SOLAR POND CONCEPTS: OLD AND NEW

T. S. JAYADEV MICHAEL EDESESS .JON HENDERSON

TO BE PRESENTED AT THE 14~~ INTERSOCIETYENERGY CONVERSION ENGINEERING CONFERENCE

Solar Energy Research lnstitute

8 8 7 - &, if, , - 1536 Cole Boulevard . : Golden, Colorado 80401 ct,+; 1- " - .\-8 8, i . ,' A Division of Midwest Research lnstitute

Prepared for the U.S. Department of Energy Contract No. EG.77C.01.4042 DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for any third party's use or the results of such use of any information, apparatus, product, or process disclosed in this report, or represents that its use by such third party would not infringe privately owned rights. not absorbed 1" the "atkr on the way down is absorbed on the dark bottom (vhich may be an artificially .blackened liner). As a result of this heat collection .at the bottom, the deeper waters become warm. SOLAR POND CONCEPTS: OLD AND NEW Higher concentrations of salt are established in the lower pond regions than in the upper regions, so T. S. Jayadev, Michael ~deses;, Jon Henderson the warmer, deep waters contain a higher density of dissolved salt than the colder waters near the sur- Solar Energy Research Institute face. This is the opposite of the situation vithout Golden, Colorado 80401 salt. Pure water, when warmed, becomes less dense. A Division of Midwest Research Institute If there were no salt in the pond there would be con- tinuous convection of the warmed water from the bottom of the pond to the cooler layers near the top. How- ever, the increased density created by the salt over- .comes the decreased density of pure water with in- creasing temperature, and thus vertical convection ABSTRACT .is prevented. Heat transfer to the surface of the pond occurs primarily by conduction, which is slow Different types of solar pon.ds have been consid- enough to enable the lower regions of ,the pond to ered from the early 1900s to the present. Salty ponds maintain a high temperature (10OeC has been measured use salt to create a nonconvecting pond. Shallow so- in actual ponds). lar ponds were investigated by Shuman and Hillsie in In practice the salty pond has three layers. In 1906 .and 1907 and are currently being studied by Law- 'the top layer vertical convection takes place due to rence Livermore Laboratories. Swedish investigators the effects of wind and evaporation. This layer are studying a combination of solar collectors and wa- serves no useful purpose and is kept as thin as prac- ter storage in a pond-cover configuration. In addi- tically possible. The next layer, which may be ap- tion, there are thermoclines created in large bodies proximately 1-n thick, contains an increasing concen- of water, as in large reservoirs. tration of salt with increasing depth and is noncon- This paper surveys the various types of solar vecting. The bottom layer is a convecting layer which ponds, classifies and models them, and then tries to ;provides. - most of the thermal storage and facilitates combine the best of the ideas to synthesize new con- heat extraction. cepts. It presents a new solar pond concept which I Variants on the simplest salty pond design have combines the good features of convecting and noncon- :been proposed to aid in controlling the boundaries of vecting (salty) ponds. ;these layers. The so-called "membrane pond" (4) con- :tains a horizontal partition to separate the lower ;convecting zone from the middle nonconvecting zone :and, possibly, a second partition slightly below the KOST LOW TEMPERATURE solar thermal systems have col- :surface of the pond to minimize the surface convecting lector and storage units as separate components. :layer. Storage is usually sufficient to last only a few days. : Salty ponds have been built and operated in such With solar ponds, it is difficult to distinguish the 'diverse locations as Israel and Canada and in Ohio and collector unit from the storage unit: the same body :New Mexico in the United States. of water serves as solar collector and storage medium. Proposed alternatives to the salty pond in the This body of water is usually large enough to provide .nonconvecting pond category are the .riscosity-stabi- long-term storage spanning seasons. I .lized pond and the gelled pond. Both of them retard Solar pond concepts my be classified in two cat-. ,internal convection by decreased fluidity of the water egories: (a) those that reduce heat loss by prevent- :in the pond and have not yet advanced significantly ing convection within the water storage medium, and beyond the conceptual stage. (b) those that reduce heat loss by covering the pond's CONVECTING PONDS - The single well-researched ex- surface. Combinations are of course possible, but one ample of the convecting pond is the shallow solar pond method is usually the primary determinant of the pond ; .proposed and designed by Lawrence Livermore Laborato- design. ,ries (11,12). The shallow solar pond is about a 10-cm If one objective of the solar pond is to capture depth of pure water enclosed in a large water bag ' and store solar thermal energy, a second objective is (typically 5 m by 60 m) with a blackened bottom, in- to do so cheaply. This means using earth as both the sulated below with foam insulation and on top with support structure for the body of water and as at glazings. The water from many such ponds is pumped least part of the insulation. A large enough body of into a large storage tank for night storage and back water embedded in the earth and made usable for ther- , into the water bags each morning, in an operating ma1 storage involves low cost for both support struc- .method called the "batch" mode. The shallow solar tures and insulation. It can provide sufficient stor- ipond may also be operated in the "flow through" mode, age to even out short-tern tenperature and insolation in which the water flows continuously through the wa- fluctuations and to furnish heat output long after ; ter bags in such a way as to maintain control over the heat collection has taken place. /outlet temperature. Especially when operated in the flow-through mode, TYPES OF SOLAR PONDS the shallow solar pond is almost the same as a flat- plate collector with water storage, the main differ- NONCONVECTING POh'DS - Most studied of the solar ence being that the solar pond collector is fixed in a ponds is the nonconvecting, salty pond (I*-10). In horizontal position and is less costly than the usual its simplest form the salty pond is merely a pond with flat-plate collector. For this reason the shallow salt dissolved in it. Salts most commonly used are solar pond is classified in the collect.or-pond storage NaCl and NgC12, although there are numerous other pos- category described below. sibilities. A second type of convecting pond will be intro- Solar radiation enters the pond, and whatever is duced in more detail in later sections of this paper. This type of pond is aore similar to the salty pond *Numhers in parentheses designate references at end of than to the shallow solar pond in that it will be em- paper. bedded in the ground and will have a similar depth.n ,. .. .. To compensate for the losses prevented by the noncon- Thus. let , vecting layer in the salty pond, the convecting pond will have glazings and special night insulation. ra(t) - Ta + Fa sin 27(t-+=) COLLECTOR-POND STORAGE COEIBINBTIONS - Several - - collector-storage combinations have been suggested in I(t) = I + I sin 2n(t-$) which the thermal storage is provided by a large pond embedded in the ground. r(r) = Z + C sin 2~(t-+~) In a Swedish design (131, a bank of tilted col- lectors is floated on a raft of insulation on top of a ' The time t and the phase angles %, @'=, are large pond. The heated water is drained into the pond measured in years. If insolation peaks in June, then and the collectors are fed with cooler water pumped *+ is approximately 0.22; if ambient temperature peaks back up from the pond. about a month afterward, then @'T is approximately A pond tested at the University of Virginia (14) 0.30. had a trickle collector mounted just above the surface Let A signify the solar collection area, To the of a square pond. Between the pond surface and the fraction of insolation transmitted to the storage area collector was a layer of foam insulation "beads" of the pond, and dVc the total heat capacity of stor- P through which the heated water trickled into the pond.. age (where d is the water density, V is the volume of An Italian proposal (15) calls for focusing col- storage, and cp is its heat capacity per unit mass). lectors to heat water to a high temperature and depos- hi energy balance yields it it in a pond of several square kilometers surface area. According to the proposal, this water can then be transported long distances in underground pipes to heat a city. When the shallow solar pond concept is compared vith the collector-pond storage designs, it is appar ent that the shallow pond is analogous to the collec- or tor, and the night storage tank is analogous to the thermal storage. . , U +U - Fig. 1 shows the classification of the different . ,t(t) + ~(t)= [TW~+(Up ) Ta - L solar pond concepts. P P g

+ TOGsin ~n(t-+~) - + UaTa sin 2t(t-+T)

- L" sin 2n(t-4L) ]

The solution to this differential equation is floating on [*I\ ! -'Jt T(t) = + +(t) - C(to) e (1

where

Fig. 1. - Solar pond taxonomy

A GENERIC MODEL

Whatever their differences, the various solar pond designs have in common a very large body of thermal storage. It may be assumed that this storage is so large that daily fluctuatinns in ambient temperature and insolation have a negligible effect on the temper- ature of storage, and that only seasonal variations in the environment need he considered. It is assumed also that the heat loss from stor- age is related linearly to the difference between the temperature of srulage and thc temperatrtrp of fhe am- bient air and to the difference between the tempera- 'Jt ture of storage and the temperature of the ground. C(to) = -i: - Fa + $(to) e o This means there must be effective heat loss coeffi- cients Ua and U such that the rate of heat loss is (T-T) + (T-) where Ta is ambient tempeyature. and to is the startup date for the pond (in years from T is ground temperature (prestimably equal to Ta, the January 1). at which time it is assumed T = T . S is g average annual ambient temperature), and T is the ten- the number of seconds in a year, if I and a are ex- perature ot the lowel convecting layer. In the con- pressed in watts. vecting pond, T is assumed to be the temperature at Note that Eq. 1 expresses tilt: pond otorage tem- any point. perature as the sum of the long term average tempera- It is reasonable to model the insolation and the ture T, a periodic temperature deviation $(t), and a ambient temperature as sine vaves, and for simplicity transient term ~(t~)e-'~. it can also be assumed that the load can be represent- Settin8 the derivative of Eq. 1 equal to zero, ed as a sine wave. one finds that in the steady state, extreme tempera- tures occur at times (1/2n) tan" [$(0.25)/$(0)]; by Inflated Plastic Film plugging these times into Eq. 1 one can'find the maxi- Liquid Foam mum and dnimum temperatures. Double Glazing

For a circular salty pond simulated by Nielsen (5), of 12-m radius and 2-m depth, wall Losses were 3573 W and floor losses were 2920 W when the pond tem- perature was 50°C and ambient temperature was 10°C. (Note that only earth insulation was used in this sim- a Plastic , ulation). Assuming that the coefficient of heat loss . . . . . , Liner :. :i ' to ambient is Ua = 3573/(50e-10') or Ua = 89.3 W/"C, :~9?:?*!?1.,. , . . . . ,;; ;;;:, .,... ;;;;:;;.::, ,::,, <;;,;.i.::,.: :.<. ~?;.::;":.;Z,;;?:*kj:&6<~<,~;,.~&~;;~*~x;k,~.;;:.::;:.: ,dAG&,<<>d &,?* x2dz&;,G;;hA{32;$*>:;;$& and the coefficient of heat loss to ground is U a 2920/(50°-lo0) = 73 Ul°C, the projected pond tempe?a- Fig. 2. Example of deep saltless pond design tures shown in Table 1 are obtained with the formulas - just developed. (The pond is assumed to have been started on April 1. Transmission through the noncon- vective layer is assumed to be .25%, ambient t mpera- houses (16). It has been found in practice to reduce tures are 10+_15"C, and insolation is 200250 W/m 3 .) the heat loss by at least 50X, although in theory an 85% reduction should be attainable. It should be not- A DEEP CONVECTING POND CONCEPT ed that the spray foam used in greenhouse experiments is a material normally used for EireFighting. It . Although the shallow convecting pond develops high seems likely that improvements could be made in the temperature water in a fairly short time, it requires material for purposes of pond insulation. pipes and plumbing to shuttle water out of the "ponds" In the morning, the spray foam insulation would each evening and storage tanks to hold the water at be allowed to settle and run off, leaving a negligible night. It also requires insulation under the water residue. The capital cost of using spray foam to pro- bags because the ground is allowed to cool off each vide sup lemental night insulation amouncs to less night after the water is removed from the. bags. than $l/mer of pond. A more economical approach is to leave the water Besides eliminating the need for pipes, pumps, in the pond at night and to provide as much extra in- and plumbing to transport the water to nighttime stor- sulation as possible on top of the pond. During the age, this "stationary" pond would not require bottom daytime, when insolation must he received through the insulation. After a warmup period the temperature of top of the pond, there is a limit as to how much top the ground would approach that of the pond water, pro- insulation can be used, and double glazing similar to viding good insulation. The only additional insula- that used in the shallow solar pond would be employed. tion that might be desired would be along the sides of But at night or during periods of low insolation addi- the pond to prevent edge losses. tional insulation could be provided. An obvious and To provide sufficient storage to even out daily simple method would be to lay extra insulation over and seasonal temperature fluctuations, the stationary the top of the pond, either automatically or manually, convecting pond would be a deep pond, not a shallow whenever insolation falls below a prescribed level. one. A more interesting and economical technique is to spray foam insulation between the glazings and between SUMMARY AND CONCLUSIONS glazing and pond when insolation drops below the pre- scribed level (Fig. 2). A spray foam has been used The salty nonconvecting pond is a viable and eco- successfully to provide night insulation for green- nomical way of collecting and storing large quantities

Table 1 - Projected Pond Temperatures Obtained Using Developed Formulas

Projected Temperature (OC) 5 kW 523 kW 523 kW Constant Summer Winter -Year Month No Load -Load Peaking Peaking July 1 51 -0 44.1 40.8 47.4 Oct. 1 66.3 56 7 53.7 59.7 Year 1 Jan. 1 Apr. 1 July 1 Oct. 1 Year 2 Jan. 1 Apr. 1 July 1 Oct. 1 Year 3 Jan. 1 Apr. 1 - Steady Minimum 49.6 State Maximum 74.4 of solar energy, especially where salt is free or in- International Solar Energy Conference in Los Angeles. , expensive. Other techniques may be more desirable Braze Research Institute Report No. R-119, 1975. vhere salt is not so readily available. The membrane pond may eliminate the need for high salt concentra- 8 V. N. Eliseev, Yu. U. Usmanov, and G. Ya. tion in the convecting layer. The shallow solar pond Umarov, "Determining ,the Efficiency of a Solar Salt uses no salt but requires considerable expense for Pond." Geliotechnika, Vol. 9, No. 1, pp. 44-46, 1973. plumbing and night storage. Combinations of collec- tors with pond storage are appropriate for some appli- 9 D. L. Styris, R. J. Zaworski, 0. K. Harling, cations and can provide water of higher temperature, and J. Leshuk, "The Nonconvecting Solar Pond Applied but they enter the realm of higher and more expensive to Building and Process Heating." USERDA Contract No. technology. Where salt is not free or inexpensive the AT(45-1)-1830. deep convecting pond may be an economically viable and environmentally acceptable alternative and deserves 10. C. F. Kooi, 'The Steady State Salt Gradient further investigation. Solar Pond.' Submitted to Solar Energy, October 1978.

REFERENCES 11. W. C. Dickinson, A. F. Clark, J. A. Day, and L. F. ~outers,"The Shallov Solar Pond Energy Conver- 1. H. Tabor, "Solar Ponds: Large Area Collectors sion System." Solar Energy, Vol. 18, pp. 3-10, 1976. for Paver Production.' Solar Energy, Vol. 7, No. 4. pp. 189-194, 1963 12. A. B. Casamajor, and R. E. Parsons, "Design Guide for Shallow Solar Ponds." Lawrence Livermore 2. H. Tabor and R. Matz, 'A Status Report on Laboratory, UCRL-52385 Rev. 1, January 8, 1979. Solar Pond Projects." Solar Energy, Vol. 9, No. 4. pp. 177-182, 1965. 13. Peter Kargen, "A Central System for Annual .. - Solar Heat." Solar Age, Vol. 3, No. 10, pp. 22-26, 3. Hershel Weinberger, ' "The Physics of the October 1978. Solar Pond." Solar Energy, Vol. 8, No. 2. pp. 45-56, 1964. 14. J. Taylor Beard, F. A. Iachetta, and L. U. Lilleleht, "Annual Collection and Storage of Solar En- 4. Ari Rabl and Carl E. Nielsen. "Solar Ponds ergy for the Heating of Buildings." University of for Space Heating." Solar Energy, Vol. 17, pp. 1-12, Virginia, Report No. UVA/532142/PUE78/102, September 1975. 11978. j 5. C. E. Nielsen, "Nonconvective Salt Gradient 15. G. Cavalleri and G. Foligno. "Proposal for Solar ponds." Solar Energy Handbook, edited by W. C. the Production and Seasonal Storage of Hot Water to Dickinson 'and P. N. Cheremisinoff, to be published by, Heat a City." Solar Energy, Vol. 19, No. 6 pp. 677- Marcel Dekker, Inc., 1979. 683, 1977.

6. F. Zangrando and H. C. Bryant, 'A 'Salt Gra- 16. John E. Groh, ''Liquid Foam--Greenhouse Insu- dient Solar Pond." Solar Age, Vol. 3, No. 4, April lation and Shading Techniques." Presented at Interna- 1978. ,I tional Symposium on Controlled-Environment Agricul- ture, Tucson, Arizona, sponsored by Environmental Re- 7. B. Saulnier, S. Savage, N. Chepurniy, "Exper- :search Laboratory, University of Arizona, April 7-8, imental Testing of a Solar Pond." Presented at the 1977. DISTRIBUTION LIST

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