RESEARCH IN SOLAR ENERGY AT THE POLITEHNICA UNIVERSITY OF TIMISOARA: STUDIES ON SOLAR RADIATION AND SOLAR COLLECTORS

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1 THERMAL SCIENCE: Year 2010, Vol. 14, No. 1, pp RESEARCH IN SOLAR ENERGY AT THE POLITEHNICA UNIVERSITY OF TIMISOARA: STUDIES ON SOLAR RADIATION AND SOLAR COLLECTORS by Ioan V. LUMINOSU a*, Coleta T. DE SABATA a, and Aldo I. DE SABATA b a Department of Physical Foundation of Engineering, Politehnica University, Timisoara, Romania b Faculty of Electronics and Telecommunications, Department of Measurements and Optoelectronics, Politehnica University, Timisoara, Romania Re view pa per UDC: : DOI: /TSCI L A short over view of a more than 30 years long his tory and re sults of re search and ther mal ap pli ca tions of so lar en ergy at the Politehnica Uni ver sity of Timisoara, Ro ma nia, are pre sented. The main di rec tions ap proached are: actinometry, stud ies on ma te ri als and green house ef fect, ther mal col lec tors, in dus trial and home ther - mal ap pli ca tions, com puter sim u la tion of phys i cal phe nom ena, and con cen tra tors. This pa per fo cuses on the con cep tion and build ing of a ded i cated lab o ra tory and on the ex per i men tal re sults that al lowed the de vel op ment of in dus trial and home ap pli - ca tions. Key words: absorbance, actinometry, exergy, thermal collectors, passive wall, transmittance Introduction A his tor i cal ac count of re search in so lar en - ergy at the Politehnica Uni ver sity of Timisoara is mo ti vated by its rel a tively long du - ra tion, the large quan tity of work and en thu si - asm in volved, and mainly by its re sults. The ac tiv i ties re lated to so lar en ergy have been per formed within the De part ment of Phys - ics fig. 1 [1-4]. A drop of the in ter est in so lar en - ergy oc curred af ter the his tor i cal events of 1989, but this at ti tude has been re con sid ered af - ter The ac tiv ity is pres ently revigorated, and, fol low ing win ning of pub lic na tional grant com pe ti tions, im por tant in vest ments are be ing made. Figure 1. The building were the Department of Physics was situated until 2007 * Corresponding author; iluminosu@upcent.ro; ioan.luninosu@et.upt.ro

2 158 THERMAL SCIENCE: Year 2010, Vol. 14, No. 1, pp Re search in so lar en ergy has been ap proached back in 1976, mo ti vated by the raise in the price of clas si cal fu els and re sources ex haus tion, pol lu tion and lo cal me te o ro log i cal and cli - mate con di tions. Access to in ter na tional mar ket was dif fi cult or some times im pos si ble in those days, so that many in stru ments and lab o ra tory equip ment had to be de signed and built lo cally. The ob jec tives in this newly ap proached re search area were: (1) con struc tion of a da ta - base with me te o ro log i cal and cli mate con di tions in the west ern part of Ro ma nia, (2) the o ret i cal and ex per i men tal re search on cap ture, stor age, and use of so lar en ergy for warm wa ter and warm air flow pro duc tion in in dus trial ap pli ca tions, (3) dry ing of ce ramic and wood prod ucts and bi tu - men, (4) ra di a tion con cen tra tion, (5) ob tain ing and use of bio mass, and (6) sim u la tion of phys i - cal phe nom ena in volved in the ther mal con ver sion of so lar en ergy. The experimental work has been fi nal ized with in dus trial and home ther mal ap pli ca tions. Research laboratory in natural conditions A re search lab o ra tory has been equipped and placed out doors on a south ori ented ter race in the court yard of the build ing (fig. 2). This newly cre - ated lab o ra tory was used by the team for per form - ing sev eral ac tiv i ties, as out lined be low. Actinometry For mea sure ments of so lar ra di a tion in ten sity, two mea sur ing de vices have been de signed and re - al ized. Solaris 1 is a com pen sa tion wattmeter in tended for in di vid ual so lar en ergy us ers [5]. The sen sor of the de vice con sists of two ther mally con duc tive plates, painted in black. One of the plates is ac tive, while the other is the ref er ence. The so lar ra di a tion falls on and heats the ac - tive plate un til ther mal equi lib rium is reached, while the ref er ence plate is pro tected from the ra - di a tion by a sil ver plated mir ror. An elec - tronic cir cuit heats the ref er ence plate un til it reaches the tem per a ture of the ac tive plate and mea sures the elec tri cal en ergy con - sumed for this pur pose. The ra di a tion in ten - sity is then cal cu lated from this en ergy, the time needed for warm ing up the ref er ence plate and the area of the cap ture sur face. The sen sor mount is rep re sented in fig. 3. The sen sor ori en ta tion can be made iden ti cal to the ori en ta tion of the so lar col lec tor whose ef fi ciency is stud ied. Figure 2. Terrace with the solar laboratory Figure 3. Mount of Solaris 1 Solaris 1 has been com pared to a cal i brated albedometer from the lo cal me te o ro log i cal sta tion. Re sults are pre sented in fig. 4. Pro cess ing of ex per i men tal data led to the fol low ing stan - dard func tion for Solaris 1 [5]:

3 THERMAL SCIENCE: Year 2010, Vol. 14, No. 1, pp Fig ure 4. Com par i son be tween Solaris 1 and an albedometer Ra di a tion in ten sity vs. hour: global ra di a tion measured with the albedometer (A) and Solaris 1 (B), di rect ra di a tion (C), dif fuse ra di a tion mea sured with the albedometer (D), and Solaris 1 (E) G = ag Solaris + b (1) where G Solaris is the global ra di a tion flux den sity mea sured by Solaris 1, G the true value of the ra di a tion flux den sity, and a and b are the con stants to be de ter mined by cal i bra tion for the geo - graph ical place where mea sure ments are per - formed. Solaris 2 is a pyranometer that mea sures the in ten sity of the global ra di a tion in the hor i zon tal plane (fig. 5) [6-10]. The ra di a tion sen sor (A in fig. 5) com prises 8 cop per cir cu lar sec tors, of 0.1 mm thick ness, painted al ter nately in white and black that con sti tute cold and warm con tacts of thermocouples mounted on their back sides and con nected in se ries. The sen sor is cov ered with a dome made of soda-lime glass for win dows, hav ing a re frac tion in dex of 1.6, and a trans mit - tance of 94% in the do main mm. This type of glass has been spe cially de signed and Figure 5. The pyranometer Solaris 2 fab ri cated for this ap pli ca tion within a chem i cal plant from the re gion. An elec tronic unit con - trols ther mally the de vice against en vi ron men tal tem per a ture vari a tions, am pli fies the sig nal pro vided by the thermocouples and dis plays the re sult of mea sure ments. The in ten sity of the out put cur rent is: a1 a2 i G C l1 l2 (2) where: a 1,2 are the ab sorp tion co ef fi cients and l 1,2 the heat trans fer co ef fi cients. A cal i bra tion curve has been ob tained by us - ing com par a tive mea sure ments with a ref er ence pyranometer from the Na tional Me te o rol ogy and Hy drol ogy In sti tute of Bu cha rest, taken at a fre quency of 1 per min ute, fig. 6. The re sults, ex pressed in W/m 2, re veal a re mark able lin ear - ity, at a slope of 45 deg. Figure 6. Calibration curve for Solaris 2

4 160 THERMAL SCIENCE: Year 2010, Vol. 14, No. 1, pp By us ing de vices such those de scribed above, mea sure ments on so lar ra di a tion have been per formed for sev eral years in Timisoara, (45 46 N, E). Av er aged re sults are pre - sented in tab. 1, where G i is the hourly av er age for the mean day of the month, and G is the monthly av er age [11]. It is rea son able to con nect ther mal so lar in stal la tions when the in ten sity of the so lar ra - di a tion is larger than the thresh old of 300 W/m 2. Ta ble 1 shows that ther mal so lar in stal la tions are ef fi cient in the pe riod April Sep tem ber for the place where mea sure ments have been per - formed. Fur ther more, it can be con cluded from tab. 1 that the ther mal so lar po ten tial of Ro ma nia is sim i lar to other coun tries from the Eu ro pean Un ion, where so lar ap pli ca tions are de vel oped. Ta ble 1. So lar ra di a tion in Timisoara G i and G Hour G i [Wm 2 ] Month G [Wm 2 ] Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec An nual av er age [Wm 2 ] 374 The dif fuse ra di a tion is con verted into heat when in ci dent on a so lar col lec tor. In cloudy (over cast) days, the flux den sity of dif fuse ra di a tion can reach 418 W/m 2, ren der ing func tional the ther mal col lec tors. A mea sur ing de vice for dif fuse ra di a tion, sketched in fig. 7, has been de signed con structed at the De part ment of phys ics and cal i brated against a sim i lar in - stru ment from the Na tional Me te o rol ogy and Hy drol ogy In sti tute of Bu cha rest [12]. The rep re - sented el e ments are: (1) and (2) the ther mal col lec tors, (3) the mag netic nee dle for in di ca tion of the south di rec tion, (4) the ali dade pro vid ing the com ple ment of the an gle be tween the pan - els and the ver ti cal, (5) the rod, (6) the de vice for mod i fy ing the an gles be tween the pan els and the ver ti cal, (7) the de vice for ro tat ing the sys tem with re spect to the hor i zon tal plane, (8) the goniometer for read ing the ro ta tion an gle in the hor i zon tal plane, (9) the me chan i cal mount, (10) the lead thread, (11) the lens for po si tion ing one of the pan els to wards the Sun,

5 THERMAL SCIENCE: Year 2010, Vol. 14, No. 1, pp and (12) the black plate where the im age of the Sun is ob - served. The ther mal col lec tors con tain a brass plate each, with di men sions of 13.5 and 7 cm and a mass m = 235 g. The con tri bu tion of the dif fuse ra di a tion to the heat col - lected by a so lar col lec tor is rel a tively low (7.4%), but it be - comes sig nif i cant when the col lect ing sur faces are large [13]. The above pre sented in stru ments are cheaper than stan - dard, com mer cial ones, of com pa ra ble ac cu racy when cal i - brated and may be con structed lo cally by so lar en ergy us ers. Studies on materials Ex per i men tal work has been per formed for mea sur ing phys i cal prop er ties of ma te ri als rel e vant for var i ous so lar en - ergy ap pli ca tions. Then, the re sults were used in the de sign of in stal la tions and so lar en ergy ap pli ca tions set ups [13]. For ex am ple, re sults of ex per i men tal stud ies on the trans - mit tances of var i ous ma te ri als in the range of wave lengths of m are synthetized in the spec tro grams of fig. 8. It can be seen that glass win dows with thick nesses in the do - main of 2-5 mm have trans mit tances greater than 70%, and a red plas tic has the larg est trans mit tance [13]. The trans mit - tance of the glass used for the dome of Solaris 2 was mea - sured to 94% in a do main of wave lengths be tween 0.4 and 1.1 mm [14]. A poly es ter sheet coated with a thin alu mi num plate was found to have a max i mum trans mit - tance of 25% around the wave length of 0.5 mm. The trans mis sion max i mum of a glass win dow plated with such a sheet is sit u ated in the blue re - gion of the spec trum, so that the trans mit ted light cre ates a pleas ant sen sa tion, as red and or ange ra - di a tions con tents are poor [15-17]. This ma te rial has been used in climatization ap pli ca tions. In an at tempt to de vise cheap so lar col lec - tors, the ca pa bil ity of heat cap ture in wa ter has been tested in two cir cum stances: (a) by us ing a re cip i ent cov ered with a 3 mm thick glass plate ex posed to so lar ra di a tion, and (b) by us ing the same setup with an ad di tional thin oil sheet on the sur face of the wa ter. It has been found that 63% of the in ci dent en ergy has been cap tured in the sec ond sit u a tion, with an op ti mum oil sheet thick ness of 1 mm, as com pared to 54% in the first sit u a tion (wa ter only) [18, 19]. Figure 7. Mount with two thermal collectors (1, 2) thermal collectors, (3) magnetic needle, (4) alidade, (5) rod, (6) device for modifying the angles, (7) device for rotating the system, (8) goniometer, (9) mechanical mount, (10) lead thread, (11) lens, (12) block plate Figure 8. Transmittances of several transparent materials vs. wavelength (examples) (1) red plastic, (2) plexiglas, (3) glass-2 mm (4) glass-3 mm, (5) glass-5 mm, (6) patterned glass-3 mm, (7) ornament 2, (8) ornament 1

6 162 THERMAL SCIENCE: Year 2010, Vol. 14, No. 1, pp Figure 9. Setup with irradiated iron plates (1) support, (2) radiation source, (3) circular support, (4) iron plate, (5) thermometer Figure 10. The slope at the origin of the heating curves (1) pigment X, (2) pigment Y, (3) pigment Z The liq uids that con tain black pig ments with an op ti mal con cen tra tion of 0.08% pro duce green house ef fect, and the slope of the heat ing curve at the or i gin is of 0.37 C/min. for a width of 2 cm of the layer of wa ter with black ink [20]. The composition of a material with superior absorbing properties has been established using the ex per i men tal setup pre sented in fig. 9 [21]. The iron plates had a thick ness of 0.5 mm, and the ir ra di ated sur face was of 25 cm 2. The op ti mal com po si tion of the ir ra di ated ma te rial has been es tab lished by rais ing the heat - ing curves of the iron plates cov ered with paints of var i ous com po si tions. The com po si tions have been var ied by mix - ing an ab sorb ing, sup port ing ma te rial, whose mass is de noted by V, with pig ments. The slopes at the or i gin of the heat ing curves vs. the pig ment masses are rep re sented in fig. 10 for three pig - ments; we de note by X, Y, and Z [%] the ra tios be tween the pig ment masses and V (curves 1, 2, and 3). Fig ure 10 re veals that the fit test ma te ri als are Y = 20% and Z = 20%. The slope is large also for Y = 50% and Y = 60%, but these high pro por - tions of pig ments raise eco nom i cal prob lems and tech no log i cal ones, as uni form spray ing of the ab sorb ing sur face be comes dif fi cult. Ther mal col lec tors Flat-plate solar collector Figure 11. Flat-plate solar collector (1) transparent plate, (2) absorber, (3) pipe for air flow, (4) insulating flame, (5) support, (6) mount wall A so lar col lec tor was built, whose struc ture is pre sented in fig. 11 [22-29]. The an gle be tween the nor mal to the col lec tor sur face and the ver ti cal is de noted as usual by s. The ab sorb ing plate plays also the role of wall for the air flow pipe. Two neighbour cor ri dors share a com mon wall, so the trans port fac tor is unity F t = 1. The air flow rate takes val ues in the in ter val of m 3 /h. The collecting surface is A = 3.8 m 2 and the length of the pipe is L cd = 15.2 m. The air en ters and leaves through square shaped holes, of side a = 0.25 m so that A = a L cd. The air is cir cu lated by a low power fan.

7 THERMAL SCIENCE: Year 2010, Vol. 14, No. 1, pp The hourly vari a tion of the av er age ef fi - ciency, mea sured at Timisoara, is pre sented in fig. 12, for s = 45 deg. The air flow rates are: 135, 108, 81, and 54 m 3 /h. The so lar ra di a tion data pre sented in tab. 1 al low for the es ti ma tion of the en ergy pro duced by a col lec tor with a given sur face. The re sults pre sented in fig. 12 re veal that high ef fi cien cies are ob tained around noon, when the an gle of in ci dence is small, and the absorbance-trans mit tance prod uct is high. The in crease of the an gle of in ci dence de ter - Fig ure 12. Mean ef fi ciency vs. hour mines a lower absorbance-trans mit tance prod - uct dur ing the rest of the day, so that the ef fi - ciency di min ishes. The curves are not sym met ric around noon, the mag ni tude of the slope in the af ter noon be ing smaller than the cor re spond ing value be fore noon, as the fluid is heated by the me tal lic pipe in the af ter noon. At high air flow rates (135 m 3 /h) the ef fi ciency ap proaches 40%. These re sults in the sim plic ity of the con struc tion rec om mend the air flow col lec tor for home and in dus trial dry ing ap pli ca tions. A ca pac ity of 800 W may be ob tained at a price of about 1200 EURO. Passive wall The pas sive (Trombe) wall is used for ac cu - mu la tion and stor age of so lar pro duced heat, for short pe ri ods of time, in or der to climatize ad ja - cent rooms [30-37]. A pas sive wall was built, hav ing an area of 8.8 m 2, and it has been in stalled on the south ern wall of a build ing hav ing a clas sic heat ing sys - tem. Dur ing ex per i ments, the room ad ja cent to the pas sive wall has been dis con nected from the heat ing sys tem. The con struc tion of the pas sive wall is pre sented in fig. 13. The wall is heated by green house ef fect, which de ter mines the heat ing of the air and the vari a tion of its den - sity. The warm air pen e trates the room via the up per hole, cools down heat ing the air in the Fig ure 13. Pas sive wall (1, 2) ad mis sion holes, (1', 2') flaps, (3, 7) ther mom e ters, (4) wall, (5) trans par ent plate, (6) solarimeter room, and then en ters the so lar trap through the lower hole, where it is heated again. The fol low ing phys i cal quan ti ties have been mea sured: flux den sity of so lar ra di a tion, air tem per a ture in side and out side, and tem per a tures of the walls at their cen ters. Some of the re - sults are pre sented in fig. 14. The pas sive wall must be ther mally in su lated at night; oth er wise im por tant heat losses oc cur. The power to the pre sented wall is: 546 Q ( t t ) t t 1 2 (3)

8 164 THERMAL SCIENCE: Year 2010, Vol. 14, No. 1, pp Figure 14. Solar radiation and temperatures of the elements of the passive wall (1) solar radiation, (2) temperature at the upper hole, (3) temperature at the lower hole, (4) ambient temperature where t 1, t 2, and t 3 are the tem per a tures at points 1, 2, and 3, re spec tively, in fig. 13. Mea sure ments have been per formed dur ing day light and 3-4 hours af ter sun set and re vealed that the power var ied be tween 82 W and 260 W. The av er age en ergy the wall pro vided to the room in the pe riod Oc to ber-no vem ber was Q u = = 6.4 MJ per day. The power in ci dent on the glass plate var ied be tween 2000 W in the morn - ing and 4800 W at noon. The av er age en ergy in - ci dent on the plate was Q i = 91 MJ per day. The av er age daily ef fi ciency of use of so lar en ergy re sulted: Qu hday, hday % (4) Q i Computer simulation of physical phenomena involved in the ther mal con ver sion of so lar en ergy The en ergy ef fi ciency of a so lar col lec tor var ies monotonically with the flow rate of the work ing agent and with the col lect ing sur face and it does not have extremum points that would al low the def i ni tion of op ti mal work ing con di tions. The exergy ef fi ciency var ies with the flow rate and col lect ing area in such a way that lo cal extremum points and a global extremum point are pres ent. These extremum points al low to de fine op ti mal work ing con di tions of the col lec tor with re spect to flow rate and col lect ing area [38-42]. Determination of optimal regime for a solar col lec tor in open cir cuit It has been shown in [41, 42] that the exergy ef fi ciency of a so lar col lec tor us ing wa ter as work ing fluid, in open cir cuit is given by: mc P ta UF A C hex 1 exp AC U mc P mc PT a tagc UF A C ln1 1 exp AC GC UT a mc P where the fol low ing sym bols are used: h ex exergy ef fi ciency, A C so lar ra di a tion col lect ing area, C p iso bar spe cific heat of the cir cu lat ing agent, G C so lar ra di a tion, ta ab sorp - tion-trans mis sion equiv a lent prod uct, U co ef fi cient of ther mal losses; F' ef fi ciency fac tor, T a am bi ent tem per a ture, and m mass flow rate of the work ing fluid. The cal cu lated exergy ef fi ciency in func tion of the col lect ing sur face area and the flow rate, ob tained by means of a spe cially writ ten com puter pro gram, is p re sented in fig. 15. (5)

9 THERMAL SCIENCE: Year 2010, Vol. 14, No. 1, pp Three kinds of quan ti ties ap pear in fig. 15: key board in - put quan ti ties, those cor re - spond ing to the cur rent po si - tion of the cur sor and co- -or di nates of the global max i - mum of the exergy ef fi ciency. In put (key board) quan ti ties are: TE = 300 K (am bi ent tem - per a ture), U = 4.77 W/m 2 K (ther mal losses co ef fi cient), HR = 500 W/m 2 (so lar irradiance), CS = 4185 J/kg/K Figure 15. The exergy efficiency h ex = f (A C, m) (spe cific heat of the work ing agent), and FF = 0.90 (col lec tor s ef fi ciency fac tor). Cal cu lated quan ti ties, cor re spond ing to the cur rent po si tion of the cur sor are: REN = (en ergy ef fi ciency), DT = 15.5 (tem per a ture rise), REX = (exergetic ef fi ciency), AREA = 1.30 m 2 (area of the col lect ing sur face), FLOW = kg/s (mass flow of the work ing agent). The co or di nates of the global max i mum (typ i cal val ues for Ro ma nia) are: AMX = 3.30 (sur face of the col lect ing area), FLMX = (mass flow of the work ing agent), RENMX = (en ergy ef fi ciency), DTMX = 63.3 (tem per a - ture rise), and REXMX = (exergetic ef fi ciency). Solar radiation Sev eral com puter pro grams run ning on per sonal com put ers of the eight ies have been writ ten and used for the dis sem i na tion of in for ma tion on so lar ra di a tion [43-47] and for ed u ca - tional pur poses, us ing com mer cial soft ware such as Turbo Pascal. Geo graph ical po si tion, sea - son and time of the day were in put data, and so lar quan ti ties such as irradiance were cal cu lated and plot ted by means of the graph i cal tools pro vided by the pack ages, us ing sim ple mod els [48-50]. Con clu sions Re search in so lar en ergy at the Politehnica Uni ver sity of Timisoara started in 1976, mo ti vated by the raise of the price of clas si cal fu els, re source ex haus tion and pol lu tion. Ini tially, it has been per formed within the De part ment of Phys ics. Co op er a tion with other de part ments of the uni ver sity fol lowed. Be sides stan dard Phys ics lab o ra to ries, an out doors re search lab o ra tory has been cre - ated and equipped. Many in stru ments had to be de signed and con structed ( home made ), as ac - cess to in ter na tional mar ket was dif fi cult or some times im pos si ble. In the field of actinometry, de sign, con struc tion and cal i bra tion of a com pen sa tion wattmeter and a pyranometer al lowed for ex tended so lar ra di a tion mea sure ments. Ex per i men tal set ups for mea sure ment of trans mit tance and absorbance of ma te ri als have been de vised; re sults of ex per i ments have been used in green house ef fect and climatization ap pli ca tions.

10 166 THERMAL SCIENCE: Year 2010, Vol. 14, No. 1, pp Sev eral types of ther mal col lec tors have been stud ied, such as flat-plate so lar col lec - tors and Trombe walls. Com puter sim u la tion of phys i cal phe nom ena was ap plied to so lar ra di a tion pre dic tion for ed u ca tional and en gi neer ing de sign pur poses and to find ing the op ti mal re gime of so lar col - lec tors in open cir cuit. The ex pe ri ence gained and data gath ered in lab o ra tory work proved to be fruit ful for the de vel op ment of home and in dus trial ap pli ca tions, such as a pi o neer ing func tional ap pli ca - tion in Ro ma nia an in dus trial hall for dry ing ce ramic prod ucts and the first func tional, so lar en - ergy based in dus trial in stal la tion for bi tu men melt ing, which has been con sid ered an in ter na - tional nov elty in the roads and high ways build ing in dus try in the eight ies. An over view of these ap pli ca tions is the sub ject of an other work. Af ter the drop in in ter est and fi nanc ing oc curred in 1990, the ac tiv ity has been re - sumed. The re search in so lar en ergy (and in re new able en er gies in gen eral) is now co or di nated at the Uni ver sity level. An im por tant na tional grant in so lar en ergy has been won in The fund ing was in tended for the cre ation of five new lab o ra to ries, ren o va tion of older as sets, cre - ation of a new mas ter di rec tion in so lar en ergy ed u ca tion, re search, and in ter na tional co op er a - tion etc. This year, based on the re search tra di tion in so lar en ergy and other re new able en er gies ap pli ca tions, such as in wind mills and microhydro plants [51], fund ing has been at tracted for build ing a new Re search in sti tute in re new able en er gies, which will func tion within the struc - ture of the Uni ver sity. Nomenclature A surface area, [m 2 ] a length, [m] a constant for the geographical place, [ ] b constant for the geographical place, [Wm 2 ] C specific heat, [Jkg 1 K 1 ] C device constant, [AK 1 ] F transport factor, [ ] F' efficiency factor, [ ] G solar radiation, [Wm 2 ] i intensity of the current, [A] L lenght, [m] m mass flow rate, [kgs 1 ] Q heat quantity, [J] Q power, [W] T temperature [K] t temperature, [ C] U coefficient of thermal losses, [Wm 2 K 1 ] X, Y, Z pigment mass in V = 1, [m 3 ] Greek let ters a absorption coefficients, [ ] h efficiency, [ ] l thermal transfer coefficients, [Wm 2 K 1 ] ta absorption-transmission equivalent product, [ ] Subscripts a C ex i p t u ambient collector exergy incident isobar transport useful Mathematical operator average References [1] De Sabata, C., Use of So lar En ergy in Ro ma nia, Past and Fu ture, Proceedings, 27 th Con gress of the Amer i - can Ro ma nian Acad emy of Arts and Sci ences, 2002, Oradea, Romania, Vol. I, 2002, pp [2] De Sabata, C., et al., So lar En ergy, Achieve ments and Fu ture Trends (in Romanian), Stud ies and Re search in Phys ics (pres ent Ro ma nian Re ports in Physics), Ac a de mia Romana, Bukarest, 34 (1982), 6, pp

11 THERMAL SCIENCE: Year 2010, Vol. 14, No. 1, pp [3] Marcu, C., Luminosu, I., Short Re port on Study and Uti li za tion of So lar En ergy at the Tech ni cal Uni ver - sity of Timisoara, De part ment of Phys ics, Buletinul Stiintific al Universitatii Politehnica din Timisoara, Matematica-Fizica (pres ently Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat - ics Phys ics Se ries), 40(54) (1995), 1, pp [4] De Sabata, C., Marcu, C., Luminosu, I., So lar Energetics at the Politehnica Uni ver sity of Timisoara, Pro - ceed ings, 2 th In ter na tional Con fer ence Renewable Sources and Environmental Electro-Technologies RSEE 98, Oradea, Romania, Analele Universitatii din Oradea, Electrotehnica, Vol. X, 1998, pp [5] De Sabata, C., et al., Solaris 1 Dif fer ential, Auto Compensated Bolometer for Solar Radiation Intensity Mea sure ment (in Romanian), Instalatii in Constructii, Ac a de mia Ro mana, Bucuresti, Septembrie 1984, pp [6] De Sabata, C., Solaris 2, Por ta ble Pyranometer (in Romanian), Instalatii in Constructii, Ac a de mia Romana, Bucharest, Septembrie-Octombrie 1986, pp [7] De Sabata, C., et al., On the Spec tral Sen si tiv ity of the Solaris 2 Pyranometer, Proceedings, Lucrarile Seminarului de Matematica Fizica (pres ently Buletinul Stiintific al Universitatii Politehnica din Timisoara, Matematica-Fizica), Noiembrie 1984, pp [8] De Sabata, C., et al., The Char ac ter is tics of the Ra di a tion Trans ducer of the Solaris 2 Pyranometer in the In fra red Range, Pro ceed ings, Na tional Con fer ence in So lar En ergy, Politehnica Uni ver sity of Timisoara, 1989, pp [9] Luminosu, I., Nagy, M., Ap pa ra tus and Instruments for Measuring Physical Quantities that Describe the Pro cesses of Heliothermal Con ver sion De vel oped at the Politehnica, Uni ver sity of Timisoara, Pro ceed - ings, Pannonian Ap plied Math e mat i cal Meetings, 2001 (PAMM), Bul le tins for Ap plied and Com puter Math e mat ics (BAM-1902/2001) (XCVI-C), Budapest University of Technology and Economics, 2001, pp [10] Terniceanu, S., Luminosu, I., A Method for Mea sur ing So lar Ra di a tion In ten sity Mean Value, Pro ceed - ings, Lucrarile Seminarului de Matematica-Fizica (pres ently Sci en tific Bul le tin of the Politehnica Uni - ver sity of Timisoara, Math e mat ics Phys ics Se ries), Noiembrie 1986, pp [11] Mihailovici, D., et al., The So lar Po ten tial in the West ern Part of Ro ma nia in 1985 (in Ger man), Pro ceed - ings, Lucrarile Seminarului de Matematica Fizica (pres ently Sci en tific Bul le tin of the Politehnica Uni - ver sity of Timisoara, Math e mat ics Phys ics Se ries), Mai-Noiembrie 1987, pp [12] Luminosu, I., Nagy, M., In stal la tions and Measurements for Determining the Power Contribution of Dif - fuse So lar Ra di a tion to the Work ing Heat of the Flat So lar Col lec tor, Proceedings, Pannonian Ap plied Math e mat i cal Meet ings, 2001 (PAMM), (Bul le tins for Ap plied and Com puter Math e mat ics (BAM-1901/2001) (XCVI-C)), Bu da pest Uni ver sity of Tech nol ogy and Eco nom ics, 2001, pp [13] De Sabata, C., et al., Lab o ra tory Re search for In creas ing Op ti cal Ef fi ciency of Plane So lar Col lec tors (in Romanian), Buletinul Stiintific al Universitatii Politehnica din Timisoara, Matematica Fizica (pres - ently Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat ics Phys ics Se ries), 24 (38) (1978), 1, pp [14] De Sabata, C., Op ti cal Prop er ties of the Glass Dome Used in the Struc ture of a Pyranometer (in Ro ma - nian), Pro ceed ings, Lucrarile Seminarului de Matematica-Fizica (pres ently Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat ics Phys ics Se ries), Noiembrie, 1983, pp [15] De Sabata, C., Luminosu, I., Ex per i men tal Re search and The o ret i cal Re marks on the Use of Some Climatization Sheets (in Ro ma nian), Buletinul Stiintific al Universitatii Politehnica din Timisoara, Matematica-Fizica (pres ently Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat - ics Phys ics Se ries), 25(39) (1980), 2, pp [16] De Sabata, C., et al., Sum mer Climatization Based on Solar Energy: Theoretical Considerations and Ex - per i men tal Re sults, Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Trans ac tions on Me - chan ics 53(67) (2008), 3, pp [17] Luminosu, I., In crease of Flat So lar Col lec tors Ef fi ciency through the Study of Phys i cal Phe nom ena In - volved in Thermo-Con ver sion (in Romanian), Ph. D. thesis, Politehnica Uni ver sity of Timisoara, Romania, 1993 [18] De Sabata, C., Craciun, P., Marcu, C., In flu ence of the Helio-Ther mal Ef fect on the Stock ing Ca pac ity of Heat Ob tained by Thermo-So lar Con ver sion (in French), Proceedings, Lucrarile Seminarului de Matematica-Fizica (pres ently Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat - ics Phys ics Se ries), Mai, 1986, pp

12 168 THERMAL SCIENCE: Year 2010, Vol. 14, No. 1, pp [19] De Sabata, C., Craciun, P., Marcu, C., Study of the Green house Ef fect Re al ized with an Oil Layer (in Ro - ma nian), Pro ceed ings, Lucrarile Seminarului de Matematica Fizica (pres ently Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat ics Phys ics Se ries), Mai, 1986, pp [20] Marcu, C., De Sabata, C., Craciun, P., Use of Some Black Flu ids for Lay ered Col lec tors (in Ro ma nian), Pro ceed ings, Lucrarile Seminarului de Matematicã Fizicã (pres ently Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat ics Phys ics Se ries), Mai, 1984, pp [21] De Sabata, C., Study of In fra red Ab sorp tion by Some Ma te ri als Used for Cap tur ing So lar En ergy (in Ro - ma nian), Buletinul Stiintific al Universitatii Politehnica din Timisoara, Matematica Fizica (pres ently Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat ics Phys ics Se ries), 24(38) (1978), 1, pp [22] Marcu, C., Luminosu, I., Ef fi cient Use of the Collector Surface in Obtaining Technological Air, Pro ceed - ings, Lucrarile Seminarului de Matematica Fizica, Universitatea Politehnica din Timisoara (pres ently Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat ics Phys ics Se ries), Mai, 1983., pp [23] Luminosu, I., Study on Tem per a ture Per for mance of a Flat Plane Sun Col lec tor in View of Ob tain ing Tech no log i cal Hot Air, Pro ceed ings, Lucrarile Seminarului de Matematica Fizica, Universitatea Politehnica din Timisoara (pres ently Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat ics Phys ics Se ries), Mai, 1983., pp [24] De Sabata, C., Marcu, C., Luminosu, I., On the En ergy Flow Rate for a Plane So lar Col lec tor That Pro - vides Tech no log i cal Hot Air, Pro ceedings, Lucrãrile Seminarului de Matematica Fizica, Universitatea Politehnica din Timisoara (pres ently Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat ics Phys ics Se ries), Mai, 1983., pp [25] Luminosu, I., Nagy, M., Ex per i men tal and Numerical Calculation Study of the Heat Loss Coefficient at the Flat-Plane Col lec tors, Pro ceed ings, Pannonian Applied Mathematical Meetings, 2003 (PAMM), Bu - da pest, 2003, (Bul le tins for Ap plied and Computer Mathematics (BAM-2097)), Tech ni cal Uni ver sity of Bu da pest, 2003, pp [26] Luminosu, I., Flat-Plate So lar Col lec tor Tech ni cal Con struc tive Pa ram e ters De ter mi na tion by Nu mer i cal Cal cu la tion, Con sid er ing Tem per a ture Criterion, Romanian Reports in Physics, Romanian Academy, 56 (2004), 1, pp [27] Luminosu, I., Marcu, C., On the Heat Loss of Flat-Plate So lar Col lec tor, Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat ics-phys ics Se ries, 46 (60) (2001), 1, pp [28] Luminosu, I., Marcu, C., The Ac tual Value of the Equiv a lent Transmittance-Absorbance Prod uct of So lar Flat Plate Col lec tors, by Com puter Program and Experimental Determinations, Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat ics - Phys ics Se ries, 44(58) (1999), 2, pp [29] Luminosu, I., New As pects in the Eval u a tion of the Ef fi ciency of a Plane So lar Col lec tor Used for Tech no - log i cal Hot Air Prep a ra tion (in French), Proceedings, Lucrarile Seminarului de Matematica Fizica, Universitatea Politehnica din Timisoara (pres ent Sci en tific Bul le tin of the Politehnica University of Timisoara, Math e mat ics Phys ics Se ries) Mai, 1985, pp [30] De Sabata, C., et al., Ther mal Phe nom ena in the Ac tive Part of Trombe Walls (in Ro ma nian), Proceedings, Lucrarile Seminarului de Matematica Fizica, Universitatea Politehnica din Timisoara (pres ently Sci - en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat ics Phys ics Se ries) Mai, 1986, pp [31] De Sabata, C., et al., Ex per i men tal Study on Trombe Wall Ef fi ciency Con cern ing Dwell ing Climatization, Pro ceed ings, Lucrarile Seminarului de Matematica-Fizica, Universitatea Politehnica din Timisoara (pres ently Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat ics Phys ics Se ries), Mai, 1986, pp [32] Luminosu, I., Ex per i men tal Stud ies and Eco nomic Con sid er ations on a Liv ing Space Heated through Pas - sive So lar Gain and through Elec tric Power, Ther mal Sci ence, 7 (2003), 2, pp [33] De Sabata, C., Marcu, C., Luminosu, I., Pas sive So lar Sys tems Con tri bu tion to the In door Ther mal Com - fort (45 N-lat. zone), Pro ceed ings, In ternational Conference Indoor Air Quality and Climate in Central and East ern Eu rope, Cluj 93 (Sci en tific Book House Pub lisher, Cluj-Napoca, Romania), 1993, pp [34] De Sabata, C., Luminosu, I., On Pos si bil i ties of Par tial Air-Con di tion ing of Dwell ings in the Tem per a ture Area Us ing Pas sive Tech niques, Pro ceedings, National Conference Solar Energy, Politehnica Uni ver - sity of Timisoara, 1989, pp

13 THERMAL SCIENCE: Year 2010, Vol. 14, No. 1, pp [35] Athanasouli, G., Massouporos, P., A Model of the Ther mal Res to ra tion Tran sient State of an Opaque Wall af ter the In ter rup tion of So lar Ra di a tion, Solar Energy, 66 (1999), 1, pp [36] Peyturaux, R., So lar En ergy (in Franch), Press. Univ. de France, Paris, 1968 [37] Lo, S. N. G., Deal, C. R., Norton, B., A School Build ing Reclad with Thermosyphoning Air Pan els, Solar En ergy, 52 (1994), 1, pp [38] Torres Reyes, E., Navarrete Gon za les, J. J., Ibarra Salazar, B. A., Ther mo dy namic Method for De sign ing Dry ers Op er ated by Flat-Plate So lar Col lec tors, Renewable Energy, 26 (2002), 4, pp [39] Torres Reyes, E., et al., A De sign Method of Flat-Plate Col lec tors Based on Min i mum En tropy Gen er a - tion, Exergy In ter na tional Jour nal, 1 (2001), 1, pp [40] Bejan, A., En tropy Gen er a tion Minimization, CRC Press, Boca Raton, Fla., USA, 1996 [41] Luminosu, I., Fara, L., De ter mi na tion of the Op ti mal Op er a tion Mode of a Flat So lar Col lec tor by Exergetic Anal y sis and Nu mer i cal Sim u la tion, Energy, 30 (2005), 5, pp [42] Luminosu, I., Fara, L., Ther mo dy namic Anal y sis of an Air So lar Col lec tor, International Journal of Exergy, 2 (2005), 4, pp [43] Luminosu, I., Tamasdan, C., Com puter Pro gram for Cal cu la tion of the Com po nents of So lar Ra di a tion on Ar bi trarily Ori ented Sur faces (in Romanian), An nals of the Uni ver sity of Oradea, Phys ics se ries, tom IV (1994), pp [44] Luminosu, I., Balan, S., Terniceanu, S., A Com puter Study on the Daily Vari a tion of the Re flec tion Co ef - fi cient and the Equiv a lent Ab sorp tion-transmission Factor (in German), Scientific Bulletin of the Politehnica Uni ver sity of Timisoara, Math e mat ics-phyics Se ries, 38(52) (1993), 2, pp [45] Luminosu, I., Bizerea, M., Popa-Luminosu, C., Pos si ble Val ues of Some Phys i cal Quan ti ties Char ac ter iz - ing the Con ver sion of So lar En ergy into Heat Ob tained by Nu mer i cal Sim u la tion (in Romanian), Pro ceed - ings, Na tional Con fer ence for In stal la tions in Civil Engineering and Ambient Comfort, Politehnica Uni - ver sity of Timisoara, 2003, pp [46] Luminosu, I., Marcu, C., The Ac tual Value of the Equiv a lent Transmittance-Absorbance Prod uct of So lar Flat Plate Col lec tors by Com puter Pro gram and Ex per i men tal De ter mi na tions, Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat ics-phys ics Se ries, 44 (58) (1999), 2, pp [47] Luminosu, I., Balan, S., Terniceanu, S., Com puter Sim u la tion of the Hourly and Daily Vari a tion of the over Tem per a ture of a So lar Col lec tor (in Ger man), Sci en tific Bul le tin of the Politehnica Uni ver sity of Timisoara, Math e mat ics-phys ics Se ries, 38(52) (1993), 2, pp [48] Hottel, H. C., A Sim ple Model for Es ti mat ing the Trans mit tance of Di rect So lar Ra di a tion through Clear Atmosphere, Solar Energy, 18 (1976), 1, pp [49] Temp, R. C., Coulson, K. L., So lar Ra diation Incident upon Slopes of Different Orientation, Solar Energy, 19 (1977), 2, pp [50] Badescu, V., Ver i fi ca tion of Some Very Sim ple Clear and Cloudy Sky Mod els to Eval u ate Global So lar Irradiance, So lar En ergy, 61 (1997), 4, pp [51] Budisan, N., et al., Re vival by Au to mation Induction Generator for Distributed Power Systems in Romanian Ac a demic Re search, Re new able Energy, 32 (2007), 9, pp Pa per sub mit ted: April 2, 2008 Pa per re vised: No vem ber 16, 2009 Pa per ac cepted: De cem ber 10, 2009