Design and experimental investigation of adryer with two types of combined solar concentratorsusing two natural heattransfer fluids

In this experimental study, a new solar dryer has been designed and manufactured by combining a concave solar concentrator with a series of convergent lenses whose concentrated radiation are reflected on the same absorber. Our main goal for this combination is to reduce the thermal losses by increasing the receiver bottom temperature. In our dryer, the receiver design uses air and water as the heat transfer fluid; these two fluids are heated and sent a heat exchanger to raise the drying air temperature. Furthermore, in order to improve the thermal performance, our approach is based on some techniques such as; the combination of two types of solar concentrators, simultaneous use of two natural heat transfer fluids, tilting concentrators, the insertion of obstacles at the absorber, and a specific solar tracker for explosive atmosphere. Our dryer has been tested for the first time in gas filling plant in Morocco for drying painted gas cylinders. The experimental results show that compared with flat-plate solar collector, the proposed dryer can significantly improve the performance in terms of air drying temperature. In fact, our solar dryer has reduced the drying time from 420 seconds to just 40 seconds and has improved drying air the temperature from 40 °C to 65 °C. Also, the health risks of workers have been reduced and the number of painted bottles has been increased by more than 43%.


I. INTRODUCTION
The most important property of the solar energy is a renewable energy resource. The average annual sun exposure in Morocco is about 2500 hours; it receives a daily solar energy of 16.2 to 27MJ/m² [1]. The drying consumes about 12 to 25% of the total primary energy demand [2]. The solar energy is an area to study extremely in drying due to they are easily applicable.Therefore,this energy appears to be the energy of the future [3,4].One of the advantages of this energy is its aptitude to be transformed into heat. The solar collector constitutes the major component of any solar system [5].Hot air is obtained from these collectors, and they are used in space heating [6,7], product drying [8,9], greenhouse heating [10] and pre-heating in ventilation systems [11,12].
The parabolic solar concentrator is often used with systems for solar absorption refrigeration that can achieve temperatures over 100°C [13,14].Thus, the environmental problems related to the emission of greenhouse gases will be reduced [15].
The activity of painting gas cylinders is expanding in Morocco. The company, Salam Gas, recorded an increase in this activity of 1700%. Despite,this important evolution, drying is done naturally and at ambient temperature. However, this traditional drying generates two major drawbacks. First, the customer expectations in terms of time, quality and quantity are not met. Second, in terms of employee health, exposure to high concentrations of solvents can cause loss of medium-term and long-term knowledge, causing behavioral changes, affecting mood and memory [16,17].
To overcome these drawbacks, in this work and for the first time in this industry, we propose a solar dryer by combining a concave solar concentrator with a series of convergent lenses whose concentrated radiation are reflected on the same absorber. Solar drying systems must be properly designed to meet the drying requirements, such as product quality and drying time. Several researchers have developed simulation models for natural convection and forced convection systems [18]. Solar air collectors are simple devices for air heating by utilizing solar energy for many applications that require low to moderate temperatures below 60°C, such as drying and space heating. The principal types were classified as one pass, double duct, and two passes [19]. Recently, studies covering different types of collectors have been undertaken by several researchers. Hot air generation and drying applications were examined with their designs [20,21]. A single-pass solar air collector [22] and a double-pass solar air collector [23,24] were designed and experimentally analyzed for their performance.
In this paper, we present the experimental studies and design of a new solar dryer by using acombined parabolicsolar concentrator with a series of convergent lenses and using a specific solar trackerfor explosive atmosphere. Our main goal for this combination is to reduceheat losses at the absorber by adding convergent lenses end has to act on the temperature difference between the bottom of the receiver and the planar absorber.The idea is to increase the receiver bottom temperature by the additional thermal energy lenses.The receiver design uses air and water as the heat transfer fluid; these two fluids are heated and sent a heat exchanger to raise the drying air temperature. According to Benghanem [25], both the orientation and tilt angles have significant effects on the magnitude of the solar radiation reaching the surface of a collector through solar tracker and forced convection. Also, to improve thermal performance and achieve air temperature that can ensure rapid drying, we used three techniques, first, we combined parabolicsolar concentrator and a series of convergent lenses for the same absorber, wealso used twonatural heat transfers fluids water and air at the same time, so these two fluids are heated and sent a heat exchangerto raise the drying air temperature. Second, the insertion of the obstacles inside the absorber, in addition while tilting the concentrators according to the latitude angle, and finally, the solar tracker realizedis specific to the explosive environment by using a pneumatic motor and a pneumatic control. Several experiments were conducted on a prototype designed and fabricated in a gas filling plant in Oujda (Morocco).The experimental results are provided and are compared with the performance of the dryer with flat collector [26].

A. System Design
In this study, we used a solar dryer equipped with acombined parabolic solar concentrator and a series of convergent lenses, forced convection, and specific solar trackingfor an explosive atmosphere zone.Maximum power into the solar collector occurs when the surface is normal to the incident solar radiation.However, it is not always possible with fixed solar collector, since the relative position of the earth to the sun varies [27].This system ( Fig.1, Fig.2 and Fig.3)was designed, fabricated and tested in a gas filling plant by the laboratoryin the Engineers National School(Mohamed First University Oujda, Morocco). The solar dryer comprised a concave solar concentratorhaving an area 6m 2 (3m x 2m) and 27 convergent lensesfor heating theair and waterat the same time. The receiver isconnected to a drying unitthroughacentrifugal fan with an airflow rate up to 2000m 3 /h (Fig.4,Tab.1). The absorber tube was fixed on galvanized sheet metal;obstacles were inserted throughout the absorber tube for improved air heat exchange. The absorberwas coated with black paint to absorb the incident solar radiation. Thedrying system used was the subject of some improvements, in particular, the insertion of obstacles on both sides of the absorber (Fig.5). The air is preheated at the absorber, afterits temperaturewill be increased at the exchanger (steam/air)at the exit of the receiver. The concave solar concentrator was tilted to an angle about 34° horizontally, which is considered an optimum angle for year-round performance of the system at Oujda [1].When solar rays hit a surface at an oblique angle, the rays are more spread out [28]. The system was oriented to face the east to maximize the incident solar radiation on the collector. The automatic operation of the dryer with the beginning of the painting cycle has led to the reduction of heat loss and improving the dryer thermal performance.A detailed numerical and experimental analysis of such an optical design system with 3m focal length and 1.8m effective aperture was performed in a previous study.

B. Hot Air/Water Production Unit
The collector is orientated in a north-south direction and tracking the sun from east to west. The stainless reflector surface of 6 m² focuses the sun's radiation ona receiver canal, called a solar absorber, along the focal line of reflector length of 3 m. A series of 27 convergent lenses focus the radiation on the opposite part of the same receiver.
The absorber channel, having a rectangular shape, was made in galvanized sheet on which was fixed the water tube and was inserted crosswise ( fig.05) and covered with a layer of black paint. The bottom of the absorbing channel is ordinary glass, allowing passage of the concentrated solar radiation to the focal plane within the channel.
Some modifications have been implemented to the absorber;the thermal oil has been replaced by air and water as HTF (heat transfer fluid), which are free of cost, nonpolluting, and the air has practically no operating temperature limit. The cross-section of the solar air/water receiver is shown schematically (fig.5).The new receiver implements a longitudinal flow design, where the temperature gain is obtained at the passage of air between obstacles, which circulates hot water.The hot air channel has a dimension of 150mm x 50mm with a face glass.
The air is preheated at the absorber with two concentrators and then it will be sent to the air/water heat exchanger ( fig.4)for superheating to a temperature of about 70°C.

C. Experimental Procedure
Painted cylinders transported by a motorized chain conveyor enter one by one into the drying unit. The fan is operated automatically with a speed adjusted to the optimum air during the drying experiments ( fig.1).The air is preheated the first time during its passage through the obstacles fixed on the planar absorber and the hot water tube, and then these two natural fluids (air and water) will be sent to a heat exchanger to raise the temperature of the drying air up to 70 °C. The air charged with solvents is discharged outside of the working environment using the paint booth extractor ( fig.1). Quality paint has been tested manually and with the specific tape.
The hot water then passes through a condenser and circulates in a closed circuit by thermo-siphon-induced flow.The height position of the condenser, relative to the concentrator, ensures free passage of water in the absorber without the need to use a circulation pump rate in the solar receiver.The condenser is equipped with a safety valve, a pressure gauge, and a tap for the makeup water.
The solar tracker used is specific to the explosive medium using a pneumatic motor and a pneumatic control.The control circuit is pneumatic and the limit has been provided at the beginning and end of the work day. Timers are adjustable, depending on the season,with the opportunity to order a manually rotating hub( fig.6).
The timing for drying should be equal or inferior to 40 seconds to obtain the desired result. The solvents content in each cylinder is about 0.01 liter. The measurement of temperatures, solar radiation, and air speed is done every 15minutes. The drying operation starts at 08:30 and stops at 15:30. Qu Qa Qp (4) Thermal losses reduce the energy received in the opening of a cylindrical parabolic concentrator; the power consumption is expressed as follows,Qp: Power corresponding to thermal losses Qa Ic. Sr. τ. α. ρ (5) The losses Qp.rat the receiver are determined by the expression: Qp.r hcv . Sr . Tr Ta Impact of the S een in Fig.9 ure has impro ovement of th peak sunlight average dryin ve and radiativ igh in addition erature.

Simultaneous
and Fig.10