Evaluation of a new heat transfer and evaporative design for a zero energy storage structure
Graphical abstract
Introduction
The quality of fruit and vegetables and their related shelf life are reduced by loss of moisture, decay, and physiological breakdown, and such deterioration is directly related to storage temperature, relative humidity, air circulation, mechanical damage, and improper postharvest sanitation (World Health Organization, 2003). Field observations over the past forty years have revealed that 40–50% of horticultural crops (which include root and tuber crops, fruit and vegetables) produced are lost before they can be consumed, mainly because of high rates of bruising, water loss, and subsequent decay due to inadequate storage facilities (Ray and Ravi, 2005). Evaporative cooling remains one of the oldest, simplest, and least expensive techniques used since ancient times to delay spoilage of stored fruit and vegetables by lowering the ambient temperature (Costelloe and Finn, 2003). Most agricultural areas in developing countries have neither grid connected electricity supplies nor adequate electric powered refrigeration facilities (Veronika, 2011). Even where both these facilities are available, most farmers do not have the financial capability to purchase, maintain, and operate expensive cooling systems. Therefore, prices of agricultural products are highest during the off-season and cheapest during the harvest season. These are common problems both for farmers and consumers. In this context, some methods of preserving fruit and vegetables, other than expensive electric powered refrigeration systems, such as storage in a ventilated shed, using zero energy storage based on an evaporative coolant system, waxing and hot water treatment have been found to be an efficient and economical means of reducing postharvest storage loss worldwide (Couey, 1989, Fallik et al., 1996, Olosunde, 2006, Lu et al., 2007, Sushmita et al., 2008, Islam and Morimoto, 2012). According to Dvizama (2010), the shelf life of fruit and vegetables is enhanced by minimizing deteriorative reactions, and availability for longer periods would reduce fluctuations in market supply and prices. With these problems in mind, a new type of “zero energy storage structure (ZESS)” was developed.
The cooling performance of the developed system was influenced by the combination of weather conditions (outside temperature, relative humidity, and air speed and direction), manipulating factor (watering), and the heat transfer properties of the heat exchanging media (wall and evaporative material). Recently, various types of evaporative cooler designs have been reported. Two-stage evaporative coolers pre-cool the air before it goes through the evaporative pad. Overall, the system is 70% effective for the indirect part and 90% effective for the direct part while the relative humidity of the cool air is 50–70% (ASHRAE Handbook, 2007). The other most common cabinet evaporative cooler is usually encapsulated by evaporatively cooled surfaces. In some cases, the curtain coolers have a sheet of canvas or strong absorbent cloth used as the evaporating surface (Dossat, 1981). The upper end of the sheet may be raised or lowered, while its bottom sits in a trough of water. Through a wick-type action, water is transported continuously from this lower end to the upper surface where it evaporates, thus cooling the space around it. According to Ganesan et al. (2004), a ZESS consisting of a brick wall (outside and inside wall) has a problem in maintaining the relative humidity level inside the storage area because of water leakage and heat loss from the system. A storage area that is too dry cannot provide the desired cooling effect and chamber that is too moist leads to fungus growth. Approximately 100 L h−1 of water was required to cool that system. So, it is necessary to reduce the water consumption rate of the ZESS.
A wide range of materials was used for this purpose and can be classified as heat exchanging medium (clay brick for inside wall, volcanic plate for outside wall), evaporating medium (sand–zeolite, gravel stone), and heat insulating material (polystyrene sheet for storage area). Volcanic plate is a good heat transfer material with high thermal conductivity (TC) and large capillary force, and allows transfer of a large amount of heat from the dry side of the wall to the wet side by conduction and maintains an adequate amount of retained water on the wet surface of the wall. The heat conducted vaporizes the water on the wet surface, thus maximizing the cooling efficiency of the system. Also, the material should be cheap and suitable for forming into various shapes. Furthermore, it should be easy to clean and replace, and be able to prevent bacterial growth on the wet surface (Liu, 2008, Dadhich et al., 2008, Odesola and Onyebuchi, 2009, Zahra and John, 1996). As a result, a well-designed, indirect evaporative cooler can maintain fruit and vegetables in a fresh condition for a longer period after harvest. In this paper, we investigated a new type of heat exchanging and evaporating medium with a solar chimney to determine the heat exchanging and evaporating capabilities favorable for storing fruit and vegetables.
Section snippets
Materials and methods
An experimental ZESS was set up at the Faculty of Agriculture, Ehime University and the experiment was conducted from 1st May, 2012 to 30th October, 2013.
Daily changes in weather conditions
As shown in Fig. 7(a) and (b), during sunny days, the maximum average outside air temperature and solar radiation was 30.2 °C and 630.6 W m−2, respectively, and the minimum average relative humidity was 29.8%. At night when solar radiation decreased to 0 W m−2, relative humidity increased to 69.6% and the outside temperature decreased to 25.4 °C. This change in relative humidity was due to air molecules that absorbed solar radiation during the daytime and thus the surrounding air temperature
Conclusion
A ZESS based on an indirect evaporative cooling mechanism was designed and developed, and then its cooling performance was evaluated under loading (with tomato and eggplant) and no loading conditions. The ZESS consists of double walls made of lava plates and bricks, an evaporative medium (gravel stone–sand–zeolite) between the double walls, a half below ground storage space, a water supply, and a natural airing system from bottom to top through the evaporative medium. Cooling of the ZESS is
Acknowledgment
The authors would like to acknowledge the financial support provided by the Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 26450354).
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