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Large Scale Dew Collection as a Source of Fresh Water Supply

DOI:10.1016/S0011-9164(00)88647-6
Authors:
Anil Rajvanshi at Nimbkar agricultural research institute
Anil Rajvanshi
  • Nimbkar agricultural research institute
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A scheme for large scale dew collection as a source of freshwater supply is outlined in the present paper. The scheme envisages bringing cold seawater (5°C) from about 500 meters depth and about 5 km from the shore, in 4, 1.22 m diameter plastic pipes. It then passes through a heat exchanger field with an area of 1.29 × 105 m2 (1.39 × 106 ft2) where it condenses 643m3 of dew over the 24 hour period. The pumping of seawater from the sea and through the field is accomplished by three 200 kW wind machines. Technical and economical feasibility of the scheme is analysed and the possibility of marine culture as a source of food is explored. The present scheme is economically not feasible as compared to a reverse osmosis facility of equivalent capacity.
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©NARI. August 2018
1
Published in Desalination, Vol. 36, No. 3, March 1981, pp. 299-306
Large Scale Dew Collection as a Source of Fresh Water Supply
(For other water related R&D done at NARI please visit this site)
Anil K. Rajvanshi
1
Director, Nimbkar Agricultural Research Institute (NARI)
Phaltan, Maharashtra, INDIA
Email: anilrajvanshi@gmail.com
ABSTRACT
A scheme for large-scale dew collection as a source of fresh water supply is outlined in the present
paper. The scheme envisages bringing cold seawater (50C) from about 500 meters depth and about 5 km
from the shore, in four, 1.22 m diameter plastic pipes. It then passes through an onshore heat exchanger
field with an area of 1.29 X 105 m2 (1.39 X 106 ft2) where it condenses 643 m3 of dew over the 24 hour
period. The pumping of sea water from the sea and through the field is accomplished by three 200 kW
wind machines. Technical and economical feasibility of the scheme is analyzed and the possibility of
marine culture as a source of food is explored. The present scheme is economically not feasible as
compared to a RO (reverse osmosis) facility of equivalent capacity.
INTRODUCTION
With increasing industrialization and population, the world’s water supplies are being taxed to
their capacity. There is already an acute shortage of potable water in developing countries. This shortage
has necessitated the use of desalination as a means of providing fresh water for drinking purposes. All of
the existing desalination plants in the world use scarce and costly fossil to fire them. Consequently, the
majority of these plants are located in Mid East countries [1]. However, the majority of the developing
countries cannot get the costly fuel to run the desalination plants, thus pointing out the need of using
alternative desalination technology.
One of the simplest desalination technologies that have received hardly any serious attention is the
large-scale dew collection. Yet this is one of the major sources of water for plants and some animals in the
coastal and inland deserts [2]. In the desert environment the dew collection takes place because of night
sky radiation cooling. However, for production of large quantities of water the night sky radiation is not
sufficient [3, 4]. An alternative method, which is proposed in the present paper, is to pass the deep sea
cold water through suitable heat exchangers for dew condensation. Obvious advantages of the scheme over
the existing desalination processes are a) no energy is expended in evaporating the sea water – the air-sea
1
This paper was written when the author was at Solar Energy and Energy Conversion Laboratory, Department of
Mechanical Engineering, University of Florida, Gainesville, Florida 32611, U.S.A.
©NARI. August 2018
2
interaction takes care of that, and b) because of assurance of constant temperature source of cold water, the
dew collection can take place continuously day and night for the whole year.
This paper discusses the technical feasibility of such a scheme and the thrust is given to presenting
an overall objective rather than detailed design calculations.
SCHEME
The scheme envisages bringing cold sea water at 4.50C (400F) from about 500 m (1600 ft) depth in
four, 1.22 m (4 ft) diameter plastic pipes to the shore. It then passes through a heat exchanger field (area of
1.29 X 105 m2), where it condenses about 643 m3 (170,000 gallons) of dew over the period of 24 hours.
The pumping of sea water from the sea and through the heat exchanger is accomplished by three, 200 kW
wind machines. After passing through the heat exchanger, the sea water goes through a series of ponds
where algae and fish are grown. It then returns to the ocean.
Figure 1 shows the schematic of the scheme and Figure 2 is an artist sketch of the scheme. Below
are detailed the different components of the system.
To sea
SEA SHORE
From Sea
Fresh water out
Fig. 1. Schematic of the scheme
Marine culture
ponds
Field of 695 heat exchangers
(Dew collectors)
Wind machine
powered pumps
©NARI. August 2018
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Fig. 2. Artist sketch of the scheme
Heat Exchanger
The condensation of dew takes place when the cold seawater passes through the heat exchanger.
Thus the choice of heat exchanger is dictated by the following considerations; a) it should not be corroded
by sea water, b) it should effectively exchange heat with environment and c) it should be inexpensive.
Based on these points the heat exchanger chosen for the present scheme is an EPDM extruded collector
[5]. These collector mats are used for medium temperature hot water heating. In this scheme the EPDM
mat (without the glazing and the back insulation) sits on the shore inclined at an angle (about 60 to 700)
such that the dew condensation takes place on both sides of the mat.
The heat exchanger area for collecting the dew is calculated from the knowledge of condensation
heat transfer coefficient. This coefficient has been obtained experimentally [4]. It should be noted that
dew condensation will take place anytime when the heat exchanger plate temperature is less than the dew
point of the ambient air. Thus even during daytime considerable condensation occurs. The heat exchanger
area Ah is therefore given as:
During night :
2hc+r Ah (Ta – Th) + 2hcond Ah (Ta – Th) = mw Cp (Tout – Tin) (1)
2hcond Ah (Ta – Th)tn = mdew hfg (2)
©NARI. August 2018
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During day :
qs Ah + 2hcond Ah (Ta – Th) + 2hc+r Ah (Ta – Th) = mw Cp (Tout – Tin) (3)
Tout + Tin
Th = ------------ (4)
2
and
2hcond Ah (Ta – Th) td = mdew hfg (5)
The values of variables used in the above equations are shown in Table 1. For the night
calculations it is assumed that the temperature of the sea water entering the heat exchanger module will be
100C (500F) and that at the exit will be 15.50C (600F). Therefore the average temperature of the heat
exchanger plate will be 12.70C (550F). Since this plant will be on shore hence an average ambient
temperature and relative humidity at night is assumed to be 23.80C (750F) and 100% respectively. The
duration of dew condensation at night is assumed to be 10 hours.
For the daytime calculations average solar radiation and ambient temperature have been assumed.
Thus over a period of 14 hours the solar radiation is assumed to have a value of about 252 W/m2 (80
BTU/ft2 hr) and ambient temperature of 26.60C (800F). These are reasonable assumptions for sea shore
©NARI. August 2018
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conditions [6]. From equation (1) the mass flow rate of sea water flowing through the heat exchangers is
calculated and is 8.31 X 106 kg/hr (18.3 X 106 lbs/hr). This value is held constant during the daytime also.
Each collector module is assumed to be 18.6 m2 (50 ft X 4 ft) in area. This choice of area has
been dictated by the ease of maintenance and installation. Thus the total number of collectors is 6950. It
should be pointed out that the value of hc+r in equations (1) and (3) is that for still air. Near the sea shore
the conditions are far different from still air. However, the collectors are arranged such that they are
normal to the direction of the wind (coming from the sea) and thus act as wind breakers, thereby justifying
the assumption of hc+r .
Sea Water Intake Pipes
The choice of sea water intake pipe is governed by the following considerations; a) it should be
noncorrosive with sea water b) it should withstand the tides and the wave motions, c) it should provide
excellent insulation for the cold water during its passage to the shore and d) it should be easily assembled.
Based on these criteria the pipe chosen for the present scheme is a 1.22 m (4 ft) diameter plastic pipe with
wall thickness of 3.7 cm (1.45 inches) [7]. Four such pipes will bring the required water to the shore.
The cold sea water of 4.50C (400F) is mostly located at a depth of about 500 m (1600 ft) [8].
Moreover, the present scheme has been designed for locations where about 500 m deep waters are
available at about 5 km or less distance from shores [9]. Heat transfer calculations for a 5 km plastic pipe
show that the temperature rise of cold water of 4.50C (400F) will be less than 0.270C (0.50F) in reaching
the shore. Thus the plastic pipe provides adequate insulation. These calculations have been performed
assuming a sea water temperature profile [8] and the flow rate of water of 2.1 X 106 kg/hr (4.6 X 106
lbs/hr).
Pumping Requirements
Table 2 shows the pressure drop in various sections of the scheme and the pumping requirements.
The total pressure drop is about 176 kPa (60 ft of water). Near the sea shore there is a constant wind and
thus it is appropriate that the wind machines be used to operate the pumps. Three 200 kW wind machines
will adequately perform the pumping of sea water through collectors. Since the wind is constant near the
shore no attempt has been made to store the water in the overhead tanks. This storage would have been
necessary to overcome the pumping loss during wind-lean periods. It is also interesting to note that any
energy input in the present scheme is that from the wind machines which make this scheme consume 57
kW hr/1000 gallons. This energy requirement compares very favorably with that used in RO units (65 kW
hr/1000 gallons), and MSF (315 kW hr/1000 gallons) [10]. This is to be expected since no energy is
expended in evaporating the water.
©NARI. August 2018
6
Mariculture ponds
The deep sea water is an excellent feed producer for mariculture crops [9]. After passing through
the heat exchanger it can easily be run into deep ponds ( 6 m deep) to produce algae, which is a rich
protein source. These algae then can be a source of food for growing fish and clams [9]. Thus besides
providing the much needed water for the locality this scheme will also provide a source of protein and
food. Based on the results of the pioneering work done in the St. Croix island by Roels and his group [9],
the present scheme will produce about 870 tons/year (wet weight) of shellfish. It should be noted that in
the existing desalination plants such a scheme (of mariculture) cannot be implemented because of lack of
deep sea water.
Finally for the sake of completeness of the study a preliminary economic analysis of the scheme
has been done. Assuming the cost of wind machines at $ 600/kw [11], the cost of heat exchangers at $
30/m2 ( $ 3/ft2) [5] and the cost of 1.22 m (4 ft) diameter plastic pipe at $ 262/m ($ 80/ft), the capital cost
of the scheme comes out to be $ 11 million. The price of this scheme should be compared with the
existing desalination plant of an equivalent capacity. For comparison we have assumed a reverse osmosis
(RO) scheme [12]. Even taking the escalating fuel prices (at 15% annual increase) into account it has been
found that RO plant will be cheaper than the existing scheme by a factor of 2.5. However, if the fuel prices
suddenly double or triple then the present scheme (dew collection) will become economically viable. It
should be pointed out that the whole purpose behind presenting the idea of dew collection is to create
awareness of the technical merits of this scheme. Nevertheless, it is felt that with better technology and
©NARI. August 2018
7
materials of various components the scheme has capability of becoming economically viable. For example
the two main components with the highest price tags are the heat exchanger field ($ 4.2 million) and sea
water intake pipes ($ 5.3 million) respectively. If the field can be made of tubular heat exchangers rather
than the flat plate (as the present scheme) considerable savings in the cost can be achieved. In the absence
of any experimental data on dew collection on tubular heat exchangers we have chosen the flat plate (for
which the data exists [4]). Similarly cheaper pipes for sea water intake will reduce the cost of the scheme.
It can also be conjectured that in the future the OTEC (Ocean Thermal Energy Conversion)
schemes may very well become floating desalination plants with the cold water from the bottom used for
dew condensation. The production of fresh water may make OTEC economically viable since the
generated commodity (in this case fresh water) can be easily transported to shore in huge plastic tankers.
Right now no viable scheme of getting the generated power from OTEC plants to the shore exists [9].
CONCLUSIONS
The following conclusions can be drawn based on the present study.
1. Large scale dew collection near the seashore for production of fresh water is technically feasible.
2. A heat exchanger field of area 1.29 X 105 m2 (1.39 X 106 ft2) can condense 643 m3 (170,000 gallons)
of dew over a period of 24 hours.
3. The cold water for dew condensation is obtained from a depth of about 600 m. The pumping of 8.32
X 106 kg/hr (18.3 X 106 lbs/hr) of this cold water is achieved by 3, 200 kW wind machines.
4. This present scheme is economically not feasible as compared to a RO facility of equivalent capacity.
Acknowledgements : The author wishes to thank Richard Dixon, Kris Kirtikara and Rolando S. Guerra at
UF in helping in the dew condensation experimental set up and data collection. And to Rahul Pisharody of
NARI for making the artist sketch of the scheme.
REFERENCES
1. P. M. Morris, “Desalination of Sea Water”, Chemistry & Industry, August 6 (1977).
2. Uwe George, In the Deserts of this Earth, Harcourt Brace Jovanovich Inc., New York (1979).
3. J. L. Monteith, “Dew”, Quarterly Journal, Royal Meteorological Society, Vol. 83, pp. 322-341
(1957).
©NARI. August 2018
8
4. A. K. Rajvanshi, “Heat and Mass Transfer in Dew Collection”, in preparation.
5. Bio Energy Systems, Inc., Personal Communication.
6. B. Y. Liu and R. C. Jordan, “Availability of Solar Energy for Flat – Plate Solar Heat Collectors”, in
Applications of Solar Energy for Heating and Cooling of Buildings (Eds. B. Y. Liu and R. C. Jordan),
ASHRAE GRP 170 (1977).
7. Steve Campbell, Personal Communication.
8. F. Kreith and J. F. Kreider, Principles of Solar Engineering, McGraw Hill Book Company, New
York (1978).
9. O. A. Roels, “Food, Energy and Fresh Water”, Mechanical Engineering, June (1980).
10. R. S. Silver and W. S. McCartney, Desalination, in The Marine Environment (Eds., Lenihan and
Fletcher), Vol. 5, Academic Press, New York (1977).
11. Anonymous, Wind Product Supplement, Solar Age, February (1980).
12. CH2M Hill, The U.S.A.I.D. Desalination Manual, Office of Engineering, U.S.A.I.D. Washington,
D.C., Contract AID/OTR-C-1618 (1980).
13. B. Gebhart, Heat Transfer, McGraw Hill Book Company, New York (1961).
NOMENCLATURE
Ah
Cp
hfg
hc+r
hcond
mdew
mw
qs
td
tn
Ta
Th
Tin , Tout
Greek letters
heat exchanger area, m2
specific heat, kJ/kg 0C
enthalpy of vaporization, kJ/kg
combined convection and radiation heat transfer coefficient, W/m2 0C
condensation heat transfer coefficient, W/m2 0C
amount of dew condensed, kg
rate of sea water flow, kg/hr
incident solar radiation, W/m2
duration of dew condensation during day, hrs.
duration of dew condensation during night, hrs.
ambient temperature, 0C
temperature of heat exchanger place, 0C
temperature of sea water entering and leaving the heat exchanger respectively, 0C
solar absorptivity of the EPDM mat
heat exchanger efficiency
A short history of water related R&D at NARI is here.
©NARI. 2018
... Dew occurs when a surface temperature is cooled (by losing heat to the sky via radiation) below the dew point temperature of the surrounding air; and thus, water condenses and accumulates on the surface [6]; usually, it occurs overnight. Under natural conditions, dew is a precious source of water for plants and animals in arid and semi-arid environments, where other sources of water such as rain and groundwater are very scarce [1,7,8]. In general, dew occurs infrequently and the harvested amounts are sometimes very small, but in some regions dew occurs frequently with significant amounts that can be utilized for human use [9][10][11][12][13]. ...
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Dew
Article
Dew formation on short grass has been studied with a balance, recording weight of condensation, and with filter papers to absorb moisture. Three regimes are distinguished: in the first, daytime evaporation continues and the grass remains dry; in the second, the surface continues to lose weight but the grass becomes wet owing to the partial condensation of water vapour evaporating from the soil; in the third, this loss of weight ceases or there is an increase in weight attributed to condensation of atmospheric water vapour. A distinction is therefore made between ‘distillation’ of water vapour from soil to grass (1–2 mg cm−2hr−1) occurring on very calm nights through a laminar layer with a transfer coefficient approaching the molecular value; and ‘dewfall,’ the turbulent transfer of water vapour from the atmosphere, negligible when the wind at 2 m falls below 0.5 m/sec but reaching 3–4 mg cm−2hr−1 with stronger winds. This distinction is supported by analysis of the surface heat-budget. On windless nights, since heat flux from the soil and net radiative loss were almost equal, the transfers of sensible and latent heat from the atmosphere were negligible and distillation was of much greater importance than dewfall. Implications for taller crops and warmer climates are briefly considered.
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Principles of Solar Engineering
Article
  • Jan 1978
The aim of the book is to present all the information necessary for the design and analysis of solar energy conversion systems. A combination of basic technical understanding and an appreciation of the economic aspects of using nonrenewable energy sources is developed. The divisions of the subject matter are as follows: introduction to solar energy and its conversion, fundamentals of solar radiation, fundamentals of fluid mechanics and heat transfer, methods of solar collection and thermal conversion, system analysis and economics of solar systems, solar heating systems, solar cooling and dehumidification, solar electric power and process heat, and a brief look at natural solar conversion systems such as wind energy, thermal ocean thermal gradient and wave power, and biomass conversion systems.
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Desalination of Sea Water
  • Jan 1977
  • Morris
P. M. Morris, "Desalination of Sea Water", Chemistry & Industry, August 6 (1977).
Heat and Mass Transfer in Dew Collection
  • A K Rajvanshi
A. K. Rajvanshi, "Heat and Mass Transfer in Dew Collection", in preparation.
Availability of Solar Energy for Flat-Plate Solar Heat Collectors
  • Jan 1977
  • Liu
B. Y. Liu and R. C. Jordan, "Availability of Solar Energy for Flat -Plate Solar Heat Collectors", in Applications of Solar Energy for Heating and Cooling of Buildings (Eds. B. Y. Liu and R. C. Jordan), ASHRAE GRP 170 (1977).
Personal Communication
  • Steve Campbell
Steve Campbell, Personal Communication.
  • Jan 1977
  • DESALINATION
  • R S Silver
  • W S Mccartney
R. S. Silver and W. S. McCartney, Desalination, in The Marine Environment (Eds., Lenihan and Fletcher), Vol. 5, Academic Press, New York (1977).
Wind Product Supplement, Solar Age
  • Feb 1980
  • Anonymous
Anonymous, Wind Product Supplement, Solar Age, February (1980).