Design and measured performance of a porous evaporative cooler for preservation of fruits and vegetables

Energy Conversion and Management 45 (2004) 2187–2195 www.elsevier.com/locate/enconman

Design and measured performance of a porous evaporative cooler for preservation of fruits and vegetables E.E. Anyanwu

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Department of Mechanical Engineering, Federal University of Technology, P.M.B. 1526, Owerri, Imo State, Nigeria Received 29 May 2003; received in revised form 23 July 2003; accepted 26 October 2003

Abstract

The design, construction and measured performance of a porous evaporative cooler for preservation of fruits and vegetables are reported. The experimental cooler, with a total storage space of 0.014 m 3 , consists of a cuboid shaped porous clay container located inside another clay container. The gap between them is filled with coconut fibre. A water reservoir linked to the cooler at the top through a flexible pipe supplied water to fill the gap, thus keeping the coconut fibre continuously wet. Results of the transient performance tests revealed that the cooler storage chamber temperature depression from ambient air temperature varied over 0.1–12 °C. Ambient air temperatures during the test periods ranged over 22–38 °C. The results also illustrate superior performance of the cooler over open air preservation of vegetables soon after harvest during the diurnal operations. Thus, the evaporative cooler has prospects for use for short term preservation of vegetables and fruits soon after harvest. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Evaporative cooler; Clay; Design; Measured performance; Preservation; Vegetables; Fruits; Temperature

1. Introduction

In hot, dry climates, such as those encountered in some parts of Nigeria, evaporation from a water water reserv reservoir oir can reach reach signifi significan cantly tly high high propor proportio tions. ns. Althou Although gh quanti quantitat tativ ivee data data on such such evaporation losses in the country is scarce, in similar climates of North America, it has been reported reported that more than 1.2 · 109 m3 evaporates annually from the reservoirs of the Colorado and

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Tel.: +243-083-231343; fax: +243-083-233228. E-mail address: [email protected] (E.E. Anyanwu).

0196-8904/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2003.10.020

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E.E. Anyanwu / Energy Conversion and Management 45 (2004) 2187–2195

Missouri rivers [1]. This effect has been exploited advantageously for space conditioning in comfort cooling and food preservation applications. Evaporative cooling is one of the oldest and simplest traditional cooling methods. It has developed from crude beginnings where water could be cooled by placing it in porous jars on roof tops at sun set. The night breeze evaporated the moisture, which seeped through the jars, making the water inside cooler [2]. Such systems do not require any external energy sources to operate and therefore are known as passive cooling systems. The principles are also employed to provide comfort in buildings in warm climates. However, they rely on weather conditions that are not predictable, and there are frequent periods of hot weather when ambient temperatures are too high and wind effects are very low but cooling is most desired. With a properly designed evaporative cooler, agricultural products, such as vegetables, fruits etc., can be maintained in fresh conditions for a longer period after harvest. In Nigeria, large quantities of these products are lost annually due to poor processing and storage facilities. This situation is even worse in the remote rural areas where the fresh food materials are produced. In such locations, non-existent or irregular grid connected electricity supplies to power refrigeration machines for cold storage and inadequate transport facilities to convey the produce to distant but lucrative markets constitute serious impediments to saving them from heat induced spoilage. Even when space conditioning machines are available, most farmers in developing countries like Nigeria do not have the financial capability to purchase and maintain them. As a result, sharp differences in food supplies exist between the harvest and off harvest periods. Thus, high market value agricultural products are usually abundant and cheap during the harvest season but scarce and expensive at other times. Cold storage powered by locally available ambient energy can assist to reverse this trend.

2. Previous evaporative cooler designs

Various types of evaporative cooler designs have been reported. The most common evaporative cooler is the cabinet cooler. Here, a convenient cabinet is usually encapsulated by evaporatively cooled surfaces. In some cases, the cabinets are constructed from metallic materials with charcoal placed adjacent to the sides [3], with the result that heat conduction takes place between the outer and inner metal container walls and combined convective and radiative heat transfer within the storage space. These result in little or no temperature differences between the evaporative cooler storage chamber and the ambient air temperature. In particular, seepage of water is inhibited by the non-porous metal container. We observe that if an evaporative cooler is designed properly such that the storage chamber temperature is consistently lower than the ambient air temperature, evaporation of water seeping into and through the walls must be the critical factor. Some evaporative coolers, also called curtain coolers, have a sheet of canvas or strong adsorbent cloth used as the evaporating surface [4]. The upper end of the sheet may be raised or lowered, while its bottom is dipped into a trough of water. Through a wick type action, water is transported continuously from this end to the upper surface where it is evaporated, thus cooling the space around it.


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3. Design and construction

In order to investigate experimentally the use of the process of evaporative cooling for preservation of fresh farm products, such as vegetables, fruits etc., a test cooler was designed and constructed at Owerri, Nigeria, as shown schematically in Fig. 1. The experimental evaporative cooler is of the cabinet box type. The cabinet is made of clay, reinforced with 3 mm thick steel wire mesh and provided with a side door for accessing the storage chamber. The overall inside dimensions of the cooler were 240 mm long · 240 mm wide · 240 mm high with a wall of 20 mm thickness. This was housed in an outer cuboid of inside dimensions 380 mm · 380 mm · 380 mm with 20 mm thick wall, such that a uniform gap of thickness 50 mm between the vertical, top and bottom side walls were filled with coconut fibre to enhance the water retention capacity of the walls. Coconut fibre is reported to have a water absorption capacity in the range of 130–180% [5]. The main advantage of this unit is that it uses simple passive cooling features to achieve low temperatures for preservation of fruits vegetables, food etc. It requires no skill in its operation and, therefore, is most suitable for rural application. All the evaporative cooler construction including the steel wire mesh, were made of locally available materials. Provision is made for dividing the cooler storage chamber into two using expanded steel wire mesh. The clay is comprised of feldspar from the tin mining site at Jos, fire clay from the coal mining sites near Enugu and Aroclay from Arondizuogu, all in Nigeria, in the ratio of 1:1:1. Fig. 2 shows a scale drawing of the evaporative cooler.

Fig. 1. Schematic illustration of the evaporation cooler.

Fig. 2. The evaporative cooler.

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A water reservoir linked to the cooler at the top through a flexible pipe supplied water always to fill the gap, thus keeping the coconut fibre continuously wet. Water seeps through the clay walls and evaporates at the wall outer surfaces. This resultant evaporation keeps the storage space temperature below ambient temperature consistently during the cooler operation.

4. Experimental observation

The experimental evaporative cooler was located under a pear tree near the school of Engineering and Engineering Technology Workshop of the Federal University of Technology, Owerri. This has the advantage of reducing considerably the effects of direct solar radiation on the test facility and exposing it to open air flow. It is positioned facing southwest, which is the most frequent wind direction at the site [6]. Experimental tests were undertaken with and without farm products in place to study the effectiveness of the design. The evaporative cooler performance tests were undertaken under no load conditions to establish its transient response to variations in prevailing weather conditions. Tests were also conducted to determine the length of storage of the fruits and vegetables in the cooler before spoilage. A control test in which the same product inside the evaporative cooler was exposed to open air conditions was used to evaluate the cooler effectiveness in preservation of the fruits and vegetables. Fresh pumpkin and tomato vegetables obtained from the University farm were used for the tests. Measurements were taken at intervals of 30 min for all the tests conducted. The cooler storage space temperatures were monitored with copper constantan thermocouples connected to a 12 way Type K digital thermometer readout with a reading accuracy of ± 0.1 °C. Wet bulb and ambient temperatures were measured with mercury in glass thermometers with a reading accuracy of ±0.5 °C. The relative humidity was monitored with a hygrometer with a reading accuracy of ±0.5%. Wind data, for the period of study, measured by the Federal University of Technology, Owerri, were used. The records were obtained with an integrating cup anemometer at half hourly intervals. These measurements provided the general trend of the prevailing conditions within the cooler. A total of 10 tests were conducted. The period of the tests covered the prevailing dry and wet seasons weather conditions of the country.

5. Results and discussion

5.1. Evaporative cooler tests results without load

The performance of the evaporative cooler without any load to be cooled was evaluated during 26–28 October 1997. The tests involved measurement of the ambient air relative humidity and temperature, and the cooling chamber temperature. Fig. 3 illustrates the variations of these recorded parameters with the time of day. The depression of the cooling chamber temperature from the ambient air temperature varies from )0.1 to 8.3 °C. Relatively high temperature depressions occurred between the 12 and 16 h local time when the ambient air temperatures were appreciably high, ambient relative humidities low and wind velocities of the order of 1.5 m/s. Thus, the evaporative cooler temperatures were consistently lower than the ambient air temperatures during


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Fig. 3. Transient performance of the evaporative cooler under no load condition.

the hottest times of the day when insolation was appreciable and cooling most needed. For other times, the ambient air relative humidity values recorded were quite high, within 70–90%, and the month of October is not usually windy at Owerri, Nigeria. Inclement weather conditions (cloud overcast and occasional rains) during this test period contributed significantly to the observed increases in relative humidity, drops in ambient air temperature and low wind speed after 16 h. The pattern of the test results for the evaporative cooler was the same during nocturnal operations (see Fig. 3). In particular, for ambient relative humidity values greater than about 80%, the cooler chamber temperature has the same order of magnitude as the ambient air temperature. These results clearly demonstrate that the evaporative cooler may be useful in our climate for short term preservation of farm products, especially during the hottest times of the day when cooling is most needed. 5.2. Evaporative cooler tests results with load

The transient responses of the evaporative cooler with load (fresh pumpkin, tomatoes etc.) are presented in Figs. 4 and 5 during the wet and dry seasons, respectively. They show the ambient relative humidity and the cooler chamber and ambient air temperature variations with the time of day. These results are representative of the series of tests undertaken during the various periods of the year. It can be seen from Fig. 4 that during the rainy (wet) season, the cooler chamber temperature was consistently below the ambient air temperature only between 8 and 18 h local time and approximately equal to it at other times. These times of marked differences and approximate equality in the recorded temperatures correspond to the diurnal and nocturnal periods of the day, respectively. However, significant depressions of the cooler chamber temperature from ambient air temperature within 5.0–8.7 °C occurred between 11 and 16 h with ambient relative humidity varying over 51.0–63%. This is desirable as the demand for cooling falls within the identified period when farm products are also harvested. Such low temperatures are

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Fig. 4. Transient performance of the evaporative cooler during the wet season.

Fig. 5. Transient performance of the evaporative cooler during the dry season.

necessary to maintain the products in fresh conditions for a significantly longer period. For most of these times, the ambient air temperatures were quite well within 28–35 °C, and the average ambient wind speed was 1.62 m/s. The differences in the diurnal and nocturnal behaviours of the evaporative cooler may be attributed to the relatively high humidity and low wind speeds characteristic of night times in the wet season. The pattern of the test results here was similar to that without load in the evaporative cooler (see Fig. 3). Figs. 6 and 7 show the frequency distribution of wind speed and relative humidity respectively, during the dry and rainy (wet) seasons. The


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Fig. 6. Frequency distribution of wind speed during the dry and wet seasons.

Fig. 7. Frequency distribution of ambient relative humidity during dry and wet season.

predominant wind speed and relative humidities are seen to be in the range 1.0–2.0 m/s and 80– 90%, respectively. Nigeria s tropical climate with distinct rainy (wet) and dry seasons is associated, respectively, with prevalent moisture laden southwesterly winds from the Atlantic Ocean and the dry continental northeasterly Harmattan wind from the Sahara Desert. These general climatic characteristics of the country could also be responsible for the observed behaviour of the cooler during the seasonal periods. It is further noted that since the mean nocturnal air relative humidity and wind speed did not vary very widely, the observed effects of their variations on the cooler performance would have been very insignificant. In such situations, the system performance would be similar to simply exposing the farm products to open air conditions. Overall the evaporative cooler temperatures during the rainy (wet) season varied over 22.3–26.8 °C compared with ambient air temperatures of 23.3–35.0 °C. Frequent rains during the wet season, generally resulted in rapid changes in the amount of water vapour entrained in the atmosphere. The level of humidity, together with the other environmental parameters of temperature and wind speed, influenced the rate of evaporation that occurred from the external surface of the cooler. Generally, when the weather is hot and humid, the rate of evaporation is reduced, resulting in poor cooling. During the dry season, it can be seen (Fig. 5) that the cooler chamber temperatures measured were consistently lower than the ambient air temperatures at all times. These depressions, varying over 3.0–12.0 °C, were expectedly higher than those of the rainy (wet) season because the cooling conditions during the dry season were generally more predictable. Õ

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The results also showed superior performance of the cooler between 11 and 18 h with temperature depressions between 6 and 12 °C most of the time. The diurnal relative humidity levels below 60% at these hottest times of the day are well within the levels necessary to facilitate evaporative cooling at the moderate wind speeds (2.0–3.5 m/s) recorded. The ambient relative humidity profile over the wet season tests was fairly constant while that of the dry season was comparatively very irregular. (see Figs. 4 and 5). Although high intermittent peak ambient relative humidity values (>60%) were recorded for both seasons, the average value of about 81% for the wet season was higher than the value of about 64% for the dry season for all the tests. The differences in the performances of the cooler during the seasons may also be largely attributable to the prevailing weather conditions. The frequency distribution of the ambient relative humidity and wind speed are presented in Figs. 6 and 7 for illustration. The results show remarkably more periods of lower humidity (<70%) and higher wind speed (>2.0 m/s) during the dry season. Conversely, more periods of higher humidity (>70%) and lower wind speed (<2.0 m/s) were recorded during the rainy (wet) season. For both periods, the peaks of the ambient air temperature are seen to correspond to the minima of the relative humidity and vice versa. Generally, the range of temperature depressions recorded compared quite well with the values of 10–13 °C reported in the literature [3]. The results clearly indicate that the evaporative cooler may be conveniently used for preservation of vegetables and fruits since their recommended temperatures for cooling application are in the range of 2–6 °C and 16–18 °C, respectively [4]. The experiments undertaken with load illustrated the performance potentials of the evaporative cooler. A control test involving exposing the load to open air for preservation was used to further evaluate the cooler s effectiveness. It was observed that the fresh pumpkin vegetable dried up after 12 h while the tomatoes indicated signs of deterioration after about 32 h. Inside the evaporative cooler the pumpkin and tomatoes remained in fresh conditions for 60 and 93 h, respectively. Õ

6. Conclusion

The results from this study demonstrate that evaporative coolers can maintain their cooling spaces at temperatures below ambient air temperatures. However, the performances of the experimental evaporative cooler were found to be significantly affected by seasonal weather conditions. Overall cooler storage chamber temperature depression from ambient air temperature reached up to 12 °C, and ambient air temperatures varied over 22–38 °C. Since significant evaporative cooler temperature depressions from the ambient air temperature always occurred during the times of the day when cooling is most desired, the cooling conditions achieved are suitable only for short term preservation of vegetables and fruits soon after harvest.

Acknowledgements

The author acknowledges the financial support of the Federal University of Technology, Owerri, under its Senate research grant scheme. The initial contributions of E.N. Okoro, and V.C. Onyema, and those of the 1997/98 and 1999/2000 academic sessions 4th year Mechanical EngiÕ


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neering (Air-conditioning and Refrigeration Option) group in data collection are warmly appreciated.

References [1] Probert D. Appl Energy 1977;3:257–65. [2] Warren R, Olivio TC. Principles of refrigeration. 2nd ed. Delman; 1979. [3] Taha AZ, Rahim AAA, Elton OMM. Renew Energy 1994;5(1):474–6. [4] Dossat JR. Principles of refrigeration. New York: John Wiley; 1981. [5] Gram, Parry, Phyner, Schaffner, Stulz, Wehrli. Report on Fibre concrete roofing by Swiss Center for Appropriate Technology (SKAT) and Intermediate Technology Energy Development Group (ITD G); 1986. [6] Anyanwu EE, Iwuagwu CJ. Renew Energy 1995;6(2):125–8.

Design and measured performance of a porous evaporative cooler for preservation of fruits and vegetables

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