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Performance analysis of a concentrated photovoltaic and thermal system
Available online at www.sciencedirect.com
ScienceDirect Solar Energy 129 (2016) 217–223 www.elsevier.com/locate/solener
Performance analysis of a concentrated photovoltaic and thermal system _ Ilhan Ceylan a, Ali Etem Gu¨rel b,⇑, Alper Ergu¨n a, Abdulsamed Tabak a a
Karabuk University, Technology Faculty, Department of Energy Systems Engineering, 78100 Karabuk, Turkey b Duzce University, Vocational School, Department of Electrical and Energy, 81010 Duzce, Turkey Received 22 July 2015; received in revised form 1 November 2015; accepted 5 February 2016
Communicated by: Associate Editor I. Farkas
Abstract The purpose of the present study is PV/T system, which designed and implemented in order to reuse of thermal energy while increasing of electric production. Solar radiation was increased with the concentrator about two folds and electrical power gains were increased. A paraffin wax was used to store latent heat as a thermal energy storage. Thermal energy was combined with a greenhouse – like air drier and used to dry the product. Thermal and electrical energy gain in the concentrated and non-concentrated panels were compared and total system efficiency was analyzed. The maximum temperature obtained at the back of the panel was calculated as 37 °C and the panel efficiency was 11% at mean solar radiation was 2000 W/m2. Ó 2016 Elsevier Ltd. All rights reserved.
Keywords: Concentrated PV; PCM; Energy storage; Solar energy
1. Introduction The most common method of producing electricity energy with solar cells in the last few years has been concentrated solar cells. Different methods were tried to increase the decreased efficiency as solar cells are heated. One of them is to cool the cells with water flux. The related literature with regard to concentrated solar cells and paraffin used as a heat storage were given as follows. In their study, Petito and Renno designed a concentrated photovoltaic and thermal (CPV/T) system to produce thermal and electricity energy for home use. The program of MATLAB was used for the technical studies and cost analysis of energy amount to be produced in different operating conditions (heat, radiation, optic type, ⇑ Corresponding author.
E-mail address: [email protected] (A.E. Gu¨rel). http://dx.doi.org/10.1016/j.solener.2016.02.010 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
the size of solar cell, reflecting factor, etc.) was carried out (Renno and Petito, 2013). In a study by Schaefer and Kerzmann, a simulation regarding linear concentrated photovoltaic solar cell was produced. In their study, they used a three-layer (GaImP/GaAs/Ge) photovoltaic cell, Fresnel lens in order to reflect sun rays on photovoltaic cell, an active water circulation for the cooling system for the purpose of increasing the efficiency and a pump was used to obtain water circulation. In the system with 6.2 kW, a thermal energy of 5.089 kW h was produced annually and 14.215 kW h electricity energy was produced in the three-layer photovoltaic panel working with a 34.75% efficiency (Kerzmann and Schaefer, 2012). Du et al. made an experimental mechanism so as to cool concentrated solar energy system. The lower parts of the experimental system was made with mirror reflectors (6 on the right, 6 on the left, 12 in total) and the upper parts was made up of a mono-crystal solar cell at the size of
_ Ceylan et al. / Solar Energy 129 (2016) 217–223 I.
218
Nomenclature PV CPV/T A c I ðtÞ t m h E_ l T Q_ u V L MC
photovoltaic concentrated photovoltaic-thermal area (m2) specific heat (kJ/(kg K)) incident total radiation (W/m2) experimental time (s) mass (kg) enthalpy (kJ/kg) rate of electrical energy (W) temperature (°C) rate of useful energy transfer (W) volume (m3) latent heat (kJ/kg) moisture content (g water/g dry matter)
gh pw i s l m mlt
greenhouse paraffin wax inlet solid liquid module melting point
Greek symbols b electrical efficiency thermal coefficient s transparency a absorptivity g efficiency q density (kg/m3) d packing factor
Subscript c cell
1.08 m 0.14 m. The whole system was designed to follow the sun in one direction (east-west). The fixed and nonconcentrated type of the same solar cell were measured comparatively during the experimental process. The output power of CPV was 4.9 times more compared to the output power of the fixed solar cell. However, while the electrical efficiency of the solar cell in the fixed panel was about 11%, it was below 9% in CPV (Du et al., 2012). RoblesOcampo et al. made the application of the CPV/T they designed in Mexico. In this system, solar rays dropping on the panel were concentrated thanks to the reflector placed under the panel and electrical efficiency was increased because of the flux (water). At the end of the measurements, electrical efficiency was calculated as 16.4% and thermal efficiency became 50% (RoblesOcampo et al., 2007). In a study by Sonneveld et al. a CPV/T system was designed to meet the need for electricity and thermal energy in greenhouses. The basic reason for the preference of this system is to prevent the shadowing caused by these panels which occupy a large surface area. In the application where two way following system was used, the electrical efficiency of the panels was 15.8%, while thermal efficiency was calculated as 65% (Sonneveld et al., 2010). In another experimental study by Kandilli (2013), solar energy was concentrated on the solar cell using a chrome concentrated layer. Thanks to ‘hot mirror’ used around the solar cell, it was aimed to allow the visible light to pass but reflect the ultraviolet rays. In that way ultraviolet rays would be reflected on the tube and the thermal efficiency of the water would be increased. In the experiments, the efficiency was calculated as 15.2% and thermal efficiency was 49.9% (Kandilli, 2013). Fazilati and Alemrajabi experimentally investigated the effects of using Phase Change Materials (PCM) as storage
medium on the performance of a solar water heater. A type of paraffin wax used as PCM in spherical capsules as storage material in the tank of solar water heater. It was observed that by using PCM in the tank the energy storage density increased in the tank up to 39% and the exergy efficiency enhanced up to 16%. It was also observed that solar water heater with PCM can supply hot water with specified temperature at 25% longer time (Fazilati and Alemrajabi, 2013). Zalewski et al. presented an experimental study of a small-scale Trombe composite solar wall. In this case, the phase change material was inserted into the wall in the form of a brick-shaped package. The energy performance of the wall from heat flux measurements and enthalpy balances were also presented (Zalewski et al., 2012). Biwole et al. (2013) investigated the use of phasechange materials (PCM) to maintain the temperature of the panels close to ambient. The main focus of the study is the computational fluid dynamics (CFD) modeling of heat and mass transfers in a system composed of an impure phase change material situated in the back of a solar panel (SP) (Biwole et al., 2013). Al-Hinti et al. presented an experimental investigation of the performance of waterphase change material (PCM) storage for use with conventional solar water heating systems. Paraffin wax contained in small cylindrical aluminum containers used as the PCM. Over a test period of 24 h, the stored water temperature remained at least 30 °C higher than the ambient temperature (Al-Hinti et al., 2010). Varol et al. experimentally investigated the performance of a solar collector system using sodium carbonate decahydrate as Phase Change Material (PCM) during March and collector efficiency compared with those of convectional system including no PCM (Varol et al., 2010). Koca et al. performed an analysis of energy and exergy for a latent heat storage system with
_ Ceylan et al. / Solar Energy 129 (2016) 217–223 I.
phase change material (PCM) for a flat-plate solar collector. The designed collector was combined in single unit solar energy collection and storage. PCMs were stored in a storage tank, which was located under the collector. A special heat transfer fluid was used to transfer heat from collector to PCM (Koca et al., 2008). Photovoltaic power is a clean energy that can be obtained directly from solar energy. This energy is regarded as a significant alternative to the solution of environmental problems resulting from traditional energy sources. Despite all these positive sides, the costs of photovoltaic systems are still too high to compete with traditional power systems. In order to be able to reduce the costs of these systems to minimum, one of the possible methods is to concentrate solar radiation and reduce the cell surface area used. However, the cell heat will increase with the concentration of sun radiation and it will decrease the cell efficiency. In order to increase cell efficiency and also prevent possible thermal damage in the cells, various cooling methods have been used (Kong et al., 2013). In this study, concentrated solar panel was designed together with heat and energy storage. Paraffin was used as a heat storage inside the solar panel which manufactured in a spherical shape out of broken cells. The temperature of the panel will increased with solar radiation dropping on the broken cells through concentrator. Increasing panel temperature was prevented using paraffin wax. The energy obtained from the solar panel was used to provide the fans in the greenhouse. The designed and manufactured concentrated solar panel with a heat storage was mounted on a greenhouse and tested in a drying application. Spinach leaves were fully dried. The dried leaves of spinach were then crumbled and beaten in a mortar with a pestle to turn into dust. Spinach dust is commonly used in cakes, soups as a child food. This study is highly different from the ones in the literature in terms of both the newly manufactured product and of experimental system. 2. Designing and operating procedure The experimental system is made up of a concentrator (1), a sphere filled with paraffin and covered with broken cells (2), a charging regulator (3), a gel accumulator (4) and a drier. The broken cells were combined with UV resistant epoxy on a sphere made of aluminum. Paraffin was
heated and filled in the sphere in a liquid form. The angle of the concentrator was manufactured in an adjustable form and the sphere was placed on the focal point. The bottom part of the sphere which had no broken cells was placed in the channel and the stored heat was transferred in the greenhouse with help of fans. The system was designed as in Fig. 1 and manufactured as in Fig. 2. As given in the figure, the heat transferred to the sphere through concentrator pass on to the paraffin in the sphere. Paraffin changes its phase at this point from solid state to liquid state and works as a heat storage. The heat stored is sent to the drying chamber with the help of a fan. In order to provide energy for the fan, broken photovoltaic cells were benefited. These photovoltaic cells could charge a gel type accumulator. The ever-charging accumulator could operate the fan. No other outside energy was given to the system. The drying operation is carried on even at time when there is no sun thanks to the charged accumulator and heat sphere (paraffin). 2.1. Operating procedure (a) As given in Fig. 3, the reflection of sun radiation on number 1 concentrator is turned into electricity energy through broken cells with a number of 7 on the sphere and stored in the gel accumulator. (b) By means of number 1 concentrator, the sun radiation focused on the sphere with a number of 7 is also stored as a heat energy in the paraffin in the sphere. (c) The heat energy stored in the sphere with a number of 7 are transferred on the greenhouse with a number of 2 inside the channel with a number of 5. (d) The energy of the circulation fans inside the channel 5 are supplied with the gel accumulator charged with broken cells. (e) The heat energy stored in the paraffin inside the sphere 7 and the electricity energy stored in the gel accumulator are used for the heat and electricity energy of the greenhouse when there is no sun.
3. Experimental analysis In order to experimentally investigate in the designed greenhouse type drier with a heat and energy sphere,
1
2
3
219
4
1. Concentrator, 2. The sphere filled with paraffin and covered with broken cells, 3. Charging regulator, 4. Gel accumulator and fans (in the box) Fig. 1. Heat-energy sphere designed and manufactured.
_ Ceylan et al. / Solar Energy 129 (2016) 217–223 I.
220
Fig. 2. Designed (a and b) and manufactured (c and d) drying system.
weight of the dried product were measured. Measured points in the drying system and measured devices properties were shown in Fig. 4 and Table 1. 4. Theoretical analysis of CPV/T system The correlation between an enthalpy and heat for paraffin wax can be expressed as follows (Lee and Lucyszyn, 2007). Z T mlt cs dT T 6 T mlt ; solid phase ð1Þ hs ¼ 1. Concentrator, 2. Greenhouse, 3. Air inlet point, 4. Air outlet point, 5. Heat depot, 6. Regulator, 7. Sphere surface (broken solar cells).
Fig. 3. Drying system with concentrated solar panel.
Z
T T
hl ¼
cl dT
T > T mlt ; liquid phase
ð2Þ
T mlt
Total enthalpy (heat energy) of paraffin wax for any temperature can be expressed as follows. htotal ¼ hs þ L þ hl
ð3Þ
Thermal properties of the paraffin wax used in the study was given in Table 2. The useful heat energy amount obtained from paraffin wax and transferred to the drying cabin can be calculated using the equation below: Q_ u;pw ¼ mpw c ðT gh T pw Þ=t
ð4Þ
The useful heat energy transferred from the greenhouse can be calculated as follows: Q_ u;gh ¼ V gh c qair ðT gh T i Þ=t 1 : solar radiation 2 : solar radiation 3 : inlet air temperature 4 : solar panel temperature
5 : outlet air temperature 6 : solar radiation 7 : drying chamber temperature
Fig. 4. Measured points in drying system with heat and energy depot.
900 g spinach was dried until it became fully dried. In the experimental system, solar radiation focusing from the top and bottom points, solar radiation on the glass surface in the drying chamber, sphere surface temperature, the temperature inside the greenhouse, blowing air temperature, inlet air temperature, blowing air velocity and the
ð5Þ
The electrical efficiency of the photovoltaic modules can be categorized in two groups as cell efficiency and module efficiency. The electrical loss from the panels are realized with temperature. Open circuit voltage and filling factor are decreased significantly with the temperature. Besides that, short circuit current is increased for a while (Mishra and Tiwari, 2013; Zondag, 2008; Ceylan et al., 2014). As a result of this important effect, the cell efficiency can be calculated as follows: gc ¼ go ½1 bðT c 25Þ
ð6Þ
In the equation, go is the efficiency under standard test conditions (STC). (I(t) = 1000 W/m2, Tc = 25 °C, AM = 1.5). Tc is the cell temperature and b is the thermal
_ Ceylan et al. / Solar Energy 129 (2016) 217–223 I.
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Table 1 Measured devices and properties. Point
Measured value
Device name
Device properties
1, 2, 6 3, 4, 7 5
Solar radiation Temperature Air velocity
PCE-SPM1 Data logging Solar Power Meter – Solarmeter 4 channel K thermometer SD logger – Termocouple Delta Ohm HD 2303.0 – Anemometer
0–2000 W/m2, ±10 W/m2 0 °C–50 °C, ±1 °C 0.1–40 m/s, ±0.01 m/s
Table 2 Thermal properties of paraffin wax (Lee and Lucyszyn, 2007).
5. Result and discussions
Property
Value
Melting point ðT m Þ, (°C) Density ðqÞ, (kg/m3) Latent heat (L), (kJ/kg) Thermal conductivity ðkÞ, (W/m K) Specific heat ðcÞ, (kJ/kg K)
47 818 (solid), 760 (liquid) 266 0.24 (solid and liquid) 2.95 (solid), 2.51 (liquid)
The cell and module efficiency obtained as a result of the tests was calculated in Eqs. (6) and (7) and given in Fig. 5. The efficiency of the concentrated modules can decrease up to 5% with heating. It is necessary to cool them in order to prevent the efficiency of these concentrated panels. In the current study, the panel temperature was measured as approximately 30 °C in the concentrated solar panel application carried out with paraffin wax. The module efficiency made of polycrystalline broken cells was also calculated with an approximation of 11%. The electrical energy gains of the concentrated solar cells was calculated in Eq. (8) and given in Fig. 6. The solar radiation on the concentrated panel increase about two folds. Electrical energy gains also increased almost twice similar to those of solar radiation. The thermal energy gains of the whole system was calculated in Eq. (9) and given in Fig. 7. A very small difference was found between the calculations made for the concentrated and non-concentrated systems. It was caused by the fact that paraffin wax cannot reach 47 °C which is the melting heat together with the concentrator. The fact that thermal heat gains are almost the same is an indication that the concentrating panel of the paraffin wax is successful in cooling. The change in cell temperature with concentrated and non-concentrated solar radiation was given in Fig. 8. Mean panel temperature was found as 30 °C. Depending on the solar radiation, the temperature of the panel in the shape
coefficient of electrical efficiency. b value changes depending on the material from which solar cells are produced. For crystal silicon almost 0.0045/K is taken, 0.0035/K is taken for CIS, 0.0025/K for CdTe and 0.002/K for a-Si (Mishra and Tiwari, 2013; Tiwari and Dubey, 2010). PV module efficiency can be calculated using Eq. (7): ð7Þ
where is sc the transparency for the PV module glass, ac is the absorptivity of the solar cell and dc is the packing factor; the values for these are taken as 0.90, 0.95 and 0.90 respectively (Dubey et al., 2009). The electrical energy gain obtained from PV module in the system can be calculated with the equation below: E_ l ¼ gm Am I ðtÞ
ð8Þ
Overall thermal energy gain in the system was calculated as follows: E_ l Q_ u;overall ¼ Q_ u;pw þ Q_ u;gh þ C power
ð9Þ
In the equation, C power is the conversion power of the thermal power plant depending on the quality of the coal. C power could be taken as 0.38 for a good quality coal with a low ash. This value is between 0.20 and 0.40 (Dubey et al., 2009; Huang et al., 2001; Mishra and Tiwari, 2013). Overall thermal energy efficiency in the system is calculated as follows: goverall ¼
Q_ u;overall Am I ðtÞ þ Agh I ðtÞ
ð10Þ
Initial moisture content of the dried products on a dry basis can be calculated as follows (Aktasß et al., 2014): MCdb ¼
Mi Md Md
ð11Þ
where M i initial mass of the dried products, M d is the product mass which in the dried state.
20
Cell efficiency Module efficiency 18
Efficiency (%)
gm ¼ gc s c ac dc
16
14
12
10
0
30
60
90 120 150 180 210 240 270 300 330 360 390
Experimental time (min.) Fig. 5. Cell and module efficiency.
_ Ceylan et al. / Solar Energy 129 (2016) 217–223 I. 2500
14 Non-concentrated PV module
10
Solar radiation (W/m²)
Electrical energy gain (W)
12
8 6 4 2 0
0
30
60
2000
40
1500
30
1000
20
500
10
0
90 120 150 180 210 240 270 300 330 360 390
0 0
30
60
Experimental time (min.) Fig. 6. PV module powers with and without concentrators.
120 150 180 210 240 270 300 330 360 390 Experimental time (min.)
90
Concentrated PV module
Concentrated PV module
80
Non-concentrated PV module
Non-concentrated PV module 70
Overall efficiency (%)
Thermal energy gain (W)
90
Fig. 8. Concentrated and non-concentrated solar radiation and cell temperature.
240
200
50
Concentrated Solar Radiation Non-concentrated Solar Radiation Cell temperature
Concentrated PV module
Cell temperature (°C)
222
160
120
80
40
60 50 40 30 20 10
0
0
30
60
0
90 120 150 180 210 240 270 300 330 360 390
0
30
60
Experimental time (min.)
Experimental time (min.)
Fig. 7. Thermal energy gains of PV module with and without concentrators.
Fig. 9. The change of the whole system thermal efficiency.
50
Ambient temperature (°C)
of a sphere full of paraffin wax increased. During the experiments, the highest solar radiation value was calculated as 2200 W and the panel temperature was 42 °C. Meanwhile outside air temperature was measured with temperature data logger and changing given in Fig. 10. The thermal efficiency of the whole system was calculated in Eqs. (9) and (10) and given in Fig. 9. Again, it was found that the concentrator had no effect on the thermal efficiency. However, the effect of the concentrator on the electrical gains is seen with a significant difference in Fig. 6. The whole system thermal energy gains increased or decreased with an inversely proportion to solar radiation. The case in Eq. (10) can be observed in Figs. 8 and 7. The mass change of the dried product depending on time was calculated in Eq. (11) in terms of dry basis and given in Fig. 11. As a result of two sequential measurement, it was carried on to dry as much as the time there is no change in the mass.
90 120 150 180 210 240 270 300 330 360 390
40
30
20
10
0
0
30
60
90 120 150 180 210 240 270 300 330 360 390
Experimental time (min.) Fig. 10. Variation of ambient temperature with experimental time.
_ Ceylan et al. / Solar Energy 129 (2016) 217–223 I.
Moisture content (g water/g dry matter)
5
4
3
2
1
0 0
30
60
90 120 150 180 210 240 270 300 330 360 390
Experimental time (min.) Fig. 11. Moisture content according to experimental time.
6. Conclusion The following results were obtained in the current study: Thermal efficiency and thermal energy gain have a similarity for the concentrated and non-concentrated panels. This shows that the paraffin used as the latent heat storage the latent heat in the panels. As the thermal energy gain of the sphere is lower because of the paraffin used as a latent heat storage, paraffin can be used to prevent the increase in the panel temperatures and decrease the efficiency. Mean concentrated panel solar radiation was measured as 2000 W/m2 and the panel efficiency was calculated as 11%. The maximum panel back temperature achieved was measured as 37 °C. The reflection of solar radiation on the panel was concentrated approximately more than twice. However, as the current did not increase twice at the same time, the efficiency for the solar panels is not alone a significant indicator. Electrical energy gains are more important indicators for these systems.
References _ Gu¨rel, A.E., 2014. Testing of a condensation-type Aktasß, M., Ceylan, I., heat pump system for low-temperature drying applications. Int. J. Food Eng. 10 (3), 521–531. Al-Hinti, I., Al-Ghandoor, A., Maaly, A., Naqeera, I.A., Al-Khateeb, Z., Al-Sheikh, O., 2010. Experimental investigation on the use of water-
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phase change material storage in conventional solar water heating systems. Energy Convers. Manage. 51 (8), 1735–1740. Biwole, P.H., Eclache, P., Kuznik, F., 2013. Phase-change materials to improve solar panel’s performance. Energy Build. 62, 59–67. _ Gu¨rel, A.E., Demircan, H., Aksu, B., 2014. Cooling of a Ceylan, I., photovoltaic module with temperature controlled solar collector. Energy Build. 72, 96–101. Du, B., Hu, E., Kolhe, M., 2012. Performance analysis of water cooled concentrated photovoltaic (CPV) system. Renew. Sustain. Energy Rev. 16 (9), 6732–6736. Dubey, S., Solanki, S.C., Tiwari, A., 2009. Energy and exergy analysis of PV/T air collectors connected in series. Energy Build. 41 (8), 863–870. Fazilati, M.A., Alemrajabi, A.A., 2013. Phase change material for enhancing solar water heater, an experimental approach. Energy Convers. Manage. 71, 138–145. Huang, B.J., Lin, T.H., Hung, W.C., Sun, F.S., 2001. Performance evaluation of solar photovoltaic/thermal systems. Sol. Energy 70 (5), 443–448. Kandilli, C., 2013. Performance analysis of a novel concentrating photovoltaic combined system. Energy Convers. Manage. 67, 186–196. Kerzmann, T., Schaefer, L., 2012. System simulation of a linear concentrating photovoltaic system with an active cooling system. Renew. Energy 41, 254–261. Koca, A., Oztop, H.F., Koyun, T., Varol, Y., 2008. Energy and exergy analysis of a latent heat storage system with phase change material for a solar collector. Renew. Energy 33 (4), 567–574. Kong, C., Xu, Z., Yao, Q., 2013. Outdoor performance of a lowconcentrated photovoltaic–thermal hybrid system with crystalline silicon solar cells. Appl. Energy 112, 618–625. Lee, J.S., Lucyszyn, S., 2007. Thermal analysis for bulk-micromachined electrothermal hydraulic microactuators using a phase change material. Sens. Actuators, A 135 (2), 731–739. Mishra, R.K., Tiwari, G.N., 2013. Energy and exergy analysis of hybrid photovoltaic thermal water collector for constant collection temperature mode. Sol. Energy 90, 58–67. Renno, C., Petito, F., 2013. Design and modeling of a concentrating photovoltaic thermal (CPV/T) system for a domestic application. Energy Build. 62, 392–402. Robles-Ocampo, B., Ruiz-Vasquez, E., Canseco-Sanchez, H., CornejoMeza, R.C., Tra´paga-Martı´nez, G., Garcı´a-Rodriguez, F.J., Vorobiev, Y.V., 2007. Photovoltaic/thermal solar hybrid system with bifacial PV module and transparent plane collector. Sol. Energy Mater. Sol. cells 91 (20), 1966–1971. Sonneveld, P.J., Swinkels, G.L.A.M., Campen, J., Van Tuijl, B.A.J., Janssen, H.J.J., Bot, G.P.A., 2010. Performance results of a solar green house combining electrical and thermal energy production. Biosyst. Eng. 106 (1), 48–57. Tiwari, G.N., Dubey, S., 2010. In: Fundamentals of Photovoltaic Modules and Their Applications, 2. Royal Society of Chemistry. Varol, Y., Koca, A., Oztop, H.F., Avci, E., 2010. Forecasting of thermal energy storage performance of phase change material in a solar collector using soft computing techniques. Expert Syst. Appl. 37 (4), 2724–2732. Zalewski, L., Joulin, A., Lassue, S., Dutil, Y., Rousse, D., 2012. Experimental study of small-scale solar wall integrating phase change material. Sol. Energy 86 (1), 208–219. Zondag, H.A., 2008. Flat-plate PV-thermal collectors and systems: a review. Renew. Sustain. Energy Rev. 12 (4), 891–959.
ScienceDirect Solar Energy 129 (2016) 217–223 www.elsevier.com/locate/solener
Performance analysis of a concentrated photovoltaic and thermal system _ Ilhan Ceylan a, Ali Etem Gu¨rel b,⇑, Alper Ergu¨n a, Abdulsamed Tabak a a
Karabuk University, Technology Faculty, Department of Energy Systems Engineering, 78100 Karabuk, Turkey b Duzce University, Vocational School, Department of Electrical and Energy, 81010 Duzce, Turkey Received 22 July 2015; received in revised form 1 November 2015; accepted 5 February 2016
Communicated by: Associate Editor I. Farkas
Abstract The purpose of the present study is PV/T system, which designed and implemented in order to reuse of thermal energy while increasing of electric production. Solar radiation was increased with the concentrator about two folds and electrical power gains were increased. A paraffin wax was used to store latent heat as a thermal energy storage. Thermal energy was combined with a greenhouse – like air drier and used to dry the product. Thermal and electrical energy gain in the concentrated and non-concentrated panels were compared and total system efficiency was analyzed. The maximum temperature obtained at the back of the panel was calculated as 37 °C and the panel efficiency was 11% at mean solar radiation was 2000 W/m2. Ó 2016 Elsevier Ltd. All rights reserved.
Keywords: Concentrated PV; PCM; Energy storage; Solar energy
1. Introduction The most common method of producing electricity energy with solar cells in the last few years has been concentrated solar cells. Different methods were tried to increase the decreased efficiency as solar cells are heated. One of them is to cool the cells with water flux. The related literature with regard to concentrated solar cells and paraffin used as a heat storage were given as follows. In their study, Petito and Renno designed a concentrated photovoltaic and thermal (CPV/T) system to produce thermal and electricity energy for home use. The program of MATLAB was used for the technical studies and cost analysis of energy amount to be produced in different operating conditions (heat, radiation, optic type, ⇑ Corresponding author.
E-mail address: [email protected] (A.E. Gu¨rel). http://dx.doi.org/10.1016/j.solener.2016.02.010 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
the size of solar cell, reflecting factor, etc.) was carried out (Renno and Petito, 2013). In a study by Schaefer and Kerzmann, a simulation regarding linear concentrated photovoltaic solar cell was produced. In their study, they used a three-layer (GaImP/GaAs/Ge) photovoltaic cell, Fresnel lens in order to reflect sun rays on photovoltaic cell, an active water circulation for the cooling system for the purpose of increasing the efficiency and a pump was used to obtain water circulation. In the system with 6.2 kW, a thermal energy of 5.089 kW h was produced annually and 14.215 kW h electricity energy was produced in the three-layer photovoltaic panel working with a 34.75% efficiency (Kerzmann and Schaefer, 2012). Du et al. made an experimental mechanism so as to cool concentrated solar energy system. The lower parts of the experimental system was made with mirror reflectors (6 on the right, 6 on the left, 12 in total) and the upper parts was made up of a mono-crystal solar cell at the size of
_ Ceylan et al. / Solar Energy 129 (2016) 217–223 I.
218
Nomenclature PV CPV/T A c I ðtÞ t m h E_ l T Q_ u V L MC
photovoltaic concentrated photovoltaic-thermal area (m2) specific heat (kJ/(kg K)) incident total radiation (W/m2) experimental time (s) mass (kg) enthalpy (kJ/kg) rate of electrical energy (W) temperature (°C) rate of useful energy transfer (W) volume (m3) latent heat (kJ/kg) moisture content (g water/g dry matter)
gh pw i s l m mlt
greenhouse paraffin wax inlet solid liquid module melting point
Greek symbols b electrical efficiency thermal coefficient s transparency a absorptivity g efficiency q density (kg/m3) d packing factor
Subscript c cell
1.08 m 0.14 m. The whole system was designed to follow the sun in one direction (east-west). The fixed and nonconcentrated type of the same solar cell were measured comparatively during the experimental process. The output power of CPV was 4.9 times more compared to the output power of the fixed solar cell. However, while the electrical efficiency of the solar cell in the fixed panel was about 11%, it was below 9% in CPV (Du et al., 2012). RoblesOcampo et al. made the application of the CPV/T they designed in Mexico. In this system, solar rays dropping on the panel were concentrated thanks to the reflector placed under the panel and electrical efficiency was increased because of the flux (water). At the end of the measurements, electrical efficiency was calculated as 16.4% and thermal efficiency became 50% (RoblesOcampo et al., 2007). In a study by Sonneveld et al. a CPV/T system was designed to meet the need for electricity and thermal energy in greenhouses. The basic reason for the preference of this system is to prevent the shadowing caused by these panels which occupy a large surface area. In the application where two way following system was used, the electrical efficiency of the panels was 15.8%, while thermal efficiency was calculated as 65% (Sonneveld et al., 2010). In another experimental study by Kandilli (2013), solar energy was concentrated on the solar cell using a chrome concentrated layer. Thanks to ‘hot mirror’ used around the solar cell, it was aimed to allow the visible light to pass but reflect the ultraviolet rays. In that way ultraviolet rays would be reflected on the tube and the thermal efficiency of the water would be increased. In the experiments, the efficiency was calculated as 15.2% and thermal efficiency was 49.9% (Kandilli, 2013). Fazilati and Alemrajabi experimentally investigated the effects of using Phase Change Materials (PCM) as storage
medium on the performance of a solar water heater. A type of paraffin wax used as PCM in spherical capsules as storage material in the tank of solar water heater. It was observed that by using PCM in the tank the energy storage density increased in the tank up to 39% and the exergy efficiency enhanced up to 16%. It was also observed that solar water heater with PCM can supply hot water with specified temperature at 25% longer time (Fazilati and Alemrajabi, 2013). Zalewski et al. presented an experimental study of a small-scale Trombe composite solar wall. In this case, the phase change material was inserted into the wall in the form of a brick-shaped package. The energy performance of the wall from heat flux measurements and enthalpy balances were also presented (Zalewski et al., 2012). Biwole et al. (2013) investigated the use of phasechange materials (PCM) to maintain the temperature of the panels close to ambient. The main focus of the study is the computational fluid dynamics (CFD) modeling of heat and mass transfers in a system composed of an impure phase change material situated in the back of a solar panel (SP) (Biwole et al., 2013). Al-Hinti et al. presented an experimental investigation of the performance of waterphase change material (PCM) storage for use with conventional solar water heating systems. Paraffin wax contained in small cylindrical aluminum containers used as the PCM. Over a test period of 24 h, the stored water temperature remained at least 30 °C higher than the ambient temperature (Al-Hinti et al., 2010). Varol et al. experimentally investigated the performance of a solar collector system using sodium carbonate decahydrate as Phase Change Material (PCM) during March and collector efficiency compared with those of convectional system including no PCM (Varol et al., 2010). Koca et al. performed an analysis of energy and exergy for a latent heat storage system with
_ Ceylan et al. / Solar Energy 129 (2016) 217–223 I.
phase change material (PCM) for a flat-plate solar collector. The designed collector was combined in single unit solar energy collection and storage. PCMs were stored in a storage tank, which was located under the collector. A special heat transfer fluid was used to transfer heat from collector to PCM (Koca et al., 2008). Photovoltaic power is a clean energy that can be obtained directly from solar energy. This energy is regarded as a significant alternative to the solution of environmental problems resulting from traditional energy sources. Despite all these positive sides, the costs of photovoltaic systems are still too high to compete with traditional power systems. In order to be able to reduce the costs of these systems to minimum, one of the possible methods is to concentrate solar radiation and reduce the cell surface area used. However, the cell heat will increase with the concentration of sun radiation and it will decrease the cell efficiency. In order to increase cell efficiency and also prevent possible thermal damage in the cells, various cooling methods have been used (Kong et al., 2013). In this study, concentrated solar panel was designed together with heat and energy storage. Paraffin was used as a heat storage inside the solar panel which manufactured in a spherical shape out of broken cells. The temperature of the panel will increased with solar radiation dropping on the broken cells through concentrator. Increasing panel temperature was prevented using paraffin wax. The energy obtained from the solar panel was used to provide the fans in the greenhouse. The designed and manufactured concentrated solar panel with a heat storage was mounted on a greenhouse and tested in a drying application. Spinach leaves were fully dried. The dried leaves of spinach were then crumbled and beaten in a mortar with a pestle to turn into dust. Spinach dust is commonly used in cakes, soups as a child food. This study is highly different from the ones in the literature in terms of both the newly manufactured product and of experimental system. 2. Designing and operating procedure The experimental system is made up of a concentrator (1), a sphere filled with paraffin and covered with broken cells (2), a charging regulator (3), a gel accumulator (4) and a drier. The broken cells were combined with UV resistant epoxy on a sphere made of aluminum. Paraffin was
heated and filled in the sphere in a liquid form. The angle of the concentrator was manufactured in an adjustable form and the sphere was placed on the focal point. The bottom part of the sphere which had no broken cells was placed in the channel and the stored heat was transferred in the greenhouse with help of fans. The system was designed as in Fig. 1 and manufactured as in Fig. 2. As given in the figure, the heat transferred to the sphere through concentrator pass on to the paraffin in the sphere. Paraffin changes its phase at this point from solid state to liquid state and works as a heat storage. The heat stored is sent to the drying chamber with the help of a fan. In order to provide energy for the fan, broken photovoltaic cells were benefited. These photovoltaic cells could charge a gel type accumulator. The ever-charging accumulator could operate the fan. No other outside energy was given to the system. The drying operation is carried on even at time when there is no sun thanks to the charged accumulator and heat sphere (paraffin). 2.1. Operating procedure (a) As given in Fig. 3, the reflection of sun radiation on number 1 concentrator is turned into electricity energy through broken cells with a number of 7 on the sphere and stored in the gel accumulator. (b) By means of number 1 concentrator, the sun radiation focused on the sphere with a number of 7 is also stored as a heat energy in the paraffin in the sphere. (c) The heat energy stored in the sphere with a number of 7 are transferred on the greenhouse with a number of 2 inside the channel with a number of 5. (d) The energy of the circulation fans inside the channel 5 are supplied with the gel accumulator charged with broken cells. (e) The heat energy stored in the paraffin inside the sphere 7 and the electricity energy stored in the gel accumulator are used for the heat and electricity energy of the greenhouse when there is no sun.
3. Experimental analysis In order to experimentally investigate in the designed greenhouse type drier with a heat and energy sphere,
1
2
3
219
4
1. Concentrator, 2. The sphere filled with paraffin and covered with broken cells, 3. Charging regulator, 4. Gel accumulator and fans (in the box) Fig. 1. Heat-energy sphere designed and manufactured.
_ Ceylan et al. / Solar Energy 129 (2016) 217–223 I.
220
Fig. 2. Designed (a and b) and manufactured (c and d) drying system.
weight of the dried product were measured. Measured points in the drying system and measured devices properties were shown in Fig. 4 and Table 1. 4. Theoretical analysis of CPV/T system The correlation between an enthalpy and heat for paraffin wax can be expressed as follows (Lee and Lucyszyn, 2007). Z T mlt cs dT T 6 T mlt ; solid phase ð1Þ hs ¼ 1. Concentrator, 2. Greenhouse, 3. Air inlet point, 4. Air outlet point, 5. Heat depot, 6. Regulator, 7. Sphere surface (broken solar cells).
Fig. 3. Drying system with concentrated solar panel.
Z
T T
hl ¼
cl dT
T > T mlt ; liquid phase
ð2Þ
T mlt
Total enthalpy (heat energy) of paraffin wax for any temperature can be expressed as follows. htotal ¼ hs þ L þ hl
ð3Þ
Thermal properties of the paraffin wax used in the study was given in Table 2. The useful heat energy amount obtained from paraffin wax and transferred to the drying cabin can be calculated using the equation below: Q_ u;pw ¼ mpw c ðT gh T pw Þ=t
ð4Þ
The useful heat energy transferred from the greenhouse can be calculated as follows: Q_ u;gh ¼ V gh c qair ðT gh T i Þ=t 1 : solar radiation 2 : solar radiation 3 : inlet air temperature 4 : solar panel temperature
5 : outlet air temperature 6 : solar radiation 7 : drying chamber temperature
Fig. 4. Measured points in drying system with heat and energy depot.
900 g spinach was dried until it became fully dried. In the experimental system, solar radiation focusing from the top and bottom points, solar radiation on the glass surface in the drying chamber, sphere surface temperature, the temperature inside the greenhouse, blowing air temperature, inlet air temperature, blowing air velocity and the
ð5Þ
The electrical efficiency of the photovoltaic modules can be categorized in two groups as cell efficiency and module efficiency. The electrical loss from the panels are realized with temperature. Open circuit voltage and filling factor are decreased significantly with the temperature. Besides that, short circuit current is increased for a while (Mishra and Tiwari, 2013; Zondag, 2008; Ceylan et al., 2014). As a result of this important effect, the cell efficiency can be calculated as follows: gc ¼ go ½1 bðT c 25Þ
ð6Þ
In the equation, go is the efficiency under standard test conditions (STC). (I(t) = 1000 W/m2, Tc = 25 °C, AM = 1.5). Tc is the cell temperature and b is the thermal
_ Ceylan et al. / Solar Energy 129 (2016) 217–223 I.
221
Table 1 Measured devices and properties. Point
Measured value
Device name
Device properties
1, 2, 6 3, 4, 7 5
Solar radiation Temperature Air velocity
PCE-SPM1 Data logging Solar Power Meter – Solarmeter 4 channel K thermometer SD logger – Termocouple Delta Ohm HD 2303.0 – Anemometer
0–2000 W/m2, ±10 W/m2 0 °C–50 °C, ±1 °C 0.1–40 m/s, ±0.01 m/s
Table 2 Thermal properties of paraffin wax (Lee and Lucyszyn, 2007).
5. Result and discussions
Property
Value
Melting point ðT m Þ, (°C) Density ðqÞ, (kg/m3) Latent heat (L), (kJ/kg) Thermal conductivity ðkÞ, (W/m K) Specific heat ðcÞ, (kJ/kg K)
47 818 (solid), 760 (liquid) 266 0.24 (solid and liquid) 2.95 (solid), 2.51 (liquid)
The cell and module efficiency obtained as a result of the tests was calculated in Eqs. (6) and (7) and given in Fig. 5. The efficiency of the concentrated modules can decrease up to 5% with heating. It is necessary to cool them in order to prevent the efficiency of these concentrated panels. In the current study, the panel temperature was measured as approximately 30 °C in the concentrated solar panel application carried out with paraffin wax. The module efficiency made of polycrystalline broken cells was also calculated with an approximation of 11%. The electrical energy gains of the concentrated solar cells was calculated in Eq. (8) and given in Fig. 6. The solar radiation on the concentrated panel increase about two folds. Electrical energy gains also increased almost twice similar to those of solar radiation. The thermal energy gains of the whole system was calculated in Eq. (9) and given in Fig. 7. A very small difference was found between the calculations made for the concentrated and non-concentrated systems. It was caused by the fact that paraffin wax cannot reach 47 °C which is the melting heat together with the concentrator. The fact that thermal heat gains are almost the same is an indication that the concentrating panel of the paraffin wax is successful in cooling. The change in cell temperature with concentrated and non-concentrated solar radiation was given in Fig. 8. Mean panel temperature was found as 30 °C. Depending on the solar radiation, the temperature of the panel in the shape
coefficient of electrical efficiency. b value changes depending on the material from which solar cells are produced. For crystal silicon almost 0.0045/K is taken, 0.0035/K is taken for CIS, 0.0025/K for CdTe and 0.002/K for a-Si (Mishra and Tiwari, 2013; Tiwari and Dubey, 2010). PV module efficiency can be calculated using Eq. (7): ð7Þ
where is sc the transparency for the PV module glass, ac is the absorptivity of the solar cell and dc is the packing factor; the values for these are taken as 0.90, 0.95 and 0.90 respectively (Dubey et al., 2009). The electrical energy gain obtained from PV module in the system can be calculated with the equation below: E_ l ¼ gm Am I ðtÞ
ð8Þ
Overall thermal energy gain in the system was calculated as follows: E_ l Q_ u;overall ¼ Q_ u;pw þ Q_ u;gh þ C power
ð9Þ
In the equation, C power is the conversion power of the thermal power plant depending on the quality of the coal. C power could be taken as 0.38 for a good quality coal with a low ash. This value is between 0.20 and 0.40 (Dubey et al., 2009; Huang et al., 2001; Mishra and Tiwari, 2013). Overall thermal energy efficiency in the system is calculated as follows: goverall ¼
Q_ u;overall Am I ðtÞ þ Agh I ðtÞ
ð10Þ
Initial moisture content of the dried products on a dry basis can be calculated as follows (Aktasß et al., 2014): MCdb ¼
Mi Md Md
ð11Þ
where M i initial mass of the dried products, M d is the product mass which in the dried state.
20
Cell efficiency Module efficiency 18
Efficiency (%)
gm ¼ gc s c ac dc
16
14
12
10
0
30
60
90 120 150 180 210 240 270 300 330 360 390
Experimental time (min.) Fig. 5. Cell and module efficiency.
_ Ceylan et al. / Solar Energy 129 (2016) 217–223 I. 2500
14 Non-concentrated PV module
10
Solar radiation (W/m²)
Electrical energy gain (W)
12
8 6 4 2 0
0
30
60
2000
40
1500
30
1000
20
500
10
0
90 120 150 180 210 240 270 300 330 360 390
0 0
30
60
Experimental time (min.) Fig. 6. PV module powers with and without concentrators.
120 150 180 210 240 270 300 330 360 390 Experimental time (min.)
90
Concentrated PV module
Concentrated PV module
80
Non-concentrated PV module
Non-concentrated PV module 70
Overall efficiency (%)
Thermal energy gain (W)
90
Fig. 8. Concentrated and non-concentrated solar radiation and cell temperature.
240
200
50
Concentrated Solar Radiation Non-concentrated Solar Radiation Cell temperature
Concentrated PV module
Cell temperature (°C)
222
160
120
80
40
60 50 40 30 20 10
0
0
30
60
0
90 120 150 180 210 240 270 300 330 360 390
0
30
60
Experimental time (min.)
Experimental time (min.)
Fig. 7. Thermal energy gains of PV module with and without concentrators.
Fig. 9. The change of the whole system thermal efficiency.
50
Ambient temperature (°C)
of a sphere full of paraffin wax increased. During the experiments, the highest solar radiation value was calculated as 2200 W and the panel temperature was 42 °C. Meanwhile outside air temperature was measured with temperature data logger and changing given in Fig. 10. The thermal efficiency of the whole system was calculated in Eqs. (9) and (10) and given in Fig. 9. Again, it was found that the concentrator had no effect on the thermal efficiency. However, the effect of the concentrator on the electrical gains is seen with a significant difference in Fig. 6. The whole system thermal energy gains increased or decreased with an inversely proportion to solar radiation. The case in Eq. (10) can be observed in Figs. 8 and 7. The mass change of the dried product depending on time was calculated in Eq. (11) in terms of dry basis and given in Fig. 11. As a result of two sequential measurement, it was carried on to dry as much as the time there is no change in the mass.
90 120 150 180 210 240 270 300 330 360 390
40
30
20
10
0
0
30
60
90 120 150 180 210 240 270 300 330 360 390
Experimental time (min.) Fig. 10. Variation of ambient temperature with experimental time.
_ Ceylan et al. / Solar Energy 129 (2016) 217–223 I.
Moisture content (g water/g dry matter)
5
4
3
2
1
0 0
30
60
90 120 150 180 210 240 270 300 330 360 390
Experimental time (min.) Fig. 11. Moisture content according to experimental time.
6. Conclusion The following results were obtained in the current study: Thermal efficiency and thermal energy gain have a similarity for the concentrated and non-concentrated panels. This shows that the paraffin used as the latent heat storage the latent heat in the panels. As the thermal energy gain of the sphere is lower because of the paraffin used as a latent heat storage, paraffin can be used to prevent the increase in the panel temperatures and decrease the efficiency. Mean concentrated panel solar radiation was measured as 2000 W/m2 and the panel efficiency was calculated as 11%. The maximum panel back temperature achieved was measured as 37 °C. The reflection of solar radiation on the panel was concentrated approximately more than twice. However, as the current did not increase twice at the same time, the efficiency for the solar panels is not alone a significant indicator. Electrical energy gains are more important indicators for these systems.
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