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Comparative study of transparent insulation materials cover systems for integrated-collector-storage solar water heaters
Solar Energy Materials & Solar Cells 58 (1999) 431}446
Comparative study of transparent insulation materials cover systems for integrated-collector-storage solar water heaters K.S. Reddy, N.D. Kaushika* Centre for Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India Received 3 June 1998; accepted 1 July 1998
Abstract The thermal performance of transparently insulated integrated-collector-storage solar water heaters is investigated theoretically as well as experimentally for a comparative study of cover systems having transparent insulation materials devices placed between the top glazing and the absorber. The data on solar transmittance, heat loss reduction characteristics and solar collector-storage e$ciencies of various con"gurations is generated for the system performance comparisons. These hot water systems exhibit average (diurnal basis) solar collection and storage e$ciencies in the range of 20}40% at a collection temperature of 40}503C. The performance of water heaters with cover system having absorber-perpendicular con"guration of TIM excel over absorber-parallel TIM covers. The e!ect of variation in the temperature of heat collected and cost of cover systems on the performance comparisons is also discussed. 1999 Elsevier Science B.V. All rights reserved. Keywords: Transparent insulation materials; Integrated-collector-storage system; Solar water heater; Absorber-parallel cover systems; Absorber-perpendicular cover systems; Honeycomb
1. Introduction Transparent or translucent insulation materials (TIM) make use of the thermal insulation properties of stagnant air layer and/or vacuum and are characterized by low heat loss coe$cients and high solar transmittance. TIM has similitude with conventional insulation materials in so far as the placement of air gap in solid mass is
* Corresponding author. Tel.: #91-11-6861977-x5006; fax: #91-11-6862037. 0927-0248/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 9 9 ) 0 0 0 1 8 - 5
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Nomenclature A C C g h h h h h h h I (t) I (t) k K l ¸ ¸ M M Nu q q q q Q (t) * Q (t) R R R R R R R R 2'+ S(t) t t 1
area of the solar absorber surface of the tank (m) speci"c heat of tank material (J/kg K) speci"c heat of water (J/kg K) acceleration due to gravity (m/s) depth of tank (m) convective heat loss coe$cient from absorber plane to cover 1 (W/m K) convective heat loss coe$cient from cover 2 to cover 3 (W/m K) radiative heat loss coe$cient from absorber plane to cover 1 (W/m K) radiative heat loss coe$cient from cover 2 to cover 3 (W/m K) radiative heat loss coe$cient from cover 3 to ambient (W/m K) convective heat loss coe$cient due to wind (W/m K) solar beam radiation at time t (W/m) solar di!use radiation at time t (W/m) conductivity of opaque insulation (W/m K) thermal conductivity of air at mean temperature ¹ (W/m K) + characteristic length of tank (m) thickness of air layer between absorber plane and cover 1 (m) thickness of air layer between cover 2 and cover 3 (m) mass of the tank (kg) mass of the water in the tank (kg) Nusselt number heat #ux from absorber plane to bottom cover of the TIM device (W/m) heat #ux across TIM device (W/m) heat #ux from top cover of TIM device to top tempered glass cover (W/m) heat #ux from top tempered glass cover to ambient (W/m) total heat loss from the system (W/m) retrieved heat #ux per unit area of heater (W/m) thermal resistance between absorber plane and cover 1 thermal resistance between cover 2 and cover 3 thermal resistance between cover 3 and ambient Rayleigh number tilt factor for beam radiation tilt factor for di!use radiation tilt factor for re#ected radiation thermal resistance across TIM device solar intensity at time t (W/m) bottom opaque insulation thickness (m) side opaque insulation thickness (m)
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¹ ¹ ¹ ¹ ¹ ¹ + ¹ ¹ ¹ ¹ ¹ (t) ¹ (t) ¹ (t!t ) G *¹ ; ; 1 ; 2 ; * ; 2'+ < Greek letters a (aq) (aq) (aq) (aq) b b k l p h e ,e ,e and e
433
time duration between two successive observations (s) temperature of cover 1 (3C) temperature of cover 2 (3C) temperature of cover 3 (3C) average ambient air temperature (3C) mean temperature of an air layer (K) average sky temperature (3C) average water temperature in the tank (3C) initial water temperature in the tank (3C) maximum water temperature in the tank (3C) ambient air temperature at time t (3C) water temperature at time t (3C) water temperatures at time (t!t ) (3C) temperature di!erence between absorber plane and cover 1 (¹ !¹ ) bottom heat loss coe$cient (W/m K) side heat loss coe$cient (W/m K) top heat loss coe$cient (W/m K) overall heat loss coe$cient (W/m K) combined conductive and radiative heat loss coe$cient across TIM device (W/m K) wind speed (m/s)
thermal di!usivity (m/s) absorptance-transmittance product for beam radiation absorptance-transmittance product for ground di!use radiation absorptance-transmittance product for sky di!use radiation e!ective absorptance-transmittance product tilt angle of water heating system (deg) Volumetric thermal expansion (K\) refractive index of cover plate kinematic viscosity (m/s) Stefan}Boltzmann constant (W/m K) angle of incidence (deg) emissivities of absorber plane, cover 1, cover 2 and cover 3
concerned. They are classi"ed into absorber-parallel, absorber-vertical, cavity and homogeneous structures based on their cellular geometry. The absorber-parallel con"guration of TIM involves multiple covers of glass/plastic "lms, which are placed parallel to absorber; this con"guration is very simple for practical realization and has been researched for long for its solar heat trap characteristics. For example
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transparent slab of methyl methacrylate (MMA) for solar heat trap was suggested and tested by Cobble [1] and Cobble et al. [2]. The concept has since been subjected to theoretical [3,4] and experimental [5] investigations for various such applications as passive solar building, #at-plate collector, integrated collector storage system. The major problem associated with this con"guration is that the number of parallel covers are to be increased to reduce heat loss, which reduces the solar transmittance signi"cantly. This leads to the consideration of absorber-perpendicular TIM which consists of a transparent honeycomb (cellular structure) immersed in an air layer. The use of the honeycomb device to reduce heat losses through the cover system of a #at plate collector is well known. In recent years transparent honeycomb insulations have been suggested and analysed for application in integrated-collector-storage (ICS) solar hot water systems [6}13]. The systems based on the storage water tank seem very suitable for domestic and industrial applications. All the above analyses have involved several approximations such as (i) the solar transmittance of TIM slab corresponding to di!use component of solar radiation is not taken into account, (ii) the beam radiation transmittance is considered only at normal incidence. Some of the above analyses have used a wrong boundary condition for heat transfer at the top surface of the TIM slab (see comments due to Kaushika and Arulanantham [14]). The resultant predictions and comparisons are therefore of debatable merit. Furthermore, the above analyses and evaluations are in general based on discrete measurements of solar transmittance and heat losses across the TIM cover system. In this paper, TIM devices are investigated for their application in cover systems of ICS water heaters. The approach involves the development of rigorous simulation model of system performance, its validation by prototype "eld experiments enabling the comparative study of TIM cover systems.
2. Experimental set-up Solar heating of a storage water tank cuboid in shape, transparently insulated at the top surface and covered with opaque insulation at all other sides is considered for TIM insulated integrated-collector-storage water heater. The experimental system has a tank of 25 l (66 cm;45 cm;8.5 cm) with rectangular cross-section and made up of 18 gauge galvanised iron sheet. It is covered with mineral wool insulation at its sides (5 cm thickness) and bottom (8 cm thickness) and encased with wooden box. It is kept in inclined position facing due south. The required inclination of the system can be adjusted manually by changing the bolt on cross-bar. The top surface of the tank is painted with black board paint to absorb the solar radiation and is transparently insulated with TIM cover to reduce the heat losses. The exploded view of the system is illustrated in Fig. 1. The temperature of water inside the tank is measured by copper}constantan (type T, copper(#)/constantan(!)) thermocouples. The copper}constantan thermocouple wires are arranged in the form of a multichannel probe wherein the junctions are placed at a separation of 10 cm to measure the vertical temperature distribution in the water heater. The thermocouple probe is connected to a SC-7501 multi logger (Iwatsu Electric Ltd, Japan). The water temperatures have
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Fig. 1. Exploded view of transparently insulated integrated-collector-storage solar water heater.
been recorded at an interval of one hour. The general schematic of the system is illustrated in Fig. 2(a). The cover system consists of TIM device compounded with air layers of near critical Rayleigh regime at its top and/or bottom.
3. Simulation model The solar radiation, after transmission through the TIM cover is absorbed by the top surface of the tank. Some part of absorbed energy is used to heat the water and the rest of energy is lost to the surroundings by conduction, convection and radiation. The energy balance for water can be written as M
d¹ (t) "S(t)!Q (t) * dt
(1)
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Fig. 2. TIM insulated ICS solar water heater (a) schematic diagram, and (b) thermal network.
or d¹ (t) #E¹ (t)"F(t). dt The total heat loss (Q (t)) from the storage water is given as * Q (t)"; [¹ (t)!¹ (t)], * *
(2)
(3)
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where ; is the overall heat loss coe$cient and is expressed as * ; "; #; #; , * 2 1 S(t)#; ¹ (t) ; * and M"M C #M C . E" *, F(t)" M M
(4)
Eq. (2) is a linear di!erential equation with integration factor e#2. Applying the initial condition ¹ (t)"¹ at t"0, at small interval of time (t), F(t) may be re garded as a constant (F). The solution is obtained as 1 ¹ (t)" F[1!e\#2]#¹ e\#2. E
(5)
The radiant energy (S(t)) reaching the absorber plane at time t is given by S(t)"I (t)R (qa) #I (t)R (qa) #[I (t)#I (t)]R (qa) . (6) The formulations of beam radiation transmittance (aq) , sky di!use radiation transmittance (aq) and ground di!use radiation transmittance (aq) , for the cover system embodying the TIM device have been developed by Symons [15] and Kaushika and Arulanantham [16], and have been adopted in the present work. The ; values may be computed by using the concept of thermal network (Fig. * 2(b)). The steady state heat loss from the absorber plane to the bottom of the TIM cover is given by q "h (¹ !¹ )#h (¹ !¹ ), where
(7)
p[(¹ #273.3)!(¹ #273.3)] h " 1 1 # !1 (¹ !¹ ) e e Nu K h " . ¸ The thermal conductivity of air, K and is given by
0.002528 ¹ + K" (¹ #200) + and Nu is the Nusselt number; Hollands et al. [17] have given a relationship for the Nusselt number in terms of Rayleigh number and collector tilt angle b as follows:
1708(sin 1.8b) Nu"1#1.44 1! R cos b
1708 > 1! # R cosb
> R cos b !1 , 5830
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where gbr*¹¸ . R" la The thermal resistance between absorber plane and bottom cover of TIM can be expressed as 1 R " . h #h
(8)
Similarly heat #ux across TIM cover may be expressed as q "; (¹ !¹ ), 2'+
(9)
where ; is the combined conductive and radiative heat loss coe$cient for TIM 2'+ cover (Arulanantham and Kaushika [18] have dealt in detail the coupled conductive}radiative heat transfer across TIM). The thermal resistance R can be written as 2'+ 1 R " . 2'+ ; 2'+
(10)
Heat #ux between the top cover of the TIM device and tempered glass cover is given as q "[h #h ](¹ !¹ ),
(11)
where Nu K h " ¸
p[(¹ #273.3)!(¹ #273.3)] and h " . 1 1 # !1 (¹ !¹ ) e e
The expression for thermal resistance (R ) between TIM cover and top tempered glass cover may be written as 1 R " . h #h
(12)
Heat loss from tempered glass cover to ambient is expressed [19] as q "[h #h ](¹ !¹ ).
(13)
The thermal resistance between top cover to the surroundings includes both convective and radiative heat loss coe$cients. The convective coe$cient (h ) can be ex pressed [20] as h "5.7#3.8 <.
(14)
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The radiative resistance from the top cover accounts for radiation exchange with the sky at temperature ¹ . So the radiation heat transfer coe$cient can be written as pe [(¹ #273.3)!(¹ #273.3)] h " . (15) (¹ !¹ ) Generally sky temperature ¹ is taken as ¹ "¹ !6. Then the resistance R is given by 1 R " . h #h So, the total thermal resistance from absorber plane to ambient is 1 R "R #R #R #R " . 2 2'+ ; 2 Heat #ux across the top cover system is then expressed as
(16)
(17)
q "; (¹ !¹ ). (18) 2 2 The conductive heat loss through the bottom and side insulation may be obtained from "rst principles. Following Langmuir et al. [21] and Boelter et al. [22], the conductive heat loss coe$cients for the bottom and sides may be expressed as
1 Ak #2.16lk #0.6t k , (19) ; " A t 1 4lhk #2.16lk #0.6t k , ;" (20) 1 A t where l"(A (for a heater of square cross-section). A The collection e$ciency is the ratio of energy collected by water mass to solar radiation received on absorber plane during time t and is given by R Q (t)dt g" . A R S(t)dt
(21)
4. Results and discussion With a view to study the relative merits of TIM cover systems, the diurnal variations of water temperature in storage tank have been studied for various TIM cover systems. Following con"gurations of TIM device placed between the top tempered glass (5 mm thickness) cover and the absorber have been considered. 1. Absorber}parallel con"guration of TIM device (C1) Air layer (near critical Rayleigh regime) } single cover, (C2) Glass sheet (3 mm thickness) [25], (C3) Polycarbonate sheet (0.5 mm thickness),
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Fig. 3. Diurnal variation of storage water temperature in the tank with absorber parallel con"guration of TIM devices: (a) air layer, (b) glass cover, (c) polycarbonate sheet, (d) double walled structured sheet of 6 mm thick, and (e) double walled structured sheet of 10 mm thick.
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441
(C4)
Double wall structured polycarbonate material (6 mm thickness, GE Plastics), (C5) Double wall structured polycarbonate material (10 mm thickness, GE Plastics), 2. Absorber}perpendicular con"guration of TIM device (C6) Cellular array(honeycomb) of 5 cm thickness, (C7) Encapsulated cellular array of 5 cm thickness compounded with 12 mm air layer, (C8) Encapsulated cellular array of 10 cm thickness compounded with 12 mm air layer. The temperature of water in the storage tank may also be predicted from the simulation model using (4)}(20). In the calculations an iterative process is used for evaluating top loss coe$cient. First a guess is made for the unknown cover temperatures, from which convective and radiative heat loss coe$cients between parallel surfaces are calculated. In the steady state q "q "q "q "q (say). The values 2 of the cover temperatures ¹ , ¹ and ¹ may, therefore, be obtained by iterating such that q "q , q "q and q "q using secant iteration method [23] to an accu racy of $0.001. The data for solar intensity [I (t) and I (t)] and ambient temperature ¹ (t) as measured on the day of experiment at New Delhi are used in the simulation. The thermophysical parameters used in the model are as follows: M "25 kg, 5 M "6 kg, C "4190 J/kg 3C, C "480 J/kg 3C, k "0.034 W/m 3C (Glass wool), t "0.08 m, t "0.05 m, which correspond to the system design characteristics dis1 cussed in [24]. The diurnal variations of simulated and observed temperatures in storage tank along with the corresponding solar data are illustrated in Fig. 3(a)}Fig. 3(e) for the absorber-parallel con"gurations of TIM. A comparison of performance characteristics in terms of solar gain e$ciency de"ned by (21) is presented in Table 1. Results indicate an excellent agreement between the experimental observations and simulation model results. The performance of solar ICS water heater with cover systems embodying a double walled structured sheet of 10 mm (GE plastics) as TIM excel over others. Table 1 Performance comparison of transparently insulated (absorber-parallel structure) solar ICS water heater Date of exp.
28-11-96 01-12-96 30-11-96 02-12-96 13-05-98
Insolation Con"guration kW h/day m of TIM device between absorber and top cover
5.28 5.96 5.91 6.32 6.15
Air layer Glass sheet Polycarbonate sheet Structured sheet (6 mm) Structured sheet (10 mm)
Performance characteristics Storage water temperature (3C)
Collection e$ciency
¹
¹
¹
g
14.2 14.0 15.3 15.5 29.0
47.7 52.8 51.2 50.5 56.0
23.1 29.5 31.7 31.4 47.0
0.1433 0.2211 0.2359 0.2140 0.3097
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Table 2 Performance comparison of transparently insulated (absorber- vertical structure) solar ICS water heater Date of exp.
11-12-96 09-12-96 05-12-96
Insolation Con"guration of (kW h/day m) TIM device between absorber plane and top cover
5.49 5.72 5.25
Cellular array (5 cm) Encap. TIM (5 cm) Encap. TIM (10 cm)
Performance characteristics Storage water temperature (3C)
Collection e$ciency
¹
¹
¹
g
9.2 9.0 11.0
47.6 45.1 39.3
31.8 33.1 30.7
0.3535 0.3582 0.3100
Fig. 4. Diurnal variation of storage water temperature in the tank with absorber perpendicular con"guration of TIM devices: (a) cellular array of 5 cm thick, (b) encapsulated cellular array of 5 cm thick, and (c) encapsulated cellular array of 10 cm thick.
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A comparison of performance characteristics of solar ICS water heater with absorber-perpendicular structures of TIM cover systems is summarised in Table 2. The diurnal variations of simulated and observed temperatures in storage tank corresponding to three con"gurations of TIM along with the corresponding solar data are illustrated in Fig. 4(a)}Fig. 4(c). The TIM made of encapsulated cellular array of 5 cm thickness compounded with 12 mm air layer corresponds to relatively high solar collection-storage e$ciency of the water heater. The validated simulation model may be used to compare the thermal performance of water heaters with di!erent TIM covers. In Fig. 5 the diurnal variations of storage water temperatures for all the con"gurations are portrayed. All the variations correspond to same radiation and atmospheric air data shown therein. The relative e!ectiveness of TIM cover systems can also be judged from their solar transmittance and heat loss reduction characteristics. For this purpose the e!ective transmittance}absorptance product (qa) of TIM cover systems are experimentally measured at normal solar angle of incidence and ; is evaluated from night-time * cooling of water temperature in storage tank. During the o!-sunshine period, the solar intensity term S(t) is zero in (5), so we have
¹ (t)"¹ (t) 1!exp
!; ¹ * M
#¹
exp
!; ¹ * . M
Fig. 5. Diurnal variation of storage water temperature for various TIM cover systems.
(22)
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Table 3 Overall heat loss coe$cient (; ) and (qa) of various TIM cover systems for ICS solar water heater * S.no. Con"guration of TIM between absorber plane and top cover
C1 C2 C3 C4 C5 C6 C7 C8
Air layer Glass sheet Polycarbonate sheet Structured sheet (6 mm) Structured sheet (10 mm) Cellular array (5 cm) Encap.TIM (5 cm) Encap. TIM (10 cm)
Theoretical (W/m K) ; 2
;
; 1
; *
; *
6.23 3.73 3.43 2.63 2.11 1.93 1.73 1.33
0.59 0.59 0.59 0.59 0.59 0.59 0.59 0.59
0.38 0.38 0.38 0.38 0.38 0.38 0.38 0.38
7.20 4.68 4.40 3.60 3.08 2.90 2.70 2.30
7.9$0.25 5.6$0.10 4.5$0.10 4.4$0.11 3.8$0.12 3.4$0.13 2.8$0.12 2.6$0.15
From (22), night time ; may be expressed as * !M ¹ (t)!¹ (t) ; " ln . * ¹ ¹ (t!1)!¹ (t)
Experimental (qa) (W/m K) at h"0
0.759 0.658 0.696 0.565 0.565 0.625 0.588 0.430
(23)
The theoretical and experimental value of overall heat loss coe$cient and transmittance}absorptance product for TIM cover systems are summarised in Table 3. The multiple wall structured sheet provide good heat loss reduction but simultaneously cut the solar radiation whereas the TIM devices made of absorber perpendicular structure provide good heat loss reduction as well as high solar transmittance resulting in relatively higher solar gain. 5. Conclusions Transparently insulated integral collection storage water heaters combine solar collection with thermal storage in same unit. Several types of these water heaters are possible with variations in their design and their thermal performance is a sensitive function of heat loss reduction and solar transmittance characteristics of the cover system embodying the TIM between the top cover and absorber. Basic objective of this study is to develop a methodology to compare the performance of these water heaters. The absorber-parallel and the absorber}perpendicular con"gurations of TIM device have been considered. The absorber - parallel con"guration of TIM Device includes (a) air layer, (b) glass sheet (3 mm), (c) polycarbonate sheet (0.5 mm), (d) double wall structured polycarbonate sheet (6 mm) and (e) double wall structured polycarbonate sheet (10 mm). The absorber-perpendicular con"guration of TIM Device includes (a) cellular array (5 cm), (b) encapsulated cellular array (5 cm) and (c) encapsulated cellular array (10 cm). The thermal performance has been investigated by prototype "eld experiments as well as by simulation model. The corresponding overall heat loss coe$cient (; ), *
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Table 4 Normalisation of solar collection-storage e$ciency with respect to system cost S.No
Con"guration of TIM device
Cost of TIM}ICS system (Rs/ 100 l)
g
g ;10\
C1 C2 C3 C4 C5 C6 C7 C8
Air layer Glass sheet Polycarbonate sheet Structured sheet (6 mm) Structured sheet (10 mm) Cellular array (5 cm) Encap. TIM (5 cm) Encap. TIM (10 cm)
7000 7400 7600 7900 8300 10 500 11 700 12 050
0.1433 0.2211 0.2359 0.2140 0.3097 0.3535 0.3582 0.3100
20.47 29.88 31.04 27.09 37.31 33.67 30.61 25.72
e!ective transmittance absorptance product (qa) and solar collection-storage e$ ciency of water heater have been evaluated. It is found that the TIM cover systems based on absorber perpendicular con"guration exhibit superior solar collectionstorage e$ciencies than those corresponding to absorber-parallel con"guration. The di!erent cover systems involve di!erent costs and di!erent "nal temperatures in the storage water tank, it therefore, makes sense to normalise solar collectionstorage e$ciencies with respect to cost of the TIM and quality of the heat. This can be accomplished by de"ning such normalised e$ciency parameter as follows: g (24) g " !, C where C is cost of the solar water heater. The results of calculation based on normalisation are given in Table 4, which indicate that the cover system corresponding to double walled structured device of 10 mm (absorber-parallel con"guration) exhibits g comparable to absorber perpen dicular con"guration. The absorber-parallel con"guration is simple in practical realisation, and, therefore, seems an attractive option for passive solar water preheaters. Acknowledgements The "nancial support provided by the Ministry of Non-conventional Energy Sources (Govt. of India), New Delhi through the research scheme No. 15/1/92-ST is duly acknowledged. References [1] M.H. Cobble, J. Franklin Instit. 278 (1964) 383. [2] M.H. Cobble, P.C. Fang, E. Lumsadine, J. Franklin Instit. 282 (1966) 102.
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[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
S.P. Sabberwal, J. Prakash, A. Seth, Energy Convers. Mgmt. 25 (1985) 249. J. Prakash, S.C. Kaushika, R. Kumar, H.P. Garg, Energy 19 (1994) 869. R.L. San Martin, G.J. Fjeld, Solar Energy 17 (1975) 345. E.I.H. Lin, Proc. Int. Solar Energy Society American Section of ISES, 1982, p. 225. U. Ortabasi, F.H. Dyksterhuis, N.D. Kaushika, Solar Energy 31 (1983) 229. N.D. Kaushika, M.B. Banerjee, Solar World Congress, Perth, Australia, 1983, p. 246. N.D. Kaushika, M.B. Banerjee, Yojana Katti, Energy 8 (1983) 883. A. Goetzberger, M. Rommel, Solar Energy 39 (1987) 211. A. Goetzberger, Int. J. Solar Energy 2 (1984) 21. Ch. Schmidit, A. Goetzberger, J. Schmidt, Solar Energy 41 (1988) 487. R. Scheafer, P. Lowrey, Solar Energy 48 (1992) 69. N.D. Kaushika, M Arulanantham, Energy 20 (1995) 829. J.G. Symons, Solar Energy 33 (1984) 637. N.D. Kaushika, M. Arulanantham, Solar Energy Mater. Solar Cells 44 (1996) 383. K.G.T. Hollands, K.N. Marshall, R.K. Wedel, Solar Energy 21 (1978) 231. M. Arulanantham, N.D. Kaushika, Appl. Thermal Eng. 16 (1996) 209. A. Du$e, W.A. Beckman, Wiley, New York, 1980. W.H. Mc Adams, 3rd ed., McGraw-Hill, New York, 1954. I. Langmuir, E.Q. Adams, F.S. Meikle, Trans. Am. Electrochem. Soc. 24 (1913) 53. L.M.K. Boelter, V.M. Cherry, H.A. Johnson, University of California Press, 1947. M.L. James, G.M. Smith, J.C. Wolford, IEP, A Dun-Donnelley Publishers, California, 1977. N.D. Kaushika, K.S. Reddy, Appl. Therm. Eng., In press. H.P. Garg, Usha Rani, Solar Energy 29 (1982) 467.
Comparative study of transparent insulation materials cover systems for integrated-collector-storage solar water heaters K.S. Reddy, N.D. Kaushika* Centre for Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India Received 3 June 1998; accepted 1 July 1998
Abstract The thermal performance of transparently insulated integrated-collector-storage solar water heaters is investigated theoretically as well as experimentally for a comparative study of cover systems having transparent insulation materials devices placed between the top glazing and the absorber. The data on solar transmittance, heat loss reduction characteristics and solar collector-storage e$ciencies of various con"gurations is generated for the system performance comparisons. These hot water systems exhibit average (diurnal basis) solar collection and storage e$ciencies in the range of 20}40% at a collection temperature of 40}503C. The performance of water heaters with cover system having absorber-perpendicular con"guration of TIM excel over absorber-parallel TIM covers. The e!ect of variation in the temperature of heat collected and cost of cover systems on the performance comparisons is also discussed. 1999 Elsevier Science B.V. All rights reserved. Keywords: Transparent insulation materials; Integrated-collector-storage system; Solar water heater; Absorber-parallel cover systems; Absorber-perpendicular cover systems; Honeycomb
1. Introduction Transparent or translucent insulation materials (TIM) make use of the thermal insulation properties of stagnant air layer and/or vacuum and are characterized by low heat loss coe$cients and high solar transmittance. TIM has similitude with conventional insulation materials in so far as the placement of air gap in solid mass is
* Corresponding author. Tel.: #91-11-6861977-x5006; fax: #91-11-6862037. 0927-0248/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 9 9 ) 0 0 0 1 8 - 5
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Nomenclature A C C g h h h h h h h I (t) I (t) k K l ¸ ¸ M M Nu q q q q Q (t) * Q (t) R R R R R R R R 2'+ S(t) t t 1
area of the solar absorber surface of the tank (m) speci"c heat of tank material (J/kg K) speci"c heat of water (J/kg K) acceleration due to gravity (m/s) depth of tank (m) convective heat loss coe$cient from absorber plane to cover 1 (W/m K) convective heat loss coe$cient from cover 2 to cover 3 (W/m K) radiative heat loss coe$cient from absorber plane to cover 1 (W/m K) radiative heat loss coe$cient from cover 2 to cover 3 (W/m K) radiative heat loss coe$cient from cover 3 to ambient (W/m K) convective heat loss coe$cient due to wind (W/m K) solar beam radiation at time t (W/m) solar di!use radiation at time t (W/m) conductivity of opaque insulation (W/m K) thermal conductivity of air at mean temperature ¹ (W/m K) + characteristic length of tank (m) thickness of air layer between absorber plane and cover 1 (m) thickness of air layer between cover 2 and cover 3 (m) mass of the tank (kg) mass of the water in the tank (kg) Nusselt number heat #ux from absorber plane to bottom cover of the TIM device (W/m) heat #ux across TIM device (W/m) heat #ux from top cover of TIM device to top tempered glass cover (W/m) heat #ux from top tempered glass cover to ambient (W/m) total heat loss from the system (W/m) retrieved heat #ux per unit area of heater (W/m) thermal resistance between absorber plane and cover 1 thermal resistance between cover 2 and cover 3 thermal resistance between cover 3 and ambient Rayleigh number tilt factor for beam radiation tilt factor for di!use radiation tilt factor for re#ected radiation thermal resistance across TIM device solar intensity at time t (W/m) bottom opaque insulation thickness (m) side opaque insulation thickness (m)
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¹ ¹ ¹ ¹ ¹ ¹ + ¹ ¹ ¹ ¹ ¹ (t) ¹ (t) ¹ (t!t ) G *¹ ; ; 1 ; 2 ; * ; 2'+ < Greek letters a (aq) (aq) (aq) (aq) b b k l p h e ,e ,e and e
433
time duration between two successive observations (s) temperature of cover 1 (3C) temperature of cover 2 (3C) temperature of cover 3 (3C) average ambient air temperature (3C) mean temperature of an air layer (K) average sky temperature (3C) average water temperature in the tank (3C) initial water temperature in the tank (3C) maximum water temperature in the tank (3C) ambient air temperature at time t (3C) water temperature at time t (3C) water temperatures at time (t!t ) (3C) temperature di!erence between absorber plane and cover 1 (¹ !¹ ) bottom heat loss coe$cient (W/m K) side heat loss coe$cient (W/m K) top heat loss coe$cient (W/m K) overall heat loss coe$cient (W/m K) combined conductive and radiative heat loss coe$cient across TIM device (W/m K) wind speed (m/s)
thermal di!usivity (m/s) absorptance-transmittance product for beam radiation absorptance-transmittance product for ground di!use radiation absorptance-transmittance product for sky di!use radiation e!ective absorptance-transmittance product tilt angle of water heating system (deg) Volumetric thermal expansion (K\) refractive index of cover plate kinematic viscosity (m/s) Stefan}Boltzmann constant (W/m K) angle of incidence (deg) emissivities of absorber plane, cover 1, cover 2 and cover 3
concerned. They are classi"ed into absorber-parallel, absorber-vertical, cavity and homogeneous structures based on their cellular geometry. The absorber-parallel con"guration of TIM involves multiple covers of glass/plastic "lms, which are placed parallel to absorber; this con"guration is very simple for practical realization and has been researched for long for its solar heat trap characteristics. For example
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transparent slab of methyl methacrylate (MMA) for solar heat trap was suggested and tested by Cobble [1] and Cobble et al. [2]. The concept has since been subjected to theoretical [3,4] and experimental [5] investigations for various such applications as passive solar building, #at-plate collector, integrated collector storage system. The major problem associated with this con"guration is that the number of parallel covers are to be increased to reduce heat loss, which reduces the solar transmittance signi"cantly. This leads to the consideration of absorber-perpendicular TIM which consists of a transparent honeycomb (cellular structure) immersed in an air layer. The use of the honeycomb device to reduce heat losses through the cover system of a #at plate collector is well known. In recent years transparent honeycomb insulations have been suggested and analysed for application in integrated-collector-storage (ICS) solar hot water systems [6}13]. The systems based on the storage water tank seem very suitable for domestic and industrial applications. All the above analyses have involved several approximations such as (i) the solar transmittance of TIM slab corresponding to di!use component of solar radiation is not taken into account, (ii) the beam radiation transmittance is considered only at normal incidence. Some of the above analyses have used a wrong boundary condition for heat transfer at the top surface of the TIM slab (see comments due to Kaushika and Arulanantham [14]). The resultant predictions and comparisons are therefore of debatable merit. Furthermore, the above analyses and evaluations are in general based on discrete measurements of solar transmittance and heat losses across the TIM cover system. In this paper, TIM devices are investigated for their application in cover systems of ICS water heaters. The approach involves the development of rigorous simulation model of system performance, its validation by prototype "eld experiments enabling the comparative study of TIM cover systems.
2. Experimental set-up Solar heating of a storage water tank cuboid in shape, transparently insulated at the top surface and covered with opaque insulation at all other sides is considered for TIM insulated integrated-collector-storage water heater. The experimental system has a tank of 25 l (66 cm;45 cm;8.5 cm) with rectangular cross-section and made up of 18 gauge galvanised iron sheet. It is covered with mineral wool insulation at its sides (5 cm thickness) and bottom (8 cm thickness) and encased with wooden box. It is kept in inclined position facing due south. The required inclination of the system can be adjusted manually by changing the bolt on cross-bar. The top surface of the tank is painted with black board paint to absorb the solar radiation and is transparently insulated with TIM cover to reduce the heat losses. The exploded view of the system is illustrated in Fig. 1. The temperature of water inside the tank is measured by copper}constantan (type T, copper(#)/constantan(!)) thermocouples. The copper}constantan thermocouple wires are arranged in the form of a multichannel probe wherein the junctions are placed at a separation of 10 cm to measure the vertical temperature distribution in the water heater. The thermocouple probe is connected to a SC-7501 multi logger (Iwatsu Electric Ltd, Japan). The water temperatures have
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435
Fig. 1. Exploded view of transparently insulated integrated-collector-storage solar water heater.
been recorded at an interval of one hour. The general schematic of the system is illustrated in Fig. 2(a). The cover system consists of TIM device compounded with air layers of near critical Rayleigh regime at its top and/or bottom.
3. Simulation model The solar radiation, after transmission through the TIM cover is absorbed by the top surface of the tank. Some part of absorbed energy is used to heat the water and the rest of energy is lost to the surroundings by conduction, convection and radiation. The energy balance for water can be written as M
d¹ (t) "S(t)!Q (t) * dt
(1)
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Fig. 2. TIM insulated ICS solar water heater (a) schematic diagram, and (b) thermal network.
or d¹ (t) #E¹ (t)"F(t). dt The total heat loss (Q (t)) from the storage water is given as * Q (t)"; [¹ (t)!¹ (t)], * *
(2)
(3)
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437
where ; is the overall heat loss coe$cient and is expressed as * ; "; #; #; , * 2 1 S(t)#; ¹ (t) ; * and M"M C #M C . E" *, F(t)" M M
(4)
Eq. (2) is a linear di!erential equation with integration factor e#2. Applying the initial condition ¹ (t)"¹ at t"0, at small interval of time (t), F(t) may be re garded as a constant (F). The solution is obtained as 1 ¹ (t)" F[1!e\#2]#¹ e\#2. E
(5)
The radiant energy (S(t)) reaching the absorber plane at time t is given by S(t)"I (t)R (qa) #I (t)R (qa) #[I (t)#I (t)]R (qa) . (6) The formulations of beam radiation transmittance (aq) , sky di!use radiation transmittance (aq) and ground di!use radiation transmittance (aq) , for the cover system embodying the TIM device have been developed by Symons [15] and Kaushika and Arulanantham [16], and have been adopted in the present work. The ; values may be computed by using the concept of thermal network (Fig. * 2(b)). The steady state heat loss from the absorber plane to the bottom of the TIM cover is given by q "h (¹ !¹ )#h (¹ !¹ ), where
(7)
p[(¹ #273.3)!(¹ #273.3)] h " 1 1 # !1 (¹ !¹ ) e e Nu K h " . ¸ The thermal conductivity of air, K and is given by
0.002528 ¹ + K" (¹ #200) + and Nu is the Nusselt number; Hollands et al. [17] have given a relationship for the Nusselt number in terms of Rayleigh number and collector tilt angle b as follows:
1708(sin 1.8b) Nu"1#1.44 1! R cos b
1708 > 1! # R cosb
> R cos b !1 , 5830
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where gbr*¹¸ . R" la The thermal resistance between absorber plane and bottom cover of TIM can be expressed as 1 R " . h #h
(8)
Similarly heat #ux across TIM cover may be expressed as q "; (¹ !¹ ), 2'+
(9)
where ; is the combined conductive and radiative heat loss coe$cient for TIM 2'+ cover (Arulanantham and Kaushika [18] have dealt in detail the coupled conductive}radiative heat transfer across TIM). The thermal resistance R can be written as 2'+ 1 R " . 2'+ ; 2'+
(10)
Heat #ux between the top cover of the TIM device and tempered glass cover is given as q "[h #h ](¹ !¹ ),
(11)
where Nu K h " ¸
p[(¹ #273.3)!(¹ #273.3)] and h " . 1 1 # !1 (¹ !¹ ) e e
The expression for thermal resistance (R ) between TIM cover and top tempered glass cover may be written as 1 R " . h #h
(12)
Heat loss from tempered glass cover to ambient is expressed [19] as q "[h #h ](¹ !¹ ).
(13)
The thermal resistance between top cover to the surroundings includes both convective and radiative heat loss coe$cients. The convective coe$cient (h ) can be ex pressed [20] as h "5.7#3.8 <.
(14)
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439
The radiative resistance from the top cover accounts for radiation exchange with the sky at temperature ¹ . So the radiation heat transfer coe$cient can be written as pe [(¹ #273.3)!(¹ #273.3)] h " . (15) (¹ !¹ ) Generally sky temperature ¹ is taken as ¹ "¹ !6. Then the resistance R is given by 1 R " . h #h So, the total thermal resistance from absorber plane to ambient is 1 R "R #R #R #R " . 2 2'+ ; 2 Heat #ux across the top cover system is then expressed as
(16)
(17)
q "; (¹ !¹ ). (18) 2 2 The conductive heat loss through the bottom and side insulation may be obtained from "rst principles. Following Langmuir et al. [21] and Boelter et al. [22], the conductive heat loss coe$cients for the bottom and sides may be expressed as
1 Ak #2.16lk #0.6t k , (19) ; " A t 1 4lhk #2.16lk #0.6t k , ;" (20) 1 A t where l"(A (for a heater of square cross-section). A The collection e$ciency is the ratio of energy collected by water mass to solar radiation received on absorber plane during time t and is given by R Q (t)dt g" . A R S(t)dt
(21)
4. Results and discussion With a view to study the relative merits of TIM cover systems, the diurnal variations of water temperature in storage tank have been studied for various TIM cover systems. Following con"gurations of TIM device placed between the top tempered glass (5 mm thickness) cover and the absorber have been considered. 1. Absorber}parallel con"guration of TIM device (C1) Air layer (near critical Rayleigh regime) } single cover, (C2) Glass sheet (3 mm thickness) [25], (C3) Polycarbonate sheet (0.5 mm thickness),
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Fig. 3. Diurnal variation of storage water temperature in the tank with absorber parallel con"guration of TIM devices: (a) air layer, (b) glass cover, (c) polycarbonate sheet, (d) double walled structured sheet of 6 mm thick, and (e) double walled structured sheet of 10 mm thick.
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441
(C4)
Double wall structured polycarbonate material (6 mm thickness, GE Plastics), (C5) Double wall structured polycarbonate material (10 mm thickness, GE Plastics), 2. Absorber}perpendicular con"guration of TIM device (C6) Cellular array(honeycomb) of 5 cm thickness, (C7) Encapsulated cellular array of 5 cm thickness compounded with 12 mm air layer, (C8) Encapsulated cellular array of 10 cm thickness compounded with 12 mm air layer. The temperature of water in the storage tank may also be predicted from the simulation model using (4)}(20). In the calculations an iterative process is used for evaluating top loss coe$cient. First a guess is made for the unknown cover temperatures, from which convective and radiative heat loss coe$cients between parallel surfaces are calculated. In the steady state q "q "q "q "q (say). The values 2 of the cover temperatures ¹ , ¹ and ¹ may, therefore, be obtained by iterating such that q "q , q "q and q "q using secant iteration method [23] to an accu racy of $0.001. The data for solar intensity [I (t) and I (t)] and ambient temperature ¹ (t) as measured on the day of experiment at New Delhi are used in the simulation. The thermophysical parameters used in the model are as follows: M "25 kg, 5 M "6 kg, C "4190 J/kg 3C, C "480 J/kg 3C, k "0.034 W/m 3C (Glass wool), t "0.08 m, t "0.05 m, which correspond to the system design characteristics dis1 cussed in [24]. The diurnal variations of simulated and observed temperatures in storage tank along with the corresponding solar data are illustrated in Fig. 3(a)}Fig. 3(e) for the absorber-parallel con"gurations of TIM. A comparison of performance characteristics in terms of solar gain e$ciency de"ned by (21) is presented in Table 1. Results indicate an excellent agreement between the experimental observations and simulation model results. The performance of solar ICS water heater with cover systems embodying a double walled structured sheet of 10 mm (GE plastics) as TIM excel over others. Table 1 Performance comparison of transparently insulated (absorber-parallel structure) solar ICS water heater Date of exp.
28-11-96 01-12-96 30-11-96 02-12-96 13-05-98
Insolation Con"guration kW h/day m of TIM device between absorber and top cover
5.28 5.96 5.91 6.32 6.15
Air layer Glass sheet Polycarbonate sheet Structured sheet (6 mm) Structured sheet (10 mm)
Performance characteristics Storage water temperature (3C)
Collection e$ciency
¹
¹
¹
g
14.2 14.0 15.3 15.5 29.0
47.7 52.8 51.2 50.5 56.0
23.1 29.5 31.7 31.4 47.0
0.1433 0.2211 0.2359 0.2140 0.3097
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Table 2 Performance comparison of transparently insulated (absorber- vertical structure) solar ICS water heater Date of exp.
11-12-96 09-12-96 05-12-96
Insolation Con"guration of (kW h/day m) TIM device between absorber plane and top cover
5.49 5.72 5.25
Cellular array (5 cm) Encap. TIM (5 cm) Encap. TIM (10 cm)
Performance characteristics Storage water temperature (3C)
Collection e$ciency
¹
¹
¹
g
9.2 9.0 11.0
47.6 45.1 39.3
31.8 33.1 30.7
0.3535 0.3582 0.3100
Fig. 4. Diurnal variation of storage water temperature in the tank with absorber perpendicular con"guration of TIM devices: (a) cellular array of 5 cm thick, (b) encapsulated cellular array of 5 cm thick, and (c) encapsulated cellular array of 10 cm thick.
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443
A comparison of performance characteristics of solar ICS water heater with absorber-perpendicular structures of TIM cover systems is summarised in Table 2. The diurnal variations of simulated and observed temperatures in storage tank corresponding to three con"gurations of TIM along with the corresponding solar data are illustrated in Fig. 4(a)}Fig. 4(c). The TIM made of encapsulated cellular array of 5 cm thickness compounded with 12 mm air layer corresponds to relatively high solar collection-storage e$ciency of the water heater. The validated simulation model may be used to compare the thermal performance of water heaters with di!erent TIM covers. In Fig. 5 the diurnal variations of storage water temperatures for all the con"gurations are portrayed. All the variations correspond to same radiation and atmospheric air data shown therein. The relative e!ectiveness of TIM cover systems can also be judged from their solar transmittance and heat loss reduction characteristics. For this purpose the e!ective transmittance}absorptance product (qa) of TIM cover systems are experimentally measured at normal solar angle of incidence and ; is evaluated from night-time * cooling of water temperature in storage tank. During the o!-sunshine period, the solar intensity term S(t) is zero in (5), so we have
¹ (t)"¹ (t) 1!exp
!; ¹ * M
#¹
exp
!; ¹ * . M
Fig. 5. Diurnal variation of storage water temperature for various TIM cover systems.
(22)
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Table 3 Overall heat loss coe$cient (; ) and (qa) of various TIM cover systems for ICS solar water heater * S.no. Con"guration of TIM between absorber plane and top cover
C1 C2 C3 C4 C5 C6 C7 C8
Air layer Glass sheet Polycarbonate sheet Structured sheet (6 mm) Structured sheet (10 mm) Cellular array (5 cm) Encap.TIM (5 cm) Encap. TIM (10 cm)
Theoretical (W/m K) ; 2
;
; 1
; *
; *
6.23 3.73 3.43 2.63 2.11 1.93 1.73 1.33
0.59 0.59 0.59 0.59 0.59 0.59 0.59 0.59
0.38 0.38 0.38 0.38 0.38 0.38 0.38 0.38
7.20 4.68 4.40 3.60 3.08 2.90 2.70 2.30
7.9$0.25 5.6$0.10 4.5$0.10 4.4$0.11 3.8$0.12 3.4$0.13 2.8$0.12 2.6$0.15
From (22), night time ; may be expressed as * !M ¹ (t)!¹ (t) ; " ln . * ¹ ¹ (t!1)!¹ (t)
Experimental (qa) (W/m K) at h"0
0.759 0.658 0.696 0.565 0.565 0.625 0.588 0.430
(23)
The theoretical and experimental value of overall heat loss coe$cient and transmittance}absorptance product for TIM cover systems are summarised in Table 3. The multiple wall structured sheet provide good heat loss reduction but simultaneously cut the solar radiation whereas the TIM devices made of absorber perpendicular structure provide good heat loss reduction as well as high solar transmittance resulting in relatively higher solar gain. 5. Conclusions Transparently insulated integral collection storage water heaters combine solar collection with thermal storage in same unit. Several types of these water heaters are possible with variations in their design and their thermal performance is a sensitive function of heat loss reduction and solar transmittance characteristics of the cover system embodying the TIM between the top cover and absorber. Basic objective of this study is to develop a methodology to compare the performance of these water heaters. The absorber-parallel and the absorber}perpendicular con"gurations of TIM device have been considered. The absorber - parallel con"guration of TIM Device includes (a) air layer, (b) glass sheet (3 mm), (c) polycarbonate sheet (0.5 mm), (d) double wall structured polycarbonate sheet (6 mm) and (e) double wall structured polycarbonate sheet (10 mm). The absorber-perpendicular con"guration of TIM Device includes (a) cellular array (5 cm), (b) encapsulated cellular array (5 cm) and (c) encapsulated cellular array (10 cm). The thermal performance has been investigated by prototype "eld experiments as well as by simulation model. The corresponding overall heat loss coe$cient (; ), *
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Table 4 Normalisation of solar collection-storage e$ciency with respect to system cost S.No
Con"guration of TIM device
Cost of TIM}ICS system (Rs/ 100 l)
g
g ;10\
C1 C2 C3 C4 C5 C6 C7 C8
Air layer Glass sheet Polycarbonate sheet Structured sheet (6 mm) Structured sheet (10 mm) Cellular array (5 cm) Encap. TIM (5 cm) Encap. TIM (10 cm)
7000 7400 7600 7900 8300 10 500 11 700 12 050
0.1433 0.2211 0.2359 0.2140 0.3097 0.3535 0.3582 0.3100
20.47 29.88 31.04 27.09 37.31 33.67 30.61 25.72
e!ective transmittance absorptance product (qa) and solar collection-storage e$ ciency of water heater have been evaluated. It is found that the TIM cover systems based on absorber perpendicular con"guration exhibit superior solar collectionstorage e$ciencies than those corresponding to absorber-parallel con"guration. The di!erent cover systems involve di!erent costs and di!erent "nal temperatures in the storage water tank, it therefore, makes sense to normalise solar collectionstorage e$ciencies with respect to cost of the TIM and quality of the heat. This can be accomplished by de"ning such normalised e$ciency parameter as follows: g (24) g " !, C where C is cost of the solar water heater. The results of calculation based on normalisation are given in Table 4, which indicate that the cover system corresponding to double walled structured device of 10 mm (absorber-parallel con"guration) exhibits g comparable to absorber perpen dicular con"guration. The absorber-parallel con"guration is simple in practical realisation, and, therefore, seems an attractive option for passive solar water preheaters. Acknowledgements The "nancial support provided by the Ministry of Non-conventional Energy Sources (Govt. of India), New Delhi through the research scheme No. 15/1/92-ST is duly acknowledged. References [1] M.H. Cobble, J. Franklin Instit. 278 (1964) 383. [2] M.H. Cobble, P.C. Fang, E. Lumsadine, J. Franklin Instit. 282 (1966) 102.
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[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
S.P. Sabberwal, J. Prakash, A. Seth, Energy Convers. Mgmt. 25 (1985) 249. J. Prakash, S.C. Kaushika, R. Kumar, H.P. Garg, Energy 19 (1994) 869. R.L. San Martin, G.J. Fjeld, Solar Energy 17 (1975) 345. E.I.H. Lin, Proc. Int. Solar Energy Society American Section of ISES, 1982, p. 225. U. Ortabasi, F.H. Dyksterhuis, N.D. Kaushika, Solar Energy 31 (1983) 229. N.D. Kaushika, M.B. Banerjee, Solar World Congress, Perth, Australia, 1983, p. 246. N.D. Kaushika, M.B. Banerjee, Yojana Katti, Energy 8 (1983) 883. A. Goetzberger, M. Rommel, Solar Energy 39 (1987) 211. A. Goetzberger, Int. J. Solar Energy 2 (1984) 21. Ch. Schmidit, A. Goetzberger, J. Schmidt, Solar Energy 41 (1988) 487. R. Scheafer, P. Lowrey, Solar Energy 48 (1992) 69. N.D. Kaushika, M Arulanantham, Energy 20 (1995) 829. J.G. Symons, Solar Energy 33 (1984) 637. N.D. Kaushika, M. Arulanantham, Solar Energy Mater. Solar Cells 44 (1996) 383. K.G.T. Hollands, K.N. Marshall, R.K. Wedel, Solar Energy 21 (1978) 231. M. Arulanantham, N.D. Kaushika, Appl. Thermal Eng. 16 (1996) 209. A. Du$e, W.A. Beckman, Wiley, New York, 1980. W.H. Mc Adams, 3rd ed., McGraw-Hill, New York, 1954. I. Langmuir, E.Q. Adams, F.S. Meikle, Trans. Am. Electrochem. Soc. 24 (1913) 53. L.M.K. Boelter, V.M. Cherry, H.A. Johnson, University of California Press, 1947. M.L. James, G.M. Smith, J.C. Wolford, IEP, A Dun-Donnelley Publishers, California, 1977. N.D. Kaushika, K.S. Reddy, Appl. Therm. Eng., In press. H.P. Garg, Usha Rani, Solar Energy 29 (1982) 467.