Simulation, design and experimental performance evaluation of an innovative hybrid solar-gas dryer

Journal Pre-proof Simulation, design and experimental performance evaluation of an innovative hybrid solar-gas dryer

Ahmed Zoukit, Hicham El Ferouali, Issam Salhi, Said Doubabi, Naji Abdenouri PII:

S0360-5442(19)31974-7

DOI:

https://doi.org/10.1016/j.energy.2019.116279

Reference:

EGY 116279

To appear in:

Energy

Received Date:

16 April 2019

Accepted Date:

02 October 2019

Please cite this article as: Ahmed Zoukit, Hicham El Ferouali, Issam Salhi, Said Doubabi, Naji Abdenouri, Simulation, design and experimental performance evaluation of an innovative hybrid solar-gas dryer, Energy (2019), https://doi.org/10.1016/j.energy.2019.116279

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Journal Pre-proof Simulation, design and experimental performance evaluation of an innovative hybrid solar-gas dryer Ahmed ZOUKIT, Hicham EL FEROUALI, Issam SALHI, Said DOUBABI, Naji ABDENOURI* Laboratory of Electric Systems and Telecommunications, Marrakech, Cadi Ayyad University BP 549, Av Abdelkarim Elkhattabi, Gueliz, Marrakech, Morocco. *[email protected]

Abstract: In this paper, a new configuration of hybrid solar-gas dryer is proposed and designed. Instead of direct heating, as usually used in gas dryers, an original indirect air heater was considered in the proposed configuration where the flue gases exhaust outside the drying chamber avoiding their diffusion in the product. This paper presents a numerical simulation of a hybrid solar-gas dryer operated under forced convection with an air mass flow rate of 0.025 kg/s. Simulations of the chamber temperature and humidity threshold and heat distribution were performed. The thermal efficiency of the dryer operated in solar mode (SM), gas mode (GM) and hybrid mode (HM) was calculated. CFD simulations revealed adequate results for efficient drying of many local products. Experimental and simulation results reveal that the temperature and relative humidity are in the suitable ranges for drying wide kind of agriproducts. Indeed, the average drying temperature and relative humidity ranged between 25-80°C and 31.3-6.2%, respectively. Simulation and experimental values are very close and the RMSE and RMSE% remained under (2.1°C, 2.7%) for the temperature and (2.5%, 2.4%) for the relative humidity. The maximum dryer efficiencies were found near 42%, 37% and 40% for SM, GM and HM, respectively.

*

Corresponding author:Pr. Naji ABDENOURI [email protected],

1

Journal Pre-proof Keywords: Solar energy, drying, hybrid solar-gas collector, CFD simulation, forced convection, dryer efficiency. Nomenclature 𝑢 E 𝛾𝑒𝑓𝑓 𝐻𝑛 𝐽𝑛 t 𝜌 P Pw Pws W g 𝛽 𝑇0 𝑇 𝜏𝑒𝑓𝑓 𝜇𝑒𝑓𝑓 𝑢t 𝐼 Pg Pc 𝜂𝑠𝑜𝑙𝑎𝑟 𝑚𝑜𝑑𝑒 𝜂𝑔𝑎𝑠 𝑚𝑜𝑑𝑒 𝜂ℎ𝑦𝑏𝑟𝑖𝑑 𝑚𝑜𝑑𝑒 𝑚 𝐶𝑓 𝑇𝑜𝑢𝑡 𝑇𝑖𝑛 G A Tch Hch RMSE RMSE%

Mean velocity component vector, m/s Specific energy of fluid, J/kg Effective thermal conductivity, W/m.K Enthalpy of species n, J/kg Diffusion flux of species n, kg/m2.s Time, min, hour Density, kg/m3 Pressure, N/m2 Water vapor pressure Saturation pressure of water vapor. Humidity ratio (kg/kg). Gravitational acceleration vector, m/s2 Thermal expansion coefficient System surrounding temperature in the work, K Mean temperature, K Effective stress tensor Effective viscosity, kg / m s Transposed mean velocity, m/s Unit tensor Gas power, W Solar-gas collector input power, W Dryer efficiency in solar mode Dryer efficiency in gas mode Dryer efficiency in Hybrid mode Air mass flow, kg/s Specific heat of flowing air, J/kg K Outlet temperature, K Inlet temperature, K Solar radiation, W/m2 solar-gas collector area, m2 Drying chamber temperature, °C Chamber relative humidity, % Root mean square error Root mean square percentage

1. Introduction Food drying is a method of food preservation which inhibits the growth of bacteria, yeasts, and mold through the removal of water. Dehydration has been widely used for this purpose

2

Journal Pre-proof since ancient times. Water is traditionally removed through evaporation (air drying, sun drying, smoking or wind drying). Food drying requires high energy consumption due to the considerable latent heat of evaporation of water and the relatively low energy efficiency of the industrial dryers [1]. The required energy for drying can take up from 7% to 15% of the corresponding total energy use [2]. Hence, using solar energy in drying applications is a potential alternative since it decreases consumption of conventional energy by about 27% to 80% [3]. Farmers in developing countries cannot afford to import expensive equipment that is either electrically or diesel engine driven [4]. Also, the access to conventional energy remains very limited in these countries. Hence, they use direct sun drying to preserve their agricultural products. This process is affordable in term of cost due to its inexpensive nature; using solar radiation. Even-though this traditional method is cheap, it suffers from many drawbacks such as the long drying process, labor cost and deterioration of the product quality due to several factors like dust, moisture, insects and microorganisms’ attacks [5-7]. Thus, farmers in developing countries face problems in drying their products fast and in suitable environmental conditions. Therefore, cost-effective and drying kinetic are the major criteria characterizing the dryer device and are the main parameters affecting the quality of the dried products. An indirect solar dryer (Figure. 1) consists of two main parts: a solar collector (Figure. 2) and a drying chamber (Figure. 3). After passing through the glass cover, the incoming solar irradiation is received by an absorber plate [8]. Then, heat is transferred from the absorber plate to the flowing air above it by convection. The heated air passes through the product which is placed in the drying chamber. By using an indirect solar dryer, the product is not directly exposed to solar irradiation to minimize discoloration and cracks on the surface of the product [9].

3

Journal Pre-proof

Figure 1. Indirect solar dryer.

Figure 2. Solar collector.

Figure 3. Drying chamber. Unfortunately, solar dryers are limited and intermittent. Their performances depend strongly on climatic conditions (solar irradiation, ambient temperature, ambient relative humidity and 4

Journal Pre-proof natural airflow) [10-12]. Hence, they cannot be fully effective without the use of a secondary heating source. This can extend the drying time, can allow the control of drying parameters and can help to overcome the intermittency of solar energy. Various types of hybrid solar dryers using electrical heaters, biomass and gas burners have been designed and developed for drying various agricultural products [13-18]. Boughali S. et al. [13] constructed a specific prototype of an indirect hybrid solar-electrical dryer for agriculture products. The proposed hybrid dryer was investigated in load conditions for a range of mass flow rate between 0.04 and 0.08 kg/m2. Biomass as a backup heating system was reported by Dhanushkodi et al. [17] in order to extend the period of drying beyond the sunshine hours. The auxiliary heating system maintains the temperature inside the drying chamber in a range of 40°C, 50°C and 60°C. Gas energy is widely used for drying either at industrial scale or by small farmers [16]. López-Vidaña EC et al. [18] investigated the thermal efficiencies of a hybrid solar-gas dryer in three operating modes: solar mode, gas mode and hybrid mode. The drying efficiencies were calculated in transitory state and were found around 86%, 71% and 24% for gas mode, hybrid mode and solar mode, respectively. A tunnel dryer using gas as auxiliary heating system was designed and investigated by Oueslaty H et al. [19]. In this dryer, the gas system was used to control the temperature at 65 °C and to maintain it without sunlight and under unfavorable conditions. Unfortunately, the hybrid solar-gas dryers that are used either by individual farmers or in industrial scale, burn the gas in the same room with the product. Figure 4 shows the hybrid solar-gas dryer used by López-Vidaña EC et al. [18]; the burner is placed inside the drying chamber. During the whole drying process, the surface of the dried product is subjected directly to the flue gases and to the emitted soot due to incomplete combustion. These flue gases are absorbed by the dried products which affect negatively their quality. In addition, the

5

Journal Pre-proof proposed dryers are complicated. They require appropriate qualifications and knowledge to control the drying process and for any maintenance.

Figure 4. Diagram of hybrid solar-gas dryer proposed by López-Vidaña et al. [18]. The energy cost, drying time and the quality of the end-dried products are reasonable factors and key questions in the overall evaluation of agricultural dryers. Furthermore, small farmers need economic and straightforward agricultural equipment requiring little investment and less maintenance. Hence, as an alternative to the conventional hybrid solar-gas dryers; we propose in this paper the use of a new configuration as shown in Figure. 5 where the gas burners are set up along the back side of the solar collector without access to the drying chamber. Thus, the absorber picks up heat from both sides: on the top side from solar irradiation and on the back side from gas burners. During daytime, the solar-gas collector receives only the solar irradiation for direct heating of the drying air. The gas heaters are utilized to maintain the drying process during off-shine hours or when the solar irradiation is not sufficiently high enough.

6

Journal Pre-proof

Figure 5. Synoptic of the designed solar-gas collector. (a) solar-gas collector; (b) Zoom of the collector frontal section. In this configuration, the air flows between the absorber and a bottom plate that acts as a heat exchanger between the gas burners and the heated air. There is no direct contact between the drying air and the flames of the gas burners. The burners can be controlled by an electric valve allowing the temperature control inside the drying chamber. The proposed hybrid solargas dryer is uncomplicated and movable. Figure 6 shows the whole structure of the proposed indirect hybrid solar-gas dryer.

Figure 6. Schematic diagram of the proposed hybrid solar-gas dryer.

7

Journal Pre-proof Numerical simulations are a valuable way for analyzing the thermal performances of drying systems and allow to ability to conduct detailed investigations of airflow and temperature distribution inside all the dryer components [20-28]. The simulations carried out in this paper were related to a no-load dryer to eliminate the impact of the moisture delivered by products for better understanding of only the dryer operation [3]. In these simulations, the temperature and the relative humidity threshold inside the drying chamber were established and the thermal efficiency of the dryer, in the three main modes (solar mode, gas mode and hybrid mode) was calculated. A prototype of the innovative dryer was designed and installed in our laboratory. The thermal performance of the experimental prototype was studied and discussed considering the results of the temperature and the relative humidity obtained in the simulation part. 2. Methodology In this attempt, in order to evaluate the performances of the proposed innovative hybrid solargas dryer the following methodology has been adopted. 

Theoretical analysis of the innovative hybrid solar-gas dryer has been presented.



By using SolidWorks software, the hybrid dryer model has been sized accordingly to

a desired temperature range varying from the ambient to 190°C. 

CFD simulations of the temperature distribution, the relative humidity and the flue

gases temperature inside the dryer have been investigated in transitory and steady state in the three operating modes: solar mode, gas mode and hybrid mode. 

The thermal efficiency of the dryer has been calculated in the steady state.



A prototype of the hybrid dryer is constructed and the experimental measurements

have been handled at different weather conditions (for various solar radiation, ambient temperature and relative humidity)

8

Journal Pre-proof 

Finally, the humidity ratio and thermal efficiencies of the dryer have been

experimentally reevaluated and compared with the simulation ones. The obtained results have been discussed and the benefits of the proposed innovative dryer prototype have been presented. 3. The hybrid dryer CFD simulation 3.1. Description of the proposed hybrid solar-gas dryer The designed dryer consists of a drying chamber which is the seat of the drying operation combined with a solar-gas collector used to generate regularly the hot air. The adopted solar air heater is a type of simple-pass finned plate. The top face is subjected to the solar irradiation as the main source of energy. The second heating system is a set of nine tubular burners located under the heat exchanger and monitored by a proportional solenoid valve. The chimney is equipped by a ventilator to ensure the evacuation of the moist drying air, improve the hot air circulation inside the chamber and promote the uniformity of the temperature in the whole volume of the chamber. The solar energy is employed for direct heating of air passing through the collector during daytime while gas is burnt to maintain the temperature inside the drying chamber during off-shine hours. The heat exchanger is used for indirect heating instead of direct gas burning inside the drying chamber as it’s usually used in conventional dryers heated by gas energy. The solar-gas collector is equipped by three apertures in the both sides allowing the entrance of the necessary air required for complete combustion and an aperture in the outlet of the collector for the exhaust of flue gases. The hybrid solar-gas dryer was simulated using SolidWorks Flow Simulation developed by Dassault systems SolidWorks crops. The software solves Navier-Stokes equations coupled to energy balance equation with the Finite Volume Method (FVM) in a computational domain consisting of rectangular prism (parallelepiped). Each element of volume is the subject of the

9

Journal Pre-proof coupled momentum and heat transfer equations resolution. In our case, the simulations were carried out to investigate the temperature, relative humidity and the airflow distribution in the solar collector ducts and inside the drying chamber. Computational heat, mass and momentum transfer were accomplished in 3-dimentional in all volumes in transient and incompressible turbulent flow modes. The stability and acceptable accuracy were obtained by using 𝑘−𝜀 turbulence model. The wall of the fins was considered thin enough to assume that the internal conduction is negligible. The transient term was considered implicit. The conservation lows for mass, momentum and energy equations in the cartesian coordinate system could be written by [29]: ∂𝜌 ∂𝑡

Continuity equation Momentum equation Energy equation

∂(𝜌𝐸) ∂𝑡

∂𝜌𝑢 ∂𝑡

+ ∇(𝜌𝑢) = 0

(1)

+ ∂𝑡(𝜌𝑢𝑢) = ― ∇𝑃 + ∇𝜏𝑒𝑓𝑓 + 𝜌𝑔𝛽(𝑇0 ― 𝑇)

(2)

∂

∂𝑇

+ ∇(𝑢(𝜌𝐸 + 𝑃)) = ∇(𝛾𝑒𝑓𝑓 ∂𝑡 ― ∑𝑛𝐻𝑛𝐽𝑛 + (𝜏𝑒𝑓𝑓.𝑢))

Where: -

𝑢 is the mean velocity component vector (m/s).

-

E is the specific energy of fluid (j/kg).

-

𝛾𝑒𝑓𝑓 is the effective thermal conductivity (W/m.K).

-

𝐻𝑛 is the enthalpy of species n (J/kg).

-

𝐽𝑛 is the diffusion flux of species n (kg/m2 s).

-

t is the time (s), 𝜌 is the density (kg/m3).

-

P is the pressure (N/m2).

-

g is the gravitational acceleration vector (m/s2).

-

𝛽 is the thermal expansion coefficient.

-

𝑇0 is the system surrounding temperature in the work (reference temperature, K). 10

(3)

Journal Pre-proof -

𝑇 is the mean temperature.

-

𝜏𝑒𝑓𝑓 is the effective stress tensor as Eq (4):

[(

𝜏𝑒𝑓𝑓 = 𝜇𝑒𝑓𝑓

∂𝑢 ∂𝑡

+

)] ―

∂𝑢𝑡 ∂𝑡

2∂𝑢𝐼 3 ∂𝑡

(4)

Where 𝜇𝑒𝑓𝑓 is the effective viscosity (kg/ (m.s)), 𝑢𝑡 is the transposed mean velocity (m/s), 𝐼 is the unit tensor. 3.2. Simulation model Figure. 7 presents the 3-D schema of the hybrid solar-gas dryer built on SolidWorks. Geometric parameters and the material properties of the hybrid solar-gas dryer are given in Table 1. This model used single component model with assumptions of incompressible and unsteady-state flow [21]. Simulations were conducted using a grid density of 1 906 000 elements distributed between the collector and the chamber. Given the complexity of the geometry, the meshing was set automatically in the drying chamber and in the collector. For example, the number of nodes inside the heat exchanger is 247x247. The high grid density requires computational resources beyond the availability in the laboratory (Intel core-i7, 3.4GHz, 32GB Ram, 1TB Hard disk).

11

Journal Pre-proof

Figure 7. 3-D model of the hybrid solar-gas dryer. (a) View on SolidWorks; (b) transparent view. Table 1. Details of components, materials, sizes and material properties of the proposed dryer.

Component Absorber Glass cover Heatexchanger Gasburner Sides insolation Bottom insolation Fins (24 fins) Air aperture Walls (Drying chamber)

Aluminium Glass Iron Iron

2m*1m 2m*1m 2m*1m 1m

Material property Thermal Density Specific heat conductivity (kg/m3) (J/(kg.K)) (W/(m.K)) 237 2700 897 1.05 2500 670 80.2 7870 447 80.2 7870 447

Mineral fiber

Thickness 50 mm

0.035

20

1030

Mineral fiber

Thickness 50 mm

0.035

20

1030

Aluminium

Thickness 1 mm Diameter 5 cm

237

2700

897

Mineral fiber

Thickness 50 mm

0.035

20

1030

Material

Size

3.3. Initial and boundary conditions The initial temperatures of the chamber walls, surface of tubular burners and inside air were assumed equal to the ambient temperature as 25°C. Boundary conditions including inlet airflow, temperature, humidity, and outlet pressure are given in Table 2. The inlet airflow was calculated from the flow rate of the ventilating fan while the inlet temperature and relative humidity were assumed to be constant at 25°C and 50% respectively. The flames of the burners are modeled as volume heat sources located under a metallic sheet. The flames’ diameter is 1cm and their number 12

Journal Pre-proof is 216 (24 flames for each burner accordingly to the number of fins). Hence, the power of each

flame is Pg/216, where Pg is the total gas power (W). The solar-gas collector is fitted with apertures in both sides and in the outlet allowing the natural circulation of the necessary air for combustion. Table 2. Boundary conditions performed in simulation. Parameter Inlet airflow (kg/s) Inlet air temperature (°C) Outlet pressure (Pa) Inlet air humidity (%)

Value 0.025 25°C 101325Pa 50

4. Simulation investigation of the dryer thermal performances Three distinct cases, based on the energetic contribution to the drying system, were investigated namely: solar mode, gas mode and hybrid mode. The solar mode is applied during daytime when solar irradiation is high enough. This mode is commonly applied for a solar dryer. The gas mode is related to complete lack of sunshine, when the drying has to be done during the night or in case where the product is likely to be damaged if we expect the favorable weather conditions. The hybrid mode is the combination of solar energy and gas energy that can be applied during cloudy days to ensure continuous drying under unfavorable weather conditions. In the presented simulations, the dryer was operated in forced convection at an air mass flow of 0.025 kg/s. This value is proved by using mathematical modeling, CFD simulations and experimental tests as reported in previous works [10,30]. 4.1. Simulation of airflow distribution inside the hybrid solar-gas dryer The airflow path lines inside the proposed hybrid solar-gas dryer at the three operating modes are predicted by CFD simulation modeling, as shown in Figure 8. There is no significant difference in the average temperature inside the drying chamber because of proper mixing of air by the mounted fan. This justifies the consideration of the average temperature inside the drying chamber for modeling and simulation. The path lines of airflow are much longer and continuous. In addition, this uniformity of drying air helps in homogeneous heat transfer in 13

Journal Pre-proof drying of products at different levels and positions in the drying chamber. Figures 8b and 8c show that the collector is much hotter in the places where the burners are located when compared with the collector in solar mode (Figure 8.a). This promotes the heat transfer to the drying air flowing through the solar-gas collector.

(a)

(b)

Burners locations

(c)

Burners locations

Figure 8. Temperature distribution and path lines of airflow inside the proposed dryer for different operating modes. (a) solar mode (900 W/m2); (b) gas mode (5 kW); (c) hybrid mode (400 W/m2, 4 kW). Figure 9 shows the path lines of drying air and flue gases. It can be inferred that the flue gases exhausted outside the drying chamber are not mixed with the drying air which is necessary to enhance the quality of the dried products. 14

Journal Pre-proof (a)

(b)

Drying air

Flue gases

Flue gases

Drying air

Combustion air input

Figure 9. Path lines of drying air and flue gases inside the hybrid solar-gas dryer. (a) Zoom of the solar-gas collector outlet; (b) Side view of the proposed dryer. 4.2. CFD simulation of temperature and relative humidity threshold inside the drying chamber In this section, the temperature and relative humidity profiles inside the drying chamber were investigated for each mode. 4.2.1. Solar mode Energy contribution during the solar mode operation was exclusively provided via the solar collector. Two distinct scenarios were investigated: the threshold of the temperature and the relative humidity inside the drying chamber was simulated at a constant solar irradiation and during whole day of dryer operation where the incident irradiation intensity follows a Gaussian form. The evolution of the solar irradiation during the whole day was measured by using a Keep and Zonen pyranometer. For the first test, constant solar radiation was considered by conducting experimental measurements from 11am to 1pm. During this period of time, the solar irradiation, ambient temperature and ambient relative humidity can be assumed constant. For the second test, the measurements of the solar irradiation were carried out during a whole clear day of operation from 9am to 6pm.

15

Journal Pre-proof At a constant solar irradiation of 960 W/m2, the average temperature and relative humidity reached inside the drying chamber were 56°C and 6.2% respectively, as shown in Figure 10. We notice that the maximum temperature was reached in less than 2 hours. Figure 11 reveals the variation of the drying temperature and relative humidity inside the drying chamber throughout the day. The used solar irradiation profile was measured in Marrakesh on March 11th, 2017. The highest temperature (56°C) and the lowest relative humidity (6.2%) were recorded between 11am and 1pm according to an irradiance of 980 W/m2. The temperature and relative humidity inside the drying chamber remained in the range 50°C-60°C and 10%6.2%, respectively, during 5h30min. This period of time is suitable for drying wide variety of products without incurring thermal damages.

(b)

(a)

Figure 10. Variation of temperature and relative humidity inside the drying chamber at a constant solar radiation (960 W/m2). (a) Drying temperature; (b) Relative humidity.

(a)

(b)

Figure 11. Variation of the temperature and the relative humidity inside the drying chamber during whole day of operation. (a) Drying temperature; (b) Relative humidity. 16

Journal Pre-proof 4.2.2. Gas mode In this mode, the only source of heat is the gas burners. The solar energy was not considered and many scenarios of gas power were specified. The values of gas power were chosen in a way to obtain a temperature range of 25-100°C inside the drying chamber. This range is more suitable to dry wide varieties of food commodities and craft products. In order to investigate the temperature inside the drying chamber at the steady state, several simulations were conducted. Hence for a gas power range of 0-8kW, the temperature increases from the ambient temperature to 102°C (Figure 12) which is considered as a suitable temperature range for many drying processes.

Figure 12. Variation of chamber temperature on the basis of gas power in the steady state. Simulations were also carried out in transitory state. Figure 13 depicts the transient drying chamber temperature and relative humidity for three gas powers (1kw, 2kW and 4kW). It was observed that the average drying temperature and relative humidity inside the drying chamber are 40.6°C, 53.7°C and 70.2°C and 38.1%, 25.3% and 14.2% for gas powers of 1kW, 2kW and 4kW, respectively. These ranges of temperature and relative humidity are highly favorable for drying wide variety of products as mentioned in [29] and justifies the choice of gas powers 1kW, 2kW and 4kW for simulation and experimental investigations. In addition, for the three cases, the maximum temperatures are reached in less than one hour which allows fast drying.

17

Journal Pre-proof (a)

(b)

Figure 13. Profile of temperature and relative humidity inside the drying chamber for different gas powers. (a) Drying temperature; (b) Relative humidity. 4.2.3. Hybrid mode If the received solar irradiation is insufficient, the auxiliary gas heating system ensures heating the drying air. In this section, several circumstances were simulated and investigated according to fluctuations of the solar irradiation complemented with gas power. Figure 14 presents the drying temperature profile and the relative humidity inside the drying chamber. The solar irradiation has an average of 250W/m2 which was typical the average solar irradiation in cloudy days in Marrakesh. The temperature and relative humidity of the drying chamber reached 61.3°C, 78.2°C and 24.2%, 18.5% respectively, for gas powers of 2kW and 4kW, respectively. Hence, despite the unfavorable weather conditions, the gas system indemnifies the continuity of the drying process. This fact, leads to obtain a high quality of end-dried products.

18

Journal Pre-proof (a)

(b)

Figure 14. Evolution of temperature and relative humidity inside the drying chamber in cloudy day with different gas powers. (a) Drying temperature; (b) Relative humidity. 4.3. Thermal efficiency of the dryer model Since the dryer is unloaded. The collector outlet temperature can be considered as the temperature inside the drying chamber [3]. Hence, the evaluation of the thermal efficiency of the dryer model refers to calculate the solar-gas collector efficiency. The thermal efficiency is defined as the ratio of the energy gain to the solar irradiation incident and gas power applied to the solar-gas collector [30]. 𝜂𝑠𝑜𝑙𝑎𝑟 𝑚𝑜𝑑𝑒 =

𝜂𝑔𝑎𝑠 𝑚𝑜𝑑𝑒 =

𝑚𝐶𝑓(𝑇𝑜𝑢𝑡 ― 𝑇𝑖𝑛) 𝐴∗𝐺

𝑚𝐶𝑓(𝑇𝑜𝑢𝑡 ― 𝑇𝑖𝑛)

𝜂ℎ𝑦𝑏𝑟𝑖𝑑 𝑚𝑜𝑑𝑒 =

𝑃𝑔

𝑚𝐶𝑓(𝑇𝑜𝑢𝑡 ― 𝑇𝑖𝑛) 𝐴 ∗ 𝐺 + 𝑃𝑔

(5)

(6)

(7)

Where 𝜂𝑠𝑜𝑙𝑎𝑟 𝑚𝑜𝑑𝑒, 𝜂𝑔𝑎𝑠 𝑚𝑜𝑑𝑒 and 𝜂ℎ𝑦𝑏𝑟𝑖𝑑 𝑚𝑜𝑑𝑒 represent the dryer efficiency in solar operation mode, gas mode and hybrid operation mode, respectively. The dryer thermal efficiency is calculated when all the used parameters are in steady state such as: the temperatures (Tout and Tin), the airflow (𝑚), the gas power (Pg) and solar irradiation (G). To do so, all the aforementioned parameters (Tin, 𝑚, 𝑃𝑔, G) are fixed, and the steady state of the output temperature (Tout) is reached after approximately 2 hours when the

19

Journal Pre-proof dryer is operated in solar mode and less than 1 hour when the dryer is operated in gas or hybrid mode. The thermal efficiency of the hybrid dryer at the three operating modes is summarized in Table 3. The calculated efficiencies in gas and hybrid modes are lower than the obtained one in solar mode. Indeed, the gas burners are located in separate places under the absorber so the gas power is not applied to the whole surface of the absorber. Table 3. Hybrid solar dryer efficiencies (Ta=25°C) Operation mode Solar mode Gas mode Hybrid mode

Chamber temperature 56.4°C 71.2°C 78.2°C

Solar irradiation 980W/m2 0 260 W/m2

Gas power 0kW 4kW 4kW

Dryer efficiency 40% 35% 37%

5. Temperature and relative humidity measurement inside the dryer 5.1. Experimental set up description The obtained simulation results were helpful to construct a hybrid solar gas dryer operating in 30-80°C temperature range. This hybrid dryer prototype consists mainly of: (i) a cubic drying chamber (1m *1m*1m) that includes four mesh trays with a surface area of 0.98m2; (ii) an electrical fan mounted in the chimney of the drying chamber; (iii) and a solar-gas collector (2m2) which is combined with the drying chamber. The solar-gas collector consists of a glass cover with a thickness of 3mm and a finned absorber (24 fins) made from aluminum (L= 2m and W=0.95m). The top side of the absorber is painted with matt black glycerophtalic lacquer that has an absorptivity of 0.95 and emissivity of 0.8. The insulation, in the bottom and edge of the solar-gas collector, is made from mineral fiber with a thickness of 5cm. A set of three burners type (Natural gas tubular burner, 30mbar) with 2 kW power on each burner, are placed between the absorber and the bottom insulation. The burners are placed equidistantly under the absorber (Figure 15).

20

Journal Pre-proof

Figure 15. Picture of the experimental prototype of the hybrid solar-gas dryer. The temperature and relative humidity inside the drying chamber were measured by (TM-110 pt100, 0.5°C accuracy) and (HM-110, 0.5% accuracy) probes, respectively. Ambient temperature and ambient relative humidity were recorded using a local weather station (Vantage Pro2). Incident global solar irradiation was measured locally using a Keep and Zonnen pyranometer. This latter was fitted on an adjustable inclination support to get the pyranometer at the same tilt angle than the collector one. The three burners are equipped by solenoid valves to control and deliver desired power. The positions of the temperature, relative humidity and air flow rate sensors is presented in Figure 16.

21

Journal Pre-proof

Figure 16. Positions of temperature, relative humidity and air flow rate sensors. A flow-meter type Kimo (90 lt/min, 16 bar) was installed in the main gas conduct to measure the whole gas flow fed to the burners. All experimental measurements were recorded by using a data acquisition platform that includes an automate type cRIO-9030, LabVIEW software and a touch panel host computer (Figure 17). For the signals’ compatibility considerations, all the used sensors and actuators deliver a continuous current of (4-20 mA).

Figure 17. Platform of data acquisition system. 22

Journal Pre-proof 5.2. Thermal flow provided by the gas combustion In most of industrial process, forced convection is widely used in heat transferred in flames with air as oxidizer (approximately more than 90%). The heat transfer between impinging premixed butane/air and impingement plate was widely studied for many geometrical forms in previous works [31-38]. These studies were performed as well by numerical simulation [31-33] and by experimental tests [32, 34, 35]. The main results were given by diagrams [32,33,36,] or by empirical formulas [35,37,38]. Hence, the heat flux between flame and plate depends on the distance between burner and plate, the source diameter (m) and the nozzle exit velocity (or the flow of the premixed air fuel). All these parameters were provided in some expressions by using dimensionless numbers (Reynolds Prandtl, Lewis and Schmidt numbers) [33-35,38]. In some works, and through a thermal camera, heat flux was deduced only from the temperature variation near the plate as function of all the previous parameters [31,32]. In these last works temperature distributions have to be determined before the heat flux deduced. In this paper, the heat flux from the flame to the impingement plate in two-dimensional and axisymmetric geometry case was expressed by using characteristics and the combustible flow, the diameter of the injector, the distance between the burner and the plate (exchanger). All these parameters were introduced in the dimensionless numbers mentioned in the follow equation (8).

𝑞 = 1.32(𝑃𝑟𝑓) ―0.6(𝑅𝑒∞) ―0.5𝐺∆𝐻[1 + (𝐿𝑒 ― 1)

[

∆𝐻𝑐ℎ𝑒𝑚 0.6 ∆𝐻

Where: 𝑃𝑟𝑓: Prandtl number 𝑅𝑒∞: Reynolds number 𝐺: mass of gas flowing per second per unit cross-sectional area of gas stream 𝐿𝑒: Lewis number

23

]]

(8)

Journal Pre-proof 𝐻𝑐ℎ𝑒𝑚: enthalpy of the fluid 𝐻: change in dissociation enthalpy 𝜇𝑐𝑝

The Prandtl number can alternatively be expressed as 𝑃𝑟 = 𝑘 Where: 𝜇: absolute or dynamic viscosity (kg/m s, lbm/(ft h)) 𝑐𝑝: Specific heat (J/kg K, Btu/(lbm oF)) 𝑘: Thermal conductivity (W/m K, Btu/(h ft2 oF/ft)) 𝜌𝑉𝐿

The Reynolds number is defined as 𝑅𝑒 = ( 𝜇 )

Where: ρ is the density of the fluid (SI units: kg/m3) u is the velocity of the fluid with respect to the object (m/s) L is a characteristic linear dimension (m) μ is the dynamic viscosity of the fluid (Pa·s or N·s/m2 or kg/m·s) ν is the kinematic viscosity of the fluid (m2/s). The Lewis number (𝐿𝑒) is a dimensionless number defined as the ratio of thermal diffusivity to mass diffusivity. It is used to characterize fluid flows where there is simultaneous heat and mass transfer. It is defined as: 𝛼

𝑆𝑐

𝐿𝑒 = 𝐷 = 𝑃𝑟 Where α : is the thermal diffusivity 𝐷: the mass diffusivity 𝑆𝑐: Schmidt number 24

(9)

Journal Pre-proof The solar collector contains 3 burners. Each burner supports a maximum flow of 13l/min through 15 flames. The diameter of each injector is 1,3 mm and distant of 4cm to the plate. The combustible used in this study is butane gas. By using equation (8) for each flame, the gas flow was adjusted to reach the desired thermal power as it’s shown in the Table 4: Table 4. Values of gas powers. Flow of the butane gas

Number of flames

Thermal power exchanged with the plate

5.6l/min

15

(1±0.015) kW

12.2l/min

15

(2±0.017) kW

23.6l/min

30

(4±0.019) kW

5.3. Investigation of the dryer performances at different operating modes Experimental measurements were conducted at different operating modes (solar mode, gas mode and hybrid mode) and at special weather conditions. The dryer operated under forced convection at airflow of 0.025 kg/s. The measured drying chamber temperature and relative humidity are compared with simulated ones by the CFD model. The root mean square error (RMSE) and the root mean square percentage (RMSE%) were used to quantify the average deviation of the simulated and measured values. The weather conditions and the experimental results were presented with the uncertainties (+/-0.5°C and 0.5%). 5.3.1. Solar mode In solar mode, experimental tests were carried out during a whole clear day of operation on March 30th, 2019 and over a constant solar irradiation period. The solar collector was oriented towards the south with a tilt angel of 30° in order to get the maximum solar radiation. The temperature and relative humidity inside the drying chamber were recorded every one minute while the ambient climatic conditions were recorded at every 25

Journal Pre-proof 5 minutes. The weather conditions during the experiments are depicted in Figure 18. The temperature grows from 24°C to 26°C and the humidity drops from 54% to 42% from the morning to the middle of the day. The two parameters meet their same morning levels in the end of the day.

Figure 18. Solar irradiation, ambient temperature and relative humidity during whole day of tests. Figure 19 shows the measured and the simulated temperature and relative humidity in the drying chamber. The highest temperature of air reached in the chamber was 60.1°C. From 10 am to 3pm, the drying chamber temperature remains between 55°C and 60°C. The temperature starts to decrease after its maximum between 11am and 1pm until it reached the ambient one at 5:30pm. At the opposite of the temperature evolution, the relative humidity decreases from the morning until 10am then it remained almost constant at 6.7% for 5 hours. At 1:30pm the relative increase till it reached 33%. An observation of the obtained experimental results shows that the drying chamber temperature and relative humidity ranged from 50°C to 60°C and 5.8% to 6.7% for 5h 40 min. These conditions are highly suitable for drying a wide variety of agriculture-products without incurring any thermal damages and without involving any other auxiliary energy source. In this case, the model describes well 26

Journal Pre-proof the behavior the drying air inside the solar dryer chamber with an RMSE and RMSE% of the temperature and the relative humidity (2.1°C, 1.97%) and (2.3%, 1.95%), respectively.

Figure 19. Experimental measurement of chamber temperature and relative humidity in solar mode (clear day). A second test was conducted in solar mode within the constant solar radiation period. The solar-gas collector was covered and then immediately exposed to solar irradiation which can be considered as a step signal. This test was performed on March 29th, 2019 between 11am and 1pm. The solar irradiation was 970W/m2, relative humidity was around 53% and the ambient temperature was about 25°C. The experimental values of the temperature and relative humidity were plotted in Figure 20. In the weather conditions bellow, the highest reached temperature in the drying was about 59.6°C which led to a minimal value of the relative humidity which equal 6.3%. The response time was about 30 minutes and the permanent regime starts after about 80min from the step irradiation. The RMSE and RMSE% for temperature and relative humidity deviation were found to be (2.2°C, 1.97%) and (2.1%, 1.86%), respectively. Within these error margins, the model can be widely adopted to predict the evolution of the dryer in solar mode.

27

Journal Pre-proof

Figure 20. Evolution of the heated air temperature and humidity following a solar irradiation step. 5.3.2. Gas mode In gas mode, three cases of gas power were considered. The experimental measurements were conducted with gas powers of 1kW (case 1), 2kW (case 2) and 4kW (case 3). The powers were estimated by measuring the gas flow delivered to the burners. All tests were carried out on three consecutive days: March 31th, 2019 (case 1), April 1st, 2019 (case 2) and April 2nd, 2019 (case 3), where the temperature and relative humidity remained almost at 25°C and 50% respectively in midday. The solar-gas collector was completely covered and shaded during the tests in order to ensure that the energy contribution has to be provided only by the gas power. The whole experiments last for approximately 65 min, when the transient period does not exceed 20 minutes. Experimental values of the temperature and relative humidity picked up in each case are presented in figure 21 with the CFD simulated ones. As it has been predicted, the air temperatures at steady state in the chamber reached 40, 53 and 70°C related to 1kW, 2kW and 4kW gas power respectively. While the relative humidity varies from 34% to 19% when the gas power varies from 1kW to 4kW. The simulated trend of temperature and relative humidity evolutions fit well the experimental 28

Journal Pre-proof behavior of these parameters in the dryer chamber. The differences are lower than (1.9°C, 2.3%) and (2.1%, 2.4%) for the temperature and relative humidity, respectively.

29

Journal Pre-proof

Figure 21. Temperature and relative humidity of air in drying chamber in gas mode. An observation of the figures proved that the highest temperatures level can be reached inside the drying chamber are 42.3°C, 54.2°C and 71,8°C, for gas powers 1kW, 2kW and 4kW, respectively. Related to these three gas powers, the chamber relative humidity reached 31.2%, 20.3% and 17.4%. In gas mode, the steady state was reached in approximately 25min, while it needs about 80 minutes in solar mode case. Thus, by using gas, the response time of the dryer was reduced and the level of the temperature was significantly elevated. The obtained range of the chamber

30

Journal Pre-proof temperatures allows to extend the use of the dryer to more products varieties especially those requiring temperatures close to 70°C. 5.3.3. Hybrid mode Experimental tests on the designed dryer were performed in hybrid mode in two cloudy days (April 2nd,3rd, 2019) with the same weather conditions. During these two days, the solar irradiation was about 256W/m2 and 268W/m2, the ambient temperature was around 25°C and the relative ambient humidity was closed to 47% at midday. Given the low solar irradiation, the hybrid mode is highly recommended. Thus, 2kW and 4kW gas powers were used respectively in the first and second day.

Figure 22. Simulation and experimental results during the hybrid tests.

31

Journal Pre-proof Figure 22 depicts the recorded values of the chamber temperature and relative humidity and the simulated ones. In case 1 (2kW gas power combined with a 2m2x256W/m2 solar irradiation) the reached values of temperature and the relative humidity were 59.4°C and 20.1%. While in case 2, (4kW gas power combined with a 2m2x268W/m2) the reached temperature and relative humidity inside the drying chamber were 77.4°C and 16.4%, respectively. By comparing the temperature in gas mode with the temperature obtained in hybrid mode, solar irradiation of about 250 W/m2 contributes to arise temperature by 5°C for 4kW and 7°C for 2kW. The deviation expressed by the model related to the experimental tests are quantified by the RMSE and RMSE% which remained under (1.8°C, 2.7%) and (2.5%, 2.1%) for the temperature and relative humidity, respectively. To operate with hybrid solar gas dryer, the whole results show that a gas energy consumption can strongly decreased by solar energy contribution and by an adequate controller. 5.4. The thermal efficiency of the dryer prototype After performing the experimental tests on the constructed prototype of the hybrid dryer, the thermal efficiencies are recalculated at each operating mode as follow: Solar mode: the experimental tests were conducted between 11am and 1pm to ensure that the parameters (G) and (Tin) are approximately constant while the airflow is maintained constant by a mounted fan in the chimney. During this period the steady state of (Tout) is reached. In gas mode, the experiments are conducted at night during 1 hour. The input temperature to the solar collector which is the ambient one (Tin) is constant while the gas power is maintained constant by feeding the burners with a constant gas flow via a proportional solenoid valve. The steady state of the collector output temperature is reached after 1 hour.

32

Journal Pre-proof Hybrid mode: the experiments were conducted between 11am and 12:15pm during approximately 70 min. Within this period the steady state of (Tout) is reached and the other parameters are considered constant. The calculated dryer thermal efficiencies were found to be 42%, 37%, and 40% for the dryer operated in solar mode, gas mode and hybrid mode, respectively. 5.5. Humidity ratio inside the drying chamber In order to investigate the water/water vapor remaining inside the drying chamber, the humidity ratio (kg/kg) has been calculated for each mode operation. The used calculating expressions are as follow:

𝑤=

621.97 × 𝑃𝑤 (𝑃 ― 𝑃𝑤)

(10)

× 10 ―3

Where: 𝑃𝑤: is the water vapor pressure. 𝑃: is the ambient pressure. Water vapor pressure 𝑃𝑤 is obtained using the following expression:

𝑃𝑤 =

𝑅𝐻 × 𝑃𝑤𝑠

(11)

100

Where Pws is the saturation pressor of water vapor. The obtained results are summarized in Table 5. Table 5. Humidity ratio inside the drying chamber at each operating mode Operating mode Solar mode Gas mode Hybrid mode

Pg = 1kW Pg = 2kW Pg = 4kW Case 1: 2kW + 256.4 W/m2 Case 2: 4kW + 268 W/m2

Humidity ratio (kg/kg) 0.007 0.016 0.019 0.038 0.024 0.046

In order to figure out the effect of water load on the performances of the hybrid dryer, this latter was loaded with 10kg of tomato as shown in Figure 23.

33

Journal Pre-proof

(b)

(a)

Figure 23. Tomato products loaded in the dryer's chamber. (a) Tomato product in the drying chamber; (b) tomato product spread in trays. Three case studies were considered (solar mode, gas mode and hybrid mode). The experiments were conducted within a sunny day (solar mode) in July 11th, 2019 during 8 hours. The maximum reached solar irradiation was 986.7W/m2. The temperature inside the drying chamber was raised from the ambient temperature (25°C) to a maximum value of 52.4°C at 1pm. In gas mode, the experiments were carried out at night. Only a power of 2kW was considered in order to obtain temperature range of 50-55°C which is usually recommended for tomato drying [29]. The maximum reached temperature was 48.2°C after 1 hour working. hybrid mode was investigated on a cloudy day (July, 15th, 2019). The experimental test was carried out with a solar irradiation of 421.6W/m2 and a gas power of 1kW. After 73 minutes, the temperature has reached its highest value (50.1°C). According to the obtained results, the thermal efficiencies of the dryer are calculated using the equations (5,6,7) mentioned in section 4.3 and the results are summarized in Table 6. Table 6. Thermal efficiency of the hybrid dryer in loaded conditions Operation mode Solar mode Gas mode Hybrid mode

Thermal efficiency Dryer prototype 37.4% 32.8% 34.4%

34

Journal Pre-proof At load condition (10kg of tomato with 94.73% humidity content), the thermal efficiency decrease by 4.6% compared to the unloaded case. 6. Results and discussions From the analysis of the experimental results presented in figures (19-22), it was inferred that the simulated and designed hybrid dryer can be used for drying wide variety of agricultural products even in sunny, at night and in cloudy days, under favorable and unfavorable conditions. The chamber temperature slightly over-estimate the measured one by 2°C, while the simulated relative humidity under-estimate the measured one by 2.3%. Otherwise, the developed CFD model gives an acceptable predicted thermal behavior of heated air by hybrid solar-gas collector. The RMSE and RMSE% remained under (2.1°C, 2.7%) and (2.5%, 2.4%) for the temperature and relative humidity validation respectively, in all operating modes of the hybrid dryer. The values of RMSE and RMSE% for all dryer operating modes (solar mode, gas mode and hybrid mode) are summarized in the Table 7. Table 7. RMSE and RMSE% values for chamber temperature and relative humidity Operating mode Solar mode Gas mode Hybrid mode

RMSE Tch (2.1°C) Hch (2.3%) Tch (1.9°C), Hch (2.1%) Tch (1.8°C) Hch (2.5%)

RMSE% Tch (1.97%) Hch (1.95%) Tch (2.3%) Hch (2.4%) Tch (2.7%) Hch (2.1%)

In solar mode, at a maximum solar radiation of 970 W/m2, the drying temperature increased from 25.3°C to 60.1°C and the relative humidity decreased from 53% to 6.3%. The steady state is reached in approximately 2 hours. In gas mode, the tests were conducted in the absence of solar radiation. In unload conditions, the chamber temperature increased from 25°C to 42.3°C, 54.2°C and 71.8°C for the gas powers of 1kW, 2kW and 4kW, respectively. The steady state was reached in less than 60 min. In hybrid mode, especially in cloudy days 35

Journal Pre-proof where there is an insufficient of solar energy, the two energy sources (solar and gas) were combined to enhance the chamber temperature to a suitable range for drying operation. A case study was investigated during two consecutive cloudy days at a solar radiation of 256W/m2 and 268W/m2. The obtained solar energy was combined with gas powers of 2kW and 4kW respectively. It was noticed that the chamber temperature increases to a maximal value of 77.4°C and the relative humidity decreases to a minimal value of 16.4% within 65 min. According to the obtained results, it can be concluded that for the products that require a temperature range of [50°C-60°C] and drying time of 5 hours, the solar mode is highly recommended, in this case, the utilized energy is free and clean. While for products that require a temperature range beyond 60°C and a drying time more than 5 hours, gas mode and hybrid mode are considered. In this case, the cost of the used energy depends on the amount of the injected gas power. Hence, the designed hybrid solar-gas dryer can be used for drying wide agricultural products that require a temperature range of [40°C-70°C]. The thermal efficiencies of the proposed dryer have been numerically and experimentally calculated in steady state. A comparison between the calculated efficiencies is presented in Table 8. The obtained results by simulations and experimental measurements are quite similar which confirm that the proposed model can be used to describe and predict the thermal properties of heated air. Table 8. Comparisons between the hybrid dryer prototype and CFD model efficiencies. Operation mode Solar mode Gas mode Hybrid mode

Dryer prototype 42% 37% 40%

Thermal efficiency Dryer CFD model 40% 35% 38%

In addition, the humidity ratio inside the drying chamber was calculated and investigated in order to figure out if there is a water/water vapor remaining inside the drying chamber. The investigations were conducted at the three main operating modes of the dryer. It was found 36

Journal Pre-proof that the humidity ratio remained under 0.046 kg/kg. The thermal efficiency of the dryer was also investigated in load conditions and it was revealed that for a load of 10kg of tomato, the thermal efficiency decreased by 4.6%. Unlike the conventional hybrid solar-gas dryers, the use of burners in the underside of the solar collector seems a viable solution instead of using burners inside the drying chamber. 7. Conclusions In this paper, an innovative hybrid solar-gas dryer has been designed, investigated and constructed. Its performances have been analyzed numerically and experimentally and the following conclusions have been drawn: 

The developed hybrid solar-gas dryer consists of a solar-gas collector and a drying

chamber. In this new configuration there is an indirect air heating instead of direct heating as used in conventional solar-gas dryers [18]. 

The flus gases exhaust from a separate outlet of the collector avoiding their mixture

with the dried product. Hence, the dried products are not subjected to the flue gases which impact their quality. 

The airflow path lines and the temperature distribution inside the drying chamber were

investigated related to many weather conditions with different thermal powers. 

The distribution of the chamber temperature is quite homogenous with a slight

difference of 1.4°C in all the chamber planes (bottom, middle and top). This uniformity leads to ensure an evenness heat transfer in drying products at different level in the drying chamber. 

Simulation results revealed that the temperature inside the drying chamber increases

from 25°C to 77.4°C and the relative humidity decreases from 50% to 6.3% for the dryer operated in the three main modes (solar mode, gas mode and hybrid mode). 

A prototype of the proposed hybrid dryer has been constructed and experimental

validations have been conducted at different weather conditions and operating modes.

37

Journal Pre-proof 

The RMSE and RMSE% have been calculated and were found to be (2.1°C, 2.7%) and

(2.5%, 2.4%) for the temperature and relative humidity, respectively. 

The thermal efficiencies of the proposed dryer have been calculated in steady state and

were found to be 42%, 37% and 40% for the dryer operated in solar mode, gas mode and hybrid mode respectively. 

The humidity ratio inside the dryer chamber has been investigated and it was found

that it remained under 0.046kg/kg for all the dryer operating modes. 

The performances of the dryer at small-water load have been evaluated in term of

thermal efficiency and it was shown that the dryer thermal efficiency decreased by 4.6% of a load of 10kg. 

The developed hybrid dryer can be easily applied to small farms in developing

countries for drying wide variety of local food commodities. 

The dryer responses to three key questions: cost efficiency, optimization of gas

energy by introducing the solar energy as the main source, fast drying time and ensuring good quality of end-dried product. 

The flues gases exhausted from the solar-gas collector are hot (80°C) and carry a

considerable amount of thermal energy which could be later channeled in a heat exchanger and used in a small recovery dryer. This option allows to extend the capacity of the dryer. Acknowledgement This work was supported by the research institute for solar energy and new energies (IRESEN) as part of the project SSH. The authors are grateful to the IRESEN institute for its cooperation. References

38

Journal Pre-proof [1] Sami S, Etesami N, Rahimi A. Energy and exergy analysis of an indirect solar cabinet dryer

based

on

mathematical

modeling

results.

Energy

2011;

36:

2847–2855.

DOI:10.1016/j.energy.2011.02.027. [2] El Hage H, Herez A, Ramadan M, Bazzi H, Khaled M. An investigation on solar drying: A

review with economic and environmental assessment. Energy 2018; 157: 815-829. DOI: 10.1016/j.energy.2018.05.197 [3] Atalay H. Performance analysis of a solar dryer integrated with the packed bed thermal energy

storage

(TES)

system.

Energy

2019;

172:

1037-1052.

DOI:

10.1016/j.energy.2019.02.023. [4] Tarigan E. Mathematical modeling and simulation of a solar agricultural dryer with backup biomass burner and thermal storage. Case Stud. Therm. Eng 2018; 12:149–165. DOI:10.1016/j.csite.2018.04.012. [5] Shobhana S, Subodh K. Solar drying for different test conditions: proposed framework for estimation of specific energy consumption and CO2 emission mitigation. Energy 2013; 51: 27-36. DOI: 10.1016/j.energy.2013.01.006. [6] Atalay H, Turhan Coban M, Kincay O. Modeling of the drying process of apple slices: Application with a solar dryer and the thermal energy storage system. Energy 2017; 134: 382391. DOI: 10.1016/j.energy.2017.06.030. [7] Akbulut A, Durmus A. Energy and exergy analysis of thin layer drying of mulberry in a forced solar dryer. Energy 2010; 35: 1754-1763. DOI: 10.1016/j.energy.2009.12.028. [8] Zoukit A, El Ferouali H, Salhi I, Doubabi S, El Kilali T, Abdenouri N. Control of a solar dryer using a hybrid solar-gas collector. Proc. 2016 Int. Conf. Electr. Sci. Technol. Maghreb, Cist. 2016. DOI: 10.1109/CISTEM.2016.8066824.

39

Journal Pre-proof [9] El Khadraoui A, Bouadila S, Kooli S, Farhat A, Guizani A. Thermal behavior of indirect solar dryer : Nocturnal usage of solar air collector with PCM. J. Clean. Prod 2017; 148:37–48. DOI:10.1016/j.jclepro.2017.01.149. [10] Zoukit A, El Ferouali H, Salhi I, Doubabi S, Abdenouri N. Takagi Sugeno fuzzy modeling of an indirect solar dryer operated in both natural and forced convection. Renewable energy 2019; 133: 849-860. DOI:10.1016/j.renene.2018.10.082. [11] El Ferouali H, Zoukit A, Zehhar N, Benkhalti F, Bouamama H, Doubabi S, Abdenouri, N. Solar drying, hygroscopic equilibrium and biochemical quality of Punica granatum Legrelliae’s

flowers.

J.

Appl.

Bot.

Food

Qual

2018;

91:

14–23.

DOI:

10.5073/JABFQ.2018.091.003. [12] El Ferouali H, Zoukit A, Salhi I, Doubabi S, Abdenouri N. Optimization study and design of a hybrid solar-electric dryer suitable to the developing countries context. Solar for Africa and Renewable Energies Journal (SARE) 2019; 1: 35-39. [13] Boughali S,Benmoussa H, Bouchekima B, Mennouche D, Bouguettaia H, BechkiD. Crop drying by indirect active hybrid solar - Electrical dryer in the eastern Algerian Septentrional Sahara. Sol. Energy 2009; 83:2223–2232. DOI:10.1016/j.solener.2009.09.006. [14] Yassen TA, Al-Kayiem HH. Experimental investigation and evaluation of hybrid solar/thermal dryer combined with supplementary recovery dryer. Sol. Energy 2016; 134:284–293. DOI: 10.1016/j.solener.2016.05.011. [15] Prasad J, Vijay V.K, Tiwari G.N, Sorayan VPS. Study on performance evaluation of hybrid drier for turmeric (Curcuma longa L.) drying at village scale. J. Food Eng 2006; 75:497–502. DOI: 10.1016/j.jfoodeng.2005.04.061. [16] Abunde Neba F, Jiokap Nono Y. Modeling and simulated design: A novel model and software of a solar-biomass hybrid dryer. Comput. Chem. Eng 2017; 104:128–140. DOI: 10.1016/j.compchemeng.2017.04.002.

40

Journal Pre-proof [17] Dhanushkodi, S., Wilson, V. H., & Sudhakar, K. (2017). Mathematical modeling of drying behavior of cashew in a solar biomass hybrid dryer. Resource-Efficient Technologies, (2016), 3(4):359-364. DOI: 10.1016/j.reffit.2016.12.002 [18] López-Vidaña EC, Méndez-Lagunas LL, Rodríguez-Ramírez J. Efficiency of a hybrid solar-gas dryer. Sol. Energy 2013; 93:23–31. DOI:10.1016/j.solener.2013.01.027. [19] Oueslaty H, Ben Mabrouk S, Marni A. Design and Installation of a Solar-gas tunnel dryer: Comparative experimental study of two scenarios of drying. In: Proceeding of The fifth International Renewable Energy Congress (IREC) 2014, 1–6, Hammamet, Tunis; 25-27 March, 2014. DOI: 10.1109/IREC.2014.6826970. [20] Farkas I, Seres I, Mészáros CS. Analytical and experimental study of a modular solar dryer. Renew. Energy 1999; 16:773–778. DOI: 10.1016/S0960-1481(98)00278-X. [21] Dejchanchaiwong R, Tirawanichakul Y, Tirawanichakul S, Tekasakul P. Single- phase and multiphase models for temperature and relative humidity calculations during forced convection in a rubber-sheet drying chamber. Maejo Int. J. Sci. Technol 2014; 8 (2):207-220. DOI: 10.14456/mijst.2014.29. [22] Tekasakul P, Dejchanchaiwong R, Tirawanichakul Y, Tirawanichakul S. Threedimensional numerical modeling of heat and moisture transfer in natural rubber sheet drying process. Drying Technology 2015; 33:1124-1137. DOI: 10.1080/07373937.2015.1014910. [23] Bisoniya T.S, Kumar A, Baredar P. Energy metrics of earth-air heat exchanger system for hot and dry climatic conditions of India. Energy and Buildings 2015; (86):214-221. DOI: 10.1016/j.enbuild.2014.10.012. [24] Mirade P.S, Daudin J.D. A numerical study of the airflow patterns in a sausage dryer. Drying Technology 2000; (18):81-97. DOI: 10.1080/07373930008917694.

41

Journal Pre-proof [25] Tekasakul P, Promtong M, Energy efficiency enhancement of natural rubber smoking process by flow improvement using a CFD technique. Appl. Energy 2008;85(9):878-895. DOI:10.1016/j.apenergy.2008.02.004. [26] Sonthikun S, Chairat P, Fardsin K, Kirirat P, Kumar A, Tekasakul P. Computational fluid dynamic analysis of innovative design of solar-biomass hybrid dryer: An experimental validation. Renewable Energy 2016; 92:185–191. DOI: 10.1016/j.renene.2016.01.095. [27] Tiwari S, Tiwari G. N. Exergoeconomic analysis of photovoltaic-thermal (PVT) mixed mode

greenhouse

solar

dryer.

Energy

2016,

114:

155–164.

DOI:

10.1016/j.energy.2016.07.132. [28] Tiwari S, Tiwari, G. N. Energy and exergy analysis of a mixed-mode greenhouse-type solar dryer, integrated with partially covered N-PVT air collector. Energy 2017; 128: 183– 195. DOI: 10.1016/j.energy.2017.04.022. [29] Tiwari, S., Agrawal, S., & Tiwari, G. N. PVT air collector integrated greenhouse dryers. Renewable

and

Sustainable

Energy

Reviews

2018;

90:

142–159.

DOI:

10.1016/j.rser.2018.03.043. [30] El Ferouali H, Zoukit A, Salhi I, El Kilali T, Doubabi S, Abdenouri N. Thermal efficiency and exergy enhancement of solar air heaters, comparative study and experimental investigation. J. Renew. Sustain. Energy 2018; 10:43709. DOI:10.1063/1.5039306. [31] Xiaochun Zhang, Longhua Hu, Wei Zhu, Xiaolei Zhang, Lizhong Yang. Flame extension length and temperature profile in thermal impinging flow of buoyant round jet upon a horizontal

plate.

Applied

Thermal

Engineering

2014.

73

(1):

15-22.

DOI:

10.1016/j.applthermaleng.2014.07.016. [32] Morad M.R, Momeni A, Ebrahimi Fordoei E, Ashjaee, M. Experimental and numerical study on heat transfer characteristics for methane/air flame impinging on a flat surface.

42

Journal Pre-proof International

Journal

of

Thermal

Sciences

2016.

110:

Pages

229-240.

DOI:

10.1016/j.ijthermalsci.2016.07.005. [33] Remie, M. Sarner, G. G. Cremers, M.F.G. Omrane, A. Schreel, K.R.A.M Alden, L.E.M. De Goey, L.P.H. Heat-transfer distribution for an impinging, laminar flame jet to a flat plate. Int. J. Heat Mass Transf 2008; 51: 3144-3152. DOI: 10.1016/j.ijheatmasstransfer.2007.08.036. [34] Vijaykumar, H. Rajendra, P. Vedula, S. Prabhu, V. Heat transfer distribution for impinging methane–air premixed flame jets. Applied Thermal Engineering 2014; 73:461-473, DOI: 10.1016/j.applthermaleng.2014.08.002. [35] Sushant, Sahu 1, Mahesh, D. Analysis of heat transfer characteristics of flame impinging to a plane surface perpendicular to flame jet axis. International Journal of Engineering Research & Technology (IJERT) 2013; 2: 578-593. [36] Baukal, C.E. Gebhart, B. A review of empirical flame impingement heat transfer correlations. Int. J. Heat Fluid Flow 1996; 17: 386-396. DOI:10.1016/0142-727X(96)00003-3. [37] Baukal, C.E. Gebhart, B. A review of semi-analytical solutions for flame impingement heat transfer. Int. J. Heat Mass Transf 1996; 39: 2989-3002. DOI: 10.1016/0142-727X(96)00003-3. [38] Subhash, Chander. Anjan, Ray. Flame impingement heat transfer: A review. Energy Conversion and Management 2005; 46: 18–19. DOI: 10.1016/j.enconman.2005.01.011.

43

Journal Pre-proof AUTHOR DECLARATION We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property. We understand that the Corresponding Author is the sole contact for the Editorial process (including Editorial Manager and direct communications with the office). He is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs. We confirm that we have provided a current, correct email address which is accessible by the Corresponding Author and which has been configured to accept email from. Signed by all authors as follows: - Ahmed ZOUKIT: March 20th, 2019 - Hicham EL FEROUALI: March 20th, 2019 - Issam SALHI: March 20th, 2019 - Said DOUBABI: March 20th, 2019 - Naji ABDENOURI: March 20th, 2019

1

Journal Pre-proof

Authorship statements Simulation, design and experimental performance evaluation of an innovative hybrid solar-gas dryer Ahmed ZOUKIT, Hicham EL FEROUALI, Issam SALHI, Said DOUBABI, Naji ABDENOURI* Laboratory of Electric Systems and Telecommunications, Cadi Ayyad University BP 549, Av Abdelkarim Elkhattabi, Gueliz, Marrakesh, Morocco. *[email protected]

Ahmed ZOUKIT, Hicham EL FEROUALI Said DOUBABI, Naji ABDENOURI: Conceptualization. -

Ahmed ZOUKIT, Hicham EL FEROUALI, Issam SALHI: Data curation.

-

Issam SALHI, Said DOUBABI, Naji ABDENOURI: Formal analysis.

-

Ahmed ZOUKIT, Hicham EL FEROUALI: Funding acquisition.

Ahmed ZOUKIT, Hicham EL FEROUALI, Issam SALHI, Said DOUBABI, Naji ABDENOURI: Investigation. -

Issam SALHI, Said DOUBABI, Naji ABDENOURI: Methodology.

-

Naji ABDENOURI: Project administration.

-

Said DOUBABI, Naji ABDENOURI: Resources.

-

Ahmed ZOUKIT, Hicham EL FEROUALI: Software.

-

Issam SALHI, Said DOUBABI, Naji ABDENOURI: Supervision.

-

Issam SALHI, Said DOUBABI, Naji ABDENOURI: Validation.

-

Ahmed ZOUKIT, Hicham EL FEROUALI: Visualization.

Ahmed ZOUKIT, Hicham EL FEROUALI, Issam SALHI, Naji ABDENOURI: Roles/Writing - original draft. Ahmed ZOUKIT, Hicham EL FEROUALI, Issam SALHI, Naji ABDENOURI: Writing - review & editing.

Journal Pre-proof

Highlights  An innovative hybrid solar-gas dryer is considered.  There is an indirect heating of drying air instead of direct heating.  An auxiliary gas heating system, placed under the solar collector, is used.  CFD modeling and simulation of the proposed hybrid dryer operated in forced convection is established.  The dryer‘s thermal performances are investigated.