SOLAR PHOTOCHEMISTRY TECHNOLOGY

Transcription

1 SOLAR PHOTOCHEMISTRY TECHNOLOGY Centro de Investigciones Energétics, Mediombientles y Tecnológics PLATAFORMA SOLAR DE ALMERÍA SUMMARY Solr photochemicl detoxifiction technologies cn provide the environmentl wste mngement industry with powerful new tool to destroy wste with clen energy from the sun. Solr collectors re trditionlly divided into three ctegories: non-concentrting, medium concentrting nd high concentrting. Concentrting solr systems mke use of direct rdition nd need solr trcking mechnisms. Non-concentrting systems re much simpler s they do not need solr trcking nd cn collect direct nd diffuse solr rdition with slightly lower yerly efficiencies. The specific hrdwre needed for solr photoctlytic pplictions is very similr to tht used for conventionl therml pplictions with the following min differences: the fluid must be exposed to the ultrviolet solr rdition, so the bsorber must be trnsprent to this rdition nd no therml insultion is required s the temperture does not ply significnt role in the photoctlytic process. Non-concentrting solr collectors re the choice for solr photoctlytic pplictions. They re more efficient thn concentrtor-bsed systems due to the use of both direct nd diffuse UV light nd their intrinsic simplicity. The CPC (sttic collectors with reflection surfce following n involute round cylindricl rector tube) re very interesting cross between trough concentrtors nd one-sun systems nd hve been found to provide the best optics for low concentrtion systems. Aluminium is the only metl surfce tht offers high reflectivity vlues in the UV spectrum. Photoctlytic rectors must be both trnsmissive nd resistnt to UV light. Common mterils tht meet these requirements re fluoropolymers, crylic polymers nd borosilicte glss nd tubulr photorectors designs re the best option. In TiO 2 heterogeneous photoctlysis, suspended ctlyst systems give efficiencies higher thn supported ctlysts. After their use, titni powders cn be gglomerted nd sedimented. Contents list 4.1 Introduction 4.2 Solr collectors for photochemicl processes 4.3 Peculirities of solr trcking nd non-trcking systems 4.4 Technologicl issues Chpter 4 1

2 4.1 INTRODUCTION Solr photochemistry technology cn be defined s the technology tht efficiently collects solr photons nd introduces them in n dequte rector volume to promote specific chemicl rections. The equipment tht performs this function is denominted solr collector. Trditionlly, solr collector systems hve been clssified into three types depending on the level of concentrtion ttined by them, which is directly relted with the chievble system temperture: Non concentrting or low-temperture, up to 150º C Medium concentrting or medium temperture, from 150º C to 400º C High concentrting or high temperture, over 400º C. Non-concentrting collectors re sttic. Usully, they re flt pltes, often imed t the sun t specific tilt, depending on the geogrphic loction. Their min dvntge is their simplicity nd low cost. An exmple is trditionl domestic hot-wter technology. Medium concentrting collectors concentrte sunlight between 5 nd 50 times, so continuous trcking of the sun is required. Prbolic Trough Collectors (PTC) nd hologrphic collectors (Fresnel lenses) re in this group. The first hve prbolic reflecting surfce tht concentrtes the rdition on tubulr receiver locted in the focus of the prbol. They my be one-xis trcking, either zimuth (est-west movement round north-south-oriented xis) or elevtion (north-south movement round n est-west-oriented xis), or two-xis trcking (zimuth + elevtion). Fresnel lens collectors consist of refrcting surfces (similr to convex lenses), which devite the rdition t the sme time they concentrte it onto focus. Figure 4.1. Medium concentrtion solr collector. One-xis prbolic trough collector (PSA, Spin) Chpter 4 2

3 High concentrting collectors hve focl point insted of liner focus nd re bsed on prboloid with solr trcking. Typicl concentrtion rtios re in the rnge of 100 to nd precision opticl elements re required. They include prbolic dishes nd solr furnces. Figure 4.2. High concentrtion solr collector. Prbolic dish solr rector (PSA, Spin) As temperture usully does not ply relevnt role in solr photochemicl processes, the ssocited technology is bsed on non-concentrting nd medium concentrting solr collectors. An importnt difference mong the two ctegories is tht non-concentrting solr technology cn profit both direct nd diffuse rdition nd concentrting solr technology only the direct one. Direct rdition is the rdition tht hs no interference with the tmosphere nd, consequently, known direction, nd cn therefore be concentrted. Diffuse rdition is the rdition tht hs interference with the tmosphere prticles nd reches the erth surfce with rndom direction. Globl rdition is composed of direct nd diffuse rdition. The concentrtion rtio (CR) cn be defined s the rtio of the collector perture re to the bsorber or rector re. The perture re is the re intercepting rdition nd the bsorber re is the re of the component (either fully illuminted or not) receiving the solr rdition. This trditionl clssifiction considers only the therml efficiency of the solr collectors. Solr therml nd thermochemicl processes re bsed on the collection nd concentrtion of lrge number of photons from ll wvelengths to chieve specific rnge of temperture, in Chpter 4 3

4 opposition to solr photochemicl processes, which re bsed on the collection of only high energy photons from short wvelengths to promote photochemicl rections. The mjority of solr photochemicl process uses the UV or ner-uv solr light (300 to 400 nm), but some photochemicl synthesis process cn bsorb useful solr light up to 500 nm nd the Photo- Fenton heterogeneous photoctlysis use sunlight up to 580 nm. Solr light of wvelength higher thn 600 nm is normlly not vlid for ny photochemicl process. Nevertheless, the specific hrdwre needed for solr photochemicl pplictions hve much in common with those used for therml pplictions. As result, both photochemicl systems nd rectors hve followed conventionl solr therml collector designs, such s prbolic troughs nd non-concentrting collectors. At this point, their designs begin to diverge, since: - the fluid must be directly exposed to solr rdition nd, therefore, the bsorber must be trnsprent to the photons, nd - temperture usully does not ply significnt role in photochemicl processes, so no insultion is required. The mjority of photochemicl processes tke plce in liquid phse, so the technology is minly ddressed to hndle photochemicl rections tht occur in wter or solvent medium. There re lso gs phse photochemicl processes nd their ssocited technology is discussed t the end of this rticle SOLAR COLLECTORS FOR PHOTOCHEMICAL PROCESSES Prbolic Trough Collectors Solr photorectors for photochemicl pplictions were originlly designed for use in linefocus prbolic-trough concentrtors. This ws in prt becuse of the historicl emphsis on trough units for solr therml pplictions. Furthermore, PTC technology ws reltively mture nd existing hrdwre could be esily modified for photochemicl processes. There re two types of PTCs: ) One-xis prbolic trough b) Two-xis prbolic trough The first engineering-scle solr photochemicl fcility to wter detoxifiction ws developed in 1989 by Sndi Ntionl Lbs (USA) using one-xis PTCs nd the second by CIEMAT in 1990 (Spin) using two-xis PTCs. Both fcilities re considerbly lrge pilot plnts (hundreds of squre meters of collecting surfce) nd cn be considered the first steps in industriliztion of the photochemicl processes. Two-xis PTCs consist of turret on which there is pltform supporting severl prllel Chpter 4 4

5 prbolic trough collectors with the bsorber in the focus. The pltform hs two motors controlled by two-xis (zimuth nd elevtion) trcking system. Thus the collector perture plne is lwys perpendiculr to the solr rys, which re reflected by the prbol onto the rector tube t the focus through which the contminted wter to be detoxified circultes. Onexis PTCs hve only one motor nd one-xis solr-trcking system; the rector tube (liner focus of the prbol) is then positioned in the sme plne contining the norml vector of the collector perture plne nd the solr vector. The ngle formed by these two vectors is clled the incident ngle of solr rdition. Sun Pth Prbolic mirror Direct Norml Rdition Focus The Solr Collector rottes long its xis trcking the sun Morning Afternoon Figure 4.3. Solr ry reflection on one-xis prbolic trough collector The eqution of the PTC prbol is: 2 x y = (4.1) 4 f where f is the focl length. If D is the perture width nd d, the rector tube dimeter, the geometric concentrtion of the collector C is: D C = (4.2) π d The bsic components of prbolic-trough collector for photochemicl pplictions re the reflecting concentrtor, the bsorber tube (photorector), the drive-trcking system nd the overll structure. After opticl losses hve been considered, the effective concentrting rtio of PTCs is usully between 5 nd 20. Typicl overll opticl efficiencies in PTC re in the rnge of 50 to 75 percent, with the following brekdown: Trcking system: 90%-95% Reflector/Concentrtor (reflectivity): 80%-90% Chpter 4 5

6 Absorber/Rector (trnsmittnce): 80%-90% Mechnicl collector errors: 90%-95% Prbolic-trough collectors mke efficient use of direct solr rdition nd, s n dditionl dvntge, the therml energy collected from the concentrted rdition could be used in prllel for other pplictions. The size nd length of the rector is smller, receiving lrge mount of energy per unit of volume, so hndling nd control of the liquid to be treted is simpler nd cheper. This cn lso be trnslted into rector ble to withstnd higher pressures One-Sun Collectors One-sun (non-concentrting) collectors (CR = 1) re, in principle, cheper thn PTCs s they hve no moving prts or solr trcking devices. They do not concentrte rdition, so the efficiency is not reduced by fctors ssocited with concentrtion nd solr trcking. Mnufcturing costs re cheper becuse their components re simpler, which lso mens esy nd low-cost mintennce. Also, the non-concentrting collector support structures re esier nd cheper to instll nd the surfce required for their instlltion is smller, becuse since they re sttic they do not project shdows on the others. Bsed on extensive effort in the designing of smll non-trcking collectors, wide number of non-concentrting solr rectors hve been developed for solr photochemicl pplictions in generl nd specilly for solr photoctlytic processes. These cn be clssified s follows: - Trickle-down flt plte, bsed on tilted plte fcing the sun over which the process fluid flls slowly; ctlyst is normlly fixed on plte surfce. - Free-flling film, similr to the trickle-down flt plte, but with higher flow rte nd normlly with ctlyst ttched to the surfce on which the process fluid circultes. It is usully open to the tmosphere. - Pressurized flt plte, consisting of two pltes between which fluid circultes using seprting wll. - Tubulr, consisting of mny smll tubes connected in prllel to mke the flow circulte fster thn flt plte. - Shllow solr ponds. Smll on-site built pond rectors hving little depth. Although one-sun designs possess importnt dvntges, the design of robust one-sun photorector is not trivil, due to the need for wether-resistnt nd chemiclly inert ultrviolet-trnsmitting rectors. In ddition, non-concentrting systems require significntly more photorector re thn concentrting photorectors nd, s consequence, full-scle systems (normlly formed by hundred of squre meters of collectors) must be designed to withstnd the operting pressures nticipted for fluid circultion through lrge field. As Chpter 4 6

7 consequence, the use of tubulr photorectors hs decided dvntge becuse of the inherent structurl efficiency of tubing; tubing is lso vilble in lrge vriety of mterils nd sizes nd is nturl choice for pressurized fluid system. Finlly, its construction must be economicl nd should be efficient with low-pressure drop Compound Prbolic Concentrtor (CPC) CPC non-imging concentrtors, extensively employed for evcuted tubes, re n interesting cross between trough concentrtors nd one-sun systems nd re good option for solr photochemicl pplictions. CPCs re sttic collectors with reflective surfce following n involute round cylindricl rector tube nd hve been found to provide the best optics for low concentrtion systems; it cn be designed with CR=1 (or ner one), then hving the dvntges of both PTCs nd one sun collectors. Figure 4.4. Solr reflection on CPC collector Thnks to the reflector design, lmost ll the UV rdition rriving t the CPC perture re (not only direct, but lso diffuse) cn be collected nd is vilble for the process in the rector. The light reflected by the CPC is distributed round the bck of the tubulr photorector illuminting most of the rector tube circumference. Due to the rtio of CPC perture to tube dimeter, the incident light on the rector is very similr to tht of one-sun photorector, being performnce close to tht of the simple tubulr photorector. As in prbolic trough, the wter is more esily piped nd distributed thn in mny one-sun designs. All these fctors contribute to excellent CPC collector performnce in solr photochemicl nd photoctlytic pplictions. Chpter 4 7

8 The explicit eqution for CPC reflector with tubulr rector cn be obtined from Figure 4.5; generic reflector point S cn be described in terms of two prmeters, ngle θ, subtended by lines originting t O (centre of the rector tube) to A nd R, nd distnce ρ, given by segment RS: θ = OA < OR (4.3) ρ = RS (4.4) RS being tngent to the rector tube t R. One importnt prmeter for CPC definition is the ngle of cceptnce 2θ, which is the ngulr rnge over which ll or lmost ll rys re ccepted (i.e., reflected into the rector tube) without moving the collector. y θ O r R θ θ C x S A B Figure 4.5. Obtining of CPC involute The solution is given in two seprte portions, n ordinry involute for A to B nd n outer portion from B to C: ρ = rθ for θ θ + π 2 prt AB of the curve (4.5) ρ θ + θ + π 2 cos( θ θ ) ( θ θ ) π = r for θ + θ θ prt BC of the curve (4.6) 1+ sin 2 2 The CPC concentrtion rtio (CR) is given by: 3π 1 C = (4.7) sinθ In the specil cse of θ =90º, CR=1 nd every CPC curve is n ordinry involute (points B nd C re coincident). Optimum CPC cceptnce hlf-ngles ( θ ) for photochemicl pplictions re obtined from 60 to 90 degrees either side of the norml. This wide Chpter 4 8

9 cceptnce ngle llows the reflector to direct both direct-norml nd diffuse sunlight onto the rector with the dditionl dvntge tht these wide cceptnce reflectors llow the reflectortube lignment errors, which is n importnt virtue for low-cost photorector rry. Figure 4.6. View of photorector rry. PSA (Spin) CPC reflectors re usully mde of polished luminium nd the structure cn be simple photorector support frme with connecting tubing. Since this type of reflector is considerbly less expensive thn tubing, their use is very cost-effective compred to deploying nonconcentrting tubulr photorectors without use of ny reflectors, but preserving the dvntges of using tubing for the ctive photorector re. 4.3 PECULIARITIES OF SOLAR TRACKING AND NON-TRACKING SYSTEMS One of the most importnt rector design issues is the selection between solr trcking nd non-trcking devices. Trcking systems purpose is to concentrte sunlight so they re normlly ssocited to concentrting systems. They cn only use direct solr irrdition becuse is the only one with known vector nd they re needed in therml pplictions when tempertures higher thn 150 C re required. Concentrting systems hve the dvntge of much smller rector-tube re, which could men shorter circuit in which to confine, control nd hndle the process fluid. Also, the lterntive of using high-qulity ultrvioletlight-trnsmitting rectors nd supported-ctlyst devices seems more logicl, both economiclly nd from n engineering point of view, if concentrting collector systems re used. Nevertheless, trcking rectors hve two importnt disdvntges compred to the nontrcking ones. The first is tht they cnnot concentrte (i.e., use) diffuse solr rdition. This is not importnt in the cse of solr therml pplictions, becuse diffuse rdition is smll frction of the totl solr rdition, but becomes primrily importnt with photochemicl Chpter 4 9

10 pplictions s the UV prt of the solr spectrum plys mjor role. The reson to this is the fct tht solr UV light is more susceptible to scttering by tmospheric gses, minly wter vpor, thn visible light (the sme mechnism sctters blue light more thn red light, which is wht cuses the sky to pper blue). Becuse of this scttering, s much s hlf of the UV rdition rrives t the erth s surfce s diffuse light, even on cler dy. Ner-UV wvelengths (from 285 to 385 nm) comprise only 2-3% of the energy in direct sunlight, but they mke up 4-6% of combined diffuse nd direct sunlight. Thin clouds, dust, nd hze reduce the direct-bem component of sunlight more thn the diffuse component. As sttic non-trcking solr collectors cn mke use of both direct nd diffuse UV rdition, their efficiency cn be noticebly higher. The second importnt disdvntge of solr trcking collectors is their higher complexity, cost nd mintennce requirements. In ddition, nontrcking collectors hve higher potentil for mnufcturing cost reduction Dily integrted irrdince x cos θ (kj m -2 ) Totl N-S E-W Lt Yerly dt nd percentul vlue: Totl 3649 kwh.m -2 (100%) N-S 3119 kwh.m -2 (85%) E-W 2792 kwh.m -2 (76%) Lt 2556 kwh.m -2 (70%) Hor 1986 kwh.m -2 (54%) Pltform Solr de Almeri Ltitude: 37,0 North Longitude: 2,3 West Hor Jn Feb Mr Apr My Jun Jul Aug Sep Oct Nov Dec 0 Months Figure 4.7. Yerly efficiency of solr trcking collectors [N-S: one xis PTC with zimuth trcking (North-South); E-W: one xis PTC with elevtion trcking (Est-West)], compred with yerly efficiency of non-trcking solr collectors [Lt: sttic flt plte with inclintion equl to locl ltitude (37º); Hor: horizontl sttic flt plte]. Totl: mximum vilble solr irrdince. Obviously, the dvntge of trcking systems is their higher efficiency in collecting solr photons s they follow the sun trjectory over the sky. In order to illustrte the reltive performnce of trcking nd non-trcking devices. Figure 4.7 shows comprtive nlysis of the efficiency of different solr collectors with regrd to direct incident rdition. The dt represented in Figure 4.7 correspond to direct rdition in n idel cloudless yer (bsed on verge meteorologicl dt on sunny dys t the Pltform Solr de Almerí, Spin, fter Chpter 4 10

11 discrding cloudy dys) nd show the energy vilble from direct rdition on the perture plne of the following collector systems: - One-xis prbolic-trough collector with zimuth trcking (Est-West movement round North-South-oriented-xis). Sun trcking collector. Mximum yerly efficiency: 85%. - One-xis prbolic-trough collector with elevtion trcking (North-South movement round n Est-West-oriented xis). Sun trcking collector. Mximum yerly efficiency: 76%. - Sttic flt plte (one sun collector) with inclintion equl to locl ltitude (37º in the cse of PSA). Non- trcking collector. Mximum yerly efficiency: 70%. - Sttic flt plte (one sun collector) with no inclintion (horizontl). Non- trcking collector. Mximum yerly efficiency: 54%. The clcultions performed re geometric nd bsed on the cosine of the incident ngle. This ngle is the formed by the solr ry with the line norml to the perture plne of the collector nd llows to know the mount of direct rdition vilble t ny given time for ech collector configurtion. In Figure 4.7, it cn be observed tht the nnul efficiency of zimuth trcking is bout 10% better thn elevtion trcking. In the first cse, this efficiency increses notbly in the summer nd decreses in the winter (identicl in the North nd South Hemispheres) wheres it is lmost constnt round the yer in the second cse. In the cse of sttic non-trcking collectors, it my be observed tht efficiencies re lower thn one-xis PTCs, ttining mximum efficiency with n inclintion (to the South in the Northern Hemisphere nd to the North in the Southern Hemisphere) from the horizontl equl to the locl ltitude. This configurtion, tht is, ngle of tilt set t the ngle of ltitude of the site, mximizes the nnul energy collection in flt-plte collector. Although the clcultions mde here re for specific loction nd ltitude, the comprisons of solr rdition collection nd conclusions obtined re qulittively vlid for ny other loction. As it cn be pprecited, differences on the yerly efficiency re not very high. This, together with the previously mentioned disdvntges of trcking systems, clerly fvors the use of sttic non-trcking rectors for solr photochemicl pplictions. 4.4 TECHNOLOGICAL ISSUES Most of the components of solr photochemicl systems re stndrd mterils with no specil requirements. The exceptions re the photochemicl rector, piping, the solr reflection surfce nd the technologicl issues relted with the employ of ctlyst or sensitizer, s lmost ll photochemicl processes use some of them to promote the chemicl rection. Chpter 4 11

12 Photochemicl Rector The requirements for solr photochemicl rector re similr to ny other photochemicl rector, with the prticulrity tht the light will cme from the sun. The photochemicl rector must contin the working fluid, including the ctlyst or sensitizer, nd must trnsmit solr UV light efficiently with miniml pressure drop cross the system. Also, it must provide good mss trnsfer from the fluid strem to n illuminted photoctlyst or sensitizer surfce in order to hve rection rte s higher s possible. As mentioned before, sttic non-trcking solr devices provides good photo-efficiencies, leding to flt-plte geometry. This geometry is widely used for solr-powered domestic hot wter heter systems in lrge prt becuse of its simple design nd it cn be lso trnslted to photochemicl processes. Adequte flow distribution inside the rector must be ssured, s non-uniform distribution leds to non-uniform residence times inside the rector, resulting in decresed performnce compred to n idel-flow sitution. When lrge rry of solr collectors re going to be used, such s the cse of tretment of wter contminnts by solr photoctlysis, the rector must be hrd enough to work under usble wter pressure. Tube configurtions clerly seem the most pproprite for fluid continment nd pumping when lrge volumes re to be processed. The choice of mterils tht re both trnsmissive to UV light nd resistnt to its destructive effects is limited. Tempertures inside solr photochemicl rectors cn esily rech 40 to 50 C, just in the cse of non-concentrting or one-sun rector, due to the bsorption of the visible portion of the solr spectrum. Therefore, photochemicl rectors must be ble to withstnd summer tempertures of round 70 to 80 C in order to insure tht there will be no dmge, which could reduce the flow. In the cse of concentrting systems, the rector temperture will be higher in function of the concentrtion degree. In ddition, rector mteril must be inert with regrd to the chemicls tht must be contined nd low ph resistnce would be needed in some specific pplictions, such s solr photoctlytic detoxifiction due to the production of inorgnic cids s rection by-products (i.e. the destruction of chlorinted hydrocrbons leds to the production of HCl). Common mterils tht meet these requirements re fluoropolymers, crylic polymers nd severl types of glss. Qurtz hs excellent UV trnsmission nd temperture nd chemicl resistnce, but the slight dvntge in trnsmission in the terrestril solr spectrum over other mterils does not justify its high cost, which mkes it completely unfesible for pplictions requiring lrge rector volumes. Fluoropolymers re good choice for photorectors due to their good UV trnsmittnce, excellent ultrviolet stbility nd chemicl inertness. Fluoropolymer mterils trnsmit light Chpter 4 12

13 s diffuse re poor IR-diffusers, but mke n excellent visible / UV diffusers. Tubulr fluoropolymers cn be extruded into tubing nd used s photorector, re very strong nd possess excellent ter resistnce nd re flexible nd lighter thn glss. One of their gretest disdvntges is tht, in order to chieve desired minimum pressure rting, the wll thickness of fluoropolymer tube my hve to be incresed, which in turn will lower its UV trnsmittnce. In ddition, due to the lck of rigidity, tube connections cn withstnd much lower pressures thn glss tubes. ETFE (ethylenetetrfluoroethylene) nd FEP (fluorinted ethylenepropylene) re good cndidtes; ETFE hs higher tensile strength thn FEP, which could men thinner-wlled tubes nd higher UV trnsmittnce, resulting in cost svings since less mteril is used nd higher photorector performnce. Acrylics could lso potentilly be used s photorector mteril. Low-cost polymers re vilble in tube form, but none possess the necessry UV nd chemicl stbility for mny photochemicl processes. QUARTZ Trsmissivity, % PYREX DURAN PTFE Commercil glss Wvelength, nm Figure 4.8. Trnsmittnce of different mterils suitble for the mnufcture of photorector tubes Glss is n lterntive for photorectors. Stndrd glss, used s protective surfce, is not be stisfctory becuse it bsorbs prt of the UV rdition tht reches it, due to its iron content. Borosilicte glss hs good trnsmissive properties in the solr rnge with cut-off of bout 285 nm. Therefore, such low-iron-content glss would seem to be the most dequte. Two undesirble effects reduce the trnsmittnce of glss rector in the solr UV spectrum: incresed bsorption in the rnge between 300 nd 400 nm nd further decrese of UVtrnsmittnce during opertion due to the dmging impct of solr rdition in the sme wvelength region (UV-solristion). Both effects re cused to lrge extent by polyvlent ions tht chnge chrge; Fe-ions in the glss chnge their chrge from Fe 2+ to Fe 3+ due to photo-oxidtion by photons, nd the oxidised Fe 3+ ion bsorbs in the UV. As result, enhncement of trnsmittnce in the nm region could only be ccomplished by Chpter 4 13

14 strong reduction in iron content, but penlised by corresponding increse in cost. Therefore, s both fluoropolymers nd glss re vlid photorector mterils, cost becomes n importnt issue. From the perspective of performnce, the choice is the mteril tht hs the best combintion of tensile strength nd UV trnsmittnce. If lrge field is being designed, lrge collector re mens lso considerble number of rectors nd, s consequence, high system pressures Reflective Surfces The opticl qulity requirements of reflective surfces for solr pplictions re usully relted to the concentrtion required by the prticulr ppliction under considertion. The higher the concentrtion desired the stricter the requirements for qulity of prmeters. Light reflected off polished or mirrored surfce obeys the lw of reflection: the ngle between the incident ry nd the norml to the surfce is equl to the ngle between the reflected ry nd the norml. When light reflects off rer surfce mirror, the light first psses through the glss substrte, resulting in reflection losses, secondry reflections, refrction, bsorption, nd scttering of light pssing through the trnsprent substrte (second-surfce mirrors). Precision opticl systems use first-surfce mirrors tht re luminized on the outer surfce to void these phenomen. When light obeys the lw of reflection, it is termed speculr reflection. Most hrd polished (shiny) surfces re primrily speculr in nture. Even trnsprent glss speculrly reflects portion of incoming light. Diffuse reflection is typicl of prticulte substnces like powders. If you shine light on bking flour, for exmple, you will not see directionlly shiny component. The powder will pper uniformly bright from every direction. Mny reflections re combintion of both diffuse nd speculr components. One mnifesttion of this is spred reflection, which hs dominnt directionl component tht is prtilly diffused by surfce irregulrities (Figure 4.9). Spred Speculr Diffuse Figure 4.9. Speculr, diffuse nd spred reflection from surfce Chpter 4 14

15 In the cse of solr photochemicl pplictions, the strictest requirements re those of PTCs, for exmple, UV-mirror mterils need to hve speculr reflectnce between nm in order to chieve concentrtion rtios from 1 to 20. The greter the errors re, nd prticulrly the reflective surfce errors, the lower the effective concentrtion rtio is. So, the reverse is lso true: the lower the effective concentrtion rtio is, the higher the opticl errors my be nd therefore, the lower the qulity of reflective surfce required. This is n importnt dditionl fctor in fvor of low or non-concentrting systems, since these lower qulity requirements (lower speculr reflectnce) re directly trnslted into lower mnufcturing cost, since the reflector element cn represent considerble frction of collector cost. Solr Irrdince (W m -2 nm -1 ) 1,5 1,4 1,3 1,2 1,1 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 4 7,30 7,11 7,03 7,00 0, Wvelength (nm) I ( λ ) dλ I ( λ ) dλ Figure Avilble solr spectrum for photochemicl processes. Reltionship of useful light for TiO 2 ( nm) nd Photo-Fenton ( nm) photoctlytic processes With regrd to the reflector/concentrtor nother importnt fctor is the reflective bse mteril. Normlly, the UV component of the spectrum is relevnt to the process nd then luminium is the best option due to its low cost nd high reflectivity. Nevertheless, there re processes which uses much wider portion of the terrestril spectrum (Figure 4.10) nd it cn use conventionl silver mirrors. Silver mirrors re specilly recommended when concentrtor devices re employed s, in these cses, higher qulity is required in the reflection surfce. However, the mjority of the solr photochemicl processes required highly reflective mteril for ultrviolet rdition. The reflectivity between 300 nd 400 nm of trditionl silver-coted mirrors is very low (reflected rdition/incident rdition) nd luminium-coted mirrors is the best option in this cse s fresh luminium metl cotings for mirrors hve the highest reflectivity. Aluminium is the only metl surfce tht is highly reflective throughout the ultrviolet spectrum. Reflectivities rnge from 92.3 percent t 280 nm. to 92.5 percent t 385 nm. Comprble vlues for silver re 25.2 percent nd 92.8 percent, respectively. A new deposited luminium surfce is frgile nd needs to be protected from wethering nd Chpter 4 15

16 brsion, but the conventionl glss cover used for silver-bcked mirrors hs the drwbck of significntly filtering UV light (n effect tht is duplicted due to the light pth through the glss). The thin oxide lyer tht forms nturlly on luminium is not sufficient to protect it in outdoor environments. Under such exposure conditions, the oxide lyer continues to grow nd UV reflectnce drops off drmticlly. The idel reflective surfce for solr photochemicl pplictions must hve high reflectivity in the UV rnge, cceptble durbility under outdoors conditions for extended service lifetimes nd resonble price. The surfces currently vilble tht best fit these requirements re electropolished nodized luminium (electrolyticlly formed luminium oxide outer lyer) nd orgnic plstic films with n luminium coting. Anodized cotings with tin oxide cn provide good protection ginst some chemicls nd good resistnce to brsion. Typiclly, thin oxide lyers (2-3 µm) re used to provide some mesure of resistnce to brsion but little protection ginst moisture or pollutnts is provided. Thicker oxide lyers (up to 50 µm) re usully specified when nodized luminium is intended for engineering/mrine pplictions but resulting in considerbly lower reflectnce. An interesting lterntive pproch is to cover the luminium with protective crylic lcquer. In both cses, compromise between outdoor resistnce nd UV reflectnce must be chieved. Another possible solution is n luminium-coted plstic film. Severl commercil coted plstic film products hve been used successfully in prbolic trough pplictions. Due to their lck of rigidity, these films must be bonded over stiff substrte nd bout two percent speculr reflectivity is lost in this process. Also, the reflectivity of ech film t the end of its lifetime (from 5 to 10 yers) would be only 88 percent of the new bonded vlue Piping Most piping my be mde of polyvinylidene fluoride (PVDF), chlorinted polyvinyl chloride (CPVC), or simply polyethylene. In ny cse, piping, s well s the rest of the mterils, must be resistnt to corrosion by the originl contminnts nd their possible by-products in the destruction process. As well s in the cse of rectors, rective mterils tht could interfere with the photochemicl process must be voided. All mterils used must be inert to degrdtion by UV solr light in order to be comptible with the minimum required lifetime of the system (10 yers). All pipes, rector nd connection devices must be strong enough to withstnd the necessry wter-flow pressure. Typicl prmeters re 2 to 4 br for nominl system pressure drop nd mximum of 5 to 7 br. Concentrting system mterils must lso be ble to withstnd possible high tempertures tht could result from bsorption of concentrted visible nd infrred light in the rector. Chpter 4 16

17 4.4.4 Rdition bsorption An importnt peculirity of solr photochemicl systems is the requirement of n intermedite element to bsorb the useful solr light. Rdition is normlly bsorbed nd trnsferred to the photochemicl process by ctlyst or sensitizer. Depending on the nture of the photoctlyst/sensitizer, the process cn be either homogeneous or heterogeneous. The ctlyst or sensitizer plys mjor role, not only becuse of its importnce to the process, but lso from technologicl point of view. This is especilly relevnt in heterogeneous photochemicl processes or when the ctlyst/sensitizer is used supported or deployed over the photorector. Supported ctlyst/sensitizer configurtions eliminte the need for ctlyst/sensitizer recupertion, but with the min objection of n importnt reduction in system efficiency. This ide requires the ctlyst/sensitizer to be nchored onto some type of inert support inside the rector. As the ctlyst/sensitizer must be exposed to sunlight nd in contct with the rection medium, the support must be configured to efficiently route the rectnts to the illuminted zone nd, t the sme time, mintin high flow rte in the fluid to ensure good mixing without significntly incresing system pressure. Supports tested so fr hve included fibreglss beds, metl fibres, steel mesh, luminium, nd mny types of plstic nd cermics such s lumin, silicon crbide nd silic, in the most diverse shpes. Some exmples of fesible techniques utilized to support the ctlyst/sensitizer re dip-coting with solvents, deposits from precursors, vpor deposition, sol-gel formtion, etc. Desirble chrcteristics of such system would include being very ctive (comprble to homogeneous systems), hve low pressure-drop, long lifetime, nd resonble cost. To present, the chievement of these chrcteristics hs not been possible. In ddition, n importnt question is how long supported ctlysts/sensitizer will lst in the fluid strem; short period of ctivity would men frequent replcement nd, consequently, n importnt rise in the overll system cost. Nevertheless, when it is very difficult to remove the ctlyst/sensitizer from the rection medium fter the completion of the process, its supporting cn not be voided. By opposition to this, homogeneous nd slurry configurtions hve the dvntge of higher throughputs low pressure-drop through the rector nd excellent fluid-to-ctlyst mss trnsfer. When the ctlyst/sensitizer cn be esily removed from the rection medium, the use of homogeneous nd slurry systems reduced in very importnt wy the size of necessry solr collector field, mking the overll system clerly more cost efficient nd competitive thn supported systems. Another importnt design prmeter, in the cse of tubulr photorectors, is the dimeter s in both homogeneous or heterogeneous processes it must be gurnteed tht ll rriving useful Chpter 4 17

18 photons re kept inside the rector nd do not go through it without intercepting rdition bsorption trget prticle. The intensity of illumintion ffects the reltionship between rection rte nd ctlyst/sensitizer concentrtion. The dispersion nd bsorption of light cuses photon density to diminish lmost exponentilly over the length of the opticl pth within ctlyst suspension. At higher light intensity, the ctlyst/sensitizer concentrtion cn be higher. When ctlyst/sensitizer concentrtion is very high, screening effect produces excessive opcity of the solution, preventing the prticles frthest in from being illuminted nd reducing system efficiency. The lower the ctlyst/sensitizer concentrtion is, the less opque the suspension. As n exmple, in the cse of titnium dioxide photoctlysis, 1 g L -1 of TiO 2 ctlyst reduces trnsmittnce to zero in 10-mm-inner-dimeter cylinder with concentrted light in prbolic trough collector. Therefore, in wider dimeter tube, only n outer lyer is illuminted. This mens tht lrger inner rector dimeter permits use of lower optimum ctlyst concentrtions. Dimeters tht re very smll do not mke sense becuse of the ssocited high pressure-drop nd very lrge dimeters imply considerble drk volume, thus reducing overll system efficiency. This mens tht the prcticl inner dimeters for tubulr photorector must be optimized to ny specific process tking into ccount ll the relevnt fctors. Finlly, in the cse of heterogeneous processes such s the TiO 2 photoctlysis, it is importnt to design the system voiding ny possibility of ctlyst settlement. To this end, the Reynold number (Re) must lwys be over 4000 in order to gurntee turbulent flow. BIBLIOGRAPHY Blke, D. M.; Mgrini, K.; Wolfrum, E.; My E.K. Mteril Issues in Solr Detoxifiction of Air nd Wter. Opticl Mterils Technology for Energy Efficiency nd Solr Energy Conversion XV, eds. Crl M. Lmpert, Clus G. Grnqvist, Michel Grtzel, nd Styen K. Deb, pp SPIE The Interntionl Society for Opticl Engineering, Blnco, J.; Mlto, S. et l. Finl Configurtion of PSA Solr Detoxifiction Loop. Pltform Solr de Almerí. Technicl Report: TR 06/ [Description of the engineering development to the instlltion of the first Europen pre-industril photoctlytic test fcility]. Fernández, P.; De ls Nieves, F.J.; Mlto, S. TiO 2 Sedimenttion Procedure. Proc. 2nd Users Europen Workshop TMR Progrmme t PSA, CIEMAT (Ed.), Mdrid, [Description of procedure to titni seprtion from slurry fter solr photoctlytic wter tretment]. Chpter 4 18

19 Jorgensen, G.; Rngprsd, G. Ultrviolet Reflector Mterils for Solr Detoxifiction of Hzrdous Wste, SERI/TP Solr Energy Reserch Institute, Golden, CO, DE [Extensive study on mterils suitble for UV solr collectors] My, E. K.; Gee, R.; Wickhm, D.T.; Lfloon, L.A.; Wright, J.D. Design nd Fbriction of Prototype Solr Receiver/Rectors for the Solr Detoxifiction of Contminted Wter, IST Corp., Golden, Colordo, Finl report to NREL. [Report on the development nd fbriction of photoctlytic wter tretment system]. Pcheco, K.; Wtt, A.S.; Turchi, C.S. Solr Detoxifiction of Wter: Outdoor Testing of Prototype Photorectors. ASME/ASES Joint Solr Energy Conference, eds. Alln Kirkptrick, nd Willim Worek, 43-49, New York, [Interesting comprtive pper of different solr collectors for photoctlytic wter tretment]. Rbl, A. Active Solr Collectors nd Their Applictions. Oxford University Press [Very useful book contining ll the bsic bckground on solr technology]. Wendelin, T. A Survey of Potentil Low-Cost Concentrtor Concepts for Use in Low- Temperture Wter Detoxifiction. ASME Interntionl Solr Energy Conference, eds. Willim B. Stine, Jn Kreider, nd Koichi Wtnbe, 15-20, New York, Chpter 4 19