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Honeycomb solar pond collector and storage system
Efwgy Printed
Vol. 8. No. II. pp. 883490. io Great Britain.
1983
HONEYCOMB
9
SOLAR POND COLLECTOR STORAGE SYSTEM
0360-SW/83 33.00 + .I0 1983 Fm-gamon Press Ltd.
AND
N. D. KAUSHIKA,~ M. B. BANJZRJEE, and YOJNA KArrt Centre of Energy Studies, Indian Institute of Technology, Hauz Khas, ND-I 10016, India (Received 2 1 September 1982)
Abstract-An analysis of a honeycomb-stabilized, saltless solar pond as a solar energy collector and long term (spanning seasons) storage system is presented. The solar pond is considered with a nonconvective zone made up of an oil layer and air honeycomb configuration. A heat flow model is developed using the two loss mechanisms (conduction and radiation). The efficiency of heat collection and the storage characteristics of the system are excellent for hot water production and process heat applications. NOTATION pond area, mr fraction of the pond area occupied by honeycomb material geometrical shape factor for radiation loss depth of the bottom convective zone, m upward heat loss coefficient, W/m*-K bottom heat loss coeBicient, W/m*-K sides heat loss coefficient, W/mZ-K radiation heat loss coefficient, W/m*-K thermal conductivity of air, W/m-K thermal conductivity of silicon oil, W/m-K thermal conductivity of side and bottom insulation, W/m-K thermal conductivity of the core material used for side and bottom insulation, W/m-K thermal conductivity of the honeycomb material, W/m-k thickness of the side and bottom insulation covers, m thickness of the core material used for side and bottom insulation, m highest number of harmonic m considered retrieved heat flux per unit of pond area, W/m* solar insolation at time 1, W/m* average value of the solar insolation W/m* amplitude of the mth harmonic of S(r) W/m* ambient air temperature at time f, K average value of T”(t), K amplitude of the mth harmonic T,,(r), K transmittivity of the oil layer for the solar incident angle 0 transmittivity of the honeycomb structure for the solar incident angle 0 average value of the sky temperature, K water temperature in the bottom convective zone at time I, K amplitude of the mth harmonic of T,(r) depth of the honeycomb structure, m total heat loss coefficient W/m*-K absorptivity of the convective zone for the solar incident angle 0 effective absorptivity-transmissitity product of nonconvective zone and including the oil-water interface heat capacity per unit volume of water, J/m3-K Stefan-Botlzamm constant emissivity of the oil surface INTRODUCTION
ponds have received worldwide attention in the last decade as an alternative source of low temperature heat for industrial process heating, space heating and electrical power generation. The most common version of the solar pond is the saline pond,12 which has many attractive features such as relatively low cost, large as well as long term storage capacity and simple technology. However, it possesses some operational problems such as rather complicated maintenance of the salt gradient, the disturbance of the salt gradient due to wave motion caused by winds, corrosive properties of salt water, etc. Recently, Ortabasi er al.3 have proposed that these problems may be circumvented in a honeycomb-stabilized, saltless solar pond. These authors have demonstrated the technical feasibility of the concept tsupported
by the Solar Energy Research Centre, University of Queensland, Brisbane 4067, Australia. 883
N. D. IC\L'SHKi et al.
884
in a laboratory model pond and have indicated that an air honeycomb structure floating on a thin layer (1 cm) of silicone oil can indeed serve as the nonconvective zone of the solar pond. In the present paper, we evaluate the solar collector and storage characteristics of a honeycomb solar pond and investigate its applications. SYSTEM
CONFIGURATION
The honeycomb solar pond is envisioned to have its nonconvective zone about 11 cm thick and a bottom convective zone of about 0.5 m depth. In a pond of such shallow depth, the side and bottom heat losses through the surrounding earth region would be very large unless insulation is provided to inhibit the loss. The configuration of a practical system would resemble a rectangle box, as is illustrated schematically in Fig, 1. The side walls and the floor are packed at the cores with insulation. The layers of thickness I3 on either side of the side and bottom insulation medium are made of material such as ferrocement to protect insulation from erosion by the ground soil and water. The bottom convective zone (water pool) is either blackened at the sides and bottom walls or else the water mass itself is blackened. HEAT
FLOW
MODEL
The incident solar radiation is intercepted and filtered out by the honeycomb before it falls on the oil layer. A part of it is reflected at the air-oil interface. The remainder is attenuated throughout the pond depth and is completely absorbed in the bottom convective zone (the zone used for heat collection, storage and extraction). The solar radiation reaching is the effective the bottom convective zone is (arM(t), where (ar)eff absorptivity-transmissivity product corresponding to the nonconvective zone and including the oil-water interface. It may be expressed as
(1)
SOIL CONVECTIVE
ZONE
Fig. I. Schematic diagram of the honeycomb solar pond.
Honeycomb solar pond collector and storage system
885
The thermal energy balance in the convective zone may be written as
The only approximation in writing Eq. (2) is the assumption that there is negligible heat storage in the insulation. The solar intensity and atmospheric air temperature variations with time are periodic and are represented by the Fourier series S(t)=Re
TA(t) = Re
(
>
(3)
TAO+ i TAmeimwr,
(4)
So+~.Smehw’
m
m
,
>
the real part of the following means quantity and where Re w = 2n/(l x 365.25 x 24 x 60 x 60) set-’ for the annual cycle variation. Consequently, T,(t) will also be periodic and T,( t ) = Re
Td + i
T,, eimt .
m
Equation (2) in combination with Eqs. (3)-(5), leads to the following expression for the heat flux: Q(t) = (zr),&, - U,(T,, - T,,,,) + (a~).,; -i
S,,, eimo’
(imop,h + U,)T,, e’““‘+ Uyi m
TAmeimor.
(6)
m
The average efficiency of heat extraction from the pond may be expressed as tl
=[SQ(f,dr]i[lS(i)dr].
(7a)
Evaluating the integrals over the annual cycle, we have
0)
rl = (ar )eir- u,(A T/S,),
where AT is the difference between the storage zone temperature and the atmospheric air temperature. If it is assumed that the honeycomb structure suppresses convection totally, the heat loss from the hot reservoir (bottom convective zone) can be evaluated by using as the two loss mechanisms only conduction and radiation. The conductive heat loss may be found from first principles; since edge effects will not be negligible in most cases, we have incorporated appropriate geometrical shape factors4y’ to allow for these effects. Following Ref. 4, the conductive heat-loss coefficients are given by h, = f [(Ak,/t,) + 2.16/k: + 0.60tJc,],
h, = f [(Ak,/t,) + 2.16& + 0.601,&],
(9)
h, = f [(4fhk,/t,) + 2.16hk, + 0.60&k,],
(10)
where k, = k,kdt, + tJ/(t,kl + tzk,), k, = A,kh + (1 - A,)k,, EGY Vol. 8 No. II-E
t, = t, + t,, r, = t, = f, + l,,
886
N. D. KAUSHIKAer al.
+ I&,), k: = 2/[( I/k,) + (l/k,)], 1 = J(A ) (for a pond with square cross-section). The radiation losses from the oil surface to the sky are calculated from the Stefan-Boltzmann law, again with the use of a shape factor to account for radiation trapping in the honeycomb structure. Following Holland6 and Tien and Yuen,’ the radiation loss coefficient is given by k, = k, = k,k.,(f, + f,)/(&,
h, = F’a
(T,” - T,4)/(T, - T,),
(11)
where l/F’= l/F+ (1 - LJ/E~. The shape factor F is taken from Ref. 8; T, is the temperature at the oil surface and may be expressed as
7’1, = Tco- (T, - T,o>(~,lk,>l([(k,l~,> + M - ’ + (OJ}.
(12)
Thus h, may be calculated from Eqs. (11, 12) by an iterative procedure similar to that of Ref. 9. Finally, the total heat loss coefficient is given by UL=h,+hB+h,+h,.
(13)
The ratio R of heat losses through the side to those through the surface is R = h,/(h, + h,). CALCULATIONS
AND
(14) RESULTS
The solar collection and storage behaviour of the pond may be judged from its year-round thermal performance. We have, therefore, made calculations for the retrieved heat flux and the efficiency of heat retrieval at constant temperature (T,, = 0) using Eqs. (6) and (7), respectively. The data on annual variation of solar intensity and atmospheric air temperature have been taken from Kaushika ef al.‘O and correspond to New Delhi (latitude 28”N) during 1974. The values of T,(8), T,(8), a(e), and the resultant (ar),g are approximately constant and correspond to a solar angle of incidence on the horizontal surface at 2 p.m. at equinox. T,(8) has been calculated by using expressions given by Holland,6 T,(8) has been derived from the solar absorption data of silicone oil supplied by Dow Corning Australia Pty. Ltd. The resultant value of (r~r)~~is 0.72. The heat-loss coefficients have been evaluated using the Eqs. (8)-(13). The values of parameters
Table 1. Variations of U, (W/m?-“K) with A and h.
50
2.18
2.23
2.30
2.37
2.43
100
2.17
2.21
2.25
2.30
2.35
200
2.16
2.19
2.22
2.25
2.29
500
2.15
2.17
2.19
2.21
2.23
1000
2.15
2.16
2.18
2.19
2.21
Honeycomb
solar
pond
collector
and storage
system
887
characterizing the pond geometry, as well as other thennophysical parameters used in these calculations, are as follows: I, = 0.10 m, l4 = 0.075 or 0.15 m, k, = 0.034 W/m-K, ES= 0.94, kz = 0.16 W/m-K tz = 0.01 m, k, = 0.035 W/m-K, (silicone oil), G = 5.669 x lo-* W/m-KJ, A, = 0.1363, A = 0.1 to 1000 m’, k, = 0.21 W/m-K, h = 0.10 to l.Om, l,=O.Ol m, r,,,,= 313-363 K, k, =0.42 W/m-K, F = 0.175 (for a cell size of 0.017 x 0.017 x 0.10 m). The variations of R and UL with geometric parameters (area, depth of the bottom convective zone) of the pond are illustrated in Figs. 2 (a, b) and Table 1, respectively. For smaller and deeper ponds, thicker side insulation is necessary to render the side losses insignificant coinpared to the heat losses through the surface. The variation of heat flux that can be retrieved from a honeycomb solar pond during year-round operation is shown in Figs. 3 (a, b). Corresponding annual average efficiencies of the honeycomb solar pond are compared with those of a salt gradient solar pond in Table 2. The honeycomb pond efficiency as a function of collector parameter (AT/So) is depicted in Fig. 4; for comparison, typical curves for the salt gradient solar pond collector’.” and the flat plate collector’* with two covers and selective coating on the absorber are also shown. The honeycomb solar pond is superior to the other collector/storage systems for heat extraction at temperatures lower than 65°C. The honeycomb solar pond is, however, less attractive for power production because organic Rankine cycle turbines are often used in power production systems. These turbines require source temperatures higher than 70°C and a source to sink temperature difference of 50°C or more.
0
10
20
30
40 POND
50
60
AREA
(r-n21
Fig. 2(a).
70
80
go
100
WI0
N. D. ~~J~HIICA
888
et al.
,
0.G I4 I 0.15 m
0.
R
0.
DEPTH
OF CONVECTIVE ZONE
0.
PONO
AREA
(rn’)
(b)
Fig. 2. Curves for the variation of R: (a) I, = 0.075 m, (b) l4 = 0.15 m.
Table 2. Heat extraction efficiencies (annual average) for a solar pond with honeycomb and salt gradient. *at extraction temperature, %
Efficiency of honeycomb solar pond,%
with edge losses
without
T
C)ptimm
efficiency salt gradient pond 5mlar ( Ref. 9))
cIf
I
k
edge 1ssses
40
55
57
39
so
45
49
35
60
35
41
32
70
25
33
30
80
14
25
28
90
4
17
27
Honeycomb
889
solar pond collector and storage system
-801
I (cl)
-25 J
F
M
A
M
J
J
A
S
0
N
MONTHS (b)
Fig. 3. Annual variation of Q(f): (a) the pond has side losses corresponding the side losses in the pond are set equal to zero.
to I, = 0.075 m, (b)
890
N. D.
et al.
khISHlKA
AT (So:220 0.91
0
W/m')
22 I
LL, I
66
0.1
0.2
0.3
88
110 I
I
0.4
0.5
J
0.6
AT /So Fig. 4. The efficiency as a function of the collector parameter AT/S,,. Curve A : flat plate collector with 2 glass covers and selective coating on the absorber. Curve B: salt gradient pond with a nonconvective zone of I m. Curve C: honeycomb solar pond with side losses corresponding to 1, = 0.075 m. Curve D: honeycomb solar pond with side losses set equal to zero.
REFERENCES I. 2. 3. 4. 5.
6. 7. 8. 9.
10. II. 12.
H. Tabor, Solar Energy 7, I89 (1963). H. Weinberger, Solar Energy 8, 45 (1964). U. Grtabasi, F. H. Dyksterhuis, and N. D. Kaushika, Solar Energy 31, 229 (1983). I. Langmuir, E. Q. Adams, and F. S. Meikle, Trans. Am. Efectrochem. Sot. 24, 53 (1913). L. M. K. Boelter, V. M. Cherry, and H. A. Johnson, Heat Transfer. University of California Press, Berkeley (1942). K. G. T. Hollands, Solar Energy 9, 1.59 (1965). C. L. Tien and W. W. Yuen, ht. J. Heat Mass Transfer 18, 1409 (1975). H. C. Hottel and J. D. Keller, Trans ASME 55, 39 (1933). M. S. Sodha, N. D. Kaushika, and S. K. Rao, Energy Res. 5, 321 (1981). N. D. Kaushika, P. K. Bansal, and M. S. Sodha, Appl. Energy 7, 169 (1980). C. F. Kooi, Solar Energy 23, 37 (1979). J. A. Duffie and W. A. Beckman, Solar Engineering of Thermal Processes. Wiley, New York (1980).
Vol. 8. No. II. pp. 883490. io Great Britain.
1983
HONEYCOMB
9
SOLAR POND COLLECTOR STORAGE SYSTEM
0360-SW/83 33.00 + .I0 1983 Fm-gamon Press Ltd.
AND
N. D. KAUSHIKA,~ M. B. BANJZRJEE, and YOJNA KArrt Centre of Energy Studies, Indian Institute of Technology, Hauz Khas, ND-I 10016, India (Received 2 1 September 1982)
Abstract-An analysis of a honeycomb-stabilized, saltless solar pond as a solar energy collector and long term (spanning seasons) storage system is presented. The solar pond is considered with a nonconvective zone made up of an oil layer and air honeycomb configuration. A heat flow model is developed using the two loss mechanisms (conduction and radiation). The efficiency of heat collection and the storage characteristics of the system are excellent for hot water production and process heat applications. NOTATION pond area, mr fraction of the pond area occupied by honeycomb material geometrical shape factor for radiation loss depth of the bottom convective zone, m upward heat loss coefficient, W/m*-K bottom heat loss coeBicient, W/m*-K sides heat loss coefficient, W/mZ-K radiation heat loss coefficient, W/m*-K thermal conductivity of air, W/m-K thermal conductivity of silicon oil, W/m-K thermal conductivity of side and bottom insulation, W/m-K thermal conductivity of the core material used for side and bottom insulation, W/m-K thermal conductivity of the honeycomb material, W/m-k thickness of the side and bottom insulation covers, m thickness of the core material used for side and bottom insulation, m highest number of harmonic m considered retrieved heat flux per unit of pond area, W/m* solar insolation at time 1, W/m* average value of the solar insolation W/m* amplitude of the mth harmonic of S(r) W/m* ambient air temperature at time f, K average value of T”(t), K amplitude of the mth harmonic T,,(r), K transmittivity of the oil layer for the solar incident angle 0 transmittivity of the honeycomb structure for the solar incident angle 0 average value of the sky temperature, K water temperature in the bottom convective zone at time I, K amplitude of the mth harmonic of T,(r) depth of the honeycomb structure, m total heat loss coefficient W/m*-K absorptivity of the convective zone for the solar incident angle 0 effective absorptivity-transmissitity product of nonconvective zone and including the oil-water interface heat capacity per unit volume of water, J/m3-K Stefan-Botlzamm constant emissivity of the oil surface INTRODUCTION
ponds have received worldwide attention in the last decade as an alternative source of low temperature heat for industrial process heating, space heating and electrical power generation. The most common version of the solar pond is the saline pond,12 which has many attractive features such as relatively low cost, large as well as long term storage capacity and simple technology. However, it possesses some operational problems such as rather complicated maintenance of the salt gradient, the disturbance of the salt gradient due to wave motion caused by winds, corrosive properties of salt water, etc. Recently, Ortabasi er al.3 have proposed that these problems may be circumvented in a honeycomb-stabilized, saltless solar pond. These authors have demonstrated the technical feasibility of the concept tsupported
by the Solar Energy Research Centre, University of Queensland, Brisbane 4067, Australia. 883
N. D. IC\L'SHKi et al.
884
in a laboratory model pond and have indicated that an air honeycomb structure floating on a thin layer (1 cm) of silicone oil can indeed serve as the nonconvective zone of the solar pond. In the present paper, we evaluate the solar collector and storage characteristics of a honeycomb solar pond and investigate its applications. SYSTEM
CONFIGURATION
The honeycomb solar pond is envisioned to have its nonconvective zone about 11 cm thick and a bottom convective zone of about 0.5 m depth. In a pond of such shallow depth, the side and bottom heat losses through the surrounding earth region would be very large unless insulation is provided to inhibit the loss. The configuration of a practical system would resemble a rectangle box, as is illustrated schematically in Fig, 1. The side walls and the floor are packed at the cores with insulation. The layers of thickness I3 on either side of the side and bottom insulation medium are made of material such as ferrocement to protect insulation from erosion by the ground soil and water. The bottom convective zone (water pool) is either blackened at the sides and bottom walls or else the water mass itself is blackened. HEAT
FLOW
MODEL
The incident solar radiation is intercepted and filtered out by the honeycomb before it falls on the oil layer. A part of it is reflected at the air-oil interface. The remainder is attenuated throughout the pond depth and is completely absorbed in the bottom convective zone (the zone used for heat collection, storage and extraction). The solar radiation reaching is the effective the bottom convective zone is (arM(t), where (ar)eff absorptivity-transmissivity product corresponding to the nonconvective zone and including the oil-water interface. It may be expressed as
(1)
SOIL CONVECTIVE
ZONE
Fig. I. Schematic diagram of the honeycomb solar pond.
Honeycomb solar pond collector and storage system
885
The thermal energy balance in the convective zone may be written as
The only approximation in writing Eq. (2) is the assumption that there is negligible heat storage in the insulation. The solar intensity and atmospheric air temperature variations with time are periodic and are represented by the Fourier series S(t)=Re
TA(t) = Re
(
>
(3)
TAO+ i TAmeimwr,
(4)
So+~.Smehw’
m
m
,
>
the real part of the following means quantity and where Re w = 2n/(l x 365.25 x 24 x 60 x 60) set-’ for the annual cycle variation. Consequently, T,(t) will also be periodic and T,( t ) = Re
Td + i
T,, eimt .
m
Equation (2) in combination with Eqs. (3)-(5), leads to the following expression for the heat flux: Q(t) = (zr),&, - U,(T,, - T,,,,) + (a~).,; -i
S,,, eimo’
(imop,h + U,)T,, e’““‘+ Uyi m
TAmeimor.
(6)
m
The average efficiency of heat extraction from the pond may be expressed as tl
=[SQ(f,dr]i[lS(i)dr].
(7a)
Evaluating the integrals over the annual cycle, we have
0)
rl = (ar )eir- u,(A T/S,),
where AT is the difference between the storage zone temperature and the atmospheric air temperature. If it is assumed that the honeycomb structure suppresses convection totally, the heat loss from the hot reservoir (bottom convective zone) can be evaluated by using as the two loss mechanisms only conduction and radiation. The conductive heat loss may be found from first principles; since edge effects will not be negligible in most cases, we have incorporated appropriate geometrical shape factors4y’ to allow for these effects. Following Ref. 4, the conductive heat-loss coefficients are given by h, = f [(Ak,/t,) + 2.16/k: + 0.60tJc,],
h, = f [(Ak,/t,) + 2.16& + 0.601,&],
(9)
h, = f [(4fhk,/t,) + 2.16hk, + 0.60&k,],
(10)
where k, = k,kdt, + tJ/(t,kl + tzk,), k, = A,kh + (1 - A,)k,, EGY Vol. 8 No. II-E
t, = t, + t,, r, = t, = f, + l,,
886
N. D. KAUSHIKAer al.
+ I&,), k: = 2/[( I/k,) + (l/k,)], 1 = J(A ) (for a pond with square cross-section). The radiation losses from the oil surface to the sky are calculated from the Stefan-Boltzmann law, again with the use of a shape factor to account for radiation trapping in the honeycomb structure. Following Holland6 and Tien and Yuen,’ the radiation loss coefficient is given by k, = k, = k,k.,(f, + f,)/(&,
h, = F’a
(T,” - T,4)/(T, - T,),
(11)
where l/F’= l/F+ (1 - LJ/E~. The shape factor F is taken from Ref. 8; T, is the temperature at the oil surface and may be expressed as
7’1, = Tco- (T, - T,o>(~,lk,>l([(k,l~,> + M - ’ + (OJ}.
(12)
Thus h, may be calculated from Eqs. (11, 12) by an iterative procedure similar to that of Ref. 9. Finally, the total heat loss coefficient is given by UL=h,+hB+h,+h,.
(13)
The ratio R of heat losses through the side to those through the surface is R = h,/(h, + h,). CALCULATIONS
AND
(14) RESULTS
The solar collection and storage behaviour of the pond may be judged from its year-round thermal performance. We have, therefore, made calculations for the retrieved heat flux and the efficiency of heat retrieval at constant temperature (T,, = 0) using Eqs. (6) and (7), respectively. The data on annual variation of solar intensity and atmospheric air temperature have been taken from Kaushika ef al.‘O and correspond to New Delhi (latitude 28”N) during 1974. The values of T,(8), T,(8), a(e), and the resultant (ar),g are approximately constant and correspond to a solar angle of incidence on the horizontal surface at 2 p.m. at equinox. T,(8) has been calculated by using expressions given by Holland,6 T,(8) has been derived from the solar absorption data of silicone oil supplied by Dow Corning Australia Pty. Ltd. The resultant value of (r~r)~~is 0.72. The heat-loss coefficients have been evaluated using the Eqs. (8)-(13). The values of parameters
Table 1. Variations of U, (W/m?-“K) with A and h.
50
2.18
2.23
2.30
2.37
2.43
100
2.17
2.21
2.25
2.30
2.35
200
2.16
2.19
2.22
2.25
2.29
500
2.15
2.17
2.19
2.21
2.23
1000
2.15
2.16
2.18
2.19
2.21
Honeycomb
solar
pond
collector
and storage
system
887
characterizing the pond geometry, as well as other thennophysical parameters used in these calculations, are as follows: I, = 0.10 m, l4 = 0.075 or 0.15 m, k, = 0.034 W/m-K, ES= 0.94, kz = 0.16 W/m-K tz = 0.01 m, k, = 0.035 W/m-K, (silicone oil), G = 5.669 x lo-* W/m-KJ, A, = 0.1363, A = 0.1 to 1000 m’, k, = 0.21 W/m-K, h = 0.10 to l.Om, l,=O.Ol m, r,,,,= 313-363 K, k, =0.42 W/m-K, F = 0.175 (for a cell size of 0.017 x 0.017 x 0.10 m). The variations of R and UL with geometric parameters (area, depth of the bottom convective zone) of the pond are illustrated in Figs. 2 (a, b) and Table 1, respectively. For smaller and deeper ponds, thicker side insulation is necessary to render the side losses insignificant coinpared to the heat losses through the surface. The variation of heat flux that can be retrieved from a honeycomb solar pond during year-round operation is shown in Figs. 3 (a, b). Corresponding annual average efficiencies of the honeycomb solar pond are compared with those of a salt gradient solar pond in Table 2. The honeycomb pond efficiency as a function of collector parameter (AT/So) is depicted in Fig. 4; for comparison, typical curves for the salt gradient solar pond collector’.” and the flat plate collector’* with two covers and selective coating on the absorber are also shown. The honeycomb solar pond is superior to the other collector/storage systems for heat extraction at temperatures lower than 65°C. The honeycomb solar pond is, however, less attractive for power production because organic Rankine cycle turbines are often used in power production systems. These turbines require source temperatures higher than 70°C and a source to sink temperature difference of 50°C or more.
0
10
20
30
40 POND
50
60
AREA
(r-n21
Fig. 2(a).
70
80
go
100
WI0
N. D. ~~J~HIICA
888
et al.
,
0.G I4 I 0.15 m
0.
R
0.
DEPTH
OF CONVECTIVE ZONE
0.
PONO
AREA
(rn’)
(b)
Fig. 2. Curves for the variation of R: (a) I, = 0.075 m, (b) l4 = 0.15 m.
Table 2. Heat extraction efficiencies (annual average) for a solar pond with honeycomb and salt gradient. *at extraction temperature, %
Efficiency of honeycomb solar pond,%
with edge losses
without
T
C)ptimm
efficiency salt gradient pond 5mlar ( Ref. 9))
cIf
I
k
edge 1ssses
40
55
57
39
so
45
49
35
60
35
41
32
70
25
33
30
80
14
25
28
90
4
17
27
Honeycomb
889
solar pond collector and storage system
-801
I (cl)
-25 J
F
M
A
M
J
J
A
S
0
N
MONTHS (b)
Fig. 3. Annual variation of Q(f): (a) the pond has side losses corresponding the side losses in the pond are set equal to zero.
to I, = 0.075 m, (b)
890
N. D.
et al.
khISHlKA
AT (So:220 0.91
0
W/m')
22 I
LL, I
66
0.1
0.2
0.3
88
110 I
I
0.4
0.5
J
0.6
AT /So Fig. 4. The efficiency as a function of the collector parameter AT/S,,. Curve A : flat plate collector with 2 glass covers and selective coating on the absorber. Curve B: salt gradient pond with a nonconvective zone of I m. Curve C: honeycomb solar pond with side losses corresponding to 1, = 0.075 m. Curve D: honeycomb solar pond with side losses set equal to zero.
REFERENCES I. 2. 3. 4. 5.
6. 7. 8. 9.
10. II. 12.
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