Experiments with solar-energy utilization at Dacca

Experiments with

Solar-Energy Utilization at Dacca V a l e n t i n e G. DeSa Director of Extension, Advisory, and Research Services, East Pakistan University of Engineering and Technology, Dacca, East Pakistan This paper presents results of studies a t Dacca to utilize solar energy for w a t e r h e a t i n g a n d refrigera t i o n . M e a s u r e d values of r a d i a t i o n i n t e n s i t y w i t h a n Eppley p y r h e l i o m e t e r were generally in agreem e n t w i t h s t a n d a r d t h e o r e t i c a l values, except in m o n t h s w h e n e i t h e r t h e diffuse r a d i a t i o n a n d s c a t t e r e d cloud reflection or t h e h u m i d i t y of t h e a t m o s p h e r e was h i g h e r t h a n average. The corr u g a t e d - s h e e t , flat-plate collector was f o u n d to be a n inexpensive a n d satisfactory solar water h e a t e r , I t was possible to predict t h e t e m p e r a t u r e rise in t h e s y s t e m w i t h t h e r m o s y p h o n circulation. An i n t e r m i t t e n t a m m o n i a - w a t e r absorption refrigera t i o n u n i t was c o n s t r u c t e d a n d tested w i t h a 4½foot parabolic reflector a n d an artificial electric h e a t e r . P e r f o r m a n c e tests i n d i c a t e d t h a t it has practical possibilities of application in r u r a l areas b u t t h a t f u r t h e r tests are needed for a n improved design of evaporator a n d absorber u n i t s ,

during the year and also due to solar disturbances. In its downward passage through the earth's atmosphere, part of the radiation is scattered and part absorbed by the constituents of the atmosphere. This depletion is substantial even on cloudless days, and with heavy clouds it may be almost complete. The solar radiation received on the earth's surface consists of a direct component (direct solar radiation) and a diffuse component (sky radiation). The sky radiation arises from the fact that the part of radiation scattered or absorbed by the atmosphere may in turn be partially re-radiated downwards towards the earth's surface. Hence the intensity of radiation received on the earth's surface will change not only diurnally, monthly, and annually but will also depend on the latitude and altitude of the place on the earth's hemisphere. Important contributions have already been made towards accurate estimation of radiation on the earth's surface on cloudless days, notably, Parry Moon's~work on standard solar radiation curves for use in engineering and Parmelee's2 data on diffuse or sky radiation. Using the analyses and data in these works, I havea estimated the theoretical total solar radiation intensities on a horizontal surface at solar noon at Dacca

T i n s paper presents results of experiments made a~5 Dacca to utilize solar energy for water heating and for refrigeration. In considering the use of solar energy, it is important to know how much of this energy is available. While not all of the available energy can be used, it does impose an upper limit. Hence, theoretical and experimental values of solar-radiation intensities at Dacca will be presented, followed by performance analyses of two types of flat-plate collectors used for water heating, namely, a f-square foot sinuosoidal tube-plate collector and a IS-square foot corrugatedsheet, fiat-plate collector. Refrigeration and air-conditioning are natural applieations of solar energy because of the coincidence of the maximum cooling load with the period of greatest solar radiation input. An intermittent ammonia-water absorption refrigeration system was designed and tested first under conditions simulating solar conditions and later under actual conditions of solar heating, SOLAR ENERGY AT DACCA Solar radiation reaches the outskirts of the earth's atmosphere with an intensity of 442.4 Btu per square foot per hour. This radiant energy varies somewhat because of the changing distance from the earth to sun Manuscript received January 16, 1964. Vol. 8, No. 3, 1964

(latitude 23°43'). The variation in maximum intensity of solar radiation throughout the year is shown in Fig. 1. The peak intensities appear constant at 329 Btu per square foot per hour during May, June, and Jul ~. I ~ 3zo ~ ~o 280 ~ ~ ~ ~ ~ 240 .'~ g ,/ - -

I

I

1

I

Maximum Intensity TotalIncident Redi•tion H o I," r i z o".,,.'1 n to.I Surface j ~. o. ~ ; ' - - " ~."

/ ~

~¢ _2~ -/

/

~ \ , . ~"r"'--r-"-~ ~ Direct Radiation Normal Incidence ~"-~ ~.,.. \"

. ~ \"

/."

~,

\

200

~ ~ 16o

•

Jan

Feb

Mar

Apr

May

dun

dul

Aug

Moxmlntensities

recorded ] I I Sep Nov Oct

Dec

Fro. 1--Observed and calculated maximum solar intensities on a horizontal surface at Dacca throughout the year. 83

The distribution of incident radiation on a horizontal surface at Dacca during the equinoxes, winter and summer solstices is shown in Fig. 2. The total daily radiation on a cloudless day amounts to 2130 Btu per square foot during the equinoxes, to 1300 Btu per square foot during the winter solstice and 2650 Btu per square foot during the summer solstice. Measurements of total incident radiation on a horizontal surface made with an Eppley pyrheliometer are given in Fig. 1. Although, in many instances, the theoretical and observed values were in fair agreement, it was observed that during some months the measured values were higher while in others lower than the cap culated values. This is accounted for by diffuse radiation and scattered cloud reflection, which amounted to 15 percent of the total radiation during April. During the monsoon months (June to August) the high humidity in the atmosphere brought about a further decrease in radiation reaching the earth's surface, FLAT-PLATE COLLECTORS The flat-plate collector is one of the simplest means of collecting solar energy for use in systems that require thermal energy at comparatively low temperatures, It consists of a flat metallic plate painted black on the surface facing the sun, insulated on the reverse to reduce heat loss and insulated in the front with transparent glass sheets for the same reason. In the northern hemisphere, the collector is set facing south and tilted

320

//~'~, o~ \~

\'~ =~ ~/ o,~:!

g24o

~,

~

~2oo--

!/ /

J

I

z

/

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o[~/

~ld~L--~/~'/~ ,~ / , ? ~ ~ , ~ / / 1 o~/z / ~ ~ ~

m oz r- 16° ~,~ 12o o ~ so

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~ ~'~) ~"'%` ~ ~ t~ ~ I~ i~

l~o

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IF

//

~\ ~

4o/~ ~.~ ,~.~.o~O'FFiSE~ L~ ~ 05 6 7

s AM

9

Io II I~ I 2 SOLAR TIME

3

4

PM

5 6 ~

Fro. 2 - - C a l c u l a t e d d i s t r i b u t i o n of solar r a d i a t i o n on a horizontal surface a t D a c c a during t h e equinoxes and solstices, 84

Absorber

surface

Glass sheets

O D Copper pipe

4"Asbestos Insulation FIG. 3--Sinusoidal t u b e - p l a t e collector.

towards the equator depending on the latitude and time of the year when most of the solar energy is required. The advantages of this type of solar collector are: absence of intricate mechanism to follow the sun, and ability to absorb both the direct and diffuse components of solar radiation. The types of flat-plate collectors used in the experiments were (a) sinuosoidal tube-type plate collector, and (b) corrugated-sheet, flat-plate collector. Sinuosoidal T u b e - P l a t e Collector The tube-plate collector is shown in Fig. 3. It consists of a ~ inch thick copper sheet six square feet in area with 0.5-inch O.D. sinuosoidal copper tubing soldered to it at 3½ inch spacing. The surface of the plate and tubes were painted black and insulation of 4 inch asbestos placed beneath the plate. Above the plate, were one or two glass sheets ~-~ inch thick of ordinary window pane quality with 1 inch spacing. The collection efficiency using two glass layers at 30 degrees south facing in November is shown in Fig. 4. The collection efficiency decreases with rise in water temperature and was about 62 percent with water temperature of 120°F. The heat collection rate, the flow of water per square f°°t per minute with different final water e°lleeti°n temperatures and number of glass sheets are shown in Fig. 5. With water-collection temperatures of 160°F a single glass sheet is adequate, while above this temperature two glass sheets give higher collection rates. This is explained by the fact that the upward heat loss from the absorber surface is the sum of the convection loss (a function of T 5/4) and the radiation loss (a function of T4). Addition of a glass layer rednces the convection loss considerably as to offset the transmissivity and absorption coefficient of solar radiation by itsinterposition. Hence there exists for a given water-collection temperature a n o p t i m u m number of glass sheets for satisfactory collection rates. Solar Energy

Month-Nov. 30 deg south facing Atm. temp. 80"F

o~ loo

~z~o ~

5" "sJ

80 60

-~. ~

2 layers of gloss ~

D ~m200

~ I00

~ ~ 20

~ -~ 50

I0~)

~,~ ~ ~

,

120

t40"F

'~

-1-

Water Collection temp,

.c E

o.,

\.~

Heat collection ~dsingle glass 0.08 .~. doubleglass (n

® 150

.g_l 40

0

Month-April 14 deg. south facing Temp.98°F

FIG. 4 (left)--Collection efficiency of sinusoidal tube collector for different collection temperatures.

0.06 0) Flow rate ~ -

~

~ 0.04 ~o

~

I~,O

'8(~

0,02 ,

22"0 °F

0

,':-

FIG. 5 ( r i g h t ) - - H e a t collection rates with single- and double-glass sheets over sinusoidal tube-plate collector.

•

Water Collection Temperature

Corrugated Sheet Flat-Plate Collector The sinuosoidal tube-plate collector had the disadvantage of being expensive. In searching for an inexpensive collector using locally available materials, the corrugated-sheet collector of the type used by Khanna 4 appeared to be the cheapest to manufacture, This plate collector is used in conjunction with a storage tank in a closed circuit so that thermosyphon action permits a natural circulation of water from col-

lector to tank. To facilitate natural convection the storage tank is placed above the top of the absorber. The corrugated-sheet solar water heater and storage tank are shown in Fig. 6. The collector consists of a corrugated galvanized sheet 22 gauge of 3 feet by 6 feet fastened at the edges to a flat galvanized sheet. The corrugations that form the top surface are blackened with a dull black paint. Two water headers are fitted to the top and bottom edges of the collector.

FIG. 6--Thermosyphon solar water heater of corrugated-sheet, fiat-plate collector type.

Vol. 8, No. 3, 1964

85

'

40o

'

FIG. 7--Heat balance for the thermosyphon system using corrugated-plate collector, October 2, 1963.

~,~>....... ~% ,,"

• ,%

'"'*--'~'~---.~.,~.?~.

• ,. AN

.'

,~o

121o

oi.~

o;.~

rINE-HOU..T,

FzG. 8--Schematic diagram of thermosyphon system•

0.13o

o,Jo

PN

2rid Oct 1963

HEAT BALANCE:- THERM(~VPHON SYSrEN

T h e d e p t h of t h e corrugations was k e p t a t ½ inch to allow a t h i n layer of w a t e r to flow b e t w e e n the sheets, T h e collector plate was housed i n a w o o d e n box covered i n f r o n t w i t h ~ inch t h i c k w i n d o w - p a n e glass a n d o n the rear with 3 inch t h i c k kapok. A 21-gallon t a n k s u i t a b l y i n s u l a t e d was used for storage. I n tests o n this flat-plate collector u s i n g w a t e r flowing t h r o u g h it c o n t i n u o u s l y , a n efficiency of 75 p e r c e n t was o b t a i n e d w i t h a t e m p e r a t u r e difference of 15°F b e t w e e n inlet a n d o u t l e t w a t e r a n d a flow rate of 0.745 p o u n d per m i n u t e per square foot of collector surface. W h e n the flow rate was reduced to 0.309 p o u n d s per m i n u t e per square foot, t h e w a t e r collection efficiency fell to 60 p e r c e n t a n d t h e t e m p e r a t u r e difference rose

to 45°F between inlet and outlet, A second series of tests were performed using the corrugated-sheet, flat-collector w i t h t h e storage t a n k for t h e r m o s y p h o n circulation. T y p i c a l record of t h e t o t a l r a d i a t i o n o n t h e collector surface a n d h e a t a b s o r b e d b y t h e w a t e r i n the s y s t e m a n d b y the a p p a r a t u s is given i n Fig. 7. D u r i n g t h e r m o s y p h o n flow, t h e c o n d i t i o n s p r e v a i l i n g are those of n o n - s t e a d y h e a t r a d i a t i o n . T o predict rise i n w a t e r t e m p e r a t u r e i n the whole s y s t e m t h e following analysis was a d o p t e d e m p l o y i n g t h e h e a t t r a n s f e r coefficients g i v e n i n the classical p a p e r b y H o t t e l a n d Woertz ~ o n " P e r f o r m a n c e of F l a t - P l a t e Collectors."

H

86

The notation used with reference to Fig. 8 is as follows: = instantaneous rate of incidence of total radiation on a horizontal surface obtained with an Eppley pyrheliometer, Btu/sq ft hr °F. = H D -F H s where H D is the direct component of radiation and H~ is the diffuse component of radiation~

~.~.~ . . . . •

,

H r = instantaneous rate of incidence of total radiation falling on the outer surface of the collector. = H D R D "-I- H s R s where RD and R s are orientation factors to convert horizontal incidence to incidence on tilted collector surface for direct and diffuse components of solar radiation respectively. H(a~), H ~(~) are average half hourly rates of incidence of total radiation. R~ = cos ~r/cos ~,, where cos Or = sin (~ - ~)sin a T cos (~ - f~)cos a cos o~ cos ~H = sin ~bsin a -I- cos ~ cos a cos = latitude of place (23°43') ~ = angle of tilt of collector, 17 degrees• ~ = hour angle from noon (15 deg per hour) a = declination of sun (-2°58 ' for Oct 2) Rs = (1 + cos f~)/2 qa = gross hourly rate of absorption of solar radiation by collector per unit of surface = H r(ra), where (ra), is the mean value of product (transmissivity of glass plate) X (absorptivity of blackened surface) X (shading factor-s); = = 0.95 while r varies from 0.82 to 0.87 with hour angle, and s-shading factor is equal to 0.945. Ao = area of collector surface, square feet. q~ = hourly rate of useful energy collection per unit of collector surface. q,,

= q° + U,~(to -

t~,)

where to = average temperature of blackened surface, °F (T~ in abs °R) t~ = temperature of ambient air, °F(T~ in abs °R) U a = U~ + U~ where Ua is the overall collector heat loss coefficient including losses through the glass and rear, based on collector plate area and temperature difference between plate and ambient air, in Btu per square foot hour °F difference. U~ = overall heat transfer coefficient from collector surface upward through glass sheet to ambient air U~ = overall heat transfer coefficient from collector surface rearwards through insulation to outside air. Solar E n e r g y

w

1 UL=

a(To 4 --

n

+

/T~-

1 +{1

T------a~ -~

c4~/~-~

+2n+f--

~

T~9

1

n}(T~ -- T,)

~,

]

n

= number of glass plates = 1

c

= convection coefficient = 0.18 for collector tilted at 17 deg = 1 -5 0.3 (wind speed mph) = 2.2 for 4 mph = 0.56 = emissivity of black surface = 0.95 = emissivity of glass = 0.96 = Stefan Boltzmann coefficient = 0.173 X 10-8 Btu per square foot hour °R4

h~ f ~ eg

2q'a

t~a, t~,~ t~t t~

= = = = =

W~ = W~ =

and is equal to 0.02 Btu per square foot hour °F per foot thick and d~ is thickness of insulation, 0.25 feet. overall heat transfer coefficient from tank to outdoor air per square foot of collector surface, and is equal to 0.348 Btu per square foot hour °F difference, overall heat transfer coefficient from pipe to outdoor air per square foot of collector surface, and is equal to 0.143 Btu per square foot hour °F difference, average plate collector temperature, °F. average ambient air temperature, °F. average temperature of water in tank, °F. average temperature of water in collector, °F. (t~ -5 t0)/2 where t~ is temperature of water at inlet, °F to is temperature of water at outlet, °F weight of water in collector, pounds = 27.14 pounds water equivalent of collector, pounds = ½(wc)i -5

(wc)~

+

~(wc)~

= 10.14 pounds where (wc)~ equals weight X specific heat of insulation, (wc)~ equals weight X specific heat of collector corrugated sheet plate. (wc), equals weight X specific heat of glass sheets. W~t = weight of water in tank and pipe = 223.66 pounds W , = water equivalent of t a n k = ½(wc)~ -5 (wc)t = 10.50 pounds. Under conditions of non-steady flow, the instantaneous rate of incidence of total radiation is equal to the sum of instarttaneous rate of increase in heat content of water and apparatus and the instantaneous rate of heat loss from the apparatus. For halfhourly intervals of time, this equation can be re-written with suffixes (1 and 2): W~ q'o = ~

W~t (two~ -

t..oO +

t~,~) + ~

-5 - ~ (t,.: -- tw.) -5 ½(UL -5 UB)

(t~2

t~,)

-5 ½ (UT -5 UP){ (twt2 -S twtt) 2

t~o,)

(t~: -52 t.~)

(Wu.c -5 Wwt

-5

We,

+

Wt.) {t~¢e - - tw,~}

Ac

8,

No. 8, 1964

there

is n e e d for

r e f r i g e r a t i o n for a i r - c o n d i t i o n i n g a n d f o o d p r e s e r v a t i o n . T h e u s e of s o l a r r a d i a t i o n itself in t h e a b s o r p t i o n refrigerating cycle has therefore much promise. While the refrigerating cycle requires thermal energy at a high-temperature level, this can be assured through t h e u t i l i z a t i o n of s o l a r c o n c e n t r a t o r s o r f l a t - p l a t e coll e c t o r s w i t h t w o to t h r e e glass l a y e r s . O f t h e d i f f e r e n t t y p e s of a b s o r p t i o n r e f r i g e r a t i n g systems, the continuous operated unit requires many mechanical components. On the other hand, the interm i t t e n t a b s o r p t i o n u n i t h a s no m e c h a n i c a l p a r t s , is c h e a p a n d e a s y t o c o n s t r u c t , a n d is m o s t s u i t a b l e for u s e i n n o n - i n d u s t r i a l i z e d areas, a l t h o u g h t h e u n i t requires constant attention during its operation. This cycle with solar heating has been experimented by ~5o

~'"~,"~'~ u_ % ~ ,,, ~ <

"~""" ~,5.,;~:~,~ ~'~"

rJ0

,,

.:;.~.~,e ..~'" i~ .'., It0

~L.,J

/ / />"

/~/..::.

,.*"

try._

2

-5 ½ (U L -5 UB -5 UT JC Up) {~cav -- taa } Vol.

sunshine,

(ta2 -s tal)}

For simplification, we can assume t h a t (t~,~ -- t~,~) is equal to ( t ~ - - t~.) and for heat losses that the temperature terms are the same. We now get: q'~ =

31.05Atw~ -5 1.896t~cl -- 83.4

=

I n a r e a s of a b u n d a n t

o_ (t~ -

HT(~)('ro~),

INTERMITTENT ABSORPTION REFRIGERATION I/NIT

rY UJ

W,

Ao (t~,~ -

=

The values of htw~ calculated from this expression for Oct. 2 using values of H obtained with an Eppley pyrheliometer, are plotted against time in Fig. 9. The measured values of 5t~o are also given in the figure, which shows reasonable agreement between the theoretical and experimental values.

UB = kJd~ = 0.08 where kc is thermal conductivity of kapok

Up =

}

(to~ -b taO. 2

The average plate temperature in half-hourly intervals was 50°F higher than the average water-collection temperature. The ambient temperature varies little and is taken in this example as 94 ° F. If ,Xty,~ denotes the increase in average watercollection temperature, then we can rewrite to obtain:

The value of UL obtained ranges from 1.182 %o 1.325 for values of (To - T~) between 56 and 98°F.

UT =

{

q'o = -~ (t~o~ - twd) -5 ½ U .(tc2 + to]) 2

F r o m H o t t e l and Woertz ~,

~o~ ~"

.

~.*o

.

.

~t ~o

. ~.3o rt~E-~S

Oct 1963

PREDICTION OF WATER

t.~

,

~..to

, .t~o P~

TEMP-THERMOSYPHON SYSTEM

FIG. 9--Calculated and experimental values of rise in water collection temperatures in the thermosyphon system for October 2. 87

FIG. lO--Experimental intermittent ammonia-water absorption refrigeration unit with parabolic solar reflector.

Trombe and Foex 6, Williams, Chung, Lof, Fester and Duflie7, Einstadt, Flannigan and Farber 8, and Chinnappa 9.

water solution. A liquid seal is provided in this vessel to prevent water vapor passing into the vessel, which is the condenser-evaporator. The generator-absorber is placed at the focus of a 4½-foot parabolic solar reflector lined with aluminized Mylar film. The bottom of the generator vessel was painted black, and its body covered with insulation during heating or regeneration. The reflector was mounted normal to the sun and was adjusted to keep radiation focussed on the generator. In some of the experilnents, solar heating was simulated by placing

Experimental U n i t The main details of the intermittent absorption refrigeration unit using ammonia-water combination is illustrated in Fig. 10. The unit consists of two vessels joined with a pipe having a valve for isolating the vessels. The larger vessel, 6 inches diameter and 9 inches long, is the generator-absorber and contains ammonia2,o j - .

.

1 ]

.

.

~

SOLAR REGENERATION CONCENTRATION 5.12%

~ I

~,.~

-

~

-

-

-

-

-

-

gEFRIGERATION = PRESS,~,

200

,

~2o~, ~ ~"

~

, /

4 ~ ~ ~ :::::...~.

Q:

o.s

~

~

'o.~ ~,

FIG. 1 1 - - D a t a of t e s t r u n on

September 4, under conditions of solar-regeneration with 53.2 percent concentration of am-

o.//"

0

15

30

45

60

75

90

f05

120

135

150

0

I0

20

30

40

50

60

TIME - MINUTES

88

Solar Energy

the generator over an electric heater. The condenserevaporator vessel was either air-cooled or cooled by immersion in a water tank. The temperatures of the generator and condenser were measured with thermometers, the pressure with a pressure gauge, and solar radiation with an Eppley pyrheliometer. To measure the mass transfer of ammonia during the cycle, the refrigeration unit was fitted with a horizontal graduated scale over which a sliding counterpoise could be moved. The unit with the scale was mounted on a hardened steel knife edge fitted to a tubular pipe stand. By moving the counterpoise, the scale could be kept balanced horizontally and the mass transfer determined, The refrigerating cycle commenced with the heating of the ammonia-water solution in the generator vessel, As heating progressed, the ammonia vapor was driven into the condenser-evaporator where it condensed, The process of regeneration continued for a period of about two hours. Upon reaching the maximum generator pressure (nearly 200 psig), the heating was stopped, the interconnecting valve closed, the insulation cover over the generator removed, and the generator vessel immersed in water. On reopening the valve, the ammonia liquid in the condenser evaporated thereby lowering the condenser-evaporator temperature. The vapor formed was re-absorbed by the weak solution in the generator. The process of refrigeration first proeeeded with rapidity, but later, as the concentration in the generator increased, the evaporator temperature slowly began to rise. P e r f o r m a n c e Results The results of the preliminary tests carried out with this unit are shown in Figs. 11 and 12, and in Table 1.

t-

SOLAR REGENERATION I CONCENTRATION62.2 % I

ELECTRICAL REGENER,~ CONCENTRATION 62 2% i

/ ,

16

,"

"

/.-Y/

12

.,/

,~" "'-

REFRIGERATIQIV

I

A

80

In these tests, the quantity of water was kept constant at 2.25 pounds and the solution concentration changed by varying the amount of ammonia in the solution. The temperature and pressure in the generator, and ammo~fia mass transferred during the processes of solar regeneration and refrigeration are shown in Fig. 11. Generator temperature of 217°F and pressure of 192.5 psig were obtained with solution concentration of 53.2 percent, and following solar regeneration lasting 145 minutes. 0.51 pounds of ammonia were condensed in the condenser-evaporator. The temperature of the condenser-evaporator during the process of refrigeration fell to 52°F. If the absorber vessel was jerked or rocked, the process of absorption was accelerated and the evaporator temperature was lowered to below freezing. The performance of this unit with the same solution concentration under conditions of solar regeneration and electrical heating is shown in Fig. 12. It demonstrates the importance of maintaining high temperatures during the period of vaporization if adequate ammonia mass transfer is to be attained; further, that solar concentrators of the parabolic reflector type are capable of delivering such temperatures. Some of the results of the preliminary experiments with different ammonia concentrations (water, 2.25 pounds in solution) under conditions of electrical heating are given in Table 1. While the weight of ammonia transferred to the condenser-evaporator increases with solution eoneentration, the lowest evaporator temperature is obtainable with lower solution concentration. With high solution concentrations, there was noticeable flooding of evaporator-condenser vessel at the end of run. The theoretical absorption refrigeration cycle with

- -~-~o_., ,. ~ ~ --~

~--

N

,%.

08

i,~_~ ,

o.6 so

.

-' ,~....

,p

0.4

/ jr-

t

40

: .... dO

55

,$ GeNI:A~Oa 7~M~114 ~1(

120

r,ME-

Vol. 8, No. 3, 196~

63.2 percent concentration of aramonia-water solution under conditions of solar heating and artifical electric heating.

-a--_4

//

0

;5

*¢~VUrES

160 0

/ 40

............. 0i80

120

20

40

160 TI~E-~NUTES

89

absorption at constant temperature was adopted for purposes of comparison of performance. The theoretical coefficient of performance (COP) was calculated from the expression derived by Chinnappa 9. The values of the theoretical COP and estimated actual COP values are shown in Table 2 for a period of regeneration lasting 100 minutes under conditions of electrical heating. Under conditions of solar regeneration, with concentrations of 53.2 and 62.2 percent, the theoretical COP's were 0.412 and 0.506, while the estimated actual COP amounted in both cases to 0.15 taking heat concentrated at the bottom of the generator into account. Further tests are planned to determine solution concentrations for optimum mass transfer and evaporator temperatures under conditions of solar regeneration. The design of the generator-absorber and the condenser-evaporator is also expected to influence considerably the performance results, likewise the mass of metal parts of the whole unit in comparison to the solar heat input. CONCLUSIONS 1. Measurements of total incident radiation on a horizontal surface are in fair agreement with theoretical values obtained from the standard method of calculation. However, during certain months the measured values were higher than these calculated because of a larger diffuse component of radiation and scattered cloud reflection. During the monsoon months, the measured values were understandably lower, since there was considerable increase in the humidity of the atmosphere. 2. The corrugated-sheet, fiat-plate collector with storage tank is as efficient and a cheaper type of solar heater than the conventional sinuosoidal tube-plate collector. With the theoretical analysis developed, it was possible to predict the temperature rise of water collection in the thermosyphon system close to experimental values. 3. The intermittent ammonia-water absorption refrigeration unit with solar regeneration has practical possibilities. Tests have indicated that required generator temperatures and mass transfer are attainable with parabolic solar concentrators and high ammoniawater solution concentrations. Further tests are needed for effective design of absorber and evaporator vessels under solar conditions,

90

TABLE 1--RESULTS WITH DIFFERENT AMMONIA CONCENTRATION

Run No. 1 - -

2

Ammonia I CO ....

tra-

tion percent 50.0

54.5

At Generator Temperature of

200°F"

__

_

Time

taken

minutes 45

65

_

L ....

i NHa Gem. I condensed press, pslg[

pounds

I 175

0.113

198

] 0.21

3

57.1

100

202

0.48

4

/ 62.2

/ 103

I 245

I 0.74

t evaporator temp. °F

56 (no jerking) 12 (jerking) 16 (jerking)

60 (no jerking)

TABLE 2--COEFFICIENTS OF PERFORMANCE Initial Concentration

50.0 54.5 62.2

TheoreticalCOP

EstimatedActualCOP

0.380 0.433 0.50

0.140 0.157 0.140

ACKNOWLEDGEMENTS The author is indebted to the following students of the East Pakistan University of Engineering and Technology: Messrs. N. Talukdar, Aziz and Maqsood for experimental work on the corrugated flat plate solar collector; Messrs. Fazle Hussain, S. Y. Farooq and P. K. Choudhury for experimental work on the

intermittent absorption refrigeration unit. The author also acknowledges the assistance given by Mr.

D.C. Dunham in making the parabolic solar collector available for the tests. REFERENCES 1. Moon, Parry, "Proposed Standard Solar Radiation Curves for Engineering Use". J1. Franklin Inst., 230, p. 583-617,

1940. Surfaces by Diffuse Solar Radiation from Cloudless Sky".

2. Parmelee, G. V., "Irradiation of Vertical & Horizontal

Trans. A S H V E , 60, p. 341, 1954. 3. DeSa, V. G., "Solar Radiation in Air-Conditioning and Flat Plate Collectors". Jl. Inst. of Engineers (Pakistan), v, p.

21-33, 1958.

4. Khanna, Mathur, Davey and Suri, "Domestic Solar Water Heater." J1. of Se. & Ind. Research, 18A, p. 51-58, 1959. 5. Hottel, H. C. and Woertz, B. B. "Performance of Flat Plate Solar Heat Collectors." Trans. ASME, 64, p. 91-104, 1942. 6. Trombe, F., and Foex, "Production of Cold by means of Solar Radiation." Solar Energy, 1, p. 51, 1957. 7. Williams, D. A., Chung, R., Lof, G. O. G., Fester, D. A., and Duffle, J. A., "Intermittent Absorption Cooling Systems with Solar Regeneration". ASME Paper No. 57-A-60, 1957. 8. Einstadt, M. M., Flannigan, F. M., and Farber, E. A.,

"Solar Air-Conditioning with Ammonia-water Absorber Refrigeration System." ASME Paper No. 59-A-276, 1959. 9. Chinnappa, J. C. V., "Experimental Study of Intermittent Vapour Absorption Refrigeration Cycle Employing Refrigerant Absorbent Systems of Ammonia-water and Ammonia Lithium Nitrate". Solar Energy, 5, p. 1-18, 1961.

Solar Energy