Potential of Nightsky Nocturnal Radiative Cooling as a Fresh Produce Preservation Method

The change in water temperature in horizontally placed pans exposed to the night-sky radiation was monitored at night between the hours of 8:00pm and 6:00am. The effect of varying the quantity of water in the pan was studied by varying the depth (1.0, 1.5, 2.0, 2.5 and 3.0mm) while effect of surface color of radiating surface was determined by placing two different coloured pans adjacent to each other. The effectiveness of this method in capturing “cold load” and the possible use of this cold energy for daytime preservation of fresh produce is analyzed and discussed.It was found that the water in the pan could be cooled to a temperature that was 9 o C below the lowest ambient dry bulb temperature. Also, the black painted surfaces cooled the water at higher rates than white surfaces and ultimately attained much lower temperatures in the long run. Within the experimental water loading depth range of 1-3 mm, the shallower depths attained lower final cold water temperature but had lower cooling efficiency in terms of total peak cold load per unit area of pan surface. The authors have shown that each square metre of pan surface can store enough “cold load” to remove the heat of respiration of a 10 kg load of vegetables or fruits that is maintained at or below the lowest ambient night dry bulb temperature. The fresh produce so maintained can therefore be about 10-15 o C below recorded daytime dry bulb temperature in the tropics which is sufficient to double the shelf life of most fresh produce commodities. Introduction The preservation of fresh produce in many rural areas of developing countries is a challenging task and much of the harvested produce in these regions is going to waste due to lack of a simple, cheap, and yet convenient method of cold storage [1]. In most cases fresh produce is stored under ambient temperature conditions leading to high amounts of losses although it is known that bringing the storage temperature of the product down by a few degrees below the ambient temperature could tremendously prolong the storage life of most perishable products [2-3]. Since most of the losses occur because of lack of resources, there is a need for research on alternative ways of cooling and maintaining low temperatures by this class of producers and consumers. It must also be understood that the practical adaptation of any such alternative will depend heavily on its simplicity and cost. In many areas of the developing world the majority of people live in rural areas where modern methods of fresh produce preservation cannot be used due to lack of electric power. For these people alternative methods of preserving fresh produce include ventilative cooling, evaporative cooling and night sky radiation cooling as discussed by Rosenbaum [4] and Kamaruddin [5]. Among these methods, the night sky radiative method presents the highest possible potential in terms of the lowest achievable temperature. This method is also well adapted for use in arid and semi-arid areas where the skies are usually clear and air is relatively dry. These factors therefore enhance the applicability of this method. Advanced Materials Research Online: 2007-06-15 ISSN: 1662-8985, Vols. 18-19, pp 325-330 doi:10.4028/www.scientific. net/AMR.18-19.325 © 2007 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific. net. (Semanticscholar.org-19/03/20,17:19:02) Nocturnal theory and usage The temperature of the sky is always lower than the temperature of the earth surface so that there is a net radiant energy transfer from earth surface to the sky. This radiant loss occurs 24 hours a day and not just at night although it is offset by solar gain during the day. Since a clear sky is colder than a cloudy sky, night sky radiation cooling is more effective under clear skies [4-6]. The rate of cooling under clear sky situation has been determined to be as high as 75 W/m 2 in the US. Total energy loss has also been found to range from 61 Wh/ m 2 for very humid regions to 110 Wh/ m 2 in relatively dry regions of the USA [6]. These figures although modest may mean a substantial saving on energy costs even in developed countries such as USA. Night sky cooling basically involves the principle that the earths surface cools as a result of the radiative transfer of long wave energy to space through the atmosphere at night when there is no solar radiation to offset the outbound radiation loss to the sky. On clear nights, radiation to the sky may take place at rates sufficient enough to cool the ground surface to temperature 10-15 degrees centigrade below the air temperature. This natural cold load can be harnessed and used to cool stored produce, life farm animals or living space. The simplest night sky radiant cooling system is a horizontal flat roof structure that is insulated during the day and uncovered during the night [6]. The roof structure may need to be in direct contact with the space below for the purpose of efficient heat transfer. There are also systems which allow cooled water to be circulated through the coils of a reconditioning fan within the space to be cooled [7]. Alternatively the cooling load may be accumulated in a fluid that flows over or below the structure to be held elsewhere in storage. The present study is aimed at determining the lowest water temperatures and accumulated cold energy potential achievable in the central regions of Kenya under nightsky radiative cooling. The study was also aimed at determining whether this cooling potential could be used to maintain suitable fresh produce conditions as a stand alone system. Materials and Methods A total of four cooling pans each measuring 1x1m and 10 cm deep were constructed out of gauge 25 mild steel sheet metal. The inner surfaces of two of the pans were painted black while those of the other two remaining pans were painted white. In order to minimize heat transfer by conduction plywood boards and saw dust fillings were used as insulating materials as illustrated in Fig. 1. The pans were then placed on a horizontal surface and a leveling device used to ensure that the pan surface was actually perpendicular to the vertical axis of a gravity pendulum. The place of installation was also chosen with care such that no vegetation or tall building could obstruct the sky. Thus the pans received maximum sky radiation and the sky view factor was close to unity [8]. During testing, all pans were carefully filled with approximately the same quantity of water just before 8:00pm. Both the ambient dry bulb temperature and the temperature at the waterpan surface interface were recorded at 8:00pm using k-type thermocouples already fixed into position and connected to a data logger (Thermodac, model no. 5001A, Eto Denki Company, Japan). The temperatures were thereafter monitored using the data logger at regular intervals of two hour until 6:00am the following morning. The average ambient relative humidity for the night was also recorded while cloud cover conditions were subjectively observed and noted. At 6:00am, the mass of water in each pan was again measured and the amount lost through evaporation was computed. The measuring procedure described above was carried out with all four pans loaded with equal amounts of water. However, the procedure was repeated with varied amounts of water ranging from one litre to three litres by increasing the quantity of water at intervals of 0.5 litres. These qualities corresponded to pan water depths of 1.0, 1.5, 2.0, 2.5 and 3.0mm of water. Results and Discussion Temperature changes during the night: The variation of ambient air and pan water temperature with time during night sky radiation cooling is presented in Fig.2 for a water loading of 1000 cm 3 /m 2 of pan surface. A cloud cover which was observed between 1:00 and 4:00 am on the second 326 Advances in Materials and Systems Technologies