Testing Of Solar Thermal Devices

“PERFORMANCE TESTING OF SOLAR THERMAL DEVICES” by Prof. S. C. Mullick Centre for Energy Studies, Indian Institute of Technology Delhi, New-Delhi-110016, India

INTRODUCTION  Solar energy – one of the most promising renewable energy source  For thermal applications – Solar collectors  Components of a flat plate collector Glass cover

Absorber plate

Side Insulation

Bottom insulation Tubes

Optical Performance Related Issues of a Solar Collector Incident Solar Energy

Reflection from Glazing

Absorption in glazing Transmitted Energy

Reflection from Absorber Absorber Plate

Glazing Thickness

Energy Absorbed Optical Efficiency = Incident Solar (Radiation) Energy

Energy Absorbed = Incident Solar Radiation Optical Losses

Thermal Efficiency

=

Useful Energy Collected Incident Solar Energy

Useful Energy Collected = Energy Absorbed Thermal Losses . "



Q u  0 I t  U L (Tp  Ta )

U L  Ut  Ub  U s

F  '

" & Qu  o 

Heat Exchange Efficiency Factor Rate of Heat Loss Per Unit Area Optical Efficiency

It 

Total Insolation

UL 

Overall Heat Loss Factor

Tp 

Average Plate Temperature

Ta 

Ambient Temperature

UT 

Top Heat Loss Factor

Ub 

Bottom Heat Loss Factor

Us 

Side Heat Loss Factor

F

'

o

Collector Parameters

UL

It

Climatic Variables

Ta

Tp

Operating Variable

Upward Heat Losses in a Single Glazed Flat Plate Collector Ta

Glass cover

hw (Convective)

Tg

hcpg

Absorber plate Bottom insulation

(Convective)

Tp

hrga

(Radiative)

hrpg (Radiative)

Under steady state conditions

• Rate of upward heat loss per unit area from absorber plate to glass cover is 4 4  ( T  T p g ) Qt''   hcpg (Tp  Tg ) 1 1  1  p g

(1)

• Rate of upward heat loss per unit area from glass cover to the atmosphere is

Qt''   g (Tg4  Ts4 )  hw (Tg  Ta ) •

Tg

(2)

can be found by solving equations (1) and (2)

• The top heat loss factor is

Qt'' Ut  (Tp  Ta )

(3)

Analytical Equations -

 Mullick and Samdarshi (1988) – Single Glazed FPC



  Tp  Tg 2

U t   hcpg  

2

 T

p

 Tg  

1/  p  1/  g  1 



1



  hw  

 g  Tg  Ts   4

Tg  Ta

1

4





 Lg / k g

Tg  Ta  hw 0.38  0.567 p  0.403  Tp / 429   Tp  Ta   Samdarshi and Mullick (1991; 1994) -- Double Glazed FPC -- FPC with N glass covers

Akhtar and Mullick (1999)  12 X 10 

f 



8

T

a

 0.2Tp   hw 

1

3



 6 X 108   0.028 T  0.5T 3  0.6 L0.2  p  p a 

Tg 

 0.3L g

T

p

 Ta  cos 

fTp  Ta 1 f

Akhar and Mullick (2007) - glass cover temperarures (Double Glazed Collector)



1



0.25



Estimation of hcpg ------------------------------1. Holland’s correlation: +

+

 Ra cos φ    1708   1708( sin 1.8φ )  Nu = 1 + 1.44 1 −  − 1  +   1 − Ra cos φ Ra cos φ 5830        for 0 ≤φ≤60 deg and 0
1.6

1/ 3

2. Buchberg correlation:  1708  Nu = 1 +1.446 1 − Ra cos φ   

Nu = 0.229( Ra cos φ )

0.252

Nu = 0.157( Ra cos φ )

0.285

+

for

1708 < Ra cosφ <5900

for

5900 < Ra cosφ <9.23x104

for 9.23X104 < Racosφ <106

Estimation of hw -----------------------• McAdams (1954) hw = 5.7 + 3.8 V

( Wind tunnel )

• Wattmuff (1977) hw = 2.8 + 3 V

( Wind tunnel )

• Test et al.(1981) hw = 8.55 + 2.56 V

( Field study )

• Mullick et al. (2007)

hw = 7.07 + 3.25 V

V  1.12m / s

Estimation of sky temperature 1. Bliss correlation: Tdp − 273   Ts = Ta 0.8 +  250  

1/ 4

2. Swinbank correlation:

Ts = 0.0552(Ta )1.5

Thermal Performance of Collector

To It

."

.

Qu mc p  To  Ti    It It

m Ti

. "

 Q u   o I t  U L (Tp  Ta   

. "

Qu







    F '   o I t  U L  T w  Ta        . "









Qu T w  Ta  ' '    F o  F U L  It It   

Performance Curve of a Solar Collector Y

F’ηo Slope= -(F’UL)

η

X

Tw – Ta

Methods for Testing a Solar Collector • 2. 2.

•

Steady-State Collector Testing in a Closed Loop Set-Up National Bureau of Standards (NBS) Procedure ASHRAE Procedure

Testing by Two Test Method Open-Loop Collector Testing Following C.S.I.R.O.

Some of the Work on Collector Testing is Done at: • CSU • JPL • IIT Delhi

General Conditions for Conducting Tests Specified by NBS • Flow Rate = 0.02 Kg/s per m2 of the collector area • Variation in inlet and Outlet Temp of Water should not exceed 0.50C each • Inlet and Outlet temp of Water should be correct to 0.10C • I > 630 W/m2 • Ti – Ta Should Correspond to 10,30,50 and 70 degrees C • Testing Time Limits – 3 hrs., Before & After Solar Noon

Closed Loop Test Set-up Tf,o It

H.E.

Tf,i

Flowmeter

Pump

Cold Water

By-pass

mf By-pass Control

Schematic Diagram Showing Open Loop Testing Const. Head

Over Flow Tf,o

Hot Water

Tf,i

Water Collection

SOLAR COOKERS

Concentrating Type Solar Cooker

Bureau of Indian Standards Testing Method • The test procedure considered is based on Thermal Test Procedures for Box-Type Solar Cookers, by Mullick et al(1987). • This standard is presented in a more technical framework than ASAE S580. • provides two figures of merit, calculated so as to be as mostly independent of environmental conditions (such as ambient temperature, insolation, etc.) as possible.

• The two figures of merit are given by the following equations. F1 =

Tps − Tas

F1 ( MC ) w  F2  ln  At 



Hs





1  Tw1  Ta   1   F1  H    1  Tw 2  Ta   1   F1  H  

Variation of Plate Temperature with Time of the Day (Cooker Without Load) 140

Temperature(0C)

130 120 110 100 90 80 10.00

11.00

12.00

13.00

14.00 Time of the Day (hrs)

Under Stagnation, the Energy Balance for Horizontally Placed Empty Solar cooker is :

ηoHS = UL (Tps- Tas) ηo UL

=

(Tps- Tas) Hs

F1 = (Tps- Tas) Hs

Variation of Temperature of Water in the Vessels With Time of the Day (with load) 100

Water Temperature (0C)

95 90 85 80 75 70 65 11.20 11.40 12.00

ττ

12.20

12.40

13.00

Time of the Day (hrs)

  MC  w  dTW Ad

o"

 Qu  UL  F o  I   Tw  Ta    o  '

  d 

Tw 2 MC  w

A



Tw1



dTw 

 1 F '  I   Tw  Ta   F1  

 F1  MC  w '

 

AF ' o

F1  MC  w '

F ' o 

A

F 'o CR 



1 I   Tw 2  Ta   F1 ln   I  1  T T  w1 a  F1

 F2

   

 Tw1  Ta  I   Tw 2  Ta  I 

  

1  Tw1  Ta I    F1  I ln   I  1  Tw 2  Ta   F I  1 

  

1 I   F1 ln   I  1  F1 

F1  MC  w A







      

      



 F1 ( MC ) w   ln  F2 A   

 boil

1  Tw 2  Ta   1   F1  I   1  Tw1  Ta   1   F1  I  

 F1 ( MC ) w  1  100  Ta    ln  1     F2 A I   F1 

• Using the Equation, a characteristic curve can be developed that describes, for a given set of conditions, how long the cooker will take to reach the reference temperature (τboil).

Characteristic Curve of a Box Type Solar Cooker τboil = ∞

500

τBoil (minutes)

400 300

200

100

0 0.05 0.06 0.07 0.08 0.09

0.10

0.11 0.12

100 – Ta H

C

0

W/M

2

Paraboloid Concentrator Solar Cooker

Performance equation of Paraboloid Concentrator Solar Cooker :

  T T   T T  a w1 a   w2     I I  F 'U L  b b    F 'o  C  1  e   o  

e 



o













Variation of F’ηo as the Bright Spot Moves Across the Bottom of Pot

F’ηo

0.60

0.50

0.40 12:30

12:40

12:50

13:00

Time of Day (hrs.)

13:10

Time for Sensible Heating of water from ambient temp. Up to 100oC



 boil



 1

  o ln  

F 'U L 1  100  Ta    1 F 'o C Ib 

 

    

Characteristic Curve of a Paraboloid Concentrator Solar Cooker

τboil = ∞

250

τBoil (minutes)

200 150

100

50

0 0.05

0.10

0.15

0.20

100 – Ta Ib

0

C

W/M

2

The General Conditions for Conducting Tests as per European Standards : • Ambient temperature: 25°C-35°C • Wind velocity < 4 m/s (at the cooker) • Global irradiance (horizontal) >800 W/m2 • Diffuse fraction < 20%

References • Akhtar N. and Mullick S. C. (1999) Approximate method for computation of glass-cover temperature and top heatloss coefficient of solar collectors with single glazing. Solar Energy 66(5), 349-354. • Akhtar N. and Mullick S. C. (2007) Computation of glasscover temperatures and top heat loss coefficient of flatplate solar collectors with double glazing. Energy, 32(7), 1067-1074. • Bliss, R. W., Jr. (1981) Atomspheric radiation near the surface of ground. Solar Energy 5, 103-120. • Buchberg H., Catton I. and Edwards D. K. (1976) Natural convection in enclosed spaces- a review of application to solar energy collection. ASME Journal of Heat Transfer 98(2), 182-188. • Hollands K.G. T., Unny T.E., Raithby G.D. and Konicek L. (1976) Free convective heat transfer across inclined layers. ASME Journal of Heat Transfer 98, 189-192.

• McAdams W. H. (1954) Heat Transmission, 3rd edition. McGraw-Hill, New York. • Mullick S. C. and Samdarshi S. K. (1988) An improved technique for computing the top heat loss factor of a flat plate collector with a single glazing. ASME J. Solar Energy Engg.110, 262-267. • Samdarshi S. K. and Mullick S.C. (1991) An analytical equation for top heat loss factor of a flat-plate solar collector with double glazing. ASME J. Solar EnergyEngg.113, 117-122. • Samdarshi S. K. and Mullick S.C. (1994) Generalized analytical equation for the top heat loss factor of a flat plate collector with N covers. ASME J. Solar Energy Engg.116, 43-46.

• Swinbank W. C. (1963) Long wave radiation from clear skies. Quarterly Journal of Royal Metrological Society 89, 339-348. • Test F. L. Lessman R. C. L. and Johary A. (1981) Heat transfer during wind flow over rectangular bodies in natural environment. ASME J. Heat Transfer, 103, 262267. • Wattmuff J. H., Charters W. W. S. and Proctor D. (1977) Solar and wind induced external coefficients solar collectors. Int. revue d’ Hellio-technique 2, p.56. • Mullick S. C., Kumar Suresh, Chourasia B. K.(2007), Wind induced heat transfer coefficient from flat horizontal surfaces exposed to solar radiation, Proceedings of Energy Sustainability (ASME), June 2730, Long Beach,California

THANKS

Existing Standards • American Society of Agricultural Engineers Standard ASAE S580 • Bureau of Indian Standards Testing Method IS 13429(Parts 1,2 &3) • European Committee on Solar Cooking Research Testing Standard

Draw backs of ASAE S580 • The single figure of merit appears to be valueless for assessing why a cooker achieved a certain performance. • Therefore, any use of the ASAE standard to analyze the performance of a cooker, rather than simply compare its performance to another cooker would be very difficult.