Application of passive radiative cooling for dew condensation

ARTICLE IN PRESS

Energy 31 (2006) 2303–2315 www.elsevier.com/locate/energy

Application of passive radiative cooling for dew condensation Daniel Beysensa,b,j,, Marc Musellic,j, Iryna Milimoukd,j, Catherine Ohayone, Simon M. Berkowiczf,j, Emmanuel Soyeuxg, Marina Miletah,j, Pascal Ortegai a

ESEME, Service des Basses Tempe´ratures, Commissariat a` l’Energie Atomique—Grenoble, 38054 Grenoble, France b ESPCI-PMMH, 10, rue Vauquelin, 75231 Paris Cedex 05, France c Universite´ de Corse, UMR CNRS 6134, 20000, Ajaccio, France d ESEME, Institut de Chimie de la Matie`re Condense´e de Bordeaux du CNRS, 87 Ave. Dr. Schweitzer, 33608 Pessac Cedex France e Laboratoire d’Hydrologie—Environnement, UFR des Sciences Pharmaceutiques, 146 rue Le´o Saignat, 33076 Bordeaux, France f Minerva Arid Ecosystems Research Centre, Hebrew University of Jerusalem, Earth Science Building, Safra Givat Ram Campus, Jerusalem, 91904, Israel g Veolia Environnement—Direction de la Recherche, du De´veloppement et de la Technologie, 36 rue de Lie`ge, 75008 Paris, France h Meteorological and Hydrological Institute of Croatia, GRIC, 10000 Zagreb, Croatia i Universite´ de la Polyne´sie Franc- aise, B.P.6570, 98702 Faaa, French Polynesia j International Organization for Dew Utilization (OPUR), 60, rue Emeriau, F-75015 Paris, France

Abstract Dew water was collected from several passive foil-based radiative condensers established in a variety of geographic settings: continental (Grenoble, in an alpine valley, and Brive-la-Gaillarde, in the Central Massif volcanic area, both in France), French Atlantic coast (Bordeaux), eastern Mediterranean (Jerusalem, Israel), and the island of Corsica (Ajaccio, France) in the Mediterranean Sea. In Ajaccio two large 30 m2 condensers have been operating since 2000. Additional semiquantitative dew measurements were also carried out for Komizˇa, island of Vis (Croatia) in the Adriatic Sea, and in Mediterranean Zadar and Dubrovnik (both in Croatia). Dew potential was calculated for the Pacific Ocean island of Tahiti (French Polynesia). The data show that significant amounts of dew water can be collected. Selected chemical and biological analyses established that dew is, in general, potable. Continued research is required for new and inexpensive materials that can enhance dew condensation. r 2006 Elsevier Ltd. All rights reserved.

1. Introduction In a world with a rapidly growing population, water scarcity will become especially severe. Arid to subhumid regions contain about 30% of the Earth’s land area as well as almost 40% of the world’s inhabitants [1]. Islands—both large and small—rural areas, and isolated regions and communities may also experience water deficiencies if wells, streams and springs are non-existent, flow seasonally, or are not readily accessible

Corresponding author. ESEME, Service des Basses Tempe´ratures, Commissariat a` l’Energie Atomique—Grenoble, 38054 Grenoble, France. Tel.: +33140795806; fax: +33140794523. E-mail addresses: [email protected], [email protected] (D. Beysens).

0360-5442/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2006.01.006

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Nomenclature Ta Tc Td N V k g h

air temperature (1C) surface condenser temperature (1C) dewpoint temperature (1C) cloud cover (oktas) wind speed (m
s1) heat transfer coefficient mass transfer coefficient dew yield (mm)

[2]. Technological approaches, such as desalination, can help meet water requirements and overcome drought episodes, though at a price that many nations cannot afford. It is thus not surprising that dew and fog water collection have been considered as inexpensive, but limited, supplemental or alternative sources of water. The team of Schemenauer and Cereceda pioneered fog collecting in Chile [3] and then promoted its application to other developing countries with suitable fog conditions [4]. Wells [5] wrote one of the earliest treatises on dew, though it took almost another 140 years before Monteith published the first quantitative scientific publications on dew formation [6,7]. Over the past 100 years, there have been several attempts to recover atmospheric water vapour. In England there were numerous observations on ‘‘dew ponds’’ forming in the countryside [8], which were later found to be capturing either mist, rain or sub-surface water flow. Jumikis [9] was fascinated by the apparently high volumes of condensed water that could be produced by the so-called ‘‘aerial wells’’ constructed by the Russian forester F.I. Zibold in Feodosia, Crimean peninsula, at the beginning of the 20th century. The air well consisted of a large heap of stones 15–40 cm in diameter, placed to form a truncated cone 20 m wide 6 m high, with a bowl-shaped depression 8 m wide on top. Reports at the time claimed that up to 360 l of condensed water/day could be produced [10]. In brief, the theory was that the voids between the stones would establish sharp differences in temperature between the stones and incoming air, hence leading to substantial condensation within the stone heaps. Subsequent attempts in [11,12], using the same approach, did not produce any appreciable quantities of condensed water except on very rare occasions. However, even these isolated events may have been related to fog interception [9, p. 93]. Beysens et al. [13] and Nikolayev et al. [14] investigated the Zibold collector and related publications. According to their calculations, the heat capacity of such condensers greatly hinders stone surface temperatures attaining or being less than the dewpoint. Hence these dew collectors were doomed to failure. The challenge, therefore, was to develop passive light-weight dew condensers that cool under a radiative process. This allows not only for portability and ease of installation, but also dispenses with the need for an external energy source. Such dew condensers are especially compatible with the low-tech requirements of developing countries. The dew yield is of course restricted by the cooling power, which lies in the range of 25–100 W/m2 for clear evening skies. Taking into account the latent heat of condensation of water (2500 J/g at 20 1C), this limits the dew water yield to under 1 l/m2 per night. In general, the meteorological conditions favouring dew formation on condensers will tend not to exceed 0.5 l/m2 per night. A collecting area of 100 m2, such as on a roof, could produce 50 l per night, which is significant where water is limited. During the past decade, studies have tried to enhance dew collection by improving the cooling material [15–17]. A foil investigated by Nilsson [16] and by Vargas et al. [18] proved able to radiate long-wave energy nearly as a black body, and also reflect visible light. They tested ways to lessen thermal influences, i.e. by insulation of the foil material on the underside, and by designing the condenser to reduce the negative effect of wind on dew formation and accumulation. This is especially important for island condensers where winds are often strong [19]. This paper reports on dew data gathered primarily from several passive foil-based radiative dew condensers established in regions where dew is frequent and collection potential high. The study sites were established in a variety of settings: continental (Grenoble, in an alpine valley, and Brive-la-Gaillarde, in the Central Massif

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volcanic area, both in France), French Atlantic coast (Bordeaux), eastern Mediterranean (Jerusalem, Israel), and the island of Corsica (Ajaccio, France) in the Mediterranean Sea. Additional semi-quantitative dew measurements were also carried out for Komizˇa, island of Vis (Croatia) in the Adriatic Sea, and in Mediterranean Zadar and Dubrovnik (both in Croatia). For these stations a semi-quantitative scaling of dew was made for a grass surface. No dew measurements were available for the Pacific Ocean island of Tahiti (French Polynesia), thus an estimate was calculated. Although the dew observation periods reported here range from only a few months to a full calendar year, the positive results should provide an incentive to further dew collection research using passive radiative condensers.

2. Experimental set-up Meteorological parameters such as air temperature, air humidity (dew point temperature), wind speed and sky radiation (cloud cover) govern dew formation [6,20]. Of the many meteorological variables that are measured, however, there are currently no universally accepted or standard methods to assess dew. Indeed, dew is not really atmospheric precipitation as its formation depends on the precise properties of the condensing surface [20]. To increase dew collection yields it is possible, however, to (i) maximize the emitting properties of a given surface, (ii) reduce the negative effect of wind velocity, (iii) diminish ground heat flux, (iv) increase dew condensation time, and (v) enhance dew drop recovery. In the present study dew collection, or its potential, was assessed in 4 ways. Dew mass condensed on a thermally isolated polymethylmethacrylate (PMMA, Plexiglas) plate was used as a reference standard in Grenoble, Ajaccio, and Bordeaux. In Ajaccio, Bordeaux, Brive-la-Gaillarde and Jerusalem, a special thermally isolated foil was used to condense dew. In Zadar, Dubrovnik, and Komizˇa, semi-quantitative measurements of dew on grass were performed with a scale ranging from 0 (no or little dewo0.05 mm), 1 (medium dew E0.05–0.15 mm), or 2 (high dew40.15 mm). No direct measurements were available for Tahiti, thus an estimate was made based on the amplitude of the difference between air temperature, Ta, and dewpoint temperature, Td, as calculated from Ta and relative humidity. The following approximate relation, with Ta and Td in 1C, provides a reasonable estimation of dew occurrence: hðT d  T a þ 3Þ if T d 4T a  3; otherwise h ¼ 0.

(1)

This relation simply stipulates that for most dew events, radiative cooling lowers the surface condenser temperature with respect to air temperature by a value less than 3 1C. As (T d 2T a ) is a weak function of Ta and a strong function of relative humidity, condition (1) can alternatively be expressed as a condition on relative humidity only; 3 1C then corresponds to a relative humidity 80% [21]. Relation (1) is based on our own experience of dew formation (see Figs. 5a and b and Ref. [21]). Strictly speaking, this relation cannot be used to evaluate the dew yield; however, it can give a rough estimation of its occurrence and amplitude in arbitrary units. The foil used in this study was a polyethylene material embedded with microspheres of TiO2 and BaSO4, based on Vargas et al. [18]. The foil improved emitting properties in the near infrared to provide radiative cooling of room temperature surfaces, and efficiently reflected visible light. Winds of about 1 m/s are favourable for replenishing humid air around a condenser. In contrast, strong winds increase heat losses and obstruct radiative cooling [22]. Numerical and experimental studies by Beysens et al. [23] were performed for a collecting surface of 1 m  0.3 m. This model condenser was made with the foil placed over an insulating 3 cm thick sheet of polystyrene foam. The model was then tested at different angles from horizontal to determine the best angle for dew drop recovery by passive gravity flow. A condenser surface angle of 301 proved optimal. In parallel, the measurements were correlated with meteorological data and compared to dew condensed on an adjacent horizontal polymethylmethacrylate (PMMA, Plexiglas) reference plate. The plate was 5 mm thick, 0.4 m  0.4 m, and placed on a thermally isolated mount formed of 12.5 mm thick aluminium foil and a 5 mm thick sheet of polystyrene foam (Fig. 1). This mounting was set on a 3 decimal electronic balance and linked to a computer.

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Fig. 1. Dew experimental set-up in Grenoble, France. P ¼ Plexiglas plate on an electronic Mettler Toledo balance. V ¼ anemometer, C ¼ dew condenser model (1 m  0.3 m) with foil surface. The angle from horizontal can be altered.

3. Results Data for the different stations are provided below. The different methods should be kept in mind when comparing the data. 3.1. Ajaccio, island of Corsica, France On the basis of the pilot study in Grenoble, two 3 m  10 m plane condensers were installed (Fig. 2) on a hillside in Ajaccio (411 550 N; 81 480 E), 400 m from the sea and at 70 m elevation. The nocturnal wind regime is predominantly from the NE (1.8 m/s average). During the day, two directions (NW and SW) are favoured. The first condenser was supported on a light cable grid (Fig. 2a), with 3 cm thick polystyrene under the foil to provide thermal isolation. At the base, a gutter funnels the water into a polyethylene storage tank. The exposed hollow part of the condenser faces the direction of the dominant nocturnal wind. A SW direction for the condenser also kept the foil surface shaded longer in the early morning, precisely when air temperature is lowest and closest to the dew-point temperature. The second condenser was similar to the former but constructed on a firm sloping surface enclosed by light building blocks (Fig. 2b). Data from the condensers were compared with the weighing reference plate set-up described above. Dewpoint temperature, air temperature, wind velocity (10 cm above the plate) and wind direction (3 m above the plate and 10 m above the ground) were measured every 15 min. During the period July 22, 2000–November 11, 2001 (478 days), we experienced 145 dew days for the reference plate (30%) and 214 dew days for the condenser (45%). The condenser yield was 767 l corresponding to an average of 3.6 l (0.12 mm) per dew day. The maximum yield in the period was 11.4 l (0.38 mm) [19]. Dew volumes produced from the reference plate and the condensers (Fig. 3) show that the yields from both 30 m2 condensers are almost the same and were considerably greater than the PMMA reference plate. The largest differences occurred for dew events with average to large yields. Fig. 4 shows that large dew yields can be obtained with such condensers even for wind speeds of 3 m/s. This is a crucial aspect for island dew collection as wind speeds are often strong. The difference between yields produced from the foil and the PMMA reference plate is about 40%. Nikolayev et al. [14] developed a model to predict the amount of condensed dew, using only simple meteorological parameters: surface (Tc) and ambient air (Ta) temperatures, air relative humidity, cloud cover (N, in oktas) and wind velocity (V). When fitting the data, however, the ideal heat and mass transfer coefficients did not represent the transfer under actual conditions. Nikolayev et al. [24] and Beysens et al. [21] thus introduced two new numerical factors that correct the transfers; k for the heat transfer and g for the mass transfer. Fitting the surface temperature data gives k, while fitting the mass data gives g. These numerical factors are constant for a given condenser and are determined experimentally. A comparison of dew yields in different locations (Ajaccio, Bordeaux, Grenoble) with the same PMMA plate condenser showed that the transfer coefficients are in the order kE3 and g ¼ 0.2. For a 301 tilted foil condenser, kE1.3 and gE1 (Fig. 5), meaning that the heat transfer is reduced and the mass transfer increased, thus leading to a higher dew yield.

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Fig. 2. The 3 m  10 m dew condensers at Ajaccio (Corsica, France). (a) Exposed condenser; (b) Enclosed condenser. F ¼ dew foil; T ¼ water collection tank; V ¼ direction of nocturnal wind.

Figs. 5a and 5b contain a fit of the condenser surface temperature ðk ¼ 1:12Þ and dew mass (g ¼ 0:74) on the PMMA plate, and the fit for the foil condenser (k ¼ 1:51, g ¼ 1:15) in Ajaccio. For the latter, we consider only two data points for the dew mass corresponding to the beginning of dew condensation, and the time of dew collection. 3.2. Island comparisons As freshwater is limited on islands, the frequency and yields of dew events are of importance. Fig. 6 compares dew yields in Ajaccio (411550 N; 81480 E, 70 m a.s.l.), Tahiti (171330 S, 1491350 W, 200 m a.s.l.), and Komizˇa (431030 N ; 161060 E, 11 m a.s.l.). Keeping in mind the limitations of the comparisons when different techniques of evaluation are used, we can estimate that Tahiti seems to have a high potential for dew collection, which can be attributed to high humidity and good atmospheric transparency, in spite of higher winds. 3.3. Continental In Fig. 7, it is clear that the yield in Grenoble (451110 N, 51420 E, 215 m a.s.l.), on a PMMA surface, is markedly lower than that of Brive-la-Gaillarde (451140 N, 11220 E, 150 m a.s.l.). Grenoble is situated within an

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30 Ajaccio July, 22 2000 - July 21, 2001

25

PMMA

MEAN : 0.073 mm SUM : 7.84 mm

P1

MEAN : 0.11mm

SUM : 17.4 mm

Count

20 15 10 5 0 0

0.05

0.1

(a)

0.15 0.2 Dew (mm)

0.25

0.3

0.35

30 Ajaccio Dec, 112001 - Dec 10, 2002

25

PMMA MEAN : 0.084 mm SUM : 9.16 mm P2

MEAN : 0.11mm

SUM : 19.9 mm

Count

20 15 10 5 0 0 (b)

0.05

0.1

0.15 0.2 Dew (mm)

0.25

0.3

0.35

Fig. 3. Comparison of dew frequency versus dew yield between the (top) Ajaccio exposed condenser, 22 July, 2000–21 July, 2001 and (bottom) Ajaccio enclosed condenser, 11 December, 2001–10 December, 2002. All data are compared to dew on the PMMA reference plate. ‘‘SUM’’ is the cumulated dew yield for the period.

industrial valley, hence lower humidity and less atmospheric transparency, while Brive-la-Gaillarde is situated in the nearby Central Massif volcanic area. 3.4. Coastal Although the analysis can be hampered by the different techniques to evaluate dew yields, in Fig. 8, the lower yield for Bordeaux (441 470 N, 01390 W, 17 m a.s.l.) as compared to the Zadar (441080 N, 151130 E, 11 m a.s.l) and Dubrovnik (421390 N, 181050 E, 10 m a.s.l) Mediterranean location, is presumably due to more and greater frequency of cloud cover. The shape of the histogram is markedly different, with many high yield events for Zadar and Dubrovnik. This was surprising and will be investigated in future with more quantitative measurements. Although the mean dew yield in Dubrovnik is slightly greater than in Zadar, the frequency of dew events was lower, thus providing less cumulated dew yield. 3.5. Eastern Mediterranean Arid and semi-arid regions are characterized by lower humidity and high atmospheric transparency. Apart from a source of drinking water, here dew can play a special role in the ecosystem as a regular source of water for small animals, plants, and biological sandcrusts. In the northern Negev desert of Israel, dew occurs on about 200 nights per year [25], and about 165 nights per year for Jerusalem. Preliminary measurements show

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0.14 Ajaccio July 22, 2000 - July 21, 2001

Mean Dew yield (mm)

0.12 0.1

PMMA PROTO1

0.08 0.06 0.04 0.02

(a)

0 0.14 Ajaccio Dec 11, 2001 - Dec 10, 2002

Mean dew yield (mm)

0.12

PMMA PROTO2

0.1 0.08 0.06 0.04 0.02 0 0 - 0.5

0.5 - 1

1 - 1.5

(b)

1.5 - 2

2 - 2.5

2.5 - 3

3 - 3.5

Wind speed (m/s)

Fig. 4. Dew yield from the exposed (a) Proto1, enclosed (b) Proto2, and PMMA plate dew condensers with respect to wind speed at 10 m.

that even in mid-summer, significant dew amounts can be collected despite the shorter evening period available for radiative cooling. Although high-pressure systems are obviously prevalent during the summer period, the proximity to the Mediterranean Sea and high temperatures allow for high humidity. The drop in air temperature at night facilitates the dew point being reached. In the Negev, the rain period is mainly confined to about 4 months (November–February), while in Jerusalem the rains occur between mid-October and mid-April. Hence, dew occurrence in the dry hot period is a welcome phenomenon. Fig. 9 contains daily dew data collected from a 1 m  1 m foil condenser in Jerusalem (311 470 N; 351 130 E, 780 m a.s.l.) for the period June–August 2003. The monthly dew totals were 1.5 l for June, 3.7 l for July, and 6.1 l for August. If we consider a roof used to collect dew water, a collecting area of 100 m2 is not unusual. This is capable of providing 20–50 l of dew water per night on a regular basis during the dry summer. 4. Chemical and bacteriological quality of water Dew water chemistry is dependent on the collector location as dew is generally the condensation of local vapour. Since dew is an outcome of a condensation process, it is generally less mineralized and less acidic than rain. 4.1. Ion measurements Dew water, because of its low yield as compared to rain, can concentrate chemical and biological pollutants and could thus also serve as a sensitive environmental indicator. As an example, Table 1 displays major ion

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0.25

12 10

(°C)

0.2

Tc (°C) fit Tc (°C) exp.

8

dew fit dew exp.

0.1 6 N (1/8)

0.0

4

wind (m/s)

1 :12 :06 :40

1 :09 :20 :00

1 :06 :33 :20

1 :03 :46 :40

1 :01 :00 :00

-0.05

0 :22 :13 :20

0

0 :19 :26 :40

0 0 :16 :40 :00

2

Time (dd:hh:mm:ss)

(a) 20

0.35 dew exp. dew fit

Tc (°C) fit Tc (°C) exp.

15

Td

0.3

(°C)

0.25 0.2

10

0.15 N (1/8)

0.1

5

wind (m/s)

Dew (mm)

Temperature, Cloudcover, Windspeed

0.15

Dew (mm)

Td

0 :13 :53 :20

Temperature, Cloudcover, Windspeed

14

0.05

(b)

-0.05

1 :12 :06 :40

1 :09 :20 :00

1 :06 :33 :20

1 :03 :46 :40

1 :01 :00 :00

0 :22 :13 :20

0 :19 :26 :40

0 :16 :40 :00

0

0 :13.53:20

0

Time (dd:hh:mm:ss)

Fig. 5. Example of a data fit for 13–14 December, 2000, Ajaccio. Data were recorded every 15 min. (a) PMMA reference plate, with k ¼ 1.12 and g ¼ 0.74. (b) Foil condenser, with k ¼ 1.51 and g ¼ 1.15. Dew mass (+) is measured at the beginning and the end of condensation. N ¼ cloud cover in oktas.

concentrations in dew and rain collected from a dew foil condenser in Bordeaux, compared to World Health Organization specifications (WHO) [26] and a French commercial spring-water (Mt. Roucous) recommended for babies. Ion concentration remains below the drinking water limit. In Ajaccio there are occasionally relatively high concentrations of Fe, between 90 and 1700 mg/l, whereas the European Union recommends 200 mg/l and the WHO recommends an upper limit of 300 mg/l for drinking water. For Al, the Ajaccio values are between 0.1 and 5.5 mg/l, whereas the European Union and WHO recommends 0.2 mg/l for drinking water. The upper value events are generally correlated with the occurrence of aerosols from the Sahara that settle over the Mediterranean area. 4.2. pH measurements The alkaline or acidic character of dew depends mainly on the ions (sulphate, carbonate) present in the local atmosphere and also the deposition of local aerosols on the condenser. Here, we report on pH data

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35 Ajaccio 01/01/02-31/12/02

30

120 100

20

Count

Count (a)

Tahiti 25/10/01-24/10/02 MEAN: 0.099 a. u. SUM: 34.0 a. u.

140

MEAN: 0.083 mm SUM: 9.27 mm

25

15

80 60

10

40

5

20

0

2311

0

0.05 0.1 0.15 0.2 0.25 0.3 Dew (mm) 100

(b)

0 0.06 012. 0.18 0.24 0.3 0.36 0.42 Dew (a. u.)

o Komiza 01/01/02-31/12/02 MEAN: 0.068mm SUM: 6.7 mm

80 Count

0

60 40 20 0 0

(c)

0.05 0.1 0.15 0.2 0.25 0.3 Dew (mm)

Fig. 6. Dew frequency versus dew yield for three island locations, Ajaccio, Tahiti and Komizˇa, with mean and cumulated values (‘‘SUM’’). (a) Ajaccio, dew on PMMA. (b) Tahiti, dew estimated from Eq. (1). ‘‘a.u.’’ ¼ arbitrary units. (c) Komizˇa, measurements of dew on grass; 0 ¼ no or little dewo0.05 mm, 1 ¼ medium dew (0.05–0.15 mm), or 2 ¼ high dew,40.15 mm. The categories of dew intensities can be made finer by averaging two values observed at 07:00 and at 21:00 the previous day.

50

70

Brive 01/01/00-31/12/00

Grenoble 01/01/00-31/12/00

60

MEAN: 0.115

MEAN: 0.073 mm

SUM: 31.6

40

SUM: 6.64 mm

30

40

Count

Count

50

30

20

20 10 10 0 (a)

0 0

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 Dew (mm)

(b)

0

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 Dew (mm)

Fig. 7. Dew frequency versus dew yield for two continental sites for January–December, 2000, with means and cumulated values (SUM). (a) Glacier alpine valley: Grenoble, dew on PMMA. (b) Central Massif area: Brive-la-Gaillarde, dew on 1 m2 foil.

obtained over 1 year in Ajaccio (Fig. 10) and Bordeaux (Fig. 11) as compared to rain. Dew pH is slightly acidic and rain is in general more acidic than dew. In Jerusalem the pH tended to range from 6.5 to 7.5.

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100

120 Bordeaux 01/01/02-31/12/02 MEAN: 0.049 mm SUM:10.66 mm

100 80

60

Count

Count

80

40

Zadar 01/01/02-31/12/02 MEAN: 0.166 mm SUM: 24.6 mm

60 40

20

20 0 -0.05

0 0

0.05

(a)

0.1 0.15 Dew (mm)

0.2

0.25

0

(b)

0.05 0.1 0.15 0.2 0.25 Dew (mm)

35 Dubrovnik 01/01/02-31/12/02 MEAN: 0.177 SUM: 7.95 mm

30 Count

25 20 15 10 5 0 0 (c)

0.05

0.1 0.15 Dew (mm)

0.2

0.25

500 450 400 350 300 250 200 150 100 50 0 6/ 1/ 20 6/ 03 8/ 20 0 6/ 15 3 /2 6/ 003 22 /2 6/ 003 29 /2 0 7/ 03 6/ 20 0 7/ 13 3 /2 7/ 003 20 /2 7/ 003 27 /2 0 8/ 03 3/ 20 0 8/ 10 3 /2 8/ 003 17 /2 8/ 003 24 /2 8/ 003 31 /2 00 3

Dew collected (ml)

Fig. 8. Dew frequency versus dew yield during 2002 for three coastal areas. The mean and cumulated values (SUM) are indicated. (a) Atlantic Ocean coast: Bordeaux, dew on PMMA. (b–c) Zadar and Dubrovnik, Mediterranean Adriatic coast, Croatia. Measurements of dew on grass; 0 ¼ no or little dewo0.05 mm, 1 ¼ medium dew (0.05–0.15 mm), or 2 ¼ high dew,40.15 mm.The categories of dew intensities can be made finer by averaging two values detected at 07:00 and at 21:00 the previous day.

Date 2

Fig. 9. Daily dew collection amounts (in ml) in Jerusalem, using a 1 m foil condenser, for June to August 2003 (4 missing data in June).

4.3. Water quality Biological analyses by Beysens et al. [27] revealed that many harmless bacteria are deposited from the ambient air. This is unavoidable as dew is collected in the open. When dew is collected with care, very little contamination by feacal bacteria is found, as in the small dew collector operated in Bordeaux. However, more significant biological pollution can be measured when such conditions are not present [28]. In Ajaccio, on the 30 m2 large collectors, the presence of microorganisms and especially of indicators of feacal contamination

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Table 1 Ion concentrations for dew and rain collected from a foil condenser in Bordeaux, compared to World Health Organization specifications, 2 and a commercial spring-water brand (Mt. Roucous). C: conductivity; Alkalinity (OH–, HCO 3 , CO3 concentrations in mEq CaCO3/L); FT: number of bacteria per ml forming colonies Measurement

Dew

Rain

Mean

Min

Max

pH in situ pH laboratory C(25 1C, mS/cm) H+(mM/l) OH(mM/l) Na+ (mg/l) K+ (mg/l) Ca2+ (mg/l) Mg2+ (mg/l) Zn2+ (mg/l) Cu2+ (mg/l)

6.26 6.00 45.1 0.00143 o104 3.60 0.41 1.47 0.36 0.036 0.0027

5.08 5.10 7.80 o104 o104 0.2 0.1 o104 o104 0.01 o104

7.22 6.90 214 0.0079 o 104 27 2 5.6 3.1 0.13 0.012

Cl (mg/l) SO2 4 (mg/l) NO 3 (mg/l) NO 2 (mg/l)

5.52 3.75 2.80 0.44

0.01 0.01 o104 o104

56 17 26 2.8

o0.04 Alkalinity (mEq CaCO3/L)

o0.04

o0.04

FT (22 1C, /ml) FT (36 1C, /ml)

2 0

4300 4300

Max (WHO)

Mt Roucous 50 km Spring from Water Atlantic Ocean

Mean

Min

Max

6

5.36 5.40 50.7 0.0113 o104 4.18 0.33 1.52 0.56 0.14 0.0088

3.23 3.90 5.5 o104 o104 0.1 0.02 o104 o104 0.025 o104

6.93 7.60 389 0.12600 o10-4 22.4 1.4 15.6 5 0.445 0.03

8.16 4.46 4.17 0.14

o104 o104 o104 o104

88 57.5 83 1.3

o0.04

o0.04

o0.04

19 0

4300 4300

25

2

50 3

2.80 0.40 1.20 0.20 o0.02 o0.002

5.82 1.05 0.175 0.912

3.20 3.3 2.30 undetectable

4.50 0.257

o0.1

(enterococcus and coliforms) listed in the European Union regulations for drinking water requires that, to be potable, dew water from large collectors must be disinfected. This can be done by simple means, e.g. chlorination. 5. On-going research Continued research is required for new and inexpensive materials that can enhance dew condensation. We are presently testing the radiative properties of different materials that could be used for passive dew condensers. Apart from the polyethylene foil used in this study (manufactured under the auspices of the OPUR association), Teflon material (PtFe), clear anti-UV PVC, and white corrugated anti-UV PVC sheets used on roofs are common materials that could be utilized for larger dew collecting surfaces. Although much more expensive than the OPUR foil, PVC has several advantages. It is more resistant to breakage, can withstand sun and heat, lightweight, does not tend to absorb dust, dirt, oil or grease, and has high resistance against many chemicals. 6. Conclusions We have demonstrated that passive radiative dew condensers can obtain significant amounts of water in a variety of geographic settings. Continued research is required for new and inexpensive materials that can enhance dew condensation. Nevertheless, dew collecting will remain limited by external factors, especially the available radiative cooling energy and local air humidity. These factors effectively limit the dew yield to 0.5 mm/night.

ARTICLE IN PRESS D. Beysens et al. / Energy 31 (2006) 2303–2315

2314

60 50

20 Ajaccio 15/03/00-14/03/01 MEAN: 5.93 VARIANCE: 0.21

Ajaccio 15/03/00-14/03/01 MEAN: 5.70

15

VARIANCE: 0.67

Count

Count

40 30

10

20 5 10 0 2.5

3.5

4.5

5.5

6.5

pH (dew)

(a)

0 2.5

8.5

7.5

3.5

4.5

5.5

6.5

7.5

8.5

pH (rain)

(b)

Fig. 10. pH histograms for Ajaccio (a) dew and (b) rain. Rain pH is slightly more acidic than dew pH and shows a larger variance.

35

12 Bordeaux 15/01/02-14/01/03

Bordeaux 15/01/02 -14/01/03 MEAN: 5.35 VARIANCE: 0.66

MEAN: 6.28

30

10

VARIANCE: 0.18

25 20

Count

Count

8

15

6

4 10 2

5 0 (a)

3

3.4 3.8 4.2 4.6 5 5.4 5.8 6.2 6.6 pH (dew, frost)

7

0

7.4

3 (b)

3.4 3.8 4.2 4.6

5

5.4 5.8 6.2 6.6

7

7.4

pH (rain, snow)

Fig. 11. pH histograms for Bordeaux (a) dew and frost, (b) rain and snow. The pH of rain is more acidic and the variance is larger than for dew.

Both chemical and biological analyses of the dew water are required to determine the quality of the water. In this respect, small-scale collectors are easier to keep clean. Operating costs of a dew collector are a function of the materials used and labour. The 30 m2 Ajaccio condenser has a cost of 0.30 Euros/l. The foil costs alone, assuming a 2-year life span, can provide dew water at a cost of about 0.04 Euros/l. An additional benefit of the dew condensers is that they can also collect rain and, to a lesser extent, fog water. It is envisaged to equip home roofs with coatings1 that favour dew condensation, thus providing an additional source of water when other sources (rain, fog) are lacking. 1 Note added in proofs: A program called Dew Equipment for Water (D.E.W.) started in April 2005 on the small island of Bisˇ evo, near the Komizˇa station. A roof has been equipped with special coating and instrumentation. Preliminary results indicate a 30% contribution of dew water to total collected water between April–August 2006, although this summer period was particularly rainy.

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