101 (2017) 713e727

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Renewable Energy

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Performance analysis of a concentrated for - 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 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 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 system which consists of daylight system, and finally developed design guidelines and a dish concentrator(s), a dual-axis 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 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 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 (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 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 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 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 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 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∅ Þsin2ð∅ 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.

between the light and the boundary is less than the critical angle. 2 2 fi fi Da 1 þ 1=tan ð∅ =2Þ sinð0:267 þ d=2Þ As a result, the ber transmits all rays that enter the ber with a d ¼ rim (8) max 2ð∅ Þ ð ð∅ ÞÞ sufficiently small angle to the fiber's axis. The limiting angle is 2 1=tan rim=2 1 1=tan rim=2 called the acceptance angle. Numerical aperture (NA) is expressed Some heat losses occur on the mirror. Energy flux coming to- like below as a measure of receiving q angle: _ max wards the cold mirror (Q m) could be expressed as follows: 1=2 ¼ q ¼ 2 2 _ ¼ _ _ NA sin max n1 n2 (15) Q m Q f Q loss (9) In solar energy application, higher NA values are preferred to _ _ ¼ ð Þþε s 4 4 get all light into the fibers. Energy flux from the FOB (Q ; ) could Q loss hwAm Tm Ta Am Tm Ta (10) out fob be given the equation given below with the assumption of the FOB _ as a single cable: where Q loss is overall heat losses for the mirror and it could be fi de ned as the sum of the radiative and convective heat losses. _ ¼ _ Qout; fob Q in; fob (16) For convective heat losses, heat transfer coefficient hw could be calculated as follows [24]: where dB is the decibel loss and L is the length of the bundle. . loss The FOB used in the experiments is composed of many fibers h ¼ð8:38 v Þþ1:76 W m2K (11) w wind explained before. Besides transmission losses, additional losses _ may occur as a result of torsion and bending causing the dual-axis The energy flux for the Stirling engine ðQ ; Þ could be found by in s sun tracking system. Fortunately, these bendings do not create the following equation: significant changes in the color of the light obtained at the out of _ ¼ _ t the FOB [25]. Q in;s Q m (12) where t is the transmittance of the mirror. 3.2. Exergy analysis _ Available power from the Stirling engineð Q out;sÞ can be calcu- lated as The maximum rate of exergy coming from the solar radiation (J) given by Petela as follows [26]: _ Q out;s ¼ 2pwT (13) 1 T 4 4 T J ¼ 1 þ a a (17) where w is the rotation speed and T is torque. 3 T 3 T Energy flux coming to the FOB could be found by where Ta is the ambient temperature and solar temperature (T) _ ¼ _ r accepted as 6000 K. Q in; fob Qm m (14) Exergy flux of solar radiation coming to paraboloidal dish can be expressed by Eq. (18): where rm is the reflectivity of the cold mirror. Light travelling in the core reflects from the core-cladding _ ¼ _ J boundary due to total internal reflection, as long as the angle Exin GbAa (18) 720 C. Kandilli, G. Külahlı / Renewable Energy 101 (2017) 713e727

The inlet and outlet exergy fluxes of the cold mirror can be conditions. In the present paper, finding out the spectral charac- found the following equations: terization of the concentrated solar irradiance and evaluating its changes on the path of the separation are very significant steps. _ ¼ _ Ta However, we need a standard solar spectrum to analyse these Exin;m Q f 1 (19) Tm changes during the experiments and understand how the spectrum is scattered along the wide range of wavelenghts. _ _ Ta The parameters measured are listed as follows: Solar global Exout;m ¼ Q m 1 (20) Tm irradiance, solar direct irradiance, and ambient temperature, wind speed, temperatures by thermocouples and thermal camera, fl Similarly, the inlet and outlet of exergy uxes of Stirling engine spectral solar irradiance, illuminance and rotation speed. The de- could be calculated as below: vices and their properties are presented in Table 2. In the present study, the experiments were conducted on the _ ¼ _ Ta Exin;s Q in;s 1 (21) roof of the test space located in Department of Mechanical Engi- Th neering, Usak University, Usak and in October 2013. Global solar irradiance, ambient temperature and wind speed data have been where T is the temperature of Stirling engine receiver. h obtained by the meteorology station established on the terrace of _ the building of Department of Mechanical Engineering. Usak, as a Ex_ ; ¼ Q ; (22) out s out s small Anatolian province, is located at a latitude of 38.41N, The inlet and outlet exergy fluxes of the FOB are given the longitude of 29.25 E and altitude of 906 m. Usak, located in the following equations: transition region from Mediterranean climate belt to continental ! climate condition, has hot and dry summers and long cold winters. The experimental results are presented to investigate the per- _ ¼ _ Ta Exin; fob Qin; fob 1 (23) formance of the LIPGECOS in this section. Tfob;in Variation of global and direct solar irradiance (W/m2) by local ! time is given in Fig. 6. Global solar irradiance on horizontal plane 2 _ ¼ _ Ta varied between 397 and 906 W/m ; while direct solar irradiance Exout; fob Q out; fob 1 (24) 2 Tfob;in varied between 658.1 and 948 W/m during the experiments. The Pyrheliometer was integrated to the LIBGECOS operating the dual given with the equation. axes solar tracking system to measure direct solar irradiance. The For any energy system, it is very important step to present the tracking system has high sensitivity; therefore the direct irradiance energy and exergy efficiencies. Overall energy and exergy effi- data has obtained with high accuracy. ciencies could be defined as follows, respectively: Variation of the rotation speed (rpm) by the hot region tem- perature (C) of the Stirling engine is plotted in Fig. 7. The rotation _ þ _ speed values of Stirling engine were obtained between 350 and Qout;s Qout; fob h ¼ (25) 1010 rpm; while the hot region temperature data of the Stirling AaGb engine was recorded between 175 and 257 C. From the graph, it is understand that the rotation speed increases as the temperature of Ex_ ; þ Ex_ ε ¼ out s out;fob the hot region increases. The data belonging the all experiment _ (26) Exin days was used to plot the graph. It should be emphasized that the wind speed is very determinative parameter to affect the temper- ature of the hot region due to the convection losses. The most essential performance parameters of a solar energy 3.3. Economic analysis system are energy and exergy efficiencies. Energy and exergy effi- ciency values were calculated by the formulas given in thermody- Net Present Value (NPV) is one of the methods that allow ana- namical modelling. Variation of energy and exergy efficiencies lysing the economic aspects of an engineering system. It is possible according to the direct normal solar radiation is shown in Fig. 8.It to calculate the present value of all annual capital expenditures and could be seen that energy and exergy efficiencies decrease while savings by NPV methods during the life time of a project. Net direct solar irradiance values increase. The energy losses increase as present value (NPV) is the sum of all the current values (costs are increasing direct solar irradiance. From the experimental data, the shown negative, and net savings are shown as positive) is obtained. average energy and exergy efficiencies of LIPGECOS system were NPV is calculated by the following formulas [27]: calculated as 0.15 and 0.09 respectively. It is observed that energy efficiencies are higher than exergy efficiencies. Cold mirror as the Xt C NPV ¼C þ i (27) key element of system has remained cool throughout the 0 ð þ Þi i¼1 1 r experiments. On the other side, some problems have been observed due to the fl In the formula, C0 is the initial investment, C is the net cash ow, glass cover of the engine. The engine should have two hot and cold r is the discount rate and t is the period. Considering that the regions to be operated effectively. Due to the glass cover, the money going out is subtracted from the discounted sum of cash concentrated energy transmits to the cold region. It causes that the fl ows coming in, the net present value would need to be positive in temperature differences between hot and cold regions decrease. order to be considered a valuable investment. We need high temperature differences to operate the Stirling en- gine at high rotating speeds. To overcome this problem, a finned 4. Experimental results copper apparatus has been designed to be absorbed the IR and UV parts of concentrated solar radiation by the engine effectively. Ac- The solar spectrum of the sky exhibits a variable composition cording to the observations, much more solar radiation has been depending on the time, region, atmospheric composition and absorbed thanks to this apparatus and the rotation speed has been C. Kandilli, G. Külahlı / Renewable Energy 101 (2017) 713e727 721

Table 2 Measurement devices and properties.

Device Properties

Meteorology station (Davis Vantage Pro 2) Wind speed range: 0.5e89 m/s with ±5% accuracy Solar irradiance range: 0e1800 W/m2 with ±5 (%) accuracy Outside temperature range: 40C to þ60 C with ±0.5 C accuracy Pyrheliometer (Kipp&Zonen-CHP1) Measure between 200 and 4000 nm solar spectrum with 7e14 mV/W/m2 sensivity Irradiance meter (Delta Ohm Lp471) Measure between 400 and 1050 nm ±5 (%) accuracy Thermal camera (Fluke Ti27) Operating range: 20 C to 600 C With thermal sensitivity of 0.05 Cat30C Spectrofotometer (Spectralight ILT950) Operating range: 350e1050 nm Thermocouples (Ordel KR6R6 K Type) Operating ranges: 0e650 C Lux meter (Lutron-LX 1108) Operating range: 40e400000 lux Resolution 0.01e100 lux Tachometer (Lutron DT 2268) Operating range: 0.5e100,000 rpm

fi 2 Fig. 6. Variation of global and direct solar irradiance (W/m2). Fig. 8. Variation energy and exergy ef ciency by direct normal irradiance (W/m ).

observation of the radiation distribution of the Sun image, tem- perature measurement and temperature control in concentrating solar energy applications. It is observed that the maximum tem- perature of Stirling engine hot region is 204.9 CinFig. 10. Tem- perature of the metal ring around the cold mirror is measured as 60.1 C. Another thermal camera image of cold mirror and the Stirling engine is shown in Fig. 11. The temperature of the cold mirror was 34.5 C, temperature of cold and hot region of the Stirling were 158.6 and 40.1, respectively. Cold mirror seems like pink and purple colors because the cold mirror surface transmits IR and UV radia- tion. A special design to adjust the location of the cold mirror and Stirling engine could be seen in Fig. 11. Cold mirror and engine components are able to move backward and forward through the prime axis of the dish in this design. The main component of LIP- Fig. 7. Variation of the rotation speed (rpm) by the hot region temperature (C) of the GECOS is the cold mirror decomposing the concentrated entire Stirling engine. solar spectrum into visible and IR-UV parts. The mirror transmits IR and UV irradiance and reflects visible radiation. Therefore the mirror does not hold any part of solar spectrum and has been really raised. However, the radiative and convective heat losses are rela- cold during the experiments, it accomplishes its function. tively high for Stirling engine due to the high temperature. Thermal camera images of the FOB are shown in Fig. 12. There is Thermal camera image for the Stirling engine with the finned the thermal image on the left side and the photo on the right side of apparatus is given by Fig. 9. Integrated apparatus is made of copper, the picture. As seen, maximum temperature of the FOB is 48 C. At black painted and finned. This apparatus provides to absorb solar the beginning of the experiments, an optical filter was placed in radiation and makes the temperature higher on the hot region of front of the FOB against the possible damage due to the high the engine. The maximum temperature is 291.1 CinFig. 9. This temperature. As mentioned before, PMMA based optical fibers can result could be evaluated as promising if we consider that the ex- resist until 70 C. In the later stages of the experiments, allowable periments conducted under windy condition and Usak province temperatures on the FOB have been measured and the optical filter does not have a remarkable solar energy potential. has been removed. Visible region radiation which directly reflected Thermal camera image of cold mirror and the Stirling engine is by the cold mirror is sent to fiber optic cable bundle. Spectrally given in Fig. 10. Thermal camera images have a very key role on decomposing of the concentrated solar radiation was succeeded in 722 C. Kandilli, G. Külahlı / Renewable Energy 101 (2017) 713e727

Fig. 9. Thermal camera image for the Stirling engine with the finned apparatus.

Fig. 10. Thermal camera images of cold mirror and the Stirling engine-1.

Fig. 11. Thermal camera images of cold mirror and the Stirling engine-2. this study. Concentrated solar radiation was divided into visible and and prevent fibers from the excessive heat load. As presented IR-UV parts by the cold mirror. Separated radiations were trans- before, the transmittance of FOB given in Fig. 4 exhibits high values ferred to the desired points and employed for both of lighting and for visible wavelengths. For this reason, it is essential to determine power generation. It is hoped that this approach can be applied to the spectral distribution of the light at the beginning and end of the different and promising researching areas in the future. FOB. In this section, the lighting part of the study should be detailed Spectral distributions at the end of the FOB with and without with different points of views. The results obtained by spectro- cold mirror are presented by Figs. 14 and 15, respectively. The test photometer, irradiance meter and light meter were presented and space has been illuminated by FOB during the experiments. The interpreted as below: room does not have any window and it is not illuminated by any Spectral distribution of the sky obtained by the spectropho- artificial light sources. The probe of spectrophotometer was located tometer (at 16:00 p.m.) is presented in Fig.13. The aims of LIPGECOS 20 cm far from the FOB in normal direction. From Fig. 15, the light are to produce power by Stirling engine and to provide indoor via the FOB is almost composed by visible wavelengths. This result illuminance via FOB. The radiation receiving by FOB should be shows that LIPGECOS has reached to its lighting aim successfully. visible part of the solar spectrum to transmit the light effectively In solar lighting applications, it is imperative to know the solar C. Kandilli, G. Külahlı / Renewable Energy 101 (2017) 713e727 723

Fig. 12. Thermal camera images of input of fiber optic cable.

Fig. 13. Spectral distribution of the sky. global illuminance and irradiance values in outdoor conditions. irradiance values increase. It is clear that the visible light provided Variation of outdoor global illuminance (klux) by global solar by FOB is predominant into solar concentrated radiation compo- irradiance (W/m2) is plotted in Fig. 16. During the experiment sition. The spectral transmittance and reflectance properties of FOB conducted in June 2013; global solar irradiance on horizontal plane and cold mirror provide very high luminous efficacy values. To varied between 397 and 906 W/m2, while the global solar illumi- supply visible light is one of the most promising and remarkable nance taken values between 51.00 and 138.80 klux. feature of LIPGECOS. While the luminous efficacy value of the most Variation of indoor global illuminance (klux) by outdoor global efficient LED sources could reach to 249 Lm/W [28]; the average solar illuminance (klux) on horizontal plane is given by Fig. 17. In- luminous efficacy of LIPGECOS was calculated as 347 Lm/W. This door illuminance provided by FOB changed between 6.00 and 26.54 result is very significant for the solar lighting applications. The lu- klux, while outdoor global illuminance taken values between 51.00 minous efficacy values could be listed for incandescent, halogen, and 138.80 klux. In the test space, the probe of light meter was compact fluorescent, old generation LEDs, new generation LEDs installed 20 cm far from the end of the FOB. and the LEDs new-developed as 14; 13; 63; 64; 94; 249 Lm/W, Variation of luminous efficacy (lm/W) by direct solar irradiance respectively. LIPGECOS has a superior luminous efficacy thanks to (W/m2) is plotted in Fig. 18. Luminous efficacy is a very indicative the spectral beam splitting approach. In the test space, only visible parameter to evaluate the lighting efficiency of any light source. light could be provided as a light source by the proposed approach. Luminous efficacy is a measure of how well a light source produces Besides, the lighting efficiency was evaluated for the present sys- visible light. It is defined as the ratio of luminous flux to power. The tem. The lighting efficiency is defined as the ratio of indoor illu- luminous efficacy values of the FOB increase, while direct solar minance (klux) to outdoor illuminance (klux) on horizontal plane. 724 C. Kandilli, G. Külahlı / Renewable Energy 101 (2017) 713e727

Fig. 14. Spectral distribution of the end of the FOB without cold mirror.

Fig. 15. Spectral distribution of the end of the FOB with cold mirror.

The average lighting efficiency was calculated as 14% ± 0.03 for shown in Table 3. In the economic analysis, the yearly mean sun- LIPGECOS. shine period was assumed as 7.5 h/day for Usak, Turkey and the cost Overview of the test space for lighting application could be seen of electricity was accepted as 0.28 $/kWh. The initial cost including in Fig. 19. This figure clearly shows that the illuminance levels the dish, the cold mirror, the FOB, the tracking system, the Stirling provided by FOB were very sufficient for the lighting application. It engine and the constructional material was $6960 for a single should be emphasized that the illuminance level has not been module. The hourly average power of the system was assumed measured on the working plane ground due to long height of the 410 W. Maintenance and repair costs for the system were ignored. It space; all indoor data has been obtained by the probes which is is calculated that the LIPGECOS has significantly long payback time located 20 cm far from end of the FOB. Successful results of this after 26 years of operation. However, it should be considered that research could be applied to different areas in the future. this result reflects the research and investigation costs. For Economical evaluation of LIPGECOS has been performed by NPV example, the cold mirror designed and sized by the authors was method given as Eq. (27). The result of economic analysis was produced in Germany. The cost of this special filtered mirror was C. Kandilli, G. Külahlı / Renewable Energy 101 (2017) 713e727 725

Fig. 16. Variation of outdoor global solar illuminance (klux) by global solar irradiance (W/m2).

Fig. 19. Overview of the test space for lighting application.

5. Conclusion

In the present study, lighting-power generation combined sys- Fig. 17. Variation of indoor global illuminance (klux) by outdoor global solar illumi- tem (LIPGECOS) based on the approach of spectral beam splitting of nance (klux). concentrating solar radiation was defined and the first experi- mental results of the system were discussed. It could be divided the concentrated solar irradiance into “heat” and “light” parts by the spectral beam splitting approach. Heating form is caused from the UV and IR parts, while the lighting form is arisen by the visible part of the solar spectra. It could be assumed that the concentrated solar energy as the sum of the heating and lighting forms of the total energy. Separated concentrated solar irradiance covering visible region can also be employed for the purpose of production elec- trical energy by the solar cells. However, considering the high cost of these techniques and materials, the efficient use of available solar cells is important. Thanks to spectral beam splitting, the existing solar cells, especially Si based, having the visible region spectral response could be operated without any excessive heating problem, and their energy efficiencies could be increased. On the other hand, IR and UV parts of the decomposed concentrated solar irradiance have a great and promising potential to be utilized for many Fig. 18. Variation of luminous efficacy (lm/W) by direct solar irradiance (W/m2). different purposes such as generation mechanical energy, $2365. Similarly, the fibre optic cables for the bundle were im- providing heating energy, applications, solar cooling, ported from Japan. The cost of the FOB was $3378. The costs of the solar distillation etc. fi cold mirror and the FOB include tariff and the other taxes. Another The spaces, which cannot be illuminated suf ciently in a natural remarkable subject related the system cost is the obtained power. way such as large commercial buildings, shopping centers, sub- The lighting power gained by the system is very sufficient and ways, car parks, operating rooms, tunnels, warehouses, mines, available. However, the power of the Stirling engine is very low. The production lines, workshops, etc., could be lighten by the proposed engine power corresponding to capacity of the system could be system. LIPGECOS can be considered as a promising and improvable much bigger. In the present study, a light and low capacity engine option to supply energy demand for the buildings in the frame of was employed to investigate spectral beam splitting of the green buildings or zero energy buildings concepts. The required fi concentrated solar energy. It is believed that an optimized system illuminance levels could be reached by the support of the arti cial operating sunbelt region could be more available economically. light sources in a hybrid structure for the cloudy sky conditions. In 726 C. Kandilli, G. Külahlı / Renewable Energy 101 (2017) 713e727

such a hybrid system, the artificial light sources could be supplied by a photovoltaic module. 0.11 314.00 0.01 313.89 0.76 239.94 435.13 For the future enhancement, if the system could be improved, 2878.27 0.11 shorter payback period could be obtained. To explain further; if the surface errors of the concentrator dish could be minimized and the

0.11 solar irradiance could be focused more precisely, in other words, if 314.00 ¡ 0.01 313.89 0.77 242.34 195.19 3151.34 0.11 the reflectivity of the dish is assumed as 0.85; if the total illumi- nation efficiency of the FOB could be increased as the 30% level by diminishing the transmission losses of the fiber optic cables, if the 0.11 47.15 whole solar irradiance could be assumed to transform the me- 3427.14 0.11 fi fi chanical energy by the Stirling engine with Carnot ef ciency; nally the capital cost of the LIPGECOS could be decreased as 40% by employing the native products instead of the imported materials, 0.11 291.91 the payback period could be reached as 12 years which is an 3705.71 0.11 acceptable value for the energy market. The remarkable results of the present study could be listed as follows: 3987.05 0.11 0.11 539.12 a. The rotation speed of the Stirling engine increases as the tem- perature of the hot region increases. b. The average energy and exergy efficiencies of LIPGECOS system 4271.21 0.11 were calculated as 0.15 and 0.09, respectively. These values 0.11 788.80 could not be considered low. As known by solar thermal spe- cialists, there is a big loss for solar concentrators in regard to energy and exergy efficiency naturally. 4558.21 0.11 c. The cold mirror does not hold any part of solar spectrum and has 0.11 1040.98 fi been really cold during the experiments; the mirror ful lls its function. The FOB transmits only the visible part of the 4848.09 0.11 concentrated solar irradiance. Spectral beam splitting of concentrated solar irradiance has been accomplished success- 0.11 1295.68 fully as intended. d. The radiative and convective heat losses for Stirling engine 5140.86 0.11 should be minimized to obtain much more solar power. e. The indoor illuminance values in test space increase, while global solar irradiance and global illuminance values increase. 0.11 1552.93 5436.56 0.11 While global solar irradiance on horizontal plane varied be- tween 397 and 906 W/m2 and outdoor global illuminance taken values between 51.00 and 138.80 klux, Indoor illuminance by FOB changes between 6 and 26.54 klux. 5735.21 0.11 fi 0.11 1812.75 f. The average luminous ef cacy of LIPGECOS was calculated as 4567891011121314 347 Lm/W. LIPGECOS has a superior luminous efficacy owing to spectral separation approach. g. The average lighting efficiency was calculated as 14% ± 0.03. 6036.85 0.11 0.11 2075.16 Experimental studies are very valuable to investigate for any energy systems. Below, suggestions drawn by the experiments are 6341.51 0.11 presented for future works: fl 0.11 2340.21 a. High re ectivity surface was employed to produce the concen- trated dish. A satellite dish presser has been utilized to obtain 6649.22 0.11 the LIPGECOS dish. For future works, the reflectivity of the surface could be increased. b. In the present study, the aim is to evaluate the spectral beam 0.11 2607.90 splitting approach. For this reason, a low capacity Stirling engine 6960.00

was employed to observe the results of the approach. The high rotation speed values were obtained during the experiments. For future works, a high capacity Stirling engine could be chosen to produce much more power. The engine should be designed according to the system size. ow 310.78 307.71 304.66 301.64 298.66 295.70 292.77 289.87 287.00 284.16 281.35 278.56 275.80 273.07 ow 270.37 267.69 265.04 262.42 259.82 257.25 254.70 252.18 249.68 247.21 244.76 fl fl c. Due to the glass cover of the Stirling engine, concentrated solar es the point of the payback time of the system.

fi irradiance reached the piston and the temperature increased on ow 313.89 313.89 313.89 313.89 313.89 313.89 313.89 313.89 313.89 313.89 313.89 313.89 313.89 313.89 ow 313.89 313.89 313.89 313.89 313.89 313.89 313.89 313.89 313.89 313.89 313.89 fl fl the cold region of the engine. For future work, glass cover for ts Per Year 314.00 314.00 314.00 314.00 314.00 314.00 314.00 314.00 314.00 314.00 314.00 ts Per Year 314.00 314.00 314.00 314.00 314.00 314.00 314.00 314.00 314.00 314.00 314.00 314.00 314.00 314.00 fi fi Stirling engine should not be employed to increase the tem- perature differences between hot and cold regions of the engine. Period (Years)Initial Investment Cost Bene Maintenance and Repair Costs 15 16 17 18 19 20 21 22 23 24 25 26 27 Interest rateNet cash 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Discount rateDiscounted net 0.99 0.99 0.98 0.97 0.96 0.95 0.94 0.93 0.92 0.91 0.91 0.90 0.89 0.88 0.87 NPV Bene Initial Investment Cost Maintenance and Repair Costs Period (Years) 0 1 2 3 Interest rateNet cash Discount rateDiscounted net 0.01 0.86 0.01 0.85 0.01 0.84 0.01 0.84 0.01 0.83 0.01 0.82 0.01 0.81 0.01 0.80 0.01 0.80 0.01 0.79 0.01 0.78 NPV Table 3 Economic analysis of the LIPGECOS by NPV method. The bold signi C. Kandilli, G. Külahlı / Renewable Energy 101 (2017) 713e727 727

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