Analysis of Solar Flat-plate Collectors

Transcription

1 1/150 Analysis of Solar Flat-plate Collectors Tomáš Matuška Faculty of Mechanical Engineering University Centre of Energy Efficient Buildings Czech Technical University in Prague Solar Thermal Collectors, , Palma de Mallorca

2 Content of lectures today 2/150 design tool KOLEKTOR for theoretical analysis of flat-plate collectors introduction into software downloading, installation, support files,... theoretical modelling of collector heat output step by step external energy balance of absorber internal energy balance of absorber what influences annual energy yields of collector examples solved with use of KOLEKTOR and annual simulation in TRNSYS

3 3/150 Design Tool KOLEKTOR 2.2 for Virtual Prototyping of Solar Flat-plate Collectors Tomáš Matuška, Vladimír Zmrhal Faculty of Mechanical Engineering University Centre of Energy Efficient Buildings Czech Technical University in Prague Solar Thermal Collectors, , Palma de Mallorca

4 Background & Motivation 4/150 Lack of software for solar thermal collector modeling: detailed parameters of solar collector detailed calculation of heat transfer user friendly general use freely available CoDePro (Madison), TRNSYS Type 103 (CSTB) wide application range for FP collectors (evacuated, building integrated, transparent cover structures, HT fluids) versatile for different heat transfer models (convection, sky radiation)

5 Model inputs and outputs 5/150 Inputs: detailed parameters of individual parts of solar collector geometry, thermophysical parameters, optical parameters climatic conditions: ambient temperature, irradiation, wind velocity operation conditions: input temperature t in, mass flowrate m, slope b Outputs: efficiency curve h (based on aperture area A a ) output temperature from solar collector t out detailed temperature distribution in collector, heat transfer coefficients

6 Principle of calculation 6/150 external balance of absorber heat flow from absorber surface to ambient temperature distribution at main collector levels (nodes p, z, b) overall U-value of collector internal balance of absorber heat flow from abs.surface to fluid heat removal factor F R absorber temperature t abs Q u AaFR G U tin ta heat gain Q u

7 External balance of absorber 7/150 heat transfer coefficients calculation based on temperatures of main nodes iteration loop for temperature of nodes (starting values based on t abs, U-value calculation, reverse calculation of temperature distribution) starting absorber temperature is estimated from input temperature (t abs = t in + 10 K)

8 Internal balance of absorber 8/150 heat transfer by fin conduction, bond conduction and forced convection in pipes iteration loop for mean fluid temperature (starting values based on t in, F R calculation, reverse calculation of mean fluid temperature), absorber temperature calculation starting mean fluid temperature is estimated from input temperature (t m = t in + 10 K)

9 Calculation procedure 9/150 external and internal balance iteration loops (5 loops enough) superior iteration loop governing output / inputs

10 Design tool KOLEKTOR 2.2 (2009) 10/150 model converted into program based on Visual Basic Studio concept of cards for data inputs selections in rolling windows default settings saving settings of parameters, loading settings installation pack 400 kb (zipped) installation for Win XP presume Microsoft.NET Framework installed new Windows include the Framework environment

11 Features and availability 11/150 user-friendly software KOLEKTOR 2.2 is useful for analysis, design and prototyping several experimental validations have been performed based on commercial collectors testing software tool KOLEKTOR 2.2 is freely available at: mathematical reference handbook as pdf theoretical background of the tool downloadable

12 Preparation for installation 12/150 local settings in Windows - change to Great Britain settings e.g. windows 8 Control Panel Language change of datum, time or number format further settings using dot in decimal values do it now...

13 Download KOLEKTOR and install now... 13/150 download kolektor22.zip unzip it Program files folder Kolektor will be created start setup.exe start installation process setup also opens the program for Windows versions higher than XP you don t need Microsoft.NET Framework should be included in...

14 Design tool KOLEKTOR 2.2 (2009) 14/150

15 Design tool KOLEKTOR /150

16 Design tool KOLEKTOR /150

17 Design tool KOLEKTOR /150

18 Design tool KOLEKTOR /150

19 Default example usual collector 19/150 download as default.kol File Open default.kol as zip file main default collector parameters solar glazing, transmittance 91 % standard flat-plate collector with selective coating: e = 5 % = 95 % cooper absorber 0.2 mm, risers 10/8 mm, distance 112 mm absorber glazing gap 20 mm, absorber insulation gap 5 mm insulation thickness 30 mm, mineral wool slope of collector 45

20 Calculation possibilities in collector 20/150 calculation for given t in analysis of heat transfer at individual parts of collector you can play with models... and see the impact

21 Calculation possibilities in collector 21/150 efficiency curve calculation range if t in from 10 C to 400 C to cover also high-temperature collectors run calculation results can be saved to file = text file, columns separated by semicolon ; get the efficiency coefficients h 0, a 1, a 2 by use of evaluation.xls important set the dots for decimals also in Excel settings

22 Solar DHW simulation in TRNSYS 22/150 download example SDHW simple solar system, modified for this Solnet course climate data Wurzburg (in data folder) daily load 200 l, cold water 10 C, hot water 55 C collector area 4 m 2, storage tank 200 l, no pipes... simplified annual energy yields for changed construction of collector reference case: annual yields of collector solar fraction 50 % 1887 kwh/a

23 23/150 Analysis of Solar Flat-plate Collectors part 1 Tomáš Matuška Faculty of Mechanical Engineering University Centre of Energy Efficient Buildings Czech Technical University in Prague Solar Thermal Collectors, , Palma de Mallorca

24 Content 24/150 how to calculate thermal output of solar collector on theoretical basis from detailed data on construction at given operation conditions with use of different alternative models of heat transfer phenomena to predict the influence of change in construction of the collector operation conditions on efficiency curve on annual energy yields of the collector

25 Solar thermal flat-plate collector 25/150 Transparent cover Absorber Header to draw the heat off Insulation Riser pipes with heat transfer fluid thermal insulation cover glazing frame Collector frame back frame full plate absorber header riser

26 Solar collector energy balance 26/150 Heat loss through glazing Useful heat removal Reflection from absorber Reflection from cover Solar irradiation Heat loss through edge and back sides of collector frame

27 Solar collector energy balance 27/150 External energy balance of absorber heat flow from absorber surface to ambient environment heat losses of the collector quality of solar collector envelope Internal energy balance of absorber heat flow from absorber surface into heat transfer fluid ability to transfer heat and remove it from collector quality of absorber construction

28 External energy flows balance of absorber 28/150 dq dt. Q s. Q l,o. Q l,t. Q u general description steady state dq/dt = 0. Q u. Q s. Q l,o. Q l,t Q s solar energy input [W] Q s = G.A c Q l,o optical losses [W] Q l,o = Q s - Q s () ef Q l,t thermal losses [W] Q l,t = U.A c (t abs t a ) Q u useful heat removed from collector [W] Q = Mc(t out t in )

29 External energy balance: efficiency 29/150 useful heat output:. Q u GA c UA t t ) ef c( abs a efficiency: Q h Q u s Q u GA c GA c UA t t ef GA c c abs a h ef U ( tabs ta ) G

30 Efficiency formulas 30/150 h ef U ( tabs ta ) G external balance, losses h F' ( t t G U m a ef ) + internal balance influence of absorber design European standards ( t t ) h F a R ef G U in + influence of flowrate, specific heat of fluid US standards

31 Solar collector efficiency (based on t abs ) 31/150 optical losses thermal losses h ef U ( tabs ta ) G efficiency

32 Reference area for calculations 32/150 gross area: A G edge side area: A b aperture area: A a maximum area = H G.L G (2H G + 2L G ).B area of glazing = H a.l a

33 Reference area - comparison 33/150 Collector testing standards relates collector efficiency to: aperture area A a absorber area A A gross area A G EN EN ISO 9806 aperture area: for comparison of collector properties or design gross area: for decision on roof production potential for comparison of collector with different active areas

34 Reference area - comparison 34/150

35 External energy balance: detailed 35/150 U p U b U p = front U-value U z U z = back U-value U b = side U-value. Q u GA a ef U p A G ( tabs ta ) UzAG ( tabs ta ) UbAb ( tabs ta )

36 Scheme of external balance 36/150

37 Wind convection 37/150 Wind induced convection heat transfer from cover to ambient: mix of natural and forced covection turbulent environment very dependent on location, terrain morphology, barriers, etc. Number of models existing for number of boundary conditions wind tunnel measurements (low turbulence) outdoor experiments (on roof, on ground, urban, countryside) Most of wind convection models empirical linear function h w a bw

38 Wind convection 38/ for list of models see Reference handbook on web McAdams Test Sharples Watmuff more realistic h w [W/m 2 K] Johnson Sparrow Theory wind tunnels 5 how the largest difference in models influence the calculation w [m/s]

39 Wind convection for w = 3 m/s 39/150 how can the model influence calculated output temperature? run KOLEKTOR programme open default.kol Design parameters Calculation card McAdam s correlation h w = 16.1 W/m 2 K t out = 55.6 C wind velocity w = 3 m/s select Calculation for given t in select McAdams... Calculate select Watmuff... Calculate Watmuff correlation h w = 11.3 W/m 2 K t out = 55.7 C

40 Wind convection for w = 3 m/s 40/150 how can the model selection influence the efficiency curve? Calculation card select McAdams select Watmuff select Efficiency curve calculation Calculate, Results export to file Calculate, Results export to file open Evaluation.xls make copies of the sheets open res files in excel, mind the semicolons as separators compare the efficiency curves based on (t m t e )/G

41 Wind convection for w = 3 m/s 41/150

42 Wind convection for w = 3 m/s 42/150 how can the model selection influence the annual energy yields? McAdams h 0 = a 1 = a 2 = Watmuff h 0 = a 1 = a 2 = open studio SDHW-SOLNET in TRNSYS make a change of coefficients in KOLEKTOR type results of collector yield find under Totals.txt compare the annual sums for both alternatives

43 Wind convection for w = 3 m/s 43/150 McAdams: Watmuff: 1887 kwh/a 1911 kwh/a 1.2 %

44 Wind convection for w = 3 m/s 44/150 selection of the wind convection models minor influence even if two models with largest difference selected valid for usual quality flat-plate collectors (as our reference is) negligible difference for high quality flat-plate collectors very critical for unglazed collectors modelling less critical for PV modules modelling reduced influence on electric yields via temperature coefficients

45 High wind velocity as test condition 45/150 w testing standard: air velocity during the testing procedure w > 3 m/s use of artifical wind needed!

46 Sky radiation 46/150 Radiation heat exchange q p1o e p1 ( T 4 p1 T 4 o ) [W/m 2 ] Sky temperature, sky emittance T 4 o e T o 4 a [K] Sky radiation heat transfer coefficient related to ambient temperature h s,p1a e p1 T T 4 p1 p1 T T 4 o a [W/m 2 K]

47 Sky temperature models 47/150 Number of models for sky temperature T o calculation based on: ambient temperature T a dew point temperature T dp water vapour pressure p d sky clearness index K o clear sky conditions cloudy sky conditions cloudy sky To T a clear sky To T 1. 5 for more see Reference handbook on web a (Swinbank, 1963)

48 Sky temperature models 48/150 Sky temperature calculation based on: T 1. 5 o T a To T a e p1 = 0.85, t p1 = 40 C, t a = 20 C t o = 3.9 C h s,p1-a = 5.38 W/m 2 K h s,p1-a = 8.97 W/m 2 K e p1 = 0.10, t p1 = 40 C, t a = 20 C h s,p1-a = 0.63 W/m 2 K h s,p1-a = 1.06 W/m 2 K considerable decrease of heat transfer coefficient if low-e coating does it mean also improvement of collector performance?

49 Coupling sky radiation + wind convection 49/150 heat transfer from glazing exterior surface: proportions q = q s (sky radiation) + q p (wind convection) w h s h p h s + h p [m/s] [W/m 2 K] [W/m 2 K] [W/m 2 K] / / / / / / / / 26.8 convection heat transfer is dominant (if wind is present...)

50 Influence of sky radiation reduction 50/150 Dh = 1 % not considered reduction of solar transmittance!!!

51 Influence of sky radiation reduction 51/150

52 Conduction through cover glazing 52/150 heat conductance h gl h v,p1p2 L gl gl [W/m 2 K] single glazing: glass: gl = 0.8 W/mK, L gl = 4 mm h gl = 200 W/m 2 K PC: gl = 0.2 W/mK, L gl = 4 mm h gl = 50 W/m 2 K practicaly negligible

53 Conduction through cover glazing 53/150 transparent insulation structures channel structures: honeycombs: aerogels: h gl = 2 to 8 W/m 2 K h gl = 1 to 2 W/m 2 K h gl = less than 1 W/m 2 K function of mean temperature

54 Conduction through cover glazing 54/150

55 Radiation between absorber and cover 55/150 e p2 e abs,p Radiation heat exchange q s,abs-p2 T 1 e p2 4 abs T 1 e abs,p 4 p2 1 [W/m 2 ] t p2 t abs

56 Radiation between absorber and cover 56/150 influence of absorber emissivity t abs 40 C t p2 20 C e abs,p e p h s,p1-a 4.7 W/m 2 K 0.6 W/m 2 K 0.3 W/m 2 K 8x 14x

57 Spectrally selective absorber 57/150 how can the absorber emissivity influence the thermal performance of the solar collector? influence on efficiency curve influence on the yields are low-emittance coating for absorbers needed? compare e abs = 0.85 e abs = 0.10 e abs = 0.05

58 Spectrally selective absorber 58/150 run KOLEKTOR programme, open default.kol Absorber card Calculation card change Front surface emissivity select Efficiency curve calculation Calculate, Export results open Evaluation.xls make copies of the sheets for three alternatives open res files in excel, mind the semicolons as separators compare the efficiency curves based on (t m t e )/G

59 Spectrally selective absorber 59/150

60 Spectrally selective absorber 60/150 how can the absorber emissivity influence the annual energy yields of collector? e abs = 0.05 h 0 = a 1 = a 2 = e abs = 0.10 h 0 = a 1 = a 2 = e abs = 0.85 h 0 = a 1 = a 2 = open studio SDHW-SOLNET in TRNSYS make a change of coefficients in KOLEKTOR type results of collector yield find under Totals.txt compare the annual sums for both alternatives

61 Spectrally selective absorber 61/150 e abs = 0.05: 1887 kwh/a e abs = 0.10: 1858 kwh/a e abs = 0.85: 1590 kwh/a -1.5 % -16 %

62 Spectrally selective absorber 62/150 new cermet coatings with extremely low-emittance are about 1.5 % better than old-fashioned galvanic coatings there could be no fear about nonuniformity of coating emittance at the whole absorber area it does not matter for annual effectivity facts above valid for domestic hot water systems significant difference for high emittance coatings very critical for high temperature operation much higher impact on the annual yields

63 Natural convection in the air gap 63/150 coupled convection and conduction heat transfer in closed gas layer h pv g Nu L L [W/m 2 K] Characteristic dimension: thickness of gas layer L Nusselt number: characterization of conduction heat transfer g /L enhancement by fluid convection (driven by buoyancy) Nu L definition: ratio of the total heat transfer to conductive heat transfer

64 Convection in the air gap - criteria 64/150 Nusselt number: Nu L = f (Gr L, Pr) Grashof number: Gr L 3 g L Dt b 2 2 g L 3 ( t h 2 T T ratio of the buoyancy to viscous force acting on a fluid h c t c ) Prandtl number: Pr c a express conformity of velocity and temperature fields, ratio of momentum diffusivity (kinematic viscosity) to thermal diffusivity

65 Convection in the air gap - criteria 65/150 Rayleigh number: Ra Gr Pr 3 βgl th a - t c ratio of buoyancy forces to thermal and momentum diffusivities of fluid for standard conditions 400 < Gr < < Pr < < Ra <

66 Structure of the convection models 66/150 Hollands: Nu L / (sin1.8 ) 1708 Ra L cos Ra cos L RaL cos 5830 Buchberg: Nu L RaL cos Nu Ra cos L 29 L Randall: Nu L Ra L cos 2 ( 45) 0. Niemann: Nu L 1 m(ra Ra L L ) K n for more see Reference handbook on web

67 Convection in the air gap optimum L 67/150 slope 45, absorber 60 C, glass 20 C application region impractical region

68 Slope dependence 68/150 absorber 60 C, glass 20 C Hollands applicable range 0 to 60

69 Slope dependence 0 69/150 absorber 60 C, glass 20 C

70 Slope dependence 45 70/150 absorber 60 C, glass 20 C

71 Slope dependence 90 71/150 absorber 60 C, glass 20 C

72 Convection in inclined air layer 72/ Ra L = 10 6 Schinkel Randall Hollands Buchberg A1-correlation 6 Nu L 5 4 Ra L = Ra L = slope [ ]

73 Slope impact on collector efficiency 73/150 increase of slope angle decrease of front heat loss increase in efficiency

74 Slope impact on collector efficiency 74/150 results from testing at cca 1000 W/m 2

75 Convection in the back air gap 75/150 in the range 90 < < 180 Nu L 1 [Nu L ( 90) 1] sin Arnold (1975) T c T h T h T c

76 Convection vs. slope 76/150 Nu = 1

77 Radiation between absorber and frame 77/150 Radiation heat exchange q s,abs-z2 T 1 e z2 4 abs T 1 e abs,z 4 z2 1 e abs,z e z2 treatment of insulations: aluminium foil applied to mineral wool t abs t z2 e z2 = 0.1 is it needed?

78 Radiation between absorber and frame 78/150 example: internal frame insulation surface emittance e z2 t abs = 60 C, t a = 20 C, back insulation 30 mm, air gap d z2 case 1: d z2 = 20 mm e z2 = 0.5 U z = 1.10 W/m 2 K e z2 = 0.1 U z = 0.96 W/m 2 K 15 % case 2: d z2 = 5 mm e z2 = 0.5 U z = 1.22 W/m 2 K e z2 = 0.1 U z = 1.17 W/m 2 K 4 %

79 Radiation between absorber and frame 79/150 back air gap 20 mm back air gap 5 mm it has almost no impact if there is aluminium foil or not e z2 = 0.5 e z2 = 0.1 e z2 = 0.5 e z2 = 0.1

80 Heat conduction through frame insulation 80/150 heat conductance of insulation layer h ins h v,z1 z2 L ins ins insulation thermal conductivity: mineral wool: PUR foam: polystyren: [W/m 2 K] = W/mK = W/mK weak resistance to thermal load [W/mK] 0,07 0,06 0,05 0,04 thermal conductivity = f (t) ins ins t t t , t m [ C] 2 2

81 Thermal conductivity of mineral wool 81/150 polynomic function

82 Insulation thickness in collector 82/150 how can the thickness of insulation influence the thermal performance of the solar collector? influence on efficiency curve influence on the yields what insulation thickness is reasonable? compare 20 mm 30 mm 50 mm

83 Insulation thickness in collector 83/150 run KOLEKTOR programme, open default.kol Glazind & insulation card Calculation card change Insulation thickness select Efficiency curve calculation Calculate, Export results open Evaluation.xls make copies of the sheets for three alternatives open res files in excel, mind the semicolons as separators compare the efficiency curves based on (t m t e )/G

84 Insulation thickness in collector 84/150

85 Insulation thickness in collector 85/150 how can the absorber emissivity influence the annual energy yields of collector? 20 mm h 0 = a 1 = a 2 = mm h 0 = a 1 = a 2 = mm h 0 = a 1 = a 2 = open studio SDHW-SOLNET in TRNSYS make a change of coefficients in KOLEKTOR type results of collector yield find under Totals.txt compare the annual sums for both alternatives

86 Insulation thickness in collector 86/ mm: 1887 kwh/a 30 mm: 1858 kwh/a 50 mm: 1950 kwh/a -3 % +3 %

87 Insulation thickness in collector 87/150 compared to 30 mm of mineral wool double thickness of insulation brings only several percents 80 to 90 % of the collector heat loss is the front side (top) loss increase of insulation on the back side cannot proportionally help change of thermal conductivity by tens of percents due degradation will not significantly change the energy yields (by percents) it does not matter for annual effectivity facts above valid for domestic hot water systems

88 88/150 Radiation between frame and adj. surface Radiation heat exchange between frame of collector and roof q s,z1-as T 1 e z1 4 z1 T 1 e as 4 as 1 [W/m 2 ] e z1 e as any treatment of external frame surface is useless wind convection frame insulation supress any influence t z1 t as = t a

89 89/150 Collector heat loss coefficient: U-value p2 p,abs p2 s,abs p2 v,p1 a p,p1 a s,p1 p h h h h h U z2 p,abs z2 s,abs z2 v,z1 a p,z1 a s,z1 z h h h h h U q p q z

90 90/150 Collector heat loss coefficient: U-value a a b b G z G p G G A U A U A U A U A U a G G b z z p a A A A A U U U U a abs a a a abs G G t t A U Q t t A U Q l,t l,t for calculations: U-value related to aperture area A a

91 Iterations for temperatures 91/150 calculation of heat transfer coefficients: temperatures t abs, t p1, t p2, t z1, t z2, (t b1, t b2 ) are required but not available at the start of calculation example of iterative determination for front heat flow: 1. estimate: t abs = t in + Dt 2. estimate: t p1, t p2 t p2 t abs t abs t 3 a t p1 t a t abs t 3 a 3. calculation of heat transfer coefficients h 4. calculation of overall heat flow rate q p 5. reverse calculation of temperatures and then repeat from (3.)

92 Iterations for temperatures 92/150 q p U p t q p abs t a q p h h t t s,absp1 p,absp1 abs p1 q q p p h h v,p1 p2 t p1 t p2 h t t s,p2a p,p2a p2 a temperature difference is proportional to heat resistance

93 1. estimation of temperatures 93/150

94 Last iteration 94/150

95 Convergence of calculations 95/150 absorber temperature 35 C ambient temperature 25 C

96 Results of external balance 96/150 U-value of solar collector dependent on: temperature of absorber ambient temperature, wind velocity, sky temperature geometry of solar collector detailed properties of collector elements, used material (conductivity), surface emittance adjacent structure properties (emittance, envelope thermal resistance)

97 97/150 Analysis of Solar Flat-plate Collectors part 2 Tomáš Matuška Faculty of Mechanical Engineering University Centre of Energy Efficient Buildings Czech Technical University in Prague Solar Thermal Collectors, , Palma de Mallorca

98 Solar collector energy balance 98/150 External energy balance of absorber heat flow from absorber surface to ambient environment heat losses quality of solar collector envelope Internal energy balance of absorber heat flow from absorber surface into heat transfer fluid ability to transfer heat and remove it from collector quality of absorber construction

99 Internal energy balance 99/150 absorber temperature

100 Energy balance of absorber fin 100/150 width W = distance between riser pipes T fin T 0 T w D e = 2a = 2 bond width fin heat transfer L fin bond heat transfer L b pipe-fluid heat transfer h i p D i bond temperature t b = t w (wall temperature)

101 Energy balance of absorber fin 101/150 solar irradiation U T a case of standard fin efficiency concept but with solar irradiation known from air-water heat exchangers theory

102 Energy balance of absorber fin element 102/150 UDx T T a GDx abs d abs dt dx x abs d abs dt dx xdx derivation of temperature distribution between tubes (in absorber fin) G D x U D x dt dx dt dx T T d d 0 a abs abs x abs abs xdx yields in solution of differential equation of 2nd order d 2 dx T 2 abs U d abs T T a S U

103 Fin efficiency F 103/150 standard fin efficiency tanh[ m( W De F (rectangular profile) m( W D )/ 2 e )/ 2] (W - D e )/2 or (W - 2a)/2... active fin length soldering ultrasonic welding

104 Fin efficiency F 104/150 standard fin efficiency tanh[ m( W De F (rectangular profile) m( W D )/ 2 e )/ 2] m U abs d abs U... collector heat loss considered from absorber surface to ambient (result from external balance) significant influence of thermal conductivity and thickness of absorber conductivity and thickness of absorber are improportional higher fin efficiency = higher heat removal from absorber

105 Fin efficiency F 105/150 reference case: fin efficiency 0.96 copper absorber (390 W/mK), d abs = 0.2 mm, fin W* = W - 2a = 100 mm heat loss coefficient U = 4 W/m 2 K aluminium (240 W/mK) d abs = 0.20 mm, W* = 79 mm d abs = 0.32 mm, W* = 100 mm steel (80 W/mK) d abs = 0.2 mm, W* = 45 mm d abs = 1.0 mm, W* = 100 mm EPDM (0.14 W/mK) d abs = 2.0 mm, W* = 6 mm

106 Plastic absorbers 106/150

107 Best absorbers fully wetted metal sheet 107/150 F = 1.0

108 Influence of material and geometry 108/ copper (Cu) 390 W/(m.K) W = 50 mm 0.8 aluminium (Al) 250 W/(m.K) 0.8 W = 125 mm steel (Fe) 100 W/(m.K) W = 200 mm h [-] h [-] (t m - t e )/G [m 2.K/W] (t m - t e )/G [m 2.K/W]

109 Efficiency factor F 109/150 how efficient is heat transfer from absorber surface to heat transfer fluid? U F' o U liquid to ambient loss abs.surface toambient loss or q F' q t c c t abs abs t m U U o heat resistances: conduction through fin conduction through bond convection inside pipe

110 110/150 Collector performance based on t m G t t U F a m h ' G t t a G t t a a m a m h h useful heat gain from collector )] ( [ ' a m u. t t U G AF Q collector efficiency analogy to experimentally obtained efficiency curve reported by testing institutes 2 out in m t t t

111 Efficiency factor F 111/150 F' W 1 U[ 2a ( W 1/ U 2a) F] 1 C b h fi 1 p D i function of fin efficiency F

112 112/150 Efficiency factor absorber designs i fi b D h C a F W a U W U F p ] ) ( [ / ' i fi D h a F W a U W U F p ] ) ( [ / ' F a W W C WU W a D h WU F b i fi p '

113 Bond conductance 113/150 bond conductance estimated from geometry and quality of contact C b a b b good metal-to-metal contact needed (welding, soldering, pressing) no clamping of absorber to pipes! bond conductance > 30 W/mK needed

114 Bond conductance 114/150 for C b < 30 W/mK significant reduction of collector performance 1,0 0,8 300 W/m.K 30 W/m.K 3 W/m.K practically no difference between high and middle quality bonds h [-] 0,6 0,4 brazed contact 0,2 0,0 0,00 0,05 0,10 0,15 0,20 (t m - t a )/G [m 2.K/W] ultrasonic welded contact

115 Bond conductance: testing 115/150 h [-] 1,0 0,8 0,6 0,4 clamped absorber touching absorber welded absorber touched by press 0,2 clamped bond 0,0 0,00 0,05 0,10 0,15 0,20 (t m - t a )/G [m 2.K/W]

116 Convection heat transfer inside pipes 116/150 forced convection heat transfer in pipes is given by: heat transfer fluid type collector flow rate concept of collector hydraulic (parallel, serial-parallel, serpentine) number of risers (flow rate distribution) diameter of risers temperature

117 Convection heat transfer inside pipes 117/150 Nusselt number as criterion, number of models are available for laminar flow (mostly present in collector), turbulent flow constant heat flux case (uniformly irradiated absorber condition) constant temperature (heat transfer with phase change) fully developed profile, entry region with developing profile of velocity and temperature h f fi NuD D i

118 Nusselt number in circular pipes 118/150 laminar flow, fully developed velocity and temperature profile, constant heat flux Nu D (Shah) laminar flow, entry region of length L, developing profile, constant heat flux Nu D 1/ L * L * L* 0.03 L* 0.03 (Shah) L* Gz 1 L / Di Re Pr D Gz... Graetz number

119 Nusselt number in circular pipes 119/150 turbulent flow Nu D A Re m D Pr n (Dittus-Boelter, Colburn, Sieder-Tate) Nu D f / 8 Re D Pr 2 2/ 3 f / 8 1/ Pr 1 (Gnielinski) where f 0.79 lnre 1. 2 D 64 for smooth pipes for more models see Reference handbook to KOLEKTOR

120 Convection heat transfer inside pipes 120/150 high heat transfer require high velocity higher flow rate low dimension low viscosity high conductivity of liquid high Reynolds number high Nusselt number

121 Convection heat transfer inside pipes 121/150 hfi [W/m 2.K] water (high-flow) water (low-flow) PPG (high-flow) PPG (low-flow) h fi = 200 to 400 W/m 2 K high flow: kg/s.m 2 low flow: kg/s.m x0,5 6x0,5 8x0,8 10x1 12x1 15x1 18x1 22x1 28x1 risers size [mm]

122 Influence of riser pipe diameter 122/150 F' W 1 U[ 2a ( W 1/ U 1 2a) F] C b h fi 1 p D i 1 h pd fi i Di 1 Nu p D f i 1 Nu p f term is dependent directly on Nu laminar flow Nu D Nu D f (constant) f ( L*) L* L D i u. D i thermal diffusivity L. pdi pl 2 D 4V 4V i 2 term 1 h i pd is independent of D in laminar flow

123 Analysis of efficiency factor F 123/150 length L = 2 m, fin width W = 100 mm, absorber thickness d = 0.2 mm, copper = 350 W/m.K, pipe D e /D i = 10/8 mm bond conductance C b = 250 W/K h i = 400 W/m 2 K U [ D e 1 ( W D e ) F ] 1 Cspoj h 1 p i D i U = 4 W/m 2.K 2,6 0,004 0,099 heat transfer coefficient: minor influence to F bond conductance: minor influence to F if h i > 200 W/m 2 K if C b > 30 W/mK geometry is principal property of absorber if low heat loss collector

124 Do we need for turbulators in pipes? 124/150 length L = 2 m, fin width W = 110 mm, absorber thickness d = 0.2 mm, copper = 350 W/m.K, pipe D e /D i = 10/8 mm bond conductance C b = 200 W/K U = 4 W/m 2 K h 1 F p i D i U = 4 W/m 2 K h i = 400 W/m 2.K h i = 600 W/m 2.K enhancement of heat transfer inside pipe by 50 % minor influence to F change by 1.3 %

125 Fully wetted absorber vs. fin&tube 125/150 how can the geometry of the absorber influence the thermal performance of the solar collector? influence on efficiency curve influence on the yields fully wetted absorber from steel type of bond middle, pipes D e /D i = 6/5 mm, bond a/b = 3/1 mm material steel, thickness 0.5 mm bond conductivity 100 W/mK number of channels 145 pcs (not 150 pcs)... W = 6.01 mm > D e

126 Fully wetted absorber vs. fin&tube 126/150 run KOLEKTOR programme, open default.kol Absorber card make the changes Material, Type of bond change Geometry, dimensions Calculation card select Efficiency curve calculation open Evaluation.xls Calculate, Export results make a copy for the wetted alternative open res file in excel, mind the semicolons as separators compare the efficiency curves based on (t m t e )/G

127 Fully wetted absorber vs. fin&tube 127/150

128 Fully wetted absorber vs. fin&tube 128/150 how can the absorber geometry influence the annual energy yields of collector? fin&tube h 0 = a 1 = a 2 = fully wetted h 0 = a 1 = a 2 = open studio SDHW-SOLNET in TRNSYS make a change of coefficients in KOLEKTOR type results of collector yield find under Totals.txt compare the annual sums for both alternatives

129 Fully wetted absorber vs. fin&tube 129/150 fin & tube 1887 kwh/a fully wetted: 1999 kwh/a +6 %

130 Fully wetted absorber vs. fin&tube 130/150 fully wetted absorber could improve the annual energy performance by more than 5 % while steel could be cheaper material than copper

131 Fully wetted absorber vs. fin&tube 131/150 did we make a mistake in calculation? can geometry influence losses? fully wetted h 0 = a 1 = a 2 = fin&tube h 0 = a 1 = a 2 = both collectors have same losses! both collectors have same U-value! a 1 +a 2 (t m -t e ) = F U F =0.996 F =0.901 U=5.39 W/m 2 K for given DT coefficients h 0, a 1, a 2 are one complex and cannot be separated

132 Vacuum tube collectors x efficiency factor 132/150 Single glass vacuum tube / flat absorber direct flow concentric direct flow heat pipe firm metal contact absorber-pipe provides high F

133 Vacuum tube collectors x efficiency factor 133/150 Double-glass (Sydney) vacuum tube all glass concentric tube (Dewar) cylindric absorber (internal) glass tube cover (external) glass tube vacuum in space between absorber coating applied on exterior surface of internal glass tube conductive heat transfer fin in contact with interior surface problematic contact absorber-pipe...

134 Sydney vacuum tube collectors 134/150

135 Thermal analysis of direct flow VTC 135/150 given by Sydney tube tube collector scheme of heat transfer h F' U t m t G a contact fin: short (W), conductive (), thick (d) bond-contact: conductive; absorber tube-fin, fin-pipes heat removal: laminar / turbulent flow in pipes

136 Influence of contact fin 136/150 Sydney vacuum tube collectors with direct flow U-pipe register G > 700 W/m 2 contact fin is a principle element in Sydney collector

137 Contact resistance in heat pipe VTC 137/150 Heat pipe (phase change of working liquid, high heat transfer coefficients) evaporator (in contact with fin transfering the heat from inner glass tube) condenser (placed into manifold box)

138 Contact resistance in heat pipe VTC 138/ %

139 Temperature distribution in flow direction 139/150 q u DY fluid flow (M/n)cT f Y (M/n)cT f Y+DY Y DY derivation of temperature distribution between tubes (in absorber fin) M n ct f y M n ct f y Dy Dyq / u 0 yields in solution of differential equation of 1rst order

140 140/150 Heat removal factor F R mathematical derivation of temperature profile: see Duffie, Beckman (2006) definition: relates the actual collector gain to gain if absorber surface at fluid inlet temperature F R A c Mc tout t G U t in in t a F R Mc A U c exp 1 c A UF' M c equivalent to effectiveness of heat exchanger, ratio of actual heat transfer to maximum possible heat transfer

141 141/150 Heat removal factor F R heat removal factor is dependent on: efficiency factor F (geometry, quality of absorber, fin efficiency F,...) collector U-value (heat losses) specific heat of fluid c (type of fluid) specific mass flow through collector M / A c (thermal capacity of flow)

142 Water vs. propylene glycol 142/150 how can the heat transfer fluid influence the thermal performance of the solar collector? influence on efficiency curve collectors are tested with water, does anything change when filled with glycol? propylene glycol / water mixture 50 % / 50 % for freezing point -32 C 8x viscous at 20 C, high influence of temperature, only 2x at 80 C lower specific heat by cca 25 % worse heat transfer

143 Water vs. propylene glycol 143/150 run KOLEKTOR programme, open default.kol Absorber card make the change of fluid Calculation card select Efficiency curve calculation Calculate, Export results open Evaluation.xls make a copy for glycol alternative open res file in excel, mind the semicolons as separators compare the efficiency curves based on (t m t e )/G

144 Water vs. propylene glycol 144/150

145 Efficiency curve 145/150 1,0 0,8 h = f (t in ) h 0,6 0,4 h = f (t m ) h = f (t abs ) 0,2 0,0 0,00 0,02 0,04 0,06 0,08 0,10 (t - t a )/G [m 2 K/W]

146 Useful heat output of solar collector 146/150 based on absorber temperature, external balance of absorber Q u A c G U t abs t a based on mean fluid temperature, experimental work Q u F' A c G U t m t a based on fluid inlet temperature, simulation tools Q u F R A c G U t in t a

147 Solar collector temperatures 147/150 mean absorber temperature t abs t in. Qu/ A F U R c (1 F R ) mean fluid temperature t m t in. Q F u/ Ac R R U (1 F F' ) based on fluid inlet temperature fluid output temperature t out 2tm tin

148 148/150 Solar collector efficiency G t t U a abs h G t t U F a m h ' G t t U F a in h R based on absorber temperature based on mean fluid temperature based on fluid inlet temperature

149 Solar collector performance calculation 149/150 calculation model climatic data optical properties Q u operation parameters h t out material properties geometry

150 Conclusions of the day 150/150 KOLEKTOR is a design tool for virtual prototyping to perform calculations of designs without need for fabrication prototypes to optimize construction and design of collector KOLEKTOR is not perfect, but relevant TRNSYS type is on the way for modelling solar thermal collectors (Slava Shemelin) extension for glazed PVT collectors (Nikola Pokorny) efforts for extremely high performing solar thermal collectors could be sometimes useless if target is annual performance and not marketing