Certificatio of ar glass Stefa Bruold (sbruold@areergy.ch), Ueli Frei (ueli.frei@areergy.ch), SPF-HSR, Oberseestrasse 10, CH-8640 Rapperswil, Switzerlad, Fax +41 (055) 2224844 Abstract The performace of a ar collector depeds o the compoets from which it is assembled. Besides the absorber, the most importat compoet is the collector cover, which i geeral is a glass pae. The optical properties of the glass pae directly affects the efficiecy of the collector ad thus the ar thermal system performace. To date, there has bee a tedecy to associate high quality ad superior optical properties with the expressio ar glass. However, a specificatio for ar glass has ot bee defied. By itroducig the ar glass certificatio system, a qualificatio of the glass by a glass efficiecy η Gl will be possible. The glass efficiecy is the product of 4 differet factors. The factors describe the ifluece of (1) the ar trasmittace, (2) the icidece agle modifier, (3) the photo-degradatio ad (4) other degradatio of the glass pae o the collector field efficiecy of a ar thermal system. Based o the glass efficiecy, the glass is assiged to a specific ar glass class reachig from 1 (highest performace) to 4 (lowest performace). Below a certai limit, the glass will ot be a certified ar glass. I order to take ito accout that treated glasses have a additioal risk of failure, these glasses are classified i separate groups. The beefit of ar glass certificatio will be twofold. First, the performace of a glass cover ca be simply evaluated by a collector maufacturer. Secod, glass covers which belog to a certai performace class ca be replaced by a equivalet or better type, without sigificatly reducig the system performace. 1. Itroductio Certificatio of ar glass [1], the glass used as a cover i ar thermal collectors, is iteded to improve trasparecy withi the market. O the oe had, the collector maufacturer should have clear criteria available to choose the most suitable glass. O the other had, malpractice i associatio with collector testig should be made more difficult. The associated label will prevet high-quality glass, which was istalled oly for collector testig purposes, from beig replaced by lower-quality glass durig productio.
I additio to complete documetatio of the glass performace, a advatage for the collector maufacturer is that differet suppliers or types of glass ca be chose. The cover i a tested collector ca be replaced with glass from the same performace class without sigificatly chagig the system performace. The performace of glass is determied by a umber of properties. I additio to the ar trasmittace for ormal icidece, the agle-depedet trasmissio properties (described by the icidece agle modifier, IAM) have a large effect. The sigificace of the IAM, which is essetially determied by the macroscopic structure, is ofte appreciably uderestimated. Aother importat factor is ageig due to photodegradatio. Additives used durig glass maufacture ca cause a sigificat reductio i the ar trasmittace whe glass is exposed to UV radiatio. If the glass pae is subjected to additioal treatmet such as etchig or coatig, the maufacturer must prove ad guaratee that the properties of the treated glass will ot chage sigificatly over 15 years of use as a collector cover. The effect of soilig is ot take ito accout. The glass efficiecy η Gl The "glass efficiecy" η GL is itroduced to allow the performace of ar glass to be evaluated simply. The glass efficiecy value describes the effect of the collector cover o the collector field yield for a referece ar system for domestic hot water, located at Rapperswil (typical Cetral Europea climate, logitude 8.82 E, latitude 47.23 N). The mai specificatios of the system studied are summarised i Table 1. Tab. 1: Data for the referece ar system used i the parameter study: c0, c1, c2 : coefficiets of the collector efficiecy curve K(IAM): icidece agle modifier (IAM) of the collector at 50 agle of icidece Collector data area 4 m 2 c 0 0.80 tilt agle 40 c 1 4.0 W / m 2 K orietatio 0 (south) c 2 0.01 W / m 2 K 2 System data tak volume 400 l locatio collector yield Rapperswil (CH) 2443 kwh ar fractio SFi app. 52% daily eergy demad 10 kwh K (IAM) 0.968 The glass efficiecy value is the product of sigle factors which describe the various glass properties. All of the factors, ad thus the glass efficiecy as well, are proportioality factors of the collector field yield:
η GL = F τ * F IAM * F UV * F DEG (1) F τ F IAM F UV trasmissio factor IAM weightig factor photodegradatio factor F DEG Degradatio factor. This factor describes the reductio i trasmissio after 15 years of ormal operatio i a flat-plate collector, without takig accout of soilig effects or photodegradatio. I geeral, F DEG = 1 ca be assumed for utreated glass (result of log-term ivestigatios of cover materials). Commet: At preset, o accelerated ageig procedures are kow to determie the degradatio factor of treated glass. However, efforts are curretly beig made withi the Solar Heatig ad Coolig Programme of the Iteratioal Eergy Agecy (IEA) to specify such a procedure. A glass efficiecy value of 1 would correspod to a fictioal collector cover with a ar trasmittace of 100 % ad a IAM correspodig to ustructured, low-iro glass, which suffered either photodegradatio or ay other type of degradatio over the course of time. The Polysu 3.3 [2] simulatio software was used to calculate the collector field yield which is 2443 kwh for the referece system. As the cover of the referece collector is a ustructured low-iro glass with a ar trasmittace of 90%, the glass efficiecy is equal to 0.90 (it will be show later that F τ = τ Sol ). Thus, it was assumed that this eergy correspods to a relative collector field yield of 0.90. All calculatios performed with differet collector covers are related to the referece system. Classificatio The followig classificatio is iteded to demostrate the glass properties i a simple form. The classificatio is desiged to meet the eeds both of the specialist (maufacturer, plaer etc.) ad the fial collector user. Each sigle glass pae which is a certified ar glass must be labelled with the class ame it belogs to. The differet performace classes are put together i Table 2 ad 3. Tab 2: Classificatio of utreated ar glass utreated glass Class U1 η GL > 0.900 Class U2 0.900 η GL > 0.885 Class U3 0.885 η GL > 0.870 Class U4 0.870 η GL > 0.850 Not ar glass 0.850 η GL
Tab. 3: Classificatio of treated ar glass treated glass (without AR) AR treated glass Class Z1 η GL > 0.900 Class X1 η GL > 0.950 Class Z2 0.900 η GL > 0.885 Class X2 0.950 η GL > 0.935 Class Z3 0.885 η GL > 0.870 Class X3 0.935 η GL > 0.920 Class Z4 0.870 η GL > 0.850 Class X4 0.920 η GL > 0.900 Not ar glass 0.850 η GL Not ar glass 0.890 η GL 2. The Trasmissio Factor F τ The trasmissio factor F τ quatifies the effect of the ar trasmittace o the collector yield of a thermal ar system. It is determied as follows: Fτ = τ (2) where τ is defied as the "direct-hemispherical ar trasmittace for ear-ormal icidece" of the collector cover. The referece ar spectrum is that for air mass 1.5, as specified for "hemispherical ar spectral irradiace" i ISO 9845-1. The relatioship preseted i equatio (2) was validated with a parameter study. The ar trasmittace τ of a ar system for domestic hot water was the parameter varied i the rage from 0.80 to 1.00. This correspods to a variatio of coefficiet c 0 of the collector efficiecy curve (table 1) betwee 0.711 ad 0.889. The results of the simulatio are summarised i Fig. 1. The calculated chages i the collector yield as a fuctio of the ar trasmittace are plotted as small circles. The assumed liear relatioship described i equatio (2) is plotted as a blue lie for compariso. The deviatio resultig from this simplified descriptio is plotted i red (dotted). As the iitial assumptio was, that the relative collector yield for τ = 0.90 correspods exactly to 90 % (see above), i.e. that the trasmissio factor should be F τ = 0.90 for τ = 0.90, the deviatio of the assumed liear depedece from the actual collector yield will be miimal for realistic trasmittace values (aroud 90 %). The deviatio betwee the liear approximatio ad the simulatio over the etire studied rage is betwee +0.4 % ad -0.05%. For ar trasmittace values of less tha 92 %, i.e. the rage for glass that has ot bee AR treated, the agreemet is still better, ±0.05 %. The costatly positive deviatio above a ar trasmittace of 92 % leads to a slight systematic overestimatio of AR-treated glass as compared to utreated glass.
Fig. 1: Relative collector yield as a fuctio of the ar trasmittace. 80 0.958 Collector Tilt Agle [ ] 60 40 0.957 0.955 0.956 0.954 20-50 -30-10 10 30 50 Orietatio [ ] Fig. 2: Trasmissio factor F τ for a collector with a AR-treated cover as a fuctio of the collector tilt agle ad orietatio.
The discussio up to ow apply for a certai orietatio of the collector field (see Tab. 1). A additioal case was ivestigated to study other orietatios. The collector yield of a collector with a AR-treated cover (τ Sol = 0.956) was compared with the yield of the referece collector which is 0.900 (τ Sol = 0.9). The orietatio of the collector field was varied betwee -60 (east) through 0 (south) to +60 (west), ad the tilt agle raged from 20 to 90. The simulatio results i a trasmissio factor F τ = 0.955 for the case already cosidered, south orietatio with a 40 tilt agle. At other orietatios, F τ varies betwee 0.954 ad 0.959. This meas that the assumptio made i equatio (2) is oly very weakly affected by the orietatio of the collector field. 3. The Degradatio Factors F UV ad F DEG The degradatio factors describe the chage i ar trasmittace due to degradatio either caused by UV radiatio (arisatio) i case of the photodegradatio factor F UV or caused by other climatical loads (i.e. such as rai, dew, temperature) i case of F DEG. They are determied as follows: F UV DEG τ UV, DEG ref τ, = (3) ref UV, DEG where τ is the ar trasmittace of the glass pae before exposure, ad τ is the ar trasmittace after the glass pae has bee exposed to the desired load. I order to determie the photodegradatio factor the glass pae has to be exposed to UV radiatio. The radiatio dose must be at least 80 kwh / m 2 for UVA ad 3 kwh / m 2 for UVB. These quatities correspod approximately to the UV exposure over a period of 1 year i Cetral Europe. For determiatio of the degradatio factor F DEG o method is available, yet. However, research groups are actually workig i the area of accelerated life testig of collector glazigs withi the frame of IEA SHCP Task 27. The degradatio factors F UV ad F DEG describe othig but a chage i the ar trasmittace τ. Thus, this quatities are liear factors of the collector field yield i aalogy to the trasmissio factor F τ. 4. The IAM Weightig Factor F IAM The IAM weightig factor F IAM quatifies the effect of the agle-depedet ar trasmittace o the collector yield of a thermal ar system. It is determied as follows:
F IAM tr lo lo tr tr lo lo tr = mi([ FVert FHor ] Str. i,[ FVert FHor ] Str. i,[ FVert FHor ] Str. out,[ FVert FHor ] Str. out ) (4) where: Axis Axis Geo F = IAM S (5) Geo IAM measured icidece agle modifier at the agular positio S coefficiet for the agular positio N = 0,..,5 idex of the agular positio Axis ( tr, lo) axis of the glass pae, to which the measured IAM refers (trasversal, logitudial) Geo ( vert, hor) orietatio of the correspodig glass pae axis (vertical, horizotal): the horizotal axis is parallel to the groud surface, the vertical axis ad the groud surface determie the tilt agle ( str. i, str. out) positio of the structured surface of the glass pae with respect to the collector Axis F Geo is thus the sum of the products of measured values of the icidece agle modifier IAM with the correspodig coefficiets S. The agles at which the IAM is measured ad the correspodig coefficiets are summarised i Table 4. Tab. 4: Agles at which the IAM is measured ad the correspodig coefficiets. N 0 1 2 3 4 5 agle [ ] 0 30 40 50 60 70 Vert S 0.6986 0.1715 0.0454 0.0389 0.0326 0.0231 Hor S 0.5530 0.2113 0.0742 0.0756 0.0697 0.0334 IAM values of referece pae accordig to Fresel 1 0.9965 0.9886 0.9678 0.9148 0.7846 Commet: By defiitio, the value for the IAM at ormal icidece is 1, i.e. IAM 0 = 1 This empirical relatioship betwee the IAM ad the collector yield was determied by sesitivity aalysis. The IAM of a glass pae was varied i arrow agular itervals ad the chage i the collector yield was observed. I this way, a weightig factor S was determied for each of these agular itervals. The total effect o the collector yield is obtaied as the sum of the weighted effects from the idividual agular itervals. The system used for simulatio agai correspods to referece ar system. The coefficiets determied by the aalysis are scaled such that F IAM assumes the value of 1 for the referece glass pae. The referece glass pae was chose to be a smooth, o-structured pae, 4 mm thick, with a refractive idex = 1.53 ad a extictio coefficiet k = 4 m -1. The IAM ca be calculated simply for this case with the help of the Fresel equatios. The calculated IAM values for the referece glass pae are also icluded i Table 4.
The practical sigificace of equatio (4) is as follows: I priciple, a glass pae ca be orietated withi a collector i four differet ways (structure iside / outside, log axis of the glass pae parallel to the collector horizotal / vertical axis). Each of these orietatios ca have a differet effect o the collector yield. The effect o the collector yield ca be calculated with the help of equatio (5), the coefficiets from Table 4 ad the measured IAM of a glass pae. The orietatio which leads to the worst result is chose as the IAM weightig factor F IAM. 1.0 0.9 0.8 0.7 1.000 1.000 0.990 0.970 0.967 0.900 valid rage referece (Fresel) 0.794 0.870 0.6 0.5 0.640 IAM 0.4 0.3 0.408 0.2 0.1 0.0 0 10 20 30 40 50 60 70 80 90 agle of icidece Fig. 3: Valid rage for calculatig the factor F IAM. The IAM values for the referece glass pae are oted withi the shaded area. The valid rage for determiig this factor from the IAM values is idicated i Fig. 3. It is evidet that the depedece of the collector yield o the orietatio of the collector plae is very closely coected with the IAM of the collector cover. This depedece is show i Fig. 4. The IAM of the glass pae chose for this example has a very strog ifluece o the collector yield. The IAM weightig factor of this pae is F IAM = 0.955. This meas that a collector equipped with this glass cover would achieve oly 95.5 % of the yield that could be obtaied with the referece pae (with F IAM = 1.0), for true south orietatio ad a tilt agle of 40. As ca be see i Fig. 4, the losses are eve greater if other orietatios are chose.
0.930 0.925 0.915 0.920 0.910 0.905 0.900 80 0.940 0.935 0.945 Collector Tilt Agle [ ] 60 40 0.950 0.955 0.945 0.950 20 0.940-50 -30-10 10 30 50 Orietatio [ ] Fig. 4: IAM weightig factor F IAM for a glass pae with a poor IAM, as a fuctio of the collector plae orietatio. 5. Coclusios The itroductio of the certificatio of ar glass offers the followig advatages: A clear defiitio of the expressio ar glass - the performace of a glass cover ca be simply evaluated by a collector maufacturer. Protectio of the customer by a stamp o the glass pae idetifyig the performace class Glass covers of a certai performace class ca be replaced by a equivalet, without sigificatly chagig the system performace. Refereces [1] Certificatio of ar glass ; Documets dowload from www.areergy.ch [2] Polysu 3.3, The simulatio program to dimesio thermal ar systems; SPF o behalf o the Budesamtes für Eergiewirtschaft (BEW), Switzerlad; Program ca be ordered via www.areergy.ch