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T distillation system enhanced with a porous evaporator and an internal condenser
Available online at www.sciencedirect.com
ScienceDirect Solar Energy 120 (2015) 593–602 www.elsevier.com/locate/solener
Modeling of a novel concentrated PV/T distillation system enhanced with a porous evaporator and an internal condenser Moh’d A. Al-Nimr a,1, Moh’d-Eslam Dahdolan b,⇑ a
Mechanical Engineering Department, Jordan University of Science and Technology, P.O. Box: 3030, Irbid 22110, Jordan b School of Engineering and Technology, Central Michigan University, Mount Pleasant, 48859 MI, USA Received 13 February 2015; received in revised form 30 July 2015; accepted 1 August 2015 Available online 5 September 2015 Communicated by: Associate Editor G.N. Tiwari
Abstract This paper presents a new concentrated PV/T system which utilizes thermal energy rejected by the PV cell to distill salty water. This PV/T system is enhanced with a porous evaporator and an internal condenser. This PV/T provides electric power and distilled water, and can operate as a passive system since circulation is driven by the thermosyphon effect. A simple steady-state mathematical model has been put to describe the performance of the system, and has been simulated using Microsoft Excel’s ‘‘Goal-Seek” data tool to find the evaporator temperature. Performance curves have been provided showing the effect of wind speed, solar intensity, ambient temperature, condenser temperature, and PV cell type on the performance of the system. (Overall efficiency, Electric power output, and Distillation rate.) Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: PV/T distillation system; Concentrated; Porous evaporator; Internal condenser
1. Introduction One of the main challenges to overcome while using PV cells is the effect of high temperature. This temperature rise affects the performance of the cells and decreases its efficiency. Controlling PV cells’ temperature can be carried out by different cooling methods. Cooling the cells by utilizing its generated heat introduced the idea of PV/T energy systems. PV/T systems are classified according to the method into air based, water based, refrigerant based PV/T systems and heat pipe based PV/T (Chandrasekar et al., 2013). ⇑ Corresponding author. Tel.: +1 989 513 7470.
E-mail addresses: [email protected] (M.A. Al-Nimr), islam1993199 [email protected], [email protected] (M.-E. Dahdolan). 1 Tel.: +962 2 7201000x22546. http://dx.doi.org/10.1016/j.solener.2015.08.006 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
PV cell cooling has been studied by many engineers throughout. Krauter has studied the effect of a thin water layer gliding on panels (Krauter, 2004). Saad and Masud have studied the improvement of PV cells efficiency by proposing water trickling cooling of the surface of the panel (Saad and Masud, 2009). Abdolzadeh and Ameri have investigated improving the performance of a photovoltaic water pumping system by spraying water over the photovoltaic cells (Abdolzadeh and Ameri, 2009). RosaClot et al. have studied the behavior of a PV cell submerged in water (Rosa-Clot et al., 2010). Royne and Dey have designed a cooling device for densely packed PV cells under high concentration based on jet impingement (Royne and Dey, 2007). Wilson have presented results of a gravityfed cooling technique applied to a wet crystalline silicon photovoltaic module under Jamaican conditions (Wilson, 2009).
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Nomenclature Ag As Ap,i D E_ gen E_ in E_ out g G hc,out heV hin hr,in hc,in ifg,w k keff L Lc m_ Nu P pc
surface area of the glass tube (m2) surface area of the still (m2) inner surface area of the porous layer (m2) the outer diameter of the glass tube (m) energy generation rate (W) energy entering the system (W) energy exiting the system (W) gravitational acceleration (m/s2) solar radiation (W/m2) external convection heat transfer coefficient (W/m2 K) evaporation heat transfer coefficient (W/m2 K) internal heat transfer coefficient (W/m2 K) internal radiation heat transfer coefficient (W/m2 K) external convective heat transfer coefficient (W/m2 K) latent heat of vaporization for water (kJ/kg) thermal conductivity (W/m K) effective thermal conductivity (W/m K) Length (m) length scale in Rac (m) mass flow rate (kg/s) Nusselt number PV cell power output (W) partial pressure of the vapor near the condenser (Pa)
Many different air based PV/T systems have been studied. An air based PV/T system has been studied theoretically by Hegazy (2000), in which thermal, electrical, hydraulic and overall performances of flat plate PV/T air collectors have been investigated. Chen et al. have described in two papers the modeling, design and performance assessment of a BIPV/T system thermally coupled with a ventilated concrete slab in a two-storey low energy house (Chen et al., 2010a,b). Sarhaddi et al. have studied a flat plate PV/T air collector’s efficiencies along with the overall exergic performance (Sarhaddi et al., 2010). Shahsavar and Ameri have modeled a direct-coupled PV/T air collector which has a thin Aluminum sheet suspended in the middle of the air channel (Shahsavar and Ameri, 2010). Brideau and Collins have developed and have experimentally validated a model for a hybrid air-based PV/T collector system with impinging jets (Brideau and Collins, 2014). Vokas et al. have studied a PV/T air system on building fac¸ade, a simulation of the system has been done to improve the output, mainly by reducing the cell temperature (Vokas et al., 2014). Feng et al. have presented a novel transparent compound parabolic concentrated (PV/ T/Day Lighted) system combined with a green building design (Feng et al., 2015).
pp Pr Q_ c;in Q_ c;out Q_ eV Q_ l;in Q_ l;out Q_ r;in Q_ r;out rc,o rp,i Rac ReD Req Tc Tp Tsky T1 V a b eg g gPV t r s
partial pressure of the vapor near the porous evaporator (Pa) Prandtl number internal convection heat transfer rate (W) external convection heat transfer rate (W) evaporation heat rate (W) internal heat losses (W) external heat losses (W) internal radiation heat transfer rate (W) external radiation heat transfer rate (W) outer radius of the condenser (m) inner radius of the porous evaporator (m) Rayleigh number Reynolds number equivalent thermal resistance (K/W) condenser temperature (K) porous evaporator temperature (K) sky temperature (K) ambient temperature (K) wind speed (m/s) thermal diffusivity (m2/s) volumetric thermal expansion coefficient (K1) emissivity system efficiency (%) PV cell efficiency (%) kinematic viscosity (m2/s) Stephan–Boltzman constant (W/m2 K4) transmissivity
Heat pipe based PV/T systems are also to be mentioned. Gang et al. have proposed a novel heat pipe PV/T system which can operate in cold regions without being frozen, the system was studied under different parametric conditions (Gang et al., 2012). Wu et al. have proposed a heat pipe PV/T hybrid system which uses a wick heat pipe to remove excessive heat from the PV cell (Wu et al., 2011). . Moradgholi et al. have proposed and studied a heat pipe based PV/T system which used a thermosyphon type heat pipe in spring and summer conditions (Moradgholi et al., 2014). Esen has fabricated a solar cooker integrated vacuum tube collector with refrigerant based heat pipes. Experimental data were obtained and presented in this study (Esen, 2004). As for water based PV/T system, Tiwari and Sodha have developed thermal model for a water based PV/T system, simulations predicted an efficiency of 58% while experiments carried by Huang et al. have shown an efficiency of 61.3% (Tiwari and Sodha, 2006b) (Huang et al., 2001). Tiwari and Sodha also have developed a thermal model for an integrated PV/T water/air heating system, thermal efficiencies for winter and summer have been obtained (Tiwari and Sodha, 2006a). Ji et al. have designed and constructed a flat-box aluminum-alloy photovoltaic and water-
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heating system designed for natural circulation, the system was able to generate electricity and hot water simultaneously (Ji et al., 2007). Zondag et al. have carried out a research in which nine designs have been evaluated, according to the study, the best efficiency has been obtained from the channel-below-transparent-PV design, but it has been suggested that the sheet-and-tube PV/T design which provides an efficiency just worse by 2% was a good design for its easier manufacturing process (Zondag et al., 2003). Another application for thermal energy supplied by PV/ T systems is water distillation. Water stills are usually combined with PV cells to make still PV/T systems. Heat removed from PV cells is utilized to evaporate water. Boubekri et al. have studied the productivity of a solar active still with a single basin liner and a single slope fitted with two reflectors coupled with a PV/T solar water heater system, the study has been also about enhancing the productivity by using a storage tank either thermal or PV/T (Boubekri et al., 2013). Dev and Tiwari have used two analytical methods to establish a characteristic equation of a hybrid PV/T active solar still based on annual experimental observations, the still is a combination of solar still and flat plate collector integrated with glass–glass photovoltaic module (Dev and Tiwari, 2010). Kumar and Tiwari have studied and compared two stills experimentally, the first is a single slope passive still and the other is a single slope PV/T active still, they have shown that the hybrid still provided higher electrical and thermal efficiency about 20% higher than the passive still (Kumar and Tiwari, 2010). Calise et al. have presented a novel solar trigeneration system provided with a PV/T water collector system and MED sea water distillation system, which has been simulated and has been economically assessed (Calise et al., 2014). Kroiß et al. have studied a seawater-proof PV/T solar collector system designed with RO distillation feature (Kroiß et al., 2014). Kumar et al. have studied a basin type hybrid PV/T active solar still. They have developed an empirical relation glass cover temperatures for known water and ambient temperatures (Kumar et al., 2010). Gaur and Tiwari have studied a hybrid active PV/T solar still which utilizes solar collectors connected in series integrated with the still basin. An optimization study to calculate the number of collectors for different heat capacity of the water has been carried out (Gaur and Tiwari, 2010). Saeedi et al. have carried out an optimization study for a PV/T active solar still. Analytical expressions have been developed to find the different components’ temperatures. Also, the PV cell electrical output was calculated, and the objective function was the overall efficiency (Saeedi et al., 2015). Finally, a concentrated solar still enhanced with a porous evaporator and an internal condenser has been studied by the authors (Al-Nimr and Dahdolan, 2015). The results have shown the performance curves of the still under steady-state conditions.
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In this paper, a novel concentrated PV/T water distillation system is presented. The system is enhanced with a porous evaporator and an internal condenser. The set system is also designed for natural passive water circulation. A mathematical model is put under steady-state conditions, and different parameters affecting the system are studied. The objectives of this paper are to present a new concept for a solar PV/T distillation system, and to study the proposed PV/T distillation system to conclude its behaviors associated with different conditions. 2. System overview 2.1. Components and mechanism The proposed system is similar to the novel solar still proposed by the same authors in reference (Al-Nimr and Dahdolan, 2015) with a major addition to the system: PV cells. According to Fig. 1, the proposed system consists of a parabolic concentrator, it concentrates solar radiation to the main PV/T component. A condenser is attached to the tubular system concentrically. The condenser is inclined by a small angle to help in collecting distilled water. The tank has two tasks: (1) to feed the porous evaporator with saline water. (2) To feed the condenser with cooling water. According to Fig. 2 the tubular part (PV/T) consists of: a glass layer (A). A porous evaporator (can be made from dark painted sponge or hay) (B). A condenser (E) filled with cooling water. This system is considered to be novel for the following reasons: (1) the concentric tubular shape. (2) The combination of solar concentration, porous evaporation, internal condensation, and PV evaporative cooling. (3) The use of thermosyphonic circulation. (4) Direct utilization of rejected heat for water distillation. Utilization of thermosyphon in solar heating have been studied by Esen and Esen. They have performed experi-
Fig. 1. System general overview showing the parabolic concentrator, the main tubular part the finned tan, and the condenser.
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Fig. 4. PV/T system simulation inputs. Fig. 2. The main tubular part. (A) glass layer, (B) PV cell, (C) Porous evaporator, (D) space filled with air vapor mixture, (E) condenser.
ments to find the thermal performance of a two-phase solar collector with different refrigerants (Esen and Esen, 2005). The mechanism of the proposed PV/T system is described as follows: concentrated solar radiation enters the PV cell (B) through the glass layer (A), electricity is generated and heat is rejected by the cell. Porous evaporator (C) becomes hot by the heat rejected from the PV cell causing water to evaporate. Water vapor is transferred to the condenser (E) through the space between the evaporator and the condenser (D) due to partial pressure difference. Vapor is condensed on the surface of the condenser then collected at the side of the still due to a really small inclination, which can make water droplets slide to that side. While cooling water circulation is carried out by thermosyphonic effect, where cold water enters the still, starts being heated up, then moves towards the cold tank while cooler water steps into its place (see Figs. 3 and 4). The condenser is fed by circulating salty water from the finned tank by the thermosyphon effect. Cooling water enters the condenser from the far side and absorbs latent heat of condensation from water vapor, acting like a counter flow heat exchanger, causing the cooling water to be heated and circulated.
2.2. Advantages The following advantages distinguish the proposed still: The system provides distilled water as well as electric power from the PV cell. Efficiency of the PV cell is prevented from decreasing since the cell is being cooled by water evaporation. Cooling water circulation inside the condenser is caused by the thermosyphon effect, hence, it can be considered as a stand-alone PV/T system or a passive PV/T system. Cooling water inside the condenser is fed from the same water tank. The porous evaporator has low total thermal capacity (less mass and lower specific heat capacity), this means lower thermal energy for higher temperature difference, and faster transient response of the evaporator. The absorbed latent heat of condensation is collected by water inside the condenser and can be utilized for other domestic applications, or can be directly supplied to the evaporator.
3. Modeling and simulation 3.1. Mathematical model A thermal model is based on simplicity is construct. Such a simple approach helps concluding the behavior of the system for the mentioned effects (which include: wind speed, ambient temperature, solar irradiance, and condenser temperature) easier and helps explaining the resulted trends more comprehensively. The following assumptions are considered while constructing the model:
Fig. 3. Description of the PV/T system mechanism.
The model is running under steady-state conditions. The glass layer and the cell have no thermal conductive resistivity (justified by the small thickness). The evaporator temperature and the glass temperature are the same due to perfect thermal contact. Properties of the air-vapor mixture in are taken as that of air at bulk temperature.
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All evaporated water is eventually collected as water distillate. Parallel evaporation and condensation, due to the concentric shape of the system. Under the above assumptions, applying the energy balance equation on the evaporator is: E_ in E_ out þ E_ gen ¼ 0
ð1Þ
here: E_ in ¼ ð1 gPV ÞasAs G
ð2Þ
E_ out ¼ Q_ l;in þ Q_ l;out
ð3Þ
ReD ¼
597
VD t
ð18Þ
Eqs. (11)–(18) help in reducing the external heat losses as a function of (Tp). To find the evaporation rate in terms of (Tp): Q_ eV ¼ heV Ap;i ðT p T c Þ
ð19Þ
where heV is given by Dunkle’s model found in reference Ahsan et al. (2013): heV ¼ ð16:27 103 Þhin
Pp Pc Tp Tc
ð20Þ
Q_ l;in Ap;i ðT p T c Þ
The internal heat losses (Q_ l;in ) are divided into radiation and convection internal losses:
hin ¼
ðT p T c Þ Q_ l;in ¼ Q_ r;in þ Q_ c;in ¼ Req
ð4Þ
P p ¼ ð0:14862ÞT p ð0:36526 102 ÞT 2p þ ð0:11242
Q_ r;in ¼ hr;in Ap;in ðT p T c Þ
ð5Þ
hr;in ¼ eg rðT p þ T c ÞðT 2p þ T 2c Þ
ð6Þ
The internal convection losses (Q_ c;in ) are given as: (Incropera et al., n.d.) 2pLk eff ðT p T c Þ Q_ c;in ¼ rp;i ln rc;o
ð7Þ
14 k eff Pr 1 ¼ 0:386 Ra4c 0:861 þ Pr k
ð8Þ
gbðT p T c ÞL3c at 43 rp;i 2 ln rc;o Lc ¼ 5 3 3 3 rp;i5 þ rc;o5 Rac ¼
ð9Þ
ð22Þ
P c ¼ ð0:14862ÞT c ð0:36526 102 ÞT 2c þ ð0:11242 103 ÞT 3c
ð23Þ
(Pp) and (Pc) in Eqs. (22) and (23) are the vapor pressures of water at the evaporator and condenser temperatures (in °C) respectively. After rewriting Eq. (1) as one function of the evaporator temperature, the evaporator temperature can be found, accordingly, outputs of the system can be calculated. To find the distillation rate: m_ ¼
Q_ eV ifg;w
ð24Þ
Table 1 Simulation inputs for the proposed still.
Q_ l;out ¼ Q_ r;out þ Q_ c;out
ð11Þ
Q_ r;out ¼ hr;out Ag ðT p T sky Þ
ð12Þ
hr;out ¼ eg rðT p þ T sky ÞðT 2p þ T 2sky Þ
ð13Þ
Q_ c;out ¼ hc;out Ag ðT p T 1 Þ
ð14Þ
NuD k
103 ÞT 3p
ð10Þ
Eqs. (4)–(10) help in rewriting internal heat as a function of the evaporator temperature (Tp). The external heat losses are given as: (Incropera et al., n.d.)
hc;out ¼
ð21Þ
ð15Þ
Parameter
Value
Outer diameter of the glass layer (D) Porous evaporator thickness (t) Outer diameter of the condenser (Dc,o) Perpendicular area of the concentrator (A) Length of the system (L) Wind speed (V) Solar intensity (G) Ambient temperature (T1) Condenser temperature (Tc) less than ambient temperature by: Absorptivity (a) Transitivity (s) Emissivity (e)
48.3 mm 2.5 mm 21.3 mm 1 m2 1m (2–20) m/s (200–1000) (W/m2) (20–25) °C (0–4) °C 0.9 0.79 0.9
Sky temperature is given by Akhtar and Mullick (2007)): T sky ¼ 0:0552 T 1:5 1 where Nu is given as (Incropera et al., n.d.): 1 58 !45 1 0:62Re2D Pr3 ReD Nu ¼ 0:3 þ 1þ 0:423 14 282; 000 1 þ Pr
ð16Þ Table 2 PV cell data used in the simulation.
ð17Þ
gT,ref bref Tref
14% 0.004 25 °C
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To find the PV cell efficiency for a certain cell temperature, Evans–Florschuetz PV efficiency correlation (Dubey et al., 2013):
practical solar still ranges (30–40%) and (4–5) litres/day (Kabeel and El-Agouz, 2011). This validation is similar to the one used in reference Al-Nimr and Dahdolan (2015).
gPV ¼ gT ;ref ð1 bref ðT c T ref ÞÞ
ð25Þ
3.2. Simulation parameters
ð26Þ
Simulation inputs are listed in Tables 1 and 2, and Fig. 5:
To find the output power of the cell: P ¼ gPV ðasAs GÞ Finally, to find the overall efficiency of the system: 4. Results and discussion
Q_ eV þ P asAs G
ð27Þ
To validate this model, a simple approach is used. After removing the PV cell from the simulation and changing the cell parameters to act like the conventional solar distillation systems (no concentration, condenser temperature slightly less than ambient), sample runs have been made, and then compared to experimental results from literature. Comparison have shown efficiencies and outputs within the
MS Excel’s Goal-Seek data tool have been used to find the porous evaporator’s temperature in Eq. (1). Then the outputs of the PV/T still are found. 4.1. Wind effect on the PV/T system Fig. 5 shows the relationship between the PV/T system performance curves with the variation of wind speed at
40
53.2
35
52.8 20-C
30
19-C 25
18-C 17-C
20
Power Output (W)
System Efficiency (%)
53 52.6 52.4
25-C
52.2
24-C
52
23-C
51.8
22-C
51.6
16-C
21-C
51.4 51.2
15 0
5
10
15
20
0
25
5
10
15
20
25
Wind Speed (m/s)
Wind Speed (m/s)
(d)
(a) 2.3
40
25-C
30
24-C 25
23-C 22-C
20
Disllaon Rate (kg/day)
System Efficiency (%)
2.1 35
21-C
1.9 1.7
20-C
1.5
19-C
1.3
18-C
1.1
17-C
0.9
16-C
0.7 0.5
15 0
5
10
15
20
0
25
5
10
15
Wind Speed (m/s)
Wind Speed (m/s)
(b)
(e)
54.2
20
25
3
54 Power Output (W)
53.8 53.6 53.4
20-C
53.2
19-C
53
18-C
52.8
17-C
52.6
16-C
52.4 52.2 0
5
10
15
Wind Speed (m/s)
(c)
20
25
Disllaon Rate (kg/day)
g¼
2.5 25-C
2
24-C 1.5
23-C 22-C
1
21-C
0.5 0
5
10
15
20
25
Wind Speed (m/s)
(f)
Fig. 5. PV/T system performance for 20 °C and 25 °C ambient temperature and 500 W/m2 solar intensity with the variation of wind speed.
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System Efficiency (%)
40 35 30 25
Poly-Si/0.14/0.004/25-C
20
a-Si/0.07/0.011/25-C
15 10 0
5
10
15
20
25
Wind Speed (m/s)
(a) 60
Power Output (W)
50 40 30
Poly-Si/0.14/0.004/25-C
20
a-Si/0.07/0.011/25-C
10 0 0
5
10
15
20
25
Wind Speed (m/s)
(b)
Disllate Rate (kg/day)
3 2.5 2 Poly-Si/0.14/0.004/25-C 1.5 a-Si/0.07/0.011/25-C 1 0.5 0
5
10
15
20
25
Wind Speed (m/s)
(c) Fig. 6. PV/T performance for different PV cell types with the variation of wind speed at 25 °C ambient temperature, 500 W/m2 solar intensity.
ambient temperature of 20 °C and 25 °C, and solar intensity of 500 W/m2. Curves (a) and (b) show the system efficiency dependence on wind speed for ambient temperatures of 20 °C and 25 °C, respectively. It is noticed that the system efficiency decreases as wind speed increases; this can be explained by the increase of heat transfer rate with the external environment. It is also noticed that the system efficiency increases as the condenser temperature decreases; this can be explained by the increase of heat transfer from the evaporator to the condenser due to higher temperature difference, hence, the increase in evaporation rate. As for the difference between these two figures, it is noticed that
599
the increase in ambient temperature increases the overall efficiency, this is explained by the decrease in external heat losses, hence, the increase in the evaporator temperature and the increase in evaporation rate, which is more significant than the decrease on the PV cell power output due to its temperature increase. Curves (c) and (d) show the dependence of the system’s electric power output from the PV cell on wind speed for ambient temperatures of 20 °C and 25 °C, respectively. It is noticed that the power output increases as wind speed increases; this can be explained by the decrease in cell temperature due to higher heat transfer rate with the external environment, since cell efficiency is inversely dependent on its temperature according to Evans–Florschuetz efficiency correlation. It is also noticed that the cell power output increases as the condenser temperature decreases; this can be explained by the decrease in cell temperature associated with the increase of heat transfer and evaporation from the evaporator towards the condenser due to higher temperature difference. As for the difference between the two figures, it is noticed that the power output decreases as the ambient temperature increases, this is explained by the increase in the PV cell temperature increase. Curves (e) and (f) show the system’s distillation rate dependence on wind speed for ambient temperatures of 20 °C and 25 °C, respectively. It is noticed that distillation rate decreases as wind speed increases. This can be explained the increase in external heat transfer with the environment. This decrease had higher impact than the increase of power output -found in Curves (c) and (d)- on the system’s overall efficiency. This can be explained by the low efficiency of the PV cells in the system and the higher solar energy input utilized for evaporation than for power generation. Finally it is noticed that the distillation rate increases as the condenser temperature decreases; again, this can be explained by the increase in internal heat transfer associated with higher temperature difference. For the differences between the two, it is noticed that the distillate output increases as the ambient temperature increases, this is due to the decrease in external heat losses, hence the increase in evaporator temperature and evaporation rate. In curves (a) and (b), it is noticed that the system efficiency increases with the increase of ambient temperature; this can be explained by the decrease in external heat transfer rate with the environment due to lower temperature difference between the still and the ambient. In curves (c) and (d), it is noticed that the electric power output of the cell decreases with the increase of ambient temperature. This can be explained by the increase of the cell temperature associated with the increase in ambient temperature, and hence, the decrease of PV cell efficiency according to Evans–Florschuetz efficiency correlation. In curves (e) and (f), it is noticed that the distillation rate of the system increases with the increase of the ambient temperature. This can be explained by the increase in evaporation associated with internal heat transfer due to higher evaporator temperature.
600
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31
100
27 25
20-C
23
19-C
21
18-C
19
17-C
Power Output (W)
System Efficiency (%)
29
25-C 24-C
60
23-C
40
22-C
20
16-C
17
80
21-C
0
15 0
200
400
600
800
1000
0
1200
200
400
1000
1200
3
32 30 25-C
28
24-C 26
23-C
24
22-C
22
21-C
Disllaon Rate (kg/day)
34
System Efficiency (%)
800
(d)
(a) 2.5 2
20-C 19-C
1.5
18-C
1
17-C
0.5
16-C
0
20 0
200
400
600
800
1000
0
1200
200
400
600
800
1000
1200
Solar Radiaon (W/m2)
Solar Radiaon (W/m2)
(e)
(b) 3.5
100 80
20-C 19-C
60
18-C
40
17-C
20
16-C
Disllaon Rate (kg/day)
120
Power Output (W)
600
Solar Radiaon (W/m2)
Solar Radiaon (W/m2)
3 2.5 25-C
2
24-C 1.5
23-C
1
22-C
0.5
21-C
0
0 0
200
400
600
800
1000
0
1200
200
400
600
800
Solar Radiaon (W/m2)
Solar Radiaon (W/m2)
(c)
(f)
1000
1200
Fig. 7. PV/T system performance for 20 °C and 25 °C ambient temperature and wind speed of 10 m/s with the variation of solar intensity.
Fig. 6 is similar to Fig. 5, however, it shows the behavior of the system using two different PV cells: (1) Poly-Si (gT,ref = 14%, bref = 0.004, Tref = 25 °C). (2) a-Si (gT,ref = 7%, bref = 0.011, Tref = 25 °C).
Evaporator Temperature (K)
314 312 310 308 306
4.2. Solar intensity effect on the PV/T system
304 302 300 0
200
400
600
800
1000
1200
Solar Intensity (W/m2)
Fig. 8. Variation of the porous evaporator temperature with solar intensity for 25 °C ambient temperature, 25 °C condenser temperature, and 10 m/s wind speed.
Fig. 7 shows the relationship between the PV/T system performance curves with the variation of solar intensity at ambient temperature of 20 °C and 25 °C, and wind speed of 10 m/s. Curves (a) and (b) show the system efficiency dependence on solar intensity for ambient temperatures of 20 ° C and 25 °C, respectively. It is noticed that the system efficiency increases for most of the solar intensity domain.
M.A. Al-Nimr, M.-E. Dahdolan / Solar Energy 120 (2015) 593–602
This can be explained by the increase in the evaporator temperature, hence, the increase in evaporation. It also can be explained by the increase in electric power out. As for the decrease in the lower portion of the domain, this can be explained by the decrease in heat transfer from the ambient to the still due to the increase of its temperature with the increase of solar intensity. Curve (c) and (d) show the system’s power output dependence with solar intensity. This curve is familiar since it is known that PV cell output is directly proportional with solar intensity by the efficiency (see Eq. (26)). Curves (e) and (f) show the dependence of the system’s distillation rate on solar intensity. It is noticed that distillation rate increases with the increase of solar intensity. This 27
System Efficiency (%)
25 23 Poly-Si/0.14/0.004/25-C
601
can be explained with the increase of evaporation associated with the increase in evaporator temperature. The ambient temperature effect is similar to its effect described previously on the wind effect. As the ambient temperature increases, the distillation output increases due to the decrease in external losses. The PV power output will also decrease due to the increase in cell temperature. And the overall efficiency will increase because of the more significant positive effect on the distillation process. Fig. 9 shows the dependence of the evaporator temperature on solar intensity. It is noticed that the temperature increases as the solar intensity increases. This is explained by the increase of thermal energy absorption associated with the increase of solar power. Fig. 9 is similar to Figs. 8 and 9, however, it shows the behavior of the system using two different PV cells: (1) Poly-Si (gT,ref = 14%, bref = 0.004, Tref = 25 °C). (2) a-Si (gT,ref = 7%, bref = 0.011, Tref = 25 °C). 4.3. Comparison
21 a-Si/0.07/0.011/25-C
19 17 15 0
500
1000
Solar Radiaon (W/m2)
(a) 100 90 Power Output (W)
80 70 60 Poly-Si/0.14/0.004/25-C
50 40
a-Si/0.07/0.011/25-C
30 20 10 0 0
500
1000
Solar Radiaon (W/m2)
(b) 3.5 3 Power Output (W)
Comparing results of the proposed PV/T system simulation with the most similar system (solar still found in reference Al-Nimr and Dahdolan (2015)), it is noticed that the behavior is very similar in general, this similarity is explained by the linear trend of the PV efficiency with the cell temperature. However, the simulation has shown an increase in the overall efficiency of the system is noticed after integrating the solar still with a PV cell. This increase is explained by using two thermal systems (solar still and PV cell) as one cascaded system, utilizing waste heat from the cell while increasing its photovoltaic efficiency.
2.5 2
Poly-Si/0.14/0.004/25-C
1.5 a-Si/0.07/0.011/25-C
1
5. Summary This paper presented a novel PV/T distillation system enhanced with a porous evaporator and an internal condenser. This proposed system has the main advantages: (1) it can operate as a passive system. (2) It provides both electric power and distilled water. (3) It has relatively faster response. A detailed overview has been provided, and a full steady-state mathematical model has been presented and simulated. The simulation has shown the effect of various parameters on the proposed system: solar intensity, wind speed, ambient temperature, condenser temperature, and PV cell used in the system. This simulation produced performance curves of the system giving a clear view on how it’s affected by the mentioned parameters. References
0.5 0 0
500
1000
Solar Radiaon (W/m2)
(c) Fig. 9. PV/T performance for different PV cell types with the variation of solar intensity at 25 °C ambient temperature, and 10 m/s wind speed.
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ScienceDirect Solar Energy 120 (2015) 593–602 www.elsevier.com/locate/solener
Modeling of a novel concentrated PV/T distillation system enhanced with a porous evaporator and an internal condenser Moh’d A. Al-Nimr a,1, Moh’d-Eslam Dahdolan b,⇑ a
Mechanical Engineering Department, Jordan University of Science and Technology, P.O. Box: 3030, Irbid 22110, Jordan b School of Engineering and Technology, Central Michigan University, Mount Pleasant, 48859 MI, USA Received 13 February 2015; received in revised form 30 July 2015; accepted 1 August 2015 Available online 5 September 2015 Communicated by: Associate Editor G.N. Tiwari
Abstract This paper presents a new concentrated PV/T system which utilizes thermal energy rejected by the PV cell to distill salty water. This PV/T system is enhanced with a porous evaporator and an internal condenser. This PV/T provides electric power and distilled water, and can operate as a passive system since circulation is driven by the thermosyphon effect. A simple steady-state mathematical model has been put to describe the performance of the system, and has been simulated using Microsoft Excel’s ‘‘Goal-Seek” data tool to find the evaporator temperature. Performance curves have been provided showing the effect of wind speed, solar intensity, ambient temperature, condenser temperature, and PV cell type on the performance of the system. (Overall efficiency, Electric power output, and Distillation rate.) Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: PV/T distillation system; Concentrated; Porous evaporator; Internal condenser
1. Introduction One of the main challenges to overcome while using PV cells is the effect of high temperature. This temperature rise affects the performance of the cells and decreases its efficiency. Controlling PV cells’ temperature can be carried out by different cooling methods. Cooling the cells by utilizing its generated heat introduced the idea of PV/T energy systems. PV/T systems are classified according to the method into air based, water based, refrigerant based PV/T systems and heat pipe based PV/T (Chandrasekar et al., 2013). ⇑ Corresponding author. Tel.: +1 989 513 7470.
E-mail addresses: [email protected] (M.A. Al-Nimr), islam1993199 [email protected], [email protected] (M.-E. Dahdolan). 1 Tel.: +962 2 7201000x22546. http://dx.doi.org/10.1016/j.solener.2015.08.006 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
PV cell cooling has been studied by many engineers throughout. Krauter has studied the effect of a thin water layer gliding on panels (Krauter, 2004). Saad and Masud have studied the improvement of PV cells efficiency by proposing water trickling cooling of the surface of the panel (Saad and Masud, 2009). Abdolzadeh and Ameri have investigated improving the performance of a photovoltaic water pumping system by spraying water over the photovoltaic cells (Abdolzadeh and Ameri, 2009). RosaClot et al. have studied the behavior of a PV cell submerged in water (Rosa-Clot et al., 2010). Royne and Dey have designed a cooling device for densely packed PV cells under high concentration based on jet impingement (Royne and Dey, 2007). Wilson have presented results of a gravityfed cooling technique applied to a wet crystalline silicon photovoltaic module under Jamaican conditions (Wilson, 2009).
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Nomenclature Ag As Ap,i D E_ gen E_ in E_ out g G hc,out heV hin hr,in hc,in ifg,w k keff L Lc m_ Nu P pc
surface area of the glass tube (m2) surface area of the still (m2) inner surface area of the porous layer (m2) the outer diameter of the glass tube (m) energy generation rate (W) energy entering the system (W) energy exiting the system (W) gravitational acceleration (m/s2) solar radiation (W/m2) external convection heat transfer coefficient (W/m2 K) evaporation heat transfer coefficient (W/m2 K) internal heat transfer coefficient (W/m2 K) internal radiation heat transfer coefficient (W/m2 K) external convective heat transfer coefficient (W/m2 K) latent heat of vaporization for water (kJ/kg) thermal conductivity (W/m K) effective thermal conductivity (W/m K) Length (m) length scale in Rac (m) mass flow rate (kg/s) Nusselt number PV cell power output (W) partial pressure of the vapor near the condenser (Pa)
Many different air based PV/T systems have been studied. An air based PV/T system has been studied theoretically by Hegazy (2000), in which thermal, electrical, hydraulic and overall performances of flat plate PV/T air collectors have been investigated. Chen et al. have described in two papers the modeling, design and performance assessment of a BIPV/T system thermally coupled with a ventilated concrete slab in a two-storey low energy house (Chen et al., 2010a,b). Sarhaddi et al. have studied a flat plate PV/T air collector’s efficiencies along with the overall exergic performance (Sarhaddi et al., 2010). Shahsavar and Ameri have modeled a direct-coupled PV/T air collector which has a thin Aluminum sheet suspended in the middle of the air channel (Shahsavar and Ameri, 2010). Brideau and Collins have developed and have experimentally validated a model for a hybrid air-based PV/T collector system with impinging jets (Brideau and Collins, 2014). Vokas et al. have studied a PV/T air system on building fac¸ade, a simulation of the system has been done to improve the output, mainly by reducing the cell temperature (Vokas et al., 2014). Feng et al. have presented a novel transparent compound parabolic concentrated (PV/ T/Day Lighted) system combined with a green building design (Feng et al., 2015).
pp Pr Q_ c;in Q_ c;out Q_ eV Q_ l;in Q_ l;out Q_ r;in Q_ r;out rc,o rp,i Rac ReD Req Tc Tp Tsky T1 V a b eg g gPV t r s
partial pressure of the vapor near the porous evaporator (Pa) Prandtl number internal convection heat transfer rate (W) external convection heat transfer rate (W) evaporation heat rate (W) internal heat losses (W) external heat losses (W) internal radiation heat transfer rate (W) external radiation heat transfer rate (W) outer radius of the condenser (m) inner radius of the porous evaporator (m) Rayleigh number Reynolds number equivalent thermal resistance (K/W) condenser temperature (K) porous evaporator temperature (K) sky temperature (K) ambient temperature (K) wind speed (m/s) thermal diffusivity (m2/s) volumetric thermal expansion coefficient (K1) emissivity system efficiency (%) PV cell efficiency (%) kinematic viscosity (m2/s) Stephan–Boltzman constant (W/m2 K4) transmissivity
Heat pipe based PV/T systems are also to be mentioned. Gang et al. have proposed a novel heat pipe PV/T system which can operate in cold regions without being frozen, the system was studied under different parametric conditions (Gang et al., 2012). Wu et al. have proposed a heat pipe PV/T hybrid system which uses a wick heat pipe to remove excessive heat from the PV cell (Wu et al., 2011). . Moradgholi et al. have proposed and studied a heat pipe based PV/T system which used a thermosyphon type heat pipe in spring and summer conditions (Moradgholi et al., 2014). Esen has fabricated a solar cooker integrated vacuum tube collector with refrigerant based heat pipes. Experimental data were obtained and presented in this study (Esen, 2004). As for water based PV/T system, Tiwari and Sodha have developed thermal model for a water based PV/T system, simulations predicted an efficiency of 58% while experiments carried by Huang et al. have shown an efficiency of 61.3% (Tiwari and Sodha, 2006b) (Huang et al., 2001). Tiwari and Sodha also have developed a thermal model for an integrated PV/T water/air heating system, thermal efficiencies for winter and summer have been obtained (Tiwari and Sodha, 2006a). Ji et al. have designed and constructed a flat-box aluminum-alloy photovoltaic and water-
M.A. Al-Nimr, M.-E. Dahdolan / Solar Energy 120 (2015) 593–602
heating system designed for natural circulation, the system was able to generate electricity and hot water simultaneously (Ji et al., 2007). Zondag et al. have carried out a research in which nine designs have been evaluated, according to the study, the best efficiency has been obtained from the channel-below-transparent-PV design, but it has been suggested that the sheet-and-tube PV/T design which provides an efficiency just worse by 2% was a good design for its easier manufacturing process (Zondag et al., 2003). Another application for thermal energy supplied by PV/ T systems is water distillation. Water stills are usually combined with PV cells to make still PV/T systems. Heat removed from PV cells is utilized to evaporate water. Boubekri et al. have studied the productivity of a solar active still with a single basin liner and a single slope fitted with two reflectors coupled with a PV/T solar water heater system, the study has been also about enhancing the productivity by using a storage tank either thermal or PV/T (Boubekri et al., 2013). Dev and Tiwari have used two analytical methods to establish a characteristic equation of a hybrid PV/T active solar still based on annual experimental observations, the still is a combination of solar still and flat plate collector integrated with glass–glass photovoltaic module (Dev and Tiwari, 2010). Kumar and Tiwari have studied and compared two stills experimentally, the first is a single slope passive still and the other is a single slope PV/T active still, they have shown that the hybrid still provided higher electrical and thermal efficiency about 20% higher than the passive still (Kumar and Tiwari, 2010). Calise et al. have presented a novel solar trigeneration system provided with a PV/T water collector system and MED sea water distillation system, which has been simulated and has been economically assessed (Calise et al., 2014). Kroiß et al. have studied a seawater-proof PV/T solar collector system designed with RO distillation feature (Kroiß et al., 2014). Kumar et al. have studied a basin type hybrid PV/T active solar still. They have developed an empirical relation glass cover temperatures for known water and ambient temperatures (Kumar et al., 2010). Gaur and Tiwari have studied a hybrid active PV/T solar still which utilizes solar collectors connected in series integrated with the still basin. An optimization study to calculate the number of collectors for different heat capacity of the water has been carried out (Gaur and Tiwari, 2010). Saeedi et al. have carried out an optimization study for a PV/T active solar still. Analytical expressions have been developed to find the different components’ temperatures. Also, the PV cell electrical output was calculated, and the objective function was the overall efficiency (Saeedi et al., 2015). Finally, a concentrated solar still enhanced with a porous evaporator and an internal condenser has been studied by the authors (Al-Nimr and Dahdolan, 2015). The results have shown the performance curves of the still under steady-state conditions.
595
In this paper, a novel concentrated PV/T water distillation system is presented. The system is enhanced with a porous evaporator and an internal condenser. The set system is also designed for natural passive water circulation. A mathematical model is put under steady-state conditions, and different parameters affecting the system are studied. The objectives of this paper are to present a new concept for a solar PV/T distillation system, and to study the proposed PV/T distillation system to conclude its behaviors associated with different conditions. 2. System overview 2.1. Components and mechanism The proposed system is similar to the novel solar still proposed by the same authors in reference (Al-Nimr and Dahdolan, 2015) with a major addition to the system: PV cells. According to Fig. 1, the proposed system consists of a parabolic concentrator, it concentrates solar radiation to the main PV/T component. A condenser is attached to the tubular system concentrically. The condenser is inclined by a small angle to help in collecting distilled water. The tank has two tasks: (1) to feed the porous evaporator with saline water. (2) To feed the condenser with cooling water. According to Fig. 2 the tubular part (PV/T) consists of: a glass layer (A). A porous evaporator (can be made from dark painted sponge or hay) (B). A condenser (E) filled with cooling water. This system is considered to be novel for the following reasons: (1) the concentric tubular shape. (2) The combination of solar concentration, porous evaporation, internal condensation, and PV evaporative cooling. (3) The use of thermosyphonic circulation. (4) Direct utilization of rejected heat for water distillation. Utilization of thermosyphon in solar heating have been studied by Esen and Esen. They have performed experi-
Fig. 1. System general overview showing the parabolic concentrator, the main tubular part the finned tan, and the condenser.
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Fig. 4. PV/T system simulation inputs. Fig. 2. The main tubular part. (A) glass layer, (B) PV cell, (C) Porous evaporator, (D) space filled with air vapor mixture, (E) condenser.
ments to find the thermal performance of a two-phase solar collector with different refrigerants (Esen and Esen, 2005). The mechanism of the proposed PV/T system is described as follows: concentrated solar radiation enters the PV cell (B) through the glass layer (A), electricity is generated and heat is rejected by the cell. Porous evaporator (C) becomes hot by the heat rejected from the PV cell causing water to evaporate. Water vapor is transferred to the condenser (E) through the space between the evaporator and the condenser (D) due to partial pressure difference. Vapor is condensed on the surface of the condenser then collected at the side of the still due to a really small inclination, which can make water droplets slide to that side. While cooling water circulation is carried out by thermosyphonic effect, where cold water enters the still, starts being heated up, then moves towards the cold tank while cooler water steps into its place (see Figs. 3 and 4). The condenser is fed by circulating salty water from the finned tank by the thermosyphon effect. Cooling water enters the condenser from the far side and absorbs latent heat of condensation from water vapor, acting like a counter flow heat exchanger, causing the cooling water to be heated and circulated.
2.2. Advantages The following advantages distinguish the proposed still: The system provides distilled water as well as electric power from the PV cell. Efficiency of the PV cell is prevented from decreasing since the cell is being cooled by water evaporation. Cooling water circulation inside the condenser is caused by the thermosyphon effect, hence, it can be considered as a stand-alone PV/T system or a passive PV/T system. Cooling water inside the condenser is fed from the same water tank. The porous evaporator has low total thermal capacity (less mass and lower specific heat capacity), this means lower thermal energy for higher temperature difference, and faster transient response of the evaporator. The absorbed latent heat of condensation is collected by water inside the condenser and can be utilized for other domestic applications, or can be directly supplied to the evaporator.
3. Modeling and simulation 3.1. Mathematical model A thermal model is based on simplicity is construct. Such a simple approach helps concluding the behavior of the system for the mentioned effects (which include: wind speed, ambient temperature, solar irradiance, and condenser temperature) easier and helps explaining the resulted trends more comprehensively. The following assumptions are considered while constructing the model:
Fig. 3. Description of the PV/T system mechanism.
The model is running under steady-state conditions. The glass layer and the cell have no thermal conductive resistivity (justified by the small thickness). The evaporator temperature and the glass temperature are the same due to perfect thermal contact. Properties of the air-vapor mixture in are taken as that of air at bulk temperature.
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All evaporated water is eventually collected as water distillate. Parallel evaporation and condensation, due to the concentric shape of the system. Under the above assumptions, applying the energy balance equation on the evaporator is: E_ in E_ out þ E_ gen ¼ 0
ð1Þ
here: E_ in ¼ ð1 gPV ÞasAs G
ð2Þ
E_ out ¼ Q_ l;in þ Q_ l;out
ð3Þ
ReD ¼
597
VD t
ð18Þ
Eqs. (11)–(18) help in reducing the external heat losses as a function of (Tp). To find the evaporation rate in terms of (Tp): Q_ eV ¼ heV Ap;i ðT p T c Þ
ð19Þ
where heV is given by Dunkle’s model found in reference Ahsan et al. (2013): heV ¼ ð16:27 103 Þhin
Pp Pc Tp Tc
ð20Þ
Q_ l;in Ap;i ðT p T c Þ
The internal heat losses (Q_ l;in ) are divided into radiation and convection internal losses:
hin ¼
ðT p T c Þ Q_ l;in ¼ Q_ r;in þ Q_ c;in ¼ Req
ð4Þ
P p ¼ ð0:14862ÞT p ð0:36526 102 ÞT 2p þ ð0:11242
Q_ r;in ¼ hr;in Ap;in ðT p T c Þ
ð5Þ
hr;in ¼ eg rðT p þ T c ÞðT 2p þ T 2c Þ
ð6Þ
The internal convection losses (Q_ c;in ) are given as: (Incropera et al., n.d.) 2pLk eff ðT p T c Þ Q_ c;in ¼ rp;i ln rc;o
ð7Þ
14 k eff Pr 1 ¼ 0:386 Ra4c 0:861 þ Pr k
ð8Þ
gbðT p T c ÞL3c at 43 rp;i 2 ln rc;o Lc ¼ 5 3 3 3 rp;i5 þ rc;o5 Rac ¼
ð9Þ
ð22Þ
P c ¼ ð0:14862ÞT c ð0:36526 102 ÞT 2c þ ð0:11242 103 ÞT 3c
ð23Þ
(Pp) and (Pc) in Eqs. (22) and (23) are the vapor pressures of water at the evaporator and condenser temperatures (in °C) respectively. After rewriting Eq. (1) as one function of the evaporator temperature, the evaporator temperature can be found, accordingly, outputs of the system can be calculated. To find the distillation rate: m_ ¼
Q_ eV ifg;w
ð24Þ
Table 1 Simulation inputs for the proposed still.
Q_ l;out ¼ Q_ r;out þ Q_ c;out
ð11Þ
Q_ r;out ¼ hr;out Ag ðT p T sky Þ
ð12Þ
hr;out ¼ eg rðT p þ T sky ÞðT 2p þ T 2sky Þ
ð13Þ
Q_ c;out ¼ hc;out Ag ðT p T 1 Þ
ð14Þ
NuD k
103 ÞT 3p
ð10Þ
Eqs. (4)–(10) help in rewriting internal heat as a function of the evaporator temperature (Tp). The external heat losses are given as: (Incropera et al., n.d.)
hc;out ¼
ð21Þ
ð15Þ
Parameter
Value
Outer diameter of the glass layer (D) Porous evaporator thickness (t) Outer diameter of the condenser (Dc,o) Perpendicular area of the concentrator (A) Length of the system (L) Wind speed (V) Solar intensity (G) Ambient temperature (T1) Condenser temperature (Tc) less than ambient temperature by: Absorptivity (a) Transitivity (s) Emissivity (e)
48.3 mm 2.5 mm 21.3 mm 1 m2 1m (2–20) m/s (200–1000) (W/m2) (20–25) °C (0–4) °C 0.9 0.79 0.9
Sky temperature is given by Akhtar and Mullick (2007)): T sky ¼ 0:0552 T 1:5 1 where Nu is given as (Incropera et al., n.d.): 1 58 !45 1 0:62Re2D Pr3 ReD Nu ¼ 0:3 þ 1þ 0:423 14 282; 000 1 þ Pr
ð16Þ Table 2 PV cell data used in the simulation.
ð17Þ
gT,ref bref Tref
14% 0.004 25 °C
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To find the PV cell efficiency for a certain cell temperature, Evans–Florschuetz PV efficiency correlation (Dubey et al., 2013):
practical solar still ranges (30–40%) and (4–5) litres/day (Kabeel and El-Agouz, 2011). This validation is similar to the one used in reference Al-Nimr and Dahdolan (2015).
gPV ¼ gT ;ref ð1 bref ðT c T ref ÞÞ
ð25Þ
3.2. Simulation parameters
ð26Þ
Simulation inputs are listed in Tables 1 and 2, and Fig. 5:
To find the output power of the cell: P ¼ gPV ðasAs GÞ Finally, to find the overall efficiency of the system: 4. Results and discussion
Q_ eV þ P asAs G
ð27Þ
To validate this model, a simple approach is used. After removing the PV cell from the simulation and changing the cell parameters to act like the conventional solar distillation systems (no concentration, condenser temperature slightly less than ambient), sample runs have been made, and then compared to experimental results from literature. Comparison have shown efficiencies and outputs within the
MS Excel’s Goal-Seek data tool have been used to find the porous evaporator’s temperature in Eq. (1). Then the outputs of the PV/T still are found. 4.1. Wind effect on the PV/T system Fig. 5 shows the relationship between the PV/T system performance curves with the variation of wind speed at
40
53.2
35
52.8 20-C
30
19-C 25
18-C 17-C
20
Power Output (W)
System Efficiency (%)
53 52.6 52.4
25-C
52.2
24-C
52
23-C
51.8
22-C
51.6
16-C
21-C
51.4 51.2
15 0
5
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15
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0
25
5
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15
20
25
Wind Speed (m/s)
Wind Speed (m/s)
(d)
(a) 2.3
40
25-C
30
24-C 25
23-C 22-C
20
Disllaon Rate (kg/day)
System Efficiency (%)
2.1 35
21-C
1.9 1.7
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1.5
19-C
1.3
18-C
1.1
17-C
0.9
16-C
0.7 0.5
15 0
5
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0
25
5
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15
Wind Speed (m/s)
Wind Speed (m/s)
(b)
(e)
54.2
20
25
3
54 Power Output (W)
53.8 53.6 53.4
20-C
53.2
19-C
53
18-C
52.8
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52.6
16-C
52.4 52.2 0
5
10
15
Wind Speed (m/s)
(c)
20
25
Disllaon Rate (kg/day)
g¼
2.5 25-C
2
24-C 1.5
23-C 22-C
1
21-C
0.5 0
5
10
15
20
25
Wind Speed (m/s)
(f)
Fig. 5. PV/T system performance for 20 °C and 25 °C ambient temperature and 500 W/m2 solar intensity with the variation of wind speed.
M.A. Al-Nimr, M.-E. Dahdolan / Solar Energy 120 (2015) 593–602
System Efficiency (%)
40 35 30 25
Poly-Si/0.14/0.004/25-C
20
a-Si/0.07/0.011/25-C
15 10 0
5
10
15
20
25
Wind Speed (m/s)
(a) 60
Power Output (W)
50 40 30
Poly-Si/0.14/0.004/25-C
20
a-Si/0.07/0.011/25-C
10 0 0
5
10
15
20
25
Wind Speed (m/s)
(b)
Disllate Rate (kg/day)
3 2.5 2 Poly-Si/0.14/0.004/25-C 1.5 a-Si/0.07/0.011/25-C 1 0.5 0
5
10
15
20
25
Wind Speed (m/s)
(c) Fig. 6. PV/T performance for different PV cell types with the variation of wind speed at 25 °C ambient temperature, 500 W/m2 solar intensity.
ambient temperature of 20 °C and 25 °C, and solar intensity of 500 W/m2. Curves (a) and (b) show the system efficiency dependence on wind speed for ambient temperatures of 20 °C and 25 °C, respectively. It is noticed that the system efficiency decreases as wind speed increases; this can be explained by the increase of heat transfer rate with the external environment. It is also noticed that the system efficiency increases as the condenser temperature decreases; this can be explained by the increase of heat transfer from the evaporator to the condenser due to higher temperature difference, hence, the increase in evaporation rate. As for the difference between these two figures, it is noticed that
599
the increase in ambient temperature increases the overall efficiency, this is explained by the decrease in external heat losses, hence, the increase in the evaporator temperature and the increase in evaporation rate, which is more significant than the decrease on the PV cell power output due to its temperature increase. Curves (c) and (d) show the dependence of the system’s electric power output from the PV cell on wind speed for ambient temperatures of 20 °C and 25 °C, respectively. It is noticed that the power output increases as wind speed increases; this can be explained by the decrease in cell temperature due to higher heat transfer rate with the external environment, since cell efficiency is inversely dependent on its temperature according to Evans–Florschuetz efficiency correlation. It is also noticed that the cell power output increases as the condenser temperature decreases; this can be explained by the decrease in cell temperature associated with the increase of heat transfer and evaporation from the evaporator towards the condenser due to higher temperature difference. As for the difference between the two figures, it is noticed that the power output decreases as the ambient temperature increases, this is explained by the increase in the PV cell temperature increase. Curves (e) and (f) show the system’s distillation rate dependence on wind speed for ambient temperatures of 20 °C and 25 °C, respectively. It is noticed that distillation rate decreases as wind speed increases. This can be explained the increase in external heat transfer with the environment. This decrease had higher impact than the increase of power output -found in Curves (c) and (d)- on the system’s overall efficiency. This can be explained by the low efficiency of the PV cells in the system and the higher solar energy input utilized for evaporation than for power generation. Finally it is noticed that the distillation rate increases as the condenser temperature decreases; again, this can be explained by the increase in internal heat transfer associated with higher temperature difference. For the differences between the two, it is noticed that the distillate output increases as the ambient temperature increases, this is due to the decrease in external heat losses, hence the increase in evaporator temperature and evaporation rate. In curves (a) and (b), it is noticed that the system efficiency increases with the increase of ambient temperature; this can be explained by the decrease in external heat transfer rate with the environment due to lower temperature difference between the still and the ambient. In curves (c) and (d), it is noticed that the electric power output of the cell decreases with the increase of ambient temperature. This can be explained by the increase of the cell temperature associated with the increase in ambient temperature, and hence, the decrease of PV cell efficiency according to Evans–Florschuetz efficiency correlation. In curves (e) and (f), it is noticed that the distillation rate of the system increases with the increase of the ambient temperature. This can be explained by the increase in evaporation associated with internal heat transfer due to higher evaporator temperature.
600
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31
100
27 25
20-C
23
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21
18-C
19
17-C
Power Output (W)
System Efficiency (%)
29
25-C 24-C
60
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40
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17
80
21-C
0
15 0
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1000
0
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1200
3
32 30 25-C
28
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23-C
24
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Disllaon Rate (kg/day)
34
System Efficiency (%)
800
(d)
(a) 2.5 2
20-C 19-C
1.5
18-C
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17-C
0.5
16-C
0
20 0
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Solar Radiaon (W/m2)
(e)
(b) 3.5
100 80
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Disllaon Rate (kg/day)
120
Power Output (W)
600
Solar Radiaon (W/m2)
Solar Radiaon (W/m2)
3 2.5 25-C
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Solar Radiaon (W/m2)
(c)
(f)
1000
1200
Fig. 7. PV/T system performance for 20 °C and 25 °C ambient temperature and wind speed of 10 m/s with the variation of solar intensity.
Fig. 6 is similar to Fig. 5, however, it shows the behavior of the system using two different PV cells: (1) Poly-Si (gT,ref = 14%, bref = 0.004, Tref = 25 °C). (2) a-Si (gT,ref = 7%, bref = 0.011, Tref = 25 °C).
Evaporator Temperature (K)
314 312 310 308 306
4.2. Solar intensity effect on the PV/T system
304 302 300 0
200
400
600
800
1000
1200
Solar Intensity (W/m2)
Fig. 8. Variation of the porous evaporator temperature with solar intensity for 25 °C ambient temperature, 25 °C condenser temperature, and 10 m/s wind speed.
Fig. 7 shows the relationship between the PV/T system performance curves with the variation of solar intensity at ambient temperature of 20 °C and 25 °C, and wind speed of 10 m/s. Curves (a) and (b) show the system efficiency dependence on solar intensity for ambient temperatures of 20 ° C and 25 °C, respectively. It is noticed that the system efficiency increases for most of the solar intensity domain.
M.A. Al-Nimr, M.-E. Dahdolan / Solar Energy 120 (2015) 593–602
This can be explained by the increase in the evaporator temperature, hence, the increase in evaporation. It also can be explained by the increase in electric power out. As for the decrease in the lower portion of the domain, this can be explained by the decrease in heat transfer from the ambient to the still due to the increase of its temperature with the increase of solar intensity. Curve (c) and (d) show the system’s power output dependence with solar intensity. This curve is familiar since it is known that PV cell output is directly proportional with solar intensity by the efficiency (see Eq. (26)). Curves (e) and (f) show the dependence of the system’s distillation rate on solar intensity. It is noticed that distillation rate increases with the increase of solar intensity. This 27
System Efficiency (%)
25 23 Poly-Si/0.14/0.004/25-C
601
can be explained with the increase of evaporation associated with the increase in evaporator temperature. The ambient temperature effect is similar to its effect described previously on the wind effect. As the ambient temperature increases, the distillation output increases due to the decrease in external losses. The PV power output will also decrease due to the increase in cell temperature. And the overall efficiency will increase because of the more significant positive effect on the distillation process. Fig. 9 shows the dependence of the evaporator temperature on solar intensity. It is noticed that the temperature increases as the solar intensity increases. This is explained by the increase of thermal energy absorption associated with the increase of solar power. Fig. 9 is similar to Figs. 8 and 9, however, it shows the behavior of the system using two different PV cells: (1) Poly-Si (gT,ref = 14%, bref = 0.004, Tref = 25 °C). (2) a-Si (gT,ref = 7%, bref = 0.011, Tref = 25 °C). 4.3. Comparison
21 a-Si/0.07/0.011/25-C
19 17 15 0
500
1000
Solar Radiaon (W/m2)
(a) 100 90 Power Output (W)
80 70 60 Poly-Si/0.14/0.004/25-C
50 40
a-Si/0.07/0.011/25-C
30 20 10 0 0
500
1000
Solar Radiaon (W/m2)
(b) 3.5 3 Power Output (W)
Comparing results of the proposed PV/T system simulation with the most similar system (solar still found in reference Al-Nimr and Dahdolan (2015)), it is noticed that the behavior is very similar in general, this similarity is explained by the linear trend of the PV efficiency with the cell temperature. However, the simulation has shown an increase in the overall efficiency of the system is noticed after integrating the solar still with a PV cell. This increase is explained by using two thermal systems (solar still and PV cell) as one cascaded system, utilizing waste heat from the cell while increasing its photovoltaic efficiency.
2.5 2
Poly-Si/0.14/0.004/25-C
1.5 a-Si/0.07/0.011/25-C
1
5. Summary This paper presented a novel PV/T distillation system enhanced with a porous evaporator and an internal condenser. This proposed system has the main advantages: (1) it can operate as a passive system. (2) It provides both electric power and distilled water. (3) It has relatively faster response. A detailed overview has been provided, and a full steady-state mathematical model has been presented and simulated. The simulation has shown the effect of various parameters on the proposed system: solar intensity, wind speed, ambient temperature, condenser temperature, and PV cell used in the system. This simulation produced performance curves of the system giving a clear view on how it’s affected by the mentioned parameters. References
0.5 0 0
500
1000
Solar Radiaon (W/m2)
(c) Fig. 9. PV/T performance for different PV cell types with the variation of solar intensity at 25 °C ambient temperature, and 10 m/s wind speed.
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