness method and obtained maximum thermal efficiency up to 72.4%. Venkatesh and Christraj [6] experimentally investigated a multipurpose solar collector system by combining a solar water collector and solar air heater. They found the maximum stagnant temperature of the multipurpose solar air heater to be 88 °C for the overall heat loss coefficient of 12.83 W/m2 K at no load condition. Moreover, they confirmed that the efficiency of the multipurpose system is higher than that of the conventional system. Shemelin and Matsuka [7] formed a mathematical model of a dual air–water solar collector (DWAC). They compared four different solar collector systems. Moreover, they found that the combining of a domestic hot water preparation system and recirculating air- heating system provides 30% higher solar energy yield then the conventional solar domestic hot-water system. Balotaki and Saidi [8] designed and analyzed a dual-purpose solar collector with rectangular fin. They found that the rectangular channel collector provides approximately 11% increment in the efficiency and outlet temperature than V corrugated and flat plate collectors. Hu et al. [9] designed a novel wavelike roof solar collector (type 1) to provide hot water and space heating. The 2
Applied Thermal Engineering 171 (2020) 115094
A. Somwanshi and N. Sarkar
insulated cover that is removed back during the day time (sunshine hours). One of the advantages of the proposed design of DCS-AWH is that the heat stored by the water in the lower tank is utilized to heat the air flowing though upper compartment during no sunshine hours. One of the additional features of the proposed design is that it contains an auxiliary water- heating element to provide hot water and air at a desired temperature or in bad weather conditions. A simple mathematical model of the proposed design has been developed and experimentally validated for the climate of Raipur, Chhattisgarh, India. The model will be helpful to predict the temperature of stored water and the exit air. The effect of covering the glass cover during night by an insulated cover has been investigated in the present study. The temperature of exit air (hot air) and the temperature of water in the tank is affected by the mass flow rate of air flowing through the upper compartment. The effect of the mass flow rate of air flowing through the upper compartment on water and air temperature was investigated. The performance of dual collector is numerically computed for a day in December (winter) and May (summer) for the composite climate of Delhi, India. The monthly hourly averages of the ambient temperature and solar radiation considered for computations where determine from a study by Tiwari [17]. The area and capacity of the proposed DCS-AWH where taken as 1.5 m2 and 105L, respectively, for computations. The dimensions and the other relevant parameters used for the computations are listed in Tables 1 and 2.
and a collector in a single unit, it is more cost-effective. The heater can simultaneously provide hot water for domestic purposes and hot air for heating rooms during winters. During summers, the hot water can be delivered to industries/hospitals, whereas the hot air can be used for drying crop. The overall annual usage of the proposed DSC-AWH is higher than that of the conventional single- purpose air -water heaters. 1.1. Introduction to the proposed heater (DSC-AWH) The proposed design is a modification of the existing design of solar collector-storage type of water heater. The device comprises two different compartments lower and upper. Both compartments are separated by a common absorber plate. The Lower compartment is used for heating water, and the upper is used for heating air. The lower compartment comprises an insulated rectangular tank containing water. The top portion of the tank is covered by an absorber plate that is painted black. The upper compartment comprises an insulated rectangular tank with a common absorber plate at the bottom and a glass cover at the top. The solar energy that is absorbed by the absorber plate is transferred to the water in the lower compartment and the air that flows through the upper compartment. A schematic of the proposed design is displayed in Fig. 1. The lower tank is connected to an overhead tank to supply water to the collector. There is a provision to collect hot water from the top of the lower tank. The inlet and outlet are provided in the upper compartment to allow the air to flow through the upper compartment. An exhaust fan is connected to the outside duct of the upper compartment to drive the airflow. To avoid the heat loss during night- time (no sunshine hours) the glass cover is covered by an
2. Mathematical analysis The following assumptions have been made in writing the energy
Fig. 1. Proposed dual collector. 3
Applied Thermal Engineering 171 (2020) 115094
A. Somwanshi and N. Sarkar
Nu = 0.0158Re0.8
Table 1 Design parameters of proposed collector. Overall size
Material Insulation Insulation thickness Transparent cover Absorber plate Area of absorber plate Frame Dimensions of air channel
Dimensions of water channel
Water tank capacity
Length Width Height Mild steel plate (0.3 cm) Glass-wool 5 cm* Glass (4 mm thick) M.S. Plate (Painted Black) 1.5 m2 Wooden Length Width Depth Air duct diameter Length Width Depth 105 L
hag = (
160 cm 110 cm 17 cm
de is the equivalent diameter and it is given by, 4ac ap
de =
ac and ap are area of cross section and wetted perimeter of upper compartment (air heating section) 150 cm 100 cm 5 cm 15 cm 150 cm 100 cm 7 cm
2.2. Air stream Considering small length ‘dx’ the steady state energy balance of air stream will be,
dTa dx = hag (Tg − Ta) L2 dx + hap (Tp − Ta) L2 dx dx
ṁ a ca
Table 2 Relevant parameters used for numerical computation.
Ta (L1) = A1 Tp + B1
Mw = 105 kg Ap = 1.5 m2
cw = 4200 J/kgK Ag = 1.5 m2
ca = 1005 J/kgK L1 = 1.5 m*
ατ = 0.85* σ = 5.67 × 10−8 W/ m2 K4*
αc = 0.05*
εeff = 0.85*
hw = 108.6W / m2K **
ρa = 1.2kg / m3
dd = 0.15m
t = 0.05m
K = 0.004W / moC
A1 =
B [1 − exp(−AL1)] A
(8)
B1 =
C [1 − exp(−AL1)] + Ta0 exp(−AL1) A
(9)
A=
hag hag L2 [hap + hag − ] ṁ a ca Ut + hrpg + h ag
(10)
hag hrpg L2 [ + hap ] ṁ a ca Ut + hrpg + h ag
(11)
L2 U T + αc S [hag ( t amb )] ṁ a ca Ut + hrpg + hag
(12)
balance equations 1. The temperatures of the glass cover, absorber and back plate vary only in the direction of air flow 2. The system is in quasi-steady state condition 3. The flow rate is assumed constant. 4. The side losses are neglected since collector area is larger than the thickness.
B=
C=
From Eqs. (1) and (7),
Tg = A2 + B2 Tp
The energy balance equations of glass cover, absorber plate, air, water and base are as follows: -
A2 = (1)
B2 =
Here Ut is the total overall heat transfer coefficient between the glass cover and ambient, hag is convective heat transfer coefficient between air and glass cover is the radiative and convective heat transfer coefficient between plate and glass cover. α c is the fraction of solar energy absorbed by glass cover, Ut will be replaced by U ′t when glass cover is covered by insulating cover during off-sunshine hours (night time)
hrpg =
− (Tg +
Tp − Tg
hrpg Ut + hrpg + h ag
Tp = A3 + B3 Tw
A3 =
εg σ [(Tg + 273) 4 − (Tamb + 267) 4]
εeff σ [(Tp +
+
(14)
(15)
(16)
From Eqs. (7), (13) and (16), we have
(3)
273) 4
hag A1 Ut + hrpg + h ag
(ατ ) S = hap (Tp − Ta) + hrpg (Tp − Tg ) + h w (Tp − Tw )
(2)
Tg − Tamb
hag B1 Ut Tamb + α c S + Ut + hrpg + hag Ut + hrpg + h ag
2.3. Absorber plate
The correlation to find hca, hramb, hrpg is given by Tiwari [17],
hramb =
(13)
Here,
2.1. Glass cover
hca = 2.8 + 3va
(7)
Here,
* Tiwari [17]. ** Garg [11].
Ut = hca + hramb
(6)
here hag and hap are convective heat transfer between air and glass cover and air and absorber plate here hag = hap . L2 is the width of flat plate collector. Eq. (6) can be written as,
* Sodha [12,13].
Ut (Tg − Tamb) = hag (Ta − Tg ) + hrpg (Tp − Tg ) + α c S
ka )0.0158Re0.8 de
(17)
(ατ ) S + hap B1 + hrpg A2 hap − A1 hap + h w + hrpg − B2 hrpg
(18)
hw hap − A1 hap + h w + hrpg − B2 hrpg
(19)
(4)
B3 =
(5)
2.4. Basin
273) 4]
The correlation used for calculating the convective heat transfer coefficient between air and glass cover is given by Kays [18],
h w (Tw − Tb) = Ub (Tb − Tamb) 4
(20)
Applied Thermal Engineering 171 (2020) 115094
A. Somwanshi and N. Sarkar
Tb = A 4 + B4 Tw
Ub Tamb Ub + h w
A4 =
(21)
A5 =
(22)
B5 =
B4 =
hw Ub + h w
(23)
1 −1 t ] + K hb
Ub = U ′t when the glass cover of the collector is covered by insulated cover of the same thickness and material. 2.5. Water
(24)
Eq. (24) can be written as,
dTw + A5 Tw = B5 dt Tw (t ) =
B5 [1 − exp(−A5 t )] + Tw0 exp(−A5 t ) A5
Ap A3 h w + Ap A 4 h w Mw c w
(28)
The design parameters of the proposed model are listed in Table 1. The schematic of the experimental set-up is displayed in Fig. 1, and the pictorial view is shown in Fig. 2. The solar radiation transmitted by a glass cover and absorbed by an absorber plate was subsequently transferred to the water and air that comes in contact. The entire unit is encased in a wooden box. Moreover, to minimize any heat transfer from the sides and the bottom, glass wool insulation layer of 5 cm thickness was placed between the walls of the tank and wooden box. The experiment was performed in January (12/01/2020) in Raipur, Chhattisgarh, India. The dual collector was placed facing south, the temperature of the inlet air, exit air, and water was recorded using calibrated resistance temperature detector (RTD). The water temperature was measured at three different depths, and the average temperature was considered. The initial temperatures (t = 0) of the absorber plate, glass cover and water were recorded before initiating the observations. The total solar radiation incident on the glass cover of the collector was measured by using a pyranometer (Kipps and Zenon). The pyranometer was placed over the glass cover in the same plane, and faced south. A fan (40 W) was connected in the exit duct to induce the air flow below the glass cover in the upper compartment. Velocity of exit air was measured using a hot wire anemometer (HTC) of an
hb = 2.8 + 3va
dTw = h w (Tp − Tw ) Ap − h w (Tw − Tb) Ap dt
(27)
3. Experimental set-up and validation
Here t and K are the thickness and thermal conductivity of insulation and hb is the convective heat transfer coefficient between bottom and air and it is given by Tiwari [17],
Mw c w
Mw c w
Eq. (26) can be solved considering the time step of one hour, Tw0 is the temperature of initial water at time t = 0. By putting the value of Tw (t ) in Eq. (7) the temperature of air exit Ta (L1) can be found out.
Ub in Eq. (20) is the bottom loss coefficient and it is given by Tiwari [17], Ub = [
Ap h w − Ap h w B3 + Ap h w − Ap B4 h w
(25)
(26)
Here,
Fig. 2. Photograph of experimental set-up. 5
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A. Somwanshi and N. Sarkar
accuracy of ( ± 0.1 m/s). The mass flow rate of air flowing through the collector was computed by equation ṁ a = ρa vad , here ρa is air density (1.2 kg/m3) and ad is the duct area. A regulator was provided with fan to control the mass flow rate of air that flows through the collector. The accuracy and the percent uncertainty of instruments are listed in Table 3. The uncertainty in determining the mass flow rate was expressed by following relation,
Δma Δv Δd = +2 ma v d To validate the proposed mathematical model, we recorded the temperature of water and exit air at the time interval of 30 min and compared it to the theoretical values obtained from numerical computation by using our model. Before initiating the observations, the lower compartment of the collector was filled with fresh water by using an inlet pipe. Moreover, the initial water, glass and plate temperatures were recorded to be 24.8 °C, 14.1 °C and 14.4 °C respectively. The mass flow rate of air flowing through the upper compartment was measured by measuring the velocity of exit air. The mass flow rate of air was 0.0178 kg/s, and the collector was covered with an insulated cover (glass wool 5 cm thick) during no sunshine hours (5.30 pm–8 am). The ambient temperature and intensity of solar radiation measured by us during the experiment is presented in Fig. 3. The theoretical and experimental values of exit air and water temperature are shown in Figs. 4 and 5. The experimental values are reasonably close to the theoretical values. The closeness of the theoretical values to the experimental values can be expressed in terms of the root mean square of percent deviation (e) and is given in a study by Jain and Tiwari [19],
e=
ei = [
Fig. 3. Ambient temperature and solar radiation during experiment.
∑ (ei )2 n
Xpre (i) − Xexp t (i) Xpre (i)
] × 100
The relationship between the theoretical and experimental values is given by a coefficient known as the coefficient of correlation (r). The experimental and theoretical values are said to be in a strong correlation, if the value of r is close to 1. The coefficient of correlation can be evaluated by following expression
r=
N ∑ Xpre Xexp −( ∑ Xpre )( ∑ Xexp )
Fig. 4. Experimental and theoretical water temperature.
2 2 N ∑ Xexp − ( ∑ Xexp )2 N ∑ Xpre − ( ∑ Xpre )2
through the upper compartment of collector, numerical computations have been made for a day in winter December (winter) for the composite climate of New Delhi, India. The design and dimensions considered for computations are same as shown in Table 1. The area and capacity of the collector were considered to be 1.5 m2 and 105L, respectively. Hourly averages of the ambient temperature and solar radiation considered for the numerical computations are presented in Fig. 6. The initial glass cover and plate temperatures were assumed to be equal to the ambient temperature. The initial water temperature was assumed to be equal to the tap water temperature (20 °C).
Here N is the number of observations. The values of the percentage deviation (e) and coefficient of correlation (r) was determined and presented in Figs. 4 and 5. The value of “e” is in between 8.01 and 10.58 and the correlation coefficient “r” is in between 0.9862 and 0.9857 with the degree of freedom (number of observation points) as 48. The value is reasonably good for solar thermal experiments. 4. Numerical computations To analyze the effect of covering with a glass cover during no sunshine hours and the effect of the mass flow rate of air flowing Table 3 Details of instruments used during experiment. Instrument
Range
Accuracy
% Uncertainty
K-type temperature sensor(Constantan) RTD(Platinum) Hot wire anemometer(HTC) Pyranometer(Kipps and Zenon) Mass flow rate
0 °C to 150 °C −50 °C to 199.9 °C 0 m/s–25.0 m/s 0 W/m2–1500 W/m2
± 0.2 °C ± 0.1 °C ± 0.1 m/s 73 µV/W/m2(Sensitivity)
2%–0.4% (10 °C–50 °C) 1%–0.2% (10 °C–50 °C) 5% (2 m/s) 10% 6.3%
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Applied Thermal Engineering 171 (2020) 115094
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drop was observed up to 38 °C. When the cover was used, the drop in the water temperature was less. The temperature dropped up to 45 °C. Similarly, the exit air temperature dropped from 52 °C to 26 °C when the cover was not used. The drop in the temperature of exit air was up to 37 °C when the cover was used. The losses in the air and water temperatures during the no sunshine hours were minimized by covering the collector with an insulated cover. The amount of heat delivered (MJ) to the water and air in a day (during winter) was computed for both the conditions with and without the insulation cover (Fig. 8). The amounts of heat delivered to the water and air when the insulation cover was used were 138 MJ and 6 MJ, respectively. The amounts of heat delivered to the water and air when the insulation cover was not used were 110 MJ and 5.4 MJ, respectively. The total heat delivered to the air and water with the insulated cover was 144 MJ and without the cover was 115.4 MJ. The total heat delivered to the air and water when a cover was used was approximately 19.9% higher than that delivered when a cover was not used.
4.2. Effect of the mass flow rate of air Fig. 5. Experimental and theoretical exit air temperature.
The mass flow rate of the air flowing below the cover plate
Fig. 6. Hourly average ambient temperature and solar radiation of December for the composite climate of Delhi (India).
4.1. Effect of cover insulation During the no sunshine hours, the heat stored by the absorber plate is lost from the top of the collector into the atmosphere. This heat loss affects the exit air and stored water temperatures. To minimize the heat loss from the top, an insulated cover with a thickness of 5 cm [9,10] was placed at the top of the glass cover during the no sunshine hours and was removed back during the sunshine hours. The effect of covering with the glass cover was investigated for a day in December in the climate of New Delhi (India). Glass cover was covered using an insulated cover from 4 pm onwards and removed early in the morning at 7am. Hourly exit air temperature and stored water temperature were numerically computed and presented in Fig. 7 (a, b). The mass flow rate of the air flow was considered to be 0.005 kg/s and the initial water temperature (t = 0) was considered to be equal to the tap water temperature (20 °C). The performance of the collector was affected during no sunshine hours (4 pm–7 am) when the collector was uncovered. In this case, considerable amount of heat was lost from the top of the collector. The performance highly improved when the collector was covered during no sunshine hours. The water temperature increased from 20 °C to a maximum value of 56 °C (4 pm). Then, a continuous
Fig. 7. Performance of dual collector with and without cover insulation. 7
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collectors conducted by Nematollahi et al. [3] and More and Pote [5]. The results are shown in Fig. 12. The average thermal efficiency in the present study is 76% which is 5.5% higher than that in the study by Nematollahi et al. [3] and about 24.6% higher than that in the study by More and Pote [5]. The maximum thermal efficiency attend is almost same as that in the study by Nematollahi et al. [3] and about 14.4% higher than that the study by More and Pote [5]. Moreover, the thermal efficiency of the dual heater is higher than that of the conventional single -purpose water heater. 6. Performance of the proposed DCS-AWH The performance of the DCS-AWH was computed for December (winter) and May (summer) for the composite climate of Delhi, India. The temperature of the stored water and exit air was computed for 24 h from 7am to the next morning. The area and capacity of the collector considered for the computations were taken as 1.5 m2 and 105L, respectively. The performance was computed at low (0.005 kg/s) and high (0.020 kg/s) mass flow rates of air flowing through the upper compartment (air-heating section). The degree of heating of air (Tae-Tai) was computed for no sunshine and sunshine hours. To minimize the heat loss during the no sunshine hours, the collector was covered with an insulating cover (4 pm–7 am). In December (winter), the ambient temperature varies between 8.0 °C − 33.4 °C, and the maximum solar radiation is 637 W/m2. Figs. 14 and 15 reveal that the maximum water temperature attained was 56 °C at a low mass flow rate (0.005 kg/s) and was 51 °C at a high mass flow rate (0.020 kg/s). The temperature of water available next morning at 7 am ranges between 44 °C and 34 °C. The maximum temperature of the exit air was between 50 °C at a low mass flow rate and 37 °C at a high mass flow rate. The average degree of heating of air during sunshine hours was 23.7 °C at a low mass flow rate and 12.5 °C at a high mass flow rate. For the no sunshine hours, the average degree of heating at a low mass flow rate was 13 °C and at a high mass flow rate it was 10.5 °C. In May (summer), the ambient temperature varied between 26.6 °C and 40.5 °C, and the maximum solar radiation was 1011 W/m2 [Fig. 13]. Figs. 16 and 17, reveal that the maximum water temperature attained was 89.6 °C at a mass flow rate of 0.005 kg/s and was 80.1 °C at a mass flow rate of 0.020 kg/s. The maximum temperature of the exit air was 81.1 °C at a mass flow rate of 0.005 kg/s and was 53.6 °C at a mass flow rate of 0.020 kg/s. The average degree of heating of air during the sunshine and no sunshine hours were 30.1 °C and 26.04 °C at
Fig. 8. Heat energy (MJ) given to air/water for a day in winter month (December) For Delhi (India).
influences the air and water temperatures. A Low mass flow rate of air increases the contact time with the heated absorber plate, thus enhances the heat transfer between the plate and air. To study the effect of the mass flow rate of air flowing, numerical computations have been done for a day. The mass flow rates considered were 0.005 kg/s, 0.075 kg/s, 0.010 kg/s, 0.015 kg/s and 0.020 kg/s. Other relevant parameters are the same as discussed earlier in the study. Figs. 9 and 10 reveal that the dual collector performs well at low mass flow rate of air. Although at a high mass flow rate, the Reynolds number increases and higher values of the convective coefficient are obtained. However, the intensity of interaction between the air and heated plate tends to decrease due to decrease in the contact time with the heated absorber plate. The maximum water temperature attained was 56 °C at a mass flow rate of 0.005 kg/s and was 51 °C at a high mass flow rate of 0.020 kg/s. The water temperature obtained next morning (7a.m) was 44 °C at low mass flow rate of 0.005 kg/s and 34 °C at a high mass flow rate of 0.020 kg/s. The exit air temperature reached a maximum value in the range of 50 °C to 37 °C for a mass flow rate between 0.005 kg/s and 0.020 kg/s. The degree of heating of air and water was higher at a lower mass flow rate of air. 5. Efficiency of the proposed DCS-AWH The instantaneous thermal efficiency of the proposed dual heater is given as follows,
ηdu =
ηw + ηa ∑ SAc dt
Here, ηw and ηa are the efficiencies of heating water and air, respectively. Ac is the collector area and dt is the time. The efficiency of heating the air and water is given as follows,
ηa =
ṁ a ca (Tae − Tai ) dt ∑ SAc dt
ηw =
Mw c w (Tw (n) − Tw (n − 1) ) ∑ SAc dt
Thermal efficiency of the dual collector was computed for the sunshine hours at different mass flow rates for a day in winter in Delhi, and the results are presented in Fig. 11. The maximum thermal efficiency of approximately 82.80% was obtained at 12 pm at a mass flow rate of 0.020 kg/s. The maximum thermal efficiency attended at a mass flow rate of 0.005 kg/s was about 82.49%. The maximum and daily average thermal efficiencies in the present study were compared with those obtained in earlier studies on dual
Fig. 9. Variation in water temperature with mass flow rate of air. 8
Applied Thermal Engineering 171 (2020) 115094
A. Somwanshi and N. Sarkar
Fig. 13. Hourly average ambient temperature and solar radiation of May for the composite climate of Delhi (India).
Fig. 10. Variation in exit air temperature with mass flow rate of air.
Fig. 14. Water temperature of winter for climate of Delhi. Fig. 11. Efficiency of dual-collector at different mass flow rate of air.
Fig. 15. Exit air temperature of winter month for Delhi.
Fig. 12. The comparison between present work and previous work.
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A. Somwanshi and N. Sarkar
foggy days. This is a main limitation of the proposed design. However, the problem can be rectified by using an additional backup electric heating coil in the system.
8. Conclusions The proposed DCS-AWH can be used to heat air and water simultaneously or separately, depending on the requirement. The design is more cost effective than the previously proposed designs. A simple mathematical model was developed and validated by experimentation. The model is helpful to analyze the performance of the dual collector for various climatic conditions. Numerical computations have been made to analyze the effect of mass flow rate of air on the heater performance. The heater performs well when air flowing through the upper compartment at a low mass flow rate. The maximum temperature of water was between 56 °C and 51 °C during December (winter). The temperature of hot water available early in the morning during winter was between 44 °C and 32 °C. In May (summer), the maximum water temperature was between 89.1 °C and 80.1 °C. The maximum temperature of exit air for winter was between 50 °C and 36 °C. Covering the collector with an insulating cover was highly recommended to avoid heat loss during the night. Covering glass cover during no sunshine hours can conserve about 19.9% of heat energy. The maximum and average thermal efficiency was 82.81% and 76% respectively. The efficiency of a dual-purpose heater was higher than that of the single purpose water heater. In the present study, computations were conducted by considering the area and length of heater to be 1.5 m2 and 1.5 m respectively. Because the design is highly scalable, one can create a design depending on the requirement.
Fig. 16. Water temperature of summer for climate of Delhi.
Declaration of Competing Interest 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. Fig. 17. Exit air temperature of summer month for Delhi.
a lower mass flow rate of 0.005 kg/s. At a mass flow rate of 0.020 kg/s, the average degree of heating during the sunshine and no sunshine hours were 7.1 °C and 11.1 °C respectively.
Acknowledgement We are grateful to Prof. M.S Sodha for suggesting the problem and helpful suggestions during work and Chattisgarh Council of Science and Technology, Raipur (CGCOST) for financial assistance. We must thank editor and reviewers for their valuable comments which helped to improve the quality of our work.
7. Limitation of the proposed design The proposed system functions inefficiently on cloudy, rainy and Appendix Flow chart showing solution procedure
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Appendix B. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.applthermaleng.2020.115094.
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