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.

Fig. 2 Scheme of distribution of the field of solar air heaters and the system of ducts. The ventilation system of the solar air collectors operates by suction with an axial fan. To prevent heat losses to the environment, the duct system was insulated with 1 inch (25.4 mm) thickness of fiberglass with a reinforced aluminium foil coating. In addition, the insulation layer was covered with corrugated aluminium sheets in order to withstand the weather conditions. Fig. 2 shows the distribution of the solar air heaters, as well as the sensors and air velocity measurement points. The ambient air enters the solar collectors by suction in forced convection produce with an axial fan (7.5 HP motor) and air passes through them increasing its temperature. The entrance of each row of collector has an air filter to prevent access to dust, insects and unwanted material. The hot air from the field of solar heaters is transported by the isolated ducts inside of the drying chamber.

2.2. Indirect air heating system

Solar air heaters are not the only solar thermal systems that can be used for drying applications. Solar collectors based on a liquid thermal fluid can also be used indirectly to heat air using a liquid-air heat exchanger. The hot water coming from the solar collector systems can be used directly in a heat exchanger or stored in a thermal tank for later use. The thermal storage tanks act as an accumulator and Page | 5 heat support system. The thermal storage allows having more uniform temperatures in the heat exchanger than the hot water coming directly from the solar collectors, as well as to have thermal energy during the night and the periods with adverse weather conditions (very cloudy and rainy days). A flat solar collector system for water heating was installed and evaluated at the drying plant. The indirect air heating system is integrated by (see Figure 3):

• Field of flat plate solar collectors. • Liquid-liquid plate heat exchanger • Thermal storage tank • Water-air finned tube heat exchanger • Centrifugal fan • Auxiliary equipment: instrumentation, pumps, expansion vessel, valves and air eliminators

Fig. 3 Indirect air heating system with the instrumentation of its components

The field of solar collectors for indirect air heating is composed of 40 flat plate solar collectors equivalent to an aperture area of 92.4 m2, distributed in four rows in parallel, each one consisted of two arrays of five collectors in parallel connected in series (10 collectors per row). These collectors are oriented towards the Equator with an inclination of 22.72 ± 0.94° and a separation between rows of 0.92 m. Fig. 4 shows the distribution of the collectors, as well as their instrumentation.

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Fig. 4 Field of flat plate solar collectors for water heating, as well as its instrumentation The field of solar collectors operates by a forced circulation system composed of a closed primary circuit and an open secondary circuit. The primary circuit was designed to operate with an ethylene glycol- water solution; while the secondary circuit with treated water (less than 50 ppm hardness). The ambient temperature in the plant drops below 0 °C during some days in winter, which is the reason to use water- ethylene glycol in the primary circuit. Each circuit operates with a 1.5 HP pump with a nominal flow rate up to 160 l/min. Primary circuit pipes are of copper, while those of the secondary circuit are polypropylene random copolymer. The primary circuit has a reverse-return configuration and an expansion vessel of 325.5 l that allows the collector field to remain in stagnation conditions without the need to drain liquid to reduce the pressure in the circuit.

The heat transfer between both circuits is done through a plate heat exchanger with a heat transfer capacity of 40kW. The heat absorbed by the secondary circuit can be used directly in the process of dehydration or storage for later use. Therefore, the secondary circuit can operate in two modes: direct or storage. In direct mode, the secondary circuit fluid absorbs the heat coming from the solar collectors through the plate heat exchanger liquid-liquid and then delivers it directly to the water-air heat exchanger. In storage mode the circuit fluid absorbs the heat and stores it in the hot water thermotank. In both operating modes, there is an automatic control device that activates the primary circuit pump and the secondary circuit pump. The tank is connected to a water pump of ¾ HP (559.275W) with nominal capacity of up to 60 l/min, which serves to supply hot water to a water-air heat exchanger. The pump is connected to a frequency inverter that allows regulating the flow of hot water that passes through the heat exchanger. A centrifugal fan (10 HP motor) sucks the air at the outlet of the water-air heat exchanger.

The hot water of the secondary circuit is stored in a horizontal atmospheric thermotank with a capacity of 6000 l (nominal). The water heating system has a differential temperature control that activates the primary and secondary circuit pumps when the difference between the temperature of the working fluid at the outlet of the collector field and the temperature of the water in the lower part of the thermo tank exceeds 8 °C. When the temperature difference falls below 4 °C the pumps switched off. In addition, the control is programmed to stop the pumps when the thermotank reaches a temperature of 90°C. Table Page | 7 1 shows the technical characteristics of the two solar collectors used in the pilot plant.

Table 1 Technical characteristics of solar collectors

Characteristics CaSolAire Flat Solar Collector Thermal fluid Air Water Dimensions Length 2100mm 2099 mm Width 1200mm 1196 mm High 93mm 95 mm Empty weight 55.35 kg 46.5 kg Gross area 2.53 m2 2.326 m2 Aperture area 2.32 m2 2.311 m2 Cover Material Tempered glass Tempered glass Transmittance 0.91 0.91 Thickness 4 mm 4 mm Absorber surface Material Steel Copper Absorptivity 0.90 0.95 Emissivity 0.80 0.05 Insulating Material Polyisocyanurate Polyurethane + mineral wool. Bottom insulation thickness 25 mm 44 mm Side insulation thickness 25 mm 25 mm

2.3. Monitoring and instrumentation

The operating variables of the solar drying system are monitored simultaneously using a data logger. The measurement instruments used, as well as their measurement range and accuracy are reported in Table 2. A Kipp & Zonen CMP3 pyranometer was installed on the solar collector plane in order to measure the solar irradiance perpendicular to the collector plane.

Table 2 Instrumentation used to measurement of variables of the solar drying system

Variable Instrument / sensor Range Accuracy Vane anemometer and Differential pressure manometer: CEM model ± 2 psi (±13.8kPa) ± 0.3 % FSO (25°C) DT-8897 Impeller flowmeters: Liquid volumetric Seametrics model SPT-100 2 l/min to 150 l/min ± 1 % FSO flow meter and SPX-100. Temperature RTD class PT-1000 -50 °C to 750 °C ± 0.5 °C Hot wire anemometer: TSI Air velocity 0 m/s to 30 m/s ± 3 % model 8345

Pyranometer: Kipp & Zonen 0 W/m2 to 2 000 Irradiance ± 2 % CMP3 W/m2 Cup anemometer: Davis Wind speed 0.5 m/s to 89 m/s ± 1 m/s model 6410 Pluviometer: Davis model 0 mm/m2 to 999.8 Rainfall ± 4 % 7852 mm/m2 Temperature and Ibérica model PCE-P18 -20 ° C to 60 °C and ± 2 % Page | 8 relative humidity 0 % to 100% ± 0.5 % and ± 2 % Water-Proof Thermometer Temperature Cole-Parmer model 90205- 5 ° C to 80 °C ± 0.1 °C (wireless) 22 Temperature Temperature data logger -35 ° C to 80 ° C ± 0.5 °C (wireless) model EL-USB-1 Full Scale Output (FSO) is the resulting output signal or displayed reading produced when the maximum measurement for a given device is applied.

3. Evaluation of the solar mode drying system

In this section, the evaluation of the nopal drying process using the solar operation mode is described. A test was carried out to determine the behaviour of the solar dehydration system on September 2018.

The test in solar operation mode consisted in using first the thermal energy of the direct air heating system and subsequently the indirect heating system in storage mode, for drying by batch of nopal. The indirect system was operated varying the mass flow rate of water that passes through the water-air heat exchanger. Table 3 shows the time intervals of each of the operating modes of the air heating system and the operation parameters. The total operating time of the solar thermal systems was 9.5 hours, with the direct mode with 63% of the total time.

Table 3 Summary of the mode of operation of the air heating system on September 26

Average Solar time Airflow Operation mode water flow (m3/h) On Off (kg/min) 09:20 15:19 Direct 4663.5 Off 15:24 15:29 Preheating ---- 15-30 15:30 16:38 Indirect 19.02 16:39 17:40 Indirect 25.81 17:41 18:23 Indirect 31.46 18:24 18:36 Indirect 6366.78 36.24 18:37 18:48 Indirect 41.01 18:49 18:52 Indirect 50.65 18:53 19:01 Indirect 55.26

3.1. Procedure used for processing experimental data

The useful energy gain supplied by solar collectors is determined by the following expression:

̇ 푄푢 = 푚̇ 퐶푝푚푓 (푇푓,표푢푡 − 푇푓,푖푛) (1)

where 퐶푝푚푓 is the mean specific heat capacity at constant pressure of the fluid, 푇푓,푖푛 the inlet fluid temperature and 푇푓,표푢푡 the outlet fluid temperature. The instantaneous efficiency of the field of solar collectors is defined as the ratio between the useful energy gain and the incident solar power Page | 9 푚̇ 퐶푝푚푓 (푇푓,표푢푡 − 푇푓,푖푛) 휂 = (2) 퐼 퐴퐶

2 where 퐴퐶 is the aperture area of solar collectors [m ] and 퐼 is the solar irradiance on the collector plane [W/m2].

3.2. Thermal evaluation of indirect air heating system

3.2.1. Solar water heating system

The evaluation of the system was made starting from the water in the tank with a very similar temperature to the ambient temperature. On September 25, 2018 the water heating system was switched on in automatic mode by means of a differential control explained in the section of indirect air heating system. The tank was filled with 6150 litres of water. During the first day of operation, the pumps of the water heating system started at 7:24 h intermittently until 7:53 h; from that time, they were kept on permanently until 15:26 h. The first day of operation, the tank reached a temperature of 71.9 °C. On the second day of operation, the pumps initially started at 8:25 h and operate intermittently according to the differential temperature control parameters and the incident solar irradiation. At 14:04 h the tank temperature reached 89.6 °C and the pumps automatically switched off; however, later the pumps switched on intermittently until they finally went off at 15:43 h. During the operation of the pumps, the pressure of the primary circuit reached up to 4 kg/cm2. Fig. 5 shows the temperature profile: at the inlet (TW-1), at the serial connection of the north row (TW-2) and at the outlet (TW-6) section of the field of flat plate solar collectors. The progression of the temperature in the hot water tank (TW-11) and the solar irradiance on the solar collectors plane (I) for the two days of operation of the system are also included.

The temperature of the tank increases 42.9 °C in the first day. During the waiting period for starting the pumps the next day (mainly at night), the tank has a thermal loss of 2.49 °C. The temperature increase on the second day of operation is 20.19 °C reaching 89.6 °C. Fig. 5 shows how the speed of the increase in water temperature in the tank is significantly lower on the second day, this is mainly because higher temperatures produce greater heat losses and, therefore, the efficiency of the solar collector decreases and the increase in temperature in the tank is slower.

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Fig. 5 Temperatures profile in the field of flat plate solar collectors, temperature of water in the tank and solar irradiance on the collector plane for September 25-26 Table 4 shows the average water mass flow rates in the primary circuit (FW-2), secondary circuit (FW-3) and the flow that passes through the north row of the collector field (FW-1). The flow FW-1 is equivalent to 22.1% of the global flow FW-2. Ideally, if the pressure drops in the collector field were balanced, there would be an equal flow between each of the rows of the collector field (25%). However, in practice it is difficult to reach. The measuring instrument and its accessories add resistance to the flow, therefore, it is expected that the flow in the row instrumented with the flow meter is lower than other rows.

Table 4 Summary of the average mass flow rates of the water heating system

Location FW-1 FW-2 FW-3 Average flow (kg/min) 24.84 112.41 73.88 Standard deviation 1.8132 0.6180 0.4007

Figure 6 shows the instantaneous efficiencies of the north and global row of the field of solar energy collectors for water heating respectively; as well as the energy accumulated in the thermal storage tank during the two days of operation. Instantaneous efficiency was determined by eq. (2) using the individual mass flow rates recorded. The area used for the efficiency of the north row was 23.11 m2, while for the global was 92.44 m2. The average heat capacity of water was 4185 J/kg K. The instantaneous efficiencies of the north row are relatively lower than the global one; this is because the water mass flow rate is lower through the north row compared to the other rows of the collector field. The efficiencies gradually decrease during the water heating because the heat losses increase with the increase of the water temperature in the storage tank. During the first day of operation, the water heating system stores 1043.39 MJ of thermal energy, 65.7 MJ is lost during the waiting period for starting the pumps the next day. The second day stores 525.76 MJ to give a total of 1503.5 MJ during the two days of operation. Therefore, the water heating system during the second day contributed only 35% of the total energy stored in the storage tank.

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Fig. 6 Instantaneous efficiencies and accumulated energy in thermal storage tank (September 25-26)

3.2.2. Extraction of the energy stored in the storage tank

During the indirect heating of air, the mass flow rate of the hot water passing through the heat exchanger of finned tubes was regulated in such a way that the temperature of the air at the outlet of the heat exchanger was maintained around 55 °C. Using a low water mass flow rate at the beginning of the operation prevents turbulent mixing inside the hot water tank; and it is expected a greater mixing inside the tank at higher mass flow rate. Fig. 7 shows the water temperature profiles at the inlet (TW-12) and outlet (TW-13), and the air temperature at the inlet (TA-34) and outlet (TA-35) section of the heat exchanger and the water temperature in the thermal storage tank (TW-11); as well as the mass flow rates of water in the heat exchanger. During the operation of the indirect heating of the air, the temperature of the water in the tank drops from 89.2 °C to 61 °C, contributing with a total of 726.78 MJ of thermal energy, equivalent to 48% of the energy stored during the two days of operation of the water heating system.

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Fig. 7 Temperature profiles in the heat exchanger and the storage tank and mass flow rates of water in the heat exchanger In a heat exchange of two fluids, the fluid of higher temperature (subscript h) gives part of its heat to the fluid with lower temperature (subscript c) and another part is lost to the environment. The thermal efficiency of a heat exchanger is determined by the following expression:

푄̇ 푚̇ 푐 퐶푝푚푓푐 (푇푓푐,표푢푡 − 푇푓푐,𝑖푛) 휂 = 푐 = (3) 푒푥푐ℎ푎푛푔푒푟 푄̇ ℎ 푚̇ ℎ 퐶푝푚푓ℎ (푇푓ℎ,표푢푡 − 푇푓ℎ,𝑖푛)

Figure 8 shows the thermal efficiencies and water and air temperature difference in the heat exchanger during the operation of the indirect air heating system. The air flow was considered constant and equal to 6366.78 m3/h. The average heat capacity for the water was 4185.3 J/kg K and for the humid air was 1010 J/kg K. The average efficiency of the exchanger was 0.8823. The total energy provided by the hot water in the exchanger was 648.81 MJ; while the energy received by the air was 571.39 MJ. Figure 8 shows the temperature difference of the water side (ΔT water) in the heat exchanger is higher than that of the air (ΔT air) during the start of indirect heating but later this behaviour is reversed at the end of the operation. This is because at the beginning of the operation there is a small mass flow rate of water that is increased stepwise in order to maintain the inlet air temperature at the drying chamber entrance. This is reflected in the temperature difference of the air side of the heat exchanger that remains above 25° C.

1.00 50 0.95 45

0.90

C) ° 0.85 40 0.80 Page | 13 35 0.75 0.70 30

Efficiency 0.65 25 0.60

0.55 20 Temperature difference ( difference Temperature 0.50 15 0.45 0.40 10 15:30 16:30 17:30 18:30 Solar time (h) η ΔT water ΔT Inter Aire

Fig. 8 Thermal efficiency and water and air temperature difference in the heat exchanger

3.3.Thermal evaluation of direct air heating system

The direct air heating system (DAHS) was operating from 9:20 to 15:19 h on September 26. Figure 9 presents the temperatures in the field of solar air heaters at the outlet section of the south (TA-10), and central (TA-16) rows, in the main duct (TA-19), ambient temperature (T amb) and solar irradiance on the collector plane (I). South row recorded average temperature 6.9 °C higher than central rows, this is explained by the air flow that passes through the collectors, which is greater in the rows near to the main duct. The average irradiance for the operating time was 924.55 W/m2 and the average ambient temperature was 24.08 °C. After 13:00 h it became cloudy and therefore the temperatures on the field decreased considerably.

Table 5 shows the minimum, maximum and average temperatures along the field of solar air heaters. For the north and south rows maximum temperatures were recorded above 80 °C in periods of time close to solar noon.

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Fig. 9 Solar irradiance on the collectors plane (I), outlet temperatures on south row (TA-10), central row (TA-16) and main duct (TA-19) and ambient temperature (T amb) on September 26 Table 5 Temperatures in the north, south and central rows in the field of solar air heaters

North Row (°C)

Sensor TA-2 TA-3 TA-4 TA-5 Maximum 47.1 55.9 81.3 81.1 Minimum 28.6 32.1 40.5 42.1 Average 39.8 47.7 68.4 69.0 South Row (°C) Sensor TA-7 TA-8 TA-9 TA-10 Maximum 49.0 60.2 77.5 81.7 Minimum 28.2 32.7 38.2 41.5 Average 40.8 51.5 64.1 69.6 Central Row (°C) Sensor TA-13 TA-14 TA-15 TA-16 Maximum 42.4 53.7 64.8 70.4 Minimum 26.3 29.3 33.5 35.8 Average 34.5 42.7 54.9 60.2

The maximum, minimum and average temperatures that were recorded in different points of the field of solar air heaters (see Figure 2) are presented in Table 6. TA-17 records the air temperature where 4 rows of collectors are joined; TA- 18 records the air temperature of 8 rows of collectors while TA-19 of the total field. The average temperatures for each of the points is always between 60 °C to 70 °C.

Table 6 Maximum, minimum and average temperatures in duct

Duct (°C) Sensor TA-17 TA-18 TA-19 Maximum 78.7 72.3 74.0 Page | 15 Minimum 40.3 37.3 38.3 Average 67.1 61.9 63.2

One of the parameters commonly used to evaluate the thermal performance of the solar collector system is the instantaneous efficiency. Using eq. (2) the instantaneous efficiencies of the global system of the field of solar heaters were determined (see Figure 10); for which the temperature registered at the inlet section of the main duct was considered as the outlet temperature (TA-19, see Figure 2), since at that point the air flows of the north and south section of solar heaters come together obtained the total flow of the solar collector system. The area used for the overall efficiency of the air heaters field was the equivalent to the effective area of the 48 air heaters, it was 111.14 m2 and a volumetric flow of 4663.5 m3/h. Figure 10 shows the instantaneous efficiencies determined in the central and south rows, the area used in both rows was 6.94 m2. The average volumetric flow of the central row is 347.07 m3/h; while the one in the south row is 232.56 m3/h. The efficiencies of the central row are superior to the south row, this difference is due to the different flows that pass through them. The increase of the air flow through the air heaters increases the efficiency of the system; however, the operating temperature at the outlet of the system decreases. The global instantaneous efficiencies are found in intermediate values between the efficiencies of the south and central rows, since the air temperature at the outlet of the solar collector field (TA-19) is intermediate to the air temperature at the outlet of the south and central row.

0.90 1200

0.80 1000

0.70

) 2 800 0.60

0.50 600

0.40

400

Irradiance (W/m Irradiance Thermal efficiency Thermal 0.30 200 0.20

0.10 0 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 Solar time (hour) Global South row Central row I

Fig. 10 Instantaneous efficiency of the field of air heaters on September 26

4. Drying chamber test with a thermal load

The drying chamber was evaluated with a load of 282 kg of Nopal (Opuntia ficus). For processing, they were washed and the thorns were removed by passing a knife through its surface. The cladodes were cut into pieces of a maximum length of 15 cm and 3 cm in width that allowed their cutting in a food processor using a 5 mm thick cutting disc. The product was placed in 300 perforated stainless steel trays Page | 16 distributed in 5 drying racks; the load density was 4.62 kg/m2. The initial moisture content of the product obtained with a moisture analyser was 85.28 % and the final was 9.13 %, both in wet basis.

푑푋 Drying rate ( ) is defined as the mass of water removed per unit of time per unit of dry matter 푑푡 and is relevant engineering and economically, since it provides information of the production capacity of a dehydration system [10]. For the presentation of the drying rate, drying curves are constructed in which the drying rate is plotted against the moisture content of the product on a dry basis (푋). The change in the moisture content of a product is presented in the curve through the drying time.

The moisture content and the drying speed are calculated by the following equations:

푚−푚 푋 = 푠 (4) 푚푠 푑푋 푋 −푋 = 푡+Δ푡 푡 (5) 푑푡 Δ푡

where 푋 is the dry base moisture content, 푚 is the mass of the sample, 푚푠 mass of the dry matter 푑푋 of the sample, is the drying rate and 푋 dry base moisture at time t. 푑푡 푡

For the construction of drying curves experimentally, product samples were placed at the inlet and outlet section of the drying chamber and periodically recorded the weight of them. If water is the only volatile substance of the product, then the loss of weight over the time corresponds to the loss of water mass, the mass of dry matter 푚푠 is assumed constant.

Figure 11 shows the location of the nopal samples whose weight change was recorded during the test. Samples I, II and III were located on the drying rack closest to the hot air inlet, while IV, V and VI on the rack closest to the air outlet in the drying chamber.

The change in moisture content (dry basis) over time for the six samples is presented in Figure 12. It is observed that samples I, II and III have a considerably faster drying rate than the samples located at the outlet section of the drying chamber. Samples I, II and III reach equilibrium after 6 hours of the start of the test, while samples IV, V and VI after 9.7 hours, this time considers the periods between thermo- solar systems change.

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Fig. 11 Position of samples of nopal and temperatures measured in the drying chamber

8

7

6

5

O O kg) / X

2 4

3 ( kg ( H

2

1

0 0 2 4 6 8 10 Drying time I II III (h) IV V VI

Fig. 12 Change of moisture dry basis samples through the time of dehydration.

The temperatures in the drying chamber during the operation of the direct heating system (DAHS) and indirect air (IAHS) are shown in Figure 13. For the direct air heating system there were periods with air temperatures at the inlet section above 60 °C, while for the indirect air heating system the temperature at the inlet section of the tunnel remained more stable and above 50 °C. For the entire test, the average temperature at the inlet section of the chamber was of 58.5 °C (Table 7).

The energy supplied to the drying chamber was also calculated; the accumulated energy is also presented in Figure 13. For the direct air heating system, the total energy contributed to the tunnel was 946.0 MJ, while for the indirect one it was 545.5 MJ, the total contribution of the thermosolar systems was 1491.5 MJ.

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Fig. 13 Temperatures in the drying chamber, inlet (TA-21), middle part (TA-24), outlet (TA-26) and accumulated energy for Direct Air Heater System (DAHS) and Indirect Air Heater System (IAHS) Table 7 Temperatures and relative humidity inside the drying chamber

Drying chamber Hi Ho Sensor TA-21 TA-24 TA-26 ° C ° C ° C % % Maximum 70.6 63.2 49.4 56.6 71.1 Minimum 41.9 24.4 20.6 6.0 15.5 Average 58.5 53.8 43.3 10.7 28.1

Taking into account the energy contributed by the thermo-solar systems to the drying chamber (1491.5 MJ), as well as the efficiency of the LPG direct fire burner (90%) and the calorific value of the LPG which is 46.16 MJ/kg of LPG [10]. The energy contributed by thermo-solar systems are equivalent to 35.9 kg of LPG that multiplied by the cost of LPG in Zacatecas by December 2018 (≈18.21 pesos/kg of LPG) gives an estimated saving of 653.7 pesos (approximately 32.7 USD) for each drying cycle of this type. 35.9 kg of LPG according to the emission factor of Mexico of 3 kg of CO2/kg of LPG [10] also gives the amounts of emissions avoided to the atmosphere, which in this case are 107.7 kg of CO2.

5. Conclusions

The results of the thermal analysis have shown the technical feasibility of the use of solar thermal technologies for direct and indirect air heating for drying products at an industrial plant. Some highlights of the work are:  Direct and indirect air heating systems are able to deliver the temperature required in the food drying process, that is, temperatures between 50 °C and 70°C.  The indirect heating system provides more stable air temperatures at the entrance of the drying chamber. However, the direct system provides higher temperatures.  The water-based solar collector field obtained higher thermal efficiencies than the air heaters field. However, the overall thermal efficiency with respect to the energy delivered to the drying

chamber decreases in cascade for the indirect system (water solar collectors) that has several stages to be able to heat the air used in the drying process.  Different kind of food products can be dried on the thermo-solar plant designed, resulting in substantial fuel savings and environmental benefits.  The development of robust solar drying technologies that adapt to the needs of the agro- industrial sector and other industrial sectors is relevant to simultaneously address the problems Page | 19 of sustainable energy supply and the loss and waste of food.  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.

Acknowledgements This work was supported by FORDECYT Project No. 190603 and SECAMPO Zacatecas.

ORCID ID of authors

O. García-Valladares: https://orcid.org/0000-0001-9478-4157 I. Pilatowsky-Figueroa: https://orcid.org/0000-0002-6492-2456 N. Ortiz-Rodríguez: https://orcid.org/0000-0002-2202-6776 C. Menchaca-Valdez: https://orcid.org/0000-0002-9853-2990

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