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V i t o s o l v i E | M A N NF.A.Q.: Flat Plate vs Va cu um Tub e - W ha t 's thebest choice for your solar project?

I n f o r m a t i o nW a t e r l o o , O n t a r i oM a r c h 2 0 0 8

As the solar industry cont inues to em erge and grow into a more v iable industry , one of the mo st c omm onmisconcept ions and confus ions re lates to the per formance di f ference between f la t p late and vacuum tubecolle ctors . Claims of vastly superior perform ance m ay lead some to believe tha t vacuum tube collectors aremagical dev ices that can defy the laws of phys ics. This can be confu s ing for custome rs wh o are try ing todecide wh ich solar techno logy is best for them . W ith many produ cts being imported into Canada fromEurope and Asia, there is often no common approval body or per formance s tandard that a l lows forobject ive comp ar ison. This ar tic le wi l l look at the construc t ion and per formance di f ferences between f la tp late and vacuum tube col lectors , and the best appl icat ions for each.Flat plate collectors are constructed very much l ike a smallgreenhouse. A square frame wi th a g lass cover houses the copperor a luminum absorber sheet through which the solar f lu id ispum ped . Minera l f iber insula tion on the back and side of the fram eis used to keep heat loss from the absorber to a minim um . Vac uumtub e collec tors do not rely on trad itiona l insulation to reduce theheat loss, but instead use the insulating properties of a vacuumsealed air space. As you kno w fro m your o ld therm os bo tt le , asealed evacuated airspace is one of nature's greatest insulators.Both co ndu ctive and con vec tive hea t losses are grea tly reduced in avac uu m. The solar absorber sheet of a vac uum t ube co llector is suspended inside of the vacu um sealed

glass tub e. These difference s in the insulating me thods are themain fundamental d i f ference between f la t and vacuum tubecolle ctors . There are 2 basic vac uum tube designs: the heat pipeand the d irect f low, wi th the main d i f ference being whether thesolar fluid goes through the header of the collector, or throughpipes ins ide each vacuum tu be. Both types of vacuum tubecollec tors have very similar perform ance charac teristics . Becauseof there simplicity, f lat plate collectors are generally much lessexpensive to produce than vacuum tube col lectors .

To tru ly understand the per formance di f ferences between the two col lector types, we must understand thefactors tha t effect solar col lector eff ic iency. Ei ther type of col lector s tar ts out wi th the same amount ofsolar radiation fall ing on it. A collecto r the n has tw o type s of losses: Op tical losses and Therm al losses.Optica l loss is solar radiation that reflec ts off the ou ter or inner layer of the glass glazing. Because of theround shape of their glass, vacuum tube collectors tend to have slightly higher optical losses than flat plate.Both types of col lectors have opt ical losses of approx ima tely 18 -22 % of the incoming solar radiat ion. Theremaining radiation that passes through the glass hits the absorber sheet where it is captured andtransferred to the solar f l u id . This absorbed radiation is no w subje ct to therm al losses. The am ount ofthermal losses wi l l vary from zero up to 6 0% . How much therm al loss a col lector exper iences is in f luencedby three factors : ambient a ir temperature, col lector f lu id temperature and the col lectors insulat ingproper t ies. Any col lector m anufa ctured w i l l have i ts eff ic iency p lotted on a col lector eff ic iency char t (Seef ig 1) . The char t shows the per formance of the col lector tak ing into account a l l o f the opt ical and thermallosses. The eff ic iency of any col lector is dynam ic, constant ly cha nging wh en the ambient a ir temperaturechanges, and with col lector f lu id temperature r is ing.Fla t p la te vs Vacuum tube.doc 1

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Now what does all this have to dowi th compar ingvacuum tube and f la t p late col lector per formance? It allcomes back to col lector thermal losses. Compar ing thecol lector eff ic iency curves of a typical f lat plate andvacuum tube col lector you can see that as thetemperature d i f ference between the ambient airandcol lector f lu id (Tambiam - T i i u i d ) increases the flat platecurve drops off at a greater rate. This is du e to thesuperior insulating properties of the vacuum air spacecompared to the mineral insula tion. So theperformance di f ference between f la t and vacuum tubecol lectors depends onwhen you are compar ing them.At t imes w i th warm air temperatures and/or low f lu idtemperatures (smal l temperature d i f ference) there isa lmos t nodif ference in eff ic iency. At t imes with coldair temperatures and/or h igh f lu id temperatures then

there can be up to a 3 0 % or higher d i f ference. In the summer months f la t and vacuum col lectors wi l lper form almost the same, whi le in the winter the vacuum tube wi l l per form better . If vacuum tube and f la tp late were the same pr ice then vacuum tube might be the preferred choice inmost appl icat ions. Butw i t hthe higher cost of vacuum tubes, then you have to look carefu l ly at the application to see if vacuum tubescan be just i f ied. Inmany solar applications especially DHW heating, simply increasing flat plate collectorarea wil l y ield thesame annual energy output as a vacuum tube col lector formuch less initial cost. Formost DHW appl icat ions where the load is re lat ive ly cons is tent all year round, flat plate collectors mostoften y ie ld thebest bang for the buck. Indoor pools also are best served byusing flat plate collectors dueto the low f lu id tempe rature requirements and constant year round loads. Vacuum tube col lectors are wel lsuited for appl icat ions that occur when ambient air temperatures are low, or where solar f lu id temperaturesare high. Combined DHW - space heat ing supplement systems wi l l o f ten use vacuum tubes to try an dcapture the max imum amount of solar radiat ion dur ing cold winte r mo nths. Another emerging ap pl icat ionfor vacuum tube col lectors is solar cooling systems which use solar thermal energy to drive anabsorpt ionchi ller syste m. Very h igh f lu id temperatures up to 90C are required, making vacuum tube col lectors the ?best col lector choice.

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Flat platecollector

10 20 30 40 50 60 70 80 90 100T e m p e r a t u r e d i f f e r e n c e IC"> b e t w e e n a m b i e n t amandc o l l e c t o r f l u i d ( T a n i h , , - T F , , i f i )

Solar system ty pical opera liig range:O Solar DHW or Solar indoor pool heattng Solar DHW w ith high volar traction Solar lor DHW and Spa ce heating ftripfemeWO Sola tor process heating or Solar air condition* g

When compar ing d i f ferent col lectors , it is also important to understand how collector areas are publishedby ma nuf actu rers. Collector areas are state d 3 w ay s: Gross area, aperture area and absorber area. W ithflat plate collectors the gross area isusually viry close to the aperture and absorber areas. How ever wi thvacu um t ube c ollect ors the absorber and aperture area" canbe much less tha n the gross area (see fig 2). ,When compar ing col lectors , and for s iz ing systems purposes, theaperture area isthe d imension to use,*

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V I E | M A N N

Unbiased per formance data is important when compar ing anyprod uct available in the mark etpla ce. W ith solar panels this cansometime s be di f f icu l t wi th produ cts coming from North Am er ica,Europe and Asia . Perform ance data in ma nufa cture rs data sheetsmay be di f f icu l t to compare due to the d i f ferent te st ing s tandardsused, and di f ferent uni ts used. One way to com pare solar productsfair ly equa lly is to use the ratings produc ed by the S olar Rating andCert i f icat ion Co rporat ion. The SRCC tests and cert i f ies col lectorand packaged system per formance for solar products sold in theUS A. All collector test results are published in the "D irecto ry ofSRCC cer t if ied Solar col lector rat in gs", w hich can be dow nloade df rom w w w .s o la r - r a t i ngs .o r g . Each collector has a CollectorThermal Per formance Rating char t , which l is ts the expected dai lyBtu prod uctio n per collec tor, und er a variety of solar radiationlevels , and with d i f ferent ambient a ir and f lu id temperaturediffere nce s. These charts wil l al low you to really see thedi f ferences in the col lector therm al losses whe n compar ing f la t andvacuu m tube col lectors . You can also see that w i th c lear skyradiat ion and warm air temp eratures the per formance of a fla t p latecol lector is tough to beat.

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There are several other factors that differentiate flat and vacuum tube collector, and these can also comeinto p lay whe n dec id ing on a col lector type . Addi t ional factors th at m ay s teer you to a f la t or vacuum tubecollecto r include: Roof area available, win d loadin g, handling and asse mb ly, and collector array piping forlarger scale syst em s. There are really no absolute rules wh en ch oosin g solar collectors, as ultim ately a flator vacuum tube col lector can be used for any solar appl icat ion. Unde rstanding the technical d i f ferencesand de-myst i fy ing some of the manufacturer c la ims and product data, wi l l help you to make the best, andmost cost effect ive choice, for your solar appl icat ion.

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