Renewable Energy 101 (2017) 713e727
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Renewable Energy
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Performance analysis of a concentrated solar energy for lighting- power generation combined system based on spectral beam splitting
* Canan Kandilli a, , Gürhan Külahlı b a Department of Mechanical Engineering, Usak University, Usak, Turkey b Department of Automotive Engineering, Faculty of Technology, Pamukkale University, Denizli, Turkey article info abstract
Article history: The novel lighting-power generation combined system (LIPGECOS) based on the approach of spectral Received 30 November 2015 beam splitting of the concentrated solar radiation was introduced and the components of the system Received in revised form consisting of paraboloidal dish, dual axes tracking system, cold mirror, fiber optic bundle and Stirling 12 September 2016 engine were explained. At the first time, a cold mirror was utilized to separate the full solar spectra into Accepted 17 September 2016 the different wavelengths experimentally. The performance analysis of LIPGECOS established at Usak University was carried out and the first experimental results were evaluated in the present study. Temperatures, rotating speed of the Stirling engine, indoor irradiance and illuminance obtained by the Keywords: Concentrated solar energy experiments were analyzed under different global and direct solar irradiance conditions. Thermal images Spectral beam splitting of the LIPGECOS were presented to examine the thermal control of the system. The energy and exergy Cold mirror efficiencies of the system were determined as 0.15 and 0.09, respectively. The average luminous efficacy Combined power generation of LIPGECOS was calculated as 347 Lm/W. A superior luminous efficacy was obtained by LIPGECOS owing Solar lighting to spectral beam splitting. The average lighting efficiency was calculated as 14% ± 0.03. It is hoped that the spectral beam splitting by the cold mirror could open new research areas for concentrated solar energy technologies. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction respect to conventional ones. Cooling loads in buildings are reduced due to the increased luminous efficacy of fiber-optic The sun is the most important source of renewable and inex- lighting system as compared with incandescent or fluorescent haustible energy. However, terrestrial solar flux has relatively low lighting [1]. Daylight can provide at a level of about 110 Lm/W of value and the solar concentrated energy should be employed to solar radiation, whereas fluorescent lamps provide about 75 Lm/W obtain higher temperatures and to increase the supplied power. of electrical input and incandescent lamps about 20 Lm/W. Thus, Concentrating solar energy systems could be classified into two efficient day lighting generates only 1/2e1/5th of the heating that different categories: Imaging and non-imaging systems. A high flux equivalent electrical lighting does; therefore, significantly and high temperatures could be obtained by a paraboloidal decreasing the building cooling load [2]. The combined savings concentrating dish which is one of the imaging solar energy from reduced lighting and cooling loads can be substantial because systems. electrical lighting can account for 25e40% of a commercial build- Nowadays, there are many application areas of the paraboloidal ing's energy requirements from the energy efficiency point of view concentrators, such as industrial heat supply, lighting and electrical [3]. Daylight is accepted as the most suitable light for good color energy. By the paraboloidal dish systems, high-temperature solar rendering and its spectral properties provide a perfect match with irradiance could be concentrated on the focal plane and this energy human visual response. At this point, integrated fiber-optic lighting could be converted into mechanical energy by Stirling heat engines. systems based on solar energy emerged as an alternative, energy The primary advantage of lighting systems with solar concen- efficient and qualitative option for the spaces with insufficient trators is their potential to reduce energy consumption with illumination, specialized safety or having a large lighting load. Since the early 1980s, many theoretical and experimental studies for the purpose of power generation and lighting based on the principle of * Corresponding author. transportation of concentrated solar radiation with fiber optic E-mail address: [email protected] (C. Kandilli). http://dx.doi.org/10.1016/j.renene.2016.09.032 0960-1481/© 2016 Elsevier Ltd. All rights reserved. 714 C. Kandilli, G. Külahlı / Renewable Energy 101 (2017) 713e727
Nomenclature h Energy efficiency r Reflectivity A Area (m2) q Admission/acceptance angle ( ) C Geometrical concentration (dimensionless) t Transmittance D Diameter (m) f Rim angle ( ) d Image diameter (m) J Maximum efficiency ratio (dimensionless) dB Decibel loss/attenuation of optical fiber (dB/m) · Ex Exergy rate (W) Subscripts F View factor (dimensionless) a Ambient f Focal length (m) b Beam G Solar irradiance (W/m2) dish Dish L Length (m) f Focal NA Numerical aperture (dimensionless) fob Fiber optic bundle n Refractive index (dimensionless) in Inlet _ loss Loss Q Energy rate (W) m Mirror h Heat transfer coefficient (W/m2K) max Maximum T Torque (Nm) min Minimum T Temperature (K, C) o Optimum v Velocity (m/s) out Outlet rim Rim Greek letters s Stirling d Dispersion angle ( ) shade Shading ε Exergy efficiency w Wind s StefaneBoltzmann constant (s ¼ 5.667 10 8 Wm 2 K 4)
cables have been carried out [4]. remote source solar lighting system using side-emitting fiber optic Latest investigations on solar lighting via fiber optics are very as illuminators to reduce the emission of greenhouse gases for remarkable. Firat and Beyene investigated seven configurations of providing illumination to these lift lobbies as a promising system. Photovoltaic (PV) energy conversion systems involved use of filters, They proved that the remote source solar lighting system could concentrator lens, fiber transport, and direct use of PV in their displace an average of 3 h of electric lighting in a day and reduce theoretical study. They found that transmission of concentrated 6.7 106 kg of carbon dioxide emission in a year [9]. In their other light onto PV cell proved to be the most efficient, but also the most study, Wong and Yang investigated the factors of solar altitude, expensive. They also showed that transmission of filtered light onto solar azimuth angle and solar irradiance, analyzed the shadowing PV cell using a plastic optical fiber (POF) bundle was the most effects caused by neighboring buildings and the supporting affordable [5]. H.J. Hun et al. introduced the applicability and framework in details, defined design parameters of the natural functional effectiveness of a daylighting system which consists of daylight system, and finally developed design guidelines and a dish concentrator(s), a dual-axis solar tracker and light guides model design as a reference for building designers in designing a including optical fiber cables. They developed simulation models remote source solar lighting system [10]. using a number of different software where Photopia provided the Fiber optic cables based on plastical materials could not resist to relevant photometric data (candela power distribution curves, high temperature for concentrated solar energy applications. CDCs) by producing a virtual luminaire of the daylighting system. It Regarding silica based fiber cables, they are not preferred for solar was found that the model based on Relux could produce more lighting applications due to lack of flexibility. On the other hand, realistic results, closer to the measured data [6]. H.J. Hun et al. also concentrated solar radiation could be transmitted to any space by performed a computational analysis on the enhancement of acrylic based fiber optic cables at low operating temperatures as daylight penetration into an unevenly lit lecture room with north- 70 C. In this context, it is very significant to the temperature facing windows with the help of Photopia and Radiance. They control for FOB in solar lighting systems. In addition, PMMA based considered two different daylighting systems, a light tube and a fibers can only transfer the visible light of the solar spectra. For this fiber-optic solar dish concentrator, as means to lead light rays into reason, a great loss is emerged for the other part of the spectra an interior space with insufficient illumination from daylight. Their including UV and IR regions. This energy loss causes excessive heat results revealed that the functional benefits of each system when on the FOB. Concentrated solar irradiance damages and melts the harvesting daylight for indoor illumination and more daylight can FOB made of PMMA at the entrance. In the present study, the be harvested by the solar tracking dish concentrator system for proposed system can eliminate this excessive heating problem and solar altitudes of less than 50 [7]. Wong and Yang proposed the prevent the FOB owing to “spectral beam splitting approach”. remote source solar lighting system which is composed of a simple As well-known, solar spectra has a wide range wavelength band heliostat and side-emitting fiber optic. They carried out simulation including from 200 nm to 2500 nm. The spectral beam splitting on the light transmission performance in the system by the could be defined as separating the solar full spectra into different ZEMAX-EE. They showed that the proposed remote source solar wavelengths by an optical device. This idea provides many advan- lighting system can be applied as an alternative lighting system to tages such as reduction of heat losses in solar energy applications illuminate the enclosed lift lobby at daytime in clear sky condition and utilizes all spectra in an efficient way. Certain portions of the [8]. Wong and Yang also designed and experimentally tested a spectrum could be employed for Concentrating Photovoltaics (CPV) C. Kandilli, G. Külahlı / Renewable Energy 101 (2017) 713e727 715 modules, such as Photovoltaic Thermal (PVT) systems, using of cell can be reduced by 20.7%, up to 10.5% of the total incident solar spectral beam splitting could also contribute to the development of energy can be recovery by the receiver, and the overall optical ef- these technologies. The spectral beam splitting approach provides ficiency in theory is about 0.764 [19]. P. Hu et al. also proposed a most effectively utilization from the entire spectrum of solar en- novel concentrating PV/Thermal (CPV/T) hybrid system with beam ergy. By application this approach to any energy conversion for splitter and fully tracked linear Fresnel reflector concentrator uti- solar energy applications, it will be possible to enhance a remark- lizing sloped panels. They investigated the relations between the able increase on the efficiency. structural parameters and the optical performances of the system. There are limited but very significant studies on spectral beam Based on the experimental data of the components, thermody- splitting for concentrating solar energy systems in the literature. namic analysis on the CPV/T hybrid system was carried out and the Imenes and Mill published a review spectral beam splitting tech- results reveal that the overall energy conversion efficiencies of the nology for increased conversion efficiency in solar concentrating proposed CPV/T hybrid system is found higher than that of the CPV systems [11]. A.G. Imenes et al. also indicated that one method of system under the same conditions [20]. Vivar et al. conducted the achieving improved electrical conversion efficiencies in solar po- most suitable commercial heat transfer fluid for a direct-absorption wer plants is to employ a spectrally selective filter that splits the beam-splitting CPV-T system, analysing the effects of high tem- collected beam into optimized components for two or more spec- perature and exposure to UV light on the optical transmittance of tral receivers. They discussed the theory and application of this the fluid under accelerated lifetime test conditions. They resulted strategy based on flux mappings produced by raytracing methods from the accelerated tests showed that the optimum fluid for our for a Multi Tower Solar Array central receiver system planned for application is the industrial grade Propylene Glycol adapted with a construction in Newcastle, Australia [12]. Rosengarten et al. per- chemically-inert red dye such as Oil Red 235 inorganic dye [21]. formed a review on spectral beam splitting for efficient conversion In the present study, lighting-power generation combined sys- of solar energy. They discussed the drawbacks associated with tem (LIPGECOS) based on the approach of spectral beam splitting of practical applications [13]. Rosengarten et al. also developed a concentrating solar radiation was introduced and the components novel spectrally splitting hybrid solar receiver by combining a of the system consisting of paraboloidal dish, dual axes solar simple dichroic filter and a liquid channel as a selective absorbing tracking system, cold mirror, fiber optic bundle and Stirling engine medium for silicon cells. In their study, a simple 5-layer dichroic were explained. The first time a cold mirror was utilized to separate filter made of titanium dioxide and silicon dioxide has been the full solar spectra into the different wavelengths for solar designed, optimized, and fabricated with a focus placed on concentrator systems in the literature. No studies on the analysis of manufacturing simplicity. They noted that the Si cells considered in a system similar to LIPGECOS have appeared in the open literature this paper can convert 26.1% of this energy into electricity which is to the authors' knowledge. In this regard, the main objectives in significantly higher than their 20.6% efficiency under the full doing the present study are as follows: (i) to define LIPGECOS based spectrum [14]. Kandilli modeled and tested a novel Concentrating on the spectral beam splitting, (ii) to present an analytical model for Photovoltaic Combined System (CPVCS) based on the spectral beam energy and exergy analyses of the systems based on the spectral splitting approach by a hot mirror, performed economical analysis beam splitting, (iii) to apply the model to the LIPGECOS, (iv) to and compared the experimental results to the data obtained by the present and to evaluate experimental results of LIPGECOS as a novel control system. As a result, energy efficiencies of concentrator, solar concentrating system, and (v) to make new suggestions to vacuum tube and overall CPVCS have been determined to be increase the systems performance for future works. The perfor- 15.35%; 49.86%; and 7.3% respectively. Similarly the exergy effi- mance analysis of LIPGECOS established in Usak University, ciencies of concentrator, vacuum tube and overall CPVCS are Department of Mechanical Engineering was carried out and the 12.06%; 2.0%; and 1.16% respectively. The cost of energy production first experimental results were evaluated in the present study. The has been stated as 6.37 $/W [15]. Crisostomo et al. designed and system description covering the system components and the fabricated SiNx/SiO2 multilayer thin film filters to act as beam spectral beam splitting was explained in the next section. Energy splitting devices in a 10 sun, linear Fresnel mirror-based, concen- and exergy models of LIPGECOS were presented in the section of trating Photovoltaic Thermal (PV/T) solar collector. The experi- “Thermodynamical Modelling” and the energy and exergy effi- mental results of this study indicated that the PV cells, illuminated ciencies were calculated. Temperatures of the system components, with the reflected light from the filters, operate on average at 9.2% rotation speed of the Stirling engine, indoor irradiance and illu- absolute higher efficiency than the same cells without the filter. minance obtained by the experimental data were evaluated under They also noted that this type of system can utilize 85.6% of the different global and direct solar irradiance data. Thermal images of incoming solar spectrum based on the measured optical properties the LIPGECOS were examined to interpret the thermal control and of the filters [16]. Crisostomo et al. also presented a general to monitor spectral separation. In the conclusion, the obtained re- methodology for the determination of the optimal spectral splitting sults were listed and suggestions for future works were discussed. parameters in hybrid PV/T collectors. The method was applied in It could be hoped that the high efficiency solar energy systems the design of a novel PV/T collector which is also modeled in detail could be developed thanks to spectral beam splitting and the LIP- using the ray tracing software Zemax 12. They noted that 47% more GECOS aiming to benefit whole spectra of the concentrated solar power can be delivered from the collector in relation to a concen- radiation. trating PV stand-alone system under the same concentration ratio [17]. W. An et al. developed a spectral splitting hybrid PV/T system 2. System description based on polypyrrole nanofluid. They found that the maximum overall efficiency of this hybrid PV/T system with polypyrrole The LIPGECOS provides the both lighting and heating energies nanofluid filter was 25.2%, which was 13.3% higher than that by a unique modular system. A cold mirror as a key element of the without filter. More importantly, the medium-temperature thermal LIPGECOS reflects back the visible region of the solar spectra and energy can be harvested in such a hybrid system [18].P.Huetal. transmits the IR and UV parts. The visible light is reflected to the described a two-stage parabolic trough concentrating photovoltaic/ FOB placed on the midpoint of the dish and then transmitted to the thermal (PV/T) system which contains a concentrator, a spectral space for lighting purposes. On the other hand, IR and UV parts of beam splitting filter, an evacuated collector tube and the solar cell the spectra are passed to the focal region of the concentrating dish components. They showed that using the filter the heat load of the by the cold mirror for heating applications. In the present study, 716 C. Kandilli, G. Külahlı / Renewable Energy 101 (2017) 713e727 obtained heating energy is employed to operate a Stirling engine. diameter’ ratio is an indicative parameter for a concentrating dish. The main aim of the study is to evaluate the spectral beam splitting This value is required to be greater than 0.40 in order to obtain the for concentrated solar energy by the cold mirror designed specially. most effective focusing. This ratio is 0.53 for the dish produced in Thus the Stirling engine was chosen in a demonstrative scale to the present study. According to theoretical calculations (Eq. (7) and facilitate the experiments. For future works, the scale could be Eq. (8)), the minimum and maximum diameters of the ideal image enlarged by employing the larger dish aperture and the higher on the focal region were determined as 1.3 and 2.5 cm, respectively. engine power. By considering the amount of energy required, the Because of surface defects of the dish and the toleration of the obtained power can be increased by increasing the number of tracking system, higher values for these diameters were inevitable. modules. The design parameters of LIPGECOS are given in Table 1. General dimension plan of the test space is given by Fig. 1. The test space has not any window or aperture, so it cannot be illumi- 2.2. Tracking system nated naturally. The dimension is defined as 6.0 3.8 1.9 m. LIPGECOS was established at outdoor terrace of the test room. Another important component is the dual-axis solar tracking Overview of the experimental system and its components are system. The solar tracking system always ensures the solar radia- shown in Fig. 2. The basic components of the system are parabo- tion's perpendicularity to the aperture area of the concentrator. In loidal concentrator dish, cold mirror, dual-axis solar tracking sys- the present study, we have two types tracking systems. The first tem, fiber optic cable bundle and Stirling engine. The system one is based on LDR (Light Dependent Resistance), and the second components and their features are briefly explained below: one is PC based control system integrated a solar algorithm soft- ware. The former dual-axis solar tracking system is produced by 2.1. Concentrator dish utilizing a PLC (Programmable Logic Controller) board and rela- tively low-cost materials. This solar tracking system consists of a Concentrator dish is a symmetric type paraboloidal dish and has PLC electronic board, four LDR sensors, and the two engines. The a diameter of 107 cm aperture. The focal length is 58.7 cm, the two satellite dish engines are adapted into the system to turn the depth is 12 cm and the rim angle is 50.25 . As known, ‘Focal length/ dish towards to the Sun. The engine under an operating voltage of
Fig. 1. General dimension plan of the test space. C. Kandilli, G. Külahlı / Renewable Energy 101 (2017) 713e727 717
Fig. 2. Overview of the experimental system and components.
Table 1 mirror transmits the IR and UV part of the radiation while it reflects The design parameters of LIPGECOS. the visible part of the spectrum. The transmittance of the cold 2 Adish(m ) 2.09 mirror for IR and UV part is assumed as 0.90. 2 Am(m ) 0.0038 The thickness and the diameter of the cold mirror designed for dBloss(dB/m) 0.30 the experiments are 2 cm and 19 cm, respectively. The mirror has f(m) 0.587 ∅ been sized taking into account the dish parameters, the location of rim ( ) 50.25 fi NA 0.51 the ber optic bundle and Stirling engine. qmax( ) 30.69 n1 1.49 2.4. Fiber optic bundle (FOB) n2 1.40 L(m) 5.00 fi r 0.75 The optical bers used in most of the studies relating to solar dish fi Cmax 1730 energy are made of plastical or glass material. Plastical optical ber Df(m) 0.0257 provides more flexible solutions than the glass optical fibers, and d ( ) 0.024 has a greater decibel loss. Due to the fact that dual-axis solar D (m) 1.07 a tracking moves all day along, the bending of plastic optical fibers ε 0.95 are unavoidable. For LIPGECOS, a bundle of flexible fiber optic ca- rm 0.60 bles of an outer diameter of 5 cm, 490 cm long, made of Polymethyl-methacrylate (PMMA) was used to transfer the 13 V has a speed of 1.9 /s. It has the ability of moving at an angle of concentrated solar radiation into the test space. For any FOB, the 180 along the horizontal axis. The low voltage values are sufficient more gaps in a bundle, the more the transmission losses. If the to run the tracking system. The tracking system consuming transmission loss is high, it will cause the excessive heating through approximately a voltage of 11.6 V and a current of 0.02 A is operated the FOB. In the present study, the FOB has been composed by 965 fi at as low as 0.23 W. pcs of 1.5 mm diameter ber optic cables in order to reduce On the other hand, the latter one has 2 stepper engines, PC- transmission losses. The refractive indexes of the core and cladding based control unit and drivers. In the present system, both two materials are 1.492 and 1.418, respectively. Numerical aperture is given as 0.51 and the corresponding maximum angle of incidence is type dual axes solar tracking systems have been tested to get direct solar irradiance for LIPGECOS. Precise direct solar irradiance values approximately 30 . have been reached by the both tracking systems. The LDR based Spectral decibel loss (dB/km) characteristic of the FOB is given in fi system has a considerable low-cost; on the other hand PC-based Fig. 4. PMMA ber optic cables have relatively high decibel losses at system has a quick-response time for solar time.
2.3. Cold mirror
The cold mirror is an optical element employed especially in the structure of the telescopes. The reflective surface of the cold mirror allows transition of IR and UV radiation and reflects the visible wavelengths. It would not be wrong to say that a cold mirror di- vides solar radiation into heat and light forms for solar engineering purposes. Owing to spectral separation characteristic of the cold mirrors, it is possible to benefit from the entire solar spectra effectively. In this study, the mirror is integrated with the parabo- loidal dish as a novel and original design. For the present study, the spectral transmittance of the cold mirror was tested by the National Metrology Institute of Laboratory of Optical Standards, Turkey. The spectral transmittance charac- teristic of the cold mirror is shown in Fig. 3. It is clear that the cold Fig. 3. Spectral transmission of cold mirror. 718 C. Kandilli, G. Külahlı / Renewable Energy 101 (2017) 713e727
650 nm and between 350 and 400 nm. Average decibel loss is a wide wavelength range. assumed as 200 dB/km by considering the datasheet of the product. In the analysis, the following assumptions were made: A PMMA fiber optic cable has a low decibel loss and high trans- mission for visible wavelengths. a. The LIPGECOS do not store energy, and then the system was assumed as steady flow. 2.5. Stirling engine b. The dual solar tracking error was taken to be zero in the theo- retical analysis. The Stirling engine used in the system is the engine including c. The electrical energy consumption of the solar tracking system single cylinder with air as operating fluid. In the current study, the for the paraboloidal dish was ignored in the calculations due to main aim is to investigate the spectral beam splitting of concen- its relatively low value. trated solar energy by the cold mirror. Therefore, the Stirling engine d. The solar beam radiation was assumed to be income normal has been employed as a receiver to figure out how the separated direction to the dish aperture. thermal energy by the cold mirror could be provided in the present e. The temperature distribution on the receiver was assumed to be study. In such a spectrally decomposing system, a Stirling engine or uniform. any other component could be sized according to the energy f. The FOB is assumed as a single cable. fl demands. g. It is assumed that the cold mirror re ects all visible wavelengths to the FOB and transmits all IR and UV wavelenghts to the Stirling engine. 3. Thermodynamical modelling h. Average values of reflectivities of the paraboloidal dish and the cold mirror, transmittance and emissivity of the cold mirror To reveal the performance of a solar energy system, to present have been considered for the analysis instead of spectrally fi energy and exergy analyses and to determine the ef ciency has a depended values. great importance. The first law of thermodynamics deals mainly with the nature of the energy conservation and energy quantity. fl _ fi The energy ux Q f on the focal plane is de ned the equation Availability of energy in the system, the nature of irreversibilities given below [22,23]: and quality of energy as exergy analysis is being presented with second law. In this section, thermodynamical modelling of LIPGE- h i _ ¼ p 2 2ð∅ Þ 2ð∅ Þ r COS was performed and the assumptions are listed. Qf f sin rim sin s dishGb (1)
r fl ∅ 3.1. Energy analysis where f is focal length, dish is re ectivity of the dish surface, rim is rim angle of paraboloidal dish, ∅s is shading angle caused by size of fl receiver and Gb is solar beam radiation comes to perpendicular the Energy ow diagram of LIPGECOS is presented in Fig. 5. In this _ design, the concentrated solar irradiance reflected by the parabo- aperture plane of the dish.Q f can also be given as following loidal dish concentrator to the focus plane is spectrally separated equation: with the help of a convex cold mirror. The visible radiation reflected _ ¼ r by the mirror is sent to the FOB placed at the center of the dish and Qf Af dishGbFCmax (2) then transmitted into the test space for solar lighting application. UV and IR radiations passing through the cold mirror reach to the Geometric concentrating ratio (Cmax) is given as follows: Stirling heat engine placed behind the mirror. The engine is oper- 2 2 ated by the effect of the heat released here by the IR and UV parts of Aa sin ð∅ Þcos ð∅ þ 0:267 þ d=2Þ C ¼ ¼ rim rim (3) max 2 the concentrated solar radiation and mechanical energy is ob- Af sin ð0:267 þ d=2Þ tained. Thus it is ensured the most effective utilization of the concentrated solar power as spectral and the use of solar energy in Diameter of the image on the focal plane (Df) can be calculated by the equation below:
ð : þ d= Þ ¼ Dasin 0 267 2 Df ð∅ Þ ð∅ þ : þ d= Þ (4) sin rim cos rim 0 267 2
The optimum focal length (fo) can be found by the dish pa- rameters as below: D ð∅ Þ ð∅ þ : þ d= Þ ¼ f sin rim cos rim 0 267 2 fo ð∅ Þ ð : þ d Þ (5) 4tan rim=2 sin 0 267 =2 For the view factor of the dish concentrator (F) can be used Eq. (6):
sin2ð∅ Þ sin2ð∅ Þ F ¼ rim s (6) 2ð∅ = Þ 4tan rim 2 The minimum and the maximum diameters of the Sun image on the focal plane can be predicted by Eqs. (7) and (8):
¼ ½ = ð∅ = Þ ð : þ d= Þ dmin 2Da 1 4tan rim 2 sin 0 267 2 (7)
Fig. 4. Spectral decibel loss of fiber optic cables employed in the study. C. Kandilli, G. Külahlı / Renewable Energy 101 (2017) 713e727 719
Fig. 5. Illustration of energy flow for LIPGECOS.