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water)
Solar Energy 193 (2019) 195–204
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Exergo-economic analysis of a serpentine flow type water based photovoltaic thermal system with phase change material (PVT-PCM/water)
T
Taher Maatallaha, Richu Zachariahb, , Fahad Gallab Al-Amria ⁎
a b
Department of Mechanical and Energy Engineering, College of Engineering, Imam Abdulrahman Bin Faisal University, P.O. Box: 1982, Dammam, Saudi Arabia Department of Mechanical Engineering, Amal Jyothi College of Engineering, Kanjirappally, Kottayam 686518, Kerala, India
ARTICLE INFO
ABSTRACT
Keywords: Photovoltaic PCM Exergy Thermal management Optimization
An investigation on PVT-PCM/water system has been carried out under the weather conditions at Kottayam (9.5425 °N, 76.8202 °E), India. In the present paper, experimentations have been done to compare the overall performance of PV and water-based PVT-PCM panels. The PVT-PCM/water system has been analysed under various outdoor environmental conditions and it has been recorded that the integration of PCM contributed to an improved thermal and overall efficiency of 26.87% and 40.59% respectively, along with a rise in electrical efficiency by 17.33% compared with conventional PV panel. It has been found that the payback time for the water-based PVT-PCM system is about 6 years on the overall exergy basis, which is 11.26% shorter compared to conventional PV panels. Also, the water-based PVTPCM system has long-term lifecycle conversion efficiency compared to conventional PV panel by about 27%.
1. Introduction Only a small fraction from the incident sunlight can be converted by PV cells into electricity, which also depends on the operating cell temperature levels. In fact, the major part of the incident solar radiation is converted into heat, elevating the PV cells’ temperature which in turn reduce the output power, the fill factor and the electrical conversion efficiency (Bahaidarah et al., 2013; Fucci et al., 2014). Indeed, 1 °C rise in cell temperature for crystalline silicon PV panels can lead to a reduction in electrical conversion efficiency of approximately 0.45% (Skoplaki and Palyvos, 2009). Different cooling techniques, including air as a heat transfer medium, have been used to decrease the heating of PV cells and control their fluctuations that cause mismatch losses, especially in PV parallel connected sub-arrays. Water cooling systems, however, are more efficient due to the high thermal capacity of water, despite the additional costs of maintenance and operating fees which can increase the payback of the system (Nižetić et al., 2018). Water-based Photovoltaic/Thermal system (PVT) combines both thermal and electrical energy outputs. PVT is used as an active water cooling system for cogenerating thermal energy, which can be used in low temperature applications, together with the significant share of electrical energy (Vokas et al., 2006). There are several models and advances of PVT modules that incorporate water in the PVT system as working fluid. (Joshi and Dhoble, 2018). The solar thermal system in water-based PVT system consists of a metal-based absorber plate and tubes embedded at the back of the PV panel
⁎
to maintain the temperature of the PV cells within an acceptable range, maximizing their electrical output. Krauter (2004) observed that the PV cell temperature dropped by 22 °C by passing water on the front side of the PV panel. Ji et al. (2006) examined the impact of residence time of water particles, in other words, mass flow rate, in a hybrid PVT collector. The temperature of PV reduced, and the heat extracted could be effectively used for thermal applications. The Phase Change Material (PCM) is an extra layer with significantly higher latent heat capacity and very low conductivity, absorbing sensible heat from the PV cells until reaching its melting point and then melts out by absorbing latent heat. PCM is packed behind PVT system and its temperature remains constant while the thermal energy is stored during phase change. The liquid temperature will increase when the PCM completes its phase change, and this process is directly related to the quantity of PCM used and its thermal conductivity. Numerical and experimental analysis on the application of PCMs for removing heat from PV and its relative improvements for the whole PVT systems have been reviewed. In fact, it has been recorded that several investigations have been carried out for controlling, stabilizing and reducing the fluctuation rates of the temperature of PV cells, on the hybrid PVT system and PV, integrating PCM. Indartono et al. (2016) considered a yellowish petroleum jelly as PCM and investigated overall efficiency of the system under Indonesian weather conditions. An improvement from 8.3% to 10.1% was recorded in photovoltaic efficiency. Mahamudul et al. (2016) examined the case of RT35 PCM and made the survey under the weather conditions of
Corresponding author. E-mail address: [email protected] (R. Zachariah).
https://doi.org/10.1016/j.solener.2019.09.063 Received 23 May 2019; Received in revised form 27 July 2019; Accepted 17 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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Nomenclature Ac Am Ce Cp CPCM CPV panel Cstructure Ea,in EBPT Ec Eex,out EPT Esol Fcr,i,n FF FSR,i,n i Imax Isc I(t) It L LCCE M m MCF n
P Pmax Pp Ppv Ps Pth Re Ss Ta Tf ON Ti To TPV UAC Voc Vmax
area of solar panel area of PV cells (m2) cost of DC electrical energy production specific heat capacity cost of PCM cost of PV panel cost of structure embodied energy energy payback time net yearly DC electrical energy obtained from PVT system annual exergy output energy production factor annual solar energy capital recovery factor fill factor sinking fund factor interest rate maximum current short circuit current solar intensity in function of time (W/m2/h) solar intensity (W/m2) life of the system (year) life cycle conversion efficiency maintenance cost of PVT system mass flow rate of water maintenance cost factor number of years
initial cost to the system maximum power cost for DC fan PV electrical power net present cost thermal power revenue generated by selling DC electrical energy salvage value ambient air temperature (°C) air temperature at outlet of PVT air panel (°C) water inlet temperature water outlet temperature PVT outlet temperature (°C) uniform end-of-year annual cost open circuit voltage maximum voltage
Symbols c
c pvcell
elec th
packing factor efficiency of solar panel electrical PV cell efficiency electrical efficiency thermal efficiency
Subscript ex
Malaysia. The results pointed out that there was a reduction of 10 °C in cell temperature by using PCM. Park et al. (2014) researched the impact of PCM layer density and conducted system analysis under South Korea's climatic circumstances. The findings showed a peak electrical efficiency improvement of 3%. Sharma et al. (2016) used RT42 PCM to make a focused PV collector and recorded a 7.7% increase in electrical efficiency under 1000 W/m2 horizontal solar radiation. Under outdoor test conditions, Huang et al. (2006) stated that, owing to a temperature gradient of 17 °C between an aluminium flat plate and an aluminium box containing PCM, the effectiveness of a PV/PCM system was increased by 7.5% at peak solar hours. Hassan (2010) tested a PV/PCM collector in Pakistan and discovered that at the peak solar hour of the day, PV cell temperature was 21.5 °C lower than the reference. Huang et al. (2011) studied the effect of fins embedded in a container covered with RT 27 and RT 35 as PCM. They discovered that fins could enhance the efficiency of the system based on crystalline PCM isolation. In addition, Huang et al. (2006) compared the results of the PVT-PCM (RT25 and GR40) integration with and without fins. They found that the use of fins could reduce the temperature of the PV cell. Hasan et al. (2010) compared PVT performance using five distinct PCMs: paraffin wax (RT20), capric–lauric acid, capric–palmitic acid, pure salt hydrate (CaCl2:6H2O) and commercial blend (SP22). Using capric–palmitic acid and CaCl2:6H2O, a peak decrease of 18 °C in PV cell temperature was achieved at 1000 W/m2. Ho et al. (2016) has numerically studied a PV-PCM system that is micro-encapsulated with MEPCM on the rear side of the PV panel and the PV module is floated over the water surface. In the case of a PV-PCM system with a PCM container thickness of 5 cm, a decrease of 5.5% in PV cell temperature and an increase in proportion of electrical conversion efficiency of 0.8% were noted. Hachem et al. (2017) studied the behaviour of PV-PCM system based on mixed PCM (petroleum jelly, copper and graphite powder) and PCM without added impurities. A typical rise in electrical efficiency of 5.8% and 3% respectively was recorded.
considering exergy as base
Atkin and Farid (2015) combined the PCM with a passive cooling system and reported an improvement of 12.97% in the PV system's electrical efficiency. Kibria et al. (2016) conducted experiments with three PCM; RT25, RT20, and RT28HC and examined the performance of PVT-PCM collector, recorded an electrical efficiency improvement of 5%. Smith et al. (2014) evaluated the outputs of PVT- PCM/water collector across several countries to suggest PCM as the best choice for tropical regions to cool PV systems. The first and second laws of thermodynamics can be combined to perform exergy analysis for obtaining the maximum work potential of a PVT system. Various researchers have conducted exergy investigations on PVT systems. The generation of entropy is directly dependent on the PVT system’s surface temperature. So the generation of entropy in PVT system can be decreased by decreasing surface temperature (Kandilli, 2019). Rajoria et al. (2012) assessed the energy and exergy gain in India under various climatic circumstances from four kinds of hybrid PVT arrays. They discovered that the integration of hybrid PVT technologies in buildings can achieve a better mix of electrical and thermal energy. Yazdanpanahi et al. (2015) carried out experimental and numerical studies on PVT/water system exergy efficiency in terms of exergy losses. They presented an optimum mass flow rate of water for maximum exergy efficiency of 13.95%. Sardarabadi et al. (2017a) compared the exergy efficiency of PVT/water, PVT/Al2O3, PVT/TiO2, and PVT/ZnO with that of a PV system without thermal collector. Among the former systems, PVT/Al2O3 system has an enhanced overall efficiency of 18.27% compared to the PV without collector. Fayaz et al. (2018) carried out experimental and numerical research on the impact of nanofluid flow rate on a water/MWCNT PVT system. They also optimized water/MWCNT nanofluid's weight fraction. Only limited research is performed in the exergy analysis of PVT PCM systems. The performance of a PVT / water system with and without PCM was contrasted by Gaur et al. (2017). It is discovered that the daily overall exergy for PVT collector with PCM is more than the collector without PCM. OM37, which has a melting point of 37 °C was 196
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used as the PCM, for which a mathematical model was developed. Hossain et al. (2019) conducted experiments on a PVT-PCM system having double side serpentine flow to study its performance in comparison with normal PV. The maximum exergy efficiency obtained was 7.09% and 12.19% for PV and PVT-PCM system respectively. Studies imply that the exergy of PVT system improved when PCM was included in nanofluid based system. PVT ZnO/ water nanofluid amalgamated with organic paraffin wax as PCM yields an overall exergy efficiency of 13.61% compared with PV and nanofluid PVT (Hosseinzadeh et al., 2018). Sardarabadi et al. (2017b) conducted exergy analysis in the PVT ZnO/ water nanofluid and PCM, recorded an increase in overall exergy of 23% compared to reference PV module. The above-mentioned investigations were commonly based on numerical analysis and conducted in European countries. Whereas, this experimental set-up has been implemented and the complete experimental procedure has been performed under Indian weather conditions. The aim of this study is to explore and compare experimentally the overall performances of the PV panel and fully serpentine flow based PVT-PCM system, under different operating conditions. The improvements made by the PVT-PCM system have been analyzed and discussed. In addition, the net annual thermal, electrical, and corresponding heat exergy improvements were evaluated. Further, embodied energy, energy payback time, lifecycle conversion efficiency, UAC and the Cost of DC electrical energy production have also been calculated and compared for both systems.
Paraffin wax RT-30 has a high latent heat of melting, high heat storage density and the ability to release the heat stored when the wax changes from solid to liquid or vice versa. By decreasing temperature variations and keeping the temperature stability of the PV cells, the PCM in the PVT-PCM panel improves the thermal system efficiency. The PCM melts by absorbing enough amount of heat, which hinders the temperature rise during the experiment. The PCM helps to lower the temperature of the PV cell, which in turn increases the PV system's electrical performance. The thermal energy stored in the PCM is used to balance the mismatch in demand and supply for heating water during nights and cloudy days and/or cooling PV cells temperature during the day and release that heat for evening use. Voltmeter and Ammeter are devised to measure open circuit voltage (Voc) and short circuit current (Isc) at different time steps. Infrared thermometers are positioned on both sides of the PV system as well as inside the PCM thermal storage, having a temperature measurement accuracy of ± 2 °C . A weather station with ambient temperature accuracy ± 0.5% is employed for the same along with a self-solar power meter with measurement range and accuracy of 2000 W/m2 and ± 10 W/m2 respectively. The measurement uncertainties of all devices are illustrated in Table 2. 2.3. Experimental procedures
2. Methodology
The experimental set-up of the water-based PVT-PCM was placed accurately in the experimentation location for preceding the different measurements under outdoor and targeted conditions. An Infrared Thermometer was used to measure the temperature of PV cells, absorber, phase change material (PCM), inlet and outlet water temperature at systematic positions and time steps while the ambient temperature was obtained with Thermistors for further use in thermal analysis. A digital solar power meter measured the solar irradiation and the anemometer measured the wind patterns. In addition, a digital flow meter was used to measure the water mass flow rate. Regulating valves have been used to regulate the water mass flow rate. The thermo-physical properties of Paraffin Wax are given in the Table 3. The paraffin wax was filled as a liquid in the cabin which is adhered to the back side of the solar panel and then cooled till it froze totally. Some free space is provided by the solidified PCM on the top end of the cabin intended to hold the expanded volume during PCM melting. The experimentations were done for the selected days in the month of May 2017 and same solid state of PCM layer was assured for each day at the start of experiments. A 5 min time step was used to log the data for the reference PV and the PVT-PCM system.
2.1. Data availability To check the feasibility of using a Phase Change Material with a PV panel, a complete prototype has been erected and its performance analysis has been carried out in Kerala (9.5425 °N, 76.8202 °E) under Indian weather conditions. Fig. 1 shows the monthly average radiation pattern measured on horizontal plane in Kerala. The monthly average temperature variation is also given in Fig. 1. The experiments were conducted under the above weather conditions and the results obtained are shown in the following sections. 2.2. Experimental setup A PVT/water system based entirely on serpentine flow, embedded with PCM, is designed and fabricated in the present study. Its performance is compared with a conventional PV system, as shown in Fig. 2. The electrical and thermal performance of the systems under consideration is explored with different water mass flow rates on chosen days. The arrangement of the aluminium absorber plate, copper serpentine tube and aluminium container filled with PCM is illustrated in Fig. 3. In the experiments C-Si based polycrystalline PV panels were used, having power capacity and electrical conversion efficiency of 100 W and 14.25%, respectively, for the comparative study of the electrical and thermal performances. The PV modules were tilted by 10°, equal to Kerala latitude, and directed towards the south direction. The PV modules were placed outdoors for nine days over a daily duration of six hours; from 10:00 A.M. to 4:00 P.M. The Short circuit current and the Open circuit voltage were measured to assure reliability in PV modules. In a water-based PVT-PCM system, a 30 mm thick aluminium container with an absorber surface area of 0.54 m2 is connected to the back side of the PV panel. The specifications of the PVT-PCM/water system are listed in Table 1. The mass flow rate is regulated and measured using a control valve and digital flow meter respectively, and four different water flow rates of 0.004 kg/s, 0.008 kg/s, 0.010 kg/s and 0.020 kg/s are considered to examine the electrical and thermal performances of water-based PVTPCM. Measurements of temperature are carried out using K-type thermocouples (accuracy ± 2.2 °C). A Schematic diagram of PVT-PCM/ water system with all dimensions in mm is shown in Fig. 4.
Fig. 1. Monthly average global solar radiation and dry bulb temperature of the experimental set up site. 197
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the following equation, th
=
m × Cp × (To
Ti )
I × Ac
(4)
where m is the water mass flow rate.Cp , Ti and To are the specific heat, the water inlet and outlet temperature respectively. 3.2. Exergoeconomic analysis of PVT-PCM system Phase change materials showed a promising sign that a significant amount of energy could be stored or released in the form of latent heat during melting and solidification without any shift in temperature. It can be noted that there have been several studies in the field of incorporating PCM into PVT systems, but it has been noted that the exergy assessment of these hybrid systems is limited. In this section, we discuss the major differences between engineering economic analyses of a typical PVT-PCM system to that of a normal PV panel. Some terminologies of economic parameters related with renewable energy systems will be discussed along with two case studies, which are used to differentiate between economic analyses of the reference PV panel and the PVT-PCM system. A new methodology has been developed for performance assessment of PVT-PCM system through total lifecycle energy effectiveness consideration in present-value terms (Mishra and Tiwari, 2013; Tiwari, 2018; Tiwari and Tiwari, 2016).
Fig. 2. The standard PV system and PVT-PCM/water system experimental setup.
3. Data reduction 3.1. Energy analysis of PVT- PCM system The PVT-PCM system's electrical and thermal efficiency was evaluated using the following equations. Electrical efficiency of PVT system can be calculated as,
=
elec
Voc × Isc × FF ×
c
I × Ac
3.2.1. Energy payback time (EPBT) EPBT is the time it takes to recover the energy that is embodied in the system. The computation of EPBT considering exergy as the base EPBTex that can be calculated as (Mishra and Tiwari, 2013; Tiwari, 2018; Tiwari and Tiwari, 2016),
(1)
where c is the packing factor. A packing factor of one corresponds to a system where the entire area is covered by PV cells. c is expressed as follows, c
=
Area of solar cells Area of PV module
EPBTex =
Pmax V × Imax = max Voc × Isc Voc × Isc
(5)
Here the PVT-PCM's embodied energy Ea,in takes into account both the system manufacturing requirements and the energy needed to produce the raw materials. The aim of any embodied energy assessment is to determine the amount of energy taken into consideration for the production of a product or element. Embodied energy analysis includes the energy used for material production, transportation, fabrication of photovoltaic cell/module and PCM, human effort, installation and maintenance of the system and finally the disposal/salvage. The parameter Eex, out stands for the overall annual exergy got from the system. The system is regarded profitable for lower values of EPBT.
(2)
Voc , Isc , I , Ac and FF are the open circuit voltage, the short-circuit current, the global solar irradiation, the PV thermal collector area and the fill factor of the PV module, respectively. FF can be calculated as follows, FF =
Ea, in Eex , out
(3)
Thermal efficiency of photovoltaic/thermal system can be found by
Fig. 3. Diagram of the water-based PVT system. 198
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Table 1 Specifications of prototype. Parameter
Unit
Value
Individual rated capacity of each module Electrical conversion of each module Open Circuit Voltage Shirt-Circuit current Total Aperture area of absorber Plate Thickness of the aluminum container Thickness of the Collector insulator Volume of PCM as liquid Tie step of measurement
Pr [W] ele [%] Voc [V] ISc [A] Height × Width [m2] ec [mm] ei [mm] VPCM [L] %
100 14.25 (at STC) 21.6 6.14 0.6565 30 0 (back insulation not there) 11 98
Table 3 Properties of Paraffin wax. Paraffin wax properties Melting point (°C) Heat of Fusion (kJ/kg) Thermal conductivity (W/m K) Specific heat capacity (kJ/kg K)
57 200–220 0.24 at 27 °C 2.1 (liquid) 2.0 (solid) 810 (liquid) 910 (solid)
Density (kg/ m3 )
3.2.3. Life cycle conversion efficiency (LCCE) LCCE is referred to as the PVT-PCM system's net output in respect of the solar input received throughout the system's lifetime. LCCE by exergy (LCCEex ) can be calculated as (Mishra and Tiwari, 2013; Tiwari, 2018; Tiwari and Tiwari, 2016): Fig. 4. Dimensions (in mm) of the PVT-PCM/water system.
LCCEex =
3.2.2. Energy production factor (EPF) EPF of PVT-PCM system gives an idea about its overall performance. On the one hand, it can be calculated annually as the reverse of EPBT as (Mishra and Tiwari, 2013; Tiwari, 2018; Tiwari and Tiwari, 2016):
EPFex =
Eex , out Ea, in
3.2.4. Economic analysis of the produced electricity The cost of DC electricity generation in €/kWh (Ce ) of the system can be computed as (Tiwari, 2018):
(6)
1 EPBTex
Ce =
(7)
Eex , out Ea, in
UAC Ec
(10)
The UAC for PVT-PCM system is calculated with respect to the present value method as (Tiwari, 2018):
UAC = P × Fcr , i, n + M × Fcr , i, n
In the energy point of view the system is worthwhile if EPFex → 1, for EPBTex → 1, otherwise it is not. On the other side, on the basis of lifetime, it can be calculated as (Mishra and Tiwari, 2013; Tiwari, 2018; Tiwari and Tiwari, 2016):
EPFex = L
(9)
Here Esol represents the annual solar energy and L the life of the system.
Or:
EPFex =
Eex , out × L Ea, in Esol × L
Ss × FSR, i, n
(11)
where P, Ss and M are the stands for net present cost, salvage value and cost for maintenance of PVT system respectively. The value of M can be calculated using the relation between P and Maintenance Cost Factor (MCF) as a product between the two (The value of MCF has been considered as 0.1). The salvage value has the following expression,
(8)
Ss = P × (1
In this case EPF should be more than one. The value of EPF should be as high as possible for the system to be cost-effective.
(12)
I) × n
where I the annual depreciation expense. The value of the capital recovery factor can be calculated as (Tiwari,
Table 2 Uncertainties of device measurement. Measurement parameter Solar radiation Current Surface Temperature Ambient Temperature Wind Speed Voltage
Device Solar Radiation Meter Ammeter Infrared Thermometer NTC Thermistor Anemometer Voltmeter
Model
Measurement range
Tenmars; TM207 Solar Power Meter Ecosense Fluke; 59MAX esp 10 K (EmbSys) Ecosense Ecosense
199
2
2000 W/m – −30 °C to +350 °C −60 °C to +150 °C 0–45 m/s –
Accuracy ± 10 W/m2 ± 0.1% ± 2 °C ± 0.5% ± 0.1 m/s ± 0.1%
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2018),
Fcr , i, n =
4.2. Comparison of electrical efficiencies of PV and PVT-PCM system
i × (1 + (1 + i )n
i )n 1
Fig. 10 highlights the variation of PVT-PCM/water system electrical efficiency for a day at different water flow rates. It is observed that the PVT-PCM/water system's electrical efficiency is always higher than that of normal PV. A maximum increment of 29.49% is noted in electrical efficiency or PV output power, with PVT-PCM/ water system. The electrical efficiency shows an average increment of around 17.33% with PVT-PCM/water system as compared to normal PV during the entire experiments. As the flow rate increases, the electrical efficiency is only slightly higher. If the flow rate is higher, the power of the pump will double (Nasrin et al., 2018). Thus, a minimum flow rate can therefore retain the PV system efficiency at a reasonable value and boost the system's life without compromising on pump power.
(13)
The sinking fund factor value can be calculated as (Tiwari, 2018),
FSR, i, n =
i (1 + i )n
1
(14)
The initial cost of the PVT-PCM system suggested can be obtained as follows (Tiwari, 2018),
P = CPV panel + Cstructure + CPCM
(15)
4. Results and discussion
4.3. Energy analysis of PV and PVT-PCM systems
Upon design, building-up and installation of the system, the experimental data of the parameters such as PV top surface, absorber layer, PCM temperatures were recorded and collected at different time intervals and dates, at various flow rates ranging from 0.004 to 0.02 kg/ s. A detailed comparative study of both systems has been given in the following section.
Fig. 11 summarizes combined energy performance indicators of the PVT-PCM system and the reference PV panel, including their thermal, electrical and overall efficiencies. The PVT-PCM showed significant reduction in heat losses, there by elevating the capacity for thermal energy storage. This resulted in the ability to generate more electrical energy compared to standard PV panels. Thus, it can be stated that the electrical production as well as the thermal energy storage capability of PV systems can be increased by controlling the temperature through latent heat of absorption in PCM.
4.1. Comparison of instantaneous temperature at different sub-layers of PV and PVT Figs. 5–9 show temperature variation at different positions of the normal PV system and water-based PVT-PCM system for a day with respect to average solar radiation without water flow and water flow of 0.004 kg/s, 0.008 kg/s, 0.01 kg/s and 0.02 kg/s. The average of the values taken for 3 consecutive days is reported. Fig. 5 shows the variation of the temperature at different sub layers of reference PV and PVT-PCM system without water flow through the serpentine tubes. It can be noted that at the starting hours, the temperature of the PV panel is more than that of PVT-PCM system, but the PVT temperature increases due course of time. This is because initially the heat from the PV panel is removed by the PCM, which melts, absorbing latent heat of fusion. But the PCM thermal storage does not remove this heat, which raises the system temperature. Fig. 6 indicates the water-based PVT-PCM system temperature variation corresponding to the average daytime solar radiation at a mass flow rate of 0.004 kg/s of water. As shown in this figure, PVTPCM panel top temperature is smaller than PV panel temperature with an average temperature reduction of 3.57%. The panel bottom temperature registered a 3.35% reduction in temperature on an average. The temperature differences in the solar radiation of both systems are shown in Fig. 7 for 0.008 kg/s water flow rate. A maximum temperature reduction of 6.84% is observed for the panel bottom temperature at 2 P.M. compared to reference PV panel. An average panel top temperature reduction of 3.57% and an average panel bottom temperature of 3.77% are observed. Fig. 8 shows the variation in temperature of the PVT-PCM with solar radiation for a day at mass flow rate of 0.01 kg/s of water. As shown in this figure, PVT-PCM system temperature is less than PVT panel temperature and the average decrease in temperature is 4.27% at the top and 3.88% at the bottom relative to standard PV system. Fig. 9 shows temperature variation at a mass flow rate of 0.02 kg/s of water for a day. Average temperature reduction reported for the PVTPCM system is 4.46% at the top and 4.41% at the bottom compared with the normal PV panel. It can be concluded that a temperature decrease is observed when the water mass flow rate increases to 0.02 kg/s. This result has also been reported by (Hasan et al., 2015; He et al., 2011; Wu et al., 2011).
4.4. Exergy and economic analysis of PV and PVT-PCM systems Exergo-economic analysis can be used to assess the PVT-PCM system's feasibility. The maximum work potential from the Table 4 details the embodied energy calculation for the PVT system. Due to greater material requirements, the embodied energy for the PVT-PCM system is obviously more than that of the PV panel. The embodied energy for the PVT-PCM system is calculated to be 624.4 kWh, expecting a life span of 20 years. Table 5 indicates the calculation of the PVT-PCM payback period. The amount of annual sunshine hours is taken to be 1800 h. The time needed to recover the PVT-PCM system's embodied energy is about 6 years, it can be regarded as a short period of time that demonstrates such system's efficiency. The PVT-PCM model therefore has a shorter payback period of 11.26% compared to conventional PV panel. This is because the embodied energy for PVT-PCM system is only 5.56% higher than the PV panel, but the annual exergy output is 18.97% higher.
Fig. 5. Trends of the instantaneous temperature of PV and PVT-PCM systems vs. solar radiation during day time without water flow. 200
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Fig. 9. Trends of the instantaneous temperature of PV and PVT-PCM systems vs. solar radiation at water flow rate of 0.02 kg/s.
Fig. 6. Trends of the instantaneous temperature of PV and PVT-PCM systems vs. solar radiation at water flow rate of 0.004 kg/s.
Fig. 7. Trends of the instantaneous temperature of PV and PVT-PCM systems vs. solar radiation at water flow rate of 0.008 kg/s.
Fig. 8. Trends of the instantaneous temperature of PV and PVT-PCM systems vs. solar radiation at water flow rate of 0.01 kg/s.
Fig. 10. Comparison of the instantaneous electrical efficiency of PV and PVTPCM panels during day time at various mass flow rates of water.
Table 6 demonstrates the annual and lifetime-based energy production factor. The production enhances accordingly for any system if EPF increases. The lifetime value of the EPF should be as large as possible in terms of the system's cost effectiveness and should be at least more than 1. The annual EPFex is 0.15 and 0.17 for the PV and PVT-
PCM respectively. EPFex is found to be 3.38 for the PVT-PCM system considering a lifetime of 20 years, which is 13.8% higher than the normal PV panel. It increases the attractiveness of the suggested system over standard systems. Table 7 illustrates the exergy-based life cycle conversion efficiency 201
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Fig. 11. Average electrical efficiency, thermal efficiency and overall efficiency measured at the system deployment site for reference PV and PVT-PCM during the days between 03 May and 19 May.
calculation for the system. Where Esol = annual irradiation × surface area of PVT = 4090 × 0.65 = 2658.5 kWh/year. The conversion efficiency for the lifecycle is a positive value, resulting in the PVT-PCM technology being very beneficial. LCCE is calculated to be 0.022 and 0.028 for PV panel and PVT-PCM system respectively. Hence, the PVT-PCM system has higher LCCE compared to conventional PV panel by about 27%. The UAC is calculated for interest rates of 5%, 10% and 15% corresponding to lifespan of 5, 10, 15 and 20 years. It's visible in the Figs. 12–14 that the UAC increases and decreases as expected for PV systems with and without PCM with an increase in interest rate and lifespan respectively. For both systems, the UAC value corresponding to the interest rate of 5% was obtained as a minimum as expected. Furthermore, UAC for PV is less than PVT-PCM in all the cases because PVT is an enhanced variant with attachments on standard PV panel, which implies that the production of PVT-PCM system requires a more quantity of material compared to PV. Fig. 15 displays the variation in the cost of DC electricity output (Ce) as a function of the number of years at interest rates of 5%, 10% and 15%. It has been seen that with an increase in lifespan, the cost of producing DC electrical energy for both PV panel and PVT-PCM system is decreasing. The value of Ce rises with higher interest rates. PVT-PCM always has a lower value for Ce owing to greater net annual DC electrical energy from PVT than PV. Compared to conventional PV panels, this makes PVT-PCM system an attractive investment option.
Table 5 Calculation of Payback period.
PV panel PVT PCM System
Ea,in (kWh)
Eex,out (kWh/year)
EPBTex (years)
591.5 624.4
88.71 105.54
6.68 5.92
Table 6 Energy production factor on annual basis based on the lifetime with PCM.
PV panel PVT PCM System
Ea,in (kWh)
E ex,out (kWh/year)
EPFex annual
Life time
EPFex life time
591.5 624.4
88.71 105.54
0.15 0.17
20 20
2.97 3.38
Table 7 Calculation of lifecycle conversion efficiency with PCM.
PV panel PVT PCM System
Ea,in (kWh)
Eex,out (kWh/year)
Life time (year)
Esol (kWh/ year)
LCCEex
591.5 624.4
88.71 105.54
20 20
2658.5 2658.5
0.022 0.028
findings were deduced from the experiment.
• The temperature of PV cells with PCM was found to be lower throughout the day than that of the system without PCM. • It has been recorded that a PVT-PCM/water system’s electrical
5. Summary and conclusions Exergo-economic analysis was performed in the present study to verify the feasibility of incorporating paraffin wax into a water-based PVT panel. A prototype of water-based PVT-PCM scheme was set up and tested under various working conditions to investigate improvements in the PV panel's electrical yield (output power) and electrical efficiency relative to conventional PV system. The following salient
• •
output is higher than that of PV panel with an average increase of approximately 17.33%. On average, the overall efficiency of the PVT-PCM system is reported an increase of 28.86% compared to conventional PV. The embodied energy of the PVT-PCM system can be recuperated in
Table 4 Calculation for embodied energy for PVT with PCM. Module efficiency (%) PV Technology P-Si
14
PCM Ein (system)
Expected life (year)
Specific energy content (kWh/m2)
Ein for 1.32m2 (kWh, 20 years)
20
910
591.5
Expected life (year)
Specific energy content (kWh/kg)
Ein for 2 kg (kWh, 20 years)
20
16.45
32.9 624.4
202
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Fig. 12. Variation of UAC with the No. of years at an interest rate of 5%.
Fig. 15. Variation of the Cost of DC electrical energy production as function of No. of years at various interest rates.
Acknowledgment
Fig. 13. Variation of UAC with the No. of years at an interest rate of 10%.
The authors wish to thank Energy Management Centre, Trivandrum (EMC/ET&R/17/SPS/01) and Management of Amal Jyothi College of Engineering for their extended financial support. References Atkin, P., Farid, M.M., 2015. Improving the efficiency of photovoltaic cells using PCM infused graphite and aluminium fins. Sol. Energy 114, 217–228. https://doi.org/10. 1016/j.solener.2015.01.037. Bahaidarah, H., Subhan, A., Gandhidasan, P., Rehman, S., 2013. Performance evaluation of a PV (photovoltaic) module by back surface water cooling for hot climatic conditions. Energy 59, 445–453. https://doi.org/10.1016/j.energy.2013.07.050. Fayaz, H., Nasrin, R., Rahim, N.A., Hasanuzzaman, M., 2018. Energy and exergy analysis of the PVT system: effect of nanofluid flow rate. Sol. Energy 169, 217–230. https:// doi.org/10.1016/j.solener.2018.05.004. Fucci, R., Lancellotti, L., Privato, C., 2014. A procedure for assessing the reliability of short circuited concentration photovoltaic systems in outdoor degradation conditions. Microelectron. Reliab. 54, 182–187. https://doi.org/10.1016/j.microrel.2013. 09.019. Gaur, A., Ménézo, C., Giroux-Julien, S., 2017. Numerical studies on thermal and electrical performance of a fully wetted absorber PVT collector with PCM as a storage medium. Renew. Energy 109, 168–187. https://doi.org/10.1016/j.renene.2017.01.062. Hachem, F., Abdulhay, B., Ramadan, M., El Hage, H., El Rab, M.G., Khaled, M., 2017. Improving the performance of photovoltaic cells using pure and combined phase change materials – experiments and transient energy balance. Renew. Energy 107, 567–575. https://doi.org/10.1016/j.renene.2017.02.032. Hasan, A., McCormack, S.J., Huang, M.J., Norton, B., 2010. Evaluation of phase change materials for thermal regulation enhancement of building integrated photovoltaics. Sol. Energy 84, 1601–1612. https://doi.org/10.1016/j.solener.2010.06.010. Hasan, A., McCormack, S.J., Huang, M.J., Sarwar, J., Norton, B., 2015. Increased photovoltaic performance through temperature regulation by phase change materials: materials comparison in different climates. Sol. Energy 115, 264–276. https://doi.
Fig. 14. Variation of UAC with the No. of years at an interest rate of 15%.
6 years, which makes it a viable solution.
• The payback period of the PVT-PCM system is 11.26% shorter than that of the conventional PV panel. • The PVT-PCM system has a lifecycle conversion efficiency of approximately 27% compared to that of the standard PV panel.
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(HPVT) water collector with different PV technology. Sol. Energy 91, 161–173. https://doi.org/10.1016/j.solener.2013.02.002. Nasrin, R., Hasanuzzaman, M., Rahim, N.A., 2018. Effect of high irradiation and cooling on power, energy and performance of a PVT system. Renew. Energy 116, 552–569. https://doi.org/10.1016/j.renene.2017.10.004. Nižetić, S., Giama, E., Papadopoulos, A.M., 2018. Comprehensive analysis and general economic-environmental evaluation of cooling techniques for photovoltaic panels, Part II: Active cooling techniques. Energy Convers. Manage. 155, 301–323. https:// doi.org/10.1016/j.enconman.2017.10.071. Park, J., Kim, T., Leigh, S.B., 2014. Application of a phase-change material to improve the electrical performance of vertical-building-added photovoltaics considering the annual weather conditions. Sol. Energy 105, 561–574. https://doi.org/10.1016/j. solener.2014.04.020. Rajoria, C.S., Agrawal, S., Tiwari, G.N., 2012. Overall thermal energy and exergy analysis of hybrid photovoltaic thermal array. Sol. Energy 86, 1531–1538. https://doi.org/10. 1016/j.solener.2012.02.014. Sardarabadi, M., Hosseinzadeh, M., Kazemian, A., Passandideh-Fard, M., 2017a. Experimental investigation of the effects of using metal-oxides/water nanofluids on a photovoltaic thermal system (PVT) from energy and exergy viewpoints. Energy 138, 682–695. https://doi.org/10.1016/j.energy.2017.07.046. Sardarabadi, M., Passandideh-Fard, M., Maghrebi, M.-J., Ghazikhani, M., 2017b. Experimental study of using both ZnO/water nanofluid and phase change material (PCM) in photovoltaic thermal systems. Sol. Energy Mater. Sol. Cells 161. https://doi. org/10.1016/j.solmat.2016.11.032. Sharma, S., Tahir, A., Reddy, K.S., Mallick, T.K., 2016. Performance enhancement of a Building-Integrated Concentrating Photovoltaic system using phase change material. Sol. Energy Mater. Sol. Cells 149, 29–39. https://doi.org/10.1016/j.solmat.2015.12. 035. Skoplaki, E., Palyvos, J.A., 2009. On the temperature dependence of photovoltaic module electrical performance: a review of efficiency/power correlations. Sol. Energy 83, 614–624. https://doi.org/10.1016/j.solener.2008.10.008. Smith, C.J., Forster, P.M., Crook, R., 2014. Global analysis of photovoltaic energy output enhanced by phase change material cooling. Appl. Energy 126, 21–28. https://doi. org/10.1016/j.apenergy.2014.03.083. Tiwari, S., 2018. Thermal modelling of photovoltaic thermal (PVT) dryer: A heat and mass transfer analysis. Indian Institute of Technology, Delhi. Tiwari, S., Tiwari, G.N., 2016. Exergoeconomic analysis of photovoltaic-thermal (PVT) mixed mode greenhouse solar dryer. Energy 114, 155–164. https://doi.org/10.1016/ j.energy.2016.07.132. Vokas, G., Christandonis, N., Skittides, F., 2006. Hybrid photovoltaic-thermal systems for domestic heating and cooling – a theoretical approach. Sol. Energy 80, 607–615. https://doi.org/10.1016/j.solener.2005.03.011. Wu, S.Y., Zhang, Q.L., Xiao, L., Guo, F.H., 2011. A heat pipe photovoltaic/thermal (PV/T) hybrid system and its performance evaluation. Energy Build. 43, 3558–3567. https:// doi.org/10.1016/j.enbuild.2011.09.017. Yazdanpanahi, J., Sarhaddi, F., Mahdavi Adeli, M., 2015. Experimental investigation of exergy efficiency of a solar photovoltaic thermal (PVT) water collector based on exergy losses. Sol. Energy 118. https://doi.org/10.1016/j.solener.2015.04.038.
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Exergo-economic analysis of a serpentine flow type water based photovoltaic thermal system with phase change material (PVT-PCM/water)
T
Taher Maatallaha, Richu Zachariahb, , Fahad Gallab Al-Amria ⁎
a b
Department of Mechanical and Energy Engineering, College of Engineering, Imam Abdulrahman Bin Faisal University, P.O. Box: 1982, Dammam, Saudi Arabia Department of Mechanical Engineering, Amal Jyothi College of Engineering, Kanjirappally, Kottayam 686518, Kerala, India
ARTICLE INFO
ABSTRACT
Keywords: Photovoltaic PCM Exergy Thermal management Optimization
An investigation on PVT-PCM/water system has been carried out under the weather conditions at Kottayam (9.5425 °N, 76.8202 °E), India. In the present paper, experimentations have been done to compare the overall performance of PV and water-based PVT-PCM panels. The PVT-PCM/water system has been analysed under various outdoor environmental conditions and it has been recorded that the integration of PCM contributed to an improved thermal and overall efficiency of 26.87% and 40.59% respectively, along with a rise in electrical efficiency by 17.33% compared with conventional PV panel. It has been found that the payback time for the water-based PVT-PCM system is about 6 years on the overall exergy basis, which is 11.26% shorter compared to conventional PV panels. Also, the water-based PVTPCM system has long-term lifecycle conversion efficiency compared to conventional PV panel by about 27%.
1. Introduction Only a small fraction from the incident sunlight can be converted by PV cells into electricity, which also depends on the operating cell temperature levels. In fact, the major part of the incident solar radiation is converted into heat, elevating the PV cells’ temperature which in turn reduce the output power, the fill factor and the electrical conversion efficiency (Bahaidarah et al., 2013; Fucci et al., 2014). Indeed, 1 °C rise in cell temperature for crystalline silicon PV panels can lead to a reduction in electrical conversion efficiency of approximately 0.45% (Skoplaki and Palyvos, 2009). Different cooling techniques, including air as a heat transfer medium, have been used to decrease the heating of PV cells and control their fluctuations that cause mismatch losses, especially in PV parallel connected sub-arrays. Water cooling systems, however, are more efficient due to the high thermal capacity of water, despite the additional costs of maintenance and operating fees which can increase the payback of the system (Nižetić et al., 2018). Water-based Photovoltaic/Thermal system (PVT) combines both thermal and electrical energy outputs. PVT is used as an active water cooling system for cogenerating thermal energy, which can be used in low temperature applications, together with the significant share of electrical energy (Vokas et al., 2006). There are several models and advances of PVT modules that incorporate water in the PVT system as working fluid. (Joshi and Dhoble, 2018). The solar thermal system in water-based PVT system consists of a metal-based absorber plate and tubes embedded at the back of the PV panel
⁎
to maintain the temperature of the PV cells within an acceptable range, maximizing their electrical output. Krauter (2004) observed that the PV cell temperature dropped by 22 °C by passing water on the front side of the PV panel. Ji et al. (2006) examined the impact of residence time of water particles, in other words, mass flow rate, in a hybrid PVT collector. The temperature of PV reduced, and the heat extracted could be effectively used for thermal applications. The Phase Change Material (PCM) is an extra layer with significantly higher latent heat capacity and very low conductivity, absorbing sensible heat from the PV cells until reaching its melting point and then melts out by absorbing latent heat. PCM is packed behind PVT system and its temperature remains constant while the thermal energy is stored during phase change. The liquid temperature will increase when the PCM completes its phase change, and this process is directly related to the quantity of PCM used and its thermal conductivity. Numerical and experimental analysis on the application of PCMs for removing heat from PV and its relative improvements for the whole PVT systems have been reviewed. In fact, it has been recorded that several investigations have been carried out for controlling, stabilizing and reducing the fluctuation rates of the temperature of PV cells, on the hybrid PVT system and PV, integrating PCM. Indartono et al. (2016) considered a yellowish petroleum jelly as PCM and investigated overall efficiency of the system under Indonesian weather conditions. An improvement from 8.3% to 10.1% was recorded in photovoltaic efficiency. Mahamudul et al. (2016) examined the case of RT35 PCM and made the survey under the weather conditions of
Corresponding author. E-mail address: [email protected] (R. Zachariah).
https://doi.org/10.1016/j.solener.2019.09.063 Received 23 May 2019; Received in revised form 27 July 2019; Accepted 17 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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Nomenclature Ac Am Ce Cp CPCM CPV panel Cstructure Ea,in EBPT Ec Eex,out EPT Esol Fcr,i,n FF FSR,i,n i Imax Isc I(t) It L LCCE M m MCF n
P Pmax Pp Ppv Ps Pth Re Ss Ta Tf ON Ti To TPV UAC Voc Vmax
area of solar panel area of PV cells (m2) cost of DC electrical energy production specific heat capacity cost of PCM cost of PV panel cost of structure embodied energy energy payback time net yearly DC electrical energy obtained from PVT system annual exergy output energy production factor annual solar energy capital recovery factor fill factor sinking fund factor interest rate maximum current short circuit current solar intensity in function of time (W/m2/h) solar intensity (W/m2) life of the system (year) life cycle conversion efficiency maintenance cost of PVT system mass flow rate of water maintenance cost factor number of years
initial cost to the system maximum power cost for DC fan PV electrical power net present cost thermal power revenue generated by selling DC electrical energy salvage value ambient air temperature (°C) air temperature at outlet of PVT air panel (°C) water inlet temperature water outlet temperature PVT outlet temperature (°C) uniform end-of-year annual cost open circuit voltage maximum voltage
Symbols c
c pvcell
elec th
packing factor efficiency of solar panel electrical PV cell efficiency electrical efficiency thermal efficiency
Subscript ex
Malaysia. The results pointed out that there was a reduction of 10 °C in cell temperature by using PCM. Park et al. (2014) researched the impact of PCM layer density and conducted system analysis under South Korea's climatic circumstances. The findings showed a peak electrical efficiency improvement of 3%. Sharma et al. (2016) used RT42 PCM to make a focused PV collector and recorded a 7.7% increase in electrical efficiency under 1000 W/m2 horizontal solar radiation. Under outdoor test conditions, Huang et al. (2006) stated that, owing to a temperature gradient of 17 °C between an aluminium flat plate and an aluminium box containing PCM, the effectiveness of a PV/PCM system was increased by 7.5% at peak solar hours. Hassan (2010) tested a PV/PCM collector in Pakistan and discovered that at the peak solar hour of the day, PV cell temperature was 21.5 °C lower than the reference. Huang et al. (2011) studied the effect of fins embedded in a container covered with RT 27 and RT 35 as PCM. They discovered that fins could enhance the efficiency of the system based on crystalline PCM isolation. In addition, Huang et al. (2006) compared the results of the PVT-PCM (RT25 and GR40) integration with and without fins. They found that the use of fins could reduce the temperature of the PV cell. Hasan et al. (2010) compared PVT performance using five distinct PCMs: paraffin wax (RT20), capric–lauric acid, capric–palmitic acid, pure salt hydrate (CaCl2:6H2O) and commercial blend (SP22). Using capric–palmitic acid and CaCl2:6H2O, a peak decrease of 18 °C in PV cell temperature was achieved at 1000 W/m2. Ho et al. (2016) has numerically studied a PV-PCM system that is micro-encapsulated with MEPCM on the rear side of the PV panel and the PV module is floated over the water surface. In the case of a PV-PCM system with a PCM container thickness of 5 cm, a decrease of 5.5% in PV cell temperature and an increase in proportion of electrical conversion efficiency of 0.8% were noted. Hachem et al. (2017) studied the behaviour of PV-PCM system based on mixed PCM (petroleum jelly, copper and graphite powder) and PCM without added impurities. A typical rise in electrical efficiency of 5.8% and 3% respectively was recorded.
considering exergy as base
Atkin and Farid (2015) combined the PCM with a passive cooling system and reported an improvement of 12.97% in the PV system's electrical efficiency. Kibria et al. (2016) conducted experiments with three PCM; RT25, RT20, and RT28HC and examined the performance of PVT-PCM collector, recorded an electrical efficiency improvement of 5%. Smith et al. (2014) evaluated the outputs of PVT- PCM/water collector across several countries to suggest PCM as the best choice for tropical regions to cool PV systems. The first and second laws of thermodynamics can be combined to perform exergy analysis for obtaining the maximum work potential of a PVT system. Various researchers have conducted exergy investigations on PVT systems. The generation of entropy is directly dependent on the PVT system’s surface temperature. So the generation of entropy in PVT system can be decreased by decreasing surface temperature (Kandilli, 2019). Rajoria et al. (2012) assessed the energy and exergy gain in India under various climatic circumstances from four kinds of hybrid PVT arrays. They discovered that the integration of hybrid PVT technologies in buildings can achieve a better mix of electrical and thermal energy. Yazdanpanahi et al. (2015) carried out experimental and numerical studies on PVT/water system exergy efficiency in terms of exergy losses. They presented an optimum mass flow rate of water for maximum exergy efficiency of 13.95%. Sardarabadi et al. (2017a) compared the exergy efficiency of PVT/water, PVT/Al2O3, PVT/TiO2, and PVT/ZnO with that of a PV system without thermal collector. Among the former systems, PVT/Al2O3 system has an enhanced overall efficiency of 18.27% compared to the PV without collector. Fayaz et al. (2018) carried out experimental and numerical research on the impact of nanofluid flow rate on a water/MWCNT PVT system. They also optimized water/MWCNT nanofluid's weight fraction. Only limited research is performed in the exergy analysis of PVT PCM systems. The performance of a PVT / water system with and without PCM was contrasted by Gaur et al. (2017). It is discovered that the daily overall exergy for PVT collector with PCM is more than the collector without PCM. OM37, which has a melting point of 37 °C was 196
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used as the PCM, for which a mathematical model was developed. Hossain et al. (2019) conducted experiments on a PVT-PCM system having double side serpentine flow to study its performance in comparison with normal PV. The maximum exergy efficiency obtained was 7.09% and 12.19% for PV and PVT-PCM system respectively. Studies imply that the exergy of PVT system improved when PCM was included in nanofluid based system. PVT ZnO/ water nanofluid amalgamated with organic paraffin wax as PCM yields an overall exergy efficiency of 13.61% compared with PV and nanofluid PVT (Hosseinzadeh et al., 2018). Sardarabadi et al. (2017b) conducted exergy analysis in the PVT ZnO/ water nanofluid and PCM, recorded an increase in overall exergy of 23% compared to reference PV module. The above-mentioned investigations were commonly based on numerical analysis and conducted in European countries. Whereas, this experimental set-up has been implemented and the complete experimental procedure has been performed under Indian weather conditions. The aim of this study is to explore and compare experimentally the overall performances of the PV panel and fully serpentine flow based PVT-PCM system, under different operating conditions. The improvements made by the PVT-PCM system have been analyzed and discussed. In addition, the net annual thermal, electrical, and corresponding heat exergy improvements were evaluated. Further, embodied energy, energy payback time, lifecycle conversion efficiency, UAC and the Cost of DC electrical energy production have also been calculated and compared for both systems.
Paraffin wax RT-30 has a high latent heat of melting, high heat storage density and the ability to release the heat stored when the wax changes from solid to liquid or vice versa. By decreasing temperature variations and keeping the temperature stability of the PV cells, the PCM in the PVT-PCM panel improves the thermal system efficiency. The PCM melts by absorbing enough amount of heat, which hinders the temperature rise during the experiment. The PCM helps to lower the temperature of the PV cell, which in turn increases the PV system's electrical performance. The thermal energy stored in the PCM is used to balance the mismatch in demand and supply for heating water during nights and cloudy days and/or cooling PV cells temperature during the day and release that heat for evening use. Voltmeter and Ammeter are devised to measure open circuit voltage (Voc) and short circuit current (Isc) at different time steps. Infrared thermometers are positioned on both sides of the PV system as well as inside the PCM thermal storage, having a temperature measurement accuracy of ± 2 °C . A weather station with ambient temperature accuracy ± 0.5% is employed for the same along with a self-solar power meter with measurement range and accuracy of 2000 W/m2 and ± 10 W/m2 respectively. The measurement uncertainties of all devices are illustrated in Table 2. 2.3. Experimental procedures
2. Methodology
The experimental set-up of the water-based PVT-PCM was placed accurately in the experimentation location for preceding the different measurements under outdoor and targeted conditions. An Infrared Thermometer was used to measure the temperature of PV cells, absorber, phase change material (PCM), inlet and outlet water temperature at systematic positions and time steps while the ambient temperature was obtained with Thermistors for further use in thermal analysis. A digital solar power meter measured the solar irradiation and the anemometer measured the wind patterns. In addition, a digital flow meter was used to measure the water mass flow rate. Regulating valves have been used to regulate the water mass flow rate. The thermo-physical properties of Paraffin Wax are given in the Table 3. The paraffin wax was filled as a liquid in the cabin which is adhered to the back side of the solar panel and then cooled till it froze totally. Some free space is provided by the solidified PCM on the top end of the cabin intended to hold the expanded volume during PCM melting. The experimentations were done for the selected days in the month of May 2017 and same solid state of PCM layer was assured for each day at the start of experiments. A 5 min time step was used to log the data for the reference PV and the PVT-PCM system.
2.1. Data availability To check the feasibility of using a Phase Change Material with a PV panel, a complete prototype has been erected and its performance analysis has been carried out in Kerala (9.5425 °N, 76.8202 °E) under Indian weather conditions. Fig. 1 shows the monthly average radiation pattern measured on horizontal plane in Kerala. The monthly average temperature variation is also given in Fig. 1. The experiments were conducted under the above weather conditions and the results obtained are shown in the following sections. 2.2. Experimental setup A PVT/water system based entirely on serpentine flow, embedded with PCM, is designed and fabricated in the present study. Its performance is compared with a conventional PV system, as shown in Fig. 2. The electrical and thermal performance of the systems under consideration is explored with different water mass flow rates on chosen days. The arrangement of the aluminium absorber plate, copper serpentine tube and aluminium container filled with PCM is illustrated in Fig. 3. In the experiments C-Si based polycrystalline PV panels were used, having power capacity and electrical conversion efficiency of 100 W and 14.25%, respectively, for the comparative study of the electrical and thermal performances. The PV modules were tilted by 10°, equal to Kerala latitude, and directed towards the south direction. The PV modules were placed outdoors for nine days over a daily duration of six hours; from 10:00 A.M. to 4:00 P.M. The Short circuit current and the Open circuit voltage were measured to assure reliability in PV modules. In a water-based PVT-PCM system, a 30 mm thick aluminium container with an absorber surface area of 0.54 m2 is connected to the back side of the PV panel. The specifications of the PVT-PCM/water system are listed in Table 1. The mass flow rate is regulated and measured using a control valve and digital flow meter respectively, and four different water flow rates of 0.004 kg/s, 0.008 kg/s, 0.010 kg/s and 0.020 kg/s are considered to examine the electrical and thermal performances of water-based PVTPCM. Measurements of temperature are carried out using K-type thermocouples (accuracy ± 2.2 °C). A Schematic diagram of PVT-PCM/ water system with all dimensions in mm is shown in Fig. 4.
Fig. 1. Monthly average global solar radiation and dry bulb temperature of the experimental set up site. 197
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the following equation, th
=
m × Cp × (To
Ti )
I × Ac
(4)
where m is the water mass flow rate.Cp , Ti and To are the specific heat, the water inlet and outlet temperature respectively. 3.2. Exergoeconomic analysis of PVT-PCM system Phase change materials showed a promising sign that a significant amount of energy could be stored or released in the form of latent heat during melting and solidification without any shift in temperature. It can be noted that there have been several studies in the field of incorporating PCM into PVT systems, but it has been noted that the exergy assessment of these hybrid systems is limited. In this section, we discuss the major differences between engineering economic analyses of a typical PVT-PCM system to that of a normal PV panel. Some terminologies of economic parameters related with renewable energy systems will be discussed along with two case studies, which are used to differentiate between economic analyses of the reference PV panel and the PVT-PCM system. A new methodology has been developed for performance assessment of PVT-PCM system through total lifecycle energy effectiveness consideration in present-value terms (Mishra and Tiwari, 2013; Tiwari, 2018; Tiwari and Tiwari, 2016).
Fig. 2. The standard PV system and PVT-PCM/water system experimental setup.
3. Data reduction 3.1. Energy analysis of PVT- PCM system The PVT-PCM system's electrical and thermal efficiency was evaluated using the following equations. Electrical efficiency of PVT system can be calculated as,
=
elec
Voc × Isc × FF ×
c
I × Ac
3.2.1. Energy payback time (EPBT) EPBT is the time it takes to recover the energy that is embodied in the system. The computation of EPBT considering exergy as the base EPBTex that can be calculated as (Mishra and Tiwari, 2013; Tiwari, 2018; Tiwari and Tiwari, 2016),
(1)
where c is the packing factor. A packing factor of one corresponds to a system where the entire area is covered by PV cells. c is expressed as follows, c
=
Area of solar cells Area of PV module
EPBTex =
Pmax V × Imax = max Voc × Isc Voc × Isc
(5)
Here the PVT-PCM's embodied energy Ea,in takes into account both the system manufacturing requirements and the energy needed to produce the raw materials. The aim of any embodied energy assessment is to determine the amount of energy taken into consideration for the production of a product or element. Embodied energy analysis includes the energy used for material production, transportation, fabrication of photovoltaic cell/module and PCM, human effort, installation and maintenance of the system and finally the disposal/salvage. The parameter Eex, out stands for the overall annual exergy got from the system. The system is regarded profitable for lower values of EPBT.
(2)
Voc , Isc , I , Ac and FF are the open circuit voltage, the short-circuit current, the global solar irradiation, the PV thermal collector area and the fill factor of the PV module, respectively. FF can be calculated as follows, FF =
Ea, in Eex , out
(3)
Thermal efficiency of photovoltaic/thermal system can be found by
Fig. 3. Diagram of the water-based PVT system. 198
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Table 1 Specifications of prototype. Parameter
Unit
Value
Individual rated capacity of each module Electrical conversion of each module Open Circuit Voltage Shirt-Circuit current Total Aperture area of absorber Plate Thickness of the aluminum container Thickness of the Collector insulator Volume of PCM as liquid Tie step of measurement
Pr [W] ele [%] Voc [V] ISc [A] Height × Width [m2] ec [mm] ei [mm] VPCM [L] %
100 14.25 (at STC) 21.6 6.14 0.6565 30 0 (back insulation not there) 11 98
Table 3 Properties of Paraffin wax. Paraffin wax properties Melting point (°C) Heat of Fusion (kJ/kg) Thermal conductivity (W/m K) Specific heat capacity (kJ/kg K)
57 200–220 0.24 at 27 °C 2.1 (liquid) 2.0 (solid) 810 (liquid) 910 (solid)
Density (kg/ m3 )
3.2.3. Life cycle conversion efficiency (LCCE) LCCE is referred to as the PVT-PCM system's net output in respect of the solar input received throughout the system's lifetime. LCCE by exergy (LCCEex ) can be calculated as (Mishra and Tiwari, 2013; Tiwari, 2018; Tiwari and Tiwari, 2016): Fig. 4. Dimensions (in mm) of the PVT-PCM/water system.
LCCEex =
3.2.2. Energy production factor (EPF) EPF of PVT-PCM system gives an idea about its overall performance. On the one hand, it can be calculated annually as the reverse of EPBT as (Mishra and Tiwari, 2013; Tiwari, 2018; Tiwari and Tiwari, 2016):
EPFex =
Eex , out Ea, in
3.2.4. Economic analysis of the produced electricity The cost of DC electricity generation in €/kWh (Ce ) of the system can be computed as (Tiwari, 2018):
(6)
1 EPBTex
Ce =
(7)
Eex , out Ea, in
UAC Ec
(10)
The UAC for PVT-PCM system is calculated with respect to the present value method as (Tiwari, 2018):
UAC = P × Fcr , i, n + M × Fcr , i, n
In the energy point of view the system is worthwhile if EPFex → 1, for EPBTex → 1, otherwise it is not. On the other side, on the basis of lifetime, it can be calculated as (Mishra and Tiwari, 2013; Tiwari, 2018; Tiwari and Tiwari, 2016):
EPFex = L
(9)
Here Esol represents the annual solar energy and L the life of the system.
Or:
EPFex =
Eex , out × L Ea, in Esol × L
Ss × FSR, i, n
(11)
where P, Ss and M are the stands for net present cost, salvage value and cost for maintenance of PVT system respectively. The value of M can be calculated using the relation between P and Maintenance Cost Factor (MCF) as a product between the two (The value of MCF has been considered as 0.1). The salvage value has the following expression,
(8)
Ss = P × (1
In this case EPF should be more than one. The value of EPF should be as high as possible for the system to be cost-effective.
(12)
I) × n
where I the annual depreciation expense. The value of the capital recovery factor can be calculated as (Tiwari,
Table 2 Uncertainties of device measurement. Measurement parameter Solar radiation Current Surface Temperature Ambient Temperature Wind Speed Voltage
Device Solar Radiation Meter Ammeter Infrared Thermometer NTC Thermistor Anemometer Voltmeter
Model
Measurement range
Tenmars; TM207 Solar Power Meter Ecosense Fluke; 59MAX esp 10 K (EmbSys) Ecosense Ecosense
199
2
2000 W/m – −30 °C to +350 °C −60 °C to +150 °C 0–45 m/s –
Accuracy ± 10 W/m2 ± 0.1% ± 2 °C ± 0.5% ± 0.1 m/s ± 0.1%
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2018),
Fcr , i, n =
4.2. Comparison of electrical efficiencies of PV and PVT-PCM system
i × (1 + (1 + i )n
i )n 1
Fig. 10 highlights the variation of PVT-PCM/water system electrical efficiency for a day at different water flow rates. It is observed that the PVT-PCM/water system's electrical efficiency is always higher than that of normal PV. A maximum increment of 29.49% is noted in electrical efficiency or PV output power, with PVT-PCM/ water system. The electrical efficiency shows an average increment of around 17.33% with PVT-PCM/water system as compared to normal PV during the entire experiments. As the flow rate increases, the electrical efficiency is only slightly higher. If the flow rate is higher, the power of the pump will double (Nasrin et al., 2018). Thus, a minimum flow rate can therefore retain the PV system efficiency at a reasonable value and boost the system's life without compromising on pump power.
(13)
The sinking fund factor value can be calculated as (Tiwari, 2018),
FSR, i, n =
i (1 + i )n
1
(14)
The initial cost of the PVT-PCM system suggested can be obtained as follows (Tiwari, 2018),
P = CPV panel + Cstructure + CPCM
(15)
4. Results and discussion
4.3. Energy analysis of PV and PVT-PCM systems
Upon design, building-up and installation of the system, the experimental data of the parameters such as PV top surface, absorber layer, PCM temperatures were recorded and collected at different time intervals and dates, at various flow rates ranging from 0.004 to 0.02 kg/ s. A detailed comparative study of both systems has been given in the following section.
Fig. 11 summarizes combined energy performance indicators of the PVT-PCM system and the reference PV panel, including their thermal, electrical and overall efficiencies. The PVT-PCM showed significant reduction in heat losses, there by elevating the capacity for thermal energy storage. This resulted in the ability to generate more electrical energy compared to standard PV panels. Thus, it can be stated that the electrical production as well as the thermal energy storage capability of PV systems can be increased by controlling the temperature through latent heat of absorption in PCM.
4.1. Comparison of instantaneous temperature at different sub-layers of PV and PVT Figs. 5–9 show temperature variation at different positions of the normal PV system and water-based PVT-PCM system for a day with respect to average solar radiation without water flow and water flow of 0.004 kg/s, 0.008 kg/s, 0.01 kg/s and 0.02 kg/s. The average of the values taken for 3 consecutive days is reported. Fig. 5 shows the variation of the temperature at different sub layers of reference PV and PVT-PCM system without water flow through the serpentine tubes. It can be noted that at the starting hours, the temperature of the PV panel is more than that of PVT-PCM system, but the PVT temperature increases due course of time. This is because initially the heat from the PV panel is removed by the PCM, which melts, absorbing latent heat of fusion. But the PCM thermal storage does not remove this heat, which raises the system temperature. Fig. 6 indicates the water-based PVT-PCM system temperature variation corresponding to the average daytime solar radiation at a mass flow rate of 0.004 kg/s of water. As shown in this figure, PVTPCM panel top temperature is smaller than PV panel temperature with an average temperature reduction of 3.57%. The panel bottom temperature registered a 3.35% reduction in temperature on an average. The temperature differences in the solar radiation of both systems are shown in Fig. 7 for 0.008 kg/s water flow rate. A maximum temperature reduction of 6.84% is observed for the panel bottom temperature at 2 P.M. compared to reference PV panel. An average panel top temperature reduction of 3.57% and an average panel bottom temperature of 3.77% are observed. Fig. 8 shows the variation in temperature of the PVT-PCM with solar radiation for a day at mass flow rate of 0.01 kg/s of water. As shown in this figure, PVT-PCM system temperature is less than PVT panel temperature and the average decrease in temperature is 4.27% at the top and 3.88% at the bottom relative to standard PV system. Fig. 9 shows temperature variation at a mass flow rate of 0.02 kg/s of water for a day. Average temperature reduction reported for the PVTPCM system is 4.46% at the top and 4.41% at the bottom compared with the normal PV panel. It can be concluded that a temperature decrease is observed when the water mass flow rate increases to 0.02 kg/s. This result has also been reported by (Hasan et al., 2015; He et al., 2011; Wu et al., 2011).
4.4. Exergy and economic analysis of PV and PVT-PCM systems Exergo-economic analysis can be used to assess the PVT-PCM system's feasibility. The maximum work potential from the Table 4 details the embodied energy calculation for the PVT system. Due to greater material requirements, the embodied energy for the PVT-PCM system is obviously more than that of the PV panel. The embodied energy for the PVT-PCM system is calculated to be 624.4 kWh, expecting a life span of 20 years. Table 5 indicates the calculation of the PVT-PCM payback period. The amount of annual sunshine hours is taken to be 1800 h. The time needed to recover the PVT-PCM system's embodied energy is about 6 years, it can be regarded as a short period of time that demonstrates such system's efficiency. The PVT-PCM model therefore has a shorter payback period of 11.26% compared to conventional PV panel. This is because the embodied energy for PVT-PCM system is only 5.56% higher than the PV panel, but the annual exergy output is 18.97% higher.
Fig. 5. Trends of the instantaneous temperature of PV and PVT-PCM systems vs. solar radiation during day time without water flow. 200
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Fig. 9. Trends of the instantaneous temperature of PV and PVT-PCM systems vs. solar radiation at water flow rate of 0.02 kg/s.
Fig. 6. Trends of the instantaneous temperature of PV and PVT-PCM systems vs. solar radiation at water flow rate of 0.004 kg/s.
Fig. 7. Trends of the instantaneous temperature of PV and PVT-PCM systems vs. solar radiation at water flow rate of 0.008 kg/s.
Fig. 8. Trends of the instantaneous temperature of PV and PVT-PCM systems vs. solar radiation at water flow rate of 0.01 kg/s.
Fig. 10. Comparison of the instantaneous electrical efficiency of PV and PVTPCM panels during day time at various mass flow rates of water.
Table 6 demonstrates the annual and lifetime-based energy production factor. The production enhances accordingly for any system if EPF increases. The lifetime value of the EPF should be as large as possible in terms of the system's cost effectiveness and should be at least more than 1. The annual EPFex is 0.15 and 0.17 for the PV and PVT-
PCM respectively. EPFex is found to be 3.38 for the PVT-PCM system considering a lifetime of 20 years, which is 13.8% higher than the normal PV panel. It increases the attractiveness of the suggested system over standard systems. Table 7 illustrates the exergy-based life cycle conversion efficiency 201
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Fig. 11. Average electrical efficiency, thermal efficiency and overall efficiency measured at the system deployment site for reference PV and PVT-PCM during the days between 03 May and 19 May.
calculation for the system. Where Esol = annual irradiation × surface area of PVT = 4090 × 0.65 = 2658.5 kWh/year. The conversion efficiency for the lifecycle is a positive value, resulting in the PVT-PCM technology being very beneficial. LCCE is calculated to be 0.022 and 0.028 for PV panel and PVT-PCM system respectively. Hence, the PVT-PCM system has higher LCCE compared to conventional PV panel by about 27%. The UAC is calculated for interest rates of 5%, 10% and 15% corresponding to lifespan of 5, 10, 15 and 20 years. It's visible in the Figs. 12–14 that the UAC increases and decreases as expected for PV systems with and without PCM with an increase in interest rate and lifespan respectively. For both systems, the UAC value corresponding to the interest rate of 5% was obtained as a minimum as expected. Furthermore, UAC for PV is less than PVT-PCM in all the cases because PVT is an enhanced variant with attachments on standard PV panel, which implies that the production of PVT-PCM system requires a more quantity of material compared to PV. Fig. 15 displays the variation in the cost of DC electricity output (Ce) as a function of the number of years at interest rates of 5%, 10% and 15%. It has been seen that with an increase in lifespan, the cost of producing DC electrical energy for both PV panel and PVT-PCM system is decreasing. The value of Ce rises with higher interest rates. PVT-PCM always has a lower value for Ce owing to greater net annual DC electrical energy from PVT than PV. Compared to conventional PV panels, this makes PVT-PCM system an attractive investment option.
Table 5 Calculation of Payback period.
PV panel PVT PCM System
Ea,in (kWh)
Eex,out (kWh/year)
EPBTex (years)
591.5 624.4
88.71 105.54
6.68 5.92
Table 6 Energy production factor on annual basis based on the lifetime with PCM.
PV panel PVT PCM System
Ea,in (kWh)
E ex,out (kWh/year)
EPFex annual
Life time
EPFex life time
591.5 624.4
88.71 105.54
0.15 0.17
20 20
2.97 3.38
Table 7 Calculation of lifecycle conversion efficiency with PCM.
PV panel PVT PCM System
Ea,in (kWh)
Eex,out (kWh/year)
Life time (year)
Esol (kWh/ year)
LCCEex
591.5 624.4
88.71 105.54
20 20
2658.5 2658.5
0.022 0.028
findings were deduced from the experiment.
• The temperature of PV cells with PCM was found to be lower throughout the day than that of the system without PCM. • It has been recorded that a PVT-PCM/water system’s electrical
5. Summary and conclusions Exergo-economic analysis was performed in the present study to verify the feasibility of incorporating paraffin wax into a water-based PVT panel. A prototype of water-based PVT-PCM scheme was set up and tested under various working conditions to investigate improvements in the PV panel's electrical yield (output power) and electrical efficiency relative to conventional PV system. The following salient
• •
output is higher than that of PV panel with an average increase of approximately 17.33%. On average, the overall efficiency of the PVT-PCM system is reported an increase of 28.86% compared to conventional PV. The embodied energy of the PVT-PCM system can be recuperated in
Table 4 Calculation for embodied energy for PVT with PCM. Module efficiency (%) PV Technology P-Si
14
PCM Ein (system)
Expected life (year)
Specific energy content (kWh/m2)
Ein for 1.32m2 (kWh, 20 years)
20
910
591.5
Expected life (year)
Specific energy content (kWh/kg)
Ein for 2 kg (kWh, 20 years)
20
16.45
32.9 624.4
202
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Fig. 12. Variation of UAC with the No. of years at an interest rate of 5%.
Fig. 15. Variation of the Cost of DC electrical energy production as function of No. of years at various interest rates.
Acknowledgment
Fig. 13. Variation of UAC with the No. of years at an interest rate of 10%.
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Fig. 14. Variation of UAC with the No. of years at an interest rate of 15%.
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