Thermal and Optical Study of Parabolic Trough Collectors of Shiraz Solar Power Plant

Proceedings of the Third International Conference on Thermal Engineering: Theory and Applications May 21-23, 2007, Amman, Jordan

THERMAL AND OPTICAL STUDY OF PARABOLIC TROUGH COLLECTORS OF SHIRAZ SOLAR POWER PLANT

A. Mokhtaria, M. Yaghoubia*, P. Kananb, A. Vadieea. R. Hessamia

a Engineering School, Shiraz University, Shiraz, Iran b Renewable Energy Organization of Iran, Tehran, Iran

Abstract In Iran an attempt is made to find possible applications of solar energy to construct the first 250 KW solar power plant in Shiraz. The power plant is comprised of two oil and steam cycles. Oil cycle includes 48 parabolic trough collectors. The present work focuses on the performance study of the parabolic trough collectors in the hot oil generation system. Analysis of optical performance and optical losses of the parabolic trough collectors (PTC) to improve the optical efficiency and to ensure the desired quality can be achieved in solar power plants are important.For thermal and optical study of the parabolic trough collectors, collector oil inlet (T if ) and outlet temperature (T fo ), ambient temperature (Ta ) were recorded with the help of thermocouples sensors. The solar beam radiation intensity was measured by a Pyrheliometer and the mass flow rate of oil by a Krohne CO. flow meter. The wind speed was measured by a vane type anemometer. All parameters were measured as a function of time. Based on these measurements the value of intercept factor and incident angle modifier of the constructed collector is determined and compared with the design condition and with optical properties of some other large size constructed commercial parabolic collectors.

Keywords: Thermal efficiency, Optical efficiency, Incident angle modifier, Responding time.

Greek Symbols Nomenclature α Absorptivity of absorber Please use standard notations. Should you choose to skip this section, all symbols must be clearly defined ρ Mass density, kg/m3 everywhere relevant in the text. Leave only one blank line between the title of this section and the first parameter τ Transmissivity of cover description. Please do not forget to delete this paragraph γ Intercept factor or the entire section accordingly. A Aperture , m2 θ Incidence angle , deg C Concentration factor σ Error

CP Specific heat , W/kg K η Efficiency f Focal length , m Subscripts 2 Ib Beam radiation , W/m a Ambient K(θ ) Incidence angle modifier f Fluid l Length of collector , m I Inlet m& Mass flow rate , kg/sec O Outlet U Useful Q Heat gain , W T Temperature , K

* Corresponding author. Tel.: +98 711 2303051 ext. 71348-51154 Fax: +98 711 6287508; E-mail: [email protected] 1. Introduction installed in Shiraz solar power plant in north south direction. The power plant is comprised of two oil and Currently, solar parabolic trough collectors (PTC) are steam cycles [8]. Oil cycle includes 48 parabolic trough employed for a variety of applications such as power collectors in 8*6 rows which are used to heat the working generation [1], steam generation [2], and some air cooling fluid (oil) as illustrated in Figure 2. The present work systems [3]. Solar thermal power plants based on PTC focuses on the performance study of the PTC in the hot oil are presently the most successful solar technologies for generation system. electricity generation, as demonstrated for more than a decade by the Solar Electric Generation Systems (SEGS) plant at Kramer Junction in California, USA [4]. The advances of technologies presented in the studies of [5], shows that a significant increase in the performance and reduction in cost is possible for parabolic trough solar thermal electric technologies as compared with the 1997 baseline technology system. Price et al. [6] reviewed the current state of art of parabolic trough solar power technology and described the R&D efforts that are in progress to enhance this technology. Their paper also shows how the economic of future parabolic trough solar power plants are expected to improve. Table 1. Parabolic trough collector specifications [8, 9]

Reflectivity of Length 25 m 0.873 Fig. 1. A row of parabolic trough collector installed in Shiraz mirror ρ solar power plant Transmissivity of Width 3.4 m 0.96 cover τ Emissivity of Aperture 3.1 m 0.25 cover Absorptivity of Focal length 88 cm 0.94 receiver α Emissivity of Outer diameter 70 mm receiver at 0.14 of receiver 300o C Outer diameter 125 mm Intercept factor 0.93 of cover Concentration 14 Oil flow rate 1.7 kg sec ratio Rim angle 90o ρτα 0.788 Solar thermal plants using PTC are basically composed of a solar collectors field and a power block. The solar collectors field is designed to collect heat from the sun which are continuously tracking the sun. Reflecting surface concentrates direct solar radiation in the optical focal line of the collector where the Heat Collecting Element (HCE) is located. The HCE absorb the Fig. 2. The collector field of 250 kW solar thermal power reflected energy and transmit it to the heat transfer fluid plant with the test loop which is pumped to the conventional power block where The general arrangement of the collectors within a electricity is generated. Characterization of optical loop of the oil cycle is shown in Figure 3. The loop consist performance and determination of optical losses of of six 25m collectors, one complete loop has total length parabolic trough collectors are very important issues in of over 150 m. This loop is made available initial for order to improve the optical efficiency of these systems testing and evaluating the performance of collectors. and to ensure the desired power quality is achieved in the solar power plants [7]. Design of collector structure is a key item in order to manufacture an easy product able to achieve a high reflecting quality and tracking precision. The collector structure is made of steel which supports the reflective elements as well as the HCEs. This structure must guarantee the parabolic shape of the mirror to obtain a good performance of the collector [1]. In Iran an investigation is made to find possible applications of solar energy to construct the first 250 kW solar power plant near the city of Shiraz. For the solar power plant in Shiraz, Fig. 3. Test loop of collector with steam generation and parabolic trough collectors 25m long, 3.4m wide with storage tank 0.88m focal length are used. More details of collector are in Table 1 [8, 9]. For normal operation September 21 at 12 2. Description of the parabolic trough collector noon is chosen as the design point. At this hour, solar radiation intensity at the plant site is 816 mW 2 [10]. In the present work, thermal performance of parabolic trough collectors system, which has been developed for Figure 1 shows the parabolic trough collector that is

66 hot oil generation, such as presented in Figure 1 is the aperture. Strong variation of the modified incident studied. The loop of PTC system contains 6 collectors for angle will affect the over all thermal performance of the hot oil generation, a hot oil expansion tank (HOET), a collector and the power plant efficiency. Duffie in circulating pump, a flow meter and other controlling 1991[17], presented a relation between Kθ)( and the devices. The parabola of the present collector with a rim collector geometry as follows: angle of 90° is very accurately constructed of steel. Glass solar reflector with a reflectance of 0.873 is used in the f ⎛ A2 ⎞ K θ ⎜11)( +−= ⎟ tanθ (4) present work. The solar receiver is made of a stainless ⎜ 2 ⎟ steel tube with an absorptance of 0.94 and thermal l ⎝ 48 f ⎠ emittance of 0.25 in 300 o C , a glass envelope and rubber In this relation f is focal length and l is length of cork seals at both ends of the glass envelope. The steel tube is coated with a heat resistant Cermet paint and is collector and A is collector aperture area. surrounded by a concentric glass cover with an annular gap of 27 mm. The space between the steel tube and Table 2. The value of Intercept factor of Shiraz collectors glass is evacuated to reduce thermal losses to the surrounding air. Oil from the expansion tank is pumped Tracking Optical Concentration Intercept through the steel tube, where it is heated and then flows period (min) factor, C factor, back into the expansion tank. The PTC rotates around the error σ γ horizontal axis of north/south axis to track the sun as it (mrad) moves through the sky during the day 1:30 11.2 14 0.94 2:30 16 14 0.93 .2.1. Performance testing of the parabolic trough collector 3. Measurements and discussions The performance of the PTC of hot oil generation The loop shown in Figure 3 is prepared for system is determined by obtaining data of collectors such measurement by cleaning the mirrors and glass cover of as stantaneous efficiency and the system efficiency for the receiver, as well as checking the system to be different combinations of incident radiation, ambient completely filled with oil and no air is present in the loop. temperature and inlet oil temperature. The collector oil Measurement includes collector oil outlet temperature Tfo inlet (T if ) and outlet temperature (T fo ) and ambient air and oil inlet temperature , oil mass flow rate and T if m& )( temperature (T ) were recorded with the help of K type a the difference in the temperature of the HCE between the thermocouples. All sensors initially calibrated with inlet and the outlet to the collector Δto )( . During accuracy of ± 5.0 o C . The solar beam radiation intensity measurements it is tried to maintain the collector aperture was measured by a Kipp & Zonen Pyrheliometer and the normal to the sun within the limits of near-normal mass flow rate of oil by a Krohne flow meter with accuracy incidence and any allowable tracking errors, as class of 1.6. The wind speed was measured by a vane applicable. The variation of collector oil outlet type anemometer with accuracy of ± m sec5.0 . All temperature,Tfo , and oil inlet temperature,T if , with time parameters were measured as a function of time. The on one day, viz. October 4, 2006 is shown in Figure 4. thermal efficiency, η of a concentrating collector operating under steady operation conditions can be described by [11]:

. Q −TTcm )( η u == fifop . IA . IA b b (1) The value of optical efficiency can be found as [12]:

ηo = []K()θ [ρ()τα n ]γ (2) The value of mirror reflectivity ρ is determined by Optical laboratory, Zanjan, Iran [15]. The glass cover transmissivityτ , and the receiver absorptivityα , are provided by earlier measurements [9]. The intercept factor Fig. 4. Variation of collector oil outlet temperature, , and γ is defined as the fraction of those rays incident on the T fo oil inlet temperature, , with time aperture that is intercept by the receiver. The value of γ T if depends on the optical errors, sun shape, rim angle and The collector oil temperature increases progressively concentration ratio [13]. The value of also depends K θ )( with time, which varies from 9:53 to 12:25 h, Iranian on the sun tracking period and it is found according to Standard Time (IST), as the oil is recirculated through a Table 2, using the proposed graph by [16]. Incidence- hot oil expansion tank of capacity 1000 liters. The mass angle modifier K()θ in Eq. (2) is predicted according to flow rate of oil through the collector is measured to be the pervious study [13] and defined by: 9 3 hrm . Figure 5, illustrates that the heat gain by a K θθ −= 00045656.0)cos()( θ − 00004078.0 θ 2 (3) single collector, which is approached to a steady state condition which is equal to the heat loss through the loop. Note that during the test only a single collector NO. 6 was Where is angle of incidence of the sun’s rays on θ tracking the sun and the rest of collectors were the collector aperture which measured from the normal to

67 unfocused. Variation of beam radiation, , and useful This analysis is applied to several collectors that have b × AI been tested at low temperature for improved accuracy. heat gain, , with time is shown in Figure 5. It is seen Qu The results are presented in a polynomial fit to the data that a fairly smooth variation of beam radiation with the according to Eq. (5). maximum (735 W/m2) occurs around noon. Small K θθ −= 000658629.0)cos()( θ − 000758124.0 θ 2 (5) variation of Qu is due to small variation of wind velocity or off tracking intervals. Generally wind velocity changes Figure 7 shows variation of the proposed relation (3), from 1.1 m sec to 2.2m sec . The collector relation (4) and the relation based on measurements (5). instantaneous efficiency is computed using Eq. (1). Figure In these relations θ various from 16.19 to 19.8 degrees 6 shows variation of the collector instantaneous efficiency and the differences demonstrates that K θ )( should be with time. determined exactly after parabolic collector set up and actual field measurements. This figure also illustrates that the actual incident angle modifier is much less than the proposed relation for the design of collector. Such behavior may attributed from mirror cleanliness, tracking error and glass cover cleanliness.

Fig. 5. Variation of beam radiation, , and useful heat b × AI gain, Qu It will be noted that the general pattern of variation of efficiency over a day is not the same as that of the useful heat gain because the value of efficiency depends on both Figure 7: Variation of incidence angle modifier of various the incident beam radiation and the useful heat gain. With models progresses of time, near the noon, incident beam radiation increase but useful heat gain has not change significantly, therefore thermal efficiency decrease. The Results of compartment characteristic of our collector useful heat gain and the collector instantaneous efficiency with the other collectors [6] are compared in Table 3. This are evaluated on hourly basis. All these parameters are table shows that the present PTC has smaller size than strongly influenced by the incident beam radiation and the other collectors and its performance should be found to follow its variation. Also corresponding optical improved to be comparable with more advanced efficiency based on relation (3) and (4) is presented in commercial PTC installed around the world. Figure 6. The optical efficiency of a parabolic trough collector decreases with incidence angle for several 3. Conclusion reasons: reduction of transmission of the glazing and the In the present work, performance of the parabolic decrease of absorption of the absorber; the increased trough collector of Shiraz power plant with hot oil width of the solar image on the receiver; and the spillover generation system is investigated experimentally over in of the radiation from troughs of finite length [14]. In order summer period. The system operates under closed loop to be able to apply test results from a short collector mode by recirculating the oil through a hot oil expansion module to collector arrays of arbitrary length, it is tank. Variations of collector oil inlet and outlet temperature necessary to separate analytically the end loss from the are measured and the maximum beam radiation during first two effects. the experimental period was 735 mW 2 . The useful heat gain and the collector instantaneous efficiency as a whole are evaluated on hourly basis. All these parameters are strongly influenced by the incident beam radiation and found to follow each other. The resulted optical and thermal efficiency of about 69% and 64% of the present collector is less than the other installed parabolic collectors. Some improvement should be made by better tracking as well as more cleaning of the mirror and glass tube cover.

Fig. 6. Variation of thermal and optical efficiency

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