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A review of solar photovoltaic panel cooling systems with special reference to Ground coupled central panel cooling system (GC-CPCS)
Renewable and Sustainable Energy Reviews 42 (2015) 306–312
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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
A review of solar photovoltaic panel cooling systems with special reference to Ground coupled central panel cooling system (GC-CPCS) Amit Sahay a,n, V.K. Sethi b, A.C. Tiwari c, Mukesh Pandey d a
Research Scholar, Department of Mechanical Engineering, University Institute of Technology, RGPV, Bhopal, Madhya Pradesh, India Ex Director-UIT & Head-School of Energy & Environment Management, University Institute of Technology, RGPV, Bhopal, Madhya Pradesh, India c Professor & Head, Department of Mechanical Engineering, University Institute of Technology, RGPV, Bhopal, Madhya Pradesh, India d Head-School of Energy & Environment Management, RGPV, Bhopal, Madhya Pradesh, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 April 2014 Received in revised form 31 August 2014 Accepted 6 October 2014
The efficiency of wafer-based crystalline as well as Thin film Solar photovoltaic cells get reduced with increase of panel temperature. It is noted that the efficiency drops by about 0.5% for increase of 1 1C of panel temperature. It is necessary to operate them at low temperatures in order to keep the PV module electrical efficiency at acceptable level. Therefore need for a low-cost cooling system for the Solar panels is felt. The cooling of Solar PV panels is a problem of great practical significance. It has the potential to reduce the cost of solar energy in three ways. First, cooling can improve the electrical production of standard flat plate PV modules. Second, cooling makes possible the use of concentrating PV systems. Finally, the heat removed by the PV cooling system can be used for domestic or industrial use. Various Solar PV panel cooling systems have been developed in past. A new system for cooling of Solar PV panels called the Ground-Coupled Central Panel Cooling System (GC-CPCS) is installed and operational at the Energy Park of Rajiv Gandhi Proudyogiki Vishwavidyalaya (RGPV), Bhopal, India. This paper discusses various Solar PV panel cooling technologies and the testing and performance of the newly developed Ground-Coupled Central Panel Cooling System (GC-CPCS). This study is unique in three ways. First, it introduces the concept of Central cooling of Solar PV panels. Second, Ground Coupled Heat Exchanger is used for the first time in any Solar PV Panel Cooling System. Third, the testing is done on site under actual solar irradiance. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Efficiency of solar PV cells Solar PV panel cooling systems Ground coupled heat exchanger (GCHEX) Central cooling systems Ground-coupled central panel cooling system (GC-CPCS) Analysis of variance (ANOVA)
Contents 1.
2.
3.
n
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 1.1. Solar PV panel cooling systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 1.2. Design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 1.3. Various panel cooling technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 1.3.1. Hybrid PV/thermal (PV/T) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 1.3.2. Microchannel cooling system [5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 1.3.3. Thermoelectric cooling system [7] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 1.3.4. Cooling using heat pipe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 1.3.5. Water film cooling system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 Ground coupled central panel cooling system (GC-CPCS) [14,15] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 2.1. Ground – coupled heat exchanger /earth tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 2.2. Central panel cooling system (CPCS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 Testing of GC-CPCS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 3.1. Smoke flow visualization [11] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 3.2. Data collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
Corresponding author. E-mail addresses: [email protected] (A. Sahay), [email protected] (V.K. Sethi), [email protected] (A.C. Tiwari), [email protected] (M. Pandey).
http://dx.doi.org/10.1016/j.rser.2014.10.009 1364-0321/& 2014 Elsevier Ltd. All rights reserved.
A. Sahay et al. / Renewable and Sustainable Energy Reviews 42 (2015) 306–312
3.2.1. Conversion efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Salient points of the observations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ANOVA analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1. ANOVA analysis for conversion efficiency and its inferences. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. 3.4.
1. Introduction An illuminated PV cell converts only a small fraction of irradiance into electrical energy. The balance is converted into heat, resulting into heating of the cell. As a result, the cell operates above ambient temperature. Keeping insolation level as constant, if the temperature is increased, there is a marginal increase in the cell current but a marked reduction in cell voltage [8]. An increase in temperature causes reduction in the band gap. This in turn causes some increase in photo-generation rate and thus, a marginal increase in current. However, the reverse saturation current increases rapidly with temperature. Due to this, the cell voltage decreases by approximately 2.2 mV per 1C rise in its operating temperature, depending on the resistivity of the silicon used: higher the silicon resistivity more marked is the temperature effect. The net effect is that the conversion efficiency decreases [9] (Fig. 1). Therefore it is beneficial to apply artificial cooling to PV modules for the improvement in power output and optimum performance parameters [2]. Various methods have been applied for this purpose viz. Hybrid PV/T system, Micro-channel cooling system, Thermo-electric cooling system, Heat pipe cooling system, Mist water cooling system, Water film cooling system. A new system is developed for Solar PV panel cooling which is referred to as Ground Coupled Central Panel Cooling System (GCCPCS) [14,15]. This system is designed, fabricated and tested at the Energy Park of Rajiv Gandhi Proudyogiki Vishwavidyalaya (RGPV), Bhopal, India [16]. This paper deals with the performance testing and statistical validation of GC-CPCS. The salient points of this system as well as this work are as follows: (a) Ground coupled Heat Exchanger [3] is used to cool the ambient air which in turn is used to cool the Solar PV panels. (b) Central cooling of panels is analogous to Central Air Conditioning system. Central cooling is well adapted for the Panel cooling system because a Solar PV power plant comprises of large number of Solar PV panels installed in tandem to each
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other. It is not economically viable to have separate cooling system for each solar panel. Hence it is convenient to cool the panels using a Central Panel Cooling System. (c) Testing is done on site under actual solar irradiance. Most of the studies are conducted in laboratory under Standard Test Condition. Solar panel manufacturers put the Solar panel in a flash that has been calibrated to deliver the equivalent of 1000 W/m2 of sunlight intensity, hold a cell temperature of 25 1C. This flash test gives the STC ratings. However STC does not resemble the ‘Real world’ conditions.
1.1. Solar PV panel cooling systems Various methods have been tried for the cooling of Photovoltaic panels. They can be classified based on the following broad parameters.
Single phase system or Two phase system Working fluid used viz. air, water With or without cogeneration Active or passive Having moving parts or no moving part
1.2. Design considerations The design considerations for cooling of PV panels draws from the pioneering work by Royne [12]. The following cooling and operational requirements are considered [14,15].
Cell Temperature Uniformity of temperature Reliability & less maintenance cost Pumping power Minimal capital cost
1.3. Various panel cooling technologies Various panel cooling technologies have been developed. The details are as follows.
Fig. 1. Variation of conversion efficiency vs. temperature of a PV cell [8].
1.3.1. Hybrid PV/thermal (PV/T) Hybrid Photovoltaic/thermal (PV/T) solar systems produce electricity and heat simultaneously and produce a higher energy conversion rate of solar radiation than typical PV modules. PV module cooling contributes to improved PV electrical efficiency and ambient air circulation is utilized as a simple mode for heat extraction. The thermal energy is put to use – typically for space heating, water preheating, ventilation, or for industrial applications such as food drying (Fig. 2). By placing a solar thermal collector behind a solar photovoltaic array, the PV cells can be cooled. At the same time, the solar collector
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Nomenclature Symbols A G I V
Area of panel (m2) Global irradiance (W/m2) Current (A) Voltage (Volt)
Greek symbols
η
Panel conversion efficiency
Abbreviations ABS Absolute time (minutes) CPCS Central Panel Cooling System GC Ground Coupled GC-CPCS Ground Coupled Central Panel Cooling System IST Indian Standard Time OFF Blower is switched OFF ON Blower is switched ON PV Photovoltaic PV/T Hybrid Photovoltaic Thermal PVT/AIR Hybrid Photovoltaic Thermal-Air PVT/WATER Hybrid Photovoltaic Thermal-Water
can harvest most of the energy that passes through the array that would otherwise be lost, recovering it for productive use [17]. Like the thermal solar collectors, PV/T, systems are also categorized according to the kind of heat removal fluid. Hence PVT/ WATER and PVT/AIR are common types, for water and air heat removal fluids respectively.
RGPV Rajiv Gandhi Proudyogiki Vishwavidyalaya STATUS State of blower, whether it is switched ON or switched OFF STC Standard Test Condition STD Local time (IST) Statistical symbols 5% F-limit F-ratio at 5% level of significance ANOVA Analysis of Variance c number of columns CF Correction Factor Cj Sum of jth column DF Degree of freedom F-ratio The ratio of the variance between samples to the variance within samples i ith row j jth column MS Mean square n Total number of observations ¼ c x r r number of rows Ri Sum of ith row SS Sum of square T ∑Xij Xij Observation (i,j)
Multiple microchannels are machined on the back of the substrates of electronic components in integrated circuits. The heat generated by the electronic components is transferred to the coolant by forced convection [6] (Figs. 3 and 4).
1.3.2. Microchannel cooling system [5] Micro heat exchangers are heat exchangers in which fluid flows in lateral confinements with typical dimensions below 1 mm. The most typical such confinement are microchannel which are channels with a hydraulic diameter below 1 mm. Microchannel heat sinks were introduced in 1981 by Tuckerman and Pease.
Fig. 3. Schematic design of micro channel cooling system displaying the heat dissipating electronic chip having micro channel heat sink and external heat exchanger.
Fig. 2. Solar photovoltaic system combined with solar thermal system in a hybrid photovoltaic thermal (PV/T) system.
Fig. 4. Micro channel carved in a silicon wafer for micro channel cooling system [5].
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Fig. 5. Schematic design of thermo electric cooling system [7].
Fig. 7. Front view of solar photovoltaic panel equipped with water film producer [4].
Fig. 6. Heat pipe cross section [13].
1.3.3. Thermoelectric cooling system [7] Thermoelectric devices are composed of two semiconductors– n-type and p-type. These two materials are connected, electrically in series but thermally in parallel [7]. Under a temperature gradient, the majority carriers diffuse from the hot side to the cold side setting up a voltage and resultant current (Fig. 5). In the reverse effect, an applied voltage forces a current through the materials causing an effective heat pump that cools one side and heats up the other. The hot side must be connected to a hest sink to dissipate the excess heat.
1.3.4. Cooling using heat pipe The term ‘heat pipe’ is a device for transferring heat from a source to sink by means of evaporation and condensation of a fluid in a sealed system. It transports heat by two-phase flow of a working fluid [13]. Heat pipes are passive thermal devices that transfer heat across long distances with a very low drop in temperature. A heat pipe is a vacuum tight device consisting of a working fluid and a wick structure. The heat input vaporizes the liquid working fluid inside the wick in the evaporator section. The vapor, carrying the latent heat of vaporization, flows towards the cooler condenser section. In the condenser, the vapor condenses and gives up its latent heat. The condensed liquid returns to the evaporator through the wick structure by capillary action. The phase change processes and two-phase flow circulation continue as long as the temperature gradients between the evaporator and condenser are maintained (Fig. 6).
1.3.5. Water film cooling system In this method of cooling of Solar PV panels a thin film of water is flown over the front side of the PV panels to decrease its temperature. By using this method reflection is also reduced and therefore the conversion efficiency will improve. The extracted thermal energy can be used in several ways, increasing total energy output of the system [4] (Fig. 7).
Fig. 8. Arrangement of solar panel, fiber sheet and nozzles [14,15].
2. Ground coupled central panel cooling system (GC-CPCS) [14,15] The proposed method suffices to cool the Solar Panels by Forced Convection of ambient air driven by a blower. The blower is run by power provided by a separate and dedicated PV panel. Air is passed through a ground-coupled heat exchanger to drop its temperature. The cooled air cools the Solar panels as it passes through the rear surface of the Solar Panels. To facilitate develop streamlines of fluid flow, fiber sheets are placed towards the rear surface of the Solar panels (Fig. 8). Natural convection is also possible in this set up. There are nine Solar Panels of 100 W each. The air is driven by a single blower. This design is similar to Central cooling system. Hence the term, Central Panel cooling System (CPCS), is conceived. The cooled air is distributed to each Solar Panels by means of a pipe. Nozzles are provided on the pipe through which the air comes out. Nozzles ensure that streamlines are developed in the desired direction. 2.1. Ground – coupled heat exchanger /earth tubes A Ground-Coupled heat exchanger [3] is an underground heat exchanger that can (a) capture heat from ground and/or
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Fig. 10. Thin-film PV panels at RGPV, Bhopal.
Fig. 9. Schematic design of ground coupled-central panel cooling system (GCCPCS) [14,15].
(b) dissipate heat to the ground. They use the Earth's near constant temperature to warm or cool air for residential or industrial use. They are also called Earth Tubes. Earth Tubes are viable and economical alternative to conventional Central Cooling Systems since there are no compressors, chemicals or burners and only blowers are required to move the air. (http://en.wikipedia.org/ wiki/Ground-coupled_heat_exchanger) [18]. 2.2. Central panel cooling system (CPCS) Here we draw analogy from the widely used Central Air Conditioning Systems. There are two broad categories of Air conditioning systems: (a) Centralized air conditioning systems (b) Decentralized air conditioning systems [1] Centralized air conditioning systems serve multiple spaces from one base location. Decentralized air conditioning systems typically serve single or small spaces from a location within or directly adjacent to the space [1]. Central Panel Cooling System (CPCS) is similar to Centralized air conditioning systems. In Centralized air conditioning systems chilled water is used as a cooling medium. The proposed Central Panel Cooling System (CPCS) is designed to use the cooled air, which comes through the ground coupled heat exchanger, as the working fluid. This complete system is termed as Ground-coupled Central Panel Cooling System (GC-CPCS). The schematic design is shown in Fig. 9. Design of underground heat exchanger, pipe, nozzles, and calculation of head losses, power of blower etc. are made. For details please refer the work by Sahay Amit et al. [14,15]. This system is installed at Rajiv Gandhi Proudyogiki Vishwavidyalaya (RGPV), Bhopal, India (Figs. 10 and 11). Here number of Solar panels is nine. The Fabrication and instrumentation schemes of the system are presented by [16]. The specifications of the Solar PV panels are as follows: Make: Schott Solar Type: Solar Thin Film Module, Single junction a-Si/ CIGS Technology Capacity: 122 W (Initial); 100 W (Stabilized) Voltage at Nominal power: 17.5 V Current at Nominal power: 5.71 A Open circuit voltage: 23.8 V Short circuit current: 6.79 A Dimensions: 1.27 m x 1.27 m Area: 1.6129 m2 Thickness: 50 mm Weight: 18 kg Number of such panels ¼09
Fig. 11. Thin-film PV panels at RGPV, Bhopal, after the installation of GC-CPCS [16].
3. Testing of GC-CPCS 3.1. Smoke flow visualization [11] Flow visualization is a technique used in Fluid Mechanics to make the flow pattern visible. In the Smoke Flow Visualization technique, smoke is advected in a real flow of gases and its temporal behavior gives information about the flow [11]. To perform this test, incense stick (Dhoop) is used to form smoke. The smoke is then fed at the inlet of the blower which is instantly transported to all the panels (Fig. 12). The smoke comes out from the entire length of the panel assembly. This clearly indicates that GCCPCS is able to uniformly distribute the cool air throughout the panels. 3.2. Data collection The state of existent convective cooling versus no convective cooling is distinguished by status of the blower as ON and OFF respectively. Therefore observations are taken by keeping the blower OFF and ON alternately allowing response time of 10 minutes. The data collected per observation are the following which are compiled in Table 1. (a) (b) (c) (d) (e) (f) (g)
Time Blower status – OFF or ON Outlet Air temperature (from ground-coupled heat exchanger) Irradiance Current Voltage Conversion efficiency
3.2.1. Conversion efficiency The power output of solar panel is given by Power output ¼ Current (I) x Voltage (V)
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(c) Therefore it is a matter of great interest to establish the causal effect of panel temperature and cooling action on the conversion efficiency. The conversion efficiency at different panel temperatures and blower status – OFF and ON – is tabulated at Table 2. (d) There is marked increase in conversion efficiency when the blower is ON.
The power input of solar panel is given by Power input ¼Irradiance (G) x Area of panel (A) Since, Efficiency ¼Output / Input Therefore, η ¼ I x V x 100=G x A
3.3. Salient points of the observations The salient points of the observations are given below.
3.4. ANOVA analysis (a) Solar Irradiance (radiation) is time varying. (b) Conversion efficiency of Solar PV panel varies with the panel temperature as well as whether the Panel cooling system is not running or running i.e. Blower is not running or running (Blower status – OFF or ON).
Analysis of Variance (ANOVA) technique is used to study whether efficiencies differ significantly due to convective cooling or Panel temperature. The basic principle of ANOVA is to test for difference among the means of the population by examining the amount of variation within each of these samples relative to the amount of variation between the samples [10]. Two-way ANOVA technique is used when the data are classified on the basis of two factors. 3.4.1. ANOVA analysis for conversion efficiency and its inferences To perform ANOVA analysis and then make ANOVA table, the following calculations are made using data from Table 2. c¼2; r¼4; n¼c x r¼ 8 T¼ ∑Xij ¼ 8.56þ 10 þ11.71 þ7.83 þ10.97 þ12.24 þ13.03 þ9.15 ¼83.49 Table 2 Efficiency data at different panel temperatures and blower status – OFF or ON; This data is used for ANOVA analysis to study causal effect of cooling on the conversion efficiency. Blower – off (Cooling system – off) Blower – on (Cooling system – on) Ri
Fig. 12. Smoke flow visualization test conducted to test the GC-CPCS [16].
Panel temp. 1C
Efficiency %
Panel temp. 1C
Efficiency %
57 49 49 53 Cj
8.56 10 11.71 7.83 38.1
55 47 47 50
10.97 12.24 13.03 9.15 45.39
19.53 22.24 24.74 16.98
Table 1 Observations. TESTING OF GC-CPCS TYPE OF TEST: TWO WAY ANOVA TEST SL NO. 5 DATE: 03-05-2014 AMBIENT TEMP.: 2 1C Apanel ¼1.6129 m2 No. of panel ¼ 9 Time ABS Minutes
Time STD IST
Blower STATUS ON/O FF
Panel temperature
Outlet air temp. 1C
Irradiance G W/m2
Current I Ampere
Voltage V Volt
Efficiency η %
1C
0 0 10 10 20 20 30 30 40 40 50 50 60 60 70
14:58 14:58 15:08 15:08 15:18 15:18 15:28 15:28 15:38 15:38 15:48 15:48 15:58 15:58 16:08
OFF ON ON OFF OFF ON ON OFF OFF ON ON OFF OFF ON ON
57 X 55 X 49 X 47 X 49 X 47 X 53 X 50
X X 27 X X X 27 X X X 27 X X X 27
460 X 360 X 400 X 330 X 340 X 310 X 520 X 444
3.1 X 3.1 X 3.1 X 3.1 X 3.1 X 3.1 X 3.1 X 3.1
184.4 X 185 X 187.3 X 189.1 X 186.5 X 189.1 X 190.7 X 190.2
8.56 X 10.97 X 10 X 12.24 X 11.71 X 13.03 X 7.83 X 9.15
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Table 3 ANOVA table (Two way) for conversion efficiency. Sources of variation Between columns (Effect of cooling action) Between rows (Effect of panel temperature) Residual
SS
DF
MS
6.643 16.89 0.513
1 3 3
6.643 5.63 0.171
CF ¼T2/n¼(83.49)2/8 ¼871.323 Total SS¼∑X2ij –CF¼(8.56)2 þ(10)2 þ(11.71)2 þ (7.83)2 þ(10.97)2 þ (12.24)2 þ(13.03)2 þ (9.15)2 -871.323¼24.046 Cj ¼[38.1, 45.39] Ri ¼[19.53, 22.24, 24.74, 16.98] SS between columns¼ ∑C2j /r-CF¼ [(38.1)2 þ (45.39)2]/4–871.323¼ 6.643 SS between rows ¼∑R2i /c-CF ¼[(19.53)2 þ(22.24)2 þ(24.74)2 þ (16.98)2]/2–871.323 ¼16.89 SS residual¼Total SS – (SS between columns þ SS between rows)¼24.046 – (6.643þ16.89) ¼ 0.513 Degree of freedom for variance between columns ¼ c – 1 ¼2 - 1¼ 1 Degree of freedom for variance between rows¼ r – 1 ¼4–1 ¼ 3 Degree of freedom for residual variance ¼ (c–1) (r–1) ¼(2–1) (4–1) ¼3 MS between columns ¼ SS between columns/Degree of freedom ¼ 6.643/1 ¼6.643 MS between rows¼ SS between rows/Degree of freedom¼ 16.89/3 ¼ 5.63 MS for residual ¼ SS residual/Degree of freedom¼ 0.513/3 ¼ 0.171 F-ratio between column¼ MS between columns / MS for residual ¼ 6.643/0.171 ¼38.85 F-ratio between rows¼ MS between rows/MS for residual¼ 5.63/0.171 ¼32.92 Using the above data ANOVA table is tabulated (Table 3). The observations of the ANOVA table are the following. (a) It is observed that the calculated F-ratio for column, 38.85, is more than the table value of F-ration at 5% level of significance. Therefore the effect of Blower status on the efficiency is significant. Hence the cooling effect is found to be significant. This validates the efficacy of GC-CPCS. (b) It is observed that the calculated F-ratio for row, 32.92, is more than the table value of F-ratio at 5% level of significance. Therefore the effect of Panel temperature on the efficiency is also significant. 4. Conclusions The panel cooling system, GC-CPCS, is tested by using Smoke Flow Visualization technique. There is marked increase in conversion efficiency due to cooling of PV panels. The data is analyzed
F-Ratio 38.85 32.92
5% F-limit F(1,3) ¼10.13 F(3,3) ¼ 9.28
using ANOVA analysis to establish the causal effect of the cooling action. The cooling system is retrofitted. If it is integrated with the design of Solar PV panels then more effective cooling is expected and hence better conversion efficiency. References [1] Bhatia A. Centralized Vs Decentralized air conditioning systems, course No : M05-012, continuing education and development, Inc., NY, 〈info@cedengineer ing.com〉. [2] Cruey Bryce, King Jordan, Tingleff. Cooling of photovoltaic cells, 〈http://bob. tingleff.com/pvcooling.pdf〉; 2006. [3] Ground-Coupled Heat Exchanger. 〈http://en.wikipedia.org/wiki/ Ground-coupled_heat_exchanger〉. [4] Hosseini R, Hosseini N, Khorasanizadeh H. An Experimental study of combining a photovoltaic system with heating system. World Renewable Energy Congress 2011 Sweden, 2011. [5] Jiang Linan et al. Closed-loop electroosmotic microchannel cooling system for VLSI circuits. IEEE T Compon Pack T 25(3): September 2002, 2002. [6] Kandlikar Satish G. (2005). High flux heat removal with microchannels – a roadmap of challenges and opportunities. Heat Transfer Eng, 26. Taylor & Francis; 2005; 5–14 (ISSN:0145-7632, 1521-0537 online.). [7] Kane Arati, Verma Vishal. Performance enhancement of building integrated photovoltaic module using thermoelectric cooling. Int J Renew Energ Res 2013;3(2):2013. [8] Katkar AA, Shinde NN, Patil PS. Performance & evaluation of industrial solar cell w.r.t temperature and humidity. Int J Res Mech Eng Technol, 2249-5762 2011;1(1) (Online). [9] Khan BH. Non-conventional energy resources. New Delhi.: Tata McGraw-Hill Publishing Company Limited; 2004. [10] Kothari CR. Research methodology methods and techniques. New Delhi: NewAge International (P) Ltd., Publishers; 2008 (ISBN(13) : 978-81-2241522-3). [11] Rathakrishnan E. Instrumentation, measurements, and experiments in fluids. CRC Press, Taylor and Francis Group; 978-08-493075-91. [12] Royne Anja. Cooling devices for densely packed high concentration PV arrays, A Thesis submitted to the University of Sydney for the degree of Master of Science, 2005. [13] Russell RF. Uniform temperature heat pipe and method of using the same, Patent no. US4320246, 1982. [14] Sahay Amit, Sethi VK, Tiwari AC. A comparative study of attributes of thin film and crystalline photovoltaic cells. VSRD Int J Mech Civil, Automobile & Production Eng 2013;3(7):2013 (July). [15] Sahay Amit, Sethi VK, Tiwari AC. Design, optimisation, and system integration of low cost ground coupled central panel cooling system (GC-CPCS). (ISSN 2277-4106). International Journal of Current Engineering and Technology 2013;3(4):1473–9 (October 2013, page no). [16] Sahay Amit, Sethi VK, Tiwari AC. Fabrication scheme, instrumentation scheme and testing of ground coupled central panel cooling system (GC-CPCS). (ISSN 2277-4106). Int J Current Eng Technol 2014;4(2):631–8 (April 2014, page no). [17] Tonui JK, Tripanagnostopoulos. Improved PV/T solar collectors with heat extraction by forced or natural air circulation. Renew Energ 2007;32:623–37. [18] Walsh Brett Vincent Analysis of ground coupled heat exchanger, an engineering project submitted to the graduate faculty of Rensselaer Polytechnic Institute, Hartford, Connecticut. December 2011, 2011.
Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
A review of solar photovoltaic panel cooling systems with special reference to Ground coupled central panel cooling system (GC-CPCS) Amit Sahay a,n, V.K. Sethi b, A.C. Tiwari c, Mukesh Pandey d a
Research Scholar, Department of Mechanical Engineering, University Institute of Technology, RGPV, Bhopal, Madhya Pradesh, India Ex Director-UIT & Head-School of Energy & Environment Management, University Institute of Technology, RGPV, Bhopal, Madhya Pradesh, India c Professor & Head, Department of Mechanical Engineering, University Institute of Technology, RGPV, Bhopal, Madhya Pradesh, India d Head-School of Energy & Environment Management, RGPV, Bhopal, Madhya Pradesh, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 April 2014 Received in revised form 31 August 2014 Accepted 6 October 2014
The efficiency of wafer-based crystalline as well as Thin film Solar photovoltaic cells get reduced with increase of panel temperature. It is noted that the efficiency drops by about 0.5% for increase of 1 1C of panel temperature. It is necessary to operate them at low temperatures in order to keep the PV module electrical efficiency at acceptable level. Therefore need for a low-cost cooling system for the Solar panels is felt. The cooling of Solar PV panels is a problem of great practical significance. It has the potential to reduce the cost of solar energy in three ways. First, cooling can improve the electrical production of standard flat plate PV modules. Second, cooling makes possible the use of concentrating PV systems. Finally, the heat removed by the PV cooling system can be used for domestic or industrial use. Various Solar PV panel cooling systems have been developed in past. A new system for cooling of Solar PV panels called the Ground-Coupled Central Panel Cooling System (GC-CPCS) is installed and operational at the Energy Park of Rajiv Gandhi Proudyogiki Vishwavidyalaya (RGPV), Bhopal, India. This paper discusses various Solar PV panel cooling technologies and the testing and performance of the newly developed Ground-Coupled Central Panel Cooling System (GC-CPCS). This study is unique in three ways. First, it introduces the concept of Central cooling of Solar PV panels. Second, Ground Coupled Heat Exchanger is used for the first time in any Solar PV Panel Cooling System. Third, the testing is done on site under actual solar irradiance. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Efficiency of solar PV cells Solar PV panel cooling systems Ground coupled heat exchanger (GCHEX) Central cooling systems Ground-coupled central panel cooling system (GC-CPCS) Analysis of variance (ANOVA)
Contents 1.
2.
3.
n
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 1.1. Solar PV panel cooling systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 1.2. Design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 1.3. Various panel cooling technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 1.3.1. Hybrid PV/thermal (PV/T) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 1.3.2. Microchannel cooling system [5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 1.3.3. Thermoelectric cooling system [7] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 1.3.4. Cooling using heat pipe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 1.3.5. Water film cooling system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 Ground coupled central panel cooling system (GC-CPCS) [14,15] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 2.1. Ground – coupled heat exchanger /earth tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 2.2. Central panel cooling system (CPCS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 Testing of GC-CPCS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 3.1. Smoke flow visualization [11] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 3.2. Data collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
Corresponding author. E-mail addresses: [email protected] (A. Sahay), [email protected] (V.K. Sethi), [email protected] (A.C. Tiwari), [email protected] (M. Pandey).
http://dx.doi.org/10.1016/j.rser.2014.10.009 1364-0321/& 2014 Elsevier Ltd. All rights reserved.
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3.2.1. Conversion efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Salient points of the observations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ANOVA analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1. ANOVA analysis for conversion efficiency and its inferences. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. 3.4.
1. Introduction An illuminated PV cell converts only a small fraction of irradiance into electrical energy. The balance is converted into heat, resulting into heating of the cell. As a result, the cell operates above ambient temperature. Keeping insolation level as constant, if the temperature is increased, there is a marginal increase in the cell current but a marked reduction in cell voltage [8]. An increase in temperature causes reduction in the band gap. This in turn causes some increase in photo-generation rate and thus, a marginal increase in current. However, the reverse saturation current increases rapidly with temperature. Due to this, the cell voltage decreases by approximately 2.2 mV per 1C rise in its operating temperature, depending on the resistivity of the silicon used: higher the silicon resistivity more marked is the temperature effect. The net effect is that the conversion efficiency decreases [9] (Fig. 1). Therefore it is beneficial to apply artificial cooling to PV modules for the improvement in power output and optimum performance parameters [2]. Various methods have been applied for this purpose viz. Hybrid PV/T system, Micro-channel cooling system, Thermo-electric cooling system, Heat pipe cooling system, Mist water cooling system, Water film cooling system. A new system is developed for Solar PV panel cooling which is referred to as Ground Coupled Central Panel Cooling System (GCCPCS) [14,15]. This system is designed, fabricated and tested at the Energy Park of Rajiv Gandhi Proudyogiki Vishwavidyalaya (RGPV), Bhopal, India [16]. This paper deals with the performance testing and statistical validation of GC-CPCS. The salient points of this system as well as this work are as follows: (a) Ground coupled Heat Exchanger [3] is used to cool the ambient air which in turn is used to cool the Solar PV panels. (b) Central cooling of panels is analogous to Central Air Conditioning system. Central cooling is well adapted for the Panel cooling system because a Solar PV power plant comprises of large number of Solar PV panels installed in tandem to each
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other. It is not economically viable to have separate cooling system for each solar panel. Hence it is convenient to cool the panels using a Central Panel Cooling System. (c) Testing is done on site under actual solar irradiance. Most of the studies are conducted in laboratory under Standard Test Condition. Solar panel manufacturers put the Solar panel in a flash that has been calibrated to deliver the equivalent of 1000 W/m2 of sunlight intensity, hold a cell temperature of 25 1C. This flash test gives the STC ratings. However STC does not resemble the ‘Real world’ conditions.
1.1. Solar PV panel cooling systems Various methods have been tried for the cooling of Photovoltaic panels. They can be classified based on the following broad parameters.
Single phase system or Two phase system Working fluid used viz. air, water With or without cogeneration Active or passive Having moving parts or no moving part
1.2. Design considerations The design considerations for cooling of PV panels draws from the pioneering work by Royne [12]. The following cooling and operational requirements are considered [14,15].
Cell Temperature Uniformity of temperature Reliability & less maintenance cost Pumping power Minimal capital cost
1.3. Various panel cooling technologies Various panel cooling technologies have been developed. The details are as follows.
Fig. 1. Variation of conversion efficiency vs. temperature of a PV cell [8].
1.3.1. Hybrid PV/thermal (PV/T) Hybrid Photovoltaic/thermal (PV/T) solar systems produce electricity and heat simultaneously and produce a higher energy conversion rate of solar radiation than typical PV modules. PV module cooling contributes to improved PV electrical efficiency and ambient air circulation is utilized as a simple mode for heat extraction. The thermal energy is put to use – typically for space heating, water preheating, ventilation, or for industrial applications such as food drying (Fig. 2). By placing a solar thermal collector behind a solar photovoltaic array, the PV cells can be cooled. At the same time, the solar collector
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Nomenclature Symbols A G I V
Area of panel (m2) Global irradiance (W/m2) Current (A) Voltage (Volt)
Greek symbols
η
Panel conversion efficiency
Abbreviations ABS Absolute time (minutes) CPCS Central Panel Cooling System GC Ground Coupled GC-CPCS Ground Coupled Central Panel Cooling System IST Indian Standard Time OFF Blower is switched OFF ON Blower is switched ON PV Photovoltaic PV/T Hybrid Photovoltaic Thermal PVT/AIR Hybrid Photovoltaic Thermal-Air PVT/WATER Hybrid Photovoltaic Thermal-Water
can harvest most of the energy that passes through the array that would otherwise be lost, recovering it for productive use [17]. Like the thermal solar collectors, PV/T, systems are also categorized according to the kind of heat removal fluid. Hence PVT/ WATER and PVT/AIR are common types, for water and air heat removal fluids respectively.
RGPV Rajiv Gandhi Proudyogiki Vishwavidyalaya STATUS State of blower, whether it is switched ON or switched OFF STC Standard Test Condition STD Local time (IST) Statistical symbols 5% F-limit F-ratio at 5% level of significance ANOVA Analysis of Variance c number of columns CF Correction Factor Cj Sum of jth column DF Degree of freedom F-ratio The ratio of the variance between samples to the variance within samples i ith row j jth column MS Mean square n Total number of observations ¼ c x r r number of rows Ri Sum of ith row SS Sum of square T ∑Xij Xij Observation (i,j)
Multiple microchannels are machined on the back of the substrates of electronic components in integrated circuits. The heat generated by the electronic components is transferred to the coolant by forced convection [6] (Figs. 3 and 4).
1.3.2. Microchannel cooling system [5] Micro heat exchangers are heat exchangers in which fluid flows in lateral confinements with typical dimensions below 1 mm. The most typical such confinement are microchannel which are channels with a hydraulic diameter below 1 mm. Microchannel heat sinks were introduced in 1981 by Tuckerman and Pease.
Fig. 3. Schematic design of micro channel cooling system displaying the heat dissipating electronic chip having micro channel heat sink and external heat exchanger.
Fig. 2. Solar photovoltaic system combined with solar thermal system in a hybrid photovoltaic thermal (PV/T) system.
Fig. 4. Micro channel carved in a silicon wafer for micro channel cooling system [5].
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Fig. 5. Schematic design of thermo electric cooling system [7].
Fig. 7. Front view of solar photovoltaic panel equipped with water film producer [4].
Fig. 6. Heat pipe cross section [13].
1.3.3. Thermoelectric cooling system [7] Thermoelectric devices are composed of two semiconductors– n-type and p-type. These two materials are connected, electrically in series but thermally in parallel [7]. Under a temperature gradient, the majority carriers diffuse from the hot side to the cold side setting up a voltage and resultant current (Fig. 5). In the reverse effect, an applied voltage forces a current through the materials causing an effective heat pump that cools one side and heats up the other. The hot side must be connected to a hest sink to dissipate the excess heat.
1.3.4. Cooling using heat pipe The term ‘heat pipe’ is a device for transferring heat from a source to sink by means of evaporation and condensation of a fluid in a sealed system. It transports heat by two-phase flow of a working fluid [13]. Heat pipes are passive thermal devices that transfer heat across long distances with a very low drop in temperature. A heat pipe is a vacuum tight device consisting of a working fluid and a wick structure. The heat input vaporizes the liquid working fluid inside the wick in the evaporator section. The vapor, carrying the latent heat of vaporization, flows towards the cooler condenser section. In the condenser, the vapor condenses and gives up its latent heat. The condensed liquid returns to the evaporator through the wick structure by capillary action. The phase change processes and two-phase flow circulation continue as long as the temperature gradients between the evaporator and condenser are maintained (Fig. 6).
1.3.5. Water film cooling system In this method of cooling of Solar PV panels a thin film of water is flown over the front side of the PV panels to decrease its temperature. By using this method reflection is also reduced and therefore the conversion efficiency will improve. The extracted thermal energy can be used in several ways, increasing total energy output of the system [4] (Fig. 7).
Fig. 8. Arrangement of solar panel, fiber sheet and nozzles [14,15].
2. Ground coupled central panel cooling system (GC-CPCS) [14,15] The proposed method suffices to cool the Solar Panels by Forced Convection of ambient air driven by a blower. The blower is run by power provided by a separate and dedicated PV panel. Air is passed through a ground-coupled heat exchanger to drop its temperature. The cooled air cools the Solar panels as it passes through the rear surface of the Solar Panels. To facilitate develop streamlines of fluid flow, fiber sheets are placed towards the rear surface of the Solar panels (Fig. 8). Natural convection is also possible in this set up. There are nine Solar Panels of 100 W each. The air is driven by a single blower. This design is similar to Central cooling system. Hence the term, Central Panel cooling System (CPCS), is conceived. The cooled air is distributed to each Solar Panels by means of a pipe. Nozzles are provided on the pipe through which the air comes out. Nozzles ensure that streamlines are developed in the desired direction. 2.1. Ground – coupled heat exchanger /earth tubes A Ground-Coupled heat exchanger [3] is an underground heat exchanger that can (a) capture heat from ground and/or
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Fig. 10. Thin-film PV panels at RGPV, Bhopal.
Fig. 9. Schematic design of ground coupled-central panel cooling system (GCCPCS) [14,15].
(b) dissipate heat to the ground. They use the Earth's near constant temperature to warm or cool air for residential or industrial use. They are also called Earth Tubes. Earth Tubes are viable and economical alternative to conventional Central Cooling Systems since there are no compressors, chemicals or burners and only blowers are required to move the air. (http://en.wikipedia.org/ wiki/Ground-coupled_heat_exchanger) [18]. 2.2. Central panel cooling system (CPCS) Here we draw analogy from the widely used Central Air Conditioning Systems. There are two broad categories of Air conditioning systems: (a) Centralized air conditioning systems (b) Decentralized air conditioning systems [1] Centralized air conditioning systems serve multiple spaces from one base location. Decentralized air conditioning systems typically serve single or small spaces from a location within or directly adjacent to the space [1]. Central Panel Cooling System (CPCS) is similar to Centralized air conditioning systems. In Centralized air conditioning systems chilled water is used as a cooling medium. The proposed Central Panel Cooling System (CPCS) is designed to use the cooled air, which comes through the ground coupled heat exchanger, as the working fluid. This complete system is termed as Ground-coupled Central Panel Cooling System (GC-CPCS). The schematic design is shown in Fig. 9. Design of underground heat exchanger, pipe, nozzles, and calculation of head losses, power of blower etc. are made. For details please refer the work by Sahay Amit et al. [14,15]. This system is installed at Rajiv Gandhi Proudyogiki Vishwavidyalaya (RGPV), Bhopal, India (Figs. 10 and 11). Here number of Solar panels is nine. The Fabrication and instrumentation schemes of the system are presented by [16]. The specifications of the Solar PV panels are as follows: Make: Schott Solar Type: Solar Thin Film Module, Single junction a-Si/ CIGS Technology Capacity: 122 W (Initial); 100 W (Stabilized) Voltage at Nominal power: 17.5 V Current at Nominal power: 5.71 A Open circuit voltage: 23.8 V Short circuit current: 6.79 A Dimensions: 1.27 m x 1.27 m Area: 1.6129 m2 Thickness: 50 mm Weight: 18 kg Number of such panels ¼09
Fig. 11. Thin-film PV panels at RGPV, Bhopal, after the installation of GC-CPCS [16].
3. Testing of GC-CPCS 3.1. Smoke flow visualization [11] Flow visualization is a technique used in Fluid Mechanics to make the flow pattern visible. In the Smoke Flow Visualization technique, smoke is advected in a real flow of gases and its temporal behavior gives information about the flow [11]. To perform this test, incense stick (Dhoop) is used to form smoke. The smoke is then fed at the inlet of the blower which is instantly transported to all the panels (Fig. 12). The smoke comes out from the entire length of the panel assembly. This clearly indicates that GCCPCS is able to uniformly distribute the cool air throughout the panels. 3.2. Data collection The state of existent convective cooling versus no convective cooling is distinguished by status of the blower as ON and OFF respectively. Therefore observations are taken by keeping the blower OFF and ON alternately allowing response time of 10 minutes. The data collected per observation are the following which are compiled in Table 1. (a) (b) (c) (d) (e) (f) (g)
Time Blower status – OFF or ON Outlet Air temperature (from ground-coupled heat exchanger) Irradiance Current Voltage Conversion efficiency
3.2.1. Conversion efficiency The power output of solar panel is given by Power output ¼ Current (I) x Voltage (V)
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(c) Therefore it is a matter of great interest to establish the causal effect of panel temperature and cooling action on the conversion efficiency. The conversion efficiency at different panel temperatures and blower status – OFF and ON – is tabulated at Table 2. (d) There is marked increase in conversion efficiency when the blower is ON.
The power input of solar panel is given by Power input ¼Irradiance (G) x Area of panel (A) Since, Efficiency ¼Output / Input Therefore, η ¼ I x V x 100=G x A
3.3. Salient points of the observations The salient points of the observations are given below.
3.4. ANOVA analysis (a) Solar Irradiance (radiation) is time varying. (b) Conversion efficiency of Solar PV panel varies with the panel temperature as well as whether the Panel cooling system is not running or running i.e. Blower is not running or running (Blower status – OFF or ON).
Analysis of Variance (ANOVA) technique is used to study whether efficiencies differ significantly due to convective cooling or Panel temperature. The basic principle of ANOVA is to test for difference among the means of the population by examining the amount of variation within each of these samples relative to the amount of variation between the samples [10]. Two-way ANOVA technique is used when the data are classified on the basis of two factors. 3.4.1. ANOVA analysis for conversion efficiency and its inferences To perform ANOVA analysis and then make ANOVA table, the following calculations are made using data from Table 2. c¼2; r¼4; n¼c x r¼ 8 T¼ ∑Xij ¼ 8.56þ 10 þ11.71 þ7.83 þ10.97 þ12.24 þ13.03 þ9.15 ¼83.49 Table 2 Efficiency data at different panel temperatures and blower status – OFF or ON; This data is used for ANOVA analysis to study causal effect of cooling on the conversion efficiency. Blower – off (Cooling system – off) Blower – on (Cooling system – on) Ri
Fig. 12. Smoke flow visualization test conducted to test the GC-CPCS [16].
Panel temp. 1C
Efficiency %
Panel temp. 1C
Efficiency %
57 49 49 53 Cj
8.56 10 11.71 7.83 38.1
55 47 47 50
10.97 12.24 13.03 9.15 45.39
19.53 22.24 24.74 16.98
Table 1 Observations. TESTING OF GC-CPCS TYPE OF TEST: TWO WAY ANOVA TEST SL NO. 5 DATE: 03-05-2014 AMBIENT TEMP.: 2 1C Apanel ¼1.6129 m2 No. of panel ¼ 9 Time ABS Minutes
Time STD IST
Blower STATUS ON/O FF
Panel temperature
Outlet air temp. 1C
Irradiance G W/m2
Current I Ampere
Voltage V Volt
Efficiency η %
1C
0 0 10 10 20 20 30 30 40 40 50 50 60 60 70
14:58 14:58 15:08 15:08 15:18 15:18 15:28 15:28 15:38 15:38 15:48 15:48 15:58 15:58 16:08
OFF ON ON OFF OFF ON ON OFF OFF ON ON OFF OFF ON ON
57 X 55 X 49 X 47 X 49 X 47 X 53 X 50
X X 27 X X X 27 X X X 27 X X X 27
460 X 360 X 400 X 330 X 340 X 310 X 520 X 444
3.1 X 3.1 X 3.1 X 3.1 X 3.1 X 3.1 X 3.1 X 3.1
184.4 X 185 X 187.3 X 189.1 X 186.5 X 189.1 X 190.7 X 190.2
8.56 X 10.97 X 10 X 12.24 X 11.71 X 13.03 X 7.83 X 9.15
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Table 3 ANOVA table (Two way) for conversion efficiency. Sources of variation Between columns (Effect of cooling action) Between rows (Effect of panel temperature) Residual
SS
DF
MS
6.643 16.89 0.513
1 3 3
6.643 5.63 0.171
CF ¼T2/n¼(83.49)2/8 ¼871.323 Total SS¼∑X2ij –CF¼(8.56)2 þ(10)2 þ(11.71)2 þ (7.83)2 þ(10.97)2 þ (12.24)2 þ(13.03)2 þ (9.15)2 -871.323¼24.046 Cj ¼[38.1, 45.39] Ri ¼[19.53, 22.24, 24.74, 16.98] SS between columns¼ ∑C2j /r-CF¼ [(38.1)2 þ (45.39)2]/4–871.323¼ 6.643 SS between rows ¼∑R2i /c-CF ¼[(19.53)2 þ(22.24)2 þ(24.74)2 þ (16.98)2]/2–871.323 ¼16.89 SS residual¼Total SS – (SS between columns þ SS between rows)¼24.046 – (6.643þ16.89) ¼ 0.513 Degree of freedom for variance between columns ¼ c – 1 ¼2 - 1¼ 1 Degree of freedom for variance between rows¼ r – 1 ¼4–1 ¼ 3 Degree of freedom for residual variance ¼ (c–1) (r–1) ¼(2–1) (4–1) ¼3 MS between columns ¼ SS between columns/Degree of freedom ¼ 6.643/1 ¼6.643 MS between rows¼ SS between rows/Degree of freedom¼ 16.89/3 ¼ 5.63 MS for residual ¼ SS residual/Degree of freedom¼ 0.513/3 ¼ 0.171 F-ratio between column¼ MS between columns / MS for residual ¼ 6.643/0.171 ¼38.85 F-ratio between rows¼ MS between rows/MS for residual¼ 5.63/0.171 ¼32.92 Using the above data ANOVA table is tabulated (Table 3). The observations of the ANOVA table are the following. (a) It is observed that the calculated F-ratio for column, 38.85, is more than the table value of F-ration at 5% level of significance. Therefore the effect of Blower status on the efficiency is significant. Hence the cooling effect is found to be significant. This validates the efficacy of GC-CPCS. (b) It is observed that the calculated F-ratio for row, 32.92, is more than the table value of F-ratio at 5% level of significance. Therefore the effect of Panel temperature on the efficiency is also significant. 4. Conclusions The panel cooling system, GC-CPCS, is tested by using Smoke Flow Visualization technique. There is marked increase in conversion efficiency due to cooling of PV panels. The data is analyzed
F-Ratio 38.85 32.92
5% F-limit F(1,3) ¼10.13 F(3,3) ¼ 9.28
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