A status report on a solar pond project

1965 Conference Paper A

Status

Report o n

a

Solar Pond Project H. T a b o r

R. M a t z

National Physical Institute of Israel

Negev Institute for Arid Zone Research, Beersheva, Israel

SHORT report ~ was given in 1959 on the first experiments conducted in Israel on solar ponds. The tempo of the research, initially rather slow, was increased in 1961 and a further account 2 was given at the U N Conference on New Sources of Energy in Rome in that year. In that account the basic problems associated with solar ponds were delineated and indications of the possible solutions given. Recently, a general account 3 of the potential of solar ponds as large-area collectors to provide low temperature heat was published indicating the likely economics and some of the more promising applications. The present paper gives an account of some of the more important investigations that have been carried out between 1959 and 1964, the results obtained, and the guide-lines for future work. Some of the problems that have been studied are: a--stability vis-a-vis temperature and density gradients; b - - e n e r g y balance; c--lower-layer flow; d--surface-layer flow; e--disturbing influences; f--diffusion rates; g--cleanliness (transparency); and h - - s a l t control. Stability and energy balance have been analysed mathematically by Weinberger 4. He has shown that density gradients readily obtainable in practice by using concentrated salt solutions--such as MgC12 or N a C l - - a t the bottom of the pond and very weak solutions at the top will provide the necessary stability against convection for a black-bottomed pond heated by solar radiation. For example, if a nearly saturated solution of MgC12 (p = 1.33) is used at the bottom, the pond will be stable up to the point at which it would start to boil (c. 117 deg C): in the case of NaC1 the maximum obtainable density is c. 1.22; this will still ensure no convection up to the boiling point provided that there is a mixed layer at the bottom of the pond. This in fact occurs with the heat-extraction technique proposed*. (The depth of the pond does not come into the dis-

cussion here inasmuch as the condition for stability is a function of temperature and density gradients.) Weinberger has also indicated the collection efficiency of the pond which, of course, depends upon the depth, the cleanliness and the temperature of withdrawal. A typical figure for a pond 100 cm deep of "No. 3 sea water" is about 25 percent (withdrawal temperature about 70 deg C). In the experimental ponds described below the transparency has been much worse than for No. 3 sea water so that lower collection efficiencies were obtained. Lower-layer flow--It is not practical to extract the heat from the bottom of the pond by means of an array of pipes constituting a heat exchanger, particularly as we are considering ponds of dimensions of the order of a square kin. An interesting possibility is the decantation of the lower layer. I t is known from hydrodynamic theory that, where a vertical density gradient exists, horizontal flow of a layer is possible without causing disturbances to layers above or below the flowing layer. Examples in nature are the flow of sea water many miles up the estuary of rivers without serious mixing with the fresh water of the river, and the flow of the Gulf Stream across the Atlantic. A study was therefore carried out in the Hydraulics Laboratory of the Technion in Haifa, under the direction of C. Elata, on the question of layer extraction from the bottom of a tank containing a salt solution with a known density gradient. (There was no superimposed temperature gradient ill these experiments because the technical difficulties of maintaining such a gradient in a laboratory experiment are very great. However, this is not essential for this study.) The results 5, which will shortly be published in English, show that stable horizontal flow of a layer at the bottom of a pond is possible and can be made to extend over any length of pond if an adequate hydraulic gradient is provided. As an example of typical numbers, for a length of 500 meters and a density gradient of 0.0033 gins per cm a, and an initial aperture height* of 15 cms, stable layer flow occurs at a rate

A

Presented at the Solar Energy Society Conference, Phoenix, Arizona, March 15-17, 1965. * This mixed layer is required to prevent a local inversion of the density gradient at the bottom that can occur during intense (noontime) periods of insolation. This phenomenon, which we term "sunstroke", will be discussed in a later report.

Vol. 9, No. 4, i965

* i.e. The vertical dimension of the inlet part at the bottom of the pond. 177

of 1.5 liters per sec per meter width (which means that the collected heat can be withdrawn with a temperature drop of about 8 deg C in the external heat exchanger.) If the density gradient is say, half that given above, the initial aperture height is increased by the factor ~/2, i.e. becomes 18.9 cm for the same flow conditions. No experiments have yet been conducted on heat extraction in an experimental pond (for reasons discussed below) but the principle has been confirmed. I t has been demonstrated in the field that a complete horizontal slice can be taken out of a pond without disturbing the liquid above or below the slice provided the necessary density gradient is present. This is an extremely valuable practical matter for it can be used to "correct" a pond, i.e. to repair a gradient that may have been disturbed for some reason, or to remove dirt of a particular density that is floating at one level of the pond. Surface-layer flow--In almost every practical application of a solar pond, it is necessary to stream a weak solution across the surface in order to maintain concentration equilibrium conditions. For example, in the simplest case where salt diffuses upwards from the bottom to the top of the pond, in addition to adding salt--or concentrated salt solution at the bottom, it is necessary to wash the surface. When the pond is used to produce salt from brines 6 a similar washing is effected but the effluent--which is richer in salt than the influent due to diffusion and surface evaporation--is not wasted but is used in the process. Experiments on surface-layer flow, like those on lower-layer flow, showed that, for the conditions likely to occur in practice, surface washing would be fully practical up to distances of several hundred meters. Disturbing infiuences--A major disturbance to the stability of a pond could be caused by waves. Elata's group also studied this question in a specially constructed wave channel, 25 meters long, 1.20 m deep and 0.60 m wide. The results, unfortunately, were not conclusive as regards naturally-produced waves but indicated reasonable mixed layer depths. The waves were produced by a plunger at one end of the tank and dissipated on a spending beach at the other end. (To produce waves by wind would have involved an extremely expensive set-up.) Mixing produced by the plunger and by the spending beach was eliminated by creating a test section in the central region of the flume separated by thin plastic membranes. I t is to be noted that wind-induced waves involve a shear that can induce reverse currents in a pond. If these currents are far below the surface, some mixing may result (although, by comparison with lower-layer flow results, this effect could be expected to be small) but the largest velocities would occur near the surface (i.e. 178

in the mixed layer) where they would not cause additional mixing. The plunger-induced waves produced a mixing zone in the upper portion of solution (which had a density gradient of 0.002 gm per cm 4) which spread downwards and appeared to reach an asymptotic depth. For a wave amplitude (crest to trough) of 2 cm, the mixing zone was about 20 cm deep. (To keep wave amplitudes down to these levels would require wind breaks and wave breaks at 50-100 meter intervals). A mixed layer at the top of a pond can have a considerable effect on pond efficiency. Not only is the conductive insulation of the layer lost if convection occurs, but also the solar radiation absorbed in the layer*--and which gives a pseudo insulating effect in a non-convecting layer--is lost. A calculation by Weinberger showed that, for a sample case of a pond used to drive a Carnot engine, a 20-cm mixed zone at the top of a 120-cm deep pond would cause a loss of output of about 30 percent. I t is interesting, and to a degree encouraging, to note that in the second experimental pond in Sdom (25 by 25 meters which had no wind or wave breaks) a mixed zone always appeared at the top but seemed to reach an asymptotic depth of 20 cm. A crude measurement on site gave the maximum wave amplitude as 2 c m .

Diffusion rates--Because the diffusion rates of many of the salts to be considered, in the concentrations and temperatures anticipated, were not available in the literature, these had to be measured. Cleanliness (transparency)--If a solar pond were used to produce low4emperature heat, particularly for power production, then, as shown by Weinberger, the pond efficiency would be sensitive to pond clarity. Where the pond is used for salt production, clarity is of somewhat lesser importance. The first experimental pond in Sdom suggested that biological pollution--at one time considered as likely to prove troublesome--would not be serious. The second source of poor transparency considered, i.e., the settling of dust of intermediate density at corresponding levels in the pond, could give trouble in small ponds but is not expected to be serious in large ponds since it is primarily a boundary effect. Furthermore, as indicated above, a dirt layer at a given density level could, if necessary, be removed from an operative pond. The third source of poor clarity is in dirt and other suspended particles introduced with the solutions. The third experimental pond, discussed below, was particularly bad in this respect. However, it is believed that for large ponds built under normal conditions the clarity will be satisfactory as experience at various salt works and at the Dead Sea works, where * The infrared radiation, which constitutes about half of solar radiation, is nearly all absorbed in the top few centimeters of any mass of waterA

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there are many square km of evaporation ponds, shows that these are remarkably clear. Salt c o n t r o l - - I n the normal conception of a solar pond, diffusion carries salt from the lower, more concentrated, layers to the upper more dilute layers from which it must be removed by washing the surface. In a typical case this diffusion amounts to about 6 mg per sq cm per day or 60 tons per sq km per day. Where concentrated brines are not available as a waste product this diffusion of salt could render the economics of the pond as a heat co]lector prohibitive. An interesting solution to this problem has been found in the "falling pond". Heat from the pond is removed by the decantation of the bottom layer and flashing it in a flash evaporator. (The vapor can be used to drive a turbine or heat another fluid). The brine remaining after the flash contains all the salt but less water and is, therefore, more concentrated than before. It is returned to the bottom to be reheated. At the same time fresh water (sea water can also be used) is added to the top. Since water is added to the top and removed at the bottom, the pond is said to be "falling", the water traveling down whilst the salt travels up through the water: if the rates are correctly adjusted the salt actually remains stationary in space and the pond can be operated without need to add any salt after the initial fil]ing. If pure water were added at the surface, it would make up for the fall and for evaporation without the need for washing. As, in practice, the water added will contain some salts, some is allowed to overflow, thereby bringing the surface to an equilibrium salt concentration. Calculations show that the rate of fall needed to balance the diffusion is a fraction of a m m per day, whereas to remove all the collected heat by flashing calls for a rate of fall of about two m m per day. This apparent conflict of requirements is solved in practice by returning most of the condensate obtained from the flash (or an equivalent alternative source of water) together with the brine being returned to the pond. Hence vis-a-vis the pond, the rate of fall is a fraction of a mm whereas vis-a-vis the flashevaporator a t w o - m m layer of the pond has been evaporated. The picture is somewhat complicated by evaporation from the surface that can be as high as 10 mm per day*. This has to be made up by adding water at the top. In practice this brings additional salts into the pond. The equilibrium concentration at the top of the pond is thus a function of the rate of surface-water addition, the rate of overflow, the diffusion rate and the rate of fall. I t follows t h a t with two variables under our control, i.e., the degree of excess washing and the rate of fall, * Measured evaporation rate from the pond surface at Atlith, June/July 1964, was about 5.5 mm per day. Vol. 9, No. ~,, 1965

it is. possible to regulate the equilibrium density graclients at the top and at the bottom. The net transfer of salts can be upwards, downwards, or zero. A new possible application emerges from operation of a falling pond when saline water is available, namely the production of salts in the final flashing operation. Experimental Ponds

Apart from a number of laboratory tank experiments carried out in the early stages and two very small field ponds of a few square meters area, three moderate size field experiments have been set up. These are referred to as No. 1 (Sdom), No. 2 (Sdom), and No. 3 (Atlith). The first pond, by the shores of the Dead Sea at Sdom, 25 by 25 meters by 1 m deep was initiated in 1959 and has already been reported 2. The method of filling was to lay down a layer of concentrated brine about 5-10 cm thick, then to add another layer less concentrated and so on. By the time the filling was completed the density steps had disappeared and a substantially smooth linear gradient obtained. A peak temperature of 96 deg C was recorded. This was extremely encouraging and the experiments were discontinued because of decay of the walls (the pond was one of about twenty that had been built 25 years earlier for evaporation experiments). The second Sdom pond was adjacent to and similar to the first except that steps were taken to reinforce the walls before filling. End brines from potash production, which comprised mainly MgC12 and KC1, were used as the concentrated solution with a rather saline well-water as the diluent. This pond was also not ideal principally because of a porous bottom, resulting in a loss or gain of liquid (and hence of heat) at the bottom of the pond dependent upon the hydrostatic relationship of surrounding ponds. The pond was in operation between June and December 1962 when it was decided to let the pond "die" and observe changes of temperature and density with time. I t was clear that a fresh pond would have to be built from scratch, despite the cost, if there were to be complete control over it. Nevertheless, many useful things were learned. Analysis of pond layers at 5-cm intervals over the depth of the gradient taken at two periods a month apart permitted the salt diffusion coefficient to be measured. This coefficient for all the salts together over a range of 30 to 60 deg C came out as 2.5 X 10-5 sq cm per sec. This compared with laboratory experiments on single MgCl~ and KC1 salts up to 60 deg C giving diffusion coefficients ranging from 2 X 10-5 to 4 X 10-5 sq cm per sec. Chemical analyses over a 196-day period permitted a materials balance to be conducted that showed how 179

TABLE Z--RADIATION REACHING VARIOUS DEPTHS Time of Depth in Measurement . _ _ P ° n d ' cm

Radiometer Reading, gcal/cm 2 sec

Surface Radiation. Eppley, gcal/cm~ sec

Surface Incident Radiation Reaching Given Depth, %

1.36 1.33 1.32

59.7 58.4 53.4

100 -

80

Z O

12.00 12.20 12.30

~ /

30

/

0.81 0. 719 0.704

12.50 13.00

40 50 60

0. 651 0. (')05 0.517

1.31

13.30 13.40

70 80

0.433 0.324

1.23

12.40

1.30 1.27

1.18

49.9 46.7 40.6

60

z

~ 4o - --.-._.__ 20

35.3 27.6 I

20

nmch solution was lost from the bottom by percolation and how nmch water was evaporated from the surface. These figures also made it possible to calculate the heat lost by percolation and by evaporation. The former was 14,800 Kcals per day, the latter 1,205,000 kcals per day. These two items together represented about 39 percent of the total incident solar energy. A coinforting fact is that although the percolation rate of 0.65 mm per day was larger than one should expect from a well-constructed p o n d - - a n d the salt loss in certain cases may be i m p o r t a n t - - t h e heat loss due to this percolation constituted less than 0.5 percent of the incident radiation. (This suggests that, where the salt loss--about 0.25 kg per sq m per day in this case--is not ilnportant as in a salt-making process, there is no need to go to excessive lengths to get the pond bottom completely impermeable.) An under-water radiometer was constructed to deternfinc radiation reaching any level in the pond. The measurements were made on 27 June 1963 between 12.00 and 13.40 hours. The results are given in Table I. The transmission values are shown in Fig. 1, which also shows Weinberger's calculated transmission values for mid-day at Sdom* for water sinfilar to "sea water No. 3 ''2. The results show that the pond solution was remarkably clear, being not much different fronl the sea-water No. 3, and this despite the dirt that could be expected to creep in in a small scale experiment in an old pan. This result is of particular significance in view of the subsequent disappointing results on pond No. 3. At this stage a decision was taken to construct a new pond under controlled conditions and fully instrumented. Because of the difficulties in finding a suitable site in the Dead Sea area (prospecting had shown that most of the available sites had underground water flow which could disturb the energy balance calculations by unknown anlounts) it was decided to build the third pond at Atlith near Half a, on the grounds of the Atlith Salt Works and with the kind permission of the Atlith Salt Company. Two other reasons These are taken from Fig. 5, Ref. 2, with the energy values expressed as percentages of incident energy. 180

I

I

J

40 60 POND DEPTH, CMS.

I

L

80

100

FIG. 1--Calculated transmission values for mid-day at Sdom. prompted the selection of the new site. One, the climatological conditions at Atlith were far more representative of other likely places in the world for solar ponds, as compared with the Mmost unique character of Sdom, 370 meters below sea level. Two, the growing recognition that a major application for solar ponds would be in salt making. The major aim of this pond would be to provide an accurate energy bMance and to test out the heat-extraction technique. Pond No. 3 was 25 by 55 meters by 1.50 meters deep and was constructed at ground level by building up the walls. I t was constructed on a material clay base varying in thickness between ½ to 1 meter, and overlying a salt mamh. A layer of 60 cm of clay soil was compacted over this to 90 percent Proctor conlpaction. The pond walls were sinfilarily compacted clay soil retained behind a wooden barrier. (See Fig. 2). It was assumed that by compacting the bottom, percolation could be reduced to negligible proportions and the expensive alternative of lining the pond with an impermeable butyl-rubber membrane was avoided. I n fact, subsequent measurements showed the percolation rate to be initially negligible, though later measurements showed percolation rates of 1.36 mm per day*, * The pond was filled in November 1963, with a brine of mean specific gravity 1.15. During a 40-day period covering November and December, a salt material balance for the contents showed a loss of 0.9 tons of salts. Since the loss of salts could have occurred only by physical percolation of the bottom solution through the pond base, this loss represented a daily percolatiou rate of 0.08 mm per day. Later measurements of pond level, made during January and February 1964, with a micronmter depth gauge in a stilling pot, showed a net drop in level of 1.65 mm per day, correcting for the 285 mm of rain that fell during the period. It may be assumed that--with negligible evaporation--this level change was attributable entirely to percolation. A further calculation of percolation made in July-August 1964, and based on a salt material balance, gave a rate of 1.36 mm per day. The sensible heat lost by percolation is approximately equivalent to 0.96 Kcals/(mm/day) (m2. ) (°C) ; consequently, for a bottom temperature of 70°C and ambient 25° (AT = 45°C), and a mean total incident radiation of 700 cals/cm2 day (June-July 19(')4at Atlith) the heat lost by percolation is less than 1 percent of incident for the above percolation rate. The salt loss is considerably more serious and for a bottom solution density of 1.2 gm per liter is of the order of 0.38 Kgm per m ~ day. Solar Energy

LIFTING M E C H A N I S M

~

~

i

~

k

SURFACE L A Y E R ~ H A N N E ~ m _

///

c(~r~pao,r~y.~['~O I'N~ERMEDIATE LAYER PIPE BOTTO

P,PE

N A T U R A L EARTH BASE

FIG. 2--Cross-section of solar pond. due to a deterioration of the coral)acting layer by the salt water. It is believed that had the original compaction been with salt water instead of fresh, this deterioration would not have occurred. Extensive instrumentation was incorporated in the design. This included buried thermocouples in the ground and the walls to permit computation of temperature gradients and heat flows; fixed-glass sampling pipettes for withdrawing samples of solution at various depths; thermocouples at various depths of the solution; radiometers for measuring solar radiation at the surface and at various depths; evaporation pans for estimating local open-surface evaporation rates; windvelocity indicator and a meteorological station. Pumps were installed to permit solutions to be added to or withdrawn from the pond. Also fixed buried horizontal perforated pipes at the two (short) ends for bottom layer flow were installed, as were adjustable horizontal perforated pipes for layer extraction and horizontal spillways for surface flushing. The pond was ready for filling in April 1964. The brines available for filling were brought from end-brine pans of the salt works but arrived dirty, and, although a pressure sand filter was installed, a really clear solution was not obtained. Attempts to coagulate and precipitate the dirt were also unsuccessful. It had been hoped that the pond would heat up as predicted and that, at say a bottom temperature of 90 deg C, heat would be extracted by lower layer flow. But there was a surprise in store. At a bottom temperature of 65 deg C bubbles began to appear and by 74 deg C these bubbles had become quite serious. These bubbles had three effects: they caused a disturbance to the density gradient; they prevented the original dirt in the pond from settling (prior to the bubbling the pond was becoming noticeably clearer); they added to the dirt by bringing up nmd from the bottom of the pond. When it was realized that no useful energy-balance studies under operating conditions could be carried out on such an unrepresentative pond, it was decided to empty the pond and determine the source of the bubbles. Vol. 9, No. ~,, 1965

An examination of the pond base showed considerable deterioration of the structure of the compacted layer for the upper 20 to 30 cm thickness. Lower down, the soil layer remained compacted, though less so than originally. Analysis of a small sample of gas obtained by outgassing samples of the soil base showed the presence of over 50 percent carbon dioxide, and H2S. The gas content of the soil was over 100 cc per gm (dry weight) indicating that considerable gas adsorption had taken place. It appeared that bacterial decomposition was occuring in the marshy ground underlying the clay bed. Samples of ground were extracted, and were found to contain relatively large amounts of organic-plant material, and to smell strongly of H2S. The ground thermal gradients, which were measured to a depth of one meter below the solution level, were extrapolated to the depth of the marshy layer, and showed that during the period of heating of the pond, the temperature had risen from about 20 to about 41 deg C. It is likely that this temperature rise favored the rate of bacterial decomposition of the plant and other organic material in the salt nlarsh.

Had the pond been lined with a membrane it is very possible that underground gas production would have lifted the membrane off the floor of the pond. Despite the disappointment in not being able to make the pond operative, nmch was learned. For example, an analysis of the temperature rise for seventeen days prior to the onset of bubbling, which amounted to 1.3 deg C per day, showed that, despite the unclear solution, the pond was collecting at the bottom (at a tenlperature of 70 deg C) about 121 percent of the solar radiation. (This compares with Weinberger's calculation of about 25 percent for a clean pond). It is, perhaps, fortunate for the solar-pond project as a whole, that this gassing phenomenon was observed, for the presence of organic sub-soils nmst be expected in areas where solar ponds might be considered. The experience of this experiment simply warns of the precautions to be taken in any future solar-pond installations. Since it was not practical to dig out the entire pond bottom and thereby to ensure no further trouble from gassing, it was decided to line the pond with butyl rubber but to design the lining to pernfit venting of any gas generated under the pond. At the time of writing this lining operation is being carried out and the "new" lined pond will be referred to as pond No. 4 (Atlith). In passing, if the intention is to use such a pond for salt production--which can be carried out at temperatures as low as 60 deg C--t he chances from gassing are far lower than for power applications where a higher operating temperature would be optimum. 181

F u t u r e Work and Potentialities Despite the setbacks, particularly with pond No. 3, the team is more than ever convinced of the potentialities of the solar pond. M a n y of the early fears have been allayed by the results of the laboratory and field studies: 1 - - I t has been demonstrated in the field that a density gradient can be set up and that this will suppress convection for bottom temperatures approaching the boiling point. 2 - - I t has been demonstrated in the laboratory that layer flow across a pond having a density gradient is feasible both for surface and submerged layers. Field tests have confirmed this feasibility, at least for distances of 55 meters. 3 - - L a b o r a t o r y and field tests show that waves will cause limited mixing of the upper zones to a depth that is not prohibitive. 4 - - I t has been shown, on paper, that the diffusion of salt upwards can be countered so that no salt need be added to a pond after the initial filling. Indeed, where saline waters are available the pond can be used to produce rather than consume salt. 5 - - T h e temperatures and energy yield predicted mathematically appear realisable. Reference 3, and, to a lesser extent, Reference 2, give an indication of the economics of solar ponds and the possible and likely applications. These can be summarized as follows: (a) The most promising application of solar ponds is for salt production 6. For typical conditions, the yield per unit area of pans would be about twice as great as with the conventional open evaporation pans. There are three reasons for this: the surface temperature in a solar pond is lower, resulting in smaller heat losses; the evaporation in open pans becomes less efficient as the brine becomes more concentrated whereas in a solar pond the evaporation of the concentrated brine is carried out in a closed flash evaporator; more days of the year can be exploited with solar ponds because the use of closed evaporators makes the salt-production process almost independent of humidity and rainfall conditions. Because the salt would be produced in closed evaporators, much higher quality salt could be expected. An estimated cost of $6 a ton is very satisfactory for

refined salt. Transportation plays a large part in the cost of salt not produced locally. Because solar-pond salt production should be possible in areas where humidity and rainfall would make open-pan production impossible, the areas where salt production would be practical are increased. (b) The production of power is less promising but certain]y not out of the question. For a sunny area and under good conditions--clean solutions, not too much wind and impermeable ground--an output of about 50 X 106 kwh per year (6000 kw installed) is to be expected from a pond of 1 sq km in size at a cost between 1 and 2 US cents per kwh, the price range depending upon the cost of the pond and its operation (the generating plant and condensers constitute about 0.7 cents of this cost). If the conditions are not very good, a cost of about 2 cents per kwh seems likely. A disadvantage is that, at sites remote from the equator there is a large difference between summer and winter. These figures mean that the pond has little chance of competing with large central fuel stations. Bu~ for units of, say, 500-5000 kw, the solar pond would compete with diesel installations. This can be quite encouraging for developing countries where there is often a need to provide small blocks of power in given areas many years before a national grid will reach such areas. REFERENCES 1. Tabor, H., "Solar Collector Developments", Solar Energy, III, No. 3, pp. 8-9, Oct. 1959. 2. Tabor, H., "Large Area Solar Collectors", UN Paper E/CONF 35/$47, 1961, Also Solar Energy, VII, No. 4, pp. 189-194, Oct. 1963. 3. Tabor, H., "Solar Ponds", Electronics and Power, pp. 296299, Sept. 1964. 4. Weinberger, H., "The Physics of the Solar Pond" Solar Energy, VIII, No. 2, pp. 45-56, April 1964. 5. Elata, C., Levin, O. and Hadar, A., "On Some Flow Problems of the Solar Pond". Reports Sl18/62, $134/62, National Research Council of Israel (In Hebrew). Elata, C. and Levin, O., "Hydraulics of the Solar Pond" To be presented at the 11th Congress of the International Association for Hydraulic Research (Sept. 1965, Leningrad). 6. Matz, R., Feist, E., and Bloch, M. R., "The Production of Salt by Means of a Solar Pond". Paper prepared for the Institute of Chemical Engineers (London) symposium on "The Application of Chemical Engineering in Newly Developing Countries".

Abstracts of Phoenix Conference Papers Available The authors' abstracts of the papers given at the annual meeting of the Solar Energy Society held in Phoenix, March 15-17 have been collected and are available from Society headquarters. To

182

obtain a set, send your order, accompanied by $2.00, to Executive Secretary, Solar Energy Society, Campus, Arizona State University, Phoenix, 95281.

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