Experimental study of active phase change cooling technique based on porous media for photovoltaic thermal management and efficiency enhancement

Experimental study of active phase change cooling technique based on porous media for photovoltaic thermal management and efficiency enhancement

Energy Conversion and Management 199 (2019) 111990 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www...

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Energy Conversion and Management 199 (2019) 111990

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Experimental study of active phase change cooling technique based on porous media for photovoltaic thermal management and efficiency enhancement

T



Yiping Wanga,b,1, Yuanzhi Gaoa,1, Qunwu Huanga, , Guohao Hua, Liqun Zhouc a

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China School of Architecture, Tianjin University, Tianjin 300072, China c College of Petrochemical Technology, Lanzhou University of Technology, Gansu 730050, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Photovoltaic Porous media Active phase change Thermal management Efficiency enhancement

The photoelectric conversion efficiency of solar cells was significantly influenced by its operating temperature which promotes the studies in thermal management for photovoltaic (PV). This paper introduced a novel cooling method based on porous media applied in PV panel for thermal management and efficiency enhancement. An active phase change (APC) cooling system has been designed and fabricated to explore the possibility of cooling the PV panel, and ethanol was considered as the working fluid. By laboratory test, average temperature, temperature distribution, electrical performance of the PV panel as well as energy and exergy efficiency were analyzed in detail. The results demonstrated that the average temperature of the PV panel can be better managed by the APC cooling method compared with the uncooled conditions at different irradiation levels. Besides, increasing the non-condensable gas flow rates has a positive effect on the temperature reduction and maximum generated power of the PV panel. The obtained three-dimensional temperature distribution map proved that the temperature distribution was well uniform with the maximum temperature difference less than 5 °C. The maximum specific power improvement and percentage improvement in power generation were 21.37 W/m2 and 19.32%, respectively.

1. Introduction In recent years, with the increasing attention paid by the international community to ecological environmental protection, energy security and climate change, development and utilization of renewable energy has become a universal consensus for all countries in the world. As an environmental-friendly renewable source, solar energy can not only decrease conventional fossil fuels (coal, oil, natural gas, etc.) consumption but also withstand damages to the environment and avoid future crisis [1,2]. As one of the most widespread applications for harvesting solar energy, PV technology has attracted increasing attention. However, the conversion efficiency of a crystalline silicon cell only about 15%–20%, and the rest is converted to thermal energy that results in undesirable consequences [3]. First, the increasing working temperature of the crystalline silicon cells results in approximately 0.45% degradation of the solar energy conversion efficiency for every degree rise in temperature [4]. Second, an unexpected permanent

structural damage of the PV module occurred if the thermal stress remains for a prolonged period which threatens the running life of PV panels [5]. Therefore, it is vital to find a highly efficient cooling method for PV thermal management to control the uprising temperature of the solar cells. Enormous innovation cooling techniques have been explored in this filed over the past decades. The recent techniques for cooling PV panel can be classified as the active and passive cooling system which mainly depends on if there is any consumption of additional energy taken into account (generally, refers to parasitic energy consumption) [6,7]. In terms of passive cooling, heat pipe cooling is a commonly used form. Wang [8] proposed an APT cooling system for cooling PV panels. The results indicated that the APT cooling system has a good temperature uniformity. When the heat flux density is 850 W/m2, the maximum surface temperature difference was less than 1 °C. Besides, Zhao [9] explored the potentials of radiative cooling for the commercial silicon PV module. The temperature reduction of the PV module was



Corresponding author. E-mail address: [email protected] (Q. Huang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.enconman.2019.111990 Received 20 June 2019; Received in revised form 20 August 2019; Accepted 24 August 2019 Available online 30 August 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature A I V Pfan Q T ΔT G U η Isc Voc Pmax FF IP ∑ Eẋ in ∑ Eẋ out ∑ Eẋ loss ∑ Eẋ pv ·

S

ηI ηII

surface area, m2 current, A voltage, V power of fan, W coolant flow rate, ml/min temperature, °C temperature difference, °C solar irradiation, W/m2 voltage, V cooling efficiency short-circuit current, A open-circuit voltage, V the maximum output power, W fill factor improvement potential, W input exergy rate, W output exergy rate, W exergy loss rate, W photovoltaic exergy rate, W

energy efficiency exergy efficiency

Abbreviation APC BIPV PV/T EVA IR NCG PV LCPV APT PCM DC UBHE SI LAX 3D

active phase change building-integrated photovoltaic photovoltaic thermal ethylene-vinyl acetate copolymer infrared non-condensable gas photovoltaic low concentrating photovoltaic atmospheric plate thermosyphon phase change materials direct current u-shaped borehole heat exchanger sustainability index long arc xenon three-dimensional

entropy generation, W/k

gen

and multi header microchannels for PV cell cooling and the results showed an increase about 28% in electrical power for the multi-header microchannel in comparison with the single-header one. Meanwhile, air-based cooling technology is another form has been broadly investigated. The results indicated by Teo [18] show that the operating temperature of the PV module could be maintained at 38 °C by using the blower and the electrical efficiency could also be kept at around 12.5%. To meet the thermal and electrical energy, a hybrid PV/T system was introduced by Mojumder [19]. The maximum thermal and PV efficiency were obtained about 56.19% and 13.75% respectively for four fins at 0.14 kg/s of mass flow rate and 700 W/m2 of solar radiation. Baloch [20] stated a converging channel cooling system to achieve low and uniform temperature on the surface of the PV panel. The experimental and numerical results show that by employing converging cooling, cell temperature was reduced significantly to 45.1 °C for June and 36.4 °C for December. Although, by applying these active cooling methods the power generation efficiency of the PV panel has been significantly improved compared with passive cooling. But few of them have considered the impact of parasitic energy consumption that must be considered in active cooling technology (or they just have considered equivalent consumption, not the real one). In our previous work [8], an APT cooling technique for low concentrating photovoltaic (LCPV) system cooling was proposed. However, at present, most of the crystalline silicon cells are non-concentrating and the practical application for cooling solar cells remains to be verified and researched. Accordingly, an APC cooling system is presented in this paper to explore the possibility of thermal management for flat PV module. Besides, most of the mentioned active or passive methods have been studied independently, and the combination of both methods was seldom investigated. Meantime, a combination of using forced convection (active) and vapor phase change cooling (passive) with the aid of porous media is not available in the open literature. The evaporation of water on a wet porous layer inside a vertical channel and using a wet porous cooling plate for building wall cooling were studied in [21–23]. However, they only considered the high boiling point coolant-water as the working fluid but were not aware of the potential for using low boiling point coolant (such as ethanol). In a word, the mentioned APC cooling method has great potential to improve the electrical efficiency of the PV panel, but only if feasibility is proven. The research presented in this manuscript is mainly aiming to explore the feasibility of using APC cooling technique and porous media

1.75 K, which indicated that radiative cooling might be suitable for solar cells working in an extraterrestrial environment. Also, evaporation cooling was another form investigated by Chandrasekar et al. [10]. They developed an evaporation cooling system with cotton wick structures for cooling standalone PV module. The results confirmed that the maximum module efficiency of 10.4% was obtained by using wick structures in combination with water while the efficiency was 9% compared to the system without cooling. Alami et al. [11] also examined the possibility of synthetic clay to the back of the module which allows a thin film of water to evaporate. The results proved the technical feasibility of the proposed approach by exhibiting a maximum increase of 19.4% to the output voltage and 19.1% to the output power. Recently, using phase change materials (PCM) for thermal regulation of PV panel is gaining more interest [12]. Nada [13] carried out an experiment for PV-building integrated system thermal regulation by using phase change materials (PCM) and Al2O3 nanoparticles. It shows that integrating the PV with pure PCM and enhanced PCM by nanoparticles can drop the temperature of the modules by 8.1 °C and 10.6 °C and increase its efficiency by 5.7% and 13.2%, respectively. Although the using of PCM technology has played a role in PV thermal management, the problem is that the higher PCM cost and disposal problem after their life cycle limited its practical application [14]. In summary, the main advantage of passive cooling is that there is no energy consumption, but its drawback is also obviously that the cooling effect is limited due to the low heat transfer rate and low thermal conductivity of coolants. Therefore, more attention should be focused on active cooling. As for active cooling, water-based cooling is one of the most widely investigated technology in field testing. Nižetić et al. [15] tested the possibility of water spray technique for simultaneously cooling both sides of the PV panel. The results show that the PV panel temperature reduced from an average 54 °C to 24 °C which leads a maximal total increase of 16.3% in electric power output and a total increase of 14.1% in electrical efficiency. Similarly, Rostami [3] presented a cooling method by using high-frequency ultrasound waves and CuO nanofluid. The results depicted that the PV module average surface temperature decreased up to 57.25% and an increase in maximum power reached 51.1% compared to the uncooled condition. Hasan et al. [16] studied the application of jet impingement for photovoltaic thermal (PV/T) collector with nanofluids. What’s more, the maximum power of PV/T with SiC nanofluid increased by 62.5% compared to the conventional PV module. Rahimi [17] reported a comparative study on using single 2

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for thermal management and efficiency enhancement of the PV module and coolant recovery. The proposed cooling system was also dedicated to presenting new horizons and alternative cooling methods in solar power generation system.

Table 1 General characteristics of the tested PV module at AM1.5G, 1000 W/m2 and 25 °C.

2. Experimental setup and approach The experimental setup of APC cooling system mainly consisted of tested PV module, solar simulator, coolant distribution and collection system along with data acquisition system, and the schematic of the experimental setup is shown in Fig. 1. 2.1. Modified PV module

Technical characteristics

Value

Model Type PV surface area Maximum power output Maximum power voltage Maximum power current Open circuit voltage Short circuit current

SUN-30 Mono-crystalline silicon 0.234 m2 30 W 17.5 V 1.71 A 21.5 V 1.94 A

reported in Fig. 2a. The detailed schematic, as shown in Fig. 2b, reveals the structure of the rear of the modified PV panel. The porous media, namely spunlaced non-woven fabric, was attached at the backside of the PV panel by using thermal conductive silica gel which is widely used in heat dissipation of high-power electronic components with the thermal conductivity of 1.5 W/(m·K). The spunlaced non-woven fabric has good affinity for ethanol, the ethanol could be evenly distributed the whole backside of the PV module due to the capillarity of the porous

A new prototype of the PV panel was fabricated, in the interest of the proposed APC cooling method. The modified PV module was designed with a unique backside structure which consisted of a commercial monocrystalline silicon PV panel (Detailed performance of the PV module parameters given by the manufacturer is shown in Table 1.) with a porous layer and an NCG flow duct made up of acrylic attaching at the rear of the panel. The schematic of the modified PV module is

Fig. 1. The diagram of the experimental facility (a) the schematic of the tested experimental setup, (b) a photograph of the actual experimental setup. 3

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Fig. 2. Schematic of the modified PV panel. (a) Three-dimensional view of the modified PV panel, (b) detailed view of the modified PV panel.

layer. An acrylic duct was attached behind the PV panel, and the specific dimensions of the duct are summarized in Table 2.

according to ASTM E927-10 with the results of 3.9%.

2.2. Coolant distribution and collection system

2.4. Measurement system The coolant flow rate was measured by a rotor flow meter (LZB3WB(F), accuracy ± 4%, Changzhou Shuang Huan Thermo-technical Instrument Co., Ltd, China). The flow rate of NCG in the duct was determined by an anemometer (TESTO405-V1, accuracy ± 0.1 m/s, Testo SE & Co. KGaA, Germany). I-V curves of the PV module were collected by an I-V curve tracer (DS-100c, current accuracy ± 0.5%, voltage accuracy ± 0.5%, Daystar, Inc, USA) and the connection elements of I–V curve tracer are shown in Fig. 3a. The data acquisition interval of I-V curves was 2 mins. The outlet temperature of the coolant in the liquid storage tank was measured by K-type thermocouple (5 TCTT-K-36-36, accuracy ± 0.1 °C, Omega Company, USA) using fourchannel K-type thermocouple collector (AZ88598, accuracy ± 0.1 °C, Heng Xin instrument company, China) and the picture is displayed in Fig. 3b and Fig. 3c. To obtain the temperature distribution of the PV module, eight K-type thermocouples were arranged on the rear of the PV module and the measured locations (white points) of K-type thermocouples are shown in Fig. 4. The temperature data were recorded at the interval of 1 s. The global pyranometer (Model: PSP; accuracy < 10 W/m2, Sensitivity, 8.43 µV/(W·m−2); 300–3000 nm) manufactured by the Eppley Laboratory, Inc, USA, was fixed on the system to measure the incident radiation. To estimate the parasitic energy consumption of the system, the current and voltage of the DC fan were monitored using a multimeter (VC890C+, current accuracy ± 1.2%, voltage accuracy ± 0.5%, Shengsheng Shengli Technology Co., Ltd, China). Infrared (IR) images of PV module could be obtained by thermal IR imager (SAT-90, imager resolution 640 × 480, accuracy ± 2 °C, Guangzhou SAT Infrared Technology Co., Ltd, China), and the temperature distribution field was built to evaluate the temperature uniformity of the tested PV module.

The coolant was fed by a liquid storage tank placed at the top of the PV module. The coolant which was driven by gravity, at a constant inlet temperature, flowed through a gate valve and rotor flow meter into the liquid distributor. The specially designed liquid distributor could guarantee uniformed distribution of ethanol liquid in porous media which is crucial to the temperature control of the PV module. The temperature of the coolant was controlled by a closed-loop temperature control system with the temperature detection accuracy of ± 0.1 °C and a 150 W heating plate. The incident radiation that was not fully utilized by PV module and converted to waste heat could be taken away when the well-distributed ethanol in the porous layer evaporated and withdraw its latent heat of vaporization away from the PV module then the unevaporated ethanol flowed into the liquid reservoir. When the PV module temperature stabilized, the temperature and electrical performance were recorded and collected. The evaporated ethanol gas was driven to the condenser by a DC fan (HL-9225S-12H, Yueqing Fenghao Electric Co., Ltd, China). Dimensions of the condenser are detailed in Table 2. And NCG flow rates in the flow duct were adjusted by changing the speed of a DC fan through a variable resistor (Kejiesheng Power Electronics Co., Ltd, China). The ethanol vapor after condensation was collected into the reservoir so the purpose of recovering working fluid and recycling utilization was achieved. To investigate the influence of NCG circulating flow rates on APC cooling system, 8.84 m3/h (low), 13.49 m3/h (medium) and 20.63 m3/h (high) were set as operated NCG inlet flow rates in the experiment, respectively. 2.3. Solar simulator Considering the unsustainable of the natural sunlight as well as flexibility in time and location, all the experiments were carried out under an indoor condition. The ambient temperature was controlled at 20 °C during the experiments. An artificial solar simulator (as shown in Fig. 1b) was manufactured to provide necessary solar irradiation in the testing of modified PV module. The solar simulator has been built with twelve long arc xenon (LAX) (XGFZ18, Shanghai Aojia Electronics Co., Ltd, China) lamps each having 1800 W fixed in an aluminum frame with 2 × 6 matrices to meet the requirement of uniformed distribution of irradiance. The non-uniformity illumination of the simulator was tested

Table 2 Major parameters of the APC cooling system.

4

Parameter

Unit

Value

Error

Acrylic duct Condenser Liquid storage tank Liquid reservoir Liquid line Vapor line

mm mm L L mm mm

514*446*160 400*250*115 2.5 1.5 Ø10*2 Ø130*2.5

±1 ±1 – – ± 0.05 ± 0.1

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Fig. 3. Measuring instruments. (a) I-V curve tracer, (b) temperature collector, (c) thermocouples.

P − Puncooled ⎞ %P(max) increase = ⎛ APC × 100 Puncooled ⎝ ⎠ ⎜



(4)

In which, %P(max) increase is the percentage increase in maximum output electrical power, PAPC and Puncooled are maximum output electrical power in APC cooling and uncooled condition, respectively. To investigate the overall performance of the PV module, the energy and exergy analysis, from the first and second law of thermodynamics, was introduced to evaluate the APC cooling system. From the first law of thermodynamics, the energy efficiency of the APC cooling system could be described as the ratio of the input energy to the output energy of the system. Thus, the energy efficiency of the APC cooling system is expressed as:

ηI = Fig. 4. Locations of thermocouples at the backside of PV module.

The following parts will be discussed in detail to evaluate the performance of the proposed APC cooling system: effect of APC cooling system on PV module temperature, electrical performance as well as energy and exergy efficiency. The average temperature of the PV module can be expressed as:

1 8

i=1

where Vm and Im are the maximum power voltage and maximum power current, respectively. Since the power output conversion is not constant even at constant solar radiation, but a maximum power point existed with a term named fill factor as follows:

(1)

FF = where Tavg - PV is the average temperature at the rear of the PV module, Ti is the temperature on the rear of PV module and i is the number of measured points. The maximum temperature difference is calculated as:

ΔT = Tmax − Tmin

TAPC − Tuncooled × 100 Tuncooled

Vm Im Voc Isc

(7)

where Voc and Isc are open circuit voltage and short circuit current of the PV module. Therefore, another form of energy efficiency equation gained when Eq. (7) substitute into Eq. (5):

(2)

ηI =

where ΔT is the maximum temperature difference among the measured points, Tmax and Tmin are maximum and minimum temperature at tested points, respectively. For the purpose of contrasting the cooling performance with and without APC cooling, the average temperature reduction efficiency of the PV module relative to the uncooled condition is defined as follow:

η=

(6)

Pm = Vm × Im

8

∑ Ti

(5)

where ηI is the energy efficiency, Pm , G , A are maximum output electrical power of PV module, incident solar radiation, the surface of PV area, respectively. The maximum output of electrical power can also be written as:

2.5. Analysis approach

Tavg − PV =

Pm G·A

Voc Isc FF G·A

(8)

Considering the parasitic energy consumed by the DC fan, the effective efficiency can be expressed as:

ηI − eff =

Voc Isc FF − Pfan G·A

(9)

And Pfan is calculated as:

Pfan = Vfan × Ifan (3)

(10)

Generally, for a PV system, the incident radiation which is not fully used by PV panel converted into heat energy then dissipated to the surroundings can be considered as useless energy. When considering the whole APC cooling system as a control volume, from viewpoint of the second law of thermodynamics, the exergy efficiency of PV panels is considered simply as the ratio of the net useful exergy output (the PV

where, η is cooling efficiency. TAPC and Tuncooled are the average temperature of the cooled and uncooled PV module, respectively. Similarly, the percentage increase in the electrical PV power output is defined as follow: 5

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electric power output) to the overall exergy input (the total incident solar irradiation). Based on this approach the heat loss from the PV panel is neglected and only the electricity generated by the PV panel is taken into consideration. So, the overall exergy balance equation can be expressed as follows [24]: ·

·

∑ Exin = ∑ Exout

(11)

or ·

·

·

∑ Exin − ∑ Exout = ∑ Exloss ·

·

(12) ·

where ∑ Ex in , ∑ Ex out and ∑ Exloss are input exergy rate, output exergy rate and exergy loss rate, respectively. For APC cooling system, the above Eq. (12) can be expressed as: ·

·

·

∑ Exin − ∑ Expv = ∑ Exloss ·

(13) ·

·

where ∑ Ex in , ∑ Ex pv and ∑ Exloss are input exergy rate, PV exergy rate and exergy loss rate, respectively. Therefore, the rate of the entropy generation of the PV system can be calculated as [25]:

Fig. 5. The average temperature variations of the PV module without cooling under different irradiation levels.

· ·

∑ Exloss S = gen Ta

3.1. Effect of APC cooling system on PV module temperature (14) 3.1.1. Average temperature The average temperature variations of PV module without cooling are shown in Fig. 5 under the irradiation of 500–900 W/m2, respectively. It was found that the average temperature increases with the increasing of experimental time and finally reaches to a steady-state condition (after nearly 55 min) with the temperature of 53.1 °C, 56.6 °C, 62.4 °C, 65.0 °C, 73.1 °C, respectively. This temperature variation trend illustrates the necessity of cooling the PV panel. Fig. 6 illustrates that with APC cooling the average temperature of PV module could be controlled at 44.0 °C compared to that of average temperature with air cooling (air flow rate of 8.84 m3/h) at 64.8 °C under the same irradiation of 900 W/m2. This result confirmed that the proposed APC cooling system is more effective compared with the conventional air cooling. (What needs to be emphasized in particular is that with the same parasitic energy consumption.) Meanwhile, it is found that while the flow rates of NCG increases from 8.84 m3/h to 20.63 m3/h, the average temperature of the PV panel decreases from

·

where S refers to the rate of the entropy generation of the PV system, gen

Ta refers to the ambient temperature. The exergy input to system represents the exergy of irradiation and is given as [26]: ·

∑ Exin = G·A ⎡⎢1 − ⎣

4

4 ⎛ Ta ⎞ 1 T + ⎛ a⎞ ⎤ 3 ⎝ Ts ⎠ 3 ⎝ Ts ⎠ ⎥ ⎦ ⎜







(15)

where Ts is the sun temperature which is taken as 5777 K (as a black body). The exergy rate of the PV system is equal to the electrical power [24]: ·

∑ Expv

= Pm = Voc × Isc × FF

(16)

Thus, taken consumed power by the DC fan into account, the exergy efficiency is arranged as:

Voc Isc FF − Pfan

ηI =

G·A ⎡1 − ⎣

4 3

Ta 4 Ts

( ) + ( ) ⎤⎦ Ta Ts

1 3

(17)

The exergetic concept of “IP” (improvement potential) which indicates the maximum improvement in the exergy efficiency for a system could be achieved when the exergy loss is minimized is expressed as [27]: ·

IP = (1 − ηI )· ∑ Exloss

(18)

Another exergetic concept, sustainability index (SI), can be determined by exergy efficiency as [28]:

SI =

1 1 − ηI

(19)

3. Results and discussion As previously described, the overall performance evaluation of the APC cooling system was assessed by PV module temperature, electrical performance, energy, and exergy analysis of the system.

Fig. 6. The average temperature variations of the PV module at the irradiation of 900 W/m2. 6

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44.0 °C to 37.9 °C. Fig. 7 shows the variations of the average temperature and cooling efficiency along with NCG flow rates under different irradiations. It is clearly seen from Fig. 7a that the average temperature of the PV module increases as the irradiation increases. Even at the irradiation of 500 W/ m2, the uncooled PV module (NCG flow rate: 0 m3/h) can easily reach up to around 55 °C. By applying APC cooling, a noticeable drop in average temperature of the PV module can be identified. For example, at the irradiation of 800 W/m2, the average temperature decreases to 40.6, 38.2, 36.3 °C with the NCG flow rates of 8.43, 13.49, 20.63 m3/h, respectively. Indeed, it is interesting to find that a significant decrease trend was obtained when the APC cooling was applied to the PV panel. This phenomenon can be attributed to the absorption of a large amount of latent heat of phase change from the backside of the PV panel during the conversion of liquid ethanol to ethanol vapor. The cooling efficiency of APC cooling at different irradiation levels is shown in Fig. 7b. As can be seen from the figure, the overall cooling efficiency exceeds 35% which indicates the proposed cooling method is significant. Similarly, the cooling efficiency is also strongly influenced by the flow rates of NCG. At the same NCG flow rate, cooling efficiency increases as irradiation level increases which can be attributed to the heat transfer driving force, temperature difference, at high irradiation level is higher compared to that of under low irradiation level. The influence of inlet coolant temperature and irradiation on the average temperature of the PV module is shown in Fig. 8. As can be concluded from the figure, the average temperature of the PV module increases with the increasing of the inlet temperature of the coolant, which can be explained that increasing the inlet temperature of coolant will reduce the temperature difference of heat transfer between PV panel and coolant. Therefore, the evaporation rate of ethanol reduced which increases the average temperature of the PV panel.

Fig. 8. The influence of inlet coolant temperature and irradiation on the average temperature of the PV module.

to the effectiveness of the APC system which showed a bright yellow spot in Fig. 9a. The existence of the coolant inlet zone was due to the temperature of coolant was lower than the temperature of the PV panel, a corresponding bright purple color spot could also be seen in Fig. 9a. Similar reasons can also explain the emergence of the border zone. The maximum lateral and axial temperature difference correspond to Fig. 9c and Fig. 9d were less than 4 °C and 5 °C which reveals good temperature uniformity distribution of PV module. Table 3 summarizes some available previous work in published literature. Noted that the maximum temperature difference is 4.8 °C at the irradiation from 500-900 W/m2 which indicates well temperature uniformity for the proposed APC cooling system compared to other research’s work (It should also be noted that the minimum temperature difference gained in jet cooling only considering a single solar cell with a small size).

3.1.2. Temperature distribution Determination of a uniform temperature distribution of the PV module is a crucial factor for establishing a reliable cooling system. The IR imager was utilized to build a temperature distribution field of PV module and the obtained IR color image is shown in Fig. 9a. In order to evaluate the temperature difference across the surface of the PV panel, a three-dimensional (3D) temperature distribution map was plotted in Fig. 9b. Of special note were “mountains” and “valleys” appeared in Fig. 9b which represent three zones (junction box zone, coolant inlet zone, and border zone, respectively.). To protect the inherent structure of the PV panel, there was no porous medium was attached in the junction box zone. Thus, the waste heat in this zone cannot be dissipated in time, and the temperature around this zone was reduced due

3.2. Effect of APC cooling system on PV module electrical performance The variation of the electrical performance of the PV panel is a vital factor to evaluate the APC cooling in this experiment. The I-V and V-P curves of the PV module under different test conditions (i.e., uncooled

Fig. 7. The temperature variations of PV panel under different test conditions. (a) The average temperature variation of the PV module with NCG flow rates, (b) the cooling efficiency of APC cooling at different irradiation levels. 7

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Fig. 9. Temperature distribution of the PV panel (a) IR color image of the PV panel, (b) three-dimensional temperature distribution map of the PV panel, (c) front view of temperature difference of the PV panel, (d) side view of temperature difference of the PV panel.

the discussion in Section 3.2, the conclusion is clear that increasing the flow rates of NCG can reduce the average surface temperature more effectively. So, the highest increase in electric power output was achieved in the case of flow rate of 20.63 m3/h and the power output is 28.4 W. The output voltage was 19.6 V for uncooled PV panel and in the case of APC cooling employed with NCG flow rate of 20.63 m3/h the output voltage reached to its highest value of 21.5 V. The calculated percentage of increase in electrical power output is illustrated in Fig. 11. The results show that by applying APC cooling the electric efficiency improved dramatically while the flow rates of NCG changed from 8.84 m3/h to 20.63 m3/h compared to the layout without cooling under different irradiation levels. The percentage of increase in the electrical PV power output reaches 19.32% with the NCG flow rate of 20.63 m3/h under the irradiation of 900 W/m2 and even the lowest increase improvement is also significant as high as 14.38% under the

condition, air cooling, APC cooling) were collected by the I-V curve tracer and the results are plotted in Fig. 10. Fig. 10a represents current versus voltage for different test condition gained at the irradiation of 800 W/m2. It is clearly found that the area enclosed by the I-V curves and the two axes (which indicates the generated electrical output power by PV panel) is gradually increasing when different cooling approaches applied to the system. Special attention should be given to air cooling (air flow rate of 8.84 m3/h) and APC cooling (NCG flow rate of 8.84 m3/h) when the same parasitic energy consumption is consumed which emphasized that the cooling efficiency of APC is more effective. The maximal electric power output versus voltage is shown in Fig. 10b. As can be seen from this figure, the maximal electric power output was 23.8 W for uncooled PV panel and 27.7 W for APC cooling under the test condition of coolant temperature of 20 °C, flow rate of 20 ml/min and NCG flow rate of 8.84 m3/h. From Table 3 Summary of maximum temperature difference for different cooling techniques. Ref.

Cooling type

Range of solar irradiation (W/m2)

Maximum temperature difference (°C)

Chandrasekar et al. [10] Luo et al. [29] Bahaidarah et al. [30] Valeh-E-Sheyda et al. [31] Rahimi et al. [36] This study

Evaporative cooling PCM cooling Jet cooling Two-phase cooling Microchannel cooling APC cooling

– 115–901 741–969 1000 1000 500–900

5.9 5.1 3.1 20 17.8 4.8

8

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Fig. 10. The electrical performance variations of PV panel under different test conditions (a) I-V characteristic for different test conditions at the irradiation of 800 W/m2, (b) V-P characteristic for different test conditions at the irradiation of 800 W/m2.

irradiation of 500 W/m2.

3.3. Energy and exergy analysis of the system In this section, energy and exergy analysis was done to evaluate the performance of the proposed APC cooling from the standpoint of the first and second law of thermodynamics. The influence of NCG flow rates and irradiation levels on PV panel electrical (energy) efficiency is presented in Fig. 12a. As can be seen from the figure, the electrical efficiency for the tested PV panel increased from 12.39% to 14.67% when the NCG flow rates changed from 0 (uncooled condition) to 20.63 m3/h. This means that the overall electrical efficiency increase in APC cooling system was 18.40% compared to the electrical efficiency of the uncooled PV panel at 900 W/m2. Meanwhile, the electrical efficiency for PV panel system increased from 13.67% to 15.98% when the NCG flow rates changed from 0 (uncooled condition) to 20.63 m3/h at a solar irradiance of 500 W/m2 which indicates overall electrical efficiency increase in APC cooling system was 16.9% compared to the uncooled PV panel. The dramatical increase trend was observed for all the tested condition when the APC cooling method was employed to the PV system. The variation of electrical efficiency with time under different experimental conditions at 900 W/ m2 is shown in Fig. 12b. An unexpectedly decrease was observed for the uncooled and air-cooled condition with time and finally reached 12.30%, 12.82% at a steady-state condition. On the contrary, by

Fig. 11. Percentage of increase in the electrical PV power output for different irradiation levels.

Fig. 12. The variations of electrical efficiency under different test conditions. (a) The influence of NCG flow rates and irradiation levels on electrical efficiency. (b) Variations of electrical efficiency with time under the irradiation of 900 W/m2. 9

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function of exergy efficiency, the same trend of change is observed. For case1 and case2, the maximum entropy generation and sustainability index were 8.4 W/k and 1.152, 8.19 W/k and 1.177 at the irradiation of 900 W/m2, respectively.

adopting the APC cooling, the declining trend of electric efficiency slowed down significantly and achieved the highest electric efficiency of 14.67% which means an increasing percentage of 19.27% and 14.43% were reached comparison with the uncooled and air-cooled condition. The performance response of PV panel in different test conditions (uncooled condition, air cooling: 8.84 m3/h, APC cooling: 20 °C, 20 ml/ min, 8.84 m3/h) are summarized in Table 4. The overall exergy efficiency, exergy loss and IP of APC cooling system were calculated for all of the test conditions then the experiment results under the irradiation of 500 W/m2 and 900 W/m2 are plotted in Fig. 13. As can be seen in Fig. 13a, the similar variation trend of exergy loss and IP of APC cooling system can be found: decreases rapidly and tends to be constant at the irradiation of 500 W/m2. The exergy loss of the whole APC cooling system decreased from 93.09 W to 90.39 W which indicates the overall exergy input to the system is effectively utilized when the proposed cooling method applied to the system. Meanwhile, the variation of IP is changed from 79.43 W to 76.26 W which also proved the validity of APC cooling. However, it is interesting to find that while increasing the flow rates of NCG, the exergy efficiency of the system decreases from 16.25% to 15.63%. This can be explained that although increasing the NCG flow rates can effectively increase the PV power generation, the power consumption (parasitic energy consumption) of DC fan will also increase (What needs to be particularly emphasized here is that we only consider the real power consumption, instead of the equivalent consumption). Even though, the minimum obtained exergy efficiency (15.63%) is well above the one for uncooled condition (14.67%). The variations of the exergy efficiency under the irradiation of 900 W/m2 are shown in Fig. 13b. A similar tendency compared to the irradiation level of 500 W/m2 was observed. The exergy efficiency sharply increases from 13.19% to 15.04% and slowly descend to 14.90% with the NCG flow rates vary from 0 to 20.63 m3/h. This variation tendency can be attributed to the increase of net PV electrical power output in contrast to the experimental condition of 500 W/m2. The exergy loss declined continuously from 170.46 W to 165.46 W which reflects that the total output exergy from the system was dramatically improved and the variation of IP also confirms this viewpoint. The variations of exergy efficiency, exergy loss and IP of the APC cooling system under different cases (case 1: uncooled condition, case 2: 20 °C, 20 ml/min, 8.84 m3/h) are plotted in Fig. 14a. The exergy efficiency of the APC cooling system is significantly higher compared to the uncooled condition, and it could be found that exergy loss and IP have approximately got linear dependence in relation to the total solar irradiance (as shown in Eqs. (20)–(23)).

3.4. Comparisons with other cooling methods To evaluate various cooling techniques proposed by researchers for PV thermal management. Two evaluation concepts, specific power improvement and percentage improvement in power generation, were adopted to make a comparison with previously published research. The evaluation of specific power improvement which is a relative obtained increase in the PV panel’s power output scaled per panel surface as suggested by [15]. Herein, the available data in the published literature are summarized in Table 5. In this research, the peak improvement of power output was 5 W which was achieved from test condition of coolant inlet temperature of 20 °C, coolant flow rate of 20 ml/min, and NCG flow rate of 20.63 m3/h compared to the uncooled PV panel under the irradiation of 900 W/m2. Hence, the specific power improvement of 21.37 W/m2 was obtained if we divide the area (the panel surface of 0.234 m2) by the increased maximum panel power output. Also, it is worth noting that the percentage improvement in power generation is 19.32% in this study which reveals satisfying results and stresses its effectiveness in efficiency enhancement compared to other research studies. 4. Conclusion In this study, a novel type of APC cooling system was designed and experimentally tested for PV panel thermal management and efficiency enhancement on a monocrystalline PV panel. Herein, the effects of APC cooling on average temperature, temperature distribution, maximum electrical power output, energy efficiency as well as exergy efficiency were analyzed in detail. Based on the obtained experimental results, the conclusions derived from the present work can be listed as follows. 1. The experiment results show that the average temperature of the PV panel could be better controlled by adopting APC cooling method compared to the uncooled condition at various irradiation levels. Meanwhile, the obtained 3D temperature distribution map confirmed a uniformity temperature distribution across the panel surface and the maximum temperature difference was less than 5 °C. 2. The cooling efficiency increases with increasing of NCG flow rates and the maximum cooling efficiency is 49.2% at the irradiation of 900 W/m2. 3. The maximum increase of 19.32% and 18.40% in electrical power output and efficiency were achieved under the irradiation of 900 W/ m2 by introducing the APC cooling system. Meanwhile, the maximum specific power improvement gained in this study is 21.37 W/ m2. 4. Compared to the exergy efficiency of 14.67% in uncooled condition, the maximum exergy efficiency of 16.25% was achieved in APC cooling system at the irradiation of 500 W/m2. This indicates that the exergy efficiency is improved about 11% by using the APC cooling method.

·

∑ Exloss - case1 = 0.193G − 3.505

(20)

·

∑ Exloss - case2 = 0.188G − 3.485

(21)

IPcase1 = 0.171G − 6.047

(22)

IPcase2 = 0.162G − 5.371

(23)

Fig. 14b shows the variations of entropy generation and sustainability index under different cases (case 1: uncooled condition, case 2: 20 °C, 20 ml/min, 8.84 m3/h). Since the sustainability index is a Table 4 PV panel performance for different test conditions at the irradiation of 700 W/m2. Applied cooling methods

Average panel temperature (°C)

Maximal power output (W)

Relative increase in power output (%)

Effective increase in power output (%)

Electrical efficiency (%)

Effective increase in electrical efficiency (%)

Uncooled condition Air cooling APC cooling

62.4 55.5 37.4

21.2 22.0 24.6

– 3.77 16.03

– 1.08 13.35

12.94 13.43 15.02

– 1.11 13.37

10

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Fig. 13. Variations of exergy efficiency, exergy loss, and IP under the solar irradiation of (a) 500 W/m2 (b) 900 W/m2.

Fig. 14. Exergy variations of the system at different solar irradiations (a) Variations of the exergy efficiency, exergy loss and IP of the system, (b) variations of the entropy generation and sustainability index of the system. Table 5 Comparisons with different cooling techniques. Ref.

Type of cooling technique

Specific power improvement gained (W/m2)

Percentage improvement in power generation (%)

Alami et al. [11] Nada et al. [13] Tina et al. [32] Moradgholi et al. [33] Nižetić et al. [15] Valeh-E-Sheyda et al. [31] Choubineh et al. [34] Yang et al. [35] This study

Evaporative cooling (Passive) PCM cooling (Passive) Immersion cooling (Passive) Air-cooled with heat pipe cooling (Passive) Water spray cooling (Active) Two-phase flow cooling (Active) Air & PCM cooling (Active) Spray cooling with UBHE system (Active) APC cooling (Active)

13.67 16.50 14.80 9.10 22.60 13.31 17.54 16.07 21.37

19.10 15.36 28.03 8.05 16.30 38.69 7.78 13.33 19.32

Acknowledgments

To sum up, the APC cooling system can effectively control the uprising temperature of PV panel compared to the uncooled condition. The aim of reducing parasitic energy consumption and recovering cooling fluid has been preliminarily achieved. And it can be concluded that the proposed APC cooling system shows great potential and gets a favorable effect on PV panel thermal management and efficiency enhancement.

Acknowledgments are given to Dr. Junpeng Huo (School of Chemical Engineering and Technology, Tianjin University, China) for his contribution to language editing and meaningful discussion in this paper. References

Declaration of Competing Interest

[1] Nematollahi O, Hoghooghi H, Rasti M, et al. Energy demands and renewable energy resources in the Middle East. Renew Sustain Energy Rev 2016;54:1172–81. [2] Sharma NK, Tiwari PK, Sood YR. Solar energy in India: Strategies, policies, perspectives and future potential. Renew Sustain Energy Rev 2012;16:933–41. [3] Rostami Z, Rahimi M, Azimi N. Using high-frequency ultrasound waves and

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 11

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