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Sede boqer shallow pond project
Energy Vol. 6, pp. 277-292 8 Pergamon Press Ltd 1981.
Printed
in Great
Britain
SEDE BOQER SHALLOW POND PROJECT A. KUDISH~ Energy Conversion Unit, Institute Ifor Desert Research, Ben-Gurion University of the Negev, Beer Sheva, Israel (Receioed 28 May 1980)
Abstract-The use of shallow solar ponds (convecting solar ponds) for the conversion of solar energy into low grade thermal energy has been a subject of intensive investigation in recent years. At the Institute for Desert Research at the Sede Boqer Campus we have been testing this concept with emphasis placed upon the utilization of locally manufactured components. The daily performance of four small module shallow solar ponds has been monitored almost continuously between August 1978 and May 1979. The ponds are each 2 x 1.3 m in size. They all have the same black PVC lower film, but differ either in the type of upper transparent film, glazing material or glazing angle. The daily performance is characterized by three factors, viz. the maximum daily water temperature achieved, the total daily thermal energy collected and the daily efficiency. Monthly average performance factors for the SSP modules have been determined. Based upon the experimental data, we conclude that the SSP system is capable of supplying - 3 GJ/m’-yr of thermal energy under climatic conditions similar to those prevailing at Sede Boqer, i.e. semi-arid zones. The economic feasibility of such a system has been analyzed in comparison with the following alternate energy sources: oil (heavy fraction), natural gas and electricity.
INTRODUCTION
The use of shallow solar ponds (SSPs) for the conversion of solar energy into low grade thermal energy has been a subject of intensive investigation for a number of years, especially by the Solar Energy Group at Lawrence Livermore Laboratory (LLL).lp4 At the Institute for Desert Research this concept has also undergone testing with emphasis placed upon the use of locally manufactured components. A shallow pond is essentially a large water bag or pillow placed within an enclosure with a clear upper glazing. A schematic diagram of an SSP is shown in Fig. 1. Water is placed within the bag, which is generally constructed from a clear upper plastic film and a black lower plastic film. The depth of the water within the bag is normally in the range of 4-15 cm. The solar energy collection efficiency is directly proportional to water depth, whereas the grade of thermal energy collected (i.e. the water temperature) is inversely proportional to water depth. Solar energy is converted to thermal energy by heating the water during the day. The water is withdrawn from the SSP before sunset (or more precisely, when the collection efficiency approaches zero) for utilization or storage. The idea of using such a simple device, viz. a water pillow, for solar energy collection is not new. The Japanese have been using numerous variations of this idea to heat water for domestic usage since the 1930s. In fact, 39 patents were issued for solar water heaters in Japan in the 1930s and another 20 were issued in the 194Os, the majority of these being for the water pillow type.5 A study of such a commercially produced Japanese pillowtype water heater has been reported in the literature.6 Harris et al.’ tested a solar water heater which was similar in design to that of the LLL group, the major difference being that their water pillow was constructed from a black butyl-rubber tube. Gopffarth et al8 tested a plastic solar water heater in which the water pillow was formed by heat sealing one black layer and one clear layer of polyethylene. They also investigated the use of tedlar as an upper glazing for the enclosure. The utilization of plastic materials as upper glazings in solar collectors has been reported in the literature numerous times, e.g. by Whillier’ and Grimmer and Moore.” TPresent address: Department of Chemical Engineering-Solar the Negev, Beer Sheva, Israel. 277
Energy Laboratory, Ben-Gurion University of
A.
KUDISH
Upper Glazing
Water bag, black bottom and clear top.
Fig. 1. Schematic diagram of an SSP.
Kudish and Wolf’ ’ have done extensive testing on a compact SSP designed for camping and military use. Their prototype design consisted of a water pillow placed within an insulated container. The container cover was insulated on the outside to maintain the water temperature for overnight storage and fitted with a mirrored surface (aluminized mylar) on the inside to function as a reflector when open and in operation during the day.
Fig. 2. Schematic diagram of the Sede Boqer SSP.
Sede Boqer shallow pond project
279
Upper glazing
Fig. 3. Cross section of the SSP enclosure.
SEDE
BOQER-SSP
INSTALLATION
The construction of the SSP installation on the Sede Boqer campus was completed in April 1978. A detailed description of it has been published previously.12 A schematic diagram of a Sede Boqer SSP is shown in Fig. 2. The water pillows were constructed from an upper clear PVC film and a lower black PVC film. The entry manifold, exit port and air vent were made from CPVC tubing. A cross section of the SSP enclosure (see Fig. 3) shows the curbing construction and the method of attaching the upper glazing to the curbing by means of the glazing anchors. The upper glazing was either Tedlar (Du Pont) or Qualex (Poligal, Ramat Hashofet, Israel). The former is in the form of a 0.1 mm thick film, whereas the latter consists of two polycarbonate panels bridged together at approx. 5 mm intervals. The spacing between the two panels is a production variable. Upon completing the construction of the SSP installation, the individual SSPs were installed within their respective enclosures and preliminary testing was initiated. A number of ponds were observed to be leaking. The origins of the leaks were found to be both in the seams joining the upper and lower films and in the films themselves. We succeeded in stopping those leaks that could be pinpointed by means of a heavy-duty adhesive (3 M Company). A second and much more serious problem developed when the seam designed to hold the exit port (viz. the seam between the pond and the exit port sleeve) failed in a number of ponds. We believe that this was the result of improperly sealed seams. The problem was further aggravated by the high temperatures attained within the ponds. Upon close examination, it was observed that these seams simply unraveled. As a result of the above-described pond failures, the actual SSP study was limited to measuring the daily performance of four small module ponds 2 x 1.3 m in size. These ponds are listed in Table 1 together with their individual PVC film types,? glazing material and glazing angle. In theory, the shallow solar ponds are designed to operate in the following manner. The ponds are filled with water from a cold water reservoir at a preset time in the morning. This time is a function of the insolation rate, in that there is a threshold value below which heat collection is practically zero. The water remains within the pond until a maximum temperature (T,,,) is attained and is then transferred to a hot water storage reservoir prior to consumption. The transfer is performed when the rate of heat loss to the surroundings is greater than the rate of heat collection. This occurrence is indicated by observing a decrease in the average pond water temperature, viz. T(T + &) < T,,,(t). At Sede Boqer, we did not utilize the hot and cold water reservoirs associated with the SSPs. There were two reasons for this: (1) the SSP site was isolated from any possible tThe film types refer to PVC films produced by Haoganplast, Israel.
A.
280 Table
1. Shallow
KIJDISH
solar pond test modules;
the film thicknesses
are listed in
parentheses.
PVC films
1 SSP No.
Transparent (upper)
Black (lower)
Glazing
Glazing
Angle
i-
consumer and thus there was no need for storage; (2) the SSPs are designed to receive cold water in the morning and it was therefore desirable to cool the previous day’s solar-heated water so that it could be recycled. In order to accomplish this, it was decided to leave the solar-heated water in the ponds overnight so that the water temperature would be lowered by means of nocturnal cooling. The daily performance of an SSP is characterized by three factors: (1) maximum daily pond water temperature obtained, (2) total daily amount of thermal energy collected, and (3) the daily collection efficiency. The maximum daily pond water temperature is a measure of the quality of the thermal energy collected, whereas the latter two factors are a measure of the quantity of thermal energy collected. In practice, there is a competition between the qualitative and quantitative factors. The higher the pond water temperature, the greater the rate of heat loss to the surroundings as a result of the greater temperature gradient between the pond water and the surroundings. This heat transfer, in turn reduces the collection efficiency and ‘the total daily amount of thermal energy collected. The controlling parameter in this competition is the water depth or volume (since the pond area is constant) within the pond. The manipulation of this parameter may be of major importance, especially in periods of low insolation rates and it has been investigated during the course of this study. EXPERIMENTAL
STUDIES
The daily performance of individual SSPs was monitored from August 1978 until May 1979 by means of a copper-constantan thermocouple, which measured the pond water temperature. The thermocouple was inserted directly into the pond water via the pond air vent. The thermocouples were connected to a multi-channel chart recorder (Chino Works, Ltd., Japan), which converts the thermocouple mv signal directly into degrees centigrade. The recorder is of the continuous scan type and provides a trace of pond water temperature as a function of time. Such a trace, replotted on a shorter time scale, is shown in Fig. 4. The ambient temperature was continuously measured by means of a copper-constantan thermocouple, which was positioned within a meteorological screen. The thermocouple was also connected to the multichannel chart recorder. The insolation rate on a horizontal surface, at the Sede Boqer campus, was measured with a Kipp & Zonen solarimeter positioned on the roof of the one-story building housing the Energy Conversion Unit. The solarimeter was connected to a Kipp & Zonen digital integrator, which printed the accumulated daily insolation rate at one-half hour intervals. The solarimeter malfunctioned in April 1979 and, therefore, during the last 2 months of testing (i.e. April and May) we were unable to determine the collection efficiency because of the absence of insolation data.
RESULTS
A computer program was written to process the large amount of experimental data that accumulated daily from the SSP measurements. The data input to the program
Me
281
Boqer shallow pond project
3600
3200
2600: AZ "E 2400, 2
400
0
2
4
6
6
IO
12
14
16
16
20
22
0 24
TIME (hr) Fig. 4. The pond water temperature as a function of time.
included the hourly pond water temperatures, ambient temperatures and insolation rate. The simplification in the calculation implied by using hourly pond water temperatures was justified by the observation that, during such a time interval, the change in pond water temperature was approximately linear (see Fig. 4). In the vicinity of the temperature maxima and minima, the time interval was shortened. The program calculated the daily performance parameters for each SSP and the results were given in both tabular and graphical form. The results of our investigation are summarized in Table 2, which lists the maximum pond water temperature (T,,,), the pond water-temperature increase (AT), the quantity of thermal energy collected (Q), and the SSP collection efficiency (q). There were four major changes that took place during the course of this investigation: (1) the water depth in the SSPs was changed from 10 to 6 cm on 15 November 1978; (2) SSP-2 was put out of commission in December 1978 as the result of a leak that could not be repaired; (3) SSP-8 was successfully repaired and put into operation during December 1978; and (4) the water depth in SSP-5 was raised from 6 to 10 cm after sunset on 16 May 1979. The rate of thermal energy collection per unit pond area for the ith hour, qi, is given by
(1) where m is the water mass within the pond (kg) and is calculated from m=pLA;
(2)
p is the water density (kg/m3); L is the depth of the water in the SSP (m); A is the SSP surface area (2.4 m’); Cp represents the specific heat of water (4.19 KJ/kg”C); and AT/At is the hourly rate of change of the SSP water temperature (“C/hr). The SSP collection efficiency was calculated on the basis of the water-pillow surface area (viz. 2.4 m2) and not for the overall enclosure area. The use of the former results in considerably higher efficiency values, since the latter is much larger (4.38 m2). The SSP hourly collection efficiency is simply ‘li
=
4ilIi
(3)
A. KUDISH
282
Table 2. Monthly average daily performance of SSP modules. Month
a)
b)
c)
d)
e)
f)
9)
h)
i)
j)
k)
August
(Zld)
September
October
(17d)
(16d)
November
November
December
January
(8d)
(6d)
(8d)
(27d)
February
March
April
MY
SSP No.
L (cm)
2
10
60.9
24.9
10,419
43.3
5
64.2
27.4
11,377
47.2
7
69.1
28.4
11,298
49.3
2
10
54.2
21.6
8,946
47.0
5
54.3
22.0
9.020
45.9
7
57.7
22.6
9,275
49.0
2
10
46.7
17.5
7,138
46.5
5
47.4
18.3
7,461
48.1
7
49.7
18.0
7.344
47.6
2
10
34.5
13.8
5,479
46.9
S
34.8
14.2
5,620
46.3
7
37.7
14.4
5,714
46.9
2
6
35.6
20.0
5,079
36.8
5
38.1
21.8
5,450
39.4
7
39.3
21.3
5,329
38.9
5
6
33.6
16.8
4,209
38.1
7
36.1
16.7
4,172
37.7
8
34.2
16.4
4,100
37.0
5
6
34.9
21.1
4,848
41.3
7
35.2
18.9
4,372
36.5
8
34.4
19.0
4,360
36.4
44.3
26.2
6.082
42.8
7 (28d)
44.3
22.8
5,291
36.9
8 (28d)
43.3
23.0
5,362
36.8
5 (27d)
(Zld)
(29d)
(14d)
5
6
6
54.2
28.9
7,245
41.8
7
54.1
26.0
6,550
37.0
8
53.8
27.0
6,699
37.8
5
6
60.7
31.5
7,833
7
60.2
27.5
7,124
8
60.3
28.8
7,195
5
6
7 8 1)
May
(6d)
69.1
38.1
12,410
67.7
32.7
10,532
68.1
34.7
11,869
5
10
62.4
28.7
12,021
7
6
68.2
33.1
13,849
8
6
68.8
35.9
14,952
where I is the hourly insolation rate (kJ/m’-hr). The corresponding above two quantitative parameters are given by
daily values for the
(14 n
V =
I n
iJll 4ii C
Ii,
i=l
where n is the number of hours during the day that the SSP was in operation, viz. qi > 0. The SSPs generally ceased operation (i.e. qi d 0) in the early afternoon. This was a consequence of the high pond water temperatures attained relative to the ambient, which resulted in a heat loss rate to the surroundings equal or greater than the rate of thermal energy collection, The fact that both the insolation rate and the ambient temperature are
Sede Boqer shallow pond project
283
decreasing at this time contributes to the occurrence of an early afternoon termination of SSP operation. As a consequence, the SSPs are unable, in a batchwise mode of operation, to utilize the total amount of sunshine available.
DISCUSSION
OF
RESULTS
In this section, the effect of varying some of the basic SSP design parameters will be discussed first and then their overall performance and economic feasibility will be analyzed. A preliminary report on the results of this study was published recently.13 Upper glazing
SSPs 2 and 7 were identical in construction, except for the transparent material which served as their upper glazing (see Table 1). The former had a Tedlar and the latter a Qualex upper glazing. The performance of these two SSPs was monitored, for comparative purposes, from August 1978 until the latter part of November 1978, when SSP-2 ceased operation, as explained previously. It is apparent from Table 2 that SSP-7 outperformed SSP-2 during this period. Only during the first half of November 1978 did they have comparable performance records. It is also important to note that it is much easier to construct an upper glazing from Qualex than from Tedlar. Qualex is produced in the form of rigid but flexible panels, whereas Tedlar is a thin film and requires closely-spaced supports. These supports also reduce the quantity of solar energy impinging on the pond surface due to their shading of the pond. Pond PVC upperjlm
All of the SSPs were constructed from a single type of black PVC bottom film, H-65 (0.40 mm thick). Two types of clear PVC film were used for the upper water bag film: (1) L-06, a 0.30 mm thick film, somewhat milky in appearance and (2), S-l 50, a 0.18 mm thick window clear film. Due to the failure of six of the ten small module SPPS, it is not possible to arrive at a definite conclusion regarding the relative merits of the two types of upper PVC tested, since the module that was to provide these data failed. There are, nevertheless, two possible SSP pairs that may be used to compare the two types of upper PVC films utilized, viz. SSP-2/SSP-5 and SSP-7/SSP-8. The problem is that in each case there is an additional parameter change (see Table 1). For SSP-Z/SSP-5, they also differ in their upper glazing angle and, for SSP-7/SSP-8, they differ in the spacing between the two polycarbonate panels of the Qualex glazing. For SSP-2/SSP-5, even if it is assumed that the upper glazing angle does not significantly affect the performance of the SSP in the range studied, there still is no conclusive evidence for the superiority of one type of upper PVC film over the other (a more detailed explanation is given in the following section). For SSP-7/SSP-8, it is expected that SSP-7 with a 6 mm spacing between panels would possess superior insulative properties in the upward direction relative to SSP-8 with a 4mm spacing between panels. These two SSPs were monitored from December 1978 until May 1979. It is apparent from the monthly average performance data summarized in Table 2 that no perceptible differences in their performance characteristics were measured. It is conceivable that the thermal difference in the upper glazing compensates for the difference, if it exists, in the upper PVC film optical and thermal properties. Upper glazing angle
The SSPs were positioned horizontally on the ground with their long dimensions along the north-south axis and the curvature of the upper glazing along the east-west axis, i.e. the axis of symmetry of the glazing was along the north-south axis. Thus, a priori, the angle of the upper glazing should have a negligible influence, provided it is not extreme, on surface reflectance, since it is in a plane that is parallel to the direction of the solar rays. SSP-5 had an upper glazing angle of 30”, whereas the other SSPs (viz. 2,7, and 8 had an angle of 21”).
284
A. KUDISH
One of the purposes of this investigation was to determine whether there exists an optimum upper glazing angle, i.e. an angle which affords the least wind resistance but, at the same time, is sufficiently sloped such that sand and dust accumulation is minimal. The rate of heat loss from the upper glazing is mainly due to forced convection, which is proportional to the wind resistance. The natural removal from the upper glazing of sand and dust particles also depends on the sweeping action of the wind. The effect of upper glazing angle on SSP performance is best analyzed by comparing the data for SSPs 2 and 5, which were both covered with a Tedlar upper glazing. In this case, the comparison is somewhat difficult. It is observed in Table 2 that SSP-5 achieved higher temperatures, larger temperature increases, and collected more thermal energy than SSP-2 during the period August 1978 until November 1978. On the other hand, with regard to collection efficiency, there is no conclusive evidence which SSP was superior. Also, even for parameters for which SSP-5 outperformed SSP-2, the differences were quite small. Thus, it is believed that a change in the upper glazing angle in the range of 20-30” has no significant influence on SSP performance. Pond water depth
The depth of water within the pond influences strongly both the quality and quantity of the thermal energy collected. The pond water temperature is inversely proportional to its depth, whereas the amount of thermal energy collected and thus the collection efficiency are proportional to temperature. In essence, the quality of the thermal energy collected is in competition with the quantity. In practice, there is generally a minimum quality or temperature requirement and this will determine the quantity of thermal energy collected. These relationships were demonstrated when the water depth was changed from 10 to 6 cm on 15 November 1978. For all three SSPs (viz. 2, 5, and 7) the performance parameters changed as predicted when comparing performance data for November, before and after the fifteenth of the month. The average daily temperature maximum increased by only l.l-3.3”C, whereas the average daily temperature rise increased by 6.2-7.6”C, (i.e. by a factor of - 1.5). The quantitative parameters, on the other hand, decreased. The average daily quantity of thermal energy collected decreased by 170-400 KJ/m’, whereas the average daily efficiency was reduced by a factor of -0.8. On the evening of 16 May 1978, the water depth in SSP-5 was changed once again. This time, it was increased from 6 to 10 cm. A comparison of the May performance data for SSP-5, before and after the sixteenth of the month, shows that the average daily maximum water temperature decreased by 6.7”C and the average daily temperature increase was reduced by a factor of 0.75. In this case, there was also a decrease in the average daily quantity of thermal energy collected instead of the expected increase. Since no insolation data were available for the months of April and May, the collection efficiency could not be calculated and no plausible reason for this behavior can be suggested. The overall effect of pond water depth on SSP behavior is also a function of the quantity of insolation available and of the ambient temperature. The former determines the quantity of solar energy available for conversion, whereas the latter determines the rate of energy loss to the surroundings for a given pond water temperature. This relationship is expressed by the simplified form of the Hottel, Whillier, and Bliss (HWB) model,’ 4 viz. 4 = Z(za), - U(T-- T,).
(4)
In Eq. (4), I is the insolation rate (KJ/hr-m2), (za), is the effective transmittance-absorptance product, U is the overall heat loss coefficient (KJ/hr-m2-‘C), T is the pond water temperature (“C), and TOequals the ambient temperature (“C). Thus, the optimal seasonal water depth is set by the minimum water temperature required in conjunction with the average seasonal insolation rate and ambient temperature.
285
Sede Boqer shallow pond project
SSP solar collector parameters (m), and U A shallow solar pond is classified as a flat plate collector and its performance is characterized by its U and (ra), values. These two parameters were determined experimentally for the SSPs by means of the HWB model [Eq. 43, which is expressed in terms of the collection efficiency, r,r,by rj = q/l = (m), - U(T - T,)/Z.
(5)
A plot of the above linear equation, viz. q as a function of (T - T,)/I, yields the value for (74, and U as the intercept and negative of the slope, respectively. The value of the intercept, (XC),,is the theoretical maximum efficiency a collector can achieve under condition of zero heat loss. It is possible to determine (7~4, and U values by using the approach of Hewettr5. This approach equates Eq. (5) to the efficiency version of Eq. (l), viz. rj = p C, L(AvAt)/l
(6)
and one obtains, after dividing through by the factor p C, L(T - T,), u
(7)
-pc,L’
Equation (7) is valid for 24 hr providing T remains significantly greater than T,, whereas Eq. (5) is valid only when q > 0. A plot of (AT/At)/T - T, vs Z/(T - Tu) yields the U and (~a), values from its intercept and slope, i.e. - U/pC,L and (za),/pC,L, respectively. Hewett also used nocturnal cooling curves to determine U values. In this case, I = 0 and Eq. (2) reduced to -AT/At
= &(T
- T,).
P
A plot of -AT/At vs (T - T,) yields a curve with a slope U/pC,L and zero intercept. The (74, and U values for the four SSP modules investigated were determined on a monthly basis using all three of the above techniques, viz. Eqs. (5) (7), and (8). Data for a number of days (generally a sampling from the beginning, middle, and end of the month) were analyzed each month in order to obtain average monthly values. The individual daily @a), and U values were calculated by means of a least-squares analysis. The monthly values are summarized for each SSP separately in Tables 3-6. In Figs 5-7, are shown a typical plot of a set of experimental data and the corresponding least-squares fit curves. It is important to note that during the winter months, it was generally very difficult to obtain reasonable (za), and U values from Eq. (5). This problem may be due, in part, to intermittent cloudiness, but this is difficult to verify since the cloudiness index Table 3. Average monthly (XC),and U values for SSP-2. The numbers in parentheses in Tables 3-6 refer to the corresponding equations in the text.
Month
ll (KJ/hr-m2-"C)
(Ta)e (5)
(7)
August (kid)
0.75 + 0.06
0.68 f 0.04
40.4 f
(5) 4.9
29.3 f 1.9
(7)
31.3 f 3.1
(8)
September (7d)
0.75 ? 0.05
0.71 _+0.02
33.7 +
5.7
28.1 f 2.8
32.1 * 2.2
27.3 t 1.7
31.4 ? 2.3
October (6d)
0.79 f 0.10
0.67 f 0.03
48.7 _f 12.8
November (Sd)
0.65 f 0.13
0.77 f 0.15
45.7 ?
6.6
28.2 f 5.1
36.0 f 5.6
Average
0.74 f 0.06
0.71 t 0.05
42.0 +
6.5
28.2 + 0.8
32.7 ? 2.2
Overall Average
0.73
34.3
286
A.
KUDISH
Table 4. Average monthly (ra), and U values for SSP-5.
Month
IJ (KJ/h4-m2-'C)
(We (5)
(7)
August (15d)
0.79 * 0.05
0.73 + 0.04
36.9 + 4.1
30.6 f 3.0
32.9 ? 3.6
September (7d)
0.74 + 0.06
0.69 k 0.02
31.1 f 4.9
28.4 ? 1.0
32.3 f 2.0
(5)
(8)
(7)
October (6d)
0.71 * 0.04
0.64 + 0.03
34.8 i 5.7
26.5 ? 2.0
29.3 r 0
November (5d)
0.57 r 0.08
0.65 _+0.13
21.7 + 9.7
24.6 2 4.1
32.6 f 6.9
17.4 i 3.4
22.5 f 1.1
December (4d)
0.46 ? 0.02
January (Sd)
0.53 ?I0.03
18.4 * 4.7
23.4 ? 2.1
February (6d)
0.56 i 0.08
0.57 f 0.08
26.9 + 2.6
25.5 t 4.9
30.7 t 2.4
March (6d)
0.65 ? 0.04
0.64 f 0.02
22.9 + 0.4
24.4 + 3.6
25.6 i 1.9
Average
0.67 k 0.09
0.61 + 0.09
29.1 t 6.2
24.5 t 4.6
28.7 ? 4.3
Overall Average
0.64
27.4
Table 5. Average monthly Month
(ra),
and U values for SSP-7 II (KJfhr-mL-"c)
(Ta)a
I
(5)
(7)
August (15d)
0.88 ? 0.07
0.78 f 0.03
36.9 ?
5.0
27.2 ? 1.6
28.8 * 1.3
September (7d)
0.83 + 0.07
0.76 + 0.02
33.5 e
5.5
26.9 +_0.7
28.1 f 2.0
October (6d)
0.72 k 0.08
0.66 * 0.02
32.1 f 10.1
22.3 + 0.8
25.1 ? 0
November (5d)
0.58 + 0.09
0.71 i 0.14
20.7 f
8.9
23.9 + 5.1
25.9 * 5.5
December (4d)
0.53 + 0.04
0.53 f a.03
21.5 i
5.2
19.3 + 2.3
18.6 i 1.6
January (Sd)
0.52 ? 0.02
0.52 + 0.03
20.0 f
5.6
February (6d)
(5)
(7)
0.51 f 0.04
(8)
15.5 ? 3.2
18.8 + 1.6
15.6 f 3.6
19.8 ? 0.7
March (6d)
0.57 f 0.06
0.57 f 0.03
21.9 ?
6.9
20.9 ? 2.9
19.6 f 1.0
Average
0.66 f 0.15
0.63 f 0.11
26.7 f
7.2
21.5 ? 4.5
23.1 f 4.3
0.65
Overall Average
23.8
Table 6. Average monthly (ra), and U values for SSP-8.
Month
U (KJ/hr-m2-'C)
(TU), (5)
(7)
December (4d)
0.53 2 0.02
0.46 f 0.02
January (8d)
0.47
0.49 f 0.03
February (6d)
0.42
0.48 + 0.03
March (6d)
0.60 f 0.11
Average
0.51 f 0.08
Overall Average
(5)
16.5 +
3.8
18.6 r 2.2
15.0 f
2.8
19.2 f 1.5
23.8
17.9 i: 1.7
20.5 f 0.9
0.57 + 0.06
26.8 f 14
22.4 + 13.9
21.8 ? 1.1
0.50 + 0.05
24.8 +_ 1.7
17.9 +
20.0 + 1.4
0.50
23.8 e
(8)
(7) 4.9
20.9
3.2
287
Sede Boqer shallow pond project
0.?-
\ r
01
0.5-
TO 05
0.2-
C).I-
(T-Tn)/I(m’-hr Fig. 5. The r] vs (T-
- “C/KJ)
T&4 curve for SSP-2 on 31 July 1978.
was not recorded at Sede Boqer. A number of trends and qualitative comparisons are apparent from the data on the four SW modules studied. (a) The (zc(), and U values for SSPs 5 and 7, which were the only SSPs continuously monitored throughout this study, exhibited minimum values during the winter months.
50
100 AA
Fig. 6. The ratio (-AT/At)/(T
150
, 200
(K J/mchr-Y)
- T.) vs I/(T - TO)curve for SSP-2 on 31 July 1978.
A.
288
KLJDISH
-( -+=$q&
3
r’
T-Tn)
U=36BKJ/mchr-“C
2
t e a!I I
I
io
2b
30
T-TA(‘C)
Fig. 7. The ratio (-AT/At) vs (T-
T,) curve for SSP-2 on 31 July 1978.
The reduction in (rtl), is expected as a result of the relatively high winter incidence angles on the upper glazing, especially since the SSPs are essentially horizontal flat plate collectors. The decrease in U values is most likely the result of the lower wind velocities prevailing at Sede Boqer during the winter. (b) There appears to be no significant difference between Tedlar and Qualex with regard to their (ra), values (see Tables 4 and 5). The monthly and overall average (Ta), values for SSPs 5 and 7 are similar. (c) There is a definite trend of lower U values for the Qualex covered SSPs (7 and 8) than for the Tedlar covered SSPs. These differences may be explained by the added insulation the air pockets in the Qualex glazing contribute, which reduce the heat loss rate in the upward direction. The (Ta), values referred to in this section include, in addition to the upper glazing (ta) values, also the (za) contribution of the clear upper PVC pond film. Thus, they are truly effective values. Based upon this analysis, it seems reasonable to ascribe to the Sede Boqer SSPmodules, average (za), and U values of 0.65 and 25 kJ/h4-m2-“C, respectively. These values are in the range of (za), and U values that characterize commercially available flat plate collectors with multiple glazings. Thermal energy collection The overall SSP efficiency would have been significantly greater had the solar heated
pond water been removed at the end of each day and the pond been filled with fresh, relatively cool water in the morning. As a result of the relatively high initial water temperatures, meaningful thermal energy collection (q > 0) would be expected to begin relatively late in the morning and to terminate relatively early in the afternoon. The monthly average daily number of hours of thermal energy collection by the SSPs is summarized in Table 7. The two preceding predictions are validated by the data in Table 7. Also recorded in the table are the monthly average values for the length of the solar day (T,) and the ratio of solar hours utilized for thermal energy collection (At/&). It is observed that this ratio varies between 58 and 65%. The efficiency of utilizing the number of solar hours available is in practice better than these values suggest. The hours during which the SSP does not collect thermal energy have relatively low insolation rates and therefore contribute less. It is also possible to increase the number of hours of thermal energy collection by operating the SSP in a flow mode, i.e. by circulating water through the SSP and thereby
289
Sede Boqer shallow pond project Table 7. Monthly average daily number of hours of thermal energy collected by SSPs. t.
SSP No.
Month
December
January
February
March
(a)
ti
8.50
15.21
8.57
6.80
15.21
8.41
6.94
14.59
7.65
6.88
14.55
7.67
7.00
14.76
7.76
2
7.00
13.92
6.92
5
7.04
14.08
7.04
7
7.18
14.18
7.00
2
7.14
13.93
6.79
5
7.14
14.00
6.86
7
7.21
14.07
6.86
5
8.00
14.14
6.14
7
8.00
14.00
6.00
a
8.00
14.00
6.00
5
7.68
14.00
6.35
7
7.88
14.23
6.35
8
7.81
14.12
6.31
5
7.22
14.12
6.90
7
7.61
14.50
6.81
8
7.46
14.54
7.08
5
7.00
14.05
7.05
7
7.09
14.32
7.23
8
7.05
14.18
7.13
6.64
14.14
7.50
6.79
14.43
7.64
6.83
14.50
7.67
6.14
14.14
8.00
6.36
14.05
7.69
6.18
14.00
7.82
- beginning (c)
of
thermal
At = tf
- ti;
energy (d)
collection;
Td = (Z/15)
d
Ate
15.18
Apri 1
collection;
P
6.64
September
November
t
6.68
August
October
a 1
(b) cos-’
Td
tf
At/T,
13.13
0.65
12.18
0.63
11.22
0.62
10.39
0.66
10.01
0.60
10.20
0.62
10.91
0.64
11.77
0.61
12.76
0.60
13.57
0.58
-
end of
thermal
(-tan+
tans),
length
energy of
solar
day.
increasing the volume of water heated per unit area of SSP. The overall effect of this mode of operation is similar to that produced by increasing the pond water depth. Economic analysis It is possible to approximate the total yearly amount of thermal energy that an SSP situated at Sede Boqer (or comparable site) is capable of collecting under normal operating conditions (viz. removing solar-heated water in the evening and filling with fresh water in the morning) in the following manner: (a) Multiply the monthly average daily quantity of thermal energy collected by the number of days in the months. (b) Assume that the monthly average daily quantity of thermal energy collected during June and July can be approximated by the average of the May and August values. This assumption is, in all likelihood, conservative. (c) Assume that the positive error introduced by step (a), which neglects days during which thermal energy collection is practically zero due to rain or heavily overcast skies, is counterbalanced by the understatement caused by the experimental procedure. Thus, under standard operating conditions (filling and emptying each day), the SSP performance would improve significantly over that measured in this study. EGY Vol. 6. No. WZi
290
A. KUDISH Table 8. Yearly total of thermal energy collected per unit area by an SSP at Sede Boqer. Month
L(Cd
QWJ/&
January
6
140
February
6
156
March
6
216
April
6
229
bY
10
373
JlLW
10
347
July
10
359
August
10
351
September
10
274
October
10
229
November
6
162
December
6
129
Total
2965
These calculations have been performed using average monthly values for the thermal energy collected by the SSPs and are reported in Table 8. The yearly total amount of thermal energy that an SSP is capable of collecting, based on these assumptions, is - 3 GJ/m2-yr. This value compares favorably with that reported by the LLL group,4 viz. 2.8 GJ/m2-yr. The annual energy equivalent per 100 m2 of shallow solar ponds providing 3 GJ/m’-yr is approx. either 83.3 MWh of electricity, 43 barrels of oil, or 6.51 x lo3 kg of natural gas (assuming an energy content of 46,000 kJ/kg). Using these values, we may determine upper limits on the cost per unit area of SSP construction for which such a system is economically feasible. It is assumed that the auxiliary system is identical to the alternative conventional system, since it must be capable of supplying 100’~ of the energy requirements during periods of consecutive non-solar winter days. Thus, the break-even point for the cost of the SSP system is given by Css, x 1 = CF QF>
(9)
where C,, is the cost of the SSP system, I is the interest and amortization factor, CF equals the cost of the alternate energy source, and QF is the quantity of the alternate energy source required. The upper limit on the cost per unit area of SSP installed for which the system is economically competitive is, of course, a function of the alternate energy source available. The economic feasibility of the SSP energy-conversion system will be analyzed with respect to oil (heavy fraction, restricted to industrial usage), natural gas and electricity. Electricity is generally utilized only for domestic hot water. We apply Eq. (9) to a 100 m2 SSP installation capable of supplying 3 GJ/m2-yr and compare it to each of the above alternate energy sources in turn. This yearly energy production per 1OOm’ SSP is approx. equivalent to either 43 bbl of oil, 6.51 x lo3 kg of natural gas or 83.3 MWh electricity. The prices quoted are local (Israel) rates for Sept. 1979. (a) Oil (heavy fraction): CF = $20/bbl; C,,, = ($8.60/m2)/Z; Css, - $58/m2. (b) Natural C,,, x I = (0.67 x 6.51 x 103)/100; gas : CF = $0.67/kg; Css, = ($43.62/m2)/Z; (c) Electricity: C, = $60/MWh; C,, x I = (60 x 83.3)/100; Cssp - $293/m’. C,,, = ($49.98/m2)/Z; C,, - $335/m2. The value for the Cssp, the cost per unit area of the SSP, is strongly dependent on the prevailing interest rate, as well as the SSP life-time. The water bag is the component with
Sede Boqer shallow pond project
291
the shortest life-time (3-5 yr), but the other components should all have life-times in the range of 10-20 yr. Here, we have assumed an interest rate of 8% and an average overall lifetime for all of the components of 10 yr. Thus, a value of I = 0.149 has been used in the analysis. It is observed, as expected, that the SSP system is least competitive with respect to oil. It could compete with natural gas and electricity if the technical problems associated with its construction (proper materials, etc.) can be solved. Another approach for determining the economic feasibility of such a system is its payback period. If we use the preceding values for energy production per unit area of SSP and cost per bbl of oil, it follows that each m2 of SSP supplies the equivalent of 0.43 bbl of oil per year (or $8.60 worth of energy). Thus, if the cost per m2 of SSP can be reduced to $86, which is a reasonable cost, the payback period becomes on the order of 10 yr or the lifetime of the system. The preceding analyses were performed without regard to the ever-increasing costs of conventional energy sources and their availability. Needless to say, the prognosis for the economic feasibility of the SSP improves as the costs for conventional fuels increase and their availability diminishes.
CONCLUSIONS
Our conclusions are briefly summarized below. (1) Qualex is preferred over Tedlar for use as an upper glazing. It exhibited superior performance characteristics and is much easier .o construct. It has a two-fold advantage with regard to construction. First, it is much e:lsier to work outdoors with a rigid panel than with a thin film, because of wind interference. Second, Tedlar, being a thin film, requires additional supports, which reduce the surface area of the pond exposed to solar radiation as the result of shading. It is still necessary to conduct aging tests on Qualex to determine the degree and rate of degradation it undergoes. (2) No definite conclusion regarding the relative merits of the two types of upper PVC films tested can be reached, since the module that was to provide these data failed. (3) A change in the upper glazing angle in the range of 20-30” has no significant influence of SSP performance in our case. This is the result of the fact that the Sede Boqer SSPs were positioned horizontally on the ground with their long dimension along the north-south axis and the curvature of the upper glazing along the east-west axis (i.e. the axis of symmetry of the glazing was along the north-south axis). (4) The pond water depth determines the quantity and quality of thermal energy collected and should be varied to obtain optimal seasonal performance. The optimum seasonal depth is defined by the minimum water temperature required in conjunction with the average seasonal insolation rate and ambient temperature. (5) The SSP solar collection parameters (ZLY),and I/ were found to be on a yearly average approx. 0.65 and 25 kJ/hr-m2-‘C, respectively. These values are similar to those characterizing most commercially available flat plate solar collectors with multiple glazings. (6) The overall SSP thermal energy-collection efficiency may be significantly improved over that measured for the Sede Boqer SSP modules, under normal operating conditions, by removing the solar heated water every evening and filling the pond with relatively cool water each morning. (7) The economic analysis has shown that the SSP is not now competitive with oil (heavy fraction), whereas it may be competitive with natural gas and electricity when the technical and material problems associated with it are overcome. (8) The payback period for the SSP system supplying 3 GJ/m2-yr and competing with oil is on the order of 10 yr if the cost per m2 of SSP can be reduced to $86. It is important to note that the energy conversion capacity per m2 of SSP can be increased significantly by using reflectors.’ I*16 An economic analysis on such a system has not been performed since experimental data on a yearly performance basis are not yet available. The LLL group has recently published a “Design Guide for Shallow Solar
292
A.
KUDISH
Ponds”, which is recommended to all those interested in this topic.4 It is believed that the prognosis for the economic feasibility of an SSP system will continue to improve as the cost for conventional fuels increases and their availability diminishes. Acknowledgements-The author would like to thank the Ministry of Commerce, Industry and Tourism for making this project possible through their funding. I wish to express my gratitude to Drs. W. C. Dickinson, A. F. Clark and the LLL group for their unselfish exchange of information and Professors A. Roy and D. Faiman of Ben-Gurion University of the Negev for their advice and suggestions during the course of this investigation. I also thank Mr. D. Pinto, Mr. Y. Machlav, and Ms. G. Loutaty for their aid in performing the experiments and correlating the data.
REFERENCES 1. W. C. Dickinson, A. F. Clark, J. A. Day, and L. F. Wouters, Solar Energy 18, 3 (1976). 2. W. C. Dickinson, A. F. Clark, and A. Iantuono, “Shallow Solar Ponds for Industrial Process Heat: The ERDA-SOHIO Project”, Lawrence Livermore Laboratory, University of California, Rep. LiCRL-78288, Livermore, California (1976). 3. Bohanan Westman Huston & Associates, Inc., “Solar Pond Facility, Bibo, New Mexico”, Lawrence Livermore Laboratory, University of California, Rep. UCRL-13680, Livermore, California (1977). 4. A. B. Casamajor and R. E. Parsons, “Design Guide for Shallow Solar Ponds”, Lawerence Livermore Laboratory, University of California, Rep. llCRL-52385 Rev. 1, Livermore, California (1979). 5. A. B. Meinel and M. P. Meinel, Applied Solar Energy. Addison-Wesley, Reading, Mass. (1976). 6. M. L. Khanna, Solar Energy 15, 269 (1973). 7. W. B. Harris, R. R. Davison, and D. W. Hood, Solar Energy 9, 193 (1965). 8. W. H. Gopffarth, R. R. Davison, W. B. Harris, and M. J. Baird, Solar Energy 12, 183 (1968). 9. A. Whillier, Solar Energy 7, 148 (1963). 10. D. P. Grimmer and S. W. Moore, “Practical Aspects of Solar Heating: A Review of Materials Used in Solar Heating Applications”, Los Alamos Scientific Laboratory, University of California, ,!,A-LIR-1752, Los Alamos, New Mexico (1975). 11. A. I. Kudish and D. Wolf, Solar Energy 21, 317 (1978). 12. A. I. Kudish, Y. Raziel and A. Roy, “Sede Boqer Shallow Solar Pond Project”, Report submitted to the Ministry of Commerce and Industry (Project No. 76/49/A), Israel (1977). 13. A. I. Kudish and A. Roy, “Sede Boqer-Shallow Solar Ponds Project”, Proc. 14th Intersociety Energy Conversion Engineering ConjI, Boston, Mass., Aug. 1979. 14. J. A. Duffie and W. A. Beckman, Solar Energy Thermal Processes. Wiley, New York (1974). 15. L. D. Hewett,“‘The January-March Performance of Shallow Solar Ponds with and without Reflectors on their North Side”, Lawrence Livermore Laboratory, University of California, Rep. LICID-17-663, Livermore, California (1977). 16. A. F. Clark, “A Horizontal Collector with a Mirror on the North Side”, Lawrence Livermore Laboratory, University of California, Rep. UCRL-77906, Livermore, California (1976).
Printed
in Great
Britain
SEDE BOQER SHALLOW POND PROJECT A. KUDISH~ Energy Conversion Unit, Institute Ifor Desert Research, Ben-Gurion University of the Negev, Beer Sheva, Israel (Receioed 28 May 1980)
Abstract-The use of shallow solar ponds (convecting solar ponds) for the conversion of solar energy into low grade thermal energy has been a subject of intensive investigation in recent years. At the Institute for Desert Research at the Sede Boqer Campus we have been testing this concept with emphasis placed upon the utilization of locally manufactured components. The daily performance of four small module shallow solar ponds has been monitored almost continuously between August 1978 and May 1979. The ponds are each 2 x 1.3 m in size. They all have the same black PVC lower film, but differ either in the type of upper transparent film, glazing material or glazing angle. The daily performance is characterized by three factors, viz. the maximum daily water temperature achieved, the total daily thermal energy collected and the daily efficiency. Monthly average performance factors for the SSP modules have been determined. Based upon the experimental data, we conclude that the SSP system is capable of supplying - 3 GJ/m’-yr of thermal energy under climatic conditions similar to those prevailing at Sede Boqer, i.e. semi-arid zones. The economic feasibility of such a system has been analyzed in comparison with the following alternate energy sources: oil (heavy fraction), natural gas and electricity.
INTRODUCTION
The use of shallow solar ponds (SSPs) for the conversion of solar energy into low grade thermal energy has been a subject of intensive investigation for a number of years, especially by the Solar Energy Group at Lawrence Livermore Laboratory (LLL).lp4 At the Institute for Desert Research this concept has also undergone testing with emphasis placed upon the use of locally manufactured components. A shallow pond is essentially a large water bag or pillow placed within an enclosure with a clear upper glazing. A schematic diagram of an SSP is shown in Fig. 1. Water is placed within the bag, which is generally constructed from a clear upper plastic film and a black lower plastic film. The depth of the water within the bag is normally in the range of 4-15 cm. The solar energy collection efficiency is directly proportional to water depth, whereas the grade of thermal energy collected (i.e. the water temperature) is inversely proportional to water depth. Solar energy is converted to thermal energy by heating the water during the day. The water is withdrawn from the SSP before sunset (or more precisely, when the collection efficiency approaches zero) for utilization or storage. The idea of using such a simple device, viz. a water pillow, for solar energy collection is not new. The Japanese have been using numerous variations of this idea to heat water for domestic usage since the 1930s. In fact, 39 patents were issued for solar water heaters in Japan in the 1930s and another 20 were issued in the 194Os, the majority of these being for the water pillow type.5 A study of such a commercially produced Japanese pillowtype water heater has been reported in the literature.6 Harris et al.’ tested a solar water heater which was similar in design to that of the LLL group, the major difference being that their water pillow was constructed from a black butyl-rubber tube. Gopffarth et al8 tested a plastic solar water heater in which the water pillow was formed by heat sealing one black layer and one clear layer of polyethylene. They also investigated the use of tedlar as an upper glazing for the enclosure. The utilization of plastic materials as upper glazings in solar collectors has been reported in the literature numerous times, e.g. by Whillier’ and Grimmer and Moore.” TPresent address: Department of Chemical Engineering-Solar the Negev, Beer Sheva, Israel. 277
Energy Laboratory, Ben-Gurion University of
A.
KUDISH
Upper Glazing
Water bag, black bottom and clear top.
Fig. 1. Schematic diagram of an SSP.
Kudish and Wolf’ ’ have done extensive testing on a compact SSP designed for camping and military use. Their prototype design consisted of a water pillow placed within an insulated container. The container cover was insulated on the outside to maintain the water temperature for overnight storage and fitted with a mirrored surface (aluminized mylar) on the inside to function as a reflector when open and in operation during the day.
Fig. 2. Schematic diagram of the Sede Boqer SSP.
Sede Boqer shallow pond project
279
Upper glazing
Fig. 3. Cross section of the SSP enclosure.
SEDE
BOQER-SSP
INSTALLATION
The construction of the SSP installation on the Sede Boqer campus was completed in April 1978. A detailed description of it has been published previously.12 A schematic diagram of a Sede Boqer SSP is shown in Fig. 2. The water pillows were constructed from an upper clear PVC film and a lower black PVC film. The entry manifold, exit port and air vent were made from CPVC tubing. A cross section of the SSP enclosure (see Fig. 3) shows the curbing construction and the method of attaching the upper glazing to the curbing by means of the glazing anchors. The upper glazing was either Tedlar (Du Pont) or Qualex (Poligal, Ramat Hashofet, Israel). The former is in the form of a 0.1 mm thick film, whereas the latter consists of two polycarbonate panels bridged together at approx. 5 mm intervals. The spacing between the two panels is a production variable. Upon completing the construction of the SSP installation, the individual SSPs were installed within their respective enclosures and preliminary testing was initiated. A number of ponds were observed to be leaking. The origins of the leaks were found to be both in the seams joining the upper and lower films and in the films themselves. We succeeded in stopping those leaks that could be pinpointed by means of a heavy-duty adhesive (3 M Company). A second and much more serious problem developed when the seam designed to hold the exit port (viz. the seam between the pond and the exit port sleeve) failed in a number of ponds. We believe that this was the result of improperly sealed seams. The problem was further aggravated by the high temperatures attained within the ponds. Upon close examination, it was observed that these seams simply unraveled. As a result of the above-described pond failures, the actual SSP study was limited to measuring the daily performance of four small module ponds 2 x 1.3 m in size. These ponds are listed in Table 1 together with their individual PVC film types,? glazing material and glazing angle. In theory, the shallow solar ponds are designed to operate in the following manner. The ponds are filled with water from a cold water reservoir at a preset time in the morning. This time is a function of the insolation rate, in that there is a threshold value below which heat collection is practically zero. The water remains within the pond until a maximum temperature (T,,,) is attained and is then transferred to a hot water storage reservoir prior to consumption. The transfer is performed when the rate of heat loss to the surroundings is greater than the rate of heat collection. This occurrence is indicated by observing a decrease in the average pond water temperature, viz. T(T + &) < T,,,(t). At Sede Boqer, we did not utilize the hot and cold water reservoirs associated with the SSPs. There were two reasons for this: (1) the SSP site was isolated from any possible tThe film types refer to PVC films produced by Haoganplast, Israel.
A.
280 Table
1. Shallow
KIJDISH
solar pond test modules;
the film thicknesses
are listed in
parentheses.
PVC films
1 SSP No.
Transparent (upper)
Black (lower)
Glazing
Glazing
Angle
i-
consumer and thus there was no need for storage; (2) the SSPs are designed to receive cold water in the morning and it was therefore desirable to cool the previous day’s solar-heated water so that it could be recycled. In order to accomplish this, it was decided to leave the solar-heated water in the ponds overnight so that the water temperature would be lowered by means of nocturnal cooling. The daily performance of an SSP is characterized by three factors: (1) maximum daily pond water temperature obtained, (2) total daily amount of thermal energy collected, and (3) the daily collection efficiency. The maximum daily pond water temperature is a measure of the quality of the thermal energy collected, whereas the latter two factors are a measure of the quantity of thermal energy collected. In practice, there is a competition between the qualitative and quantitative factors. The higher the pond water temperature, the greater the rate of heat loss to the surroundings as a result of the greater temperature gradient between the pond water and the surroundings. This heat transfer, in turn reduces the collection efficiency and ‘the total daily amount of thermal energy collected. The controlling parameter in this competition is the water depth or volume (since the pond area is constant) within the pond. The manipulation of this parameter may be of major importance, especially in periods of low insolation rates and it has been investigated during the course of this study. EXPERIMENTAL
STUDIES
The daily performance of individual SSPs was monitored from August 1978 until May 1979 by means of a copper-constantan thermocouple, which measured the pond water temperature. The thermocouple was inserted directly into the pond water via the pond air vent. The thermocouples were connected to a multi-channel chart recorder (Chino Works, Ltd., Japan), which converts the thermocouple mv signal directly into degrees centigrade. The recorder is of the continuous scan type and provides a trace of pond water temperature as a function of time. Such a trace, replotted on a shorter time scale, is shown in Fig. 4. The ambient temperature was continuously measured by means of a copper-constantan thermocouple, which was positioned within a meteorological screen. The thermocouple was also connected to the multichannel chart recorder. The insolation rate on a horizontal surface, at the Sede Boqer campus, was measured with a Kipp & Zonen solarimeter positioned on the roof of the one-story building housing the Energy Conversion Unit. The solarimeter was connected to a Kipp & Zonen digital integrator, which printed the accumulated daily insolation rate at one-half hour intervals. The solarimeter malfunctioned in April 1979 and, therefore, during the last 2 months of testing (i.e. April and May) we were unable to determine the collection efficiency because of the absence of insolation data.
RESULTS
A computer program was written to process the large amount of experimental data that accumulated daily from the SSP measurements. The data input to the program
Me
281
Boqer shallow pond project
3600
3200
2600: AZ "E 2400, 2
400
0
2
4
6
6
IO
12
14
16
16
20
22
0 24
TIME (hr) Fig. 4. The pond water temperature as a function of time.
included the hourly pond water temperatures, ambient temperatures and insolation rate. The simplification in the calculation implied by using hourly pond water temperatures was justified by the observation that, during such a time interval, the change in pond water temperature was approximately linear (see Fig. 4). In the vicinity of the temperature maxima and minima, the time interval was shortened. The program calculated the daily performance parameters for each SSP and the results were given in both tabular and graphical form. The results of our investigation are summarized in Table 2, which lists the maximum pond water temperature (T,,,), the pond water-temperature increase (AT), the quantity of thermal energy collected (Q), and the SSP collection efficiency (q). There were four major changes that took place during the course of this investigation: (1) the water depth in the SSPs was changed from 10 to 6 cm on 15 November 1978; (2) SSP-2 was put out of commission in December 1978 as the result of a leak that could not be repaired; (3) SSP-8 was successfully repaired and put into operation during December 1978; and (4) the water depth in SSP-5 was raised from 6 to 10 cm after sunset on 16 May 1979. The rate of thermal energy collection per unit pond area for the ith hour, qi, is given by
(1) where m is the water mass within the pond (kg) and is calculated from m=pLA;
(2)
p is the water density (kg/m3); L is the depth of the water in the SSP (m); A is the SSP surface area (2.4 m’); Cp represents the specific heat of water (4.19 KJ/kg”C); and AT/At is the hourly rate of change of the SSP water temperature (“C/hr). The SSP collection efficiency was calculated on the basis of the water-pillow surface area (viz. 2.4 m2) and not for the overall enclosure area. The use of the former results in considerably higher efficiency values, since the latter is much larger (4.38 m2). The SSP hourly collection efficiency is simply ‘li
=
4ilIi
(3)
A. KUDISH
282
Table 2. Monthly average daily performance of SSP modules. Month
a)
b)
c)
d)
e)
f)
9)
h)
i)
j)
k)
August
(Zld)
September
October
(17d)
(16d)
November
November
December
January
(8d)
(6d)
(8d)
(27d)
February
March
April
MY
SSP No.
L (cm)
2
10
60.9
24.9
10,419
43.3
5
64.2
27.4
11,377
47.2
7
69.1
28.4
11,298
49.3
2
10
54.2
21.6
8,946
47.0
5
54.3
22.0
9.020
45.9
7
57.7
22.6
9,275
49.0
2
10
46.7
17.5
7,138
46.5
5
47.4
18.3
7,461
48.1
7
49.7
18.0
7.344
47.6
2
10
34.5
13.8
5,479
46.9
S
34.8
14.2
5,620
46.3
7
37.7
14.4
5,714
46.9
2
6
35.6
20.0
5,079
36.8
5
38.1
21.8
5,450
39.4
7
39.3
21.3
5,329
38.9
5
6
33.6
16.8
4,209
38.1
7
36.1
16.7
4,172
37.7
8
34.2
16.4
4,100
37.0
5
6
34.9
21.1
4,848
41.3
7
35.2
18.9
4,372
36.5
8
34.4
19.0
4,360
36.4
44.3
26.2
6.082
42.8
7 (28d)
44.3
22.8
5,291
36.9
8 (28d)
43.3
23.0
5,362
36.8
5 (27d)
(Zld)
(29d)
(14d)
5
6
6
54.2
28.9
7,245
41.8
7
54.1
26.0
6,550
37.0
8
53.8
27.0
6,699
37.8
5
6
60.7
31.5
7,833
7
60.2
27.5
7,124
8
60.3
28.8
7,195
5
6
7 8 1)
May
(6d)
69.1
38.1
12,410
67.7
32.7
10,532
68.1
34.7
11,869
5
10
62.4
28.7
12,021
7
6
68.2
33.1
13,849
8
6
68.8
35.9
14,952
where I is the hourly insolation rate (kJ/m’-hr). The corresponding above two quantitative parameters are given by
daily values for the
(14 n
V =
I n
iJll 4ii C
Ii,
i=l
where n is the number of hours during the day that the SSP was in operation, viz. qi > 0. The SSPs generally ceased operation (i.e. qi d 0) in the early afternoon. This was a consequence of the high pond water temperatures attained relative to the ambient, which resulted in a heat loss rate to the surroundings equal or greater than the rate of thermal energy collection, The fact that both the insolation rate and the ambient temperature are
Sede Boqer shallow pond project
283
decreasing at this time contributes to the occurrence of an early afternoon termination of SSP operation. As a consequence, the SSPs are unable, in a batchwise mode of operation, to utilize the total amount of sunshine available.
DISCUSSION
OF
RESULTS
In this section, the effect of varying some of the basic SSP design parameters will be discussed first and then their overall performance and economic feasibility will be analyzed. A preliminary report on the results of this study was published recently.13 Upper glazing
SSPs 2 and 7 were identical in construction, except for the transparent material which served as their upper glazing (see Table 1). The former had a Tedlar and the latter a Qualex upper glazing. The performance of these two SSPs was monitored, for comparative purposes, from August 1978 until the latter part of November 1978, when SSP-2 ceased operation, as explained previously. It is apparent from Table 2 that SSP-7 outperformed SSP-2 during this period. Only during the first half of November 1978 did they have comparable performance records. It is also important to note that it is much easier to construct an upper glazing from Qualex than from Tedlar. Qualex is produced in the form of rigid but flexible panels, whereas Tedlar is a thin film and requires closely-spaced supports. These supports also reduce the quantity of solar energy impinging on the pond surface due to their shading of the pond. Pond PVC upperjlm
All of the SSPs were constructed from a single type of black PVC bottom film, H-65 (0.40 mm thick). Two types of clear PVC film were used for the upper water bag film: (1) L-06, a 0.30 mm thick film, somewhat milky in appearance and (2), S-l 50, a 0.18 mm thick window clear film. Due to the failure of six of the ten small module SPPS, it is not possible to arrive at a definite conclusion regarding the relative merits of the two types of upper PVC tested, since the module that was to provide these data failed. There are, nevertheless, two possible SSP pairs that may be used to compare the two types of upper PVC films utilized, viz. SSP-2/SSP-5 and SSP-7/SSP-8. The problem is that in each case there is an additional parameter change (see Table 1). For SSP-Z/SSP-5, they also differ in their upper glazing angle and, for SSP-7/SSP-8, they differ in the spacing between the two polycarbonate panels of the Qualex glazing. For SSP-2/SSP-5, even if it is assumed that the upper glazing angle does not significantly affect the performance of the SSP in the range studied, there still is no conclusive evidence for the superiority of one type of upper PVC film over the other (a more detailed explanation is given in the following section). For SSP-7/SSP-8, it is expected that SSP-7 with a 6 mm spacing between panels would possess superior insulative properties in the upward direction relative to SSP-8 with a 4mm spacing between panels. These two SSPs were monitored from December 1978 until May 1979. It is apparent from the monthly average performance data summarized in Table 2 that no perceptible differences in their performance characteristics were measured. It is conceivable that the thermal difference in the upper glazing compensates for the difference, if it exists, in the upper PVC film optical and thermal properties. Upper glazing angle
The SSPs were positioned horizontally on the ground with their long dimensions along the north-south axis and the curvature of the upper glazing along the east-west axis, i.e. the axis of symmetry of the glazing was along the north-south axis. Thus, a priori, the angle of the upper glazing should have a negligible influence, provided it is not extreme, on surface reflectance, since it is in a plane that is parallel to the direction of the solar rays. SSP-5 had an upper glazing angle of 30”, whereas the other SSPs (viz. 2,7, and 8 had an angle of 21”).
284
A. KUDISH
One of the purposes of this investigation was to determine whether there exists an optimum upper glazing angle, i.e. an angle which affords the least wind resistance but, at the same time, is sufficiently sloped such that sand and dust accumulation is minimal. The rate of heat loss from the upper glazing is mainly due to forced convection, which is proportional to the wind resistance. The natural removal from the upper glazing of sand and dust particles also depends on the sweeping action of the wind. The effect of upper glazing angle on SSP performance is best analyzed by comparing the data for SSPs 2 and 5, which were both covered with a Tedlar upper glazing. In this case, the comparison is somewhat difficult. It is observed in Table 2 that SSP-5 achieved higher temperatures, larger temperature increases, and collected more thermal energy than SSP-2 during the period August 1978 until November 1978. On the other hand, with regard to collection efficiency, there is no conclusive evidence which SSP was superior. Also, even for parameters for which SSP-5 outperformed SSP-2, the differences were quite small. Thus, it is believed that a change in the upper glazing angle in the range of 20-30” has no significant influence on SSP performance. Pond water depth
The depth of water within the pond influences strongly both the quality and quantity of the thermal energy collected. The pond water temperature is inversely proportional to its depth, whereas the amount of thermal energy collected and thus the collection efficiency are proportional to temperature. In essence, the quality of the thermal energy collected is in competition with the quantity. In practice, there is generally a minimum quality or temperature requirement and this will determine the quantity of thermal energy collected. These relationships were demonstrated when the water depth was changed from 10 to 6 cm on 15 November 1978. For all three SSPs (viz. 2, 5, and 7) the performance parameters changed as predicted when comparing performance data for November, before and after the fifteenth of the month. The average daily temperature maximum increased by only l.l-3.3”C, whereas the average daily temperature rise increased by 6.2-7.6”C, (i.e. by a factor of - 1.5). The quantitative parameters, on the other hand, decreased. The average daily quantity of thermal energy collected decreased by 170-400 KJ/m’, whereas the average daily efficiency was reduced by a factor of -0.8. On the evening of 16 May 1978, the water depth in SSP-5 was changed once again. This time, it was increased from 6 to 10 cm. A comparison of the May performance data for SSP-5, before and after the sixteenth of the month, shows that the average daily maximum water temperature decreased by 6.7”C and the average daily temperature increase was reduced by a factor of 0.75. In this case, there was also a decrease in the average daily quantity of thermal energy collected instead of the expected increase. Since no insolation data were available for the months of April and May, the collection efficiency could not be calculated and no plausible reason for this behavior can be suggested. The overall effect of pond water depth on SSP behavior is also a function of the quantity of insolation available and of the ambient temperature. The former determines the quantity of solar energy available for conversion, whereas the latter determines the rate of energy loss to the surroundings for a given pond water temperature. This relationship is expressed by the simplified form of the Hottel, Whillier, and Bliss (HWB) model,’ 4 viz. 4 = Z(za), - U(T-- T,).
(4)
In Eq. (4), I is the insolation rate (KJ/hr-m2), (za), is the effective transmittance-absorptance product, U is the overall heat loss coefficient (KJ/hr-m2-‘C), T is the pond water temperature (“C), and TOequals the ambient temperature (“C). Thus, the optimal seasonal water depth is set by the minimum water temperature required in conjunction with the average seasonal insolation rate and ambient temperature.
285
Sede Boqer shallow pond project
SSP solar collector parameters (m), and U A shallow solar pond is classified as a flat plate collector and its performance is characterized by its U and (ra), values. These two parameters were determined experimentally for the SSPs by means of the HWB model [Eq. 43, which is expressed in terms of the collection efficiency, r,r,by rj = q/l = (m), - U(T - T,)/Z.
(5)
A plot of the above linear equation, viz. q as a function of (T - T,)/I, yields the value for (74, and U as the intercept and negative of the slope, respectively. The value of the intercept, (XC),,is the theoretical maximum efficiency a collector can achieve under condition of zero heat loss. It is possible to determine (7~4, and U values by using the approach of Hewettr5. This approach equates Eq. (5) to the efficiency version of Eq. (l), viz. rj = p C, L(AvAt)/l
(6)
and one obtains, after dividing through by the factor p C, L(T - T,), u
(7)
-pc,L’
Equation (7) is valid for 24 hr providing T remains significantly greater than T,, whereas Eq. (5) is valid only when q > 0. A plot of (AT/At)/T - T, vs Z/(T - Tu) yields the U and (~a), values from its intercept and slope, i.e. - U/pC,L and (za),/pC,L, respectively. Hewett also used nocturnal cooling curves to determine U values. In this case, I = 0 and Eq. (2) reduced to -AT/At
= &(T
- T,).
P
A plot of -AT/At vs (T - T,) yields a curve with a slope U/pC,L and zero intercept. The (74, and U values for the four SSP modules investigated were determined on a monthly basis using all three of the above techniques, viz. Eqs. (5) (7), and (8). Data for a number of days (generally a sampling from the beginning, middle, and end of the month) were analyzed each month in order to obtain average monthly values. The individual daily @a), and U values were calculated by means of a least-squares analysis. The monthly values are summarized for each SSP separately in Tables 3-6. In Figs 5-7, are shown a typical plot of a set of experimental data and the corresponding least-squares fit curves. It is important to note that during the winter months, it was generally very difficult to obtain reasonable (za), and U values from Eq. (5). This problem may be due, in part, to intermittent cloudiness, but this is difficult to verify since the cloudiness index Table 3. Average monthly (XC),and U values for SSP-2. The numbers in parentheses in Tables 3-6 refer to the corresponding equations in the text.
Month
ll (KJ/hr-m2-"C)
(Ta)e (5)
(7)
August (kid)
0.75 + 0.06
0.68 f 0.04
40.4 f
(5) 4.9
29.3 f 1.9
(7)
31.3 f 3.1
(8)
September (7d)
0.75 ? 0.05
0.71 _+0.02
33.7 +
5.7
28.1 f 2.8
32.1 * 2.2
27.3 t 1.7
31.4 ? 2.3
October (6d)
0.79 f 0.10
0.67 f 0.03
48.7 _f 12.8
November (Sd)
0.65 f 0.13
0.77 f 0.15
45.7 ?
6.6
28.2 f 5.1
36.0 f 5.6
Average
0.74 f 0.06
0.71 t 0.05
42.0 +
6.5
28.2 + 0.8
32.7 ? 2.2
Overall Average
0.73
34.3
286
A.
KUDISH
Table 4. Average monthly (ra), and U values for SSP-5.
Month
IJ (KJ/h4-m2-'C)
(We (5)
(7)
August (15d)
0.79 * 0.05
0.73 + 0.04
36.9 + 4.1
30.6 f 3.0
32.9 ? 3.6
September (7d)
0.74 + 0.06
0.69 k 0.02
31.1 f 4.9
28.4 ? 1.0
32.3 f 2.0
(5)
(8)
(7)
October (6d)
0.71 * 0.04
0.64 + 0.03
34.8 i 5.7
26.5 ? 2.0
29.3 r 0
November (5d)
0.57 r 0.08
0.65 _+0.13
21.7 + 9.7
24.6 2 4.1
32.6 f 6.9
17.4 i 3.4
22.5 f 1.1
December (4d)
0.46 ? 0.02
January (Sd)
0.53 ?I0.03
18.4 * 4.7
23.4 ? 2.1
February (6d)
0.56 i 0.08
0.57 f 0.08
26.9 + 2.6
25.5 t 4.9
30.7 t 2.4
March (6d)
0.65 ? 0.04
0.64 f 0.02
22.9 + 0.4
24.4 + 3.6
25.6 i 1.9
Average
0.67 k 0.09
0.61 + 0.09
29.1 t 6.2
24.5 t 4.6
28.7 ? 4.3
Overall Average
0.64
27.4
Table 5. Average monthly Month
(ra),
and U values for SSP-7 II (KJfhr-mL-"c)
(Ta)a
I
(5)
(7)
August (15d)
0.88 ? 0.07
0.78 f 0.03
36.9 ?
5.0
27.2 ? 1.6
28.8 * 1.3
September (7d)
0.83 + 0.07
0.76 + 0.02
33.5 e
5.5
26.9 +_0.7
28.1 f 2.0
October (6d)
0.72 k 0.08
0.66 * 0.02
32.1 f 10.1
22.3 + 0.8
25.1 ? 0
November (5d)
0.58 + 0.09
0.71 i 0.14
20.7 f
8.9
23.9 + 5.1
25.9 * 5.5
December (4d)
0.53 + 0.04
0.53 f a.03
21.5 i
5.2
19.3 + 2.3
18.6 i 1.6
January (Sd)
0.52 ? 0.02
0.52 + 0.03
20.0 f
5.6
February (6d)
(5)
(7)
0.51 f 0.04
(8)
15.5 ? 3.2
18.8 + 1.6
15.6 f 3.6
19.8 ? 0.7
March (6d)
0.57 f 0.06
0.57 f 0.03
21.9 ?
6.9
20.9 ? 2.9
19.6 f 1.0
Average
0.66 f 0.15
0.63 f 0.11
26.7 f
7.2
21.5 ? 4.5
23.1 f 4.3
0.65
Overall Average
23.8
Table 6. Average monthly (ra), and U values for SSP-8.
Month
U (KJ/hr-m2-'C)
(TU), (5)
(7)
December (4d)
0.53 2 0.02
0.46 f 0.02
January (8d)
0.47
0.49 f 0.03
February (6d)
0.42
0.48 + 0.03
March (6d)
0.60 f 0.11
Average
0.51 f 0.08
Overall Average
(5)
16.5 +
3.8
18.6 r 2.2
15.0 f
2.8
19.2 f 1.5
23.8
17.9 i: 1.7
20.5 f 0.9
0.57 + 0.06
26.8 f 14
22.4 + 13.9
21.8 ? 1.1
0.50 + 0.05
24.8 +_ 1.7
17.9 +
20.0 + 1.4
0.50
23.8 e
(8)
(7) 4.9
20.9
3.2
287
Sede Boqer shallow pond project
0.?-
\ r
01
0.5-
TO 05
0.2-
C).I-
(T-Tn)/I(m’-hr Fig. 5. The r] vs (T-
- “C/KJ)
T&4 curve for SSP-2 on 31 July 1978.
was not recorded at Sede Boqer. A number of trends and qualitative comparisons are apparent from the data on the four SW modules studied. (a) The (zc(), and U values for SSPs 5 and 7, which were the only SSPs continuously monitored throughout this study, exhibited minimum values during the winter months.
50
100 AA
Fig. 6. The ratio (-AT/At)/(T
150
, 200
(K J/mchr-Y)
- T.) vs I/(T - TO)curve for SSP-2 on 31 July 1978.
A.
288
KLJDISH
-( -+=$q&
3
r’
T-Tn)
U=36BKJ/mchr-“C
2
t e a!I I
I
io
2b
30
T-TA(‘C)
Fig. 7. The ratio (-AT/At) vs (T-
T,) curve for SSP-2 on 31 July 1978.
The reduction in (rtl), is expected as a result of the relatively high winter incidence angles on the upper glazing, especially since the SSPs are essentially horizontal flat plate collectors. The decrease in U values is most likely the result of the lower wind velocities prevailing at Sede Boqer during the winter. (b) There appears to be no significant difference between Tedlar and Qualex with regard to their (ra), values (see Tables 4 and 5). The monthly and overall average (Ta), values for SSPs 5 and 7 are similar. (c) There is a definite trend of lower U values for the Qualex covered SSPs (7 and 8) than for the Tedlar covered SSPs. These differences may be explained by the added insulation the air pockets in the Qualex glazing contribute, which reduce the heat loss rate in the upward direction. The (Ta), values referred to in this section include, in addition to the upper glazing (ta) values, also the (za) contribution of the clear upper PVC pond film. Thus, they are truly effective values. Based upon this analysis, it seems reasonable to ascribe to the Sede Boqer SSPmodules, average (za), and U values of 0.65 and 25 kJ/h4-m2-“C, respectively. These values are in the range of (za), and U values that characterize commercially available flat plate collectors with multiple glazings. Thermal energy collection The overall SSP efficiency would have been significantly greater had the solar heated
pond water been removed at the end of each day and the pond been filled with fresh, relatively cool water in the morning. As a result of the relatively high initial water temperatures, meaningful thermal energy collection (q > 0) would be expected to begin relatively late in the morning and to terminate relatively early in the afternoon. The monthly average daily number of hours of thermal energy collection by the SSPs is summarized in Table 7. The two preceding predictions are validated by the data in Table 7. Also recorded in the table are the monthly average values for the length of the solar day (T,) and the ratio of solar hours utilized for thermal energy collection (At/&). It is observed that this ratio varies between 58 and 65%. The efficiency of utilizing the number of solar hours available is in practice better than these values suggest. The hours during which the SSP does not collect thermal energy have relatively low insolation rates and therefore contribute less. It is also possible to increase the number of hours of thermal energy collection by operating the SSP in a flow mode, i.e. by circulating water through the SSP and thereby
289
Sede Boqer shallow pond project Table 7. Monthly average daily number of hours of thermal energy collected by SSPs. t.
SSP No.
Month
December
January
February
March
(a)
ti
8.50
15.21
8.57
6.80
15.21
8.41
6.94
14.59
7.65
6.88
14.55
7.67
7.00
14.76
7.76
2
7.00
13.92
6.92
5
7.04
14.08
7.04
7
7.18
14.18
7.00
2
7.14
13.93
6.79
5
7.14
14.00
6.86
7
7.21
14.07
6.86
5
8.00
14.14
6.14
7
8.00
14.00
6.00
a
8.00
14.00
6.00
5
7.68
14.00
6.35
7
7.88
14.23
6.35
8
7.81
14.12
6.31
5
7.22
14.12
6.90
7
7.61
14.50
6.81
8
7.46
14.54
7.08
5
7.00
14.05
7.05
7
7.09
14.32
7.23
8
7.05
14.18
7.13
6.64
14.14
7.50
6.79
14.43
7.64
6.83
14.50
7.67
6.14
14.14
8.00
6.36
14.05
7.69
6.18
14.00
7.82
- beginning (c)
of
thermal
At = tf
- ti;
energy (d)
collection;
Td = (Z/15)
d
Ate
15.18
Apri 1
collection;
P
6.64
September
November
t
6.68
August
October
a 1
(b) cos-’
Td
tf
At/T,
13.13
0.65
12.18
0.63
11.22
0.62
10.39
0.66
10.01
0.60
10.20
0.62
10.91
0.64
11.77
0.61
12.76
0.60
13.57
0.58
-
end of
thermal
(-tan+
tans),
length
energy of
solar
day.
increasing the volume of water heated per unit area of SSP. The overall effect of this mode of operation is similar to that produced by increasing the pond water depth. Economic analysis It is possible to approximate the total yearly amount of thermal energy that an SSP situated at Sede Boqer (or comparable site) is capable of collecting under normal operating conditions (viz. removing solar-heated water in the evening and filling with fresh water in the morning) in the following manner: (a) Multiply the monthly average daily quantity of thermal energy collected by the number of days in the months. (b) Assume that the monthly average daily quantity of thermal energy collected during June and July can be approximated by the average of the May and August values. This assumption is, in all likelihood, conservative. (c) Assume that the positive error introduced by step (a), which neglects days during which thermal energy collection is practically zero due to rain or heavily overcast skies, is counterbalanced by the understatement caused by the experimental procedure. Thus, under standard operating conditions (filling and emptying each day), the SSP performance would improve significantly over that measured in this study. EGY Vol. 6. No. WZi
290
A. KUDISH Table 8. Yearly total of thermal energy collected per unit area by an SSP at Sede Boqer. Month
L(Cd
QWJ/&
January
6
140
February
6
156
March
6
216
April
6
229
bY
10
373
JlLW
10
347
July
10
359
August
10
351
September
10
274
October
10
229
November
6
162
December
6
129
Total
2965
These calculations have been performed using average monthly values for the thermal energy collected by the SSPs and are reported in Table 8. The yearly total amount of thermal energy that an SSP is capable of collecting, based on these assumptions, is - 3 GJ/m2-yr. This value compares favorably with that reported by the LLL group,4 viz. 2.8 GJ/m2-yr. The annual energy equivalent per 100 m2 of shallow solar ponds providing 3 GJ/m’-yr is approx. either 83.3 MWh of electricity, 43 barrels of oil, or 6.51 x lo3 kg of natural gas (assuming an energy content of 46,000 kJ/kg). Using these values, we may determine upper limits on the cost per unit area of SSP construction for which such a system is economically feasible. It is assumed that the auxiliary system is identical to the alternative conventional system, since it must be capable of supplying 100’~ of the energy requirements during periods of consecutive non-solar winter days. Thus, the break-even point for the cost of the SSP system is given by Css, x 1 = CF QF>
(9)
where C,, is the cost of the SSP system, I is the interest and amortization factor, CF equals the cost of the alternate energy source, and QF is the quantity of the alternate energy source required. The upper limit on the cost per unit area of SSP installed for which the system is economically competitive is, of course, a function of the alternate energy source available. The economic feasibility of the SSP energy-conversion system will be analyzed with respect to oil (heavy fraction, restricted to industrial usage), natural gas and electricity. Electricity is generally utilized only for domestic hot water. We apply Eq. (9) to a 100 m2 SSP installation capable of supplying 3 GJ/m2-yr and compare it to each of the above alternate energy sources in turn. This yearly energy production per 1OOm’ SSP is approx. equivalent to either 43 bbl of oil, 6.51 x lo3 kg of natural gas or 83.3 MWh electricity. The prices quoted are local (Israel) rates for Sept. 1979. (a) Oil (heavy fraction): CF = $20/bbl; C,,, = ($8.60/m2)/Z; Css, - $58/m2. (b) Natural C,,, x I = (0.67 x 6.51 x 103)/100; gas : CF = $0.67/kg; Css, = ($43.62/m2)/Z; (c) Electricity: C, = $60/MWh; C,, x I = (60 x 83.3)/100; Cssp - $293/m’. C,,, = ($49.98/m2)/Z; C,, - $335/m2. The value for the Cssp, the cost per unit area of the SSP, is strongly dependent on the prevailing interest rate, as well as the SSP life-time. The water bag is the component with
Sede Boqer shallow pond project
291
the shortest life-time (3-5 yr), but the other components should all have life-times in the range of 10-20 yr. Here, we have assumed an interest rate of 8% and an average overall lifetime for all of the components of 10 yr. Thus, a value of I = 0.149 has been used in the analysis. It is observed, as expected, that the SSP system is least competitive with respect to oil. It could compete with natural gas and electricity if the technical problems associated with its construction (proper materials, etc.) can be solved. Another approach for determining the economic feasibility of such a system is its payback period. If we use the preceding values for energy production per unit area of SSP and cost per bbl of oil, it follows that each m2 of SSP supplies the equivalent of 0.43 bbl of oil per year (or $8.60 worth of energy). Thus, if the cost per m2 of SSP can be reduced to $86, which is a reasonable cost, the payback period becomes on the order of 10 yr or the lifetime of the system. The preceding analyses were performed without regard to the ever-increasing costs of conventional energy sources and their availability. Needless to say, the prognosis for the economic feasibility of the SSP improves as the costs for conventional fuels increase and their availability diminishes.
CONCLUSIONS
Our conclusions are briefly summarized below. (1) Qualex is preferred over Tedlar for use as an upper glazing. It exhibited superior performance characteristics and is much easier .o construct. It has a two-fold advantage with regard to construction. First, it is much e:lsier to work outdoors with a rigid panel than with a thin film, because of wind interference. Second, Tedlar, being a thin film, requires additional supports, which reduce the surface area of the pond exposed to solar radiation as the result of shading. It is still necessary to conduct aging tests on Qualex to determine the degree and rate of degradation it undergoes. (2) No definite conclusion regarding the relative merits of the two types of upper PVC films tested can be reached, since the module that was to provide these data failed. (3) A change in the upper glazing angle in the range of 20-30” has no significant influence of SSP performance in our case. This is the result of the fact that the Sede Boqer SSPs were positioned horizontally on the ground with their long dimension along the north-south axis and the curvature of the upper glazing along the east-west axis (i.e. the axis of symmetry of the glazing was along the north-south axis). (4) The pond water depth determines the quantity and quality of thermal energy collected and should be varied to obtain optimal seasonal performance. The optimum seasonal depth is defined by the minimum water temperature required in conjunction with the average seasonal insolation rate and ambient temperature. (5) The SSP solar collection parameters (ZLY),and I/ were found to be on a yearly average approx. 0.65 and 25 kJ/hr-m2-‘C, respectively. These values are similar to those characterizing most commercially available flat plate solar collectors with multiple glazings. (6) The overall SSP thermal energy-collection efficiency may be significantly improved over that measured for the Sede Boqer SSP modules, under normal operating conditions, by removing the solar heated water every evening and filling the pond with relatively cool water each morning. (7) The economic analysis has shown that the SSP is not now competitive with oil (heavy fraction), whereas it may be competitive with natural gas and electricity when the technical and material problems associated with it are overcome. (8) The payback period for the SSP system supplying 3 GJ/m2-yr and competing with oil is on the order of 10 yr if the cost per m2 of SSP can be reduced to $86. It is important to note that the energy conversion capacity per m2 of SSP can be increased significantly by using reflectors.’ I*16 An economic analysis on such a system has not been performed since experimental data on a yearly performance basis are not yet available. The LLL group has recently published a “Design Guide for Shallow Solar
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Ponds”, which is recommended to all those interested in this topic.4 It is believed that the prognosis for the economic feasibility of an SSP system will continue to improve as the cost for conventional fuels increases and their availability diminishes. Acknowledgements-The author would like to thank the Ministry of Commerce, Industry and Tourism for making this project possible through their funding. I wish to express my gratitude to Drs. W. C. Dickinson, A. F. Clark and the LLL group for their unselfish exchange of information and Professors A. Roy and D. Faiman of Ben-Gurion University of the Negev for their advice and suggestions during the course of this investigation. I also thank Mr. D. Pinto, Mr. Y. Machlav, and Ms. G. Loutaty for their aid in performing the experiments and correlating the data.
REFERENCES 1. W. C. Dickinson, A. F. Clark, J. A. Day, and L. F. Wouters, Solar Energy 18, 3 (1976). 2. W. C. Dickinson, A. F. Clark, and A. Iantuono, “Shallow Solar Ponds for Industrial Process Heat: The ERDA-SOHIO Project”, Lawrence Livermore Laboratory, University of California, Rep. LiCRL-78288, Livermore, California (1976). 3. Bohanan Westman Huston & Associates, Inc., “Solar Pond Facility, Bibo, New Mexico”, Lawrence Livermore Laboratory, University of California, Rep. UCRL-13680, Livermore, California (1977). 4. A. B. Casamajor and R. E. Parsons, “Design Guide for Shallow Solar Ponds”, Lawerence Livermore Laboratory, University of California, Rep. llCRL-52385 Rev. 1, Livermore, California (1979). 5. A. B. Meinel and M. P. Meinel, Applied Solar Energy. Addison-Wesley, Reading, Mass. (1976). 6. M. L. Khanna, Solar Energy 15, 269 (1973). 7. W. B. Harris, R. R. Davison, and D. W. Hood, Solar Energy 9, 193 (1965). 8. W. H. Gopffarth, R. R. Davison, W. B. Harris, and M. J. Baird, Solar Energy 12, 183 (1968). 9. A. Whillier, Solar Energy 7, 148 (1963). 10. D. P. Grimmer and S. W. Moore, “Practical Aspects of Solar Heating: A Review of Materials Used in Solar Heating Applications”, Los Alamos Scientific Laboratory, University of California, ,!,A-LIR-1752, Los Alamos, New Mexico (1975). 11. A. I. Kudish and D. Wolf, Solar Energy 21, 317 (1978). 12. A. I. Kudish, Y. Raziel and A. Roy, “Sede Boqer Shallow Solar Pond Project”, Report submitted to the Ministry of Commerce and Industry (Project No. 76/49/A), Israel (1977). 13. A. I. Kudish and A. Roy, “Sede Boqer-Shallow Solar Ponds Project”, Proc. 14th Intersociety Energy Conversion Engineering ConjI, Boston, Mass., Aug. 1979. 14. J. A. Duffie and W. A. Beckman, Solar Energy Thermal Processes. Wiley, New York (1974). 15. L. D. Hewett,“‘The January-March Performance of Shallow Solar Ponds with and without Reflectors on their North Side”, Lawrence Livermore Laboratory, University of California, Rep. LICID-17-663, Livermore, California (1977). 16. A. F. Clark, “A Horizontal Collector with a Mirror on the North Side”, Lawrence Livermore Laboratory, University of California, Rep. UCRL-77906, Livermore, California (1976).