Radiative- and artificial-cooling enhanced dew collection in a coastal area of South Australia

Article (PDF Available)inUrban Water Journal 11(3) · April 2014with 93 Reads
DOI: 10.1080/1573062X.2013.765494
Abstract
Dew yield can be increased by artificial cooling at a cost of energy consumption. To examine how much enhancement in dew yield can be achieved by artificial cooling, and how this enhancement varies with meteorological conditions and collector materials, a dew collection experiment was performed over a month (April–May) in a coastal area of South Australia. Four collectors made of two different materials (aluminium and Teflon) were tested. Evaluated over the whole dew collection period, without artificial cooling, the Teflon collector is on average 120% more efficient than the aluminium collector. With artificial cooling, the Teflon collector is about 20% more efficient than the aluminium collector. The artificial cooling enhances dew formation close to 45% for the Teflon collectors, while the enhancement is over 150% for the aluminium collectors. The enhancement magnitude is dependent on meteorological conditions.
Figures - uploaded by Huade Guan
Author content
All content in this area was uploaded by Huade Guan
This article was downloaded by: [Flinders University of South Australia], [Dr Huade Guan]
On: 19 May 2013, At: 19:11
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,
37-41 Mortimer Street, London W1T 3JH, UK
Urban Water Journal
Publication details, including instructions for authors and subscription information:
http://www.tandfonline.com/loi/nurw20
Radiative- and artificial-cooling enhanced dew
collection in a coastal area of South Australia
Huade Guan b b , Megan Sebben b b & John Bennett a
a School of the Environment, Flinders University , Australia
b National Centre for Groundwater Research and Training , Australia
Published online: 16 May 2013.
To cite this article: Huade Guan , Megan Sebben & John Bennett (2013): Radiative- and artificial-cooling enhanced dew
collection in a coastal area of South Australia, Urban Water Journal, DOI:10.1080/1573062X.2013.765494
To link to this article: http://dx.doi.org/10.1080/1573062X.2013.765494
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any substantial or systematic
reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to
anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contents
will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should
be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims,
proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in
connection with or arising out of the use of this material.
RESEARCH ARTICLE
Radiative- and artificial-cooling enhanced dew collection in a coastal area of South Australia
Huade Guan
a,b
*, Megan Sebben
a,b
and John Bennett
a
a
School of the Environment, Flinders University, Australia;
b
National Centre for Groundwater Research and Training, Australia
(Received 25 May 2012; final version received 26 December 2012)
Dew yield can be increased by artificial cooling at a cost of energy consumption. To examine how much enhancement in
dew yield can be achieved by artificial cooling, and how this enhancement varies with meteorological conditions and
collector materials, a dew collection experiment was performed over a month (April May) in a coastal area of South
Australia. Four collectors made of two different materials (aluminium and Teflon) were tested. Evaluated over the whole
dew collection period, without artificial cooling, the Teflon collector is on average 120% more efficient than the aluminium
collector. With artificial cooling, the Teflon collector is about 20% more efficient than the aluminium collector. The artificial
cooling enhances dew formation close to 45% for the Teflon collectors, while the enhancement is over 150% for the
aluminium collectors. The enhancement magnitude is dependent on meteorological conditions.
Keywords: dew collection; artificial cooling; radiative cooling; Teflon; aluminium; South Australia
1. Introduction
Dew plays important environmental roles, such as
providing moisture for plants and animals, contributing
to rock weathering, enhancing soil biochemical functions,
and assisting the spreading of plant disease (Agam and
Berliner 2006). For some vegetation species in semiarid
environments, direct foliar absorption of dew reduces
drought stress to these species (Munne-Bosch et al. 1999).
Even without foliar adsorption, dewfall in dry season is
reported to strengthen tree survival by reducing transpira-
tion in the morning (Barradas and Glez-Medellin 1999). In
urban environments, dew formation can enhance the
removal of atmospheric contaminants (Richards 2004,
Okochi et al. 2005). However, dew formation is reduced or
absent in urban areas in comparison to the natural
environments, due to a common urban heat island
phenomenon and a reduced atmospheric moisture supply
(Richards 2004, Ye et al. 2007).
In some situations, dew may become a significant
potable water source, complementary to conventional
water supply (Muselli et al. 2002, Sharan et al. 2007). Over
the last decade there has been an increased research interest
in dew, including collector design (Nilsson et al. 1994,
Muselli et al. 2002, 2006a, Jacobs et al. 2008, Clus et al.
2009), dew chemistry (Takenaka et al. 2003, Muselli et al.
2006b, Lekouch et al. 2010), dew modelling (Jacobs et al.
2002, Beysens et al. 2005, Richards 2009), and dew in
natural environments (Ye et al. 2007). It is now known that,
by radiative cooling, maximum dew yield is 0.8 mmday
21
(Sharan et al. 2007). The maximum radiative-cooling dew
collection achieved is 0.6 mmday
21
in a semiarid area
(Berkowicz et al. 2004).
Comparing to rainfall, dew formation is more
temporally reliable, because occurrence of meteorological
conditions for dewfall is more frequent than for rainfall
(Muselli et al. 2002). Thus, dew has potential to be a useful
complementary source to conventional water supply.
However, the dew yield from radiative cooling, in which
the net energy loss rate of the surface resulting from the
surface emitting thermal radiation less the down-welling
thermal radiation absorbed by the surface at each specific
time interval, is usually less than 10 mmyr
21
(Beysens
et al. 2005, Sharan et al. 2007). This low yield limits its
usefulness for potable water supply. Dew yield can be
increased by artificial cooling at a cost of energy
consumption. In situations where water shortage is more
critical and serious than energy shortage, artificial-cooling
enhanced dew collection may become a societally and
even economically feasible solution (Al-Jalil et al. 2007).
Artificial cooling, together with desiccant, has been
applied in dehumidifiers, in which fans are used to inject
airflow to the device (Liu et al. 2009). However, few
studies have examined the effects of artificial cooling on
dew formation. In this paper, we report the results of a dew
collection experiment with artificial cooling by using ice
bricks, in comparison to those from radiative cooling. Our
objectives are to show (1) how much enhancement in dew
yield is achieved by artificial cooling, and (2) how this
q2013 Taylor & Francis
*Corresponding author. Email: huade.guan@flinders.edu.au
Urban Water Journal, 2013
http://dx.doi.org/10.1080/1573062X.2013.765494
Downloaded by [Flinders University of South Australia], [Dr Huade Guan] at 19:11 19 May 2013
enhancement varies with meteorological conditions and
collector materials. In this study, we use six ice bricks to
provide sufficient and identical cooling sources for the dew
collectors, to examine the maximum enhancement that can
be achieved, and the micrometeorological factors influen-
cing this enhancement.
2. Method
2.1 The experimental site and dew collectors
The study site (35.068S, 138.668E) is located in the
Adelaide Hills of South Australia, about 15 km from the
coast, at an elevation of 400 m above sea level. Climate at
this site is of Mediterranean type, with a dry summer in
December through February, and a wet winter in June to
August. The annual precipitation is estimated to be about
900 mm, based on two nearby Australian Bureau of
Meteorology stations (Belair and Stirling), which is larger
than the nearby semiarid Adelaide plain of an annual
precipitation around 550 mm. Annual mean daily mini-
mum temperature is 98C, and mean daily maximum
temperature is 198C.
The experiment was conducted in a domestic backyard
consisting mainly of lawn on a sandy soil base (Figure 1). It
was well sheltered from the wind with eucalyptus forest to
the east, fences and vegetation to the north and south, and a
house ten meters to the west. Four dew collectors, two with
a 2 mm-thick polytetrafluoroethylene (Teflon
w
) surface
and two with a 3 mm-thick aluminium surface, were used in
the study. Teflon has been commonly used for dew
collection (Takenaka et al. 2003); aluminium is among the
most frequently used materials, and has been used for dew
collection experiments (Takenaka et al. 2003). The
collector sheet, 30 £60 cm
2
, was placed on a 7 cm
polystyrene foam, which was mounted on a wood frame,
so that the surface was tilted at a 308angle (Beysens et al.
2003) (Figure 1). The average collector surface height was
about 40 cm. An aluminium gutter was attached to the
down-slope end of the collector surface, with a 58angle to
facilitate dew water collection (Figure 1). The collectors
were placed at the site to maximise the sky-view angle, and
oriented so that their rear open side faced the house
10 meters away to the west. For one of the two Teflon and
aluminium collectors, an opening in the polystyrene foam
was created for the artificial cooling source (Figure 1).
Hereafter, the four dew collectors are referred to as TF1
(Teflon collector), TF2 (Teflon collector with artificial
cooling), AL1 (aluminium collector), AL2 (aluminium
collector with artificial cooling).
Six frozen bricks (Willow
TM
Ice Brick) were placed in
the opening between the collector sheet and the
polystyrene foam, with about 0.5 cm air space between
the bricks and the collector sheet when the bricks were in
place. The frozen bricks were inserted to the collector
shortly after sunset each day, and removed in the morning
to be refrozen. During the experiments, it was observed
that the cooling energy had not been used up when dew
was collected in the morning. This ensured that the ice
bricks provided sufficient cooling sources to enhance dew
yield throughout the night. To estimate how much energy
has been absorbed by the ice brick during the night, one
frozen brick and one used brick were put into a water-filled
thermal-isolated containers, respectively. The frozen brick
absorbed 131 kJ energy from the water, while the used
brick absorbed 31 kJ energy from the water, with slightly
different equilibrium water temperature. Based on this
measurement, if the cooling potential of six frozen bricks
released over the night were used for vapour condensation
only, it could lead to about 1.3 mm dew formation. This is
over three times the maximum dewfall collected during
the experimental period, suggesting that the six ice bricks
did provide sufficient cooling sources to examine the two
objective questions.
2.2 Data collection
Dew collector surfaces were cleaned after sunset every day
during the experimental period, with a graduated cylinder
placed under the gutter outlet (Figure 1). In the morning
before sunrise, each collector was scraped using a rubber
kitchen scraper, and then dew volume was measured
from the graduated cylinder. Some residual dew water was
Figure 1. The layout of four collectors (TF1: Teflon with
radiative cooling, TF2: Teflon with artificial cooling, AL1:
aluminium with radiative cooling, AL2: aluminium with artificial
cooling) at a domestic backyard (a), and the details of one
collector (AL1) (b). The collector surface is 30 £60 cm
2
, and
about 40 cm above ground at the middle point. During the
experiment, the frozen bricks were inserted to the socket right
under the collector surface for TF2 and AL2.
H. Guan et al.2
Downloaded by [Flinders University of South Australia], [Dr Huade Guan] at 19:11 19 May 2013
inevitably left on the dew collector surface, leading to
some uncertainty in the result analysis. The remaining dew
water failed to be collected was estimated to be in an order
of 0.01 mm. This problem in the result analysis is more
significant for the low dew yield nights.
Air temperature and relative humidity were continu-
ally measured at two heights: collector height (40 cm),
and 2-meter height, using HOBOware Pro Temperature
and Relative Humidity data loggers. The HOBO logger
has an accuracy of ^0.358C for temperature, and ^2.5%
for relative humidity. The resolution is 0.038C for
temperature, and 0.03% for relative humidity. The air
temperature and relative humidity data were logged at a
10-minute interval. Temperature of each collector surface
was measured continually, using Thermochron iButton
(Dallas Semiconductor, Dalla, Texas; http://www.ibutton.
com/) (Johnson et al. 2005). The temperature logger
(iButton: DS1921G, diameter 15 mm, thickness 5 mm)
used in this study has a resolution of 0.58C and accuracy
of 18C. The temperature logger was attached to the
underside of the collecting surface, in the centre and 2/3
up from the lower edge. For the aluminium collectors, this
was done directly on the underside of the collector surface
because aluminium has good thermal conductivity. For the
Teflon collectors, the Teflon sheet was carved to a
minimal thickness (0.5 mm) at the point where the
temperature logger was placed, to minimise the difference
between the measured temperature and that of the Teflon
collector surface.
Cloud cover data were obtained from the Bureau of
Meteorology, which were recorded by visually estimating
the cloud fraction in the sky, reported in eighths, every three
hours at the Adelaide Airport (17 km from the study site).
Four readings of the total cloud amount (at 20:30, 23:30,
02:30, and 05:30) occurring within the dew collection
period were used to estimate potential dew formation.
2.3 Potential dewfall modelling
Dew formation occurs when the surface temperature is
below dew point temperature of the surrounding air. In this
dew-forming duration, dewfall is controlled by two
factors: the supply rate of air moisture to the dew collector
surface, and the net energy loss rate from the collector
surface. For a radiative cooling collector, potential dewfall
is the dew formation rate constrained by radiative cooling.
By comparing the actual dewfall and the potential dewfall,
one can find out whether air moisture is a constraining
factor for the specific dew collection period.
A simple potential dewfall model by Jacobs et al.
(2002) is adopted.
D*
f¼D
Dþ
g
Q*2G
l
ð1Þ
where D
f
*is the potential dew formation in (kg/m
2
/s),
Drepresents the slope of the saturated vapor pressure
versus temperature curve,
g
is the psychrometric constant
(<66 Pa/K), lis latent heat of vaporisation corrected for
the temperature effect (J/kg), Q*is net radiant flux away
from the surface (W/m
2
), and Gis net ground heat flux to
the surface (W/m
2
). For TF1 and AL1, Gis assumed to be
zero because the surface is thermally isolated from the
collector frame by the polystyrene foam (Figure 1). In this
equation, sensible heat flux is not considered. The sensible
heat flux usually adds energy to the surface during the
night, which decreases the dew yield potential. Without
including this term, Equation (1) tends to overestimate the
maximum possible dew yield to the surface. Nevertheless,
it is still valid to provide an upper-bound estimate of
potential dewfall (Jacobs et al. 2002).
The net radiation energy loss from the surface during
the night time is calculated by
Q*¼Qout 21Qin ð2Þ
where Q
out
is emitted thermal radiation from the surface,
Q
in
is the down-welling thermal radiation to the surface,
and 1is surface emissivity, quantitatively equivalent to
surface absorptance for thermal radiation. The surface
emitted thermal radiant flux density is calculated by
Qout ¼1
s
T4
cð3Þ
where
s
is the Stefan-Boltzmann constant, and T
c
is the
collector surface temperature in Kelvin degree. The down-
welling thermal radiant flux density is calculated using
Qin ¼½Fv1vþFsð
w
þð12
w
Þ1sÞ
s
T4
að4Þ
where F
v
and F
s
are the fractions of vegetation and sky in
the collector’s hemispheric view,
w
is the sky cloud
fraction, 1
v
and 1
s
are the emissivity values of vegetation
and clear sky, respectively, T
a
is the air temperature at
2 metres height, in Kelvin degree.
The following emissivity values are used in this study.
The emissivity is assumed to be unity for cloud (Rooney
2005), 0.95 for vegetation, 0.94 for Teflon (estimated from
(Baldridge et al. 2009)). The clear sky emissivity is
calculated using Equation (5) (Rooney 2005).
1s¼1:24 e
Ta

1=7
ð5Þ
where eis vapour pressure in milibars (hPa), T
a
is the air
temperature in Kelvin degree. Because the emissivity of
aluminium surface is small (Baldridge et al. 2009), and
strongly altered after the surface is covered with dew
drops, it was calibrated with the observations in the study.
It is found that on a homogeneous surface, such as the
collectors used in this study, dew drops cover a nearly
Urban Water Journal 3
Downloaded by [Flinders University of South Australia], [Dr Huade Guan] at 19:11 19 May 2013
constant fraction of the surface, depending on the surface
wettability (Beysens 1995, Zhao and Beysens 1995).
Consistently, we found that an overall emissivity of 0.6
was good to upper bound the observed dew yield on the
AL1 collector over the experiment period.
3. Results and discussion
Dew collection experiments were performed over a month
in autumn from April 24 through to May 23 of 2009.
Fourteen dew events were successfully recorded (Table 1).
For the remaining days, either no dew formed, or the dew
was mixed with rain water. For a couple of nights during
which a close-to-saturation relative humidity occurred, fog
may have formed. It is difficult to tell fog deposition
from dew formation on the collector surface. However, if
fog deposition was dominant, the difference in ‘dew’ water
from the four collectors should have become very small.
This situation is not observed from the data (Table 1).
Thus, fog deposition, if it did occur, should not overshadow
our analysis of dew formation between the four collectors.
Although the data period is short, the results show some
consistent and interesting patterns. For the two radiative-
cooling collectors, TF1 resulted in larger dewfall rates
than AL1 (Figure 2), which is consistent with earlier
findings (Takenaka et al. 2003). Averaged over the 14
events, the dewfall of TF1 is 0.18 ^0.09 (one standard
deviation) mm/night, in comparison to 0.08 ^0.07 mm/
night of AL1.
For each night, potential dewfall rate is calculated for
each 10-minute interval, according to Equation (1). Latent
heat was calculated as a function of temperature, and a value
of 66 Pa/K was used for the psychrometric constant. The
sum of potential dewfall, over the intervals with the surface
temperature equal to or below the collector-level dew
temperature, is regarded as the total potential dewfall rate of
the night. The results are summarised in Figure 2. The
difference between the actual and potential dewfall rates are
larger for TF1 (Figure 2a) than for AL1 (Figure 2b). For
AL1, the maximum dewfall rate from radiative cooling
rarely exceeded 0.2 mm/night (Figure 2b), the potential dew
formation is a good approximation for most nights with dew
yield. This indicates that the constraint from the air moisture
supply rate is not dominant. For TF1, the potential dewfall
rate was around 0.3 mm/night, the constraint from the air
moisture suppling rate could have become dominant. This is
indicated from that the actual dewfall fell significantly from
the potential rate (Figure 2a). However, it is also likely that
during these nights, sensible heat added energy to the
condenser surface (Nikolayev et al. 1996, Clus et al. 2009),
reducing potential dewfall which was not included in
Equation (1) calculated potential dewfall rate. For a night
without constraint in the moisture supply rate (Event 8,
more evidence shown later), the calculated potential dew
yield is a good approximation for the actual dew formation.
Table 1. Summary of 14 dew events resulting from the four collectors during 6 pm 6 am of each event night, together with average air temperature, dew point temperature and
relative humidity measured at 40 cm above ground over the same period for each event.
Dewfall (mm)
Date
Event
number
Mean air
temperature (8C)
Dew point
temperature (8C)
Mean
RH (%) TF1 TF2 AL1 AL2
29-Apr 1 8.0 5.6 85.0 0.083 0.200 0.004 0.096
30-Apr 2 7.8 5.8 87.2 0.149 0.210 0.000 0.160
1-May 3 9.9 8.9 93.7 0.233 0.328 0.112 0.324
3-May 4 5.7 4.9 94.5 0.269 0.301 0.155 0.180
4-May 5 7.6 6.8 94.6 0.231 0.278 0.162 0.299
5-May 6 8.0 7.1 93.9 0.208 0.268 0.141 0.277
6-May 7 7.3 6.4 94.3 0.204 0.256 0.107 0.259
12-May 8 8.0 7.2 94.9 0.370 0.373 0.172 0.289
13-May 9 8.1 7.1 93.7 0.176 0.267 0.082 0.254
18-May 10 10.1 9.1 92.9 0.072 0.216 0.006 0.184
19-May 11 8.0 6.5 90.0 0.144 0.211 0.000 0.175
20-May 12 7.1 6.4 95.1 0.199 0.251 0.139 0.161
21-May 13 11.3 10.4 94.3 0.163 0.342 0.072 0.289
22-May 14 11.9 6.7 70.1 0.000 0.072 0.000 0.000
H. Guan et al.4
Downloaded by [Flinders University of South Australia], [Dr Huade Guan] at 19:11 19 May 2013
During this night, the sensible heat gain on the condenser
surface should be negligible.
To investigate what meteorological conditions favour
dew formation on the radiation-cooling collectors, dew
yield data are plotted with specific humidity and vapour
pressure deficit (VPD) (Figure 3). VPD is the difference
between the saturated vapour pressure and the actual air
vapour pressure at the mean collector height. It is
calculated as the saturated vapour pressure multiplied by
(1- relative humidity), where the saturated vapour pressure
is calculated according to the Clausius-Clapeyron
equation. Specific humidity is a measure of water vapour
concentration in the air, while vapour pressure deficit
shows how easily the water vapour can be condensed to
dew. The results indicate that dew yield from the radiative-
cooling collectors is significantly dependent on vapour
pressure deficit (Figure 3b), but not on the specific
humidity (Figure 3a). There seems to be a vapour pressure
deficit threshold of 0.9 hPa, above which no dew forms on
the aluminium surface (Figure 3b). This is probably
because that by radiative cooling only, the aluminium
collector cannot lower the surface temperature down to the
dew point of a 0.9-hPa-VPD air. A similar VPD threshold
is observed for the Teflon surface (Figure 3b), below
which dew yield from the collector surface increases with
a decrease of VPD, and above which dew yield becomes
insensitive to VPD.
With artificial cooling, the dew formation rates for both
collector types (TF2 and AL2) increase (Figure 2). The
ratio of dew formation on the artificial cooling collector
over the corresponding radiative cooling collector is used
as a measure of artificial cooling enhancement in dew
collection. The magnitude of this enhancement is larger for
the aluminium collector than the Teflon collector. The ratio
of total dew yield of the 14 events is 1.42 for TF2/TF1, and
2.56 for AL2/AL1.
To examine the meteorological conditions that
influence artificial-cooling enhancement, the values of
TF2/TF1 and AL2/AL1 are plotted with specific humidity
(Figure 4), and VPD (Figure 5). Because dew collection
error is amplified in the ratio for low dew-yield nights, data
Figure 2. Measured dew formation per night from the radiative-
cooling and artificial-cooling collectors, in comparison to
potential dew formation (D
f
*) calculated for the radiative-
cooling collector: (a) the Teflon collectors (TF1: radiative
cooling, TF2: artificial cooling), and (b) the aluminium collectors
(AL1: radiative cooling, AL2: artificial cooling). The potential
dewfall is missing for the first three events due to the lack of air
temperature measurements for these days.
Figure 3. The amount of dew yield from the two radiative
cooling collectors (TF1: Teflon, AL1: aluminium) vs. average
specific humidity (a), and average vapor pressure deficit (b), of
the dew collection period (6 pm6 am) measured at the dew
collector height. The function fits in (b) are statistically
significant at 99.5% and 99.8% confidence levels, respectively.
Urban Water Journal 5
Downloaded by [Flinders University of South Australia], [Dr Huade Guan] at 19:11 19 May 2013
of some low dew-yield nights are not used for the analysis
of artificial-cooling enhancement, and thus not included in
Figures 4 and 5. They are events 1, 10, and 14 for the
Teflon collectors, and events 1, 2, 10, 11, and 14 for the
aluminium collectors.
For both the Teflon and aluminium collectors, artificial
cooling enhancement increases with specific humidity
(Figure 4). At a low specific humidity (e.g., 5.5 g/kg), the
artificial cooling enhancement is small for both aluminium
and Teflon surfaces. With a larger specific humidity, more
vapour in the air is available to be condensed, and thus the
artificial cooling mechanism becomes more efficient in
enhancing dew formation. This specific humidity depen-
dence is more significant for the aluminium collector than
the Teflon collector.
Two mechanisms may account for the difference in this
specific humidity dependence between the aluminium and
Teflon collectors. The first mechanism is related to the
change of surface emissivity after dew forms on the
aluminium surface. Aluminium surface has a emissivity
below 0.1, while water approximately has an emissivity
value of 0.98 (Baldridge et al. 2009). After dew forms, the
apparent surface emissivity becomes the average of the two
(aluminium and water) weighted by the surface fraction
covered by dew. The larger specific humidity is, the quicker
dew forms on the artificial cooling surface, increasing the
surface emissivity of the aluminium collector. This
mechanism is evident by comparing the surface tempera-
ture of TF1 and AL1 in a high dew-yield night (Figure 6)
and a low dew-yield night (Figure 7). In the high dew yield
night, the difference in surface temperature between the
two radiative-cooling collectors was 348C before 20:00,
which was owing to the low emissivity of the aluminium
surface. After dew formed, starting around 20:30 (inferred
from the surface temperature being below the dew point),
the difference in temperature between the two collectors
became smaller than 18C. Between 1:30 and 4:00, a large
amount of dew formed resulted from a more humid air,
which is inferred from a slightly elevated dew point and a
quick increase of the surface temperature to the dew point.
After this, both AL1 and TF1 were covered with dew water,
and thus not much emissivity difference exists between
them, leading to a negligible surface temperature
difference after 4:00 (Figure 6). In the low dew yield
night (Figure 7), with little dew formed on AL1 owing to
Figure 5. The dewfall ratio of the artificial cooling collector
over the corresponding radiative cooling collector as a function
of average vapour pressure deficit of the dew collection period
(6 pm6 am) at the dew collector height for the Teflon collectors
(a), and aluminium collectors (b). The funciton fit is statistically
significant at a confidence level of 99.97% for (b), but not
significant for (a).
Figure 4. The dewfall ratio of the artificial cooling collector
over the corresponding radiative cooling collector as a function
of average specific humidity of the dew collection period
(6 pm6 am) at the dew collector height for the Teflon collectors
(a), and aluminium collectors (b). The linear funciton fit is
statistically significant at a confidence level of 99.6% for (a), and
99.8% for (b).
H. Guan et al.6
Downloaded by [Flinders University of South Australia], [Dr Huade Guan] at 19:11 19 May 2013
the low emissivity, the surface temperature kept around
108C, close to the ambient air temperature. While even
with some dew being formed during the night, which
tended to increase surface temperature, the TF1 surface
temperature kept decreasing to around 88C before sunrise.
The contrast of the difference in surface temperature
between AL1 and TF1 during the high and low dew yield
nights, indicates that the aluminium surface emissivity
altered by dew can significantly enhance radiative cooling
and thus dew formation. This mechanism is more
significant for the nights of high specific humidity.
Because the emissivity of Teflon is very close to that of
water, the effect of dew-altered surface emissivity is not
important.
The other possible mechanism is related to the
different efficiency of artificial cooling in reducing surface
temperature. Thermal conductivity of aluminium is 1000
times that of Teflon. The frozen bricks may have lead to a
larger heat loss rate on the aluminium surface, resulting in
a larger dew formation enhancement from the air of a
larger specific humidity. If this happened, the surface
temperature of AL2 should have decreased quicker than
that of the TF2. However, this is not observed from the
surface temperature measurements in the low dew yield
night (Figure 7). This is probably because the cooling
source (frozen bricks) did not directly contact the
aluminium sheet, reducing the heat conducting rate from
the aluminium surface. Thus, this second mechanism is
less likely.
The effects of vapour pressure deficit on artificial
cooling enhancement are quite different between the
aluminium and Teflon collectors. Artificial cooling
enhancement increases with ln(VPD) significantly for the
aluminium collector, but far less significantly for the Teflon
collector (Figure 5). The contribution of artificial cooling
becomes negligible when VPD is close to 0.5 hPa. This is
probably because at such a low VPD, radiative cooling of
the Teflon and dew-covered aluminium surface is sufficient
to bring the collector surface temperature below the dew
point temperature (e.g., event 8, Figure 6).It is also probably
because during the low VPD nights, wind speed was very
low at the well-sheltered study site, which limited the
moisture supply rate to the dew collectors.Unfortunately, no
wind measurement was conducted in the field. Future work
with more meteorological monitoring for a longer time is
needed to examine this potential effect. In this regard, it is
worth to note that at some situations, a natural air convection
of 0.5 m/s can be resulted from the temperature difference
between the collector surface and the surrounding air
(Beysens et al. 2005, Clus et al. 2009).
Figure 6. Measured surface and air temperatures during a
selected high-yield night (Event 8: May 11 12) for the radiative-
cooling and artificial-cooling collectors: (a) Teflon (TF1 and
TF2), and (b) Aluminium (AL1 and AL2). Dew point was
measured at the collector height (0.4 metre above the ground).
Figure 7. Measured surface and air temperatures during a
selected low-yield night (Event 10: May 17 18) for the
radiative-cooling and artificial-cooling collectors: (a) Teflon
(TF1 and TF2), and (b) Aluminium (AL1 and AL2). Dew point
was measured at 2 meters above ground.
Urban Water Journal 7
Downloaded by [Flinders University of South Australia], [Dr Huade Guan] at 19:11 19 May 2013
Artificial cooling enhances dewfall by the following
three possible mechanisms. First, artificial cooling source
increases dew-forming duration. This is clearly observed in
a low dew-yield night (Figure 7). For the Teflon collectors,
artificial cooling almost doubled the dew-forming duration
(Figure 7a). For the aluminium collectors, artificial cooling
increased the dew-forming duration from about zero to
almost the whole night (Figure 7b). Second, artificial
cooling increases net energy loss rate from the surface by
thermal conduction. This is evident from the observed
lower surface temperature of the artificial cooling
collectors than the corresponding radiative cooling
collectors (Figures 6 and 7). The lower surface temperature
simultaneously reduces the emitted radiation energy loss
from the surface, which may, to some extent, compensate
the artificial cooling effect. However, the results of this
study do not provide evidence to either prove or disprove
this possibility. Third, for a collector surface of a low
emissivity, dew that has formed under the artificial cooling
condition increases the surface emissivity, and conse-
quently enhances radiative cooling. This mechanism is
important for collectors with a low surface emissivity, such
as the aluminium surface in this study. It implies that
without artificial cooling, wetting the condenser of a low
emissivity at some micrometeorological conditions, may be
useful to initiate dew formation and enhance dew yield.
With artificial cooling, is radiative cooling still useful
for dew formation? The results of this study support the
contribution of radiative cooling. Averaged over the 14
dew yield nights, with artificial cooling, the Teflon
collector (TF2) collects 22% more dew than the aluminium
collector (AL2) (Figure 8a). Since the air moisture supply
to the two collectors was similar, the difference in dewfall
could most likely be due to the difference in cooling power
between the two collectors. This cooling power results
from the sum of artificial cooling and radiative cooling. As
discussed earlier, AL2 most likely had an artificial-cooling-
induced heat loss rate similar to TF2. A larger TF2 dewfall
rate than that from the AL2 surface was most likely
attributed to a larger radiative cooling from the TF2 surface
than the AL2 surface.
The importance of radiative cooling in dew formation
on TF2 and AL2 are further supported by comparison of
the ratio of dew yield of TF2 and AL2 with the specific
humidity and vapour pressure deficit (Figure 8b and c).
The dew yield ratio of AL2/TF2 increases with specific
humidity to around unity at a specific humidity of 6.5 g/kg.
After this point, AL2/TF2 becomes stable. This informs
that at a lower specific humidity, artificial cooling does not
quickly force dew to form on the aluminium surface. In
this situation, the difference in surface emissivity results in
a lower dew yield in AL2. At a specific humidity larger
than 6.5 g/kg, dew quickly forms on the artificial-cooling
aluminium surface, which makes its surface emissivity
similar to the Teflon surface, and thus similar dew
formation on AL2 and TF2. By a similar mechanism, the
dew-yield ratio of AL2/TF2 decreases with vapour
pressure deficit, which is evident in Figure 8c. The three
outliers at the lower VPD end are event 4, 8, and 12. They
shared a common meteorological condition: high relative
humidity (95%). It is likely that during these nights, the
strong down-welling radiation, due to a high relative
humidity and possible fog formation, limited the passive
cooling for the aluminium condenser which had an overall
emissivity slightly lower than the Teflon condenser.
Figure 8. (a) Measured dew formation per night on the two
artifical-cooling collectors (TF2: Teflon, AL2: aluminium), (b)
the ratio of dewfall on AL2 over that on TF2, in comparsion to
the ratio between two radiative-cooling collectors (AL1:
aluminium, TF1: Teflon), vs. average specific humidity of the
dew collection period (6 pm 6 am), (c) same as (b), but vs.
average vapour pressure deficit.
H. Guan et al.8
Downloaded by [Flinders University of South Australia], [Dr Huade Guan] at 19:11 19 May 2013
In the experiments, the average dew yield from the
artificial-cooling collectors for the 14 events is 0.23 mm/
night. This is far smaller than the artificial cooling potential
of the six frozen bricks, which is equivalent to 1.3 mm/
night. This indicates that most of the heat sink from the
cooling source is wasted. With what power and at what
time the artificial cooling should be applied to enhance dew
formation more efficiently at a low cost of energy
consumption is an interesting issue to be further explored.
4. Conclusions
To examine artificial-cooling-enhanced dew collection,
dew collecting experiments were performed over a month
(AprilMay) in a coastal area of South Australia, with four
collectors of two materials (aluminium and Teflon). The
results show that artificial cooling enhances dew formation
nearly 45% for the Teflon collectors, while the enhance-
ment is over 150% for the aluminium collectors. The
enhancement magnitude is dependent on meteorological
conditions, increasing with specific humidity for both the
Teflon and aluminium collectors, and with vapour pressure
deficit for the aluminium collector. In addition to
increasing the dew formation duration and heat loss rate
from the collector surface, the two mechanisms which
contributes to dew formation enhancement for both
aluminium and Teflon collectors, artificial cooling also
increases the aluminium surface emissivity through dew
formation that would not otherwise occur.
Without artificial cooling, the Teflon collector is on
average 120% more efficient than the aluminium collector
over the whole dew collection period. While with artificial
cooling, the Teflon collector is only 20% more efficient
than the aluminium collector over the same period. The
difference between the two artificial cooling collectors
becomes more significant when the air is of a low specific
humidity (,6.5 g/kg), and/or of a high vapour pressure
deficit (.0.8 hPa). The results also show that radiative
cooling still contributes to dew formation on the artificial
cooling collectors. But the material type becomes less
important owing to that the high emissivity of dew water
compensates for the low surface emissivity. These results
provide useful information for designing collecting devices
or roofs to enhance dew formation in urban environments.
Acknowledgements
Julie Guerin, Amanda Treijs, Tess Stevens, and Chuanyu Zhu
assisted in some data collection. Craig T. Simmons and Erick
Bestland provided valuable comments in preparing the
manuscript.
References
Agam, N. and Berliner, P.R., 2006. Dew formation and water
vapor adsorption in semi-arid environments - a review.
Journal of Arid Environments, 65 (4), 572– 590, doi:
10.1016/j.jaridenv.2005.09.004.
Al-Jalil, H., Amayreh, J., and Al-Widyan, M., 2007. Feasibility
of collecting ambient air moisture by forced condensation.
Ama-Agricultural Mechanization in Asia Africa and Latin
America, 38 (1), 5154.
Baldridge, A.M., Hook, S.J., Grove, C.I., and Rivera, G., 2009. The
aster spectral library version 2.0. Remote Sensing of
Environment, 113 (4), 711– 715, doi: 10.1016/j.rse.2008.11.007.
Barradas, V.L. and Glez-Medellin, M.G., 1999. Dew and its
effect on two heliophile understorey species of a tropical dry
deciduous forest in Mexico. International Journal of
Biometeorology, 43 (1), 1 7, doi: 10.1007/s004840050109.
Berkowicz, S.M., Beysens, D., Milimouk, I., Heusinkveld, B.G.,
Muselli, M., Wakshal, E., and Jacobs, A.F.G., 2004. Urban
dew collection under semi-arid conditions: Jerusalem. In:H.
Rautenbach and J. Olivier, eds. Proceedings of the Third
International Conference on Fog, Fog Collection and Dew,
October 1115, 2004. Cape Town, South Africa: Paper E4.
Beysens, D., 1995. The formation of dew. Atmospheric Research,
39 (13), 215– 237, doi: 10.1016/0169-8095(95)00015-J.
Beysens, D., Milimouk, I., Nikolayev, V., Muselli, M., and
Marcillat, J., 2003. Using radiative cooling to condense
atmospheric vapor: A study to improve water yield. Journal
of Hydrology, 276 (1-4), 1 11, doi: 10.1016/S0022-1694(03)
00025-8.
Beysens, D., Muselli, M., Nikolayev, V., Narhe, R., and
Milimouk, I., 2005. Measurement and modelling of dew in
island, coastal and alpine areas. Atmospheric Research,73
(1-2), 1–22, doi: 10.1016/j.atmosres.2004.05.003.
Clus, O., Ouazzani, J., Muselli, M., Nikolayev, V.S., Sharan, G.,
and Beysens, D., 2009. Comparison of various radiation-
cooled dew condensers using computational fluid dynamics.
Desalination, 249 (2), 707–712, doi: 10.1016/j.
desal.2009.01.033.
Jacobs, A.F.G., Heusinkveld, B.G., and Berkowicz, S.M., 2002.
A simple model for potential dewfall in an arid region.
Atmospheric Research, 64 (1-4), 285– 295, doi: 10.1016/
S0169-8095(02)00099-6.
Jacobs, A.F.G., Heusinkveld, B.G., and Berkowicz, S.M., 2008.
Passive dew collection in a grassland area, The Netherlands.
Atmospheric Research, 87 (3-4), 377– 385, doi: 10.1016/j.
atmosres.2007.06.007.
Johnson, A.N., Boer, B.R., Woessner, W.W., Stanford, J.A.,
Poole, G.C., Thomas, S.A., and O’Daniel, S.J., 2005.
Evaluation of an inexpensive small-diameter temperature
logger for documenting ground water-river interactions.
Ground Water Monitoring and Remediation, 25 (4), 68 74,
doi: 10.1111/j.1745-6592.2005.00049.x.
Lekouch, I., Mileta, M., Muselli, M., Milimouk-Melnytchouk, I.,
Sojat, V., Kabbachi, B., and Beysens, D., 2010. Comparative
chemical analysis of dew and rain water. Atmospheric
Research, 95 (2-3), 224234, doi: 10.1016/j.atmosres.
2009.10.002.
Liu, X.H., Chang, X.M., Xia, J.J., and Jiang, Y., 2009.
Performance analysis on the internally cooled dehumidifier
using liquid desiccant. Building and Environment, 44 (2),
299308, doi: 10.1016/j.buildenv.2008.03.009.
Munne-Bosch, S., Nogues, S., and Alegre, L., 1999. Diurnal
variations of photosynthesis and dew absorption by leaves in
two evergreen shrubs growing in Mediterranean field
conditions. New Phytologist, 144 (1), 109 119, doi:
10.1046/j.1469-8137.1999.00490.x.
Muselli, M., Beysens, D., Marcillat, J., Milimouk, I., Nilsson, T.,
and Louche, A., 2002. Dew water collector for potable water in
Urban Water Journal 9
Downloaded by [Flinders University of South Australia], [Dr Huade Guan] at 19:11 19 May 2013
Ajaccio (Corsica Island, France). Atmospheric Research,64
(1-4), 297312, doi: 10.1016/S0169-8095(02)00100-X.
Muselli, M., Beysens, D., and Millmouk, I., 2006a. A
comparative study of two large radiative dew water
condensers. Journal of Arid Environments, 64 (1), 54 76,
doi: 10.1016/j.jaridenv.2005.04.007.
Muselli, M., Beysens, D., Soyeux, E., and Clus, O., 2006b. Is dew
water potable? Chemical and biological analyses of dew
water in Ajaccio (Corsica Island, France). Journal of
Environmental Quality, 35 (5), 18121817, doi: 10.2134/
jeq2005.0357.
Nikolayev, V.S., Beysens, D., Gioda, A., Milimouk, I.,
Katiushin, E., and Morel, J.P., 1996. Water recovery from
dew. Journal of Hydrology, 182 (1-4), 19 35, doi:
10.1016/0022-1694(95)02939-7.
Nilsson, T.M.J., Vargas, W.E., Niklasson, G.A., and Granqvist,
C.G., 1994. Condensation of water by radiative cooling.
Renewable Energy, 5 (1-4), 310 317, doi: 10.1016/0960-
1481(94)90388-3.
Okochi, H., Kataniwa, M., Sugimoto, D., and Igawa, M., 2005.
Enhanced dissolution of volatile organic compounds into
urban dew water collected in Yokohama, Japan. Atmospheric
Environment, 39 (33), 6027– 6036, doi: 10.1016/j.atmo-
senv.2005.05.025.
Richards, K., 2004. Observation and simulation ofdew in rural and
urban environments. Progress in Physical Geography,28(1),
76 94, doi: 10.1191/0309133304pp402ra.
Richards, K., 2009. Adaptation of a leaf wetness model to
estimate dewfall amount on a roof surface. Agricultural and
Forest Meteorology, 149 (8), 1377 1383, doi: 10.1016/j.
agrformet.2009.02.014.
Rooney, G.G., 2005. Modelling of downwelling long-wave
radiation using cloud fraction obtained from laser cloud-base
measurements. Atmospheric Science Letters, 6, 160 163,
doi: 10.1002/asl.110.
Sharan, G., Beysens, D., and Milimouk-Melnytchouk, I., 2007. A
study of dew water yields on galvanized iron roofs in Kothara
(north-west India). JournalofAridEnvironments,69(2),
259– 269, doi: 10.1016/j.jaridenv.2006.09.004.
Takenaka, N., Soda, H., Sato, K., Terada, H., Suzue, T., Bandow,
H., and Maeda, Y., 2003. Difference in amounts and
composition of dew from different types of dew collectors.
Water Air and Soil Pollution, 147 (1-4), 51 60, doi: 10.1023/
A:1024573405792.
Ye, Y.H., Zhou, K., Song, L.Y., Jin, J.H., and Peng, S.L., 2007.
Dew amounts and its correlations with meteorological
factors in urban landscapes of Guangzhou, China. Atmos-
pheric Research, 86 (1), 21– 29, doi: 10.1016/j.
atmosres.2007.03.001.
Zhao, H. and Beysens, D., 1995. From droplet growth to film
growth on a heterogeneous surface - condensation associated
with a wettability gradient. Langmuir, 11 (2), 627– 634, , doi:
10.1021/la00002a045.
H. Guan et al.10
Downloaded by [Flinders University of South Australia], [Dr Huade Guan] at 19:11 19 May 2013
  • ... 120,121 The method uses an engineered cold surface to cool the adjacent air mass below the dew point to produce water droplets via con- densation. 17,18,121,122 A sorption-based approach for atmospheric water vapor harvesting is also gaining popularity (Figure 5), in which a water sorbent, such as metal-organic framework (MOF), anhydrate salts, deliquescent salts, is used to harvest water vapor from air, and it is then heated with assistance from photothermal material to release and subsequently condense the water. [122][123][124][125] Effective photothermal materials, such as carbon nanotubes, carbon black particles, and graphene, have been utilized to directly tap sunlight to drive the water vapor release out of the water vapor sorbents. ...
    ... Materials for daytime radiative cooling that help reduce the amount of energy needed for cooling buildings also find use in atmospheric water capture and dew collection. 16,18 On the other hand, light and heat trapping/spreading concepts and composite materials developed for solar water desalination technologies are now being adapted to engineer ice-phobic surfaces that use solar energy to pre- vent and mitigate ice formation. 133,134 New com- posite material systems are emerging to replace the commonly used table salt in cloud-seeding applications. ...
    Article
    Full-text available
    he water and energy sectors of an economy are inextricably linked. Energy is required in water production, distribution, and recycling, while water is often used for energy generation. In many geographical locations, the energy-water nexus is exacerbated by the shortage of both fresh water resources and energy generation infrastructure. New materials, including metamaterials, are now emerging to address the challenges of providing renewable energy and fresh water, especially to off-the-grid communities struggling with water shortages. Novel nanomaterials have fueled recent technology breakthroughs in solar water desalination, fog and dew collection, and cloud seeding. Materials for passive thermal management of buildings and individuals offer promising strategies to reduce the use of energy and water for heating and cooling. While many challenges remain, emerging materials and technologies improve sustainable management of water and energy resources.
  • ... Ambient dew samples at RMNP were gathered using a dew collector with a design similar to Guan et al. (2014) . The collector was built in-house and consists of a wooden base that supports a 7 cm thick polystyrene foam block with an area of 48 × 60 cm. ...
    ... This results in dew formation on the Teflon ® surface which can be manually collected into clean sample bottles the following morning using a pre-cleaned scraper and funnel . The emissivity of Teflon ® is 0.94 (Baldridge et al., 2009) and is very similar to that of vegetation (0.95) (Guan et al., 2014). The dew collector was deployed before dusk on nights that had a forecast favourable for dew formation (high relative humidity , light winds, and clear skies). ...
    Article
    Full-text available
    Several field studies have proposed that the volatilization of NH3 from evaporating dew is responsible for an early morning pulse of ammonia frequently observed in the atmospheric boundary layer. Laboratory studies conducted on synthetic dew showed that the fraction of ammonium (NH4+) released as gas-phase ammonia (NH3) during evaporation is dependent on the relative abundances of anions and cations in the dew. Hence, the fraction of NH3 released during dew evaporation (Frac(NH3)) can be predicted given dew composition and pH. Twelve separate ambient dew samples were collected at a remote high-elevation grassland site in Colorado from 28 May to 11 August 2015. Average [NH4+] and pH were 26 µM and 5.2 respectively and were on the lower end of dew [NH4+] and pH observations reported in the literature. Ambient dew mass (in g m−2) was monitored with a dewmeter, which continuously measured the mass of a tray containing artificial turf representative of the grass canopy to track the accumulation and evaporation of dew. Simultaneous measurements of ambient NH3 indicated that a morning increase in NH3 was coincident in time with dew evaporation and that either a plateau or decrease in NH3 occurred once the dew had completely evaporated. This morning increase in NH3 was never observed on mornings without surface wetness (neither dew nor rain, representing one-quarter of mornings during the study period). Dew composition was used to determine an average Frac(NH3) of 0.94, suggesting that nearly all NH4+ is released back to the boundary layer as NH3 during evaporation at this site. An average NH3 emission of 6.2 ng m−2 s−1 during dew evaporation was calculated using total dew volume (Vdew) and evaporation time (tevap) and represents a significant morning flux in a non-fertilized grassland. Assuming a boundary layer height of 150 m, the average mole ratio of NH4+ in dew to NH3 in the boundary layer at sunrise is roughly 1.6 ± 0.7. Furthermore, the observed loss of NH3 during nights with dew is approximately equal to the observed amount of NH4+ sequestered in dew at the onset of evaporation. Hence, there is strong evidence that dew is both a significant night-time reservoir and strong morning source of NH3. The possibility of rain evaporation as a source of NH3, as well as dew evaporation influencing species of similar water solubility (acetic acid, formic acid, and HONO), is also discussed. If release of NH3 from dew and rain evaporation is pervasive in many environments, then estimates of NH3 dry deposition and NHx ( ≡ NH3 + NH4+) wet deposition may be overestimated by models that assume that all NHx deposited in rain and dew remains at the surface.
  • ... Radiative cooling towards the night sky drives natural atmospheric moisture extraction via the formation of dew on surfaces ( Muselli et al., 2006;Nikolayev et al., 1996;Sharan, 2013). The maximum expected yield of radiative dew harvesting is ~ 0.8 L/(m 2 d) ( Beysens et al., 2013) but empirical studies of passive dew capturing reveal much lower and varying water yield ( Beysens et al., 2005;Guan et al., 2014;Jacobs et al., 2002Jacobs et al., , 2008Nilsson, 1996). In general, the process is limited by the rate of radiative heat exchange, the weather and the surface properties ( Beysens, 2016). ...
    Article
    The enormous amount of water vapor present in the atmosphere may serve as a potential water resource. An index is proposed for assessing the feasibility and energy requirements of atmospheric moisture harvesting by a direct cooling process. A climate-based analysis of different locations reveals the global potential of this process. We demonstrate that the Moisture Harvesting Index (MHI) can be used for assessing the energy requirements of atmospheric moisture harvesting. The efficiency of atmospheric moisture harvesting is highly weather and climate dependent, with the smallest estimated energy requirement found at the tropical regions of the Philippines (0.23 kW/L). Less favorable locations have much higher energy demands for the operation of an atmospheric moisture harvesting device. In such locations, using the MHI to select the optimal operation time periods (during the day and the year) can reduce the specific energy requirements of the process dramatically. Still, using current technology the energy requirement of atmospheric moisture harvesting by a direct air cooling process is significantly higher than of desalination by reverse osmosis.
  • ... Ambient dew samples at RMNP were gathered using a dew collector with a design similar to 19 Guan et al. (2014) bottles the following morning using a pre-cleaned scraper and funnel. The emissivity of Teflon ® 26 is 0.94 (Baldridge et al., 2009) and is very similar to that of vegetation (0.95) (Guan et al., 27 2014). ...
    Article
    Full-text available
    Several field studies have proposed that the volatilization of NH3 from evaporating dew is responsible for an early morning pulse of ammonia frequently observed in the atmospheric boundary layer. Laboratory studies conducted on synthetic dew showed that the fraction of ammonium (NH4+) released as gas-phase ammonia (NH3) during evaporation is dependent on the relative abundances of anions and cations in the dew. Hence, the fraction of NH3 released during dew evaporation (Frac(NH3)) can be predicted given dew composition and pH. Twelve separate ambient dew samples were collected at a remote high elevation grassland site in Colorado from 28 May to 11 August, 2015. Average [NH4+] and pH were 26 μM and 5.2, respectively, and were on the lower end of dew [NH4+] and pH observations reported in the literature. Ambient dew mass (in g m−2) was monitored with a dewmeter, which continuously measured the mass of a tray containing artificial turf representative of the grass canopy to track the accumulation and evaporation of dew. Simultaneous measurements of ambient NH3 indicated that a morning increase in NH3 was coincident in time with dew evaporation, and that either a plateau or decrease in NH3 occurred once the dew had completely evaporated. This morning increase in NH3 was never observed on mornings without surface wetness (neither dew nor rain, representing one-quarter of mornings during the study period). Dew composition was used to determine an average Frac(NH3) of 0.94, suggesting that nearly all NH4+ is released back to the boundary layer as NH3 during evaporation at this site. An average NH3 emission of 6.2 ng m−2 s−1 during dew evaporation was calculated using total dew volume (Vdew) and evaporation time (tevap), and represents a significant morning flux in a non-fertilized grassland. Assuming a boundary layer height of 150 m, the average mole ratio of NH4+ in dew to NH3 in the boundary layer at sunrise is roughly 1.6 ± 0.7. Furthermore, the observed loss of NH3 during nights with dew is approximately equal to the observed amount of NH4+ sequestered in dew at the onset of evaporation. Hence, there is strong evidence that dew is both a significant night-time reservoir and strong morning source of NH3. The possibility of rain evaporation as a source of NH3, as well as dew evaporation influencing species of similar water solubility (acetic acid, formic acid, and HONO) is also discussed. If release of NH3 from dew and rain evaporation is pervasive in many environments, then estimates of NH3 dry deposition and NHx (≡NH3 + NH4+) wet deposition may be overestimated by models that assume that all NHx deposited in rain and dew remains at the surface.
  • Article
    The enormous amount of water vapor present in the atmosphere may serve as a potential water resource. An index is proposed for assessing the feasibility and energy requirements of atmospheric moisture harvesting by a direct cooling process. A climate-based analysis of different locations reveals the global potential of this process. We demonstrate that the Moisture Harvesting Index (MHI) can be used for assessing the energy requirements of atmospheric moisture harvesting. The efficiency of atmospheric moisture harvesting is highly weather and climate dependent, with the smallest estimated energy requirement found at the tropical regions of the Philippines (0.23 kW/L). Less favorable locations have much higher energy demands for the operation of an atmospheric moisture harvesting device. In such locations, using the MHI to select the optimal operation time periods (during the day and the year) can reduce the specific energy requirements of the process dramatically. Still, using current technology the energy requirement of atmospheric moisture harvesting by a direct air cooling process is significantly higher than of desalination by reverse osmosis.
  • Article
    An innovative atmospheric moisture harvesting system is proposed, where water vapor is separated from the air prior to cooling and condensation. The system was studied using a model that simulates its three interconnected cycles (air, desiccant, and water) over a range of ambient conditions, and optimal configurations are reported for different operation conditions. Model results were compared to specifications of commercial atmospheric moisture harvesting systems and found to represent saving of 5-65% of the electrical energy requirements due to the vapor separation process. We show that the liquid desiccant separation stage that is integrated into atmospheric moisture harvesting systems can work under a wide range of environmental conditions using low grade or solar heating as a supplementary energy source, and that the performance of the combined system is superior.
  • Article
    Full-text available
    Over the last 20 years, dew harvesting has evolved to fruition due to a better understanding of its physics, thermodynamics, and the radiative cooling process of condensing substrates. Although resultant yields are relatively small, dew positions itself as a viable water resources supplement because it occurs naturally and frequently in many locations globally, particularly in the absence of precipitation or when more traditional water sources are subject to depletion. Moreover, dew water is generally potable, especially in rural locations, where it is most beneficial. This review summarizes dew harvesting research achievements to date including formation processes, collection in various environments, prediction models, water quality, and applications. The paper concludes with outlining existing gaps and future research needs to improve the understanding and performance of dew harvesting in the context of adaptation to climate change.
  • Article
    This work investigated the feasibility of collecting ambient air moisture. A dehumidifying unit of 215 watts was used to condense and collect moisture. A rain gage recorded the collected moisture. Dry bulb temperature and relative humidity (RH) were recorded plus the instantaneous voltage and current. A data logger recorded data on an hourly basis. There was positive correlation between ambient RH and moisture collection rate. Up to 137.2 ml of water/hr were collected with efficiency of 0.822 liters/kW hr at 14.0 degrees C and RH of 87.4 % resulting in a cost that compares favorably with the local rate of bottled water.
  • Article
    Dew has broad relevance to physical geography and many human activities. This paper reviews the observation and simulation of rural and urban dew, and the implications of dew as a climatic phenomenon. There is no universal protocol for measuring dew (i.e., condensation on cooled surfaces), its component fluxes (dewfall and distillation) or guttation, which is 'dewdrops' exuded from leaves. The many methods that exist to measure dew and surface moisture range from simple visual assessment, to electronic wetness sensors, lysimetry and remote sensing. Most studies of dew are rural; urban dew data are rare and studies seldom address dew in patchy landscapes, e.g., a forest clearing. Hardware dew models are rare. Numerous numerical models exist to simulate or forecast dew on rural crops but few address dew for surfaces other than leaves, e.g., a cocoa pod or road. There is a general consensus in the literature that dew is reduced or absent in cities because of the urban heat island effect and reduced vapour supply. However, little data exist to test this. Given the broad relevance of dew to many topics, future studies of dew in complex landscapes, including urban areas, would prove valuable.
  • Article
    In order to improve the yield of dew condensation from atmospheric vapor, two large (30m2 in area) insulated plane radiative condensers, inclined at 30°, were installed in Ajaccio (Corsica island, France; latitude 41°55′N, longitude 8°48′E). Prototype P1 was elevated such that the underside was open and exposed. Prototype P2, however, was enclosed on all sides and closer to the ground. Both used a special radiative foil that enhances dew formation. The period of observation for P1 was July 22, 2000–November 11, 2001, and for P2 was December 10, 2001–December 10, 2003. All data were compared with respect to the same horizontal calibration plate of polymethylmethacrylate (Plexiglas) placed at 1m above the ground on a sensitive recording balance. Water yield of both prototypes were compared and correlated against meteorological data (cloud cover, relative humidity, wind speed, condenser temperature and air temperature). Both prototypes exhibit improved performances when compared with the calibration plate: more dew days (+16% and +15% for P1 and P2, respectively); decrease of the humidity threshold (−3% and −4.4% for P1 and P2); increase of dew yields for wind speeds up to 3ms−1. A model of the mass and thermal exchanges with the ambient air was used. Two adjustable parameters (heat and mass transfer coefficients) are used in the model. The values of these parameters were found larger than the values obtained in continental sites where dew forms with weak wind, thus emphasizing the peculiarities of dew formation in windy islands. When data are reduced with the calibration PMMA data, prototype P1 provided average water yields slightly larger than the enclosed prototype P2, a result that can be attributed to the influence of surface thermal radiation.
  • Article
    A dew collection project was carried out in Kothara, NW India, during the dry season between October 2004 and May 2005. One of the goals was to determine the amount of dew water that could be collected with little investment by adapting plain, uninsulated, corrugated galvanized iron roofs that are common in most rural regions of India. During the study period, the cumulative dew yield on an 18m2 double—sloped (30°) test roof was 113.5L (6.3mm). The west-facing side gave 35% higher water yields than the east-facing side. The use of thermal insulation and more IR radiative materials would have increased this yield by 40% (8.9mm or 160L). The cumulative dew water yield remains modest when compared with the average annual rainfall (300mm). But dew occurs far more frequently than rain and is available precisely during the dry season when water is most scarce. Dew events were correlated with meteorological data; relative humidity (the most important parameter) is strongly correlated with the monsoon.
  • Article
    Most leaf wetness and dew studies are rural, however, there is growing scientific interest in urban dew. This study investigates the feasibility of adapting an existing leaf wetness (dew) model for urban use. The approach predicts dewfall amount for (a) a maple leaf, and (b) an asphalt-shingle roof, using a computed surface energy balance (which includes a roof subsurface heat flux), latent heat fluxes, and an inferred surface water balance. Simulations are run and verified to a first-order using data collected in Vancouver, BC, Canada, in 1996. Too few data are available to adequately test the ‘leaf’ model. The skill of the ‘roof’ model to predict dewfall amount is confirmed by several statistical indices, e.g. a root mean square error of 0.04mm per night, and Willmott's index of agreement of 0.87. Skill is seen also in the timing of dewfall onset and maxima. It seems feasible to estimate dewfall on an artificial surface, such as a roof, using a ‘leaf wetness’ approach, with suitable modifications. Approaches could readily be applied to other surfaces, and are a first step towards a comprehensive urban dew model.
  • Article
    The aim of this study was to investigate dew amounts in different urban landscapes and to examine the correlations between dew amounts and meteorological factors in Guangzhou, China. Results indicated that the dew amounts at fine night were different from landscapes to landscapes. A significant difference was found in average dew amounts between forest landscape and residential landscape (forest landscape and commercial landscape, industrial landscape and residential landscape). The highest mean dew amounts in urban area were observed in forest landscape (0.034 mm night−1), followed by industrial landscape (0.022 mm night−1), commercial landscape (0.013 mm night−1) and residential landscape (0.009 mm night−1), respectively. For maximum dew amounts, a similar relationship like the mean dew mounts was found in urban measured landscapes, whose values in turn were 0.104 mm, 0.08 mm, 0.03 mm and 0.109 mm, respectively. Both the mean dew amounts night−1 and maximum dew amounts in urban landscapes were significantly less than those (0.077 mm in mean value and 0.224 mm in maximum value) in their countryside. The mean dew amounts correlated positively with mean relative humidity; meanwhile it correlated negatively with daily evaporation and urban heat island, respectively. It was therefore concluded that urban forest landscape was an important site for dew deposit, urban environment was not favorable for dew condensation, and relative humidity, daily evaporation and urban heat island might be the most important three meteorological factors responsible for the dew amounts in urban area.
  • Article
    A method is described for the estimation of downwelling long-wave radiation in both cloudy and clear-sky conditions. This method uses an estimate of cloud fraction, obtained from the output of a laser cloud-base recorder, to modify the clear-sky emissivity as calculated from standard near-surface measurements.
  • Article
    Atmospheric humidity can be condensed as dew and used for example in small-scale irrigation. In arid locations, the most favourable conditions for dew collection persist in the late night and around sunrise. We study the possibility to use a dew collector for condensing atmospheric water vapour by exploiting the effect of radiative cooling. In particular, we study pigmented polymer foils with high solar reflectance and high thermal emittance. Suitable pigments are a mixture of TiO2 and BaSO4 particles or a novel SiO2/TiO2 composite. We calculate the condensation rate under different climatic conditions and report on initial field tests.
  • Article
    Dehumidifier is one of the most important components in liquid desiccant air-conditioning systems. Previous study shows that the internally cooled dehumidifier may have better mass transfer performance than the adiabatic unit. The effect of flow pattern, especially the flow direction of air to desiccant on the internally cooled dehumidifier performance is numerically analyzed in detail. The result shows that counter-flow configuration of air to desiccant has better dehumidification performance, and parallel-flow configuration performs the poorest with the same conditions, due to more uniform mass transfer driving force expressed in the counter-flow configuration. The decrease of the desiccant concentration is the main factor that influences the internally cooled dehumidifier's performance, while the increase of the desiccant temperature is the main performance restricting factor in adiabatic dehumidifier. Internally cooled dehumidifier has better mass transfer performance compared with adiabatic dehumidifier plus external heat exchanger.
  • Article
    Condensation experiments on a cold solid substrate with a gradient of contact angle (theta) are reported. The gradient is sufficiently small so that the driving force due to the imbalance of the interfacial tensions at the contact line is negligible throughout most stages of the experiment. The condensation proceeds by forming spherical droplets near the hydrophobic side (theta similar or equal to 100 degrees) and by forming quasi-films near the hydrophilic side (theta similar or equal to 0 degrees). In the middle part is the crossover region, where the condensation proceeds by forming islands of nonspherical shapes. It is found that the hysteresis of contact angle due to the pinning of contact lines by surface heterogeneity plays an important role in the growth of the condensation pattern. The effects of the wettability gradient and the contact-angle hysteresis on the nucleation rate, the growth rate of islands, the dynamics of coalescence, and the growth morphology are studied and discussed. An important result is that the growth remains self-similar with the same growth law as for perfect hemispherical drops (with negligible pinning force), although the droplets exhibit complicated shapes.