Thermal gradient humidification-dehumidification desalination system

A solar energy desalination process utilizing solar radiation directly for the evaporation of salt water is described. Ambient air takes on water vapor as the air passes through an evaporative medium. It is then directed between a saline water-covered, solar absorbing surface and a solar collecting housing. The resulting heated and moisture-saturated air is cooled in a heat exchange means where condensation of fresh water occurs. Simultaneously, cool salt water is utilized as the cooling water in the heat exchange means, and takes on the heat of condensation given up by the condensing vapor. The heated salt water from the heat exchange means is partially directed over the solar absorbing surface, and at least a portion of it is also directed to wet the evaporative medium. Several optional sub-processes are described for operation of the system during periods of reduced insolation, and an alternative process is described for operation of the process on a floating platform.

BACKGROUND OF THE INVENTION 
This invention relates to the field of purification of salt water by the 
humidification of air with that water, followed by dehumidification of the 
air to produce fresh water. The invention is especially useful for the 
continuous desalination of seawater in locales where the temperature of 
the ocean drops sharply with increasing depth. 
Man's requirements for water are becoming increasingly evident as the 
earth's population increases, agricultural needs grow, and industries 
expand. Fresh water represents less than 3% of the water on earth. Of this 
3%, nearly 75% is "trapped" as ice throughout the world, but predominantly 
at the polar ice caps. The remaining 97% of the earth's water is in the 
form of salt water or brackish water. The quest for fresh water has turned 
to desalination technology to convert the great oceans and vast inland 
brackish water reserves to fresh water. 
Fresh water is obtained from salt water by separating the fresh water from 
an ever increasing concentration of salt water or by separating the salt 
from an ever increasing fresh water solution. Some of the processes which 
have been employed to accomplish this include simple evaporation, 
distillation, multieffect evaporation, multistage flash evaporation, thin 
film distillation, reverse osmosis, freeze crystallization, ionic 
separation and electrodialysis. Most of the existing desalination plants 
which employ these processes use fossil fuel as the energy source. 
Solar distillation, which is an evaporation /condensation process, is based 
on the absorption of the sun's radiant heat on the dark coated bottoms of 
shallow trays filled with seawater (or brackish water). The water vapor 
which is formed is subsequently condensed on the cooler undersurface of 
transparent material located immediately above the pan. Convective cooling 
of the transparent cover by the atmosphere removes the heat of 
condensation. The transparent collector cover is sloped to allow the 
condensate to run off into collecting troughs at the base of the 
collector. 
A variation on the solar still is illustrated in Hodges et al., "An 
Integrated System for Providing Power, Water and Food for Desert Coasts," 
6(1) HortScience 10 (1971). In this system, a single stream of hot 
seawater is sprayed down through a packed tower countercurrent to a 
rapidly rising stream of air. This fills the air with a salt-free vapor 
which moves up through a duct into a second, condenser tower. Seawater 
enters the cycle at the bottom of the condenser and is pumped up within 
the tubes of a heat exchanger. Vapor forced over from the evaporator forms 
on the tubing as fresh water condensate and rains down to the base, where 
it is collected. The seawater spiralling up through the condenser and 
heated by the latent heat of vaporization is then conducted to a seawater 
heater, out of contact with the air stream, before entering the evaporator 
tower. The heat source for seawater heating is described as heat from a 
solar collector or waste heat from generators. 
U.S. Pat. No. 4,172,767 to Sear describes a modified solar still which 
makes use of the difference in temperature between water on the surface of 
the sea and water at some greater depth. Seawater is collected in a tank 
and evaporated with the aid of accumulated rays from the sun. A blower 
forces the moisture-laden air above the heated seawater through a pipe 
into the depth of the sea. Moisture is condensed as the heat of 
condensation of the vapor in the air is dissipated in the seawater, and 
the potable water is then pumped up to the surface. Other patents relating 
to the use of ocean thermal gradients to produce potable water by 
condensation of water from a vapor-laden air stream include U.S. Pat. Nos. 
4,186,311; 3,928,145; 4,151,046; 3,986,936; 2,820,744; 4,110,172; 
4,187,151 and 3,257,291. U.S. Pat. No. 4,041,707 discusses cooling of warm 
surface air by passing it below the ocean surface, but does not suggest 
use of a moisture recovery system. Publications which pertain to 
compression of air using wave power or to related technology include U.S. 
Pat. Nos. 610,790; 875,042; 926,408; 1,267,936; 4,022,549 and 4,152,895. 
Many of the processes currently employed for desalination are burdened with 
expensive requirements for mechanical equipment or energy from external 
sources. Evaporative desalination is characterized by high thermal energy 
consumption. While this consumption may be reduced by employing a 
multiple-effect concept, the number of effects employed in a particular 
operation is then determined by the trade-off of additional heat exchanger 
costs versus energy cost. In a vapor compression-evaporation process, 
approximately four-fifths of the energy requirement is for mechanical 
compression, and the remainder is for boiling water circulation. In the 
electrodialysis process, electrical requirements are proportional to the 
salt concentration of the water being purified, so the process is 
generally used only for desalting of brackish water. Separation of salt 
from saline water via an ion exchange resin is generally limited to small 
scale desalination projects since the chemicals required to regenerate the 
resins would become prohibitively expensive in large scale application. 
No reference in the prior art envisions the combination of energy recycle 
and low operating energy requirements which characterize the inventive 
process. 
SUMMARY OF THE INVENTION 
The instant invention comprises an apparatus and process for the 
evaporative purification of salt water and other impure water to provide 
fresh water. The system provides for the production of fresh water by a 
unique means such that the energy efficiency of the process is especially 
high and the corresponding cost per gallon of fresh water advantageously 
low. All raw materials for the process are available on site, the system 
has low power requirements and is simple to operate. The apparatus and 
process of the invention are useful for the purification of salt water, 
brine, brackish water, and other impure water which is capable of 
purification by traditional evaporation-condensation processes. The term 
"salt water" as used throughout the specification and claims hereof is 
thus intended to encompass salt water, including seawater, brine, brackish 
water, and other impure water which is capable of purification by 
traditional evaporation-condensation processes. 
The inventive apparatus is an apparatus for the evaporative desalination of 
salt water to provide fresh water comprising: 
heat exchange means having a liquid-receiving section with entrance and 
exit ports and a vapor-receiving section for passing cold salt water 
contained in the liquid receiving section into heat exchange contact with 
a flow of moisture laden comparatively warm air contained in the vapor 
receiving section so that fresh water may be condensed from the moisture 
laden comparatively warm air and the heat of condensation of that fresh 
water transferred to the comparatively cold salt water, the temperature of 
which will become comparatively warm; 
means, communicating with the exit port of the liquid-receiving section of 
the heat exchange means, for at least partially segregating the flow of 
salt water of comparatively moderate temperature into at least a first and 
second stream; 
means, having air inlet and air outlet ports, for contacting the first 
stream of the salt water of comparatively warm temperature with the flow 
of moist air which is subsequently passed into heat exchange relationship 
with the comparatively cold salt water and means for simultaneously 
segregating from the atmosphere and heating by solar means both the salt 
water of comparatively warm temperature and the flow of moist air, to 
cause evaporation of water into the air so that it becomes moisture laden; 
an evaporative medium positioned prior to the air inlet of the means for 
contacting the first stream of the salt water of comparatively warm 
temperature with the flow of moist air, so that ambient air may be passed 
through the evaporative medium to absorb moisture and become the flow of 
moist air which is contacted with the stream of salt water while both are 
heated by solar means; and means for conveying at least a portion, and 
preferably all, of the salt water of comparatively warm temperature into 
contact with the evaporative medium in order to wet the medium. 
More specifically, the apparatus preferably comprises, first, heat exchange 
means having a liquid-receiving section and a vapor-receiving section for 
passing cold salt water into heat exchange contact with a flow of moisture 
laden comparatively warm air. Fresh water may thus be condensed from the 
moisture laden comparatively warm air as the heat of condensation of the 
water vapor which becomes fresh water is transferred to the comparatively 
cold salt water, the temperature of which becomes comparatively warm. 
The liquid receiving section communicates with at least two conduits, each 
of which is designed to receive a portion of the flow of salt water of 
comparatively warm temperature. The first such conduit further comprises 
sides and a base having a solar absorptive surface over which the water 
flows, but is open at its top to a chamber intended to contain moist air 
and to contact this air with the upper surface of the salt water of 
comparatively warm temperature which flows through the conduit. (The moist 
air is subsequently passed into heat exchange relationship with the 
comparatively cold salt water.) The conduit and chamber are housed within 
a solar collector housing so that the water and air may be simultaneously 
heated, and water evaporated into the air to make it moisture laden. 
The second conduit and exit of the first conduit communicate with an 
evaporative medium positioned at the ambient air inlet to the 
vapor-containing chamber. The evaporative medium may thus be constantly or 
continuously wetted with salt water of comparatively warm temperature, and 
means are provided to force ambient air through the evaporative medium 
where it will pick up moisture before entry into the chamber. 
The inventive process is a process for the evaporative purification of salt 
water to provide fresh water comprising, first, passing comparatively cold 
salt water into heat exchange contact with a flow of moisture laden 
comparatively warm air so that fresh water is condensed from the moisture 
laden comparatively warm air and the heat of condensation of that water 
vapor (which becomes fresh water) is transferred to the comparatively cold 
salt water, the temperature of which becomes comparatively warm. The flow 
of salt water of comparatively warm temperature is then at least partially 
segregated into at least two streams. One such stream is contacted with 
the flow of moist air which is subsequently passed into heat exchange 
relationship with the comparatively cold salt water, and both this salt 
water of comparatively warm temperature and the flow of moist air are 
simultaneously heated by solar means to cause evaporation of water into 
the air so that it becomes moisture laden. An evaporative medium is wetted 
with at least a portion of the salt water of comparatively warm 
temperature, and ambient air is passed through the wetted evaporative 
medium so that the air absorbs moisture and becomes the flow of moist air 
which is contacted with the first stream of salt water while both are 
heated by solar means. Operation of the inventive system is preferably 
continuous. 
Alternatively, the inventive apparatus and process may include 
modifications permitting operation of the system on a floating platform. 
In its preferred form, the inventive apparatus further comprises a warm 
brine collection means for storage of the warm salt water exiting the 
process, and the process further comprises a sub-process for recycling 
this warm water through the system to produce additional fresh water at 
night or on cloudy days when solar heating is not available. The apparatus 
and process are preferably applied to the desalination of seawater or 
brackish water. 
In its most preferred form, the apparatus and process are modular, within a 
system of repeating units as described above. Since the performance of 
each module is substantially independent of the other modules, servicing 
and maintenance operations may be carried out without shutting down the 
entire system. Design around a standard module size also allows a wide 
range of production capacities, ranging, for example, from 10 to over 
10,000 tons of fresh water per day. Assembly of a desalination plant is 
simplified by the modular approach, since modules or parts of modules can 
be fabricated at a remote site, brought in to the assembly site, and 
assembled with a minimum of skilled local labor. 
The power requirements for parasitic pumps and blowers for the process are 
comparatively low, and are preferably supplied by a photovoltaic 
array/battery storage system. By using manifolds for air and water 
distribution in the design, a minimum number of pumps and blowers is 
required, thus simplifying the mechanical systems. This is a significant 
advantage in many applications of the invention, because skilled labor is 
often difficult to obtain in exactly those locales where demand for such a 
desalination system is greatest.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the apparatus of the invention comprises heat exchange 
means 1 having a liquid-receiving section 2 and a vapor-receiving section 
3 for passing cold salt water contained in the liquid receiving section 2 
into heat exchange contact with a flow of moisture laden comparatively 
warm air contained in the vapor receiving section 3 so that fresh water 
may be condensed from the moisture laden comparatively warm air. In the 
figure, the fresh water drips down and is collected in a fresh water 
storage means 4. The heat of condensation of the condensing water is 
transferred to the comparatively cold salt water, the temperature of which 
thus becomes comparatively warm. 
The liquid receiving section 2 of heat exchange means 1 communicates with 
at least two conduits 5, 6 each of which is designed to receive a portion 
of the flow of salt water of comparatively warm temperature. The first 
such conduit 5 further comprises sides 7 and a base 8 having an absorptive 
surface 9 over which the salt water of comparatively moderate temperature 
flows. The conduit 5 preferably communicates at its other extreme with an 
evaporative medium 12. This conduit 5 is open at its top to a chamber 10 
through which moist air flows in contact with the upper surface of the 
salt water of comparatively warm temperature flowing through conduit 5. 
(The moist air is subsequently passed into heat exchange relationship with 
the comparatively cold salt water.) The conduit 5 and chamber 10 are 
housed within a solar collector housing 11, preferably comprising a 
transparent cover or glazing, so that the water and air may be 
simultaneously heated by solar means, and water evaporated into the air to 
make it moisture laden. The conduit 5, chamber 10 and solar collector 
housing 11 are collectively referred to as the evaporator section of the 
inventive apparatus. 
The second conduit 6 which communicates with the liquid receiving section 2 
of heat exchange means 1 communicates at its other extreme with an 
evaporative medium or evaporative pad 12, which additionally functions as 
a heat exchange means. The evaporative medium 12 is positioned at the 
ambient air inlet 13 to the vapor-containing chamber 10. The evaporative 
medium 12 is continuously wetted by the comparatively warm salt water from 
conduits 5 and 6, so that salting out on the medium does not occur. One 
apparatus for accomplishing this wetting is illustrated in the figure, 
with pump 14 designed to pump salt water of comparatively warm temperature 
through conduit 15 to the top of the evaporative medium 12, through which 
the water descends. Means are provided to force ambient air through the 
evaporative medium 12 where it will pick up moisture and thermal energy 
before entry into the chamber 10. In the figure, this means is in the form 
of a fan or blower 16 positioned at the air outlet of the vapor receiving 
station 3 of heat exchange means 1. 
Alternative means for either completely or partially segregating the 
streams of comparatively warm salt water are contemplated. The function of 
the segregating means is to limit the mass of water on which the solar 
energy impinges so that the energy is imparted only to that mass which is 
required to humidify the air, and not largely wasted in heating other 
water. Referring to FIG. 4, instead of employing rigid conduits to 
segregate the streams, for example, a solar absorbing surface 9 
essentially floating on the bulk of the comparatively warm water 54 and 
with a thin film of water 52 on its upper surface, may be utilized. The 
solar absorbing means 9 need not be continuous, and may only partially or 
intermittently segregate the water to be evaporated from the bulk of the 
warm water stream. One means for accomplishing this is illustrated in FIG. 
5, i.e., floating discontinuous strips of solar absorbing material 9. It 
is also contemplated that the configuration of the solar absorbing surface 
9 may be other than flat. 
When the apparatus is applied, as is preferable, to the desalination of 
seawater or brackish water, the cold water to be desalinated will enter 
the system through conduit 17, which communicates with the liquid 
receiving section 2 of heat exchange means 1, and will be forced through 
the system by conventional pump means, illustrated in the figure as a 
single pump at 18. 
The process of the invention may be described as follows. 
Comparatively cold salt water 50 is passed into heat exchange contact with 
a flow of moisture laden comparatively warm air 51 so that fresh water is 
condensed from the moisture laden comparatively warm air and the heat of 
condensation given up upon formation of that fresh water is transferred 
insofar as practicable to the comparatively cold salt water the 
temperature of which becomes comparatively warm. The transfer of the heat 
of condensation from the condensing vapor to the salt water is not 
complete, because of the limitations inherent in commercially available 
heat exchangers, but is substantial. The recycle of this heat within the 
process provides a significant advantage in terms of increased fresh water 
production for the same input of energy from external sources. 
The flow of salt water of comparatively warm temperature is then at least 
partially segregated into at least two streams. One such stream 52 is 
contacted with the flow of moist air 53 which is subsequently passed into 
heat exchange relationship with the comparatively cold salt water, and 
both the salt water of comparatively warm temperature and the flow of 
moist air are simultaneously heated by solar means to cause evaporation of 
water into the air so that it becomes moisture laden. This contacting is 
preferably countercurrent, since the evaporation rate is proportional to 
temperature differentials between the air and water streams and to the 
relative velocity of the air with respect to the water. The optimum length 
of conduit 5 is determined by the cost of additional length vs additional 
fresh water produced. 
An evaporative medium 12 is continuously wetted with at least a portion and 
preferably all of the salt water of comparatively warm temperature 52 and 
54, and ambient air 55 is passed through the wetted evaporative medium 12 
so that the air absorbs moisture and becomes the flow of moist air 53 
which is contacted with the first stream of salt water 52 while both are 
heated by solar means. The preferred ratio of salt water directed through 
conduit 6 versus salt water directed through conduit 5 is about 5:1. By 
passing only a portion, rather than all, the salt water from the heat 
exchange means 1 into conduit 5, the water that does enter this conduit is 
heated more quickly, since the same solar input is impinging on a smaller 
mass of water. 
In a preferred embodiment of the invention, illustrated with reference to 
the FIG. 1, ambient air 55 is drawn from the atmosphere by a blower 16 
into an evaporative/heat exchange medium 12. A pump 14 is used to draw 
seawater 54 of comparatively warm temperature from conduits 5 and 6 
through the riser pipe 15 into the evaporative/heat exchange medium 12. 
The evaporative pad warms and humidifies the air as it is drawn into the 
chamber or tunnel 10. The evaporation chamber 10 is enclosed by a 
transparent glazing 11 as its housing on the top and a solar 
absorption/evaporation surface 9, preferably black in color, on the bottom 
8. The glazing may be either a non-concentrating or concentrating cover. 
In the case of a concentrating glazing, the solar absorption/evaporation 
surface 9 must be sized such that it does not exceed the solar 
concentrated radiation area at the absorber level. A thin film of warm 
seawater 52 from the condenser or heat exchange means 1 is introduced into 
conduit 5 and flows above the absorption/evaporation surface 9. As the 
seawater 52 travels down the conduit 5 open at its top to chamber 10, it 
is heated and partially evaporated by direct solar radiation. The 
now-saturated hot air 51 enters the condenser or heat exchange means 1 and 
is drawn through. The cooling water 50 for the condenser or heat exchange 
means 1 is pumped from an offshore subsea location. This indirect "solar 
energy" heating of the water and air is important in both equipment cost 
and energy savings. 
The fresh water which is condensed in the condenser or heat exchange means 
1 is collected in a fresh water storage reservoir or means 4. The 
processed brine overflows into a warm brine collection means 19. Overflow 
from the warm brine collection means or pond may be directed back into the 
sea or brackish water source. The warm brine 56 may be used at night to 
evaporate more water to augment daytime fresh water production. This 
results in excellent second law thermodynamic efficiency because 
successive moisture removal is achieved as the temperature of the water is 
degraded. 
As a result of the solar-assisted operation of the system, two sources of 
preheated salt water will be available for preparation of additional fresh 
water during periods of reduced insolation, i.e., at night or on cloudy 
days. All references to nighttime operation herein are equally applicable 
to operation on cloudy days. One source is the hold-up of salt water 52 of 
comparatively moderate temperature in the evaporator conduit itself; the 
second is the accumulation from solar-assisted operation of water 
discharged from the process into warm brine collection means 19. When 
sunlight is not available, three options are available to take advantage 
of the energy retained in these two reservoirs of warm water. 
Referring to FIGS. 1 and 2, the first and second options make use of the 
housing 11, which is cooled by nighttime air and radiative cooling to the 
night sky, as a condensing surface. In the first option, the sensible heat 
of the salt water 52 contained in the conduit 5 generates vapor which will 
humidify the air and condense on the underside of the cold housing surface 
11. This condensed water will run down the sloped, preferably glass 
surface of the housing 11 and accumulate in a drain 20 provided along the 
inside lower edge of the housing 11. The drain 20 communicates with the 
fresh water storage means 4 into which the fresh water flows. 
In the second option, water 56 in the brine collection means 19 is 
permitted to circulate through conduit 21 and over the evaporative medium 
12 while nighttime air 55 is drawn through the medium 12. This air then 
passes over the salt water of comparatively moderate temperature retained 
in the first conduit 5. The sensible cooling of these two brine sources 
provides the energy required to vaporize the water. Condensed water will 
once again run down the sloped surface of the housing 11 and accumulate in 
the drain 20 which communicates with the fresh water storage means 4. 
In the third alternative for nighttime operation, warm brine 56 from the 
brine collection means 19 is circulated over the evaporative pad 12 and is 
also recycled into conduit 5 via conduit 22. Implementation of this third 
alternative may or may not be desirable depending on temperature of the 
recycled water. Condensation may be effected on the glazing alone, or by 
operation of the heat exchange means 1. Warm air, saturated with water 
vapor, would be pulled through the heat exchange means 1 by the exhaust 
fan 16, and the brine would be discharged to the sea via conduit 23, by 
passing, as a result of closure of a valve, the brine collection means 19. 
The fan and pumps would operate at reduced output. The cold seawater 
circulated through the heat exchange means 1, depending on relative 
temperatures, may be discharged immediately to the sea through conduit 24 
or may be combined with the warm brine entering conduit 5. 
As a further alternative illustrated in FIG. 3, the apparatus and process 
may be located on a conventional floating platform 25, floating on the 
surface of the salt water 26. This embodiment parallels the embodiments of 
the invention as operated on land with two significant differences. First, 
since the air on the surface of the salt water should already be largely 
saturated with moisture under ambient conditions, the evaporative medium 
may not be required. Secondly, the salt water of comparatively warm 
temperature exiting the heat exchange means 1 which does not pass over the 
solar absorbing surface 9 is commingled with the seawater on which the 
surface 9 rests or floats, as is the remainder of the salt water of 
comparatively warm temperature after it has passed over the solar 
absorbing surface. Convection currents operate to keep the warmest 
seawater in the locale immediately under the absorbing surface 9. 
Several embodiments of the inventive apparatus and process will be 
illustrated through the example of a seawater desalination plant located 
on the seashore in a desert locale. Typical values of various operating 
parameters for a single modular section during daytime performance are set 
forth in Table I. 
TABLE I 
______________________________________ 
Dew Point (D.P.) of Inlet Air 
32.degree. C. 
Length of First Conduit (Evaporator 
Section) 200 m 
Width of First Conduit (Evaporator 
Section) 0.6 m 
Height of First Conduit (Evaporator 
Section) 0.3 m 
Average Daily Insolation 
5 kWh/m.sup.2 
Average Solar hour/day 
8 h 
Average Insolation/hr kWh/m.sup.2 
0.625 
Density of Dry Air at 30.degree. C. 
1.169 kg/m.sup.3 
Depth of First Conduit 
.06 m 
Daily Average Solar Collection 
Efficiency (heat used in process/total 
solar radiation at surface of module 
glazing) (n.sub.c) = .65 
Specific Heat of Air, C(a) = [.2385 + w (.48)] w = water content in 
##STR1## 
Specific Heat of Water, C(s) 
##STR2## 
Density of Seawater, D(s) 
1.025 kg/l 
Air Flow Rate, F(a) 
##STR3## 
Mass Flow Rate of air, m(a) 
##STR4## 
Specific Heat of Fresh Water C(w) 
##STR5## 
Average Ambient Air Temperature 
During Solar Hours (yearly basis) 
35.degree. C. 
Average Nighttime Ambient Temperature 
20.degree. C. 
Average Daytime Dew Point (D.P.) at 
32.degree. C. 
Average Nighttime Dew Point (D.P.) at 
18.degree. C. 
Temperature of Cold Seawater 
22.degree. C. 
______________________________________ 
Ambient air enters the system and passes through an evaporative pad in 
which the seawater of comparatively warm temperature gives up heat and 
water vapor to the air. This step takes advantage of the heat remaining in 
the brine and provides warm, saturated air at the exit of the evaporative 
pad. The spent brine, at 39.degree. C., is directed to the warm brine pond 
where it can be reused to generate fresh water during nighttime operation. 
At the point where the air exits the evaporative pad and enters the chamber 
above the solar evaporative pond, the dry bulb temperature is 42.2.degree. 
C., the dew point is 42.2.degree. C., the enthalpy of the air is 
5.65.times.10.sup.-2 kWh/kg and the energy flow rate associated with the 
air flow is 79.26 kWh/hr. 
Solar insolation passes through the cover glazing of the solar evaporative 
pond where some of it is absorbed. The remaining solar radiation is 
absorbed by the dark-colored channel (solar absorption surface) over which 
the seawater passes. The seawater picks up heat from the dark-colored 
channel and its temperature rises. From the point where the air enters the 
evaporation chamber, the saturated air traverses the length of the solar 
evaporative pond picking up heat and humidity from the solar heated water 
above the absorbers. It gains heat from the seawater through convective 
heat transfer. (Some heat is lost from the air to the cover glazing 
through similar convective heat transfer mechanisms.) The seawater cools 
while it supplies the latent heat of vaporization for the water vapor 
added to the air stream. 
The interplay of combined heat and mass transfer from the solar input 
results in a warm, nearly saturated air stream at the exit from the solar 
evaporative pond. At this point, the dry bulb temperature of the air is 
55.degree. C., the dew point of the air is 52.degree. C., the enthalpy of 
the air is 9.10.times.10.sup.-2 kWh/kg, and the energy flow rate 
associated with the air flow is 127.7 kWh/hr. 
From this point, the warm, humidified air is directed past the condenser 
cooled by cold seawater where a substantial amount of the water contained 
in the air stream is recovered in the form of fresh water. The fresh water 
produced in the condenser drains to a fresh water holding tank for later 
use. At the exit from the condenser, the dry bulb temperature of the air 
is 25.degree. C., the dew point of the air is 25.degree. C., the enthalpy 
of the air is 26.3.times.10.sup.-3 kWh/kg, and the energy flow rate 
associated with the air flow is 36.9 kWh/hr. At the inlet for cold 
seawater to the condenser, the flow rate of the seawater is 2612.3 kg/h at 
a temperature of 22.degree. C. The exit temperature of the seawater from 
the condenser is 52.degree. C. This warmed seawater travels the length of 
the module toward the air inlet gaining solar energy. 
The heat gain of the seawater passing through the condenser is equal to the 
reduction in enthalpy of the air passing through the condenser. The heat 
removed in the condenser shows up primarily as the latent heat of 
vaporization of the water vapor into the air stream, plus a change in the 
sensible heat content of the air stream. Thus, in the condenser, 91.2 
kWh/hr of energy is moved from the air stream and transferred to the 
seawater. This results in a fresh water production rate of 109.4 kg/h, or 
875.2 kg per eight hour day per module. Nighttime production easily 
provides an additional 125 kg/day for a total of 1000 kg fresh water per 
day per module. 
As a result of the daytime operation, there will be, as indicated above, 
two preheated volumes of seawater. One is the hold-up of salt water of 
comparatively warm temperature maintained in the evaporator channel itself 
which amounts to 7380 kg at a temperature of about 52.degree. C. A second 
amount is the accumulation from the daytime operation that is held in a 
warm brine pond. This is equal to 19,884 kg, but at a temperature of 
39.degree. C. 
Employing the first (completely passive) option for nighttime operation, 
the sub-process is tantamount to a conventional solar still. Values 
reported on such operation indicate that over a 14 hour period, a 
production of approximately 1.7 kg/m.sup.2 can be expected. With a total 
area of 120 m.sup.2, over the same 14 hour period, 200 kg will be 
produced. 
The second option takes advantage of the availability of the active 
componentry necessary for daytime operation. In this case, the water in 
the storage pond (39.degree. C.) will be allowed to circulate over the 
evaporator pad while nighttime air is drawn through the pad. This air then 
passes over the evaporator pond (52.degree.). The sensible cooling of 
these two brine sources provides the energy required to vaporize the water 
that is transferred into the cooler night air with a dew point of about 
18.degree. C. This air, saturated with water vapor is directed over the 
condenser by the exhaust blowers and the brine is discharged to the sea. 
As a result, about 650 kg of product water would be produced during 14 
hours of nighttime operation. 
Values for energy use by the desalination system are presented for a solar 
input of 3.25 kWh/m.sup.2 /day and parasitic blower and pump electrical 
loads of 7.23.times.10.sup.-2 kWh (electric) per square meter per day (at 
0.5 kW for eight hour daytime production and 0.33 kW for fourteen hour 
nighttime production from the 120 square meter desalination system). 
Performance of the desalination system is summarized in Table II. 
TABLE II 
______________________________________ 
SUMMARY OF DAILY PERFORMANCE OF 
DESALINATION SYSTEM 
OPTIONS FOR NIGHTTIME 
OPERATION 
OPTION I OPTION II 
PASSIVE TIALLY 
SYSTEM ACTIVE SYSTEM 
______________________________________ 
Daytime fresh water 
7.29 kg/m.sup.2 
7.29 kg/m.sup.2 
production 
Nighttime fresh water 
2.00 kg/m.sup.2 
5.00 kg/m.sup.2 
production 
Total fresh water production 
9.29 kg/m.sup.2 
12.29 kg/m.sup.2 
Parasitic Electrical energy 
1.3 kWh/m.sup.3 
2.8 kWh/m.sup.3 
requirements 
______________________________________ 
Total daily fresh water production is 12.29 kg/m.sup.2 /day using option II 
and 9.29 kg/m.sup.2 /day for option I. For the 120 square meter module of 
this example, daily fresh water production ranges from 1475 kg/day to 
1114.8 kg/day. Solar energy requirements range from 0.27 kWh/g for option 
II to 0.36 kWh/kg for option I. Parasitic electrical energy requirements 
supplied by the photovoltaic array with battery storage are quite low 
because blower and pumping loads are quite modest. Parasitic electrical 
energy requirements range from only 1.3.times.10.sup.-3 kWh/kg for option 
I to 2.8.times.10.sup.-3 kWh/kg for option II. 
Although option II indicates greater power consumption per cubic meter of 
product water, it must be noted that the fresh water produced is 32% 
greater. This could be reflected in a smaller plant size. 
The relatively low power requirements on the order of 1.3 to 2.8 
kWh/m.sup.3 of fresh water produced for operation of the pumps and blowers 
is preferably met by photovoltaic cell generation of electricity. Other 
alternatives, which may be used individually or in combination, include 
operation of a Rankine cycle based on the difference between the 
temperature of the surface salt water and the temperature of the water at 
some depth, concentration of solar energy on a boiler to produce steam 
which powers a turbine and generator, gravity return of the reject brine 
to the salt water source serving to operate a hydropower turbine, wind 
power, wave power, osmotic power, or hydropower generated by the level 
differential between a pond filled at high tide and the level of seawater 
at low tide. Electrical storage batteries may be employed to provide the 
power requirement for nighttime operation and to restart the equipment 
during the day. For infrequent occasions of inclement weather when several 
starts are required before the batteries can be recharged, make-up power 
may be provided by combustion engine driven motor generator sets or by 
purchased electric utility power. 
While the details of the inventive apparatus and process may be varied 
within the general inventive concept, specifics of the preferred mode of 
operation will now be described, assuming six thousand modules associated 
in a single desalination plant. A number of modules associated in a single 
desalination plant are illustrated in FIG. 6, wherein reference numerals 
and the elements to which they refer correspond to like numerals and 
elements in FIG. 1. 
The modularized components include: 
Evaporative Pad 
Pumps for Evaporative Pad 
Evaporator Section 
Condensers 
Fans 
Fresh Water Storage 
Mechanical Tube Cleaning System 
Items which more economically service the system in larger units include: 
Cold Water Pipe 
Cold Water Pump 
Control Room 
Operations Building 
Product Water Pump 
It appears that the inventive process requires less power than existing 
desalination systems. Total power requirements are only in the range of 
1.3 kW-hr per cubic meter of product water. However, the process is very 
sensitive to systems pressure drop. Large hourly flows of air 
(8.4.times.10.sup.6 kg) and water (1515.times.10.sup.6 kg) cause large 
increases in power consumption for even minor rises in pressure drop. A 
practical objective for power requirements is 1.0 MW distributed as 
follows: 
______________________________________ 
Power Consumption Pressure Drop 
______________________________________ 
Fans 230 kW 7.6 mm water 
Pumps 420 kW 5.2 m water 
Other 350 kW -- 
Total 1000 kW 
______________________________________ 
Another unique characteristic of the inventive process is the relative 
simplicity of its major fluid systems. Air systems may have no controls or 
valves. Flow is fixed by the relatively low resistances through the 
evaporative pads, evaporator sections, and condensers. 
The main water system also operates with a minimum number of controls or 
control valves. The cold water pump provides sufficient head to force the 
water through the condensers. The water flows through the solar pond and 
back to the sea by gravity. 
PUMPS AND SCREENS 
Cold Water Pumps 
The cold water pumps are located in the basement of the cold water pump 
house near the shore. Vertical pumps are used with their barrels located 
below the low tide level to preclude a need for priming. To minimize 
friction drop and pressure losses, the system has no valves. Velocity is 
held below 1.2 m/sec, and isolation of the individual modules when 
required is with a simple flange blind. The pump discharge head is 
determined in part by the elevation of the inventive apparatus above sea 
level. Total head is in the range of 8-12 meters. This allows low speed 
and promotes long impeller and bearing life. For purposes of water power 
consumption calculation, it is assumed that the elevation of ponds is 1.5 
m above sea level. 
Three 50 percent capacity pumps are provided so that 100 percent output can 
be maintained when any one pump is down for repairs. 
A trash screen is located on the end of the cold water pipe. Fine cleaning 
is done on shore using a sand bed type filter, traveling screen or other 
type cleaner. 
Pumps for the Evaporative Pad (Evaporator Pumps) 
Evaporator pumps are provided for each of the modules to pump water from 
the solar ponds over an air-water contacting evaporative medium. Pump head 
is low (2.5-3.0 meters) and flow is approximately 2.6.times.10.sup.5 
kg/hr. Because the contacting medium requires flow through small passages, 
it is necessary to strain the water at the pump suction. Redundant or 
self-cleaning screens are necessary to assure continuous operation without 
plugging. 
The pumps operate at low speed and with materials of construction selected 
to resist seawater corrosion. Twenty-year life with minimum maintance 
should be readily achieved. 
Fans 
Fan operation is second only to water pumping in consumption of power. 
Sixty fans are provided each with a capacity of approximately 2,440 cu m 
per minute at approximately 50 mm water column and each requiring a 30 kW 
motor. The objective is to obtain a pressure drop distribution as follows: 
______________________________________ 
Evaporator 1.40 
Entrainment Separator 1.25 
Pond Friction 2.50 
Condenser 2.50 
TOTAL 7.65 mm 
______________________________________ 
Fans handle humid coastal air and are subject to nighttime dew point 
condensation and to airborne salt from winds over the sea. Conditions are 
mildly corrosive and carbon steel construction coated with an organic 
polymer should provide 20-year life. 
Cold Water Pipe and Warm Water Discharge 
The cold water pipe, 2.25 m in diameter, supported on the bottom and 
anchored against local current, is run from the coldest available point to 
the desalination plant located on shore. At the velocity of the cold water 
flowing in this size pipe, heating from the warm outside water near the 
surface and from the buried section of the pipe on land has only a small 
warming effect on the water. On shore, the 2.25 m cold water pipe is 
buried and run into a manifold for distribution to the modules. 
The intake of the cold water pipe is located at a sufficient distance above 
the sea bottom to preclude entry of agitated mud and sand. A screen 
located over the opening prevents entry of fish and sea plants. 
Warm discharge water from the inventive apparatus is returned to the sea in 
an open trench. Concentration and temperature of the discharged brine 
depend on the option choosed for the night time desalination process and 
the ambient conditions. The increase in salt water salinity would be in 
the range of 3% to 5%. 
Materials of construction for optimal life and cost include steel, 
concrete, concrete lined steel, polyethylene pipe, fiberglass reinforced 
plastic pipe and plastic pipe liners. 
Evaporative Pad 
The evaporative pad serves to recover heat from the warm seawater before it 
is discharged from the system, and to transfer it to the entering air. To 
keep pressure losses low, the velocity of air entering the evaporator is 
held to approximately 30 meters per minute. The air entry is through an 
opening approximately 1.2-1.5 m high extending the full width of the 
module. A bird screen or other low pressure drop device may be added over 
the opening if required. 
Many types of contacting devices which will achieve good heat transfer 
between air and water are available. Standard chemical plant type packing 
such as Pall rings, Raschig rings, Intalox saddles and other free packing 
materials may be used. Low pressure drop preformed plastic shapes with 
many tortuous paths integrally molded into large block assemblies are also 
available for this task. 
Biofouling is controlled by chlorination. Massive kill doses are applied 
for short periods to one module at a time (of the sixty which make-up the 
plant) so that the overall level in the plant effluent will always be 
within acceptable limits. Salting out, if it occurs, is washed out by 
circulating fresh water over the contact material at night or during 
scheduled maintenance. 
Because of the low air velocities through the evaporative pad, salt water 
entrainment is negligible. Entrainment which does occur settles out over 
the 200 m air passage over the solar pond in the evaporator section. If 
too much salt carries over to the condenser, an extrainment separator 
located downstream of the condenser may be employed. 
Evaporator Section-Solar Pond Enclosure 
The enclosure for the solar pond consists of four basic components: (1) the 
housing or cover; (2) the support structure; (3) the absorptive surface or 
absorber and; (4) the first conduit for salt water of relatively warm 
temperature. 
The cover has two basic functions. It must allow the energy required for 
evaporation (sunlight) to reach the feed water and must keep this energy 
available for the evaporative process. Keeping the energy available 
involves limiting reradiation, convection and conduction heat transfer and 
ambient air infiltration. 
In addition to these basic functions, the cover must be an effective 
surface for nighttime condensation, must have the potential for 20-year 
life and must be easily cleaned and maintained. 
Glass appears to best meet all these criteria. It has a transmissivity of 
approximately 85% in the visible and ultraviolet range and is virtually 
opaque to infrared reradiation. It is impervious to air and water and can 
be mounted with any number of leak-tight sealant-gasket combinations. 
Glass is an effective condensing surface, and can be treated to be an 
excellent surface. (A good condensing surface is conducive to liquid 
formation and runoff without droplet formation.) Glass is also very hard, 
abrasion resistant and durable with a life of well over 20 years barring 
catastrophic occurrences. Its hard, smooth surface is conductive to simple 
cleaning. Its availability and relatively low cost make replacement viable 
when failures do occur. 
The most viable alternatives to glass are plastic film or sheet. Most of 
these materials have about the same transmissivity as glass in the visable 
and ultraviolet range but also have a high value of transmissivity for 
infrared radiation, thus allowing the pond heat to reradiate. Plastics, in 
general, are poor surfaces for condensation and must be treated for 
acceptable service. The most serious drawback for plastics is the low 
tolerance to heat and ultraviolet radiation. Generally, thickness must be 
increased to yield a durable surface. This increases weight, decreases 
transmissivity and increases cost. 
The support structure for the cover must be strong enought to support the 
glazing under all types of weather conditions. It must accomplish this 
while keeping the size of the members to a minimum. This not only reduces 
the weight (i.e., cost), but also limits the interference of the structure 
with solar radiation needed by the pond water. The structure must be 
corrosion resistant to provide low maintenance and a 20-year life. It 
should also be easily and cheaply assembled and installed. 
These criteria appear to be best met by a framework of fiberglass 
reinforced plastic structural members. This material is strong, 
lightweight, noncorrosive and cost competitive with metals having similar 
properties (i.e., aluminum, stainless steel). 
The inventive apparatus and process as set forth in this embodiment employ 
a black absorber sheet approximately one-half inch below the surface of 
the pond water. The solar radiation absorber is a vital part of the 
desalination system. For the system to work well, the solar radiation must 
be converted to usable long-wave length energy (heat) and transferred to 
the feed water. This absorber must be durable in salt water and resistant 
to biofouling. Also, it should be inexpensive and easily replaced if 
damage or severe biofouling occur. 
One absorber which fits these criteria is a semirigid plate of black 
polyethylene or polypropylene. The plate is simply supported by "legs" of 
the desired length. The plastic material is unaffected by temperatures in 
the plant operating range and is resistant to ultraviolet radiation damage 
when blackened as required. Using mass production fabrication techniques, 
large, strong absorber plates can be produced quickly and cheaply. 
The final component of the solar pond is the first conduit for salt water 
of relatively moderate temperature. This is the reservoir for the feed 
water. It must be water tight, resistant to salt water attack and 
biofouling, and be easily cleaned. The base should also limit heat loss to 
the earth below and have a 20-year life. 
The best alternative appears to be a concrete or packed earth base with a 
heavy plastic liner. 
Condenser 
The heat exchange means or condenser is a critical design element because 
of its potentially adverse impact on plant power consumption and the 
difficulty in controlling corrosion and biofouling. The latter problems 
are controlled by mechanical cleaning, careful selection of materials and 
chlorination. 
Low pressure drop, i.e., low velocity, is necessary to keep power 
requirements low. But low velocities decrease the rate of heat transfer 
and allow some types of crustaceans to adhere to tube walls. Compromise 
between the cost of the heat exchangers and the cost of the power supply 
system is necessary. 
The condenser configuration may be a conventional single pass straight tube 
design with water flow inside the tubes and air outside, finned tubes, or 
pressed frames and brazed sections. One or more condenser units may be 
employed. 
Biofouling Control 
Desalination systems are potentially quite vulnerable to reduced 
performance due to biofouling. There are two basic form of fouling of 
concern: Microbiofouling, the formation of a "slime" layer, on surfaces 
exposed to water; and macrobiofouling, the growth of larger marine 
organisms such as barnacles, mussels or hydroids. Microfouling is always a 
precursor to macrofouling. It is not always the case, however, that 
macrofouling will develop on all exposed surfaces. Where there is a 
continuous fluid flow through a system at rates above 1 meter per second, 
macrofouling organisms and their larvae do not attach surfaces. Thus, even 
though a wetted surface will develop a microfouling layer, fluid velocity 
can avert macrofouling growth. Usually, though, a system cannot be 
designed to assure a continuous minimum flow velocity everywhere. 
Maintenance shutdowns, for example, are necessary but will allow 
macrofoulers to attach and grow during subsequent operations. 
The proposed system has several areas were fouling is of concern. 
Intake screens. Two kinds of intake screens are employed: an outer velocity 
cap screen designed to prevent ingestion and entrainment of fish and 
similarly sized marine bicta; and an inner fine screen to capture smaller 
organisms (such as shrimp). 
Velocity cap designs are intended to reduce water velocity and to direct 
intake flows along horizontal gradients. Fish are capable of resisting 
large horizontal velocities, but do not seem to be able to evade vertical 
velocity streams. In any case, velocity caps are susceptable to 
macrofouling. 
The inner intake screens are finer in mesh size. A variety of designs are 
well developed for use in power plant service. Traveling screens of 
appropriate design are replaced and cleaned without interrupting flow. In 
addition, chlorine is injected into the intake system before flow reaches 
the inner screens. This reduces or even eliminates macrofouling of the 
screens. 
Condenser tubing. Macrofouling will eventually plug condenser tubes, and 
must be prevented by strong measures. Microbiofouling layers, if 
unchecked, build up on condenser tubes, reducing effectiveness of heat 
transfer, and thus reducing plant efficiency. A combination of chlorine 
injection and mechanical cleaning with a system using circulating sponge 
rubber balls or brushes (using reverse flows to move brushes through the 
tubes) prevents macrofouling and controls microfouling to tolerable 
levels. 
Solar heating pond. Given the operating temperatures reached in the pond, 
macrofouling organisms are in general killed, but microfouling bacteria 
may survive. Periodic antifouling measures include chlorination and/or 
periodic stopping of pond flow to allow the water temperature to build up 
to levels that will be lethal to microorganisms (e.g., 60.degree. C.). 
Finally, heavier doses of chlorine are required in some circumstances to 
purify the water and make it environmentally suitable for returning it to 
the sea. 
Evaporative Pad: The heated water from beneath the solar pond passes 
through the matrix of the evaporative pad before being discharged. 
Biofouling is generally controlled upstream of this point. 
Chlorination 
Chlorination is necessary to prevent biofouling of the condenser heat 
exchanger internal tubing surfaces, to prevent algae and growth in the 
solar pond and product water pools and to assure a low bacterial count in 
the product water. 
The quantity of chlorine required for treating seawater varies considerably 
with the water intake condition and normally ranges 1-10 ppm. However, 
this demand does not have to be met if chlorine is to act only as a 
deterrent. Very low sublethal doses of chlorine if injected continuously 
repel hard-shelled organisms and also inhibit microorganisms. A dose of 
0.05 ppm is adequate for partial control, with higher concentrations 
injected periodicaly to produce a kill in selected areas. For a chlorine 
concentration of 0.05 ppm for the inventive apparatus and process, 10 kg 
of available chlorine per day is required for the seawater system. Less 
than 1 kg is sufficient for the product water pools. 
Because power is at a premium, chlorine is purchased as concentrated sodium 
perchlorate solution rather than generated on site. 
Four pump transfer and metering units are provided, two each for the 
seawater and potable water systems. One set in each system is a redundant 
standby to assure continuous operation during unanticipated daytime 
maintenance. 
Chlorinator transfer and metering systems are well developed and reliable 
components are described in the literature. 
Mechanical Tube Cleaning 
To keep the heat transfer surfaces in the condenser free from fouling, in 
addition to chlorination, a mechanical cleaning system is provided. 
Separate mechanical cleaners are installed at each of the modules. 
Both mechanical and chemical tube cleaning processes have been used to 
remove fouling from tube surfaces that come in contact with seawater. Of 
the mechanical systems available, Amertap is the system most easily 
adaptable to the instant process. Most of the equipment required for this 
operation is external to the heat exchangers. It consists of a ball 
storage tank, which may include counters and graders to detect ball wear; 
ball feed pumps and lines, collection screens to prevent the release of 
rubber balls into the water stream, and lines to return balls from the 
collection screens to the ball storage tanks. 
The balls are randomly distributed to all the tubes in a given heat 
exchanger. In usual applications, the number of balls in circulation 
through a heat exchanger at any given time corresponds to about 10 percent 
of the number of tubes. This provides an average tube cleaning frequency 
of about once every 5 to 10 minutes. The number of balls in the system can 
be adjusted, within certain limits, to provide the desired cleaning 
frequency. 
The power requirement for the Amertap recirculating pumps is less than 0.5 
kW per module. 
Fresh (Product) Water Storage 
The product water storage consists of 60 modularized pools located below 
the condenser piping systems. Each pool has a capacity of 1000 cu m (ten 
days production), and is roughly 1.5 m deep by 40 m long by 16 m wide. The 
pools feed a common buried water header. 
Solar Power Subsystem 
Requirements for the solar power subsystem are: 
Normal operating power must be derived from direct or indirect solar energy 
sources and must be sufficient for running all site auxiliary systems. 
The system must be capable of normal restarts from the solar energy source 
or from stored energy obtained from a solar source. 
A fossil fuel back-up power system can be provided for emergency 
conditions. 
The energy delivery system and controls and control room equipment are 
designed along conventional lines. 
The power system selected for the instant example of the inventive process 
is a solar cell collector system with battery storage and with DC/AC 
converters to operate motors, provide lighting and other site power uses. 
It should be understood that the foregoing disclosure emphasizes certain 
specific embodiments of the invention and that all modifications or 
alternatives equivalent thereto are within the spirit or scope of the 
invention as set forth in the appended claims.