Numerical simulation of PV cooling by using single turn pulsating heat pipe

International Journal of Heat and Mass Transfer 127 (2018) 203–208

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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt

Numerical simulation of PV cooling by using single turn pulsating heat pipe Hossein Alizadeh a, Roghayeh Ghasempour a, Mohammad Behshad Shafii b, Mohammad Hossein Ahmadi c,⇑, Wei-Mon Yan d,e,⇑, Mohammad Alhuyi Nazari a a

Faculty of New Sciences and Technologies, Tehran University, A.C, Tehran, Iran Faculty of Mechanical Engineering, Sharif University of Technology, Tehran, Iran Faculty of Mechanical Engineering, Shahrood University of Technology, Shahrood, Iran d Department of Energy and Refrigerating Air-Conditioning Engineering, National Taipei University of Technology, Taipei 10608, Taiwan e Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors, National Taipei University of Technology, Taipei 10608, Taiwan b c

a r t i c l e

i n f o

Article history: Received 19 April 2018 Received in revised form 21 June 2018 Accepted 21 June 2018

Keywords: PV cooling Pulsating heat pipe Solar energy Heat transfer

a b s t r a c t Electrical efficiency of photovoltaic (PV) modules depends on their working temperature. Effective cooling is required in order to achieve higher performance. Pulsating heat pipes (PHPs) are compact heat transfer devices with high effective thermal performance due to the two-phase heat transfer mechanism. Since the lower temperature of PV modules leads to higher electricity generation and better efficiency, PHPs can be applied for PV cooling. In this work, the PV cooling by applying a single turn PHP is numerically investigated. In addition, a copper fin with the same dimensions as the PHP for cooling the PV panel is simulated to compare the performance of the PHP with a solid metal like copper. Results indicated that PHPs are an appropriate option for PV cooling and has the capability to increase PV modules efficiency. It was found that a PV panel using the PHP may have approximately 18% enhancement in electrical power generated compared with that without any cooling system. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Due to environmental problems of using fossil fuels and completion of these fuels [1], renewable energies are developing significantly in recent years [2–4]. Solar energy is one of the popular type of renewable energies which are applied for various purposes such as heating, desalination, and electricity generation [5,6]. Solar energy can be applied directly using solar PV modules or indirectly via thermal power plants, for electricity generation [7,8]. Both of these approaches are widely used and several previous studies have focused on their performance enhancement [9,10]. Enhancement in the efficiency of electricity generation systems would lead to obtaining electricity at more affordable price and lower environmental unfavorable effect. PV solar cell performance depends on the operating temperature. Generally, increase in solar cell temperature causes a decrease in efficiency [11]. To improve the efficiency of PV solar cells, the cooling of PV is one of the ⇑ Corresponding authors at: Department of Energy and Refrigerating Air-Conditioning Engineering, National Taipei University of Technology, Taipei 10608, Taiwan (W.M. Yan). E-mail addresses: [email protected] (M.H. Ahmadi), wmyan@ ntut.edu.tw (W.-M. Yan). https://doi.org/10.1016/j.ijheatmasstransfer.2018.06.108 0017-9310/Ó 2018 Elsevier Ltd. All rights reserved.

methods [12]. There are various PV cooling methods [13]. For example, Akbarzadeh and Wadowski [14] used thermosyphon for PV cooling. Results indicated that it is possible to achieve higher efficiency using this approach. Aldossary et al. [15] examined the PV cooling by using water channels. Most of the methods applied to PV cooling are active cooling since the passive cooling had some disadvantages such as inadequate heat dissipation at high temperatures [15]. Heat pipes are passive cooling devices with high effective thermal performance which can be applied for PV panel cooling. There are several types of heat pipes. Pulsating heat pipes (PHPs) are more applicable in devices which have compact sizes and high heat fluxes such as electronic systems [16,17]. Pulsating heat pipes (PHPs) are widely used for heat transfer purposes due to appropriate performance in heat dissipation [18–20]. The PHPs consist of a capillary tube with several turns [21,22]. The internal diameter of the PHPs must be small enough for slug-plug regime formation [23]. There are two major types of PHPs: closed loop and open loop [24]. In closed loop PHPs, two ends of capillary tubes are connected to each other while are separated in open loop ones [16]. Several parameters are influential in their thermal performance including filling ratio, working fluid, inclination angle and etc. Previous studies have been conducted to improve their

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Nomenclature convective heat transfer coefficient (W/m2 K) length (m) PHP effective length Nusselt number heat transfer through PHP (W/m2) sun radiation heat input (W/m2) electrical power (W/m2) convective heat transfer (W/m2) radiative heat transfer (W/m2) heat flux (W/m2) Prandtl number Rayleigh number

hh l leff Nu qPHP qs qel qh qh q Pr Ra

R T

q e0 b

thermal resistance (K/W) temperature (K) density (kg/m3) qs PV panel efficiency

Subscripts ad adiabatic a ambient c condenser e evaporator s PV panel surface

thermal performance by applying different approaches such as using nanofluids [25] and changes in structure [26]. The main mechanism of heat transfer in the PHPs is two-phase heat transfer, evaporation in evaporator and condensation in the condenser [27]. The driving force for fluid motion is pressure difference between condenser and evaporator which is attributed to boiling phenomena [24]. In addition to electronic systems, the PHPs are also employed in solar water heaters, phase change materials and solar desalination systems [28,29]. In the present study, the thermal performance of a PHP for PV cooling is investigated numerically with using the results of Saha et al. [30]. Details about the PHP geometry and thermal performance are presented in next section.

thermal conductivity in the numerical simulation. The thermal resistance as a function of temperature difference is represented in Eq. (3).

R ¼ 0:00000539905ðDTÞ3 þ 0:001124605ðDTÞ2  0:07636998ðDTÞ þ 2:102284

2. Analysis In this work, a pulsating heat pipe (PHP) is employed according to the data of Saha et al. [30]. The inner diameter of the PHP is 4 mm and it was made of quartz. The dimensions of the PHP were shown schematically in Fig. 1. The total length of the PHP was 150 mm and divided into three parts including condenser, adiabatic and evaporator with 30, 60 and 50 mm in length, respectively. PV panels must be installed in an orientation in which the solar irradiation is normal to it. Kacira et al. [31] investigated the performance of a PV panel in various inclined angles. They found that the best performance was obtained in the range of 20–40 inclination angle and the highest radiation was noted in 30° inclination angle. Besides, they indicated that the PHP had its best thermal performance in 40% filling ratio. Therefore, in this work, the thermal resistances in 40% filling ratio at the tilt angle of 30° for PV panel is considered for simulation. To model the transport of the PHP, effective thermal conductivity of PHP should be calculated. Eq. (1) stands for effective thermal conductivity of PHP based on its geometry.

Q ¼ Ac  qPHP ¼

DT DT ¼ l eff R

ð1Þ

keff Ac

where the effective length of the PHP is:

R leff ¼

Le

ð

Rx 0

q0e dxÞdx þ Lad qc;max þ qc;max

R Lc

Rx ð Le þLad q0e dxÞdx

ð2Þ

Since thermal resistance of PHPs is a function of temperature difference between evaporator and condenser, a correlation is obtained to indicate the relationship between temperature difference and thermal resistance and used for evaluating the effective

Fig. 1. Schematic diagram of the PHP experimental set-up [30].

ð3Þ

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Energy balance is applied to obtain the temperature of the PV surface. As shown in Fig. 2, a part of the absorbed energy is transferred by radiation and convection heat transfer. The remained part results in electricity generation and increase in internal energy of the panel. Energy balance in PV panel is:

qC p d

dT s ¼ qs  qel  qh  qr  qPHP dt

ð4Þ

In which e1 is the emissivity of the PV panel surfaces. Convective heat transfer coefficient in Eq. (8) is calculable by Eq. (10) [33].

( NuL ¼

0:825 þ

)2

0:387Ra1:6 L ½1 þ ð0:492=PrÞ9=16 

8=27

ð10Þ

Convective heat transfer coefficient on the back side of the PV panel is obtained by using Eq. (10), While, the convection heat transfer on the upper side was estimated by using Eq. (11) [24].

where qs is received solar radiation which can be calculated as follows.

hh ¼ 2:8 þ 3:8uwind

qs ¼ e0 Q s

Eq. (4) can be rewritten as a differential transport equation as shown below:

ð5Þ

where Q s is normal radiation to the solar panel and e0 is the PV cell absorption rate. In the energy equation, qel is the generated power by the PV panel due to the solar irradiation. The generated electricity can be obtained by applying Eq. (6).

qel ¼ bQ s

ð6Þ

In above equation, b is the PV panel efficiency. Ibrahim [32] showed that the efficiency of the PV panels depends on the working temperature. The relationship between efficiency and the surface temperature of PV panel is correlated as:

b ¼ 0:1757 T s þ 21:737

@ ! ðqhÞ þ r  ð v qhÞ ¼ r  ðkrTÞ þ Sh @t

In the energy equation, qh and qr are heat transfer due to the convection and the radiation heat transfer, respectively. The qh and qr are obtained by using Eqs. (8) and (9).

ð8Þ

qr ¼ 2e1 rsb ðT 4s  T 4a Þ

ð9Þ

Fig. 3. PV panel temperature contours for different solar radiations.

Fig. 2. Energy conservation for PV panel.

Fig. 4. Comparison of present study with previous results.

Table 1 Specifications of the PV panel. Power

5 (w)

Power tolerance

±5%

Operating voltage

18 (V)

21.6 (V)

Operating current

277 (mA)

Open circuit voltage Short circuit current

Type of solar cell Monocrystalline silicon solar cell Irradiance = 1000 (W/m2); Module temperature = 298 K; Am = 1.5

ð12Þ

In this work, the above equation is integrated and discretized over the subdivided domain for evaluating the variable of temperature at the geometric center of each control volume. In the first step, the temperature of PV panel is investigated at various solar radiations. A commercial PV panel is selected in the simulation. The panel consists of three layers including glass cover

ð7Þ

qh ¼ hh ðT s  T a Þ

ð11Þ

301 (mA)

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Fig. 5. The 3-dimensional model and grid structure of the PV panel with installed PHP.

325

Cooling with PHP

320

Temperature (K)

with 3.2 mm thickness, photovoltaic cell with a conductive layer in its both sides and PVF layer which is used as insulation at the bottom of the panel. The thickness of PVF layer is 2 mm. The dimensions of the panel, electrical characteristics, and its working temperature is presented in Table 1. Predicted temperature contours at PV panel domain for five various radiations are illustrated in Fig. 3. In this work, the ambient temperature of 291 K is assumed. It is clearly found in Fig. 3 that higher temperature counters are noted for higher solar radiations. To validate the simulated results, the predicted results are compared with analytical and experimental studies, as depicted in Fig. 4. An overall inspection of Fig. 4 discloses that the predicted results agree well with the previous results from the analytic model [34] and the experiment [35]. The geometry of the PV panel with installed PHP was designed and meshed to investigate the effect of using PHP in PV cooling. The 3-dimensional model of the panel and the grid structure of the numerical domain is shown in Fig. 5. In this work, grid independency was investigated for evaluating the numerical simulation and its accuracy. In this procedure, the temperature of PV surface was investigated based on the irradiation for various mesh sizes. Conducting the grid independency and comparing the results obtained from the numerical modeling and the observed insensitivity in the PV panel temperature at different mesh densities, a mesh with approximately 2,300,000 elements has been applied.

Cooling with Copper Fin

315 310 305 300 295 290

0

1000

2000

3000

4000

5000

6000

7000

8000

Time (s) Fig. 6. Transient average temperatures of the PV cooled by PHP and copper fin.

3. Results and discussion To investigate the performance of the PHP in PV cooling, a transient numerical simulation is conducted by considering the heat flux of 1000 W/m2 and the ambient temperature of 291 K. The temperature of condenser section of the PHP is assumed to be ambient temperature. Moreover, another simulation by using the copper tube as a fin for cooling PV panel is also performed for comparison. Fig. 6 compares the predicted results for the mentioned simulations. It is clear in Fig. 6 that PV cooling with PHP or copper tube fin results in 16.1 K or 4.9 K reductions in the panel temperatures, respectively. Based on predicted results, it is found that the PC cooling with using the PHP leads to much a high reduction in temperature due to its high effective thermal performance compared with copper tube fin. Besides, it was also noted that PC cooling with applying PHP leads to shorter time to reach a steady working temperature, compared with that with using copper tube fin. Temperature contours of the panel using PHP for cooling are illustrated in Fig. 7. The highest temperature on the surface of the panel is about 309.4 K which is at the corners of the panel.

Fig. 7. Temperature contour of the PV panel surface for 1000 (W/m2) solar radiation.

The minimum temperature of the panel is near the vicinity of the PHP and is about 305.3 K. To investigate the performance of the PHP cooling system under any environmental conditions, the cooling of PV panel is modeled by assuming adiabatic boundary condition for PV panel walls and a transient calculation with two types of cooling by the copper fin and the PHP. Fig. 8 presents the simulated results under this condition. The left vertical axis indicates the temperature of the PV

207

700

6

315

600

5

310

500 400

305

300 300

Cooling with Copper Fin

295

Cooling with PHP

290 10

100

1000

200 100 0 100000

10000

Electrical Power (W)

320

Temperature (K)

Temperature (K)

H. Alizadeh et al. / International Journal of Heat and Mass Transfer 127 (2018) 203–208

PHP

4 3 2 1 0

600

700

800 900 Solar Irradiation (W/m2)

1000

Time (s) Fig. 10. Electrical power produced in different scenarios. Fig. 8. Transient adiabatic temperature of PV panel cooled by the PHP and copper fin.

4. Conclusion surface in the case of cooling the panel with PHP and the right axis shows the temperature of the PV panel with copper fin cooling. In Fig. 8, the simulated results indicate that in the case of cooling by the PHP, better PV panel cooling is noted by PHP. In addition, it is clearly found that the temperature of the PV panel surface does not dramatically increase with time for PHP cooling. However, in the case of cooling with copper fins, the temperature increases significantly with time. These results conclude that the PHP cooling is more effective for transmitting the generated heat by the PV panel to the working fluid. To examine PHP cooling performance under various conditions, the simulation is conducted for five solar radiations. Fig. 9 shows temperature differences in PV cooling by using the PHP and copper tube with the same dimensions. As illustrated in Fig. 9, using PHP cooling results in much higher temperature difference compared with that using the copper tube. In addition, it is found that the deviation of the temperature difference for both cooling methods increases with increasing solar radiation. It stands for better thermal performance of the PHP at higher heat inputs. By increasing the heat input to the PHP evaporator section, the frequency and amplitude of the vapor slug increase which would result in a higher heat transfer coefficient [30]. Besides, the increase in the heat input can produce more extensive vortexes in the working flow, which can further increase the heat transfer coefficient [30]. It is observed noted from Eq. (7), a high electricity generation is noted for a low PV panel temperature. Therefore, the generated electricity in the investigated scenario is presented in Fig. 10. As shown in Fig. 10, the higher generated power with PHP cooling is found. Using PHP cooling for 1000 W/m2 solar irradiation leads to 18% increase in electrical power generation, while only 6% increase for copper fin cooling.

17 PHP Copper

14

Ts-T

11 8 5 2 550

650

750

850

950

1050

Solar Irradiation (W/m2) Fig. 9. Temperature difference of the PV panel cooled by the PHP and copper fin uder various solar radiations.

In this work, PV cooling by using a single loop pulsating heat pipe (PHP) is numerically investigated in details. Thermal performance of the PHP is selected based on the best tilt angle of the PV panel and the optimum filling ratio of 40%. The surface temperatures of the non-cooled PV panel are investigated numerically and the results show a good agreement with the previous results. Besides, a transient simulation has been conducted for investigating the performance the PV panel with PHP cooling and it is shown that PHP cooling will reduce the surface temperature of the PV panel by 16.1 K. Whereas, using a copper fin reduces the PV panel temperature by only 4.9 K. The results also demonstrate that utilizing PHP cooling in comparison with a solid copper fin would be more effective for thermal cooling. In addition, using PHP for cooling PV panel enhances the cooling process and reduces the inefficiencies in a shorter time. The performance of the PHP cooling augments the generated electrical power up to 18% for 1000 W/ m2 solar irradiation. Conflict of interest The authors declared that there is no conflict of interest. Acknowledgements The authors appreciates the financial support from Ministry of Science and Technology, Taiwan, under grant number MOST 1062221-E-027-102 –MY2 and MOST 107-3113-E-008-003. References [1] T.E. Amin, G. Roghayeh, R. Fatemeh, P. Fatollah, Evaluation of nanoparticle shape effect on a nanofluid based flat-plate solar collector efficiency, Energy Explor. Exploit. 33 (2015) 659–676. [2] L.D.B. Pestaño, W.I. Jose, Production of solid fuel by torrefaction using coconut leaves as renewable biomass, Int. J. Renew. Energy Dev. 5 (2016) 187–197. [3] M. Mohammad, R. Ghasempour, F.R. Astaraei, E. Ahmadi, A. Aligholian, A. Toopshekan, Optimal planning of renewable energy resource for a residential house considering economic and reliability criteria, Int. J. Electr. Power Energy Syst. 96 (2018) 261–273. [4] M.S. Sadaghiani, M.H. Ahmadi, M. Mehrpooya, F. Pourfayaz, M. Feidt, Process development and thermodynamic analysis of a novel power generation plant driven by geothermal energy with liquefied natural gas as its heat sink, Appl. Therm. Eng. 133 (2018) (2018) 645–658. [5] J.T. Dellosa, Potential effect and analysis of high residential solar photovoltaic (PV) systems penetration to an electric distribution utility (DU), Int. J. Renew. Energy Dev. 5 (2016) 179–185. [6] D. Tarwidia, D.T. Murdiansyah, N. Ginanjar, Performance evaluation of various phase change materials for thermal energy storage of a solar cooker via numerical simulation, Int. J. Renew. Energy Dev. 5 (2016) 199–210. [7] S.D. Nazemi, M. Boroushakib, Design, Analysis and optimization of a solar dish/stirling system, Int. J. Renew. Energy Dev. 5 (2016) 33–42. [8] A. Naseri, M. Bidi, M.H. Ahmadi, Thermodynamic and exergy analysis of a hydrogen and permeate water production process by a solar-driven transcritical CO2 power cycle with liquefied natural gas heat sink, Renew. Energy 113 (2018) 1215–1228.

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