[PDF]Solar Distillation for Small-Scale Water Demands

MICROFICHE
REFERENCE
LIBRARY

A project of Volunteers in Asia



Solar Dist illation as a Means of Meeting
gmall-Sc ale Water Demands

Published by:

United Nations

New Yo*:k, NY 10017 USA

Paper copie? arc $ 7.00. Ask for publication
number 70.II.B.1 when ordering.

Available from:

United Nations
Publications Sales
Room A-3315

New York, NY 10017 USA

Reproduced by permission of the Department
of Public Information, the United Nations.

Reproduction of this microfiche document in any
form is subject to the same restrictions as those
of the original document.




Department of Economic and Social Affairs



SOLAR DISTILLATION

as a means of meeting
small-scale water demands




UNITED NATIONS
New York, 1970



NOTE



Symbols of United Nations documents are composed of capital
letters combined with figures. Mention of such a symbol indicates a
reference to a United Nations document.



ST/ECA/121



UNITED NATIONS PUBLICATION
Sales No.: E.70.ILB.1



Price: $U.S. 2.00
(or equivalent in other currencies)



CONTENTS



Chapter Page

Preface , V ;

Explanatory notes -viii

I. Summary and findings ]_

A. Current technology and costs ±

5. Potential scope of solar distillation 5

C. Steps for evaluation of solar distillation .....<,.. 3

II. £asin-type solar stills 5

A. Still design 5

B. Theory of solar-still operation 10

C. Still size, rain-water catchment and product storage ... 15

D. Installation and operation of solar stills 20

E. Economics of solar distillation 2J

III. Trends in solar distillation yj

A. Evolution of basin-type stills 37

B. Collection of solar energy: solar ponds 30

C. Combined energy-source systems yo

D. Multiple-purpose systems 30

E. Integrated -function plants 40

References and selected bibliography . „ 59

ANNEXES

I. Alternative solar-distillation processes 6^4

II. The theory of solar-still operation 75

III. Water storage requirements and costs 85



-iii-



LIST OF TABLES



Page

1. Existing large solar stills, 1969 ......... 8

2. Typical solar-still production . . 16

3. Example of preliminary estimate of solar-still yield 17

h. Capital costs of solar stills 26

5* Estimate of hourly output, q e , of solar stills, using

characteristic chart „ 83

6. Estimate of solar-still output and storage requirements 85

LIST OF FIGURES

1. Solar still, Las Marinas, Spain „ . 1+2

2. Schematic cross -section of basin-type solar still 1*3

3. Schematic diagram of solar distillation plant, showing

major items of equipment 1+1+

If. Plan and sections of basin-type solar still, Eaytona Beach,

United States of America 1+5

5- Beam and glass seal details of basin-type solar still,

Daytona Beach, United States of America 1+6

6. Plan and sections of solar still, Las Marinas, Spain If 7

7. Details of lower beam for solar still, Las Marinas, Spain .1+8

8. Section of basin-type solar still, Commonwealth Scientific

and Industrial Research Organization, Australia 1+9

9. Section and frame details of basin-type solar still, Technical
University of Athens, Greece ....... . 50

10. Schematic design for demonstration unit, small-scale basin-type

solar still, for use on Pacific Islands .............. 51

11. Schematic sections of plastic -covered basin-type solar stills ... 52

12. Diagrammatic section of solar still, showing significant energy
transport streams to, from and within the still 53

13. Effect of ambient temperature and loss efficiency on predicted

still output at H s = 2,555 BTU square foot" 1 day -1 ........ 5I+

ll+. Effect of solar radiation and loss coefficient on predicted
still output at T Q = 60°F, C^ g = 16 and wind velocity of

5 miles per hour 55



-iv-



LIST OF FIGURES (continued)

Page



15. Comparison of predicted and experimental still performances,
Gfiffith, Australia 55

16. Comparison of results of extended tests of capacity of basin-
type solar stills with results obtained from calculations .... 57

17. Typical solar-still daily production as a function of radiation

v and average air temperature 58

18. Compression-distillation unit using electric energy from solar-
power plant 66

19. Multiple-effect evaporator heated by steam from focusing solar
collector 67

20. Multiple -effect evaporator heated by sterol from flat -plate

collector 68

21. Multiple -effect solar still employing glass covers and

copper condenser-evaporator plates 69

22. Glass-covered evaporating pan with reflecting surfaces ..... 70

23. Extruded plastic still with "blackwick for evaporation and

cooling 71

2k. Tilted -tray solar still 72

25. Tilted wick-type solar still 73

26. Experimental multiple-stage flash solar distiller, Puerto Penasco,
Sonera, Mexico 7I

27. Heat fluxes for a solar still „ . . . . , . 76

28. Evaporative heat transfer, q e , versus cover temperature,

Tg, for different values of brine temperature, T w 78

29* Cover heat loss,
values of ambient temperature, T a , and wind velocity 79

30. Characteristic chart for thermal performance of a solar still . . 30



-v-



PREFACE



This study is part of a programme of studies undertaken by the Resources
and Transport Division of the United Nations Secretariat which is concerned with
questions of applying new technologies and development methods to the water
resources problems in developing countries. Particular attention has been
focused in recent years on the application of water desalination as a means of
meeting real water needs and stimulating development in areas suffering from a
shortage of fresh water.

This report is intended to define the conditions under which solar
distillation may provide an economic solution to the problems of fresh water
shortage in small coiuiuunities . In particular, the purposes of the study are:
(a) to review the current status of solar distillation; (b) to outline the
general classes of situations in which it may represent the best solution to
water supply problems; (c) to provide a method for potential users to estimate
performance and costs of current still designs in their areas; (d) to note
practical problems of solar-still design and operation; and (e) to recognize
some possible changes in solar-distillation technology and economics i^hich may
affect the applicability of the process in future.

The study does not consider the conversion of solar energy into other forms,
such as the generation of electrical energy, which may subsequently be used in
desalination processes.

The proposals for the present study which were contained in the report of
the Secretary-General, "Water desalination with special reference to
developments in 1965" (E/4l42), were approved by the Economic and Social Council
in resolution 111k (XL) of 7 March I966.

Accordingly, a panel of experts was convened at United Nations Headquarters
from 12 to 19 October I968, composed of the following persons:

VJV. Baum, Physico Technological Institute, Turkmenian Academy of Sciences,
Ashkhabad Turkmen SSR

A. A. Eelyannis, Technical University of Athens, Greece

J.A. Duffie, University of Wisconsin, Madison, Wisconsin, United States
of America

E.D. Howe, Sea Water Conversion Laboratory, University of California,
Berkeley, California, United States of America

G.O.G. Lof , Consulting Chemical Engineer, Denver, Colorado, United States
of America

R.N. Morse, Commonwealth Scientific and Industrial Research Organization,
Melbourne, Australia

E. Tabor, National Physical Laboratory of Israel, Jerusalem, Israel

-vi-



Assists through the provision of material and substantive conoents
was also re ■: iied by C. Gome 11a, Societe d 'etudes pour le traiteinent et
I 1 utilisation des eux (SEIUEE), Paris , France. Staff nembers of the Resources
and Transport Division of the United Nations participated in the .meetings and
in the preparation of this report. The United Nations Secretariat is
particularly indebted to J. A. ruffle, who assisted in the preparation of this
report .

Additional United Nations publications in the field of desalination are
list ? ] in the bibliography.



-vii-



EXPLANATORY NOTES



Reference to "gallons" indicates United States gallons, and to "dollars",
United States dollars, unless otherwise ststed .

The abbreviation "gpd" indicates gallons per da;y .

The folio-wing table will allow conversion into other units :



To convert

gallons (U.S.)
gallons (U.S.)
1,000 gallons (U.S.)
square feet
gallons /square foot
dollars/square foot
pounds /square foot
JTU square foot- 1 day" 1
J3TU hour"" 1 square foot" 1

BTU square foot" 1 °p" 1



-1



imperial gallons
cubic metres
cubic metres
square metres
cubic metres/square metre
dollars/square metre
kilo grammes /square metre ^
calorie square centimetre"



-1



day



Multiply by



0.833
0.00379
3.79
0.0929
.okoB

10.77
4,88
0.271



-1



square centimetre



calorie hour
o c -i

calorie square centimetre



-1



-1



o c -l



The nomenclature used in annex II to this report is given below:



"wg
T ga

H s
h w



PwsPwg
%

1e
Iga
^r'%

t

Tg } T w
(¥g>r

°£w
Tlo

CT



thermal capacity of water, still and ground, JTU °F sq. ft.
convective heat transfer coefficient glass cover to air

BTU hour" 1 sq. ft." 1 J , " 1 J having a value of 2.6 for a wind

velocity of 5 mph and 7.2 for 20 mph
solar radiation on horizontal surface BTU hour" 1 sq. ft."
latent heat of vaporization of water ETU/lb.

thermal loss coefficient base of still to surroundings BTU hour

sq. ft." 1 °F
partial pressure of water vapour at Tw T wg psi
heat loss from base of still BTU hour -1 sq. ft." 1
heat transfer brine to glass by evaporation BTU hour - sq. ft.
heat transfer glass cover to surroundings BTU hour" 1 sq. ft." 1
heat transfer brine to glass by radiation and convection

BTU hour - 1 sq. f t . " 1
time

ambient temperature °F ,

temperature of glass cover and saline water °P
solar absorptance and transmittance of glass cover
solar absorptance of brine and trough system
drainage efficiency of still

Stefan-Boltzmann constant 17.2 x 10~ BTU hour" sq



0.487
0.487



-1



-1



-1



ft.



-1



-viii-



I. SUMMARY OF FINDINGS



Solar energy represents a vast energy resource which is most available
in many areas where population density may be low and where conventional energy
resources may be expensive. Its use for operation of desalination processes
for production of fresh water is technologically feasible.



A. Current technology and costs

The most advanced solar still currently in use is the basin-type still ,
a century-old concept which has been modified and adapted to modern materials
and applications. An 11,500-sq. ft. basin-type solar still installed at
Las Marinas , Spain, is shown in the frontispiece; and a sectional diagram of
a basin-type still is shown in figure 2.*

At the current time, all solar stills can be viewed as being in various
stages of development, rather than as an established technology. Some still
designs, however, are in advanced stages of development, and considerable
operating experience is available upon which to judge their utility and costs.
Still productivity can be predicted with some confidence for those designs in
which problems of mechanical failure or corrosion are minimized. The most
advanced designs are those which use standard materials of construction, such
as glass, concrete, asphalt and corrosion-resistant metals. Consideration is
also being given to designs that rely in part on new plastic materials. With
most designs there are possibilities of construction using, to some degree,
locally available materials and labour in the area of application. Construction
maintenance and operation do not require high levels of skill in working with
complex machinery.

Solar distillation should be considered a possible method for water-
supply under the following circumstances:

(a) Natural fresh water is not available and saline water is available;

(b) The climate is good (i.e., the solar radiation levels are high);

(c) The potable water needs for the community or user are below about
50,000 gP d;

(d) Reasonably level Ir^.nd is available for solar-still sites

(e) Such land is in isolated locations where inexpensive power and
highly trained manpower are not always available.



* Figures 2-17 may be found at the end of the report.



-1-



The unique characteristics and problems of solar distillation must be
taken into account when evaluating it as a possible method of water-supply in
comparison with other methods.

Being based on a time -variable energy source, solar distillation provides
a variable output. Summer product-water yields are typically three to four times
winter yields. If water needs do not follow the same patterns with time,
product-water storage or auxiliary supply, or a combination, must be provided.

Solar distillation is a capital-intensive process, requiring relatively
large capital investment per unit of capacity and, in properly designed and
constructed systems, a minimum of operating and maintenance costs. Product-
water costs thus depend primarily upon still productivity, capital cost of the
installation, its service life, and amortization and interest rates.

Solar-still productivity is conveniently referred to, in round numbers,
as being, typically, 0.1 gal sq. ft." 1 day" for a good day. It is, however,
highly dependent upon solar radiation and less dependent upon air temperature
and other factors. On clear winter days a well-designed still will yi.^ld
perhaps 0.03 and on clear, hot summer days, perhaps 0.12 gal sq. ft, -1 day"^" .

Summing up these yields over a year, experience shows that annual still
yields of about 25 gal/sq. ft. are obtained, with some variations due to climate
and still design.

The unit capital of solar stills "built in recent years has been $2.0C-$0.6o/
sq. ft. Most of the estimates based on current designs of durable stills show
costs of materials and labour for still construction to be in the neighbourhood
of $l/sq. ft.

The projected lifetime for stills constructed of concrete, glass and other
long-lived materials is twenty years or more. Other still designs have been
developed using some materials with shorter service lives which must be
periodically renewed.

Assuming favourable interest rates, such as those granted to a public -utility
type of venture, and service lives as noted, one obtains water costs of $3-$6 per
1,000 gallons. Variations within and from this range are experienced because
of rainfall collection, storage costs and unpredicted factors affecting still
productivity .

While this cost is high, when measured by the usual standards of large-scale
water-supplies, it is based on solar plants with outputs of 25,000-50,000 gpd,
or less. Experience thus far with other desalination processes in this small
size range has snown product -water costs to be as high or higher, particularly
when energy sources have to be especially provided. In plants larger than this,
it is clear that other desalination processes can produce water more economically
than can solar distillation. It may also be observed that the costs of solar
stills do diminish as significant developments occur; however, the costs of
competitive processes may also decrease accordingly.

Flexibility in choice of sise over a significant range, without
significantly affecting unit costs, is an important feature of the solar still.



-2-



B. Potential scope of solar distillation



There are other solar processes which may broaden the scope of solar
distillation in future, depending upon the success of development studies jn
che processes and upon further developments in other areas of research not
directly related to solar distillation. Some of the areas of current or possible
research and development on solar distillation are described below.

Further refinements in basin still design, to improve performance and
reduce costs, are being studied. Use of new materials with unique properties
and of locally available materials and evolutions of design are potentially
important. Some research is directed to development of very small units, in

the range of 500 gpd or less .

\

The basin-type solar still combines the solar-energy collection function
and the distillation function in a single unit. Separation of these functions
would allow regenerative, or multi-stage processes, to be solar-operated. This
development is, of necessity, dependent upon very significant progress in
solar-collector technology (i.e., a "breakthrough").

Combined energy source systems, in which solar energy to the still is
augmented by waste heat from, for example, intermittently operated diesel or
gasoline engines, may reduce the cost of product water from the still.

Multiple-purpose systems, producing some combination of water, salt and
possibly power, can be conceived. Within the limits set by relative markets for
these commodities, such systems could be significant, but only after very
considerable further research and development .

Multiple -function plants, in which water production is integrated with
water use, are also being considered, for example, in an integrated system for
>>>