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Performance testing and evaluation of solid absorption solar cooling unit
PII: S0038-092X(97)00010-8
Solur ~51:nur,qv Vol. 61, No. 2, pp. 127. 140. 1997 0 1997 Published by Elsevw ScienceLtd All rights reserved. Printed in Great Britain 0038-092X197 $17.00 $-0.00
PERFORMANCE TESTING AND EVALUATION OF SOLID ABSORPTION SOLAR COOLING UNIT *+ J. BLUMENBERG, ** H. J. KAVASCH ** and T. ROETTINGER * Centre for Energy Studies, Indian Institute of Technology, Hauz Khas. New Delhi 1IO 016. India
N. K. BANSAL,
**
** Institute for Thermodynamics, Technical University, Munich, Germany Received 17 August 1995; revised version accepted 22 January 1997 Communicated by VOLKER WlTTER Abstract--Based on solid-vapour intermittent absorption system, DORNIER a German firm designed and fabricated a solar cooling unit, which utilizes thermal energy supplied by heat pipe vacuum tube solar collectors through thermosyphonic flow of water. The unit of 1.5 kWh/day cooling capacity uses ammonia as a refrigerant and IMPEX material as absorbent and does not have any moving part requiring no auxiliary energy. The IMPEX material (80% SrCl, and 20% Graphite) has high heat and mass transfer coefficient as well as high absorption capacity. Detailed experiments were performed on a unit in Delhi under real field conditions followed by theoretical analysis. Theoretical maximum overall COP of the unit is 0.143, and it depends upon the climatic conditions. Under field conditions, it was found that if the maximum daytime ambient temperature was 30°C and night time temperature 2o”C, it took three sunny days to freeze water in the cooling box. After the second day, the temperature inside the cooling box remained 1‘C. The overall COP was found to be 0.081 only. The automatic control valve based on mechanical/thermal principles however has defects and the problem of corrosion of the sealings needs to be solved. In climates where day time temperatures are high (Delhi summer 43”C47”C during the day, 3O”C-35°C during the night) and solar radiation relatively low (4-5 kWhjm2d) because of pollution and sand in the atmosphere, it is most unlikely that pressure in the ammonia circuit can reach values at which ammonia vapours start to condense. The unit. needs to be redesigned for such conditions. 0 1997 Published by Elsevier Science Ltd
generation is a major problem. This problem is less in H,O-LiBr and NH,-NaSCN systems. In H,O-LiBr systems however, the crystal formation is a major problem. In NH,-NaSCN combination, the viscosity of mixture creates problems in the flow of solution. The solid absorbent-refrigerant combination has no such problems as with a liquid absorbent system. The development of such systems is however restrictive because of other kinds of problems inherent in the solid absorbents. Solid absorbents are subject to considerable swelling during the absorption phase which may result in close packing after a few cycles. The thermal conductivity of salts is quite low and therefore operation of the system for moderate temperature differences is very difficult. Chinnapa ( 1961) made systematic investigations for intermittent vapour absorption refrigeration cycle employing NH3-Hz0 and NH,-LiN03 systems. The investigations show that NH,-LiNO, system is as good as NH,-H,O. Sargent and Beckman (1968) found that NH,-NaSCN combination gave nearly the same COP as obtained by using NH,-Hz0 system. A critical survey of intermittent absorption systems was made by Nielsen ( 198 1) based
1. INTRODUCTION
The concept of solar powered absorption cooling was suggested more than 35 yr ago as a solution to refrigeration problems in the desert areas and other remote areas where electricity was not easily available. As a result of the energy crisis of 1970 these systems again attracted the attention of scientists and engineers for developing absorption cooling systems which can work efficiently and economically by using solar heat. Most of the work on absorption refrigeration was done on the continuous type of systems needing additional power for operating pumps. An intermittent absorption refrigeration employing cooling storage is more appropriate for remote area applications. Lots of experimental as well as theoretical work has been done on such systems based on refrigerant-liquid absorbent combinations like NH,-H,O, NH,NaSCN, H,O-LiBr etc. For an NH,-H,O absorption cooling system, transfer of water vapours to the condenser during the process of ‘Author to whom all correspondence should be addressed: E-mail, [email protected]; Fax.. +91-l l-6862037; Tel., +91-l l-666979 Ext. 5002. 127
on NH,-H20, NH,-LiNO,, NH,-CaC12 and NH,-SrCl, combinations. He concludes that the solid absorbents like CaC1, or SrCl, should be used for solar refrigeration. NH,-LiNO, poses problems of viscosity, while CaCl, shows swelling during the absorption phase and a problem of particle migration. Stanish and Pearlmutter ( 1981) showed the possibility of using salt hydrants as absorbents in heat pump cycles. Parametric effects on the performance of solar powered NH,-CaCI, system were investigated by Iloeja (1983). Studies conducted by Worse-Schmidt (1983) show that using a solid absorption cycle, solar driven refrigerating plants can be built very simply and they operate reliably over years without maintenance beyond occasional replenishment of water in the condenser water bath. Performance results of intermittent solar refrigeration in Egyptian climate show technical feasibility of NH,~-H,O systems ( El-Shaarwi and Ramadan, 1986 1. Mello and Hahne (1993) studied experimentally the dependence of the operating pressure of the absorption and desorption processes of ammonia on CaCl, as a function of the temperature difference starting from the equilibrium temperature. The results show slight influence on the ammonia absorption rates for pressures above 10 bar. Test results showed a good potential for chemical reactions to be used in combination with solar energy (Mello and Hahne. 1993). Under a project sponsored by the German Ministry of Research and Technology. DORNIER ( Kleinemeyer, 1995 ) developed an ammonia-strontium chloride based unit using heat pipe evacuated tube collectors. One of the units delivered to India was tested under actual field conditions. In this article we report the performance of this unit besides giving details of design calculations for various components of the system.
the reactor (generator cum absorber), desorbing the refrigerant vapours, which in turn condense and the liquid refrigerant IS stored within the evaporator and the storage tank (Fig. I ). After the sunset. the reactor cools down and the absorption process starts. The pressure rn the system lowers and the evaporation vt ammonia/refrigerant will take place in the evaporator producing cooling and consequently ice is formed in the water basin. The ammonia vapours are absorbed by the salt present in the reactor. The ice produced during the night time keeps the box cool during the day. 3. THE SOLAR
3. I
COOLING
UNIT
SpeciJcations
The solar cooling unit manufactured by DORNIER (Fig. 2) has its specifications given in Table 1, The main parts of the system are: (1) The reactor (generator cum absorber) filled with 7.2 kg SrCl, and 1.8 kg Graphite is heated up by solar energy and cooled down to ambient temperature, generating and absorbing maximum 9 1 of ammonia. (2) The condenser, which liquifies the generated ammonia. (3) The evaporator producing cold energy and ice with an integrated additional storage for ammonia. control valve which operates (4) Automatic only twice a day. (5) Vacuum tube solar collectors with water circuit for heating the reactor. During night water circulates through the reactor only to cool it down. 3.2. The reactor Two important parameters which effect the system performance are the heat transfer coefficient (described by the thermal conductivity and heat transfer coefficient on the wall)
2. PRINCIPLE
OF INTERMITTENT SOLID ABSORBENT-VAPOUR ABSORPTION SYSTEM
The ability of a liquid or a solid medium to absorb and generate specific vapours depending on its temperature constitutes the basis of absorption refrigeration. An intermittent solar powered system operates on diurnal cycle with one generation cycle per day. In the morning, the absorbent will contain maximum amount of refrigerant. During the day, the heat drawn from a solar collector system is utilized to heat
Fig. 1. Basic operation
of a solid absorbent system.
solar cooling
Performance testing and evaluation of solid absorption solar cooling unit
Fig. 2. Photographic view of the solar refrigerator Table I. Specifications of the solar cooling unit
# Cooling unit size
300 I
ii- Total energy to be produced at 5 kWh/m* day insolation
1.5 kWh (0.440.5 kWh/day is the nominal energy required to keep the temperature within the cooling unit constant, I kWh/day is the net energy corresponds to 57 1of water to be cooled from 30°C to 15°C or 10.75 kg ice to be produced) 40-45°C during day time, 30°C during night time Ice storage of max 4 kWh (about 33 kg of ice) to hold the refrigeration temperature constant at about I-5°C. The maximum ice storage can keep the refrigerator cold for about 5 days with no or less solar energy supply Stainless Steel 2.1 m’; (No. 13; one used for automatic control valve)
f
Ambient conditions Internal ice storage
NH, circuit pipe material
; Vacuum tube solar collector area # No moving parts except one valve in water circuit which -
also operates only twice in a day
and the mass transfer coefficient. The salt SrCl, has itself very low conductivity values of about k = 0.1-0.3 W/mK and a low heat transfer coefficient of about h= lo-20 W/m2K. In practice it is well known that the mass transfer using salt without additives is less and can partially or totally block the reaction after certain number of cycles. This is the reason why this technology could not progress much despite general advantages. In the new technology used in the system, the specially pretreated Graphite
.__
(system STELF, patented world wide) handled and prepared with the salt is used in the reactor. Graphite is temperature stable, does not react with ammonia and has a high conductivity. Besides these properties, graphite improves the vapour flow and thus enhances the mass transfer. Further, the salt is pressed in order to enlarge the storage capacity and the heat transfer and to diminish the specific reaction volume without effecting the mass transfer in specific limits. It is seen that the heat conductiv-
ity can be varied between k =4-25 W/mK while the heat transfer coefficient on the wall is increased to about h = 500- 1200 W/m2K with the new technology. Test results concerning the mass transfer also show good performance. In the reactor, the mixture of 80% SrCl, and 20% Graphite known as IMPEX is in the form of cylindrical pieces. Seventeen such pieces of IMPEX are put in the reactor for the present cooling unit. 3.3. Solar generator Solar generator is a system, which consists of thirteen vacuum tube collectors “HPSEIDO” produced by Beijing Solar Energy Research Institute, China. Collectors are essentially heat pipes in which water is used to transfer heat from the absorber to the top of the collector. When the collector is irradiated by the sun. water at the lower end of the pipe starts to evaporate and diffuses to the colder end at the top. Here the vapour condenses and the resulting fluid flows back to the bottom. The efficiency equation of the collector specified by the Mess Report ( 1994) Solar Energy Integration and Research Centre Rapperswil, Switzerland is: ‘7= c,, - c, .\-- C’,.\-?I
(1)
c0 =0.762, C’, =2.21 ( W/m’K ) and where, C; =0.004( W/m2K2) Graphically, the efficiency curve is given in Fig. 3. 3.4. The ammonia circuit (Fig. 4) As discussed earlier during the day the ammonia is generated in the reactor and leaves it at a high temperature and pressure. In the condenser it is cooled down and hence liquihed. This condensed ammonia is stored in the ammo-
nia storage tank and evaporator. The \atetv valve is also mounted in the circuit which exhausts the ammonia to the atmosphere 11 ~hc pressure rises too high (above 30 bar). Durinr! the night the evaporated ammonia is transferred back to the reactor through the same circuit The pressure in the ammonia circuit is high (15-20 bar) during day-time and low (ahot:! 3 bar) during night-time. As the graphite is inert to ammonia so the properties of SrCl,-Graphite (STELF) mixture can be considered as that of SrCl,. One mole of SrCl, can absorb 8 moles oi’ NH,, but generateidesorb only 7 moles under the design conditions of the system, so one mole always remains absorbed in SrCl,. The mixture of SrCl, and NH, is called “Ammoniacate” Theoretically the mass of ammonia required at the time of fresh charging the new system can be calculated by using the following equation. SrC1,.8NH,++SrClz.
lNH,+7NH,
where, 158.62 kg of SrCl, = I mole of SrC12. Realising the fact that only 7 molecules of NH, are desorbed during the generation process. maximum of 5.4016 kg of NH, is needed for recharging the system
The water circuit (see Fig. 5) has two functions in the solar cooling unit. The first is to transfer heat energy from the solar collector to the reactor during day-time and the second function is to help in dissipation of the absorbed heat produced during the night to the atmosphere. so that cooling of the reactor takes place easily. For thermosyphonic how of water, the
X-Value
Fig. 3. Efficiency curve of the heat pipe solar collectors
produced
by Beijing Solar Energy
Research
Institute.
China
Performance
testing and evaluation
of solid absorption solar cooling unit
Vacuum fibe
Fig. 4. Schematic
of solar cooling
unit produced
Cdlector
by DORNIER
Vacuum colkctw
Fig. 5. Heat transfer
reactor was installed at 2.15” inclination with horizontal. Water in the circuit is at saturation corresponding to ambient temperature. In the morn-
I31
GmbH
Germany.
wbes
loop.
ing the automatic control valve closes. The solar collector transfers heat energy to water pipe A causing the formation of vapour bubbles in the pipe A. The vapours cannot reach the absorp-
tion heat exchanger because of siphon c’, but start collecting at B. The pressure in the circuit is now increasing and water in the circuit is forced into the heat exchanger until it is filled completely. The absorption heat exchanger is now blocked and cannot transfer heat during day-time to the atmosphere. The new water level in “D” is below the reactor, so the water circuit will work as a regular heat pipe. When the solar insolation decreases in the evening and control valve opens automatically and also the pressure in the circuit drops again. The water level in “D” will rise to a higher level. The steam formed in the hot reactor will pass to the absorption heat exchanger through the control valve and condenses there. Heat of energy is rejected to the condensation atmosphere.
perature Tc = -. 7 C and using the expression:,
3.6. Cooling box
From the expressions of the specific heats C, and C, and the enthalpies of evaporator, condenser and evaporation given in the appendix, the various values are obtained as:
The cooling box of the unit is made from the cabinet of a common refrigerator with more insulation on the wall. The total volume of the box is 260 1.The heat exchanging tubes carrying ammonia are dipped into water contained in a plate type of heat exchanger kept inside the box. The total volume of water which can be filled between the plates is 33 1.
yields p, = 3.238 bar Similarly; -2793.3 T, = _~._.._-~._ lnp,-11.676
= 273
and for T, = 55°C. p2 = 23.567 bar. The average generator temperature Tg and the absorber temperature T, are calculated as 114.56”C and 62.73”C respectively. Corresponding to Fig. Al and Appendix A, the other values of temperatures are calculated as T, = 125.56”C, & = 103.56”C, T, = 51.73”C and ii;, = 77.65”C.
C, = 0.687 Wh/kgK, C; = 0.294 WhlkgK, h, = 46.29 Whjkg. h, = 128.12 Whikg.
4. THEORETICAL
ANALYSIS
h, = 356.83 Wh,fkg
The reaction during the desorbing phase of ammonia is given as follows; SrCl,SNH, t - +
SrCl, 1NH, +
7NH3
(X)
(Y)
(Z)
(158.62+ 136)
(158.62+17)
(119)
294.62 A mixture of “Ammoniacate”. w.r.t. &Cl,
175.62
119
SrCl, and NH, is called Fractions of various masses
m, =294.62/158.62=
1.857
m,=175.62/158.62=
1.107
m, = 119/158.62=0.75
Fraction of ammonia (w.r.t. SrCl,) in the reactor at saturation m3 = 136/l 58.62 =0.857
The temperature/pressure relationships and other thermodynamic properties of NH,, SrCl, and SrCl,. 8NHs mixtures are given in Appendix A. Considering the evaporation tem-
At the average temperature of the evaporator T,, the latent heat of evaporation is calculated as: T, and the condenser temperature h, = 321.75 Whlkg.
the rated cooling capacity From Q, = 1500 Wh/d and Q, = M4. h, one obtains M4 = 4.204 kg. However, considering the average temperature and mean enthalpy, the mass of the desorbed ammonia is (Swartman, 1971) given by; M-I
M3=
exp (h, ~-11,)/h,, yielding M3 = 5.421 kg M SrCl* = M3/mz = 7.228 kg
Therefore, 7.228 kg of SrCl, is required for 1.5 kWh/d of cooling capacity. Mass of ammonia needed at first charging of the system MS =ms x MSrC12=6.194 kg.
For recharging, the amount of ammonia is equal to M,=5.421 kg.
I’erfomance
testing and evaluation
Te
Ta Tc
of solid absorption
?a
Td
Tg
solar cooling
unit
i 33
Tg
Tem~ufe Fig. 6. Pressure-temperature
Quantities of various salts and mixture are obtained as MX
= m, MSrClz
= 13.42 kg
MY
=mY . MSrCl2
=8.001 kg
= mgr Msra 2 = 1.807 kg Total mass of salt mixture in the reactor = MSrC,, + MBr = 9.025 kg Enthalpy of refrigeration h, =(46.2251 -O.O976T,) x 16.3388 wh/kg of NH, = 572.56 Wh/kgofNH,
Q,=M,C,V,
the
-C)+M,C,V,-G)
+M,&,t?‘,-L)+M,h, = 3663.39Wh COP= QJQ,
cycle.
It takes about 3 days to get the water totally frozen in the storage (33 1 of water).
5. EXPERIMENT
MSr
The energy required for generating, amount of ammonia is calculated as
curve for SrCI,~-NH,
= 0.409
The efficiencies of heat collection and subsequent transfer to the reactor is approximately 35%. If the daily solar insolation is 5 kWh/m’d, the required collector area is 2.1 m2 and the overall COP (= Qe/Ztotal) is 0.143.
The various parameters measured during the experimentation on the solar cooling unit (see Fig. 4) include the global solar radiation I, ammonia pressure p, reactor temperature r,, ambient temperature TX, ice storage temperature T, and the cooling box temperature T4. The solar radiation was measured by using class I Kipp and Zonen pyranometer transducer and temperatures by using thermocouple. All sensors were connected to a 6 colour data printer for continuous monitoring. The unit was installed outdoors on a horizontal and stable foundation with evacuated tube collectors pointing towards the north at the inclination of 30”. Initially the unit has nitrogen filled in the heat exchange circuit connected to solar collectors, because of the danger of freezing during the transportation. Before starting the operation, therefore, it is absolutely essential that N, is released and distilled water is filled in. After releasing the nitrogen carefully, the heat exchanger circuit is evacuated to about 1 mbar. When the pressure is just below 1 mbar, the vacuum pump is removed and inlet valve is
Delhi,
:I
India,
28.-29.05.1994
70
y 60 700 * 600 $
2 50 E; f 40
Delhi,
b
India,
28.-29.05.1994
140
20 16
,I '20 Y . 0 100 5 B 80 z g 60
16 14
LO F
20
Fig. 7. Temperatures,
radiation
and pressure
connected to a vessel containing more than 3.3 1 of distilled water. Inlet valve is opened and as soon as 3.3 1 of water is sucked, it is closed again. The heat exchange circuit is evacuated again to the saturation pressure of water. The unit was received with all ammonia absorbed in the reactor. 6. RESULTS
AND DISCUSSIONS
The performance of the unit was studied theoretically first for climatic conditions prevailing in Delhi. The corresponding ambient temperatures and solar radiations are given in Appendix B (Figs. Bl and B2) in terms of hourly values of ambient temperatures and solar radiation on an inclined surface. Tables in
measured
on the solar cooling
unit
Appendix C give the calculated values of various parameters, which include the mass of ammonia desorbed and the values of COP. The values show that, in the climatic conditions of Delhi, the total amount of ammonia i.e. 5.4 kg is dissolved by noon. The results also show that the pressure is not high enough in the ammonia circuit for condensation to take place. The condensation of ammonia is therefore difficult particularly in the hot summer months. In order to understand the system, a detailed description of the initial unsuccessful results as well as the final successful results are included here. Measurements performed on two successive days in May 1994 are given in Fig. 7. The results show that (1) The temperature in the cooling box (T3)
Performance testing and evaluation of solid absorption solar cooling unit
135
-Tl
Delhi, India, 28.7.-01.08.1994
-
T2
-T3
-10 8 @a c
z co: c
g ti 0
:: id
z =: =: m ij
B
=:
::
ti
6;
0
_
0
7
-
N
c
cl
id
z 6
=: =: uj ti
0
CI
-
Fig. 8. Experimental observations during the phase of electrical heating.
was nearly equal to the ambient temperature (T,) during the night. (2) The pressure in the ammonia circuit was never above 13.5 bar ( 3) The reactor temperature (r,) never fell below 80°C The results obviously show that there was no refrigeration inside the cooling box. The ammonia vapours did not condense because the pressure never reached the condensing value of 20 bar at 45’C. As the reactor temperature was also high, the absorption of ammonia vapours also did not take place. Efforts were made to recharge the system completely by emptying and filling the ammonia again and by replacing the distilled water also. Also the monsoon season began in Delhi and therefore the water in the reactor loop was also heated electrically. The measurements of 3 days are shown in Fig. 8. The figure shows that the cooling system is functioning. In four days, all the water in the cooling box was frozen and the temperature in the cooling box (r,) dropped to -4°C. The electrical system was replaced by the solar system after the monsoon period. The results of measurements made in October 1994 are shown in Fig. 9. The cooling produced was equivalent to 1.23 kWh and the solar radiation falling on the collector surface was 15.2 kWh giving an overall COP,,, of 0.08 1.
It is seen that the experimentally obtained of 0.081 is less than the theoretically obtained COP,,, of 0.143. The reason for this lies in the fact that the theory has been developed for average conditions. The system however works only dynamically. The generation of ammonia takes place only after the reactor has reached a certain temperature. The early hours of solar radiation are therefore used to warm the reactor only and do not contribute to the average performance of the system. The steady state theoretical analysis therefore will always predict a higher COP,,, than the measured values. COP,,,
7.
CONCLUSIONS
Principally, the refrigeration unit of the DORNIER works. If the maximum day-time temperature is 30°C and night 20°C it took three sunny days to freeze the water in the cooling box. The temperatures inside the cooling box remained at 1“C after the second sunny day. The prototype unit, however, has defects. The automatic control valve does not work satisfactorily and the sealings get damaged because of corrosion by ammonia. May and June months in Delhi are characterized by high temperatures (43”C-47°C
136
160 -
Fig. 9. Experimental
observations
in October
during the day and 3O”C35”C during the night). The solar radiation is low (between 5 and 6 kWh/m’d) because of pollution and sand in the atmosphere. It is most unlikely that pressure in the ammonia circuit reaches the condensing pressures. For an ambient temperature of 45°C and an average condensing surface temperature of 5°C above ambient, a pressure of roughly 21 bar is required for condensation of ammonia. During measurements, a maximum of 18 bar was recorded. It is therefore necessary to redesign the system for such higher temperatures besides solving other technical problems.
‘
ABBREVIATIONS 1 2 3 4 a
h I WI
M3 4
MS P P PS PI PZ
2 Q", RE RG RI T, 7-2 T3 T4
z g
specific heat (Wh/kgK)
COP coefficient of performance (dimensionless) with solar COPSOI coefficient of performance
energy
(dimensionless) enthalpy (Wh/kg) solar radiation (W/m’) mass fraction mass of desorbed ammonia (kg) fraction of ammonia at saturation mass of ammonia need for first charging pressure (bar) pressure in the ammonia circuit (bar) partial vapour pressure of Ammonia at saturation (bar) condensing pressure (bar) pressure in the reactor (bar) cooling produced (kwh/d) energy required for generating ammonia ( Wh) heat extracted from the solar collector and delivered to distilled water in the heat exchange circuit (Whld) refrigeration effect (kWh/d) heat supplied to the collector field (kWh/d) total solar energy on collector (kWh/d) reactor temperature (“C) ambient temperature (“C) cool box temperature (“C) ice/water temperature (“C)
system
absorber temperature (C) ambient temperature (‘C) condenser temperature (“C) desorber temperature (‘C ) evaporator temperature (‘C ) generator temperature ( ‘C) (T- T,/‘I) solar collector field efficiency (dimensionless) No. of moles (moles)
zl NOMENCLATURE
1994 with solar heating
firn - (bar) 0 x
Y z
reactor ambient cooling box ice absorber condenser desorber evaporator latent evaporation generator graphite at mean temperature average overall SrCl, .8NH, SrC1, 1NH, 7NH,
REFERENCES Chinnapa J. C. V. (1961) Experimental study of the Intermittent Vapour Absorption Refrigeration Cycle Employing the Refrigerant-Absorbent Systems of Ammonia Water and Ammonia Lithium Nitrate. Solar Energy 5, l-18. El-Shaarwi M. A. 1. and Ramadan R. A. (1986) Solar Refrigeration in the Egyptian Climate, Solar Energy 37(S), 3477361. Iloeja 0. C. (1983/84) Parametric Effects on the Performance of a Solar Powered Solid Absorption Refrigerator, ISES Congress. Kleinemeyer M. (1995) Documentation of the Development of a Solar Powered Solid Absorption Refrigerator, DORNIER GmbH, Friedrichschafan, Germany. Mello P. and Hahne E. (1993) Effects of Operating Pressure in a Solid Absorption Refrigerator, App. Solar Energy, pp. 780-784.
137
Performance testing and evaluation of solid absorption solar cooling unit MESS REPORT (1994) Pruef und Forschungsstelle Solarenergie Technikum Rappaerswil Switzerland, Test Collector, No. 103. Nielsen P. B. (1981) A Critical Survey of Intermittent Absorption Systems for Solar Refrigeration, In?. Congr. of Refrigeration Moscow, Session 1 B 2.1, pp. 659-667. Sargent S. L. and Beckman W. A. (1968) Theoretical Performance of an Ammonia-Sodium Thiocyanate Intermittent Absorption Refrigeration Cycle. Solar Energy 12,
Stanish M. A. and Pearlmutter D. D. (1981) Salt Hydrates as Absorbents in Heat Pump Cycles. Solar Energy 26, 333-339.
Swartman Robert K. and Swaminathan C. ( 1971) Solar Powered Refrigeration,
Mechanical Engg.
83, 22-24,
June. Worse-Schmidt P. ( 1983) Solar Refrigeration for Developing Cycle, Int. J. Ambient Energy, 4(3). July.
137-146.
APPENDIX A.1.
Thermodynamic
properties
A
of the reactor
containing
SrCI,
and graphite
30 z? 3 z g a
20
10 8 6 5 4 3 2
1.0 0.8 0.6 0.5 0.4 0.3
2.5
T-' x 103[K-'1
Temperature Fig. Al. Equilibrium curves of Ammonia and SrCI,. As Graphite is inert, all chemical properties of the SrCI, can be extended to the SrCI,-Graphite mixture. Temperature limits depending on the pressure exist for both absorption and generation. As shown in Fig. 10, Strontium-Chloride absorbs ammonia below a fixed equilibrium temperature (“Mono”) and generates it above a fixed equilibrium temperature (“Okta”). There are two marked points in the diagram. process during the dav). If the (1) Point A (generation ._ condenser temperature is 40°C a pressure of 15 bar is required to initiate the condensation of ammonia. At this pressure the equilibrium temperature of the Ammoniacate is about 100°C. That means that the reactor temperature has to be higher than 100°C for the initiation of the generation of ammonia. The used solar vacuum tube collectors can provide the heat for the required temperature even at a comparatively low irradiation. (2) Point B (adsorption process during night)If an evapora_I
tion temperature of -7°C is required, the pressure has to be 3 bar. Hence for the absorption of ammonia the reactor temperature should be below the corresponding equilibrium temperature of 62°C. The graphs are approximated with the following equations: T
-2793.3 = -273 ‘Ond Inp - 11.676
T,,, =
-4983.3 lnp- 16.018
- 213
Equations for the reaction times and sorption ( Kleinemeyer. 1995)
,Qf=,$f,.(l-emk~‘) k= -lln(OS) --0
masses
138
'. h
where, M
absorbed/generated mass (kg) mass of NH, at the state of saturation time factor (min) time (min) half-life period min)
M,
k
; -0
(6.2I kg)
A. 1. I. Other physicul values Specific
Heat
( Kleinemeyer,
Capacities 1995):
of
the
reactor
components
c PS.CI>HVHJ =(576.296+3.124T+0.07T2+0.00005T-l)i3600 ~=(649.8+3.524T+0.0217’Z+0.00016T-7)/3600 8VH
CPM,
CP__ %<.I,
= 0.224
“NH, C,rsphlfc = ( ‘3.4CPM,,
“NH,+ l~8CP
C&l2 lNHJ tirrPh,ls=(7.9C,M?, “NHz + ‘.8L.,h,,.)‘9.7 C, specific heat capacity T temperature (‘C) Specific reaction enthalpies of SrCl, and NH, /1,=(46.2251 -0.09767;) x 16.3388 h, reaction enthalpy ( Wh/kg NH,)
Bansal C’Iui r, reactlon temperature ( ‘C) This enthalpy already contains the specific heat of thr. generated as well as absorbed ammonia Further properties of SrCl,-Graphite. ( I) Even at considerably high temperatures (150'C~ the evaporation pressure of the salt can be neglected. (2) The swelling caused by the absorption of ammoma 1s very low compared with other salts used for solid absorption refrigeration (CaCI,). This fact is extremely important for the design of the reactor geometry. .4dvantapes of the SrCI,-Graphlte mixture compared to pure SrCI, ii) As the heat conductivity 01 the mixture IS higher than that of the pure salt, less energy is required to heat up the reactor to equilibrium temperature. This energy cannot be used for the generation process and there. fore lost. (2) The long-term stability of the reactlon kinetics IS also enhanced by the addition of the Graphite to the salt (3) As Graphite is inert, the addition of the Graphite does not influence the reaction velocity of the mixture and the physical values of the SrCI,.
APPENDIX B B.I. Hourly mean ambient temperature and solar radiation on 30” south jhcing tilted surfhce
45
40 i-
I
30 25 20
.A /:. .
15 10
API: -
May
-
Jun
14
16
;
5 0 6
I
I
8
10
0
12
14
16
18
I
I
~;L.i__.L
6
8
10
45
___
i
~...
12
18
~-.--_...II__~
40
06
8
10
12
14
16
0' 6
18
I
I
I
8
10
12
mean ambient
Nov
/ -
Dee -2
14
Time (Hours)
Time (Hours) Fig. Bl. Hourly
I -
temperature
in Delhi.
16
16
Performance testing and evaluation of solid absorption solar cooling unit
139
1.2
1.0 “E 3 5 g ._ z :: E
0.8 0.6 0.4 0.2 \
0.0
_-L
4
12
8
16
20
0.0 -
_. 24
-’ 4
**. 1 Jul
-
Sep(
“E
1.0 0.8 0.6
0.0 /4
12
8
16
20
J
4
24
16
f.1:\. ..
2 5
Au(l
12
8
20
24
..
act
-
NOY
-0ec
a. .
12
8
16
20
Time (Hours)
Time (Hours)
Fig. B2. Hourly mean solar insolation on south facing 30” tilted surface.
APPENDIX
C
C. 1. Theoretical performance of solar cooling unit Table Cl. Theoretical results (January to March) T, = - 8.93 Time January 10.00 11.00 12.00 13.00 14.00 15.00 16.00 February 10.00
I I .oo 12.00 13.00 14.00 15.00 16.00 March
Il.00 IL00 13.00 14.00 15.00 16.00
r amb.
I’,
13.50 15.80 18.10 19.80 20.90 21.30 20.90
&
M3
QE
COP
T,
P,
T,
R,
rl
6.86 7.42 8.01 8.47 8.77 8.89 8.77
96.00 110.00 116.00 120.00 119.00 115.00 114.00
8.15 13.76 17.02 19.54 18.89 16.44 15.87
18.64 35.52 42.94 47.95 46.69 41.69 40.45
3.874 5.680 7.864 9.670 II.264 12.510 13.297
0.485 0.477 0.516 0.457 0.424 0.352 0.120
1.462 2.324 3.451 4.275 4.951 5.390 5.484
I .95l 3.248 5.117 6.520 7.680 8.410 8.564
0.629 0.976 1.490 1.857 2.200 2.462 2.521
0.430 0.420 0.432 0.434 0.444 0.457 0.460
0.162 0.172 0.189 0.192 0.195 0.197 0.190
16.00 18.30 20.50 22.10 23.20 23.60 23.20
7.47 8.06 8.66 9.12 9.45 9.57 9.45
96.00 110.00 116.00 120.00 119.00 115.00 114.00
8.15 13.76 17.02 19.54 18.89 16.44 15.87
18.64 35.52 42.94 47.95 46.69 41.69 40.45
4.315 6.233 8.224 10.141 11.848 13.211 14.1I?
0.512 0.502 0.500 0.482 0.455 0.398 0.216
1.859 2.821 3.816 4.740 5.517 6.059 6.254
2.591 4.081 5.738 7.319 8.649 9.545 9.866
0.835 I .227 1.671 2.085 2.478 2.794 2.904
0.449 0.435 0.438 0.440 0.449 0.461 0.464
0.194 0.197 0.203 0.206 0.209 0.212 0.206
24.30 26.70 28.30 29.70 30.20 29.70
9.78 10.55 Il.08 11.57 11.74 11.57
110.00 116.00 120.00 119.00 115.00 114.00
13.76 17.02 19.54 18.89 16.44 15.87
35.52 42.94 47.95 46.69 41.69 40.45
6.808 8.902 10.922 12.724 14.188 15.204
0.533 0.531 0.515 0.494 0.451 0.317
3.420 4.531 5.572 6.461 7.120 7.443
5.083 6.955 8.750 10.269 11.346 II.878
1.528 2.025 2.493 2.942 3.322 3.496
0.447 0.447 0.447 0.455 0.466 0.470
0.224 0.227 0.228 0.231 0.234 0.230
COP,,,
24
Table C2. Theoretical rrsults (.4pr1l to June) .-__ -..-..-..-_-_.. T&!
I>?
i
7,;
-8.93 ,____ . . .._____-____-.
R,
ETA
R,,
/MS
QF
0.552
<‘OF’
._ c ‘UP,,,
April I I .oo 12.00 13.00 14.00 15.00 16.00 May 1 I .OO 12.00 13.00 14.00 15.00 16.00 June
I1 .oo 12.00 13.00 14.00 15.00 16.00
30.30 32.70 34.50 35.70 36.20 35.70
11.78 12.66 13.36 13.84 14.04 13.84
110.00 116.00 120.00 119.00 115.00 114.00
13.76 17.02 19.54 18.89 16.44 Ii.87
35.51 42.94 47.95 46.69 41.69 40.45
7.008 9.1 IO 11.143 12.965 14.456 15 525
0.549 0.535 0.517 0.480 0.373
3.742 4.896 5.984 6.929 7.642 x.040
5.621 7.575 9.459 I 1.067 12.231 12.889
I.690 3.206 2.695 3.170 1.581 3 793
0.452 0.451 0.450 0.458 0.469 0.47?
II.241 Ii.242 0.242 0.244 0.248 0 244
35.10 37.30 39.00 40.10 40.50 40.10
13.60 14.50 15.23 15.71 15.89 15.71
110.00 116.00 120.00 119.00 115.00 Il4.00
13.76 17.02 19.54 18.89 16.44 15.87
35.52 42.94 47.95 46.69 41.69 40.45
6.896 8.940 10.909 12.688 14.150 15.21?
0.560 0.556 0.542 0.525 0.492 0.396
3.803 4.940 6.007 6.942 7.661 8.084
5.723 7.650 9.499 I I.094 12.263 12.962
I .720 2.227 2.706 3.178 3.590 3.x15
0.452 0.451 0.450 0.458 0.469 0.472
0.249 0.249 0 248 0.250 0.254 0 _‘
35.50 37.30 38.70 39.60 39.90 39.60
13.76 14.50 15.10 15.49 15.62 IS.49
110.00 116.00 120.00 119.00 115.00 114.00
13.76 17.02 19.54 18.89 16.44 15.87
35.52 42.94 47.95 46.69 41.69 40.45
6.777 8.772 10.697 12.438 13.875 14.927
0.557 0.552 0.536 0.519 0.485 0.388
3.735 4.835 5.867 6.770 7.467 7.875
5.610 7.472 9.258 10.799 11.934 12.610
I.686 7.176 2.637 3.093 3.494 7.71 I
0.452 0.450 0.450 0.457 0.468 0.47 1
r1.249 0.248 (I.247 0.249 11.252 d.149
Table C3. Theoretical Time
Tamb
PS
32.10 33.40 34.40 35.10 35.30 35.10
12.44 12.93 13.32 13.60 13.68 13.60
30.70 32.00 32.90 33.50 33.70 33.50 30.40 31.90 33.10 33.80 34.10 33.80
results (July to September)
T, = - 8.93
P2
I,
R,
ETA
&;
M,
c)c
COP
L’OPS,,
110.00 116.00 120.00 119.00 115.00 114.00
13.76 17.02 19.54 18.89 16.44 15.87
35.52 42.94 47.95 46.69 41.69 40.45
5.754 X.761 10.697 12.442 13.875 14.914
0.548 0.541 0.524 0.504 0.465 0.358
3.659 4.745 5.759 6.638 7.304 7.677
5.482 7.319 9.073 10.573 11.658 12.274
1.648 2.131 2.585 3.029 3.413 3.612
0.450 0.449 0.449 0.456 0.467 0.471
0.244 0.243 0.242 0.243 0.246 0.242
I I .92 12.40 12.74 12.97 13.04 12.97
110.00 116.00 120.00 119.00 115.00 114.00
13.76 17.02 19.54 18.89 16.44 15.87
35.52 42.94 47.95 46.69 41.69 40.45
6.785 8.835 10.813 12.585 14.028 15.055
0.548 0.542 0.524 0.502 0.46 1 0.344
3.647 4.757 5.793 6.684 7.348 7.701
5.462 7.338 9.132 10.651 Il.732 12.315
1.642 2. I37 2.601 3.051 3.435 3.625
0.450 0.449 0.449 0.456 0.467 0.471
0.242 0.242 0.241 0.242 0.245 0.241
11.81 12.36 12.81 13.08 13.20 13.08
110.00 116.00 120.00 119.00 115.00 114.00
13.76 17.02 19.54 18.89 16.44 15.87
35.52 42.94 47.95 46.69 41.69 40.45
6.535 8.564 10.519 12.258 13.663 14.626
0.545 0.539 0.522 0.498 0.454 0.318
3.433 4.526 5.547 6.413 7.051 7.358
5.104 6.946 8.707 10.187 11.229 11.735
1.534 2.023 2.480 2.918 3.287 3.454
0.447 0.447 0.447 0.455 0.466 0.469
0.235 0.236 0.236 0.238 0.241 0.236
TL?
July 11.00 12.00 13.00 14.00 15.00 16.00 August 11.00 12.00 13.00 14.00 15.00 16.00 September 11.00 12.00 13.00 14.00 15.00 16.00
Solur ~51:nur,qv Vol. 61, No. 2, pp. 127. 140. 1997 0 1997 Published by Elsevw ScienceLtd All rights reserved. Printed in Great Britain 0038-092X197 $17.00 $-0.00
PERFORMANCE TESTING AND EVALUATION OF SOLID ABSORPTION SOLAR COOLING UNIT *+ J. BLUMENBERG, ** H. J. KAVASCH ** and T. ROETTINGER * Centre for Energy Studies, Indian Institute of Technology, Hauz Khas. New Delhi 1IO 016. India
N. K. BANSAL,
**
** Institute for Thermodynamics, Technical University, Munich, Germany Received 17 August 1995; revised version accepted 22 January 1997 Communicated by VOLKER WlTTER Abstract--Based on solid-vapour intermittent absorption system, DORNIER a German firm designed and fabricated a solar cooling unit, which utilizes thermal energy supplied by heat pipe vacuum tube solar collectors through thermosyphonic flow of water. The unit of 1.5 kWh/day cooling capacity uses ammonia as a refrigerant and IMPEX material as absorbent and does not have any moving part requiring no auxiliary energy. The IMPEX material (80% SrCl, and 20% Graphite) has high heat and mass transfer coefficient as well as high absorption capacity. Detailed experiments were performed on a unit in Delhi under real field conditions followed by theoretical analysis. Theoretical maximum overall COP of the unit is 0.143, and it depends upon the climatic conditions. Under field conditions, it was found that if the maximum daytime ambient temperature was 30°C and night time temperature 2o”C, it took three sunny days to freeze water in the cooling box. After the second day, the temperature inside the cooling box remained 1‘C. The overall COP was found to be 0.081 only. The automatic control valve based on mechanical/thermal principles however has defects and the problem of corrosion of the sealings needs to be solved. In climates where day time temperatures are high (Delhi summer 43”C47”C during the day, 3O”C-35°C during the night) and solar radiation relatively low (4-5 kWhjm2d) because of pollution and sand in the atmosphere, it is most unlikely that pressure in the ammonia circuit can reach values at which ammonia vapours start to condense. The unit. needs to be redesigned for such conditions. 0 1997 Published by Elsevier Science Ltd
generation is a major problem. This problem is less in H,O-LiBr and NH,-NaSCN systems. In H,O-LiBr systems however, the crystal formation is a major problem. In NH,-NaSCN combination, the viscosity of mixture creates problems in the flow of solution. The solid absorbent-refrigerant combination has no such problems as with a liquid absorbent system. The development of such systems is however restrictive because of other kinds of problems inherent in the solid absorbents. Solid absorbents are subject to considerable swelling during the absorption phase which may result in close packing after a few cycles. The thermal conductivity of salts is quite low and therefore operation of the system for moderate temperature differences is very difficult. Chinnapa ( 1961) made systematic investigations for intermittent vapour absorption refrigeration cycle employing NH3-Hz0 and NH,-LiN03 systems. The investigations show that NH,-LiNO, system is as good as NH,-H,O. Sargent and Beckman (1968) found that NH,-NaSCN combination gave nearly the same COP as obtained by using NH,-Hz0 system. A critical survey of intermittent absorption systems was made by Nielsen ( 198 1) based
1. INTRODUCTION
The concept of solar powered absorption cooling was suggested more than 35 yr ago as a solution to refrigeration problems in the desert areas and other remote areas where electricity was not easily available. As a result of the energy crisis of 1970 these systems again attracted the attention of scientists and engineers for developing absorption cooling systems which can work efficiently and economically by using solar heat. Most of the work on absorption refrigeration was done on the continuous type of systems needing additional power for operating pumps. An intermittent absorption refrigeration employing cooling storage is more appropriate for remote area applications. Lots of experimental as well as theoretical work has been done on such systems based on refrigerant-liquid absorbent combinations like NH,-H,O, NH,NaSCN, H,O-LiBr etc. For an NH,-H,O absorption cooling system, transfer of water vapours to the condenser during the process of ‘Author to whom all correspondence should be addressed: E-mail, [email protected]; Fax.. +91-l l-6862037; Tel., +91-l l-666979 Ext. 5002. 127
on NH,-H20, NH,-LiNO,, NH,-CaC12 and NH,-SrCl, combinations. He concludes that the solid absorbents like CaC1, or SrCl, should be used for solar refrigeration. NH,-LiNO, poses problems of viscosity, while CaCl, shows swelling during the absorption phase and a problem of particle migration. Stanish and Pearlmutter ( 1981) showed the possibility of using salt hydrants as absorbents in heat pump cycles. Parametric effects on the performance of solar powered NH,-CaCI, system were investigated by Iloeja (1983). Studies conducted by Worse-Schmidt (1983) show that using a solid absorption cycle, solar driven refrigerating plants can be built very simply and they operate reliably over years without maintenance beyond occasional replenishment of water in the condenser water bath. Performance results of intermittent solar refrigeration in Egyptian climate show technical feasibility of NH,~-H,O systems ( El-Shaarwi and Ramadan, 1986 1. Mello and Hahne (1993) studied experimentally the dependence of the operating pressure of the absorption and desorption processes of ammonia on CaCl, as a function of the temperature difference starting from the equilibrium temperature. The results show slight influence on the ammonia absorption rates for pressures above 10 bar. Test results showed a good potential for chemical reactions to be used in combination with solar energy (Mello and Hahne. 1993). Under a project sponsored by the German Ministry of Research and Technology. DORNIER ( Kleinemeyer, 1995 ) developed an ammonia-strontium chloride based unit using heat pipe evacuated tube collectors. One of the units delivered to India was tested under actual field conditions. In this article we report the performance of this unit besides giving details of design calculations for various components of the system.
the reactor (generator cum absorber), desorbing the refrigerant vapours, which in turn condense and the liquid refrigerant IS stored within the evaporator and the storage tank (Fig. I ). After the sunset. the reactor cools down and the absorption process starts. The pressure rn the system lowers and the evaporation vt ammonia/refrigerant will take place in the evaporator producing cooling and consequently ice is formed in the water basin. The ammonia vapours are absorbed by the salt present in the reactor. The ice produced during the night time keeps the box cool during the day. 3. THE SOLAR
3. I
COOLING
UNIT
SpeciJcations
The solar cooling unit manufactured by DORNIER (Fig. 2) has its specifications given in Table 1, The main parts of the system are: (1) The reactor (generator cum absorber) filled with 7.2 kg SrCl, and 1.8 kg Graphite is heated up by solar energy and cooled down to ambient temperature, generating and absorbing maximum 9 1 of ammonia. (2) The condenser, which liquifies the generated ammonia. (3) The evaporator producing cold energy and ice with an integrated additional storage for ammonia. control valve which operates (4) Automatic only twice a day. (5) Vacuum tube solar collectors with water circuit for heating the reactor. During night water circulates through the reactor only to cool it down. 3.2. The reactor Two important parameters which effect the system performance are the heat transfer coefficient (described by the thermal conductivity and heat transfer coefficient on the wall)
2. PRINCIPLE
OF INTERMITTENT SOLID ABSORBENT-VAPOUR ABSORPTION SYSTEM
The ability of a liquid or a solid medium to absorb and generate specific vapours depending on its temperature constitutes the basis of absorption refrigeration. An intermittent solar powered system operates on diurnal cycle with one generation cycle per day. In the morning, the absorbent will contain maximum amount of refrigerant. During the day, the heat drawn from a solar collector system is utilized to heat
Fig. 1. Basic operation
of a solid absorbent system.
solar cooling
Performance testing and evaluation of solid absorption solar cooling unit
Fig. 2. Photographic view of the solar refrigerator Table I. Specifications of the solar cooling unit
# Cooling unit size
300 I
ii- Total energy to be produced at 5 kWh/m* day insolation
1.5 kWh (0.440.5 kWh/day is the nominal energy required to keep the temperature within the cooling unit constant, I kWh/day is the net energy corresponds to 57 1of water to be cooled from 30°C to 15°C or 10.75 kg ice to be produced) 40-45°C during day time, 30°C during night time Ice storage of max 4 kWh (about 33 kg of ice) to hold the refrigeration temperature constant at about I-5°C. The maximum ice storage can keep the refrigerator cold for about 5 days with no or less solar energy supply Stainless Steel 2.1 m’; (No. 13; one used for automatic control valve)
f
Ambient conditions Internal ice storage
NH, circuit pipe material
; Vacuum tube solar collector area # No moving parts except one valve in water circuit which -
also operates only twice in a day
and the mass transfer coefficient. The salt SrCl, has itself very low conductivity values of about k = 0.1-0.3 W/mK and a low heat transfer coefficient of about h= lo-20 W/m2K. In practice it is well known that the mass transfer using salt without additives is less and can partially or totally block the reaction after certain number of cycles. This is the reason why this technology could not progress much despite general advantages. In the new technology used in the system, the specially pretreated Graphite
.__
(system STELF, patented world wide) handled and prepared with the salt is used in the reactor. Graphite is temperature stable, does not react with ammonia and has a high conductivity. Besides these properties, graphite improves the vapour flow and thus enhances the mass transfer. Further, the salt is pressed in order to enlarge the storage capacity and the heat transfer and to diminish the specific reaction volume without effecting the mass transfer in specific limits. It is seen that the heat conductiv-
ity can be varied between k =4-25 W/mK while the heat transfer coefficient on the wall is increased to about h = 500- 1200 W/m2K with the new technology. Test results concerning the mass transfer also show good performance. In the reactor, the mixture of 80% SrCl, and 20% Graphite known as IMPEX is in the form of cylindrical pieces. Seventeen such pieces of IMPEX are put in the reactor for the present cooling unit. 3.3. Solar generator Solar generator is a system, which consists of thirteen vacuum tube collectors “HPSEIDO” produced by Beijing Solar Energy Research Institute, China. Collectors are essentially heat pipes in which water is used to transfer heat from the absorber to the top of the collector. When the collector is irradiated by the sun. water at the lower end of the pipe starts to evaporate and diffuses to the colder end at the top. Here the vapour condenses and the resulting fluid flows back to the bottom. The efficiency equation of the collector specified by the Mess Report ( 1994) Solar Energy Integration and Research Centre Rapperswil, Switzerland is: ‘7= c,, - c, .\-- C’,.\-?I
(1)
c0 =0.762, C’, =2.21 ( W/m’K ) and where, C; =0.004( W/m2K2) Graphically, the efficiency curve is given in Fig. 3. 3.4. The ammonia circuit (Fig. 4) As discussed earlier during the day the ammonia is generated in the reactor and leaves it at a high temperature and pressure. In the condenser it is cooled down and hence liquihed. This condensed ammonia is stored in the ammo-
nia storage tank and evaporator. The \atetv valve is also mounted in the circuit which exhausts the ammonia to the atmosphere 11 ~hc pressure rises too high (above 30 bar). Durinr! the night the evaporated ammonia is transferred back to the reactor through the same circuit The pressure in the ammonia circuit is high (15-20 bar) during day-time and low (ahot:! 3 bar) during night-time. As the graphite is inert to ammonia so the properties of SrCl,-Graphite (STELF) mixture can be considered as that of SrCl,. One mole of SrCl, can absorb 8 moles oi’ NH,, but generateidesorb only 7 moles under the design conditions of the system, so one mole always remains absorbed in SrCl,. The mixture of SrCl, and NH, is called “Ammoniacate” Theoretically the mass of ammonia required at the time of fresh charging the new system can be calculated by using the following equation. SrC1,.8NH,++SrClz.
lNH,+7NH,
where, 158.62 kg of SrCl, = I mole of SrC12. Realising the fact that only 7 molecules of NH, are desorbed during the generation process. maximum of 5.4016 kg of NH, is needed for recharging the system
The water circuit (see Fig. 5) has two functions in the solar cooling unit. The first is to transfer heat energy from the solar collector to the reactor during day-time and the second function is to help in dissipation of the absorbed heat produced during the night to the atmosphere. so that cooling of the reactor takes place easily. For thermosyphonic how of water, the
X-Value
Fig. 3. Efficiency curve of the heat pipe solar collectors
produced
by Beijing Solar Energy
Research
Institute.
China
Performance
testing and evaluation
of solid absorption solar cooling unit
Vacuum fibe
Fig. 4. Schematic
of solar cooling
unit produced
Cdlector
by DORNIER
Vacuum colkctw
Fig. 5. Heat transfer
reactor was installed at 2.15” inclination with horizontal. Water in the circuit is at saturation corresponding to ambient temperature. In the morn-
I31
GmbH
Germany.
wbes
loop.
ing the automatic control valve closes. The solar collector transfers heat energy to water pipe A causing the formation of vapour bubbles in the pipe A. The vapours cannot reach the absorp-
tion heat exchanger because of siphon c’, but start collecting at B. The pressure in the circuit is now increasing and water in the circuit is forced into the heat exchanger until it is filled completely. The absorption heat exchanger is now blocked and cannot transfer heat during day-time to the atmosphere. The new water level in “D” is below the reactor, so the water circuit will work as a regular heat pipe. When the solar insolation decreases in the evening and control valve opens automatically and also the pressure in the circuit drops again. The water level in “D” will rise to a higher level. The steam formed in the hot reactor will pass to the absorption heat exchanger through the control valve and condenses there. Heat of energy is rejected to the condensation atmosphere.
perature Tc = -. 7 C and using the expression:,
3.6. Cooling box
From the expressions of the specific heats C, and C, and the enthalpies of evaporator, condenser and evaporation given in the appendix, the various values are obtained as:
The cooling box of the unit is made from the cabinet of a common refrigerator with more insulation on the wall. The total volume of the box is 260 1.The heat exchanging tubes carrying ammonia are dipped into water contained in a plate type of heat exchanger kept inside the box. The total volume of water which can be filled between the plates is 33 1.
yields p, = 3.238 bar Similarly; -2793.3 T, = _~._.._-~._ lnp,-11.676
= 273
and for T, = 55°C. p2 = 23.567 bar. The average generator temperature Tg and the absorber temperature T, are calculated as 114.56”C and 62.73”C respectively. Corresponding to Fig. Al and Appendix A, the other values of temperatures are calculated as T, = 125.56”C, & = 103.56”C, T, = 51.73”C and ii;, = 77.65”C.
C, = 0.687 Wh/kgK, C; = 0.294 WhlkgK, h, = 46.29 Whjkg. h, = 128.12 Whikg.
4. THEORETICAL
ANALYSIS
h, = 356.83 Wh,fkg
The reaction during the desorbing phase of ammonia is given as follows; SrCl,SNH, t - +
SrCl, 1NH, +
7NH3
(X)
(Y)
(Z)
(158.62+ 136)
(158.62+17)
(119)
294.62 A mixture of “Ammoniacate”. w.r.t. &Cl,
175.62
119
SrCl, and NH, is called Fractions of various masses
m, =294.62/158.62=
1.857
m,=175.62/158.62=
1.107
m, = 119/158.62=0.75
Fraction of ammonia (w.r.t. SrCl,) in the reactor at saturation m3 = 136/l 58.62 =0.857
The temperature/pressure relationships and other thermodynamic properties of NH,, SrCl, and SrCl,. 8NHs mixtures are given in Appendix A. Considering the evaporation tem-
At the average temperature of the evaporator T,, the latent heat of evaporation is calculated as: T, and the condenser temperature h, = 321.75 Whlkg.
the rated cooling capacity From Q, = 1500 Wh/d and Q, = M4. h, one obtains M4 = 4.204 kg. However, considering the average temperature and mean enthalpy, the mass of the desorbed ammonia is (Swartman, 1971) given by; M-I
M3=
exp (h, ~-11,)/h,, yielding M3 = 5.421 kg M SrCl* = M3/mz = 7.228 kg
Therefore, 7.228 kg of SrCl, is required for 1.5 kWh/d of cooling capacity. Mass of ammonia needed at first charging of the system MS =ms x MSrC12=6.194 kg.
For recharging, the amount of ammonia is equal to M,=5.421 kg.
I’erfomance
testing and evaluation
Te
Ta Tc
of solid absorption
?a
Td
Tg
solar cooling
unit
i 33
Tg
Tem~ufe Fig. 6. Pressure-temperature
Quantities of various salts and mixture are obtained as MX
= m, MSrClz
= 13.42 kg
MY
=mY . MSrCl2
=8.001 kg
= mgr Msra 2 = 1.807 kg Total mass of salt mixture in the reactor = MSrC,, + MBr = 9.025 kg Enthalpy of refrigeration h, =(46.2251 -O.O976T,) x 16.3388 wh/kg of NH, = 572.56 Wh/kgofNH,
Q,=M,C,V,
the
-C)+M,C,V,-G)
+M,&,t?‘,-L)+M,h, = 3663.39Wh COP= QJQ,
cycle.
It takes about 3 days to get the water totally frozen in the storage (33 1 of water).
5. EXPERIMENT
MSr
The energy required for generating, amount of ammonia is calculated as
curve for SrCI,~-NH,
= 0.409
The efficiencies of heat collection and subsequent transfer to the reactor is approximately 35%. If the daily solar insolation is 5 kWh/m’d, the required collector area is 2.1 m2 and the overall COP (= Qe/Ztotal) is 0.143.
The various parameters measured during the experimentation on the solar cooling unit (see Fig. 4) include the global solar radiation I, ammonia pressure p, reactor temperature r,, ambient temperature TX, ice storage temperature T, and the cooling box temperature T4. The solar radiation was measured by using class I Kipp and Zonen pyranometer transducer and temperatures by using thermocouple. All sensors were connected to a 6 colour data printer for continuous monitoring. The unit was installed outdoors on a horizontal and stable foundation with evacuated tube collectors pointing towards the north at the inclination of 30”. Initially the unit has nitrogen filled in the heat exchange circuit connected to solar collectors, because of the danger of freezing during the transportation. Before starting the operation, therefore, it is absolutely essential that N, is released and distilled water is filled in. After releasing the nitrogen carefully, the heat exchanger circuit is evacuated to about 1 mbar. When the pressure is just below 1 mbar, the vacuum pump is removed and inlet valve is
Delhi,
:I
India,
28.-29.05.1994
70
y 60 700 * 600 $
2 50 E; f 40
Delhi,
b
India,
28.-29.05.1994
140
20 16
,I '20 Y . 0 100 5 B 80 z g 60
16 14
LO F
20
Fig. 7. Temperatures,
radiation
and pressure
connected to a vessel containing more than 3.3 1 of distilled water. Inlet valve is opened and as soon as 3.3 1 of water is sucked, it is closed again. The heat exchange circuit is evacuated again to the saturation pressure of water. The unit was received with all ammonia absorbed in the reactor. 6. RESULTS
AND DISCUSSIONS
The performance of the unit was studied theoretically first for climatic conditions prevailing in Delhi. The corresponding ambient temperatures and solar radiations are given in Appendix B (Figs. Bl and B2) in terms of hourly values of ambient temperatures and solar radiation on an inclined surface. Tables in
measured
on the solar cooling
unit
Appendix C give the calculated values of various parameters, which include the mass of ammonia desorbed and the values of COP. The values show that, in the climatic conditions of Delhi, the total amount of ammonia i.e. 5.4 kg is dissolved by noon. The results also show that the pressure is not high enough in the ammonia circuit for condensation to take place. The condensation of ammonia is therefore difficult particularly in the hot summer months. In order to understand the system, a detailed description of the initial unsuccessful results as well as the final successful results are included here. Measurements performed on two successive days in May 1994 are given in Fig. 7. The results show that (1) The temperature in the cooling box (T3)
Performance testing and evaluation of solid absorption solar cooling unit
135
-Tl
Delhi, India, 28.7.-01.08.1994
-
T2
-T3
-10 8 @a c
z co: c
g ti 0
:: id
z =: =: m ij
B
=:
::
ti
6;
0
_
0
7
-
N
c
cl
id
z 6
=: =: uj ti
0
CI
-
Fig. 8. Experimental observations during the phase of electrical heating.
was nearly equal to the ambient temperature (T,) during the night. (2) The pressure in the ammonia circuit was never above 13.5 bar ( 3) The reactor temperature (r,) never fell below 80°C The results obviously show that there was no refrigeration inside the cooling box. The ammonia vapours did not condense because the pressure never reached the condensing value of 20 bar at 45’C. As the reactor temperature was also high, the absorption of ammonia vapours also did not take place. Efforts were made to recharge the system completely by emptying and filling the ammonia again and by replacing the distilled water also. Also the monsoon season began in Delhi and therefore the water in the reactor loop was also heated electrically. The measurements of 3 days are shown in Fig. 8. The figure shows that the cooling system is functioning. In four days, all the water in the cooling box was frozen and the temperature in the cooling box (r,) dropped to -4°C. The electrical system was replaced by the solar system after the monsoon period. The results of measurements made in October 1994 are shown in Fig. 9. The cooling produced was equivalent to 1.23 kWh and the solar radiation falling on the collector surface was 15.2 kWh giving an overall COP,,, of 0.08 1.
It is seen that the experimentally obtained of 0.081 is less than the theoretically obtained COP,,, of 0.143. The reason for this lies in the fact that the theory has been developed for average conditions. The system however works only dynamically. The generation of ammonia takes place only after the reactor has reached a certain temperature. The early hours of solar radiation are therefore used to warm the reactor only and do not contribute to the average performance of the system. The steady state theoretical analysis therefore will always predict a higher COP,,, than the measured values. COP,,,
7.
CONCLUSIONS
Principally, the refrigeration unit of the DORNIER works. If the maximum day-time temperature is 30°C and night 20°C it took three sunny days to freeze the water in the cooling box. The temperatures inside the cooling box remained at 1“C after the second sunny day. The prototype unit, however, has defects. The automatic control valve does not work satisfactorily and the sealings get damaged because of corrosion by ammonia. May and June months in Delhi are characterized by high temperatures (43”C-47°C
136
160 -
Fig. 9. Experimental
observations
in October
during the day and 3O”C35”C during the night). The solar radiation is low (between 5 and 6 kWh/m’d) because of pollution and sand in the atmosphere. It is most unlikely that pressure in the ammonia circuit reaches the condensing pressures. For an ambient temperature of 45°C and an average condensing surface temperature of 5°C above ambient, a pressure of roughly 21 bar is required for condensation of ammonia. During measurements, a maximum of 18 bar was recorded. It is therefore necessary to redesign the system for such higher temperatures besides solving other technical problems.
‘
ABBREVIATIONS 1 2 3 4 a
h I WI
M3 4
MS P P PS PI PZ
2 Q", RE RG RI T, 7-2 T3 T4
z g
specific heat (Wh/kgK)
COP coefficient of performance (dimensionless) with solar COPSOI coefficient of performance
energy
(dimensionless) enthalpy (Wh/kg) solar radiation (W/m’) mass fraction mass of desorbed ammonia (kg) fraction of ammonia at saturation mass of ammonia need for first charging pressure (bar) pressure in the ammonia circuit (bar) partial vapour pressure of Ammonia at saturation (bar) condensing pressure (bar) pressure in the reactor (bar) cooling produced (kwh/d) energy required for generating ammonia ( Wh) heat extracted from the solar collector and delivered to distilled water in the heat exchange circuit (Whld) refrigeration effect (kWh/d) heat supplied to the collector field (kWh/d) total solar energy on collector (kWh/d) reactor temperature (“C) ambient temperature (“C) cool box temperature (“C) ice/water temperature (“C)
system
absorber temperature (C) ambient temperature (‘C) condenser temperature (“C) desorber temperature (‘C ) evaporator temperature (‘C ) generator temperature ( ‘C) (T- T,/‘I) solar collector field efficiency (dimensionless) No. of moles (moles)
zl NOMENCLATURE
1994 with solar heating
firn - (bar) 0 x
Y z
reactor ambient cooling box ice absorber condenser desorber evaporator latent evaporation generator graphite at mean temperature average overall SrCl, .8NH, SrC1, 1NH, 7NH,
REFERENCES Chinnapa J. C. V. (1961) Experimental study of the Intermittent Vapour Absorption Refrigeration Cycle Employing the Refrigerant-Absorbent Systems of Ammonia Water and Ammonia Lithium Nitrate. Solar Energy 5, l-18. El-Shaarwi M. A. 1. and Ramadan R. A. (1986) Solar Refrigeration in the Egyptian Climate, Solar Energy 37(S), 3477361. Iloeja 0. C. (1983/84) Parametric Effects on the Performance of a Solar Powered Solid Absorption Refrigerator, ISES Congress. Kleinemeyer M. (1995) Documentation of the Development of a Solar Powered Solid Absorption Refrigerator, DORNIER GmbH, Friedrichschafan, Germany. Mello P. and Hahne E. (1993) Effects of Operating Pressure in a Solid Absorption Refrigerator, App. Solar Energy, pp. 780-784.
137
Performance testing and evaluation of solid absorption solar cooling unit MESS REPORT (1994) Pruef und Forschungsstelle Solarenergie Technikum Rappaerswil Switzerland, Test Collector, No. 103. Nielsen P. B. (1981) A Critical Survey of Intermittent Absorption Systems for Solar Refrigeration, In?. Congr. of Refrigeration Moscow, Session 1 B 2.1, pp. 659-667. Sargent S. L. and Beckman W. A. (1968) Theoretical Performance of an Ammonia-Sodium Thiocyanate Intermittent Absorption Refrigeration Cycle. Solar Energy 12,
Stanish M. A. and Pearlmutter D. D. (1981) Salt Hydrates as Absorbents in Heat Pump Cycles. Solar Energy 26, 333-339.
Swartman Robert K. and Swaminathan C. ( 1971) Solar Powered Refrigeration,
Mechanical Engg.
83, 22-24,
June. Worse-Schmidt P. ( 1983) Solar Refrigeration for Developing Cycle, Int. J. Ambient Energy, 4(3). July.
137-146.
APPENDIX A.1.
Thermodynamic
properties
A
of the reactor
containing
SrCI,
and graphite
30 z? 3 z g a
20
10 8 6 5 4 3 2
1.0 0.8 0.6 0.5 0.4 0.3
2.5
T-' x 103[K-'1
Temperature Fig. Al. Equilibrium curves of Ammonia and SrCI,. As Graphite is inert, all chemical properties of the SrCI, can be extended to the SrCI,-Graphite mixture. Temperature limits depending on the pressure exist for both absorption and generation. As shown in Fig. 10, Strontium-Chloride absorbs ammonia below a fixed equilibrium temperature (“Mono”) and generates it above a fixed equilibrium temperature (“Okta”). There are two marked points in the diagram. process during the dav). If the (1) Point A (generation ._ condenser temperature is 40°C a pressure of 15 bar is required to initiate the condensation of ammonia. At this pressure the equilibrium temperature of the Ammoniacate is about 100°C. That means that the reactor temperature has to be higher than 100°C for the initiation of the generation of ammonia. The used solar vacuum tube collectors can provide the heat for the required temperature even at a comparatively low irradiation. (2) Point B (adsorption process during night)If an evapora_I
tion temperature of -7°C is required, the pressure has to be 3 bar. Hence for the absorption of ammonia the reactor temperature should be below the corresponding equilibrium temperature of 62°C. The graphs are approximated with the following equations: T
-2793.3 = -273 ‘Ond Inp - 11.676
T,,, =
-4983.3 lnp- 16.018
- 213
Equations for the reaction times and sorption ( Kleinemeyer. 1995)
,Qf=,$f,.(l-emk~‘) k= -lln(OS) --0
masses
138
'. h
where, M
absorbed/generated mass (kg) mass of NH, at the state of saturation time factor (min) time (min) half-life period min)
M,
k
; -0
(6.2I kg)
A. 1. I. Other physicul values Specific
Heat
( Kleinemeyer,
Capacities 1995):
of
the
reactor
components
c PS.CI>HVHJ =(576.296+3.124T+0.07T2+0.00005T-l)i3600 ~=(649.8+3.524T+0.0217’Z+0.00016T-7)/3600 8VH
CPM,
CP__ %<.I,
= 0.224
“NH, C,rsphlfc = ( ‘3.4CPM,,
“NH,+ l~8CP
C&l2 lNHJ tirrPh,ls=(7.9C,M?, “NHz + ‘.8L.,h,,.)‘9.7 C, specific heat capacity T temperature (‘C) Specific reaction enthalpies of SrCl, and NH, /1,=(46.2251 -0.09767;) x 16.3388 h, reaction enthalpy ( Wh/kg NH,)
Bansal C’Iui r, reactlon temperature ( ‘C) This enthalpy already contains the specific heat of thr. generated as well as absorbed ammonia Further properties of SrCl,-Graphite. ( I) Even at considerably high temperatures (150'C~ the evaporation pressure of the salt can be neglected. (2) The swelling caused by the absorption of ammoma 1s very low compared with other salts used for solid absorption refrigeration (CaCI,). This fact is extremely important for the design of the reactor geometry. .4dvantapes of the SrCI,-Graphlte mixture compared to pure SrCI, ii) As the heat conductivity 01 the mixture IS higher than that of the pure salt, less energy is required to heat up the reactor to equilibrium temperature. This energy cannot be used for the generation process and there. fore lost. (2) The long-term stability of the reactlon kinetics IS also enhanced by the addition of the Graphite to the salt (3) As Graphite is inert, the addition of the Graphite does not influence the reaction velocity of the mixture and the physical values of the SrCI,.
APPENDIX B B.I. Hourly mean ambient temperature and solar radiation on 30” south jhcing tilted surfhce
45
40 i-
I
30 25 20
.A /:. .
15 10
API: -
May
-
Jun
14
16
;
5 0 6
I
I
8
10
0
12
14
16
18
I
I
~;L.i__.L
6
8
10
45
___
i
~...
12
18
~-.--_...II__~
40
06
8
10
12
14
16
0' 6
18
I
I
I
8
10
12
mean ambient
Nov
/ -
Dee -2
14
Time (Hours)
Time (Hours) Fig. Bl. Hourly
I -
temperature
in Delhi.
16
16
Performance testing and evaluation of solid absorption solar cooling unit
139
1.2
1.0 “E 3 5 g ._ z :: E
0.8 0.6 0.4 0.2 \
0.0
_-L
4
12
8
16
20
0.0 -
_. 24
-’ 4
**. 1 Jul
-
Sep(
“E
1.0 0.8 0.6
0.0 /4
12
8
16
20
J
4
24
16
f.1:\. ..
2 5
Au(l
12
8
20
24
..
act
-
NOY
-0ec
a. .
12
8
16
20
Time (Hours)
Time (Hours)
Fig. B2. Hourly mean solar insolation on south facing 30” tilted surface.
APPENDIX
C
C. 1. Theoretical performance of solar cooling unit Table Cl. Theoretical results (January to March) T, = - 8.93 Time January 10.00 11.00 12.00 13.00 14.00 15.00 16.00 February 10.00
I I .oo 12.00 13.00 14.00 15.00 16.00 March
Il.00 IL00 13.00 14.00 15.00 16.00
r amb.
I’,
13.50 15.80 18.10 19.80 20.90 21.30 20.90
&
M3
QE
COP
T,
P,
T,
R,
rl
6.86 7.42 8.01 8.47 8.77 8.89 8.77
96.00 110.00 116.00 120.00 119.00 115.00 114.00
8.15 13.76 17.02 19.54 18.89 16.44 15.87
18.64 35.52 42.94 47.95 46.69 41.69 40.45
3.874 5.680 7.864 9.670 II.264 12.510 13.297
0.485 0.477 0.516 0.457 0.424 0.352 0.120
1.462 2.324 3.451 4.275 4.951 5.390 5.484
I .95l 3.248 5.117 6.520 7.680 8.410 8.564
0.629 0.976 1.490 1.857 2.200 2.462 2.521
0.430 0.420 0.432 0.434 0.444 0.457 0.460
0.162 0.172 0.189 0.192 0.195 0.197 0.190
16.00 18.30 20.50 22.10 23.20 23.60 23.20
7.47 8.06 8.66 9.12 9.45 9.57 9.45
96.00 110.00 116.00 120.00 119.00 115.00 114.00
8.15 13.76 17.02 19.54 18.89 16.44 15.87
18.64 35.52 42.94 47.95 46.69 41.69 40.45
4.315 6.233 8.224 10.141 11.848 13.211 14.1I?
0.512 0.502 0.500 0.482 0.455 0.398 0.216
1.859 2.821 3.816 4.740 5.517 6.059 6.254
2.591 4.081 5.738 7.319 8.649 9.545 9.866
0.835 I .227 1.671 2.085 2.478 2.794 2.904
0.449 0.435 0.438 0.440 0.449 0.461 0.464
0.194 0.197 0.203 0.206 0.209 0.212 0.206
24.30 26.70 28.30 29.70 30.20 29.70
9.78 10.55 Il.08 11.57 11.74 11.57
110.00 116.00 120.00 119.00 115.00 114.00
13.76 17.02 19.54 18.89 16.44 15.87
35.52 42.94 47.95 46.69 41.69 40.45
6.808 8.902 10.922 12.724 14.188 15.204
0.533 0.531 0.515 0.494 0.451 0.317
3.420 4.531 5.572 6.461 7.120 7.443
5.083 6.955 8.750 10.269 11.346 II.878
1.528 2.025 2.493 2.942 3.322 3.496
0.447 0.447 0.447 0.455 0.466 0.470
0.224 0.227 0.228 0.231 0.234 0.230
COP,,,
24
Table C2. Theoretical rrsults (.4pr1l to June) .-__ -..-..-..-_-_.. T&!
I>?
i
7,;
-8.93 ,____ . . .._____-____-.
R,
ETA
R,,
/MS
QF
0.552
<‘OF’
._ c ‘UP,,,
April I I .oo 12.00 13.00 14.00 15.00 16.00 May 1 I .OO 12.00 13.00 14.00 15.00 16.00 June
I1 .oo 12.00 13.00 14.00 15.00 16.00
30.30 32.70 34.50 35.70 36.20 35.70
11.78 12.66 13.36 13.84 14.04 13.84
110.00 116.00 120.00 119.00 115.00 114.00
13.76 17.02 19.54 18.89 16.44 Ii.87
35.51 42.94 47.95 46.69 41.69 40.45
7.008 9.1 IO 11.143 12.965 14.456 15 525
0.549 0.535 0.517 0.480 0.373
3.742 4.896 5.984 6.929 7.642 x.040
5.621 7.575 9.459 I 1.067 12.231 12.889
I.690 3.206 2.695 3.170 1.581 3 793
0.452 0.451 0.450 0.458 0.469 0.47?
II.241 Ii.242 0.242 0.244 0.248 0 244
35.10 37.30 39.00 40.10 40.50 40.10
13.60 14.50 15.23 15.71 15.89 15.71
110.00 116.00 120.00 119.00 115.00 Il4.00
13.76 17.02 19.54 18.89 16.44 15.87
35.52 42.94 47.95 46.69 41.69 40.45
6.896 8.940 10.909 12.688 14.150 15.21?
0.560 0.556 0.542 0.525 0.492 0.396
3.803 4.940 6.007 6.942 7.661 8.084
5.723 7.650 9.499 I I.094 12.263 12.962
I .720 2.227 2.706 3.178 3.590 3.x15
0.452 0.451 0.450 0.458 0.469 0.472
0.249 0.249 0 248 0.250 0.254 0 _‘
35.50 37.30 38.70 39.60 39.90 39.60
13.76 14.50 15.10 15.49 15.62 IS.49
110.00 116.00 120.00 119.00 115.00 114.00
13.76 17.02 19.54 18.89 16.44 15.87
35.52 42.94 47.95 46.69 41.69 40.45
6.777 8.772 10.697 12.438 13.875 14.927
0.557 0.552 0.536 0.519 0.485 0.388
3.735 4.835 5.867 6.770 7.467 7.875
5.610 7.472 9.258 10.799 11.934 12.610
I.686 7.176 2.637 3.093 3.494 7.71 I
0.452 0.450 0.450 0.457 0.468 0.47 1
r1.249 0.248 (I.247 0.249 11.252 d.149
Table C3. Theoretical Time
Tamb
PS
32.10 33.40 34.40 35.10 35.30 35.10
12.44 12.93 13.32 13.60 13.68 13.60
30.70 32.00 32.90 33.50 33.70 33.50 30.40 31.90 33.10 33.80 34.10 33.80
results (July to September)
T, = - 8.93
P2
I,
R,
ETA
&;
M,
c)c
COP
L’OPS,,
110.00 116.00 120.00 119.00 115.00 114.00
13.76 17.02 19.54 18.89 16.44 15.87
35.52 42.94 47.95 46.69 41.69 40.45
5.754 X.761 10.697 12.442 13.875 14.914
0.548 0.541 0.524 0.504 0.465 0.358
3.659 4.745 5.759 6.638 7.304 7.677
5.482 7.319 9.073 10.573 11.658 12.274
1.648 2.131 2.585 3.029 3.413 3.612
0.450 0.449 0.449 0.456 0.467 0.471
0.244 0.243 0.242 0.243 0.246 0.242
I I .92 12.40 12.74 12.97 13.04 12.97
110.00 116.00 120.00 119.00 115.00 114.00
13.76 17.02 19.54 18.89 16.44 15.87
35.52 42.94 47.95 46.69 41.69 40.45
6.785 8.835 10.813 12.585 14.028 15.055
0.548 0.542 0.524 0.502 0.46 1 0.344
3.647 4.757 5.793 6.684 7.348 7.701
5.462 7.338 9.132 10.651 Il.732 12.315
1.642 2. I37 2.601 3.051 3.435 3.625
0.450 0.449 0.449 0.456 0.467 0.471
0.242 0.242 0.241 0.242 0.245 0.241
11.81 12.36 12.81 13.08 13.20 13.08
110.00 116.00 120.00 119.00 115.00 114.00
13.76 17.02 19.54 18.89 16.44 15.87
35.52 42.94 47.95 46.69 41.69 40.45
6.535 8.564 10.519 12.258 13.663 14.626
0.545 0.539 0.522 0.498 0.454 0.318
3.433 4.526 5.547 6.413 7.051 7.358
5.104 6.946 8.707 10.187 11.229 11.735
1.534 2.023 2.480 2.918 3.287 3.454
0.447 0.447 0.447 0.455 0.466 0.469
0.235 0.236 0.236 0.238 0.241 0.236
TL?
July 11.00 12.00 13.00 14.00 15.00 16.00 August 11.00 12.00 13.00 14.00 15.00 16.00 September 11.00 12.00 13.00 14.00 15.00 16.00