Solar Thermal Dehydrating Plant for Agricultural Products Installed in Zacatecas, México
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WEENTECH Proceedings in Energy 5 (2019) 01-19 Page | 1 4th International Conference on Energy, Environment and Economics, ICEEE2019, 20-22 August 2019, Edinburgh Conference Centre, Heriot-Watt University, Edinburgh, EH14 4AS, United Kingdom Solar thermal dehydrating plant for agricultural products installed in Zacatecas, México O. García-Valladares*, I. Pilatowsky-Figueroa, N. Ortiz-Rodríguez, C. Menchaca-Valdez Instituto de Energías Renovables, Universidad Nacional Autónoma de México, privada Xochicalco s/n, centro, CP 62580, Temixco, Morelos, México. *Corresponding author’s mail: [email protected] Abstract This paper presents a hybrid thermo-solar plant for the dehydration of foods, built in Zacatecas, Mexico. The plant is integrated by a semi- continuous drying chamber with a capacity of up to 2 700 kg of fresh product. The thermal energy required for the drying process is provided by two solar powered thermal systems: an air heating system with 48 collectors (111.1 m2) and a water heating system with 40 solar water heaters (92.4 m2), a thermal insulated storage tank, and a fin and tube heat exchanger. The plant also has a fossil energy backup system (LPG) to heat air. The monitoring system measures and records 55 process variables (temperature, relative humidity, water pressure, solar irradiance, air velocity, volumetric and mass flow rates). Experimental results obtained with the solar air heaters and water collectors are reported. The average efficiency of the solar air heaters field was ≈45% with a maximum increase of the air temperature of 40.8 °C and for the water solar collectors field the average efficiency was ≈50% and the water in the storage tank reaches 89.6 °C in two days of operation. A batch test was carried out on September 26 with the thermos-solar systems for the dehydration of 282 kg of Nopal (Opuntia ficus-indica) with an initial moisture content of 85.28 % (wet basis), reaching a final moisture content of 9.13 % (wet basis) in 9.7 hours. Both thermo-solar systems are able to deliver the temperature required in the food drying process (temperatures above 50 °C). According to the results obtained, different kind of food products can be dried on the thermo-solar plant designed, resulting in substantial fuel savings and environmental benefits. The development and implementation of solar drying systems technology, initially at demonstration scale, in the Mexican industrial sector could allow a technical and economic maturation of the technology, and the benefits could be presented in the short term. Keywords: Solar Energy; Solar drying; Solar air heaters; Flat plate solar collectors; Experimental test Copyright © 2019 Published by WEENTECH Publishers. This is an open access article under the CC BY License (http://creativecommons.org/licenses/BY/4.0/). All Peer-review process under responsibility of the scientific committee of the 4th International Conference on Energy, Environment and Economics, ICEEE2019 https://doi.org/10.32438/WPE.1119 Manuscript History Receipt of completed manuscript: 05 January 2019 Receipt of Revised Manuscript: 11 April 2019 Date of Acceptance: 30 May 2019 Online available from: 03 September 2019 1. Introduction The loss (that take place at the production, storage, processing and distribution stages) and waste of food (food of good quality for consumption but does not get consumed because it is discarded) have a negative impact on the environment due to the use of water, land, energy and other natural resources to produce food that nobody will consume. According to the Food and Agriculture Organization (FAO) Page | 2 studies, it is estimated that around 30% of cereals are lost and wasted each year; 40-50% of tubers, fruits and vegetables; 20% of oilseeds, meat and dairy products; and 35% of fish. Food losses and waste depend on the specific conditions and local situation of each country or culture [1]. In developing countries food losses are from 10 to 40%, due to various reasons such as lack of adequate technology, inadequate cultivation and fertilization, lack of marketing channels, crop losses and lack of storage facilities [2-4]. In Mexico, the national average of loss and waste was 37.11% (2013), among the most lost and wasted agricultural foods are guava (57.73%), mango (54.54%), avocado (53.97%), banana (53.76%) and nopal (53.26%) [5]. The Confederación Nacional de Agrupaciones de Comerciantes de Abasto (CONNACA), in Mexico, calculates the greatest food losses in the post-harvest stage due to the lack of adequate packaging or transportation [6]. One of the main areas of action to reduce food losses and waste is the improvement of conservation technologies. However, solutions to reduce losses usually involve greater energy use, especially in the conservation of food products. Of course, from an environmental point of view, the negative impacts of measures to reduce food losses and waste should be less than the benefits. Therefore, the technological proposals to reduce the loss and waste of food should be focused on integral and sustainable solutions, such as solar drying that uses the energy from the sun to remove moisture from the products by heat and mass transfer mechanisms. In many countries, the use of solar thermal systems in agriculture to conserve vegetables, fruits, coffee and other crops have proven to be practical, economical and with an environmentally responsible approach [7]. Most of the numerous designs of solar dryers, which are available, are mainly used for the drying of various crops, either for domestic use or for small-scale industrial production [8]. There are few works related to the development and research of demonstrative solar drying systems with a focus on high capacity agro-industrial applications and with long life materials. Among the various types of solar dryers, the indirect type forced convection dryers have been reported superior in drying speed and drying quality [3-4]; they are also the most suitable for drying large quantities [8]. The indirect solar drying is a fairly new technique, not yet standardized or widely commercialized, involving some thermal energy collection devices and special techniques dryers. The technique of indirect solar drying has almost only advantages: the drying times are low, greater control in the final humidity of the product, without losses by the inclement of the time, a capacity of compact load and greater productivity (kg/h). Its only disadvantage is the high cost of initial capital for the drying chamber, the field of the solar collectors and all the necessary auxiliary equipment, such as ducts, pipes, fans, instruments of control and measurement, and more or less qualified personnel to operate the drying process [8]. In view of this scenario, a demonstration pilot hybrid solar drying (solar – liquefied petroleum gas (LPG)) plant in the state of Zacatecas, Mexico, which operates by forced convection in a type tunnel drying chamber, was developed and installed. In this work, the thermal analyses of the two solar thermal technologies that are coupled to the drying chamber are presented. 2. General description The plant is made of an industrial structure of 400 m2, inside which is a semi-continuous horizontal drying tunnel with a capacity of up to 2700 kg of fresh product (depending on the product to be dried and the final presentation), a food processing area, a control laboratory and an office. In the upper part of the drying tunnel, there is a backup energy system, the air-water heat exchanger, a Page | 3 centrifugal fan (coupled to the heat exchanger and backup energy system) and an axial fan (coupled to the system of solar air heaters). In the outdoor area: there is a system for washing and disinfecting the product, a liquefied petroleum gas (LPG) tank (as part of the energy backup system) and two solar thermal systems: a direct and an indirect air heating system. The indirect system has a tank for the storage of hot water. Also, there is a meteorological station for the measurement of climatological variables. Fig. 1 shows schematically the distribution of the components that integrates the drying system of the plant, as well as some of the sensors installed for its evaluation. Fig. 1 Schematic representation of the drying plant and some of the sensors installed in the plant The integration of the different technologies offers a system of generation of versatile thermal power, capable of adapting quickly and easily to different modes of operation. Following are some of the operation schemes of the thermal power generation system: conventional, solar and hybrid. The conventional mode consists of the operation of the backup energy system with liquefied petroleum gas (LPG), where the combustion gases are mixed with a mass of fresh air before entering the drying chamber. The solar mode consists in taking advantage of the solar energy captured by the direct and indirect heating systems of air, either independently or in combination. The hybrid mode consists of the operation of the solar and conventional mode combined, in order to perform a continuous drying process. 2.1. Direct air heating system The technology of solar heating of air has advantages in comparison with the solar heaters of liquids, since they do not present problems of freezing or evaporation, leaks, damages and risk for the environment or health due to the use of dangerous working fluids. However, solar air heaters are limited 3 3 due to their small volumetric heat capacity compared to water (air=1.21 kJ/m K, water=4186 kJ/m K) Page | 4 [9]. The direct air heating system is integrated by 48 air heaters with a total aperture area of 111.1 m2 distributed in an arrangement of three collectors in series by 16 in parallel (see Fig. 2). The collectors are oriented towards the Equator with an inclination of 23.49 ± 0.84° with respect to the horizontal and with a space between rows of 0.71 m to avoid shading between them and to allow the transit during maintenance.