Performance Analysis of Parabolic Trough 2 Collector in Hot Climate

4 Adel A. Ghoneim*, Adel M. Mohammedein, Kandil M. Kandil 5 Applied Sciences Department, College of Technological Studies, PAAET, 6 P.O. Box 42325, Shuwaikh 70654, Kuwait 7 8 9 . 10 ABSTRACT 11 12 In most existing buildings cooling and heating loads lead to high primary energy consumption and consequently high CO2 emissions. These can be substantially decreased with suitable energy concepts using appropriate integrated renewable systems. In the present study a numerical model is developed to study the effect of different collector parameters and operating conditions on the performance of parabolic trough solar collector (PTSC) in Kuwait climate. The proposed model takes into consideration the thermal interaction between absorber-envelope, and envelope-envelope for thermal radiation losses which have been neglected in existing models. A review of the equations for convective heat transfer losses was performed as well and new equations were developed and used in the present model. The effects of heat conduction in the collector tube wall and mixed convection in the inner tube; which have been neglected in previous studies are included in the proposed model. In addition a case study is carried out adapting parabolic trough collectors to satisfy nominal space heating load, water heating load and cooling load of a typical Kuwaiti dwelling. Finally, the environmental impact of solar heating and cooling systems under Kuwait climate conditions is investigated. Present results indicate that convection loss from the absorber tube to supporting structures is the largest among the other losses (conduction and radiation). Also, at noon time PTSC has the smallest angle of incidence and the highest efficiency and when the annulus between the receiver surface and the glass envelope is in vacuo, conduction and convection across the annulus are effectively eliminated. In addition, space heating load and domestic water heating load can be completely provided by PTSC. The minimum required collector area is about 82 m to supply cooling loads of a typical residential house under all climatic conditions in Kuwait. A CO2 emission reduction of about 12.3 tonne/year can be achieved as a result of adapting parabolic trough solar collector in the house.


17
Energy consumption worldwide is increasing rapidly due to increasing global population and 18 industrialization processes in many countries. The production and use of conventional fossil 19 fuel energy resources account for a high percentage of air pollution leading to harmful 20 impact on our environment. In contrast, renewable energy sources can be adapted to 21 produce energy sources with little-if any pollution. Widespread commercialization of 22 renewable energy systems, especially in hot weather regions like Kuwait can significantly 23 reduce energy consumption of conventional fuels. In return, this will help greatly in reducing 24 pollution and maintain our environment healthy and clean. Concentrating solar power (CSP) 25 technologies now constitute feasible commercial options for large scale power plants as well 26 as for smaller electricity and heat generating devices. There are currently four basic 27 commercially available CSP technologies. These plants typically consist of three main 28 circuits: the Solar Field, through which the heat transfer fluid (HTF) circulates, the Power 29 Block, which circulates water and steam, and the Thermal Storage System (TES) system. 30 The HTF and water-steam circuits and the HTF and TES circuits can exchange energy at 31 the corresponding heat exchangers. 32 Parabolic trough collector is a type of high temperature solar concentrator collector. These 33 collectors have a linear parabolic shaped reflector that focuses sun radiation on a receiver at 34 the focus of the reflector. Parabolic troughs are long parallel rows of curved glass mirrors. 35 Mirrors are usually coated silver or polished aluminum focusing the sun's energy on an 36 absorber pipe located along its focal line. Parabolic trough collectors use higher 37 temperatures to heat up a fluid in absorber tubes arranged in the focal line. The heat transfer 38 fluid (HTF), oil, is circulated through the pipes. Under normal operation the heated HTF 39 leaves the collector with a specified collector outlet temperature and is pumped to a central 40 power plant area. There, the HTF is passed through several heat exchangers where its 41 energy is transferred to the power plant's working fluid, which is usually steam. The heated 42 steam is used in turn to drive a turbine generator to produce electricity. Thermal storage can 43 be used to provide electricity during peak hours or when sun intensity is low. Solar collector 44 tube is a key component in parabolic trough solar thermal generation system. It is used to 45 convert solar radiation to thermal energy. Optimizing its performance and improving its 46 efficiency has important effects on the thermal-electricity conversion efficiency. 47 Kalogirou et al. [1] studied the performance of parabolic trough collectors using a theoretical 48 model. They predicted the quantity of steam generated by the system. The optimum flash 49 vessel diameter and inventory obtained from this analysis are 65 mm and 0.7 1, respectively. 50 Current state of the art of parabolic trough solar power technology is reviewed by Price et al. 51 [2]. They described the research efforts that are in progress to enhance this technology. 52 They claimed that since the last commercial parabolic trough plant was built, substantial 53 technological progress has been realized. The review presented shows how the economics 54 of future parabolic trough solar power plants is expected to improve. Riffelmann

Solar Radiation
Prediction of collector performance requires knowledge of the absorbed solar energy by the 165 collector absorber plate. The solar energy incident on a tilted collector consists of three 166 different components: beam radiation, diffuse radiation, and ground-reflected radiation. The 167 details of the calculation depend on which diffuse-sky model is utilized. Using an isotropic 168 sky model, the absorbed radiation on the absorber plate can be given by: 169 where the subscripts b, d, and g represent beam, diffuse, and ground-reflected radiation, 171 respectively. G is intensity of radiation on a horizontal surface, ( ) τα the transmittance-172 absorptance product that represents the effective absorptance of the cover-plate system, β 173 the collector slope, g ρ is the diffuse reflectance of ground and the geometric factor R b is the 174 ratio of beam radiation on the tilted surface to that on a horizontal surface. The angle of 175 incidence (θ) represents the angle between the beam radiation and plane normal to the 176 surface. In addition to losses due to the angle of incidence, there are other loss from the 177 collector due to the angle of incidence. The loss occur because of additional reflection and 178 absorption by the glass envelope as the angle of incidence increases. The incidence angle 179 modifier (IAM) corrects for these additional reflection and absorption losses. 180 The calculation can be simplified by defining equivalent angles that give the same 181 transmittance for diffuse and ground-reflected radiation. Performing the integration of the 182 transmittance over the appropriate angle of incidence with an isotropic sky model and 183 equivalent angle of incidence for diffuse radiation yields: 184 The heat collection element (HCE) consists of an absorber surrounded by a glass envelope. 206 The absorber is typically a stainless steel tube with a selective absorber surface which 207 provides the required optical and radiative properties. Selective surfaces combine a high 208 absorptance of solar radiation with low emittance for the temperature range in which the 209 surface emits radiation. The glass envelope is an anti reflective evacuated glass tube which 210 protects the absorber from degradation and reduces heat loss. The vacuum enclosure is 211 used primarily to reduce heat loss at high operating temperatures and to protect the solar-212 selective absorber surface from oxidation. The heat transfer model is based on an energy 213 balance between the heat transfer fluid and the environment. 214 215 The solar energy reflected by the mirror is absorbed mainly by the glass envelope Q e-abs and 216 the absorber surface Q a-abs . Part of the energy taken by the absorber is transferred to the 217 heat transfer fluid by forced convection V a-f ;conv , the remaining energy is transferred back to 218 the glass envelope by radiation Q a-e;rad and natural convection Q a-e;conv and lost through the 219 support brackets by conduction Q cond;bracket as well. The heat loss from the absorber in the 220 form of radiation and natural convection is conducted by the glass envelope and lost to the 221 environment by convection Q e-sa;conv and radiation Q e-s;rad , together with the energy absorbed 222 by the glass envelope Q e-abs . In order to obtain the partial differential equations that govern 223 the heat transfer phenomena, an energy balance is applied over a section of the solar 224 receiver. The detailed analysis of the partial differential equations and method of solution 225 can be found in Rohsenow et al. [29]. After applying the energy balance on a control volume, 226 assuming unsteady state and incompressible fluid, the following partial differential equation 227 can be obtained: 228 where f m & is the fluid mass flow rate, A i,a is the internal cross sectional area of the absorber, 231 T f is the fluid temperature, and Q ' a-f;conv is the heat transfer by convection from the absorber to 232 HTF per unit length and is given by where Nu f is the Nusselt number, k f is the thermal conductivity of HTF, and T a is the absorber 237 wall temperature.
The energy absorbed in the solar receiver is affected by optical properties and imperfections 320 of the solar collector ensemble. For a concentrating collector, the effective optical efficiency 321 is defined as long as the direct beam radiation is normal to the collector aperture area. When 322 the beam radiation is not normal, the angle of incidence modifier (IAM) is included to account 323 for optical and geometric losses due to angles of incidence greater than 0º. The IAM 324 depends on geometrical and optical characteristics of the solar collector. 325 326 327 328 329

331
The set of partial differential equations (PDE) was discretized for steady state conditions by 332 using the finite difference method and taking into account the dependence of thermal 333 properties on temperature. Turning to the heat transfer fluid, discretization by backward 334 differencing creates a set of algebraic equations. For the absorber and the envelope, the 335 discretization is carried out using the central difference and thus, another set of algebraic 336 equations is obtained. Finally, the boundary conditions for each element are set down. The 337 proposed model includes the thermal interaction between absorber-envelope, and envelope-338 envelope. These thermal radiation losses were not included in existing models. To account 339 for the thermal interaction between adjacent surfaces, a comprehensive radiative analysis 340 was implemented for heat losses in the absorber and the glass envelope. area. The thermal loss from the collector receiver is related to the operating temperature. 358 The The incidence angle modifier, K ατ (θ i ), enables the performance of the collector to be 410 predicted for solar angles of incidence other than 0° (normal). Simulations using the present 411 numerical model are carried out setting up a value of θ i and then calculating K ατ . 412 Angle of incidence modifier represents the ratio between thermal efficiency values at a 413 specified value and the peak efficiency of the collector at zero incidence. Results of 414 simulation are presented in Figure 3. Regression analyses provided the following equations 415 for K ατ as a function of θ i : 416 Coefficients of determination for equations (20) and (21) are 0.958 and 0.961. Up to an 420 angle of incidence of approximately 25° the glass-s hielded receiver performed slightly better, 421 but beyond that its performance declined more rapidly and was inferior to that of the 422 unshielded receiver. 423 In Figure 3, the calculated value of K ατ was 0.74 for the glass-shielded receiver at the 424 maximum tested incidence angle of 54°, an 8.9% lowe r than that for the unshielded receiver. 425 At the same maximum angle of incidence, the simplified cosine model under-predicted K ατ 426 by about 25% for the unshielded receiver and by 17.7% for the glass shielded unit. Two 427 factors are primarily responsible for the decline in performance of a PTSC with increasing θ i ,: 428 the geometric reduction in irradiance falling on the aperture as θ i increases or 'cosine effect' 429 and the change in optical efficiency (due to differences in light interaction) with the reflective 430 surface of the collector, the glass shield (if present) and the absorber. Nothing can be done 431 to account for the first effect besides of tilting or rotating the PTSC constantly so as to keep it 432 perpendicularly oriented to the sun. 433 The thermal losses of the collector receiver depend on operating temperature. The thermal 438 losses through PTSC change in different ways depending on the receiver's configuration and 439 operational conditions. As shown in Figure 4, the convection loss from the absorber tube to 440 supporting structures is the largest. Follows the radiation loss from glass envelope to 441 ambient air; while the smaller loss is the convection loss from glass envelope to ambient air. 442 Thermal losses are always present if there is a temperature difference between receiver and 443 ambient whenever solar radiation is available or not. Increased solar radiation results in increased solar energy absorbed by the collector. It 450 should be noted that thermal losses also increase due to the increased collector 451 temperature. However, this increase is smaller than the enhanced absorbed solar energy. 452 As shown from Figure  The angle of incidence modifier is a very significant factor impacting on the solar efficiency. It 458 is defined as the ratio between the transmittance-absorptance product for the angle of the 459 incidence of radiation and that at normal incident radiation. It is approximately equal to the 460 cosine of the angle of incidence. The angle of incidence modifier depends on time of day, 461 date, the location and orientation of the aperture, and whether the collector is stationary or 462 tracks the sun movement about one or two axes. Collector efficiency reaches a maximum 463 value only at zero angle of incidence. The efficiency of a PTSC decreases when the solar 464 beam angle of incidence increases as shown in Figure 6. The final (end) effect of the angle 465 of incidence is to reduce radiation arriving at the absorber tube. Therefore, at noon time 466 PTSC has the smallest angle of incidence and the highest efficiency.

484
A case study is carried out to investigate the performance of solar heating and cooling 485 systems in Kuwait. The solar system is designed to satisfy a significant part of the nominal 486 space heating load, water heating load and cooling load of a typical Kuwaiti house. A 487 schematic diagram of the system studied is represented in Figure 8. The thermal performance of a solar system is usually measured by the solar fraction (F). 555 Solar fraction is defined as the fraction of the load met by solar energy. Figure 9 shows the 556 variation of the solar fraction of space heating (F s ), domestic water heating (F D ), and cooling 557 load (F Ac ) with collector area. As seen from the figure, the solar space heating load and 558 domestic water heating load are completely satisfied for areas around 29 m 2 . Conversely, 559 the space cooling load requires much greater areas. The minimum required collector area is 560 about 82 m 2 which can supply the cooling load of a typical residential house for sunshine 561 hours under all hot climatic conditions in Kuwait, with a maximum cooling load of 562 approximately 21 KW (6 ton refrigeration). The coefficient of performance (COP) of the absorption chiller is approximately 0.66 which is 569 within the accepted practical values of the conventional lithium bromide system. 570 A program is written to determine the yearly reduced CO 2 emission as a result of integrating 571 parabolic trough collectors on the house roof. The annual variation of CO 2 decreased 572 emission vs. collector area is presented in Figure 10. This paper investigates the performance of parabolic trough collectors as well as solar 601 heating and cooling systems in Kuwait climate. A theoretical model that takes into 602 consideration the factors ignored in previous models is proposed. Based on present results, 603 the following conclusions can be drawn: 604 • The performance of parabolic trough collectors can be significantly enhanced by 605 optimizing its parameters as well as operating conditions. 606 • Convection loss from the absorber tube to the supporting structures is the largest 607 among the other losses (conduction and radiation). 608 • The angle of incidence modifier is an important factor impacting on the solar 609 efficiency. 610 • At noon time, PTSC has the smallest angle of incidence and the highest efficiency.

611
• When the annulus between the receiver surface and the glass envelope is in vacuo,

612
conduction and convection across the annulus are effectively eliminated. 613 • Space heating load and domestic water heating load can be completely provided by 614 parabolic trough collectors. 615 • The minimum required parabolic trough collector area is about 82 m 2 to supply 616 cooling loads of a typical residential house under all climatic conditions in Kuwait. 617 • Total reduction in CO 2 emission may reach to some 12.3 tonne/year.

618
• The results of the present study should encourage widespread utilization of solar 619 energy systems which will help in keeping our environment healthy and clean. 620